1,721,370 research outputs found
Feature Selection with Non Linear PCA: A Neural Network Approach
Full text linkMachine learning consists in the creation and development of algorithms that allow a machine to learn itself, gradually improving its behavior over time. This learning is more effective, the more representative is the features of the dataset used to describe the problem. An important objective is therefore the correct selection (and, possibly, reduction of the number) of the most rele- vant features, which is typically carried out through dimensional reduction tools such as Principal Component Analysis (PCA), which is not linear in the more general case. In this work, an approach to the calculation of the reduced space of the PCA is proposed through the definition and implementation of appropriate models of artificial neural network, which allows to obtain an accurate and at the same time flexible reduction of the dimensionality of the problem
Luminescence Investigation of Direct and Indirect Excitons Bound to Deep-Neutral Acceptors in e-GaSe
No full textWe report on our investigations of excitonic luminescence spectra of Bridgmann grown GaSe crystals from the liquid N2 temperature up to 300 K and at weak laser excitation intensity. We measured the spontaneous luminescence due to direct and indirect excitonic recombinations. Moreover, a detailed analysis of the intensity of the lines composing the emission spectrum versus the temperature and the excitation intensity permits to detect and assign two lines of the excitonic
luminescence as due to the recombination of direct as well as indirect excitons bound to localized centres in the forbidden energy gap. On the basis of recent theories on bound exciton-impurity complexes we found that both direct and indirect excitons are bound to neutral and deep acceptor centres. From the measured dissociation energy of these bound excitons we could estimate the ionization energy of the acceptor levels, which were already identified independently in GaSe by electrical transport measurements
Kinetics of Radiative Recombinations in GaSe and Influence of Cu doping on the Luminescence Spectra
Full text linkSpontaneous photoluminescence (PL) spectra of Cu-doped and undoped -GaSe have been investigated in the temperature range from 80 to 300 K and at low laser-excitation intensity (P) from 10-3 to 10 W cm-2. The main modification of the spectra in doped crystals with respect to those of undoped samples is the appearance of two bands in the extrinsic part of the PL spectrum and centered at 655 and 678 nm, respectively. The luminescence at energies below the excitonic recombinations (extrinsic bands) is enhanced by doping. Also indirect free- and bound-excitonic lines are also strongly influenced by the impurity concentration; in fact, their emission intensity, which depends linearly on P in undoped crystals, shows a quadratic dependence in doped samples. The temperature dependence of the PL spectra gives the thermal activation energy of some extrinsic bands, which results equal the ionization energy of the acceptor levels involved in the extrinsic transitions. A simple kinetic model of the radiative recombination is proposed; it accounts for the experimental data of the excitation intensity dependence of the free- and bound-excitonic lines. This model can also explain the different temperature dependence of the PL intensity of these lines: linear for the free-excitonic emissions, exponential for the bound-excitonic recombinations. Some radiative transitions from donor levels located in the energy gap of GaSe are analyzed and a scheme of donor and acceptor states involved in the PL spectra is proposed
Direct and Indirect Excitonic Emission in GaSe
No full textPhotoluminescence spectra of undoped crystals of the layer semiconductor GaSe have been measured from 80 K up to room temperature. The direct and indirect excitonic emissions are clearly observed in the intrinsic part of the spectrum. Moreover, a detailed analysis of the luminescence intensity has been made as a function of exciting power and temperature, allowing one to ascribe some lines of the intrinsic part of emission spectrum to direct and indirect excitons bound to localized impurity levels in the energy gap of GaSe
Role of the deposition parameters and aging on the optical and photoluminescence properties of C70 films
No full textWe have investigated the influence of the growth parameters (substrate temperature and deposition rate) and the aging process on the optical
properties of C70 thin films, by means of absorption and photoluminescence spectra. The Urbach energy, obtained from the absorption spectra,
indicates that the substrate temperature influences the film optical properties more than the deposition rate. The luminescence spectra suggest the
important role of the disorder in the radiative efficiency. The main structures of the emission spectra have been assigned to an intramolecular
polaron–exciton. The analysis of the temperature dependence of the photoluminescence spectra of the as-deposited samples shows that the
vibronic transitions are dominant at low temperature, whereas the singlet purely electronic recombination (due to Frenkel-type exciton) is visible at
a sufficiently high temperature. On the contrary, in the aged samples this purely electronic transition is well resolved from low to high temperature.
This anomalous behaviour is discussed and attributed to the disorder introduced in the film.
© 2007 Elsevier B.V. All rights reserved
Effects of Structural Disorder on the Optical and Vibrational Properties of CdSXSel-X Alloy Thin Films Deposited by Laser Ablation
No full textOptical Spectroscopy and Structural Properties of Synthetic and Natural Eumelanin
Full text linkOptical properties of synthetic and natural eumelanin are presented and compared, in order to investigate the structural organization of eumelanin, which is related to the function of this biopolymer. Synthetic eumelanin is produced by oxidation of tyrosine
with hydrogen peroxide, whereas natural eumelanin is extracted from Sepia Officinalis
and from Rana Esculenta. Vibrational spectroscopy techniques (as Raman scattering and infrared absorption) show that both types of biopolymer include chemical functional groups characteristic of the monomeric units of eumelanin, although natural eumelanin includes also protein-related groups, proportionally to the protein content. X-ray diffraction spectra are in agreement with the hypothesis that eumelanin monomers assembly themselves and form protomolecules consisting of stacked layers (distant 3 – 4 Å each other) of indolic sheets. Absorption measurements, characterized by a monotonic increase of optical density from near-IR to UV range, support the model that eumelanin consists of a distribution of aggregates of oligomeric structures having different size and chemical composition. The estimated values of the optical gap indicate that the natural eumelanins are characterized by a larger structural disorder than the synthetic one. Fluorescence spectra confirm that the biopolymer consists of ensembles of chemically distinct oligomer systems, which can be selectively excited. This result is also supported
by Dynamic Light Scattering measurements, which permit to visualize the distribution of particles size. In fact, the nanoaggregate systems of natural eumelanin have a larger size than those of synthetic eumelanin. This might be related to the biological functions of such a biopolymer, particularly as far as photoprotective action is concerned
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