Selection Role of Metal Oxides into Transition Metal Dichalcogenide

Jan 8, 2018 - School of Material Science and Engineering, State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou Uni...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 9645−9652

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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*,†,‡,∥ †

Department of Material Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC 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 § Institute of Materials Research and Engineering (IMRE), A*STAR, 2 Fusionopolis Way, Innovis, Singapore 138634, Singapore ∥ Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, ROC ⊥ National Nano Device Laboratories, No. 26, Prosperity Road 1, Hsinchu 30078, Taiwan, ROC ‡

S Supporting Information *

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 addressed the 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 nonuniform 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 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 predeposited oxide the selenization/sulfurization temperature can be largely decreased, avoiding major substrate damage and allowing direct device fabrication. The direct relationship 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. Because of 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. KEYWORDS: transition metal dichalcogenide, metal oxides, tungsten oxide, tungsten diselenide, role of metal oxide selection, selenization

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INTRODUCTION

mechanical exfoliation of the TMDC monolayer from the bulk TMDCs through scotch tape can achieve much better quality of TMDCs, the lack of mass production and precise control of the number 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

Synthesis of a few TMDC monolayers has drawn lots of attention due to their remarkable properties when approaching 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 © 2018 American Chemical Society

Received: November 24, 2017 Accepted: January 8, 2018 Published: January 8, 2018 9645

DOI: 10.1021/acsami.7b17861 ACS Appl. Mater. Interfaces 2018, 10, 9645−9652

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic of selenization processes for the formation of large area WSe2. (a) Representation of the oxide deposition using two different physical vapor deposition methods (PVD), including e-beam evaporation and sputtering. (b) Schematic of the customized selenium furnace for the reduction of WOx into WSe2..

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 to 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 predeposited metal together with a finely tuned process is the key to achieve a low temperature process that allows the direct fabrication of field-effect transistors (FETs) without further transfer. The influence on different qualities of metal oxides prepared by e-beam evaporation and sputtering processes into metal-Se2 after selenization was investigated where the conversion of WOx into WSe2 is used as the 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 WSe2 from 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 (E-WOx) and reactive sputtering (S-WOx) were used to understand the effects of oxygen 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 synthesized by E-WOx (E-WSe2) possesses smaller domains with a metallic characteristic, while the layered WSe2 materials synthesized by 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- and bilayers toward multilayers. These findings offer great insight into the formation of a few WSe2 monolayers from the reduction of WOx that can be extended to other TMDCs in

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 quality,13 while the lack of uniform deposition at the 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 a 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 high temperature growth makes the direct growth of the selenium-based TMDCs on insulators impossible, requiring an 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 number of layers in a large area since the uncontrolled overlayer formation usually leads to the formation of pyramid structures.21,22 Alternatively, a postselenization process, including the predeposition of metal oxides followed by selenization, offers a much better thickness control, while the uniformity in a large area is a key issue for further consideration. In this regard, we had previously reported alternative methods for the synthesis of layered WSe2 through postselenization of the predeposited WOx thin films by laser irradiation and microwave heating processes, displaying particular physical characteristics such as control of the layer orientation. However, scalability is still a major concern.23,24 Recently, a plasma-assisted selenization process has been demonstrated to achieve the growth of seleniumbased TMDCs such as WSe2 at a temperature as low as 250 °C in a large area.25 However, no detailed studies on selecting the quality of the metal oxides converted into metal-Se2 were addressed. Despite the large uniformity, the poor quality of film processes at the low temperature may hamper their use in 9646

