M. E. Alvarez-Ramos

Hollow Au–Ag bimetallic nanoparticles with high photothermal stability

Noble metal nanoparticles have received much attention due to their interesting properties that make them useful in different technical fields. Metallic nanoparticles with optical properties in the near infrared region of the electromagnetic spectrum are of great importance for biological applications, in particular photothermal therapy, as it is greatly enhanced by metallic nanoparticles. However, despite the large amount of work that has been done with metallic nanoparticles for thermal therapy, there is a reduced amount of scientific reports about the photothermal stability of most studied nanoparticles. In this work, hollow Au–Ag bimetallic nanoparticles were synthesized via galvanic replacement reaction, with optical properties that can be tuned systematically along the visible and near infrared region of the spectrum, by changing the pH before the synthesis of the templates and by controlling the amount of gold added for the synthesis of the nanoshells. The synthesized nanoparticles exhibit good photothermal properties when illuminated with an 808 nm laser light. An increase of temperature of nearly 20 °C is achieved after 15 minutes of irradiation. Moreover, the Au–Ag nanoparticles show good reusability even after ten heating/cooling cycles. The nanoparticles also retain their optical properties after 12 hours of continuous irradiation and are able to maintain their photothermal characteristics of increasing the temperature at the same levels during the entire process.

Experimental and theoretical investigation on the molecular structure, spectroscopic and electric properties of 2,4-dinitrodiphenylamine, 2-nitro-4-(trifluoromethyl)aniline and 4-bromo-2-nitroaniline

2,4-Dinitrodiphenylamine (I), 2-nitro-4-(trifluoromethyl)aniline (II) and 4-bromo-2-nitroaniline (III) have been investigated by DFT and experimental FTIR, Raman and UV-Vis spectroscopies. The gas-phase molecular geometries were consistent with similar compounds already reported in the literature. From the vibrational analysis, the main functional groups were identified and their absorption bands were assigned. Some differences were found between the calculated and the experimental UV-Vis spectra. These differences were analyzed and explained in terms of the TD-DFT/B3LYP limitations, which were mainly attributed to charge-transfer (CT) effects. These findings were in agreement with previous works, which reported that TD-DFT/B3LYP calculations diverge from experimental results when the electronic transitions involve CT. Despite this, TD-DFT/B3LYP calculations provided satisfactory results and a detailed description of the electronic transitions involved in the absorption bands of the UV-Vis spectra. In terms of the NLO properties, it was found that compound (I) is a good candidate for NLO applications and deserves further study due to its good β values. However, the β values for compounds (II) and (III) were negatively affected compared to those found on o-nitroaniline.

Enhancing the power conversion efficiency of solar cells employing down-shifting silicon quantum dots

We report the synthesis and characterization of silicon quantum dots that exhibit down-shifting, photo luminescent characteristics. We also discuss the fabrication and characterization of single crystal Silicon (c-Si) Solar cells with and without the influence of the previously mentioned QDs. The incorporation of these nanostructures triggers improvements in the performance of the fabricated photovoltaic devices, especially in the open circuit voltage (Voc) and short circuit current density (Jsc). Specifically, the experimental results showed increments in the Voc from 532.6 to 536.2 mV and in the Jsc from 33.4 to 38.3 mA/cm2. The combined effect of those improved Voc and Jsc values led to an increment in the power conversion efficiency (PCE) from 11.90 to 13.37%. This increment represents an improvement of the order of 12.4% on the power conversion efficiency of this type of solar cells. The observed results could be conducive to promoting the proliferation of photovoltaic structures.

Enhanced conversion efficiency in Si solar cells employing photoluminescent down-shifting CdSe/CdS core/shell quantum dots

Silicon solar cells have captured a large portion of the total market of photovoltaic devices mostly due to their relatively high efficiency. However, Silicon exhibits limitations in ultraviolet absorption because high-energy photons are absorbed at the surface of the solar cell, in the heavily doped region, and the photo-generated electron-hole pairs need to diffuse into the junction region, resulting in significant carrier recombination. One of the alternatives to improve the absorption range involves the use of down-shifting nano-structures able to interact with the aforementioned high energy photons. Here, as a proof of concept, we use downshifting CdSe/CdS quantum dots to improve the performance of a silicon solar cell. The incorporation of these nanostructures triggered improvements in the short circuit current density (Jsc, from 32.5 to 37.0 mA/cm2). This improvement led to a ∼13% increase in the power conversion efficiency (PCE), from 12.0 to 13.5%. Our results demonstrate that the application of down-shifting materials is a viable strategy to improve the efficiency of Silicon solar cells with mass-compatible techniques that could serve to promote their widespread utilization.

