Stacking CVD-Grown MoS₂ Monolayers for High-Performance 2D Optoelectronics

Due to excellent optoelectronic properties, 2D transition metal dichalcogenides (TMDCs) are being pursued for various applications such as transistors, flexible electronics, and optoelectronics. TMDCs are also potential candidates for flexible and high-specific-power photovoltaics (PV). A single MoS₂ monolayer can absorb up to 10% of light at wavelengths in the visible and IR spectrum and has a direct bandgap of 1.85 eV. Bulk MoS₂ lacks many of these desirable properties, as it has an indirect bandgap of ~1.2 eV. Similarly, the photoluminescence quantum yield (PLQY) in the case of bulk MoS₂ is ~0 as compared to monolayer MoS₂, which has achieved PLQY as high as 95% [1]. We introduce a scalable transfer process to stack CVD-grown large-area MoS₂ monolayer films to obtain enhanced absorption while retaining monolayer-like properties including direct bandgap and high PLQY. Key optoelectronic properties are measured before and after the transfer to ensure the film’s quality remains intact throughout the process.
Large-sized (>cm²) and uniform-quality MoS₂ monolayer films were synthesized on a sapphire substrate using chemical vapor deposition. Square-shaped patterns were created in the films using electron beam lithography and etching to better track and analyze the efficiency of the transfer process in the same locations. Optoelectronic properties were then measured before transferring to another sapphire substrate using the proposed transfer technique. Our new transfer approach utilizes an aluminum mesh sandwiched between two Poly (methyl methacrylate) (PMMA) layers spin-coated on the donor substrate to support the film during transfer from donor to receiver, preserving the geometry and preventing wrinkle formation, tears, and gaps.
Optical images of the transferred film verified that we were able to retain the shape and geometry of the patterns. The absorption was mapped at the patterns for incident light of 660 nm wavelength and compared with the absorption map obtained from the same patterns before the transfer. The average absorption obtained after the transfer was 5.39% as compared to 5.85% in the case of the pre-transferred film. Atomic force microscopy indicates an increase in thickness after transfer from 0.67 nm to 1.43 nm due to slightly increased spacing between film and substrate. Raman spectra showed a peak difference (E2g – A1g) of 20.49 cm-1 for the as-grown monolayer film, while after transferring and measuring at the same spot a peak difference of 20.25 cm-1 was observed, indicating that the Raman modes were also preserved during the transfer. Similarly, PL measurements showed that the film retained up to 90-95% of its original PL intensity when measured at the same spot after the mesh transfer, showing that the film retained its direct bandgap behavior as well.
Stacks of monolayers were then created, and the above-mentioned measurements were performed again; the results showed absorption of 12%, 17%, and 22% for 2, 3, and 4-layered stacks respectively, while retaining monolayer-like Raman modes, maintaining the E2g – A1g peak difference under 21.5 cm-1 for the stacked monolayers. Similarly, a consistent increase in the PL intensity was seen by adding more monolayer films, thus indicating that a direct bandgap is retained as opposed to a transition to reduced PL and indirect bandgap as seen in as-grown multilayer films [2].
Ongoing work includes the fabrication of field-effect transistors to measure the mobility of transferred and stacked monolayers. We are also fabricating large-area, MoS₂-based PV utilizing these stacks to improve optical absorption and photocurrent while also employing a vertical carrier transport architecture to improve PV performance. Simulations using the transfer matrix method indicate a PV current density of 18 mA/cm2 is achievable by stacking 50 MoS₂ monolayers.
References:
Science 350.6264 (2015): 1065-1068.
Applied Surface Science 487 (2019): 1356-1361.