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Chemical Vapor Deposition Growth of Degenerate pType Mo-Doped ReS2 Films and Its Homojunction Jing-Kai Qin, Wen-Zhu Shao, Cheng-Yan Xu, Yang Li, Dan-Dan Ren, Xiao-Guo Song, and Liang Zhen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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ACS Applied Materials & Interfaces

Chemical Vapor Deposition Growth of Degenerate p-Type Mo-Doped ReS2 Films and Its Homojunction Jing-Kai Qin,† Wen-Zhu Shao,† Cheng-Yan Xu,*,†,‡ Yang Li,† Dan-Dan Ren,† Xiao-Guo Song,‡ and Liang Zhen*, † †

School of Materials Science and Engineering, Harbin Institute of Technology,

Harbin 150001, China ‡

Shandong Provincial Key Laboratory of Special Welding Technology, Harbin

Institute of Technology at Weihai, Weihai 264209, China E-mail: [email protected]; [email protected]

ABSTRACT Substitutional

doping

of

transition

metal

dichalcogenides

(TMDs)

two-dimensional materials has proven to be effective in tuning their intrinsic properties, such as band gap, transport characteristics and magnetism. In this study, we realized substitutional doping of monolayer rhenium disulfide (ReS2) with Mo via chemical vapor deposition. Scanning transmission electron microscope (STEM) demonstrated that Mo atoms are successfully doped into ReS2 by substitutionally replacing Re atoms in the lattice. Electrical measurements revealed the degenerate p-type semiconductor behavior of Mo-doped ReS2 field effect transistors (FETs), in agreement with density functional theory (DFT) calculations. The p-n diode device based on doped ReS2 and ReS2 homojunction exhibited gate-tunable current rectification behaviors, and the maximum rectification ratio could reach up to 150 at Vd=−2V/+2V. The successfully synthesis of p-type ReS2 in this study could largely 1

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promote its application in novel electronics and optoelectronics devices. KEYWORDS: ReS2; substitutional doping; DFT calculations; homojunction; p-n diode

1. Introduction Transition metal dichalcogenides (TMDs) two-dimensional (2D) materials have attracted considerable attentions due to their distinct properties and promising applications in electronics and optoelectronics1-5. For semiconductor materials, controlled modification of carrier type and density is essential, which could enable fabricating functional devices with extended applications by tuning their physical and chemical properties6-9. In this regard, alloying and substitutional doping have been proven to be effective in realizing the versatile change of band structures of 2D materials6, 10. Substitution of host atoms with dopants could provide strong covalent bonding of atoms and realize the stabilization of doping. Several alloyed TMDs, such as Co1-xMoxS2, Mo1-xMnxSe2, and MoS2xSe2(1-x), were successfully synthesized using chemical vapor deposition method, and their intrinsic properties were tuned by introducing dopants6, 8-9. For example, the incorporation of Se atoms into monolayer MoS2 could lead to a significant change of band gap, with the PL peak continuously tuned from 668 to 795 nm with different Se compositions6. Recently, rhenium disulfide (ReS2) has received lots of attentions among the TMDs families5, 11-12. The interlayer decoupling effect and lack of interlayer registry make its bulks behave electronically and vibrationally as same as decoupled

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monolayer13. Similar to other semiconductor materials, controllable n-type or p-type conduction of ReS2 is indispensable, especially when fabricating rectification devices based on homojunction structures. For 2D materials, although van der Waals heterojuction devices based on different materials is an alternative solution, their type-II band offset still largely affect the devices properties. For example, the photoexcitons recombination across the spatially indirect band edges in CdS/MoS2 heterostructure could lead to extension as well as red shift of the emission peaks in photoluminescence spectrum, which would degrade its optoelectronic performance under visible spectrum14. The limited splitting of Fermi levels at the interface could also result in a decreased photovotages15. Recent investigations indicate that ReS2 exhibits n-type transport behavior16-18, and in order to extend its application in functional devices, especially in rectifying devices, it would be necessary to realize the p-type doping of ReS2. Molybdenum (Mo) and rhenium (Re) are located in the adjacent groups with nearly identical covalent radius (Mo = 135 pm; Nb = 137 pm) and chemical valence states19, and it is possible for Mo atoms to replace Re cations in the host lattice to realize substitutional doping20. In addition, Mo owns one less valence electron than Re, and DFT calculations have demonstrated that Mo is a promising hole donor to change the carrier type of pristine ReS221. Chemical vapor deposition (CVD) is a feasible method to obtain monolayer ReS216, 18, 22, and it could also be used to realize the doping of ReS2 nanoparticles and vertical nanosheets by introduing dopants during growth process23-24. However, the synthesis of uniform few-layer ReS2 films with Mo doping which is suitable for electronic application has

