Direct evidence of tungsten clustering in W0.02V0.98O2 thin films and its effect on the metal-to-insulator transition

Substitutional tungsten doping of VO2 thin films and its effect on the metal-to-insulator transition are investigated by Cs-corrected scanning transmission electron microscopy (STEM), and ab initio simulations. The W0.02V0.98O2 thin films deposited on (0 0 1) sapphire are studied in both planar and transverse geometries. The tungsten atoms are distinguishable from the V atoms in the Z-sensitive high angle annular dark field STEM image and their nature is further confirmed by electron energy loss spectroscopy. The W dopants are found to form local clusters at first neighbor, preferentially along the 〈0 1 0〉R directions. Ab initio modeling for this 2 at.% W doped VO2 confirms the experimentally found W clustering mechanism to be the most stable substitutional configuration and demonstrates that the binding energy of such a cluster is 0.18 eV. Driving forces for short range ordering are also obtained along the 〈0 1 1〉R and 〈1 1 0〉R directions. However, strong energetic penalty is found for the 〈0 0 1〉R direction. Simulations indicate that the clustering helps to stabilize the tetragonal structure, while a diluted W dopant induces more structural distortion and V-V pairing. This suggests that the clustering mechanism plays a critical role in the transition temperature evolution with the W dopants.