M. Nirmal

Magnetic circular dichroism study of CdSe quantum dots

We study the magnetic circular dichroism (MCD) of exciton states near the band edge of CdSe nanocrystallites (quantum dots). The experiment probes the difference between left and right circularly polarized transitions in the presence of an external magnetic field. Analysis of the MCD signal determines the sign and magnitude of the exciton g-factor which is shown to be highly sensitive to the energy band parameters used in the effective mass approximation. The observation of theoretically predicted changes in the sign of the exciton g-factor between the first two transitions is in agreement with recent theory describing the presence of fine structure underlying the optical transitions of CdSe nanocrystallites.

Size-dependent optical spectroscopy of II-VI semiconductor nanocrystallites (quantum dots)

We use low temperature (10K) optical hole-burning and fluorescence line narrowing spectroscopy to investigate the electronic properties of CdSe nanocrystallites (quantum dots) as a function of crystallite diameter (20–80Å). We discuss how the homogeneous linewidth of the HOMO-LUMO transition, the energy shift between the absorbing and emitting state, and the LO phonon frequency vary with nanocrystallite size.

Surface electronic properties of CdSe nanocrystallites

The surface electronic properties of CdSe nanocrystallites have been probed using low temperature and Zeeman spectroscopies. Fluorescence line narrowed spectra show dramatic changes between 1.75 and 10 K and also as a function of applied magnetic field. These effects are attributed to the localization of the photogenerated charge carriers on the surface. A simple model has been constructed to calculate the charge distribution within the nanocrystallite.

Synthesis and structural characterization of II–VI semiconductor nanocrystallites (quantum dots)

A methodology for the production of II–VI semiconductor nanocrystallites employing organometallic precursors has been developed. The rapid pyrolysis of reagents in a coordinating solvent provides temporally discrete nucleation. Subsequent controlled growth allows the production of macroscopic quantities of nanocrystallites with consistent structure, surface derivatization and a high degree of monodispersity. The samples produced are structurally characterized with a combination of X-ray and Electron Beam based techniques.

Size-dependent spectroscopy and photodynamics of some II-VI semiconductor nanocrystallites (quantum dots)

The relaxation dynamics of photoexcited CdSe nanocrystallites (quantum dots) are dominated by the surface. Surface electronic properties of CdSe nanocrystallites have been probed using low temperature fluorescence line narrowing and time resolved luminescence. We find that the surface structure creates a random potential for the hole with a size dependent barrier for site to site hopping.