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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.

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.

In 2013, the international Commission on Atmospheric Chemistry and Global Pollution (iCACGP) and the International Global Atmospheric Chemistry (IGAC) Project Americas Working Group (iCACGP/IGAC AWG) was formed to build a cohesive network and foster the next generation of atmospheric scientists with the goal of contributing to a scientific community focused on building collective knowledge for the Americas. The Latin America–Caribbean (LAC) region shares common history, culture, and socioeconomic issues but, at the same time, it is highly diverse in its physical and human geography. The LAC region is unique because approximately 80% of its population lives in urban areas, resulting in high-density hotspots of urbanization and vast unpopulated rural areas. In recent years, most countries of the region have experienced rapid growth in population and industrialization as their economies emerge. The rapid urbanization, the associated increases in mobile and industrial sources, and the growth of the agricultural activities related to biomass burning have degraded air quality in certain areas of the LAC region. Air pollution has negative implications for human health, ecosystems, and climate. In addition, air pollution and the warming caused by greenhouse gases could impact the melting of Andean glaciers, an important source of freshwater. To better understand the links between air pollution and climate, it is necessary to increase the number of atmospheric scientists and improve our observational, analytical, and modeling capacities. This requires sustained and prioritized funding as well as stronger collaboration within the LAC region.

In recent years, the need for sustainable clean energy sources has been increasing. Solar cells are a promising alternative to such energy problem. However, high solar cells efficiencies have not been achieved yet. Graphene Carbon Quantum Dots (GCQDs) are a type of nano structures with great potential to improve solar cells efficiencies. GCQDs optical properties are not fully understood. To study the optical properties of GCQDs, a synthesis of GCQDs through a pyrolysis method using Citric Acid as a carbon source had been realized. Changing the pyrolisis exposition time, samples with different GCQD sizes were obtained. The absorption spectra of the samples was measured. In addition, calculations of the absorption spectra and Density of States (DoS) based on Density Functional Theory (DFT) using the software Quantum Espresso were performed. A good agreement between the experimental and theoretical results was obtained. The absorption spectra and the simulations shown two absorption bands. Based on the DoS calculations, the origin of the lower and higher energy absorption bands could be associated to transitions between carbon-oxygen $(\pi-\pi^{*})$ and carbon-carbon $(\pi-\pi^{*})$ states, respectively.

5 páginas, 4 figuras., GaAs/InxGa1−xP quantum wells, with x=0.541 and 0.427, have been investigated by continuous wave and time resolved photoluminescence. Spatial localization of excitons due to alloy compositional defects seem to be responsible of the observed phenomenology: non-exponential decay behavior at long times and slow decrease of the effective recombination time by increasing temperature. A two-class exciton kinetic model, based on a mobility edge varying with the temperature explain the experimental results., This work has been supported by the Spanish Ministerio de Ciencia y Tecnolog ıa under project number TIC99-1035-C02. The main author (C.R.) also thanks the spanish Ministerio de Asuntos Exteriores (MUTIS program).

3 páginas, 1 figura.-- Comunicación presentada al Proceedings of the International Conference on Superlattices, Nano-structures and Nano-devices ICSNN 2002 o-structures and Nano-devices ICSNN 2002., The temperature dependence of the effective mobility edge in semiconductor quantum wells containing disorder is reported for the first time. It is deduced by reproducing the experimental continuous wave and time resolved luminescence spectra by means of a two-class exciton kinetic model, which considers the co-existence of free and localized excitons and introduces a mobility edge defined by a Fermi function. The mobility edge varies faster than the PL peak energy when the temperature increases, passing from the high energy side of the PL band at low temperatures (recombination of localized excitons dominates) to the low energy side above 125 K (recombination of quasi-free excitons dominates)., This work has been supportedby the Spanish CICYT under the project no. TIC99-1035.

