Temperature dependence of phase diagrams and dynamics in nanocrystal assembly by solvent evaporation

Abstract

We provide a systematic study of the phase diagram and dynamics for single component nanocrystals (NCs) by a combination of a self-consistent mean-field molecular theory (MOLT-CF) and molecular dynamics (MD) simulations. We first compute several thermodynamic functions (free energy, entropy, coefficient of thermal expansion and bulk modulus) as a function of temperature by both MOLT-CF and MD. While MOLT-CF correctly captures the trends with temperature, the predicted coefficients of thermal expansion and bulk moduli display quantitative deviations from MD and experiments, which we trace back to the mean-field treatment of attractions in MOLT-CF. We further characterize the phase diagram and calculate the dependence on temperature of the bcc to fcc transition. Our results reveal that differences in entropic and enthalpic contributions to the free energy oscillate as a function of NC separation and are correlated to a geometric quantity: the volume of overlap between the ligand layers in different particles. In this way, we generally show that bcc is favored by enthalpy, while fcc is by entropy, in agreement with previous experimental evidence of fcc stabilization with increasing temperature, but contrary to what is expected from simpler particle models, where bcc is always entropically favored. We also show that the lowest relaxation times drastically increase in the latest stages of solvent evaporation. Overall, our results demonstrate that MOLT-CF provides an adequate quantitative model describing all phenomenology in single component NCs.