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Synthesis of Atomically Thin Transition Metal Ditelluride Films by Rapid Chemical Transformation in Solution Phase Anupam Giri, Heeseung Yang, Woosun Jang, Junghyeok Kwak, Kaliannan Thiyagarajan, Monalisa Pal, Donghyun Lee, Ranbir Singh, Chulhong Kim, Kilwon Cho, Aloysius Soon, and Unyong Jeong Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00684 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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Chemistry of Materials

Synthesis of Atomically Thin Transition Metal Ditelluride Films by Rapid Chemical Transformation in Solution Phase Anupam Giri, †, ‡ Heeseung Yang, ǁ, ‡ Woosun Jang, ǁ Junghyeok Kwak, † Kaliannan Thiyagarajan, † Monalisa Pal, † Donghyun Lee, § Ranbir Singh, ⊥ Chulhong Kim, § Kilwon Cho, ⊥ Aloysius Soon ǁ, * and Unyong Jeong †* †

Department of Materials Science and Engineering, Pohang University of Science and Technology, 77 CheongamRo, Nam-Gu, Pohang 790-784, Korea. ǁDepartment

of Materials Science Engineering, Yonsei University, 134 Shinchon-dong, Seoul, Korea.

§

Department of Creative IT Engineering, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang 790-784, Korea.

⊥Department

of Chemical Engineering, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang 790-784, Korea.

ABSTRACT: The controlled synthesis of large-area, atomically thin molybdenum and tungsten ditelluride (MoTe2 and WTe2) crystals is crucial for their emerging applications based on the attractive electronic properties. However, the solution phase synthesis of high-quality and large-area MoTe2 or WTe2 ultrathin films have not been achieved yet. In this study, we synthesized for the first time, large-area atomically thin MoTe2 and WTe2 films in solution phase, through rapid crystal formation directly on a conducting substrate. For the synthesis, we developed a new Te precursor. The crystal growth involves an in-situ chemical transformation from Te nanoparticles into MoTe2 or WTe2 thin films. The synthesis enables precise control of the number of atomic layers over a large area, from a monolayer to multilayers. Micropatterned MoTe2 thin films are also readily synthesized in-situ using the same process. The photodetector made of 3-layer semiconducting MoTe2 thin films exhibits high photoresponsivity (Rλ) over a broad spectral range (300-1100 nm) with a maximum in the near-IR region, including a Rλ = 30 mA W–1 even at λ = 1.10 μm and a fast photoresponse (87 μs). Our synthesis method presents a crucial step in the solution phase synthesis of metal telluride ultrathin films and paves the way for their large-scale emerging applications.

1. INTRODUCTION Atomically thin two-dimensional (2D) layers of transition metal dichalcogenides (TMDCs), MX2 (M = Mo, W; X = S, Se, Te), are being intensively studied as emerging materials because of their unique optoelectronic properties,1,2 and their potential uses as catalyst,3,4 lithium battery anode,5 and photothermal agent.6 Among the TMDCs, the properties of MoTe2 differ from those of their lighter chalcogen analogs.7 For instance, MoTe2 is stable in both the semiconducting 2H phase and the semi-metallic 1T′ phase,8 although MoS2 and MoSe2 are stable only in the 2H phase, and WTe2 is stable in the 1T′ phase.9 The reversible structural phase transition between the semiconducting and semi-metallic phases in atomically thin MoTe2 films have attracted large interest very recently.10,11 Moreover, WTe2 and MoTe2 monolayers are predicted to be an excellent platform for studying fundamental physical phenomena such as superconductivity,8,12 spintronics,12 and high-efficiency thermoelectricity.13,14 Mass production of high-quality, large-area, atomically thin MoTe2 and WTe2 films is highly required

to provide a practical material platform for the fundamental studies and promising applications. The synthesis of MTe2 thin films is particularly challenging compared to that of MS2 and MSe2 because of synthetic issues involving lack of precursors, fast oxidation,15 volatility,16 thermal instability,17 and phase targeting.18 Recently, successful chemical vapor deposition (CVD) growth of single-layer and multilayer MTe2 thin films in the 2H and 1T′ phase was reported.9,16,19,20 However, so far, there have been no success in one-step solution phase synthesis of MTe2, no report even in the two-step process including solutionphase coating and thermolysis at a high temperature. A solution-based MTe2 thin film growth on a substrate, if possible, is highly desirable as this process would be relatively simple and large area production can be readily achieved. The synthesis of MTe2 thin films in solution phase is a challenging task because of the lack of Te precursors. In this study, we suggest a reactive Te precursor, produced by the reaction of an imidazolium fluoroborate salt with tellurium tetrachloride (TeCl4) for the robust synthesis of MTe2 thin films.

