Electrical Transport Properties of Polymorphic MoS2 - ACS Nano

Electrical Transport Properties of Polymorphic MoS2

Jun Suk Kim,†,‡ Jaesu Kim,†,‡ Jiong Zhao,† Sungho Kim,† Jin Hee Lee,†,‡ Youngjo Jin,†,‡ Homin Choi,†,‡ Byoung Hee Moon,† Jung Jun Bae,† Young Hee Lee,*,†,‡ and Seong Chu Lim*,†,‡ †

Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 16419, Republic of Korea Department of Energy Science, Sungkyunkwan University (SKKU), Suwon 16419, Republic of Korea

‡

S Supporting Information *

ABSTRACT: The engineering of polymorphs in twodimensional layered materials has recently attracted significant interest. Although the semiconducting (2H) and metallic (1T) phases are known to be stable in thinfilm MoTe2, semiconducting 2H-MoS2 is locally converted into metallic 1T-MoS2 through chemical lithiation. In this paper, we describe the observation of the 2H, 1T, and 1T′ phases coexisting in Li-treated MoS2, which result in unusual transport phenomena. Although multiphase MoS2 shows no transistor-gating response, the channel resistance decreases in proportion to the temperature, similar to the behavior of a typical semiconductor. Transmission electron microscopy images clearly show that the 1T and 1T′ phases are randomly distributed and intervened with 2H-MoS2, which is referred to as the 1T and 1T′ puddling phenomenon. The resistance curve fits well with 2D-variable range-hopping transport behavior, where electrons hop over 1T domains that are bounded by semiconducting 2H phases. However, near 30 K, electrons hop over charge puddles. The large temperature coefficient of resistance (TCR) of multiphase MoS2, −2.0 × 10−2 K−1 at 300 K, allows for efficient IR detection at room temperature by means of the photothermal effect. KEYWORDS: molybdenum disulfide, phase transition, Li intercalation, variable range-hopping transport, IR detection

T

technique used to fabricate TMDs materials demonstrates new pathways for controlling the junction barrier. Li-treatment of MoS2 results in the coexistence of various MoS2 phases and consequently develops significantly different electronic and magnetic properties.19,20 Therefore, studying the transport properties of polymorphic MoS2 is a key factor for understanding metallicity in two-dimensional materials, although such metallic properties have not yet been thoroughly explored. In this study, Li-treated MoS2 showed various phases, including 2H, 1T, and 1T′, which were confirmed by highresolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). Furthermore, characteristic vibrational modes of zigzag-Mo atoms, indicating the presence of 1T and 1T′-MoS2, were probed using Raman spectroscopy. In multiphase MoS2, electron transport is governed by hopping among localized metallic domains of 1T phases that are separated by 2H phases. Therefore, no gating effect was observed due to metallic 1T phases. However, the resistance of multiphase MoS2 decreases with temperature, which results from the increased hopping transport of electrons

ransition metal dichalcogenides (TMDs) composed of MX2, where M is a metal, such as Mo, W, and Ti, and X is a chalcogenide, such as S, Se, and Te, can be synthesized with a wide variety of electrical properties ranging from those of semiconductors1−4 and metals1,2,5,6 to those of superconductors.6−8 MoS2 specifically has been intensively investigated due to the presence of its direct bandgap and high electron mobility.9 These are promising features for various types of van der Waals materials; such materials hold specific advantages over graphene, including the presence of a bandgap that is critical for electronic and optoelectronic devices. MoS2-based field-effect transistors (FETs) exhibit n-type behavior with a large contact resistance.9−13 For this reason, various metal electrodes have been studied in an effort to reduce contact resistance10,14,15 and to establish ambipolar MoS2 FETs,16 which are crucial for complementary metal− oxide−semiconductor (CMOS) technologies. However, the contact between metal and MoS2 does not follow Schottky-type behavior; instead, it displays a Fermi-level pinning effect.10 Metallic MoS2 with a lithium-solution treatment has recently been introduced for the reduction of contact resistance,17,18 which is dissimilar to the conventional methods, such as chemical doping and ion implantation, used for the reduction of contact resistance in other devices. The phase engineering © 2016 American Chemical Society

