Continuously Tuning Electronic Properties of Few-Layer Molybdenum

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Continuously Tuning Electronic Properties of FewLayer Molybdenum Ditelluride with In Situ Aluminum Modification Towards Ultrahigh Gain Complementary Inverters Dianyu Qi, Cheng Han, Ximing Rong, Xiu-Wen Zhang, Manish Chhowalla, Andrew T. S. Wee, and Wenjing Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04416 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 22, 2019

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Continuously Tuning Electronic Properties of FewLayer

Molybdenum

Ditelluride

with

In

Situ

Aluminum Modification Towards Ultrahigh Gain Complementary Inverters Dianyu Qi1,2, Cheng Han1,2*, Ximing Rong3, Xiu-wen Zhang3, Manish Chhowalla4, Andrew T. S. Wee2,5*, Wenjing Zhang1*

1SZU-NUS

Collaborative Innovation Center for Optoelectronic Science & Technology,

International Collaborative Laboratory of 2D Materials for Optoelectronics Science and Technology of Ministry of Education, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China

2Department

of Physics, National University of Singapore, 2 Science Drive 3, Singapore

， 117551, Singapore.

ACS Paragon Plus Environment

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3Shenzhen

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Key Laboratory of Flexible Memory Materials and Devices, College of

Electronic Science and Technology, Shenzhen University, Shenzhen 518060, China.

4Department

of Materials Science and Metallurgy, 27 Charles Babbage Road,

Cambridge, CB3 0FS, UK

5Centre

for Advanced 2D Materials, National University of Singapore, Block S14, 6

Science Drive 2, Singapore 117546, Singapore.

Abstract

Semiconducting molybdenum ditelluride (2H-MoTe2), a two-dimensional (2D) transition metal dichalcogenide (TMD), has attracted extensive research attention due to its favorable physical properties for future electronic devices, such as appropriate bandgap, ambipolar transport characteristic, and good chemical stability. The rational tuning of its electronic properties is a key point to achieve MoTe2-based complementary electronic and optoelectronic devices. Herein, we demonstrate the dynamic and effective control of the electronic properties of few-layer MoTe2, through the in situ


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surface modification with aluminum (Al) adatoms, with a view towards high-performance complementary inverter devices. MoTe2 is found to be significantly electron doped by Al, exhibiting a continuous transport transition from p-dominated ambipolar to n-type unipolar with enhanced electron mobility. Using a spatially-controlled Al doping technique, both p- and n-channels are established on a single MoTe2 nanosheet, which gives complementary inverters with a record-high gain of ~195 that stands out in the 2D family of materials due to the balanced p- and n-transport in Al-modified MoTe2. Our studies coupled with the tunable nature of in situ modification enable MoTe2 to be a promising candidate for high-performance complementary electronics.

Keywords MoTe2, aluminum, electron doping, bandgap tuning, complementary inverter

The emergence of two-dimensional (2D) layered materials initiates the application of a large variety of atomically-thin materials that are stacked by van de Waals interaction in the next generation nanoelectronics,1-3 which provides the great opportunity to


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overcome the short-channel effects and dangling-bond-induced traps occurring in conventional silicon(Si)-based complementary metal oxide semiconductor (CMOS) devices. Graphene, the forefather of 2D materials, possesses the ultrahigh charge carrier mobility4 and a wealth of outstanding fundamental properties,5 but the absence of a finite bandgap seriously limits its application in logic electronics. Recently, black phosphorus (BP) fast emerges in 2D family, simultaneously showing a sizable bandgap6 and high carrier mobility.7, 8 However, few-layer BP degrades rapidly in ambient air and its device performance is highly restricted by the poor stability.9, 10 Transition metal dichalcogenides (TMDs) are a well-known class of 2D materials with the stoichiometry of MX2, where one layer of transition metal atoms (M) from group IVVII is sandwiched by two layers of chalcogen atoms (X). Compared with graphene and BP, TMDs are featured by good chemical stability and moderate bandgap, making them the promising candidates for electronic and optoelectronic devices.1 Among TMDs family, molybdenum ditelluride (MoTe2) has recently attracted great research interests and efforts owing to its favorable optical and electronic properties. It has been reported that chemical or physical treatments can trigger a structural phase transition in MoTe2


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between semiconducting hexagonal (2H) and metallic monoclinic (1T’) phases, such as alkali metal intercalation, 11 electrostatic doping,12 laser irradiation13 and Ca2N electride contact.14,

15

Like most of TMDs, 2H-MoTe2 (simplified as MoTe2 afterwards) has a

direct bandgap of ~1.1 eV for monolayer and experience a direct-to-indirect band crossover to ~0.9 eV for bulk.16, 17 This gap is quite close to that of Si and comparably smaller than that of mostly-studied TMDs like MoS2 and WSe2, which facilitates the efficient control of charge carriers in MoTe2 by electrostatic gating. Together with the weak Fermi level pinning effects at metal/MoTe2 interface,18 MoTe2-configured field-effect transistors (FETs) are able to achieve either unipolar (n- or p-type) or ambipolar transport characteristics by selecting proper metal contacts with the mobility falling in the range of 0.3-30 cm2V-1s-1 for holes and 0.03-30 cm2V-1s-1 for electrons, respectively.19-22 For the future application of MoTe2 in complementary electronics and optoelectronics, it is of great necessity to effectively tune the electronic properties of MoTe2 in a controlled and non-destructive manner, including the modulation of carrier type, concentration, mobility, and even electronic band structure.


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Electrostatic modulation by applying an external electrical field is demonstrated to be a useful method to either dope or engineer the bandgap of 2D materials.23-26 In TMDsbased devices, the transport properties highly depend on the Schottky junction formed between TMDs and metal, making the contact engineering be another approach to tailor the device performance.

