Intersublevel optical transitions in InAs nanocrystals probed by photoinduced absorption spectroscopy: The role of thermal activation

Optical transitions between the quantized sublevels of colloidally synthesized InAs nanocrystals have been revealed using infrared photoinduced absorption spectroscopy. Two different groups of intersublevel transitions were observed. Using a correlation between the measured transition energies and experimental data obtained by scanning tunneling microscopy and photoluminescence excitation spectra, we have identified one group to be related to interconduction transitions while the other is related to intervalence transitions. However, in contrast to conduction intersublevel transitions that appear at all temperatures, we have found that the valence intersublevel transitions are thermally activated and cannot be observed at temperatures below 100 K. This behavior can be explained by assuming a three-level system model for the valence band. We propose two possible explanations for our findings; the first is based on the assumption of well-defined envelope-state symmetry relations while the second attributes the thermal activation process to the presence of shallow surface localized hole states.