Density functional calculations on silicon clusters show that strain effects on the band gap display qualitatively new trends for dots smaller than $\sim 2\mathrm{nm}$. While the bulk indirect band gap increases linearly with increasing strain, this trend is reversed for small clusters $(⩽1\mathrm{nm})$. In the intermediate $1–2\mathrm{nm}$ size range, strain appears to have almost no effect. These results follow from the fact that the bonding/antibonding character of the HOMO and the LUMO change nonmonotonically with size. Since the strain level of the surface atoms dominate this behavior, they strongly stress the role of surface passivation on experimentally measured band gaps.

• Received 22 May 2006

DOI:https://doi.org/10.1103/PhysRevB.74.035339