High-throughput dry transfer and excitonic properties of twisted bilayers based on CVD-grown transition metal dichalcogenides

van der Waals (vdW) layered materials have attracted much attention because their physical properties can be controlled by varying the twist angle and layer composition. However, such twisted vdW assemblies are often prepared using mechanically exfoliated monolayer flakes with unintended shapes through a time-consuming search for such materials. Here, we report the rapid and dry fabrication of twisted multilayers using chemical vapor deposition (CVD) grown transition metal chalcogenide (TMDC) monolayers. By improving the adhesion of an acrylic resin stamp to the monolayers, the single crystals of various TMDC monolayers with desired grain size and density on a SiO2/Si substrate can be efficiently picked up. The present dry transfer process demonstrates the one-step fabrication of more than 100 twisted bilayers and the sequential stacking of a twisted 10-layer MoS2 single crystal. Furthermore, we also fabricated hBN-encapsulated TMDC monolayers and various twisted bilayers including MoSe2/MoS2, MoSe2/WSe2, and MoSe2/WS2. The interlayer interaction and quality of dry-transferred, CVD-grown TMDCs were characterized by using photoluminescence (PL), cathodoluminescence (CL) spectroscopy, and cross-sectional electron microscopy. The prominent PL peaks of interlayer excitons can be observed for MoSe2/MoS2 and MoSe2/WSe2 with small twist angles at room temperature. We also found that the optical spectra were locally modulated due to nanosized bubbles, which are formed by the presence of interface carbon impurities. The present findings indicate the widely applicable potential of the present method and enable an efficient search of the emergent optical and electrical properties of TMDC-based vdW heterostructures.


S1. AFM characterization of dry-transferred WS2 homobilayer.
Figure S1a shows the optical and AFM images of dry transferred monolayer WS2 on WS2.The AFM image was taken in the dashed area.The AFM image shows a flat surface for the dry transferred WS2 even after the cleaning.To evaluate the interlayer coupling, the height of the bottom and top transferred WS2 was measured as shown in Figure S1b,c.A comparable height of the bottom and top transferred WS2 was observed (0.8−0.9 nm), suggesting a well coupling between the 2D interface.

S2. PL spectra of hBN-encapsulated CVD-grown TMDC monolayers.
We have investigated the quality of CVD-grown TMDC monolayers encapsulated in hBN because hBN encapsulation has been used to probe intrinsic transport and optical properties [1][2][3] .Figure S2a,b shows the optical images and PL intensity maps of CVD-grown monolayer MoS2 before and after hBN encapsulation.The PL of hBN-encapsulated MoS2 is 40 times brighter than that of as-grown MoS2 on the SiO2/Si substrate.This suggests the suppression of non-radiative relaxation, which could be due to the surface states of SiO2.In the PL maps, a triangular bright region is observed in the center of the MoS2 crystal after encapsulation.This reflects the difference in crystallinity.All encapsulated samples, including MoS2, WS2, MoSe2, and WSe2 monolayers, show a high-energy shift and linewidth narrowing of PL peaks compared to the as-grown samples (Figure S2c).This indicates the suppression of inhomogeneous tensile strain as observed for the suspension process 4 .The high quality and the suppression of inhomogeneous broadening were also confirmed for the low-temperature PL spectrum of hBN-encapsulated monolayer MoSe2 (Figure S2d).The PL spectrum measured at 8 K shows two sharp peaks corresponding to the emission from neutral excitons (X 0 ) and trions (T) 5 .The widths of these peaks are 8.5 meV for X 0 and 7.2 meV for T, which are comparable to the values for the exfoliated flakes encapsulated by hBN 2 .Similar studies on the hBN encapsulation of CVD-grown TMDCs have reported the exciton-exciton annihilation and low-temperature PL for hBN-encapsulated monolayered WS2 3,6 , and the low-temperature PL and reflectance spectra for WSe2 7 .Together with these studies, the present results provide a basis for the intrinsic optical properties of CVD-grown TMDCs.

