Selection Role of Metal Oxides into Transition Metal Dichalcogenide

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Selection Role of Metal Oxides into Transition Metal Dichalcogenide Monolayers by a Direct Selenization Process Wei-Sheng Lin, Henry Medina, Teng-Yu Su, Shao-Hsin Lee, Chia-Wei Chen, Yu-Ze Chen, Arumugam Manikandan, Yu-Chuan Shih, Jian-Hua Yang, Jyun-Hong Chen, Bo-Wei Wu, Kuan-Wei Chu, Feng-Chuan Chuang, Jia-Min Shieh, Chang-Hong Shen, and Yu-Lun Chueh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17861 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Selection Role of Metal Oxides into Transition Metal Dichalcogenide Monolayers by a Direct Selenization Process Wei-Sheng Lin1+, Henry Medina1,3+, Teng-Yu Su1, Shao-Hsin Lee1, Chia-Wei Chen1, Yu-Ze Chen1, Arumugam Manikandan1, Yu-Chuan Shih1, Jian-Hua Yang1, Jyun-Hong Chen1, Bo-Wei Wu1, 4, Kuan-Wei Chu4, Feng-Chuan Chuang4, Jia-Min Shieh5, Chang-Hong Shen5* and Yu-Lun Chueh1, 2, 4* 1

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

30013, Taiwan, ROC 2

School of Material Science and Engineering, State Key Laboratory of Advanced Processing and

Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou City 730050, Gansu, P.R. China. 3

Institute of Materials Research and Engineering (IMRE), A*STAR, 2 Fusionopolis Way, Innovis,

Singapore 138634, Singapore 4

Department of Physics, National Sun Yat-Sen University, Kaohsiung, 80424, Taiwan. ROC

5

National Nano Device Laboratories, No. 26, Prosperity Road 1, Hsinchu 30078, ROC

+

These authors contributed equally to this work

*Corresponding author: [email protected] and [email protected]

Keywords: transition metal dichalcogenide, metal oxides, tungsten oxide, tungsten diselenide, Role of metal oxide selection, selenization Abstract- Direct reduction of metal oxides into a few transition metal dichalcogenide (TMDCs) monolayers has been recently explored as an alternative method for large area and uniform deposition. However, not many studies have been addressed on characteristics and requirement of the metal oxides into TMDCs by the selenization/sulfurization processes, yielding a wide range of outstanding properties to poor electrical characteristics with non-uniform films. The large difference implies that the process is yet not fully understood. In particular, the selenization/sulfurization at low temperature leads to poor crystallinity films with poor electrical performance, hindering its practical development. A common approach to improve the quality of

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the selenized/sulfurized films is by further increasing the process temperature, thus requiring additional transfer in order to explore the electrical properties. Here, we show that by finely tuning the quality of the pre-deposited, oxide the selenization/sulfurization temperature can be largely decreased, avoiding major substrate damage and allowing direct device fabrication. The direct relations between the role of selecting different metal oxides prepared by e-beam evaporation and reactive sputtering and their oxygen deficiency/vacancy leading to quality influence of TMDCs was investigated in detail. Due to its outstanding physical properties, the formation of tungsten diselenide (WSe2) from the reduction of tungsten oxide (WOx) was chosen as a model for proof of concept. By optimizing the process parameters and the selection of metal oxides, layered WSe2 films with controlled atomic thickness can be demonstrated. Interestingly, the domain size and electrical properties of the layered WSe2 films are highly affected by the quality of the metal oxides, for which the layered WSe2 film with small domains exhibits a metallic behavior and the layered WSe2 films with larger domains provides clear semiconducting behavior. Finally, an 8” wafer scale-layered WSe2 film was demonstrated, giving a step forward in the development of 2D TMDC electronics in the industry.


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Introduction Synthesis of a few TMDC monolayers has drawn lots of attention due to their remarkable properties when approaching to the monolayer.1-4 In particular, tungsten diselenide has shown exceptional valley polarization,5 light emission at room temperature,6 outstanding hole mobility,7 and excellent electrode for CO2 reduction in ionic liquids.8 Up to date, several methods on the formation of TMDCs have been proposed, including mechanical and chemical exfoliation of bulk TMDCs into layered counterparts.9-10 Although the mechanical exfoliation of TMDC monolayer from the bulk TMDCs through a scotch tape can achieve much better quality of TMDCs, the lack of mass production and precise control of numbers of layers is a critical issue.11-12 The chemical exfoliation of the bulk TMDCs via ion intercalation provides the fastest way of mass production. However, damage of phase in TDMCs, leading to a high density of defects after the chemical exfoliation may deteriorate intrinsic behaviors, limiting its applications. Therefore, chemical vapor deposition (CVD) is the most common approach used for the growth of TMDCs with good quality13 while the lack of a uniform deposition in a wafer scale with large domain size and layered controllability is a major drawback, which has to be taken into account.14 High growth temperature over 700 °C is imperative for the growth of high quality TMDCs in the conventional CVD process, although a few studies have reported the low temperature growth of sulfur-based TMDCs such as MoS2 at the growth temperature as low as 500 °C. The selenium-based TMDCs such as WSe2 below 700 °C are yet to be achieved.15-16

