Postsynthetic Route for Modifying the MetalInsulator Transition of VO<sub>2</sub> by Interstitial Dopant Incorporation

The thermally driven orders-of-magnitude modulation of resistance and optical transmittance observed in VO<sub>2</sub> makes it an archetypal first-order phase transition material and underpins functional applications in logic and memory circuitry, electromagnetic cloaking, ballistic modulation, and thermochromic glazing to provide just a few representative examples. VO<sub>2</sub> can be reversibly switched from an insulating to a metallic state at an equilibrium transition temperature of 67 °C. Tuning the phase diagram of VO<sub>2</sub> to bring the transition temperature closer to room temperature has been a longstanding objective and one that has tremendous practical relevance. Substitutional incorporation of dopants has been the most common strategy for modulating the metalinsulator transition temperature but requires that the dopants be incorporated during synthesis. Here we demonstrate a novel postsynthetic diffusive annealing approach for incorporating interstitial B dopants within VO<sub>2</sub>. The postsynthetic method allows for the transition temperature to be programmed after synthesis and furthermore represents an entirely distinctive mode of modulating the phase diagram of VO<sub>2</sub>. Local structure studies in conjunction with density functional theory calculations point to the strong preference of B atoms for tetrahedral coordination within interstitial sites of VO<sub>2</sub>; these tetrahedrally coordinated dopant atoms hinder the rutile → monoclinic transition by impeding the dimerization of V–V chains and decreasing the covalency of the lattice. The results suggest that interstitial dopant incorporation is a powerful method for modulating the transition temperature and electronic instabilities of VO<sub>2</sub> and provides a facile approach for postsynthetic dopant incorporation to reach a switching temperature required for a specific application.