Thermoelectric Properties of Bi1-xSbx Nanowire Arrays
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Thermoelectric Properties of Bi1-xSbx Nanowire Arrays Yu-Ming Lin,1 Stephen B. Cronin,2 Oded Rabin,3 Jackie Y. Ying,4 and Mildred S. Dresselhaus1,2 1 Department of Electrical Engineering and Computer Science, 2Department of Physics, 3 Department of Chemistry, 4Department of Chemical Engineering Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT We present here a thermoelectric transport property study of Bi1-xSbx alloy nanowires embedded in a dielectric matrix. Temperature-dependent resistance measurements exhibit nonmonotonic trends as the antimony mole fraction (x) increases, and a theoretical model is presented to explain the features that are related to the unusual band structure of Bi1-xSbx systems. Seebeck coefficient measurements are performed on nanowires with different diameters and compositions, showing enhanced thermopower over bulk Bi. The magneto-Seebeck coefficient of these nanowires also exhibits an unusual field dependence that is absent in bulk samples. INTRODUCTION Bismuth (Bi) is a very attractive material for the study of low-dimensional systems due to its unique band structure and its very small electron effective masses [1]. Since the quantum confinement effects are inversely proportional to the effective masses, these effects are more readily observed in Bi than in other materials at a given size. In addition, bulk Bi has very long carrier mean free paths, making it an ideal material to study transport behaviors in lowdimensional systems. Bi is a group V semimetal, in which the electrons are distributed in three highly anisotropic carrier pockets at the L points of the Brillouin zone, and the holes are contained in one pocket at the T point [1]. The small energy overlap (38 meV at 77 K) in bulk Bi between the L-point conduction band and the T-point valence band is predicted to vanish in Bi nanowires when the wire diameter is smaller than ~ 50 nm, thus causing a semimetal-tosemiconductor transition [2]. Experimental results for this quantum-confinement-induced semimetal-semiconductor transition in Bi nanowires have been previously reported [3,4]. In addition to band shifts due to quantum confinement effects, it is also possible to controllably alter the energy-band structure and transport properties of Bi by alloying with antimony (Sb) [5,6], which is also a group V element. Bi1–xSbx alloys form substitutional solid solutions over the whole range of concentrations x, and the lattice constants and structural parameters obey Vegards’ law for x ≤ 0.3 [7], which is the range of predominant interest to thermoelectric applications. In antimony, the electron pockets are at the L points of the Brillouin zone, while holes are at the H points. It is noted that for 0.07 < x < 0.22, the alloys become semiconductors with a direct (0.09 < x < 0.16) or indirect band gap [8]. Therefore, Bi1-xSbx alloy nanowires constitute a promising 1D system in which the desired band structure and many related properties can be achieved by combining the quantum confinement effect and the Sb alloying effect.
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