Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Substrate Effects on Growth Dynamics of WTe2 Thin films

Substrate Effects on Growth Dynamics of WTe2 Thin films IntroductionTwo‐dimensional (2D), layered transition metal dichalcogenides (TMDCs) of the form MX2 where M is a transition metal and X is a chalcogen (S, Se, and Te), have been extensively investigated in the last decade due to their reduced dimensionality, correlated electronic phases, and layer‐dependent electrical and chemical properties, which make them attractive for next‐generation electronics,[1] photodetectors,[2] energy storage devices,[3] and catalysis.[4] In recent years, much effort has been put into the scale‐up of TMDCs. Large‐area thin films have been achieved through methods including chemical vapor deposition (CVD),[5,6] metalorganic CVD,[7] atomic layer deposition (ALD),[8] sputtering,[9] molecular beam epitaxy,[10] and others.[11] Of these methods, one promising synthesis route is the direct conversion of metal or metal‐oxide precursor thin films to TMDC films through annealing in a chalcogen‐rich environment in the presence of Ar and H2.[12–14] This direct conversion process has the advantage of thickness‐controlled growth of large‐area TMDC thin films,[15] direct synthesis of lateral and vertical heterostructures,[16] and growth of horizontally or vertically aligned 2D thin films.[17–19] Of the 2D chalcogen thin films, large‐area telluride thin films such as MoTe2, PtTe2, and WTe2, have proven the most difficult to achieve due to the low reactivity between transition metal atoms and tellurium.[11] Nevertheless, these 2D tellurides exhibit many correlated electron transport phenomena, such as large magnetoresistance,[20] superconductivity,[21,22] a quantum spin Hall state,[23] and a type‐II Weyl semi‐metal state,[24] making them attractive for low‐power electronics and interconnect applications. Despite the attractive transport properties and potential device applications, the synthesis of the telluride thin films is much less studied than the semiconducting sulfides and selenides. To enable the scale‐up of these materials for future technologies, understanding of the growth dynamics for large‐area, layer‐controlled synthesis of transition metal telluride thin films is necessary.We have previously demonstrated the large‐area growth of semiconducting 2H MoTe2 thin films with large grains and layer control through the tellurization of atomic layer deposition (ALD) grown amorphous MoOx thin films.[15] We also showed that the choice of growth substrate impacts the phase stability and large‐scale uniformity of MoTe2 monolayer films.[25] The large grains in 2H MoTe2 films were obtained by first synthesizing 1T′ MoTe2 films from the starting oxide film, and then converting the 1T′ MoTe2 to 2H MoTe2. In contrast to MoTe2, WTe2 is stable at room temperature in the Td phase, which is structurally identical to the 1T′ phase in‐plane but with different layer stacking.[26] WTe2 can be stabilized in the 1T′ phase through high‐pressure[27,28] or temperature.[29] The distinction between the 1T′ and Td phases can be made by the splitting of the A12$A_{\bf 1}^2$ peak in Raman spectroscopy.[27,28] There are several synthesis studies on WTe2 which have investigated the reactivity of W with Te through direct deposition of W metal or amorphous WOx.[30–35] In this work, we investigated the role of substrate on the growth of large area, thickness‐controlled WTe2 films, which was not investigated in these prior studies. We synthesize WTe2 by tellurizing ALD‐grown WOx precursor thin films of varying thicknesses using sapphire and SiO2 substrates and focus on the growth of WTe2 with thickness <5 nm to study the effects of the growth substrate on the resulting morphology of WTe2. We find that monolayer WTe2 flakes are observed on sapphire but not on SiO2 for the same WOx film thickness, which is similar to our previous results for the growth of MoTe2 thin films.[15,25] However, unlike MoTe2 that formed continuous films with thickness control, we observe the formation of WTe2 flakes on both sapphire and SiO2 instead of continuous films. We attribute this to the lower reactivity of W with Te and weaker substrate interactions between W and sapphire than between Mo and sapphire, resulting in a greater likelihood of isolated flake formation.Results and DiscussionWTe2 was synthesized by converting WOx films deposited by ALD on c‐plane sapphire and wet thermal oxide SiO2/Si substrates (Methods). For ALD of WOx, W(CO)6 and H2O were used as precursors following the existing literature.[36] As demonstrated in our previous work on the growth of large‐area MoTe2,[15] ALD allows for the deposition of uniform metal‐oxide films across a large‐area, resulting in thickness‐controlled 2D telluride thin films, with repeatable growth results. WOx films with ALD cycles of 300, 200, 50, 30, and 20 were grown to ensure a wide range of thicknesses for WTe2 down to a monolayer. The WOx films were tellurized in a H2/Ar atmosphere at 600 °C (Figure 1a). For WTe2 converted from 300 cycle‐thick WOx on both the sapphire and SiO2 substrates, transmission electron microscope (TEM) images show what appears to be continuous, polycrystalline films with grain size in the range of 10–200 nm (Figure 1b). The acquired selected area electron diffraction pattern matches the expected Td phase of WTe2 (Figure 1c), and the stoichiometry of the film is similar to bulk exfoliated flakes based on energy dispersive X‐ray spectroscopy (EDX) (Figure 1d).1FigureConversion of WOx to WTe2. a) Growth schematic for the synthesis of WTe2 converted from ALD grown WOx on sapphire and SiO2 substrates. b) TEM image of WTe2 converted from 300 cycle‐thick WOx on SiO2. Scale bar, 100 nm. c) Selected area electron diffraction image from b. Dotted semicircles denote diffraction rings for Td WTe2 with Miller indices starting from inner most: (002), (023)/(113), (124)/(116), and (146). d) EDX spectra taken from WTe2 converted from 300 cycle‐thick WOx on SiO2.Raman spectra of Td WTe2 converted from WOx films of varying thicknesses on sapphire and SiO2 are shown in Figure 2. For WTe2 flakes exfoliated from bulk, the A12$A_{\bf 1}^2$, A15$A_{\bf 1}^5$, and A18$A_{\bf 1}^8$ Raman modes shift going from bulk to the monolayer limit, with the A12$A_{\bf 1}^2$ peak showing the largest shift from ≈208.5 to ≈213.5 cm−1.[37] For WTe2 converted on SiO2, the A12$A_{\bf 1}^2$ peak shifts from 208.74 to 210.26 cm−1 as the WOx source thickness decreases from 300 to 50 cycles, where 210.26 cm−1 corresponds to 4–5 layer WTe2.[37] WTe2 converted on sapphire shows a shift in A12$A_{\bf 1}^2$ from 208.4 cm−1 to 214.6 cm−1 as the WOx source thickness decreases from 300 to 30 cycles (Figure 2a). The A12$A_{\bf 1}^2$ peak location of 214.6 cm−1 suggests monolayer formation.