Structural, absorption and optical dispersion characteristics of rhodamine B thin films prepared by drop casting technique

The drop casting technique has been successfully used to deposit highly uniform and good adhesion rhodamine B (Rh.B) thin films. The structural and morphological properties of Rh.B were studied by X-ray diffraction (XRD), and transmission electron microscopy (TEM), respectively. The molecular structure and electronic transitions of Rh.B were investigated by Fourier-transform infrared (FTIR) and ultraviolet-visible-near infrared (UV-VIS-NIR) spectra, respectively. The calculated Stokes shift between the excitation and emission of Rh.B reflects the displacement in potential surface between the ground and the excited states. The important absorption parameters such as molar extinction coefficient (ε_molar), the oscillator strength (f), and the electric dipole strength (q²) were also reported. The analysis of the spectral behavior of the absorption coefficient in the intrinsic absorption region reveals an indirect allowed transition with a band gap of 1.97 eV and associated phonons of 75 meV. The dispersion of the refractive index is discussed in terms of the single oscillator Wemple-Didomenico (WD) model. The single oscillator energy (E_o), the dispersion energy (E_d), the high frequency dielectric constant (ε_∞), the lattice dielectric constant (ε_L) and the ratio of the free charge carrier concentration to the effective mass (N / m^*) were estimated. From the optical constants analysis, the optical conductivity, volume and surface energy loss functions could also be calculated.