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Thermoelectric and structural characterization of individual nanowires and patterned thin films.

机译：单个纳米线和图案化薄膜的热电和结构表征。

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摘要

This dissertation presents the development of methods based on microfabricated devices for combined structure and thermoelectric characterizations of individual nanowire and thin film materials. These nanostructured materials are being investigated for improving the thermoelectric figure of merit defined as ZT=S2sigmaT/&kgr;, where S is the Seebeck coefficient, sigma is the electrical conductivity, &kgr; is the thermal conductivity, and T is the absolute temperature. The objective of the work presented in this dissertation is to address the challenges in the measurements of all the three intrinsic thermoelectric properties on the same individual nanowire sample or along the in plane direction of a thin film, and in correlating the measured properties with the crystal structure of the same nanowire or thin film sample. This objective is accomplished by the development of a four-probe thermoelectric measurement procedure based on a micro-device to measure the intrinsic &kgr;, sigma, and S of the same nanowire or thin film and eliminate the contact thermal and electrical resistances from the measured properties. Additionally the device has an etched through hole that facilitates the structural characterization of the sample using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS).;This measurement method is employed to characterize individual electrodeposited Bi1-xTex nanowires. A method based on annealing the nanowire sample in a forming gas is demonstrated for making electrical contact between the nanowire and the underlying electrodes. The measurement results show that the thermoelectric propertied of the nanowires are sensitive to the crystal quality and impurity doping concentration. The highest ZT found in three nanowires is about 0.3, which is still lower than that of bulk single crystals at the optimum carrier concentration. The lower ZT found in the nanowires is attributed to the high impurity or carrier concentration and defects in the nanowires.;The micro-device is further modified to extend its use to characterization of the in-plane thermoelectric properties of thin films. Existing practice for thermoelectric characterization of thin films is obtaining &kgr; in the cross plane direction using techniques such as the 3o method or time domain laser thermal reflectance technique whereas the sigma and S are usually obtained in the in-plane direction. However, transport properties of nanostructured thin films can be highly anisotropic, making this combination of measurements along different directions unsuitable for obtaining the actual ZT value. Here, the micro-device is used to measure all three thermoelectric properties in the inplane direction, thus obtaining the in-plane ZT. A procedure based on a nano-manipulator is developed to assemble etched thin film segments on the micro-device. Measurement results of two different types of thin films are presented in this dissertation.;The first type is mis-oriented, layered thin films grown by the Modulated Elemental Reactant