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We show that superlattice (SL) of PbS quantum dots (QD) can be easily prepared by drop casting of colloidal QD solution onto glass substrate and the ordering level can be controlled by the substrate temperature. A QD solution was dropped on glass and dried at 25, 40, 70 and 100oC resulting in formation of different SL structures. X-ray diffractograms (XRD) of deposited films show a set of sharp and intense peaks that are higher order satellites of a unique peak at 1.8 degrees (two theta), which corresponds, using the Bragg’s Law, to an interplanar spacing of 5.3 nm. The mean particles diameter, calculated through the broadening of the (111) peak of PbS using the Scherrer’s formula, were in agreement with the interplanar spacing. Transmission electron microscopy (TEM) measurements were also used to study the SL structure, which showed mainly a face centered cubic (FCC) arrangement of the QD. The photoluminescence (PL) spectrum of QD in the SL showed a shift toward lower energy compared to one in solution. It can be attributed to the fluorescence resonant energy transfer (FRET) between neighbors QD´s. Moreover, we observed greater redshift of PL peak for film with lower drying temperature, suggesting that it has a more organized structure.

Aqueous colloidal silver nanoparticles have substantial potential in biological application as markers and antibacterial agents and in surface-enhanced Raman spectroscopy applications. A simple method of fabrication and encapsulation into an inert shell is of great importance today to make their use ubiquitous. Here we show that colloids of silver-core/silica-shell nanoparticles can be easily fabricated by a laser-ablation-assisted chemical reduction method and their sizes can be tuned in the range of 2.5 to 6.3 nm by simply choosing a proper water-ethanol proportion. The produced silver nanoparticles possess a porous amorphous silica shell that increases the inertness and stability of colloids, which decreases their toxicity compared with those without silica. The presence of a thin 2 to 3 nm silica shell was proved by EDX mapping. The small sizes of nanoparticles achieved by this method were analyzed using optical techniques, and they show typical photoluminescence in the UV-vis range that shifts toward higher energies with decreasing size.

The results of investigating the optical properties, chemical composition, and crystal structure of tungsten-oxide films annealed in vacuum and air at 700°C are presented. The films are deposited by means of reactive dc magnetron sputtering. The samples involving single films, as well as heterostructures with tungsten- and titanium-oxide films, located on quartz glass substrates are examined. It is ascertained that, in different samples, annealing leads to different changes in the optical properties, chemical composition, and crystal structure of the films.

The article is devoted to the use of a computational statistical experiment using the Monte Carlo method when assessing the stability of the technical characteristics of an automobile generating installation in mass production. The algorithm calculates the stability of the performance of a batch of generators selected at random. The Monte-Carlo method consists in the multiple calculation of parameters according to a certain formalized scheme, which is the mathematical description of this process. The process under study is to determine the effect of technological errors in the manufacture of dimensional parameters, subject to certain statistical laws of distribution, on the electromagnetic characteristics of the generator

In recent years, there has been a dramatic progress in the photonics field of disordered media, ranging from applications in solar collectors, photocatalyzers, random lasing, and other novel photonic devices, to investigations into fundamental topics, such as localization of light and other phenomena involving photon interactions. Anderson localization of light is an open researcher frontier, which has greatly attracted the attention of researchers in the past few decades. In this work, we study the transport of light in a strongly disordered optical medium composed by core-shell nanoparticles (TiO 2 cSilica) suspended in ethanol solution. We demonstrate the crossover from a diffusive transport to a localization transition regime as TiO 2 cSilica nanoparticle concentration is increased. A striking phenomenon of enhanced absorption, mainly near the input border, arises at the localization transition, from which an increase of refractive index was inferred. An increase of the density of localized states and absorption near the input border is reported when the incidence angle is increased. The specular reflection, measured for the photons that enter the sample, is considerably lower than the effective internal reflection undergone by the coherently backscattered photons in the exact opposite direction, indicating a nonreciprocal propagation of light (parity-symmetry breaking). A theoretical simulation, performed through random-matrix theory, agrees satisfactorily with the experimental results, showing the generality of this approach to address transport phenomena.

The first- and second-order Raman spectra of carbon nano-onions (CNOs), produced via annealing of detonation nanodiamonds with a mean grain size of ∼5 nm in the argon ambience at the maximal temperature of annealing process (TMAX) varying from 1500 to 2150 °C, are analyzed together with the high-resolution transmission electron microscopy (HRTEM) images. The combined analysis provides a deep insight into the annealing-induced atomic-scale structural modifications of the CNO nanoparticles. The Raman and HRTEM data unambiguously demonstrate the reduction in the number of defects in the CNO structure, as well as indicate the conversion from the diamond sp3-bonded carbon phase to the sp2-bonded carbon phase with increasing TMAX and its almost full completion for TMAX = 1600 °C.

