# Fileset [SI_v6_F.pdf](https://mdr.nims.go.jp/filesets/5f08108e-c858-4c19-8f87-44a782c54871/download) ## Creator [Ryotaro Sakakibara](https://orcid.org/0000-0001-7150-2831), [Kaito Hirata](https://orcid.org/0000-0003-1489-2938), [Yasufumi Takahashi](https://orcid.org/0000-0003-2834-8300), Wataru Norimatsu, [Yasumitsu Miyata](https://orcid.org/0000-0002-9733-5119) ## Rights This document is the Accepted Manuscript version of a Published Work that appeared in final form in Nano Letters, copyright © 2025 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.nanolett.5c02492.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/) ## Other metadata [Improved Strain Engineering of Monolayer Transition Metal Dichalcogenides via Van der Waals Epitaxy on Graphene/SiC(0001)](https://mdr.nims.go.jp/datasets/071d4447-b277-4f81-bcfb-dad9ea6ed4d6) ## Fulltext 1 Supporting Information for Improved Strain Engineering of Monolayer Transition Metal Dichalcogenides via van der Waals Epitaxy on Graphene/SiC(0001) Ryotaro Sakakibara1,2*, Kaito Hirata3,4, Yasufumi Takahashi4,5,6, Wataru Norimatsu7, and Yasumitsu Miyata1,2* 1 Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan 2 Department of Physics, Tokyo Metropolitan University, Hachioji, 192-0397, Japan 3 Department of Physical Science and Engineering, Nagoya Institute of Technology, Nagoya 466-8555, Japan 4 Department of Electronics, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan 5 Research Institute for Quantum and Chemical Innovation, Institutes of Innovation for Future Society, Nagoya University, Nagoya 464-8601, Japan 6 WPI Nano Life Science Institute (WPI-NanoLSI), Kanazawa University, Kanazawa 920-1192, Japan 7 Faculty of Science and Engineering, Waseda University, Tokyo 169-8555, Japan *Correspondence should be addressed to R.S. (SAKAKIBARA.Ryotaro@nims.go.jp) and Y.M. (MIYATA.Yasumitsu@nims.go.jp) Contents - Experimental details - Supplementary notes - Supplementary Figures S1–S13 - References 2 Experimental details Preparation of graphene/SiC substrate A nominally on-axis 4H-SiC(0001) substrate (CREE inc.) was cleaned by ultrasonication in acetone and ethanol, and then immersed in a hydrofluoric acid solution to remove the oxide layer on the surface. The pre-treated SiC substrate was then annealed at 1720°C for several minutes in an Ar gas flow atmosphere at a flow rate of 100 sccm. This process resulted in the formation of 1–2 layers of epitaxial graphene over the entire substrate surface.1,2 CVD growth of WSe2 and MoS2 Salt-assisted CVD growth was conducted using a two-zone electric furnace.3-5 For WSe2 growth, a ceramic boat containing Se beads was placed upstream, while KBr powder (25 mg), WO3 powder (25 mg), and a growth substrate were positioned downstream inside a quartz tube. The quartz tube was filled with a N2/H2 gas mixture (0.1% H2) at a flow rate of 500 sccm. The upstream and downstream furnaces were heated to 420°C and 840°C, respectively, and maintained at these temperatures for 2 min. The system was then rapidly cooled to room temperature using an electric fan. For MoS2 growth, S flakes, KBr powder (3 mg), and MoO2 powder (30 mg) were used as precursors. The upstream and downstream furnaces were set to 180°C and 780°C, respectively. N2 carrier gas was supplied at a flow rate of 250 sccm and the growth duration was 3 min. Characterizations PL and Raman measurements were carried out at room temperature using a Renishaw inVia spectrometer. A 532 nm excitation laser with a spot size of about 1 μm2 was used. Cross-sectional STEM observation was carried out using a JEM-ARM200F microscope operated at an acceleration voltage of 200 kV. HAADF imaging and EDS measurements were performed. A thin specimen for observation was prepared using the focused ion beam method. To evaluate the catalytic reactivity of the MoS2 samples, HER measurements were carried out using a home-made SECCM.6 HER current mapping images were acquired in hopping mode, with a nanopipette aperture radius of about 50 nm. The threshold current for distance control between the nanopipette and the sample, as well as the hopping amplitude, were set to 0.3 pA and 1 μm, respectively. Further details of the HER measurements are provided in the previous study.6 Atomic force microscopy (AFM) observation was carried out in the dynamic force mode to obtain topographic images of the sample surfaces. 3 Supplementary notes Strain direction in monolayer WSe2 The strain in our synthetic WSe2 monolayers is considered isotropic rather than uniaxial, for the following two reasons. First, WSe2 grown on graphene/SiC shows no evidence of E' mode splitting in the Raman spectra (Figure S5), which is a feature typically associated with uniaxial strain.7 Second, the in-plane TEC of SiC is isotropic, owing to the three-fold symmetry of the atomic arrangement on the SiC(0001) plane. The same reasoning applies to WSe2 grown on other substrates, including graphite, SiO2, and sapphire. It is worth noting that directional strain control may be possible by using substrates with anisotropic TEC. On the limitation of wavenumber resolution in our Raman spectroscopy setup As described in the main text, the effect of strain in WSe2 is clearly observed using PL measurements, while it is difficult to detect using Raman spectroscopy. Specifically, the E' mode of WSe2 on graphene/SiC exhibited nearly uniform characteristics across the grain, and its Raman shift is indistinguishable from that of WSe2 on graphite (Figures S4 and S5). According to a previous study, the Raman shift rate induced by biaxial tensile strain