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Dori M.,University of Patras | Seintis K.,University of Patras | Stathatos E.,Technological and Educational Institute of Patras | Tsigaridas G.,National Technical University of Athens | And 6 more authors.
Chemical Physics Letters | Year: 2013

In this Letter, the electron injection in TiO2 films sensitized with six fluorene sensitizers is studied by femtosecond time resolved fluorescence spectroscopy using nanocrystalline Al2O3 films as reference. The sensitizers are dipolar organic molecules with the fluorene group utilized as a conjugated bridge. The electron injection efficiency is correlated to the structure, conjugation length and excited state potential of the sensitizers. One of the sensitizers has been studied using different amounts of cheno-deoxy cholic acid as co-adsorbent. In order to correlate the efficiency of electron injection with the device performance, quasi solid-state solar cells have been fabricated and characterized.

Sellountos E.J.,University of Lisbon | Tsinopoulos S.V.,Technological and Educational Institute of Patras | Polyzos D.,University of Patras | Polyzos D.,Institute of Chemical Engineering And High Temperature Chemical Processes
CMES - Computer Modeling in Engineering and Sciences | Year: 2012

A Local Boundary Integral Equation (LBIE) method for solving two dimensional problems in gradient elastic materials is presented. The analysis is performed in the context of simple gradient elasticity, the simplest possible case of Mindlin's Form II gradient elastic theory. For simplicity, only smooth boundaries are considered. The gradient elastic fundamental solution and the corresponding boundary integral equation for displacements are used for the derivation of the LBIE representation of the problem. Nodal points are spread over the analyzed domain and the moving least squares (MLS) scheme for the approximation of the interior and boundary variables is employed. Since in gradient elasticity the equilibrium equation is a partial differential equation of forth order, the MLS is ideal for solving these problems since it holds the C (1) continuity property. The companion solution of displacements is explicitly derived and introduced in the LBIEs for zeroing the tractions and double tractions on the local circular boundaries. Two representative numerical examples are presented to illustrate the method, demonstrate its accuracy and assess the gradient effect in the response. Copyright © 2012 Tech Science Press.

Giannopoulos G.I.,Technological and Educational Institute of Patras | Georgantzinos S.K.,University of Patras | Katsareas D.E.,University of Patras | Anifantis N.K.,University of Patras
CMES - Computer Modeling in Engineering and Sciences | Year: 2010

A hybrid finite element formulation, combining nanoscopic and macroscopic considerations is proposed, for the prediction of the elastic mechanical properties of single walled carbon nanotube (SWCNT)-based composites. The nanotubes are modeled according to the molecular mechanics theory via the use of spring elements, while the matrix is modeled as a continuum medium. A new formulation concerning the load transfer between the nanotubes and matrix is proposed. The interactions between the two phases are implemented by utilizing appropriate stiffness variations describing a heterogeneous interfacial region. A periodic distribution and orientation of the SWCNTs is considered. Thereupon, the nanocomposite is modeled using a three dimensional finite element unit cell, which is subjected to longitudinal as well as transverse loadings in order to obtain the mechanical properties in these directions. The Halpin-Tsai equations are used to extract the mechanical properties for randomly oriented SWCNTs. The formulation is validated through comparison of the predicted mechanical responses to corresponding solutions, obtained from the literature. © 2010 Tech Science Press.

Fakis M.,University of Patras | Hrobarik P.,TU Berlin | Hrobarik P.,Slovak Academy of Sciences | Stathatos E.,Technological and Educational Institute of Patras | And 2 more authors.
Dyes and Pigments | Year: 2013

The steady-state spectroscopic properties and ultrafast relaxation dynamics of the very promising indoline-based sensitizer dye D149 are reported in various organic solvents. For the study of relaxation dynamics of both S 2 and S 1 states, time resolved femtosecond upconversion spectroscopy was used. Quantum chemical calculations of excitation energies and dipole moments at the DFT level as well as using high-level correlated second-order approximate coupled-cluster model with singles and doubles (CC2) have been also performed. The difference between the first excited and ground state dipole moments was found experimentally to be ∼13D in agreement with calculations. According to our time resolved results, the population of the S 2 state has shown a rapid internal conversion to S 1, taking place within 0.15-0.45 ps, depending on the solvent. This is followed by a slower decay component indicating that some S 2 species relax on a long timescale. As the detection is shifted to the transition from the S 1 state, the overall decay dynamics is retarded, while a slow fluorescence rise is observed at long wavelengths, indicating a time-dependent spectral shift. The lifetime of S 1 was found to depend on the solvent used, being 630 ps in toluene, 720 ps in chloroform, 670 ps in THF, 700 ps in dichloromethane, 220 ps in acetonitrile and 190 ps in EtOH. The time resolved emission spectra as well as the solvent response function, C(t), obtained in the aprotic solvents, have shown that as the solvent polarity increases a more rapid spectral shift is observed. An exception is found in the case of the protic solvent EtOH due to specific H-bonding effects. The time resolved emission spectra in all solvents experience no significant change of shape and width with time, except from THF, where the initially formed Intramolecular Charge Transfer state is observed at early times. © 2012 Elsevier Ltd. All rights reserved.

