Howard University Ajibola Adeyeye Tosin Odunfa Phylicia Rottman February Conference Alternative Energy, Turkmenistan Johns Hopkins University C.L. Chien Peter Searson Nanostructured Thermoelectric Materials for Solar Power Utilization Collaborations Boston College Moldova Magnet Labs.

Motivation: Population + Standard-of-living (Energy) = Carbon emissions World Numbers: Average world energy consumption rate = 15 TW = 1.5 x W. World human population = 6.7x 10 9

US Energy use is Joule. ~50% ends as waste heat.

Thermoelectrics is an electronic heat-to-electric converter.

Anisotropic nanostructured thermoelectric materials. REFERENCES A.A. Snarskii and L.P Bulat, in Thermoelectric Handbook, Micro to Nano, edited by D.M. Rowe (CRC Taylor and Francis, Boca Raton, USA, 2006), chapter 45. V.P. Babin, T.S. Gudkin, Z.M. Dashevskii, L.D. Dudkin, E.K. Iordanishvili, V.I. Kaidanov, N.V. Kolomoets, Q.M. Narva, and L.S. Stilbans. Sov. Phys. Semicond. 8, 478 (1974). Schematic view of the operation of an anisotropy thermoelectric device.

Surface States of the thermoelectric Bi Spin Orbit interaction Bi Angle Resolved Photoemission (ARPES) experiments find that Bi surfaces support states that are not present in the bulk. Density roughly charges/cm 2. “Surface states” because the energy of the states is independent of the momentum perpendicular to the surface. References: Yu. M. Koroteev, G. Bihlmayer, J.E. Gayone, E.V. Chulkov, S. Blugel, P.M. Echenique, and Ph. Hofmann, Phys. Rev. Lett. 93, (2004). The surface states are “magnetic” because there is splitting of the surface bands according to spin value. Spin polarized surface states are interesting for thermoelectrics because they carry substantial “spin entropy”. Connects with current interest in spintronics. If nanowires is covered with surface states with a surface density of 5x10 12 cm -2, carriers in surface states are the majority and bulk states the minority for diameters of around 50 nm.

SEM side-view of a 30-nm Bi wire array. The wires appear as the light areas; the dark areas are insulator. Charge carriers in Bi Nanowires A. Nikolaeva, D. Gitsu, L. Konopko, M.J. Graf and T.E. Huber. Physical Review B77, (2008). Open URL

Arxiv cond.mat Submitted to archival journal. Observation of 3D behavior in surface states of bismuth nanowires and the evidence for bulk Bi charge fractionalization T. E. Huber, A. Nikolaeva, L. Konopko, and M. J. Graf

Summary: -Electric energy and thermal-to-electric conversion. -We study an example. Bismuth (a thermoelectric material). -The Shubnikov-de Haas method shows that there is, in addition to the Fermi surface of the bulk states, a NEW Fermi surface of Bi in nanowires. The NEW Fermi surface is spherical; the carriers effective mass is 0.3 m e. The volume of the Fermi surface is consistent with ARPES. The surface carriers have “metallic densities”. - The bulk states undergo a semimetal-to-semiconductor transition. Thermopower for large diameters (d>= 60 nm) supports this model. - Thermopower data for small diameters (d< 60 nm) supports model of surface carriers. -SURPRISE!: Thermoelectrics are correlated electron systems with strong spin(orbit) effects.