Disorder effects produced by the Mn and H incorporations on the optical absorption edge of Ga1-xMnxAs:H nanocrystalline films

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

The optical absorption edge of nanocrystalline Ga(1-x)Mn(x)N films deposited by reactive sputtering

We have focused on the optical absorption edge of nanocrystalline Ga(1-x)Mn(x)N (0.00 <= x <= 0.18) films deposited by reactive RF magnetron sputtering. The films obtained are nanocrystalline with grain sizes of about 25 nm, having wurtzite structure and strong orientation texture in the c-axis direction. The optical characterizations of the absorption edges were obtained in the 190-2600 nm spectral range. The increase of the Mn content causes an increase of the absorption coefficient which can be clearly noticed at low energies, and a quasi-linear decrease of the optical gap. Broad absorption bands observed around similar to 1.3 and similar to 2.2 eV were associated with transitions between the Mn acceptor level and the valence and conduction bands, respectively. The observed changes in the optical properties due to the Mn incorporation observed in these nanocrystalline films are similar to those reported for ferromagnetic GaMnN single-crystal films.

Optical Behavior of Conjugated Pt-Containing Polymetallaynes Exposed to Gamma-Ray Radiation Doses

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Structural and optical properties of chlorinated plasma polymers

Amorphous hydrogenated chlorinated carbon (a-C:H:Cl) films were produced by the plasma polymerization of chloroform-acetylene-argon mixtures in a radiofrequency plasma enhanced chemical vapor deposition system. The main parameter of interest was the proportion of chloroform in the feed, R(C), which was varied from 0 to 80%. Deposition rates of 80 nm min (1) were typical for the chlorinated films. Infrared reflection-absorption spectroscopy revealed the presence of C-Cl groups in all the films produced with chloroform in the feed. X-ray photoelectron spectroscopy confirmed this finding, and revealed a saturation of the chlorine content at similar to 47 at.% for R(C)>= 40%. The refractive index and optical gap, E(04), of the films were roughly in the 1.6 to 1.7, and the 2.8 to 3.7 eV range. These values were calculated from transmission ultraviolet-visible-near infrared spectra. Chlorination leads to an increase in the water surface contact angle from similar to 40 degrees to similar to 77 degrees. (C) 2011 Elsevier B.V. All rights reserved.

Fast optical preparation, control, and readout of a single quantum dot spin

We propose and demonstrate the sequential initialization, optical control, and readout of a single spin trapped in a semiconductor quantum dot. Hole spin preparation is achieved through ionization of a resonantly excited electron-hole pair. Optical control is observed as a coherent Rabi rotation between the hole and charged-exciton states, which is conditional on the initial hole spin state. The spin-selective creation of the charged exciton provides a photocurrent readout of the hole spin state.

Enhancement of optical absorption by modulation of the oxygen flow of TiO2 films deposited by reactive sputtering

Oxygen-deficient TiO2 films with enhanced visible and near-infrared optical absorption have been deposited by reactive sputtering using a planar diode radio frequency magnetron configuration. It is observed that the increase in the absorption coefficient is more effective when the O-2 gas supply is periodically interrupted rather than by a decrease of the partial O-2 gas pressure in the deposition plasma. The optical absorption coefficient at 1.5 eV increases from about 1 x 10(2) cm(-1) to more than 4 x 10(3) cm(-1) as a result of the gas flow discontinuity. A red-shift of similar to 0.24 eV in the optical absorption edge is also observed. High resolution transmission electron microscopy with composition analysis shows that the films present a dense columnar morphology, with estimated mean column width of 40nm. Moreover, the interruptions of the O-2 gas flow do not produce detectable variations in the film composition along its growing direction. X-ray diffraction and micro-Raman experiments indicate the presence of the TiO2 anatase, rutile, and brookite phases. The anatase phase is dominant, with a slight increment of the rutile and brookite phases in films deposited under discontinued O-2 gas flow. The increase of optical absorption in the visible and near-infrared regions has been attributed to a high density of defects in the TiO2 films, which is consistent with density functional theory calculations that place oxygen-related vacancy states in the upper third of the optical bandgap. The electronic structure calculation results, along with the adopted deposition method and experimental data, have been used to propose a mechanism to explain the formation of the observed oxygen-related defects in TiO2 thin films. The observed increase in sub-bandgap absorption and the modeling of the corresponding changes in the electronic structure are potentially useful concerning the optimization of efficiency of the photocatalytic activity and the magnetic doping of TiO2 films. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4724334]