Furthermore, the diffusion of

chalcogenide atoms into the substrate during the high temperature growth makes the direct growth of the selenium-based TMDCs on insulators impossible, requiring additional transfer process for the following device fabrication processes.13, 17-20 For the conventional CVD process, the most pressing difficulty depends on how to control the numbers of layers in a large area since the uncontrolled over layer formation usually leads to the formation of pyramid structures.21-22 Alternatively, a post-selenization process, including the



pre-deposition of metal oxides followed by the selenization, offers a much better thickness control while the uniformity in a large area is a key issue for the further consideration. With this regard, we had previously reported alternative methods for the synthesis of layered WSe2 through the post selenization of the pre-deposited WOx thin films by laser irradiation and microwave heating processes, displaying particular physical characteristics such as control of the layer orientation while the scalability is still a major concern.23-24 Recently, a plasma-assisted selenization process has been demonstrated to achieve the growth of selenium-based TMDCs such as WSe2 at a temperature as low as 250 oC in large area.25 However, no detailed studies on selecting quality of the metal oxides converted into metal-Se2 were addressed. Despite the large uniformity, the poor quality of films processes at the low temperature may hamper their use for electrical applications.26 The most common approach to improve the electrical properties of the selenized/sulfurized films is by further increasing the process temperature, limiting the substrates used for the process mostly due substrate damage and requiring further transfer in order to unveil its electrical properties.26-27 In this regard , we show that the quality of the pre-deposited metal together with a finely tuned process are the key to achieve a low temperature process that allow the direct fabrication of field-effect transistors (FETs) without the further transfer. The influence on different quality of metal oxides prepared by e-beam evaporation and sputtering processes into metal-Se2 after the selenization was investigated where the conversion of WOx into WSe2 is used as material model. In addition, different gas ratios of N2 and H2 as well as the selenization temperature were also discussed. Interestingly, at a temperature over 600 °C, the forming gas ratio is not critical to influence the formation of the WSe2 from the WOx. At the selenization temperature of 500 °C, an increase in the amount of hydrogen is required in order to achieve the full selenization of WOx confirmed by Raman and XPS. Furthermore, two different WOx films deposited by e-beam evaporation (EWOx) and reactive sputtering (S-WOx) were used to understand the effects of oxygen


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vacancies/point defects on the formation of the layered WSe2 structure from the reduction of WOx films. After the selenization process, the difference in oxygen deficiency reflected in the physical and electrical properties of the reduced WSe2 monolayers were investigated. The layered WSe2 films synthetized by E-WOx (E-WSe2) possesses smaller domains with a metallic characteristic while the layered WSe2 materials synthetized by the S-WOx (S-WSe2) display a semiconducting behavior with larger domain size confirmed by TEM and supported by electrical measurements. Finally, transfer-free back gate devices directly fabricated on a SiO2/Si substrate from the S-WSe2 deposited on a wafer scale were demonstrated with the precise thickness controllability from mono-, bi-layers toward multilayers. These findings offer a major insight about the formation of a few WSe2 monolayers from the reduction of WOx that can be extended to other TMDCs in a large area and the development of 2D TMDC electronics in large scale as well.

Experimental Section Metal oxide deposition: Two different methods were used for the deposition of WOx films including RF-Sputtering with WO3 targets (99.95 %) operated under 15 mTorr in an Ar/O2 ratio of 20 with a fixed power of 50 W and the E-beam deposition at a pressure of 10-6 Torr using WO3 as a source (99.95 %) at a deposition rate of 0.01 nm/sec. In both cases, the time was controlled to achieve the different oxide thicknesses. Selenization process: After the deposition of the metal oxide films, the films were transferred to the selenization chamber. The pressure was set in the range of 1 to 10 Torr with the forming gas containing a ratio of H2: 100 sccm and N2: 20 sccm, respectively. Then, selenium (5N) was first heated to 300 ºC and maintained during the entire process. On the same time, the temperature of the target substrate was ramped to 500 ºC/600 ºC within 1 hr. Upon reaching the set point for the substrate temperature, the temperature was maintained for 1 hr. Finally, after the process is



complete, the hydrogen flow was turned off, and the substrate and Se were left to cool naturally until the temperature decreases to below 50 ºC. Then, the samples were retrieved and unloaded from the chamber. Fabrication of FET devices: For the back gate devices, patterned WOx films were reduced directly on 300 nm-thick dry SiO2 grown on p+ Si wafers. Ti 10 nm/ Pt 100 nm electrodes were later deposited by using standard photolithography methods. The geometry of the channel was set to W= 15 µm and L= 5µm. Characterizations: Micro Raman spectroscopy with spot size smaller than 1 µm was recorded using a 514 nm laser. Microstructure analysis was investigated from a high-resolution transmission electron microscopy (HRTEM, JEOL, JEM-3000F FEGTEM, 300 kV) with a pointto-point resolution of 0.17 nm equipped STEM and energy disperse ve spectrometer (EDS). The chemical bonding between tungsten and oxygen on the surface and the interior of the film were certified by X-ray photoelectron spectroscopy (PHI 5000 VersaProbe II). The electrical properties of the fabricated devices were measured with a semiconductor parameter analyser (Keithley 4200) at room temperature. Results and discussion Figure 1 shows a schematic of the steps used to form a few WSe2 monolayers with controlled thicknesses. First, tungsten oxide (WOx) was deposited by two different physical vapour deposition methods, including evaporation and sputtering as presented in Figure 1(a), respectively. For reference along the manuscript, the evaporated-WOx will be represented as the E-WOx while the sputtered-WOx will be denoted as the S-WOx. Later, the WOx films are selenized in a customized cold-wall furnace. The schematic of the selenization furnace is described in Figure 1(b). After the deposition of 5 nm-thick oxide films by both methods, X-ray electron spectroscopy (XPS) was used to highlight the differences. In the W 4f spectra as shown in Figure S1(a), the peaks located at~38 and 36 eV are the footprint of W 4f5/2 and W4f7/2 peaks of 6 ACS Paragon Plus Environment

