Surface-Decorated Silicon Nanowires: A Route to High-$ZT$ Thermoelectrics

Based on atomistic calculations of electron and phonon transport, we propose to use surface-decorated silicon nanowires for thermoelectric applications. Two examples of surface decorations are studied to illustrate the underlying ideas: nanotrees and alkyl functionalized silicon nanowires. For both systems we find (i) that the phonon conductance is significantly reduced compared to the electronic conductance leading to high thermoelectric figure of merit $ZT$, and (ii) for ultrathin wires, surface decoration leads to significantly better performance than surface disorder.

Figure 1

Sketch of electronic density of states (top) in the alkyl functionalized SiNW (middle) and in the nanotree (bottom). The trunk in the nanotree is broader than the branch and thus has a smaller band gap. Also, the alkyl molecular states are located deep inside the bands. For both structures, electrons and holes close to the band edges are therefore only weakly scattered while phonons are strongly scattered.

Figure 2

Hole and electron transmissions for pentyl functionalized, $D=12\AA$ SiNW (a)–(b) and for a nanotree with branch length $L_B=15.4\AA$ (e)–(f). Panels (c)–(d) and (g)–(h) show the PDOS on the wire and on the pentyl and branch, respectively. The energy scales are relative to the valence band edge $E_v$ for the holes (left column) and relative to the conduction band edge $E_c$ for electrons (right column). The DFT band gap of the $D=12\AA$ wire is 1.65 eV, while the TB band gap of the $D=20\AA$ wire is 1.77 eV.

Figure 3

Thermal conductance ratio $\kappa /{\kappa }_0$ (a) for alkyl functionalized SiNWs with different alkyl lengths and (b) for nanotrees with different branch lengths $L_B$. The nanotree trunks are again $D=20\AA$ with 12 Å diameter branches. The insets show the phonon transmission function at low energies. Fano-like resonant scattering is observed in both systems.

Figure 4

Thermoelectric figure of merit $ZT$ for $p$-type (a) and $n$-type (b) wires. $N$ is the number of pentyl molecules (squares), nanotree branches (circles), and silicon surface vacancies (triangles) in the wire.