Transfer-Free Growth of Atomically Thin Transition Metal Disulfides

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Transfer-free Growth of Atomically Thin Transition Metal Disulfides using a Solution Precursor by a Laser Irradiation Process and their Application in Low-power Photodetectors Chi-Chih Huang, Henry Medina, Yu-Ze Chen, Teng-Yu Su, Jian-Guang Li, Chia-Wei Chen, Yu-Ting Yen, Zhiming Wang, and Yu-Lun Chueh Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00033 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 25, 2016

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Transfer-free Growth of Atomically Thin Transition Metal Disulfides using a Solution Precursor by a Laser Irradiation Process and their Application in Low-power Photodetectors Chi-Chih Huang1†, Henry Medina1†, Yu-Ze Chen1, Teng-Yu Su1, Jian-Guang Li1, Chia-Wei Chen1, YuTing Yen1, Zhiming M. Wang2 and Yu-Lun Chueh1* 1

Department of Materials Science and Engineering, National Tsing Hua University, No. 101, Section

2, Kuang-Fu Road, Hsinchu, Taiwan. 2

Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of

China, People’s Republic of China. *E-mail: [email protected] †

These authors contributed equally to this work.

Abstract: Although chemical vapor deposition is the most common method to synthesize transition metal dichalcogenides (TMDs), several obstacles, such as the high annealing temperature restricting the substrates used in the process and the required transfer causing the formation of wrinkles and defects, must be resolved. Here, we present a novel method to grow patternable two-dimensional (2D) transition metal disulfides (MS2) directly underneath a protective coating layer by spin-coating a liquid chalcogen precursor onto the transition metal oxide layer, followed by a laser irradiation annealing process. Two metal sulfides, molybdenum disulfide (MoS2) and tungsten disulfide (WS2), are investigated in this work. Material characterization reveals the diffusion of sulfur into the oxide layer prior to the formation of the MS2. By controlling the sulfur diffusion, we are able to synthesize continuous MS2 layers beneath the top oxide layer, creating a protective coating layer for the newly formed TMD. Air-stable and low-power photo-sensing devices fabricated on the synthesized 2D WS2 without the need for a further transfer process demonstrate the potential applicability of TMDs generated via a laser irradiation process.

Keywords: Laser irradiation, Transition metal dichalcogenides, patternable, photodetector

1 ACS Paragon Plus Environment

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Recently, two-dimensional (2D) materials have attracted substantial attention because of their outstanding physical properties compared to their bulk counterparts.1 Graphene is the most wellknown 2D material. Its handling and synthesis have greatly evolved in recent years; nevertheless, despite its large mobility, graphene lacks a bandgap, limiting its development for several applications.2, 3

Similar to graphite, transition metal dichalcogenide (TMD) materials possess a lamellar structure. A

TMD monolayer has a structure composed of a transition metal atom sandwiched between two chalcogen atoms, but it should be clarified that a TMD monolayer is, in fact, a 3-atom-thick structure.4-6 Because of their strongly in-plane- and weakly out-of-plane bonding through van der Waals forces, TMDs can be exfoliated into a few TMD monolayers and even into single TMD monolayers.5,

7, 8

The physical properties of bulk TMDs have been studied since 1960.4,

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The

availability of cutting-edge technologies for nanoscale material growth and their special material properties upon quantum confinement have opened up novel applications for 2D TMDs in nanoelectronics. TMDs based on molybdenum and tungsten (MoX2 and WX2, X=S, Se, Te) possess remarkable electronic and optical properties because of their intrinsic bandgap, which shows a gradual transition from indirect to direct and begins to display photoluminescence when the thickness is less than 4 TMD monolayers, reaching a direct bandgap at the monolayer.10-12 In particular, MoS2 and WS2 (MS2) display bandgap energies of 1.9 and 2.1 eV, respectively, at the monolayer.12 Interest in these 2D materials is rapidly increasing because of their performance in low-power electronics and their particular direct bandgap feature in the visible light range, which has great potential applications in optoelectronic devices. Although the synthesis of 2D TMDs is evolving rapidly based on our previously acquired knowledge in graphene, several drawbacks remain to be addressed. For example, while mechanical exfoliation has proven to be a powerful tool to create TMD monolayers for material characterization, it is not suitable for mass production.5 Chemical exfoliation has also been demonstrated,7, 8, 13-16 but the poor quality of the resulting TMDs limits its usage for device fabrication in electronic and optical applications. Recently, it has been reported that chemical vapor deposition (CVD) methods are capable of growing atomically thin films of TMDs with large areas and decent quality, but this process requires high temperatures to achieve the sulfurization/selenization of the various metal precursors.13-16