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W6+ oxygen state whereas the other two small peaks located at ~36.8 and 34.8 eV belong to the doublet of W5+ oxygen state.28-29 The W6+ state is the mark of tungsten in tungsten trioxide (WO3) configuration with perfect stoichiometry and W5+ is associated to the oxygen loss, indicating an under stoichiometry of oxygen in the oxide film, sometimes referred as WO3-x rather than WO3.2829

In addition, the O 1s spectra show a major peak located at 531 eV and a small shoulder located

at 532.3eV (Figure S1b). The peak located at 531 eV is related to the expected stoichiometric oxygen in WO3 (lattice peak) and the peak located at 532.3 eV (non-lattice peak) is associated with hydroxyl groups (-OH) adsorbed at the surface due to oxygen vacancies and defects inside the film.28 To assess compositions of oxygen in WOx films, the ratios between the areas of nonlattice peak (Onon_L) compared to the total areas of oxygen contribution (OL+ Onon_L) are utilized as a criteria to characterize the amount of oxygen vacancies in the as-grown WOx under different deposition conditions. Noted that the higher the ratio value is, the higher the amount of defects/vacancies in the oxide is. As a result, in the case of E-WOx, the measured ratio of the nonlattice oxygen is around 29 %. For the sputtered-WO3 films deposited in Ar atmosphere without oxygen, the non-lattice oxygen peak ratio also reached higher values, indicating a large amount of oxygen vacancies (22.3 %). Oxygen was inserted in the chamber during sputtering a smaller O1snon_L/O1sL+non_L ratio as an indication of a stoichiometry closer to that of WO3 was achieved. After the process optimization (see experimental details), the smallest O1snon_L/O1sL+non_L ratio of 13.2 % was obtained as the Ar/O2 ratio was set 20. This condition will be later referred in the manuscript as S-WOx. Note that Raman spectra of E-WOx and S-WOx show no identifiable peaks, indicating poor crystallinity After identifying distinct difference between the two methods used for oxide deposition, we further address the selenization process (Figure 1b) and the corresponding optical image of the selenization furnace is shown in Figure S2. In an initial state prior to establish a comparison between the oxide deposition methods, the selenization parameters were finely tuned as shown in



Figure 2. Raman and XPS spectroscopy were further used to optimize and characterize the formation of WSe2 after the process where the S-WOx films were first used for the purposes of optimization. Figures 2(a), 2(c), and 2(e) show the Raman spectra of the S-WOx films after the selenization under different process time, substrate temperatures and forming gas ratios, respectively. The peak at 250 cm-1 corresponds to out-of-plane vibration of the selenium (Se) atoms (A1g). The shoulder at lower Raman shift can be fitted by a peak at 248 cm-1 attributed to the in-plane vibrational mode (E12g) while the peak fitted at 260 cm-1 refers to the second-order vibration of the longitudinal acoustic phonon at the M point (2LA(M)).30 Due to the close location of E12g, A1g and 2LA(M) peaks are difficult for precise deconvolution of E12g, and 2LA(M) peaks. Instead, we mainly refer to the A1g peak for later analysis. The first order Raman peak of silicon located at 520 cm-1 was also used as a reference in order to estimate the relative intensity of A1g peaks of WSe2.31 Furthermore, Figures S3(a), 2(b) and 2(c) summarize the Raman data from Figures 2(a), 2(c) and 2(e), comparing the relative intensity to silicon and the full width at half maxima (FWHM) of A1g. In addition, from the W 4f spectra as shown in Figures 2(c), 2(f) and 2(i), the transformation ratio from the WOx into the WSe2 was estimated. The transformation ratio is defined as the area under the 4Wf7/2 and 4Wf5/2 contributed curves of WSe2 compared to the total area under the curve of the W 4f spectra (contributed areas of WOx and WSe2 peaks). As a result, values closer to 100 % implies a better transformation of the oxide into WSe2. For reference, the forming gas ratios in Figures 2(e) and 2(f) indicate different as gas flow ratios of H2/N2 in the unit of sccm. Figures 2a show the Raman results in a selenization temperature of 600 °C and a forming gas ratio of 20/100 sccm for H2/N2. The Raman spectra of WSe2 at A1g reduces as the selenization time increases and the spectra looks quite similar in terms of relative intensity, indicating a small decrease on the FWHM of A1g after the selenization process of 60 min. By comparing the transformation ratio based on W 4f spectra after the selenization process of 30 min (Figure 2c), the film only reaches the transformation of ~78 % from the WOx into the WSe2 compared with that of 88 % reached after the selenization process of 60 min. However, it remains 8 ACS Paragon Plus Environment

