Enhancement of the thermoelectric figure-of-merit in nanowire superlattices
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Enhancement of the thermoelectric figure-of-merit in nanowire superlattices Chumin Wang1, J. Eduardo González1 and Vicenta Sánchez2 1 2
Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, D.F., México Departamento de Física, Facultad de Ciencias, Universidad Nacional Autónoma de México, D.F., México
ABSTRACT Based on the Kubo-Greenwood formula, the thermoelectric effects in periodically and quasiperiodically segmented nanowires are studied by means of a real-space renormalization plus convolution method, where the electrical and lattice thermal conductivities are respectively calculated by using the tight-binding and Born models; the latter includes central and non-central interactions between nearest-neighbor atoms. The results show a significant enhancement of the thermoelectric figure-of-merit (ZT) induced by the structural disorder and/or the reduction of nanowire cross-section area. In addition, we observe a maximum ZT in both the chemicalpotential and temperature spaces. INTRODUCTION Thermoelectric devices that make a direct conversion between thermal and electrical energies have attracted great attention in the last years. In particular, nanowire superlattices have a band structure by design, which with a properly placed chemical potential (µ) by applying a gate voltage could significantly enhance the thermoelectric power [1]. Furthermore, the phonon scattering at nanowire surface and compositional interfaces leads to a lower thermal conductivity and then an increase of the thermoelectric figure-of-merit (ZT). For example, M2O3(ZnO)n (M=In, Ga, Fe) segmented nanowires were built from pure ZnO nanowires through a thermal metal evaporation followed by an annealing in O2 at 1173K. The resulting randomly segmented nanowires reveal improved thermoelectric effects in comparison with the pure ones [2]. In addition, the grain morphology in modulated telluride nanowire heterostructures can be modified by tuning the ratio between PbTe and Bi2Te3 in order to achieve a largely reduced thermal conductivity and then an enhanced ZT [3]. On the other hand, the Kubo formalism provides a microscopic description of the electronic and phononic transport in solids [4]. Recently, we have developed a real-space renormalization plus convolution method [5] for the Kubo-Greenwood formula, which has the advantage of being computationally efficient, without introducing additional approximations, and capable to analyze aperiodic nanowires with truly macroscopic length. In this work, we report a comparative study of thermoelectricity in periodically and quasiperiodically segmented nanowires with different square cross-sections.
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THE MODEL The dimensionless thermoelectric figure-of-merit (ZT) is defined as ZT = S 2σ T (κ el + κ ph ) , where S is the Seebeck coefficient