Received: April 4, 2016 Accepted: July 11, 2016 Published: July 11, 2016 7500

DOI: 10.1021/acsnano.6b02267 ACS Nano 2016, 10, 7500−7506

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Figure 1. (a) Ids−Vgs curve of a 2H-MoS2 device measured at Vds = 1.0 V. Inset is an optical image of the device. Scale bar is 20 μm. (b) Ids−Vds curve of 2H-MoS2 device measured at various gate voltages. (c) Schottky barrier height (ESch) at the junction between 2H-MoS2 and metal electrode (Cr/Au) as a function of the gate voltage.

among metallic phases. Near 300 K, variable range-hopping (VRH) transport of multiphase MoS2 showed a large temperature coefficient of resistance (TCR) of (−2 to −4) × 10−2 K−1. Such a large TCR of Li-treated MoS2 was adopted to demonstrate the detection of IR ranging from 2−14 μm, which was not feasible with pristine 2H-MoS2.

RESULTS AND DISCUSSION All devices were fabricated using several layers of MoS2 flakes (3−7 layers) obtained through mechanical exfoliation onto SiO2 (300 nm)/Si (500 μm) substrates. The device electrode was then patterned by e-beam lithography; Cr (5 nm)/Au (50 nm) was deposited using thermal and e-beam evaporation at a pressure of 1 × 10−6 Torr. The channel length of exfoliated MoS2 was approximately 3 μm. The 2H-MoS2 device was then annealed in vacuum chamber at 200 °C for 2 h to improve adhesion between metal electrode and MoS2. The electrical properties of MoS2 before the phase transition were investigated inside a closed-cycle refrigerator (CCR) at a base pressure of 2 × 10−6 Torr. The temperature was lowered from 300 to 100 K. The 2H-MoS2 FET showed a large oncurrent at a positive gate bias in Figure 1a, indicating that electrons were the majority carrier. This was further proven by observing Ids−Vds as a function of gate bias, as shown in Figure 1b. Figure 1c shows that the Schottky-barrier height (SBH, ESch). At a given Vgs, the Arrhenius plot, ln(Ids/T2/3) vs 1/T, was obtained and we estimated the slope of the plot to extract Schottky barrier height.21 The SBH of 2H-MoS2 device ranged from 57 meV at 0 Vgs to 40 meV at 50 Vgs. In order to study the transport properties of Li-treated MoS2, the 2H-MoS2 device was treated with lithium solution under inert atmosphere. (See Materials and Methods.) During the lithium treatment, the Li-ions diffused between layers, inflating the interlayer space (see Supporting Information, Section 1), transferring charges, and leading to a phase transition of MoS2 from the 2H phase to the 1T phase.22−25 Then the sample was washed several times with hexane to remove residual organic molecules and unreacted lithium ions; it subsequently was cleaned again with deionized water and isopropyl alcohol (IPA) to remove the remaining lithium ions. Then the sample was dried at room temperature in a vacuum for 12 h. After the Li treatment, a no-gate dependence of Ids was observed, as shown in Figure 2a, which shows a stark difference compared to the I−V curve in Figure 1a. The absence of a gating effect after the Li treatment may imply an emergence of a different phase of MoS2. In Figure 2b, all the Ids−Vds curves are linear, regardless of gate bias. The existence of different

Figure 2. (a) Ids−Vgs curves of a multiphase MoS2 device measured at Vds = 0.01, 0.1, and 1.0 V. (b) Ids−Vds curves of a multiphase MoS2 device measured with different gate voltages: −90, 0, and 90 V. HR-TEM images of (c) a monolayer MoS2 and (d) a bilayer MoS2 after lithium treatment.

