STOICHIOMETRY OF CD(S,SE) NANOCRYSTALS BY ANOMALOUS SMALL-ANGLE X-RAY-SCATTERING

In Cd(S,Se)-doped glasses the optical properties are strongly dependen t on the size of the nanocrystals, but can be also largely modified by changes in the crystal stoichiometry; however, the information on bot h stoichiometry and size is difficult to obtain in crystals smaller th an 10 nm. The intensity scattered at small angles is classically used to get information about nanoparticles sizes. Moreover the variation o f amplitude of this intensity with the energy of the x ray-''the anoma lous effect''-near the selenium edge is related to stoichiometry. Anom alous small-angle x-ray scattering has been used as a tentative method to get information about stoichiometry in nanocrystals with size lowe r than 10 nm. Experiments have been performed on samples treated for 2 days at temperatures in the range 540-650 degrees C. The samples trea ted at temperatures above 580 degrees C contain crystals with size lar ger than 4 nm. For all these samples the anomalous effect has nearly t he same amplitude, and we found the stoichiometry x=0.4 for the CdSxSe 1-x nanocrystals. This agrees with the previous results obtained by sc anning electron microscopy and Raman spectroscopy. The results are als o confirmed by measurements of the position of the optical absorption edge and by wide-angle x-ray scattering experiments. For the sample tr eated at 560 degrees C, the nanocrystal size is 3 nm and the stoichiom etry x=0.6 is deduced from the anomalous effect. For samples treated a t lower temperatures the anomalous effect is not observable, indicatin g an even lower selenium content in the nanocrystals (x>0.7). We obser ved differences in the Se content of nanocrystals for different heat t reatments of the same initial glass. These results may be very helpful to interpret the change in the optical properties when the temperatur e of the treatments decreases in the range 560-590 degrees C. In this temperature range, compositional effects seem to be of the same order of magnitude as the effects of the quantum confinement. (C) 1995 Ameri can Institute of Physics.