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without major changes once the selenization time is further increased to 90 min. Previous reports of WSe2 synthesized on SiO2 by the standard CVD process also shows similar transformation ratios,19 which is usually attributed to the polycrystalline nature of the films because the oxidation process is initiated at the grain boundaries possibly caused by the substrate.32 Contrary to the selenization time, the selenization temperature has distinct difference for the transformation of the WSe2 from the WOx as confirmed by Raman spectra (Figure 2b). An increase in the ratio of A1g/Si and the decrease in FWHM attributed to lower electron-phonon scattering and larger phonon coherence-length because of the better crystallinity of the WSe2 converted from the WOx as the selenization temperature increases.33 In terms of the forming gas ratio, A1g intensity ratio and FWHM show a slight improvement with the increased H2 concentration. W 4f spectra also follow a similar trend, achieving the best WSe2 transformation using forming gas ratios of 100/20 and 120/0. Interestingly, we found that at the selenization temperature of 600 °C, the gas forming ratio has a minor influence compared to the major role observed when the temperature was further decreased to 500 °C (Figure S4). Nevertheless, at the selenization temperatures over 600 °C, the transformation ratio is the highest based on XPS results. Consequently, the selenization time of 1hr at the selenization temperature of 600 °C with the forming gas ratio of H2/N2, 100/20 provides the optimal condition with the largest intensity of A1g with the smallest FWHM, yielding the almost complete transformation ratio from the WOx into the WSe2 confirmed by W 4f spectra in XPS spectra. Furthermore, 5 nm-thick S-WOx and E-WOx films were compared using the optimized selenization parameters. Figures 3(a) and 3(b) present the corresponding Raman spectra of both films after the selenization process. In general, the highest A1g intensity ratios of the selenized SWOx films (S-WSe2) compared to that of the selenized E-WOx (E-WSe2) are similar. However, a large variation in the A1g intensity ratio of the E-WSe2 was found and the FWHM of the A1g in SWSe2 is smaller compared with E-WSe2 in average as an indication of the better crystallinity.33



Although W 4f spectra do not show major difference as shown in Figure 3(c), the transformation ratio of the S-WSe2 reaches into 90 %, suggesting almost full selenization under the identical selenization conditions. By using XPS, the Se/W ratio was estimated and presented in Table S2. The Se/W ratio in both cases is higher than 2, showing an excess of selenium. Unfortunately, both ratios are very close to each other to correlate this result with the quality of the selenized films. Atomic force microscope (AFM) was employed to analyze the surface roughness of E-WSe2 and S-WSe2 as shown in Figures S5(a) and S5(b). In both cases, the roughness is close to 0.6 nm, which is the same as that of the original SiO2 substrate, providing an evidence of highly uniform films even after the selenization process. High Resolution Transmission Electron Microscopy (HRTEM) images were used to further characterize crystallinities of E-WSe2 and S-WSe2 as shown in Figures 3(d) and 3(e) and insets in Figures 3(d) and 3(e) show the fast Furrier transfer (FFT) diffraction patterns of the selected areas. A circular pattern from the FFT diffraction pattern converted from the E-WSe2 indicates the small domains with random orientations while a hexagonal pattern with well-defined symmetry spots from the FFT diffraction pattern converted from the S-WSe2 presents an evidence of better crystallinity of S-WSe2, with which atomic lattice image is shown in the upper inset of Figure 3(e). The results are also confirmed by the A1g modes in Raman spectra with the smaller FWHM. Note that hexagonal stacking of lattice also consists of hard sphere model where yellow and blue colors represent W and Se atoms. In order to investigate and evaluate the electrical characteristics of E-WSe2 and S-WSe2, transfer-free back gate field effect transistors (FET) were prepared as the schematic shown in Figure 4(a). 300 nm-thick SiO2/p+ Si was used as a dielectric layer and back gate electrode, respectively. As an advantage of our method, the WOx film can be first pre-patterned as the channel. After the selenization, Pt electrodes were deposited as source and drain (Experimental section for details). For the case of the S-WSe2, the transfer characteristics are displayed in Figures 4(b) and 4(c). The Ids-Vds curve at different Vbg biases clearly shows an increase in


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conductance under a negative back gate bias, indicating a P-type semiconducting behavior in agreement with previous reports by the CVD grown-WSe2.34-35 WSe2 is expected to show ambipolar behaviour; however, the P-type behaviour of our devices is greatly influenced by the metal used as a contact.18, 36 We do not discard that our synthesized WSe2 films show n-type behavior by using the correct electrode materials and strong gating.37-39 In addition, the mobility was calculated by µ=(L/WCox Vds) (△Ids/△Vbg) at the linear region of △Ids/△Vbg where Cox, W and L are gate capacitance (300 nm-thick SiO2), channel width and length of the device, respectively.10,