Photo-mediated Seedless Synthesis of Silver Nanoparticles Using CW-Laser and Sunlight Irradiation

Noble metal nanoparticles, such as silver nanoparticles (AgNPs), have found technological applications due to their localized surface plasmon resonance (LSPR). In this context, a great quantity of synthetic methods for the preparation of AgNPs have been developed, including photo-chemical synthesis [1]. The photo-chemical synthesis is an advantageous technique because it is simple and environmentally friendly [2]. In this work, synthesis of AgNPs was performed via direct photo-reduction process of the silver nitrate and sodium citrate solutions without the previous addition of silver seeds.

Comparison of spatially and temporally resolved diffuse transillumination measurement systems for extraction of optical properties of scattering media.

This paper discusses the main differences between two different methods for determining the optical properties of tissue optical phantoms by fitting the spatial and temporal intensity distribution functions to the diffusion approximation theory. The consistency in the values of the optical properties is verified by changing the width of the recipient containing the turbid medium; as the optical properties are an intrinsic value of the scattering medium, independently of the recipient width, the stability in these values for different widths implies a better measurement system for the acquisition of the optical properties. It is shown that the temporal fitting method presents higher stability than the spatial fitting method; this is probably due to the addition of the time of flight parameter into the diffusion theory.

Characterization of the Molecular Crystal L-Alaninium Oxalate by Raman Microscopy, Optical Microscopy and X-ray Powder Diffraction

There are studies that claim that molecular materials formed by combination of amino acids with other organic or inorganic compounds exhibit good NLO properties, low laser damage threshold and acceptable mechanical and thermal properties [2]. For example, oxalates of amino acids have displayed good NLO properties and efficient second harmonic generation (SHG). In particular, L-alaninium oxalate (LAO) is a molecular crystal with formula C3H8NO2 ·C2HO4 , that has attracted attention due to its good NLO performance [3]. For the present work we carried out a structural characterization of the multi-component molecular complex LAO, which has been synthetized by the slow evaporation technique and mechanochemical route.

Monitoring the degradation of lubricating oil by means of surface plasmon

Extend the lubricant oil life is pursuit for scientific society, it represent a preservation of natural sources and contamination for oil wasted, furthermore, the reduction of costs to industrial application and time of machinery operation it would be extended as well. Lubricant oil analysis play an important role in the proper functioning of industrial machinery such turbines, compressors and any engine. In this work, are present preliminary results of monitoring the degradation of engine oil by surface plasmon detector, it which will be correlated with FT-IR technique by norm ASTM-E-2412, this technique is the most used to interpret the oil quality [1, 2]. FT-IR can detect the evolution of the functional groups that are present in the lubricating oil and through them can predict the condition of the oil, the most representative groups for this characterization correspond to oxidation, nitration and sulfation [3], Therefore these will be correlated with the results of the measurement of the surface plasmon extinction band of silver nanoparticles deposited on polystyrene cells by the laser irradiation technique and measured in a UV-VIS Ocean optics USB 2000+.

Study of Alkali Halide Solid Solutions by Scanning Electron Microscopy and X-ray Diffraction

Alkali halide materials are ionic crystals with the chemical formula MX, where M is an alkali metal (Li, Na, K, Rb, Cs) and X is a halogen (F, Cl, Br, I). They can form solid solutions by substitution between the alkali metals, the halogens or both [1]. Solid solutions are widely studied with the aim to understand the phase selection during solidification. The objective of the present work is to determine the phases present in the final microstructure of a crystalline material obtained from the melt of various alkali halide salts.

Influence of photo-luminescent CdSe/CdS core shell quantum dots in solar cell efficiency

We report the synthesis and characterization of CdSe/CdS core-shell quantum dots (CdSe/CdS-QDs) that exhibit absorption in the UV range of the solar spectrum and emit photons with wavelengths centered around 625 nm, a wavelength that is well suited for silicon absorption and electron-hole pair generation. We also report the fabrication and characterization of single crystal silicon (c-Si) solar cells with and without the aforementioned photo luminescent, down-shifting CdSe/CdS- QDs. The incorporation of these nanostructures triggered improvements in the performance of the devices, particularly in the open circuit voltage (Voc) and short circuit current density (Jsc) for which the measured values showed an increase from 543 to 546 mV and from 32.5 to 37.0 mA/cm2, respectively. The combined effect of the improved values led to an increment in the power conversion efficiency (PCE) from 12.01 to 13.54%. This increase represents a 12.7% improvement in the PCE of the fabricated devices. The effort described herein is considered a good fit to the generalized trend to improve the efficiency of solar cells with mass-compatible techniques that could serve to promote their widespread utilization.