Thermoelectric Nanowires by Electrochemical Deposition

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THERMOELECTRIC NANOWIRES BY ELECTROCHEMICAL DEPOSITION Oded Rabin (a), Yu-Ming Lin (b), Stephen B. Cronin (c), Mildred S. Dresselhaus (b,c) Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. (a) Dept. of Chemistry. (b) Dept. of Electrical Engineering and Computer Science. (c) Dept. of Physics. ABSTRACT Nanowires made of thermoelectric-relevant materials were grown by electrochemical deposition. Their diameter and ordering are dictated by the porous alumina template that is fabricated on the working electrode prior to the deposition. The composition of the nanowires is controlled by the composition of the electrolyte and the deposition potential. This technique offers unique opportunities regarding the range of geometries and materials that can be employed. The structural and transport properties of these wires will be presented, and comparison will be made to nanowires synthesized by other techniques. INTRODUCTION Thermoelectric devices make use of the coupling of the electrical current to the thermal current in the body of the material for power generation or for cooling purposes [1]. Such a device is composed of junctions of thermoelectric materials, which are characterized by the thermoelectric figure of merit, Z, a function of the electrical conductivity (σ), the Seebeck coefficient (S), and the thermal conductivity (κ) of the materials (Equation 1). The development of materials of high Z is a focus of on-going research. New advances in the fabrication of nanostructures have shown that with low-dimensional structures, superior thermoelectric properties can be achieved [2]. The modification of the electronic density of states upon reducing the dimensions from bulk to a quantum structure can have a positive effect on the value of the product σS2 under the appropriate conditions. Moreover, the thermal conductivity is reduced due to the boundary scattering of phonons at the interfaces between the nanostructures and the surrounding matrix. Thus, superlattices [3] and quantum-dot superlattices [4] have been reported to have high values of Z. Another system of practical interest is an array of quantum wires. The phenomena that increase the value of Z are expected to be more pronounced in quantum wires than in quantum wells due to their lower dimensionality. In addition, arrays of nanowires are normally oriented for thermal transport perpendicular to a surface, while quantum well structures are more adequate for in-plane transport. σ ⋅S2 Z= (Equation 1) κ

G8.20.1

COLD JUNCTION anodic alumina

p

p-type nanowire

n

n-type nanowire

HOT JUNCTION metal connect silicon wafer

Figure 1: Schematic structure of a nanowire-based thermoelectric device Arrays of nanowires are most conveniently fabricated by template synthesis. A template is a porous matrix whose pores with diameters in the nanometer scale are filled with a second material to form embedded nanowires. Porous anodic alumina (PAA) is a commonly used template for the fabrication of nanostructures [5,6]. The self-assembled ordered triangular array o