OPTICAL-PROPERTIES OF LITHIUM TUNGSTEN BRONZE THIN-FILMS

The optical absorption spectra of fine grained polycrystalline thin films of lithium tungsten bronze are reported. The results are similar to the previously reported data for hydrogen and sodium tungsten bronzes: pointing to the secondary importance of the nature of the guest atom. The visible blue color band, peaking in the near infrared, is attributed to electron transitions from the conduction band to higher lying states. The absence of free carrier absorption in material, which is very small grained polycrystalline material (tens of nm and less) is tentatively attributed to a modified electron-phonon interaction. The main gap of the bronze, which is indirect, is effectively increased by electron filling of the conduction band, this effect is overwhelmed by bandgap shifts associated with crystallographic changes, which are a function of the guest atom type and concentration.