Valence band and catalytic activity of Au nanoparticles in Fe2O3/SiO2/Si(100) environment

A 10-nm-thick gold film was evaporated onto a SiO₂/Si(100) substrate and was implanted by Ar⁺ ions at 40 keV and 10¹⁵ at/cm² dose creating island like Au nanoparticles. An 80-nm-thick gold film was also deposited the onto same substrate and considered as reference.Fe₂O₃ film was deposited onto the gold nanoparticles and the gold/oxide interface was modeled. The valence band and the structure were measured by means of photoelectron spectroscopy (XPS) and by transmission electron microscopy (TEM), respectively. The catalytic activity was detected by CO oxidation, which was higher after the deposition of Fe₂O₃ layer onto Au nanoparticles than that on a continuous Au film. This observation was correlated to the nanosize and the redistributed valence band density of states of gold in the Au/Fe₂O₃ interface.     

Numerical study on optical properties of Ag/SiO2SiO2/Ag sandwich nanocrescents substrate

A novel structure of a metal/dielectric/metal (Ag/SiO₂SiO₂/Ag) sandwich nanocrescent has been proposed and studied. We make a detailed numerical analysis on the extinction efficiency and LSPR property of the sandwich nanocrescent by using the finite difference time domain (FDTD) method. It clearly demonstrates that a comparable field enhancement can be achieved by varying the thickness of the SiO₂SiO₂ layer at different incident polarizations. Excited in the Y-polarization, the maximum electric field enhancement factor reaches 600 at the peak wavelength 1108.9?nm, which is six times higher than previous reported single layer nanocrescent. The refractive index sensitivity of this new sandwich nanocrescent is 375.5?nm/RIU (refractive index unit). The structure is shown to produce a high local field enhancement as well as wide plasmon resonance tunabilities. Besides, compared with adjusting the shape and size of the single layer nanocrescent structure, it is much more convenient and easier to change the thickness of the sandwich nanocrescent. Due to its excellent properties, this structure is very suitable for LSPR and SERS nanosensing substrate.     

Self-consistent performance modeling for dual band MIS UV photodetectors based on Si/SiO2 multilayer structure

In this paper, we present a self-consistent theoretical model for a metal-insulator semiconductor (MIS) dual band ultraviolet (UV) photodetector with a modified structure implying an arbitrarily defined insulating potential barrier as its active region. Utilizing our proposed model, the dark and photocurrent density-voltage (J-V) characteristics of MIS UV photodetectors with multi-quantum wells of silicon (MQWs) are calculated. We demonstrate that dark current is reduced in the suggested structure, because the electron-tunneling probability becomes unity at energies coincident with the peak detection wavelength. This is due to the resonant tunneling and decreases at energies that are significantly smaller than this optimum value. In consequence, the number of carriers contributing to the dark current, which have a broad energy distribution at high temperatures, will decrease. It is also shown that the designed structure could detect two individual UV wavelengths, simultaneously. The width of each Si quantum well has been considered at around 1.2 nm, in order to observe these two absorption peaks in the middle and near UV regions of photon spectrum (about 365 nm, 175 nm).     

Extension of optical properties of ZnO/SiO2 materials induced by incorporation of Au or NiO nanoparticles

Incorporating noble metal nanoparticles (NPs) and oxides has been proved to be an effective method to tune the optical properties of silica based materials. In this paper the optical and photocatalytic properties have been studied for ZnO/SiO₂ modified with Au or NiO nanoparticles. Changes in the optical properties of semiconductor ZnO particles have been observed due to the deposition of coloured Au and NiO nanoparticles by reducing the band gap energy and thus extending light absorption to visible domain. The excellent surface characteristics of NiO/ZnO/SiO₂ and Au/ZnO/SiO₂ favour the adsorption behaviour of these materials and limit the recombination of electron–holes pairs. Crystal Violet degradation under VIS light proved to have higher efficiency in the presence of Au/ZnO/SiO₂ (97%) than for NiO/ZnO/SiO₂ (60%).     

