The optical constants of rapidly grown silver sulfide films approximately 400 A thick have been determined in the wavelength range 0.28–2.5 μ using three independent methods. A fourth method was used to obtain values of n and k for thin (40 A), naturally grown silver sulfide films on opaque silver in the wavelength range 0.34–0.60 μ. Although these results are probably not representative of bulk silver sulfide, they should be approximately correct for tarnish films on silver. Thus, they can be used to calculate the reflectance as a function of wavelength for silver covered with tarnish films of various thicknesses. The measured reflectances of two such films were in reasonable agreement with the calculations, and the reflectance of heavily tarnished silver was essentially the same as that of fresh silver in the infrared. The measured optical constants of silver sulfide can also be used to calculate the thickness of thin tarnish films from ellipsometric measurements, and to calculate the shift in the surface plasma-resonance frequency when silver is covered with a thin tarnish film.

Optical Constants of Silver Sulfide Tarnish Films

A light scattering method is described which permits determination of the absorption index of colloidal particles. This technique requires transmission and scattering measurements but not particle size information provided that the particles are small compared to the wavelength (Rayleigh scatterers). It has been applied to hematite (α - Fe 2 O 3 ) pigment particles over the visible wavelengths. The results were checked by comparing measured values of the Rayleigh ratio for a dispersion of larger particles of known particle size distribution with calculations based upon the newly determined absorption index. The agreement was within the experimental error. When literature values of the absorption index were used in such calculations, there were significant differences from the measured values. Color calculations based upon tristimulus color theory which utilized the new values of the absorption index agreed with qualitative observation of color, both in dilute sols where single scattering predominates and in multiply scattering concentrated sols.

Absorption index and color of colloidal hematite

Capacity measurements and cyclic voltammetry have been used to conduct a systematic investigation of the potential distribution at the semiconductor-organic electrolyte interface in the dark and under illumination with the n-type synthetic layered compounds MoS2, MoS2, and WSe2. From the obtained flat band potentials it could be concluded that the valence band edges vs. Ag/Ag+ are located in acetonitrile at ca. 1.0 V (MoS2), 0.7 V (MoSe2), and 0.4 V (WSe2). As theoretically expected, the oxidized component of redox systems with Fermi levels near or below the valence band edge can inject holes thus inducing an inverted region and generating a p-n junction in the solid. The cathodic current in the cyclic voltammograms of such redox systems occurs via hole injection with consecutive recombination of the injected holes while less oxidizing redox systems can only be reduced via the conduction band. — Since the semiconductors used in these experiments do not corrode in presence of a high surface concentration of holes, such a contact with an oxidizing redox electrolyte offers most favourable conditions for the application in a photoelectrochemical solar cell. The inversion layer has a maximal band bending and promises to obtain the highest possible photovoltage, which is however limited by the width of the band gap.



Mit Hilfe von Kapazitatsmessungen und zyklischer Voltametrie wurde die Potentialverteilung an der Grenzflache Halbleiter/organischer Elektrolyt im Dunkeln und unter Belichtung untersucht. Aus der Lage der Flachbandpotentiale synthetischer Schichtgitterkristalle von MoS2, MoSe2 und WSe2 in Acetonitril-Elektrolyten konnten die Lagen der Valenzbandkanten ermittelt werden. Sie liegen bei 1,0 V (MoS2), 0,7 V (MoSe2) und 0,4 V (WSe2) gegenuber einer Ag/Ag+-Elektrode in Acetonitril. Entsprechend der theoretischen Erwartung konnen Redoxsysteme mit Fermi-Niveaus nahe oder unterhalb der Valenzbandkante Defektelektronen in der Oberflache dieser Halbleiter injizieren. Dadurch entsteht eine Inversionsschicht und somit ein p-n-Ubergang im Festkorper. Der kathodische Strom in den zyklischen Voltamogrammen mit solchen Redoxsystemen erfolgt durch Injektion von Defektelektronen, wonach diese in der Inversionsschicht mit Elektronen rekombinieren. Dagegen konnen weniger oxidierende Redoxsysteme nur uber das Leitungsband reduziert werden. — Die Halbleiter, die hier untersucht wurden, korrodieren in diesem Medium nicht, obwohl die Oberflache stark p-leitend ist. Solch ein Kontakt mit einem oxidierenden Redoxsystem bietet daher besonders gunstige Voraussetzungen fur die Verwendung in photoelektrochemischen Solarzellen. Die Inversionsschicht besitzt eine maximale Bandverbiegung und fuhrt daher zur Erzeugung der grostmoglichen Photospannung, die allerdings durch die Weite der Bandlucke begrenzt wird.

Photoelectrochemical Reactions and Formation of Inversion Layers at n-Type MoS2-, MoSe2-, and WSe2-Electrodes in Aprotic Solvents

Some investigative studies of pigmented nevi with cutaneous microscopy.

Resonant optical antennas provide unprecedented opportunities to control light on length scales far below the diffraction limit. Recent studies have demonstrated that nanostructures made of multilayer transition metal dichalcogenides (TMDCs) can exhibit well-defined and intense Mie resonances in the visible to the near-infrared spectral range. These resonances are realizable because the TMDC materials exhibit very high in-plane refractive indices, in fact higher than what is found in typical high-index dielectric materials like Si or GaAs. However, their out-of-plane refractive indices are comparatively low. Here we experimentally and theoretically investigate how this unusually large material anisotropy influences the optical response of individual TMDC nanoresonators made of MoS2. We find that anisotropy strongly affects the far-field optical response of the resonators, as well as complex interference effects, such as anapole and resonant Kerker conditions. Moreover, we show that it is possible to utilize the material anisotropy to probe the vectorial nature of the nanoresonator internal near fields. Specifically, we show that Raman spectra originating from individual MoS2 nanoresonators exhibit mode-specific anisotropic enhancement factors that vary with the nanoresonator size and correlate with specific modes supported at resonance. Our study indicates that exploring material anisotropy in novel high-index dielectrics may lead to new nanophotonic effects and applications.

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Rene Bailly

Papers

Infrared light for mineral determination

A New Tool for Infrared Studies.