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not been reported. In this work, we demonstrated the substitutional doping of Mo atoms in monolayer ReS2 films on mica substrate via chemical vapor deposition by introducing vapor-phase MoO3 precursor. The substitution of Mo dopants was confirmed by scanning transmission electron microscope (STEM). The chemically inert and atomically smooth surface of mica substrate plays a crucial role in the growth process, where the monolayer films could grow along the in-plane direction with very flat surface. The electrical transport measurement shows the Mo doping of ReS2 leads to a degenerate p-type conduction behaviors, which is further demonstrated by DFT calculations. We also fabricated a p-n diode device based on homojunction of n-type multilayer ReS2 films and p-type monolayer RexMo1-xS2 films, which exhibits gate-tunable and current-rectifying transport characteristics. 2. Experimental section 2.1 Epitaxial growth and transfer of monolayer ReS2 Monolayer RexMo1-xS2 was grown by CVD in a two temperature-zone tubular furnace, in which a 1-inch diameter quartz tube is placed. Sulfur powder was placed in the low temperature zone, which was heated to 200 ºC at a ramping rate of 5 ºC/min and maintained for 10 min. 20 mg Re power (Alfa Aesar, purity: 99.99%) and 50 mg MoO3 powder (Alfa Aesar, purity: 99.9%) were placed on the other ceramic boat with freshly cleaved fluorophlogopite mica above it. The temperature of zone II was heated to 600 ºC with 15 ºC /min ramping rate and maintained for 10 min. Argon with 40 sccm was used as the carrier gas to convey vapor species to the downstream

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mica substrates. As for the synthesis of undoped ReS2, 50 mg Te powder (Alfa Aesar, purity: 99.99%) was used as catalyst instead of MoO3, leaving other setting parameters unchanged. Monolayer ReS2 could be obtained with growth time of 10 min, and by prolonging the growth time to 30 min, multilayer films of ReS2 could be obtained. In the transfer process, monolayer ReS2 on mica substrate was covered with polymethyl methacrylate (950 PMMA, Sigma-Aldrich) by spin coating (3000 rpm for 2 min), and baked at 150 °C for 10 min. Then, the mica was etched by hydrofluoric acid (20 wt %) solution. After carefully cleaned with distilled water, ReS2 was transferred onto Si/SiO2 substrate, followed by the removal of PMMA using acetone for 10 h. 2.2 Characterization The morphology of synthesized samples was characterized by optical microscope (Zeiss Imager A2m) and atomic force microscope (Bruker Dimension ICON-PT). Transmission electron microscope (Talos F200x, FEI) combined with HAADF-STEM modules was employed to indentify the doping heteroatom in monolayer ReS2 films. Raman spectra were recorded using Argon ion laser Raman spectrometer (LabRAM ploRA,Horiba JobinYvon) with a 532 nm laser, the spectral resolution of Raman is 1 µm. 2.3 Device fabrication and electrical property measurements FET devices were fabricated by transferring ReS2 films onto freshly clean SiO2/Si substrate with 300 nm-thick SiO2 layer. Cr/Au electrodes with thicknesses of

