SYNTHESIS OF POROUS QUANTUM-SIZE CDS MEMBRANES - PHOTOLUMINESCENCE PHASE-SHIFT AND DEMODULATION MEASUREMENTS

Optically transparent CdS membranes have been synthesized. Colloidal Q-CdS particles (particle diameter dp< 40 Å) carrying short phosphate chains and excess Cd2+ ions have been directly converted into a microporous membrane form. By controlling ionic strength and particle concentrations, one can link self-organized fusion-free aggregates and avoid formation of powder-like flocculation products. The resultant unsupported membranes exhibit different mechanical properties (e.g., rigidity, delayed elasticity, and solubility) in the presence of water depending upon the preparation method employed. The membrane form of CdS has distinctively different photophysical properties than the precursor colloidal form. Conversion of weakly red luminescent colloids (broad band at 700 nm) into membranes activated an intense room temperature band edge luminescence (BEL) (narrow bands between 450 and 500 nm) attributed to the recombination of excitons and/or shallowly trapped electron/hole pairs. A specific solvent effect has been observed indicating the major role of water and the related acid-base chemistry in producing radiationless recombination centers at the particle surface. In membrane/solvent experiments, water was found to be a very efficient quencher of the emission while alcohols and acetonitrile did not induce any changes in luminescence properties. A novel Multi-Harmonic Fourier Transform (MHFT) technique was used to determine the average lifetimes of the BEL decay process (≈10 ns in the colloidal form and ≈70 ns in the corresponding membrane form). Simultaneously detected (multiple-frequency domain) phase/modulation data have been used to describe the complex BEL decay, and a three-exponential law best fits the data. © 1990, American Chemical Society. All rights reserved.