Nanocrystal growth in alkali halides observed by exciton spectroscopy

Abstract

We have developed a method to study the growth of CuCl and CuBr nanocrystals in alkali halides by in situ absorption spectroscopy. Exciton lines are used as a signature of crystallinity and a broad absorption peak measures the number of ${\mathrm{Cu}}^{+}$ ions. The mean radius $R$ and the concentration $x_{\mathrm{cr}}$ of nanocrystals are determined simultaneously, which in turn enables the determination of the particle density $N$ of crystals as a function of time. Neither the size-distribution function nor $R\left(t\right)\mathrm{}$ follow the prediction of the classical growth theory. $x_{\mathrm{cr}}\mathrm{}\left(t\right)\mathrm{}$ reveals two different growth mechanisms, one for high and one for low growth temperatures. $N\left(t\right)\mathrm{}$ demonstrates that the growth of big crystals at the expense of small crystals happens from the beginning of the growth process. This result contradicts the traditional view of Ostwald ripening as a final stage of precipitation and stresses the dynamical character of cluster growth. The experimental data are described by a set of empirical parameters that can be used as a guide for controlled growth of CuCl nanocrystals in NaCl. The smallest nanocrystals detected by exciton spectroscopy consist of 50 unit cells. The kinetic properties of KCl and NaBr crystals doped with CuCl and CuBr are very similar to those observed in NaCl doped with CuCl.