Very High Thermopower of Bi Nanowires with Embedded Quantum Point Contacts Eyal Shapira, † Amir Holtzman, † Debora Marchak, and Yoram Selzer* School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel ABSTRACT: Quantum confinement effects in bismuth (Bi) nanowires (NWs) are predicted to impart them with high thermopower values and hence make them efficient thermo- electric materials. Yet, boundary scattering of charge carriers in these NWs operating in the diffusion transport regime mask any quantum effects and impede their use for nanoscale thermoelectric applications. Here we demonstrate quantum confinement effects in Bi NWs by forming in their structure ballistic quantum point contacts (QPCs) leading to excep- tionally high thermopower values (S > 2 mV/K). The power factor, S 2 G, of the QPCs is maximized at G ∼ 0.25G 0 (where G 0 is the quantum of conductance) within agreement with a one-band model with step edge characteristics. KEYWORDS: Thermopower, Seebeck coefficient, quantum point contact, quantum confinement, nanowires T hermoelectric (TE) materials convert thermal gradients and electric fields for power conversion and for refrigeration, respectively. 1,2 TE devices are all-solid-state units with no moving fluids and mechanical parts and thus are highly reliable and ideal for integration into small scale applications with moderate power demands. Their performance is limited to a fraction of the Carnot efficiency and is described by a figure of merit ZT defined as = κ +κ S GT ZT 2 e L (1) where S is the Seebeck coefficient (thermopower), T is an average temperature, G is the electrical conductance, and κ el , κ L , are the electronic and lattice components of the thermal conductance, respectively. Currently state of the art devices have ZT ∼ 1.5, making them even less efficient than household refrigerators. It has been proposed that band structure engineering by the introduction of low dimensional (2D and 1D) structures, such as superlattices and nanowires (NWs) can enhance the performance of thermoelectric materials. 3,4 Specifically, high ZT values have been theoretically envisioned for bismuth (Bi) NWs. 5,6 Bi has a long electron Fermi wavelength (λ F ∼ 26 nm) and therefore it should be easy to make Bi NWs that reach the 1D conduction limit 6 with this material. It also has low bulk thermal conductivity (8 W m -1 K -1 ), which is expected to become even lower in a NW geometry due to acoustic phonons confinement and scattering with boundaries. 7 While high thermopower values have been measured for two-dimensional electron gas in SrTiO 3 , 8 realization of similar values due to quantum confinement effects in individual and well charac- terized NWs have never been achieved. Here we demonstrate quantum confinement effects by exploiting the long electron mean free path (100 nm at 300 K and ∼0.4 mm at 4 K) of Bi and by fabricating NWs with embedded ballistic quantum point contacts (QPCs). QPCs are short conducting constrictions with a characteristic width comparable to the Fermi wavelength of their charge carriers, characterized by quantized conductance at integer multiples of G 0 = 2e 2 /ℏ, where e is electron charge and ℏ is the Plank constant. 9,10 On the basis of the Landauer-Bü ttiker formalism, the thermopower in the ballistic regime is approximately (see further discussion below) ≈− − S E E eT 1 F (2) where E 1 is the threshold energy of the lowest one-dimensional sub-band or mode in the QPC, and E F is the Fermi energy. QPCs based on GaAs two-dimensional electron gas structures have a typical band gap in the order of tens of microvolts, and therefore could have high S values only at very low temperatures (T < 1 K). 10 Short molecular junctions operating in the ballistic regime with a HOMO-LUMO gap in the order of 1 eV, could potentially have a high S value in the order of mV/K at room temperature. 11 However, current lack of capability to effectively tune the Fermi energy position within their gap prohibits maximization of S 2 G and hence their use for thermoelectric applications. Ballistic Bi constrictions with bandgap in the hundreds-of-millivolts range offer the possibility Received: November 1, 2011 Revised: December 12, 2011 Published: January 3, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 808 dx.doi.org/10.1021/nl2038425 | Nano Lett. 2012, 12, 808-812