Direct evidence of tungsten clustering in W$_{\text{0.02}}$V$_{\text{0.98}}$O$_{\text{2}}$ thin films and its effect on the metal-to-insulator transition
Substitutional tungsten doping of VO$_{\text{2}}$ thin films and its effect on the metal-to-insulator transition are investigated by means of X-ray diffraction, Cs-corrected scanning transmission electron microscope (STEM) and ab-initio simulations. The W$_{\text{0.02}}$V$_{\text{0.98}}$O$_{\text{2}}$ 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 (HAADF) STEM image and their nature is further confirmed by electron energy loss spectroscopy (EELS). The W dopants are found to substitute in the V sites and form local clusters at first neighbour, preferentially along the <010>$_{\text{R}}$ directions. Ab-initio modeling for this 2 at.% W doped VO$_{\text{2}}$ confirms the experimentally found W clustering mechanism to be the most stable substitutional configuration and it demonstrates that the binding energy of such cluster is 0.18 eV. Driving forces for short range ordering are also obtained along the <011>$_{\text{R}}$ and <110>$_{\text{R}}$ directions. Strong energetic penalty is found for the <001>$_{\text{R}}$ direction. Simulations indicate that the clustering helps in stabilizing 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.