DOI: 10.1021/acsami.7b17861 ACS Appl. Mater. Interfaces 2018, 10, 9645−9652

Research Article

ACS Applied Materials & Interfaces

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. Note that the higher the ratio value is, the higher is the amount of defects/vacancies in the oxide. 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 nonlattice oxygen peak ratio also reached higher values, indicating a large amount of oxygen vacancies (22.3%). Oxygen was inserted in the chamber during sputtering, and a smaller O 1snon_L/O 1sL+non_L ratio as an indication of stoichiometry closer to that of WO3 was achieved. After the process optimization (see experimental details), the smallest O 1snon_L/O 1sL+non_L ratio of 13.2% was obtained as the Ar/O2 ratio was set to 20. This condition will be later referred to in the article as S-WOx. Note that Raman spectra of E-WOx and S-WOx show no identifiable peaks, indicating poor crystallinity. After identifying a 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 establishing 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. Figure 2a, c, and e shows the Raman spectra of the S-WOx films after selenization under different process times, substrate temperatures, and forming gas ratios, respectively. The peak at 250 cm−1 corresponds to an 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 Because of the close location of E12g, A1g and 2LA(M) peaks are difficult for the 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 S3a, 2b, and 2c summarize the Raman data from Figure 2a, c, and 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 Figure 2c, f, and 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% imply a better transformation of the oxide into WSe2. For reference, the forming gas ratios in Figure 2e and f indicate different gas flow ratios of H2/N2 in sccm units. Figure 2a shows the Raman results at 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 look 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

a large area and the development of 2D TMDC electronics in large scale as well.

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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 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/s. 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. Then, selenium (5 N) was first heated to 300 °C and maintained during the entire process. At the same time, the temperature of the target substrate was ramped to 500 °C/600 °C within 1 h. Upon reaching the set point for the substrate temperature, the temperature was maintained for 1 h. 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 °C. 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 = 5 μm. 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 microscope (HRTEM, JEOL, JEM-3000F FEGTEM, 300 kV) with a point-to-point resolution of 0.17 nm equipped STEM and an energy dispersive 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 analyzer (Keithley 4200) at room temperature.

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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 vapor deposition methods, including evaporation and sputtering as presented in Figure 1a. For reference in this article, the evaporated-WOx will be represented as E-WOx, while the sputtered-WOx will be denoted as S-WOx. Later, the WOx films are selenized in a customized cold-wall furnace. The schematic of the selenization furnace is described in Figure 1b. 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 S1a, the peaks located at ∼38 and 36 eV are the footprint of W 4f5/2 and W 4f7/2 peaks of the W6+ oxygen state, whereas the other two small peaks located at ∼36.8 and 34.8 eV belong to the doublet of the 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 with oxygen loss, indicating an under stoichiometry of oxygen in the oxide film, sometimes referred to as WO3‑x rather than WO3.28,29 In addition, the O 1s spectra show a major peak located at 531 eV and a small shoulder located at 532.3 eV (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 (nonlattice peak) is associated with hydroxyl groups (−OH) 9647

DOI: 10.1021/acsami.7b17861 ACS Appl. Mater. Interfaces 2018, 10, 9645−9652

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ACS Applied Materials & Interfaces

Nevertheless, at selenization temperatures over 600 °C, the transformation ratio is the highest based on XPS results. Consequently, the selenization time of 1 h at the selenization temperature of 600 °C with a 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. Figure 3a and b presents the corresponding Raman spectra of both

Figure 2. (a) Raman spectra and (b) XPS W 4f spectra after the selenization at 600 °C at different periods of time. (c and d) Raman spectra and W 4f spectra at different substrate temperatures for 1 h. Similarly, in e and f are the Raman spectra and W 4f spectra after the selenization at 600° for 1 h under different forming gas ratios.

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 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 differences 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 a 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 are observed.33 In terms of the forming gas ratio, A1g intensity ratio and fwhm show a slight improvement with 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).

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 E-Wse2 and SWse2, respectively. The upper right inset in e shows the IFFT image of the diffraction pattern, displaying the hexagonal characteristic of WSe2.

films after the selenization process. In general, the highest A1g intensity ratios of the selenized S-WOx 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 S-WSe2 is smaller compared with E-WSe2 on average as an indication of the better crystallinity.33 Although W 4f spectra do not show a major difference as shown in Figure 3c, the transformation ratio of the S-WSe2 reaches 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 Figure S5a and b. In both cases, the roughness is close to 0.6 9648

DOI: 10.1021/acsami.7b17861 ACS Appl. Mater. Interfaces 2018, 10, 9645−9652

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ACS Applied Materials & Interfaces

Figure 4. Electrical characterization of S-WSe2. (a) Photograph and 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.

