Low-Temperature and Ultrafast Synthesis of Patternable Few-Layer

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Low Temperature and Ultrafast Synthesis of Patternable FewLayers Transition Metal Dichacogenides with Controllable Stacking Alignment by Microwave-Assisted Selenization Process Yu-Ze Chen, Henry Medina, Sheng-Wen Wang, Teng-Yu Su, Jian-Guang Li, WenChun Yen, Kai-Yuan Cheng, Hao-Chung Kuo, Guozhen Shen, and Yu-Lun Chueh Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04579 • Publication Date (Web): 06 Jan 2016 Downloaded from http://pubs.acs.org on January 8, 2016

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Chemistry of Materials

Low Temperature and Ultrafast Synthesis of Patternable Few-Layers Transition Metal Dichacogenides with Controllable Stacking Alignment by Microwave-Assisted Selenization Process Yu-Ze Chen1,5, Henry Medina1, Sheng-Wen Wang2, Teng-Yu Su1, Jian-Guang Li1, Wen-Chun Yen1, Kai-Yuan Cheng3, Hao-Chung Kuo2, Guozhen Shen4 and Yu-Lun Chueh1* 1

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu

30013, Taiwan. 2

Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung

University, Hsinchu 30010, Taiwan 3

Institute of Electronic Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan.

4

State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors,

Chinese Academy of Sciences, Beijing 100083, China 5

Department of Chemistry and Biochemistry, University of California, Los Angeles,

California 90095, United States *E-mail: [email protected]

Abstract- To date, chemical vapor deposition (CVD) process is the most popular approach due to its high yield and quality. Nevertheless, the need of high temperature and the relative long process time within each cycle hinders for commercial development in terms of production cost. In this work, we demonstrate a fast (< 3 min) and feasible approach to synthesize few-layers of WSe2 and MoSe2 on arbitrary substrates by a microwave-assisted selenization process. The TMDs can be patterned by standard photolithography. Furthermore, controllable layered growth from horizontal to vertical alignment can be achieved, leading to an enhanced chemical catalytic activity due to large edge sites (exposed areas). As a proof, a highly sensitive NO gas sensor based on vertical WSe2 was fabricated. Moreover, our microwave-assisted selenization process can be further extended to achieve other 2D-TMD materials due to the simplicity of the process.

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Introduction A decade ago, scientists successfully extracted out single carbon layer, namely graphene, from graphite by scotch tape.1 Immediately, graphene gained tremendous attention due to the significant performance over its bulk counterpart.1-4 Due to its the inherent zero bandgap impeding its application in electronic devices,5 scientist are devoting themselves to explore other potential two dimensional materials.6 Apart from the single element group, such as graphene, germanene,7-9 phosphorene10,11 and silicene,12,13 there is an increasing interest to discover binary component alloys with one atomic thicknesses. Among all layered structural binary component alloys, transition metal dichalcogenides (TMDs) are the most attractive binary materials.14 Contrary to the gapless band structure of graphene, two dimensional TMDs are recognized as one of the potential successors in the semiconductor field owing to the nature of their bandgap, ranging from 1.5~3 eV.14 Interestingly, scientists had uncovered evidence that the electronic structure of TMDs follows a layer-dependent relation15,16 and exfoliated monolayer MoS2 and WSe2-based transistors have been demonstrated an impressive carrier mobility (200~500 cm2V-1.s-1) with a high on/off ratio.17,18 Great efforts on the synthesis of TMDs had been proposed, including conventional top-down method, such as mechanical exfoliation19, lithium-based intercalation20-22 and plasma-thinning process23. However, long process time, low yield and difficulty of layer control in the large area will be main drawbacks. Contrary, bottom-up synthesis methods of TMDs had been proposed as well, such as the thermolysis of WSe224 and direct deposition of MoS2 via PLD process25 but the high temperature required for thermolysis and the expensive PLD system further hinder its popularization in the industry. Recently, the sulfurization or selenization through CVD processes had successfully achieved large-area of monolayer TMDs with improved quality while high temperatures (900K~1300K) remain to be required with the much longer process time.26-28 In addition, laser annealing, a fast and low cost process, is an alternative synthesis method, which has been demonstrated. Yet, more


