Photostimulated Changes Research Articles

Optical triggering of a metal-insulator transition in neodymium nickelate films

We have studied photostimulated changes in the electrical resistivity of thin epitaxial films of neodymium nickelate NdNiO3 around its sharp metal-to-insulator transition. It is found that intense light irradiation strongly affects the film resistivity below the transition temperature, where the resistivity is repeatedly switched from high insulating values into low metal-phase levels. By varying the irradiation and thermal conditions, we establish that this strong photoresistive effect is likely caused by light-absorption induced heating rather than electronic excitations. Further analysis of the temperature-dependent resistivity predicts a significant drop in the film thermal conductivity at the metal-to-insulator transition. The simultaneous abrupt change in two physical parameters: electrical resistivity and thermal conductivity, is responsible for a considerable magnitude of the observed photoresistive effect. This effect can be utilized in novel electronics applications, such as thin-film photodetectors or photoswitches.

Metal-organic framework single crystal infrared photodetector

Metal–organic frameworks (MOFs) are an emerging class of functional materials possessing the coupled optical and electrical responses necessary for the operation of modern optoelectronic devices. Yet, the realization of near infrared MOF-based photodetector is still a challenge. Here, we report on the MOF single crystal photodetector with 600–1000 nm spectral range, overlapping with the efficient optical absorption of the MOF (HKUST-1), converting the light energy to electric one. We detect that 0.1 s response rate of the detector turned out to be higher than the previously announced MOF-based photodetectors, that is due to the fairly fast and reversible process of desorption of coordinated water molecules underlying the mechanism of charge carrier generation. We also confirm the 1 mJ cm−2 fluence for the threshold photodetection due to a photostimulated change in the optical properties of the MOF, rather than its degradation, which paves the way to fast and multiple detection of a low-intensity infrared laser light on the MOF single crystal.

The effect of surface plasmon-polaritons on the photostimulated diffusion in light-sensitive Ag–As4Ge30S66 structures

The effect of surface plasmon-polaritons (SPPs) excited at the interface between the profiled surface of the silver layer (in the form of a diffraction grating) and the As4Ge30S66 layer on the photostimulated diffusion of silver into chalcogenide has been studied. The gratings with the period a = 519 nm and modulation depth h/a ≈ 0.037 (where h is the grating depth) were formed on chalcogenide photoresist films by using interferential lithography and covered with the 80-nm-thick aluminum layer, 85-nm-thick silver layer, and thin As4Ge30S66 layer. Photostimulated changes in this structure were studied measuring the angular dependences of specular reflection (Rp) of p-polarized light with the wavelength 632.8 nm. It was found that as a result of exposure, “degradation” (broadening, increase in reflection at the minimum) of the minimum in the angular dependence of Rp (which is associated with the SPP resonance) occurs faster, when the samples are irradiated at the angle corresponding to SPP excitation. This observation indicates acceleration of the photostimulated diffusion process in this structure under the plasmon field action.

Photostimulated changes in egg production and reproductive performance of the Guinea fowl in a sub-humid tropical environment.

The effect of photo-induced changes on egg production and reproductive performance of female guinea fowl was studied by exposing the birds to three photo-regimens, namely natural daylight all day for 12 weeks (NDL), 8 hours of light per day for 8 weeks (8L-16D) plus supplementary light for 16 hours per day for 4 weeks (16L-8D) (RSL), and provided natural daylight all day for 8 weeks plus supplementary light at 16 hours per day for 4 weeks (SL). Weight gain during the pre-laying period was similar (P>0.05) in all the treatment groups between 20-36 weeks of age. The reverse was the case during the laying period between 36-52 weeks, which coincided with the age at sexual maturity. Feed intake was similar (P>0.05) for the NDL and SL groups during the pre-laying period but the RSL group had significantly lower (P<0.05) feed intake than the others. Overall laying mortality was lowest in the RSL birds. There was no significant difference (P>0.05) in body weight for all the groups. Age at first egg was significantly lower (P<0.05) in the RSL (207 d) than in the NDL (249 d) and SL (235 d) birds which were similar (P>0.05). The guinea fowls which received 8 weeks of restricted light (RSL) responded more rapidly in terms of day-to-first-egg after exposure to 16 hours of light and achieved significantly higher (P<0.05) level of egg production than the other two groups. The overall economy of production, in terms of cost, revealed that RSL birds performed better than the NDL and SL, in that it cost N36.58 and 127.05 more to produce each NDL and SL guinea fowl at point of lay respectively. The results indicate that different light regimes altered the time during which egg production response took place and apparently affected the genetic potentials of the groups with regard to egg mass and total number of eggs laid during the period covered by this trial.