A new approach for electrical and thermal improvement of contacts between carbon nanostructures (multi-wall carbon nanotubes - MWCNTs and multi-layer graphene - MLG) and metal electrodes by localized laser heating is presented. The nanostructures were deposited over electrodes using the dielectrophoresis (DEP) technique. A focused laser beam was used for direct heating the samples in ambient atmosphere. The Raman spectroscopy was used to determine the process temperature by observations of the graphitic G-line downshift. In the laser annealing experiments, the G-line position was found first to downshift linearly with laser power indicating gradual heating of the sample proportional to the absorbed power. However, with increasing power the shift was found to saturate at levels that depend on the metal and the contact area. This saturation was attributed to gradual increase of the contact area and improvement of the thermal contacts between the nanostructures and metal electrode that can occur during sample heating. The maximum sample temperature in the beginning of the annealing process was always higher for MLG samples, due to smaller area of contact established between rigid multi-layer graphene and initially rough metal surface. The final result is the increased heat losses to the electrodes and, subsequently, the reduction of the samples temperature. The main advantage of this method, when compared with traditional and rapid thermal annealing, is that the thermal treatment is localized in a small pre-determined region, allowing individually controlled annealing process.

The thermal resistance of the contact between a metal and carbon nanotubes (CNTs) and between CNT layers is estimated by the local heating of CNT arrays (films) of different thicknesses using a focused laser beam and measuring their local temperature via Raman spectroscopy. It is demonstrated that thermal contacts between nanotubes, and also between CNTs and an electrode, can be formed by means of laser annealing.

We demonstrated the efficiency of the Raman spectroscopy method in the study of the process of the formation of the amorphous zinc aluminotitanate (ZAT) phase during the phase decomposition of the titanium-containing zinc aluminosilicate glasses doped with cobalt oxide. The quantitative dependences of the variation of the intensity of the Raman bands characteristic for amorphous and crystalline phases on the temperature of the thermal treatment and the cobalt oxide concentration have been obtained. The speed of phase decomposition with the separation of the nanosize crystals of zinc aluminate spinel (gahnite) and ZAT phase increases at low-temperature thermal treatments with increasing cobalt oxide concentration. The addition of cobalt oxide increases the amount of the amorphous ZAT phase separated during phase decomposition and increases its thermal stability during high-temperature treatments. It has been shown that the Co2+ ions enter the zinc aluminate spinel (gahnite) crystals precipitated during phase decomposition. It has been supposed that the composition of the residual glass is close to that of the quartz glass and it contains a very small amount of titanium-oxygen tetrahedra.

The evolution of structure, phase composition and spectroscopic properties of CoO-doped (up to 5 mol%) titania-containing zinc aluminosilicate glasses with their heat-treatment has been studied using Raman scattering, small angle X-ray scattering, X-ray diffraction analysis and optical absorption spectra. Addition of cobalt oxide was observed to facilitate amorphous phase separation of the parent glass and gahnite, ZnAl2O4, crystallization. Cobalt oxide entered phases formed during low-temperature heat-treatments (720 °C), i.e., amorphous phase, enriched in ZnO, Al2O3 and TiO2 and crystalline phase of gahnite. The absorption of these glass-ceramics was defined mainly by tetrahedral Co2+ ions located in gahnite nanocrystals. As the temperature was increased further, traces of anosovite solid solution appeared and then decomposed. Even after high-temperature heat-treatments, a certain portion of Co2+ ions remained in amorphous zinc aluminotitanate phase and in octahedral sites of inversed gahnite spinel. In glass-ceramics, the residual high silica amorphous phase contained a small quantity of [TiO4] centers, which content was smaller in Co:ZAS samples as compared with non-doped glass-ceramics.

A characteristic feature near ∼255 nm has been revealed in the optical absorption spectra of nano-graphite aqueous suspensions. This is a broad absorption peak, which is due to the surface plasmon resonance in nanographite particles. The peak shape and intensity depend on the structural quality and aggregate state of onion-like nanographite particles in suspension.

