Atomistic simulations of coherent thermoelectric transport in ultrathin pristine and decorated noble metals nanowires.

Nanowires (NWs) are envisioned to be an important component of novel nanoscale electronic devices. Here, we perform atomistic simulations of coherent thermoelectric (TE) transport in free-standing ultrathin NWs of noble metals (viz. Au, Ag, Cu, Pt) cut along the [111] crystallographic direction using a non-equilibrium Greens function based first principles approach. Effect of decorating these NWs with Au or Pt atoms in an fcc environment, is also explored. Results of the calculated electronic structure show that all considered NWs, except the Pt and Pt-decorated NWs, are metallic and non-magnetic. Interestingly, Cu@Pt NW is found to be a magnetic semiconductor with an unequal band gap in ↑ − and ↓ − spin energy bands, while Pt NW and other Pt-decorated NWs (viz. Ag@Pt and Au@Pt) behave like a magnetic half-metal. To explore the TE potential of as-considered NWs, electrical conductance G , thermal conductance κ T , and thermopower S are computed in the linear response regime. Notably, the metallic NWs exhibit a quantized G. Among these, pristine NWs show a quantized value 3 G 0 up to a temperature of 200 K, while Cu@Au NW preserves its quantized value 5 G 0 even at T exceeding 300 K. In thermal transport, valence electrons make a dominant contribution to the room-temperature κ T of pristine NWs. The phononic contribution κ p h , however, goes up following decoration, with Pt-decorated NWs showing a pronounced increment (by a factor of ∼ 2). But, these NWs undergo a simultaneous reduction in the electronic contribution κ e l owing to half-metallic conduction, resulting in a net suppression of total thermal conductance κ T. Moreover, Pt-decorated NWs show a manifold enhancement in zero-bias S , with Ag@Pt exhibiting the highest value ∼ 119 μ V/K. However, S can be augmented considerably by adjusting the chemical potential μ of electrodes, and an overall best S ∼ − 890 μ V/K is found in Au NW for μ = 0. 575 eV. In this way, TE figure of merit Z T is tunable in pristine NWs over the range 1 − 3. 7. Correspondingly, the decorated NWs exhibit somewhat smaller S and Z T. Furthermore, spin-asymmetric electronic transport in Pt and Pt-decorated NWs results in net spin conductance G s and spin thermopower S s. It is found that the spin figure of merit Z s T , as high as 3.5, can be attained in Pt NW by controlling μ. Thus, noble metals NWs can be tailored for both thermoelectric and spin-thermoelectric applications. We have also investigated TE transport for the pristine NWs in the [110] direction, and found S and Z T to be comparatively much smaller. Such NWs may be useful as interconnects for low noise applications. • Thermoelectric transport in free-standing NWs of noble metals cut along the [111] direction. • Decoration of Cu NW with Pt makes it a magnetic semiconductor. • An overall best S ∼ − 890 μ V/K in Au NW for μ = 0. 575 eV. • ZT in pristine NWs over the range 1 – 3.7. • A decent Z c T ∼ 0. 48 and Z s T ∼ 0. 33 in Ag@Pt NW for μ = 0. [ABSTRACT FROM AUTHOR]