Effective band gap of Si nanocrystals embedded in SiO2 matrix

Using a formulation of the Hartree-Fock formalism with the potential morphing method in the effective mass approximation, we calculate the effective band gap of Si nanocrystals embedded in SiO2 matrix without the existence of polysilane, as a function of their diameter in the size range 1-3.5 nm. Our results are in better agreement with the experimental data, in comparison with other existing theoretical data. For diameter smaller than 2 nm our results have the same tendency with the existing theoretical results, e.g., the discrepancy between theory and experiment seems to be essential.     

Colloidal Quantum Dot Infrared Lasers Featuring Sub-Single-Exciton Threshold and Very High Gain

The use of colloidal quantum dots (CQDs) as a gain medium in infrared laser devices has been underpinned by the need for high pumping intensities, very short gain lifetimes, and low gain coefficients. Here, PbS/PbSSe core/alloyed-shell CQDs are employed as an infrared gain medium that results in highly suppressed Auger recombination with a lifetime of 485 ps, lowering the amplified spontaneous emission (ASE) threshold down to 300 µJ cm⁻², and showing a record high net modal gain coefficient of 2180 cm⁻¹. By doping these engineered core/shell CQDs up to nearly filling the first excited state, a significant reduction of optical gain threshold is demonstrated, measured by transient absorption, to an average-exciton population-per-dot 〈N^th〉_g of 0.45 due to bleaching of the ground state absorption. This in turn have led to a fivefold reduction in ASE threshold at 〈N^th〉_ASE = 0.70 excitons-per-dot, associated with a gain lifetime of 280 ps. Finally, these heterostructured QDs are used to achieve near-infrared lasing at 1670 nm at a pump fluences corresponding to sub-single-exciton-per-dot threshold (〈N^th〉_Las = 0.87). This work brings infrared CQD lasing thresholds on par to their visible counterparts, and paves the way toward solution-processed infrared laser diodes.     

Chemical investigation and antimicrobial properties of mastic water and its major constituents

Mastic water is a commercial flavouring obtained during the steam distillation of mastic resin (the resin of Pistacia lentiscus var. chia) for the production of mastic oil. The mastic water extracts were analysed by GC−MS. The major compounds identified were verbenone, α-terpineol, linalool and trans-pinocarveol. Overall the composition was found to be very different from that of mastic oil. Additional GC−MS revealed the enantiomeric ratio of the chiral constituents of mastic water. The antimicrobial activity of mastic water extract, as well as that of its major constituents, was examined against Escherichia coli, Staphylococcus aureus and Candida spp. including ATCC wild clinical and food-borne strains. Linalool and α-terpineol were found to be the most potent antimicrobial constituents. Finally the stability of mastic water at different temperatures was studied, showing no change in the GC−MS profile of the organic extract for a period of 4 months at storage temperatures up to 4 °C.     

Policy plan for the use of biomass and biofuels in Greece: Part I: Available biomass and methodology

Greece is a country in which biomass can play an important role in achieving the 2010 national target of 20% electricity production from Renewable Energy Sources (RES) Residues from traditional agricultural activities, but also the cultivation of new energy crops, can lead to a high level of availability and utilization of biomass energy products in Greece. In order for biomass to make a substantial contribution to the 2010 national target, set by the EU RES Directive 2001/77, as well as the corresponding 2020 target, to be set by the upcoming RES Framework Directive, a national policy plan for biomass and biofuels has to be formulated. This plan will assess the available biomass quantities and the potential and routes, by which this biomass can be converted to final energy products. The aim of the present paper is to discuss the current situation and prospects of biomass availability and exploitation in Greece and to develop a coherent and reliable methodology, in order to assess the conversion of the country’ s biomass potential into useful energy products.     

Development of an investment decision tool for biogas production from agricultural waste

Anaerobic digestion is a very promising solution for the treatment of agricultural waste, preventing pollution and leading to efficient energy production. Since this technology is available to each farmer in a different way depending on the location and the scattering of the primary sources, it is essential to clarify the best conditions adapted to local situations to treat the targeted residues and make this information accessible to farmers. In particular the possibility of codigestion seems to be very attractive for farmers who will be able to treat their own waste together with other organic substrates. Their profit in this case is double since they treat properly their own residues, taking advantage of the selling of heat and electricity as well as the utilisation of a stabilised biofertiliser. The aim of this paper is to present an investment decision kit for economic evaluation of biogas plant projects based on agricultural feedstocks.     

