Flexible Molybdenum Disulfide (MoS2) Atomic Layers for Wearable

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Review 2

Flexible Molybdenum Disulfide (MoS) Atomic Layers for Wearable Electronics and Optoelectronics Eric Singh, Pragya Singh, Ki Seok Kim, Geun Young Yeom, and Hari S. Nalwa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19859 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

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ACS Applied Materials & Interfaces

Flexible Molybdenum Disulfide (MoS2) Atomic Layers for Wearable Electronics and Optoelectronics Eric Singh,† Pragya Singh,‡ Ki Seok Kim,§ Geun Young Yeom,*,§,# and Hari Singh Nalwa*,⊥ †Department

of Computer Science, Stanford University, Stanford, California 94305, United

States ‡Department

of Electrical Engineering and Computer Science, National Chiao Tung University,

Hsinchu 30010, Taiwan (R.O.C.) §School

of Advanced Materials Science and Engineering, Sungkyunkwan University,

2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 16419, South Korea #SKKU

Advanced Institute of Nano Technology, Sungkyunkwan University,

2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 16419, South Korea ⊥Advanced

Technology Research, 26650 The Old Road, Suite 208, Valencia, California 91381,

United States

Corresponding Authors

*E-mail: [email protected] (G. Y. Yeom). *E-mail: [email protected] (H. S. Nalwa).

1

ACS Paragon Plus Environment

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ABSTRACT Flexible, stretchable, and bendable materials, including inorganic semiconductors, organic polymers, graphene, and transition metal dichalcogenides (TMDs), are attracting great attention in such areas as wearable electronics, biomedical technologies, foldable displays, and wearable pointof-care biosensors for healthcare. Among a broad range of layered TMDs, atomically-thin layered molybdenum disulfide (MoS2) has been of particular interest, due to its exceptional electronic properties, including tunable bandgap and charge carrier mobility. MoS2 atomic layers can be used as a channel or a gate dielectric for fabricating atomically-thin field-effect transistors (FETs) for electronic and optoelectronic devices. This review briefly introduces the processing and spectroscopic characterization of large-area MoS2 atomically-thin layers. The review summarizes the different strategies in enhancing the charge carrier mobility and switching speed of MoS2 FETs by integrating high- dielectrics, encapsulating layers, and other 2D van der Waals layered materials into flexible MoS2 device structures. The photoluminescence (PL) of MoS2 atomic layers has, after chemical treatment, been dramatically improved to near-unity quantum yield. Ultra-flexible and wearable active-matrix organic light-emitting diode (AM-OLED) displays and wafer-scale flexible resistive random-access memory (RRAM) arrays have been assembled using flexible MoS2 transistors. The review discusses the overall recent progress made in developing MoS2 based flexible FETs, OLED displays, non-volatile memory (NVM) devices, piezoelectric nanogenerators (PNGs), and sensors for wearable electronic and optoelectronic devices. Finally, it outlines the perspectives and tremendous opportunities offered by a large family of atomically-thin-layered TMDs. KEYWORDS: molybdenum disulfide (MoS2), flexible electronics, flexible field-effect transistors (FETs), wearable organic light-emitting diode (OLED), flexible memory devices, flexible piezoelectric nanogenerators (PNGs), sensors 2


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TABLE OF CONTENTS 1. INTRODUCTION 2. LARGE-AREA MoS2 ATOMIC LAYERS 3. MoS2 ATOMIC LAYERS FOR FLEXIBLE ELECTRONICS 4. MoS2-BASED FETs 5. STRATEGIES FOR BOOSTING THE FIELD-EFFECT MOBILITY OF MOS2 FETs 5.1 EFFECT OF CHEMICAL DOPING ON MOBILITY 5.2 EFFECT OF CONTACT ELECTRODES ON MOBILITY 5.3 EFFECT OF ORGANIC GATE DIELECTRICS ON MOBILITY 5.4 EFFECT OF HIGH- DIELECTRICS ON MOBILITY 5.5 EFFECT OF 2D/2D CONTACTS ON MOBILITY 6. MoS2-BASED FLEXIBLE FETs 7. MoS2-BASED WEARABLE OLED DISPLAYS 8. MoS2-BASED FLEXIBLE MEMORY DEVICES 9. MoS2-BASED FLEXIBLE PIEZOELECTRIC NANOGENERATORS 10. MoS2 FET-BASED FLEXIBLE SENSORS 11. CONCLUSION AND PERSPECTIVE

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1. INTRODUCTION After the mechanical exfoliation of atomic-thick layered graphene in 2004,1 another analogous class of atomically-thin layered materials 2D materials that include TMDs,2−13 transition-metal carbides,

nitrides,

and

carbonitrides

referred

to

as

MXenes,14−17

black

phosphorus/phosphorene,18−22 hexagonal boron nitride (h-BN),23−26 borophene,27−29 silicene,30−33 germanene,30,

33−35

stanene,36−40 and graphynes and graphdyines,41−45 has emerged that show

promising applications from nanoelectronics to nanomedicine. Here phosphorene, borophene, silicene, germanene, and stanene are monoelemental analogues of 2D graphene. Atomically-thin monolayer and few-layer TMDs [MX2 =X–M–X, where M refers to a transition metal atom, such as Mo, W, Zr, Hf, Nb, Ta, Ti, Ni, V, and Re, and X is a chalcogen atom, either S, Se, or Te] exhibit a very interesting and wide range of electrical properties from semiconducting (MoS2, MoSe2, WS2, and WSe2), to semimetallic (MoTe2, WTe2, and TiSe2), to metallic (NbS2, TiS2, NiS2, and VSe2), to superconducting (NbSe2 and TaS2) regimes.46−50 One of the major differences is that TMDs have sizable bandgaps compared to the zero bandgap in graphene, which property makes TMDs more interesting and suitable for the development of new electronic and optoelectronic devices. The h-BN, a wide bandgap (5.9 eV) layered 2D material, received much attention as a gate dielectric for graphene-based electronic devices, due to its super smooth surface, low dielectric constant, and high thermal and mechanical stability.51−55 The large-area graphene electronic devices fabricated on the h-BN substrate were found to exhibit several-fold larger carrier mobilities and better device performance, compared to graphene electronics devices on silicon dioxide (SiO2) substrates.24, 56−60 Monolayer to few-layered TMDs have been prepared by a number of methods, including micromechanical exfoliation by peeling off atomic layers from their corresponding bulk crystals,61, 62 consecutively depositing layer-on-layer by the chemical vapor 4


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deposition (CVD) process,63 chemical route,64 isolating layers through lithium intercalation,65 and liquid exfoliation of layers in organic solvents under the sonification process.66−68 The various properties of TMDs depend upon the number of layers, and their nanostructures and morphologies, which are important for application viewpoints.69, 70 Flexible electronic and optoelectronic devices are becoming important, and include wearable biosensors for monitoring vital signs and physical activities in the healthcare industry,71−77 foldable displays,78,

79

bioinspired prosthetics,80,

81

and soft robotics.82−86 Therefore, along with the

emergence of the Internet of things (IoT) technology, the market for wearable consumer electronic products, such as smartwatches and point-of-care biosensors for healthcare, is continuously growing around the world, and is worth billions of dollars. FETs-based products have been commonly used for fabricating wearable large-area electronic devices and sensors on flexible polymer substrates or integrated into textile clothing. For example, an electronic artificial skin was developed from super-flexible organic FETs, where all thin films were made of light-weight soft materials, except the electrodes.87 The bendable and stretchable materials for wearable electronics include fiber-based electronics,88, 89 textile-based triboelectric nanogenerators,90, 91 light-emitting diodes (LEDs) from conducting polymers92 and fullerene nanocomposites,93 ionic gel paper for electroluminescent devices,94 ultra-high frequency diodes from silicon/cellulose nanocomposite,95 bendable solar cells,96, 97 polypyrrole-based artificial muscle,98 elastomeric robotic tentacles99 and prosthetic tactile,100 CNT-based yarns for sensors101 and supercapacitors,102 silver nanowires (AgNWs)/polymer composites for smart clothing103 and gold nanowires (AuNWs) for pressure sensors,104 and textile-based electrochemical sensors,105 carbon nanoparticles coated textile fiber/Ecoflex composite-based wearable strain sensors and heaters,106 hydrogel for all-gel-state supercapacitors,107 polypyrrole-MnO2-coated CNT-based textile supercapacitors,108 Cu@Cu4Ni 5



nanowires/polydimethylsiloxane (PDMS) composites for OLEDs,109 textile-based microbial fuel cells,110 inorganic111, 112 and organic semiconductors,113−115 and flexible and stretchable electronics. Graphene, graphene oxide (GO), reduced graphene oxide (rGO) and their based nanocomposites with metallic nanoparticles, conducting polymers, fabrics, CNTs, metal oxides, etc., have been studied for fabricating wearable electronic devices including flexible FETs,106−121 displays,122−124 memory devices,125 strain sensors,126−130 gas and chemical sensors,131−135 radio frequency identification (RFID) tag sensors for IoT,136 supercapacitors,137−150 solar cells,151−161 nanogenerators,162−166 electromagnetic interference shielding,167−168 and lithium rechargeable batteries.169,170 While similar types of device applications in flexible and wearable electronics have been envisaged for the atomically-thin TMDs, TMDs have a long way to go to become the equivalent counterparts of what have been accomplished for graphene-based materials in the field of flexible and wearable electronics over the last decade. Among the diverse range of TMDs, rapid progress has been made in studying atomically-thin-layered MoS2, due to its excellent electrical, optical, mechanical, and thermal properties, in order to integrate these unique properties of MoS2 thin film into the next generation of electronic and optoelectronic devices. In this review, we briefly introduce MoS2 atomically-thin-layered film from its processing, to spectroscopic characterization, to potential applications. The large-area flexible MoS2 nanosheets prepared by roll-to-roll transfer are discussed. The various important properties of thin-layered MoS2 and graphene have been compared in view of their roles in developing flexible and wearable electronics and optoelectronics. The strategies for significantly improving the charge carrier mobility of MoS2 FETs by integration of high- dielectrics, as well as enhancing the PL quantum yield of MoS2 atomic layers after chemical treatment, are analyzed. The research articles published on flexible MoS2 atomically-thin layers remain scattered throughout the literature; therefore, this review 6


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article specifically summarizes the flexible MoS2 atomic layers that have been used for developing wearable electronic and optoelectronic devices that include flexible FETs, OLED displays, RRAM devices, PNGs, and sensors. The review is concluded with an outlook and future challenges for fabricating novel electronic devices from the vast variety of atomically-thin-layered TMDs.

2. LARGE-AREA MoS2 ATOMIC LAYERS The emergence of high-resolution microscopic techniques, such as scanning electron microscope (SEM), transmission electron microscope (TEM), and atomic force microscopy (AFM), has made it possible to visualize materials and their based devices at nanoscale. This has opened tremendous possibilities over the past two decades of using diverse nanomaterials, such as organic conjugated polymers, CNTs, metal nanoparticles, graphene, TMDs, and the vast variety of inorganic/organic hybrid nanocomposites based on them, in research fields ranging from nanoenergy, to nanoelectronics, to nanomedicines.171−175 Atomically-thin MoS2 nanosheets have been prepared on different types of rigid and flexible substrates using mechanical exfoliation,61,

176, 177

electrochemical exfoliation,65, 178 thermal vapor sulfurization of Mo metal179−181 or CVD-growth using molybdenum oxide (MoO3),182−187 and by the decomposition of thiomolybdates.188−190 Liu et al.191 prepared atomically-thin-layered MoS2 nanosheets using the electrochemical exfoliation of bulk MoS2 crystals. The electrochemically exfoliated MoS2 nanosheets showed lateral sizes in the (5–50) μm range, much larger compared with liquid-phase, mechanically, and chemically exfoliated MoS2 nanosheets. The back-gate FET fabricated with monolayer MoS2 nanosheet showed the field-effect mobility (μFE) of 1.2 cm2/Vs and on/off current ratio of 106, comparable to MoS2 nanosheets prepared by micromechanical exfoliation method. Layer controlled CVD is one of the most suitable techniques for preparing large-area MoS2 nanosheets for electronics. Among 7



all TMDs, overall, MoS2, WS2, MoSe2, and WSe2 have received much attention, where MoS2 specifically is one of the most studied TMDs for electronics and otherwise. Tongay et al.192 prepared monolayer MoS2, MoSe2, and WSe2 nanosheets by mechanically exfoliating flakes from their bulk crystals. The monolayers of MoS2, MoSe2, and WSe2 were characterized by Raman spectroscopy, AFM, and PL spectroscopy methods (Fig. 1). The height of about 0.7 nm obtained by AFM images confirmed monolayers of (1L) MoS2, MoSe2, and WSe2. Raman spectra of monolayer MoS2, MoSe2, and WSe2 differ with their corresponding few-layers. The intensity of PL peaks was found to be dramatically enhanced from the bulk crystals to monolayer MoS2, MoSe2, and WSe2, because of the indirect-to-direct bandgap transition. A strong PL peak was observed at 1.84 eV for monolayer MoS2, 1.56 eV for MoSe2, and 1.65 eV for WSe2 at room temperature (RT).

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Figure 1. (a) AFM images of mechanically exfoliated monolayer MoS2, MoSe2, and WSe2. (b) Raman spectra of monolayer MoS2, MoSe2, and WSe2 where the solid lines represent monolayers and the dashed lines correspond to few-layers (c) Normalized PL spectra of monolayer MoS2, MoSe2, and WSe2 record at room temperature. Reprinted with permission from ref 192. Copyright 2013 Nature Publishing Group.

The optical and electrical properties of MoS2 films can be tailored by chemical doping. Neupane et al.193 reported the effect of methanol doping on the optical and electrical properties of monolayer TMDs. Figure 2 shows the PL intensity maps, and PL and Raman spectra of mechanically exfoliated pristine (0 h) and 16 h methanol treated monolayer (1L), bilayer (2L), and trilayer (3L) MoS2 films. The monolayer, bilayer, and trilayer thickness of MoS2 films were determined by analyzing the frequency difference (∆) between the E12g and A1g vibration modes in Raman spectra. A redshift of the PL peak was observed in the 16 h methanol-treated MoS2 films, which ranged from (7 to 4 nm) in 1L-MoS2 to 3L-MoS2. The Raman spectra of 16 h methanoltreated MoS2 films showed redshifts of the A1g vibrational peaks by (1.0, 0.7, and 0.5) cm−1 from the (1, 2, and 3)L domains, respectively. Interestingly the size of the peak shifts in both PL and Raman spectra of 16 h methanol-treated MoS2 films were found to be lower for mechanically exfoliated 1L-MoS2 compared with CVD-grown 1L-MoS2, arising from the smaller number of structural defects in exfoliated 1L-MoS2 than CVD-grown 1L-MoS2 film. A redshift of the A1g Raman mode by ~1.5 cm−1 was observed in 16 h methanol-treated 1L-MoS2, compared with pristine 1L-MoS2 film. In addition to PL and Raman spectra, the effects of methanol doping have also been evidenced by imaging and the field-effect mobility of 1L-MoS2 due to reduced structural defects.

9



Figure 2. (a) PL intensity maps of mechanically exfoliated one-layer (1L), two-layer (2L) and three-layer (3L) MoS2 films before and after methanol treatment for 16h. (b) Averaged PL spectra of pristine and 16h methanol-treated 1L, 2L, and 3L MoS2 (c) Averaged Raman spectra of pristine and methanol-treated 1L, 2L, and 3L MoS2 films. The dotted lines show the positions of the A peaks in PL spectra and A1g peaks in Raman spectra of the pristine MoS2 films. Reprinted with permission from ref 193. Copyright 2017 American Chemical Society.

The production of large-area flexible nanosheets of 2D materials is highly desirable for wearable electronics. Bae et al.194 prepared 30 inch flexible graphene films using roll-to-roll CVD method, which showed optical transparency of 97.4 % at 550 nm. In another attempt, Kobayashi et al.195 reported the production of 100 m-long flexible and transparent graphene films by the rollto-roll CVD method on a copper foil, and subsequently transferred graphene films onto a polyethylene terephthalate (PET) flexible substrate. The large-area layered MoS2 nanosheets are highly desirable for flexible electronic and optoelectronic devices, and attempts have been made by several research groups to prepare large-area monolayer and few-layer MoS2 nanosheets on different substrates using CVD, chemical synthesis, pulsed laser deposition, etc.196−201 Lim et al.202 reported roll-to-roll production of large-area MoS2 nanosheets onto flexible PET substrate, where 10


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the number of MoS2 layers was controlled by tuning the concentration of ammonium tetrathiomolybdate (NH4)2MoS4. Figure 3 shows the entire roll-to-roll transfer and production processing, photographs, optical transmittance, and Raman spectra of layer-controlled MoS2 nanosheets. The 50 cm long MoS2 nanosheets chemically synthesized on Ni foils showed optimum stoichiometry and uniformity of thickness for the entire nanosheet. The optical transmittance of roll-to-roll produced MoS2 layers was (91.8, 87.9, and 66.4) % for (0.2, 0.5, and 1.25) wt % (NH4)2MoS4 at 550 nm, respectively. Raman spectra and X-ray photoelectron spectroscopy (XPS) confirmed the chemical and structural features of MoS2 layers. The applications of MoS2 nanosheets in electronic and optoelectronic devices, as well as in hydrogen evolution catalysis, were demonstrated. The roll-to-roll produced MoS2-based FETs showed an electron mobility of 0.6 cm2/Vs, and a current on/off (IOn/IOff) ratio of 10³. The visible-light photodetectors developed from MoS2 showed photoresponsivity of 22 mA/W at 40 V. The MoS2 nanosheets on Ni foils showed catalytic activity having overpotential of 165 mV/dec. The production of large-area flexible MoS2 nanosheets will open tremendous opportunities for developing novel wearable electronic and optoelectronic devices.

