氧化鐵奈米顆粒之電子傳輸與低磁場磁電阻研究

Abstract

本論文主旨為研究氧化鐵奈米顆粒(Fe3O4 nanoparticles (NPs))與二氧化矽包覆氧化鐵奈米顆粒(Fe3O4@SiO2 NPs)在電荷傳輸上的分析與比較，以及在低磁場的作用下，兩種材料在變溫時的磁電阻表現。本實驗用電子束微影及熱蒸鍍製作完整的奈米電子元件，以熱退火減少接點電阻，並使用原子力顯微鏡測量樣品的幾何面積與高度，最後利用兩點量測法，在不同溫度下量測電流-電壓曲線來觀察導電行為。常溫下Fe3O4@SiO2 NPs的電導率比Fe3O4 NPs低了一個數量級。根據三維Mott變程跳躍傳輸理論，對所有樣品的電導率與溫度關係圖做擬合分析，得到Fe3O4@SiO2 NPs的特徵溫度約1 X 108 K，比氧化鐵奈米顆粒的2 X 107 K大5倍左右。為瞭解材料的局域長度，本實驗利用場效電晶體的操作模式，使用液態離子(ion liquid)當作閘極以施予偏壓，測得兩種氧化鐵樣品皆呈現電洞傳輸模式，且可以獲得Fe3O4 NPs和Fe3O4@SiO2 NPs的遷移率分別是1 X 10-2 cm2 V-1 s-1和1 X 10-3 cm2 V-1 s-1，再由電導率可以估計電荷密度大小，最後從變程跳躍傳輸理論的特徵溫度求出局域長度與跳躍距離，其結果顯示強局域化特性。 將磁場方向固定垂直於樣品表面，量測磁場變動下的電阻變化，觀察到特殊的正磁阻效應。由實驗數據觀察到高溫呈現負磁阻效應，推測電阻變化與鐵磁性樣品的磁矩受到磁場影響產生偏轉有關；低溫呈現正磁阻效應，推論磁場對局域態之波函數產生縮減的影響，電子傳輸因所需的平均跳躍距離增加而變得困難。最後結合磁電阻效應與三維變程跳躍傳輸，觀察到B.I. Sjklovskii提出的電阻變化率與磁場平方成正比的趨勢。

In this work, electron transport in Fe3O4 and Fe3O4@SiO2 nanoparticles (NPs) are investigated and differentiated. The two kinds of nanoparticles, showing superparamagnetism, are confirmed by superconducting quantum interference device. The X-ray diffraction (XRD) data of both Fe3O4 and Fe3O4@SiO2 NPs exhibit the crystalline structure of Fe3O4. The two kinds of NPs were each dispersed in n-Hexane solution. The solution was dropped on a Si/SiO2 substrate so as to make two-probe devices for electrical property measurements. The temperature and electrical field dependent conductivity of Fe3O4 and Fe3O4@SiO2 NPs are systematically studied. The conductivity of Fe3O4 NPs can be fitted by the theory of three-dimensional Mott’s variable range hopping transport. The model can describe well the temperature behavior from 80 K to 300 K for Fe3O4 NPs but it can only fit to the data at high temperatures for Fe3O4@SiO2 NPs. The estimated temperature parameter ( ) of Fe3O4@SiO2 NPs is much higher than Fe3O4 NPs. On the other hand, from the field effect behavior, we obtained that the mobility of Fe3O4@SiO2 NPs is smaller than that of Fe3O4 NPs. The small mobility magnitudes confirm that both Fe3O4 and Fe3O4@SiO2 NPs are in the strong localization regime. Beside electron transport properties, we carried out magnetoresistance (MR) measurements. We discovered negative MR at high temperature and positive MR at low temperature for both Fe3O4 and Fe3O4@SiO2 NPs. The negative MR is due to the polarized magnetic moment of the magnetic NPs which could reduce spin scattering in electron transport. The positive MR could account for the shrinkage of localized electron wave functions under a magnetic field.