Researchers from the NOMAD laboratory at the Fritz Haber Institute were engaged in describing how surfaces change when in contact with reactive gas phases under different conditions of temperature and pressure. To do this, they developed the so-called grand canonical method of replica exchange (REGC). The results were published in the journal Review of physical sheets June 17.
“Exchanging replicas” means that there are many replicas prepared for the silicon surface in contact with different hydrogen atmospheres. These copies are exchanged with each other during the simulation. “Grand canonical” means that the silicon surface in each replica exchanges deuterium atoms or molecules with the deuterium gas reservoir it touches, eventually reaching equilibrium with the deuterium gas reservoir.
Knowledge of the morphology and structural evolution of material surfaces in a given reactive atmosphere is a prerequisite for understanding the mechanism of, for example, heterogeneous catalysis and electrocatalysis reactions due to the structure-property-power relationship. In general, the reliable tracking of phase equilibria is of technological importance for the intelligent design of surface properties. Phase transitions are indicated by features of the reaction function (for example, heat capacity). FHI researchers solved this problem by developing a grand canonical replica exchange (REGC) method combined with molecular dynamics. The approach not only captures the restructuring of the studied surface under different reactive conditions, but also determines the lines of phase transitions of the surface, as well as triple and critical points.
The dissociative adsorption of molecular hydrogen on silicon surfaces has become a critical criterion in the study of adsorption systems and has important applications such as surface passivation. The REGC approach is demonstrated using a silicon surface in contact with a deuterium atmosphere. In the range from 300 to 1000 Kelvin, the REGC approach identifies 25 different thermodynamically stable surface phases. Most of the phases identified, including some phase transitions between order and disorder, have not been observed experimentally before. It is also shown that the dynamic formation or breaking of Si-Si bonds is the driving force behind the phase transition between the experimentally confirmed adsorption schemes.
The REGC method allows combining traditional concepts of condensed state statistical mechanics with state-of-the-art electronic structure calculations to predict the stability phase diagrams of real systems. In addition, this approach has significant implications for surface restructuring calculations in surface science and is potentially relevant to various important applications such as heterogeneous catalysis, electrocatalysis, and surface segregation.