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Figure 3. (a) Schematic illustration of the roll‐to‐roll transfer and production of layer‐controlled MoS2. (b) Photograph of layer-controlled MoS2 nanosheets transferred onto the flexible PET substrates where 0.2, 0.5 and 1.25 wt% of (NH4)2MoS4 was used. (c) Optical transmittance of layer-controlled MoS2 nanosheet transferred onto glass substrates prepared with different concentration of (NH4)2MoS4. Inset shows the transmittance different solution concentration at 550 nm. (d) Raman spectra of layer-controlled MoS2 nanosheet transferred onto a SiO2/Si substrate at 514 nm excitation wavelength. Inset is a photograph of MoS2 synthesized with 1.25 wt% of (NH4)2MoS4 transferred onto a SiO2/Si substrate. Reprinted with permission from ref 202. 12


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Copyright 2018 Wiley−VCH.

Like 2D graphene-based materials, monolayer and few-layer MoS2 thin films have been used in a very broad range of applications, including FETs,203−205 LEDs,206, 207 photodetectors,208−210 bulk-heterojunction

(BHJ),211

dye-sensitized

solar

cells,212

supercapacitors,213−215

photocatalysis,216−220 biosensors,221−225 chemical sensors,226−229 and lithium-ion batteries.230 Furthermore, MoS2 shows excellent electrical and transport properties, and has been use as a gate material. Atomically-thin layers of TMDs are also quite sensitive to trivial environmental changes due to their unique properties; therefore, their interactions and sensitivity toward other chemical and biological species, and response to pressure, light illumination, and human body motions can be utilized for developing different types of sensors. The sensitivity of TMDs can be further enhanced either by forming their hybrid nanocomposites with other synergistic materials, or conveniently attaching chemical and biological functionalities onto their edges and surfaces.

3. MoS2 ATOMIC LAYERS FOR FLEXIBLE ELECTRONICS The concepts and current understandings of the electronic, physical, and chemical properties of single-atom-layer graphene have been applied as a reference to experiment with other 2D atomically-layered materials, including TMDs, phosphorene, and MXenes. Table 1 compares the important optical, electrical, mechanical, and thermal properties of graphene231−240 and MoS2.241−255 Both atomically-thin-layered 2D graphene and MoS2 impart some general properties. An apparent difference is in the bandgap, which is zero for graphene,1, 231 while monolayer MoS2 is a direct bandgap semiconductor, having ~1.9 eV bandgap.241 The lack of bandgap in graphene 13



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is not beneficial from the viewpoint of electronic and optoelectronic applications, where MoS2 is a more interesting material for similar types of applications, due to its finite bandgap. Graphenebased photodetector shows a relatively low photoresponsivity of 0.5 mA/W because of its zero bandgap,233 compared with the ultrahigh photoresponsivity of 880 A/W at 561 nm wavelength for the monolayer MoS2,243 which of the monolayer MoS2-based photodetector is 106 higher than that of graphene, due to the existing direct bandgap. The direct bandgap of ~1.9 eV for monolayer MoS2 gives rise to high absorption coefficient; therefore, the monolayer MoS2 is highly sensitive to photons, and is a better light absorber than graphene.241 Graphene exhibits several orders of magnitude higher carrier mobility1, 231, 234 than that of layered MoS2 films.244−246 The Young’s modulus of monolayer graphene235 was found to be almost twice that of monolayer MoS2247 under similar nanoindentation measurements. The elastic bending modulus of 9.61 eV for monolayer MoS2248 is about 7-fold larger than the 1.4 eV for monolayer graphene,236 because monolayer MoS2 consists of a sulfur–molybdenum–sulfur (S-Mo-S) trilayer atomic structure (~0.65 nm) that inherits more atomic interactions than a single atomic layer graphene. The optical transmittance from a single atomic layer graphene corresponds to 97.7 %,232 while for a bilayer of MoS2, it is 96.7 %.242 The thermal conductivity of single- and few-layer graphene varies from (1,500–5,300) W/mK,237,

238

comparatively much higher than that of thin-layered MoS2.249,

250

The thermal

stability up to 1,050 oC252 and refractive index of 4.49 at 651 nm253−255 of MoS2 are much higher than those of graphene. The structure-property relationships, electronic, optoelectronic, carrier dynamics, and mechanical properties of 2D materials, including graphene and TMDs, have been analyzed and compared in some of the recently published reviews.256−260

Table 1. A comparison of the optical, electrical, mechanical and thermal properties of atomically 14


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thin layered MoS2 and graphene. Properties Bandgap (eV) Optical Transparency (%) Optical Absorption Mobility (cm2/Vs) Young Modulus (GPa) Breaking Strength (GPa) Bending Modulus (eV)

Graphene 0 (Dirac cone) 97.7 0.5 mA/W (2~3.5)×105 1000 130 1.4 Thermal Conductivity (W/mK) 5300 1500 (few-layer) Electrical Conductivity (S/cm) 2000 Thermal Stability (oC) 601 Refractive index 3.135 (670 nm)

ref. 1, 231 232 233 231, 234 235 235 236 237 238 239 239 240

MoS2 1.8 (parabolic) 96.7 (2L) 880 A/W 10-500 270 16-30 9.61 34.5 (RT) 52 (few-layer) 0.1-1(few-layer) 1050 4.49 (651 nm)

ref. 241 242 243 244−246 247 247 248 249 250 251 252 253−255

The synergistic effect between high flexibility, bendability, and durability of polymer substrates, as well as the desired electrical and mechanical properties of materials, determines the performance of flexible devices for wearable electronics.261−265 Organic polymers, such as polyimide (PI), PET, poly(ethylene napthalate) (PEN), PDMS, polycarbonate (PC), latex rubber, polyurethane (PU), etc., inherently possess high mechanical flexibility and stretchability; therefore, these flexible polymers have been commonly used as thin-film substrates for fabricating flexible electronic devices, because of their light-weight, smooth surface morphology, high thermal stability, mechanical durability, optical transparency, and chemical inertness. In addition, poly(vinylidene fluoride) PVDF, teflon, polyester, nylon, PU, textiles, and different types of papers have also been used as flexible substrates for developing flexible, stretchable, and foldable wearable electronic devices. For example, stretchability as high as 1,000 % has been reported for the vinyltriethoxysilane nanoparticles (VS-NPs)/polyacrylamide (PAM) hydrogel electrolytebased supercapacitors.266 Stretchability of 350 % for AuNWs/latex rubber,267 500 % for multiwalled CNTs (MWCNTs)/Ecoflex,268 and 800 % for graphene/rubber269 has been reported for 15



nanomaterials/flexible polymer nanocomposites, which is motivating the development of flexible and stretchable sensors for wearable electronics. The high flexibility and stretchability of the polymer substrates and their strong binding with nanomaterials, such as metal nanoparticles, CNTs, graphene, and other nanostructures, contribute to the flexibility and durability of electronic devices and sensors. The mechanical strain exerted on atomically-thin layers during the bending of flexible substrate plays an important role in determining the performance of flexible thin film devices in wearable electronics. The effect of mechanical strains on the electronic structure and optical properties of atomically-thin-layered TMDs has been reported.269,

270

Desai et al.271 demonstrated a significant increase in the PL

intensity of (2–4) layer WSe2 up to 2 % uniaxial tensile strain, originating from the indirect-todirect bandgap transition induced under strain. The PL intensity of WSe2 bilayers was found to be 35 times larger, and comparable to unstrained monolayer WSe2. When the energy difference between the direct and indirect bandgaps was compared with MoS2, it was found to be smaller for WSe2 multilayers than for MoS2 multilayers.272 Sahin et al.273 reported the PL peak for both bulk and bilayer WSe2 at 1.57 eV, while for monolayer WSe2 peak, it was 1.64 eV. In contrast with the WSe2, MoS2 bilayer showed an indirect bandgap at 1.6 eV, whereas monolayer MoS2 showed a direct bandgap at 1.9 eV. The significantly enhanced PL emission showed that the bandgap in WSe2 multilayers can be modulated under strain to adjust the carrier mobility, and such strained multilayers can be potentially used for developing flexible optoelectronic devices. In another study, Conley et al.274 reported the effect of (0–2.2) % uniaxial tensile mechanical strain on the phonon spectra and band structures of monolayer and bilayer MoS2. The direct-to-indirect transition of the optical bandgap at 1 % applied strain was observed in monolayer MoS2, as evidenced by the decrease of PL intensity under the applied strain. This study showed strain-induced bandgap 16


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engineering of monolayer MoS2. The applied mechanical strains on atomically-thin layers modulate various properties, including the optical bandgap, electronic structure, transport, magnetic, and optical properties of MoS2 nanosheets, due to the altered atomic distance.275−280 The bandgap of atomically-thin-layered MoS2 can be tailored by applying strains for fabricating flexible electronic devices, in which the binding between the MoS2 and flexible polymer substrate plays an important role. Yang et al.281 studied the effect of in-plane uniaxial tensile strain on the bandgap of exfoliated MoS2 nanosheets transferred onto the PDMS flexible substrate. Figure 4 shows the progression of the Raman and PL spectra of the exfoliated MoS2 nanosheets on flexible PDMS substrate under different applied in-plane uniaxial tensile strains. The bandgap of exfoliated MoS2 nanosheets gradually decreases as a function of the applied inplane uniaxial tensile strain during mechanical bending of PDMS substrate. The shifts in E12g modes and the A1g modes of the exfoliated MoS2 nanosheets are clearly indicated under applied tensile strain. The MoS2 nanosheets showed the PL peak at 880.1 nm (1.409 eV) before bending the PDMS substrate, which peak shifts to (901.8 and 908.8) nm ((1.375 and 1.363) eV) under (0.47 and 1.21) % applied in-plane uniaxial tensile strain, respectively, during the bending process. A direct band-to-band transition in PL peak was observed at 695.1 nm (1.784 eV) in exfoliated MoS2 nanosheets, which is similar to that of the as-synthesized MoS2 nanosheets. The PL spectroscopy clearly shows a decrease in the optical bandgap under applied tensile strain on MoS2 nanosheets. Both Raman and PL spectra clearly indicate that the optical properties of MoS2 nanosheets change under applied tensile strains. Exploiting the good mechanical properties of MoS2, flexible and bendable devices, including flexible sensors for the detection of folic acid,282 humidity,283 strain sensors,284 solar cells,285, 286 supercapacitors,287, 288 and batteries,289, 290 have been developed. The MoS2/flexible polymers nanocomposite-based electronic devices and their durability will be 17



discussed throughout this review article.

Figure 4. (a) Raman spectra of the exfoliated MoS2 nanosheets deposited on flexible PDMS substrate under different applied tensile strains. (b) The frequencies of E12g modes and the A1g modes under different applied tensile strains extracted from (a). (c) PL spectra of exfoliated MoS2 nanosheets flexible PDMS substrate under 0% (black line), 0.47% (red line), and 1.21% (blue line) in-plane uniaxial tensile strain. (d) The PL peaks of exfoliated MoS2 nanosheets extracted from (c). Reprinted with permission from ref 281. Copyright 2014 Nature Publishing Group.

4. MoS2-BASED FETs Atomically-thin-layered dichalcogenides, such as MoS2, MoSe2, WS2, WSe2, TiS2, SnS2, ZrS2, and HfS2, show very interesting electronic properties,291−295 and show potential for developing novel FETs, LEDs, photodetectors, and luminescent display devices. Atomically layered TMDs 18


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have also demonstrated great potential in developing BHJ and dye-sensitized solar cells.211−212 MoS2 has gained more attention, due to its large intrinsic bandgap, making it suitable for developing a variety of electronic devices. Radisavljevic et al.244 reported the fabrication of single-layer MoS2 transistor on SiO2 substrate. Figure 5 shows a schematic of monolayer MoS2 FET and the current (Ids)–back-gate voltage (Vbg) curve of MoS2 FET recorded at RT. The monolayer MoS2 FET exhibited mobility of (0.1–10) cm2/Vs, similar to the values reported by Novoselov et al.2 When the 30 nm thick layer of high-κ hafnium dioxide (HfO2) was deposited as a top-gate dielectric on single-layer MoS2 conductive channel, the HfO2/MoS2/SiO2 FETs showed high μFE of 217 cm2/Vs, a low subthreshold swing (SS) value of 74 mV/dec, and a current IOn/IOff ratio of 1 × 108 at RT. The enhancement in mobility after deposition of a high-κ HfO2 dielectric was caused by the suppressed Coulomb scattering generated by high-κ dielectric material. This particular study created much interest in the scientific community in developing MoS2 monolayer and multilayer based transistors. Lopez-Sanchez et al.243 reported the mobility of 4.0 cm2/Vs, and high photoresponsivity of 880 A/W for the exfoliated monolayer MoS2, which was 105 times higher than the earlier reported photoresponsivity value of 7.5 mA/W for monolayer MoS2 phototransistor at an optical power of 80 μW,296 and the photoresponsivity of >100 mA/W for the multilayered MoS2 phototransistors.297 These outstanding electronic and optoelectronic properties generated much interest in monolayer MoS2 thin films. Furthermore, Schmidt et al.246 reported the electronic properties of CVD-grown monolayer MoS2, where the field effect mobilities of (45 and 500) cm2/Vs at RT and low temperature, respectively, were observed for the FET devices. Radisavljevic and Kis298 reported the tailoring of mobility in monolayer MoS2 FETs using different dielectric configuration, and mobility was found to be temperature-dependent. Wu et al.299 developed FETs with single-crystal MoS2 channels prepared by CVD. The mobility of 17.3 cm2/Vs and the on/off 19



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current ratio of 4 × 108 for a bilayer back-gated MoS2 FET were observed, which were found to be higher compared to those of FETs prepared by polycrystalline MoS2 films. By managing the grain boundaries and the contamination during the transfer process, the high-quality MoS2 atomic layers deposited on SiO2 showed electronic properties comparable to exfoliated high-quality MoS2 flakes. Raman spectroscopy was used to determine the numbers of layers of individual MoS2 grains. The peak frequency difference (Δ) of (18 and 22) cm-1 between E12g and A1g modes evidenced the grains as single-layer and bilayer MoS2 crystals, respectively. The PL spectrum measured at a laser excitation wavelength of 532 nm exhibited emission peaks at (676 and 630) nm for a bilayer MoS2 crystal. Das et al.300 studied several metal contacts with different work function, and reported the μFE of 10 nm thick MoS2 layer as (21, 90, 125, and 184) cm2/Vs for Pt, Ni, Ti, and Sc contacts, respectively. The Schottky barrier heights of different metal contacts measured were (230, 150, 50, and 30) meV for Pt, Ni, Ti, and Sc, respectively. The μFE increased from (184 to 700) cm2/Vs for Sc as the source/drain contact, after applying a 15 nm thick layer of high-κ dielectric Al2O3 on top of the MoS2 transistor. The field-effect mobility of MoS2 FET measured with flake thickness of (2 to 70) nm, was found to be layer thickness dependent, where a layer thickness in the (6–12) nm range appeared to maximize the device performance. Bao et al.301 reported the carrier mobility of multilayer MoS2 FETs on SiO2 and polymethyl methacrylate (PMMA) substrates. MoS2 FETs on SiO2 substrate showed the mobility of (30–60) cm2/Vs, which was found to be independent of film thickness ((15–90) nm), whereas the mobility of MoS2 FETs on PMMA substrate increased with layer thickness, yielding high electrons mobility up to 470 cm2/Vs, and holes mobility up to 480 cm2/Vs at 50 nm thickness. This study demonstrated the effect of substrate on the mobility of the bulk MoS2 FETs, with long-range disorder at the PMMA interface, while with short-range disorder for the SiO2, due to the chemical interactions. As reported above, the mobility of MoS2 20


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FETs depends upon a number of factors that include layer preparation technique, substrate, temperature, metal contacts, gate dielectric, dielectric configuration, and number of MoS2 layers.