Nanographite (NG) particles were produced by annealing of superpurified detonation nanodiamonds (grain size ~ 5 nm) at 1600 °C. The aim of this research was to provide Raman characterization of nanographites obtained and to investigate characteristic features of UV optical absorption in NG suspensions caused by the excitation of surface plasmon resonance (SPR) and its dependence on disorder and defectiveness of graphene shells during their transformation. The 1-st and 2-nd order Raman spectra of the NG samples excited at 514 nm were analyzed. Two different approaches applied for evaluation of the in-plane NG crystallite sizes by using the D- and G-band intensities ratio gave quite different results (~ 3.5 nm and ~ 5.5 nm) reflecting, most likely, a complicated NG structure. The changes in both intensity and position of SPR absorption peak for water suspension of NG particles may originate in structural imperfections and/or changes in aggregation of NG particles.

There has been a growing interest in disordered optical media in recent years due to their potential applications in solar collectors, random lasers, light confinement, and other advanced photonic functions. This paper studies the transport of light for different incidence angles in a strongly disordered optical medium composed of core-shell TiO2@Silica nanoparticles suspended in an ethanol solution. A decrease of optical conductance and an increase of absorption near the input border are reported when the incidence angle increases. The specular reflection, measured for the photons that enter the sample, is lower than the effective internal reflection undergone by the coherently backscattered photons in the exact opposite direction, indicating a nonreciprocal propagation of light. This study represents a novel approach in order to understand the complex physics involved at the phase transition to localization.

The conditions of formation of deep periodic trenches by photoelectrochemical etching of n-Si (100) with linear seeds on the surface are analyzed. Criteria for the proper choice of the period of seed grooves and the etching current density in relation to the doping level of the substrate are formulated. Corrugation of walls is a characteristic feature of the obtained structures; this corrugation is caused by traces of merged macropores. Atomic-force microscopy is used to study roughness of the side-walls in relation to the etching conditions; the current density at which one can obtain the smoothest side-walls is determined. The roughness of the side walls in structures with periods of 7 and 9 μm on Si with the resistivity of 15 Ω cm amounts to ∼40 nm. It is shown that additional treatment of the structures in alkaline solutions can decrease the side-wall roughness by approximately a factor of 2.

This article reports on the in situ analysis of nickel silicide (NiSi) thin films formed by thermal processing of nickel thin films deposited on silicon substrates. The in situ techniques employed for this study include micro-Raman spectroscopy (microRS) and X-ray diffraction (XRD); in both cases the variations for temperatures up to 350 degrees C has been studied. Nickel silicide thin films formed by vacuum annealing of nickel on silicon were used as a reference for these measurements. In situ analysis was carried out on nickel thin films on silicon, while the samples were heated from room temperature to 350 degrees C. Data was gathered at regular temperature intervals and other specific points of interest (such as 250 degrees C, where the reaction between nickel and silicon to form Ni(2)Si is expected). The transformations from the metallic state, through the intermediate reaction states, until the desired metal-silicon reaction product is attained, are discussed. The evolution of nickel silicide from the nickel film can be observed from both the microRS and XRD in situ studies. Variations in the evolution of silicide from metal for different silicon substrates are discussed, and these include (100) n-type, (100) p-type, and (110) p-type silicon substrates.

Using Raman spectroscopy method, the attempt was made to quantify the SiO2 content in stuffed β-quartz derivative phases precipitated in lithium aluminosilicate glass-ceramics. The initial glasses were prepared by a conventional melt quenching technique, annealed and heat-treated in a temperature range of 660–850 °C. The unit-cell parameters were determined by X-ray diffraction (XRD) analysis. In Raman spectra, the one-mode behavior of complex mixed crystals of a series of stuffed β-quartz derivative phases x(LiAl)(1 − x)SiO2 was determined. It was demonstrated that the linear dependence of the frequency of the high-frequency Raman band with SiO2 content can be used for quantifying the SiO2 concentration in the crystalline phase with β-quartz structure. It was found that the linear dependence of the unit cell’s volume of β-quartz derivative phases on the SiO2 content can be used for quantifying the SiO2 content in the crystalline phase with β-quartz structure as well. The results obtained by both independent methods are in a good agreement.