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Because few substrates can sustain the temperatures required for CVD, a transfer process of the TMDs to an additional substrate is usually required and can easily lead to the formation of wrinkles on the transferred TMD film.17 Recently, Park and coworkers18 developed a CVD process to achieve the synthesis of MoS2 and WS2 monolayers at a temperature of 550 °C. However, the precise control of these metal organic precursors, complex equipment design and long deposition times exceeding 24 hr represent limitations to this technique. In addition, to achieve satisfactory electrical measurements, the devices must be measured in vacuum or following an additional encapsulation process. Previously, we demonstrated the synthesis of graphene and a few WSe2 monolayers directly onto insulators.19, 20 Both methods made use of laser irradiation and had good versatility to achieve 2D materials with controllable thicknesses. Building on this experience, here, we developed a novel approach for the formation of TMDs by spin-coating a liquid precursor onto a metal oxide layer, followed by a laser irradiation annealing process. Two TMDs consisting of a few WS2 and MoS2 monolayers, as confirmed by Raman and X-ray absorption spectrometry and transmission electron microscopy (TEM), were observed by irradiation with a laser pulse with controlled power. Interestingly, the formation of the TMD monolayers occurred beneath the metal oxide layer instead of on its top; namely, the metal oxide capped the TMD configuration, thereby avoiding the instability and degradation of the electrical measurement caused by oxygen and water absorption, which causes the need for further annealing to achieve water desorption and measurement in vacuum.18, 21 The TMDs’ detailed formation mechanism and the growth parameters, such as the laser power duration, were proposed and investigated. The patternability of the TMDs created using our approach was also demonstrated. We fabricated air-stable and low-power photodetectors based on our capped WS2 without the necessity of an additional transfer step, which represents a further step in the development of TMDs for practical applications. Figure 1a shows a schematic of the steps used to grow MS2, where the W and Mo metal layers are represented by the symbol “M”. Figure 1a1 displays the basic layer structures deposited by e-beam evaporation. First, the nickel layer was evaporated onto the bottom of the structure as the laser absorption layer. Then, a thin titanium dioxide (TiO2) layer was deposited as a barrier layer, and a transition metal oxide (MOx M=W, Mo) thin film was applied on top of the layered structure. This


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transition metal oxide layer has dual functions as both the metal source for the synthesis of the MS2 and the capping layer. Then, sulfur in oleylamine was spin-coated (Figure 1a2) on top of the MOx layer. Finally, the structure was irradiated by a λ=808-nm laser (Figure 1a3) at room temperature. A low level of vacuum was needed to pump out the gases produced by the evaporated oleylamine (bp. ~365 °C) because of local heating caused by the laser irradiation. Optical microscopy (OM) images of tungsten oxide (Figure 1b) and molybdenum oxide (Figure 1c) reveal a color difference in the laserirradiated areas, suggesting that a chemical reaction occurred. Raman spectroscopy using a 514-nm excitation laser was preferred because of its versatility in identifying TMD materials.22-24 Normally, the Raman spectra of MoS2 and WS2 display two characteristic features, including the E12g peak, related to the in-plane vibrational mode and the A1g peak associated with the out-of-plane vibration of sulfur atoms.25 Figures 1d and 1e show the Raman spectra at the surface of the tungsten oxide taken from outside and inside the irradiated areas, respectively. Figures 1f and 1g similarly display the spectra of molybdenum oxide taken from outside and inside the dark irradiated areas. Remarkably, the Raman spectra inside the laser-irradiated area present the two characteristic E12g and A1g phonon modes of WS222 (Figure 1c) and MoS223 (Figure 1g), and no peaks could be detected for the WO3 and MoO3 films before the laser irradiation process in the same range because of the poor crystallinity of the film deposited by electron beam evaporation (Figures 1d and 1f). In the Raman spectra, the positions and distances between the E12g and the A1g peaks can be further used to roughly determine the number of monolayers of both WS222 and MoS2.24 Therefore, the Raman spectrum of the laser-annealed area shown in Figure 1e presents the E12g and A1g peaks located at 355.7 cm-1 and 418.6 cm-1, which correspond to bi- and tri-layer WS2, respectively.22 The E12g peak at 384 cm-1 and the A1g peak at 405.7 cm-1 in Figure 1g similarly coincide with the positions expected for a few MoS2 monolayers.26 Based on Beer-Lambert law calculations and light reflections, we previously demonstrated the large power absorption of nickel compared to other metals, such as molybdenum and copper, at the wavelength of 808 nm.19 Here, we analyzed the function of TiO2 as a barrier to prevent the internal diffusion of sulfur during the laser irradiation process. First, we prepared samples without the barrier layer. Figure S1a shows the OM image of the top view of the sample without the TiO2 layer as a barrier layer after laser irradiation. Clearly, the result is completely different from that shown in Figure