40

Based on the transfer characteristic as shown in Figure 4(c), the estimated

mobility is ~0.2 cm2 V-1s-1, which is still comparable with that of other 2-terminal FETs prepared by exfoliated WSe241 and CVD-grown WSe242 measured on air at room temperature. Interestingly, devices prepared by the evaporated WOx (E-WSe2) shows the slight gate dependence, displaying mostly metallic behavior as shown Figure S6. Interestingly, no indication of the formation of metallic phases (1T structure) can be formed from XPS nor Raman.9, 43 Therefore, we suggest that the observed metallic behavior on WSe2 formed by selenization of the E-WSe2 can be caused due carrier transport at the grain boundaries with decreased gap states.44 Another important aspect of the sulfurization/selenization of metal oxide for the formation of TMDC monolayers is the thickness control with excellent uniformity in the large area. To demonstrate this concept, we further attempt to synthesize the S-WSe2 on an 8” wafer. By carefully controlling the oxide thickness, numbers of WSe2 can be finely tuned. Figures 5(a) to 5(c) display the cross-sectional HRTEM images of 1, 5 and 7-nm-thick S-WOx films after the selenization process. In particular, due to the transition from indirect to direct, bilayer WSe2 shows two peaks located at 1.61 and 1.59 eV attributed to the A exciton and indirect bandgap transition (Figure S7).45, 46 For thicker films, no PL was observed probably due to weak intensity of the PL with increased number of monolayers. The Raman spectroscopy provides a fast and non-invasive method to investigate the quality of the WSe2 by analyzing the A1g peak in order to examine the uniformity of the layered WSe2 film in a large area. Figure 5(d) shows the photograph of 7 to 8 S-WSe2 monolayers 11 ACS Paragon Plus Environment


deposited on an 8” SiO2/Si wafer. The inset in Figure 5(d) presents the Raman mapping of A1g in a 2 x 2 µm area as a proof of the uniformity in the local area. Furthermore, Figure 5(e) shows the Raman spectra taken at the positions marked in Figure 5(d), with which no variation in the A1g intensity at all points as a clear evidence of the macroscopic uniformity. Note that the excellent thickness control at few nanometers required for the oxide deposition and the uniform heating and selenium gas distribution necessaries to achieve uniform a few WSe2 monolayers in the large area. In addition to SiO2 and sapphire, WSe2 was reduced in a broad range of substrates such as quartz, HfO2 and Au thin films as a proof of concept in Figure S8. We noted that the intensity of the Raman peaks varies between substrates, suggesting that additional tuning should be used for each specific target substrate.

Conclusion An analysis of the effectiveness of different metal oxide deposition methods and their influence on the physical and electrical properties of WSe2 after selenization was presented. In addition, further optimization of the selenization of tungsten oxide shows the effect of temperature, selenization time and forming gas ratio on the quality of the resulting films. Interestingly, a small variation in the physical properties of the tungsten oxide films will strongly affect the electrical properties of WSe2 displaying metallic behavior with small domains when deposited by e-beam evaporation with the increased amount of oxygen vacancies to the semiconductor behavior with the good field effect behaviors when the oxide was deposited by the reactive sputtering, providing the less oxygen deficiency confirmed by XPS. Large uniformity of WSe2 films with monolayer thickness control was also achieved, offering several advantages such as low cost and transfer-free process. More importantly, this approach can be also applied to other kinds of TMDCs materials, giving a step forward in the development of 2D TMDC technology.


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Supporting Information XPS of E-WOx and S-WOx; photograph of the customized large area selenization furnace chamber; Raman and XPS spectra of WSe2 selenized at different temperature, selenization time and under different forming gas ratio. AFM and low magnification TEM of E-WSe2 and S-WSe2. Transfer characteristic of E-WSe2.

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, 105-2633-M-007-003, 1052119-M-009-009 and the National Tsing Hua University through Grant no. 106N509CE1. Y.L. Chueh greatly appreciates the use of the facility at CNMM.H. Medina gratefully acknowledges the support from the Institute of Materials Research and Engineering (IMRE) under the Agency for Science, Technology, and Research (A*STAR) through the project No. IMRE/15-2C0115.

References 1.Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H., The Chemistry of Twodimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263275. 2.Zhang, X.; Qiao, X. F.; Shi, W.; Wu, J. B.; Jiang, D. S.; Tan, P. H., Phonon and Raman Scattering of Two-dimensional Ttransition Metal Dichalcogenides from Monolayer, Multilayer to Bulk Material. Chem. Soc. Rev. 2015, 44, 2757-2785. 3.Duan, X.; Wang, C.; Pan, A.; Yu, R.; Duan, X., Two-dimensional Transition Metal Dichalcogenides as Atomically Thin Semiconductors: Opportunities and Challenges. Chem. Soc. Rev. 2015, 44, 8859-8876. 4.Schmidt, H.; Giustiniano, F.; Eda, G., Electronic Transport Pproperties of Transition Mmetal Dichalcogenide Field-effect Devices: Surface and Interface Effects. Chem. Soc. Rev. 2015, 44, 7715-7736.



