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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 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.

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.