Experimental results on TiO 2 , HfO 2 , In 2 O 3 , and ZnO have confirmed that ferromagnetism (FM) is certainly possible in undoped oxide thin films. As for the TiO 2 , In 2 O 3 , and HfO 2 films, FM is most probably due to oxygen vacancies. Additionally, our X-ray magnetic circular dichroism (XMCD) measurements on TiO 2 films doped with transition -metal elements (Cr, Mn, Co) show that these contribute only with a paramagnetic component to the total magnetization, thus implying that FM in these films must originate from the TiO 2 host matrix. As for ZnO, our data have revealed that the FM in this compound does not originate from oxygen vacancies but more likely from defects on Zn sites. We propose a model that is based on an electronic structure calculation using the tight binding method in the confinement configuration: vacancy site in TiO 2 , HfO 2 , In 2 O 3 films could create spin splitting and high spin state, so that the exchange interaction between the electrons surrounding the oxygen vacancy with the local field of symmetry could lead to a FM ground state of the systems. The theoretical calculations give the results of 3.18 µ B per vacancy for TiO 2 , 3.05 µ B /vac for HfO 2 and 0.16 µ B /vac for In 2 O 3 . It also proves that the mechanism for ZnO system must be different, that FM cannot stem from oxygen vacancies but from other sources. The model strongly suggests that confinement effects should play a key role in shaping up magnetic properties of low dimension systems.

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A simple theoretical model for the origin of spontaneous polarization in nanocrystals is developed. We propose that the origin of the spontaneous polarization is in the boundary layer between “cap” and the nanocrystal, and the internal electric field in the dot is due to the piezoelectric effect caused by the strain existing in the interface region of materials with different lattice constants. The model, based on spherical rotation symmetry without inversion [SO(3)], employs a distribution of polarization with symmetry which is a subgroup of SO(3) consistent with the hexagonal structure of wurtzite structure. We predict the internal electric field and compare with experimental data.

A model is proposed to study the hybrid exciton in a quantum dotdendrimer systems. The semiconductor organic hybrid exciton is studied using a "real space" Green's function method and a diagrammatic technique. The energy of the hybrid exciton as well as the Green function matrix elements have been calculated for different quantum dot-dendrimer systems, and the method can be applied for systems with different structures. Using the double-time Green's functions the optical processes can be calculated. The optical properties of the systems are controllable by the size and structure of the QD-dendrimer systems.

We show that an array of semiconductor quantum dots in an organic host leads to a large two-photon absorption. The optical nonlinearity depends on the semiconductor, the dot size, and the dot-to-dot spacing. Using numerical simulations, we demonstrate that a large optical limiting is possible using thin films of this hybrid material.

We consider the effect of an extra electron in a doped quantum dot ZnS : M n 2+ . The Coulomb interaction and exchange interaction between the extra electron and the states of the Mn ion will mix the wavefunctions, split the impurity energy levels, break the previous selection rules and change the transition probabilities. Using this model of an extra electron in the doped quantum dot, we calculate the energy and the wave functions, the luminescence efficiency and the transition lifetime and compare with the experiments. Our calculation shows that two orders of magnitude of lifetime shortening can occur in the transition 4 T 1 − 6 A 1 , when an extra electron is present.PACS numbers: 73.20. Dx, 78.60.J, 42.65, 71.35 1 I.Introduction. In contrast to undoped materials, the impurity states in a doped nanocrystal play an important role in the electronic structure, transition probabilities and the optical properties. In recent years, attempts to understand more about these zero-dimensional nanocrystal effects have been made in several labs by doping an impurity in a nanocrystal, searching for novel materials and new properties, and among them Mn-doped ZnS nanoparticles have been intensively studied [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Among many bulk wide band gap compounds, manganese is well known as an activator for photoluminescence (PL) and electroluminescence (EL) and the Mn 2+ ion d-electrons states act as efficient luminescent centers while doped into a semiconductors.In 1994 Bhargava and Gallagher [1,2] reported the first realization of a ZnS semiconductor nanocrystal doped with Mn isoelectronic impurities and claimed that Mn-doped ZnS nanocrystal can yield both high luminescence efficiency and significant lifetime shortening. The yellow emission characterized for Mn 2+ in bulk ZnS [16][17][18], which is associated with the transition 4 T 1 − 6 A 1 , was reported to be observed in photoluminescence (PL) spectra for the Mn 2+ in nanocrystal ZnS. In nanocrystals, however, the PL peak for the yellow emission is reported slightly shifted toward a lower energy (in bulk ZnS:Mn it peaks arount 2.12 ev, in nanocrystal ZnS:Mn it peaks at 2.10 eV). Also, the reported linewidth of the yellow emision in the PL spectrum for a nanocrystal is larger than for the bulk. Most strikingly, the luminescence lifetime of the Mn 2+ 4 T 1 − 6 A 1 transition was reported to decrease by 5 orders of magnitude, from 1.8 ms in bulk to 3.7 ns and 20.5 ns in nanocrystals while maintaining the high (18%) quantum efficiency.In ref.[6] the authors suggested that the increase in quantum efficiency as well as the lifetime shortening is the result of strong hybridization of s-p electrons of the ZnS host and d-electrons of the Mn impurity due to confinement, and also of the modification of the crystal field near the surface of the nanocrystals. Stimulated by this dramatic result, many other laboratories are trying to synthesize the Mn-doped ZnS nanocrystals and considerable attention has been paid to optical properties o...

We calculate the imaginary partofthethird order optical non-linearity for an array of semiconductor quantum dots in an organic host and show that it leads to large two-photon absorption. The calculated two-photon absorption is greater than currently measured materials. The large non-linearity results from a hybrid exciton formed in the inorganic-organic medium. The band gap of the semiconductor dot determines the spectral region of the resonances that vary from the visible to the near, mid and far infrared regions. We show that relatively small changes in the ratio ofthe quantum dot size to the quantum dot-to-dot spacing result in significant changes in the non-linearity. We briefly describe applications in communications, optical filters, and bio photonics for thin films comprising these hybrid excitons.

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