5.Singh, A.; Tran, K.; Kolarczik, M.; Seifert, J.; Wang, Y.; Hao, K.; Pleskot, D.; Gabor, N. M.; Helmrich, S.; Owschimikow, N.; Woggon, U.; Li, X., Long-Lived Valley Polarization of Intravalley Trions in Monolayer WSe2. Phys. Rev. Lett. 2016, 117, 257402. 6.Withers, F.; Del Pozo-Zamudio, O.; Schwarz, S.; Dufferwiel, S.; Walker, P. M.; Godde, T.; Rooney, A. P.; Gholinia, A.; Woods, C. R.; Blake, P.; Haigh, S. J.; Watanabe, K.; Taniguchi, T.; Aleiner, I. L.; Geim, A. K.; Fal'ko, V. I.; Tartakovskii, A. I.; Novoselov, K. S., WSe2 LightEmitting Tunneling Transistors with Enhanced Brightness at Room Temperature. Nano Lett. 2015, 15, 8223-8228. 7.Movva, H. C.; Rai, A.; Kang, S.; Kim, K.; Fallahazad, B.; Taniguchi, T.; Watanabe, K.; Tutuc, E.; Banerjee, S. K., High-Mobility Holes in Dual-Gated WSe2 Field-Effect Transistors. ACS Nano 2015, 9, 10402-10410. 8.Asadi, M.; Kim, K.; Liu, C.; Addepalli, A. V.; Abbasi, P.; Yasaei, P.; Phillips, P.; Behranginia, A.; Cerrato, J. M.; Haasch, R.; Zapol, P.; Kumar, B.; Klie, R. F.; Abiade, J.; Curtiss, L. A.; Salehi-Khojin, A., Nanostructured Transition Metal Dichalcogenide Electrocatalysts for CO2 Reduction in Ionic Liquid. Science 2016, 353, 467-470. 9. Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M., Photoluminescence from chemically exfoliated MoS2. Nano Lett. 2011, 11, 5111-5116. 10. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single-layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147-150. 11. Lin, M. W.; Kravchenko, II; Fowlkes, J.; Li, X.; Puretzky, A. A.; Rouleau, C. M.; Geohegan, D. B.; Xiao, K., Thickness-dependent Charge Transport in Few-layer MoS2 Field-effect Transistors. Nanotechnology 2016, 27, 165203. 12. Kim, J. H.; Kim, T. H.; Lee, H.; Park, Y. R.; Choi, W.; Lee, C. J., Thickness-dependent Electron Mobility of Single and Few-layer MoS2 Thin-film Transistors. AIP Adv. 2016, 6, 065106. 13. Huang, J. K.; Pu, J.; Hsu, C. L.; Chiu, M. H.; Juang, Z. Y.; Chang, Y. H.; Chang, W. H.; Iwasa, Y.; Takenobu, T.; Li, L. J., Large-area Synthesis of Highly Crystalline WSe(2) Monolayers and Device Applications. ACS Nano 2014, 8, 923-930. 14. Ling, X.; Lee, Y. H.; Lin, Y.; Fang, W.; Yu, L.; Dresselhaus, M. S.; Kong, J., Role of the Seeding Promoter in MoS2 Growth by Chemical Vapor Deposition. Nano Lett. 2014, 14, 464472. 15. Kang, K.; Xie, S.; Huang, L.; Han, Y.; Huang, P. Y.; Mak, K. F.; Kim, C. J.; Muller, D.; Park, J., High-mobility Three-atom-thick Semiconducting Films with Wafer-scale Homogeneity. Nature 2015, 520, 656-660. 14 ACS Paragon Plus Environment

Page 14 of 25

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


16. Gong, Y.; Lin, Z.; Ye, G.; Shi, G.; Feng, S.; Lei, Y.; Elias, A. L.; Perea-Lopez, N.; Vajtai, R.; Terrones, H.; Liu, Z.; Terrones, M.; Ajayan, P. M., Tellurium-Assisted Low-Temperature Synthesis of MoS2 and WS2 Monolayers. ACS Nano 2015, 9, 11658-11666. 17. Browning, P.; Eichfeld, S.; Zhang, K.; Hossain, L.; Lin, Y.-C.; Wang, K.; Lu, N.; Waite, A. R.; Voevodin, A. A.; Kim, M.; Robinson, J. A., Large-area Synthesis of WSe2 from WO3 by Selenium–oxygen Ion Exchange. 2D Materials 2015, 2, 014003. 18. Zhou, H.; Wang, C.; Shaw, J. C.; Cheng, R.; Chen, Y.; Huang, X.; Liu, Y.; Weiss, N. O.; Lin, Z.; Huang, Y.; Duan, X., Large Area Growth and Electrical Properties of P-type WSe2 Atomic Layers. Nano Lett. 2015, 15, 709-713. 19. Chen, J.; Liu, B.; Liu, Y.; Tang, W.; Nai, C. T.; Li, L.; Zheng, J.; Gao, L.; Zheng, Y.; Shin, H. S.; Jeong, H. Y.; Loh, K. P., Chemical Vapor Deposition of Large-Sized Hexagonal WSe(2) Crystals on Dielectric Substrates. Adv. Mater. 2015, 27, 6722-6727. 20. Chen, L.; Liu, B.; Ge, M.; Ma, Y.; Abbas, A. N.; Zhou, C., Step-Edge-Guided Nucleation and Growth of Aligned WSe2 on Sapphire via a Layer-over-Layer Growth Mode. ACS Nano 2015, 9, 8368-8375. 21. Zhu, Y.; Yang, J.; Zhang, S.; Mokhtar, S.; Pei, J.; Wang, X.; Lu, Y., Strongly Enhanced Photoluminescence in Nanostructured Monolayer MoS2 by Chemical Vapor Deposition. Nanotechnology 2016, 27, 135706. 22. Wang, L.; Chen, F.; Ji, X., Shape Consistency of MoS2 Flakes Grown Using Chemical Vapor Deposition. Appl. Phys. Express 2017, 10, 065201. 23. Chen, Y. Z.; Medina, H.; Su, T. Y.; Li, J. G.; Cheng, K. Y.; Chiu, P. W.; Chueh, Y. L., Ultrafast and Low Temperature Synthesis of Highly Crystalline and Patternable Few-layers Tungsten Diselenide by Laser Irradiation Assisted Selenization Process. ACS Nano 2015, 9, 4346-4353. 24. Chen, Y.-Z.; Medina, H.; Wang, S.-W.; Su, T.-Y.; Li, J.-G.; Yen, W.-C.; Cheng, K.-Y.; Kuo, H.C.; Shen, G.; Chueh, Y.-L., Low-Temperature and Ultrafast Synthesis of Patternable FewLayer Transition Metal Dichacogenides with Controllable Stacking Alignment by a Microwave-Assisted Selenization Process. Chem. Mater. 2016, 28, 1147-1154. 25. Medina, H.; Li, J.-G.; Su, T.-Y.; Lan, Y.-W.; Lee, S.-H.; Chen, C.-W.; Chen, Y.-Z.; Manikandan, A.; Tsai, S.-H.; Navabi, A.; Zhu, X.; Shih, Y.-C.; Lin, W.-S.; Yang, J.-H.; Thomas, S. R.; Wu, B.-W.; Shen, C.-H.; Shieh, J.-M.; Lin, H.-N.; Javey, A.; Wang, K. L.; Chueh, Y.-L., Wafer-Scale Growth of WSe2 Monolayers Toward Phase-Engineered Hybrid WOx/WSe2 Films with Sub-ppb NOx Gas Sensing by a Low-Temperature Plasma-Assisted Selenization Process. Chem. Mater. 2017, 29, 1587-1598.