Surface-plasmon-enhanced photoluminescence of CdS nanoparticles with Au/SiO2 nanocomposites

To examine the effect of metal nanostructures on the photoluminescence of CdS nanoparticles, Au/SiO₂ nanocomposites were prepared with back-reflection geometry. The contribution from the prolonged optical-path lengths was excluded, and the luminescence enhancement was attributed only to surface-plasmon resonance. The optimum nanostructures for the PL enhancement were examined, and correlated with the wavelength of the surface-plasmon peak of the Au/SiO₂ nanocomposites and the PL emission peak of CdS nanoparticles.     

Au/(Si/SiO2)/p型Si结构的可见电致发光研究

Ｓｉ／ＳｉＯ２薄膜采用射频磁控溅射技术制备,当正向偏压大于５Ｖ时即可观测到来自不同Ｓｉ层厚度的Ａｕ／（Ｓｉ／ＳｉＯ２）／ｐ－Ｓｉ结构在室温下的可见电致发光,其发光谱峰位均位于６６０ｎｍ处,测得的各种储藏坟下的发光峰位不随正向偏压的升高而移动。突验结果表明光发射主要来自于ＳｉＯ２层中的发光中心上的复合发光。     

Fabrication of Au/SiO2 Nanocomposite Films by Self-Assembly Multilayer Method

Gold colloid was prepared by chemical reduction of hydrogen tetrachloroaurate, polyelectrolyte/gold nanoparticle/silica nanoparticie composite films were fabricated via an electrostatic self-assembly multilayer method, and composite films of gold nanoparticle dispersed in silica matrix were formed by heat-treating the polyelectrolyte/gold nanoparticle/silica nanoparticle composite films to eliminate the polyelectrolyte. The obtained composite films were investigated with UV-vis, TEM, AFM and XRD. The results show that the self-assembly multilayer method is a promising process to produce composite films of gold nanoparticle-dispersed in organic and/or inorganic matrixes.     

Au and NiO nanocrystals doped into porous sol-gel SiO(2) films and the effect on optical CO detection

Thin-film composites comprised of NiO and NiO/Au nanoparticles in a porous SiO(2) matrix have been prepared using the sol-gel technique. When at elevated temperatures (200?°C< T<350?°C) and exposed to carbon monoxide, the films undergo reversible changes in optical transmittance at wavelengths in the visible-near IR region. For NiO composite films heated at 330?°C and exposed to 1% CO in air, there is an increase in transmittance which approaches 2-4% over most of the visible range. For NiO/Au composite films the transmittance increase exhibits a wavelength dependence, with a maximum change which is close to 6% at λ≈630?nm and which is close to zero in the Au plasmon resonance range (λ≈550?nm).     

Properties of sensors based on shear-horizontal surface acoustic waves in LiTaO3/SiO2 and quartz/SiO2 structures

Acoustic wave devices based on waveguide modes with shear-horizontal polarization, i.e., Love modes, are very promising for sensor application, especially in liquid environments. They can be used for the determination of liquid density and viscosity as well as for chemical sensors. Up to now, several systems have been reported based on standard ST quartz. Those devices lack temperature stability, which is essential for field application. Thus, temperature-compensated systems based on different Y-rotated quartz and lithium tantalate (LiTaO3) plates with SiO2 guiding layers have been investigated. Temperature behavior as well as relevant acoustic properties were considered. Furthermore, experimentally determined data for device sensitivity are compared with theoretical predictions from numerical calculations.     