Indirect optical transitions from carriers trapped on the delta doping and on the parabolic quantum well

In this work, doped AlGaAs/GaAs parabolic quantum wells (PQW) with different well widths (from 1000 angstrom up to 3000 angstrom) were investigated by means of photoluminescence (PL) measurements. In order to achieve the 2DEG inside the PQW Si delta doping is placed at both side of the well. We have observed that the thickness of this space layer plays a major rule on the characteristics of the 2DEG. It has to be thicker enough to prevent any diffusions of Si to the well and thin enough to allow electrons migration inside the well. From PL measurement, we have observed beside the intra well transitions, indirect transitions involving still trapped electron on the delta doping and holes inside the PQW. For the thinness sample, we have measured a well defined PL peak at low energy side of the GaAs bulk emission. With the increasing of the well thickness this peak intensity decreases and for the thickest sample it almost disappears. Our theoretical calculation indicated that carriers (electron and holes) are more placed at the center of the PQW. In this way, when the well thickness increases the distance between electrons on the delta doping and holes on the well also increases, it decreases the probability of occurrence of these indirect optical transitions. (C) 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of Universidade Federal de Juiz de Fora, Brazil.

Strutural and optical approach of CdS@ZnS core-shell system

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Electronic structure and optical properties of BaMoO4 powders

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Structural and optical properties of CaTiO3 perovskite-based materials obtained by microwave-assisted hydrothermal synthesis: An experimental and theoretical insight

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Design of a simple detection cell with extended optical path length for capillary electrophoresis: Application to multiresidue pesticide analysis

Absorbance detection in capillary electrophoresis (CE), offers an excellent mass sensitivity, but poor concentration detection limits owing to very small injection volumes (normally I to 10 nL). This aspect can be a limiting factor in the applicability of CE/UV to detect species at trace levels, particularly pesticide residues. In the present work, the optical path length of an on-column detection cell was increased through a proper connection of the column (75 mu m i.d.) to a capillary detection cell of 180 mu m optical path length in order to improve detectability. It is shown that the cell with an extended optical path length results in a significant gain in terms of signal to noise ratio. The effect of the increase in the optical path length has been evaluated for six pesticides, namely, carbendazim, thiabendazole, imazalil, procymidone triadimefon, and prochloraz. The resulting optical enhancement of the detection cell provided detection limits of ca. 0.3 mu g/mL for the studied compounds, thus enabling the residue analysis by CE/UV.

Indicadores de cálcio e de voltagem codificados geneticamente na detecção de potenciais de ação e inputs sinápticos em cultura de neurônios hipocampais