18, 27-29

However, both strategies require a sophisticated device

fabrication process and the efficiency is limited by the quality of dielectric or contact interfaces. Surface modification, based on coating a specific adlayer on the surface, provides a strong and non-destructive doping capability on 2D materials with the ease of device fabrication. To date, a variety of surface species has been employed on TMDs to modify their electronic properties.30 Very recently, metal oxides (Al2O331 and MgO32), benzyl viologen (BV)33 and oxygen33, 34 have been reported to n- or p-type dope MoTe2 and further improve its carrier mobility, which leads to the construction of complementary devices on a single MoTe2 nanosheet, such as p-n homojunction-based diodes and logic inverters. Nevertheless, these doping methods and device characterizations involve a complicated chemical environment during atomic-layerdeposition (ALD), air exposure or solution processes, which fails to create good


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interfaces between surface dopants and MoTe2 and thus results in the limited device performance. In order to realize MoTe2-based complementary devices with highperformance, the development of in situ modification scheme under vacuum conditions is highly desirable. On the other hand, beside the modulation of carrier polarity and mobility, the control of electronic band structure, e. g. bandgap, is also very crucial for MoTe2 in functional optoelectronic applications. Few-layer BP has recently exhibited a tunable bandgap by electrostatic gating24 and potassium functionalization;35, 36 while a continuous bandgap decrease induced by a vertical electrostatic field has been theoretically predicted in bilayer TMDs37 and further verified in bilayer MoS2 devices.23 So far, however, the effective control of bandgap for MoTe2 has not been experimentally fulfilled. Here, we report an effective surface functionalization scheme via the in situ deposition of aluminum (Al) adatoms in vacuum to continuously tune the electronic properties of few-layer MoTe2. Al is found to strongly n-dope MoTe2, leading to the carrier modulation over a wide range from hole-domination to pure electron. The electron mobility of MoTe2 was obviously enhanced by one order to ~12.8 cm2V-1s-1 after Al deposition. By


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integrating the p-channel and Al-modified n-channel in an individual MoTe2 flake, we achieve the complementary inverter devices with the highest gain of ~195 ever reported in 2D materials, arising from the obtained symmetric p- and n-transport of MoTe2 after doping. Results and Discussion Ultrathin MoTe2 flakes were isolated on a heavily p-doped Si substrate with 300 nm SiO2, and subsequently configured as FET devices using palladium (Pd)/gold (Au) as metal contacts (details in Experimental Section). Figure 1a displays the atomic force microscopy (AFM) image of an as-fabricated MoTe2 FET, where the line profile indicates a 4.4 nm-thick MoTe2 flake, corresponding to ~6 atomic layers. First-order Raman spectrum of the flake in the same device (Figure 1b) shows three characteristic peaks nearly located at 174, 236 and 290 cm-1 that arise from A1g, E2g and B2g vibration modes in MoTe2 lattice, respectively, where B2g mode refers to the out-of-plane vibration, and A1g, E2g represent the in-plane vibration modes. Considering the fact that the intensity ratio of B2g to E2g decreases with the increase of MoTe2 thickness (see


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Supporting Information Figure S1), the B2g/E2g ratio of ~0.17 in Figure 1b corresponds to ~6 layers of MoTe2, in consistence with previous reports.38 All the electrical measurements of as-made MoTe2 devices were implemented in high vacuum conditions (~10-8 mbar) to eliminate the impact of oxygen and moisture in air on the device interfaces. The typical transfer characteristic (Isd-Vg) of MoTe2 FETs at room temperature is illustrated in Figure 1c. Applying a gate voltage (Vg) ranging from -80 to 50 V, the source-drain current (Isd) increased from OFF to ON state along with both negative and positive sweeping of Vg, corresponding to the hole and electron transport, respectively, which shows an current on/off ratio of ~105 (inset of Figure 1c). Furthermore, the on-current of hole branch is over one order of magnitude higher than that of electron branch, indicating an hole-dominated ambipolar transport characteristic. By extrapolating the linear region of both hole and electron branch, the threshold voltage (Vth) was determined to be -50.8 V for holes and 29.9 V for electrons. Thus, the carrier concentration driven by a specific Vg can be estimated by the formula: n  Ci Vg  Vth  e

(1)


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, where Ci denotes the capacitance per unit area between MoTe2 and back gate defined by Ci   0 r d (  r and d are the dielectric constant and thickness of SiO2, respectively), and e is the elementary charge. For example, the electron concentration at Vg = 30 V was derived to be ~0.7 × 1010 cm-2. Similarly, the field-effect mobility of MoTe2 was further extracted from the linear regime of transfer plot using the equation below:



L dI sd WCiVsd dVg

(2)

where dI sd dVg represents the slope of linear region in the transfer plot, L and W are the length and width of conduction channel, respectively. The hole and electron mobility are calculated to be ~26.1 and ~1.0 cm2V-1s-1, respectively, suggesting the limited electron transport compared to the hole side. Figure 1d exhibits the current-voltage (Isd-

Vsd) output characteristics of the same device. An excellent linearity was observed for both hole (Vg from -80 to 0 V) and electron (Vg from 10 to 50 V) transport, yielding the ohmic contact between MoTe2 and Pd/Au electrodes.


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Aluminum, with a low work function (~4.2 eV) and good chemical reactivity, serves as an effective electron donor to manipulate the electronic properties of 2D materials.39, 40 In order to explore the surface modification of Al on tuning the transport properties of MoTe2, Al was in situ evaporated onto MoTe2 devices in high vacuum for electrical measurements. The nominal thickness of the deposited Al adatoms or layers was detected by a quartz crystal microbalance (QCM) in front of MoTe2 devices (see Experimental Section). The morphology of MoTe2 surface deposited by 16 Å Al is presented in Figure 2a, which indicates the formation of Al particles or clusters with the height of several nanometers rather than a continuous film. Figure 2b demonstrates the typical transfer characteristic evolution of MoTe2 FETs in logarithmic scale as a function of Al thickness. The pristine MoTe2 shows a current minimum (i. e. charge-neutrality point) nearly located at zero gate voltage, suggesting a nearly neutral conduction without electrostatic gating. With increasing Al thickness, this point dramatically shifted to negative gate voltage, in particular by over 70 V at 4 Å, and rapidly moved beyond the sweeping compliance of Vg with further deposition. This suggests a significant ntype doping on MoTe2, which leads to the continuous transition of its transport behavior