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10/100 nm were deposited by thermal evaporation using metal shadow masks. The transfer and output characteristics of FETs, rectifying behaviors of p-n diode were recorded using Keithley 4200 semiconductor characterization system on a Lakeshore probe station. 2.4 Density functional theory calculations First, we constructed a 2×2 super cell containing 4 Re and 8 S atoms to calculate the band structure of pristine ReS2. A vacuum layer of 15 Å is placed above the monolayer to avoid the interaction between the adjacent layers, and cut off energy of 500 eV and a precise 9×9×1 k-point sampling grid are used. All the atoms in the supercell are allowed to relax until residual forces have converged to less than 2.0×10−3 eV/ Å, and the total energy to less than 1.0×10−4 eV during the structural optimization. Then, electronic properties of Mo doped ReS2 are investigated by replacing a Re atom with Mo atom in a large super lattice (4×4×1), and the impurity concentration is set to 12.5%. 3. Results and discussion Figure S1 in Supporting Information illustrates the furnace set up for the growth of monolayer RexMo1-xS2 by chemical vapor deposition. Mica is used as the growth substrate, because its atomic flatness, surface chemical inertness and hexagonally arranged in-plane lattice characteristics make it easy for van der Waals (vdW) epitaxy growth18, 25-27. Re metal could react slowly with S vapor and form ReS2 crystal under 500–800 ºC16. However, the high melting point of Re (~3180°C) would largely hinder the nucleation and growth efficiency of ReS2 during the growth process. In this work,

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MoO3 plays dual roles during the growth process, acting as Mo doping resources and catalyst for Re evaporation, as following chemical equation shown25.

Re + 2S + 7MoO3 → ReO3 + 7MoO 2 + 2SO 2

(1)

The high melting Re metal could be oxidized by MoO3 into volatile ReO3 species when using high ratio of MoO3, and then ReO3 species could be reduced by sulfur vapor into ReS2. By this way, the nucleation and growth efficiency of ReS2 could be largely improved. Figure 1a schematically illustrates the surface reaction during the epitaxial growth process of RexMo1-xS2 on mica substrates, small ReS2 domains absorb on the mica surface and continue to expand into large-area films. Meanwhile, the volatile MoO3 is reduced to Mo4+ cations by sulfur vapor, which are incorporated into ReS2 lattice, leading to substitutional Mo doping of ReS2 films. Monolayer RexMo1-xS2 films on mica substrate were characterized by optical microscopy, as shown in Figure 1b. AFM topography image exhibits homogeneous color contrasts, indicating highly uniform thickness of RexMo1-xS2 films. The height profile at the edge area gives a thickness of 0.78 nm, corresponding to monolayer structure of ReS2 (Figure 1c). Then the films are transferred onto SiO2/Si substrate for optical microscope characterization. Clear transition from a complete full-covered layer to isolated triangular-like flakes could be also observed (Figure 1d and Figure S2 in Supporting Information).

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Figure 1. (a) Schematic diagram of the growth process of epitaxy Mo-doped ReS2 films. The reduced Mo4+ cations are incorporated into monolayer ReS2 films. (b) Optical microscope image of as-grown films on mica substrate. (c) AFM topography image of monolayer RexMo1-xS2 films grown on mica. Inset is the height profile. (d) Optical microscope image of the samples after transferred on SiO2/Si substrate.

As for the synthesis of undoped ReS2, Te was used as catalyst instead of MoO3 as previous reported18. After 10 min of growth, monolayer ReS2 films were grown on mica substrate, as shown in Figure S3 in Supporting Information. Prolonging the growth time to 30 min, continuous multilayer ReS2 films could also be obtained (Figure S4 in Supporting Information). It is worth noting that Te catalyst does not introduce doping in ReS2 films due to its extreme weak oxidability18. XPS spectrum in Figure S5 in Supporting Information presents no Te peaks (located at 573 and 583.4 eV). Re and Te could form Re–Te binary eutectic, whose eutectic point could be lowered down to 430 ºC when Te–Re weight ratio is up to 90%. By this approach, Re could easily volatilize into vapor phase, and further react with sulfur into