Trabajo presentado en el 7th International Workshop on Nonlinear Optics and Excitation Kinetics in Semiconductors, NOEKS 2003, celebrado en Karlsruhe (Alemania), del 23 al 28 de febrero de 2003, Continuous wave and time-resolved photoluminescence studies as a function of temperature have been performed on disordered InxGa 1-xP/GaAs quantum wells. Simulations of stationary and transient spectra, by using a two-exciton kinetic model, allow the observation of a red shift of the effective mobility edge when temperature increases. From the temperature dependence of the radiative lifetime it can be also deduced that independent localization of electrons and holes seems to be the most likely mechanism for exciton localization in our samples. On the other hand, values ranging from 0.8 to 5.6 ps have been obtained for the radiative lifetime at k∥ = 0., This work has been partially supported by the spanish CICYT under project TIC99-1035.

6 páginas, 3 figuras., In this work, a study of the relaxation mechanisms in InAs self-assembled quantum dots have been performed by means of photoluminescence (PL), resonant PL (RPL) and PL excitation. The observed phenomenology is different for samples with different InAs coverage. At low InAs coverages, two resonant Raman scattering lines are observed when the excitation energy is chosen inside the emission band. These lines are related to the TO and LO GaAs phonons. Multi-phonon relaxation of carriers (GaAs phonons) is clearly observed in RPL and excitation PL in the lowest InAs coverage sample. Neither such mechanism nor resonant Raman scattering is present in samples with larger InAs coverages, for which the average quantum dot size and areal density remain essentially constant., This work has been partially supported by the spanish Ministerio de Ciencia y Tecnolog ıa under project number TIC99-1035-C02, Ministerio de Asuntos Exteriores (MUTIS program) and the Generalitat Valenciana.

6 páginas, 3 figuras., InxGa1—xP/GaAs (x = 0.541 and 0.427) heterostructures, grown by Atomic Layer Molecular Beam Epitaxy (ALMBE) on low temperature substrates, have been characterised by pressure-dependent and time-resolved photoluminescence experiments. The excitonic optical transitions and recombination dynamics are both influenced by the particular band alignments of these systems. The valence band offset has been found to have approximately the same absolute value (ΔEVB ≈ 380 meV), independent of the In content of the alloy in the barrier, whereas the conduction band offset varies appreciably depending on the alloy band gap. The huge valence band offset implies a strong asymmetry in the confinement of carriers, affecting the exciton recombination dynamics in the quantum wells., This work has been supported by the projects TIC96-1020-C02 and HA1997-0129 of the Ministry of Education of Spain.

5 páginas, 4 figuras.-- PACS: 73.21.La;73.63.Kv;78.55.Cr;78.67.Hc;S7.12.-- Trabajo presentado en la 7th International Conference on Optics and Excitons in Confined Systems (OECS7)., Resonant photoluminescence experiments have been performed on self-assembled quantum dots. The emission bands measured in the investigated samples can be deconvoluted in several Gaussian components, which could be related to different size families of dots. The experimental results reveal the importance of GaAs phonons for the carrier relaxation of excess energy, specially for the sample with low dot density. With increasing temperature, electrons from smaller dots can be activated to reach the wetting layer electron states, from which they can be trapped at larger dots (below 100 K) or escape to the GaAs barriers (above 100 K)., This work has been partial supported by the Spanish Ministerio de Ciencia y Tecnologı´a under project number TIC99-1035-C02, Ministerio de Asuntos Exteriores (MUTIS program) and the Generalitat Valenciana

4 páginas, 2 figuras.-- PACS: 78.67.Hc; 73.21.La; 78.55.Cr.-- Proceedings of the International Conference on Superlattices, Nano-structures and Nano-devices ICSNN 2002., We have performed photoluminescence experiments in samples containing self-assembled quantum dots with different spacer layer thicknesses. A strong filtering effect produced by the GaAs spacer layer on the dots size being stacked is observed for spacers thinner than 10 nm. This effect produces a blue shift of the emission band from stacked dots and a simultaneous line width narrowing. At the same time, given the existence of a broad dot size distribution in the first layer, bigger dots can evolve towards InAs cylinder-like structures, whose emission occurs at appreciably lower energies as compared to the emission band associated to dot stacks (with some GaAs separation)., This workhas been supported by the Spanish CICYT under the Project No. TIC99-1035-C02 and European Commission GROWTH Program NANOMAT Project, Contract No. G5RD-CT-2001-00545.