In-situ TEM Study of Bismuth Nanostructures
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1044-U03-04
In-situ TEM Study of Bismuth Nanostructures Xiaoting Jia1, Vincent Berube2, Shuo Chen3, Bed Poudel4, Son Hyungbin5, Jing Kong5, Yang Shao-Horn3, Ren Zhifeng4, Gang Chen3, and Mildred S Dresselhaus2,5 1 Material Sciences and Engineering, MIT, 77 Massachusetts avenue room 13-3021, Cambridge, MA, 02139 2 Physics, MIT, Cambridge, MA, 02139 3 Mechanical Engineering, MIT, Cambridge, MA, 02139 4 Physics, Boston College, Chestnut Hill, MA, 02467 5 Electrical Engineering and Computer Science, MIT, Cambridge, MA, 02139 ABSTRACT Nanostructured thermoelectric materials have attracted lots of interest in recent years, due to their enhanced performance determined by their thermoelectric dimensionless figure of merit. However, because of equipment limitations, not much work has been done on combining simultaneous transport measurements and structural characterization on individual nanostructured thermoelectric materials. With an integrated TEM-STM system, we studied the structural behavior and electrical properties of bismuth (Bi) nanobelts and nanoparticles. Results showed that clean Bi nanostructures free of oxides can be produced by in-situ high temperature electro-migration and Joule annealing processes occurring within the electron microscope. Preliminary electrical measurements indicate a conductivity of two orders of magnitude lower for Bi nanoparticles than that for bulk Bi. Such in-situ studies are highly advantageous for studying the semimetal-semiconductor transition and how this transition could enhance thermoelectric properties. INTRODUCTION The anisotropic electronic structure of Bi provides some directions along the constant energy surfaces for the three electron pockets at the L point of the Brillouin zone which have very low effective mass components giving rise to high mobility carriers. At the same time, the constant energy surfaces have other directions with heavy masses giving rise to a high density of states effective mass. For both of these reasons, very good thermoelectric properties should be obtained for carrier transport along the low mass direction. However, as the size of a Bi nanowire or nanoparticle become smaller, the lowest quantized level in the conduction band moves up in energy and the highest quantized level in the valence band moves down, so that eventually these levels cross and a semimetal to semiconductor transition occurs. This transition occurs for larger diameter nanostructures as the temperature is decreased or as Sb is added in the structure up to about 10%. In the semiconducting state, transport by electrons and not by holes can be achieved by n-type doping, thereby allowing us to capture the benefits of the anisotropic constant energy surfaces of the L point carriers in Bi. The heavy atomic mass of Bi also results in a low thermal conductivity which is also desirable for good thermoelectric performance. Studies at the single nanowire and single particle level are necessary to probe the anisotropic aspects of these anisotropic constant energy surfaces. Experiments inside
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