| Research Abstract |
Semiconductor particles with sizes of a few nanometers show completely different properties from bulk materials by quantum confinement effect The purpose of this study is to show the optical absorption characteristics and nonradiative processes with the use of photoacoustic (PA) spectroscopy together with photoluminescence (PL) and photocurrent (PC) spectroscopies in (1) CdS and CdSe nanocrystals in a glass matrix, (2) nanoporous TiO_2 electrodes, and (3) ZnS nanocrystals doped with Mn^<2+> ions (ZnS : Mn).
(1) Different sizes of CdS and CdSe nanocrystals in a glass matrix were prepared. The lowest excited energy of the nanocrystals by quantum confinement effect shifts to lower energy region with the increase of nanocrystal size. The thermal activation energy in CdSe nanocrystals is larger mat that in CdS. The ultrafast photothermal signals in CdSSe nanocrystals showed tri-exponential behavior (the direct recombination, trapping from excited states, and subsequent recombination processe
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s, respectively).
(2) The effect of voltage on a concentrated elctrolyte applied to nanoporous TiO_2 electrodes was studied The PA intensities with the applied voltage treatments are higher than that without the treatments below the band-gap region, indicating the increase of carrier concentration. The PC intensities with the treatments are higher than that without the treatments above the band-gap region, indicating the increase of carrier concentration. The increase in PC signal intensity and a shift to lower energy are observed with the increase of CdS coating, indicating the transfer of photoexcited electrons of CdS nanocrystals to the conduction band of TiO_2.
(3) The peak position in the PA difference spectrum increases with the decrease of Mn^<2+> ion concentration, indicating the hybridization of the s-p states of the ZnS nanocrystal and the d state of the Mn^<2+> impurity. The peak intensity of the PL spectrum shows a maximum at a certain value of Mn^<2+> ion concentration (concentration quenching). Less
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