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Figure 1. Reactive Te precursor for the synthesis of metal ditellurides. a-b) Te precursor and its transformation into Te nanorods 11 19 under microwave irradiation. c) B and F NMR characterization of the as-synthesized Te precursor in CDCl3 at 25°C.

MoTe2 and WTe2 thin films were synthesized by directly growing the crystals on a Si substrate in a solution phase. The synthesis is based on the rapid chemical transformation of the reactive Te precursor. To synthesize a largearea MTe2 thin film directly on solid substrate, we exploited the microwave-assisted thin film formation we previously developed.21 The number of Te-Mo-Te triple layer was manipulated precisely over the whole substrate. This study clearly visualizes the mechanism of thin film formation which includes rapid sequential chemical transformation of reduced elements. Micro-patterning of metal telluride films through soft lithography is also demonstrated. We utilized the semiconducting MoTe2 thin films to fabricate a near-IR photodetector. The photocurrent profile exhibits high photoresponsivity (Rλ) over a broad spectral range (300-1100 nm) with a maximum in the near-IR region, including a Rλ = 30 mA W–1 even at λ = 1.10 μm and a fast photoresponse (87 μs). This near-IR absorbance is the promising unique property of the semiconducting MoTe2 film synthesized in this study. The reasons are theoretically explained by the grain-size dependent photoresponse.

2. RESULTS AND DISCUSSION The solution phase synthesis of MTe2 thin films is especially problematic, because, preparation of an appropriate Te precursor dissolved in a desired solvent is challenging due to its oxidative instability. Moreover, Te analogs of ligands that are often used as S- or Se-containing precursors are either unknown or not readily available. Recently, a colloidal synthesis of 1T′-MoTe2 nanostructures was achieved using trioctylphosphine telluride as a Te precursor in an oleylamine-oleic acid system through a hot injection method.22 However, this procedure is not compatible for the direct growth of 2D MTe2 thin films on a substrate. Thus, a new Te precursor for solution phase synthesis of MTe2 thin films is required. Here, we developed

a reactive Te precursor, produced by the reaction of an imidazolium fluoroborate salt with tellurium tetrachloride (TeCl4) for the robust synthesis of MoTe2 and WTe2 thin films. Figure 1a-b shows the Te precursor and its transformation to Te nanorods (NRs) under microwave irradiation for 90 s. The preparation of the Te precursor is remarkably simple. TeCl4 (75 mmol) and [BMIM]+[BF4]were introduced in a round-bottom ﬂask filled with Ar under magnetic stirring, and heated to 60 °C. The reaction mixture turned into clear light yellow solution in 1 h. The reaction formed [BMIM]+[BF4]- -TeCl4 complex, where the [BF4]- moieties are orderly bound due to complexation with TeCl4 molecules. This new complex was used as the reactive Te precursor for the synthesis of MTe2. Figure 1c compares the nuclear magnetic resonance (NMR) spectra of the pure electrolyte and the Te precursor. We did not notice any significant differences in the 1H (Figure S1) and 13C NMR (Figure S2) patterns of [BMIM]+ in the parent electrolyte and upon complexation with TeCl4. In contrast, significant differences were observed between the 11B and 19F NMR patterns of [BF4]- in the electrolyte (Figure 1c). In the 11B NMR spectra of the complex, a new peak (δ = −1.2 ppm, marked with a black arrow) was observed along with the characteristic peak of [BF4]- at δ = -1.0 ppm. This new peak is attributed to [BF4]molecules, which are orderly bound due to complexation with TeCl4 molecules.23,24 The two signals in the 19F NMR spectra of [BMIM]+[BF4]- (blue line) arise by coupling of the 19F nucleus with the two boron isotopes, 10B (δ = −151.51 ppm ) and 11B (δ = −151.57 ppm), present in the [BF4]- anion.25 Upon complexation with TeCl4, the 19F NMR spectrum showed new signals at δ = -150.19 ppm and δ = -150.24 ppm, which are ascribed to deshielding due to the interaction between B-F and Te+4 as indicated in the chemical structure of the Te precursor.24,26,27


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Figure 2. Synthesis of MoTe2 nanosheets. TEM and EELS mapping image of the aliquots taken at different reaction times of 18, 45, 60, and 90 s, during the reaction of Te precursor with MoCl5 in the bulk solution under microwave irradiation.

On the basis of the NMR results, [BF4]- has a strong affinity to TeCl4 and the relatively larger Te+4 ions preferentially accommodate the smaller [BF4]- molecules within their coordination sphere. Note that TeCl4 can form a clear and stable complex only with [BF4]- containing liquid electrolytes, among the various combinations of liquid electrolytes and Te precursors tested for the synthesis.

