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

在這本論文當中，我們主要研究三個在奈米結構的物理現象。首先是硒化鉛量子點的自組裝行為，接著是硒化鉛量子點陣列中的電荷集體傳輸行為，最後一部份探討的是氧化鋅摻雜鈷奈米線中的鐵磁性。 由於溶凝膠技術的發展，科學家們可以大量地製造出尺寸為數奈米到數十奈米的半導體量子點，並以此研究奈米尺度下的各種物理現象，諸如量子侷限效應、庫倫阻滯效應、人造原子能態、以及自組裝行為。為了瞭解量子點自組裝行為的各種機制以及量子點間由於相互藕合所產生的集體效應，我們利用一種簡單的方法來製造二維量子點島嶼。此方法僅須將含有量子點的溶液滴在預先加熱好的基板上，待溶劑揮發後，量子點即自組裝而形成各種大小的二維島嶼。觀察二維島嶼的島嶼密度和溫度及覆蓋度的關係，我們可研究各種島嶼成長的機制，包括spinodal相分離、尺度函數及擴散限制聚集。將實驗的統計數據和各種成長的理論進行擬合後，可以推算出量子點間相互之間的鍵結能和形成穩定核種的臨界尺寸，並且決定出各種成長圖形在相圖中的位置。經由控制基板的溫度和溶液的濃度，我們可以在spinodal分解和擴散限制聚集兩種機制之間相互調制而產生出不同形貌的二維量子點島嶼。這兩種機制的相互作用使得spinodal圖形的特徵波長和碎形島嶼的維度發生變化。我們利用上述的方法在石墨和金的平面上製造出各種大小和形狀的二維自組裝量子點島嶼。 二維或三維量子點陣列的電性傳輸行為至今仍是一個熱門的主題。有許多的理論工作在討論量子點陣列當中的庫倫作用力和集體庫倫阻滯的現象，而相關的實驗工作也同時在進行，特別是針對半導體量子點。為了研究陣列當中量子點相互的耦合作用，我們在金的平面上製造出不同大小的二維硒化鉛量子點陣列，再利用掃描穿隧電流顯微鏡量測室溫及液態氮溫度下的穿隧電流能譜。我們首先利用雙穿隧接面模型對各種大小陣列的穿隧電流能譜進行擬合，並且求得量子點陣列和基板之間的等效電容。結果顯示此等效電容並不隨著陣列的大小而成正比的增加，以此整個陣列並不能被視為一個整體導電的島嶼。我們計算了探針和陣列之間、陣列和基板之間以及在陣列當中兩子點相互之間的電容，發現可以用集體傳輸的理論對電壓-電流曲線進行分析。電流曲線隨著電壓呈現指數的變化，並且此指數會隨陣列的尺寸增加，顯示量子點彼此之間具有電容式耦合。 在氧化鋅摻雜鈷奈米線中的鐵磁性的主題中，我們利用氣相傳輸法製備出各種不同尺寸的氧化鋅奈米線，其直徑可由奈米金催化顆粒的大小來控制。接著以離子佈植法將鈷離子摻雜入奈米線而形成稀磁性半導體奈米線，並利用掃描式電子顯微鏡及穿透式電子顯微鏡觀察奈米線的外觀形貌及內部的晶格結構，再以超導量子干涉儀進行磁性的量測。摻雜鈷後的奈米線呈現微弱的鐵磁性，而且飽和磁化量和矯頑力隨著奈米線的直徑而增加。在經過真空環境下的退火處理後，氧化鋅摻雜鈷奈米線呈現出明顯的室溫鐵磁性，由穿透式電子顯微鏡的影像顯示此鐵磁性並非來自於鈷的金屬團簇。我們進一步發現真空退火處理對較細的奈米線會有較大的鐵磁增強效果。另外，由零場冷卻和帶場冷卻磁化率曲線的分歧以及隨溫度變化的矯頑力等現象顯示此奈米線具有超順磁的特性亦即表示單磁疇的存在。我們推測此奈米線鐵磁性的來源是由於真空退火所產生的氧空缺將奈米線中的鈷離子耦合起來而產生。利用磁力顯微鏡對單一的奈米線進行觀察，我們發現奈米線的磁疇結構分為橫向磁化及縱向磁化兩種。而單一奈米線的磁化強度可由雙磁偶極模型估算出。

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.