8


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continuous ReS2 films. The type of substrate plays a crucial role in the phase structure of products. Mica substrate could lower the energy barrier for Re and S atoms migration due to the chemical inertness and atomically surface flatness, which could facilitate the in-plane growth of ReS226. As for SiO2/Si substrate under the same growth condition, numerous nanosheets with random orientation appear instead of ReS2 films (Figure S6 in Supporting Information). Raman spectrum shows that they are mixture containing ReS2, MoS2 and MoO2 (Figure S7 in Supporting Information). Different from mica, SiO2 substrate could not provide such low energy barrier for atoms migration on surface18, thus the uniform in-plane growth of films are hindered. Raman spectroscopy has proven to be a powerful approach for probing the doping level in TMDs6, 27. Figure 2a shows Raman spectrum of monolayer RexMo1-xS2 on SiO2/Si substrate. Due to the low crystal symmetry of ReS2, 14 Raman-active vibration modes could be identified in the range of 100–500 cm−1. In the present study, we only pay attention to the strongest modes in the range of 120 and 240 cm−1, which could reflect the doping effect of ReS2. Two dominate Raman peaks located at 152.4 and 213.8 cm−1 are detected, which are assigned to Eg and Ag-like vibration modes of ReS2. Compared with undoped monolayer ReS2, all the modes of monolayer RexMo1-xS2 exhibit slightly shift towards high frequency, with the maximum shift up to 2.2 cm-1 (Figure 2b). As previously reported, Raman shift introduced by doping could be attribute to the electron-phonon coupling in TMDs28-29. In our experiment, Mo doping could lead to the increase of hole carrier density, which could further

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interact with incoming photons and result in the Raman shift towards high frequency, similar to the change in Raman spectrum induced by gate-voltage tuning in graphene and MoS2 28, 30.

Figure 2. (a) Raman spectrum of monolayer RexMo1-xS2 on SiO2 substrate. (b) Comparison of Raman spectra between RexMo1-xS2 and ReS2 monolayers.

X-ray photoelectron spectroscopy (XPS) analysis is used to characterize chemical stoichiometry change and valence states after doping. Figure 3a-c presents Mo 3d, S 2p and Re 4f spectra of RexMo1-xS2 films, with ReS2 samples for comparison. XPS survey spectrum of RexMo1-xS2 films is shown in Figure S8 in Supporting Information. As shown in Figure 3a, in the undoped ReS2 films, only one peak located at 227.3 eV is detected in the range from 220 to 232 eV, which is associated with S 2s of Re-S bonding. After doping, Mo4+ 3d core-level peaks at 227.5 and 230.5 eV are clearly observed, indicating the formation of Mo-S bonding, and S 2s peaks shifts toward lower energy to 225.1 eV. It is worthy noted that the Mo 3d related peaks shift toward to lower energy by 1.5 eV compared with pristine MoS2 (3d5/2~229 eV and 3d3/2~232 eV), and the core-level peaks of S and Re also exhibit the same tendency with shift around 1.0 and 1.4 cm-1 eV. This is attributed to downshift of Fermi level (EF) in ReS2 10


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incorporated with Mo elements, which is similar to previous studies on Nb-doped MoS210. These evidences suggest that the electronic structure of the ReS2 is altered, and the Mo doping is atoms substitution rather than physical absorption. By fitting the peaks area in XPS spectra, the compositions of the films are estimated to be Re0.9Mo0.1S2, corresponding to Mo doping concentration of about 10%. Different from 2H phase MoS2, Re atoms tend to agglomerate along b-direction in ReS2, leading to a distorted 1T crystal structure with very low symmetry. It would take much more energy for Mo4+ to be incorporated into ReS2 with high doping concentration due to the large crystal structure change. Thus, the maximum concentration almost does not change and remains at around 10% with more weight of MoO3.

Figure 3. XPS spectra of element core levels in monolayer RexMo1-xS2 and undoped monolayer ReS2. (a) Mo 3d; (b) S 2p; (c) Re 4f.

To further identify the microstructures and compositions of monolayer RexMo1-xS2 film, we transfer the samples directly on carbon supported Cu grid using traditional PMMA wet transfer method for TEM characterization. Figure 4a shows a low-magnification TEM image of monolayer RexMo1-xS2 films, and the holes are probably introduced during the transfer process. High-angle annular dark field scanning transmission electron microscope (HAADF-STEM) image could provide 11