Effects of gamma irradiation on electroluminescence spectra from Au/amorphous Si/SiO2 superlattices/p-Si structures

Amorphous-Si/SiO₂ superlattices have been grown using the two-target alternation magnetron sputtering technique. The thickness of the SiO₂ layers in all the superlattices was 2.0 nm and that of the Si layers in nine types of the superlattice ranged from 0.6 to 3.8 nm in steps of 0.4 nm. Visible electroluminescence (EL) spectra of Au/amorphous-Si/SiO₂ superlattices/p-Si structures with Si layers of nine different thicknesses showed a peak located at 650 nm. After gamma irradiation, the EL peak increased 2.5 times in intensity. Moreover, a strong new 470 nm blue peak emerged from the EL spectra in all the Au/amorphous-Si/SiO₂ superlattices/p-Si structures. The experimental results indicate that the EL recombination process mainly originates from luminescence centers in the SiO₂ layers rather than from the Si layers in the superlattices.     

Quasi-freestanding,striped WS2 monolayer with an invariable band gap on Au(001)

Revealing the structural/electronic features and interfacial interactions of monolayer MoS2 and WS2 on metals is essential to evaluating the performance of related devices.In this study,we focused on the atomic-scale features of monolayer WS2 on Au(001) synthesized via chemical vapor deposition.Scanning tunneling microscopy and spectroscopy reveal that the WS2/Au(001) system exhibits a striped superstructure similar to that of MoS2/Au(001) but weaker interfacial interactions,as evidenced by experimental and theoretical investigations.Specifically,the WS2/Au(001) band gap exhibits a relatively intrinsic value of ～ 2.0 eV.However,the band gap can gradually decrease to ～ 1.5 eV when the sample annealing temperature increases from ～370 to 720 ℃.In addition,the doping level (or Fermi energy) of monolayer WS2/Au(001) varies little over the valley and ridge regions of the striped patterns because of the homogenous distributions of point defects introduced by annealing.Briefly,this work provides an in-depth investigation into the interfacial interactions and electronic properties of monolayer MX2 on metal substrates.     

Synthesis and structural, optical and electrical properties of TiO2/SiO2 nanocomposites

Sole components of titania (TiO₂), silica (SiO₂) nanoparticles, and binary TiO₂–SiO₂ nanocomposites with various molar ratios of silica contents were prepared by modified sol–gel method. The samples were calcined at 500 °C for 5 h and characterized by X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), UV–Vis spectroscopy, Brunauett–Emmett–Teller (BET), and photoconductivity. The crystallite size for TiO₂/SiO₂ nanocomposites was calculated using Scherrer’s formula and found to be 5 nm for TiO₂ nanoparticles. The binary oxide shows the anatase type of TiO₂ at the mole ratio up to 80 mol% of TiO₂ added. The band gap for as-synthesized nanocomposites was calculated and it was found that the band gap decreases with increase of SiO₂ content and then decreases with excess SiO₂ content. FTIR confirms that both TiO₂ and SiO₂ phases have been formed. The BET surface area for TiO₂/SiO₂ nanocomposite is found to be 303 m²/g, and pore size distribution has an average pore diameter about 10 nm for 40 mol% of TiO₂ added. It also confirms the absence of macropores and the presence of micro and mesopores. The field-dependent dark and photoconductivity studies reveal that the dark and photocurrent increase linearly with applied field confirming the ohmic nature of the electric contacts. The dark and photocurrent increase slightly with increase of SiO₂ content and decrease with excess amount of SiO₂.     

On the optical properties of Mn/SiO and SiO thin films

The ultraviolet, visible and infrared properties of unannealed and annealed amorphous Mn/SiO cermet thin films (300 to 1000 nm thick) prepared by vacuum evaporation at 5.0×10^–4 Pa are investigated. The ultraviolet and visible results are analysed assuming optical absorption by indirect transitions. A systematic reduction of the optical energy gap and an increase in the width of the band-tail region is observed with increasing metallic content. The effects of the ratio deposition rate/residual pressure and substrate temperature on the optical properties of SiO _x (1<x<2) thin films are also investigated.     