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1b. The concentric circles surrounding the exposed area containing the small particles suggest that a large was damaged by the laser irradiation. X-ray photoelectron spectroscopy (XPS) was used to further analyze the sample at the surface of and inside the nickel layer. To identify the Ni layer, an in situ Ar sputterer was applied to the center of the irradiated area until the Ni signal became strong. Figures S1b and S1c show the corresponding Ni 2p and S 2p spectra of both areas (surface and Ni layer). Sulfur (S) and nickel (Ni) peaks can be clearly observed in both areas, indicating the diffusion of S atoms into the Ni layer after the laser irradiation process. Ni also diffused into the WO3 oxide layer. Because of their chemical stability, nitrides27 are normally used as a barrier layer. We preferred to use TiO2 as the barrier layer over other materials because of its photocatalytic properties when combined with TMDs28 and its low sulfur diffusion, even at relatively high temperatures.29, 30 Figure 2a shows the OM image of a sample with TiO2 as a barrier layer between the Ni and WOx layers. The laser spot can be clearly identified after irradiation, but no concentric rings are observed. Note that the duration and power of the laser were exactly the same as those in Figure S1. Figures 2b-2d show the XPS spectra at the center of the irradiated area and at three different depths (surface, TiO2 and Ni layers). The Ni 3p, Ti 2p and S 2p binding spectra were used as references to identify the presence of Ni, Ti and S at three different depths. At the surface, only the sulfur signal at 163.9 eV corresponding to neutral sulfur31 was obtained because of the presence of residual sulfur (Figure 2b). Then, Ar sputtering was applied to remove the top WOx/WS2 layer until the Ti signal was identified (Figure 2c). We identified this region as the interface between the WOx/WS2 and TiO2 layers. Figure 2c shows a clear sulfur signal, indicating that sulfur indeed penetrated the WOx layer. It should be noted that the peak becomes broad and its position shifts slightly to a lower binding energy because of the formation of WS2.

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By further sputtering until the Ni and Ti peaks were clearly identified (corresponding to

the interface between TiO2 and Ni), the sulfur signal completely disappeared, demonstrating the excellent performance of TiO2 as a barrier layer (Figure 2d) Figure 3a shows a low-magnification TEM image of the laser annealing region, in which thin film stacking can be clearly observed. The darker region at the bottom corresponds to the Ni layer, which is under a lighter layer with a thickness of ~10 nm that is attributed to TiO2; on the top, a darker region exists that is assigned to the WOx layer. According to Figure 3(b), the internal spacing was 0.63 nm,


