Thermoelectricity in periodic and quasiperiodically segmented nanobelts and nanowires
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Thermoelectricity in periodic and quasiperiodically segmented nanobelts and nanowires J. Eduardo Gonzalez1, Vicenta Sanchez2, and Chumin Wang1 1 2
Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, D.F., Mexico. Departamento de Fisica, Facultad de Ciencias, Universidad Nacional Autonoma de Mexico, D.F., Mexico.
ABSTRACT Thermoelectric properties of segmented nanowires and nanobelts are studied by means of the Kubo-Greenwood formula and a real-space renormalization plus convolution method. The tightbinding and Born models are respectively used for the calculation of electronic and lattice thermal conductivities. In particular, we investigate the thermoelectric figure of merit (ZT) of periodic and quasiperiodically segmented nanowires with two different cross sections, where the segments of the quasiperiodic one are ordered following the Fibonacci sequence. The results show an increase of ZT when the cross section area of nanowires diminishes. In addition, we present results of ZT in segmented nanobelts with an inhomogeneous cross section. For both nanowires and nanobelts, the quasiperiodicity seems to be an important enhancing factor of ZT. INTRODUCTION Energy conversion through thermoelectric devices constitutes a sustainable alternative for the generation of electricity. Their performance can be measured by using the dimensionless figureof-merit defined as (1) ZT V S 2T (N el N ph ) , where S is the Seebeck coefficient, σ is the electrical conductivity, κel and κph are the electronic and phononic thermal conductivities, respectively [1]. The inherent correlation between these thermoelectric quantities makes difficult to improve the value of ZT. Recently, nanowire heterostructures, such as M2O3/ZnO (M = In, Ga Fe) with compositional segmentation [2], have demonstrated a significant improvement of ZT, mainly due to the phonon scattering at composite interfaces. Likewise, nanobelt heterostructures have been made [3] and the power factor ( V S 2 ) of Sb2Te3 nanobelts was found larger than the corresponding bulk value [4]. On the theoretical side, thermoelectric properties can be determined by using the Boltzmann equation [5]. For aperiodic nanostructures, the atomic-scale modeling requires innovative approaches, since the long-range structural disorder makes useless the Bloch theorem and reciprocal space. In this article, we report a quantum mechanical study of thermoelectric properties in periodic and quasiperiodically segmented nanowires and nanobelts by means of the Kubo-Greenwood formula and a real-space renormalization plus convolution method [6], which has the advantage of being computational efficient and able to address a macroscopic number of atoms in a non-perturbative way. THE MODEL In the Boltzmann formalism, the thermoelectric quantities can be calculated by means of the Kubo-Greenwood formula as [6,7]
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