Z-contrast with atomic lateral resolution8, 31 (Z = atomic number). Figure 4b shows HAADF-STEM image of monolayer RexMo1-xS2. Re atomic chains could be observed, and the estimated lattice spacing of the (100) and (010) planes are 3.1 and 3.4 Å, respectively18. The corresponding fast-Fourier transform (FFT) pattern (inset in Figure 4b) shows a set of lattice with an angle between a [100] and b [010] axes of 60°, further demonstrating the high quality of the obtained monolayer RexMo1-xS2. The contrast-corrected STEM image (Figure 4c) reveals clear DS-chains formed by Re atoms or Mo dopants (brighter sites). The atomic number (Z) of Mo is 42, much smaller then Re (Z = 75), and thus the intensity of Mo atoms are expected to be about 60% of Re atoms in HAADF image. The substituted Mo atoms are indicated by red circles in Figure 4c. The statistical histogram of site intensities labeled with red arrow is plotted in Figure 4d, where the intensity ratio for Re/Mo is around 1.6/1, indicating that Mo atoms occupy Re sites and successful substitutional doping. Figure S9 in Supporting Information shows EDX spectrum obtained from the RexMo1-xS2 films, peaks associated with Mo located at 17.3 and 19.5 eV could be clearly detected (the Re and Cu elements are from the products and grid of copper, respectively). The Mo mole fraction [Mo/(Mo+ Re)] is about 0.08, indicating the compositions of the films are Re0.92Mo0.08S2, consistent with XPS results.

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Figure 4. (a) Bright-field TEM image of monolayer RexMo1-xS2 films. (b) HAADF-STEM image of RexMo1-xS2 films (boxed area in (a)) with the corresponding FFT pattern. (c) Filtered HAADF-STEM image of monolayer RexMo1-xS2, where the atoms marked with red circle indicate the Mo atoms. (d) Intensity distribution of Re atoms along the red arrow in (c). The intensity of Mo is about 60% of Re. Mo exhibits approximately 62% lower intensity.

FET devices based on multilayer ReS2 and monolayer RexMo1-xS2 were fabricated to evaluate the electrical transport properties. Cr/Au (10/100 nm) is deposited as source and drain electrodes, with p+ doped silicon as back gate. The gate-dependent current of the monolayer RexMo1-xS2 device with different drain voltages were plotted in Figure 5a. With the increase of Vg, the source-drain current (Ids) decreases slightly, suggesting that the monolayer RexMo1-xS2 exhibit degenerate p-type transport behaviors. In addition, the leakage current of the device maintains at a high level without saturation when tuning the Vg from –60 to 60 V, indicating the p-type doping introduced by Mo substitution is degenerated. Figure 5b shows the output characteristics of the device at different gate voltages. The increasing current level with decreasing Vg also confirms the p-type transport behavior of monolayer RexMo1-xS2-based FETs. We remark that previous reported ReS2 devices, no matter the fabrication process (by exfoliation or CVD method), all exhibit n-type transport characteristics22, 32. In this study, after Mo substitution doping, the carrier type of ReS2 changes from n-type to p-type. In addition, according to the theoretical prediction, Schottky barrier (ϕB) at Cr/ReS2 contact interface could reach up to 1.5 eV, estimated from the work function of Cr (4.6 eV) and the VBM of ReS2 (6.1 eV)33.

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This would lead to nonlinear output characteristics of the device. However, in our study, Ids linearly increases with Vds, suggesting the Ohmic contact at the Cr/RexMo1-xS2 interface. This could be explained by the tunneling transport mechanism. As shown in Figure 5e, the degenerate p-type doping of ReS2 could result in a very narrow depletion width at the contact area29, 34, where the tunneling current would play dominant role in the carrier transport process instead of thermionic emission of electrons under bias voltage. We also fabricated four terminal device, and measured the gate depended sheet resistance (Rs) and contact resistance (Rc) as plotted in Figure S10 in Supporting Information. Compared to Rs, Rc is about one order smaller, and it does not change with the increase of bias voltage, indicating the excellent contact between electrode and channel materials. Thus, we can draw the conclusion that the carriers follow F-N tunneling transport mechanism due to the thinning of interfacial barrier, which is introduced by the degenerate doping of ReS2. The electrical properties of pristine multilayer ReS2 films were also investigated for comparison. Figure 5c and d displays the typical transfer and output characteristics curves for the undoped ReS2 device, which exhibits obvious n-type transport behaviors. Source-drain current (Ids) exhibits nonlinear relationship with Vds, indicating Schottky barrier exists at Cr/ReS2 interface. As shown in Figure 5f, the Schottky barrier is 0.4 eV for n-type ReS2 and Cr electrode, estimated from Cr work function 4.6 eV and ReS2 CBM 4.2 eV33. The field-effect mobility of carrier is extracted based on the equation:

µ = ( L / WCgVds )(∆I ds / ∆Vg )

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(2)

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where L, W and Cg represent the FET channel length, width and gate dielectric capacitance, respectively. The carrier mobility of multilayer ReS2 films could reach to 0.52 cm2 V-1, and the ON/OFF ratio is up to 103. Compared with single-crystal monolayer ReS2 FET in previous reports11, 18, the device based on multilayer ReS2 films exhibits a depredated electrical performance, which is attributed the carrier scattering effect in grain boundary35.

Figure 5. (a) Source-drain current (Ids) versus gate voltage (Vg) characteristics of monolayer RexMo1-xS2 FET device at different drain voltages (Vds). (b) Corresponding output curves with different Vg . Inset is optical microscopy image of the device on 300 nm SiO2/Si substrate. Scale bar is 20 µm.(c) Ids versus Vg curves for the device based on undoped multilayer ReS2 films with thickness of 4.2 nm 15



(Vds=1 V). (d) Output curves for the same device. (e) Interface band diagram between Cr and RexMo1-xS2 after contact under negative Vds. (f) Interface band diagram between Cr and ReS2 after contact under negative Vds.

In order to further understand the origin of p-type electronic behaviors after doping, density functional theory (DFT) calculations were performed to predict the band structure of RexMo1-xS2. Different from the stable hexagonal (2H) phase structure of other TMDs (MoS2, WS2 etc), ReS2 exhibits a stable octahedral (1T) phase structure, and the Peierls distortion of ReS2 would result in the buckled S layers and zigzag Re chains along b[010] direction (Figure 6a). The calculation results show that the single-layer and bulk ReS2 have identical band structure, both of them have direct bandgap of 1.34 eV (bulk) and 1.41 eV (monolayer). The conduction band maximum (CBM) and valence band minimum (VBM) are both located at the Γ symmetry points (Figure 6c), consistent with previous reports21, 36. The partial DOS (PDOS) analysis shows that valence band maximum (VBM) and conduction band minimum (CBM) are mainly composed of Re-d and S-p states, while other states play a minor role (Figures S11 and S12 in Supporting Information). Upon Mo doping, the band gap structure changes from direct (Eg=1.41 eV) to in-direct (Eg=1.15 eV), where the impurity levels are introduced slightly above the VBM (Figure 6d). The Fermi energy also shifts below the VBM due to the one electron removal from the metal atoms, leading to a p-type doping of ReS2. We also calculated the formation energy of Mo substitutional doping. Under Mo-rich conditions the calculated formation energy is 0.143 eV, and it would become more negative in S-rich limit condition. The results

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suggest that Mo substitutional doping in ReS2 could be realized theoretically. The calculation data can well explain our experiment results, where the RexMo1-xS2 devices exhibit p-type, semi-metal transport characteristics with very low ON/OFF ratio.

Figure 6. (a) Top view (up) and side view (down) of distorted 1T phase of ReS2. Blue balls represent Re atoms and yellow balls represent S atoms. (b) A 4 × 4 supercell containing three Re atoms, one Mo atom and eight S atoms, with one Re atoms substituted by Mo atoms. The doping concentration is 12.5%. (c) Calculated band structures of monolayer ReS2. (d) Calculated band structures of ReS2 after Mo doping.