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from p-dominated ambipolar, cross balanced ambipolar, and eventually to unipolar ntype in the limited range of Vg, as clearly observed in the linear plots (Figure 2c). More importantly, the on-current as well as the slope of n-branch was obviously enhanced after Al decoration, revealing the greatly improved electron transport. The electron concentration and field-effect mobility of MoTe2 were extracted by the aforementioned calculations and plotted with respect to Al thickness in Figure 2d. The electron concentration estimated at 30 V Vg was sharply increased from ~0.7 × 1010 to ~2.2 × 1012 cm-2 by over two orders of magnitude; while the Al-modified device presents almost one order mobility enhancement from ~1.0 to ~12.8 cm2V-1s-1 after coating 32 Å Al. This enhanced electron transport mainly originates from two aspects: (1) the increased electron density may fill up the electron trapping sites existing on the surface of MoTe2,

e. g. intrinsic defects or trapped O2 in air;41, 42 (2) the n-doping can reduce the effective Schottky barrier height at Pd/MoTe2 interface due to the narrowing of Schottky junctions where electrons undergo a thermal-assisted tunneling process during transport. On the basis of transport modulation of Al on MoTe2, a spatially-controlled Al doping technique was developed to fabricate the complementary logic inverter devices on a


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single MoTe2 flake. The device structure is schematically illustrated in Figure 3a. Fewlayer MoTe2 was transferred onto a 5 nm-thick hexagonal boron nitride (h-BN) sheet that serves as the dielectric with an underlying graphene layer as gate electrode. Two FET channels were constructed on the MoTe2 flake in series, where one channel was capped by a photoresist mask (e. g. polymethyl methacrylate (PMMA)), used as the pFET, and the n-FET was realized in the uncapped channel by Al doping. Figure 3b and c show the optical microscopy images of a vertically-stacked MoTe2/BN/Graphene structure and fabricated inverter devices on it, respectively. For the inverter measurements, the input voltage (VIN) was applied to the bottom gate, while three top contacts sequentially served as power supply (VDD), output voltage (VOUT) and grand (GND), respectively. The typically-measured transfer characteristics of two parallel FETs on an individual MoTe2 flake are exhibited in Figure 3d. The uncapped FET modified by 0.2 Å Al shows a clear negative shift compared to the capped FET with the enhanced electron transport. This leads to the intersection of the n-branch of Almodified channel with the p-branch of resist-covered channel in their subthreshold region, where the conductance ratio between two branches is suddenly reversed, thus


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generating an inverted output signal VOUT with respect to VIN. Figure 3e demonstrates the output characteristics of the MoTe2 inverter at VDD = 1, 2 and 3 V. For all VDD, the output voltage experiences a violent transition from “high” (~VDD) to “low” (~0 V) state with a steep slope within the VIN ranging from 0 to 2 V. The gain of the inverter, defined by the slope of output curves (dVOUT/dVIN), was plotted as a function of VIN in Figure 3f, which shows a Dirac-δ function like behavior with the highest value of ~27, 138 and 195 for 1, 2 and 3 V VDD, respectively. To further examine the accuracy of reached high gain, over ten MoTe2 inverters were fabricated and measured under the same conditions. The statistical analysis provides a slight error bar on the gain value for each

VDD, as presented in the inset of Figure 3f, suggesting the extraction of high gain in our devices to be reliable and repeatable. To the best of our knowledge, the gain of ~195 at 3 V VDD obtained here is the highest value ever reported in the 2D materials-based complementary inverters, and a performance comparison with previous literatures are listed in Table 1. The realization of this record-high gain is mainly attributed to the symmetric ambipolar transport of MoTe2 induced by Al modification, which results in the nearly identical


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subthreshold swing (SS) of ~250 mV/decade for both n-branch of Al-doped channel and

p-branch of capped channel at the intersection. Most of inverters consisting of 2D materials always focus on how to obtain the SS as low as possible when pursuing high gain, while the balance between n- and p-transport is overlooked. Compared with the exact SS value, the balance between p- and n-branch plays a more important role in achieving the ideal performance for inverters, as previously revealed in organic electronics,43-45 and therefore Al-modified MoTe2 with balanced p- and n-transport offers an ideal platform for building up the complementary inverters with high-performance. Alternatively, the use of ultra-thin h-BN as the high-k dielectric also greatly facilitates the electrostatic tuning of charge carriers in MoTe2 channel by the gate bias, and thus enhances the gain of our inverters. In Figure 2b, we surprisingly notice that the minimal current at charge-neutrality point of MoTe2 gradually increased from 1.2 × 10-10 to 1.0 × 10-9 A after the deposition of 4 Å Al, and further kept the trend of evolution with higher doping levels, as plotted in the inset of Figure 2d. The clear increase of off-current probably originates from the decrease of transport bandgap for MoTe2, which can be quantified by temperature-


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dependent measurements as follows24, 46. The minimal conductivity at charge-neutrality point is expressed by:

 m  eni  e  h 

(3)

where ni is the intrinsic and thermally excited carrier density for both electrons and holes, and µe and µh are the electron and hole mobility, respectively. Under thermaldynamic equilibrium at the temperature of T, ni is proportional to the term of T 3 2 e

 Eg 2 kBT

,

where Eg and kB refer to the energy gap and Boltzmann constant, respectively. Considering that the carrier mobility remains unchanged for the T varying around room temperature in a small range (Figure S7), we simply establish the temperaturedependence of σm by the relation:  m  T 3 2 e

 Eg 2 kBT

. Thus, the bandgap Eg can be

directly determined by the linear fitting of ln  m T 3/2  with respect to 1/T. For the estimation of bandgap in Al-modified MoTe2, we conducted the temperature-dependent transport measurements on MoTe2 FETs accompanied with the in situ deposition of Al, and the obtained transfer characteristics for pristine, 2 Å and 4 Å Al-coated MoTe2 were presented in Figure 4a. The minimal conductance of pristine device varied by two


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orders of magnitude as temperature increased from 140 to 280 K, and this variation was apparently decreased in logarithmic scale with the increasing Al coverage. The collected σm and T values were plotted as ln  m T 3/2  versus 1/T, as shown in Figure 4b, hence giving rise to the extraction of Eg for each case. It is worth noting that all the plots show good linearity for T varying from 260 to 300 K, and start to deviate the linear fitting with T < 260 K, which agrees well with the temperature region on which the carrier mobility was nearly reserved. The measured bandgap of MoTe2 was clearly reduced from ~0.52 to ~0.24 eV after 4 Å Al modification, and further Al deposition results in the shift of charge-neutrality point beyond the measurable Vg range, making it difficult to evaluate the bandgap at higher doping level, but the tendency of gap reduction is clear and continuous. The estimated bandgap for pristine MoTe2 is lower than the generallyreported optical bandgap of few-layer MoTe2 (~0.9-1.0 eV), which is probably due to the existence of Schottky barrier between MoTe2 and metal contacts as well as the intrinsic fitting error of this extraction strategy.46 For the underlying mechanism of bandgap tuning, we propose that deposited Al adatoms tend to bond with MoTe2 on the surface


