Strain-tunable Molecular Doping In Hydrogenated Germanene

Theoretical and experimental studies show that the fully-hydrogenated germanene is a non-magnetic semiconductor with a band gap of 1.59 eV,while the one side half-hydrogenated germanene corresponds to a ferromagnetic semiconductor with a band gap of 0.50 eV.Its excellent electronic properties are expected to be demonstrated in various applications,such as optoelectronic devices,sensors,field effect transistors and so on.How to effectively regulate the charge transport and doping is a difficult problems for researchers in the practical applications.Conventional doping techniques of ion implantation and migration will inevitably damage the material,resulting in degraded device performances.Therefore,it become a significant research topic in the international frontier to search for more effective and convenient semiconductor doping methods.At present,the study of realizing effective molecular doping through the interaction between electron acceptor molecules(or electron donor molecules)and materials attracts considerable research attention.In this thesis,by first-principles calculations we thoroughly study the effects of tetrathiafulvalene(TTF)and tetracyanoquinodimethane(TCNQ)molecular adsorption on the electronic properties of hydrogenated germanene.The results show that,as a typical electron acceptor,the TCNQ molecular adsorption both makes fully-hydrogenated germanene and half-hydrogeanted germanene changing from semiconductor to metal.In contrast,as a typical electron donor,the TTF molecular adsorption only introduce a weak n-type doping in fully-hydrogenated germanene.In order to regulate molecular doping in hydrogenated germanene,the biaxial strain are applied in fully hydrogenated germanene and half-hydrogenated germanene.The results indicate that the band gap of fully-hydrogenated germanene and n-type doping caused by TTF molecular adsorption can be linearly tuned under strain.For the case of TCNQ molecular adsorption,when the tensile strain increases up to 2.5%,the fully-hydrogenated germanene/TCNQ transfers from metal to semiconductor.More importantly,an effective p-type doping is realized.Compared with the fully-hydrogenated germanene,the half-hydrogenated germanene is extremely sensitive to strain.Under the biaxial tensile strain of 0.25%,the ferromagnetic semiconductor-metal transition induced by TCNQ molecular adsorption can besurprisingly overturned.In particular,the p-type ferromagnetic semiconductor character can be maintained in the range of 0.25%~1.25%.Further increasing the strain up to 1.5%,it would be returned from a ferromagnetic semiconductor to a metal again.This study on the geometric structures,charge transfer or distribution and the electronic properties of hydrogenated germanene after the adsorption of different molecule explains the varieties of the electronic properties of hydrogenated germanene induced by molecular adsorption,and then reveal the mechanism of strain-tunable molecular doping in hydrogenated germanene.The research is very helpful for subsequent experiments,such as molecular sensors and so on.