Excitation of size selected nanocrystallites in porous silicon

We prepared silicon that exhibits green-to-red rainbow luminescence along the sample, reflecting gradients in the crystallite size. The excitation has a size-dependent feature, riding a smooth bulklike continuum. For the size-dependent contribution, we measured absorption band edges of 3.75 and 3.0-3.25 eV at the meniscus and opposite end. Excitation in the meniscus, monitored at the edge of the blue emission, isolates ultrasmall sizes, with "excitation coefficients" quadratic with energy, and emission encompassing much of the visible spectrum. Results are discussed in terms of quantum-confinement-induced restructuring of the diamondlike bonds to form radiative Si-Si surface states.