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ABSTRACT Intensity, temperature and frequency-dependent photoabsorption spectroscopy are used to probe trap states in CdS0 .44 Se0 . 6 nanoparticles in glass. The photoabsorption signal from 6.2 nm radius particles is found to be a combination of two effects: bleaching, due to phasespace filling, and an electric-field effect. Frequency-dependent photoabsorption data show that the electric-field effect is caused by a long-lived (r=30 ps at T=293 K) trap state and that the bleaching occurs on a faster time scale. The trap activation energy obtained from the temperature dependence of the lifetime is 0.23 ±0.06 eV. Only bleaching is observed in the smaller nanocrystallites.

INTRODUCTION Nanometer-sized semiconductor particles (or "quantum dots") in glass provide a vehicle to study electronic states which are confined in all three dimensions. The goal of these studies is to understand not only the effect of confinement on the electronic energy levels, but also the nature of the electron-hole excited states: spatial distribution of the wavefunctions,

recombination dynamics and interactions with other quasiparticles such as phonons and polarons. CdSxSei-x nanoparticles have been the principal model system for optical studies of quantum confinement since the early work of Brus and collaborators, 1 2' and Ekimov and collaborators. 3 - 5 Carrier recombination in these systems have typically been studied on a nano- and pico-second time scale by using time-resolved photoluminescence and time-delayed differential transmission spectroscopy,6- 8 and studies of defects have been accomplished with steady-state spectroscopies. 9 However, the milli- to micro-second time-scale dynamics have not been studied extensively. Frequency-dependent photomodulation is a useful spectroscopic technique to study defect dynamics in this time regime and has been used to study deep trap levels in III-V compounds.' 0 1 2 Of course, this technique is particularly useful for insulators (such as nanoparticles in glass) where deep-level transient spectroscopy cannot be used. In this study we report intensity, frequency and temperature dependent photoabsorption (PA) data for 6.2 nm radius CdS0.44 Seo.5 6 nanoparticles in glass. The data confirm the existence of a charged impurity state with lifetime r=30 us (at room temperature) with an activation energy of 0.23 ±0.06 eV.

241 Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society

EXPERIMENT The samples were prepared by thermal treatment of commercially available Schott RG630 13 filter glass. Raman spectra show that particle composition is unaffected by this treatment. Details of sample preparation and characterization are published elsewhere."' The particle size was measured by transmission electron microscopy. The samples are polished to a thickness of 70-120 pm and placed in a variable-temperature liquid nitrogen cryostat. Photoabsorption spectroscopy is a pump-probe experiment which measures the normalized difference in light transmission through the sample (Tpump on - Tpump of I)/Ta