Technique (MERT). Three different structures of such thin films are characterized, namely WSe2, Wx(WSe2) y and (PbSe0.99)x(WSe2)x superlattice films. All three structures exhibit in-plane &kgr; values much higher than their cross-plane e values, with an increased anisotropy compared to bulk single crystals for the case of the WSe2 film. The increased anisotropy is attributed to the in-plane ordered, cross-plane disordered nature of the mis-oriented, layered structure. While the WSe 2 film is semiinsulating and the Wx(WSe2) y films are metallic, the (PbSe0.99)x(WSe 2)x films are semiconducting with its power factor (S 2sigma) greatly improved upon annealing in a Se vapor environment.;The second type of thin films is semiconducting InGaAlAs films with and without embedded metallic ErAs nanoparticles. These nanoparticles are used to filter out low energy electrons with the introduction of Schottky barriers so as to increase the power factor and scatter long to mid range phonons and thus suppress &kgr;. The in-plane measurements show that both the S and sigma increase with increasing temperature because of the electron filtering effect. The films with the nanoparticles exhibited an increase in sigma by three orders of magnitude and a decrease in S by only fifty percent compared to the films without, suggesting that the nanoparticles act as dopants within the film. On the other hand, the measured in-plane &kgr; shows little difference between the films with and without nanoparticles. This finding is different from those based on published cross-plane thermal conductivity results.
机译：本文介绍了基于微细加工装置的方法的发展，这些方法用于单个纳米线和薄膜材料的组合结构和热电表征。这些纳米结构材料正在研究中，以改善热电品质因数，定义为ZT = S2sigmaT /＆kgr;其中S为塞贝克系数，sigma为电导率，＆kgr；是导热系数，T是绝对温度。本文提出的工作目的是解决在单个纳米线样品上或沿着薄膜的平面内方向上测量所有三个固有热电特性的挑战，以及将测量的特性与晶体相关联的挑战。同一纳米线或薄膜样品的结构。通过基于微型设备的四探针热电测量程序的开发来实现此目标，该程序可测量同一纳米线或薄膜的本征＆kgr，sigma和S，并消除被测物的接触热阻和电阻属性。此外，该设备还具有蚀刻通孔，可使用透射电子显微镜（TEM）和能量色散X射线光谱仪（EDS）方便地对样品进行结构表征;该测量方法用于表征单个电沉积的Bi1-xTex纳米线。演示了一种基于在成形气体中对纳米线样品进行退火的方法，该方法用于使纳米线与下面的电极进行电接触。测量结果表明，纳米线的热电性质对晶体质量和杂质掺杂浓度敏感。在三根纳米线中发现的最高ZT约为0.3，在最佳载流子浓度下，仍低于整体单晶的ZT。纳米线中较低的ZT归因于纳米线中的高杂质或载流子浓度和缺陷。;对微型器件进行了进一步修改，以扩展其用途，以表征薄膜的面内热电特性。薄膜热电表征的现有实践正在获得＆kgr;使用3o方法或时域激光热反射技术在横向方向上进行扫描，而sigma和S通常在平面内方向上获得。然而，纳米结构薄膜的传输特性可能是高度各向异性的，这使得沿着不同方向的这种测量组合不适合获得实际的ZT值。在此，微型器件用于测量面内方向上的所有三个热电特性，从而获得面内ZT。开发了一种基于纳米操纵器的程序，以将蚀刻的薄膜片段组装在微型设备上。本文介绍了两种不同类型薄膜的测量结果。第一种是通过调制元素反应器技术（MERT）生长的取向错误的层状薄膜。表征了这种薄膜的三种不同结构，即WSe2，Wx（WSe2）y和（PbSe0.99）x（WSe2）x超晶格薄膜。所有这三个结构均显示面内＆kgr; WSE2膜的情况下，其值远高于其横面e值，与块状单晶相比，各向异性增加。各向异性的增加归因于取向错误的分层结构的面内有序，跨面无序性质。虽然WSe 2薄膜是半绝缘的，而Wx（WSe2）y薄膜是金属的，但（PbSe0.99）x（WSe 2）x薄膜是半导体的，其功率因数（S 2sigma）在Se蒸气环境中退火后大大改善了第二类薄膜是带有和不带有嵌入的金属ErAs纳米粒子的半导体InGaAlAs薄膜。这些纳米粒子用于通过引入肖特基势垒来滤除低能电子，从而提高功率因数并散射中至高范围的声子，从而抑制＆kgr;。平面内测量表明，由于电子滤波效应，S和sigma均随温度升高而增加。与没有纳米颗粒的膜相比，具有纳米颗粒的膜表现出σ增加了三个数量级，而S仅降低了百分之五十，这表明纳米颗粒在膜内充当了掺杂剂。另一方面，测得的面内＆kgr;在具有和不具有纳米颗粒的薄膜之间显示出很小的差异。此发现与基于已发布的跨平面热导率结果的发现不同。

著录项

  • 作者

    Mavrokefalos, Anastassios Andreas.;

  • 作者单位

    The University of Texas at Austin.;

  • 授予单位 The University of Texas at Austin.;
  • 学科 Mechanical engineering.;Condensed matter physics.;Materials science.
  • 学位 Ph.D.
  • 年度 2008
  • 页码 108 p.
  • 总页数 108
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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