Preserved Conductance in Covalently Functionalized Silicon Nanowires

We study by means of ab initio simulations the Landauer conductance of covalently functionalized silicon nanowires. We show that in the case of alkyl side chains, the most common linkers, silicon nanowires remain quasiballistic over a large energy range. More reactive side molecules, such as alkenyl or phenyl conjugated radicals, amino and alkoxide groups, are less favorable as they induce resonant backscattering in the valence bands mainly. Such results provide strong support for the use of selectively functionalized nanowires in (opto)electronic devices and molecular sensors.

Figure 1

(a) Symbolic representation of the hexane radical (hexyl) functionalized SiNW. A plot of the integrated charge density for states with energy in the range of Fig. 2 is provided (contour at 10% of the maximum density value). (b) Symbolic representation of the alkenyl ($n=6$) functionalized wire with a plot of the side chain “HOMO” level [see peak at $\sim -0.8\mathrm{eV}$ in Fig. 3c].

Figure 2

Transmission for alkyl-functionalized SiNWs. The index ($n$) indicates the number of carbon atoms in the alkyl chain. In (a) and (b), the ($n=4$) butyl chain is grafted on the $\{111\}$ and $\{100\}$ facets of the wire.

Figure 3

(a) $e\mathrm{DOS}$ for the hexane ($n=6$) isolated molecule. (b)–(f) Local density of states projected on the side chains’ orbitals: (b) hexyl, (c) alkenyl, (d) phenyl, (e) alkoxide, and (f) amino radicals. The two thick vertical lines indicate the SiNWs’ band gap. Note that the two graphs are not to scale. $e\mathrm{DOS}$ in the lower panel have been normalized to the number of valence electrons in the side chain for the sake of comparison.

Figure 4

Transmission for SiNWs functionalized with conjugated (a),(g) -$(\mathrm{CH}{)}_4$-H, (b),(h) -$(\mathrm{CH}{)}_6$-H alkenyl radicals, (c),(i) a phenyl group, (d),(j) a -${\mathrm{OC}}_3{\mathrm{H}}_7$ alkoxide group, (e),(k) an amino group, and (f),(l) a residual chlorine atom. The color code corresponds to that of Fig. 3 (lower panel). The stars indicate some typical resonant backscattering drop of conductance. The effect of passivating the Si-connected C atom of the -$(\mathrm{CH}{)}_6$-H alkenyl group by a hydrogen is indicated (see arrow).