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development of this technique is required.29 Recently, TMDs films with largely exposed areas from edge sites have been proved to be catalytically active, showing interesting results in electrochemical applications, such as hydrogen evolution (HER)30-33 and oxygen reduction reaction (ORR). 34 However, due to the high energy of the edges where the dangling bonds inherently exist, the formation of basal plane along (001) plan is more likely preferred. However, the formation of vertical TMDs with largely exposed areas is difficult due to a limited portion of edge sites. To date, only a few works utilizing rapid heating of metals32, 35, 37 or template-assisted approach36 had been reported for wall formation of TMDs to increase exposed areas from edge sites, which is attributed to the curvature of the substrate32 or to the anisotropic diffusion of sulfur atoms.35 In this regard, we propose an extremely fast approach to synthesize atomically thin TMD films with controllable thickness from the direct transformation of WO3 into WSe2 and MoO3 into MoSe2 by the microwave-assisted selenization process. The detailed material characterizations including Raman, photoluminescence and X-ray absorption spectra were measured and discussed. In addition, different growth parameters by the microwave-assisted selenization process, including different power and process time were reported. The patternable few-layers WSe2 and MoSe2 by standard photolithography methods were demonstrated and are able to be extended on arbitrary substrates, exhibiting the capability of heterostructures. Surprisingly, TMDs with controlled layer orientation, from horizontal (planar) to vertical layer structure driven by the microwave treatment highlight the uniqueness of the microwave-assisted selenization process. A highly sensitive NO gas sensor based on vertical WSe2 was demonstrated in comparison to planar few-layers WSe2. Most importantly, we provide a different point view based on our experimental results to build up the formation mechanism of the vertically aligned TMDs. Moreover, our microwave-assisted selenization process can be further extended to achieve other 2D-TMD materials due to the simplicity of the process.



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Experimental Section Sample preparation and Microwave assisted-selenization process. First, 2 nm-thick WO3 was deposited on thermally oxidized 50/300 nm SiO2/Si, sapphire and quartz by electron beam evaporation. Second, WO3/SiO2/Si and Se ingots were placed into 0.5 inch quartz tube and vacuumed to a 10-4 torr. Then, quartz tube was sealed by oxy-propane flame. The microwave system used in present study has a frequency of 2.45 GHz. A 2 in SiC wafer acts as the microwave absorber layer to generate heat. Once the microwave assisted-selenization process was finished, the quartz tube was removed from SiC and rapidly cooled by air. Characterizations. Micro-Raman spectroscopy (HORIBA, LabRAM, HR800) equipped with a 632.8 nm laser was conducted to examine the quality and uniformity of WSe2 and MoSe2 as well as photoluminescence (PL) spectrum. Morphologies, lattice spacing with different thickness of WSe2 were obtained by high resolution transmission electron microscopy (HRTEM, JEOL, JEM-3000F FEGTEM, 300kV). X-ray photoemission spectroscopy equipped with a monochromatic Al Kα X-ray source (XPS, Ulvac-PHI 1600) was operated to obtain the bonding information. XRD spectrum (Rigaku, TTRAXIII, 1.54 Å) was employed to obtain to the crystalline structure. Finally, the electrical measurements were characterized with a Keithley 4200-SCS semiconductor parameter analyzer. Results and Conclusion Figure 1 schematically illustrates how to grow few-layers TMDs by the microwave-assisted selenization process. First, WO3 and MoO3 films were deposited by electron beam evaporation on arbitrary substrates and placed in a quartz tube together with pure Se (Methods in supporting information). Then, the quartz tube was sealed under a vacuum pressure of ~ 10-4 torr without the introduction of any protective gas as shown in Figure 1a. The detailed sample preparation processes were provided in the experimental section. A microwave system with a frequency of 2.45 GHz was used as the energy source while SiC substrate serves as absorber material, interacting with the microwaves as illustrated


