Surface Reconstruction

Oxide surfaces are important for applications in catalysis and thin film growth, as well unique physics phenomenon. Comparatively little is known about atomic arrangements at oxide surfaces at present, The determination of the atomic structure and the retrieval of information about reconstruction and bonding of metal oxide surfaces is challenging owing to the highly defective structure and insulating properties of these surfaces. Transmission electron microscopy (TEM) offers extremely high spatial resolution (less than one ångström) and the ability to provide systematic information from both real and reciprocal space. Strontium titanate (SrTiO3) has a wide range of applications in the electronic industry and attracts growing world-widely interest recently because of novel discoveries at its surfaces, interfaces and with selected dopants. The understanding of some of the structural properties of SrTiO3 and its optical properties have been lagging due to limited characterization techniques available to study single monolayers and dopants in this material. The versatile TEM technique, i.e. NCSI-HRTEM, STEM, EDS, EELS, DPC, Ptychography, in-situ… …, provide infinite possibility for revealing abundant information of surface at atomic scale. The puzzle of surface would be addressed piecemeal.

Two Dimensional Electron Gas

The 2D electron gas (2DEG) emerged at the interface of different oxide materials is one of the most interesting phenomena  in oxide community. The origin of 2DEG has remained highly controversial since its discovery at LaAlO3/SrTiO3 (LAO/STO) interface. Various models are proposed, which include electronic reconstruction via surface-to-interface charge transfer and defect-mediated doping involving cation intermixing or oxygen vacancy (VO) formation. Since the highly concentrated electrons at the interfaces accompanied by structural and chemical modification, it is essential to collect other comprehensive data as well as measure the electron distribution for correct understanding of the system. The breaking of symmetry across an oxide heterostructure causes the electronic orbitals to be reconstructed at the interface into energy states that are different from their bulk counterparts. The detailed nature of the orbital reconstruction critically affects the spatial confinement and the physical properties of the electrons occupying the interfacial orbitals. Using 2DEG forming at LaAlO3/SrTiO3 interfaces with different crystal symmetry, we characterized selective orbital occupation and spatial quantum confinement of electrons using inline electron holography. Furthermore, comprehensive scanning transmission electron microscopy analyses, in conjunction with density functional theory calculations, demonstrate that VO forming at the LAO/STO surface above the critical thickness (tc) cancels the polar field by doping the interface with 2DEG. The antiferrodistortive (AFD) octahedral rotations in LAO, which are suppressed below the tc, evolve with the formation of VO above the tc. The present study reveals that local symmetry breaking and shallow donor behavior of VO induce the AFD rotations and relieve the electrical field by electron doping the oxide heterointerface.