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In this work we have analysed the conditions to grow epitaxial layers by Liquid Phase Epitaxy (LPE) from ternary Ga-As-P liquid phases on GaAs and GaP under initial conditions that are far away of the thermodynamic equilibrium. First, it is shown that the liquid-solid (L-S) interfaces are “stable” for all compositions of the liquid phases exclusively in contact with the GaP substrates. At the same time the elastic energy generated in mismatched GaAsyP1-y layers induces a reduction in the As content of the layer. Then, it should be expected that highly lattice mismatched epitaxial layers could be grown with small elastic energy, so that beyond certain stress the layers are initially non-planar having spatially separated but simultaneous local centres of nucleation and dissolution pits. These processes should cause a change on the composition of the liquid phase and, as consequence, the formation of the epitaxial islands with a composition gradient along its thickness. Our estimations show that in the case of contact of a Ga-As liquid phase with a GaP substrate the P content in the epitaxial islands increases with its thickness. The thermodynamic analysis was done with the CALPHAD method using SGTE data.
Cadmium sulfide quantum dots (CdS QDs) are semiconductor nanoparticles having sizes in the order of nanometers. They are materials that have outstanding properties for down conversion applications. These nanostructures have been used in the fabrication of white light emitting diodes (WLEDs) in the last years. However, inhomogeneous deposition of CdS QD conversion materials allows unwanted UV light escape. In addition, low efficiency due to strong self-quenching effect, incompatibility between CdS QD solution/crystal polyester resin matrix and reabsorption are common problems that need to be solved. In this work, we try to address the incompatibility between the CdS QD solution/crystal polyester resin matrix by using a solvent exchange procedure. To block the unwanted UV-light escape, we coated our devices with a mixture of graphene carbon quantum dot (GCQD) solution/crystal polyester resin matrix. The QDs and the WLED prototypes were characterized by absorption and photoluminescence (PL) spectroscopy. The QDs embedded in the matrix shown a good homogeneous dispersion. On the other hand, the mixture shown a rapid solidification. These facts indicate a good compatibility between the CdS QDs and the crystal polyester resin. We also observed a considerable reduction of unwanted near UV-light. White light emission from WLED devices with common crystal polyester resin and low-cost materials has been achieved.
Graphene Carbon Quantum Dots (GCQDs) are multi-layered carbon nanostructures that have attracted considerable attention due to its unique properties. Many technological applications, such as batteries, biological imaging, capacitors, solar cells, light emitting diodes, among others, could benefit from the low toxicity and the chemical and physical stability of these nanostructures. Despite much research, many optical properties, such as absorption and photoluminescence, of GCQDs are not completely understood yet. GCQD absorption spectra show a number of different bands whose origin is still on discussion. Many interpretations are made considering a single graphene layer. In this work, GCQD samples synthesized by the pyrolysis of citric acid was characterized by absorption spectroscopy measurements and Density Functional Theory simulations considering multi-layered structures. Density of States and electronic response functions calculations were also performed. From the results of these calculations, the absorption band associated to a π-π* (CC) transition could be also associated to a transition between different graphene layers.
A significant area of research is biomedical applications of nanoparticles which involves efforts to control the physicochemical properties through simple and scalable processes. Gold nanoparticles have received considerable attention due to their unique properties that they exhibit based on their morphology. Gold nanospheres (AuNSs) and nanorods (AuNRs) were prepared with a seed-mediated method followed of polyethylene glycol (PEG)-coating. The seeds were prepared with 0.1 M cetyltrimethyl-ammonium bromide (CTAB), 0.005 M chloroauric acid (HAuCl4), and 0.01 M sodium borohydride (NaBH4) solution. Gold nanoparticles with spherical morphology was achieved by growth by aggregation at room temperature, while to achieve the rod morphology 0.1 M silver nitrate (AgNO3) and 0.1 M ascorbic acid solution were added. The gold nanoparticles obtained by the seed-mediated synthesis have spherical or rod shapes, depending on the experimental conditions, and a uniform particle size. Surface functionalization was developed using polyethylene glycol. Morphology, and size distribution of AuNPs were evaluated by Field Emission Scanning Electron Microscopy. The average size of AuNSs, and AuNRs was 7.85nm and 7.96 x 31.47nm respectively. Fourier transform infrared spectrometry was performed to corroborate the presence of PEG in the AuNPs surface. Additionally, suspensions of AuNSs and AuNRs were evaluated by UV-Vis spectroscopy. Gold nanoparticles were stored for several days at room temperature and it was observed that the colloidal stability increased once gold nanoparticles were coated with PEG due to the shield formed in the surface of the NPs and the increase in size which were 9.65±1.90 nm of diameter for AuNSs and for AuNRs were 29.03±5.88 and 8.39±1.02 nm for length and transverse axis, respectively.
