Interfacial Polarization Induced By Strain Engineering At Ferroelectric Surfaces And Exploration On Optically Promoted Pyroelectric Applications

Symmetry is a core concept that constitutes modern physics,and crystal material inversion symmetry breaking often makes the material polar.Ferroelectric crystal materials lack center of symmetry and possess spontaneous polarization,exhibiting a series of important physical effects such as ferroelectricity,piezoelectricity,pyroelectricity,electro-optics,etc.It has major applications in the fields of national economy,national defense and military,information and communication,medical care and health,etc.The existence of polarization of ferroelectric materials makes them very sensitive to external fields(such as electric,light,heat,strain,acoustics,etc.)and surface/interface structural characteristics.The control of material polarization can be realized by stress field combined with material microstructure defect regulation,such as:epitaxial strain,composition,ion doping,mechanical boundary conditions,etc.In recent years,some novel improvement methods have emerged,such as large spontaneous polarization induced by interphase strain,and considerable piezoelectricity and pyroelectricity induced by interfacial asymmetric Schottky junction.The traditional improvement method of pyroelectricity shifts the ferroelectric-to-paraelectric phase transition closer to room temperature through chemical substitution to obtain a large pyroelectric coefficient.The application of pyroelectric effect is mainly focused in high-performance pyroelectric detectors.In recent years,pyroelectric energy harvesting and pyroelectric manipulate microfluids have also begun to attract attention and research.In this dissertation,periodic nanoindentations are introduced into the ferroelectric crystal material lithium niobate(LiNbO3)based on strain engineering,and the effects on the crystal structure and interfacial polarization including piezoelectric and pyroelectric properties are investigated.A ferroelectric crystal material lithium tantalate(LiTaO3)is also used to explore and develop the application of an optically-induced pyroelectric effect to manipulate microfluidics.In the first chapter,we have reviewed the basic properties of ferroelectric materials,the phenomenological theory and soft mode theory of ferroelectric phase transitions,the frontier research hotspots of ferroelectric materials,and the improvement methods of piezoelectric and pyroelectric properties and applications.In the second chapter,we have calculated the spontaneous polarization and piezoelectric coefficient of LiNbO3 based on first principles.The main contents are as follows:(1)We have introduced density functional theory,modern polarization theory,and VASP software.(2)Based on first-principles,the Berry phase method was used to calculate the spontaneous polarization of LiNbO3.The dependence of the spontaneous polarization and strain along the z-axis was obtained.It was found that when the elastic deformation changed by 2%,the spontaneous polarization changed by about 4 μC/cm2.(3)The piezoelectric stress tensor e33 of LiNbO3 is obtained and the elastic matrix under elastic deformation is calculated.The dependence of the piezoelectric coefficient d33 on the tensile deformation is obtained.When the tensile deformation increases by 2%,the d33 increases from 9.6 to 10.3 pC/N.The above indicates the limited improvement of the spontaneous polarization and piezoelectric coefficient of LiNbO3 by elastic deformation.In the third chapter,we have studied the effects of nanoindentation on LiNbO3 crystal structure,piezoelectric coefficient d33 and pyroelectric coefficient p.The main contents are as follows:(1)Experimental tests show that periodic nanoindentation increases the d33 of LiNbO3 by 130%and the pyroelectric coefficient p by 110%.(2)We tested the mechanical properties(Young’s modulus and hardness)of LiNbO3 crystals using a nanoindenter.Characteristic points of elastoplastic transition(pop-in phenomenon)are observed,indicating that plastic deformation starts here.High-resolution transmission electron microscopy(HRTEM)analysis of the cross-section of single indentation which prepared by Nano indentation instrument revealed a possible NbO2 phase.The amorphous region still has a certain short-range periodicity,indicating that ferroelectric nanodomains may be generated.(3)Micro-region Raman spectroscopy analysis of a single indentation showed that the indentation caused residual stress.The Raman peaks appear as possible components of the NbO2 phase or crystal orientation rotation.The nonlinear optical second-harmonic(SHG)signal enhancement around the indentation indicates polarity enhancement or crystal orientation rotation.(4)Using a patterned sapphire substrate as a template,LiNbO3 wafer was mechanically pressed to prepare a large-area periodic nanoindentation structure.X-ray diffraction(XRD)analysis showed that the indentation caused partial amorphization.Micro-region Raman analysis shows that the compressive stress runs through the thickness of the crystal along the zaxis.Oxygen vacancy defects were found by X-ray photoelectron spectroscopy(XPS),and the possibility of forming a dipole with Li vacancy defects was discussed.The hysteresis loop results show that the coercive field decreases and the remanent polarization of indented LiNbO3 nearly doubles,indicating that the indentation increases the internal defects and polarization of the crystal.(5)In the fourth chapter,we have explored and developed an application of optically induced pyroelectric effect to manipulate water droplets by using the ferroelectric crystal material LiTaO3.The main contents are as follows:(1)A method for manipulating water droplet by optically-induced LiTaO3 pyroelectric effect was developed.A rapid wettability transition of the contact angle from 151.0° to 74.5° in a few seconds was achieved,and the reversibility conditions for the wettability transition were investigated and realized.(2)For the first time,the influence mechanism of interfacial charge distribution on wetting behavior was investigated.It realizes the control of the movement posture of droplet deformation,spreading and wetting,and droplet jumping.Based on the interface localized charge effect,Taylor cones and Plateau-Rayleigh instability phenomena were observed,and the deformation,jumping,horizontal movement and splitting of droplets under the self-focusing effect were realized.(3)The conductivity effect of droplet wetting was investigated and it was found that droplet conductivity has a significant effect on wettability.(4)A laser droplet pipette was designed and manufactured,which realized the loss-free pipetting and transfer of droplets;realized the pinning and static control of rolling droplets on the inclined surface;realizes the control of water droplet movement by electric heating.Finally,we summarized the research work of this dissertation and prospected the future research direction.The innovation of this dissertation are as follows:(1)A strategy to improve the interface polarity by plastic deformation was proposed,and the regulation mechanism of nanoindentation on the piezoelectric and pyroelectric properties of lithium niobate was studied,and the piezoelectric and pyroelectric properties were significantly improved.The test shows that the piezoelectric coefficient is increased by 130%,and the pyroelectric coefficient is increased by 110%,and the influence mechanism of elastic deformation,second phase,nano-domain and other factors on the improvement of piezoelectric performance is discussed.This is a strategy for plastic deformation to enhance interfacial polarity and has the potential to enhance properties of materials whose chemical composition has been fixed.(2)We have explored and developed a microfluidic manipulation technology based on the pyroelectric effect.The influence mechanism of interfacial charge distribution on wetting was investigated.Taylor cones,Plateau-Rayleigh instability phenomena were observed.A variety of attitude manipulations such as spreading,deformation,jumping,splitting,and translation of water droplet microfluidics are realized.The mechanism of the effect of droplet conductivity on droplet wetting was investigated.(3)A laser pipette for loss-free pipetting of droplets was fabricated using the pyroelectric effect.Expanded the application of pyroelectric effect.