Finally, 2D nanosheets of MoTe2 were obtained within 90 s. Elemental analysis and TEM studies indicate that the 2D MoTe2 nanosheets are formed following a nonstoichiometric nucleation and growth process, where stoichiometric metal chalcogenides are obtained through a surface nucleation and detachment process, regardless of the starting composition and dimensions.28

The reaction pathway producing the (W)MoTe2 thin films involve in-situ rapid chemical transformation from Te nanocrystals into the MoTe2 or WTe2 thin film. To trace the reaction kinetics, we studied the synthesis in bulk electrolyte medium. Under microwave irradiation, the Te precursor formed amorphous Te nanoparticles within 18 s and then crystalline Te NRs within 90 s (Figure S3 and Figure 1b). Meanwhile, the MoCl5 precursor alone mixed in the electrolyte formed amorphous Mo aggregates within 30 s and then changed into Mo nanoparticles within 60 s (Figure S4), indicating a slower reduction rate of MoCl5 compared to the Te precursor. Time-dependent changes in the products are shown using X-ray powder diffraction (XRD) analysis (Figure S5). When MoCl5 was mixed with the Te precursor and microwave irradiation was applied, 2D nanosheets of MoTe2 was obtained within 90 s. The MoTe2 formation was confirmed by XRD (Figure S6). Figure 2 presents the transmission electron microscope (TEM) image and the corresponding electron energy loss spectroscopy (EELS) mapping image of the sample corresponding to different reaction times during the synthesis of MoTe2 nanosheets. EELS mapping, energydispersive X-ray spectroscopy (EDS) and TEM images confirm that Te nanoparticles and amorphous Mo rich aggregates coexisted during the first 18 s of the reaction (Figure S7). The Te nanoparticles crystallized into NRs, and at the same time, the amorphous Mo reacted at the Te surface to form a Mo-rich shell within 45 s (Figure S8). After 60 s, all rod-like Te structures disappeared and rough microplate-like structures appeared, with an average elemental composition of Mo:Te = 1:0.9 (Figure S9).

Figure 3a illustrates the mechanism of the thinfilm formation. The precursor electrolyte solution containing a metal precursor (MoCl5 or WCl4) and a Te precursor, newly synthesized in this study, was sandwiched between a conductive silicon wafer as a bottom substrate and a glass as top substrate. And then, microwave was irradiated for 30–45 s to the sample. We used a p++ Si wafer (electrical conductivity = 1000 S/m, thickness = 0.5 mm) to effectively absorb the microwave, so that the interface between the substrate and the precursor solution was selectively heated. This confined interfacial heating facilitates selective decomposition of the precursor at the substrate surfaces and results in fast thin film growth only on the substrate without forming crystals in the bulk solution. Washing the electrolyte solution in EtOH: H2O (70:30 v/v %) mixture solvent produced a MoTe2 or WTe2 thin film. The microwave power should be adjusted so that the substrate temperature (Tsub) was higher than the critical temperature (Tc) of precursor thermolysis, whilst the precursor solution temperature (Tsol) was lower than Tc. The microwave penetration depth (δ) of the p++ silicon wafer is governed by the microwave frequency and the electrical conductivity of the target substrate, so it was the control variable required to attain the surface crystallization condition.21,29 Theoretically, the temperature of p++ silicon wafer substrate increase at a rate of 7.8 oC s-1 under 700 W microwave irradiation.21 The thin film formation on the conducting substrate is analogous to the mechanism of the chemical transformation in the bulk solution phase as shown in Figure 2. A difference is that the thin film growth includes



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Figure 3. Mechanistic details of (W)MoTe2 thin film formation and characterization of the synthesized thin film. a) Scheme of the thin film formation by rapid solution phase chemical transformation of Te precursor over a conducting substrate under microwave irradiation. b) Optical microscopic image of monolayer MoTe2 thin film. Inset shows the AFM image and the height profile of the MoTe2 thin film. c) HRTEM image of the MoTe2 thin film. d) STEM-EDS elemental mapping of the MoTe2 thin film showing the homogeneous distributions of Mo (green) and Te (red) over the whole sample area. e) EDS analysis of the MoTe2 thin film. f-h) Cross-sectional TEM images of the monolayer, few-layer, and multilayer MoTe2 thin films. Their corresponding OM images and digital image of the whole specimen (upper inset, scale bar: 0.5 cm) are exhibited together.

