Nanoscale Interface Control for Highly-Luminescent Nanophosphors

Abstract

Nanophosphors have been extensively investigated during the last decade due to their size dependent physical properties which is different from the bulk materials. For display devices, nanophosphors have potential advantages over traditional micron-sized phosphors. Nanophosphors offer higher packing densities, lower scattering of light, and quantum-size effect. However, nanoparticles have generally showed poor luminescence efficiency, due to the surface adsorbates and defects in nanocrystals. Therefore, I have studied on the nanoscale interface modification to develop highly-luminescent nanoparticles. Two fundamental factors are crucial for the novel properties of semiconductor nanoparticles. The first is the large surface to volume ratio: The surface states are likely to trap electrons and/or holes, and induce the nonradiative recombination of these charge carriers, leading to the reduction in the luminescent and photovoltaic efficiency. The second approach is taking advantage of the surface-plasmon resonance from metal nanostructures to semiconductors. The interaction of semiconductor nanoparticles with surface plasmons has an enhanced emission by electromagnetic-field amplications, and also has a suppression of emission by the energy transfer between semiconductor and metal nanoparticles. The PL enhancement of CdS nanoparticles by surface-plasmon resonance (Chap. 2, 3, 4), the surface-passivation effects by UV irradiation with oxygen bubbling (Chap. 5), and the photoluminescence enhancement using Li-addition (Chap. 6) will be introduced. Especially, to examine the influence of metal nanoparticles on the photoluminescence of semiconductors, colloidal mixtures of CdS and Au nanoparticles were prepared with different CdS/Au fractions. Compared to the cadmium-sulfide nanocrystals (quantum efficiency  9%), the CdS/Au mixtures showed enhanced luminescence properties (quantum efficiency  18%). The existence of an optimum ratio of metal to semiconductor nanoparticles for the photoluminescence intensity indicates that interactions between the metal and semiconductor nanoparticles induced by surface-plasmon resonance occur constructively at appropriate distances. In order to explain the origin of PL enhancement of CdS nanoparticles, a calculation was carried out for spherical Au nanoparticles with which the field enhancement by Au nanoparticles is found to insufficient to improve the PL quantum efficiency. In this respect, the PL enhancement can be understood in terms of increased radiative recombination rates due to surface plasmon resonance between the excitons in semiconductor nanoparticles and the electron plasma in metal nanoparticles, as confirmed by time-resolved PL.