觀察奈米結構之物理性質: I.自組裝硒化鉛量子點陣列之電荷集體傳輸行為 II.氧化鋅摻雜鈷奈米線之鐵磁性

Abstract

In the thesis we study three physical phenomena in nanostructures: self-assembly of PbSe quantum dot array, collective transport in PbSe quantum dot arrays, and ferromagnetism in Zn1-xCoxO nanowires. Semiconductor quantum dots with diameters of several to tens of nanometers have been largely synthesized through colloidal techniques for nanoscience exploration of quantum confinement, Coulomb staircase, and artificial-atom states in individual quantum dots and self-assembling growth behavior. To learn about the underlying physics of self-assembly, growth mechanisms, and coupling-induced collective properties, here we report a facile way of preparing nanocrystal-assembled 2D islands by drop-casting nanocrystal suspension on a hot substrate. Growth mechanisms such as scaling function, spinodal decomposition phase separation, and diffusion-limited aggregation are investigated based on the observation of quasi-monolayer coverage. After a curve fitting to several theoretical growth models, the pair bond (interaction) energy, critical nucleus size, and the phase of growth patterns were determined. Moreover, by heating the substrate and controlling the concentration of nanocrystal suspension, the spinodal decomposition and diffusion-limited aggregation can be tuned to modulate growth patterns of 2D nanocrystal islands. The interplay of these two mechanisms results in a variation of wavelength in spinodal growth patterns and of fractal pattern dimensions. By using this experimental approach, various sizes and shapes of nanocrystal-assembled 2D islands can be deposited on a flat surface of either graphite or gold. Although charge transport of three-dimensional quantum dot arrays has been attempted for study on the micron scale, the electrical properties of a nanoscale array, self-assembled from a single quantum dot through a bottom-up procedure, have not been explored yet. Inter-dot Coulomb interactions and collective Coulomb blockade were theoretically argued to be a newly important topic, and experimentally identified in semiconductor quantum dots. To study the interdot coupling, we control growth parameters to self-assemble different sizes of PbSe quantumdot arrays on flat gold surface for scanning tunneling spectroscopy measurements at both room and liquid-nitrogen temperatures. The current-voltage curves of the arrays are analyzed using a double-barrier tunnel junction model to acquire the shunt capacitance between the array and the gold substrate. The increment of this capacitance is small as the particle number increases extremely from 1 to 80. Thus the array cannot be taken as a simple semiconductor island. The tip-to-array, array-to-substrate, and interdot capacitances are evaluated and the tunneling spectra of quantum-dot arrays are analyzed by the theory of collective Coulomb blockade. The current–voltage of PbSe quantum-dot arrays conforms properly to a scaling power law function. The dependence of tunneling spectra on the sizes (numbers of quantum dots) of arrays is reported and the capacitive coupling between quantum dots in the arrays is explored. In the topic of ferromagnetism in Zn1-xCoxO nanowires, diameter controllable ZnO nanowires have been fabricated by thermal evaporation (vapor transport) with various sizes of gold nanoparticles as catalysts. Diluted magnetic semiconductor (DMS) Zn1-xCoxO nanowires were then made by high energy Co ion implantation. The morphology and crystal structure of the nanowires were inspected by use of scanning and transmission electron microscopes. Magnetic properties of the Zn1-xCoxO nanowires were measured by employing a SQUID magnetometer. The as-implanted Zn1-xCoxO nanowires displayed weak ferromagnetism and size dependent behavior has been observed in the magnetic field and temperature dependences of magnetization. The shrinkage of the nanowire diameter reduced the spontaneous magnetization as well as the hysteresis loops. After high-vacuum annealing, Zn1-xCoxO nanowires exhibited strong ferromagnetic ordering at room temperature. Electron microscopy analysis was used to ensure the absence of Co nanocrystals in the annealed nanowires. The effect of annealing on the creation of a strong ferromagnetic state is much more pronounced in thinner nanowires. From field cooled and zero-field cooled magnetization and coercivity measurements between 2 and 300 K, superparamagnetic features were observed in the Zn1-xCoxO nanowires. We argue that the generation of point defects by vacuum annealing is the origin for the enhanced ferromagnetism in the Zn1-xCoxO nanowires. We employed magnetic force microscopy to verify the ferromagnetism in individual Zn1-xCoxO nanowires. Two kinds of domain structure, transverse and longitudinal, were observed in ferromagnetic nanowires and the magnetic dipole moment of individual nanowires was estimated by fitting to a two magnetic point dipole moment model.