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where Al donates electron into Mo or Te orbitals. This localized electron donation leads to the formation of local electrostatic gating on MoTe2, thus decreasing its transport bandgap due to the Stark effect.35 We further performed the first-principles DFT calculations on the electronic structures of MoTe2 with and without Al coating to understand the modulation of its electronic properties. Here, Al coating ratio γ is defined as:

  N Al N Te

(4)

where NAl and NTe represent the number of deposited Al atoms and Te atoms on the surface, respectively. In this work, we focus on two ratios of γ = 0.25 and 0.75 on 6layer MoTe2 for calculations, as illustrated in Figure 4c, and the structural details (the variation of bond length and bond angle) on the surface are discussed in Figure S8. Figure 4d-f show the electronic density of states (DOS) for pristine MoTe2 and MoTe2 with γ = 0.25 and 0.75, respectively. Pristine MoTe2 exhibits a Fermi level nearly located at the valence band maximum (VBM). Upon the Al modification, the entire band downward shifts with reference to the Fermi level, making the Fermi level move into the


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conduction band (CB). This reveals a significant n-doping effect on MoTe2, in consistence with our experimental results, which can be understood by the electron donation of Al to MoTe2. On the other hand, the calculated electronic bandgap of MoTe2 was clearly decreased after Al coating, which is ascribed to the Al-induced structural distortion and orbital hybridization in MoTe2, as supported by the partial DOS (PDOS) plots in Figure S9. The electron density difference for Al-coated MoTe2 is illustrated in the inset of Figure 4e and f. The charge distribution of Mo, Te and Al atoms presents a vertical polarization at γ = 0.25, and as the Al coverage increases to γ = 0.75, the charge at the interface becomes chaotic and no longer vertical or polarized. For both coverages, the density difference comes from Mo and top-layer Te atoms, and no contribution from the lower Te, which suggests that the orbital hybridization between Al and MoTe2 only occurs on the surface.

Conclusion In summary, we clearly demonstrate a continuous and effective modulation on the electronic properties of few-layer MoTe2 via the in situ surface modification with Al. Al


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modification can significantly increase the electron concentration and mobility of MoTe2, thus leading to a continuous transition of its transport behavior from p-dominated ambipolar to unipolar n-type. Based on the obtained symmetric p- and n-branch in Almodified MoTe2 FETs, the high-performance complementary inverter devices were achieved on an individual MoTe2 flake, showing an ultra-high gain of ~195 that is highest value ever reported in 2D family. Our results promise a facile methodology to effectively tailor the ambipolar transport as well as electronic band properties of MoTe2, and thus pave the way for its future applications in high-performance electronics and optoelectronics.

Experimental Section Sample preparation and device fabrication: Few-layer MoTe2 flakes were mechanically exfoliated from bulk crystals (HQ graphene) by using scotch tapes, and subsequently transferred onto a degenerately p-doped Si wafer with 300 nm oxides for FET fabrication. For the preparation of stacked structures, a graphene sheet was initially isolated onto a SiO2/Si substrate. BN and MoTe2 flakes were then exfoliated on a


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viscoelastic stamp (polydimethylsiloxane, PDMS), and sequentially transferred onto graphene to form the vertically-stacked MoTe2/BN/Graphene structure, followed by vacuum annealing at 200 oC for 2 h to remove interlayer bubbles.36 Conventional electron beam lithography (EBL) technique (Raith, PIONEER Two) was used to pattern the electrodes exactly on the MoTe2 nanosheet, and 10/70 nm Pd/Au was subsequently thermally evaporated onto substrates as metal contacts. After liftoff in acetone, the asfabricated MoTe2 devices were wire-bonded onto a lead chip carrier for electrical measurements. To fabricate inverter devices, a second EBL process was conducted on the MoTe2 channel to build up the photoresist (PMMA) mask precisely on the desired position. The spatially-masked devices were also bonded to a chip carrier before loading to the vacuum chamber.

In situ device characterization: The as-prepared devices were loaded into a custom-built high vacuum (~10-8 mbar) system for the in situ electrical characterizations. Al source was thermally evaporated from a high-temperature effusion cell at ~900 oC onto the device surface under vacuum conditions. A quartz crystal microbalance (QCM) exactly located in front of the sample was utilized to monitor and calibrate the evaporation rate


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for the precise control of nominal thickness of Al, where the rate can be controlled within 1 Å/min. The sample stage was equipped with cycling cooling media (e. g. liquid nitrogen) and electrical connections, making the temperature-dependent transport measurements can be in situ performed in vacuum without exposure to air. An Agilent 2912A source measure unit (SMU) outside was used to characterize the device performance.

In situ UPS/XPS characterization: In situ UPS and XPS were carried out on bulk MoTe2 in a home-made ultrahigh vacuum (UHV) system (~10-10 mbar) with He I (21.2 eV) and Mg Ka (1252.6 eV) as excitation sources, respectively. The Fermi level of the system was calibrated to a clean Au substrate, and a similar effusion cell was also employed to evaporate Al to the sample surface. Applying a bias of -5 V, the work function of samples was determined by the secondary electron cutoff at low kinetic energy region. The nominal thickness of deposited Al layer was estimated by the attenuation of Mo and Te 3d core levels and further calibrated by QCM. Theoretical calculations: First-principles DFT calculations were performed using the Vienna ab-initio simulation package (VASP) code.47, 48 The projector augmented wave


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(PAW) potential method was used to describe the interactions between ions and electrons. The exchange–correlation interactions between electrons were treated within the generalized gradient approximation (GGA–PBE).

