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1 Supporting Information Large-Area, Transfer-Free, Oxide-Assisted Synthesis of Hexagonal Boron Nitride Films and Their Heterostructures with MoS2 and WS2 Sanjay Behura, Phong Nguyen, Songwei Che, Rousan Debbarma, and Vikas Berry* Department of Chemical Engineering, University of Illinois at Chicago, IL 60607, United States * Table of Contents: 1. Surface characteristics of piranha treated SiO 2/Si surfaces 2. Chemical vapor deposition thermal processing conditions for the growth of hexagonal-boron nitride 3. Relationship between mass transport coefficient and diffusion coefficient 4. Grain size of hexagonal-boron nitride 5. Raman characteristics of hexagonal-boron nitride growth on Si surfaces 6. X-ray diffraction and X-ray photoelectron spectroscopic characteristics of hexagonal-boron nitride surfaces 7. Optimizing the chemical vapor deposition parameters for the control of layer numbers of hexagonal-boron nitride 8. Chemical vapor deposition processes for the growth of transition metal dichalcogenides on hexagonal boron nitride surfaces 7.1 Low-Pressure and Moderate-Temperature Synthesis of MoS 2/h-BN Heterostructure 7.2 Atmospheric-Pressure and Low-Temperature Synthesis of WS 2/h-BN Heterostructure 9. Raman characteristics of MoS 2/h-BN Heterostructure 10. Raman characteristics of WS 2/h-BN Heterostructure S1

2 1. Surface characteristics of piranha treated SiO 2/Si surfaces: The surface characterizations of piranha treated SiO 2/Si substrates were performed via X-ray photoelectron spectroscopy (XPS) and contact angle measurement. XPS analysis is essential to understand the surface chemistry of the SiO 2/Si and to identify any functional groups added on the surfaces. The XPS survey spectrum of SiO 2/Si shows Si 2s, Si 2p, O 1s and O 2s states (Figure S-1 (a)), the Si 2p state (103.7 ev) 1 displayed in Figure S-1 (b), and the O1s state (532.7 ev) 1 displayed in Figure S-1 (c). Furthermore, the degree of wettability of the SiO 2/Si surface was verified by the measurement of contact angle, a non-destructive surface analysis tool. As presented in Figure S-1 (d), a contact angle of 41 o is measured by placing water micro-droplet on the surface of SiO 2/Si, which clearly indicates that the surface is hydrophilic. This is also to be noted that the wettability of SiO 2/Si surface depends on its roughness, homogeneity and different chemistries. The measurement of contact angle of 41 o indicates that the surface of SiO 2/Si contains oxide groups even after piranha treatment. Figure S-1: Surface characteristics of piranha treated SiO 2/Si. XPS spectra corresponds to (a) survey scan, (b) Si 2p, (c) O 1s and contact angle corresponds to (d) SiO 2/Si substrate. S2

3 2. Chemical vapor deposition thermal processing conditions for the growth of hexagonal-boron nitride: The thermal chemical vapor deposition (CVD) parameters for the direct growth of hexagonal boron nitride (h-bn) is displayed in Figure S-2. Figure S-2: The CVD thermal processing conditions for the growth of h-bn. 3. Relationship between mass transport coefficient and diffusion coefficient: Flux transport: F mass transport= h g (C g - C S). (1) Where h g is mass transfer coefficient, C g is the concentration of gas in the bulk, and C S is the concentration of the active species at the surface. Fick s first law of diffusion: F = -D g C = D g (C g - C S)/ δ (2) Where D g is diffusivity factor, C g is the concentration of gas in the bulk, C S is the concentration of the active species at the surface, and δ is the boundary layer thickness. We have the relationship as: F = h g (C g-c S) = D g (C g-c S)/ δ h g = D g/ δ (3) S3

