Observation of quantum confinement in the occupied states of diamond clusters

Condensed-phase hydrogen-terminated diamond clusters (diamondoids) have been studied with soft-x-ray emission and x-ray-absorption spectroscopy. The occupied and unoccupied electronic states measured with these methods imply an increasing highest occupied molecular orbital--lowest unoccupied molecular orbital gap with decreasing diamondoid size, with the shifting entirely in the occupied states, in contrast to other semiconductor nanocrystals. These experimental results are compared with theoretical calculations on the electronic structure of diamondoids.