Figure 5. (A) Schematic illustration of monolayer MoS2 FET showing electrical connections of the device. A 6.5 Å thick single-layer of MoS2 as a conductive channel is deposited on 270 nm thick SiO2 substrate which acts as a back gate. The monolayer MoS2 is covered with 30 nm thick high-κ HfO2 top-gate dielectric. (B) Current (Ids) versus back-gate voltage (Vbg) curve of MoS2 FET recorded at room temperature with 10 mV applied bias voltage (Vds). Inset shows the Ids–Vds curve for 0, 1 and 5 V back-gate voltage. Reprinted with permission from ref 244. Copyright 2011 Nature Publishing Group. 21



The mobilities of TMDs at RT and low temperatures have been theoretically calculated by several research groups. Jin et al.302 calculated the mobilities of monolayer MX2 (M=Mo,W; X=S,Se) using density functional theory (DFT), and predicted the intrinsic mobilities of (270, 90, 540, and 270) cm2/Vs at RT for MoS2, MoSe2, WS2, and WSe2, respectively. The comparative trend of mobility was WS2 > MoS2 = WSe2 > MoSe2. The intrinsic hole mobility of monolayer WS2 was found to be comparable to that of bulk Silicon (450 cm2/Vs) at RT. Zhang et al.303 calculated the electron mobilities of MX2 (M = Mo, W, Sn, Hf, Zr, Pt; and X = S, Se, Te). The calculated electron mobilities were (237, 199, 228, 968, 734, 47, 59, 1795, 2916, 202, 154, 278, 64, and 67) cm2/Vs for MoS2, MoSe2, MoTe2, WS2, WSe2, HfS2, HfSe2, PtS2, PtSe2, PtTe2, SnS2, SnSe2, ZrS2, and ZrSe2, respectively. Both PtS2 and PtSe2 showed the highest electron mobility among all TMDs. The mobility of WS2 was found be 968 cm2/Vs, the highest among TMDs having 1H-structure. The estimated phonon limited mobility of PtSe2 with 1T-structure was found to be 2,916 cm2/Vs at RT, the highest among 14 TMDs. Here, WS2 has a direct bandgap of 1.99 eV, while PtSe2 has an indirect bandgap of 1.25 eV. The electron mobility trend emerged as WS2 > WSe2 > MoS2 > MoSe2, similar to that reported by Jin et al.302 The electron mobilities of MoS2 and WSe2 are similar to the experimental results. Yin et al.304 calculated the carrier mobility of Janus MoSSe structure, and reported low mobility in monolayer MoSSe, but quite high electron and hole mobilities of (1,194 and 5,894) cm2/Vs for bilayer or trilayer Janus MoSSe structures, respectively. Kaasbjerg et al.305 studied electron-phonon interaction in single-layer MoS2 using first principles with density-functional-based technique. The deformation potential coupling and Frohlich interaction yielded the RT phonon-limited mobility of 410 cm2/Vs. These authors306 also theoretically predicted the acoustic phonon limited mobility exceeding 105 cm2/Vs at temperatures 22


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< 10 K in n-doped monolayer MoS2, and carrier densities as high as 1011 cm−2. Acoustic phonon limited mobilities of n-type MoS2 were predicted as a function of temperature, and for carrier densities ranging (1010 to 3×1013) cm−2 (Fig. 6). The mobility of MoS2 does not increase over 7×103 cm2/Vs at 100 K. The dependence of the mobility on carrier density in n-type MoS2 is associated with Fermi velocity and transport in the Bloch-Gruneisen region.

Figure 6. Acoustic phonon limited mobility of n-doped MoS2 as a function of carrier density (n) at temperatures of 4, 20, and 50 K. The color dots depict the quantum-classical crossover from the non-degenerate to a degenerate carrier density distribution where the mobility is not carrier density dependent. Reprinted with permission from ref 306. Copyright 2013 American Physical Society. The charge carrier mobility of MoS2 FETs depends upon various factors, including the preparation of MoS2 atomic layers either by exfoliation or CVD, the number of atomic layers (thickness), and the contact electrodes.307−335 Table 2 summarizes the field-effect mobility (μFE) and the current IOn/IOff ratio of MoS2 FETs fabricated on SiO2/Si substrates. Some important results are discussed herein. Schmidt et al.246 developed back-gated FETs from CVD-grown monolayer MoS2 on a Si/SiO2 substrate. Raman spectrum showed a difference of 21 cm−1 between E12g and A1g vibrating modes, which is a characteristic of the formation of monolayer MoS2. Monolayer 23



MoS2 FETs developed without any high-κ dielectric encapsulation, exhibited RT field-effect mobility of 45 cm2/Vs, and low temperature mobility of 500 cm2/Vs. Lin et al.316 used exfoliated MoS2 flakes of different thicknesses to fabricate FETs, and measured mobility as a function of temperature. The MoS2 FETs showed thicknesses-dependent mobility for (5 to 10)-layer ((3.6 – 7) nm thickness) of MoS2. The maximum mobility of 70 cm2/Vs was measured for 5-layer FETs at 295 K. The mobility of MoS2 FETs increases with increasing temperature, as well as layer thickness. Temperature-dependent conductivity measurements for different gate voltages were revealed. A metal-to-insulator transition was observed during temperature-dependent electrical conductivity measurements for MoS2 FETs having less than 10 layers. Wang et al.312 developed NAND gate, logic inverter, static random access memory (SRAM) cell, and 5-stage ring oscillator operating at 1.6 MHz based on mechanically exfoliated bilayer MoS2 by applying direct-coupled FET logic (DCFL) technology. The bilayer MoS2 FETs showed on-state current density of 23 μA/μm, and transconductance (gm) exceeding 12 μS/μm at Vds of 1.0 V. The subthreshold of 88 mV/dec and on/off current ratio of 107 for Vds over 0.5 V were measured for bilayer MoS2 FETs, and after depositing a 20 nm thick high-κ dielectric HfO2 layer on the top of MoS2 film, the μFE of (5–15) cm2/Vs before passivation increased to 313 cm2/Vs.

Table 2. Field-effect mobility (μFE) and the current IOn/IOff ratio of MoS2 based FETs fabricated on SiO2/Si substrates under different preparation methods. ------------------------------------------------------------------------------------------------------------MoS2 Channel Layer Mobility (μFE) IOn/IOff ref. & Preparation method (cm2/Vs) Ratio ------------------------------------------------------------------------------------------------------------WSe2 Crystal 500 104 307 6 MoSe2 Monolayer (CVD) 20-80 10 308 HfS2 few-layer (exfoliated) 2.4 107 309 MoS2 Bulk 200 310 MoS2 (exfoliated) 3.0 2 6 MoS2 Monolayer (exfoliated) 10 10 296 24


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MoS2 (exfoliated) MoS2 Bilayer (exfoliated) MoS2 Multilayer (exfoliated) MoS2/TiO2/Co MoS2 Multilayer (exfoliated) (MoS2 thickness = 5.7 nm) (MoS2 thickness = 7.0 nm) (MoS2 thickness = 32 nm) (MoS2 thickness = 39 nm) (MoS2 thickness = 55 nm) (MoS2 thickness =70 nm)

1.5 5−15 12 76

102 107 105 106

311 312 313 313

9.9 1.6 36.8 31.9 38.7 42

4×06 103 107 105 7×104 104

314 314 314 314 314 314

MoS2 Five-layer (exfoliated) MoS2 Monolayer (exfoliated) MoS2 Flakes (Plasma printing) MoS2 Flakes MoS2 thermolysis MoS2 Monolayer (CVD)

70 315 44 316 6-44 105~107 317 44-86 2×102 318 6.0 106 319 45 246 500 (10 K) 106 246 MoS2 Monolayer (CVD) 6.0 105 320 MoS2 Monolayer (CVD)/Al2O3 16.1 109 320 4 MoS2 Monolayer (CVD) 0.02 10 321 MoS2 Few-layer (CVD) 0.04 322 6 MoS2 Monolayer (CVD) 4.3 6.0×10 323 MoS2 Mono-/Few-layer (CVD) 0.003−0.03 103 324 MoS2 Monolayer (CVD) 0.03−0.23 105 325 MoS2 Bilayer (CVD) 17.3 4.0×108 326 MoS2 sputtering/CVD 12.24 1.56×106 327 5 7 MoS2 Monolayer (CVD) 3-8 10 ~10 328 MoS2 Monolayer (CVD) 30 106 329 7 MoS2 Monolayer (CVD) 1.2 10 330 MoS2 Monolayer (exfoliated) 230 105 331 5 Bilayer (exfoliated) 450 10 331 Trilayer (exfoliated) 820 105 331 3 MoS2 flake(exfoliated) 3.71 10 332 MoS2 Monolayer (CVD) 43 333 MoS2 Monolayer (CVD) 500 (10 K) 106 246 MoS2 Monolayer (exfoliated) 65 105 334 MoS2 Monolayer (e-beam lithography) 0.1~1.5 335 MoS2 Monolayer (e-beam lithography)/PE 150 106 335 -------------------------------------------------------------------------------------------------------------

5. STRATEGIES FOR BOOSTING THE FIELD-EFFECT MOBILITY OF MoS2 FETs 25



The field-effect mobility of MoS2 based FETs is rather low, and therefore several strategies have been explored, including chemical doping, the nature of contacts, the use of high-κ dielectrics such as Al2O3 and HfO2, polyanions, fluoropolymers, and other 2D materials, in order to improve the mobility of MoS2-based FETs. 5.1 EFFECT OF CHEMICAL DOPING ON MOBILITY Chemical doping has been generally used for improving the electronic properties of inorganic and organic semiconductors, as well organic polymers. Yang et al.336 used chloride doping on WS2 and MoS2 FETs. Figure 7 shows a schematic of chlorine (Cl)-doped few-layer WS2 or MoS2 back-gate FET and their transfer length method (TLM) resistances as a function of gap space. The chlorine doping was accomplished by soaking the WS2 or MoS2 flakes in 1,2-dichloroethane solution over 12 h. The contact metal Ni (30 nm thickness)/Au (60 nm thickness) was deposited after e-beam lithography. The 90 nm SiO2 was used as a back-gate oxide, and p+2 Si as the back-gate contact. The contact resistance (Rc) of (0.7 and 0.5) kΩ·μm for WS2 and MoS2, respectively, was measured at the back-gate bias (Vbg) of 50 V. The Cl-doped WS2 showed the transfer length (Lt) of 132 nm, specific contact resistivity (ρc) of 9.2 × 10–6 Ω·cm2, and doping density of 6.0 × 1011 cm–2; whereas the Cl-doped MoS2 showed Lt of 60 nm, ρc of 3 × 10–7 Ω·cm2, and doping density of 9.2 × 1012 cm–2. The Rc in both WS2 and MoS2 decreased significantly after chlorine doping, and the effective electron density of ((2.3 and 2.9) × 1013) cm–2 for WS2 and MoS2, respectively, was observed at Vbg of 50 V. The Rc in the few-layer WS2 and MoS2 was reduced to (0.7 and 0.5) kΩ·μm, respectively, after chloride doping. The few-layer WS2 FETs showed the drain current of 380 μA/μm, current IOn/IOff ratio of (4×106 and 3×107) at the drain bias of (2 and 0.05) V, respectively, and μFE of 60 cm2/Vs. The stability of Cl-doped WS2 and MoS2 FETs was studied in vacuum and air. The MoS2 FET 26


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device displayed stability both in vacuum or air for four weeks. The transfer characteristics curves of the MoS2 FET devices remained the same after 4 weeks as those of the pristine device. There was no increase in the off current, and contact retained a low resistance, although there was a slight decrease in the drain current. In the case of WS2 FETs, both Rc and the channel sheet resistance increased, indicating potential degradation of the FET device under atmospheric conditions.

Figure 7. (a) Schematic of chlorine (Cl)-doped few-layer WS2 or MoS2 back-gate FET. (b) TLM resistances of Cl-doped WS2 and MoS2 FETs as a function of gap space (nm) measured at a Vbg of 50 V. Reprinted with permission from ref 336. Copyright 2014 American Chemical Society.

The n-doping of few-layer MoS2 and WSe2 using potassium (K) as a surface charge transfer dopant has been demonstrated to yield high electron densities of (1.0 × 1013 and 2.5 × 1012) cm–2 for MoS2 and WSe2, respectively.337 The top-gated WSe2 and MoS2 FETs with selective K-doping at the source/drain metal contacts exhibited low resistance. Top-gated few-layer MoS2 FETs after K-doping of metal contacts displayed an electron mobility of 25 cm2/V·s. The few-layer WSe2 FETs showed electron mobility of 110 cm2/V·s and current IOn/IOff ratio of 104, after selectively K-doping for 120 min at the metal contacts. Polyethyleneimine (PEI) doping of multilayer MoS2 FETs resulted in a 1.2 times reduction in Rc changing from (5.06 to 4.57) Ω·mm, a 2.6 times reduction in sheet resistance, which decreased from (19.9 to 7.65) kΩ/cm2, and 50 % enhancement in field-effect mobility. The threshold voltage (Vth) shifted negatively upon the PEI doping.338 The 27



current IOn/IOff ratio of MoS2 FETs was reduced from (105 to 102) after PEI doping. The ON-current of the MoS2 FET device increased from (10.25 to 17.61) mA/m after PEI doping, which is a 70 % improvement in the ON-current. The n-type few-layer MoS2 FETs with graphene/Ti heterocontacts showed a drain current over 160 mA/mm at 1 μm gate length, current IOn/IOff ratio of 107, and mobility of 50.4 cm2/Vs.339 The intrinsic field-effect mobility was found to decrease from (50.4 to 28.7) cm2/Vs at (300 to 400) K, because of the electron-phonon scattering. A 3.3 times increase in Rc changing from (12.1 to 3.7) Ω·mm and 2.1 times increase in ON-resistance was measured for the MoS2/graphene/Ti contacts, compared with MoS2/metal FETs having no graphene layer. The electrical and optical properties of monolayer MoS2 have also been improved by the in situ rhenium (Re) doping.340

5.2 EFFECT OF CONTACT ELECTRODES ON MOBILITY The low Rc at the metal-semiconductor interface is an important factor for the realization of highperformance FETs, and therefore several different types of metal-to-MoS2 contacts leading to low Schottky barrier have been studied.341 Molecular doping has also been used to reduce the Rc in MoS2 FETs. Das et al.342 studied the effect of several metal contacts with different work function on the field-effect mobility of MoS2 multilayers. The Schottky barrier heights of (230, 150, 50, and 30) meV were calculated for platinum (Pt), nickel (Ni), titanium (Ti), and scandium (Sc) at the metal-to-MoS2 contact interface, respectively, which have low work functions of (5.9, 5.0, 4.3, and 3.5) eV for Pt, Ni, Ti, and Sc, respectively. The current density increased from (0.10 to 0.70 to 1.0 to 4.8) μA/μm for Pt, Ni, Ti, and Sc, respectively, at a drain voltage (Vds) of 1 V, indicating that lower Schottky barrier heights lead to lower Rc. The field-effect mobility of 10 nm thick MoS2 layer was (21, 90, 125, and 184) cm2/Vs for Pt, Ni, Ti, and Sc contacts, respectively. The field28


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effect mobility increased from (184 to 700) cm2/Vs for Sc as the source/drain contact at RT, after applying a 15 nm thick layer of a high-κ dielectric Al2O3 on top of the back-gated MoS2 transistor. For 5 μm channel length, the gm of 4.7 μS/μm at a Vds of 1.0 V and the saturation current density of 240 μA/μm were measured. The field-effect mobility of MoS2 FET measured with flake thickness of (2 to 70) nm was found to be layer thickness dependent, where a layer thickness in the (6–12) nm range appeared to maximize the device performance. Kang et al.343 fabricated (1– 5)-layer thick back-gated MoS2 FETs on Al2O3/Si substrate using molybdenum (Mo) as the contact metal. The mobilities of (13, 14 and 27) cm2/Vs were recorded using two-terminal measurements for single-, double-, and tri-layer MoS2 FETs, respectively, at a drain voltage (Vds) of 1 V. The 4layer MoS2 FETs showed lower Rc of 2 kΩ·mm and mobility of 27 cm2/Vs. On/off current ratios exceeding 103 were measured at the (Vds) of 0.1 V. The back-gated FET configuration to evaluate the impact of Ti as a contact metal with exfoliated monolayer MoS2 was employed by Liu et al.344 The monolayer MoS2 FET having Ti (10 nm)/Au (100 nm) contact showed an n-type behavior with current IOn/IOff ratio over 106 between Vds of (0.1 to 5.0) V. The Schottky barrier varied from (0.3 to 0.35) eV between monolayer MoS2 and Ti as calculated from six different FETs, which is larger compared with the Schottky barrier between multilayer MoS2 and Ti, because electron affinity is lower for monolayer MoS2 than for multilayer MoS2. The Rc of monolayer MoS2 FETs strongly depends upon the backgate voltage (Vbg), which is reduced from (25 to 7) kΩ·mm at Vbg of (-5.0 to 30) V at current (Ids) below 20 μA. The minimum Rc of 1.3 kΩ·mm was measured with Ti contact at Ids of 150 μA and Vbg of 30 V. This lower Rc yielded ON-current value of 200 μm/μA at Vbg of 30 V and Vds of 8 V, SS of 410 mV/dec, and mobility of 44 cm2/Vs for monolayer MoS2 FETs onto a SiO2 substrate. In another study, Wang et al.345 measured μFE of (46.7, 36.4, and 26.2) cm2/Vs for MoS2 FETs with 29



(Ni, Au, and Pd) metal contacts which increased to (73, 43.1, and 23.7) cm2/Vs, respectively, after depositing an h-BN interfacial contact layer. The h-BN/Ni/Au contact-based MoS2 FETs had a low Schottky barrier height of 31 meV, as well as a low Rc of 1.8 kΩ·mm, which yielded the highest mobility of 321.4 cm2/Vs at 77 K. Yang et al.346 studied the mobility in exfoliated multilayer MoS2 FETs without any high-κ dielectric, but with different contact electrodes (Ni, Ti/Ni), where mobility of (9.9, 1.6, 18.3, 36.8, 38.7, and 42) cm2/Vs was observed for MoS2 layer thickness of (5.7, 7.0, 12, 32, 55, and 70) nm, respectively, depending upon the electrode contacts.