4 4. Grain-Size of h-bn: To determine the grain size of h-bn, we synthesized lateral heterostructures of h- BN (incompletely-grown) and graphene; and performed Raman scanning. [Process: A controlled experiment of CVD synthesis of graphene directly on h-bn is performed to find the domain size of h-bn. In this experiment, copper metal of thickness nm is deposited on the incompletely grown h-bn. The Cu/h-BN/SiO 2/Si substrate is placed in a CVD chamber and heated to 900 o C for the growth of graphene at low pressure (1 Torr) and for very short duration of time (2 min). The methane to hydrogen gas composition is maintained to be 150:50 sccm] As it can be observed from the figure below, the graphene film formed at the grain boundary of h-bn film. The Raman spectrum (Figure S-3 (a)) in blue corresponds to graphene film which is selectively grown at the h-bn grain boundary (blue circle in the figure S-3 (b)) and the Raman spectrum of h-bn (Figure S-3 (a)) is denoted in green circle (Figure S-3 (b)). Figure S-3 (b) corresponds to h-bn s E 2g-band s spatial mapping. The grain boundary of h-bn film provides a lower nucleation energy barrier than the in-plane epitaxial growth on h-bn film. Hence graphene nucleates at these grain boundaries. As shown in the figure below, the domain size varies from 10 to 20 µm. This study outlines a fundamentally novel chemical mechanism towards achieving h-bn based heterostructures. Figure S-3: (a) Raman spectra corresponding to h-bn (green color) and graphene (blue color), whereas the corresponding domains are noticed in the green and blue circle regions in (b), which is the Raman spatial mapping for h-bn s E 2g-band. S4

5 5. Raman characteristics of h-bn growth on Si surfaces: Figure S-4 (a) and (b) are the optical image and Raman spectrum of post h-bn growth on Si surface, respectively. There is clear evidence of absence of h- BN. Figure S-4: Raman spectroscopic analysis of post h-bn growth Si surface. (a) Optical microscopic view of the Si surface and (b) Raman spectra corresponding to different regions marked blue, red and black color. 6. X-ray diffraction and XPS characteristics of h-bn surfaces: The XPS spectrum for O 1s peak in h-bn film is shown in Figure S-5 (a). We noticed two pronounced binding states in the O 1s spectrum for h-bn film: the peak at ev is due to O-Si binding, whereas the peak at ev corresponds to O-B-N binding. 2 In addition, another weak intensity peak appears at ev, which can be attributed to the adsorbed oxygen from the atmosphere. The presence of O-B-N binding state in the O 1s and B 1s spectrum confirms the proposed oxide-assisted growth method for the h-bn on silicon-based oxide substrates. The XPS survey scan for h-bn films is presented in Figure S-5 (b) indicating the presence of B 1s, N 1s, and O 1s binding states. The adventitious carbon peak (C 1s) is also observed. X-ray diffraction (XRD) of h-bn film on SiO 2/Si substrate is displayed in Figure S-5 (c), which clearly indicates the majority of crystal orientation are in the direction of (002) centered at 24 o. 3 The estimated interlayer spacing according to Bragg s law is 0.37 nm. S5

6 Figure S-5: (a) XPS spectrum for O 1s in which O-B-N binding state is also noticed. (b) XPS survey scan for h-bn films indicating the presence of B 1s, N 1s, O 1s peaks. The adventitious carbon peak (C 1s) is also observed. (c) XRD analysis of h-bn film on SiO 2/Si surfaces. 7. Optimizing the CVD parameters for the control of layer numbers: The growth process for the 2DNs and their heterostructures in this procedure has two parallel routes: (a) nucleation and planar growth in the x-y plane and (b) epitaxial growth in the z-direction. Since the planar growth is catalytic and faster than epitaxial, the x-y plane coverage occurs before epitaxial increase in number of layers. Therefore, at the point when x-y plane just completes, there are some minimum epitaxial layers deposited at each processing condition. The authors performed reaction to control the layer numbers of h-bn by controlling the reaction time (10 sec, 1 min and 5 min). The growth performed for 10 sec resulted in nominally monolayer h-bn, whereas we found the h-bn of 7 and 20 nm for 1 and 5 min growth, respectively. This implies that between 10 sec and 1 min, there is a window for achieving thinner structures (between 1 layer and 7 nm). The authors envision that further engineering of the catalytic parameters (reaction time, surface oxide concentration and reaction temperature) can enable more control on film thickness. Figure S-6 (a) presents the Raman E 2g peak for 10 sec, 1 min and 5 min growth conditions which correspond to a monolayer, 7 nm and 20 nm h-bn films on SiO 2/Si surfaces. These results are further supported by analysis of Raman peak positions and atomic force microscopy (AFM). It is seen in the Figure S-6 (b) that the E 2g phonon mode intensity increases with the increase of film thickness (also increase of growth time). The inset of Figure S-6 (a) is the phonon mode corresponding to Raman E 2g peak. Moreover, it is also noticed that the peak position for 10 sec growth is shifted upwards and for 1 min growth shifts to downwards with respect to 5 min growth, further confirming that the film grown for 10 sec results atomically thin structure (Figure S-6 (b)). These results are in good agreement with the previous report. 4 Figure S-6 (c and d) and (e, and f) presents the AFM images and corresponding line scan of the h-bn synthesized for 1 and 5 min, respectively. S6