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in Figure 1b. When an alternating electromagnetic field was applied to SiC absorber, dipoles are prone to be rotated in order to be aligned with the direction of the electromagnetic field. Thus, high frequency alternating electromagnetic field such as microwave (2.45 GHz) can violently trigger the ration of dipoles in the SiC absorber in a short time. As a result, the temperature of the SiC absorber can rapidly ramp up to further anneal the vacuum-sealed tube with the controllable heating temperature by adjusting the microwave power (Figure 1c). When the heating temperature of tube reaches a certain level, Se ingots would be vaporized, followed by reacting with WO3 or MoO3 films, leading to the formation of WSe2 or MoSe2 via chemical reduction by 2WO3(s) + 4Se(g) → 2WSe2(s) + 3O2(g) or 2MoO3(s) + 4Se(g) → 2MoSe2(s) + 3O2(g) (Figure 1d). The corresponding photograph of the quartz tube containing a sample size with 1 cm x 0.7 cm after the microwave-assisted selenization process is shown in Figure S1. Note that the amount of Se for the microwave-assisted selenization process is a key factor to influence the formation of TMDs. During the cooling process, the residual Se vapors, namely unreacted Se, normally condense at the colder end of the quartz tube (Figure S1). We found that the excess amount of the residual Se vapors would result in higher partial pressure, giving rise to the leakage of Se vapors out of tube and contaminating the surface of TMDs while insufficiently residual Se can result in incomplete selenization of metal oxide films into TMDs. Figure 2a displays the characteristic Raman spectra of WSe2 and MoSe2, and the peak positions were perfectly in accordance with previous works.38 For WSe2, the most notorious peak is located at 260 cm-1, corresponding to the double resonance of the longitudinal acoustic phonon 2LA(M) while another peak can be deconvoluted at 250 cm-1, corresponding to an out-of-plane lattice vibration mode (A1g).29 However, in-plane lattice vibration mode (E12g) is too close to A1g, making the fitting of E12g and A1g being difficult when using the 633 nm-excitation laser.38 For MoSe2, the major peak located at 243 cm-1 was found, which corresponds to A1g mode while a small peak located at 290 cm-1 corresponding to the E12g is



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barely observed due to the strong intensity of the A1g peak.39 Furthermore, the photoluminescence (PL) spectrum as shown in Figure 2b revealed the excited photon energy with 1.6 eV for MoSe2 and 1.65 eV for WSe2, respectively, again confirming the formation of a few-layers TMDs.40 To further understand the bonding information of the few-layers WSe2 and MoSe2, X-ray photoemission spectroscopy (XPS) was conducted, respectively. Figures 2c1-2c2 display the corresponding W 1s and Se 3d of WSe2 synthesized by the microwave-assisted selenization process. After the transition of the WO3 film into few-layers WSe2, W 4f7/2 and W 4f5/2 peaks suffered a chemical shift to 31.8 and 33.8 eV, respectively, which is in agreement with previous works.41 In addition, the Se 3d spectra contain two peaks located at 54 and 55 eV, which are also consistent with Se 3d5/2 and Se 3d3/2 peaks of few-layers WSe2.42 On the other hand, Figures 2c3-c4 reveal the corresponding XPS of MoSe2 transformed from MoO3 after the microwave-assisted selenization process, with which the positions of Mo 3d5/2 and 3d3/2 peaks at 229.2 and 232.2 eV were found, respectively. As for Se 3d peak, two shoulders can be deconvoluted at 54 and 55 eV, corresponding to Se 3d5/2 and Se 3d3/2, respectively. As a result, both Mo 3d and Se 3d spectra are in accordance with previous works.43 Moreover, Raman mapping images as shown in Figure S2 reveal the uniform layered structure of WSe2 as well as MoSe2 whose the 2LA(M) peak in WSe2 and the A1g peak in MoSe2 were selected for image mapping process. Briefly summarizing the above results, our microwave-assisted selenization process proves to be a universal approach to synthesize few-layers 2D TMDs and the process can be extended to other metals and chalcogenide materials. We suggest that any individual TMDs should be tuned independently to achieve the best quality. For that reason, we only choose WSe2 synthesized by the microwave-assisted selenization process for further process optimization in present study. Figure 3 displays the plot of time and power required for the optimized synthesis of the few-layers WSe2 film on a SiO2/Si substrate by the microwave-assisted selenization process. In order to obtain an optimized growth condition,


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Raman spectroscopy was applied to analyze each sample synthesized under a variety set of conditions, including exposure time and microwave irradiation power. Surprisingly, we found that the few-layers WSe2 synthesized by the microwave-assisted selenization process could be achieved less than 3 min while it can be achieved within a narrow range of power as marked as hollow circles in the pink region. As the applied power < 100 W, it is insufficient to trigger the selenization process as marked as hollow triangles in the blue region. As the applied power > 500 W, the excess energy causes the evaporation of the WO3/WSe2 away from the substrate, resulting in WSe2 flakes or in an empty region as marked as crosses in the yellow region. The corresponding temperature at a specific irradiation power was calibrated by a thermocouple inside the chamber as plotted in Figure S3 where the lowest synthesized temperature is ~625 oC, which is quite low compared with the WSe2 growth temperature in other standard synthesis methods (from 835 to 1060 oC).44, 45 Similar to other CVD processes, our microwave-assisted selenization process can be applied to synthesize the few-layers TMDs on arbitrary substrates so that samples can be directly used for device fabrication, being free of the damage caused by hand-worked transfer process. To shed light on this issue, the few-layers WSe2 prepared by the microwave-assisted selenization process were grown on four kinds of substrates, including 300 nm-thick SiO2/Si, 50 nm-thick SiO2/Si, sapphire and quartz, respectively, as shown in Figure 4a whose contrast from each kind of samples can be clearly seen. Apparently, they are fairly uniform, regardless of different kinds of substrates. The corresponding Raman spectra of four kinds of substrate conditions were shown in Figure 4b, and all samples show the general features of the WSe2 peaks while the quality is clearly different among them. It should be noted that the full width at half maximum (FWHM) of the Raman spectra directly corresponds to the convolution of the A1g and 2LA(M) modes. Therefore, the smallest FWHM of ~ 12 cm-1 can be observed for the growth of the WSe2 with the best crystalline quality prepared on the sapphire substrate while the broadest peak (~ 30 cm-1) with much worse crystalline quality occurs in the quartz



