Homogeneous optical properties of semiconductor nanocrystals

In between the molecular and bulk forms of matter, semiconductor nanocrystals are novel materials with interesting optical and electronic properties. We present a study of the homogeneous optical properties of two nanocrystal systems. First, the homogeneous absorption of InP nanocrystals is studied via hole burning experiments. The optical spectrum consists of a HOMO-LUMO transition with a 10 meV width and a second electronic transition shifted by 0.11 eV. The optical transitions are assigned within a three valence-band model. The CdS/HgS/CdS quantum-dot/quantum-well system is also investigated and a transmission electron microscopy study shows that the growth of the HgS well region and the CdS outer layer is epitaxial. Selective optical techniques are used to study the electronic level structure. In hole burning, a discrete transition (width of 7 meV) with pronounced phonon side bands at a frequency of 250 cm^-1 is observed. In fluorescence, the line narrowed spectrum also shows phonon replicas at a similar frequency. The measurements provide direct evidence for charge localization in the low band gap HgS well region within this colloidally synthesized nanoheterostructure.