[37] Additionally, the A24$A_{\bf 2}^4$ Raman mode centered at ≈109 cm−1 is forbidden in the monolayer limit, and we do not observe this peak for WTe2 converted from 30 cycle WOx on sapphire, further indicating monolayer thickness. We note the absence of Raman signals for WTe2 converted from 30 cycle WOx film on SiO2, which could indicate that it is more favorable to form WTe2 on sapphire than on SiO2 at the monolayer limit. For both substrates, WTe2 was not detected from 20 cycle WOx after the conversion reaction. Overall, Raman measurements show that WTe2 grown on sapphire appears to be thinner than on SiO2 for the same starting oxide thicknesses as the A12$A_{\bf 1}^2$ peak shifts with decreasing thickness of the WOx precursor film for the sapphire substrate but not for SiO2.2FigureRaman spectra of WTe2 films of varying thicknesses. Raman scans of WTe2 converted from WOx films of 300, 200, 50, and 30 cycles grown on sapphire a) and SiO2 b). Bulk exfoliated WTe2 flake (≈30 layers) included for reference. c,d) Peak location shifts for A12\[A_1^2\] (black), A15$A_1^5$ (red), and A18\[A_1^8\] (blue) Raman modes on sapphire c) and SiO2 d). WTe2 were converted at 600 °C for 2 h.The morphology of WTe2 was further investigated using scanning electron microscopy (SEM) (Figure 3). The uniformity of the starting WOx films was verified before tellurization (Figure S1, Supporting Information). Surprisingly, the SEM images show that the conversion does not result in flat and uniform WTe2 films, but rather clusters of WTe2 flakes on both SiO2 and sapphire. As the starting oxide thickness decreases from 300 ALD cycles (Figure 3a,d) to 50 cycles (Figure 3c,f) these clusters become smaller and more spread out (Table S1, Supporting Information). From the intensity contrast in SEM, WTe2 flakes converted on sapphire from 50 cycle WOx appear thinner than WTe2 converted on SiO2 from the same 50 cycle WOx, consistent with the Raman analysis.3FigureSEM images of WTe2 converted from WOx ALD thin films. SEM images of WTe2 converted from (left to right) 300, 200, and 50 cycle ALD WOx on sapphire (a,b,c), and SiO2 (d,e,f). Scale bars, 500 nm. The dark regions represent thin WTe2 flakes in c).For WTe2 converted from 30 cycle WOx on sapphire and SiO2, the charging effect from the insulating substrates made SEM characterization difficult. Thus, morphology and thickness were measured directly through atomic force microscopy (AFM) (Figure 4). For WTe2 converted from 30 cycle WOx on sapphire (Figure 4a), AFM verifies the presence of monolayer flakes (Figure 4a, inset; Figure 4c, purple). In contrast to WTe2 monolayer flakes on sapphire, the conversion reaction from 30 cycle WOx on the SiO2 substrate shows a smooth surface without flakes and a RMS roughness of 0.2 nm (Figure 4b). Coupled with the lack of Raman signal, we conclude that there is no WTe2 present. The morphology of WTe2 converted from thicker WOx films was also investigated with AFM (Figure S2, Supporting Information). For WTe2 converted from thicker WOx films, the WTe2 flakes have a wider distribution of thicknesses; for example, for WTe2 converted from 50 cycle WOx on sapphire, the AFM height profile shows many thin (monolayer to trilayer) flakes but also flakes with thicknesses greater than 4 layers (Figure 3c, green; Figure S2b, Supporting Information). In contrast, the WTe2 flakes converted from 50 cycle WOx on SiO2 (Figure 4c, orange; Supplementary Figure 2a) are much thicker on average than those converted on sapphire with the majority of flakes having thickness greater than 4 layers.4FigureAFM height profiles of WTe2 converted from 50 and 30 cycle WOx. a) AFM image of WTe2 converted from 30 cycle WOx on sapphire with height profile of a monolayer flake (inset). Line scan indicated by solid orange line. b) AFM image of WTe2 converted from 30 cycle WOx on SiO2. Scale bars, 200nm. c) Individually measured flake heights for WTe2 converted from 30 cycle WOx on sapphire (purple), 50 cycle WOx on sapphire (green), and 50 cycle WOx on SiO2 (orange).The thicknesses of WTe2 flakes measured using AFM are consistent with the interpretation of the SEM images and Raman spectra, which show that the sapphire substrate allows for the growth of thinner WTe2 flakes, achieving monolayer WTe2 flakes converted from the 30 cycle WOx films, while the SiO2 substrate does not. This is consistent with our previous work on MoTe2 that showed continuous bilayer films converted from 14 cycle MoOx on sapphire but 4–5 layer‐thick flakes on SiO2.[15] We attributed the difference in the morphology of MoTe2 to the increased wetting of Mo atoms to the more reactive sapphire surface, a result that has also been demonstrated in the growth of WS2.[8] The main distinction between our previous results on MoTe2 and this work on WTe2 is the formation of flakes on sapphire for WTe2 instead of uniform films, which we observed during the growth of MoTe2. The starting morphology (Figure S1, Supporting Information) and W oxidation state (Figure S3, Supporting Information) are the same for WOx films of the same starting thickness on sapphire and SiO2, ruling out effects from differing precursor film quality. One explanation is potential differences in substrate interactions between Mo and W on sapphire. In a recent study, Lalithambika et al. investigated the growth of MoS2 and WS2 on c‐plane sapphire using density functional theory.[38] They found that the desorption barrier is smaller for W than for Mo, and the barrier for surface diffusion is 1.5 times larger for W than for Mo, resulting in step velocities for W two orders of magnitude smaller than Mo. This resulted in the growth of much smaller WS2 flakes than MoS2 flakes (200 nm for WS2 vs 70 µm for MoS2), and longer time for complete surface coverage of WS2 compared to MoS2. The lack of diffusion of W on sapphire compared to Mo can therefore explain the differences in morphology between MoTe2 and WTe2 and why a continuous WTe2 film is not formed, even when converted from the thickest starting WOx film (300 cycles) (Figure 3a,d; Figure S2d, Supporting Information).SEM and AFM characterization clearly shows WTe2 flakes, which is not consistent with the apparent WTe2 film observed using TEM (Figure 1b). Annular dark‐field scanning‐TEM (STEM) images of WTe2 converted from 50 cycle WOx on sapphire shows that some support structure exists to connect the flakes (Supplementary Figure 4). EDX from those support regions show a W signal (Figure S4b, Supporting Information), indicating that these regions may be an unreacted W or WOx layer. To test this hypothesis, depth profile measurements with x‐ray photoelectron spectroscopy (XPS) (Figure 5) were conducted to determine if the films are fully converted to WTe2. Ar etching with a power of 1 kV was used, and W 4d and Te 3d scans were taken at 90 s intervals. For