5.3 EFFECT OF ORGANIC GATE DIELECTRICS ON MOBILITY In fabricating MoS2 FETs, in general SiO2, Al2O3, or HfO2 are used as gate dielectrics; however, a new class of organic material-based gate dielectrics has recently been employed to enhance the performance of FETs. Liu et al.347 studied the PL of mechanically exfoliated monolayer MoS2 and WS2 on PMMA, polyvinyl alcohol (PVA), PDMS, fluoropolymer CYTOP, CYTOP/SiO2, SiO2, Al2O3, and h-BN (annealed). The energy difference between absorption and emission peaks (Stokes shift) of excitons of around (25 and 15) meV was observed for MoS2 and WS2, respectively. A negligible Stokes shift was noticed for monolayer MoS2 and WS2 on CYTOP/quartz substrates. The PL spectrum of monolayer MoS2 and WS2 on CYTOP substrate showed full-width at halfmaximum (FWHM) at (45 and 27) meV, respectively, compared to (80 and 50) meV for monolayer MoS2 and monolayer WS2, respectively, on SiO2/Si or quartz substrates. The back-gated FETs from monolayer MoS2 and WS2 were fabricated on a fluoropolymer CYTOP substrate, and compared with SiO2/Si substrate. The FETs based on monolayer MoS2/CYTOP/SiO2/Si exhibited n-type transconductance; but no p-type conductivity and μFE of 20 cm2/Vs were recorded with a two-terminal configuration. In comparison, MoS2/SiO2/Si FETs showed mobility of 20 cm2/Vs. 30


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The monolayer WS2/CYTOP FET showed weaker n-type conductivity, electron mobility of 15 cm2/Vs, and hole mobility of 3 cm2/Vs; in comparison, monolayer WS2/SiO2/Si FETs showed μFE of 12 cm2/Vs. The variation in mobility was between (1–40) cm2/Vs, and the threshold voltage was ±5 V for different FETs. This study showed the renormalization of bandgap, carriers, and Pauli blocking via substrate-induced doping. Yoo et al.348 also used fluoropolymer CYTOP as a gate dielectric for fabricating multilayer MoS2 FETs, which showed Vth of 5.7 V, SS of 2.2 V/dec, and μFE of 82.3 cm2/Vs. Vth of -7.8 V, SS of 2.7 V/dec, and μFE of 42.7 cm2/Vs were obtained for MoS2 FETs made with the cross-linked poly(4-vinylphenol) (PVP) as a gate dielectric. The electron-withdrawing fluorine groups in CYTOP induced surface dipoles that enhance the surface potential, causing a reduction in carriers in the MoS2 channel, which positively shifts threshold voltage. This indicates that the gate dielectric can control the performance of multilayer MoS2 FETs. The gold-coated Si substrate functionalized with 4-heptadecafluorooctylbenzene diazonium tetrafluoroborate (C8F17BD) has been used as an organic dielectric for developing MoS2 FETs.349 Source/drain contacts (Cr/Au) were patterned on the mechanically exfoliated MoS2 flakes. MoS2 FETs had a double-gate geometry, having a local gate on the electrografted dielectric, and a backgate of the Si p-doped wafer covered with 150 nm thick SiO2 layer. MoS2 FETs showed reduced gate bias swing to 1 V, very small gate-leakage current, subthreshold as low as 110 mV/dec, on/off current ratio >104 for gate voltage (Vg) of 1.5 V, and suppressed transfer characteristics related hysteresis. This shows that the organic gate dielectric could be suitable for fabricating highperformance MoS2 FETs. Kobayashi et al.350 developed ferroelectric field-effect transistors (FeFETs) using MoS2 channel, and vinylidene fluoride (VDF)-trifluoroethylene (TrFE) copolymer as a back-gate dielectric. A hysteresis behavior in the Ids–Vg curve evidenced interactions between 31



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MoS2 and ferroelectric dielectric. The MoS2/VDF-TrFE based FeFET showed n-type characteristics, and μFE of 625 cm2/Vs, on/off current ratio of 8 × 105, and memory window width of 16 V. In another study, Chen et al.351 fabricated few-layer MoS2 FET using a high-κ ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene) P(VDF-TrFE-CFE) (ε = ~50) as a gate dielectric material. The four-layer MoS2 FeFET with P(VDF-TrFE-CFE) top gate dielectric layer showed the current IOn/IOff ratio of 3.27 × 106, and μFE of 51.94 cm2/Vs. Kim et al.352 used poly(2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane-co-cyclohexyl

methacrylate)

[p(V4D4-co-CHMA)] copolymer as a gate dielectric in MoS2 FETs. The initiated chemical vapor deposition (iCVD) grown p(V4D4-co-CHMA) copolymer thin films showed a dielectric constant of 2.30, and 3.8 × 10–11 A/cm2 leakage current at 1 MV/cm. The p(V4D4-co-CHMA) gate dielectrics based MoS2 FETs showed electron mobility of 35.1 cm2/Vs, an SS of 0.2 V/dec, a current IOn/IOff ratio of 2.6 × 106, and better stabilities, compared to MoS2 FETs with SiO2 backgate dielectrics. Choi et al.353 fabricated few-layer MoS2 FET using polyanionic poly(styrene sulfonic acid) (PSSH) as the top-gate dielectric. The MoS2 FET showed high electron mobility of >100 cm2/Vs at RT, due to the creation of an electric double layer (EDL) by the polyanionic PSSH proton conductor, which aided the injection of electrons into the few-layer MoS2 channel. Figure 8 shows the chemical structures of fluoropolymer CYTOP, C8F17BD, and P(VDF-TrFE) copolymer, which have been used as gate dielectrics for MoS2 FETs.

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Figure 8. (a) Chemical structures of fluoropolymer CYTOP,347 (b) (C8F17BD),349 and (c) Poly[vinylidene fluoride (VDF)-trifluoroethylene (TrFE)] copolymer used as organic gate dielectrics for fabricating MoS2 FETs.350

5.4 EFFECT OF HIGH- DIELECTRICS ON MOBILITY Low and high dielectric constant materials have been used for a variety of applications in electronic and optoelectronic devices.354 The mobility of MoS2 FETs is too low for high-speed electronic device applications; therefore, high-κ dielectric materials have been used to enhance the carrier mobility. The high-κ dielectric layer of ZrO2 (ε = ~22) was used for fabricating MoS2 transistors.355 The μFE of MoS2 TFTs was enhanced by 2.5 times to 50.1 cm2/Vs by using the sol– gel processed high-κ ZrO2 insulator compared with MoS2/SiO2/Si FET structures having μFE of 19.4 cm2/Vs, due to the suppressed Coulomb electron scattering and reduced interface trap concentration by high-κ ZrO2 gate dielectric layer. The use of sol–gel processed high-κ ZrO2 insulator and the laser direct writing (LDW) process of developing electrodes contributed to high 33



field-effect mobility and resolution up to ~10 nm. Kim et al.356 reported a very high mobility of up to 1,000 cm2/Vs at RT for monolayer MoS2 FETs using SrTiO3 dielectric. Here, the mobility of monolayer MoS2 FETs is significantly enhanced by high-κ SrTiO3 material, due to the dielectric screening effect. Shin and Son357 fabricated ferroelectric PbTiO3 thin film gated monolayer MoS2 FETs for ferroelectric non-volatile random access memory (FeRAM) devices. The epitaxial PbTiO3 thin film was pulsed laser deposited onto the Nb-doped SrTiO3 substrate, and a mechanically exfoliated monolayer MoS2 nanosheet was transferred to the surface of the PbTiO3/Nb:SrTiO3 substrate. The monolayer MoS2/PbTiO3/Nb:SrTiO3-based FET showed a mobility of 327 cm2/V·s at RT. The FeRAM device fabricated using MoS2/PbTiO3/Nb:SrTiO3 FET showed a broad memory window and the resistance variation of 17 kΩ, due to the high remnant polarization of PbTiO3 thin film. Min et al.358 demonstrated that the high-κ dielectric induced dielectric screening effect has a negative impact on mobility with increasing MoS2 thickness. The monolayer MoS2 FETs with top-gate high-κ Al2O3 dielectric showed the mobility of 170 cm2/Vs with a low SS of 90 mV/dec. However, the bilayer and trilayer MoS2 FETs exhibited mobilities of (25 and 15) cm2/Vs with the large SSs of (0.5 and 1.1) V/dec, respectively. This study supports that the effect of high-κ gate dielectric in increasing mobility is significantly more dominant in monolayer MoS2 FETs compared with those of multilayer MoS2 FETs. Lee et al.359 reported the mobility of exfoliated monolayer, bilayer, and trilayer MoS2 FETs with high-κ Al2O3 dielectric as (80, 27, and 10) cm2/Vs, respectively. The current IOn/IOff ratio ranged (106 to 107), depending upon the number of layers in MoS2 FETs. This study also showed that the effect of high-κ gate dielectrics on the mobility of MoS2 FETs also strongly depends upon the MoS2 layer thickness, where the interface between the MoS2 layers and high-κ dielectric plays an important role in molecular interactions. Zou et al.360 reported top-gated and bottom-gated FETs of multilayer 34


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MoS2 conductive channel encapsulated by stacked Al2O3 (9 nm)/HfO2 (6 nm). The top-gated FET showed mobility of 100 cm2/Vs, current IOn/IOff ratio of 108, and SS of 93 mV/dec. Top-gated FET also showed enhanced reliability, because 6 MV cm−1 gate-biased stressing caused Vth shift of (10−3 – 10−2) V MV–1 cm–1. The improvement of electrical properties in top-gated MoS2 FET with encapsulated MoS2 channel by stacked Al2O3/HfO2 occurred due to the new device configuration and dielectric environment. In another study, Liu et al.361 reported top-gated MoS2 based negative capacitance (NC)-FETs having SS value of 42.5 mV/dec, current IOn/IOff ratio of 4 × 106, and gm of 45.5 μS/μm for less than 100 nm MoS2 channel length. The top-gated MoS2 NC-FETs are composed of suspended Ag nanowires as a gate electrode, and poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF–TrFE)] as the gate dielectric layer. The back-gated MoS2 FETs having 300 nm thick SiO2 dielectric layer showed the mobility of 109 cm2/Vs, and the current IOn/IOff ratio of 106 for the channel length of 2 m. The top-gated MoS2 NC-FETs showed the current IOn/IOff ratio of over 108 for the 80 nm channel length at RT. The lowest SS value of 37.2 mV/dec has been observed at RT with Vds of 0.1 V, showing that the role of P(VDF–TrFE) ferroelectric in the negative capacitance contributes to the performance of MoS2 FETs. The MoS2 NC-FETs having AgNWs/HfO2/P(VDF-TrFE) stack layers showed the current IOn/IOff ratio of 3 × 105 and SS value of 107.5 mV/dec for the 50 nm channel length. The insertion of an HfO2 passivation layer further enhanced the stability of the NC gate stacked-structure, and inhibited the P(VDF–TrFE) ferroelectric from fatigue. These MoS2 based NC–FETs are clearly different from those of traditional MOSFETs. The SS showed a nonlinear behavior as a function of temperature, because of the P(VDF–TrFE) ferroelectric gate stack. The MoS2 based NC–FETs can be used for low-power electronic products. Xu et al.362 used ferroelectric HfZrOx in the gate dielectric to develop few-layer MoS2 channel-based NC-FETs 35



with configuration of Al2O3/ITO)/HfZrOx/ITO/SiO2/Si stack, which exhibited SS of 47 mV/dec. In another study, NC-FETs with MoS2 channel and hafnium zirconium oxide (HfZrO2) ferroelectric in the gate stack showed SS value of sub-60 mV/dec.363 The SS of MoS2 NC-FET was found to be two orders of magnitude higher than that of MoS2 FET. The MoS2 NC-FETs show great promise for low-power electronic devices. The high-κ Al2O3 has been used as a gate dielectric for developing MoS2 FETs. Yu et al.364 studied single-layer CVD-grown MoS2 thin film, and CVD-grown graphene/MoS2 and MoS2/Ti structures for FETs. MoS2/graphene FETs consist of MoS2 channel, high-κ dielectric Al2O3 as top gate dielectric, and graphene film as source, drain, and gate electrodes, and Si as back gate. The MoS2/Ti FETs were fabricated with MoS2 channel, Al2O3 as a top dielectric layer, and graphene as the top gate. For both types of FETs, the Vds bias was 0.5 V. MoS2/graphene FETs showed gm of 0.5 μS/μm at Vds of 7 V, SS of 150 mV, and current IOn/IOff ratio of more than 103. For MoS2/Tibased FETs, the threshold voltage was not changed, and the transconductance or field-effect mobility was found to decrease with decreasing temperature. The Schottky barrier height of MoS2/graphene junction decreased from (110 to 0) meV after changing the back-gate voltage from (0 to 35) V, however the barrier height in MoS2/Ti junction has changed from (50 to 40) meV after the back-gate was varied from (0 to 80) V. Graphene, due to its large carrier density, performs better than Ti in making contact with MoS2. Li et al.365 studied interactions between MoS2 and high-κ dielectrics by fabricating back-gated multilayer MoS2 FETs on high-κ Al2O3 dielectric coated Si substrates. The μFE (20.5 cm2/Vs) and SS were enhanced in MoS2/Al2O3/Si FETs compared with MoS2 FETs on SiO2 (7.4 cm2/Vs for MoS2/SiO2/Si), due to the dielectric screening effect. The μFE was found to increase further when a high-κ HfO2 layer was deposited on the top of MoS2/Al2O3/Si, as a result of reduced Rc between the MoS2 and metal. The SS of MoS2/Al2O3/Si 36


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FET device was 233 mV/dec compared with 2,617 mV/dec for MoS2/SiO2/Si FET. Multilayer MoS2 FETs before and after depositing the HfO2 passivation layer were compared. The μFE of MoS2 FETs was improved after depositing HfO2 passivation layer, which increased from (20.5 to 25.2) cm2/Vs for HfO2/MoS2/Al2O3/Si FETs. The current IOn/IOff ratio decreased from (107 to 105) for (MoS2/Al2O3/Si to HfO2/MoS2/Al2O3/Si) FETs, while the SS value increased from (233 to 549) mV/dec after depositing a HfO2 layer. It has been observed that deposition of a high-κ top oxide increases the field-effect mobility of MoS2 FETs, due to the reduction of Coulomb impurities (CI) scattering. Yu et al.366 studied the experimental and theoretical electron transport properties of thiol-treated monolayer MoS2 FETs fabricated on SiO2, Al2O3, and HfO2 substrates having the dielectric constant of (3.9, 10, and 16.5), respectively. The four-probe field-effect mobility of monolayer MoS2 FETs on SiO2, Al2O3, and HfO2 substrates was measured as a function of temperature ((20–300) K), where mobility was found to increase with the dielectric constant of the substrate. The monolayer MoS2 FETs showed room-temperature mobility of 150 cm2/Vs, after suppressing the effect of CI. The CI-limited mobility (μCI) is also impacted by dielectric constant () and carrier density (n). The back-gated monolayer MoS2 FET on SiO2 substrate showed room-temperature mobility of 80 cm2/Vs, being limited by CI scattering. The MoS2 FETs-based on HfO2 (Al2O3) substrates, showed the mobilities of (148 (591) and 847 (113)) cm2/Vs at RT and 20 K, respectively. Cui et al.367 measured the Hall mobility (μHall) of the van der Waals heterostructure having h-BN-encapsulated and graphene contacted MoS2 channel devices as a function of temperature and carrier density. The hBN/MoS2/graphene/h-BN stacked van der Waals FET devices on SiO2/Si substrates were fabricated. Impurity-limited mobilities (μimp) of (1,020, 7,300, and 34,000) cm2/Vs for CVD-grown monolayer, 4-layer, and exfoliated 6-layer MoS2 devices, respectively, were recorded at low 37



temperature. The quantum mobilities of (1,400, 3,100, and 10,000) cm2/Vs for single-layer, 4layer, 6-layer MoS2 devices, respectively, were also predicted. Lui et al.368 also demonstrated that CI scattering can be suppressed after sandwiching a single-layer MoS2 channel between two h-BN layers, resulting in mobility over 1,000 cm2/Vs at low temperatures. The MoS2 FETs have been used for developing gigahertz (GHz)-radio frequency (RF) transistors. Sanne et al.369 developed the GHz-RF transistors using CVD-grown monolayer MoS2. The Raman spectrum exhibited E12g peak at 383.5 cm−1, and A1g peak at 403.1 cm−1, confirming the CVD-grown monolayer MoS2 based on the difference between the two vibrating modes (A1g – E12g) of 19.6 cm−1. A 30 nm thick layer of high-κ dielectric HfOx (k=18) was deposited by atomic layer deposition (ALD) as the top gate dielectric, and the gate length (Lg) was 250 nm for all FETs. MoS2 FETs showed current densities higher than 200 μA/μm, gm of 38 μS/μm, and low-field mobility of 55 cm2/Vs. The intrinsic cutoff frequency (fT) and maximum frequency of oscillation (fmax) of (6.7 and 5.3) GHz, respectively, were measured for a FET having a gate length of 250 nm. The voltage gain (Av) was 6 dB at 100 MHz, which is a minimum frequency, but Av was sustained until a frequency of 3 GHz was reached. The extracted saturation velocity from the transit frequency was 1.1×106 cm/s. A common-source amplifier with 14 dB voltage gain and a RF mixer with -15 dB conversion gain were demonstrated from CVD-grown monolayer MoS2 FETs. Krasnozhon et al.370 fabricated top-gated MoS2 transistors operating at GHz frequencies. RF FETs were prepared using single layer, bilayer, trilayer, and multilayer MoS2 crystals. FETs had an Lg of 240 nm, 50 nm long underlap regions on both sides, and channel widths in the (9–21.5) μm range. A layer of high-κ dielectric HfO2 (30 nm thick) was deposited by ALD method. The trilayer MoS2 showed the fT = 6 GHz. The variation of intrinsic fT as a function of the number of MoS2 layers was studied. There is an increase in the cutoff frequency with increasing number of MoS2 38


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layers from single layer to trilayer. The fT of more than 1 GHz was observed for MoS2 FETs after de-embedding. The fT values of 2 GHz for Vds = 2 V and Vg = –3 V for single-layer MoS2 transistor, 3 GHz for bilayer, and a highest fT of 6 GHz were measured for trilayer MoS2 transistor for Vds = 2.5 V and Vg = –1.5 V; whereas, the fT of 5.5 GHz for the multilayer MoS2 transistor was recorded at Vds = 2.5 V and Vg = –1.0 V. Mason’s unilateral gain (U) plotted as a function of frequency showed an fmax of 8.2 GHz for a trilayer MoS2 device.

