Thermoelectric Transport Properties of Individual Bismuth Nanowires
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Thermoelectric Transport Properties of Individual Bismuth Nanowires Stephen B. Cronina, Yu-Ming Linb, Oded Rabinc, Marcie R. Blackb, Gene Dresselhausd, and Mildred S. Dresselhausa,b a Department of Physics, bDepartment of Electrical Engineering and Computer Science, c Department of Chemistry, and dFrancis Bitter Magnet Laboratory Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT We developed a method for making 4-point contacts to Bi nanowires with a thick oxide coat using a combination of lithographic and focused ion beam (FIB) techniques. The resistivity of Bi nanowires with diameters in the range 70-200nm is found to increase with decreasing wire diameter. In contrast to bulk Bi, the temperature dependence of the resistivity is found to decrease monotonically with increasing temperature. The results are explained on the basis of increased scattering in the nanowire and the known temperature dependence of the electronic properties of bulk Bi. A large magneto-resistance was also measured, indicating a high crystalline quality of the nanowires. A large spread in the measured values of the resistivity indicates significant systematic error in the measurement technique. Possible sources for error are discussed.
INTRODUCTION Bulk bismuth is a poor thermoelectric material because, as a semimetal, the coexistence of holes and electrons approximately cancels their respective contributions to the Seebeck effect. However, it has been predicted that if the overlap of the conduction and valence bands (38meV at 77K) could be removed, Bi could then have a ZT as high as 1.8 at room temperature [1]. One way of reducing the band overlap is by alloying with antimony and by applying a magnetic field. It is predicted that ZT~1 can be achieved over the entire temperature range 100-300K in bulk Bi-Sb alloys by applying a magnetic field [1]. However, the large magnetic fields needed to achieve this effect, ~1.7 Tesla, are not practical for commercial applications. Another way of removing the band overlap in Bi is by utilizing the quantum confinement effect in low dimensional systems, such as thin films or nanowires, to shift the band edges apart to form a semiconductor. Inside a nanowire, the energy bands split up into subbands with the approximate energy separation ≈ h 2π 2 m*d 2 , where m* and d are the effective mass and wire diameter, respectively. Because of the very small effective masses of the electrons in Bi, the quantum energy level separations are very large. For small enough diameters, the shift in subbands is large enough such that the lowest conduction subband and the highest valence subband no longer overlap, and the material forms a semiconductor. The critical diameter for this semimetal-to-semiconductor transition is calculated to be 49nm (at 77K) for the observed wire growth crystalline orientation of our fabrication technique [2]. The Bi nanowires used in this study are prepared by filling an electrochemically grown porous alumina template with molten Bi at high pressure. Details of this process are
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