WTe2 on sapphire and SiO2, both W and Te peaks initially show purely W and Te metal peaks, with the W peaks matching that of bulk exfoliated WTe2 (Figure S5a, Supporting Information). After ≈270–360 seconds of etching, we no longer observe any Te signal from either substrate, suggesting that WTe2 is completely etched away. However, a W signal persists until ≈450 s of etching. This suggests that there is an interfacial layer between the substrates and WTe2 (Figure S5b, Supporting Information), which is W and not WOx due to the lack of doublet peaks in the W 4d region. This interfacial layer is also shown to exist for WOx films that were not successfully converted to WTe2 such as 30 cycle WOx on SiO2 (Figure S6, Supporting Information).5FigureXPS depth profile for WTe2 converted from 200 cycle WOx. XPS depth profiles were taken for WTe2 converted from 200 cycle WOx on sapphire a,b), and SiO2 c,d). Starting from the bottom, W 4d and Te 3d scans were taken at 90 s etching intervals. Colored lines indicate when Te (green, red) and W (blue) signals disappear on both substrates.This residual W layer can be attributed to the lower reactivity of W with Te. A previous study by our group on the direct synthesis of WTe2 from sputtered W metal films found that the use of H2 as a carrier gas to form the H2Te intermediate was necessary to achieve a WTe2 film due to the increased reactivity of H2Te with W versus Te with W.[30] This is because the Gibbs free energy of reaction for WTe2 (−26.2 kJ mol−1 at 1100 K)[39] is much smaller than those for other TMDCs such as WS2 (−151.83 kJ mol−1 at 1100 K)[40] and MoTe2 (−64.3 kJ mol−1 at 1100 K),[41] which have both been fully converted from WOx[42] or MoOx[15] without any obvious interfacial layers between the converted films and substrates.ConclusionIn investigating the substrate interactions involved during synthesis of Td WTe2, we find that like MoTe2, sapphire is more advantageous for forming WTe2 in a layer‐by‐layer manner with thinner WTe2 flakes being observed on sapphire versus SiO2 for the same starting oxide thicknesses, culminating in the formation of monolayer WTe2 flakes from 30 cycle WOx on sapphire. We note this is likely due to better diffusion of W on sapphire versus on SiO2, which has been observed in other TMDCs as well.[8] Unlike MoTe2 however, we observe the formation of WTe2 flakes instead of films on both sapphire and SiO2 due to the increased diffusion barrier for W compared to Mo on substrates.[38] Additionally, the weak reactivity of W with Te compared to other TMDCs causes the formation of a thin unreacted W layer at the interface between the WTe2 flakes and the growth substrate, which likely also enhances flake formation due to the difficulty of nucleating WTe2 at the interface. This work shows that the substrate has a definitive impact on the growth of transition metal telluride films synthesized through direct conversion of metal and metal–oxide precursor films, especially at the thin film limit. It also shows that while direct synthesis is a viable method for the layer‐by‐layer growth of TMDCs, more work needs to be done regarding the reactivity of W with Te. Previous attempts have included increasing the Te supply by converting WOx films on NiyTex substrates,[31] using H2O to convert WO3 to WO2(OH),[35] making the precursor film more reactive to form WTe2, and utilizing H2Te.[30] Nevertheless, these methods have proven ineffective for synthesizing large‐area uniform WTe2 at the thin film limit. To achieve layer‐by‐layer growth of high‐quality WTe2 thin films, future studies may consider investigating different growth substrates that allow for enhanced diffusion of W, along with Te sources that exhibit greater reactivity with W.Experimental SectionAtomic Layer Deposition of WOx Films and Substrate PreparationSilicon wet thermal oxide substrates (500 nm, University Wafer) were treated with O2 plasma for 30 min, while the single‐side polished c‐plane sapphire substrates (450 µm, Cryscore Optoelectronic Limited) were cleaned in a modified version of the RCA process.[43] The first step was a soak in an ethanol bath for 12 h at room temperature, followed by a rinse with DI H2O. Samples were then sonicated for 30 min at room temperature in a 1:20:79 solution of detergent:ethanol:DI H2O. Samples were then soaked in a 3:1 H2SO4:H2O2 piranha solution for 20 min at 80 °C, followed with a DI H2O rinse. The substrates were dried with high‐purity nitrogen gas between each step. Atomic layer deposition of WOx films was performed in a FijiTM 200 Gen 2 Plasma Atomic Layer Deposition System at a substrate temperature of 200 °C. W(CO)6 and H2O were used as precursors in accordance with a previously published procedure.[36] Films of different thicknesses were synthesized using cycle numbers of 20, 30, 50, 200, and 300.Conversion of WOx to WTe2WTe2 was synthesized through the annealing of WOx thin films of various thicknesses grown by ALD in a tellurium atmosphere. Te powder (2 g, Sigma–Aldrich, 99.999%) was placed in a 2 inch quartz tube at zone 1 of a two‐zone furnace (MTI OTF‐1200X‐II), while WOx thin films deposited on SiO2 and sapphire substrates were placed downstream in the second zone of the tube furnace. After purging the tube many times with Ar to ensure no residual oxygen was present, the two zones were heated to 570 °C for Te powder and 600 °C for WOx films in 15 min and held there for 2 h. A mixture of H2/Ar was‐flowed at 100/10 sccm at atmospheric pressure during the reaction. After the synthesis was completed, the chamber was purged with 200 sccm Ar gas for 10 min, and then rapidly cooled to room temperature by opening the furnace cover. WOx films with the same starting thickness on different substrates were converted during the same tellurization reaction under the same conditions.Structural and Chemical CharacterizationPlan‐view TEM images were taken using a FEI Titan Themis transmission electron microscope with an image corrector at the Advanced Science Research Center at the City University of New York. To avoid sample damage, TEM images were taken at 80 kV. STEM images and EDX were taken at 200 kV using a Tecnai Osiris microscope at the Yale Institute for Quantum Engineering (YINQE). All plan‐view WTe2 samples were transferred onto carbon‐covered Quantifoil TEM grids with equally spaced 2 µm holes using a hydrofluoric acid lift‐out method. To characterize the surface morphology of the WTe2 samples, SEM and EDX were taken using a Hitachi SU8230 UHR cold field emission scanning electron microscope (Yale West Campus Materials Characterization Core). AFM was conducted using a Cypher ES microscope from Asylum Research. The microscope was operated in peak‐force tapping mode (Yale West Campus Imaging Core). Raman spectroscopy (WITec Confocal Raman Microscope alpha300R) at the Advanced Science Research Center at the City University of New York was used to verify film uniformity and characterize the thickness and phase of WTe2; The laser used was 532 nm, with a laser power ranging from 20–50 mW and a diffraction grating of 1800 g mm−1. Conversion of the films from WOx to WTe2 was characterized through X‐ray photoelectron spectroscopy (XPS) (PHI VersaProbe II) (Yale West Campus Materials Characterization Core).AcknowledgementsD.H. was supported by a NASA Space Technology Research Fellowship (No. 80NSSC19K1131). J.L.H and G. J. were supported by the Moore foundation EPiQS Synthesis Investigator Award. The following user facilities were acknowledged for instrument use, scientific and technical assistance: the Yale West Campus Materials Characterization Core (MCC), the Yale Institute for Nanoscience and Quantum Engineering (YINQE), the Yale West Campus Imaging Core, and the Imaging Facility of Advanced Science Research Center at the Graduate Center of CUNY.Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.S.‐J. Liang, B. Cheng, X. Cui, F. Miao, Adv. Mater. 2019, 32, 1903800.J. Jiang, Y. Wen, H. Wang, L. Yin, R. Cheng, C. Liu, L. Feng, J. He, Adv. Electron. Mater. 2021, 7, 2001125.B. Xu, S. Qi, M. Jin, X. Cai, L. Lai, Z. Sun, X. Han, Z. Lin, H. Shao, P. Peng, Z. Xiang, J. E. ten Elshof, R. Tan, C. Liu, Z. Zhang, X. Duan, J. Ma, Chin. Chem. Lett. 2019, 30, 2053.D. Deng, K. S. Novoselov, Q. Fu, N. Zheng, Z. Tian, X. Bao, Nat. Nanotechnol. 2016, 11, 218.J. Zhou, J. Lin, X. Huang, Y. Zhou, Y. Chen, J. Xia, H. Wang, Y. Xie, H. Yu, J. Lei, D. Wu, F. Liu, Q. Fu, Q. Zeng, C.‐H. Hsu, C. Yang, L. Lu, T. Yu, Z. Shen, H. Lin, B. I. Yakobson, Q. Liu, K. Suenaga, G. Liu, Z. Liu, Nature 2018, 556, 355.X. Zhang, T. H. Choudhury, M. Chubarov, Y. Xiang, B. Jariwala, F. Zhang, N. Alem, G.‐C. Wang, J. A. Robinson, J. M. Redwing, Nano Lett. 2018, 18, 1049.M. Chubarov, T. H. Choudhury, D. R. Hickey, S. Bachu, T. Zhang, A. Sebastian, A. Bansal, H. Zhu, N. Trainor, S. Das, M. Terrones, N. Alem, J. M. Redwing, ACS Nano 2021, 15, 2532.B. Groven, A. Nalin Mehta, H. Bender, J. Meersschaut, T. Nuytten, P. Verdonck, T. Conard, Q. Smets, T. Schram, B. Schoenaers, A. Stesmans, V. Afanas'Ev, W. Vandervorst, M. Heyns, M. Caymax, I. Radu, A. Delabie, Chem. Mater. 2018, 30, 7648.J.‐H. Huang, K.‐Y. Deng, P.‐S. Liu, C.‐T. Wu, C.‐T. Chou, W.‐H. Chang, Y.‐J. Lee, T.‐H. Hou, Adv. Mater. Interfaces 2017, 4, 1700157.Q. He, P. Li, Z. Wu, B. Yuan, Z. Luo, W. Yang, J. Liu, G. Cao, W. Zhang, Y. Shen, P. Zhang, S. Liu, G. Shao, Z. Yao, Adv. Mater. 2019, 31, 1901578.W. Zhuang, Z. Chen, X. Wang, Adv Phys X 2022, 7, 2034529.H. Wang, D. Kong, P. Johanes, J. J. Cha, G. Zheng, K. Yan, N. Liu, Y. Cui, Nano Lett. 2013, 13, 3426.D. Kong, H. Wang, J. J. Cha, M. Pasta, K. J. Koski, J. Yao, Y. Cui, Nano Lett. 2013, 13, 1341.M. S. Shawkat, S. bin Hafiz, M. M. Islam, S. A. Mofid, M. M. al Mahfuz, A. Biswas, H. S. Chung, E. Okogbue, T. J. Ko, D. Chanda, T. Roy, D. K. Ko, Y. Jung, ACS Appl. Mater. Interfaces 2021, 13, 15542.D. J. Hynek, R. M. Singhania, S. Xu, B. Davis, L. Wang, M. Yarali, J. v. Pondick, J. M. Woods, N. C. Strandwitz, J. J. Cha, ACS Nano 2021, 15, 410.J. M. Woods, Y. Jung, Y. Xie, W. Liu, Y. Liu, H. Wang, J. J. Cha, ACS Nano 2016, 10, 2004.S. L. Shang, G. Lindwall, Y. Wang, J. M. Redwing, T. Anderson, Z. K. Liu, Nano Lett. 2016, 16, 5742.Y. Jung, J. Shen, Y. Liu, J. M. Woods, Y. Sun, J. J. Cha, Nano Lett. 2014, 14, 6842.M. Wang, T. J. Ko, M. S. Shawkat, S. S. Han, E. Okogbue, H. S. Chung, T. S. Bae, S. Sattar, J. Gil, C. Noh, K. H. Oh, Y. Jung, J. A. Larsson, Y. Jung, ACS Appl. Mater. Interfaces 2020, 12, 10839.M. N. Ali, J. Xiong, S. Flynn, J. Tao, Q. D. Gibson, L. M. Schoop, T. Liang, N. Haldolaarachchige, M. Hirschberger, N. P. Ong, R. J. Cava, Nature 2014, 514, 205.E. Sajadi, T. Palomaki, Z. Fei, W. Zhao, P. Bement, C. Olsen, S. Luescher, X. Xu, J. A. Folk, D. H. Cobden, Science 2018, 362, 922.D. Kang, Y. Zhou, W. Yi, C. Yang, J. Guo, Y. Shi, S. Zhang, Z. Wang, C. Zhang, S. Jiang, A. Li, K. Yang, Q. Wu, G. Zhang, L. Sun, Z. Zhao, Nat. Commun. 2015, 6, 7804.S. Wu, V. Fatemi, Q. D. Gibson, K. Watanabe, T. Taniguchi, R. J. Cava, P. Jarillo‐Herrero, Science 2018, 359, 76.A. A. Soluyanov, D. Gresch, Z. Wang, Q. Wu, M. Troyer, X. Dai, B. A. Bernevig, Nature 2015, 527, 495.D. J. Hynek, R. M. Singhania, J. L. Hart, B. Davis, M. Wang, N. C. Strandwitz, J. J. Cha, CrystEngComm 2021, 23, 7963.C. H. Lee, E. C. Silva, L. Calderin, M. A. T. Nguyen, M. J. Hollander, B. Bersch, T. E. Mallouk, J. A. Robinson, Sci. Rep. 2015, 5, 10013.Y. Zhou, X. Chen, N. Li, R. Zhang, X. Wang, C. An, Y. Zhou, X. Pan, F. Song, B. Wang, W. Yang, Z. Yang, Y. Zhang, AIP Adv. 2016, 6, 075008.P. Lu, J.‐S. Kim, J. Yang, H. Gao, J. Wu, D. Shao, B. Li, D. Zhou, J. Sun, D. Akinwande, D. Xing, J.‐F. Lin, Phys. Rev. B 2016, 94, 224512.Y. Tao, J. A. Schneeloch, A. A. Aczel, D. Louca, Phys. Rev. B 2020, 102, 060103(R).Y. Zhou, H. Jang, J. M. Woods, Y. Xie, P. Kumaravadivel, G. A. Pan, J. Liu, Y. Liu, D. G. Cahill, J. J. Cha, Adv. Funct. Mater. 2017, 27, 1605928.S. Song, Y. Sim, S.‐Y. Kim, J. H. Kim, I. Oh, W. Na, D. H. Lee, J. Wang, S. Yan, Y. Liu, J. Kwak, J.‐H. Chen, H. Cheong, J.‐W. Yoo, Z. Lee, S.‐Y. Kwon, Nat. Electron. 2020, 3, 207.C. Wang, Y. Sun, S. Huang, Q. Xing, G. Zhang, C. Song, F. Wang, Y. Xie, Y. Lei, Z. Sun, H. Yan, Phys. Rev. Appl. 2021, 15, 14010.Y. Zhang, Z. Wang, J. Feng, S. Ming, F. Qu, Y. Xia, M. He, Z. Hu, J. Wang, Y. Zhang, Z. Wang, J. Feng, S. Ming, F. Qu, Y. Xia, M. He, Z. Hu, J. Wang, J. Semicond. 2022, 43, 102002.C. H. Naylor, W. M. Parkin, Z. Gao, H. Kang, M. Noyan, R. B. Wexler, L. Z. Tan, Y. Kim, C. E. Kehayias, F. Streller, Y. R. Zhou, R. Carpick, Z. Luo, Y. W. Park, A. M. Rappe, M. Drndić, J. M. Kikkawa, A. T. C. Johnson, 2D Mater. 2017, 4, 021008.G. Zhou, H. Gao, J. Li, X. He, Y. He, Y. Li, G. Hao, Nanotechnology 2022, 33, 175602.K. Zhang, C. McCleese, P. Lin, C. Xin, M. G. Morales, W. Cao, F. J. Seo, C. Burda, H. Baumgart, Conf. Metab. Aspects Convalescence, Trans., 17th 2015, 69, 199.Y. C. Jiang, J. Gao, L. Wang, Sci. Rep. 2016, 6, 19624.A. Lalithambika, V. Kranthi Kumar, M. Jain, S. Raghavan, Cryst. Growth Des. 2022, 22, 4708.P. A. G. O'Hare, G. A. Hope, J. Chem. Thermodyn. 1992, 24, 639.P. A. G. O'Hare, J. Chem. Thermodyn. 1987, 19, 675.P. A. G. O'Hare, G. A. Hope, J. Chem. Thermodyn. 1989, 21, 701.A. L. Elías, N. Perea‐López, A. Castro‐Beltrán, A. Berkdemir, R. Lv, S. Feng, A. D. Long, T. Hayashi, Y. A. Kim, M. Endo, H. R. Gutiérrez, N. R. Pradhan, L. Balicas, T. E. Mallouk, F. López‐Urías, H. Terrones, M. Terrones, ACS Nano 2013, 7, 5235.D. Zhang, Y. Wang, Y. Gan, Appl. Surf. Sci. 2013, 274, 405. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Advanced Materials Interfaces Wiley

Substrate Effects on Growth Dynamics of WTe2 Thin films

Download PDF
7 pages

Loading next page...
