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.

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