标题: | 奈米线之制备及其热、电传导与热力学性质之研究 Fabrication and Investigation of transport and thermodynamic properties of nanowires |
作者: | 欧敏男 Ou, Min-Nan 杨宗哲 陈洋元 Yang, Tzong-Jer Chen, Yang-Yuan 电子物理系所 |
关键字: | 奈米线;热传导;磁异向性;nanowire;thermal conductivity;magnetic anisotropic |
公开日期: | 2008 |
摘要: | 许多实验结果显示低维度材料具有与块状材料迥异的特性,这些性质的改变可能肇因于材料尺寸的缩减。以一维量子线之奈米线为例,其电子能态的分布受到量子局限限效应的改变,或因表面原子的比例增加,这些都可能是导致材料热、电、磁等性质改变的原因。为了研究奈米线的各种物理性质,我们分别利用自组构与微影制程这两种方法制备群聚之奈米线阵列及单一镍奈米线。我们以电化学沉积法让材料填充于阳极氧化铝(AAO)模板的孔洞中,制作奈米线阵列。以此方法我们制作六角形排列之铁(Fe)与三碲化二铋(Bi2Te3)奈米线阵列,用以研究其群聚之磁性与电性。其中铁奈米线之线径约60及200奈米,而三碲化二铋奈米线之线径则约60奈米。X光绕射显示,60奈米的铁与三碲化二铋奈米线阵列均具有较好的结晶性,而线径约200奈米之铁奈米线阵列,并无此明显特征。进一步的磁性量测结果显示,60奈米铁之饱合场较小且矫顽场较大,显示60奈米铁确实具有较强之磁异向性能,此外其在小外加磁场中磁阻的变化与外加磁场大小呈现平方关系,与材料之退磁场效应相关。 金属的许多特性均与电子的行为相关,如金属的热传导主要来自电子的贡献,因此金属材料的电导率与热导率比值遵守Wiedemann-Franz 定律,研究单一镍奈米线之热导性与电导性可以获得电子与声子在一维材料中的传输行为,并进一步瞭解电子在材料中的特性。我们整合光与电子束微影制程、薄膜沉积及蚀刻技术用以制备一根悬桥结构之镍奈米线,其截面为100奈米厚及180奈米宽之矩形,长约35微米。磁阻之量测显示单一之镍奈米线仍保有铁磁性,其矫顽场约500 Oe。我们用自行开发的电性量测系统与三倍频技术(3ω technique)量测此镍奈米线在温度15~300 K之间的电阻率、热传导率与比热,其室温电阻率约为36 μΩ-cm,残余电阻率约17 μΩ-cm,因此其RRR ~ 2 (Residual Resistivity Ratio),较一般块材小,显示此奈米线中具有许多杂质与缺陷,使电子受到严重的散射。热导率量测之结果亦显示奈米线之室温热导率约为块材的20%,且其值随温度下降而下降,明显与块材不同。热传导率与电导率的量测结果显示奈米线其电声子传导特性仅于75-300 K之间符合接近Wiedemann-Franz 定律,显示镍奈米线中热流的传播比电流更受到压抑。 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. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#GT009221809 http://hdl.handle.net/11536/76235 |
显示于类别: | Thesis |
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