oriented attachment and reorganization of the primary building blocks. The optical microscopy (OM) images in Figure 3b show a continuous, uniform MoTe2 thin film over a large area. For better optical observation of the film, we transferred the film onto a Si wafer with a thick oxide layer (300 nm). From the topological height profile in the atomic force microscopy (AFM) image shown in the inset of Figure 3b, the thickness of the MoTe2 film was ∼0.84 nm, which is similar to that of single-layer MoTe2.30 The high-resolution TEM (HRTEM) image of the MoTe2 film shows periodic arrangement of the atoms with a lattice spacing of 0.22 nm (Figure 3c), which is consistent with the (114) planes of MoTe2. High-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) and EDS elemental mapping revealed a uniform spatial distribution of Mo and Te over the entire detection range of the thin film (Figure 3d). The EDS analysis confirmed the formation of MoTe2 (Figure 3e) with Mo and Te as the principal elements in a stoichiometric composition (Mo: Te = 1: 2). To manipulate the number of Te−Mo−Te triple atomic layers over the whole substrate in a controllable manner from monolayer to multilayer MoTe2, we adjusted the precursor concentration in the electrolyte solution

while keeping the other experimental conditions, including substrate size, volume of the precursor solution, and microwave power and exposure time, the same. Figures 3f–h shows the cross-sectional TEM images of the MoTe2 thin films with a thickness of monolayer, 3-layers, and 5layers, respectively. Corresponding OM images are shown together. The uniformity of the film thickness over the entire area of the specimen (inset image) is evident from the homogeneous color in the camera images. X-ray photoelectron spectroscopy (XPS) was used to examine the chemical compositions and valence states of the MoTe2 thin film. The high-resolution spectrum of each element of MoTe2 is shown in Figures 4a-b. The binding energy peaks of Mo 3d at 228.1 eV (3d5/2) and 232.2 eV (3d3/2) are assigned to the Mo−Te bonds of MoTe2 crystals (Figure 4a).31 The peaks at 572.8 and 583.3 eV, which are consistent with the binding energies of Te 3d5/2 and 3d3/2, respectively, can also be attributed to the Mo−Te bonds of MoTe2 crystals (Figure 4b).31 Additionally, the [Mo]:[Te] atomic ratio was found to be 1:2.06, indicating that the MoTe2 thin film was stoichiometric. We used thin-film XRD to obtain further information regarding the crystal structure of the 5-layer film (Figure 4c).


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Figure 4. Characterization of the MoTe2 thin films. a) XPS spectra of Mo 3d core levels for the MoTe2 thin films. b) Highresolution Te 3d XPS spectra of the same region on the MoTe2 thin film. c) XRD patterns of the MoTe2 thin film. d) Raman spec-1 trum of MoTe2 thin film, and inset shows the Raman intensity mapping of Ag peak at 161.6 cm (scale bar: 2 µm).

Figure 5. Characterization of the WTe2 thin film. a) Optical microscopic image of 3-layer WTe2 thin film. b) AFM image and the height profile of the WTe2 thin film. c) Cross-sectional TEM image of the monolayer WTe2 thin film. d) XRD patterns of the WTe2 thin film. e) XPS spectra of W 4d core levels for the WTe2 thin film. f) High-resolution Te 3d XPS spectra of the same re7 -1 gion on the WTe2 thin film. g-h) Raman spectrum of WTe2 thin film and Raman intensity mapping of A 1 peak at 162 cm for WTe2, respectively.

All the observed peaks are indexed to monoclinic MoTe2 (JCPDS file No. 71–2157), confirming the formation of 1T′MoTe2. The lack of peak-to-peak resolution in the XRD diffractogram might be due to the low-symmetry monoclinic crystal structure of 1T′-MoTe2. Peak broadening took place because of the presence of nanoscale crystal domains. An estimation of the grain size from the fullwidth at half-maximum (FWHM) of the (002) reflection plane yields a value of ~8 nm. Unfortunately, the XRD pattern from the monolayer film was too weak to analyze. We performed Raman spectroscopy of the 5-layer MoTe2

thin film to further confirm their crystal phases. Figure 4d shows the characteristic Raman spectra of the 1T′ phase of MoTe2 thin films with a prominent Ag mode peak at ≈161.6 cm−1 and a lower intensity peak at ≈264 cm−1.9,19 The inset image is the Raman intensity mapping for the Ag peak at 161.6 cm-1. We obtained the same intensity mapping in the entire specimen, indicating the formation of homogeneous film over large area. lowed

This chemical transformation approach also althe formation of WTe2 thin films.



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Figure 6. a) Schematic presentation of the micro-patterning strategy of MoTe2 thin film on Si substrate. AFM images (b and d) and the corresponding height profile (c and e) along the dotted line drawn across the image of the thin (3-4 nm) and thick (6-7 nm) MoTe2 line patterned film, respectively.