5.5 EFFECT OF 2D/2D CONTACTS ON MOBILITY 2D/2D contacts are another strategy for improving the charge carrier mobility of MoS2 FETs. Experimentally high mobilities in TMDs have been demonstrated by Chuang et al.,371 where lowresistance 2D/2D source/drain electrode contacts were used for fabricating WSe2, MoS2, and MoSe2 based FETs. Figure 9 shows the schematics and side view of 3.5 nm thick WSe2 FET with 2D/2D contacts and degenerately p-doped 2D WSe2 (Nb0.005W0.995Se2) contact electrodes, and source-drain current (Ids) versus back-gate voltage (Vbg) plots. The WSe2 channel is encapsulated between h-BN layers. The gate h-BN dielectric is a 40 nm thick layer on SiO2 substrate. The WSe2 FETs yielded the current IOn/IOff ratio of >109 at RT at Vds of -1 V and SS value of 460 mV/dec for WSe2 channel of (10.8 µm × 3.0 μm) length by width. The SS value was further reduced to 63 mV/dec with a top-gate WSe2 FET having h-BN gate dielectric. Few-layer WSe2 FETs developed using undoped 2D WSe2 channel (CVD) and degenerately p-doped 2D WSe2 source/drain electrodes (exfoliated) showed low Rc of 0.3 kΩ·μm, current IOn/IOff ratio as high as >109 at RT, and drive currents over 320 μA/μm. Figure 10 shows the two-terminal conductivity (σ) of WSe2 and MoS2 as a function of temperature, and the back-gate voltage for low-resistance 2D/2D contacts. The WSe2 channel was (14.8 μm × 4.7 μm × 3.5 nm) in length by width by thickness. 39



The MoS2 channel was (13.0 μm × 2.7 μm × 6.8 nm) in length by width by thickness. The twoterminal μFE of WSe2 and MoS2 devices with 2D/2D contacts was measured as a function of temperature. The field-effect hole mobilities for both WSe2 and MoS2 FETs increased with decreasing temperature. The maximum μFE of ((2.1 and 2.8) × 103) cm2/Vs for WSe2 and for MoS2, respectively were measured at 5 K, while the RT field-effect hole mobility of WSe2 was 2.2 × 102 cm2/Vs. The hole mobility for MoS2 FET increased from (1.8 × 102 to 2.8 × 103) cm2/Vs at RT to 5 K. The high mobilities in p-type WSe2 and MoS2 FETs are associated with low-resistance 2D/2D contacts, as well as h-BN channel passivation. This new 2D/2D ohmic contact strategy is suitable for p-type and n-type FETs.

Figure 9. (a) Schematics of WSe2 FET with 2D/2D contacts and side view depicting degenerately p-doped 2D WSe2 (Nb0.005W0.995Se2) contact electrodes with WSe2 channel. (b) Source-drain current (Ids) versus back-gate voltage (Vbg) plots yielding the current on/off ratio of >109 for WSe2 FET at room temperature. Reprinted with permission from ref 371. Copyright 2016 American Chemical Society.

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Figure 10. (a) Two-terminal conductivity (σ) of WSe2 as a function of temperature and the backgate voltage (Vbg) at Vds = −50 mV. (b) Temperature-dependent σ of MoS2 as a function of backgate voltage at Vds = −10 mV. Temperature-dependent two-terminal field-effect hole mobilities in (c) WSe2 and (d) MoS2. Reprinted with permission from ref 371. Copyright 2016 American Chemical Society. The van der Waals MoS2/WSe2 heterojunctions were also studied by Nourbakhsh et al.372 The heterojunction FET having few-layer WSe2 stacked on multilayer MoS2 by dry transfer methods showed room-temperature negative differential resistance (NDR), which indicated lateral band-toband tunneling. A positive-to-negative transconductance was observed in the MoS2/WSe2 heterojunction transistors. Figure 11 shows the schematics of few-layer MoS2 and WSe2 FETs, MoS2/WSe2 heterojunction FET, and their transfer characteristics (Id–Vg). The MoS2 FET exhibited an n-type semiconducting behavior, whereas WSe2 FET showed both n- and p-type 41



behavior, leading to ambipolar transfer characteristics. The transfer characteristics (Id–Vg) of a back-gated in-series MoS2/WSe2 FET exhibited three distinct regions; region I (Vg < –53 V), region II (–53 V < Vg < –30 V) where current peak appeared at –42 V (assigned to a p-n junction), and region III (Vg > –30 V), where the current gradually increases as Vg increases (modeled by an n–n heterostructure). In another study, Shin et al.373 reported a van der Waals Schottky junction between conducting NbS2 and semiconducting MoS2 for developing vertical, as well as in-plane, current devices. The Schottky diode shows an ideality factor of (1.8–4.0), and a current IOn/IOff ratio of (103–105). The metal semiconductor field-effect transistors (MESFETs) exhibited small gate hysteresis, low threshold gate voltage of (–0.5 to –1.0) V, and SS of (60–80) mV/dec, arising from low-density traps at the NbS2/n-MoS2 interface. The mobility of MESFETs was affected by the source/drain contact for MoS2 channel, where the highest RT mobility of 800 cm2/Vs was recorded with graphene source/drain contact.

Figure 11. (a) Schematic illustration of back-gated few-layer MoS2- and WSe2-FET and transfer characteristics (Id−Vg). The inset is an optical image of a stacked MoS2/WSe2 FET. (b) Schematic 42


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illustration and transfer characteristics (Id−Vg) of the MoS2/WSe2 heterojunction FET. The inset shows that the transconductance peaked in the region II. Reprinted with permission from ref 372. Copyright 2016 American Chemical Society.

The electronic charge transport and charge carrier mobility of 2D TMDs have been discussed in detail in the literature.374−376 The charge carrier mobility of MoS2 FETs depends upon a number of factors, as discussed above. Table 3 summarizes the field-effect mobility and the current IOn/IOff ratio of MoS2 FETs integrated with high- gate dielectrics. The charge carrier mobility of MoS2 FETs ranges (4–1,460) cm2/Vs, while the current IOn/IOff ratio ranges (104 – 109), depending upon the nature of the high- gate dielectrics, such as electrolyte,311 fluoropolymers,350, 351 Al2O3,358, 365, 377−379 373

HfO2,244, 312, 365, 369, 381 SiNx,382 ZrO2,384 and SrTiO3,385 in addition to the 2D/2D contacts,371,

by the number of MoS2 atomic layers, contact electrodes, and measurement temperature. All of

these factors play an important role in engineering the charge carrier mobility of MoS2 FETs.

Table 3. The effect of high- gate dielectrics on the field-effect mobility (μFE) and the current IOn/IOff ratio of MoS2 FETs. --------------------------------------------------------------------------------------------------------------------MoS2 Channel Mobility (μFE) IOn/IOff Drain Voltage ref. 2 Layer & Preparation [cm /Vs] Ratio (Vd), [V] --------------------------------------------------------------------------------------------------------------------MoS2/P[VDF-TrFE]a copolymer 625 8 × 105 3 350 b MoS2/P(VDF-TrFE-CFE) 51.94 3.27 × 106 1 351 HfO2/MoS2 Monolayer/SiO2 MoS2 Bilayer/HfO2 MoS2 Monolayer/PEOc/LiClO4d MoS2 (23 Layers)/Al2O3 MoS2 (exfoliated)/Al2O3 MoS2 Multilayer (exfoliated)/Al2O3 MoS2 Monolayer (exfoliated)/Al2O3 MoS2 Bilayer (exfoliated)/Al2O3 MoS2 Trilayer (exfoliated)/Al2O3 MoS2 Monolayer (CVD)/SiO2

108 107 106 108 107 107 106 105

217 313 150 517 700 100 170 25 15 54 43


0.01 0.5 0.3 2 0.2 0.2, 0.4, 1 0.5 0.5 0.5 0.1, 1

244 312 311 377 378 379 358 358 358 380


MoS2 Monolayer (CVD)/PIe MoS2 Monolayer (CVD)/Al2O3 MoS2 Monolayer (CVD)/HfO2 MoS2 Monolayer (CVD)/HfO2 MoS2 Multilayer/SiO2 substrate MoS2 Multilayer/PMMA substrate MoS2 Multilayer (exfoliated) SiOx/SiNx MoS2 Multilayer (exfoliated)/SiNx MoS2 Multilayer (exfoliated)/SiO2 MoS2 Multilayer Flakes MoS2 Multilayer Flakes/Al-IZO MoS2 Multilayer Flakes/Al2O3/Si HfO2/MoS2 Multilayer Flakes/Al2O3/Si MoS2 Multilayer Flakes/SiO2 ZrO2/MoS2 Monolayer (CVD)/SiO2 MoS2 Monolayer (exfoliated)/SrTiO3

22 30 24 63 60 480 12 4 5 1.4 33.6 20.5 25.2 7.4 12.1 1460

107 107 105 105 104 105 104 107 107 105 107 107 107

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0.05, 0.5, 1, 2 0.1, 1 0.1, 1 0.01 1 1 1 1 1 0.6 0.6 0.6 5 -

380 381 381 369 301 301 382 382 382 383 383 365 365 365 384 385

2D/2D Contacts WSe2/p-WSe2 (CVD/exfoliated)

250 (RT) 109 371 9 620 (80 K) 10 371 MoS2/p-MoS2 (CVD/exfoliated) 200 (RT) 106 371 700 (80 K) 106 371 NbS2/n-MoS2/Graphene 800 103−105 1, 2, 3 373 --------------------------------------------------------------------------------------------------------------------a(VDF-TrFE) is vinylidene fluoride-trifluoroethylene. bP(VDF-TrFE-CFE) is poly(vinylidene fluoride-trifluoroethylene-chlorofloroethylene). c(PEO) is poly(ethylene oxide). d(LiClO4) is lithium perchlorate. e(PI) is polyimide.

One of the concepts for using 2D TMDs for electronics is to create novel materials that could match the electrical properties and processing speed of conventional inorganic semiconductors. Cao et al.386 used quantum transport simulations to compare the performance of Si and monolayer MoS2-based conventional and junction less (JL)-FETs, by setting 5.9 nm transistor channel length parameter, according to the International Technology Roadmap for Semiconductors (ITRS). The only difference between conventional and JL-FETs was that the JL-FETs channel as well as their source/drain regions had the same impurity doping density. The high- HfO2 (dielectric constant = 23.5) was used as the gate oxide in a double-gate FET topology during the quantum simulation. 44


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The thickness of Si film was set to 2 nm, much smaller than that of 5.9 nm projected by ITRS.387 A comparison was made between the transfer characteristics of 2 nm Si and monolayer MoS2 based conventional and JL-FETs, and their SS as a function of doping density. The doping density and current level were kept the same for all FETs. The doping density is 5 × 1020 /cm3 for both Si and monolayer MoS2 with both conventional and JL-FET structures. The lowest doping density was restricted to ~ 5 × 1019 /cm3, in view of the source/drain contact and ohmic contacts. MoS2 FETs showed steeper subthreshold slopes in comparison with Si FETs, due to smaller thickness. The thickness of monolayer MoS2 is 0.65 nm. The SS was found to degrade with increasing doping density for all Si and MoS2-based FETs, while SS was improved for JL-FETs, because the SS values are lower for JL-FETs compared with conventional FETs at higher doping density. These parameters indicate the superiority of JL-FET structures over conventional FET structures. MoS2based JL-FETs can be used for low-standby-power (LSTP) applications, where high carrier mobility is not required. The degradation of mobility in MoS2 JL-FETs due to impurity doping could an issue for device applications. Fiori et al.388 studied saturation velocity in few-layer MoS2 transistors, where saturation velocity (0.28 × 107 cm/s) was found to be comparable to silicon, but lower than that of graphene (5.5 × 107 cm/s) and semiconductors, such as InSb, GaP, GaN, and InGaAs.389 Overall, the 2D materials show great potential for electronic applications.390 Figure 12 shows a comparison of the charge carrier mobilities, current IOn/IOff ratios, and bandgap for different types of semiconducting materials.391 The field-effect mobility, current IOn/IOff ratio, and bandgap of a single-layer MoS2 compare well with inorganic semiconductors (Ge, InP, Si, GaAs, SiC), single-walled carbon nanotubes (SWCNTs), graphene, and other 2DTMDs.392−399 Like graphene, TMDs including MoS2 also inherit mechanical strength and flexibility for developing ultrathin electronic and optoelectronic devices. TMD-based FETs also 45



indicate great promise for developing large-area electronic devices. The FETs from CVD grown single-crystal MoS2 channels were developed by Wu et al.400 Raman spectra of single-layer and bilayer MoS2 grains were recorded, which showed two characteristic Raman active modes related to the in-plane and out-of-plane vibrations of sulfide groups. Raman spectroscopy was used to determine the numbers of layers of individual MoS2 grains. The peak frequency differences (Δs) of (18 and 22) cm-1 between E12g and A1g modes evidenced the grains as single-layer and bilayer MoS2 crystals, respectively. The PL spectrum measured at a laser excitation wavelength of 532 nm exhibited emission peaks at (676 and 630) nm, respectively, for a bilayer MoS2 crystal. The mobility of 17.3 cm2/Vs and the current Ion/Ioff ratio of 4×108 for a bilayer back-gated MoS2 FET were observed, and were found to be higher compared to FETs prepared by CVD polycrystalline MoS2 films. By managing the grain boundaries and the contamination during transfer process, the high-quality MoS2 atomic layers deposited on SiO2 showed electronic properties comparable to exfoliated high quality MoS2 flakes. Yu et al.401 also compared and analyzed the carrier mobility of TMDs with conventional inorganic semiconductors, including Si, Ge, and III–V compound semiconducting materials, organic semiconductors, and black phosphorus, and pointed out the better stability and suitability of TMDs in electronic and optoelectronic devices than those of conventional bulk semiconductors, which eventually degrade, due to the scattering of surface roughness.

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Figure 12. A comparison of field-effect mobility, current IOn/IOff ratio and bandgap of a singlelayer MoS2 with other organic materials, inorganic semiconductors (Ge, InP, Si, GaAs, SiC), SWCNTs, graphene and other 2D-TMDs. (a) Field-effect mobility versus current IOn/IOff ratio and (b) Field-effect mobility versus bandgap. The current IOn/IOff ratio in the range of 104−107 is desirable for electronic applications as suggested by the International Technology Roadmap for Semiconductors.392 The properties of single-layer MoS2 are compared with bilayer graphene,393 10 nm thick multilayered black phosphorus film394 SWCNTs,395 few-layered TMDs films396 organic semiconductors397 and traditional semiconductors such as Si, Ge, InP, GaAs.398,399 Reprinted with permission from ref 391. Copyright 2015 American Chemical Society.

6. MoS2-BASED FLEXIBLE FETs 2D materials have been viewed as playing an important role in developing flexible electronics and optoelectronics. This section discusses the flexible FETs developed from MoS2 atomic layers, and their applications in fabricating flexible LED displays, NVM devices, and PNGs. The ionic liquid (IL)-gated FETs having bilayer and few-layer MoS2 were fabricated by Perera et al.402 The ILgated FET devices showed electron mobility of 60 cm2/Vs at 250 K, significantly higher than that of Si-back-gated devices without ionic liquid gate (5 cm2/Vs). The mobility of IL-gated FETs was found to increase from (100 to 220) cm2/Vs at (180 to 77) K, as measured by the four-terminal 47



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method, due to the limited phonon scattering. IL-gated FETs showed an ambipolar behavior with current IOn/IOff ratio >107 for electrons and 104 for holes, and SS of 50 mV/dec at 250 K. Pu et al.403 fabricated flexible MoS2 Thin-Film electric double-layer transistors (EDLTs) using ion gel films as a gate dielectric prepared from ethyl propionate solution of poly(styrene-blockmethylmethacrylate-block-styrene) and the ionic liquid namely 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)

imide

([EMIM][TFSI]).