phases of MoS2 has been identified using HR-TEM, which is presented in Figure 2c,d. For the microscopy, a chemically exfoliated MoS2 monolayer was used, which was regenerated using the chemical processes adopted for metallic MoS2 FET. The areas colored in blue in Figure 2c indicate the 1T phases after the Li treatment. (See Supporting Information, Sections 2−3 for more detailed TEM characterizations.) The 1T-MoS2 phases are randomly distributed and intervened with 2H-MoS2 phases. On the micrograph, the physical separation among 1T islands was approximately a few to several nanometers. A newly emerged 1T phase after Li treatment goes through another transition into a semimetallic 1T′ phase, which is marked by yellow in Figure 2d. Thus, Li-treated MoS2 resembles to the metallic granular electrically insulated by oxide or semiconductor materials.26 After the removal of the lithium ions, Li-treated MoS2 layers restack and undergo an additional phase transition from the 1T phase to the 1T′ phase.27,28 The second transition from the 1T phase to the 1T′ phase is considered to be initiated by restacking; the 1T′ phase can be stabilized by the negative chemical species that are introduced during the rinsing process. In addition, it is known that the 1T′ phase is energetically more stable than the 1T phase.29,30 (See 7501

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Figure 3. (a) Ids−Vgs characteristics of the multiphase MoS2 device at Vds = 0.1 V from temperatures between 100 and 300 K. (b) Plots of resistance (Y1) and TCR (Y2) vs temperature characteristics of a multiphase MoS2 device. (c) Ln(G) vs T−1/3 of a multiphase MoS2 device at Vds = 0.1 V. The inset shows ln(G) vs T−1/2 at Vds = 0.1 V. (d) ln(W) vs ln(T) plot of multiphase MoS2. (e) Schematics of variable rangehopping transport in polymorphic MoS2.

involved with electron conduction in a response to temperature change, and a fit of ln(G) to T−1/3 demonstrates a clear linear relationship as shown in Figure 3c. This strongly supports that electron conduction occurs through the hopping process on a two-dimensional plane through localized metallic states, whose temperature dependence of conductance G is described as ⎛ T 1/3 ⎞ ⎟, where T G ≅ exp⎜ − T1 1 is characteristic temper⎝ ⎠ 33−36 Although a similar transport, nearest-neighbor ature. hopping (NNH), has been observed in pristine MoS2 through defect states of S vacancy even near room temperature.34 In this transport regime, the conductance shows a stronger temperature dependence, ln(G) ≅ T−1. However, Li-treated MoS2 shows VRH, which has not yet been reported from Li-treated MoS2. Because the fit of ln(G) to T−1/d, where d is dimensionality, does not clearly differentiate the dimensional dependence. For example, a fit of ln(G) to T−1/2, corresponding for Efros−Shklovskii (ES) hopping transport,37 still looks linear, as shown in the inset of Figure 3c. Therefore, we introduce, in order to confirm 2D Mott VRH,

