Structural and optical properties of thermally evaporated methylene blue
trihydrate thin films up to a film thickness of 165 nm have been
investigated. The lattice constants and the crystallite size were
calculated. The optical parameters were obtained using spectrophotometric
measurements of the transmittance and reflectance at normal incidence of
light in the wavelengths range 200–2500 nm. Below the absorption edge,
subgap absorption in the energy range 0.75–1.4 eV was observed and
interpreted in terms of defect states induced absorption phenomena and act
as deep traps for charge carriers. The type of optical transition was found
to be an indirect allowed transition. The values of the optical gap
Eopt, the transport gap Et, the exciton binding energy EB, the Urbach's energy Eu and the dispersion parameters for as-deposited,
annealed and irradiated films with different doses of γ-irradiation
were estimated and discussed their influenced by heat treatment and
irradiation.