Low Dimensional Growth Behavior and Nonlinear Optical Effects of Perovskite Niobates Crystal

Author:Zhou Fei

Supervisor:zhu jing chuan lai zhong hong


Degree Year:2017





Perovskite oxides,as one kind of typical dielectric and ferroelectric material,have been widely studied due to their important applications in sensors,actuators,and information storage devices,and have also shown great potential to be used in energy conversion and storage.Currently,researches on such materials are focus on bulk materials.There are scarce reports on their two dimensional(2D)counterpart,because there remains a bigger challenge in fabricating free-standing 2D perovskite oxides.However,recent advancements in nanotechnology has led to increasingly stringent requirements in device miniaturization,which in turn necessitates low-dimensional structures.Perovskite niobates was studied in this thesis.The intrinsic physical properties of Ag Nb O3 were researched by theoretical calculation.Its surface plasma resonance properties were calculated and verified by experiment.The 2D models of KNb O3were built based on KNb O3 crystal structure.The physical properties of 2D KNb O3with different facet orientations with respect to dimension reduction were calculated.The 2D growth model was constructed and the growth mechanism was investigated.Based on the 2D growth model,the 2D growth process was designed and followed by 2D KNb O3 fabrication.Photoluminescence(PL)experiments verified the relationship of the electronic structure of materials to dimension reduction.The nonlinear optical properties were investigated in 2D regime.A novel research method initiated by theoretical calculation and design of a 2D growth of perovskite oxides and followed by experimental fabrication and properties characterization,was developed.The electronic structure,bond and electron transition properties of AgNb O3were studied systematically by density functional theory(DFT).Results showed that the calculated bandgap of Ag Nb O3 by using functional of LDA+Hubbard U was2.75 e V,which was close to the experimental value(2.8 eV).The valence band was constructed by the hybridizing of Ag 4d-O 2p electrons.The overlapped electron clouds of Ag-Ag bond showed a metal-like property,indicating potential optical absorption property by localized surface plasmon resonances(LSPRs).The Ag Nb O3powders were synthesized by solid-state method.Results of theoretical calculation were proved by optical absorption experiments.The intrinsic physical relationship of LSPRs was built by Drude dispersion model.The hydrothermal method was adopted to explore the technology of morphology controlling of materials.Research showed that well dispersed cubic particles of pure phase AgNb O3 can be obtained with a PH value of 3.2 by hydrothermal assistance.Absorption tests indicated that the optical harvesting properties of AgNb O3 can be improved effectively by microstructure modification.The research results of Ag Nb O3 bulk materials indicated the validity of first principle calculation in properties prediction of perovskite niobates.The intrinsic and anisotropic properties of KNb O3 crystal were investigated based on DFT.The variation law of electronic structure of 2D KNb O3 respect to dimension reduction was studied by DFT calculation,together with the prediction of their intrinsic physical properties.Results of DFT calculation showed that different surface energies of each facet affected the self-driven growth process directly,and the degeneracy of conduction band would decrease with quantum confinement showed up by dimension reduction.Meanwhile,it expanded the bandgap.Materials transformed from insulator to metal when thickness approached nanometers.2D KNb O3 with(100)facet showed two dimensional free electron gas(2DFEG),while(111)and(120)facets showed two dimensional free hole gas(2DFHG).Based on previous calculation results and the environment of hydrothermal reaction,the precursor model of K(Nb O2)for 2D growth was built.Surface energy calculation showed that the drivenforce of(001)facet was much larger than(100)facet,theoretically predicted the possibility of 2D growth of materials induced by local energy instability.The 2D growth process was designed based on above 2D growth model.The effects of thermodynamics and dynamics to the growth controllability of two dimensions were studied.The process described as:the liquid precursor was synthesized under the reaction situation of 160 ℃/12 h by hydrothermal method.Then,the 2D hydrate K(Nb O2)OH·x H2O were obtained by in-situ growth under60℃ in open air.The in-situ growth process listed as:the observed 2D sheet preferred to grow laterally after nucleation,approaching its lateral size limit within the limited time.After which,it grew in the vertical direction layer-by-layer.Layer stacking started from energy singularity point,meanwhile maintained the tendency of 2D growth.Finally,2D KNb O3 with perovskite structure was obtained after annealing at 600 ℃.The phase transition from as grown hydrate precursor to perovskite structure was verified by the characterizations of crystal structures,elements and valences.Photoluminescence measurements were carried on materials,which revealed the variation law of electronic structure and electronic states transition respect to dimension reduction.Research showed that strong Raman induced PL phenomenon was existed in hydrate K(Nb O2)OH·x H2O,and its intensity kept the linear increase relationship with incident power.Its intrinsic PL spectrum revealed its potential application in white light emission source.After annealing,perovskite KNb O3showed potential for blue light emission device application.The PL peak could be divided into two part,interband transition(Nb 4d-O 2p)and self-constrained excitons(STEs).The intrinsic peak blue shifted from 3.07 eV of 22 nm sample to3.43 e V of 0.9 nm sample,indicating an obvious quantum confinement effects.The relative intensity of intrinsic peaks showed restoring behavior.Analysis believed that the restoring behavior is contributed by the increase of quantum confined electronic states with dimension reduction,validating the existence of2DFEG/2DFHG in nanometers size.The second harmonic generation(SHG)and its thickness dependent properties of materials were investigated by home-built nonlinear optical test system.The intrinsic relation of phonon-exciton-photon interaction was investigated.The behavior of exciton enhanced second order nonlinear response was studied.Results showed that STEs resonance would enhance SHG of material effectively.With thickness decrease,the oscillation strength of STEs increased in a 2D quantum well,where oscillator strength was propositional to 1/r,leading to a linear increase of second order nonlinear susceptibility with thickness decreasing.The second order nonlinear susceptibility was elevated by more than 50 times compared with bulk materials when the thickness of 2D well is 0.9 nm.The phonon-exciton-photon interaction physical model was built by virtue of fourth order time dependent perturbation theory.Experiments indicated that the splitting and shifting of SHG peaks were induced by the perturbation of low frequency vibration modes(TO)of KNb O3 crystal lattice.