Structural and optical properties of Tris(8-hydroxyquinoline) aluminum (III) (Alq<inf>3</inf>) thermal evaporated thin films

Abstract

X-ray diffraction (XRD), transmission electron microscope (TEM) micrographs and optical properties of Tris(8-hydroxyquinoline) aluminum (III) (Alq 3) have been studied. XRD of powder Alq3 showed that the material has a polycrystalline nature with triclinic structure. The crystal structure and morphology of the as-deposited and annealed (at 473 K for 2 h.) Alq3 thin films indicated that the as-deposited film is amorphous in nature, while the annealed film has a polycrystalline nature with amorphous background. The molecular structure of the Alq3 was confirmed by the analysis of (FTIR) spectra. The optical constants such as the refractive index, n, the absorption index, k and the absorption coefficient, α, of both the amorphous and polycrystalline Alq3 films were determined using spectrophotometric measurements of transmittance (T) and reflectance (R) in the wavelength range (200-2500 nm). The analysis of the data showed an indirect allowed transition energy gaps Egind of 2.66 eV and 2.28 eV for the as-deposited and the annealed Alq3 thin films, respectively. As well as another probability of direct allowed transition was carried out with energy gaps Eg1d of 2.82 eV, and Eg2d of 4.14 eV for the as-deposited film and Eg1d of 2.59 eV, Eg2d of 3.88 eV for the annealed films, respectively. Some optical parameters namely molar extinction coefficient (εmolar), oscillator strength (f) and electric dipole strength (q2) have been evaluated. According to the single oscillator model (SOM), some related parameters such as oscillation energy (E0), the dispersion energy (Ed), the optical dielectric constant (ε∞), the lattice dielectric constant (εL) and the ratio of free carrier concentration to its effective mass (N/m*) were estimated. Graphical representation of both the surface and volume energy loss functions and the real optical conductivity as a function of photon energy supports the existence of the mentioned optical transitions. © 2010 Elsevier B.V. All rights reserved.