G. Polupan

Complex nature of the red photoluminescence band and peculiarities of its excitation in porous silicon

Abstract Photoluminescence and photoluminescence excitation spectroscopies, scanning electron microscopy, and atomic force microscopy were used to study the photoluminescence mechanism in porous silicon. The dependences of photoluminescence parameters on electrochemical etching regimes, excitation light wavelength, and vacuum ageing have been investigated. We show that intensive and broad “red” luminescence band (∼600–800 nm) is non-elementary, and can be decomposed into three elementary bands. The mechanisms of the elementary bands are discussed.

Carrier dynamics in InAs quantum dots embedded in InGaAs/GaAs multi quantum well structures

Ground and multi excited state photoluminescence, as well as its temperature dependence, in InAs quantum dots embedded in symmetric InxGa1-xAs/GaAs (x = 0.15) quantum wells (DWELL) have been investigated. The solution of the set of rate equations for exciton dynamics (relaxation into QWs or QDs and thermal escape) solved by us earlier is used for analysis the variety of thermal activation energies of photoluminescence thermal quenching for ground and multi excited states of InAs QDs. The obtained solutions were used at the discussion of the variety of activation energies of PL thermal quenching in InAs QDs. It is revealed three different regimes of thermally activated quenching of the QD PL intensity. These three regimes were attributed to thermal escape of excitons: i) from the high energy excited states of InAs QDs into the WL with follows exciton re-localization; ii) from the InxGa1-xAs QWs into the GaAs barrier and iii) from the WL into the GaAs barrier with their subsequent nonradiative recombination in GaAs barrier.

Photoluminescence of Ge nano-crystallites embedded in silicon oxide

Photoluminescence (PL), transmission electron microscopy, Raman scattering spectra and extended X-ray absorption fine structure have been investigated in silicon oxide films enriched by Ge in as-grown state and after their thermal annealing at 800 °C. The dependences of PL peculiarities on the concentration of Ge, as well as on the existence (or absence) of Ge nano-crystallites in silicon oxide films are analyzed.

Development of High Efficiency Two-Phase Thermosyphons for Heat Recovery

Due to high fuel prices, it has become necessary to investigate new methods for saving and more efficient use of energy, emphasizing the use of energy remaining in the waste gases of combustion equipment. For this reason, in the last five decades there has been an important technological development in heat transfer equipment, to promote changes in configuration and applying heat transfer systems with high effectiveness. One example is the use of twophase thermosyphons (Azada et al., 1985; Faghri, 1995; Gershuni et al., 2004; Noie, 2005; Peterson, 1994; Reay, 1981).

Effect of Maximum Temperature and Heating-Cooling Repeated Cycles on Thermal Contact Resistance of a Composite Tube

Experimental research results of the operational parameter effect on Thermal Contact Resistance (TCR) in a copper-aluminum L-type finned tube are presented. The investigated operational parameters were the maximum operational temperature and the number of repeated heating-cooling cycles. The TCR was experimentally determined by measuring the total heat supply, core tube wall and inner fin surface temperatures for steady-state and natural-convection conditions. In addition, the specimen was tested through up to 200 heating-cooling cycles. The experimental results showed a TCR increase of 81% at the same time as the average temperature difference between the hot inner flow and cooling air increased from 30°C to 130°C; over the maximum operational temperature (120°C), the TCR increased faster than before; and, after the heating-cooling cycle testing the TCR presented an increase of 31% in respect with the initial value. Such findings may be useful as a reference for preliminary thermal design and as recommendations for optimal operation of heat exchangers based on copper-aluminum L-type finned tubes.

Study of the 90° Elbows Performance as Phase Separators in an Air-Water Two-Phase Flow

In order to observe the 90° elbows performance as phase separators in an air-water two-phase flow, experimental results for the phase split which occurs at a 90° branched elbow are presented. The branched elbow geometry was varied in order to have three (branch diameter / elbow diameter) ratios and three branch inclination angles. Also the pressure was monitored at different points of the elbow with ramification in order to examine the pressure drop effect. The flow pattern upstream was mainly slug flow. First, the analysis of the main independent variables effect, (superficial velocities, branch inclination angle, ratio of diameters and pressure gradients) was carried out, then a correlation for the phase split was developed and, finally a comparison was made with data of phase separation in T junctions obtained by Azzopardi [1] and Soliman [2], as a result, a better behavior as phase separator was found for the elbow.Copyright