Figure 5a shows an OM image of the WTe2 thin film, showing a continuous and uniform film over a large area. The thickness of the film as measured by AFM was found to be 2.3 nm (Figure 5b), similar to that of 3-layer WTe2 thin film. Also, the possibility of monolayer WTe2 thin film formation was confirmed from cross-sectional TEM image (Figure 5c). Figure 5d shows the XRD pattern of a WTe2 thin film sample, where all Bragg peaks in the pattern can be well indexed to the orthorhombic structure of WTe2 (JCPDS file No. 81–1908). The prominence of the (002) peak in the XRD pattern clearly indicates the presence of a well-stacked layered structure. The calculated crystallite size from the FWHM of the most intense (002) peak of the WTe2 thin film was found to be 7.2 nm. Moreover, in case of WTe2 thin films, the binding energy peaks of W 4d and Te 3d (Figure 5e-f) are also consistent with the reported value,32 and W: Te ratio calculated from W 4d and Te 3d regions in Figure 5e-f confirms a W:Te ratio of 1:1.94, which is reasonably consistent with bulk WTe2 crystals. The satellite peaks could be due to the surface oxidation of the samples because of the high surface energy of the ultrathin films.33 Figure 5g shows the characteristic Raman spectra of WTe2 showing the two main vibrational modes, namely A71 and A91, observed at 162 and 211.2 cm-1, respectively, in agreement with previous reports on 1T′-WTe2 ﬂakes.9,34 Moreover, the corresponding Raman intensity mapping for A71 peak at 162 cm-1 indicating the formation of homogeneous film over large area (Figure 5h).

We further extended the synthesis procedure to the in-situ formation of micro-patterned MoTe2 thin films on a Si substrate. Figure 6a schematically describes the fabrication process. We used a line-and-space pattern of poly(ε-caprolactone) (PCL) fabricated using capillary force lithography.35After Oxygen plasma treatment on the polymer pattern, the precursor solution was coated and covered with a cover glass. After waiting for the solution to wet the surface completely, the cover glass was removed and the PCL pattern was selectively dissolved using toluene, leaving only the precursor electrolyte pattern on the substrate. After being covered with another Si substrate of same size and conductivity, the patterned precursor was put under microwave irradiation to produce the MoTe2 micro-patterned film. The residual electrolyte solution was removed by washing with ethanol and H2O. An AFM image of the patterned thin film (Figure 6b) and the corresponding thickness profile (Figure 6c) along the dotted line shows that the pattern thickness was approximately within 3-4 nm. Moreover, as shown in figure 6d-e, the thickness of the line patterned thin films can also be tuned (~6-7 nm) by adjusting the precursor concentration in the electrolyte solution. Figure S10a-b shows the corresponding FESEM image of the thin (~ 3 nm) and thick (~ 6 nm) line pattern of MoTe2 thin films, respectively. Our DFT calculations predict that in the case of a 2H MoTe2 monolayer consisting of small grains, the optical responses in the near-IR region increase dramatically compared to pristine MoTe2 (Figure 7).


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Figure 7. Topological defect structures of monolayer 2H MoTe2 thin film and their corresponding calculated optical absorbance. Atomic structure of defect-free mono-layer MoTe2 (a), grain void-type defect structure (b), and grain line-type structure (c). Blue and brown spheres denote Mo and Te atoms, respectively. DFT-calculated imaginary part of dielectric function (ε2) (d) and energy loss spectra (ELS) of defect-free and defected 2H MoTe2 (e).

To examine and model the topological defects in the forms of grain boundaries in pristine monolayer 2H MoTe2 (Figure 7a), we have chosen 2D atomic geometries comprising mainly of line defects with different domain contact angles between neighboring hexagonal crystalline domains, namely, corner-to-corner (grain void-type, Fig. 7b) or edge-to-edge (grain line-type, Figure 7c) arrangements. The optical responses are deliberated from the imaginary part of the frequency-dependent dielectric function (ε2, Figure 7d) and the energy loss spectra (ELS, Figure 7e) in the near-IR region. The enhanced optical response in the near-IR region may originate from the introduction of surface states along the nano-grain boundaries (which are otherwise absent in the pristine 2H MoTe2 film). We further varied both the grain size and widths of the defects in both void-type and line-type defected 2H MoTe2 (Figure S11). We found the enhancement in the near-IR optical response to persist once the grains are in nanoscale. We identified 2H MoTe2 through AFM, Raman spectroscopy, and temperature-dependent transport property measurements for the 3-layer MoTe2 (Figures 8ac) and 5-layer MoTe2 film (Figures 8d-f). Figure 8a shows the AFM image and the corresponding height profile of 3layer MoTe2 film. The Raman peak of the same film is shown in Figure 8b. The presence of E2g peak along with the A1g peak of the film clearly confirms the 2H phase of MoTe2.18 The 3-layer thin films showed an exponential increase in conductance with increasing temperature

