Influence of the substrate on the spectroscopic characteristics of the fluorescence of adsorbed dye molecules

Abstract

UDC 535.37 It was previously shown that various electronic [i, 2] and even phase [3] transitions in a solid substrate can be sensitized with the aid of dye molecules adsorbed on the surface of the solid. On the other hand, the heterogeneity of the surface and the presence of charges have a strong influence on the spectroscopic characteristics of the luminescence of adsorbed dye molecules. Local nonuniform fields can cause displacement of the maxima in the fluorescence spectrum and changes in the coefficient of anisotropy of the fluorescence [i, 4], and they also have a strong influence on the vibrational levels of the molecule, causing changes in the form of the spectrum [5]. Inhomogeneous broadening is observed along with the homogeneous broadening of the spectra. In the general case the magnitude of the inhomogeneous broadening of a spectrum depends both on the heterogeneity of the surface and on the molecular environment of the probe under consideration, particularly on the solvation shell of the molecule being studied. It is ultimately determined by the local electric field in which the molecule is found at the moment when the quantum of light is emitted and thus depends on the depth of the surface potential well, the magnitude of the dipole moment of the photoexcited molecule, and the degree of restructuring of its environment at that moment in time. In the present work we investigated the influence of nonuniformity in the distribution of the local charged centers on the surface and the corresponding fluctuations in the surface electrostatic potential on the spectroscopic characteristics of the fluorescence of adsorbed dye molecules. Structures consisting of a semiconductor, a dielectric, and adsorbed dye molecules were investigated. Thermally oxidized crystals (iii) of high-resistance silicon (p ~ 1-2 k~'cm) and germanium (p ~ 30 ~.cm) served as the substrates. The thickness of the SiO 2 (GeO 2) layer on the surface, which was measured with the aid of an ellipsometer, was equal to 5 nm. On some of the Ge samples the oxide was created by means of the molecular stratification of a film of Cr203. Some of the Si samples were subjected to surface implantation by argon ions with an energy equal to 40 keV. The following dyes were used: erythrosine, Rhodamine B, Acridine Yellow, and Coumarin 47. The dye molecules were adsorbed from alcoholic solutions. The concentration of the adsorbed molecules was evaluated with the aid of a piezoelectric resonance balance, and it was equal to 2"10 ~s molecules/cm 2. The charge of the traps in the dielectric SiO 2 (GeO 2) was measured by determining the field effect on a large sinusoidal signal. The fluorescence spectra were recorded on a KSVU-12 automated spectrofluorometer with a thermostated vacuum measuring cell and on an Hitachi MPF-4 fluorometer. The fluorescence spectra of both samples which were evacuated at a pressure P = 10 -5 Pa and samples which were hydrated as a result of the admission of water vapor to a pressure P = 2400 Pa at T = 300 K were studied over a broad temperature range. The adsorption of all the dyes investigated was accompanied by a shift to a longer wavelength (-10-20 nm) in comparison to their solutions with a concentration C = i0 -s M. Variation of the temperature also caused appreciable displacement of the maximum in the fluorescence spectrum.