Paspalakis E.,University of Patras | Boviatsis J.,Technological and Educational Institute of Patras
Nanoscale Research Letters | Year: 2012

We study the creation of high-efficiency controlled population transfer in intersubband transitions of semiconductor quantum wells. We give emphasis to the case of interaction of the semiconductor quantum well with electromagnetic pulses with a duration of few cycles and even a single cycle. We numerically solve the effective nonlinear Bloch equations for a specific double GaAs/AlGaAs quantum well structure, taking into account the ultrashort nature of the applied field, and show that high-efficiency population inversion is possible for specific pulse areas. The dependence of the efficiency of population transfer on the electron sheet density and the carrier envelope phase of the pulse is also explored. For electromagnetic pulses with a duration of several cycles, we find that the change in the electron sheet density leads to a very different response of the population in the two subbands to pulse area. However, for pulses with a duration equal to or shorter than 3 cycles, we show that efficient population transfer between the two subbands is possible, independent of the value of electron sheet density, if the pulse area is π. © 2012 Paspalakis and Boviatsis.

Fakis M.,University of Patras | Stathatos E.,Technological and Educational Institute of Patras | Tsigaridas G.,University of Patras | Giannetas V.,University of Patras | Persephonis P.,University of Patras
Journal of Physical Chemistry C | Year: 2011

In this work, the femtosecond upconversion spectroscopy was used for the study of ultrafast decay and electron transfer dynamics of the very promising all-organic sensitizer indoline dye, D149, in inert environments as well as on TiO2 substrates. The inert environments were toluene and acetonitrile solutions as well as poly(methyl methacrylate) (PMMA) films and Al 2O3 nanostructured substrates. These were used to investigate relaxation and aggregation induced decays. TiO2 substrates were used to probe interfacial electron transfer dynamics. The excitation was at the S2 band while the dynamics of both S 2 and S1 bands were detected. The dynamics of the S 2 band in solutions and PMMA film, show a rapid S2-S 1 internal conversion, ranging from 0.20 to 0.45 ps depending on the environment. This component becomes 0.10 ps in Al2O3 and in TiO2. The dynamics of the S1 band probe both hot and cold S1 species. In solutions and PMMA, the hot species relax to cold ones within a few tens of picoseconds while the cold ones decay to the ground state within a few hundreds of picoseconds. In Al2O3, the dynamics become much faster because of aggregation induced quenching. In TiO2, the S1 dynamics exhibit three mechanisms of 0.1, 1.3, and 8.6 ps time constants indicating a multiphasic injection mechanism. The electron transfer quantum yield and rate from the S1 band were calculated, using the time-resolved data, and were found to be 0.90 and 3.82 × 1011 s-1, respectively. Finally, quasisolid- state dye-sensitized solar cells, with D149 as sensitizer, were constructed, and their efficiency was found 5.66%. Similar cells, using the ruthenium complexes N719 and black dye as sensitizers, were also constructed for comparison. It was found that D149 showed higher efficiency than black dye but lower than N719. © 2011 American Chemical Society.

Kouni S.N.,University of Ioannina | Giannopoulos S.,University of Ioannina | Ziavra N.,Technological and Educational Institute of Ioannina | Koutsojannis C.,Technological and Educational Institute of Patras
American Journal of Otolaryngology - Head and Neck Medicine and Surgery | Year: 2013