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which corresponds to a WS2 bilayer structure.34, 35 and is consistent with the Raman spectra. Note that the two perpendicular lattice distances (0.38 nm and 0.32 nm) in the upper region indicated in Figure 3(b) correspond to the (002) and (110) planes of the monoclinic WO3 phase, respectively.36 In addition, the lattice distance of 0.35 nm found in the bottom region is in agreement with the (101) TiO2 lattice plane.37 As a result, we can conclude that the TMD was indeed located at the interface between WOx and TiO2. In addition to the TEM lattice distance, we confirmed that the TMD bilayer was WS2 based on Raman spectra. Figure S2 shows the Raman shift of the WOx after laser irradiation, which displays only the characteristic peaks of WS2.22 We discarded the possibility of TiS2 formation because the corresponding Raman peaks would have been located at 227 cm-1 and 337 cm-1 for the E12g and A1g bands, respectively.38-40 Interestingly, the TEM results confirm that the bilayer WS2 was indeed formed at the interface between the two oxide layers, in which WOx acts as a cap and supports the good sulfur-blocking behavior of TiO2. Figures 3c and 3d present the XPS spectra of tungsten and sulfur at the interface between the two oxide layers. As expected from the TEM and Raman results, the W 4f distribution (Figure 3c) shows a mixture of tungsten oxides and WS2. For easy identification, the peaks in the W 4f spectra shown in Figure 3c are labelled from W1 to W6. The peaks at 35.5 eV (W4) and 37.1 eV (W5) correspond to the W 4f7/2 and W 4f5/2 binding energies of WO3, respectively.41, 42

The peak at 34.6 eV (W3) is attributable to WO2,43 whereas the three peaks at 31.7 eV (W1), 33.9

eV (W2) and 38.0 eV (W6) correspond to the W 4f7/2, W 4f5/2 and W 5p3/2 binding energies of WS2, respectively.32, 33 The S 2p core-level spectra shown in Figure 3d exhibit two peaks at 162.2 eV (S 2p3/2) and 163.2 eV (S 2p1/2), which are related to WS2.32, 33 A clear photo-luminescence (PL) signal appears at approximately 1.99 eV, as displayed in Figure 3e, confirming the formation of bilayer WS2.44 Finally, based on Raman and XPS spectra, TEM and PL results, we can verify the synthesis of bilayer WS2 underneath the WOx capping layer by our laser irradiation method. Figure 4a shows a schematic diagram of the proposed formation mechanism. We suggest that several steps occurred over only a few minutes during the laser irradiation. Figure 4a1 shows the schematic of the layered structure after sulfur spin-coating but before laser irradiation. When the laser was turned on, the Ni layer was locally heated because of the large absorption at the specific wavelength of the laser used for our irradiation process (λ= 808 nm).19 Here, oleylamine was used as a


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solvent to dissolve the sulfur and as a catalyst because of its high reactivity with metal ions, which leads to metastable compounds that can function as secondary precursors and thus be easily decomposed and diffused into the film.45-47 After a slight increase in temperature during the laser irradiation process, the sulfur rapidly diffused into the tungsten oxide because of the concentration difference, whereas oleylamine, whose boiling point is approximately ~365 °C, was easily evaporated (Figure 4a2). The sulfur diffusion was stopped at the TiO2 layer because of its excellent sulfurblocking ability (Figure 4a3). Then, when the temperature became sufficiently high, the WOx layer reacted with sulfur, finally transforming into bilayer WS2. The thermal gradient inside the top oxide layer was responsible for the preferential formation of WS2 underneath the WOx. Because of the low thermal conductivity of tungsten oxide,48 a higher temperature was expected at the interface between the two oxides than at the sample surface (Figure 4a4). To demonstrate that sulfur diffusion occurs before the formation of the WS2, a sample was prepared under low power irradiation. Figure S3 presents the W 4f and S 2p binding energies at the sample surface and deeper in the film at the interface between the WOx and TiO2 layers after the laser irradiation process with low power and a short irradiation time. Clear sulfur peaks were found at both depths and had similar intensities, implying a uniform distribution along the film, whereas the W4f binding energy spectra showed only the distinctive peaks associated with WO3. Furthermore, no Raman peak was observed in the range expected for the formation of WS2. These results confirm the fast sulfur diffusion prior to the formation of WS2. In addition, the synthesis conditions were optimized by controlling the power and irradiation time. Figure 4b presents the results obtained using various laser powers and exposure times. The black circles represent the failed cases under high power with a long irradiation time. Note that the red stars indicate the successful synthesis of WS2, whereas the orange triangles indicate the conditions under which no change was observed after the laser irradiation process. All cases were verified by OM and Raman results. The OM images in Figures 4c, 4d and 4e correspond to damaged (black circle), reacted (red star) and unreacted (orange triangle) areas, respectively. Under extremely high power, all the layers in the center of the irradiated area were completely removed by the laser, leaving only the glass substrate (Figure 4c). Note that samples prepared under these conditions still displayed Raman peaks at 350 cm-1 (E12g) and 420 cm-1 (A1g), corresponding to bulk WS223 (Figure S4).