7 Figure S-6: (a) Raman E 2g peak of h-bn films grown for 10 sec, 1 and 5 min with the inset showing the picture of phonon mode corresponding to Raman peak. (b) Position of Raman E 2g peak at different growth time. AFM topography and line scan for h-bn synthesized for 1 min (c and d) and for 5 min (e and f). S7

8 8. CVD processes for the growth of transition metal dichalcogenides on hexagonal boron nitride surfaces: Spilt-tube 3-zone CVD furnace (MTI Corporation) is used for the synthesis of h-bn and large-area heterostructured films with MoS 2 and WS Direct Synthesis of MoS 2/h-BN Heterostructures: For the MoS 2/h-BN heterostructure, the substrate (i.e. h-bn/sio 2/Si) is placed in the 3 rd zone near the molybdenum oxide (MoO 3) powder. During the sulfurization process, samples were kept at 800 o C for min under argon and/or hydrogen flow and maintaining a vacuum of 10 Torr. The sulfur (S) vapors were generated from S powders placed up-stream in a lower temperature region (250 o C) as described in the Figure S Direct Synthesis of WS 2/h-BN Heterostructures: For the WS 2/h-BN heterostructure, the substrate (i.e. h-bn/sio 2/Si) is placed in the 3 rd zone near the tungsten carbonyl (W(CO) 6) powder. During the sulfurization process, samples were kept at 500 o C for min under argon and/or hydrogen flow and maintaining atmospheric pressure. The S vapors were generated from S powders placed up-stream in a lower temperature region as described in the Figure S-7. Figure S-7: The CVD set-up for direct growth of MoS 2 and WS 2 on h-bn/sio 2/Si substrates. S8

9 9. Raman characteristics of MoS 2/h-BN Heterostructure: Figure S-8 presents the MoS 2/h-BN heterostructure. (a) Optical microscopy image with the inset shows the Raman E 2g phonon mode of underneath h-bn. The Raman position mapping for (b) MoS 2 (E 2g), (c) MoS 2 (A 1g) and (d) h-bn (E 2g). Figure S-8: MoS 2/h-BN Heterostructure. (a) Optical microscopy image with the inset shows the Raman E 2g phonon mode of underneath h-bn. The Raman position mapping for (b) MoS 2 (E 2g), (c) MoS 2 (A 1g) and (d) h-bn (E 2g). S9

10 10. Raman characteristics of WS 2/h-BN Heterostructure: Figure S-9 shows the WS 2/h-BN heterostructure. (a) Optical microscopy image with the inset shows the Raman E 2g phonon mode of underneath h-bn. The Raman position mapping for (b) WS 2 (E 2g), (c) WS 2 (A 1g) and (d) h-bn (E 2g). Figure S-9: WS 2/h-BN Heterostructure. (a) Optical microscopy image with the inset shows the Raman E 2g phonon mode of underneath h-bn. The Raman position mapping for (b) WS 2 (E 2g), (c) WS 2 (A 1g) and (d) h-bn (E 2g). References (1) Choi, Y.-R.; Zheng, M.; Bai, F.; Liu, J.; Tok, E.-S.; Huang, Z.; Sow, C.-H. Scientific reports 2014, 4, (2) Schild, D.; Ulrich, S.; Ye, J.; Stüber, M. Solid State Sciences 2010, 12, (3) Bhimanapati, G. R.; Kozuch, D.; Robinson, J. A. Nanoscale 2014, 6, (4) Gorbachev, R. V.; Riaz, I.; Nair, R. R.; Jalil, R.; Britnell, L.; Belle, B. D.; Hill, E. W.; Novoselov, K. S.; Watanabe, K.; Taniguchi, T.; Geim, A. K.; Blake, P. Small 2011, 7, 465. S10