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

Abstract

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.

Fig. 1. Twisted multilayers fabricated from CVD-grown TMDC monolayers. (a) Schematic of CVD growth of monolayer TMDC single crystals and assemblies into vdW twisted multilayers. Optical images of CVD-grown (b) WS₂ and (c) WSe₂ monolayer grains, and (d) stacked bilayers of WS₂ and WSe₂. The scale bars are 50 μm. (e) Optical images of 10-layer twisted MoS₂ in each fabrication step. The scale bars are 10 μm. Arrows indicate newly stacked monolayer MoS₂ single crystals in each fabrication step.

Fig. 2. Dry transfer process with the acrylic resin stamp. (a) Schematic of the transfer system used in the present study. (b) Optical images of MoS₂ grains in contact with the stamp (top) at room temperature and (bottom) during heating. (c) Schematic of the transfer process including (i) the contact of the stamp with the sample, (ii) heating to melt the stamp, (iii) cooling to solidify the stamp, and (iv) lifting the stamp to pick up TMDC monolayers. Optical images of CVD-grown grains of MoS₂ (d) before and (e) after stamp lifting. The scale bars are 200 μm. White dotted lines indicate the areas in contact with the stamp.

Fig. 3. Evaluation of interlayer interaction for twisted bilayer MoS₂. (a) Optical images of four representative twisted bilayers with different twist angles for CVD-grown MoS₂. Scale bars are 5 μm. (b) Room-temperature PL spectra and (c) PL peak positions for direct and indirect optical transitions for 16 twisted bilayer MoS₂ with different twist angles. Black line shows the PL peak of the A exciton of MoS₂, whereas the red line indicates the PL peaks derived from an indirect gap.

Fig. 4. PL properties of MoS₂/MoSe₂ heterobilayers with various twist angles. (a) Optical images of hBN encapsulated MoS₂/MoSe₂ heterobilayers. (b) PL intensity image (smaller white triangles are 1L MoSe₂ and larger triangle is 1L MoS₂, and the dark triangles within MoS₂ are twisted area). Scale bars are 10 μm. (c) Room-temperature PL spectra of the twisted area with various twist angles. The dashed line indicates the trend of the interlayer exciton peak. (d) PL peak positions of the intralayer exciton from MoS₂ and MoSe₂, and the interlayer exciton from the MoSe₂/MoS₂ heterobilayer with different twist angles. (e) Fitting results of the PL spectra of the hBN encapsulated MoSe₂/MoS₂ twisted bilayers at 3°, 28°, and 60° twist angles.

Fig. 5. Cathodoluminescence (CL) analysis of hBN-encapsulated monolayer MoSe₂. (a) STEM image of hBN-encapsulated monolayer MoSe₂ suspended on a TEM grid. (b) CL map at 1.57 eV of the same area as (a). Scale bars in (a) and (b) are 1 μm. (c) CL spectra recorded at the positions indicated by P1, P2, P3, and P4 in (a). The inset shows a schematic of the CL measurement under electron beam excitation.

Fig. 6. Cross-sectional structural analysis of hBN-encapsulated MoS₂ stacked in a vacuum. (a) Cross-sectional STEM image with bubbles. (b) EDS mapping of the selected area (dotted rectangle) in (a). (c) Line elemental analysis along the red line in (a). Light red and green regions are MoS₂ and the bubble area, respectively.