Supporting Information, Sections 2−5 for in-depth characterizations of TEM, XPS, and Raman spectroscopy.) Although Li treatment of 2H-MoS2 leaves metallic 1T and semimetallic 1T′ puddles that are connected by 2H phases, Ids−Vgs curves in Figure 2a indicate the dominant role of metallic phases of 1T and 1T′ in device transport. No gating behaviors are observed consistently at low temperatures, as shown in Figure 3a. The transport properties are measured down to 100 K because the feasibility of Li-treated MoS2 is tested for IR detection in the later part of present manuscript. The operational temperature of most IR detectors is above 100 K. No gating effect implies that the population of the metallic phase may exceed for the threshold for a percolation transport that will be discussed further later. However, unlike the metallic characteristics seen in Figure 2a, the current-temperature (Ids−T) curves measured from 100 to 300 K in increments of 20 K, presented in Figure 3a, exhibit increasing Ids in proportion to temperature, indicating semiconducting behavior. Such a temperature dependence of resistance of our sample shows a similar behavior to that of inhomogeneous mixture of metal and nonmetals. The current increases over 2 orders of magnitude in this temperature range, whereas no gate modulation of Ids occurs at each temperature step in Figure 3a. It is possible that the patches of 2H-MoS2 are responsible for the resistance lowering through the thermal excitation of valence electrons over the bandgap, because 2H-MoS2 still occupies a large portion in Li-treated MoS2 flakes, which is determined by TEM and XPS investigations. We rule out this possibility because the bandgap energy of 2H-MoS2, approximately 1.8 eV,4,31 is too large to explain the corresponding increase of Ids with temperature. The resistance of multiphase MoS2, obtained as a function of temperature, decreases from 57 to 0.3 MΩ when the temperature changes from 100 to 300 K, with a negative TCR ranging from −0.08 K−1 at 100 K to −0.016 K−1 at 300 K, as shown in Figure 3b. This value is quite large compared to the TCR of typical metals, such as Au, Fe, Mo, and Al.32 The variation in the TCR indicates that different energy levels are

( )

a parameter W =

d(ln(σ )) 38 . d(ln(T ))

Electrical conductivity, σ, can be 1/(d+1)

expressed as σ = σ0e−(T1/T) , where σ0 is electrical conductivity at T0, a reference temperature. Therefore, a more precise determination of the temperature dependence of electrical conductivity can be made by extracting the slope from a plot of ln(W) vs ln(T), as shown in Figure 3d. The slope of the curve in Figure 3d is approximately −0.37. The negative value explains the semiconducting behavior, as electrical resistance decreases with temperature, and 0.37 signifies 2D Mott-type electron hopping, whose theoretical value is 0.33. Therefore, although multiphase MoS2 exhibits metallic properties without any gate modulation, the resistance−temperature (R−T) behavior is semiconductor-like due to electron hopping through semiconducting 2H-MoS2 puddled to the metallic 1TMoS2, as schematically shown in Figure 3e. The transport behavior of Li-treated MoS2 has a similarity to a metal alloyed 7502

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Figure 4. Electronic density of states of (a) 1T-MoS2 and (b) 1T′-MoS2. (c) Plot of electron-hopping distance vs temperature in polymorphic MoS2.

magnitude larger than that of the 1T′ phase. These results lead us to postulate that although Li treatment of MoS2 contributes to the existence of polymorphic phases, electron conduction by hopping mostly occurs through metallic 1T, rather than through 1T′ phases. Furthermore, the previous reports on 2H-MoS2 have shown VRH transport, which is mediated by S vacancy.34 However, the absence of any gating effect of Litreated MoS2 indicates negligible contributions of 2H-MoS2 to the transport shown in Figure 3a. Here, 2H-MoS2 is expected to operate as a thin insulating layer between 1T phases. In order to extract hopping parameters among 1T metallic puddles, a slope S of the plot , in Figure 3c is estimated. The slope S is equivalent to T−1/3 1 ⎡ 3 ⎤ which is further described as T1 = ⎢⎣ k N(E )L 2 ⎥⎦, where kB is the B F l Boltzmann constant, N(EF) is the number of states near EF, and Ll is the localization length.46 The localization length of the electronic wave function in our polymorph MoS2 becomes Ll = 7.75 Å. The hopping distance of electrons depends on the temperature because the tunneling of electrons among 1T puddles is promoted by lattice vibrations that allow for hopping between the states in the 1T phase. At a given temperature T,