Raman

spectroscopy

and

AFM

measurements confirmed the MoS2 films as trilayers grown by CVD. Figure 13 shows a schematic of a flexible MoS2 EDLT fabricated using ion gel as a gate dielectric on a PI substrate, and drain current-gate voltage curves of the MoS2 EDLT for 0.75 mm bending radius, and the dependence of the drain current and carrier mobility on bending radius. The ion gel films showed a capacitance of 4.67 μF/cm2 on the PI substrate at 1 Hz. The mobility of MoS2 EDLT on the flexible PI substrates was 3.01 cm2/Vs between Vg of (1.1 and 1.5) V with current IOn/IOff ratio of 103. The mobility of MoS2 EDLT was 12.5 cm2/Vs between the Vg of (1.5 and 2.0) V. MoS2 EDLTs fabricated on flexible PI showed excellent mechanical flexibility on bending, because no degradation was noticed for a curvature radius of 0.75 mm. The mechanical flexibility of MoS2 EDLTs was found to be better, compared with FETs based on graphene and CNTs.

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Figure 13. (a) Schematic and photograph of a flexible MoS2 thin film EDLT fabricated using ion gel as a gate dielectric on a PI substrate. (b) Drain current-gate voltage curves of the MoS2 EDLT for 0.75 mm bending radius of 0.75 mm and before and after conducting the bending test experiments. The inset is the photograph of MoS2 thin film at the 0.75 mm bending radius. (c) The drain current (red line) and carrier mobility (blue line) as a function of bending radius. The inset shows the bending measurements. Reprinted with permission from ref 403. Copyright 2012 American Chemical Society.

The effect of MoS2 atomic layer thickness on charge carrier mobility has been studied. Pak et al.404 reported thickness-modulated few-layer MoS2 films ranging from (single to five)-layer (L1 to L5) films of (0.72 to 12.69) nm thickness. The Raman peak differences (Δ) (of 19.7, 21.2, 23.4, 24.7, and 27) cm-1 confirmed the formation of (single, two, three, four, and five)-layer (L1–L5), respectively. The PL spectra showed bandgap at 1.89 eV confirming a single-layer MoS2 film. The optical transmittance of 95 % was measured for a single-layer MoS2, which decreased with increasing MoS2 film thickness. The bilayer MoS2 films showed > 90 % optical transparency, mobility of 12.24 cm2/Vs, and current IOn/IOff ratio of 1.57 × 106, whereas three-layer MoS2 film 49



exhibited mobility of 0.44 cm2/Vs, and current IOn/IOff ratio of 5.7 × 104. The single-layer MoS2 transferred onto a flexible PET substrate indicated high optical transparency and flexibility for wearable electronics. In another study,405 CVD-grown monolayer MoS2 FETs showed (13.9 ± 2) cm2/Vs and current IOn/IOff ratio of < 105, with high mechanical durability under 1 % strain. Yoon et al.406 prepared flexible and optically transparent MoS2 FETs using exfoliated MoS2 film as channel, and CVD-grown graphene as source/drain electrodes on a flexible PET substrate. The graphene/MoS2/c-PVP/ITO/PET configuration FETs showed ~74 % optical transmittance, mobility of ~4.7 cm2/Vs, and current IOn/IOff ratio of >104. The mobility and Vth shift measured up to 10,000 bending cycles at a bending radius of 2.7 mm showed some degradation in electrical performance of FET devices as a function of bending cycles, which was recovered upon annealing. The flexible FETs developed from multilayer MoS2 on a flexible PI substrate with embedded AgNWs showed mobility of 141 cm2/Vs and current IOn/IOff ratio of 5 × 105.407 The MoS2 FETs showed no noticeable degradation under repeated bending cycles with (5 and 10) mm bending radii. The high mechanical stability combined with transparency make MoS2 FETs suitable for flexible electronics. Large-area flexible MoS2 FETs are important for wearable electronics. In this context, Shinde et al.408 developed the superhydrophilic surface of SiO2/Si substrate by functionalizing with hydroxyl (OH) functional groups. In this process, large-area MoS2 layers were grown by metalorganic chemical vapor deposition (MOCVD) technique on the hydroxyl functionalized SiO2/Si substrate, and thereafter PI thin film was spin-coated over the wafer-scale MOCVD-grown MoS2 layers. The PI thin film acted both as a carrier layer, as well as a flexible substrate. The large-area 3600 MoS2 FET devices were developed on PI substrate without any damage over a 4 inch wafer, which showed high flexibility in crumpled form. The rollable MOS2 inverter with 50


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NAND and NOT gates showed high mechanical flexibility of electronic circuits. Figure 14 shows a schematic of the fabrication process of large-area flexible MoS2 FETs on flexible PI thin film substrate, photographic images of crumpled, bendable, and rollable MoS2 circuits, and the electrical and mechanical properties of flexible NAND and NOT logic gates under different bending radii. The flexible MoS2 FETs fabricated on PI thin film substrate showed electron mobility of 6.7 ± 20 cm2/Vs, current IOn/IOff ratio of 105, and Vth of 2.5 V; however, the carrier mobility decreased by 20 % after 100 bending cycles, and by changing the bending radius from (12 to 3.2) mm. The flexible MoS2 FETs integrated NAND and NOR logic gates also demonstrated strong mechanical properties.

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Figure 14. (a) Schematic depicting fabrication process of large-area flexible MoS2 FETs on flexible PI thin film substrate. (b) Photograph of large-area crumpled MoS2 FET device. Scale bar = 1 cm. (c) Photograph of 4 x 4 cm bendable and rollable MoS2 circuits wrapped around a glass pipette. Magnified circuit structure of NOT and NAND devices. Scale bar = 1 cm. (d–f) Schematic and electrical characteristics of the MoS2 inverter. Both NAND and NOT gates were fabricated on the flexible PI thin film substrate applying water-assisted transfer method. (g) Voltage transfer characteristics (VTCs) of the MoS2 inverter at a supply voltage (VDD) of 5 V in flat state and bending radii of 3.2, 6.4, 12 mm. The inset shows the corresponding gain at different bending radii. (h) The time-dependent output electrical characteristic of MoS2 based NAND gate at different 52


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bending radii, flat and recovery states. The inset indicates the change of output voltage at different bending radius, flat and recovery states. Reprinted with permission from ref 408. Copyright 2018 Wiley-VCH.

The role of 2D graphene in improving the electrical properties and flexibility of MoS2 FETs has been investigated. Zhu et al.409 used single-atom thick graphene monolayer as a bottom gate to promote the growth of high-κ HfO2 dielectric layer and decrease the gate leakage in monolayer MoS2 FETs. The high displacement fields over 8 V/nm were applied to graphene gate for electrostatically doping MoS2, which decreased the Rc between monolayer MoS2 and Ni/Au electrodes to 2.3 kΩ·μm. Figure 15 shows a schematic of the MoS2 FET device with graphene bottom local-gate, transferred CVD-grown MoS2 layer onto the graphene/HfO2, field-effect mobilities between (0.4 and 1.0) μm channel length, current IOn/IOff ratio at (14 and 50) nm short channel lengths, photographs of a MoS2 FET fabricated on flexible PEN substrate, and transport properties. The monolayer MoS2 FETs showed an SS of 64 mV/dec with a channel length of 1.5 μm at RT with very low hysteresis, Vth of ~0.5 V, and channel conductance of >100 μS/μm. The quality of high-κ HfO2 dielectric layer was significantly improved by the graphene bottom gate, where the gate leakage current of HfO2 (16 nm)/TiO2 (1.2 nm) gate stack was below 10 pA, before the dielectric breakdown at 10 V. The breakdown field with 16 nm thick HfO2 dielectric layer was 0.58 V/nm, which further increased to 1 V/nm as the thickness of HfO2 layer was reduced to 4 nm. The μFE of MoS2 FETs varied from (11.4 to 21.4) cm2/Vs, as the channel length increased from (0.4 to 1.0) μm. The current IOn/IOff ratio of MoS2 FETs changed over (2–3) orders of magnitude as a function of VDS at (14 and 50) nm channel lengths, but remained over 105 for VDS of below 2.0 V. The 14 nm MoS2 FET devices showed SS of 86.5 mV/dec and the current IOn/IOff ratio of 3.3 × 106 for VDS of 0.1 V, whereas the SS of 73 mV/dec and current IOn/IOff ratio of 3.7 × 107 were 53



recorded for 50 nm channel length at VDS value of 1.6 V. The flexible monolayer MoS2 FETs fabricated onto PEN substrate showed an SS of 75.5 mV/dec and the current IOn/IOff ratio of 4.8 × 107. The MoS2 FET devices showed no noticeable degradation up to channel length of 14 nm. The graphene bottom-gate MoS2 FETs with high optical transparency can be used for displays and photodetectors.

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Figure 15. (a) Schematic illustration of MoS2 FET having graphene bottom local-gate. (b) Optical microscope image of the CVD-grown MoS2 layer transferred onto the graphene/HfO2. (c) Fieldeffect mobilities as a function of channel length. (d) current IOn/IOff ratio as a function of VDS at channel lengths (Lch) of 14 and 50 nm. (e) Optical microscope photographs of a MoS2 FET device fabricated on transparent flexible PEN substrate. The HfO2 layer is 16 nm thick. Both MoS2 and graphene gate are high transparent. (f) Transport properties showing a SS of 75.5 mV/dec. Reprinted with permission from ref 409. Copyright 2018 American Chemical Society.

Wearable electronics need flexibility in MoS2-based RF circuits. Cheng et al.410 used mechanically exfoliated MoS2 flakes for fabricating flexible RF circuits from self-aligned MoS2 FETs. Figure 16 shows a schematic of a dual-channel self-aligned MoS2 FET, TEM images of self-aligned MoS2 FETs with source/drain electrodes, and HR-TEM image showing the interface between MoS2 and the transferred high-κ dielectric Al2O3 gate stack. The HR-TEM confirmed eight-layer MoS2 flakes. Figures 16 (d–f) show the mobility and current IOn/IOff ratio of back-gated MoS2 transistor as a function of MoS2 thickness, the Ids–VTG characteristics for top-gated MoS2 transistor at different bias voltage, and the On-state current and transconductance top-gated MoS2 transistor, respectively. The flexible MoS2 FET circuits were fabricated on a layer of SU-8 substrate. The mobility of the MoS2 FETs reached 170 cm2/Vs at 0.5 V of drain voltage. The dual-channel self-aligned MoS2 FET showed an intrinsic fT and fmax of (1.3 and 1.5) GHz at a gate length of 68 nm at drain bias of 5.0 V and top-gate voltage of 2.7 V for multilayer MoS2 flakes mechanically exfoliated on a silicon substrate. The fT and fmax of (10.2 and 14.5) GHz were measured for 68 nm channel length MoS2 FET on quartz substrate with source-drain bias of 5 V and top-gate voltage of 3.5 V. When MoS2 FETs were fabricated on PI substrate, the fT and fmax of (4.7 and 5.4) GHz were noted for the similar channel length under the source-drain bias and top-gate voltage of (8.0 and 3.7) V, respectively. The current IOn/IOff ratio of FETs ranged (106–107), depending upon different substrates. The performance of MoS2 FETs on flexible substrate remained almost unchanged up to 1,000 bending 55



cycles under bending radius of 5 mm. The 13 nm thick layer of high-κ dielectric Al2O3 was deposited on MoS2 flakes by ALD method. The SS of 60 mV/dec at RT, peak gm of 60 μS/μm for drain voltage of (Vds) of 5 V, and saturation velocity of 1.8×106 cm/s were measured. Figures 16 (g–i) also show small-signal current gain |h21| as a function of frequency for MoS2 FETs with a channel length of (216, 116, and 68) nm, and the fT and fmax as a function of gate length measured for twenty MoS2 FETs. At a d.c. bias of 5 V, the intrinsic cutoff frequencies of (13.5, 26 and 42) GHz and fmax of (16, 34, and 50) GHz were measured at gate bias of (2.1, 2.3, and 1.9) V for channel length of (216, 116, and 68) nm, respectively. The fmax depends on the gate length for MoS2 FETs. Figures 16 (j–l) show a photograph of flexible MoS2 transistors, and the Ids-VTG characteristics of a top-gated flexible MoS2 FET as a function of bending cycles. The top-gated MoS2 FETs on a flexible substrate having a channel length of 116 nm showed no degradation of Ids–VTG characteristics under 2.0 V bias voltage at a bending radius of 5 mm up to 1,000 bending cycles. However, slight changes were observed for the On-state-current and transconductance as a function of bending cycles.

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Figure 16. (a) Schematic representation of a dual-channel self-aligned MoS2 FET showing source/drain and transferred gate stacks. The inset is the schematic cross-section of self-aligned MoS2 FET device. (b) The cross-sectional TEM image of a self-aligned MoS2 FET device. Scale bar is 50 nm. (c) HR-TEM image showing interface between MoS2 and the transferred high-κ dielectric Al2O3 gate stack. Scale bar is 3 nm. (d) The mobility and current IOn/IOff ratio of backgated MoS2 transistor as a function of MoS2 thickness. (e) The Ids-VTG characteristics for a 116 nm channel length top-gated MoS2 transistor of 116 nm channel length at different bias voltage. (f) The On-state current and transconductance top-gated MoS2 transistor. (g) Small-signal current gain |h21| as a function of frequency for three MoS2 FETs with a channel length of 216 (blue), 57



116 (red) and 68 nm (black). (h) Maximum available gain (MAG) as a function of frequency for similar MoS2 FETs. (i) The fmax as a function of gate length measured for twenty MoS2 FETs. (j) Photograph of MoS2 transistors on a flexible substrate. (k) The Ids-VTG characteristics of a topgated MoS2 FET on a flexible substrate up to 1000 bending cycles. (l) On-state-current (Ids) and transconductance (gm) as a function of bending cycles. Reprinted with permission from ref 410. Copyright 2014 Nature Publishing Group.

For practical applications, the mechanical flexibility of MoS2 FETs should be tested under different bending radii and the number of bending cycles. Chang et al.381 fabricated flexible MoS2 transistors using atomic-layer-deposited high-κ HfO2 and Al2O3 films for low-power systems. Figure 17 shows the AFM height profile, Raman spectra of the MoS2 flake, a schematic and an optical microscopic image of the MoS2 FET, and photograph of the flexible MoS2 FET. AFM analysis indicated a thickness of 15 nm for the MoS2 flake, while Raman spectroscopy showed a peak difference of 25 cm-1 between the E12g and A1g peaks, estimating a multilayer with four or more layers in a MoS2 flake. Figure 18 shows the mechanical flexibility of MoS2-based FETs with high-κ gate dielectrics, Al2O3 and HfO2 in terms of normalized low-field mobility, and current IOn/IOff ratio as a function of bending radius for HfO2 and Al2O3 gate dielectrics. Degradation was observed below 1 mm bending radius, arising from the failure of the gate dielectric. MoS2 FETs fabricated with Al2O3 as a high-κ gate dielectric on flexible PI substrate showed low-field mobility of 30 cm2/Vs with Y-function method, a current IOn/IOff ratio over 107, and a subthreshold slope at 82 mV/dec. MoS2 FETs were found to be mechanically robust up to a bending radius of 2 mm for Al2O3, and 1 mm for HfO2 dielectric. The appearance of cracks was found to be slower in HfO2 film, in comparison with Al2O3 dielectric deposited on flexible substrates. MoS2 FETs on HfO2 retained 70 % of their properties and the current IOn/IOff ratio of 104, even after deformation at a bending radius of 1 mm, whereas MoS2 FETs with Al2O3 dielectric showed a significant 58


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degradation below 2 mm bending radius, due to the structural damage of the dielectric layer. The MoS2 FET devices also showed thickness-dependent mobility within the (7.9–21.0) nm thickness range, where maximum mobility was observed at 7.9 nm thickness, and thereafter, mobility decreases with increasing film thickness. Both Raman spectrum and AFM analysis showed that the MoS2 remained intact, while FET devices failure was caused by the dielectrics. The Raman spectrum of MoS2 FET measured after the bending test showed no changes in the vibration modes. Stretching experiments were performed to observe crack density dependence on tensile strain for both Al2O3 and HfO2 dielectrics, which indicated the critical crack onset strain as (1.69 and 1.72) % for Al2O3 and HfO2, and the saturation of crack density at (145 and 164) mm-1 for Al2O3 and HfO2, respectively. The crack propagation velocity was noticed as (28.4 and 4.9) μm/s for Al2O3 and HfO2, respectively.

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Figure 17. (a) AFM height profile showing the thickness of around 15 nm for the MoS2 flake. (b) Raman E12g and A1g vibration peaks showing a difference of 25 cm–1 for the MoS2 flake which confirmed the multilayer film. (c) The schematic illustration of the MoS2 FET on a flexible PI substrate. (d) The optical microscopic image of the MoS2 FET after the source/drain patterned using e-beam lithography. The channel length of MoS2 FET device is 1 μm and the MoS2 flake of about 10 nm thickness are shown by the dark blue color. (e) Photograph of the flexible MoS2 FET on a PI film. Reprinted with permission from ref 381. Copyright 2013 American Chemical Society.

