Monte Carlo simulation of phase separation in ${\mathrm{K}}_x{\mathrm{C}}_{60}$

Kinetic Monte-Carlo methods have been used to simulate phase transformations in ${\mathrm{K}}_x{\mathrm{C}}_{60}$ layers. It has been found that the phase composition is controlled by a balance between the Madelung energy and the energy of the electronic-shells interaction. A density-functional calibrated tight-binding method is used to calculate electronic effects. Our calculations point out a phase separation in the fulleride layer into K-rich $\left({\mathrm{K}}_3{\mathrm{C}}_{60}\right)$ and K-depleted areas at room temperature. The two-phase system is formed as a fine mixture which is shown to be stable against aggregation. The average diameter of ${\mathrm{K}}_3{\mathrm{C}}_{60}$ particles is about 20 to 40 nm and depends on the K content. This phase separation explains nanostructuring effects observed in electrochemically doped potassium fulleride layers. The size of the simulated ${\mathrm{K}}_3{\mathrm{C}}_{60}$ particles correlates with the size of experimentally observed clusters. Since ${\mathrm{K}}_3{\mathrm{C}}_{60}$ particles are metallic, the system considered can serve as an array of nanoelectrodes.