Figure 5. Thickness control and uniformity. (a, b, and c) Cross-sectional TEM of 1−2, 7−8, and ∼10 S-WSe2 monolayers, respectively. (d) Photograph of a few S-WSe2 monolayers deposited on an 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.

symmetry spots from the FFT diffraction pattern converted from the S-WSe2 presents evidence of better crystallinity of SWSe2, with which an atomic lattice image is shown in the upper inset of Figure 3e. The results are also confirmed by the A1g modes in Raman spectra with the smaller fwhm. Note that hexagonal stacking of the lattice also consists of a 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 shown in the schematic in

nm, which is the same as that of the original SiO2 substrate, providing evidence of highly uniform films even after the selenization process. High resolution transmission electron microscopy (HRTEM) images were used to further characterize the crystallinities of E-WSe2 and S-WSe2 as shown in Figure 3d and e, and the insets in Figure 3d and e show the fast Fourier 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 9649

DOI: 10.1021/acsami.7b17861 ACS Appl. Mater. Interfaces 2018, 10, 9645−9652

ACS Applied Materials & Interfaces Figure 4a. 300 nm-thick SiO2/p+ Si was used as a dielectric layer and back gate electrode. As an advantage of our method, the WOx film can be first prepatterned as the channel. After selenization, Pt electrodes were deposited as the source and drain (see Experimental Section for details). For the case of the S-WSe2, the transfer characteristics are displayed in Figure 4b and c. The Ids − Vds curve at different Vbg biases clearly shows an increase in 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 behavior; however, the P-type behavior 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) (ΔIds/ΔVbg) at the linear region of ΔIds/ΔVbg where Cox, W, and L are gate capacitance (300 nm-thick SiO2), channel width, and length of the device, respectively.10,40 On the basis of the transfer characteristic as shown in Figure 4c, 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 in air at room temperature. Interestingly, devices prepared by the evaporated WOx (E-WSe2) show the slight gate dependence, displaying mostly metallic behavior as shown in Figure S6. Interestingly, no indication of the formation of metallic phases (1T structure) can be formed from XPS or Raman.9,43 Therefore, we suggest that the observed metallic behavior on WSe2 formed by selenization of the E-WSe2 can be caused by 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, the number of WSe2 can be finely tuned. Figure 5a to c displays 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 the weak intensity of the PL with increased number of monolayers. Raman spectroscopy provides a fast and noninvasive 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 5d shows the photograph of 7 to 8 S-WSe2 monolayers deposited on an 8′′ SiO2/Si wafer. The inset in Figure 5d presents the Raman mapping of A1g in a 2 × 2 μm2 area as a proof of the uniformity in the local area. Furthermore, Figure 5e shows the Raman spectra taken at the positions marked in Figure 5d, with which no variation in the A1g intensity at all points as a clear evidence of the macroscopic uniformity. Note that excellent thickness control at few nanometers is required for oxide deposition, and uniform heating and selenium gas distribution are necessary to achieve a few uniform WSe2 monolayers in a 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.

Research Article

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CONCLUSION

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ASSOCIATED CONTENT

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 an increased amount of oxygen vacancies to the semiconductor behavior with good field effect behaviors when the oxide was deposited by reactive sputtering, providing 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 a 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.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b17861. XPS of E-WOx and S-WO x ; 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 ratios; AFM and low magnification TEM of E-WSe2 and S-WSe2; and transfer characteristic of EWSe2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(C.-H.S.) E-mail: [email protected]. *(Y.-L.C.) E-mail: [email protected]. ORCID

Henry Medina: 0000-0003-1461-5703 Arumugam Manikandan: 0000-0001-7284-2002 Yu-Lun Chueh: 0000-0002-0155-9987 Author Contributions #

W.-S.L. and H.M. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the Ministry of Science and Technology through grant numbers 104-2628-M-007-004MY3, 104-2221-E-007-048-MY3, 105-2633-M-007-003, and 105-2119-M-009-009 and the National Tsing Hua University through grant number 106N509CE1. Y.-L.C. greatly appreciates the use of the facility at CNMM. H.M. gratefully acknowledges the support from the Institute of Materials Research and Engineering (IMRE) under the Agency for Science, Technology, and Research (A*STAR) through project number IMRE/15-2C0115. 9650

DOI: 10.1021/acsami.7b17861 ACS Appl. Mater. Interfaces 2018, 10, 9645−9652

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.7b17861 ACS Appl. Mater. Interfaces 2018, 10, 9645−9652