Reactions at the Al/SiO2/SiC layered interface

Electron spectroscopy and thermodynamic modelling have been used to examine reactions at the Al/SiO₂/SiC layered interfaces at 800 °C. The reactions have been examined as a function of oxide thickness. Three regimes have been isolated: (i) where there is no oxide present aluminium and SiC react to produce Al₄C₃ and free silicon; (ii) where there is a thin oxide present the initial products are aluminosilicates and amorphous alumina; however, once the SiO₂ is consumed, Al₄C₃ emerges as a product; (iii) where a thick oxide is present only aluminosilicate and alumina are formed.     

Effect of composition on optical properties of co-evaporated Mn/SiO x,Cr/SiO x and Cu/SiO x cermet thin films

Optical absorption spectra of amorphous Mn/SiO _x , Cr/SiO _x , and Cu/SiO _x cermet films, 300 nm thick, with compositions from 0 to 25 at% Mn, Cr and Cu, respectively, prepared by co-evaporation at 293 Kin vacuo have been investigated. The linearity of ()^1/2 versus graphs in the high absorption region for all the cermet films indicates that indirect photon transitions in k-space are involved in the absorption process. Taue's rule is also confirmed. In all cases the optical energy gap decreases significantly with increasing metallic content of the films and the width of the tail of localized states increases.     

Preparation and optical properties of Au/Teflon nanocomposites

Using thermal deposition technique, we have prepared Au/Teflon thin films on glass substrates. To control the film microstructure, both TEM and AFM characterization have been carried out. The visible optical transmission spectra have been measured in-situ. The peak positions and linewidths in the spectra have been identified in terms of the gold nanoparticle shape, size, and arrangement. The results obtained provide evidence for formation of both sphere-like and cylinder-like gold nanoparticles in the films. The fraction of each kind of the particles depends on whether the plasma treatment has been used. The treatment is shown to enhance the fraction of the cylinder-like particles.     

Fabrication and optical properties of ZnS:Mn2+ quantum dots/SiO2 nanocomposites

ZnS:Mn²⁺ quantum dots (QDs)/SiO₂ nanocomposites were successfully synthesized by stöber method. The results showed that the Mn²⁺ ions were substitutionally incorporated into the ZnS host and the average size of the ZnS:Mn²⁺ (5 %) QDs was about 5.5 nm. The yellow–orange emission from the Mn^{2+ 4}T₁–⁶A₁ transition was observed in the photoluminescence spectra, the peak intensity increased as the Mn²⁺ doped ratio increased, and showed a maximum when the concentration of the Mn²⁺ ions kept at 3 %. As the hydrolysis time of tetraethyl orthosilicate increased, the intensity of the yellow–orange emission reached the highest value when t = 4 h for the ZnS:Mn²⁺ (5 %) QDs/SiO₂ nanocomposites.     

Dual surface plasmon resonances in Zn nanoparticles in SiO(2): an experimental study based on optical absorption and thermal stability

Metallic zinc nanoparticles (NPs) of 5-15?nm in diameter, formed in silica glass (SiO(2)) by Zn ion implantation of 60?keV, showed a strong ultraviolet absorption peak at around 4.8?eV, which has been assigned as the surface plasmon resonance (SPR) of Zn NPs, and another small peak at 1.2?eV, which has never been reported before. To identify the origin of the 1.2?eV peak, the correlations of thermal stability between the two peaks and Zn NPs were evaluated under annealing both in a vacuum (pure thermal stability) and in oxygen gas (thermal oxidation stability). The well-correlated stability between the 1.2?eV peak, the 4.8?eV peak and Zn NPs indicates that the 1.2?eV peak is not ascribed to radiation-induced defects but to the Zn NPs. The 1.2?eV peak can be ascribed to an SPR of Zn NPs in SiO(2), because the peak satisfies the criterion of the SPR of metallic NPs. Since the 4.8?eV peak is also expected to satisfy the criterion, Zn NPs in SiO(2) have two SPRs at 1.2 and 4.8?eV.