CVD Growth of Nanostructures from Bi 2 Te 3

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0886-F05-04.1

CVD Growth of Nanostructures from Bi2Te3 N. Gothard, B. Zhang, J. He, and Terry M. Tritt Dept. of Physics and Astronomy, Clemson University, Clemson, SC, USA ABSTRACT Research into thermoelectric materials has recently undergone a push into lower dimensional materials in the hopes that quantum confinement effects will enhance the performance of these structures. It has already been demonstrated that 2D superlattice materials show enhanced properties [1]. More recently, materials known to have good thermoelectric properties, such as Bi2Te3 or PbTe, have been grown in low dimensional morphologies. We investigate synthesis techniques for growing low dimensional structures of Bi-Te materials with the aim of incorporating them into a composite material alongside bulk Bi2Te3. INTRODUCTION AND BACKGROUND The efficiency of a thermoelectric device is typically described in terms of the dimensionless figure of merit, or ZT

α 2σ ZT = κ

1

Here σ is the electrical conductivity, κ is the total thermal conductivity, comprised of the sum of electrical and lattice contributions, (κ E and κ L, respectively) and α is the Seebeck coefficient, or thermopower. The thermopower is given as the voltage differential per temperature gradient:

α ≡ dV dT

2

Thermoelectric (TE) materials have long been investigated under the auspices of increasing the electrical conductivity through doping or by lowering the lattice contribution to the thermal conductivity by introducing a phonon scattering mechanism into the crystal structure. Such mechanisms could include the introduction of a “rattling” atom into cage-like crystal structures or lowering the grain size in polycrystalline samples. Further, consideration of the Mott equation,

( )

 1 ∂n E  n E ∂E

α ∝

()

 E f

3

suggests that an enhancement of the thermopower should be found in systems that exhibit sharp peaks in their electronic density of states, such as the Van Hove singularities observed in one dimensional systems. Further, calculations have shown that significant

0886-F05-04.2

enhancement of the ZT occurs in 1D materials due to quantum confinement of phonons, but only when the dimension is reduced to the range of ~1nm [1]. Bi2Te3 has long been one of the most promising thermoelectric materials for room temperature applications. The unit cell of Bi2Te3 consists of covalently bonded layers of Bi and Te atoms that are stacked one upon the other and joined by Van der Waals forces. Such a structure promotes good in-plane electrical conductivity while inhibiting propagation of phonons between the layers, in accordance with the phonon-glass electron-crystal model of TE materials [2]. It has been shown that structures that exhibit similar naturally layered structures, such as MoS2 [3] and TiS2 [4], may be synthesized as nanotubes under the proper conditions. Already, Bi2Te3 has been grown in 1D morphology via a template-based method [5-6], as well as by electrodeposition [7] and hydrothermal synthesis [8]. In this study, we investigate whether the chemical vapor

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