from 210 K (10-13 A at 1.5 V) (Figure 8c). This nonlinear current (I)–voltage (V) behavior with temperature (inset of Figure 8c) further indicates that the MoTe2 thin films are in semiconducting 2H phase. The 5-layer MoTe2 film estimated from the AFM image (Figure 8d) showed the Raman spectrum of the 1T' phase (Figure 8e).9 A steady increase in conductance with temperature in the temperature-dependent transport (Figure 8f) showed the characteristics of semimetallic behavior. Inset shows the plot of current value at 1.5 V with temperature. Similarly, as shown in Figure S12, the multilayer WTe2 thin film also showed the semimetallic behavior. Combined with the XRD and Raman results for the 5-layer MoTe2 film, the IV profile indicates that the film transformed from the semiconducting to the semimetallic phase as its thickness increased.18 However, since the energy difference between semiconducting and semimetallic phase of MoTe2 is very small, a subtle difference in the synthetic parameters may lead to the formation of one phase over another. Thus, a substantial amount of semiconducting domain may still exist in the 5-layer MoTe2. 1T' phase is a major phase of the 5-layer or thicker film and 2H phase is the main phase of the 3-layer or thinner film. OM image of the 3-layer and 5-layer MoTe2 devices fabricated over 300 nm SiO2/Si substrate for temperature-dependent transport property measurements, are shown in Figure S13a-b, respectively. To authenticate the theoretical prediction of the emergence of IR optical absorbance in the 2H MoTe2 of small grains, we fabricated near-IR photodetector.



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Figure 8. Thickness dependent characteristics of the 3-layer and 5-layer MoTe2 thin films. (a-c) 3-layer MoTe2 film: (a) AFM, (b) Raman spectrum, (c) Temperature dependent transport properties. (d-f) 5-layer MoTe2 film: (d) AFM, (e) Raman spectrum, (f) Temperature dependent transport properties. Insets in Figure c and f show the current plots at 1.5 V.

Figure 9. a) Spectral photoresponsivity curve of the photodetector. b) Current (I)–Voltage (V) characteristic of the photodetector in the dark and under light illumination (λ =1064 nm) at different laser powers from 5 to 300 µW. c) Photo-switching stability of the photodetector under repeated on–off illumination from 633, 860 and 1064 nm laser sources. The bias voltage (2V) and illumination power (100 µW) were kept constant. d) Time-resolved photocurrent measurement shows a response time (τr) of 87 µs and a decay time (τd) of 456 µs.

As, MoTe2 has relatively narrow band gap (~1 eV) compared to MoS2 and MoSe2 (> 1.5 eV), it is a good candidate for near-IR photodetector. However, most of the MoTe2 device showed maximum spectral responsivity in the visible spectrum range.36-39 Very recently a bilayer MoTe2

photodetector with a calculated responsivity of 4.8 mAW-1 at a wavelength of 1160 nm was reported, but it was a waveguide-integrated photodetector and the responsivity needed much progress.40 Apparently, there is an emerging


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demand for developing IR photodetectors with high responsivity and low device complexity. To fabricate the photodetector device, 3-layer 2H MoTe2 film was transferred over a HfO2 (20 nm)/Si substrate and Au electrodes were thermally evaporated through a mask with a 75 μm-long channel. OM image of the photodetector device is shown in Figure S13c. Monochromatic laser light illumination was directed vertically onto the device and the current change between the Au electrodes was monitored. Figure 9 demonstrates the optoelectronic characteristics of the MoTe2 thin film photodetector. Figure 9a shows the broad spectral (300–1100 nm) photoresponsivity (Rλ) of the photodetector. It was calculated from the measured EQE (Figure S14) of the device with 2V bias and at zero gate voltage. Our photodetector was spectrally selective to near-IR light, with a maximum of Rλ = 53.1 mA W−1 at 870 nm, and about 30 mA W–1 at λ = 1.10 μm. This high photoresponsivity in the near-IR spectral region clearly substantiate the predicted defect-induced optical property of the 2H MoTe2 thin film. Figure 9b shows the photocurrent as a function of the laser illumination power in the range 5–300 µW at a fixed wavelength of 1064 nm. No gate voltage was applied. The device current of 72 nA (at 3V bias) in dark, increased up to 1.5 µA at 50µW laser illumination. Moreover, as shown in Figure 9c, the 3-layer MoTe2 photodetector showed a stable photo-switching characteristic under alternating dark (OFF state) and illuminated conditions (ON state) at 100 µW laser (633, 860 and 1064 nm) illumination and a bias of 2V. Unlike the reported photogating and photoconductive-type detectors based on few-layer MoTe2 transistors 38,41,42 and state-of-the-art MoS2 photodetectors, 43,44 the 3-layer MoTe2 thin film photodetector shows a far higher response speed (τr = 87 µs), as shown in Figure 9d.

