We study the light-matter coupling by Raman scattering in colloidal suspensions composed by core-shell TiO2@Silica (Rutile@Silica) nanoparticles suspended in ethanol and water solutions. Strong enhancement of the Raman signal per particle is observed as [TiO2@Silica] is increased above a threshold, being stronger in ethanol suspensions. Moreover, above this [TiO2@Silica] threshold, the optical transmittance of the ethanol suspension starts to be considerably lower than in water, despite scattering strength being higher in water. These results are attributed to localization of light induced by strong correlation in the scatterers' position as a consequence of the long-range Coulomb interaction between the TiO2@Silica nanoparticles. Light diffraction in TiO2@Silica suspensions (water and ethanol) shows strong correlation in the scatterers' position (structure seemingly cubic), being stronger in ethanol than in water (longer-range Coulomb interaction). As a result, we demonstrate in these colloidal suspensions for the first time, to our knowledge, strongly enhanced light-matter coupling through correlation-induced localization with klT much higher than unity and in an ordered colloidal-photonic structure. This strong enhancement of light-matter coupling by localization of light opens an avenue for manufacturing powerful sensing tools.

Significant drop of thermal contact resistance between a graphene flake and metal electrodes was shown as a result of laser annealing. The improvement of thermal contacts of initially rough metal electrodes is attributed to local melting of the metal surface caused by laser heating followed by increasing of real metal-graphene contact area.

The effect of surface-enhanced Raman spectroscopy (SERS) was investigated in N719 dye thin films deposited on silicon wafer with a thin film of silver nanoparticles (Ag-NPs) fabricated by laser ablation in an aqueous solution, using a NdYAG laser (λ = 1064nm). Optical absorption spectroscopy of the Ag-NPs colloidal solution shows an absorption peak at λ = 400nm, associated with a localized surface plasmon resonance in the Ag-NPs. Scanning electron microscopy (SEM) reveals that these NPs have an approximately spherical shape, with their diameter being tunable by laser power intensity. Raman spectroscopy measurements were performed using low laser power to avoid damage to the N719 dye films. Thus, a small Raman signal is obtained. The Raman intensity was greatly increased when the N719 film was deposited on a substrate with a thin film of Ag-NPs due to the SERS effect. The process was also used in Rhodamine-B to clearly demonstrate the SERS effect obtained by the use of these NPs produced by laser ablation.

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.

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 (TiO2@Silica) suspended in ethanol solution. We demonstrate the crossover from a diffusive transport to a localization transition regime as TiO2@Silica 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 non-reciprocal 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.

[This corrects the article DOI: 10.1021/acsomega.7b01791.].

In the last few years, research on dye-sensitised devices has been focused on the development of solar cells, based on CH 3 NH 3 PbX 3 (X = I − , Br − , Cl − ) composites with perovskite structure. The deposition of perovskite thin films is usually carried out by solution-based processes using spin-coating techniques that result in the production of high quality films. Solar cells made by this method exceed 20% efficiency, with the potential for use in large scale production through ink print or screen printing techniques. As an alternative route, perovskite thin films can be deposited through thermal evaporation. A new method is proposed to produce CH 3 NH 3 PbI 3 , based on a radio-frequency (rf) -sputtering technique that results in a high reproducibility of the films and is compatible with roll-to-roll processes. We deposited thin films of lead-sulphide (PbS) and converted them into perovskite by placing the films in an iodine atmosphere, followed by dipping in a solution of methylammonium iodide (CH 3 NH 3 I). The conversions to PbI 2 and CH 3 NH 3 PbI 3 were confirmed by elemental analyses, absorption, and photoluminescence spectroscopy. Structural properties were revealed by X-ray diffraction and infrared and Raman spectroscopy.

This paper studies the transport of light for different incidence angles in a strongly disordered optical medium composed by core-shell nanoparticles (TiO2@Silica) suspended in ethanol solution. A decrease of optical conductance and an increase of absorption near the input border are reported when the incidence angle is increased. We associated this phenomenon to an increase of the density of localized states (localization increase) near the input border, which could be explained by a large increase of internal reflection with the incidence angle, which in turn would be a direct consequence of the enhancement of the effective refractive index near the input border by localization itself. 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 non-reciprocal propagation of light (mirror-symmetry breaking). This study represents a novel approach in order to understand the complex physics involved in a strongly disordered optical medium at the critical regime of approaching localization.

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.

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.

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.

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.

The first- and second-order Raman spectra of carbon nano-onions (CNOs), produced via annealing of detonation nanodiamonds with a mean grain size of ∼5nm 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.

We investigate electrical photoresponse of multilayer graphene decorated with CdSe/ZnS quantum dots. It was found that photoresponse of these hybrid structures depends on quantum dot photoluminescence quantum yield. We demonstrate influence of external factors (light exposure and treatment with ammonia vapors) on photoluminescence quantum yield of quantum dots and electrical photoresponse of the hybrid structures.

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 C-line downshift. In the laser annealing experiments, the C-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 MW 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. (C) 2014 Elsevier B.V. All rights reserved.

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.

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.