 
/lp/wiley/substrate-effects-on-growth-dynamics-of-wte2-thin-films-bdJBUm4Y4g

References (37)

Publisher
Wiley
Copyright
© 2023 Wiley‐VCH GmbH
eISSN
2196-7350
DOI
10.1002/admi.202202397
Publisher site
See Article on Publisher Site

Abstract

IntroductionTwo‐dimensional (2D), layered transition metal dichalcogenides (TMDCs) of the form MX2 where M is a transition metal and X is a chalcogen (S, Se, and Te), have been extensively investigated in the last decade due to their reduced dimensionality, correlated electronic phases, and layer‐dependent electrical and chemical properties, which make them attractive for next‐generation electronics,[1] photodetectors,[2] energy storage devices,[3] and catalysis.[4] In recent years, much effort has been put into the scale‐up of TMDCs. Large‐area thin films have been achieved through methods including chemical vapor deposition (CVD),[5,6] metalorganic CVD,[7] atomic layer deposition (ALD),[8] sputtering,[9] molecular beam epitaxy,[10] and others.[11] Of these methods, one promising synthesis route is the direct conversion of metal or metal‐oxide precursor thin films to TMDC films through annealing in a chalcogen‐rich environment in the presence of Ar and H2.[12–14] This direct conversion process has the advantage of thickness‐controlled growth of large‐area TMDC thin films,[15] direct synthesis of lateral and vertical heterostructures,[16] and growth of horizontally or vertically aligned 2D thin films.[17–19] Of the 2D chalcogen thin films, large‐area telluride thin films such as MoTe2, PtTe2, and WTe2, have proven the most difficult to achieve due to the low reactivity between transition metal atoms and tellurium.[11] Nevertheless, these 2D tellurides exhibit many correlated electron transport phenomena, such as large magnetoresistance,[20] superconductivity,[21,22] a quantum spin Hall state,[23] and a type‐II Weyl semi‐metal state,[24] making them attractive for low‐power electronics and interconnect applications. Despite the attractive transport properties and potential device applications, the synthesis of the telluride thin films is much less studied than the semiconducting sulfides and selenides. To enable the scale‐up of these materials for future technologies, understanding of the growth dynamics for large‐area, layer‐controlled synthesis of transition metal telluride thin films is necessary.We have previously demonstrated the large‐area growth of semiconducting 2H MoTe2 thin films with large grains and layer control through the tellurization of atomic layer deposition (ALD) grown amorphous MoOx thin films.[15] We also showed that the choice of growth substrate impacts the phase stability and large‐scale uniformity of MoTe2 monolayer films.[25] The large grains in 2H MoTe2 films were obtained by first synthesizing 1T′ MoTe2 films from the starting oxide film, and then converting the 1T′ MoTe2 to 2H MoTe2. In contrast to MoTe2, WTe2 is stable at room temperature in the Td phase, which is structurally identical to the 1T′ phase in‐plane but with different layer stacking.[26] WTe2 can be stabilized in the 1T′ phase through high‐pressure[27,28] or temperature.[29] The distinction between the 1T′ and Td phases can be made by the splitting of the A12$A_{\bf 1}^2$ peak in Raman spectroscopy.[27,28] There are several synthesis studies on WTe2 which have investigated the reactivity of W with Te through direct deposition of W metal or amorphous WOx.[30–35] In this work, we investigated the role of substrate on the growth of large area, thickness‐controlled WTe2 films, which was not investigated in these prior studies. We synthesize WTe2 by tellurizing ALD‐grown WOx precursor thin films of varying thicknesses using sapphire and SiO2 substrates and focus on the growth of WTe2 with thickness <5 nm to study the effects of the growth substrate on the resulting morphology of WTe2. We find that monolayer WTe2 flakes are observed on sapphire but not on SiO2 for the same WOx film thickness, which is similar to our previous results for the growth of MoTe2 thin films.[15,25] However, unlike MoTe2 that formed continuous films with thickness control, we observe the formation of WTe2 flakes on both sapphire and SiO2 instead of continuous films. We attribute this to the lower reactivity of W with Te and weaker substrate interactions between W and sapphire than between Mo and sapphire, resulting in a greater likelihood of isolated flake formation.Results and DiscussionWTe2 was synthesized by converting WOx films deposited by ALD on c‐plane sapphire and wet thermal oxide SiO2/Si substrates (Methods). For ALD of WOx, W(CO)6 and H2O were used as precursors following the existing literature.[36] As demonstrated in our previous work on the growth of large‐area MoTe2,[15] ALD allows for the deposition of uniform metal‐oxide films across a large‐area, resulting in thickness‐controlled 2D telluride thin films, with repeatable growth results. WOx films with ALD cycles of 300, 200, 50, 30, and 20 were grown to ensure a wide range of thicknesses for WTe2 down to a monolayer. The WOx films were tellurized in a H2/Ar atmosphere at 600 °C (Figure 1a). For WTe2 converted from 300 cycle‐thick WOx on both the sapphire and SiO2 substrates, transmission electron microscope (TEM) images show what appears to be continuous, polycrystalline films with grain size in the range of 10–200 nm (Figure 1b). The acquired selected area electron diffraction pattern matches the expected Td phase of WTe2 (Figure 1c), and the stoichiometry of the film is similar to bulk exfoliated flakes based on energy dispersive X‐ray spectroscopy (EDX) (Figure 1d).1FigureConversion of WOx to WTe2. a) Growth schematic for the synthesis of WTe2 converted from ALD grown WOx on sapphire and SiO2 substrates. b) TEM image of WTe2 converted from 300 cycle‐thick WOx on SiO2. Scale bar, 100 nm. c) Selected area electron diffraction image from b. Dotted semicircles denote diffraction rings for Td WTe2 with Miller indices starting from inner most: (002), (023)/(113), (124)/(116), and (146). d) EDX spectra taken from WTe2 converted from 300 cycle‐thick WOx on SiO2.Raman spectra of Td WTe2 converted from WOx films of varying thicknesses on sapphire and SiO2 are shown in Figure 2. For WTe2 flakes exfoliated from bulk, the A12$A_{\bf 1}^2$, A15$A_{\bf 1}^5$, and A18$A_{\bf 1}^8$ Raman modes shift going from bulk to the monolayer limit, with the A12$A_{\bf 1}^2$ peak showing the largest shift from ≈208.5 to ≈213.5 cm−1.[37] For WTe2 converted on SiO2, the A12$A_{\bf 1}^2$ peak shifts from 208.74 to 210.26 cm−1 as the WOx source thickness decreases from 300 to 50 cycles, where 210.26 cm−1 corresponds to 4–5 layer WTe2.[37] WTe2 converted on sapphire shows a shift in A12$A_{\bf 1}^2$ from 208.4 cm−1 to 214.6 cm−1 as the WOx source thickness decreases from 300 to 30 cycles (Figure 2a). The A12$A_{\bf 1}^2$ peak location of 214.6 cm−1 suggests monolayer formation.