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Figure 18. A study of the mechanical flexibility of MoS2 FETs with Al2O3 and HfO2 gate dielectrics. (a) A photograph showing flexible MoS2 FET at a bending radius of 5 mm. (b) Normalized low-field mobility as a function of bending radius for HfO2 and Al2O3 gate dielectrics. (c) Normalized ON current (VD = 1 V) as a function of bending radius for HfO2 and Al2O3 gate dielectrics. The bending radius is 1 mm for HfO2 and 2 mm for Al2O3 gate dielectric. (d) current IOn/IOff ratio as a function of bending radius. Significant degradation occurred below a bending radius of 1 mm with HfO2 dielectric due to the failure of gate dielectric. Reprinted with permission from ref 381. Copyright 2013 American Chemical Society.

Both 2D h-BN dielectric and few-layer graphene gate have been explored for developing flexibility MoS2 FETs. Lee et al.411 fabricated flexible FETs with MoS2 channels using h-BN dielectric and few-layer graphene gate electrodes. MoS2 FETs showed field-effect mobilities up to 45 cm2/Vs for a trilayer MoS2 on h-BN dielectric, lower than 10 V as operating gate voltage, and retaining of performance up to strain of 1.5 %. The mobilities of the MoS2 FETs show dependence as a function 61



of the number of MoS2 layers, and on different substrates. A monolayer MoS2 on h-BN showed the mobility of 10 cm2/Vs compared to 1 cm2/Vs for a monolayer MoS2 on SiO2. FETs with MoS2 channel, h-BN dielectric, and few-layer graphene gate showed mobilities of (12 and 24) cm2/Vs for a monolayer and bilayer, respectively, and the absence of hysteresis. MoS2 FETs fabricated on SiO2 and h-BN substrates showed an increase in field-effect mobility as a function of the number of MoS2 layers. The monolayer MoS2 FETs fabricated SiO2-supported h-BN showed mobility of 7.6 cm2/Vs, compared to the mobility of 0.5 cm2/Vs for SiO2. The current IOn/IOff ratios of both types of MoS2 FETs were (104–106). Figure 19 shows a photograph of the flexible and transparent MoS2 FET fabricated with MoS2 channel, h-BN dielectric, and few-layer graphene gate on the PEN substrate, bending up to 1.5 % strain. The monolayer MoS2 FET on PEN substrate has 95 % transparency, mobility of 29 cm2/Vs, and low operating gate voltage of 5 V, before mechanical bending.

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Figure 19. (a) Photograph of the flexibility and transparent MoS2 FET fabricated with MoS2 channel, h-BN dielectric, and few-layer graphene gate on the PEN substrate. (b) Reflection mode and (c) transmission-mode optical micrographs of the flexible MoS2 FETs. Dashed lines indicate the border of h-BN, MoS2 and graphene. (d) Transfer curves of the flexible MoS2 FET under different bending conditions from zero to 1.5% strain. (e) Relative field-effect mobility of the flexible MoS2 FET as a function of strain. Reprinted with permission from ref 411. Copyright 2013 American Chemical Society.

The flexible monolayer MoS2 FETs have been used to develop RF transistors for GHz applications. Chang et al.380 developed flexible nanodevices using CVD-grown large-area monolayer MoS2 on SiO2/Si substrate. Figure 20 shows the optical images, electrical characteristics of flexible MoS2 FETs, and mechanical bending testing, performance, and comparison of low-power RF TFTs in a GHz range. The low field mobility of MoS2 FETs on PI surface was in the range (19–31) cm2/Vs, depending upon variations in the devices. The CVD-grown flexible monolayer MoS2 FET on PI substrate showed Rc of 9.4 kΩ·μm and a mobility of 22 cm2/Vs. After HfO2-encapsulation, CVDgrown MoS2 FETs showed the current IOn/IOff ratio of higher than 105, Rc of 2.7 kΩ·μm, and mobility of 54 cm2/Vs measured by the Y-function method. The transit frequency and maximum frequency of oscillation of MoS2 FETs indicate their figures of merit in terms of RF applications. MoS2 RF TFT at VG = - 1.0 V and VD = 2.0 V showed an extrinsic transit frequency and fmax of (2.7 and 2.1) GHz, which increased to fT and fmax of (5.6 and 3.3) GHz, respectively, after the RF de-embedding procedure. The RF performance of MoS2 device corresponds to fTLg of 2.8 GHz.m, and veff of 1.8 × 106 cm/s. Three-point bending test performed on flexible MoS2 transistors showed robustness after 10,000 cycles of mechanical bending. MoS2 FETs retained their performance after tensile strain of 1 % was applied. The fabrication of a common-source amplifier with 15 dB gain, mixer with maximum conversion gain of –17 dB, and a wireless AM receiver with carrier 63



frequency of 1.5 MHz demodulating signal at 500 kHz was also demonstrated using flexible CVDgrown MoS2 FETs. Tsai et al.414 used flexible MoS2 FETs to develop a piezoresistive strain sensor. The strain sensitivity of MoS2 can be adjusted over one order of magnitude with the Fermi level tuning of MoS2 through applied gate biasing. The gate-tunable gauge factor (GF) of –40 was measured for 2 nm thick layer, which is comparable to polycrystalline Si (-30). The GF of MoS2 FET was found when the device was switched off (Vbg < 10 V), and reached as high as –40 at Vbg = 20 V subthreshold regime, and then decreased again close to Vbg > 20 V, due to the combined effect of the strain-induced bandgap and the evolution of Fermi level with change in Vbg. The mechanical stability of flexible MoS2 FETs on the Al2O3–ITO–PET substrate was examined under different bending radius and (20, 60, and 80) bending cycles, where the effect of bending on the FET device was found to be negligible. MoS2 FETs showed good mechanical stability, even after 180 bending cycles recorded over several days. The piezoresistivity in MoS2 originated from the change in strain-induced bandgap, as evidenced by the optical reflection spectroscopy. Amani et al.415 developed flexible integrated circuits using CVD-grown MoS2 and graphene layers on PI substrate to use for FETs, logic gate, NVM devices, and photodetector. The MoS2 FETs showed maximum mobility of 18.9 cm2/Vs, current IOn/IOff ratio of >107, and photoresistivity of 33.2 A/W at -15 V back gate voltage. Salvatore et al.416 used PI foil for developing flexible MoS2 FETs, which showed mobility of 19 cm2/Vs, current IOn/IOff ratio of >106, SS value of 250 mV/dec, low gate leakage current of 0.3 pA/μm, and mechanical stability at a bending radius of 5 mm for 10 bending cycles. Low-frequency electronic noise has also been studied for monolayer and multilayer MoS2 FETs. Sangwan et al.417 measured low-frequency noise in single-layer MoS2 FETs. The single-layer MoS2 FETs show n-type behavior with μFE of 34.1 cm2/Vs and current IOn/IOff ratio over 5 × 105 for gate voltage (Vg) = (60 to –60) V. The current noise spectral density 64


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of a single-layer-MoS2 FET shows a 1/fβ behavior, with β = 1.07 for a frequency of 8 kHz. All MoS2 FETs followed 1/fβ behavior with β = 1.0 at RT. Xie et al.418 investigated low-frequency noise in bilayer MoS2 flakes-based transistors as a function of back gate voltages and temperatures of (80 to 470) K. MoS2 has longer trap decay times because of van der Waals bonding in 2D materials, compared to silicon, 3D material, which gives rise to different noise dependence based on carrier density. Due to the weak van der Waals forces between the surface traps and 2D materials, the annealing process was found to reduce the trap density, resulting in a significant decrease in low-frequency noise. Low-frequency electronic noise in MoS2 FETs has been reported by other research groups.419, 420

Figure 20. (a) Electrical characteristics and optical image (inset) of flexible monolayer MoS2 FETs (Lg = 500 nm) at 300 K. (b) ID–VD characteristics of flexible MoS2 FETs. (c) Mechanical bending testing (inset) where tensile strain of 1% was used and optical image of the bending fixture. (d) Optical image (inset) of the RF TFT, Intrinsic transit frequency of 5.6 GHz was obtained for 18 μm device width. (e) The power gain as a function of frequency where an intrinsic fmax of 3.3 GHz 65



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was achieved. (f) A comparison of RF TFTs with MoS2 FETs. The fTL and veff values for flexible substrates and exfoliated MoS2 values on a rigid substrate. Data were taken from ref.410 (right triangles) for MoS2 FETs, ref.369 (down triangle) for MoS2 FETs, ref.370 (up triangles) for MoS2 FETs, ref.412 (circle) for indium gallium zinc oxide (IGZO) TFTs and ref.413 (square) for polycrystalline silicon (poly-Si) TFTs. Reprinted with permission from ref 380. Copyright 2015 Wiley-VCH.

Table 4. summarizes the charge carrier mobility () and current IOn/IOff ratio of flexible MoS2 FETs integrated with high- gate dielectric materials including h-BN, Al2O3 and HfO2. MoS2 FETs were fabricated on flexible polymer substrates. The charge carrier mobility of MoS2 FETs ranges between 3.0 to 200 cm2/Vs depending upon the preparation method, nature of high- gate dielectrics, and the number of atomic layers.380,403,405−411,414−416 Table 4. Mobility and current IOn/IOff ratio of flexible MoS2 FETs developed using flexible polymer substrate and high- gate dielectrics. MoS2 Channel thickness Exfoliated multilayer Exfoliated multilayer

Flexible Substrate PETa PI

Gate Mobility (μ) Dielectric [cm2/Vs] PVPb 4.7 SU-8/Al2O3 141

Ion/Ioff Ratio 104 5×105

Exfoliated trilayer bilayer monolayer

PENc PEN PEN

h-BN h-BN h-BN

45 24 12

Exfoliated multilayer

PI

Al2O3

Lithography multilayer

SU-8

E-beam evaporated trilayer

Drain (Vd) Voltage [V]

Ref.

0.5 1

406 407

105 107 -

0.05 0.05 0.05

411 411 411

19

106

2

416

Al2O3

170

107

0.5

410

Al2O3-ITO-PET

Al2O3

7

104

10

414

CVD-grown trilayer CVD-grown trilayer CVD-grown monolayer CVD-grown monolayer

PI PI PI

ion-gel HfO2 HfO2

12.5 3.0 54 18.9

105 103 >105 107

0.1 0.1 0.1, 1 -

403 403 380 415

CVD-grown monolayer

PET

HfO2

13.9±2

3.2×105

1

405

11.4−21.4

±2×105 105

1

409

CVD-grown monolayer

PEN

HfO2 66


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MOCVD-grown monolayer PI Al2O3 6.7±20 105 1 408 --------------------------------------------------------------------------------------------------------------------aPET is polyethylene terephthalate, bPVP is poly(4-vinylphenol), cPEN is polyethylene naphthalate.

7. MoS2-BASED WEARABLE OLED DISPLAYS Different types of materials, including inorganic semiconductors, liquid crystals, organic conjugated oligomers and polymers, up-conversion phosphor materials, CNTs, and graphene have been applied in developing flexible display technologies.421−431 Withers et al.432 developed LEDs using MoS2 and WS2 for single quantum-well (SQW) and multiple quantum-well (MQW) structures using graphene-based transparent conductive layer and h-BN layer based tunnel barriers. The 3-stacked MoS2, 3MQW MoS2, and 4MQW MoS2 heterostructure showed quantum efficiency (QE) of (3.54, 6.0, and 8.4) %, respectively. Atomically-thin layered TMDs including MoS2,433−437 WS2,438,

439

and WSe2,440,

441

have been studied for their optoelectronic applications in LED

displays, due to their high charge carrier mobility. While TMDs show potential for optoelectronic applications, their photoluminescence quantum yield (PL QY) is rather low. Therefore, defect engineering, oxygen bonding, and chemical doping are some of the strategies that have been used to increase the PL QY in MoS2 thin films.442−445 Amani et al.446 chemically treated exfoliated monolayer MoS2 with organic superacid bis(trifluoromethane)sulfonimide (TFSI), and reported a dramatic 95 % increase in PL QY yield of monolayer MoS2. The PL peak intensity of monolayer MoS2 increased 190-fold after TFSI chemical treatment, compared to exfoliated pristine monolayer MOS2. In another comparative study, monolayer MoS2, WS2, MoSe2, and WSe2 were chemically treated by TFSI under similar conditions, while both WS2 and MoS2 achieved near-unity QY after TFSI chemical treatment; 67



however, the QYs of both MoSe2 and WSe2 were not improved.447 TFSI treatment was found to be effective in the passivation/repairing of defects only in sulfur-based TMDs (WS2 and MoS2), but not effective in selenide-based TMDs (MoSe2 and WSe2). The enhancement in PL QY of TSFI treated monolayer MoS2 was found to be depleted upon exposure to organic solvents and water. Therefore, MoS2 monolayers were encapsulated with chemically resistant and hydrophobic fluoropolymer CYTOP to induce environmental stability, and then the encapsulated CYTOPMoS2 monolayers were treated with TSFI.448 This encapsulation/passivation strategy yielded nearunity QY in monolayer MoS2. Figure 21 shows a schematic of the encapsulation of monolayer MoS2 by fluoropolymer CYTOP and subsequent passivation by TFSI, and PL and optical images, as well as maximum PL QY measured 1 × 10−2 Wcm−2 incident laser power for pristine MoS2 flakes after exfoliation, after encapsulating MoS2 flakes by CYTOP, and thereafter chemical treatment by TFSI solution. The PL QY of the CYTOP-MoS2 monolayers increased by 2 orders of magnitude after TSFI treatment. Furthermore, high PL QY was also retained by CYTOP encapsulated patterned monolayer MOS2 after TFSI treatment and rinsing with water, methanol, and acetone; therefore, this technique can be used in fabricating optoelectronic devices, such as LEDs and solar cells.

68


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Figure 21. (a) Schematic showing encapsulation of monolayer MoS2 by fluoropolymer CYTOP and subsequent passivation by organic superacid bis(trifluoromethane)sulfonimide (TFSI). (b) Histogram showing the PL QY of pristine MoS2 flakes after exfoliation, after encapsulating MoS2 flakes by CYTOP, and thereafter passivating by TFSI. (c−e) PL mapping images and corresponding optical images of monolayer MoS2 (c) after exfoliation, (d) encapsulating by CYTOP (CYTOP-MoS2), and (e) after treatment by TFSI (CYTOP-MoS2 treated). Reprinted with permission from ref 448. Copyright 2017 American Chemical Society.

MoO3 nanoparticles/MoS2 nanosheets hybrid has been used as a hole injection layer (HIL) in fabricating OLEDs.449 The performance of OLED device was significantly enhanced by MoS2/MoO3 HIL, and showed a higher performance than that of OLEDs having PEDOT:PSS/MoO3 nanoparticles as HILs, due to the smooth surface of MoS2/MoO3 HIL and high 69



electrical conductivity. MoS2/MoO3 HIL-based OLED devices also showed long-term stability, compared with PEDOT:PSS-based OLED devices. Flexible OLED display devices have been fabricated from CVD-grown MoS2 FETs on PI substrate.450 The MoS2 OLED devices showed driving circuit of 10 μA, and a current IOn/IOff ratio of 109. OLEDs are most sought for wearable electronic displays. Choi et al.451 developed an ultraflexible AM-OLED based wearable display using MoS2 thin-film transistor (TFT) containing a high-κ dielectric Al2O3 passivation layer. The Al2O3 encapsulated MoS2 TFTs inherit reduced electrical resistance at the MoS2/metal interface region. The MoS2 backplane circuitry based flexible OLED display was developed using a metal-organic CVD-grown bilayer MoS2 thin film on a flexible PET substrate of 6 mm thickness. The top-gated MoS2 TFT is encapsulated between two high-κ dielectric Al2O3 passivation layers, which give rises to carrier mobility, due to the ntype doping of the MoS2 thin film channel and the source/drain electrodes contact regions, allowing the operation of flexible OLED display. Figure 22 shows schematics of the MoS2 TFTbased flexible AM-OLED display, a display attached on a wrist to human skin, and its ultraflexibility on a polymer substrate. The wafer-scale top-gated MoS2 TFTs showed mobility ranging (17 to 20) cm2/Vs, current IOn/IOff ratio of 106, low hysteresis of < 0.75 V, and positive Vth of (5 ± 2) V with TFT device yields of > 95 %. The Al2O3 encapsulated MoS2 TFTs showed 28 times higher mobility than that of conventional back-gated TFTs. The OLED showed the highest luminance of 408 cdm-2 at VGate of 9.0 V; however, the luminance of each pixel can be tailored in the (0–480) cdm-2 range by the gate control, indicating the suitability of MoS2 TFTs for display applications. The OLED also showed a response time of 2.5 ms, and fast IOn/IOff states under repeated VGate of (0±10) V. Figure 23 shows a bilayer MoS2 backplane circuitry based ultraflexible AM-OLED display attached to the human wrist, due to the high mechanical strength of the bilayer 70


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MoS2 film. The AM-OLED display showed stable operation while attached to the human wrist. Four representative letters “M”, “O”, “S”, and “2” were sequentially switched on the human skin with fast response to systematic corresponding switching of the program codes for each alphabetic letter. The AM-OLED displays did not show any device failure after peeling-off from the PDMScoated carrier glass substrate. The AM-OLED displays showed nearly 10 % variation in pixel current under repeated bending at the 0.7 mm bending radius, without affecting the device performance. The ultraflexibility of AM-OLED display originated from the low mechanical stiffness of the whole assembled device.

