In this work, we demonstrate that the optical bandgap of WO3 films can be continuouslycontrolled through uniaxial strain induced by low-energy helium implantation.The insertion of He into epitaxially grown and coherently strained WO3 films canbe used to induce single axis out-of-plane lattice expansion of up to 2%. Ellipsometricspectroscopy reveals that the optical bandgap is reduced by about 0.18 eVper percent expansion of the out-of-plane unit cell length. Density functional theorycalculations show that this response is a direct result of changes in orbital degeneracydriven by changes in the octahedral rotations and tilts.
Andreas Herklotz, Stefania Rus, Santosh KC, Valentino Cooper, Amanda Huon, Er-Jia Guo, and Thomas Ward. "Symmetry driven control of optical properties in WO3 films" APL Materials (2017). doi:10.1063/1.4989395