Quantum confinement in CdSe nanocrystallites

Abstract

Quantum confinement increases the spacing between energy levels as the nanocrystallite size is decreased. Its qualitative features hold both for states localized near the center of a nanocrystallite and those near the surface, such as states due primarily to dangling bonds. However, different quantitative features are expected because of the different size constraints on each of these states. Since the majority of atoms in a typical nanocrystallite are on the surface, contrasting confinement effects between these two types may prove useful in predicting how surface state dependent properties, such as optical absorption, change with the size of the nanocrystallite. By applying first principles pseudopotential methods to indium doped, uncapped CdSe nanocrystallites containing 17 and 34 atoms, we identify center and surface localized states. Using the lowest occupied energy state as a reference, the energy of a state localized near the center is found to increase 24 mRy from the 34 to 17 atom nanocrystallite. An equivalent surface state within the two cases studied is not found, but the energy level spacing is speculated to increase on the order of 100 mRy between the 34 and 17 atom cases based on states that are highly local to the surface, but not equivalent. Furthermore, we find it's necessary for the impurity to sit at the center of the nanocrystallite in order for the impurity states to be electrically active.