Size, Temperature, Pressure And Multiple Field Coupling Effect On Physical Properties Of ZnO

Extensive researches revealed that ZnO with variations of size (in nanometerscale), temperature, and pressure show the unusual behaviors in elasticity, band gap,Raman shift, and melting point temperature. It is hard to explain the unusualbehaviors by existing theory models, which means that existing theories can notprovide theoretic direction for the design of ZnO devices. Therefore, it is highlydesired to improve existing theories or develop a new theoretical model to reveal thekey factors and the physical mechanism behind them. This dissertation focuses onsize, temperature, and pressure dependence of the elastic modulus, band gap, Ramanshift, and melting point of ZnO based on the bond-order-length-strength (BOLS)correlation mechanism and the local bond average (LBA) approach. A set ofanalytical expressions connecting the elasticity, band gap, Raman shift, and melt pointdirectly to the bond identities of the ZnO and their response to the intrinsiccoordination imperfection and the applied stimulus (temperature and pressure) arebuilt, and size, temperature, and pressure dependence of the elastic modulus, band gap,Raman shift, and melting point of ZnO are unified to the change of the bondparameters due to the alteration of coordination environment and external stimulations,and the respondend the change of atomic cohesive energy, energy density, crystalpotential, and electron-phonon coupling. Finally, we revealed the key factors andfound the physical mechanism of the size, temperature, and pressure dependence ofthe elasticity, band gap, Raman shift, and melting point temperature for ZnO. Theobtained results in this dissertation are summarized as follows:1. It has been clear that: the elastic modulus of ZnO nanostructures increases withthe decrease of feature size, which originates from the energy density gain due to thebond contract, bond strength gain of surface atoms. Size induced the changeamplitude of the elastic modulus is determined by surface-to-volume ratio.High-temperature or low-pressure causes the decrease of the elastic modulus due tothe decrease of the energy density. The bond nature indicator is acquired by thesize-dependence of Young′smodulus while the bond energy and Debye temperatureare acquired by the temperture-dependence of Young′smodulus.2. It is demonstrated that the band gap of ZnO is related to the crystal energy,andproportional to the single bond energy. The bond contract, bond strength gain of surface atoms increase the crystal energy and the effect of the electron-phononcoupling which will cause the expansion of band gap, the blue shift ofphotoabsorption and photoluminescence as the size of ZnO nanostructures decreases.High-temperature or low-pressure causes the decrease of the band gap due to thedecrease of the single bond energy.3. Raman shift is determined by the atomic coordination number, bond length, andsingleH bond energy. The optical Raman modes peaks shift towards lower frequency asthe size of ZnO nanostructures decreases due to the competition among thecoordination number imperfect, bond contract, bond strength gain of surface atoms.The low-frequency Raman (LFR) acoustic modes originate from the intergraininteraction which increases with miniaturization of ZnO nanoparticles, so that theLFR peaks shift towards higher frequency. High-temperature or low-pressure causesthe optical Raman modes peaks to shift towards lower frequency due to from bondexpansion and bond weakening.4. The relation between the melting point and size, pressure is built. Size andpressure dependence of the melting point of ZnO is predicted based on the obtainedbond nature indicator and the single bond energy. Besides, size induced the decreaseand pressure induced the increase are analysed.