Experimental Research On The Ferroelectricity Of Twisted Bilayer Graphene

Ferroelectric materials have important applications in functional devices such as capacitors,ferroelectric field-effect transistors,and nonvolatile-random access memory devices,due to their spontaneous and electrically switchable polarization characteristics.However,the size effect of traditional bulk ferroelectric materials makes it difficult to achieve sub-nanometer level ferroelectric materials in thickness,which greatly hinders the integration of ferroelectric devices.Therefore,it is urgent to find a ferroelectric material system that can break through the size effect limitation.With the advancements in two-dimensional materials,searching for two-dimensional ferroelectrics has become one of the key targets.Experimentally there are two methods to achieve two-dimensional ferroelectric materials: 1)two-dimensional intrinsic ferroelectric materials exfoliated from the bulk ferroelectric ones,and 2)ferroelectric polarization induced at the interfaces of the heterostructure consisting of two-dimensional van der Waals building blocks,which provides more possibilities for extending the two-dimensional ferroelectric families.Basing on the twisted bilayer graphene(TBLG)encapsulated by the hexagonal boron nitride(h-BN)devices,we reported the electronic properties induced by the interface ferroelectric polarization.High quality h-BN encapsulated TBLG with a twisted angle of30 degrees devices were fabricated by using the dry transfer technique with the exfoliated graphene and h-BN.The standard four-probes method was used to characterize the electronic properties of the TBLG devices at different temperature.A robust resistance hysteresis was observed at the top interface between TBLG and h-BN,which can be persisted from the extreme low temperature(40 m K)up to room temperature.The hysteresis can be explained by the extra carrier induced by the interface ferroelectric polarization.The interface polarization was estimated to be 0.7 p C/m,which was of the same order with the theoretical predicted ones of the two-dimensional sliding ferroelectrics.By stacking the non-polarized graphene and h-BN together,we have achieved a heterostructure with spontaneous ferroelectric polarized interface.Our results can help to enrich the two-dimensional ferroelectric families,and to open more possibilities by coupling the novel properties of two-dimensional materials with the ferroelectrics.This paper is organized as follows: 1)the development of two-dimensional materials is introduced,including the structural and physical properties of h-BN and graphene.And the twistronics consisting of two-dimensional materials by twisting a specific angle and two-dimensional sliding ferroelectrics were also introduced;and 2)the preparation,processing,and the characterization experimental equipment and corresponding experimental technique related to this research were introduced;and 3)the electrical transport characteristics of the h-BN encapsulated TBLG sample at 40 m K was presented,and the interface ferroelectric polarization of the device was observed at the top interface of h-BN and TBLG;and 4)the electrical transport characteristics of the h-BN encapsulated TBLG sample under different temperatures were investigated,and the interface ferroelectric polarization of the device can be maintained from extremely low temperature environment to room temperature environment;and 5)summary and outlook on the current work.