26. Vangelista, S.; Cinquanta, E.; Martella, C.; Alia, M.; Longo, M.; Lamperti, A.; Mantovan, R.; Basset, F. B.; Pezzoli, F.; Molle, A., Towards a Uniform and Large-scale Deposition of MoS2 Nanosheets via Sulfurization of Ultra-thin Mo-based Solid Films. Nanotechnology 2016, 27, 175703. 27. Lin, Y. C.; Zhang, W.; Huang, J. K.; Liu, K. K.; Lee, Y. H.; Liang, C. T.; Chu, C. W.; Li, L. J., Wafer-scale MoS2 Thin Layers Prepared by MoO3 Sulfurization. Nanoscale 2012, 4, 66376641. 28. Vasilopoulou, M.; Soultati, A.; Georgiadou, D. G.; Stergiopoulos, T.; Palilis, L. C.; Kennou, S.; Stathopoulos, N. A.; Davazoglou, D.; Argitis, P., Hydrogenated Under-stoichiometric Tungsten oxide Anode Interlayers for Efficient and Stable Organic Photovoltaics. J. Mater. Chem. A 2014, 2, 1738-1749. 29. Weinhardt, L.; Blum, M.; Bär, M.; Heske, C.; Cole, B.; Marsen, B.; Miller, E. L., Electronic Surface Level Positions of WO3Thin Films for Photoelectrochemical Hydrogen Production. J. Phys. Chem. C 2008, 112, 3078-3082. 30. Terrones, H.; Del Corro, E.; Feng, S.; Poumirol, J. M.; Rhodes, D.; Smirnov, D.; Pradhan, N. R.; Lin, Z.; Nguyen, M. A.; Elias, A. L.; Mallouk, T. E.; Balicas, L.; Pimenta, M. A.; Terrones, M., New First Order Raman-active Modes in Few Layered Transition Metal Dichalcogenides. Sci. Rep. 2014, 4, 4215. 31. Parker, J. H.; Feldman, D. W.; Ashkin, M., Raman Scattering by Silicon and Germanium. Phys. Rev. 1967, 155, 712-714. 32. Liu, Y.; Tan, C.; Chou, H.; Nayak, A.; Wu, D.; Ghosh, R.; Chang, H. Y.; Hao, Y.; Wang, X.; Kim, J. S.; Piner, R.; Ruoff, R. S.; Akinwande, D.; Lai, K., Thermal Oxidation of WSe2 Nanosheets Adhered on SiO2/Si Substrates. Nano Lett. 2015, 15, 4979-4984. 33. Pradhan, N. R.; Rhodes, D.; Memaran, S.; Poumirol, J. M.; Smirnov, D.; Talapatra, S.; Feng, S.; Perea-Lopez, N.; Elias, A. L.; Terrones, M.; Ajayan, P. M.; Balicas, L., Hall and Fieldeffect Mobilities in Few Layered P-WSe2 Field-effect Transistors. Sci. Rep. 2015, 5, 8979. 34. Campbell, P. M.; Tarasov, A.; Joiner, C. A.; Tsai, M. Y.; Pavlidis, G.; Graham, S.; Ready, W. J.; Vogel, E. M., Field-effect Transistors Based on Wafer-scale, Highly Uniform Few-layer P-type WSe2. Nanoscale 2016, 8, 2268-2276. 35. Liu, B.; Ma, Y.; Zhang, A.; Chen, L.; Abbas, A. N.; Liu, Y.; Shen, C.; Wan, H.; Zhou, C., High-Performance WSe2 Field-Effect Transistors via Controlled Formation of In-Plane Heterojunctions. ACS Nano 2016, 10, 5153-5160.