Made available in DSpace on 2015-10-21T20:15:17Z (GMT). No. of bitstreams: 0 Previous issue date: 2015-06-23. Added 1 bitstream(s) on 2015-10-22T09:48:15Z : No. of bitstreams: 1 WOS000356663700001.pdf: 1328411 bytes, checksum: 2d01feaf14a69ea302c9623eac68234d (MD5) Graphene, in single layer or multi-layer forms, holds great promise for future electronics and high-temperature applications. Resistance to oxidation, an important property for high-temperature applications, has not yet been extensively investigated. Controlled thinning of multi-layer graphene (MLG), e.g., by plasma or laser processing is another challenge, since the existing methods produce non-uniform thinning or introduce undesirable defects in the basal plane. We report here that heating to extremely high temperatures (exceeding 2000 K) and controllable layer-by-layer burning (thinning) can be achieved by low-power laser processing of suspended high-quality MLG in air in "cold-wall" reactor configuration. In contrast, localized laser heating of supported samples results in non-uniform graphene burning at much higher rates. Fully atomistic molecular dynamics simulations were also performed to reveal details of oxidation mechanisms leading to uniform layer-by-layer graphene gasification. The extraordinary resistance of MLG to oxidation paves the way to novel high-temperature applications as continuum light source or scaffolding material. Univ Estadual Campinas, Ctr Semicond Components, BR-13083870 Campinas, SP, Brazil Univ Estadual Campinas, Inst Fis Gleb Wataghin, BR-13083970 Campinas, SP, Brazil UNESP, Univ Estadual Paulista, Dept Fis, IGCE, BR-13506900 Rio Claro, SP, Brazil UNESP, Univ Estadual Paulista, Dept Fis, IGCE, BR-13506900 Rio Claro, SP, Brazil

We present technique of obtaining complex hybrid structures combining the multi-walled carbon nanotubes or multi-layer graphene and luminescent hydrophobic semiconductor core/shell quantum dots CdSe/ZnS. As a result, a formation of quantum dot decorated carbon nanotubes and graphene films is evidenced by 2D microluminescence and micro-Raman mapping of quantum dots and nanocarbons, respectively, where a spatial correlation between the luminescence and Raman signals is found.

We investigate electrical photoresponse of multilayer graphene decorated with CdSe/ZnS quantum dots. Itwas found that photoresponse of these hybrid structures depends on quantum dot photoluminescence quantumyield. We demonstrate inuence of external factors (light exposure and treatment with ammonia vapors) onphotoluminescence quantum yield of quantum dots and electrical photoresponse of the hybrid structures.Keywords: Graphene, quantum dots, hybrid structures, photocurrent, ammonia sensor 1. INTRODUCTION Hybrid structures based on graphene and semiconductor quantum dots is a hot research spot during the lastyears. 1{5 Such structures do not combine unique properties of graphene and quantum dots only, but possessemergent properties. For example, matching remarkable electronic properties of graphene 6,7 and outstandingoptical properties of semiconductor quantum dots (QD) 8{10 allows formation of hybrid structures with enhancedphotoresponse. 5 Such structures are promising as light harvesting systems

The quality of thermal contacts between a graphene flake and metal electrodes was shown to improve strongly by laser annealing. The improvement of thermal contacts to initially rough metal electrodes is attributed to local melting of the metal surface under laser heating, and increased area of real metal-graphene contact. Improvement of the thermal contacts was observed also between multi-layer graphene and silicon oxide surface, with more efficient heat transfer from graphene as compared with the graphene-metal case.

The accuracy of thermal conductivity measurements by the micro-Raman technique for suspended multi-layer graphene flakes has been shown to depend critically on the quality of the thermal contacts between the flakes and the metal electrodes used as the heat sink. The quality of the contacts can be improved by nonlocal laser annealing at increased power. The improvement of the thermal contacts to initially rough metal electrodes is attributed to local melting of the metal surface under laser heating, and increased area of real metal-graphene contact. Improvement of the thermal contacts between multi-layer graphene and a silicon oxide surface was also observed, with more efficient heat transfer from graphene as compared with the graphene-metal case.

We report on the enhancement of the electrical photoresponse in a hybrid structure composed of multi-layer graphene flakes covered by a layer of CdSe/ZnS quantum dots (QDs) and placed between metal electrodes. The rate of the photoexcitation energy transfer from QDs to graphene, (0.5–2)×109 s−1 which controls the photoelectrical response of the structure, was found from the analysis of photoluminescence intensities and decay times for QDs in solution, on a bare glass substrate and on the surface of multilayer graphene, and in the presence of ammonia vapors.

Lenses with variable focus may find extensive applications in industrial production of optical devices. The variable-focus lenses considered in this work may be used most efficiently in laser systems wherein the radiation initially has linear polarization. Consideration is being given to the principles of operation of switchable lenses. The aberration of such lenses is analyzed, the data on control elements having a large angular aperture are presented, and a practical example of a variable-focus lens is given.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.