K-enriched WO3 nanobundles: high electrical conductivity and photocurrent with controlled polarity.

Potassium ions are successfully intercalated into WO3 nanobundles with the integrity of the pseudo-orthorhombic structure remaining intact. The nanobundles display a 5-fold increase in the electrical conductivity. It changes from a value of 10(-4) Sm(-1) for pure WO3 to 40 Sm(-1) upon potassium intercalation. The electrical conductivity also increases by ~200 times as temperature increases from 23 to 200 °C whereby analysis shows a thermal activation energy of ~1 eV. Density functional theory calculations show that K ions cause the reduction of the surrounding W atoms and lead to an increase in the electron population in the conduction band. Hence, the conductivity of the K-WO3 nanobundles is greatly enhanced. The calculated band structure also shows a gap of 1 eV that is consistent with the measured thermal activation energy. Upon illumination of focused laser beam, individual and isolated nanobundle displays significant photon induced current (9 nA) without external bias at low laser power (2 mW); the amplitude and polarity of photocurrent could be controlled by location of laser spot.