Page 16 of 25

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


36. Fang, H.; Chuang, S.; Chang, T. C.; Takei, K.; Takahashi, T.; Javey, A., High-performance Single Layered WSe(2) P-FETs with Chemically Doped Contacts. Nano Lett. 2012, 12, 37883792. 37. Smyth, C. M.; Addou, R.; McDonnell, S.; Hinkle, C. L.; Wallace, R. M., Contact Metal– MoS2Interfacial Reactions and Potential Implications on MoS2-Based Device Performance. The Journal of Physical Chemistry C 2016, 120, 14719-14729. 38. Smyth, C. M.; Addou, R.; McDonnell, S.; Hinkle, C. L.; Wallace, R. M., WSe2-contact Metal Interface Chemistry and Band Alignment Under High Vacuum and Ultra High Vacuum Deposition Conditions. 2D Materials 2017, 4. 39. Liu, W.; Kang, J.; Sarkar, D.; Khatami, Y.; Jena, D.; Banerjee, K., Role of Metal Contacts in Designing High-performance Monolayer N-type WSe2 Field Effect Transistors. Nano Lett. 2013, 13, 1983-1990. 40. Radisavljevic, B.; Kis, A., Mobility Engineering and a Metal-insulator Transition in Monolayer MoS(2). Nat. Mater. 2013, 12, 815-820. 41. Li, H.; Lu, G.; Wang, Y.; Yin, Z.; Cong, C.; He, Q.; Wang, L.; Ding, F.; Yu, T.; Zhang, H., Mechanical Exfoliation and Characterization of Single- and Few-layer Nanosheets of WSe(2) , TaS(2) , and TaSe(2). Small 2013, 9, 1974-1981. 42. Huang, J.; Yang, L.; Liu, D.; Chen, J.; Fu, Q.; Xiong, Y.; Lin, F.; Xiang, B., Large-area synthesis of monolayer WSe2 on a SiO2/Si substrate and its device applications. Nanoscale 2015, 7, 4193-4198. 43. Fan, X.; Xu, P.; Li, Y. C.; Zhou, D.; Sun, Y.; Nguyen, M. A.; Terrones, M.; Mallouk, T. E., Controlled Exfoliation of MoS2 Crystals into Trilayer Nanosheets. J. Am. Chem. Soc. 2016, 138, 5143-5149. 44. Huang, Y. L.; Ding, Z.; Zhang, W.; Chang, Y. H.; Shi, Y.; Li, L. J.; Song, Z.; Zheng, Y. J.; Chi, D.; Quek, S. Y.; Wee, A. T., Gap States at Low-Angle Grain Boundaries in Monolayer Tungsten Diselenide. Nano Lett. 2016, 16, 3682-3688. 45. Zeng, H.; Liu, G. B.; Dai, J.; Yan, Y.; Zhu, B.; He, R.; Xie, L.; Xu, S.; Chen, X.; Yao, W.; Cui, X., Optical Signature of Symmetry Variations and Spin-valley Coupling in Atomically Thin Tungsten Dichalcogenides. Sci Rep 2013, 3, 1608. 46. Tonndorf, P.; Schmidt, R.; Bottger, P.; Zhang, X.; Borner, J.; Liebig, A.; Albrecht, M.; Kloc, C.; Gordan, O.; Zahn, D. R.; Michaelis de Vasconcellos, S.; Bratschitsch, R., Photoluminescence Emission and Raman Response of Monolayer MoS(2), MoSe(2), and WSe(2). Opt Express 2013, 21, 4908-4916.



Figure Captions Figure 1. Schematic of selenization processes for the formation of large area WSe2. (a) The representation of the oxide deposition using two different physical vapour deposition methods (PVD), including e-beam evaporation and sputtering. (b) The schematic of the customized selenium furnace for the reduction of WOx into WSe2. Figure 2. (a) Raman spectra and (b) XPS W 4f spectra after the selenization on 600 °C at different periods of time. (c) and (d) Raman spectra, and W 4f spectra at different substrate temperatures for 1 hr. Similarly, in (e) and (f) are the Raman spectra and W 4f spectra after the selenization at 600 ° for 1 hr under different forming gas ratios. Figure 3. (a) and (b) Raman spectra and the corresponding statistical analysis of A1g and FWHM of WOx films after the selenization process. (c) XPS W 4f spectra of selenized films. (d) and (e) HRTEM images with their corresponding FFT diffraction patterns of EWse2 and S-Wse2, respectively. The upper right inset in (e) shows the IFFT image of the diffraction pattern, displaying the hexagonal characteristic of WSe2. Figure 4. Electrical characterization of S-WSe2. (a) A photograph and the schematic of the back gate FET device fabricated for measurements. (b) Ids vs. Vds and (c) the transfer characteristic Ids vs. Vbg of the S-WSe2 transfer-free back gate transistor.


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Figure 5. Thickness control and uniformity. (a), (b) and (c) the cross-sectional TEM of 1~2, 7~8 and ~10 S-WSe2 monolayers, respectively. (d) A photograph of a few S-WSe2 monolayers deposited on a 8” SiO2/Si wafer. The right upper inset shows the A1g Raman mapping of the selected area. (e) Raman spectra taken at the selected points displayed in (d).



Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Selection Role of Metal Oxides into Transition Metal Dichalcogenide

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