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Further TEM analysis (Figure S5) revealed that both oxide layers disappeared and that intermittent WS2 sheets remained on top of the Ni film. We believe that the missing oxide layer may have been caused by the high temperature experienced by the Ni layer subjected to high laser power, whereas the WS2 remains because of its high temperature stability. Using a moderate power and irradiation time, the characteristic Raman signal of WS2 was detected in the color-changed area (Figure 4d). Using a low power and short irradiation time, no WS2 signal was observed, and the corresponding OM image displayed in Figure 4e shows no difference before and after the laser irradiation process. Finally, WS2 was successfully synthesized at powers between 3 and 7 watts with irradiation times of less than 10 minutes using this procedure. Thus, this technique is an ultrafast process requiring no additional annealing or cooling time. For device fabrication, MS2 should be patterned. Using our laser annealing process, MS2 can be defined by controlling the laser spot movement, although the scalability will be restricted to the resolution of the laser beam size. Previous works proposed an approach to define graphene by patterning nickel as the metal catalyst, regardless of the resolution of the laser beam size.19, 49 In this case, the metal oxide layer was patterned as the precursor of the MS2 material. Figure 5a shows the schematic of the patterned sample structure. Photolithography was used to define the WOx array after nickel and TiO2 deposition (Figure 5b). Raman mapping of the patterned WS2 after laser irradiation revealed a clear letter with sharp edges (Figure 5c). The Raman spectra obtained inside and outside of the letter region are presented in Figure 5d. In this technique, the resolution is restricted by the resolution of the lithography method used to define the array rather than the laser beam size. Thus, we believe that using our synthesis method, nanoscale devices may be achievable. To probe the device-fabrication performance of the laser irradiation process, we further attempted to fabricate photodetectors from our synthesized bilayer WS2. As mentioned previously, we selected this layer to take advantage of not only its ability to aid in the synthesis of the MS2 but also its excellent optical properties, which could be exploited for photoelectric devices. Figure 6a shows a schematic of the cross-section of the proposed device, in which the nickel layer acts as the back electrode, WS2 is the absorber layer sandwiched by two high work-function oxides (WO3 and TiO2) acting as coupling layers to enhance the photocatalytic behavior, and silver was deposited as a top


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electrode. Previous works have reported enhanced visible light absorption on quantum-sized MS2 (M=Mo or W) nanoclusters attached to TiO2 particles. The photocatalytic activity results from the electron transfer from MS2 to TiO2 and the interlayer coupling increasing the charge carrier separation.28 Similarly, Huo et al. proposed that WS2 nanosheets functionalized with WO3 particles could significantly improve the photocatalytic activity in the visible range.50 The uniqueness of the one-step formation of WOx/WS2/TiO2 layers after our laser irradiation process should be noted. The corresponding photocurrent-time (I-t) curve was measured under red (633 nm) laser irradiation with an applied bias of 1 V, as shown in Figure 6b. Interestingly, the noise was at the picoampere level, resulting in an estimated signal-to-noise ratio (S/N) of 100. To estimate the photodetector’s detection limit, we further reduced the applied bias to 0.1 V. The corresponding photocurrent-time (I-t) curves were measured under green (532 nm) and red (633 nm) laser irradiation with an applied bias of 0.1 V, as shown in Figures 6c and 6d. Clear S/N values exceeding 10 were observed, demonstrating the potential of these devices for low-power electronics. Figure S6 shows the effect of different biases on the S/N. The observed switching behavior was attributed to the increased current under a constant bias when lights with different wavelengths were turned on. Current-voltage (I-V) curves in the dark, under green light (532 nm) and under red light (633 nm) are shown in Figure 6e. The clear current increase under light illumination resulted from the photocatalytic activity of the absorber layer and indicates good sensitivity to both green and red light. Note that the power densities for the red (18 mW/mm2) and green lasers (10 mW/mm2) were different. To make a fair comparison, Figure 6f displays the responsivity of the device under both wavelengths, calculated as the variation of the photocurrent (∆Iph) divided by the incident power density given by Rph = ∆Iph/Pin. We suggest that the higher responsivity under a negative bias might result from the enhanced electron injection using silver as a lower work-function metal relative to nickel. The Vbias was defined as the applied voltage between the top (Ag) and bottom (Ni) electrodes, using the potential at the bottom electrode as a reference (VRef) (Figure 6a). Figure S7 displays the band alignment for the WS2-based photodetector. For WS2, the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energies have been reported to be 3.6 and 5.5 eV, respectively.51 Silver possesses a lower work function (~4.3 eV)52 than nickel (~5.3 eV)53, which explains its better electron injection under a


