Growth And Properties Of Freestanding Transition Metal Oxide Films

2D materials have recently aroused significant interest due to their remarkable electronic properties and potential for novel electronic applications.In the conventional2D materials,such as graphene and TMDs,these properties are largely controlled by the weakly interacting electrons of the s and p orbital character.In contrast,the strongly interacting electrons of the d orbital character in transition metal oxide perovskites give rise to a rich spectrum of exotic phases,including high temperature superconductivity,colossal magnetoresistance,Mott metal-insulator transitions,and multiferroicity.As a strongly correlated analog of the conventional 2D materials,2D transition-metal oxide perovskites would open the door to a rich spectrum of exotic 2D correlated phases that have not yet been explored.Besides,2D materials exhibit super-flexibility and nanoscale mechanical bending is expected to sustain controllable strain engineering.In combination with the intrinsic properties of perovskite oxides,the expectation is that such strains and strain gradients can potentially stimulate strong correlated physical phenomena and performances via electromechanical coupling effects(i.e.,piezoelectricity and flexoelectricity)in these novel freestanding 2D complex oxide systems.In this thesis,I focus on the growth and properties of freestanding transition metal oxide films.I successfully fabricated 2D perovskites films of which novel properties were found.Main work and conclusions are summarized as follows:(1)Using reactive molecular beam epitaxy,I explored and optimized the growth condition of Bi FeO₃(BFO)films.High crystalline quality BFO films grown on various substrate were obtained.For film growth under continuous co-deposition,it is critical to oxidize the volatile species.Novel polarization states are produced in BFO films by chemical doping in a controllable way.Using aberration corrected STEM and XEDS with atomic resolution,we investigate the complex atomic configurations at the interface of BFO/(001)Sr Ti O₃.We observe an intriguing rock-salt like Bi O₂ double layer at the interface grown on Ti O₂-terminated Sr Ti O₃(STO)substrate.The underlying mechanism for the formation of the novel interfacial structure may be related to the Ti-rich reconstruction of STO.(2)We report the fabrication of freestanding perovskite films with high crystalline quality almost down to a single unit cell.Using water-soluble Sr₃Al₂O₆ as the sacrificial buffer layer,I synthesize freestanding STO and BFO ultrathin films by reactive molecular beam epitaxy and transfer them to different substrates such as crystalline silicon wafers and holey carbon films.We find that freestanding BFO films exhibit an unexpected giant tetragonality and polarization when approaching the ultimate 2D limit.The results demonstrate the absence of critical thickness for stabilizing the crystalline order in the freestanding ultrathin oxide films.The ability to synthesize and transfer crystalline freestanding perovskite films without thickness limitation onto any desired substrate opens a new field for the 2D correlated electronic phases and interfacial phenomena that technically have not yet been accessible.(3)Freestanding perovskite oxides exhibit an exquisite flexibility and capability to accommodate a giant strain gradient in a controllable way.We reveal the super-flexibility and high lattice deformation tolerance(strained up to 8%)of freestanding BFO and STO films,which result in extremely high strain gradient of?4×10⁷ m^-1 in the bent state.These extreme strain states substantially enhance polarization states in BFO and STO films,measured at atomic scale to be as high as 250 and 86?C/cm²,respectively.This enhancement is attributable to the giant flexoelectric effect induced by large strain gradients.The in-situ modulation of local polarization by giant strain gradient is further demonstrated in BFO.Flexoelectricity(strain gradient)offers another path toward achieving the manipulation of electrical behaviors in these strongly correlated 2D materials,as future potential building blocks in multifunctional flexible electronics.