with a semiconductor in which metal grains are surrounded by thin insulating layers. The alloy shows metal−insulator transitions depending on their relative composition ratio.39 Our TEM observation in Figure 2c,d reveals a similar compositional structure to a semiconductor-alloyed metal. The randomly scattered metallic puddles in a few layers of Litreated MoS2 are expected to screen the gate electric field, which contributes no gating effect. The electrical resistance decreases as the temperature increases due to the enhanced hopping among the metallic grains bounded by the thin semiconducting layers. Another possible electron conduction for ln(G) to T −1/3 relation is the transport through inhomogeneities in Li-treated MoS2 including charge puddles, S vacancies, and defects. In order to examine the influence of them on the transport, Ids ∼ Vαds characteristics of Li-treated MoS2 are plotted in log−log scale as a function of temperature because the value of α changes with the types of inhomogeneities associated with the transport. For instance, α = 1.0 indicates an ohmic conduction, but α = 1.3 and α = 2.0 means the conduction of charged puddles and trap-free space charge limited current, respectively. Within the temperature range from 100 to 300 K, α remains a unity, indicating no contribution from those disorders [Supporting Information, Section 6]. The results clearly concord with a linearity of ln(G) ∼ T−1/3 in Figure 3c that indicates a single transport mechanism, Mott VRH. This is different from the previous study that showed additional transport mechanism as the temperature becomes low.40,41 This strongly supports the VRH among metallic 1T-MoS2 near the room temperature. However, when the temperature drops near 30 K, a nonlinear behavior in Ids ∼ Vαds with α = 1.3 appears in the plot, log(Ids) ∼ log(Vds) [see Supporting Information, Section 7], a sign of contribution of disorders.42 We performed density-functional theory (DFT) calculations using the generalized-gradient approximation (GGA) with the Perdew−Burke−Ernzerhof (PBE96) functional43,44 and the projector-augmented wave (PAW) method, as implemented in VASP.45 Monolayers of MoS2 were built in a unit cell of each phase of 2H, 1T, and 1T′, with a vacuum distance of 1.6 nm along the c axis and a cutoff energy for the plane-wave basis set to 350 eV. In most calculations, 25 × 25 × 5 sampling of the Brillouin zone was used. All calculations were spin-polarized; both the positions of atoms and the shape of the unit cell were fully relaxed to obtain the optimized lattice structure. Our calculations show the number of energy states at EF of the 1T and 1T′ phases of MoS2, whereas a narrow energy gap (Eg < 30 meV) at EF is found from 1T′ phase one, as shown in Figure 4a,b. In addition, the density of states at EF in the 1T phase, approximately 1 × 1015 eV−1 · cm−2, is almost 2 orders of

the optimal hopping distance R0 is expressed as R 0 =

Ll 3

1/3

( TT ) 1

.

Within the temperature range between 300 and 100 K, R0 is estimated between 3.19 and 4.60 nm. The distribution of 1T islands in the TEM images shown in Figure 2c reveals that the average separation among them varies approximately from a few to several nanometers. Our R0 is consistent with the TEM observations. The transport nature of Li-treated MoS2 is driven by the tunneling of electrons among 1T puddles with nearly the same EF level. At low temperatures, hopping to higher energy states is not feasible because of low thermal energy; electron hopping is allowed through mostly similar energy levels. Thus, hopping distance at a certain 1T puddle depends on the location of the next 1T puddle that has similar EF levels. Because different energy states are distributed at each 1T site, the hopping distance is supposed to change. The activation energy for the tunneling increases at high temperatures. Tunneling occurs through 1T puddles with a wide energy window, which can decrease the hopping distance, as shown in Figure 4c. In addition to the localization distance Ll, the Coulomb gap Δ that forms at EF due to Coulomb interaction between electrons is estimated because it characterizes ES hopping mechanism. No Coulomb gap exists in Mott VRH, unlike ES hopping. Because the Coulomb gap in 2D space can be expressed Δ ≈ e4N(EF),47 the Coulomb gap of Li-treated MoS2 is negligibly small, ΔMoS2 < 1 mK. Therefore, the Mott VRH 7503

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surfaced in our samples at the temperature far below 100 K. Therefore, VRH at 300 K observed from Li-treated MoS2 is attributed to the hoping of electrons via metallic 1T phases. A large TCR of −0.08 to −0.016 K−1 is obtained from polymorphic MoS2. We demonstrated that, at room temperature, an IR bolometer, using Li-treated MoS2, showed resistance changes sharply based on temperature variations.