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negative Vbias. Lower work-function metals, such as Mg, may also induce better electron injection, but their electrical instability under atmospheric conditions will degrade the device performance. In contrast, Ag offers a higher oxidation resistance, with stable performance over time. As proof, the airstability of the photodetector was tested by measuring the responsivity for up to 80 days, where the responsivity current was normalized based on the photoresponse obtained at the first time measurement, as shown in Figure S8. Clearly, no major changes in the responsivity occurred, confirming the air-stability of the WOx/WS2/TiO2 layer-based device synthesized by our novel laser irradiation method. Note that red light (633 nm) results in better responsivity than green light (532 nm) because of the high external quantum efficiency (EQE) of our bilayer WS2 at 633 nm, which may be attributed to the phonon energy affinity with the direct bandgap energy of WS2 at the K point.54 In our case, the 633-nm incident phonon energy (1.95 eV) almost matched the direct bandgap energy of our bilayer WS2, which is located at 1.99 eV, as confirmed by PL spectra and shown in Figure 3e. An additional device without laser irradiation was also examined and showed no photoresponse at 633 nm or 532 nm, verifying that the photoresponse was indeed caused by the WS2 absorption layer (Figure S9). In addition to the red and green lasers, a blue laser with a wavelength of 410 nm was also applied to these samples (Figure S10). A strong photoresponse at this wavelength can be clearly observed, which was most likely caused by several factors, such as the presence of the WOx layer, TiO2 layer and/or metal semiconductor interface. Several studies based on WO3 thin films have confirmed an optical bandgap between 2.6 eV and 3.1 eV,

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which matches the photon energy at 410 nm.

Nevertheless, after exposure to the blue laser, the dark current (Idark) increased, but the photocurrent (Iph) decreased, implying that some damage of the photodetector occurred because of the high power density of the blue laser (40 mW/mm2) compared to those of the green and red lasers, leading to overheating. To compare the response times between the different excitation lights, the dynamic variation in the photocurrent after illumination using different wavelengths (Figure S11) was fitted following a first-order system as57 ∆Iph(t) = Idark + A e(t/τ) where A is a constant value, and τ is the time constant for the photodetector’s response at different excitation wavelengths. Rise and fall times between 10 and 90 %58 were also measured and are


