The Synthesis And Pressure Effects Of Nano Functional Materials

High pressure science has been an important field in modern science and technology.As another condition in addition to temperature, pressure affects many propertiesof materials to a large extent. The primary effect it brings is to reduce the distancebetween molecules or atoms of the materials, which results in the variation of structure(crystal structure, molecular structure, the alignment of atoms), and induces further aseries of changes on, such as, the energy band structure, the electron orbital configurationand density of states of electrons. Meanwhile, high pressure (or together withhigh temperature) can be a highly effective method on synthesizing various functionalmaterials. For those reasons above, investigations of physical properties and synthesizingfunctional materials under high pressure condition is of great importance. Thisthesis concerns thus these two aspects: The pressure effects on physical properties ofseveral kinds of nanocrystallines are studied, and the ZnO-based nano-sized functionalmaterial is synthesized under high temperature and high pressure. The thesis consistsof seven chapters.Chapter One briefly introduces high pressure techniques and research methods aswell as presents a review of high pressure effects and research progress in related fields.Chapter Two gives an introduction to nanomaterials and nanotechnology and,mainly summarizes high pressure studies on physical properties of nanomaterials ofdifferent dimensions.Chapter Three studies the pressure dependence of photoluminescence of CdSe,CdZnSe alloy and CdSe/ZnS core/shell quantum dots (QDs) synthesized via a hotsolutionmethod. High pressure technique was used to compare the gap turning and thestructural properties of these QDs. It is found that a phase transition from wurtzite torocksalt occurs at about 5.9 GPa for CdSe QDs, and 5.4 GPa for CdZnSe alloy QDs.The existence of Zn element plays an important role on the structural stability of CdSeQDs. The directΓenergy gaps of wurtzite QDs were found to increase linearly withpressure before phase transition; the pressure coefficient is about 35.4 meV/GPa for CdZnSe, and, 28.4 meV/GPa for CdSe QDs. The larger value of gap pressure coefficientfor CdZnSe alloy QDs is attributed to the alloying effect for strengthening theanion-cation s - s orbital coupling and weakening p - d orbital coupling in the alloy.It is found that CdSe/ZnS core/shell QDs transform to rocksalt at about 7 GPa, indicatingthat the wide gap semiconductor shell contributes significantly to the structuralstability of CdSe core. Furthermore, the luminescence peak is nonlinearly blueshiftedwith increasing pressure, therefore, the surface modification and alloying play differentroles in the modulation of the energy gap of CdSe QDs.In Chapter Four, the 5.6 nm, 8.6 nm, 19.6 nm TiO₂ and Eu³⁺-doped TiO₂ nanocrystallinesare synthesized by the sol-gel method and hydrothermal treatment. The structuralstability of these nanocrystallines on pressure is investigated by Raman and photoluminescencespectroscopies. A phase transition is observed at 8.2-10.6 GPa forEu³⁺-doped TiO₂ from Raman spectra. This is also confirmed by the fact that the PLintensity ratio, I₂(⁵D₀-⁷F₂)/I₁(⁵D₀-⁷F₁), reduces with increasing pressure and has ainflection at 8 GPa. For TiO₂ of different sizes, Raman spectra show that, the sampleswith sizes smaller than 10 nm transform to amorphous structure and another 19.6 nmTiO₂ transforms to baddeleyite structure at 10.3-13.3 GPa. This indicating that the sizeeffect plays an importmant role on pressure induced phase transition of nanomaterials.In Chapter Five, the fluorescence spectra of both Rhodamine 101 powder and solution(mixture of methanol, ethanol and water as solvent) have been measured underhigh pressure. It has been found that the emission property of Rhodamine 101 powderis quite different from that in solvent under high pressures. For Rhodamine 101 powder,the fluorescence intensity drops quickly with the increase of pressure and disappearsalmost at 8 GPa, while the emission peaks shift to lower frequency dramatically (about100 nm within 8 GPa). For Rhodamine 101 solution, the fluorescence intensity decreasesslowly with the increase of pressure; the emission intensity at 13 GPa is about10% of that at the ambient pressure. Moreover, the emission peak position decreasesmore slowly (about 50 nm within 13 GPa). The change of molecular structure andspecial solvent effect are the primary factors that influence the emission character ofRhodamine 101 powder and solution under high pressure, respectively.In Chapter Six, a mass production of single crystal ZnO nanowires with wurtzite structure were synthesized by a simple and rapid method based on thermal evaporationwithout the use of noble metal catalyst. The defect and near band edge emissionat room temperature are considered to come from the singly ionized oxygen vacancyand the overlap of free exciton and free-to-band emission respectively by EPR and lowtemperature photoluminescence spectra. The coupling parameters of exciton-acousticphonon and of exciton-longitudinal-optical phonon were determined as 63.44μeV/Kand 898 meV, and 251 K for Einstein temperature by fitting the temperature dependenceof free exciton emission peak energy and FWHM to empirical formulas. Theflower-like ZnO nanostructure are synthesized via hydrothermal method without anysurfactant. The growth mechanism is discussed and the optical properties are alsoinvestigated in detail by absorption, luminescence and Raman spetra. Magnetic Co²⁺-doped ZnO nanorods are fabricated via direct hydrothermal synthesis. The measurementsof XRD and absorption spectra demonstrate the presence of cobalt in the +2state in a tetrahedral crystal field. The room-temperature ferromagnetism is attributedto inherent property of the materials caused by the presence of cobalt in high-spinconfiguration in a tetrahedral crystal field.Chapter Seven summarizes the contents of six chapters above.