Prediction of anomalous redshift in semiconductor clusters

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Abstract
Nonlocal and energy dependent pseudopotentials are employed to investigate the effect of size on spectroscopic transitions in silicon clusters. The symmetry allowed Franck–Condon electronic transition in clusters is to the red of the corresponding transition in bulk silicon, a feature opposite to that observed in other semiconductor clusters. This redshift increases with decreasing cluster size and it is sufficiently pronounced to be experimentally observable. Furthermore, this unusual quantum confinement effect on the excited electronic states is displayed by large clusters whose ground state structures have converged to the bulk limit. The symmetry forbidden non-Franck–Condon electronic transitions do not exhibit such an anomalous redshift. Finally, this study reveals that three-dimensional confinement does not make silicon, which is an indirect gap semiconductor, into a direct gap material.