Comparative investigations of surface structure, photoluminescence and its excitation in silicon wires

Abstract The dependence of the photoluminescence (PL) and PL excitation spectra on the porous silicon top surface structure and the oxide composition on it has been investigated. Researches were carried out using the following methods: PL, PL excitation, electron paramagnetic resonance, atomic force microscope and X-ray photoelectron emission spectroscopy. Results indicate a direct correlation between the suboxide content and roughness structure on the surface with PL intensity. No correlation was noted between the PL intensity and the concentration of Si dangling bonds (non-radiative recombination centers). These results have given further support to a suboxide-related color center on the Si/SiO x interface as the source of the intense red luminescence of silicon wires.

Emission transformation in CdSe/ZnS quantum dots conjugated to biomolecules

The variation of photoluminescence (PL) spectra in CdSe/ZnS quantum dots (QDs) at the conjugation to antibodies (ABs) has been investigated and discussed in this paper. Two types of CdSe/ZnS QDs with different CdSe core sizes (5.4 and 6.4nm) and emissions (605 and 655nm) were studied before and after the conjugation to anti-Interleukin-10 (IL-10) and anti-Pseudo rabies virus (PRV) ABs. The PL high energy shift and asymmetric shape of PL bands have been detected in bioconjugated QDs. Note that the bioconjugation impact on spectral characteristics of CdSe/ZnS QD emission has been not studied yet in details. The surface enhanced Raman scattering (SERS) effect is revealed in bioconjugated CdSe/ZnS QDs. The SERS effect testifies that the excitation light used at the Raman study generates the electric dipoles in AB molecules. At the same time, the permanent position of LO-phonon Raman lines in Raman spectra of nonconjugated and bioconjugated QDs confirms that QD materials do not change at the bioconjugation. It is shown as well that the compressive strains do not play any role in the PL high energy shift in bioconjugated QDs. PL spectra of pure anti IL-10 ABs, anti PRV ABs, a phosphate buffer saline (PBS) and PL spectrum dependences versus excitation light intensities have been investigated as well. Finally, the PL spectrum transformation in bioconjugated QDs is attributed to varying the quantum confinement effect in CdSe/ZnS QDs and the energy band profiles in QD cores. Both these effects are stimulated by the electromagnetic field of excited AB dipoles. The obtained results can be useful for sensitivity improving the QD bio-sensors.

Estudio Numérico del Efecto de la Presión en el Proceso de Mezcla Metano-Oxígeno en un Arreglo de Chorros 4-Lug Bolt

Numerical results of methane-oxygen mixing process in a contained 4-Lug-Bolt jets array system under the influence of pressure variation are presented. The analysis was divided in two stages, low and high pressure, under the flammability limits criteria. At low pressures the density was modeled with the ideal-gas equation and for high pressures with the Redlich-Kwong equation. The predictions show that the gas mixture is highly influenced by recirculation between oxygen and methane jets, as well as a dragging effect of oxygen over the methane jet. Moreover, the pressure has no influence on the lower flammability limit, whereas it does affect the upper flammability limit. Despite these effects on flammability limits, the pressure shows no significant effect on the mixing of methane-oxygen by diffusion.

Some reasons of emission variation in InAs quantum dot-in-a-well structures

Photoluminescence (PL) and X ray diffraction have been studied in InAs quantum dots (QDs) embedded in symmetric In0.15Ga1-0.15As/GaAs quantum wells (dot-in-a-well, DWELL) with QDs grown at different temperatures. The density of QDs decreases from 1.1×1011 down to 1.3×1010 cm-2 with increasing the QD growth temperatures from 470 to 535°C. The QD density decreasing in DWELLs is accompanied by the non monotonous variation of QD parameters. The PL intensity increases and the PL peak shifts to low energy in structures with QDs grown at 490 and 510°C. On the contrary the structures with QDs grown at 525 and 535°C are characterized by lower PL intensities and PL peak positions shifted to higher energy. The method of X-ray diffraction has been applied with the aim to study the variation of elastic strain in DWELL structures with QDs grown at different temperatures. It was shown that the minimum of elastic strain corresponds to DWELL with QDs grown at 490-525 °C. For lower (470 °C) and higher (535 °C) QD growth temperatures the level of compressive strain increased in DWELLs. The reasons of strain variation are discussed as well.