Differential Reflectance Spectroscopy in Analysis of Surfaces

Abstract

Differential reflectance spectroscopy (DRS) is a surface analytical technique. It uses ultraviolet (UV), visible, or infrared (IR) light as a probing medium. The interaction of light with ‘strongly absorbing materials’, such as metals, alloys, and semiconductors, occurs in the first 10–20 nm. Thus, the differential reflectometer probes 50–100 atomic layers into nontransparent solid surfaces. Because of the specific probing depth of light, DRS fills the gap between other surface techniques such as ion scattering, Auger spectroscopy, and ESCA (electron spectroscopy for chemical analysis), which probe 1, 5, or even 20 monolayers, and X‐ray diffraction (XRD) that probes as deep as 1–50 µm into a bulk material. The information gained by DRS is somewhat different from that obtained by the other surface techniques mentioned. A ‘differential reflectogram’ reveals details about the electron structure around the Fermi surface. Specifically, the instrument allows the exact measurement of the energies, which electrons absorb from photons as they are raised into a higher, allowed energy states. As each material has a specific electron band structure, the measurement of the characteristic energies for electron ‘interband transitions’ serves as a means for identifying these materials. Furthermore, investigation of any shift in these characteristic energies, which may be caused by the addition of solute elements to a solvent, by transformations, lattice defects, ordering, or the like, provides a deeper insight into the nature of the solid state. The application of DRS is, of course, not restricted to strongly absorbing materials such as metals, alloys, or semiconductors. Its strength has likewise been demonstrated in the identification and characterization of transparent or semitransparent surface layers as observed in thin‐film corrosion products on metal substrates. DRS is used to scan two samples whose properties differ slightly. Thus, the main advantage of DRS over conventional optical techniques lies in its ability to eliminate any undesirable influences of oxides, windows, electrolytes, instrumental variations, and the like, owing to the differential nature of the technique. No vacuum is needed unless measurements in the vacuum UV are desired. Thus, the formation of a surface layer due to environmental interactions can be studied in situ. The measurements are fast: a complete differential reflectogram, that is, an automatic scan from the UV through the visible into the IR region, is accomplished in about 3 min.

Electrical Conductivity and Optical Properties of Pulsed Laser Deposited LaNi5 Nanoscale Films

“…Because these nanoscale films are insular in shape, for the main part of the thicknesses considered in this study, the substrate’s reflectivity must be removed. This is done using single-beam differential reflectance spectroscopy measurements [ 39 ]. Small isles deposited for low thickness films increase in area as thickness increases and an ever-smaller substrate area remains uncovered, until a continuous film with metallic properties is obtained, at around 180 nm, as is deduced from the conductivity data.…”

“…The electron energy spectrum in metal alloys strongly influences their optical properties and can be studied when most of the free electrons are bound in the surface states or captured at the interfaces. Therefore, relative differential reflectance spectroscopy measurements were made for films with different thicknesses in the 0.185–25 μm spectral domain, leading to the identification of the allowed electronic interband transitions [ 39 ]. These results are presented and discussed in this work only for the most important UV-VIS-NIR domain and for the films up to 42 nm, where the effects were most noticeable.…”

Differential reflectivity spectroscopy on single patch nanoantennas

“…Juan Uriel Esparza, 1 Amit Raj Dhawan, 1 Rafael Salas-Montiel, 2 Willy Daney de Marcillac, 1 Jean-Marc Frigerio, 1 Bruno Gallas, 1 and Agnès Maître 1, a) This document provides supporting information concerning the influence of radius size and edge defects on the DRS and absorption spectra. In addition, we provide the gold permittivity used for calculations.…”

“…The comparison of the sample under investigation with a reference, under same conditions of illumination, provides sensitive spectral information, so that, even the smallest inhomogeneity change between samples may lead to a different reflectogram signature. As a consequence, DRS has been used for several purposes such as the study of intrinsic and doping materials (n-or p-type) 1 , the formation of metallic clusters during thin vapor deposition [2][3][4] , the anisotropy in metallic surface structure 5,6 and analysis of multi-polar effects in alkali metals such as potassium nanoparticles 7,8 . In addition, DRS technique can also probe the optical properties of individual plasmonic nanostructures 9 .…”

Standoff detection of explosive materials by differential reflection spectroscopy

“…It essentially measures the optical absorption of a material. The bench top apparatus currently in use is depicted in Figure 1 and briefly described below 37,38 . The optical properties of an explosive are determined by its electron structure that is, the arrangement of electrons in the molecular orbitals.…”