Optical efficiency of Ag and Au nanoparticles

In this paper, we present the results of calculations aimed at the optical radiation efficiency of Ag and Au nanoparticles, which is defined by the ratio of the scattering cross-section to the extinction one. The calculations were performed using Mie theory formalism for surrounding medium of various refractive indexes. It has been shown that silver nanoparticles exhibit substantially larger optical efficiency in a broad spectral range as compared to gold nanoparticles. The optical efficiency for silver nanoparticles with diameters over 90 nm exceeds 90%

Experimental study and theoretical analysis of photoelectric characteristics of AlxGa₁₋xAs–p-GaAs–n-GaAs-based photoconverters with relief interfaces

We studied experimentally the photoelectric characteristics of the AlxGa₁₋xAs–p-GaAs–n-GaAs structures with relief interfaces. A theoretical analysis of spectral dependences of internal quantum efficiency of short-circuit current in the above solar cells (SC) was performed. In particular, the low-energy spectral region (where absorption is weak) was considered. A comparison was made between the experimental and theoretical photocurrent spectral curves. From it, we determined a number of parameters of the AlxGa1–xAs and GaAs р-layers, as well as of the n-GaAs layers. Some recommendations concerning the ways to increase photocurrent and extend photosensitivity spectral region were developed for technologists. A theoretical analysis of a “spotty” model for open-circuit voltage formation in relief AlxGa₁₋xAs–p-GaAs–n-GaAs-based SC was made. This model enables one to give a qualitative explanation for decrease of open-circuit voltage in relief SC as compared to the case of flat interface

Comparison of properties inherent to thin titanium oxide films formed by rapid thermal annealing on SiC and porous SiC substrates

The comparative analysis of optical characteristics inherent to TiO2/SiC and TiO2/por-SiC/SiC structures has been performed. It has been shown that, in these structures regardless of the substrate structure, formation of TiO2 layers with approximately the same width 60 nm takes place. In this case the TiO2 film composition is close to the stoichiometric one. At the same time, the presence of an additional porous layer in the TiO2/por-SiC/SiC structure leads to blurring the oxide film – substrate interface but promotes an increase in the intensity of the Raman scattering signal from the oxide film

Effect of nanosize metal overlayer on C₆₀ thin film optical parameters near fundamental absorption edge

The effect of nanosize metal overlayer, both evaporated on C₆₀ films (Bi, In) and attached as nanoparticles of (Ag, Au), on the optical parameters of C₆₀ films near the fundamental absorption edge has been studied. The values of direct band gap (Eg), the optical gap (E₀) in the framework of Tauc model and the Urbach tail parameter (EU) were determined from the absorption coefficient (α) spectra plotted in coordinates (αhν)², (αhν)¹/² ln(α) vs hν, respectively. Parameters obtained testify diminishing the structural disorder in C₆₀ thin films with nanosize metal overlayer at optimal ratio of C₆₀ to metal layer thicknesses

Characterization of nanoscaled films on flat and grating substrates as some elements of plasmonics

The optical properties of multilayer structures consisting of dielectric, conductivity-oxide and nanoscaled metal layers, deposited on the planar substrates (witness samples) and surface relief ones (diffraction gratings) with micro- and nanoscale sizes, are investigated by AFM, spectral ellipsometry (SE), and photometric techniques. The SE-measured parameters are related to actual characteristics of the layers when specified the model of their near-surface regions. Using a parametrization of the layer dielectric function versus the wavelength and a fitting procedure, the dielectric parameters are determined. It is shown that the optical constants are affected by both the substrate morphology and the adjacent medium. Preliminary data about the influence of isolated particle plasmon excitations in 2D-substrates with the top nanoscaled Au layer on its optical properties are presented

Au/GaAs photovoltaic structures with single-wall carbon nanotubes on the microrelief interface

The effect of single-wall carbon nanotubes nanolayer on photoelectric properties of Au/n-GaAs photovoltaic structure with a microrelief interface has been investigated. Microrelief interfaces of dendrite-like and quasi-grating type aimed at enhancement of photocurrent have been prepared by the wet chemical anisotropic etching of GaAs. Carbon nanotubes obtained using the arc-discharge method were deposited on GaAs surface modified with poly(vinylpyridine) by dip-coating repeated several times. Optical, photoelectric and electrical properties of Au/GaAs structures have been studied in dependence on the averaged thickness of nanotubes nanolayer. Considerable photocurrent enhancement has been determined for structures with both the flat and textured interface but with a different optimal thickness of nanotubes layer. The effect was concluded to be caused by increasing the lateral photocurrent component due to enlargement of the current collection area and increase of the light trapping

Effect of chemical modification of thin C₆₀ fullerene films on the fundamental absorption edge

Fullerene C₆₀ films were grown using physical vapor deposition on Si substrates at room temperature. Then chemical modification with cross-linking these films was performed using the reaction with 1,8-octanediamine (DA) or octane-1,8- dithiol (DT). These chemically cross-linked C₆₀ films are capable of stable binding the Ag or Au nanoclusters. Optical properties of the obtained nanostructured hybrid films were investigated by both reflectance spectroscopy and spectral ellipsometry within the spectral range 1.55 to 5.0 eV at various angles of incidence. From the spectral dependences of the extinction coefficient in the region of optical absorption edge, the physical nature of the fundamental allowed direct band-gap transitions between HOMOLUMO states Eg, the optical absorption edge near the intrinsic transition Eo, and exponential tail of the density-of-states caused by defects have been determined. Influence of chemical modification and decoration of metal nanoparticles on the above mentioned parameters has been analyzed

Optical characterization of thin Au films by standard and polaritonic ellipsometry

This work is aimed at optical characterization of thin Au films by multiple-angle-of-incidence reflectance ellipsometry at the fixed wavelength (632.8 nm) in standard and attenuated total reflection (ATR) modes in contact with different dielectric media (water, alcohol and air). The comparative experimental evaluation of the precision of determined optical parameters (refractive index n and absorption coefficient k) and thickness for both ellipsometric modes has been studied by calculation of their sensitivity correlation coefficients. Medium- and mode-related optical constants of Au films were revealed. In the ATR mode the effective optical parameters were higher then in the standard mode, n decreased and k increased with increasing the refractive index of adjoining medium. This effect must be taken into consideration in polaritonic optoelectronic and optochemical sensor technique

Transformation of thin gold films morphology and tuning of surface plasmon resonance by post-growth thermal processing

The effect of vacuum heat treatment in the temperature range between 100 and 400 °C on the morphology and optical properties of thermally evaporated thin gold films with mass thickness from 8 to 80 nm was studied by AFM, TEM and optical spectroscopy in the spectral region of localized (surface) plasmons excitation. Our investigations have shown that change of film morphology induced by thermal annealing depends strongly on film thickness. Thus, optical behavior of thin metal films is modified due to being induced by recrystallization film permittivity changes as well as transformation of film morphology. The considered processes might be crucial for thin film optical properties determination and must be taken into account for the choice of adequate thin film optical model. Besides that, post-fabrication thermal annealing enables a wide modification of thin metal films morphology and surface plasmon resonance parameters, offering new possibilities for metal substrates design enhancing surface electromagnetic fields and therefore SERS, SEIRA, luminescence effects