In the current work, we compared the green synthesis of silver nanoparticles (AgNP) using plant extracts, a promising methodology against the use of chemical reducers, such as oleic acid and oleylamine. The advantages of green synthesis are one-step method, economic and ecological while the advantages of classic synthesis methods are high nanoparticle performance, homogeneity in size and smaller average sizes. With this work we want to demonstrate that plant extracts with specific mixtures of chemical compounds can obtain smaller average sizes with greater homogeneity in nanoparticles compared to the use of classical synthesis. Myrtillocactus geometrizans was used as a polar plant extract, which was selected by the chemical components contained in the extract. Phytosterols, oleic acid and betalains contained in Myrtillocactus geometrizans are biomolecules responsible for the reduction and stability of AgNP below 5 nm. TEM analysis of the green synthesis of nanoparticles revealed the formation of spherical particles with an average diameter of 5 nm and with preferential crystallographic directions of the silver plane .
A student's t-test was applied in carbon nanospheres synthesis from cis-1,4-polyisoprene considering the green chemical principles. The synthesis was carried out by Chemical Vapor Deposition method with a quartz tube reactor using an AISI 304 steel bar as catalyst. It was possible to obtain two types of different samples, one from the surface of the steel bar (catalyst) and another from the quartz tube surface (without catalyst) in the same experiment. Carbon spheres were observed in both samples by micrographs obtained by FESEM. The Raman and FTIR spectroscopies shown characteristic bands of this carbon structures (G and D). The results obtained by student's t-test proved a statistical significance between spheres means of samples collected from steel bar and quartz tube surface.
The control over the materials structure is crucial for the modulation of its properties, in order to achieve this control is important to know the formation mechanism of the material as function of parameters used. For this purpose, the effect of temperature (120, 140, 160 and 180 °C) on the hydrothermal synthesis of zinc sulphide is evaluated and a proposal of the sequence of reactions formation of zinc sulphur is presented. ZnS nanostructures with blend-phase were obtained, the temperature increment induces the growth of the nanostructure ranged between .62 and 12.72 nm, furthermore, increase the crystallinity of the ZnS nanostructures. The proposed reactions suggest the formation of a complex of thioacetamide with the Zn+2 and its subsequent decomposition into ZnS.
In this work, the first results of the effects of temperature during the production of Se2- ions and the effect during the interaction of Cd2+ and Se2- ions in the synthesis process of CdSe nanoparticles are presented. The synthesis of CdSe was carried out by the colloidal technique, in the first one we used a temperature of 63 °C to produce Se2- ions and in the second one an interaction temperature of 49 °C. The samples were characterized using a Scanning Electron Microscope (SEM) and a Scanning Tunneling Microscope (STM). From the SEM micrographs it was possible to identify the thorns formation and irregular islands. STM micrographs reveal elliptical shapes with a regular electron cloud profile.
Gold nanoparticles (AuNPs) have been classified as one of the most attractive nanotechnologies, thanks to their potential or already implemented applications; therefore, biological methods for their synthesis have been widely investigated. This study explores the synthesis of AuNPs using the extract of Anemopsis californica, and determinates the effect of the solvent used (water, methanol, and isopropanol) to obtain the AuNPs. Biogenic nanoparticles were analysed through UV-Vis spectroscopy and transmission electron microscopy (TEM, HRTEM, and SAED). Significant differences in polydispersity and morphology of AuNPs among the different methods used were found; the aqueous extract and the extract based on methanol formed nanotriangles and polyhedral nanoparticles; the shape of the nanoparticles is predominantly polyhedral when isopropanol is used as the solvent. The as obtained nanoparticles were placed on glass slides to perform Surface-Enhanced Raman Scattering (SERS) experiments, an amplification of the methylene blue Raman signal was observed when triangular nanoparticles cover the biogenic SERS substrate.
Methane dehydroaromatization (MDHA) is a direct activation approach to covert methane to value-added chemicals in a single step. This requires no intermediate step, making it a commercially economic approach. Mo supported on HZSM-5/MCM-22 is a well-studied catalyst for this reaction, where Mo sites are responsible for activating methane to C2Hy dimers, which can oligomerize on HZSM-5 Bronsted acid sites to produce aromatics. Challenges for these bifunctional catalysts involve rapid coking and low product yield. In this study, a novel catalytic approach is introduced using group VIB metals (Cr, Mo, W) supported on sulfated zirconia (SZ) solid acid. It is believed that the Bronsted acidity of SZ should help to convert the dimers generated from metal sites to ethylene and aromatics like benzene.
Here, fresh Mo, W and Cr were doped into SZ and characterized using pyridine DRIFTS, ammonia TPD, BET and SEM-EDS.. Catalytic activity for MDHA was ranked as Mo>W>Cr. Mo/SZ showed greater selectivity towards ethylene and benzene, followed by W/SZ, which was selective primarily towards ethylene. Cr/SZ showed the least activity under similar reaction conditions, producing only a small amount of ethylene. Higher catalytic activity for Mo/SZ was possibly due to reduced Mo oxide sites, found from XANES analysis, as well as higher acidity, observed from TPD. Deactivation was mainly due to coking, observed from subsequent TPO analysis. Further investigation is necessary to enhance the activity of this novel catalytic approach before considering for potential industrial applications.