3. CONCLUSIONS In summary, we have developed a solution phase synthesis method for atomically thin MoTe2 and WTe2 films grown directly on substrate, for the first time. The thin film formation reaction involves rapid in-situ chemical transformation of a newly developed reactive Te precursor, assisted by confined interfacial heating of the target substrate. The number of Te-Mo-Te layers in the MoTe2 thin film was controlled from the monolayer to multilayer. The film was homogeneous in chemical composition and thickness over the entire substrate. When combined with soft lithography, a micro-patterned MoTe2 thin film can be readily achieved in-situ on a substrate. A photodetector fabricated from the semiconducting MoTe2 thin films exhibited maximum spectral responsivity in the near-IR region and an ultrafast photoresponse. This rapid chemical transformation approach is promising for direct fabrication of large-area thin films and can be extended to the synthesis of other metal telluride thin films for fundamental studies and potential applications.

4. EXPERIMENTAL SECTION

4.1. Synthesis of MoTe2 nanosheets in bulk solution. MoCl5 (34 mM) was added to the Te precursor (75 mM) followed by magnetic stirring for 2-3 hrs until the solution turned into greenish color. Afterwards, the system was degassed for 3 hrs under vacuum. In a typical small-scale synthesis, 2 ml of the reaction mixture was taken in a glass vial and was heated in a single-mode variable-power 700 W domestic microwave oven for 90 s. The actual power selected for all the experiments was 117 W. The mixture was allowed to cool to room temperature naturally. The reaction mixture was diluted with ethanol, the precipitate was collected and exhaustively washed with ethanol and water mixture (70:30 v/v %). To investigate the growth mechanism, small amount of the reaction mixture was transferred to a cold vial kept in an ice bath to quench the reaction at different time intervals of 18 s, 45 s, 60 s and 90 s.

4.2. Synthesis of MoTe2 and WTe2 thin films directly over conducting substrate. To synthesize (W)MoTe2 thin films, the precursor electrolyte solution containing metal precursors (WCl4 or MoCl5) and the Te precursor was sandwiched between a p++ Si wafer (as bottom substrate) and a glass substrate, followed by microwave irradiation for 30-45 s (depending on the substrate size), at 117 W power. After microwave irradiation, the reaction mixture was allowed to cool to room temperature naturally and the wafer was washed with ethanol-water mixture (70:30 v/v %) for several times and finally with acetone, before drying in vacuum desiccator.

4.3. Thickness-controlled synthesis and transfer of the thin films. To control the number of Te−Mo−Te triple atomic layer over the whole substrate, we adjusted the concentrations of the precursors in the liquid electrolyte solvent. The relative concentration of the metal precursor and Te precursor was fixed at 25:55 mM, and the stock solution was diluted further with the liquid electrolyte solvent. All the other experimental conditions were same, such as substrate size, volume of the precursors solution, microwave exposure time, microwave power, and position of the sample on the microwave glass turntable plate. To transfer the (W)MoTe2 thin films we first spin-coated PMMA on top of the sample. After baking over hot plate at 150 oC for 15 min, the PMMA-coated sample was completely immersed into 30 % KOH solution to etch the SiO2 until the PMMA with (W)MoTe2 film floated on the surface. Afterward, a new substrate (based on requirements), was used to fish out the PMMA supported (W)MoTe2 thin film. Finally, after several washing with DI water (to remove KOH), the PMMA film was dissolved by DCM.45

4.4. Micro patterning of MoTe2 thin film. PCL line pattern was generated by capillary force lithography from spin-coated PCL thin films on the O2 plasma (200W, 90 s) treated Si substrate (1.5 cm2). For the line pattern, dimension of the PDMS stamp used was, line/space = 1.7 μm/1.7 μm. Before spreading the precursor solution, PCL pattern was treated with O2 plasma (200 W, 90 seconds) in order to make the surface hydrophilic. MoTe2 precursor solution (1-2 µL) (Mo: Te = 10: 22 mM) was dropped on the pattern surface. The solution was evenly spread on the entire polymer pattern by placing a cover glass. The sample was put in ambient condition (at 25 oC) for 30 min. After carefully removing the cover glass, to remove the PCL pattern, sample was washed three times in toluene by dipping vertically into toluene solution. Then the substrate was annealed at 180 oC for 30 min over a hot plate, followed by cooling to room temperature. Afterwards another Si wafer of same size was placed downward over the resulting precursor line pattern and the sandwiched precursor line pattern was then thermally heated under microwave irradiation for 15-30 s at 117 W power. Finally, the patterned thin film was washed with ethanol and DI water alternatively for several times.