49, 50

The plane wave cut-off

energy of 320 eV was utilized for all structural optimization and electronic calculations. 6 layers of 2 × 2 MoTe2 supercell were used to investigate the effect of Al coating on the electronic properties of MoTe2 and its layer-dependence, and the K-point mesh was generated as 9 × 9 × 1 according to the Monkhorst–Pack scheme51 for the Brillouin zones. At least 15 Å vacuum was included in all the unit cells to avoid weak interactions between layers, and the van der Waals interaction was taken into account using the DFT-D2 correction. Calculations on structural relaxation and electronic properties were obtained after full geometry relaxation with a force convergence criterion of 10-5 eV. Other characterizations: Atomic force microscopy (AFM) was conducted in ambient environment by tapping mode on Bruker Dimension FastScan. Raman spectroscopy was performed on WITEC-Alpha-300R, and the laser wavelength was 532 nm and the power was 30 µW.


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ASSOCIATED CONTENT

The authors declare no conflict of interest.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at

DOI:

Raman spectra of MoTe2 flakes with different Al thickness, transfer characteristics of Almodified MoTe2 FET via backward gate sweeping, UPS/XPS spectra of Al-modified bulk MoTe2, additional information of DFT calculations on Al-coated MoTe2 (PDF).

AUTHOR INFORMATION Corresponding Author Wenjing Zhang, Email: [email protected]

Andrew Thye Shen Wee, Email: [email protected]

Cheng Han, Email: [email protected]

Author Contributions


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‡D. Qi and C. Han contributed equally to this work.

ACKNOWLEDGMENT

This work was supported by National Science Foundation of China (No. 51472164), the 1000 Talents Program for Young Scientists of China, Shenzhen Peacock Plan (No. KQTD2016053112042971), Fundamental Research Foundation of Shenzhen (No. JCYJ20170817100405375), the Educational Commission of Guangdong Province project (No. 2015KGJHZ006), the Science and Technology Project of Guangdong Province (No. 2016B050501005), the Educational Commission of Guangdong Province (No. 2016KCXTD006), the Natural Science Foundation of SZU (No. 000050 and No. 2017029), the Shenzhen Science and Technology Innovation Commission (No. ZDSYS201707271554071), MOE Tier 2 grant (R 144000382112), A*STAR Pharos Program Grant (No. 1527300025), cleanroom facility support from the NUS Centre for Advanced 2D Materials (CA2DM) and the Photonics Center of Shenzhen University.


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REFERENCES 1.

Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S.,

Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides.

Nat. Nanotechnol. 2012, 7, 699-712. 2.

Geim, A. K.; Novoselov, K. S., The Rise of Graphene. Nat. Mater. 2007, 6, 183-

191. 3.

Liu, H.; Du, Y.; Deng, Y.; Ye, P. D., Semiconducting Black Phosphorus:

Synthesis, Transport Properties and Electronic Applications. Chem. Soc. Rev. 2015, 44, 2732-2743. 4.

Mayorov, A. S.; Gorbachev, R. V.; Morozov, S. V.; Britnell, L.; Jalil, R.;

Ponomarenko, L. A.; Blake, P.; Novoselov, K. S.; Watanabe, K.; Taniguchi, T.; Geim, A. K., Micrometer-Scale Ballistic Transport in Encapsulated Graphene at Room Temperature. Nano Lett. 2011, 11, 2396-2399. 5.

Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K.,

The Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81, 109-162. 6.

Li, L.; Kim, J.; Jin, C.; Ye, G. J.; Qiu, D. Y.; da Jornada, F. H.; Shi, Z.; Chen, L.;

Zhang, Z.; Yang, F.; Watanabe, K.; Taniguchi, T.; Ren, W.; Louie, S. G.; Chen, X. H.; Zhang, Y.; Wang, F., Direct Observation of the Layer-Dependent Electronic Structure in Phosphorene. Nat. Nanotechnol. 2016, 12, 21-25. 7.

Li, L.; Yang, F.; Ye, G. J.; Zhang, Z.; Zhu, Z.; Lou, W.; Zhou, X.; Li, L.; Watanabe,

K.; Taniguchi, T.; Chang, K.; Wang, Y.; Chen, X. H.; Zhang, Y., Quantum Hall Effect in Black Phosphorus Two-Dimensional Electron System. Nat. Nanotechnol. 2016, 11, 593597. 8.

Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y.,

Black Phosphorus Field-Effect Transistors. Nat. Nanotechnol. 2014, 9, 372-377. 9.

Favron, A.; Gaufrès, E.; Fossard, F.; Phaneuf-L’Heureux, A.-L.; Tang, N. Y. W.;

Lévesque, P. L.; Loiseau, A.; Leonelli, R.; Francoeur, S.; Martel, R., Photooxidation and Quantum Confinement Effects in Exfoliated Black Phosphorus. Nat. Mater. 2015, 14, 826-832.


26

Page 27 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

10.

Hu, Z.; Li, Q.; Lei, B.; Zhou, Q.; Xiang, D.; Lyu, Z.; Hu, F.; Wang, J.; Ren, Y.;

Guo, R.; Goki, E.; Wang, L.; Han, C.; Wang, J.; Chen, W., Water-Catalyzed Oxidation of Few-Layer Black Phosphorous in a Dark Environment. Angew. Chem. Int. Ed. 2017, 56, 9131-9135. 11.

Tan, S. J. R.; Abdelwahab, I.; Ding, Z.; Zhao, X.; Yang, T.; Loke, G. Z. J.; Lin, H.;

Verzhbitskiy, I.; Poh, S. M.; Xu, H.; Nai, C. T.; Zhou, W.; Eda, G.; Jia, B.; Loh, K. P., Chemical Stabilization of 1T' Phase Transition Metal Dichalcogenides with Giant Optical Kerr Nonlinearity. J. Am. Chem. Soc. 2017, 139, 2504-2511. 12.

Wang, Y.; Xiao, J.; Zhu, H.; Li, Y.; Alsaid, Y.; Fong, K. Y.; Zhou, Y.; Wang, S.;

Shi, W.; Wang, Y.; Zettl, A.; Reed, E. J.; Zhang, X., Structural Phase Transition in Monolayer MoTe2 Driven by Electrostatic Doping. Nature 2017, 550, 487-491. 13.

Cho, S.; Kim, S.; Kim, J. H.; Zhao, J.; Seok, J.; Keum, D. H.; Baik, J.; Choe, D.-

H.; Chang, K.; Suenaga, K., Phase Patterning for Ohmic Homojunction Contact in MoTe2. Science 2015, 349, 625-628. 14.

