奈米線之製備及其熱、電傳導與熱力學性質之研究

Abstract

許多實驗結果顯示低維度材料具有與塊狀材料迥異的特性，這些性質的改變可能肇因於材料尺寸的縮減。以一維量子線之奈米線為例，其電子能態的分佈受到量子侷限限效應的改變，或因表面原子的比例增加，這些都可能是導致材料熱、電、磁等性質改變的原因。為了研究奈米線的各種物理性質，我們分別利用自組構與微影製程這兩種方法製備群聚之奈米線陣列及單一鎳奈米線。我們以電化學沉積法讓材料填充於陽極氧化鋁(AAO)模板的孔洞中，製作奈米線陣列。以此方法我們製作六角形排列之鐵(Fe)與三碲化二鉍(Bi2Te3)奈米線陣列，用以研究其群聚之磁性與電性。其中鐵奈米線之線徑約60及200奈米，而三碲化二鉍奈米線之線徑則約60奈米。X光繞射顯示，60奈米的鐵與三碲化二鉍奈米線陣列均具有較好的結晶性，而線徑約200奈米之鐵奈米線陣列，並無此明顯特徵。進一步的磁性量測結果顯示，60奈米鐵之飽合場較小且矯頑場較大，顯示60奈米鐵確實具有較強之磁異向性能，此外其在小外加磁場中磁阻的變化與外加磁場大小呈現平方關係，與材料之退磁場效應相關。 金屬的許多特性均與電子的行為相關，如金屬的熱傳導主要來自電子的貢獻，因此金屬材料的電導率與熱導率比值遵守Wiedemann-Franz 定律，研究單一鎳奈米線之熱導性與電導性可以獲得電子與聲子在一維材料中的傳輸行為，並進一步瞭解電子在材料中的特性。我們整合光與電子束微影製程、薄膜沉積及蝕刻技術用以製備一根懸橋結構之鎳奈米線，其截面為100奈米厚及180奈米寬之矩形，長約35微米。磁阻之量測顯示單一之鎳奈米線仍保有鐵磁性，其矯頑場約500 Oe。我們用自行開發的電性量測系統與三倍頻技術(3ω technique)量測此鎳奈米線在溫度15~300 K之間的電阻率、熱傳導率與比熱，其室溫電阻率約為36 μΩ-cm，殘餘電阻率約17 μΩ-cm，因此其RRR ~ 2 (Residual Resistivity Ratio)，較一般塊材小，顯示此奈米線中具有許多雜質與缺陷，使電子受到嚴重的散射。熱導率量測之結果亦顯示奈米線之室溫熱導率約為塊材的20%，且其值隨溫度下降而下降，明顯與塊材不同。熱傳導率與電導率的量測結果顯示奈米線其電聲子傳導特性僅於75-300 K之間符合接近Wiedemann-Franz 定律，顯示鎳奈米線中熱流的傳播比電流更受到壓抑。

There are experiments revealing changes of physical properties in low dimensional materials likely due to size reduction. In quasi one dimension nanowires, the quantum confinement and surface effect may affect their magnetism, transport, and thermodynamic properties. To evaluate these one-dimensional properties a bottom-up method was used to fabricate iron and Bi2Te3 nanowires in an AAO template using chemical electrodeposition. The average diameters of two highly ordered iron nanowires are about 60 and 200 nm, respectively, and that of Bi2Te3 nanowires is 60 nm. Magnetization measurements show a larger anisotropic magnetization in both 60-nm nanowires. It is illustrated by the formation of magnetic easy axis and preferred crystal orientation of [110] along the longitudinal axes of nanowires. The quadratic magnetic field dependence of normalized magnetoresistance (MR) at low field is attributed to the additional effect of demagnetization in low dimensional systems. A top-down fabrication method was employed to create single nickel nanowires for the direct study of transport and thermodynamic properties in one wire. Optic, e-beam lithography, thermal evaporation, and etching techniques were applied to construct individual and sagging nickel nanowires on a silicon wafer. The thermal conductivity of the sagging nickel nanowire was measured between 15 and 300 K. The room temperature and 0.5 K electrical resistivity are about 36 μΩ-cm and 17 μΩ-cm respectively, giving a low residual resistivity ratio (RRR) of only 2. As compared to the bulk Ni，this result indicates that the conductive electrons are strongly scattered by defects and impurities. The temperature dependence of thermal conductivity and Lorenz number also significantly differ from that of the bulk. Transport measurement data on the nickel nanowire show that at 75- 300 K, it follows the Wiedemann-Franz law, whereas the agreement break down below 75 K indicating that the thermal current is more suppressed than the electrical current in the one dimension system.