hopping transport is dominant at room temperature, as seen in Figure 3c. In addition, the conductance-temperature dependence in Mott VRH is also subject to change upon the average diameter of metallic 1T-MoS2, density of states at EF, and temperature.48,49 The large TCR found in multiphase MoS 2 can be advantageous for IR detection by converting the incident light into heat, resulting in a significant decrease in resistance. For the detection of midrange IR, the change in current flow observed in multiphase MoS2 is monitored at a given voltage during the IR exposure. In Figure 5, the responsivity

MATERIALS AND METHODS Fabrication Procedure for a Multiphase MoS2 Device. A 1.6 M n-butyl lithium solution (Sigma-Aldrich) was used to induce a phase transition of MoS2. The 2H-MoS2 devices were immersed in 10 mL of 1.6 M n-butyl lithium in a sealed flask at room temperature under argon (99.999%). In particular, the flake sizes less than ∼5 μm were selectively used for the uniform distribution of Li-ions inside the MoS2 flakes. After 12 h, the sample was washed with n-hexane (anhydrous) several times to remove the residual organic molecules and the unreacted lithium ions. Then, the device was rinsed further with deionized water in order to remove the remaining lithium ions. Finally, the device was cleaned with isopropyl alcohol (IPA), which was followed by drying at room temperature under vacuum conditions for 12 h. ASSOCIATED CONTENT

Figure 5. Responsivity of Li-treated MoS2 upon IR exposure.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02267. More information on the interlayer spacing of multiphase MoS2, structural characterizations using TEM and XPS, temperature dependence measurement, low temperature measurement, IR source and additional references (PDF)

ΔA

R resp = ΔW of Li-treated MoS2 is about 0.5 μA/W,50 and is highly reproducible over several IR exposures from 2 to 14 μm. The emission spectrum of the IR source is given in Figure S8. (See Supporting Information, Section 8 for more detailed information.) Because of the high resistivity of Li-treated MoS2 , approximately 62.5 MΩ, the Johnson noise, expressed as Vn = 4kBTR reaches approximately 1 × 10−6 V·Hz−1/2, where R is the electrical resistance, and T is the absolute temperature. Thus, noise-equivalent power (NEP), a minimum detectable power, is 27 nV·Hz−1/2. The device performance can be significantly enhanced after achieving effective thermal isolation from the substrate. A detection of the spectral range from 2 to 14 μm is not feasible with pristine MoS2, which has a bandgap of 1.8 eV, but various phases that emerged following the Li treatment generated paths that, once the MoS2 was heated due to the incoming IR, conducted electrons through hopping. The slow response is attributed to the gradual rise in temperature of the MoS2 and SiO2 substrate system.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Institute for Basic Science (IBS-R011-D1) and the Human Resources Development program (No. 20124010203270) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) through a government grant funded by Korea government Ministry of Trade, Industry, and Energy.

CONCLUSION In summary, Li treatment of MoS2 implants polymorphic structures blended with semiconducting 2H phases, 1T′ phases with narrow bandgaps, and 1T phases with zero bandgaps. However, the group of the different phases in Li-treated MoS2 showed no gating effect, an electrical property of metal, although the resistance dropped as the temperature increased, as is true of a semiconductor. This baffling transport behavior arises from electrons that hop over to the metallic 1T phase puddles surrounded by the semiconducting 2H phase. The electrical conduction mechanism of multiphase MoS2 is well explained by variable-range electron hopping. The transport parameters such as TCR, Ll, T1 are analogous to VRH in pristine MoS2,51 The defect or disorder-mediated transport

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DOI: 10.1021/acsnano.6b02267 ACS Nano 2016, 10, 7500−7506

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DOI: 10.1021/acsnano.6b02267 ACS Nano 2016, 10, 7500−7506