Electronic Transport Of Strain-Engineered Two Dimensional Materials

Owing to low dimensionality and special crystal structure,graphene and graphene-like two dimentional(2D)materials possess excellent physical and chemical properties and have become a hot topic of condensed matter physics in the past ten years.The properties of 2D materials could be effectively modulated by strain,which makes them ideal candidates for future strain electronics.In this thesis,we studied piezoconductive properties of suspended graphene and strain engineering of Fe3GeTe2 thin film.We also studied the low-temperature elec-tronic transport of graphene/MoS2 heterostructure.In chapter one,according to 'complex materials on demand' paradigm,we give a brief introduction to current research on 2D materials,including:valley electronics of 2D materials,ferromagnetic 2D materials;strain engineering 2D materials;transistors and photodetector based 2D heterostructures.In chapter two,we give a detailed introduction on the development of mechanical exfoliation techniques and several transfer techniques,as well as the interlayer impurity and degradation issues in 2D heterostructure devices.In chapter three,we have systematically studied the piezoconductive effect of suspended graphene with different number of layers using pressure modulated conduc-tance microscope.Positive piezoconductive effect is observed in bi-and multi-layer graphene.We proposed the model of strain-induced competition between interlayer coupling and intralayer transport mechanism to explain this phenomenon,and further confirmed this mechanism by numerical calculations based on non-equilibrium Green's function method.Numerical calculation results also explain well why the tri-layer graphene shows the most pronounced positive effect in our experiments.In order to realize strain engineering in ultra-low temperature electronic transport study.In chapter four,we have built in-situ strain applying platform and developed 2D material flexible device fabrication technique.The system could be used to study the electronic properties of strain-engineered 2D materials,with the maximum ten-sile strain up to 3%.We fabricated thin Fe3GeTe2 devices with hall bar geometry on flexible polyimide substrate.We observed that the ferromagnetic-paramagnetic tran-sition temperature increases and the hall magnetoresistance hysteresis loop enlarges with increasing tensile strain.This indicates tensile strain could effectively enhance ferromagnetism of Fe3GeTe2 thin films.The study of 2D material heterostructure and its interface is also of great signifi-cance.In chapter five,we have fabricated graphene/MoS2 heterostructure using PVA pick-up method.Hole conduction behavior of MoS2 in the overlap zone was observed in low temperature electronic transport studies.We also observed raman peak blue shift and significant decrease of photofluorescence spectra peak of MoS2 in the overlapped region,suggesting charge transfer behavior at the interface.