Investigation of inter-layer charge transfer process in optically excited graphene and transition metal dichalcogenide-based heterostructures

Abstract

Light-matter interaction of two-dimensional (2D) materials and their van der Waals hybrids have gained significant interest in the past few years as they offer an intriguing platform for studying fundamental physical phenomena and exploring potential technological applications. Owing to their excellent light absorption properties, semiconducting transition metal dichalcogenides (TMDC) are considered ideal for partnering with graphene to form a 2D hybrid for optoelectronic investigations. While the high photo-responsivity arising from the physical separation of photo-generated carriers across the hetero-interface has been the focus of optoelectronics investigations of graphene-TMDC-based heterostructures, understanding of the underlying mechanism that governs the inter-layer charge transfer process is limited, which is however essential for designing and developing advanced devices. In this thesis, we study the optoelectronics of graphene and TMDC-based heterostructures, mainly focusing on developing a comprehensive understanding of the physics that actuates the charge carrier transfer process and its dynamics at the heterostructure interface. In the first part of the thesis, we describe the temperature-dependent photo-response measurements on single-layer and bilayer graphene-TMDC heterostructure-based field effect transistors (FETs). We observe that the charge-transfer process is temperature dependent, and the energy scales defined by the band alignment at the interface govern the charge-transfer dynamics. The experiments also capture the role of TMDC trap states in the inter-layer charge transfer process. Furthermore, the performance figures of the graphene-TMDC van der Waals hybrid evaluated at room temperature suggest a higher bandwidth compared to many of its contemporary FET-based photodetectors, along with a significant gain. In the second half of the thesis, we have leveraged WSe2 proximitized twisted bilayer graphene (tBLG) to perform optoelectronic measurements, where the misorientation angle of the tBLG layer was chosen to lie close to the magic angle of 1.1○. We show that the photoresponse is extremely sensitive to the band structure of tBLG. Strong suppression of photoresponse is observed on tuning the Fermi energy inside the low energy moiré flat bands. However, as the Fermi level is tuned beyond the moiré bands, photo-gating-mediated photoresponse prevails. Our observations suggest that the screening effects from moiré flat bands strongly affect the charge transfer process at the WSe2/tBLG interface, which is further supported by time-resolved photo-resistance measurements. Finally, we present the low-frequency 1/f noise measurements performed on optically excited small-angle WSe2/tBLG heterostructures. The reduced screening inside the moiré band gap is manifested as an enhancement in noise magnitude, suggesting the sensitivity of noise measurements to the band structure of the underlying system.