Speaker
Description
IR Specular Reflectance (IR-SR) spectroscopy is an external reflectance technique widely used in the investigation of various materials. The IR light is specularly reflected, as described by the Fresnel equations, and contains information about the optical, dielectric, and vibrational properties of the material, which can be extracted using dispersion or Kramers–Kronig analysis. Particularly useful, however, is the application of IR-SR in the investigation of thin and ultrathin films. In the latter case (film thickness up to 100 nm), a simplification of the reflectance function can be introduced, whereby the reflectance function can be approximated as a reflectance–absorbance function; that is, the absorbance of the film can be evaluated through reflectance measurements. The reflectance–absorbance (IR-RA) spectrum, calculated from the ratio-reflectance (IR-RR) spectrum for a system consisting of soda-lime-silicate glass coated with an ultrathin siloxane film, can reveal the vibrational properties of the film, including the wavenumber positions and profiles of the IR bands, from which information on the assignment of the Si–O–Si vibrations can be obtained. To demonstrate, however, that this method provides reliable results, they must be verified using an independent method. For this purpose, the construction of two-trace two-dimensional correlation spectroscopy (2T2D-COS) maps from the originally recorded IR-SR spectra was proposed. The resulting maps, particularly the asynchronous maps, show peaks that correspond well to the band maxima observed in the IR-RA spectra of the film. It will be shown that this approach can be used not only for films with thicknesses close to 100 nm, but also for films only a few nanometers thick. The 2T2D-COS maps can also be constructed from IR-RA spectra, in which case they provide information on whether two films have the same structure. In addition to its application in IR reflectance spectroscopy, Raman spectroscopy of thin films can also be used to distinguish bands belonging to the glass substrate from those originating from the film. However, it remains unclear whether 2T2D-COS can be successfully applied when the intensities of the substrate and film bands change randomly and vary from one measurement to another, particularly in cases where spectral normalization is not possible. To address this problem, two-dimensional correlation spectroscopy (2D-COS) was applied to a series of Raman confocal spectra recorded from the same silane-modified glass sample at the same laser position. The measurements were performed on a thicker part of the film (≈1 μm), while the perturbation introduced was the position of the laser focus, i.e., the depth of measurement within the film. In this way, the asynchronous map made it possible to distinguish some of the overlapping bands and assign them either to the glass substrate or to the film.
Keywords: soda-lime-silicate glass; siloxane, IR Specular Reflectance; Raman spectroscopy; Two-trace two-dimensional correlation spectroscopy; Two-dimensional correlation spectroscopy