Weak d 0 ferromagnetism: Zn vacancy condensation in ZnS nanocrystals.

We provide the explanation of the large discrepancy of three orders of magnitude between the experimentally measured and theoretically calculated magnetic moments in ZnS nanocrystals. We assume that the condensation of Zn vacancies into a single droplet takes place. The energy calculations reveal that the droplet phase is more favorable than the uniformly distributed vacancy configuration. The other assumption made is that a small magnetic moment could arise at the interface between the ZnS crystal and vacancy cluster. The calculations however dismiss this hypothesis because the magnetization of the layered system also vanishes. Thus we suggest that the experimentally low magnetization values could be explained from one of the two following pictures: (a) there are two phases where the vacancy cluster with the zero magnetic moment coexists along with the other phase, in which there are uniformly distributed Zn vacancies with low concentrations or (b) there is only a single vacancy phase-a vacancy droplet being in the metastable state with a weak nonvanishing magnetic moment.

Large enhancement in photocurrent by Mn doping in CdSe/ZTO quantum dot sensitized solar cells.

We find a large enhancement in the efficiency of CdSe quantum dot sensitized solar cells by doping with manganese. In the presence of Mn impurities in relatively small concentrations (2.3%) the photoelectric current increases by up to 190%. The average photocurrent enhancement is about 160%. This effect cannot be explained by a light absorption mechanism because the experimental and theoretical absorption spectra demonstrate that there is no change in the absorption coefficient in the presence of the Mn impurities. To explain such a large increase in the injection current we propose a tunneling mechanism of electron injection from the quantum dot LUMO state to the Zn2SnO4 (ZTO) semiconductor photoanode. The calculated enhancement is approximately equal to 150% which is very close to the experimental average value of 160%. The relative discrepancy between the calculated and experimentally measured ratios of the IPCE currents is only 6.25%. For other mechanisms (such as electron trapping, etc.) the remaining 6.25% cannot explain the large change in the experimental IPCE. Thus we have indirectly proved that electron tunneling is the major mechanism of photocurrent enhancement. This work proposes a new approach for a significant improvement in the efficiency of quantum dot sensitized solar cells.

Optical spectra of CdMnSe of nano-ferro- and antiferro-magnets.

We study optical transitions in CdSe quantum dots doped by Mn atoms. At low concentrations the transitions are spin-forbidden. Nevertheless, strong light absorption was experimentally found. To explain this effect we propose a new mechanism that includes two or more Mn atoms closely placed to each other containing the electrons with opposite spin projections. In this case the spin-flip is unnecessary. In addition we study absorption from quantum dots containing two Mn atoms with different multiplicities. We find that the strongest absorption from the gap is for an antiferromagnetic arrangement. The obtained results confirm the experimental concentration dependencies.

Spectroscopic and electronic structure properties of CdSe nanocrystals: spheres and cubes.

In this work we study the electronic structure of CdmSem quantum dots of various sizes and different shapes such as spheres and cubes using DFT, TDDFT, and CIS methods. This work requires a careful computational analysis where a proper exchange-correlation functional has to be chosen to fit the experimental optical gap. We find some differences in the optical and HOMO-LUMO gap values between spheres and cubes. In general, the gaps for the cubes have higher values than those for the spheres. We also calculate optical absorption spectra using the data for energy levels and oscillator strengths for optical transitions. We find that DFT yields some discrepancy in the density of states for the spheres and cubes. However, the density of states calculated by TDDFT and CIS provide better agreement. The results of the calculation can be useful for quantum dots synthesized in laser ablation experiments.