Excited states and optical absorption of small semiconducting clusters: Dopants, defects and charging

Spatially confined semiconductors form an exciting type of material with potential for numerous applications, many of which rely on the material's optical properties and excitation dynamics. For successful realization of the applications a detailed analysis of excitations in these materials is needed, extending beyond the ideal case and including various defects, such as photoionization, doping and surface dangling bonds. High-level ab initio electronic structure calculations on two representative classes of semiconducting clusters have lead to much progress on achieving a thorough understanding of the excitation properties of these materials when they are altered with charging, doping or dangling bonds. The calculations show that the defects introduce new intra-band transitions, blue-shift the optical absorption spectra and push the normal excitonic and multiexcitonic transitions to higher energies. Generally, doping and charging have similar effects on the excited state properties, while introduction of dangling bonds cause less severe changes, since the latter defect can be partially accommodated by reorganization in the local bonding pattern.