To explore the application of p-type RexMo1-xS2 films on homojunction-based rectifying devices, we fabricate a p-n diode based on homojunction of multilayer ReS2 and monolayer RexMo1-xS2. Rectifying devices based on homojunction structure could effectively avoid the problems in devices introduced by type II band structure of different materials10. First, monolayer RexMo1-xS2 was transferred onto 300 nm SiO2/Si substrate using traditional wet transfer method, followed by the transfer of ReS2 films using the same procedure. Cr/Au electrodes (10/100nm) were deposited as

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metal contacts. The device was annealed at 300 ºC for 2h to enhance the interlayer coupling between the p-n interface and electrode contact37. Figure 7a shows the schematics of the device and the electrical connections. A back-gate voltage (Vg) is applied to the p+-doped silicon, with a bias voltage (Vds) applied across the diode. Figure 7b is optical image of the fabricated device, in which the blue region represents the multilayer ReS2 films, while the dark purple region is the monolayer RexMo1-xS2. As shown in Figure 7c, the thicknesses of multilayer ReS2 films and monolayer RexMo1-xS2 are 3.5 and 1.0 nm, respectively. Wrinkles appeared on the surface of monolayer RexMo1-xS2 may arise from the wet transfer process, which are often observed in previous studies38. As shown in Figure 7d, the p-n homojunction diode exhibits obvious rectifying transport behaviors. The rectification ratio R is defined as the ratio of Iforward/Ireverse current. By using –20 V back gate voltage, the rectification ratio R can reach up to 150 at Vd=−2V/+2V. The good diode characteristic is due to the homogeneous p-n junction using the same material29,

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. Without the interface traps and interface

resistance, the diode performance could be significantly improved. In addition, the degree of rectification could be largely tuned by adjusting the applied back gate voltage. Obviously, the rectification ratio R would decrease from 150 to 20 as the back gate voltage changes from −20 to 20 V. We have proved that the undoped multilayer ReS2 films exhibit excellent gate tunable transport properties, whereas the p-type monolayer RexMo1-xS2 is degenerated with semi-metal behaviors. Typically, the transport properties of heavily-doped semiconductor are lack of sensitivity to

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electrostatic field from the back gate40, while the carrier density could be efficiently tuned by back gate in lightly-doped ones. In other words, the rectifying behavior of the diode is predominantly governed by the undoped ReS2 films side, in which the carrier density could enhance rapidly with increasing the back gate voltage, resulting in enhanced current flow. The rectification ratio R measured in our study is comparable with previous reported O2 plasma treated MoS2 p-n homojuction diode, in which the value is about 240 at Vd=−1V/+1V41.

Figure 7. (a) Schematic of ReS2/RexMo1-xS2 homojunction diode. (b) Optical microscope image of the device. Blue regions represent the multilayer ReS2 film, and the dark purple region is monolayer RexMo1-xS2. (c) AFM topography image of the junction region. Inset is the height profile showing the thickness of monolayer RexMo1-xS2 (~ 1.0 nm) and multilayer ReS2 films (~ 3.5 nm). (d) Gate voltage (Vg) dependence of channel current (Ids) of the device at different bias voltages (Vds). (e) I−V characteristic at different back-gate voltages. (f) Rectification ratio as a function of back gate voltage Vg.

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4. Conclusions In summary, by using CVD approach, we realize the growth of monolayer Mo doped ReS2 films on mica substrate. Upon substitutional Mo doping, the intrinsic n-type conduction property of ReS2 is changed to p-type, and the semi-metal transport behavior indicates the doping is degenerate. DFT calculations confirm the Mo substitution doping would introduce impurity levels close to VBM, which could change the band gap type and lead to a p-type doping. The electrical properties of p−n homojunction diode based on ReS2 and RexMo1-xS2 are investigated, which exhibit gate-tunable current rectification behavior. The successfully synthesis of p-type ReS2 in this study could largely promote its application in novel electronics and optoelectronics devices.

ASSOCIATED CONTENT Supporting Information Available Schematic of the furnace set-up, optical microscopy image and Raman spectrum of triangular-like RexMo1-xS2, optical microscopy image, Raman spectrum, TEM analysis and XPS spectrum of undoped ReS2 films, SEM images and Raman spectrum of composites nanosheets, EDX analysis of

RexMo1-xS2 films, DFT calculations of

RexMo1-xS2. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] E-mail: [email protected] 20


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Notes The authors declare no competing financial interest. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51572057) and Shandong Provincial Key Lab of Special Welding Technology, Harbin Institute of Technology at Weihai. The authors thank W. Zheng for FETs measurements and P. Miao for Raman spectrum analysis. References 1.

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Films and Their Homojunction - ACS Publications - American

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