一維鎳奈米柱，鎳-氧化鎳、鎳-二氧化鈦核殼奈米結構之製造與研究

Abstract

本論文是研究結合陽極氧化鋁模板(AAO)及無電鍍技術(electroless plating)在矽基板上製作出規則排列的鎳奈米柱陣列(Ni nanorod array)。研究中發現Ni nanorod array之磁性表現與其微結構有直接之關係。穿透式電子顯微鏡(TEM)的分析結果發現Ni nanorod array之結構為微晶結構(nanocrystalline)其平均晶粒尺寸2~3nm，此微晶結構導致Ni nanorod array表現出超順磁(superparamagnetic)的磁性行為。Ni nanorod array經400℃退火後發現磁性表現由超順磁轉變為鐵磁(ferromagnetic)行為，利用TEM觀察Ni nanorod array之暗視野影像(dark field image)發現經退火後Ni nanorod array之微結構由微晶轉變為片狀(laminar)之晶粒結構，由橫截面的暗視野影像發現片狀結構之成長方向平行Ni nanorod array之長軸。研究結果發現微結構的改變驅動超順磁轉變為鐵磁的磁性相變化行為。 研究中的另一主題是將Ni nanorod array置於大氣氣氛下退火後製作出鎳-氧化鎳之核-殼奈米柱陣列(Ni-NiO core-shell nanorod array)。HRTEM發現經大氣下退火後Ni nanorod表面生成多晶結構(polycrystalline)之氧化鎳殼層，鎳柱與氧化鎳殼層之界面發現有第二相析出。不同的退火條件造成氧化鎳殼層厚度及鎳柱微結構的改變而此改進而影響Ni- NiO core-shell nanorod array之電性及光感測的表現。鎳-氧化鎳所形成的異質接面表現出整流特性，研究中發現此接面在未施加外加偏壓情況下，紫外光(ultra-violet)照射時會有顯著的光電流(short-circuit photocurrent)產生，此結果顯示Ni- NiO core-shell nanorod array有做為紫外光感測器的潛力。 研究中的最後主題是利用原子層沉積技術(ALD)在Ni nanorod上沉積二氧化鈦(TiO2)進而製作出鎳-二氧化鈦之核-殼奈米柱陣列(Ni-TiO2 core-shell nanrod array)。TEM分析結果得知利用ALD所沉積之二氧化鈦殼層為多晶結構且厚度相當均勻其厚度為11nm。研究中發現鎳-二氧化鈦(Ni-TiO2)之異質接面具有整流特性且在未施加外加偏壓情況下，紫外光照射時會有顯著的光電流產生，此結果顯示Ni-TiO2 core-shell nanrod array有做為紫外光感測器的潛力。此外將Ni-TiO2 core-shell nanrod array與TiO2 nanotube array兩者做紫外光感測效果上之比較，結果發現接面面積越大則產生之光電流越大。

This dissertation investigates the fabrication and characterization of 1-D nanorods. This dissertation divided into three parts. The first part is study on the microstructures and magnetic properties of nickel nanorods using an anodic alumina oxide template and electroless deposition were investigated. The as-deposited nanorods were found to contain nanocrystalline grains with an average size of ~ 2-3 nm. The temperature-dependent magnetic hysteresis curves indicated a superparamagnetic behavior of the as-deposited rods as a result of the reduction of ferromagnetic crystallites. The superparamagnetic (SM) Ni nanorods transformed into ferromagnetic (FM) ones when annealed at 400 □C. Results from dark-field transmission electron microscopy (TEM) reveal that the microstructure of the rods tends to form a laminar structure with grain growth parallel to the long axis of the rods, together with the enhancement of ferromagnetic ordering along the same direction. The results suggest that the obtained SM-FM phase transition is microstructure-driven. The second part relate to the fabrication of highly ordered Ni-NiO core-shell arrays were fabricated by directly annealing Ni arrays at a desired temperature, naturally forming a nano-size semiconductor (NiO)-metal (Ni) heterojeuction at the surfaces of the nanorods. High resolution TEM reveal the developing mechanism of core shells, showing an involvement of a second phase segregation at Ni/NiO interface upon annealing. Results suggest that the increase of annealing time leads to the modifications of microstructure that are responsible for the electrical and photoconductive properties of the devices. The developed nanojunction enables the device’s functionalities including current-rectifying and short-circuit photocurrent generating, showing an environmentally friendly nature. These make the device worthy of being considered an alternative for a UV photodetector. The third part relate to the fabrication of well-aligned Ni-TiO2 core-shell nanorod arrays on Si substrate by electroless deposition using anodic alumina oxide and atomic layer deposition technique. Results from TEM and high-resolution TEM indicated that the as-prepared samples were a vertically well-aligned Ni-TiO2 core-shell nanorod array, and the outer TiO2 shell was polycrystalline anatase phase with a thickness about 11nm. The asymmetry of the current-voltage curve reveled that a schottky barrier formed between the Ni core and TiO2 shell. The enhanced separation of photogenerated holes and electrons was demonstrated by photoluminescence and photocurrent measurement. Under UV light irradiation, the short-circuit photocurrent of the Ni-TiO2 core-shell nanorod arrays was eight times large than that of the TiO2 nanotube arrays.