[37] Additionally, the A24$A_{\bf 2}^4$ Raman mode centered at ≈109 cm−1 is forbidden in the monolayer limit, and we do not observe this peak for WTe2 converted from 30 cycle WOx on sapphire, further indicating monolayer thickness. We note the absence of Raman signals for WTe2 converted from 30 cycle WOx film on SiO2, which could indicate that it is more favorable to form WTe2 on sapphire than on SiO2 at the monolayer limit. For both substrates, WTe2 was not detected from 20 cycle WOx after the conversion reaction. Overall, Raman measurements show that WTe2 grown on sapphire appears to be thinner than on SiO2 for the same starting oxide thicknesses as the A12$A_{\bf 1}^2$ peak shifts with decreasing thickness of the WOx precursor film for the sapphire substrate but not for SiO2.2FigureRaman spectra of WTe2 films of varying thicknesses. Raman scans of WTe2 converted from WOx films of 300, 200, 50, and 30 cycles grown on sapphire a) and SiO2 b). Bulk exfoliated WTe2 flake (≈30 layers) included for reference. c,d) Peak location shifts for A12\[A_1^2\] (black), A15$A_1^5$ (red), and A18\[A_1^8\] (blue) Raman modes on sapphire c) and SiO2 d). WTe2 were converted at 600 °C for 2 h.The morphology of WTe2 was further investigated using scanning electron microscopy (SEM) (Figure 3). The uniformity of the starting WOx films was verified before tellurization (Figure S1, Supporting Information). Surprisingly, the SEM images show that the conversion does not result in flat and uniform WTe2 films, but rather clusters of WTe2 flakes on both SiO2 and sapphire. As the starting oxide thickness decreases from 300 ALD cycles (Figure 3a,d) to 50 cycles (Figure 3c,f) these clusters become smaller and more spread out (Table S1, Supporting Information). From the intensity contrast in SEM, WTe2 flakes converted on sapphire from 50 cycle WOx appear thinner than WTe2 converted on SiO2 from the same 50 cycle WOx, consistent with the Raman analysis.3FigureSEM images of WTe2 converted from WOx ALD thin films. SEM images of WTe2 converted from (left to right) 300, 200, and 50 cycle ALD WOx on sapphire (a,b,c), and SiO2 (d,e,f). Scale bars, 500 nm. The dark regions represent thin WTe2 flakes in c).For WTe2 converted from 30 cycle WOx on sapphire and SiO2, the charging effect from the insulating substrates made SEM characterization difficult. Thus, morphology and thickness were measured directly through atomic force microscopy (AFM) (Figure 4). For WTe2 converted from 30 cycle WOx on sapphire (Figure 4a), AFM verifies the presence of monolayer flakes (Figure 4a, inset; Figure 4c, purple). In contrast to WTe2 monolayer flakes on sapphire, the conversion reaction from 30 cycle WOx on the SiO2 substrate shows a smooth surface without flakes and a RMS roughness of 0.2 nm (Figure 4b). Coupled with the lack of Raman signal, we conclude that there is no WTe2 present. The morphology of WTe2 converted from thicker WOx films was also investigated with AFM (Figure S2, Supporting Information). For WTe2 converted from thicker WOx films, the WTe2 flakes have a wider distribution of thicknesses; for example, for WTe2 converted from 50 cycle WOx on sapphire, the AFM height profile shows many thin (monolayer to trilayer) flakes but also flakes with thicknesses greater than 4 layers (Figure 3c, green; Figure S2b, Supporting Information). In contrast, the WTe2 flakes converted from 50 cycle WOx on SiO2 (Figure 4c, orange; Supplementary Figure 2a) are much thicker on average than those converted on sapphire with the majority of flakes having thickness greater than 4 layers.4FigureAFM height profiles of WTe2 converted from 50 and 30 cycle WOx. a) AFM image of WTe2 converted from 30 cycle WOx on sapphire with height profile of a monolayer flake (inset). Line scan indicated by solid orange line. b) AFM image of WTe2 converted from 30 cycle WOx on SiO2. Scale bars, 200nm. c) Individually measured flake heights for WTe2 converted from 30 cycle WOx on sapphire (purple), 50 cycle WOx on sapphire (green), and 50 cycle WOx on SiO2 (orange).The thicknesses of WTe2 flakes measured using AFM are consistent with the interpretation of the SEM images and Raman spectra, which show that the sapphire substrate allows for the growth of thinner WTe2 flakes, achieving monolayer WTe2 flakes converted from the 30 cycle WOx films, while the SiO2 substrate does not. This is consistent with our previous work on MoTe2 that showed continuous bilayer films converted from 14 cycle MoOx on sapphire but 4–5 layer‐thick flakes on SiO2.[15] We attributed the difference in the morphology of MoTe2 to the increased wetting of Mo atoms to the more reactive sapphire surface, a result that has also been demonstrated in the growth of WS2.[8] The main distinction between our previous results on MoTe2 and this work on WTe2 is the formation of flakes on sapphire for WTe2 instead of uniform films, which we observed during the growth of MoTe2. The starting morphology (Figure S1, Supporting Information) and W oxidation state (Figure S3, Supporting Information) are the same for WOx films of the same starting thickness on sapphire and SiO2, ruling out effects from differing precursor film quality. One explanation is potential differences in substrate interactions between Mo and W on sapphire. In a recent study, Lalithambika et al. investigated the growth of MoS2 and WS2 on c‐plane sapphire using density functional theory.[38] They found that the desorption barrier is smaller for W than for Mo, and the barrier for surface diffusion is 1.5 times larger for W than for Mo, resulting in step velocities for W two orders of magnitude smaller than Mo. This resulted in the growth of much smaller WS2 flakes than MoS2 flakes (200 nm for WS2 vs 70 µm for MoS2), and longer time for complete surface coverage of WS2 compared to MoS2. The lack of diffusion of W on sapphire compared to Mo can therefore explain the differences in morphology between MoTe2 and WTe2 and why a continuous WTe2 film is not formed, even when converted from the thickest starting WOx film (300 cycles) (Figure 3a,d; Figure S2d, Supporting Information).SEM and AFM characterization clearly shows WTe2 flakes, which is not consistent with the apparent WTe2 film observed using TEM (Figure 1b). Annular dark‐field scanning‐TEM (STEM) images of WTe2 converted from 50 cycle WOx on sapphire shows that some support structure exists to connect the flakes (Supplementary Figure 4). EDX from those support regions show a W signal (Figure S4b, Supporting Information), indicating that these regions may be an unreacted W or WOx layer. To test this hypothesis, depth profile measurements with x‐ray photoelectron spectroscopy (XPS) (Figure 5) were conducted to determine if the films are fully converted to WTe2. Ar etching with a power of 1 kV was used, and W 4d and Te 3d