Kim, S.; Song, S.; Park, J.; Yu, H. S.; Cho, S.; Kim, D.; Baik, J.; Choe, D.-H.;

Chang, K. J.; Lee, Y. H.; Kim, S. W.; Yang, H., Long-Range Lattice Engineering of MoTe2 by a 2D Electride. Nano Lett. 2017, 17, 3363-3368. 15.

Lee, K.; Kim, S. W.; Toda, Y.; Matsuishi, S.; Hosono, H., Dicalcium Nitride as a

Two-Dimensional Electride with an Anionic Electron Layer. Nature 2013, 494, 336-340. 16.

Lezama, I. G.; Arora, A.; Ubaldini, A.; Barreteau, C.; Giannini, E.; Potemski, M.;

Morpurgo, A. F., Indirect-to-Direct Band Gap Crossover in Few-Layer MoTe2. Nano Lett. 2015, 15, 2336-2342. 17.

Aslan, O. B.; Ruppert, C.; Heinz, T., Optical Properties and Band Gap of Single-

and Few-Layer MoTe2 Crystals. Nano Lett. 2014, 14, 6231-6236. 18.

Nakaharai, S.; Yamamoto, M.; Ueno, K.; Tsukagoshi, K., Carrier Polarity Control

in α-MoTe2 Schottky Junctions Based on Weak Fermi-Level Pinning. ACS Appl. Mater.

Interfaces 2016, 8, 14732-14739. 19.

Lin, Y. F.; Xu, Y.; Wang, S. T.; Li, S. L.; Yamamoto, M.; Aparecido-Ferreira, A.;

Li, W.; Sun, H.; Shu, N.; Jian, W. B., Ambipolar MoTe2 Transistors and Their Applications in Logic Circuits. Adv. Mater. 2014, 26, 3263-3269.


27

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20.

Page 28 of 39

Pradhan, N. R.; Rhodes, D.; Feng, S.; Xin, Y.; Memaran, S.; Moon, B.-H.;

Terrones, H.; Terrones, M.; Balicas, L., Field-Effect Transistors Based on Few-Layered

α-MoTe2. ACS Nano 2014, 8, 5911-5920. 21.

Xu, H.; Fathipour, S.; Kinder, E. W.; Seabaugh, A. C.; Fullerton-Shirey, S. K.,

Reconfigurable Ion Gating of 2H-MoTe2 Field-Effect Transistors Using Poly(ethylene oxide)-CsClO4 Solid Polymer Electrolyte. ACS Nano 2015, 9, 4900-4910. 22.

Ignacio Gutiérrez, L.; Alberto, U.; Maria, L.; Enrico, G.; Christoph, R.; Alexey, B.

K.; Alberto, F. M., Surface Transport and Band Gap Structure of Exfoliated 2H-MoTe2 Crystals. 2D Mater. 2014, 1, 021002. 23.

Chu, T.; Ilatikhameneh, H.; Klimeck, G.; Rahman, R.; Chen, Z., Electrically

Tunable Bandgaps in Bilayer MoS2. Nano Lett. 2015, 15, 8000-8007. 24.

Deng, B.; Tran, V.; Xie, Y.; Jiang, H.; Li, C.; Guo, Q.; Wang, X.; Tian, H.; Koester,

S. J.; Wang, H.; Cha, J. J.; Xia, Q.; Yang, L.; Xia, F., Efficient Electrical Control of ThinFilm Black Phosphorus Bandgap. Nat. Commun. 2017, 8, 14474. 25.

Doganov, R. A.; O’Farrell, E. C. T.; Koenig, S. P.; Yeo, Y.; Ziletti, A.; Carvalho,

A.; Campbell, D. K.; Coker, D. F.; Watanabe, K.; Taniguchi, T.; Neto, A. H. C.; Özyilmaz, B., Transport Properties of Pristine Few-Layer Black Phosphorus by van der Waals Passivation in an Inert Atmosphere. Nat. Commun. 2015, 6, 6647. 26.

Oostinga, J. B.; Heersche, H. B.; Liu, X.; Morpurgo, A. F.; Vandersypen, L. M. K.,

Gate-Induced Insulating State in Bilayer Graphene Devices. Nat. Mater. 2007, 7, 151157. 27.

Fang, H.; Tosun, M.; Seol, G.; Chang, T. C.; Takei, K.; Guo, J.; Javey, A.,

Degenerate n-Doping of Few-Layer Transition Metal Dichalcogenides by Potassium.

Nano Lett. 2013, 13, 1991-1995. 28.

Das, S.; Chen, H.-Y.; Penumatcha, A. V.; Appenzeller, J., High Performance

Multilayer MoS2 Transistors with Scandium Contacts. Nano Lett. 2013, 13, 100-105. 29.

Chuang, S.; Battaglia, C.; Azcatl, A.; McDonnell, S.; Kang, J. S.; Yin, X.; Tosun,

M.; Kapadia, R.; Fang, H.; Wallace, R. M.; Javey, A., MoS2 p-Type Transistors and Diodes Enabled by High Work Function MoOx Contacts. Nano Lett. 2014, 14, 13371342.


28

Page 29 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

30.

Hu, Z.; Wu, Z.; Han, C.; He, J.; Ni, Z.; Chen, W., Two-Dimensional Transition

Metal Dichalcogenides: Interface and Defect Engineering. Chem. Soc. Rev. 2018, 47, 3100-3128. 31.

Lim, J. Y.; Pezeshki, A.; Oh, S.; Jin, S. K.; Lee, Y. T.; Yu, S.; Hwang, D. K.; Lee,

G. H.; Choi, H. J.; Im, S., Homogeneous 2D MoTe2 p–n Junctions and CMOS Inverters Formed by Atomic-Layer-Deposition-Induced Doping. Adv. Mater. 2017, 29, 1701798. 32.

Luo, W.; Zhu, M.; Peng, G.; Zheng, X.; Miao, F.; Bai, S.; Zhang, X. A.; Qin, S.,

Carrier Modulation of Ambipolar Few-Layer MoTe2 Transistors by MgO Surface Charge Transfer Doping. Adv. Funct. Mater. 2017, 28, 1704539. 33.

