Стратегічні пріоритети подолання демографічної кризи в Україні

We report a computational study on the spontaneous self-assembly of spherical particles into twodimensional crystals. The experimental observation of such structures stabilized by spherical objects appeared paradoxical so far.We implement patchy interactions with the patches point-symmetrically (icosahedral and cubic) arranged on the surface of the particle. In these conditions, preference for self-assembly into sheet-like structures is observed. We explain our findings in terms of the inherent symmetry of the patches and the competition between binding energy and vibrational entropy. The simulation results explain why hollow spherical shells observed in some Keplerate-type polyoxometalates (POM) appear. Our results also provide an explanation for the experimentally observed layer-by-layer growth of apoferritin - a quasi-spherical protein

Mechanism and modeling of nanorod formation from nanodots

A population balance model based on Smoluchowski aggregation kinetics is developed to explain the formation of nanorods from a colloidal suspension of spherical nanoparticles (nanodots). Our model shows that linear pearl-chain aggregates form by the oriented attachment (OA) of nanodots during the early stages of synthesis, since it occurs with a time scale smaller than the coalescence time scale of nanodots present within an aggregate. The slower coalescence step leads to the transformation of the linear pearl-chain aggregate into a smooth nanorod over a longer time scale of many hours, as observed in experiments. The attachment kinetics is modeled by a modified Brownian collision frequency, with the latter decreasing with nanorod length, leading to the experimentally observed slower growth in nanorod length at longer times. The collision frequency also includes the effects of attractive dipole-dipole and van der Waals interactions between nanodots, which are primarily responsible for OA. Our model predictions are general, and they compare favorably with available experimental data in the literature on the distribution of the aspect ratio (length to diameter) of ZnO and ZnS nanorods over different time scales

Model for core-shell nanoparticle formation by ion-exchange mechanism

Core-shell nanoparticles can be synthesized by partial exchange of the cation of the core nanoparticles by a second cation of the desired shell compound. Process time and cation concentration can be varied to control the thickness of the shell. A mathematical model is presented that describes the process of ion-exchange from preformed spherical nanoparticles. The overall process consists of diffusion of the cation from the solution to the core - nanoparticle surface, then solid-state diffusion of the cation across the shell, and finally the cation exchange reaction. We find that the second step of diffusion in the shell is rate-controlling. Our model compares well with the available experimental data of CdS-PbS core-shell nanoparticle formation at various [Ph2+]/[Cd2+] molar ratios and at different process times. The model can be useful for a priori specification of experimental conditions required for a desired shell thickness