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 W(0.02)V(0.98)c thin films deposited on (001) 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 < 010 >(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 < 011 >(R) and < 110 >(R) directions. However, strong energetic penalty is found for the < 001 >(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. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.