Transition state redox during dynamical processes in semiconductors and insulators

Activation barriers associated with ion diffusion and chemical reactions are vital to understand and predict a wide range of phenomena, such as material growth, ion transport, and catalysis. In the calculation of activation barriers for non-redox processes in semiconductors and insulators, it has been widely assumed that the charge state remains fixed to that of the initial electronic ground state throughout a dynamical process. In this work, we demonstrate that this assumption is generally inaccurate and that a rate-limiting transition state can have a different charge state from the initial ground state. This phenomenon can significantly lower the activation barrier of a dynamical process that depends strongly on charge state, such as carbon vacancy diffusion in 4H-SiC. With inclusion of such transition state redox, the activation barrier varies continuously with Fermi level, in contrast to the step-line feature predicted by the traditional fixed-charge assumption. In this study, a straightforward approach to include the transition state redox effect is provided, the typical situations where the effect plays a significant role are identified, and the relevant electron dynamics are discussed. Modeling of reaction dynamics can be improved by rethinking a traditional belief about the charge of a transition state. Chemists usually assume that for a non-redox reaction, the state with the highest energy along a reaction path, or the transition state, has the same charge as the initial state. But Guangfu Luo, Thomas Kuech and Dane Morgan from the University of Wisconsin-Madison in the USA have now demonstrated that a transition state with unfixed charge is more realistic for dynamical processes involving semiconductors or insulators. Through atomistic simulations of dynamics, such as vacancy diffusion in gallium arsenide, the team reports that allowing a change of charge during a dynamical process can reduce its energy barrier relative to that predicted by the traditional approach and predict a more realistic continuous variation of energy barrier with Fermi level. Ab initio calculations reveal that the rate-limiting transition state during a dynamical process in a semiconductor or an insulator can have a different charge state from that of the initial ground state, rather than the same charge state as generally assumed. Such “transition state redox” effect may significantly lower the activation barrier and can be realized through the hopping of an initial excited state and possibly an electron exchange with the bulk during the dynamical process.