Recently, genetically encoded optical indicators have emerged as noninvasive tools of high spatial and temporal resolution utilized to monitor the activity of individual neurons and specific neuronal populations. The increasing number of new optogenetic indicators, together with the absence of comparisons under identical conditions, has generated difficulty in choosing the most appropriate protein, depending on the experimental design. Therefore, the purpose of our study was to compare three recently developed reporter proteins: the calcium indicators GCaMP3 and R-GECO1, and the voltage indicator VSFP butterfly1.2. These probes were expressed in hippocampal neurons in culture, which were subjected to patchclamp recordings and optical imaging. The three groups (each one expressing a protein) exhibited similar values of membrane potential (in mV, GCaMP3: -56 ±8.0, R-GECO1: -57 ±2.5; VSFP: -60 ±3.9, p = 0.86); however, the group of neurons expressing VSFP showed a lower average of input resistance than the other groups (in Mohms, GCaMP3: 161 ±18.3; GECO1-R: 128 ±15.3; VSFP: 94 ±14.0, p = 0.02). Each neuron was submitted to current injections at different frequencies (10 Hz, 5 Hz, 3 Hz, 1.5 Hz, and 0.7 Hz) and their fluorescence responses were recorded in time. In our study, only 26.7% (4/15) of the neurons expressing VSFP showed detectable fluorescence signal in response to action potentials (APs). The average signal-to-noise ratio (SNR) obtained in response to five spikes (at 10 Hz) was small (1.3 ± 0.21), however the rapid kinetics of the VSFP allowed discrimination of APs as individual peaks, with detection of 53% of the evoked APs. Frequencies below 5 Hz and subthreshold signals were undetectable due to high noise. On the other hand, calcium indicators showed the greatest change in fluorescence following the same protocol (five APs at 10 Hz). Among the GCaMP3 expressing neurons, 80% (8/10) exhibited signal, with an average SNR value of 21 ±6.69 (soma), while for the R-GECO1 neurons, 50% (2/4) of the neurons had signal, with a mean SNR value of 52 ±19.7 (soma). For protocols at 10 Hz, 54% of the evoked APs were detected with GCaMP3 and 85% with R-GECO1. APs were detectable in all the analyzed frequencies and fluorescence signals were detected from subthreshold depolarizations as well. Because GCaMP3 is the most likely to yield fluorescence signal and with high SNR, some experiments were performed only with this probe. We demonstrate that GCaMP3 is effective in detecting synaptic inputs (involving Ca2+ influx), with high spatial and temporal resolution. Differences were also observed between the SNR values resulting from evoked APs, compared to spontaneous APs. In recordings of groups of cells, GCaMP3 showed clear discrimination between activated and silent cells, and reveals itself as a potential tool in studies of neuronal synchronization. Thus, our results indicate that the presently available calcium indicators allow detailed studies on neuronal communication, ranging from individual dendritic spines to the investigation of events of synchrony in neuronal networks genetically defined. In contrast, studies employing VSFPs represent a promising technology for monitoring neural activity and, although still to be improved, they may become more appropriate than calcium indicators, since neurons work on a time scale faster than events of calcium may foresee

Synthesis, optical and ferroelectric properties of PZT thin films: experimental and theoretical investigation

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Optical and structural properties of PEO-like plasma polymers

This paper deals with the study of optical, structural and biocompatible properties of PEO-like plasma polymerized films resulting from RF excited diethylene glycol dimethyl ether (CH3O(CH2CH2O)(2)CH3 diglyme) glow discharges. The study was carried out using visible-ultraviolet and FTIR spectroscopies and contact angle measurements. FTIR spectra of plasma polymerized diglyme showed a stronger presence of ethylene glycol groups in film structure for lower RF power levels. The contact angle measurements for water revealed an increasing from 30degrees to 62,5degrees when the RF power was varied from 2 to 45 W, indicating the decreasing of the hydrophilic character of diglyme films with the increasing of RF power. This trend is in agreement with FTIR results. The data from visible-ultraviolet reflectance and transmittance spectra revealed alterations on optical properties of plasma polymerized diglyme films. The film's optical gap varied from 3.8 to 3 eV for RF power running from 5 to 45 W.

Optical and electrical properties of polymerizing plasmas and their correlation with DLC film properties

Diamond-like carbon (DLC) films were grown from radiofrequency plasmas of acetylene-argon mixtures, at different excitation powers, P. The effects of this parameter on the plasma potential, electron density, electron temperature, and plasma activity were investigated using a Langmuir probe. The mean electron temperature increased from about 0.5 to about 7.0 eV while the mean electron density decreased from about 1.2x10(9) to about 0.2x10(9) cm(-3) as P was increased from 25 to 150 W. Both the plasma potential and the plasma activity were found to increase with increasing P. Through actinometric optical emission spectrometry, the relative concentrations of CH, [CH], and H, [H], in the discharge were mapped as a function of the applied power. A rise in [H] and a fall in [CH] with increasing P were observed and are discussed in relation to the plasma characteristics and the subimplantation model. The optical properties of the films were calculated from ultraviolet-visible spectroscopic data; the surface resistivity was measured by the two-point probe method. The optical gap, E(G), and the surface resistivity, rho(s), fall with increasing P. E(G) and rho(s) are in the ranges of about 2.0-1.3 eV and 10(14)-10(16) Omega/square, respectively. The plasma power also influences the film self-bias, V(b), via a linear dependence, and the effect of V(b) on ion bombardment during growth is addressed together with variation in the relative densities of sp(2) and sp(3) bonds in the films as determined by Raman spectroscopy.