S3. Characterizations of hBN-encapsulated MoSe2/WSe2 twisted bilayers.
Figure S3a,b shows the optical images and PL intensity maps of hBN-encapsulated MoSe2/WSe2 twisted bilayers on an SiO2/Si (sample #1) and on a TEM grid (sample #2).The PL intensity maps at 1.62 eV show the dark triangular regions corresponding to the MoSe2/WSe2 twisted bilayers.The PL spectra were obtained from the twisted bilayers with different twist angles as shown in Figure S3c.Room temperature interlayer excitons were observed at ~ 1.3 eV for the twist angles of 1 and 60 degrees for sample #1 and 1 and 2 degrees for sample #2.This indicates a well interlayer coupling 8,9 , which is a similar result with the hBN-encapsulated MoSe2/MoS2 twisted bilayers as described in the main text.

S4. Characterizations of hBN-encapsulated MoSe2/WS2 twisted bilayers.
Figure S4a shows the optical micrograph image of hBN encapsulated MoSe2/WS2 twisted bilayers.The corresponding PL image is shown in Figure S4b, and the green dashed lines indicate the grain edges of monolayer WS2.Tens of small MoSe2/WS2 twisted bilayers can be observed within large WS2 grain.Figure S4c shows a typical broadband PL spectrum from the twisted bilayers.PL peaks of MoSe2 in the twisted bilayers show a red shift compared with that of monolayer MoSe2 as shown in Figure S4d.Note that the twist angle dependence is not as large as reported results 8 .

S5. Twist-angle dependence of PL peak energies.
For the hBN-encapsulated MoSe2/MoS2 and MoSe2/WSe2 samples, PL peaks of interlayer exciton were observed at room temperature.Furthermore, the PL peak energies of interlayer exciton showed a twist angle dependence as shown in Figure S5a.This twist angle dependent interlayer exciton property is likely due to the indirect transition 10 , interlayer coupling effect 11 , or the effect of atomic reconstruction 12 .For the MoSe2/WS2, the PL peak energies showed a red shift compared to the A exciton of monolayer MoSe2 (Figure S4d).As shown in Figure S5b, the PL peaks depend on the twist angle (red circles), which results from the hybridization effect of interlayer and intralayer excitons because of the degenerated conduction band edges of MoSe2 and WS2 13 .For the MoSe2/MoS2, the A exciton showed a red shift with a twist angle of 60°.A similar red shift was observed in the previous study, where the effect of atomic reconstruction was discussed 12 .Further study is necessary to investigate the origin of each combination in the future, and it is beyond the scope of the current study.

S6. Characterizations of hBN-encapsulated monolayer MoS2.
To further characterize the sample quality, hBN-encapsulated monolayer MoS2 was prepared by the present transfer method as shown in Figure S6a.A PL intensity map at 1.81 eV (under a 532 nm laser excitation) was obtained in the red dashed region as shown in Figure S6b.The PL map shows many quenched regions.The AFM image showed dot-like bubbles at the same locations with PL quenched regions (Figure S6c).In Figure S6d, the PL spectra measured at the bubbles show variations in PL intensity and peak energy.This is likely due to the bandgap modulation by lattice strain 14 and the impurity-assisted nonradiative relaxation around the bubbles.

Figure S1 .
Figure S1.AFM observations of the dry-transferred WS2 homobilayer.(a) Optical image of homobilayer WS2 and its AFM topography image in the dashed area.Height profiles of (b) the bottom WS2 and (c) the top transferred WS2.

Figure S3 .
Figure S3.Characterizations of hBN-encapsulated MoSe2/WSe2 twisted bilayers.Optical micrograph images and PL intensity maps of the red dashed region at 1.62 eV (WSe2) of the fabricated samples on (a) an SiO2/Si substrate and (b) a SiN TEM grid.(c) PL spectra of MoSe2/WSe2 twisted bilayers with different twist angles measured on the SiO2/Si substrate and the TEM grid.

Figure S4 .
Figure S4.(a) Optical micrograph and (b) PL images of hBN encapsulated MoSe2/WS2.In (b), the green dashed lines indicate the gain edge of bottom monolayer WS2.(c) Typical PL spectrum obtained from the twisted bilayer region.(d) Comparison of PL spectra obtained from the twisted region with seven different twist angles and monolayer MoSe2.

Figure S6 .
Figure S6.(a) Optical micrograph image of hBN-encapsulated monolayer MoS2.(b) PL intensity map (@1.81 eV) and (c) AFM image of the red dashed area in (a).(d) Typical PL spectra taken from the marked area in (b).