Transfer method effects on graphene/α-germanium selenide heterostructures: Toward strain and interfacial interactions in bandgap modulation

Two-dimensional van der Waals (vdW) materials with an intrinsic bandgap offer promising platforms for integrating graphene into functional heterostructures. In this study, we investigated the relative roles of strain and interfacial interactions in modulating the electronic band structure of graphene within a vdW heterostructure. Our model system is graphene on α-germanium selenide (α-GeSe). By comparing solvent and thermal transfer methods, we explore how fabrication conditions impact the structure and band characteristics of the resulting heterostructures. Scanning probe microscopy and first-principles calculations reveal that solvent-based transfer preserves the substrate integrity and the intrinsic electronic structure of graphene, yielding a novel parallelogram moiré pattern. In contrast, the thermal transfer induces oxidation of Ge and formation of rhombus-shaped etch pits, which locally strain the graphene layer and result in a measurable bandgap. Meanwhile, spatial doping variations are primarily governed by charge transfer effects due to interface interaction. Our findings demonstrate that strain, rather than interlayer coupling, is the dominant mechanism for bandgap modulation in graphene/α-GeSe heterostructure. This work underscores the critical role of transfer techniques in engineering the properties of 2D van der Waals heterojunctions and establishes α-GeSe as a promising, tunable substrate for future graphene-based electronic and optoelectronic applications.