NANOCRYSTAL GROWTH IN ALKALI-HALIDES OBSERVED BY EXCITON SPECTROSCOPY

We have developed a method to study the growth of CuCl and CuBr nanocr ystals in alkali halides by in situ absorption spectroscopy. Exciton l ines are used as a signature of crystallinity and a broad absorption p eak measures the number of Cu+ ions. The mean radius R and the concent ration x(cr) of nanocrystals are determined simultaneously, which in t urn enables the determination of the particle density N of crystals as a function of time. Neither the size-distribution function nor R(t) f ollow the prediction of the classical growth theory, x(cr)(t) reveals two different growth mechanisms, one for high and one for low growth t emperatures. N(t) demonstrates that the growth of big crystals at the expense of small crystals happens from the beginning of the growth pro cess. This result contradicts the traditional view of Ostwald ripening as a final stage of precipitation and stresses the dynamical characte r of cluster growth. The experimental data are described by a set of e mpirical parameters that can be used as a guide for controlled growth of CuCl nanocrystals in NaCl. The smallest nanocrystals detected by ex citon spectroscopy consist of 50 unit cells. The kinetic properties of KCl and NaBr crystals doped with CuCl and CuBr are very similar to th ose observed in NaCl doped with CuCl.