Spin-polarizable excitonic luminescence in colloidal Mn2+-doped CdSe quantum dots.

The photoluminescence of colloidal Mn2+-doped CdSe nanocrystals has been studied as a function of nanocrystal diameter. These nanocrystals are shown to be unique among colloidal doped semiconductor nanocrystals reported to date in that quantum confinement allows tuning of the CdSe bandgap energy across the Mn2+ excited-state energies. At small diameters, the nanocrystal photoluminescence is dominated by Mn 2+ emission. At large diameters, CdSe excitonic photoluminescence dominates. The latter scenario has allowed spin-polarized excitonic photoluminescence to be observed in colloidal doped semiconductor nanocrystals for the first time.

Giant excitonic Zeeman splittings in colloidal Co2+ -doped ZnSe quantum dots.

Colloidal Co(2+):ZnSe diluted magnetic semiconductor quantum dots (DMS-QDs) were prepared by the hot injection method and studied spectroscopically. Ligand-field electronic absorption and magnetic circular dichroism (MCD) spectra confirm homogeneous substitutional speciation of Co(2+) in the ZnSe QDs. Absorption spectra collected at various times throughout the syntheses reveal that dopants are absent from the central cores of the QDs but are incorporated at a constant concentration during nanocrystal growth. The undoped cores are associated with dopant exclusion from the ZnSe critical nuclei. Analysis of low-temperature electronic absorption and MCD spectra revealed excitonic Zeeman splitting energies (DeltaE(Zeeman)) of these Co(2+):ZnSe QDs that were substantially smaller than anticipated from bulk Co(2+):ZnSe data. This reduction in DeltaE(Zeeman) is explained quantitatively by the absence of dopants from the QD cores, where dopant-exciton overlap would be greatest. Since dopant exclusion from nucleation appears to be a general phenomenon for DMS-QDs grown by direct chemical methods, we propose that DeltaE(Zeeman) will always be smaller in colloidal DMS-QDs grown by such methods than in the corresponding bulk materials.

Spectroscopy of photovoltaic and photoconductive nanocrystalline Co2+-doped ZnO electrodes.

Photocurrent and photoconductivity measurements have been used in combination with absorption and magnetic circular dichroism (MCD) spectroscopic measurements to elucidate the mechanism of photoinduced carrier generation in nanocrystalline Co(2+):ZnO electrodes. These experiments allowed direct observation of two broad Co(2+) charge transfer (CT) bands extending throughout the visible energy range. The lower energy CT transition is assigned as a Co(2+) --> conduction band excitation (ML(CB)CT). Sensitization of this ML(CB)CT level by (4)A(2) --> (4)T(1)(P) ligand-field excitation is concluded to be responsible for the distinctive structured photocurrent action spectrum of these electrodes at ca. 14 000 cm(-1). The higher energy CT transition is assigned as a valence band --> Co(2+) excitation (L(VB)MCT) and is found to have an internal quantum efficiency for charge separation that is approximately four times larger than that of the ML(CB)CT excitation. The different internal quantum efficiencies for the two CT excitations are related to differences in excited-state wave functions arising from configuration interaction with the 1S excitonic levels of ZnO. Whereas the ML(CB)CT excited state is best described as a localized Co(3+) + e(-)(CB) configuration, the L(VB)MCT excited state (Co(+) + h(+)(VB)) has a 4-fold greater admixture of delocalized excitonic (Co(2+) + h(+)(VB) + e(-)(CB)) character in its wave function, a conclusion supported by quantitative analysis of the CT absorption intensities. Practical factors controlling the overall photovoltaic efficiencies of the photoelectrochemical cells, including electrode conductivity and porosity, were also examined.