scans were taken at 90 s intervals. For WTe2 on sapphire and SiO2, both W and Te peaks initially show purely W and Te metal peaks, with the W peaks matching that of bulk exfoliated WTe2 (Figure S5a, Supporting Information). After ≈270–360 seconds of etching, we no longer observe any Te signal from either substrate, suggesting that WTe2 is completely etched away. However, a W signal persists until ≈450 s of etching. This suggests that there is an interfacial layer between the substrates and WTe2 (Figure S5b, Supporting Information), which is W and not WOx due to the lack of doublet peaks in the W 4d region. This interfacial layer is also shown to exist for WOx films that were not successfully converted to WTe2 such as 30 cycle WOx on SiO2 (Figure S6, Supporting Information).5FigureXPS depth profile for WTe2 converted from 200 cycle WOx. XPS depth profiles were taken for WTe2 converted from 200 cycle WOx on sapphire a,b), and SiO2 c,d). Starting from the bottom, W 4d and Te 3d scans were taken at 90 s etching intervals. Colored lines indicate when Te (green, red) and W (blue) signals disappear on both substrates.This residual W layer can be attributed to the lower reactivity of W with Te. A previous study by our group on the direct synthesis of WTe2 from sputtered W metal films found that the use of H2 as a carrier gas to form the H2Te intermediate was necessary to achieve a WTe2 film due to the increased reactivity of H2Te with W versus Te with W.[30] This is because the Gibbs free energy of reaction for WTe2 (−26.2 kJ mol−1 at 1100 K)[39] is much smaller than those for other TMDCs such as WS2 (−151.83 kJ mol−1 at 1100 K)[40] and MoTe2 (−64.3 kJ mol−1 at 1100 K),[41] which have both been fully converted from WOx[42] or MoOx[15] without any obvious interfacial layers between the converted films and substrates.ConclusionIn investigating the substrate interactions involved during synthesis of Td WTe2, we find that like MoTe2, sapphire is more advantageous for forming WTe2 in a layer‐by‐layer manner with thinner WTe2 flakes being observed on sapphire versus SiO2 for the same starting oxide thicknesses, culminating in the formation of monolayer WTe2 flakes from 30 cycle WOx on sapphire. We note this is likely due to better diffusion of W on sapphire versus on SiO2, which has been observed in other TMDCs as well.[8] Unlike MoTe2 however, we observe the formation of WTe2 flakes instead of films on both sapphire and SiO2 due to the increased diffusion barrier for W compared to Mo on substrates.[38] Additionally, the weak reactivity of W with Te compared to other TMDCs causes the formation of a thin unreacted W layer at the interface between the WTe2 flakes and the growth substrate, which likely also enhances flake formation due to the difficulty of nucleating WTe2 at the interface. This work shows that the substrate has a definitive impact on the growth of transition metal telluride films synthesized through direct conversion of metal and metal–oxide precursor films, especially at the thin film limit. It also shows that while direct synthesis is a viable method for the layer‐by‐layer growth of TMDCs, more work needs to be done regarding the reactivity of W with Te. Previous attempts have included increasing the Te supply by converting WOx films on NiyTex substrates,[31] using H2O to convert WO3 to WO2(OH),[35] making the precursor film more reactive to form WTe2, and utilizing H2Te.[30] Nevertheless, these methods have proven ineffective for synthesizing large‐area uniform WTe2 at the thin film limit. To achieve layer‐by‐layer growth of high‐quality WTe2 thin films, future studies may consider investigating different growth substrates that allow for enhanced diffusion of W, along with Te sources that exhibit greater reactivity with W.Experimental SectionAtomic Layer Deposition of WOx Films and Substrate PreparationSilicon wet thermal oxide substrates (500 nm, University Wafer) were treated with O2 plasma for 30 min, while the single‐side polished c‐plane sapphire substrates (450 µm, Cryscore Optoelectronic Limited) were cleaned in a modified version of the RCA process.[43] The first step was a soak in an ethanol bath for 12 h at room temperature, followed by a rinse with DI H2O. Samples were then sonicated for 30 min at room temperature in a 1:20:79 solution of detergent:ethanol:DI H2O. Samples were then soaked in a 3:1 H2SO4:H2O2 piranha solution for 20 min at 80 °C, followed with a DI H2O rinse. The substrates were dried with high‐purity nitrogen gas between each step. Atomic layer deposition of WOx films was performed in a FijiTM 200 Gen 2 Plasma Atomic Layer Deposition System at a substrate temperature of 200 °C. W(CO)6 and H2O were used as precursors in accordance with a previously published procedure.[36] Films of different thicknesses were synthesized using cycle numbers of 20, 30, 50, 200, and 300.Conversion of WOx to WTe2WTe2 was synthesized through the annealing of WOx thin films of various thicknesses grown by ALD in a tellurium atmosphere. Te powder (2 g, Sigma–Aldrich, 99.999%) was placed in a 2 inch quartz tube at zone 1 of a two‐zone furnace (MTI OTF‐1200X‐II), while WOx thin films deposited on SiO2 and sapphire substrates were placed downstream in the second zone of the tube furnace. After purging the tube many times with Ar to ensure no residual oxygen was present, the two zones were heated to 570 °C for Te powder and 600 °C for WOx films in 15 min and held there for 2 h. A mixture of H2/Ar was‐flowed at 100/10 sccm at atmospheric pressure during the reaction. After the synthesis was completed, the chamber was purged with 200 sccm Ar gas for 10 min, and then rapidly cooled to room temperature by opening the furnace cover. WOx films with the same starting thickness on different substrates were converted during the same tellurization reaction under the same conditions.Structural and Chemical CharacterizationPlan‐view TEM images were taken using a FEI Titan Themis transmission electron microscope with an image corrector at the Advanced Science Research Center at the City University of New York. To avoid sample damage, TEM images were taken at 80 kV. STEM images and EDX were taken at 200 kV using a Tecnai Osiris microscope at the Yale Institute for Quantum Engineering (YINQE). All plan‐view WTe2 samples were transferred onto carbon‐covered Quantifoil TEM grids with equally spaced 2 µm holes using a hydrofluoric acid lift‐out method. To characterize the surface morphology of the WTe2 samples, SEM and EDX were taken using a Hitachi SU8230 UHR cold field emission scanning electron microscope (Yale West Campus Materials Characterization Core). AFM was conducted using a Cypher ES microscope from Asylum Research. The microscope was operated in peak‐force tapping mode (Yale West Campus Imaging Core). Raman spectroscopy (WITec Confocal Raman Microscope alpha300R) at the Advanced Science Research Center at the City University of New York was used to verify film uniformity and characterize the thickness and phase of WTe2; The laser used was 532 nm, with a laser power ranging from 20–50 mW and a diffraction grating of 1800 g mm−1. Conversion of the films from WOx to WTe2 was characterized through X‐ray photoelectron spectroscopy (XPS) (PHI VersaProbe II) (Yale West Campus Materials Characterization Core).AcknowledgementsD.H. was supported by a NASA Space Technology Research Fellowship (No. 80NSSC19K1131). J.L.H and G. J. were supported by the Moore foundation EPiQS Synthesis Investigator Award. The following user facilities were acknowledged for instrument use, scientific and technical assistance: the Yale West Campus Materials Characterization Core (MCC), the Yale Institute for Nanoscience and Quantum Engineering (YINQE), the Yale West Campus Imaging Core, and the Imaging Facility of Advanced Science Research Center at the Graduate Center of CUNY.Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.S.‐J. Liang, B. Cheng, X. Cui, F. Miao, Adv. Mater. 2019, 32, 1903800.J. Jiang, Y. Wen, H. Wang, L. Yin, R. Cheng, C. Liu, L. Feng, J. He, Adv. Electron. Mater. 2021, 7, 2001125.B. Xu, S. Qi, M. Jin, X. Cai, L. Lai, Z. Sun, X. Han, Z. Lin, H. Shao, P. Peng, Z. Xiang, J. E. ten Elshof, R. Tan, C. Liu, Z. Zhang, X. Duan, J. Ma, Chin. Chem. Lett. 2019, 30, 2053.D. Deng, K. S. Novoselov, Q. Fu, N. Zheng, Z. Tian, X. Bao, Nat. Nanotechnol. 2016, 11, 218.J. Zhou, J. Lin, X. Huang, Y. Zhou, Y. Chen, J. Xia, H. Wang, Y. Xie, H. Yu, J. Lei, D. Wu, F. Liu, Q. Fu, Q. Zeng, C.‐H. Hsu, C. Yang, L. Lu, T. Yu, Z. Shen, H. Lin, B. I. Yakobson, Q. Liu, K. Suenaga, G. Liu, Z. Liu, Nature 2018, 556, 355.X. Zhang, T. H. Choudhury, M. Chubarov, Y. Xiang, B. Jariwala, F. Zhang, N. Alem, G.‐C. Wang, J. A. Robinson, J. M. Redwing, Nano Lett. 2018, 18, 1049.M. Chubarov, T. H. Choudhury, D. R. Hickey, S. Bachu, T. Zhang, A. Sebastian, A. Bansal, H. Zhu, N. Trainor, S. Das, M. Terrones, N. Alem, J. M. Redwing, ACS Nano 2021, 15, 2532.B. Groven, A. Nalin Mehta, H. Bender, J. Meersschaut, T. Nuytten, P. Verdonck, T. Conard, Q. Smets, T. Schram, B. Schoenaers, A. Stesmans, V. Afanas'Ev, W. Vandervorst, M. Heyns, M. Caymax, I. Radu, A. Delabie, Chem. Mater. 2018, 30, 7648.J.‐H. Huang, K.‐Y. Deng, P.‐S. Liu, C.‐T. Wu, C.‐T. Chou, W.‐H. Chang, Y.‐J. Lee, T.‐H. Hou, Adv. Mater. Interfaces 2017, 4, 1700157.Q. He, P. Li, Z. Wu, B. Yuan, Z. Luo, W. Yang, J. Liu, G. Cao, W. Zhang, Y. Shen, P. Zhang, S. Liu, G. Shao, Z. Yao, Adv. Mater. 2019, 31, 1901578.W. Zhuang, Z. Chen, X. Wang, Adv Phys X 2022, 7, 2034529.H. Wang, D. Kong, P. Johanes, J. J. Cha, G. Zheng, K. Yan, N. Liu, Y. Cui, Nano Lett. 2013, 13, 3426.D. Kong, H. Wang, J. J. Cha, M. Pasta, K. J. Koski, J. Yao, Y. Cui, Nano Lett. 2013, 13, 1341.M. S. Shawkat, S. bin Hafiz, M. M. Islam, S. A. Mofid, M. M. al Mahfuz, A. Biswas, H. S. Chung, E. Okogbue, T. J. Ko, D. Chanda, T. Roy, D. K. Ko, Y. Jung, ACS Appl. Mater. Interfaces 2021, 13, 15542.D. J. Hynek, R. M. Singhania, S. Xu, B. Davis, L. Wang, M. Yarali, J. v. Pondick, J. M. Woods, N. C. Strandwitz, J. J. Cha, ACS Nano 2021, 15, 410.J. M. Woods, Y. Jung, Y. Xie, W. Liu, Y. Liu, H. Wang, J. J. Cha, ACS Nano 2016, 10, 2004.S. L. Shang, G. Lindwall, Y. Wang, J. M. Redwing, T. Anderson, Z. K. Liu, Nano Lett. 2016, 16, 5742.Y. Jung, J. Shen, Y. Liu, J. M. Woods, Y. Sun, J. J. Cha, Nano Lett. 2014, 14, 6842.M. Wang, T. J. Ko, M. S. Shawkat, S. S. Han, E. Okogbue, H. S. Chung, T. S. Bae, S. Sattar, J. Gil, C. Noh, K. H. Oh, Y. Jung, J. A. Larsson, Y. Jung, ACS Appl. Mater. Interfaces 2020, 12, 10839.M. N. Ali, J. Xiong, S. Flynn, J. Tao, Q. D. Gibson, L. M. Schoop, T. Liang, N. Haldolaarachchige, M. Hirschberger, N. P. Ong, R. J. Cava, Nature 2014, 514, 205.E. Sajadi, T. Palomaki, Z. Fei, W. Zhao, P. Bement, C. Olsen, S. Luescher, X. Xu, J. A. Folk, D. H. Cobden, Science 2018, 362, 922.D. Kang, Y. Zhou, W. Yi, C. Yang, J. Guo, Y. Shi, S. Zhang, Z. Wang, C. Zhang, S. Jiang, A. Li, K. Yang, Q. Wu, G. Zhang, L. Sun, Z. Zhao, Nat. Commun. 2015, 6, 7804.S. Wu, V. Fatemi, Q. D. Gibson, K. Watanabe, T. Taniguchi, R. J. Cava, P. Jarillo‐Herrero, Science 2018, 359, 76.A. A. Soluyanov, D. Gresch, Z. Wang, Q. Wu, M. Troyer, X. Dai, B. A. Bernevig, Nature 2015, 527, 495.D. J. Hynek, R. M. Singhania, J. L. Hart, B. Davis, M. Wang, N. C. Strandwitz, J. J. Cha, CrystEngComm 2021, 23, 7963.C. H. Lee, E. C. Silva, L. Calderin, M. A. T. Nguyen, M. J. Hollander, B. Bersch, T. E. Mallouk, J. A. Robinson, Sci. Rep. 2015, 5, 10013.Y. Zhou, X. Chen, N. Li, R. Zhang, X. Wang, C. An, Y. Zhou, X. Pan, F. Song, B. Wang, W. Yang, Z. Yang, Y. Zhang, AIP Adv. 2016, 6, 075008.P. Lu, J.‐S. Kim, J. Yang, H. Gao, J. Wu, D. Shao, B. Li, D. Zhou, J. Sun, D. Akinwande, D. Xing, J.‐F. Lin, Phys. Rev. B 2016, 94, 224512.Y. Tao, J. A. Schneeloch, A. A. Aczel, D. Louca, Phys. Rev. B 2020, 102, 060103(R).Y. Zhou, H. Jang, J. M. Woods, Y. Xie, P. Kumaravadivel, G. A. Pan, J. Liu, Y. Liu, D. G. Cahill, J. J. Cha, Adv. Funct. Mater. 2017, 27, 1605928.S. Song, Y. Sim, S.‐Y. Kim, J. H. Kim, I. Oh, W. Na, D. H. Lee, J. Wang, S. Yan, Y. Liu, J. Kwak, J.‐H. Chen, H. Cheong, J.‐W. Yoo, Z. Lee, S.‐Y. Kwon, Nat. Electron. 2020, 3, 207.C. Wang, Y. Sun, S. Huang, Q. Xing, G. Zhang, C. Song, F. Wang, Y. Xie, Y. Lei, Z. Sun, H. Yan, Phys. Rev. Appl. 2021, 15, 14010.Y. Zhang, Z. Wang, J. Feng, S. Ming, F. Qu, Y. Xia, M. He, Z. Hu, J. Wang, Y. Zhang, Z. Wang, J. Feng, S. Ming, F. Qu, Y. Xia, M. He, Z. Hu, J. Wang, J. Semicond. 2022, 43, 102002.C. H. Naylor, W. M. Parkin, Z. Gao, H. Kang, M. Noyan, R. B. Wexler, L. Z. Tan, Y. Kim, C. E. Kehayias, F. Streller, Y. R. Zhou, R. Carpick, Z. Luo, Y. W. Park, A. M. Rappe, M. Drndić, J. M. Kikkawa, A. T. C. Johnson, 2D Mater. 2017, 4, 021008.G. Zhou, H. Gao, J. Li, X. He, Y. He, Y. Li, G. Hao, Nanotechnology 2022, 33, 175602.K. Zhang, C. McCleese, P. Lin, C. Xin, M. G. Morales, W. Cao, F. J. Seo, C. Burda, H. Baumgart, Conf. Metab. Aspects Convalescence, Trans., 17th 2015, 69, 199.Y. C. Jiang, J. Gao, L. Wang, Sci. Rep. 2016, 6, 19624.A. Lalithambika, V. Kranthi Kumar, M. Jain, S. Raghavan, Cryst. Growth Des. 2022, 22, 4708.P. A. G. O'Hare, G. A. Hope, J. Chem. Thermodyn. 1992, 24, 639.P. A. G. O'Hare, J. Chem. Thermodyn. 1987, 19, 675.P. A. G. O'Hare, G. A. Hope, J. Chem. Thermodyn. 1989, 21, 701.A. L. Elías, N. Perea‐López, A. Castro‐Beltrán, A. Berkdemir, R. Lv, S. Feng, A. D. Long, T. Hayashi, Y. A. Kim, M. Endo, H. R. Gutiérrez, N. R. Pradhan, L. Balicas, T. E. Mallouk, F. López‐Urías, H. Terrones, M. Terrones, ACS Nano 2013, 7, 5235.D. Zhang, Y. Wang, Y. Gan, Appl. Surf. Sci. 2013, 274, 405.

Journal

Advanced Materials Interfaces – Wiley

Published: Apr 1, 2023

Keywords: 2D materials; atomic layer deposition; thin‐film synthesis; WTe 2

There are no references for this article.