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displayed in Table S1. The results clearly show that the shortest response time was achieved at 633 nm. We suggest that the enhanced response time was caused by the higher mobility of WS2 acting as absorber layer at 633 nm compared to the low mobility of WO3 as the absorber layer at 410 nm. All measurements were performed at room temperature and in air without vacuum. We should also note that because of the vertical structure, the device can be operated at a very low bias (0.1 V) compared to the normal operating voltage applied in other works using WS258, 59, demonstrating this device’s potential for low-power applications. In summary, a novel and facile method to synthesize a few monolayers of MS2 (M=W and Mo) within a few minutes by combining the spin coating of liquid precursors on a metal oxide layer and a laser irradiation annealing process is presented here. MO3 (M=W and Mo) was both a precursor for the formation of MS2 but also acted as a capping layer to protect against contact with air. We also used TiO2 as a barrier layer to prevent internal sulfur diffusion into Ni or Ni into the MO3. Raman and XPS analyses supported the presence of WS2, and TEM was used to confirm the number of monolayers (bilayer MS2) and the successful synthesis under the oxide protective layer. XPS results collected under different irradiation powers provided evidence for the diffusion of sulfur into the oxide layer until reaching the barrier layer prior to the formation of the MS2. This rapid sulfur diffusion at moderate temperatures explains the formation mechanism of MS2 under the oxide layer. Precise adjustments of the laser power and irradiation time were found to be fundamental to the process. Using standard photolithography methods to define the channel, followed by the deposition of silver electrodes, resulted in the successful synthesis of a transfer-free photo sensor capable of being operated at very low power with good sensitivity and the ability to be measured in air. This laser irradiation process has several advantages: It is inexpensive, does not require an additional transfer step, facilitates rapid synthesis and direct patterning, and generates a material that is stable in air. Our approach can also be extended to other TMD materials.

Supporting Information Methods; XPS of the structure without barrier layer. Raman spectra of WS2 after laser irradiation. XPS of sulfur diffusion into the WO3 layer. Cross-sectional TEM view and Raman after high power


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irradiation. Band alignment of the WS2 based photodetector. Photoresponse of WS2 photodetector at 410 nm illumination. Time constant estimation and time response at different wavelengths. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements The research is supported by the Ministry of Science and Technology through Grant through grants no 104-2628-M-007-004-MY3, 104-2221-E-007-048-MY3, 104-2633-M-007-001, 104-2622-M-007002-CC2, and the National Tsing Hua University through Grant no. 104N2022E1. Y.L. Chueh greatly appreciates the use of facility at CNMM, National Tsing Hua University through Grant No. 104N2744E1.

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Figure Captions


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Figure 1 (a) Schematic illustration of the experimental steps. Optical images of laser-irradiated WO3 and MoO3 samples are shown in (b) and (c), respectively. (d) and (e) show Raman spectra of the bright and dark areas shown in (b) corresponding to WO3 before and after laser irradiation. Similarly, (f) and (g) present Raman spectra of the pristine and irradiated MoO3 areas shown in (c).

Figure 2 (a) Optical microscope image of the laser-irradiated-sample with a TiO2 barrier layer. Ni 2p, Ti 2p and S 2p XPS signals at (b) WO3, (c) TiO2 and (d) Ni layers.

Figure 3 (a) Low- and (b) high-magnification cross-sectional TEM images of the irradiated sample. (c) W 4f and (d) S 2p XPS spectra at the interface between the WO3 and TiO2 layers. (e) Photoluminescence signal of WS2 synthesized by laser irradiation

Figure 4 (a) Schematic representation of the mechanism of WS2 formation. (b) Various samples’ results obtained under different laser powers and with different exposure durations. OM images of (c) sample damage resulting from excessive duration or power, (d) a sample obtained using the optimized conditions for WS2 synthesis and (e) an irradiated but unreacted sample (short duration/low power).

Figure 5 (a) Schematic diagram of patterned WS2. (b) OM image of WS2 patterned by photolithography and the corresponding Raman mapping image of the E2g band. (c) and (d) Raman image and spectra inside and outside of the N letter.

Figure 6 (a) Cross-sectional schematic diagram of the sandwiched device. (b) Current-time curve under red illumination with an applied bias of 1 V. (c) and (d) Current-time curves measured under green and red light illuminations with an applied bias of 0.1 V. (e) I-V curves measured with and without light illumination. (f) Responsivity curves under green and red lights.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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TiO2

300

Step 1

400

450

300

300

400

400

450

Raman shift (cm )

A1g

350

350

-1

-1

S in oil Step 2

350

Raman shift (cm )

WS2 E2g

Intensity (a.u.)

Ni

Intensity (a.u.)

WO3 or MoO3

Intensity (a.u.)

TOC

Intensity (a.u.)

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450 -1

A1g

MoS2

350

E2g

375

400

425

450

-1

Raman shift (cm )

Raman shift (cm )

Step 3 200 µm


200 µm

Transfer-Free Growth of Atomically Thin Transition Metal Disulfides

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