4.5. Characterization of the bulk powder samples and thin films. The samples were analyzed using optical microscope (Olympus BX-51), X-ray diffraction (XRD, RIGAKU D/MAX-2500/PC) with Cu Kα radiation (λ = 0.1542 nm), field emission scanning electron microscope (FE-SEM, S-4200, Hitachi), Scanning transmission electron microscopy (STEM, JEM2100F and JEM-2011HC, JEOL) at an accelerating voltage of 200 kV, and energy dispersive X-ray spectrometry (EDS, INCA Xsight 7421, Oxford Instruments). X-ray photoelectron spectroscopy (XPS) measurements were carried out in K-alpha (Thermo VG, U.K.) equipped with monochromated Al X-ray sources (Al Kα line: 1486.6 eV run at 12 kV and 3 mA, fixed analyzer transmission). The topologies of the thin film were examined by atomic force microscopy (AFM, Dimension 3100, Digital Instruments Co.). Raman spectra were obtained by a Micro-Raman Spectrometer (Witec Alpha 300 RA Confocal Raman), with the wavelength and spot size of the laser excitation of 532 nm and 1 µm, respectively. A Focused Ion Beam/EBSD (Helius, Pegasus, FEI) was used to prepare cross-sectional TEM specimens. Nuclear magnetic resonance spectra (1H, 13C NMR) were recorded on Bruker DRX 600 MHz NMR Spectrometer. 19F and 11B NMR spectra were measured with a 500 MHz Bruker Avance DRX Spectrometer.

4.6. Device fabrication. To perform the temperature dependent electrical measurements, MoTe2 and WTe2 thin films were transferred on a 300 nm SiO2/ Si substrate followed by metal deposition (50 nm Au) to define a 75 µm channel lengths. MoTe2 photodetector devices were fabricated using the synthesized 3layer semiconducting MoTe2 thin films. MoTe2 thin films were transferred to 20 nm thick ALD deposited HfO2 on a p++ doped Si wafer. To define the device pattern over the thin film, standard photolithography technique was used and 50 nm thick Au electrode pads were then deposited over the thin film.46 Finally, the device was thermally annealed at 200 oC for 30 min, to reduce the contact resistance. 4.7. Low-temperature electrical measurements. Lowtemperature I-V measurements were conducted by supporting the devices using a standard sample holder in a liquid N2 cryostat (JANIS VPF-100). The temperature (temperature ranges from 78 to 390 K) of the devices were controlled by connecting the cryostat to a Lake Shore 335 temperature controller unit, and dark I-V characteristics were measured using a Keithley 2636 source meter.

4.8. Photocurrent measurements. The current-voltage (I-V) and the current-time (I-t) characteristic of the photodetectors were measured by semiconductor parameter analyzer (Agilent 4156A) and Keithley 2400 source meter controlled by a LabView program, using either a 633 nm He-Ne laser, 860 nm broadband super luminescent light emitting diode (SLED, Superlum) and 1064 nm pulsed laser source (Teem photonics). A laser power meter (PM100D, THORLABS) was used to measure the exact power of the incident laser illumination. The incident photonto-current conversion efficiency (EQE) was measured using a photo-modulation spectroscopy setup (Merlin, Oriel IQE-200) with monochromatic light from a xenon lamp. The power density of the monochromatic light was calibrated using a Si photodiode certified by the National Institute for Standards and Technology.

4.9. Study of defect structure dependent optical property. Density-functional theory (DFT) calculations are done with the Vienna Ab initio Simulation Package (VASP), under the formalism of projector-augmented wave (PAW) approach.47-49 For all theoretical calculations, we adopted 500 eV of planewave kinetic energy cutoff, and the exchange-correlation functional due to Perdew, Burke, and Ernzerhof (PBE) is used.50 For the

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Brillouin zone integration, Γ-centered k-grid with the reciprocal sampling distance of 0.15 Å-1 is used. Optical response calculation of 2H-MoTe2 and derived defect structures is conducted under the independent-particle approximation (IPA) within the random-phase approximation (RPA),51 where the excitonic and local-field effects are neglected. Energy loss spectra (ELS) is obtained by the following relation, ELS = ε2/ε21+ε22, where ε1 and ε2 denotes the real and imaginary part of dielectric function, respectively.

■ ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. 1

Additional data supporting this publication including H and 13 C-NMR spectra of the Te precursor, XRD, FESEM and EDX mapping of the time dependent samples, FESEM images of the micro patterned thin film, calculated absorbance spectra of pristine and small grain MoTe2, and EQE of the near-IR photodetector device.

■ AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected].

Author Contributions ‡

A. Giri and H. Yang contributed equally to this work.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT We are grateful for the financial support from the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIP) under Project No. NRF2015R1A2A1A10054164 and from the Center for Advanced Soft Electronics funded by the Ministry of Education, Science and Technology as a “Global Frontier Project” (CASE2015M3A6A5072945). Computational resources are supported by the Korea Institute of Science and Technology Information (KISTI) supercomputing center (KSC-2017-C3-0007).

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