is approximately -1 cm-1/%.8 Therefore, for the estimated 0.4% strain in our WSe2/graphene/SiC sample, a Raman shift of around 0.4 cm-1 would be expected. This shift is comparable to the wavenumber resolution of our current setup (~0.4 cm-1), making it difficult to resolve such small shifts. Higher-resolution measurements using finer gratings may allow for a more accurate strain evaluation. In contrast, for MoS2 grown on graphene/SiC, a clear shift of the E' mode relative to that on graphite is observed (Figure S12). This can be attributed to the higher strain sensitivity of the E' mode in MoS2, which exhibits a Raman shift rate of approximately -4 cm-1/%.8 Origin of the interaction between TMDs and graphene/SiC substrate As mentioned in the main text, a strong interaction between TMDs and substrate is necessary to introduce thermal strain. Here, we discuss a possible origin of the interaction between TMDs and graphene/SiC substrate. One possible scenario is the formation of covalent bonds between dangling bonds at the TMD edges and reactive sites on the underlying graphene or SiC. Specifically, TMD edges may be bound to graphene defects generated during the CVD process, or to SiC surface dangling bonds exposed through holes in graphene layer. These local anchor points could mediate strain transfer upon cooling. 4 Effect of the step-terrace structure and graphene layer on the PL properties of WSe2 Figure S7 shows AFM topography images of WSe2 grown on graphene/SiC. The step-terrace structure of the SiC substrate surface is clearly observed,1 and the WSe2 grains covered multiple steps with heights of 2–5 nm. Here, the PL peak energy and FWHM maps of WSe2 shown in Figures 2b and 2c of the main text do not exhibit any distribution correlated with the step-terrace structure. This indicates that the strain distribution is not related to the underlying morphology. In contrast, the PL intensity map in Figure 2a of the main text shows regions with lower intensity. As shown in Figure S8, this intensity distribution appears to correlate with the FWHM map of the graphene 2D band. Since the FWHM of the 2D band positively correlates with the number of graphene layers,9,10 it is suggested that the PL intensity of WSe2 tends to decrease on regions with fewer graphene layers. 5 Figure S1. Optical images of WSe2 crystals grown on (a) graphene/SiC, (b) graphite, (c) bare SiC, (d) SiO2, and (e) sapphire substrates. Figure S2. (a) Cross-sectional HAADF-STEM image of WSe2/graphene/SiC. (b) HAADF-STEM image of the same field of view as (a) with reduced contrast. (c) Bright-field STEM image of the same sample. (d) In-plane view of the WSe2 lattice, initially grown on an SiO2/Si substrate and later transferred onto a TEM grid. 6 Figure S3. (a) PL intensity, (b) peak energy, and (c) peak FWHM maps of monolayer WSe2/SiO2. (d) PL spectra measured at three different points. (e) PL intensity, (f) peak energy, and (g) peak FWHM maps of monolayer WSe2/sapphire. For the PL peak of monolayer WSe2/sapphire, a peak derived from charged exciton (trion) is prominent on the low-energy side, so the spectrum was fitted with two Voigt function components.11 The map data are for the neutral free exciton. (h) PL spectra measured at three different points. Figure S4. (a) E' Raman mode intensity, (b) Raman shift, and (c) FWHM maps of monolayer WSe2 grown on graphene/SiC. (d) Typical Raman spectrum of monolayer WSe2 grown on graphene/SiC. 7 Figure S5. Typical Raman spectra of monolayer WSe2 grown on graphene/SiC, graphite, SiO2, and sapphire substrates. Raman shift due to tensile strain was recognized only on SiO2. Figure S6. Raman spectra of graphene/SiC after the CVD growth of WSe2 (top) and a bare SiC substrate for reference (bottom). 8 Figure S7. (a,b) AFM topography images of WSe2 crystals grown on graphene/SiC. (c,d) Height profiles acquired along the red lines in (a) and (b), respectively. The step-terrace structure of the SiC substrate surface is clearly visible. In (d), the thickness of monolayer WSe2 (~0.8 nm) is confirmed. The finger-like structures can also be seen, which may have formed during thermal decomposition of SiC.12 Figure S8. (a) PL intensity map of monolayer WSe2 grown on graphene/SiC. (b) Raman FWHM map of the graphene 2D band acquired in the same region as (a), where the position of the WSe2 crystal is outlined by dashed lines. By comparing the two map images, it can be said that the inhomogeneity of the PL intensity correlates with the 2D FWHM, which reflects the number of graphene layers.9,10 9 Figure S9. Optical images of MoS2 crystals grown on (a) graphene/SiC and (b) graphite substrates. Figure S10. (a,b) AFM topography images of MoS2 crystals grown on graphene/SiC. (c,d) Height profiles acquired along the red lines in (a) and (b), respectively. In (d), the thickness of monolayer MoS2 (~0.7 nm) is confirmed. 10 Figure S11. (a) Typical PL spectra and (b) plot of PL peak FWHM versus peak energy for monolayer MoS2 grown on graphene/SiC and graphite. In contrast to the case of WSe2, tensile-strained MoS2 shows a broadening of linewidth with a redshift of the PL peak. This behavior can be explained by enhanced intervalley scattering arising from strain-induced band structure modulation in MoS2.13,14 Figure S12. (a) Typical Raman spectra and (b) Plot of Raman A1' position versus E' position for monolayer MoS2 grown on graphene/SiC and graphite. The slight redshift of the E' mode in monolayer MoS2 on graphene/SiC indicates the presence of tensile strain.15 11 Figure S13. 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