van Thanh, VuongVuongvan ThanhVan, NguyenduyNguyenduyVanvan Truong, DoDovan TruongTUAN HUNG NGUYEN2025-09-242025-09-242022https://www.scopus.com/inward/record.uri?eid=2-s2.0-85122832894&doi=10.1016%2Fj.apsusc.2021.152321&partnerID=40&md5=0258c3cefe7c7cfc2ff2eb2309e4fc60https://scholars.lib.ntu.edu.tw/handle/123456789/732305We investigate the mechanical, electronic, and optical properties of monolayer GeX (X= S, Se, and Te) with γ structure based on density-functional theory calculations. We find that the mechanical anisotropy of γ-GeS is higher than those of γ-GeSe and γ-GeTe, because of its strongest ionic bond. In the unstrained case, γ-GeX is an indirect-gap semiconductor with the Mexican-hat (MH) dispersion in the valence band. By applying tensile and biaxial strains, both energy band gap and valley positions are controlled. On the other hand, by applying an external electric field, the energy band gap is decreased to zero because of the downward interlayer band. We also find that the in-plane optical absorptions of monolayer γ-GeX in the visible-light region are comparable to that of the monolayer transition metal dichalcogenides such as MoS<inf>2</inf>. Due to the unique structure in the z direction, the monolayer γ-GeX also shows a high value of the out-of-plane optical absorptions. The strain engineering significantly modifies the optical absorption in the visible light, while the effect of the external electric field on the optical properties is weak. Our results will be helpful to design the electro-optical devices based on monolayer materials with MH band.Energy Band GapExternal Electric FieldMexican-hat BandOptical AbsorptionStrain EngineeringΓ-gexDensity Functional TheoryElectric FieldsEnergy GapGermanium CompoundsLayered SemiconductorsLightLight AbsorptionMolybdenum CompoundsOptical PropertiesStrainTransition MetalsDensity-functional Theory CalculationsEffects Of StrainsElectronic And Optical PropertiesEnergy BandgapsExternal Electric FieldMechanical AnisotropyMexican-hat BandStrain EngineeringStructure-basedΓ-gexMonolayersjournal article10.1016/j.apsusc.2021.1523212-s2.0-85122832894