Purpose and background: Acoustic signals are transmitted through the external and middle ear mechanically to the cochlea where they are transduced into electrical impulse for further transmission via the auditory nerve. The auditory nerve encodes the acoustic sounds that are conveyed to the auditory brainstem. Multiple brainstem nuclei, the cochlea, the midbrain, the thalamus, and the cortex constitute the central auditory system. In clinical practice, auditory brainstem responses (ABRs) to simple stimuli such as click or tones are widely used. Recently, complex stimuli or complex auditory brain responses (cABRs), such as monosyllabic speech stimuli and music, are being used as a tool to study the brainstem processing of speech sounds. We have used the classic 'click' as well as, for the first time, the artificial successive complex stimuli 'ba', which constitutes the Greek word 'baba' corresponding to the English 'daddy'. Patients and methods: Twenty young adults institutionally diagnosed as dyslexic (10 subjects) or light dyslexic (10 subjects) comprised the diseased group. Twenty sex-, age-, education-, hearing sensitivity-, and IQ-matched normal subjects comprised the control group. Measurements included the absolute latencies of waves I through V, the interpeak latencies elicited by the classical acoustic click, the negative peak latencies of A and C waves, as well as the interpeak latencies of A-C elicited by the verbal stimulus 'baba' created on a digital speech synthesizer. Results: The absolute peak latencies of waves I, III, and V in response to monoaural rarefaction clicks as well as the interpeak latencies I-III, III-V, and I-V in the dyslexic subjects, although increased in comparison with normal subjects, did not reach the level of a significant difference (p < 0.05). However, the absolute peak latencies of the negative wave C and the interpeak latencies of A-C elicited by verbal stimuli were found to be increased in the dyslexic group in comparison with the control group (p = 0.0004 and p = 0.045, respectively). In the subgroup consisting of 10 patients suffering from 'other learning disabilities' and who were characterized as with 'light' dyslexia according to dyslexia tests, no significant delays were found in peak latencies A and C and interpeak latencies A-C in comparison with the control group. Conclusions: Acoustic representation of a speech sound and, in particular, the disyllabic word 'baba' was found to be abnormal, as low as the auditory brainstem. Because ABRs mature in early life, this can help to identify subjects with acoustically based learning problems and apply early intervention, rehabilitation, and treatment. Further studies and more experience with more patients and pathological conditions such as plasticity of the auditory system, cochlear implants, hearing aids, presbycusis, or acoustic neuropathy are necessary until this type of testing is ready for clinical application. © 2013 Elsevier Inc. All rights reserved.

Diker H.,Ege University | Diker H.,DYO Boya Fab. San. Tic. A.S. | Varlikli C.,Ege University | Stathatos E.,Technological and Educational Institute of Patras
International Journal of Energy Research | Year: 2014

Nitrogen-doped TiO2 nanocrystalline particles are synthesized by a microwave-assisted hydrothermal growth method using different amines (Dipropylamine, Diethanolamine and Ammonium hydroxide) as nitrogen sources. Characterization of the nanoparticles was performed with X-ray diffraction, UV-vis diffuse reflectance spectroscopy, Field Emission Scanning Electron Microscopy and X-ray Photoelectron Spectroscopy. The prepared N-doped TiO2 nanoparticles exhibit pure anatase phase with average diameter of 9nm and reduced optical energy gap compared to undoped TiO2. Immobilization of N-doped and pure TiO2 nanoparticles on SnO2:F conductive glass substrates was successfully performed by using doctor-blade technique and paste of the aforementioned nanoparticles. A series of N-doped TiO2 photoelectrodes with varying N dopant source and concentrations were fabricated for quasi-solid state dye-sensitized solar cells. The N-doped solar cells achieve an overall conversion efficiency ranging from 4.0 to 5.7% while undoped TiO2 showed 3.6%. The basic difference to the electrical performance of the cells is focused to the enhancement in the current density of N-doped TiO2-based cells which was from 11% to 58% compared with undoped TiO2 cells. Current densities were directly proportional with nitrogen doping level in TiO2 lattice which differs depending on the amine source nature such as basicity differences, hydrogen bonding abilities and steric inherences. © 2013 John Wiley & Sons, Ltd.

Asian I.,Izmir Institute of Technology | Marinakis V.,Technological and Educational Institute of Patras
Communications in Theoretical Physics | Year: 2011

Recently, many important nonlinear partial differential equations arising in the applied physical and mathematical sciences have been tackled by a popular approach, the so-called Exp-function method. In this paper, we present some shortcomings of this method by analyzing the results of recently published papers. We also discuss the possible improvement of the effectiveness of the method. © 2011 Chinese Physical Society and IOP Publishing Ltd.

Marinakis V.,Technological and Educational Institute of Patras
Advances in Mathematical Physics | Year: 2010

We show that a nonlinear equation that represents third-order approximation of long wavelength, small amplitude waves of inviscid and incompressible fluids is integrable for a particular choice of its parameters, since in this case it is equivalent with an integrable equation which has recently appeared in the literature. We also discuss the integrability of both second- and third-order approximations of additional cases. Copyright © 2010 V. Marinakis.

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