Quantum dots: the confined systems!
There has been great interest in the optical properties of quantum confined systems like quantum well, quantum wire and quantum dots, because they differ strongly from the corresponding bulk crystals. I have been involved with the investigation of such systems using Raman and photoluminescence studies. One of the interesting samples is the sharp-edge filter glass, which is made by embedding CdSSe quantum dots in a glass matrix. The filter glasses have a variety of colors depending upon the size of the dot.
Raman studies of Quantun dots:
Low-frequency Raman spectra from such systems show confined acoustic phonons in the region below 50 cm-1 (in some cases, even below 5 cm-1) depending upon particle size, which correspond to the symmetric and quadrupolar acoustic vibrations of the quantum dots. The frequency positions of these elastic vibrations are inversely proportional to the particle size, and hence they are very close to the laser line for the sample with larger particles. A set of very careful experiments is needed to observe these modes, which have not been seen distinctly in the past. In our experiments, we have been able to measure Raman spectra downto about 3 cm-1 in both Stoke's and anti-Stoke's spectral ranges. We have observed both polarized and depolarized phonon modes and have also been able to observe some of the overtones of the polarized mode. We have also built a theoretical model to describe the vibrations of these quantum dots, which includes the effect of surrounding glass matrix. In the high-frequency range, Raman spectra show surface-phonon modes that cannot be seen in bulk materials.
Photoluminescence studies of Quantun dots:
Photoluminescence from filter glasses containing CdSSe quantum dots embedded in a glass matrix has contributions from recombinations at the band-edge and from shallow and deep traps. Apart from a blue shift due to the confinement, the band-edge luminescence shows an additional shift that depends on the probing laser power. The results are analysed in terms of laser induced local heating and band-filling process, which shift the band gap in opposite directions. The shallow trap luminescence shows large contributions from the surface states, which was studied by invoking photodarkening effects. By controlling the illumination power, it was possible to create a single exciton, and to study the threshold conditions for the same. The deep trap luminescence depends weakly on the particle size and shows its origin in the glass matrix.