Qu, D.; Liu, X.; Huang, M.; Lee, C.; Ahmed, F.; Kim, H.; Ruoff, R. S.; Hone, J.;

Yoo, W. J., Carrier-Type Modulation and Mobility Improvement of Thin MoTe2. Adv.

Mater. 2017, 29, 1606433. 34.

Chang, Y.-M.; Yang, S.-H.; Lin, C.-Y.; Chen, C.-H.; Lien, C.-H.; Jian, W.-B.;

Ueno, K.; Suen, Y.-W.; Tsukagoshi, K.; Lin, Y.-F., Reversible and Precisely Controllable

p/n-Type Doping of MoTe2 Transistors through Electrothermal Doping. Adv. Mater. 2018, 30, 1706995. 35.

Kim, J.; Baik, S. S.; Ryu, S. H.; Sohn, Y.; Park, S.; Park, B.-G.; Denlinger, J.; Yi,

Y.; Choi, H. J.; Kim, K. S., Observation of Tunable Band Gap and Anisotropic Dirac Semimetal State in Black Phosphorus. Science 2015, 349, 723-726. 36.

Han, C.; Hu, Z.; Gomes, L. C.; Bao, Y.; Carvalho, A.; Tan, S. J. R.; Lei, B.; Xiang,

D.; Wu, J.; Qi, D.; Wang, L.; Huo, F.; Huang, W.; Loh, K. P.; Chen, W., Surface Functionalization of Black Phosphorus via Potassium Toward High-Performance Complementary Devices. Nano Lett. 2017, 17, 4122-4129. 37.

Ramasubramaniam, A.; Naveh, D.; Towe, E., Tunable Band Gaps in Bilayer

Transition-Metal Dichalcogenides. Phys. Rev. B 2011, 84, 205325. 38.

Yamamoto, M.; Wang, S. T.; Ni, M.; Lin, Y.-F.; Li, S.-L.; Aikawa, S.; Jian, W.-B.;

Ueno, K.; Wakabayashi, K.; Tsukagoshi, K., Strong Enhancement of Raman Scattering from a Bulk-Inactive Vibrational Mode in Few-Layer MoTe2. ACS Nano 2014, 8, 38953903.


29

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

39.

Page 30 of 39

Ao, Z. M.; Jiang, Q.; Zhang, R. Q.; Tan, T. T.; Li, S., Al Doped Graphene: A

Promising Material for Hydrogen Storage at Room Temperature. J. Appl. Phys. 2009,

105, 074307. 40.

Liu, Y.; Cai, Y.; Zhang, G.; Zhang, Y.-W.; Ang, K.-W., Al-Doped Black

Phosphorus p–n Homojunction Diode for High Performance Photovoltaic. Adv. Funct.

Mater. 2017, 27, 1604638. 41.

Zhu, H.; Wang, Q.; Cheng, L.; Addou, R.; Kim, J.; Kim, M. J.; Wallace, R. M.,

Defects and Surface Structural Stability of MoTe2 under Vacuum Annealing. ACS Nano 2017, 11, 11005-11014. 42.

Chen, B.; Sahin, H.; Suslu, A.; Ding, L.; Bertoni, M. I.; Peeters, F. M.; Tongay, S.,

Environmental Changes in MoTe2 Excitonic Dynamics by Defects-Activated Molecular Interaction. ACS Nano 2015, 9, 5326-5332. 43.

Wang, S. D.; Kanai, K.; Ouchi, Y.; Seki, K., Bottom Contact Ambipolar Organic

Thin Film Transistor and Organic Inverter Based on C60/Pentacene Heterostructure.

Org. Electron. 2006, 7, 457-464. 44.

Liu, Y.-Y.; Song, C.-L.; Zeng, W.-J.; Zhou, K.-G.; Shi, Z.-F.; Ma, C.-B.; Yang, F.;

Zhang, H.-L.; Gong, X., High and Balanced Hole and Electron Mobilities from Ambipolar Thin-Film Transistors Based on Nitrogen-Containing Oligoacences. J. Am. Chem. Soc. 2010, 132, 16349-16351. 45.

Zeng, W.-J.; Zhou, X.-Y.; Pan, X.-J.; Song, C.-L.; Zhang, H.-L., High

Performance CMOS-like Inverter Based on an Ambipolar Organic Semiconductor and Low Cost Metals. AIP Adv. 2013, 3, 012101. 46.

Dong, H. K.; Cho, S.; Kim, J. H.; Choe, D. H.; Sung, H. J.; Kan, M.; Kang, H.;

Hwang, J. Y.; Kim, S. W.; Yang, H., Bandgap Opening in Few-Layered Monoclinic MoTe2. Nat. Phys. 2015, 11, 482-487. 47.

Kresse, G.; Furthmüller, J., Efficient Iterative Schemes for Ab Initio Total-Energy

Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. 48.

Kresse, G.; Hafner, J., Ab Initio Molecular Dynamics for Liquid Metals. Phys.

Rev. B 1993, 47, 558-561. 49.

Blöchl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50,

17953-17979.


30

Page 31 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

50.

Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation

Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 51.

Monkhorst, H. J.; Pack, J. D., Special Points for Brillouin-Zone Integrations.

Phys. Rev. B 1976, 13, 5188-5192. 52.

Liu, T.; Xiang, D.; Zheng, Y.; Wang, Y.; Wang, X.; Wang, L.; He, J.; Liu, L.; Chen,

W., Nonvolatile and Programmable Photodoping in MoTe2 for Photoresist-Free Complementary Electronic Devices Adv. Mater. 2018, 30, 1870402. 53.

Koenig, S. P.; Doganov, R. A.; Seixas, L.; Carvalho, A.; Tan, J. Y.; Watanabe, K.;

Taniguchi, T.; Yakovlev, N.; Castro Neto, A. H.; Özyilmaz, B., Electron Doping of Ultrathin Black Phosphorus with Cu Adatoms. Nano Lett. 2016, 16, 2145-2151. 54.

Kim, J. S.; Jeon, P. J.; Lee, J.; Choi, K.; Lee, H. S.; Cho, Y.; Lee, Y. T.; Hwang,

D. K.; Im, S., Dual Gate Black Phosphorus Field Effect Transistors on Glass for NOR Logic and Organic Light Emitting Diode Switching. Nano Lett. 2015, 15, 5778-5783. 55.