On the base stations antenna system design for mobile communications

The design of a base station antenna for mobile communications is presented. The orthogonal method (OM) is applied under constraints on nulls of the radiation pattern. In the synthesis process the mutual coupling between the elements of the antenna array could be taken into account. Beam tilting and direction of arrival are also considered. The whole design is completed by implementing a special architecture of the antenna. Finally, some examples show the applicability of our technique.     

In vitro activity of vitamins, flavonoids, and natural phenolic antioxidants against the oxidative deterioration of oil-based systems

It is well-known, that lipid antioxidants can retard the oxidative rancidity of foods caused by atmospheric oxidation, and thus protect oils, fats, and fat-soluble components from their quality degradation. In the last few years, much emphasis has been put on the promotion and use of natural antioxidants, commonly occurring in many fruits and vegetables and thereby produced from various natural extracts. This review gives a summary of previously reported work together with more recent trends in the field of natural antioxidants. Focus is given on the mechanism of actions and the inhibitory effect of certain vitamins against the oxidative degradation of oil-based systems. Moreover, the use of natural phenolics (flavonoids, olive-oil penolics, herb extracts etc.) as antioxidants in numerous lipid food applications is discussed.     

Wireless Convergence Architecture: A Case Study Using GSM and Wireless LAN

The evolution of wireless networks has motivated the expansion of the static business environment to a mobile and wireless one. However, current and forthcoming wireless technologies are characterized by different attributes, regarding coverage area, offered bandwidth and delay. The transparent conjunction of various wireless technologies into a single mobile terminal can further boost the wireless explosion. This paper presents the Wireless Convergence Architecture (WCA) that incorporates different wireless interfaces under the same mobile terminal. Depending on the location and the availability of the underlying wireless network, automatic and seamless switching is performed to the most appropriate wireless network interface. Special care is taken to maintain continuous connectivity at the transport layer (TCP). WCA introduces software components both at the terminal and network side. A specific implementation is presented, based on two complementary wireless technologies – in terms of coverage area – an IEEE 802.11-compliant in the short local and a GSM in the wide area. To demonstrate the operational correctness of the architecture, experiments were performed using standard and proprietary networking applications.     

Activity of natural carotenoid preparations against the autoxidative deterioration of sunflower oil-in-water emulsions

The activity of natural carotenoid extracts against the autoxidative deterioration of sunflower oil-in-water emulsions was estimated in terms of primary (lipid hydroperoxides) and secondary (“off flavour” volatiles) oxidation products. At a concentration of 2 g l⁻¹, the tested carotenoids did not inhibit the production of hydroperoxides but they significantly retarded the formation of volatile aldehydes during the storage of simple tween-stabilised emulsions at 30 °C. In another series of experiments, several carotenoid extracts (paprika, annatto and marigold preparations) containing mainly polar carotenoids, added at an active concentration of 1 g l⁻¹, exerted a strong activity against hydroperoxides and TBARs during the accelerated oxidation (60 °C) of homogenised protein-based emulsions. On the contrary, carotene preparations rich in hydrophobic α- and β-carotenes and lycopene did not significantly differ from the control emulsion. Therefore, the carotenoid structure modulated their antioxidant effect, while concentration and emulsion structure may also affect carotenoid activity in protein dispersed systems.     

A variational Bayesian methodology for hidden Markov models utilizing Student's-t mixtures

The Student's-t hidden Markov model (SHMM) has been recently proposed as a robust to outliers form of conventional continuous density hidden Markov models, trained by means of the expectation-maximization algorithm. In this paper, we derive a tractable variational Bayesian inference algorithm for this model. Our innovative approach provides an efficient and more robust alternative to EM-based methods, tackling their singularity and overfitting proneness, while allowing for the automatic determination of the optimal model size without cross-validation. We highlight the superiority of the proposed model over the competition using synthetic and real data. We also demonstrate the merits of our methodology in applications from diverse research fields, such as human computer interaction, robotics and semantic audio analysis.