Coherent transport and thermoelectric effects in pristine and binary alloyed noble metals nanoribbons using first principles

Noble metals exhibit, in nano-scale regime, unusual physical properties in comparison to their bulk form. Here, we perform first principles simulations on freestanding pristine (viz. Au, Ag, Cu) and binary alloyed (viz. AuAg, AuCu) noble metals nanoribbons (NRs) in honeycomb geometry, using density functional theory as implemented in SIESTA code. Further, electrical and thermal transport through NRs is studied in coherent realm using TransSIESTA- a non-equilibrium Greens function based module of SIESTA, and the Landauer–Büttiker formalism, with special focus on tuning of thermopower S and thermoelectric efficiency ZT. Phonons’ contribution to heat transport is also included, and the required phonon spectra is calculated using empirical inter-atomic potentials. Interestingly, we find that both electron and phonon dispersions show marked dependence on edge geometry (armchair (AC) or zigzag (ZZ)), width, and composition of NRs. While the ZZ-edged pristine NRs remain metallic, their AC counterparts turn semiconducting, with alloying inducing band gap even in ZZ NRs and additional gaps in phonon spectra. Consequently, alloyed NRs show significantly reduced electronic and phonon conductance. In contrast, in pristine ZZNRs, electrical conductance G remains quantized up to a temperature of at least 200K, with the quantization window, however, shrinking fast on increasing width of NR. Though we find NRs to have only small S and ZT, these can be augmented several times by adjusting the electrochemical potential μ of electrodes to lie near either edge of energy gap, if any, about Fermi level EF or some other energy. In this way, a maximum |S| of ∼120μVK−1 can be achieved in pristine NRs (AuAC), whereas in alloyed NRs (AuCuZZ), tuned |S| can be as high as ∼871μVK−1. Likewise, ZT is found to be higher in alloyed configurations, with an excellent value of ∼3.6 in AuCuZZ. In pristine systems, ZT remains less than 0.5, with AuAC showing the highest value of ∼0.47. Both S and ZT are, however, considerably suppressed with increasing width of NRs. Strong energy dependence of electronic transmission near EF causes a violation of the Wiedmann–Franz law for these NRs. •Tuning of thermoelectric transport in noble metals nanoribbons (NRs).•Quantization of electrical conductance in zigzag pristine NRs.•Alloying induces semiconducting conduction and extra gaps in phonon spectra.•Room-temperature |S|∼871μVK−1 and ZT∼3.6 in AuCu NR.