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substrate. The results indicate that the different quality of the TMDs film can be significantly affected by the crystallinity and roughness of the substrates. It is why a much better growth condition of the TMDs can be achieved on the sapphire as the grown substrate. Next, based on the capability of synthesizing TMDs on arbitrary substrates, we fabricate heterostructures, namely P-N junction, composed of the p-type WSe2 layer converted from the WO3 film on a n-type SiC substrate in single step process (Upper inset in Figure 4c). The Raman spectrum presented in Figure S4 proves the successful synthesis of the WSe2 on the SiC substrate through the microwave-assisted selenization process. In addition, the crystal structures of resulting WSe2 were measured by XRD as shown in Figure S5. Furthermore, to confirm the formation of WSe2/SiC PN-junction, the electrical performance was measured as shown in Figure 4c. Clearly, I-V characteristics exhibited the rectifying behavior with a turn-on voltage of ~2V and the ideality factor of ~2 extracted from in bottom inset of Figure 4c. The larger ideality factor may be ascribed from several reasons, including (1) the poor contact issue between electrode and WSe2 layer, (2) the defects formed due to lattice mismatch at the interface between the two materials and (3) the defects formed in WSe2 due to the non-uniform and imperfect deposition of the WO3 film after the microwave-assisted selenization process. We believe that further improvement of the quality of the as-deposited WO3 film using improved deposition methods can achieve much better crystalline qualify of TMDs and less defects at the interface between two junctions, resulting in much better ideality factor. From our previous research, we demonstrate the direct patterning of WSe2 from WO3 pre-pattern process by the laser annealing process,29 providing a fast and facile approach for device fabrication compared to other methods.23, 46, 47 Similarly, we follow the same approach for the microwave-assisted selenization process. Figures 4d and 4e display the optical image of the as-deposited WO3 with different font sizes of “NTHU”, ranging from 20 to 100 µm before and after the microwave-assisted selenization process, respectively. Apparently, the initial light green color of the letters turned into dark green after the


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microwave-assisted selenization process, suggesting that a chemical reaction occurred while no color change was observed in other areas. In addition, the pattern was precisely kept as it was initially predefined. The corresponding Raman mapping image was presented in Figure 4f. Figure 4g shows the corresponding Raman spectra taken from the point marked in Figure 4f and the area displayed in green corresponds to the 2LA(M) mode used as reference for the Raman mapping. Distinctly, the Raman results confirm the successful transformation of the WO3 film into the layered structure WSe2. We believe that the pattern ability combined with the low temperature nature of the microwave-assisted selenization process is fundamental to overcome the problems related to device fabrication compared with other methods.28, 46 Another unique feature of our microwave-assisted selenization process is the precise thickness control and stacking alignment of TMDs by precise control of the thickness of metal oxide layer. Differing from previous report that controlling the thickness of metal seed layers can result in tunable thickness of TMDs.30 In this work, different thicknesses of WO3 from ~20, ~10 and ~3 nm were prepared on SiO2/Si and sapphire substrates, respectively. Note that ~3 nm-thick WO3 film will be the island shape on the SiO2/Si substrate due to larger surface energy while it can be uniform film on the sapphire substrate. Cross-sectional TEM images of layer structure WSe2 with thicknesses of ~20, ~8 and ~2 nm transferred from the WO3 films and an internal spacing of 0.69 nm corresponding to (001) plan as shown in each inset of Figures 5a-5c were confirmed after the microwave-assisted selenization process. Interestingly, stacking orientation along horizontal alignment was observed as the rough thickness of WO3

Low-Temperature and Ultrafast Synthesis of Patternable Few-Layer

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