From atoms to nanoparticles: changing the shape

U. V. Desnica, P. Dubcek, M. Buljan, I.D. Desnica-Frankovic R. Boskovic Institute, Zagreb, Croatia

S. Bernstorff, Sincrotrone Trieste, Italy

Due to the large optical non-linearity as well as fast response times, composites containing II-VI binary semiconductor nanocrystallites in light-atom matrix show promise for very interesting applications in optical devices. CdS nanocrystals (NCs) were formed in various substrates by ion implantation, using high and equal doses of Cd and S atoms up to, 1x10¹⁷ cm^-2, and subsequent annealing up to T_a = 1000 ^oC.

Fusion of atoms into CdS NCs within substrate, growth of the size of NCs, their size distribution, and the formation of 3D ensemble of NCs in the 200 nm thin implanted layer were tracked as a function of ion dose and T_a. The formation of CdS phase from constituent atoms was checked by several methods: low-angle-incidence X-ray Diffraction, Raman spectroscopy, UV-Visible Transmittance and Reflectance measurements. The shape of obtained CdS NCs, however, was strongly dependent on the choice of targeted substrate, selected in this work as either amorphous (SiO₂) or crystalline cubic (silicon) or crystalline hexagonal (g-Al₂O₃).

The morphology of CdS NCs was studied by 2D grazing incidence small angle x-ray scattering (GISAXS), and Transmission Electron Microscopy (TEM). 2D GISAXS pattern comprised quasi-isotropic half-rings in SiO₂, elliptical half-rings in Al₂O₃, and streaks inclined under characteristic angles in silicon. The analyses of these patterns were performed using Guinier approximation as well as the local mono-disperse approximation (LMA). It was found that spherical NCs were formed in amorphous SiO₂ and elliptic, rod/like NCs in hexagonal Al₂O₃. In both types of samples a well correlated ensemble of NCs was distributed evenly in the implanted layer. In the Si substrate, however, the faceted, plate-like NCs of CdS are formed, nestled preferentially in the <111> planes of monocrystalline Si. The observed, very different self-organization of Cd and S-implanted ions into variously shaped and variously distributed CdS nanocrystals, depending of the target substrate, was analyzed in terms of minimizing total energy for the second phase formation and available space inside diverse substrate materials.