71



Figure 22. (a) Schematic illustration of MoS2 backplane circuitry based flexible OLED display. The high mobility bilayer MoS2 TFT contains a high-κ dielectric Al2O3 passivation layer which induces n-type doping of the MoS2 channel and S/D electrodes contact regions (top); ultrathin AM-OLED display having MoS2 TFT-based backplane array (middle). AM-OLED display is attached on wrist to human skin (bottom). (b) Schematic of ultrathin AM-OLED display having different layers. The AM-OLED display system is about 7 mm thick. (c) Photograph of AM-OLED display fabricated on a flexible polymer substrate showing ultraflexibility. The inset shows AMOLED display circuit in the flat state. Reprinted with permission from ref 451. Copyright 2018 American Association for the Advancement of Science.

Figure 23. (a) Photograph of MoS2 backplane circuitry based ultrathin structured flexible AMOLED display attached on the human wrist while the AM-OLED display is operated; (b) Current mapping data obtained during the display of the letter “M” where the current of ON pixel is depicted by green dots and OFF pixel by black dots. (c) Photograph of dynamic operation of AM72


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OLED display on the human wrist with an external circuit where representative letters “M,” “O,” “S,” and “2” are sequentially switched on the skin corresponding to the active-matrix line addressing. (d) Photograph showing peel-off process of AM-OLED display from a carrier glass substrate. The flexible AM-OLED display is folded during peel-off process due to the ultraflexibility of entire display system. Reprinted with permission from ref 451. Copyright 2018 American Association for the Advancement of Science.

8. MoS2-BASED FLEXIBLE MEMORY DEVICES Non-volatile semiconductor memories are one of the key components of all electronic devices, such as computers, cell phones, digital televisions, cameras, optical disks, flash memory chips, and global positioning systems (GPS).452 Atomically-thin layered MoS2 has been used to develop NVM devices.453−459 In addition, synergistic nanocomposites of MoS2 thin films have been prepared with other van der Waals heterostructures, ferroelectrics, metal nanoparticles, and organic polymers; therefore, different types of NVM devices have been fabricated using MoS2/graphene nanocomposites,460−462 MoS2 QDs/h-BN nanocomposite,463 black phosphorus/hBN/MoS2heterostructures,464 MoS2/PbS van der Waals heterostructures,465 MoS2/PdNPs hybrid,466 MoS2/PbTiO3 nanocomposite,467 MoS2/[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) nanocomposites,468 MoS2/PVA nanocomposite,469 and MoS2/polyacrylonitrile hybrid.470 MoS2-based flexible NVM devices have been developed using organic polymers as a matrix in nanocomposites, or as flexible polymer substrates. Yeon et al.471 used exfoliated nanosheets of TMDs with PVA film to fabricate memory devices. The Ga-doped ZnO/MX2–PVA/Ag device configuration showed bistable and write-once-read-many-times flexible memory devices with a current IOn/IOff ratio of 3 × 103 at 2.0 V for MoS2, and at –1.0 V for WS2. Bhattacharjee et al.472 developed flexible NVM devices by dispersing MoS2/graphene composite in the PMMA matrix. The MoS2/graphene memory devices showed rewritable memory with high resistance (Roff/Ron) 73



ratio > 104 after 10 days, and reproducibility up to 104 cycles. The flexible MoS2/graphene memory devices fabricated on a PET substrate were examined for (40–5) mm bending diameters for up to 100 bending cycles, which showed the mechanical stability of the memory devices. Liu et al.473 developed flexible non-volatile rewritable memory devices using MoS2/PVP nanocomposites. The rGO/MoS2–PVP/Al configured device showed non-volatile rewritable memory effect. The 1T phase MoS2 can be transformed into the 2H phase MoS2 nanosheets through synthetic routes. Kappera et al.474 reported phase-related low-resistance contacts for MoS2 transistors. The metals contacts deposited on the semiconducting MoS2 2H phase showed high-resistance between (0.7–10) kΩ·μm, where the metallic MoS2 1T phase of MoS2 can be generated on semiconducting MoS2 2H phase nanosheets, which brings down resistances to between (200–300) Ω·μm at zero gate bias. FETs fabricated with MoS2 1T phase electrodes showed carrier mobility values of 50 cm2/Vs, SS values below 100 mV/dec, current IOn/IOff switching ratios of > 107, and drive currents of 100 μA μm−1. The performance of FET was partially impacted by the deposition of different metals. The 1T/2H interface engineering of MoS2 resulted in increased electrical characteristics. Zhang et al.475 demonstrated the effect of MoS2 1T and 2H structural phase transition on the resistive switching properties of MoS2 based flexible memory devices, using the write-once readmany times memory effect in Al/1T@2H-MoS2-PVP/ITO/PET, and rewritable memory effect in Al/2H-MoS2-PVP/ITO/PET in flexible MoS2 memory devices. The resistive switching properties of flexible MoS2-based NVM devices can be tailored by adjusting the 1T and 2H phase structures in the MoS2 nanosheets. This study showed the role of MoS2 structural phase transition in the resistive switching properties of the memory devices. Son et al.476 have developed wafer-scale flexible RRAM arrays with MoS2 nanosheets. The MoS2 nanosheet-based RRAM arrays exhibited 10,000 times higher current IOn/IOff ratio compared 74


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with exfoliated MoS2 nanosheets. The MoS2 RRAM arrays were integrated with pressure sensors, as well as with quantum-dot light-emitting diodes (QLEDs). Figure 24 shows the photograph of wafer-scale MoS2 RRAM arrays, Raman spectra of MoS2 nanosheets recorded at five different points, flexible MoS2 RRAM arrays, cumulative probability as a function of resistance, and switching voltage for RRAM devices developed using synthesized and exfoliated MoS2 nanosheets, and bending test for the flexible RRAM devices and the integration of MoS2 RRAM arrays with QLEDs and pressure sensor, and the cross-sectional structures of mechanical data storage and QLED-based optical data reading systems. The current IOn/IOff ratio of the RRAM arrays developed from synthesized MoS2 nanosheets was found to be much higher, when compared with exfoliated MoS2 nanosheets at the same operational voltages. The mechanical data storage device consists of a (15 × 15) MoS2 RRAM array, a pressure-sensitive rubber layer, an electrically conducting rubber layer, and Al electrodes. When voltage is applied to the electrically conductive rubber layer, and subsequently an external pressure is exerted by using the pen, the electrical resistance of the pressure-sensitive rubber layer was found to decrease, and the RRAM cells in the low-resistance region are changed from the high-resistance state (HRS) to the lowresistance state (LRS), which can be read electrically at –0.1 V voltage. The QLED-based optical data reading system consists of a (15 × 15) MoS2 RRAM array, 20 nm thick poly[(9,9dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)diphenylamine)] (TFB) layer, and 25 nm thick poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS) layer as the hole-transport layer (HTL), a 35 nm thick CdSe quantum dots (QDs) layer, and a 45 nm thick layer of ZnO nanocrystals. When a (15 × 15) QLED array is incorporated with the Al top electrode (TE) of the RRAM array, the memory cells in the LRS enable QLED pixels to turn on at 4 V, whereas the RRAM cells in the HRS prevent the current flow to the other QLEDs. The flexible MoS2 75



RRAM devices retained their performance, even after bending with a 3 mm radius. Therefore, the flexible MoS2 RRAM arrays can be used for developing wearable data storage systems having mechanical writing, as well as optical reading, capabilities.

Figure 24. (a) Photograph of wafer‐scale MoS2 RRAM arrays consisting of 9 crossbar arrays (10 × 10 for each array) and 18,566 individual cells. (b) Raman spectra of MoS2 nanosheet on a SiO2 wafer, demonstrating uniformity at five different points including top (T), left (L), center (C), right (R), and bottom (B). (c) MoS2 based RRAM arrays fabricated on a flexible PET substrate. The cumulative probability in percentage as a function of resistance (d) and switching voltage (e) for RRAM devices fabricated using synthesized MoS2 nanosheets (red, 50 cells) and exfoliated MoS2 nanosheets (blue, 50 cells). (f) The electrical resistance versus inverse of the bending radius curves for MoS2 RRAM devices indicating that both the low-resistance state (LRS) and the high76


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resistance state (HRS) are retained after bending the flexible RRAM devices with a 3 mm radius. Inset shows maximum principal strain versus bending radius plot. (g) Schematic illustration showing the integration of MoS2 RRAM arrays with QLEDs and pressure sensor. (h) Crosssectional structure of MoS2 RRAM arrays based mechanical data storage device. (i) Schematic illustrations of the QLED-based optical data reading system and (j) cross-sectional structure of MoS2 RRAM arrays into optical data reading system. Reprinted with permission from ref 476. Copyright 2016 Wiley-VCH.

9. MoS2-BASED FLEXIBLE PIEZOELECTRIC NANOGENERATORS PVDF and its copolymers with trifluoroethylene (TrFE), nylons, cyanopolymers, biopolymers (silk, cellulose, chitin, proteins, polypeptides, DNA), polyureas and polythioureas, liquid crystals, and ceramics, such as lead zirconate titatnate (PZT), barium titanate (BaTiO3), and ZnO, show piezoelectric and ferroelectric properties.477, 478 The strong piezoelectric effect displayed by these piezoelectric materials allows the conversion of mechanical energy into electrical energy; therefore, a large family of piezoelectric materials can be used in developing PNGs. Flexible PNGs for wearable electronics have been developed using textile fibers,90,

91, 479

silk fibroin-based

biodegradable composite,480 graphene162−165 and graphene/PEDOT:PSS hybrid composites films,166 CNTs,482 PVDF,482 PVDF-niobate nanocomposite with nylon nanofiber/AgNW,483 poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) nanofiber,484 P(VDF-TrFE) nanofibers and PDMS/MWCNTs composite,485 MWCNTs mesh/PDMS films,486 porous PDMS,487 and metallic MXenes thin films,17 porous aerogels and electrospun nanofibers,488 BaTiO3 nanofibers,489

BaTiO3/poly(vinylidene

fluoride-co-hexafluoropropene)

P(VDF-HFP)

nanocomposites,490 ZnO nanorods,491 lithium-doped ZnO nanowires/PDMS composites,492 and GaN.493 Piezoelectricity has been observed in atomic layered TMDs, which include MoS2,494, 495 MoTe2,496 and WSe2/MoS2 heterostructures.497 Therefore, layered TMDs, such as WSe2,498 MoS2,499 MoS2/PVDF nanofibers,500,

501

and MoS2/graphene nanocomposites502 and MoS2/PU 77



nanocomposites,503 have been explored in fabricating flexible PNGs for energy harvesting by blending their strong piezoelectric effect with electronic properties. CVD-grown MoS2 monolayer generally inherits intrinsic sulfur (S) vacancy. Han et al.504 used H2S gas at 1,000 °C for 30 min over CVD-grown pristine MoS2 monolayer to fill S atoms into the S vacancy positions. Both pristine MoS2 monolayer and S-treated MoS2 monolayer were used in fabricating PNGs, and the performance of PNGs was compared before and after S-treatment, to examine the effect of point-defect passivating S vacancy. The S vacancy sites on MoS2 surface tend to form a covalent bond with S atom during S-treatment, which influences the charge-carrier density of MoS2 monolayer. Figure 25 shows a schematic of the MoS2 monolayer-based PNG, photograph of a flexible PNG device, Raman spectra, and lateral piezoelectric response of the pristine MoS2 monolayer and S-treated MoS2 monolayer, measured using lateral piezoresponse force microscopy (PFM) methods. The recorded piezoelectric coefficients (d11) of the pristine MoS2 monolayer and S-treated MoS2 monolayer were found to be ((3.06 ± 0.6) and (3.73 ± 0.2)) pm/V, respectively, and are higher when compared with the d11 value of 2.3 pm/V for a-quartz. This shows that S-treatment increased the d11 value of the pristine MoS2 monolayer. The PNG fabricated with the S-treated MoS2 monolayer when compared with the pristine MoS2 monolayer showed piezoelectric output current of (100 cf. 30) pA, and output voltage of (22 cf. 10) mV, respectively. The S-treated monolayer showed 3 times higher piezoelectric output peak current and 2 times higher voltage than the pristine MoS2 monolayer based PNG. The power output of Streated MoS2 PNG compared with the pristine MoS2 PNG reached (0.73 cf. 0.07) pW, which is a 10-fold increase. The carrier density of 2.19 × 1012 cm-2 for the pristine MoS2 device decreased to 6.11 × 1011 cm-2 after S-treatment, possibly due to the free charge carrier trapped by S atom after S-treatment. This indicates that sulfur S-treatment reduces free charge carrier and prevented the 78


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screening effect; therefore, the output peaks of current, voltage, and power of S-treated MoS2 PNG were significantly increased, compared with the pristine MoS2 PNG. The difference in d11 of PNGs was analyzed using PFM of the pristine MoS2 and the S-treated MoS2 monolayers. PL, XPS, and Kelvin probe force microscopy (KPFM) were also used to characterize the pristine MoS2 monolayer and S-treated MoS2 monolayer.

Figure 25. (a) Schematic illustration of the MoS2 monolayer-based PNG. (b) Photograph of the MoS2 monolayer-based PNG device on flexible PET substrate. Inset shows an optical microscope image of the Au electrode configuration. (c) Raman spectra of the pristine MoS2 monolayer and S-treated MoS2 monolayer. (d) Lateral piezoelectric response of the pristine MoS2 monolayer and 79



S-treated MoS2 monolayer, and a-Quartz, using lateral PFM methods. The recorded piezoelectric coefficient (d11) of the pristine MoS2 monolayer (red circle) and S-treated MoS2 monolayer (blue triangle) and a-quartz (black square). Reprinted with permission from ref 504. Copyright 2018 Wiley-VCH.

Figure 26 shows a schematic of the intrinsic sulfur (S) vacancy in the nanosheet of the pristine MoS2 monolayer and the passivation of S vacancy after the S-treatment, and the PL spectra and XPS of the nanosheets of the pristine MoS2 monolayer and S-treated MoS2 monolayer. Although a sharp absorption peak at 1.89 and another peak at 1.98 eV were observed in the PL spectra for the pristine MoS2 monolayer and S-treated MoS2 monolayer, the intensity of the PL peak was much higher for S-treated MoS2, compared with the pristine MoS2. The stoichiometry of monolayer MoS2 nanosheets was studied by the XPS, where S/Mo ratios of (8:1 and 2:1) were measured for the pristine and S-treated monolayer MoS2, respectively, which indicates the presence of S atom vacancies in the pristine MoS2. The nanosheet of the pristine monolayer MoS2 becomes stoichiometric monolayer MoS2 nanosheet, after filling the S atom vacancies by the Streatment. The binding energy of S-treated monolayer MoS2 nanosheet shifts to 229.49 eV, which is lower than that of 229.86 eV for the pristine MoS2. The filing of S vacancies in the pristine MoS2 after the S-treatment reduces free-charge carrier, which impacts the performance of the PNG device. The defects in MoS2 atomic layers play a significant role in controlling the electrical and chemical properties of MoS2-based materials.505−511

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Figure 26. (a) Schematic illustration of the intrinsic S vacancy in the pristine MoS2 monolayer, and the passivation of S vacancy after the S-treatment. (b) PL spectra of the nanosheets of the pristine MoS2 monolayer and S-treated MoS2 monolayer. The XPS of the Mo 3d and S 2s corelevels (c) nanosheets of the pristine MoS2 monolayer and (d) S-treated MoS2 monolayer. Reprinted with permission from ref 504. Copyright 2018 Wiley-VCH.

10. MoS2 FET-BASED FLEXIBLE SENSORS Atomically-thin layers of MoS2 exhibit high sensitivity toward a large number of chemical and biological species, pollutants, tactile pressure, and human body motions, therefore, MoS2 thin films have been investigated for developing different types of sensors.221−229 Kim et al.547 developed flexible strain-pressure sensors from MoS2 nanoflakes, graphene foam porous networks and Ecoflex nanocomposite, and which showed a pressure detection range of 0.6-25.4 kPa, high 81



sensitivity of 6.06 kPa–1, response time of 200 ms, and mechanical stability over 4000 bending cycles. The applications of MoS2-based FETs in fabricating sensors have also been explored. Sarkar et al.221 demonstrated MoS2-based FET biosensors for the detection of pH changes. The MoS2-based pH sensor showed a wide pH detection range of 3–9 and 74-fold higher sensitivity than that of graphene biosensor. The ultrahigh sensitivity of 196 was observed for the 100 fM streptavidin solution and 713 for 1 unit of pH change. Shan et al.548 developed a biosensor using a bilayer MoS2 back-gate FET for the detection of glucose. The MoS2 FET biosensor showed the sensitivity of 260.75 mA mM−1, a wide linear detection range from 300 nM to 30 mM, and the very low detection limit of 300 nM with a fast response time of

Flexible Molybdenum Disulfide (MoS2) Atomic Layers for Wearable

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