Radisavljevic, B.; Whitwick, M. B.; Kis, A., Integrated Circuits and Logic

Operations Based on Single-Layer MoS2. ACS Nano 2011, 5, 9934-9938. 56.

Wang, H.; Yu, L.; Lee, Y. H.; Shi, Y.; Hsu, A.; Chin, M. L.; Li, L. J.; Dubey, M.;

Kong, J.; Palacios, T., Integrated Circuits Based on Bilayer MoS2 Transistors. Nano

Lett. 2012, 12, 4674-4680. 57.

Yu, L.; El-Damak, D.; Radhakrishna, U.; Ling, X.; Zubair, A.; Lin, Y.; Zhang, Y.;

Chuang, M.-H.; Lee, Y.-H.; Antoniadis, D.; Kong, J.; Chandrakasan, A.; Palacios, T., Design, Modeling, and Fabrication of Chemical Vapor Deposition Grown MoS2 Circuits with E-Mode FETs for Large-Area Electronics. Nano Lett. 2016, 16, 6349-6356. 58.

Tosun, M.; Chuang, S.; Fang, H.; Sachid, A. B.; Hettick, M.; Lin, Y.; Zeng, Y.;

Javey, A., High-Gain Inverters Based on WSe2 Complementary Field-Effect Transistors.

ACS Nano 2014, 8, 4948-4953. 59.

Yu, L.; Zubair, A.; Santos, E. J. G.; Zhang, X.; Lin, Y.; Zhang, Y.; Palacios, T.,

High-Performance WSe2 Complementary Metal Oxide Semiconductor Technology and Integrated Circuits. Nano Lett. 2015, 15, 4928-4934. 60.

Pezeshki, A.; Hosseini Shokouh, S. H.; Jeon, P. J.; Shackery, I.; Kim, J. S.; Oh,

I.-K.; Jun, S. C.; Kim, H.; Im, S., Static and Dynamic Performance of Complementary


31

ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 39

Inverters Cased on Nanosheet α-MoTe2 p-Channel and MoS2 n-Channel Transistors.

ACS Nano 2016, 10, 1118-1125. 61.

Pu, J.; Funahashi, K.; Chen, C.-H.; Li, M.-Y.; Li, L.-J.; Takenobu, T., Highly

Flexible and High-Performance Complementary Inverters of Large-Area Transition Metal Dichalcogenide Monolayers. Adv. Mater. 2016, 28, 4111-4119. 62.

Zhao, M.; Ye, Y.; Han, Y.; Xia, Y.; Zhu, H.; Wang, S.; Wang, Y.; Muller, D. A.;

Zhang, X., Large-Scale Chemical Assembly of Atomically Thin Transistors and Circuits.

Nat. Nanotechnol. 2016, 11, 954-959. 63.

Huang, M.; Li, S.; Zhang, Z.; Xiong, X.; Li, X.; Wu, Y., Multifunctional High-

Performance van der Waals Heterostructures. Nat. Nanotechnol. 2017, 12, 1148-1154. 64.

Su, Y.; Kshirsagar, C. U.; Robbins, M. C.; Haratipour, N.; Koester, S. J.,

Symmetric Complementary Logic Inverter Using Integrated Black Phosphorus and MoS2 Transistors. 2D Mater. 2016, 3, 011006.


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Figure 1. (a) AFM image of an as-fabricated MoTe2 FET device. The line profile suggests the thickness of MoTe2 flake to be ~4.4 nm (~6 layers), and the scale bar is 1 µm. (b) Raman spectrum of the 4.4 nm-thick flake in the above device on SiO2/Si substrate. (c) Typical transfer characteristic (Isd-Vg) of MoTe2 FETs at Vsd = 1 V. Inset: logarithmic plot of the transfer curve. (d) Output (Isd-Vsd) characteristics of the same device with Vsd ranging from 0 to both 1 and -1 V at different gate voltages for hole (red,

Vg from -80 to 0 V) and electron (blue, Vg from 10 to 50 V) transport, respectively.


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Figure 2. (a) AFM image of 16 Å Al-coated MoTe2 surface. The scale bar is 200 nm, and the inset gives the height profile of the black line. (b) Transfer characteristic evolution of a MoTe2 FET in logarithmic scale measured at Vsd = 1V with the gradual deposition of Al. (c) Linear plot of the same transfer curves. Inset: schematic illustration of Al-modified MoTe2 FETs. (d) The plot of extracted electron concentration at Vg = 30 V and electron mobility as a function of Al thickness.


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Figure 3. (a) Schematic illustration of complementary logic inverters established on a single MoTe2 flake. (b-c) Optical microscopy images of a vertically-stacked MoTe2/BN/Graphene structure and as-fabricated MoTe2 inverters on it. The scale bar is 5 µm, and the number 1-5 indicates the constructed five devices. (d) Transfer characteristics (Vsd = 1 V) of PMMA-coated p-channel and Al-modified (0.2 Å) n-channel in a MoTe2 inverter. (e) Output characteristics of the same device at VDD = 1, 2 and 3 V, respectively. (f) The extracted gain values as function of VIN for each VDD. The inset figure shows the statistically-measured gain over ten devices with an error bar with respect to VDD.


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Figure 4. (a) Temperature-dependent transfer characteristics (140-280 K) of a MoTe2 FET modified by 0, 2 and 4 Å Al, respectively. (b) Corresponding ln  m T 3/2  versus 1/T plots for Al-modified devices. The linear fitting of plots above 260 K extracts the bandgap. (c) Structures of Al on 6-layer MoTe2 with different coating ratios of γ = 0.25 and 0.75. (d-f) Calculated electronic density of states (DOS) for pristine (d) and Alcoated MoTe2 at γ = 0.25 (e) and 0.75 (f), and the insets shows the charge variation after Al coating (yellow and green color represent positive and negative of electron charge, and the electron isosurface level was set at 5 × 10-4 eV/Å3).


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Table 1. The comparison of inverter performance with previous literatures

Materials

MoTe2

BP

VDD [V]

Gain

Reference

0.1

Continuously Tuning Electronic Properties of Few-Layer Molybdenum

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