Impact of Surface Plasmon Polaritons on Silver Photodiffusion into As2S3 Film

The effect of the surface plasmon polariton excitation at the silver–chalcogenide glass interface on the photostimulated diffusion of silver into chalcogenide was studied. The test sample was a two-layer Ag–As2S3 structure deposited on the diffraction gratings with a 899-nm period made using interference photolithography. Photostimulated changes were registered by measuring the kinetics and the angular dependences of the specular reflection of the 632.8-nm wavelength p-polarized light. It was found that the process of photostimulated diffusion of silver into the chalcogenide layer significantly accelerates at its initial stage when the surface plasmon polariton is excited at the Ag–As2S3 interface during irradiation. Thus, it promotes the increase in photosensitivity of the structure—when irradiated at an angle corresponding to the excitation of the plasmon, the entire photodoping process completes much faster.

Neuromorphometric changes associated with photostimulated migratory phenotype in the Palaearctic-Indian male redheaded bunting.

Neural substrates, including brain areas, differential gene expression and neuroendocrine basis, of migration are known. However, very little is known about structural changes in the brain that underlie the development and cessation of migration in long-distance avian migrants. Towards this, we investigated neuromorphological changes in the higher-order associative areas in male redheaded bunting (Emberiza bruniceps), which is a Palaearctic-Indian night migrant with wintering grounds in India. Photosensitive birds (8L:16D; SD) were exposed to stimulatory long days (16L:8D; LD), with controls retained on non-stimulatory short days. LD birds depicted shifts to, and sustained night-time activity as recorded by actograms. LD birds demonstrated increased body mass, fat deposition and testicular volume in keeping with the migratory phenotype. When LD birds had exhibited 10.0 ± 2.4 cycles of Zugunruhe (intense nighttime activity in captives, akin to night migratory flight in the wild), bird brains were fixed by transcardial perfusion, and changes in the neuronal morphometry of pallial, sub-pallial and hypothalamic brain regions studied using rapid Golgi technique with modifications, as used and validated in our laboratory. There were significant differences in both area and perimeter of soma in the visual hyperpallium apicale implicated in migratory orientation and the neuroendocrine control region for timing of migration, the mediobasal hypothalamus. We attribute these neuromorphometric changes in the soma area and perimeter to the photostimulated changes associated with the development of migration and reproductive phenotypes in redheaded buntings. It is suggested that changes in the neuronal plasticity in brain control regions parallel photoperiod-induced physiological responses.

Plasmon-Stimulated Photodoping in the Thin-Layer As2S3–Ag Structure

The effect of the excitation of surface plasmon polaritons at the silver–chalcogenide glass interface upon the photostimulated diffusion of silver into chalcogenide was studied for the first time. A high-frequency aluminum diffraction grating with a period of 248.5 nm and a deposited two-layer Ag–As2S3 structure was used to excite plasmons. The process of photostimulated diffusion of silver into the chalcogenide layer is accelerated (that is, the photosensitivity of such a structure increases) when a surface plasmon polariton is excited during exposure at the Ag–As2S3 interface. The photostimulated changes in the optical characteristics of the structure, including at the initial stage of the photodiffusion process, were monitored by recording the dynamics of changes in the characteristics of plasmon excitation with exposure time.

Emission Kinetics of Surface (Bi)Excitons in ZnO Thin Films

The kinetics of near-edge photoluminescence (PL) in ZnO nanofilms prepared by the atomic layer deposition has been investigated. It is established that the kinetics of near-edge PL in 4-nm films is determined to a great extent by surface 2D-exciton (SX) and biexciton (SXX) complexes. The contribution from surface biexcitons is estimated based on a photostimulated change in the surface potential in ZnO films with different thicknesses. Ultrafast dynamics of surface biexcitons in thin films are revealed. It is shown that biexcitons localized near the surface have the shortest radiative lifetime (less than 100 ps) among all bound exciton complexes, which is explained by the large oscillator strength.

Плазмон-стимулированное фотолегирование в тонкослойной структуре As-=SUB=-2-=/SUB=-S-=SUB=-3-=/SUB=---Ag

Abstract The effect of the excitation of surface plasmon polaritons at the silver – chalcogenide glass interface on photostimulated diffusion of silver into the chalcogenide was studied for the first time. To excite plasmons, a high-frequency aluminum diffraction grating with a period of 248.5 nm was used, onto which a two-layer Ag–As2S3 structure was deposited. It was found that the process of photostimulated diffusion of silver into the chalcogenide layer is accelerated (that is, the photosensitivity of such a structure increases) when the surface plasmon polariton is excited at the Ag – As2S3 interface during the exposure. The dynamics of photostimulated changes in the optical characteristics of the structure, including the initial stage of the photodiffusion process, was registered by recording the changes in the plasmon excitation characteristics with the exposure time.

Кинетика излучения поверхностных (би) экситонов в тонких пленках ZnO

AbstractThe kinetics of near-edge photoluminescence (PL) in ZnO nanofilms prepared by the atomic layer deposition has been investigated. It is established that the kinetics of near-edge PL in 4-nm films is determined to a great extent by surface 2D-exciton (SX) and biexciton (SXX) complexes. The contribution from surface biexcitons is estimated based on a photostimulated change in the surface potential in ZnO films with different thicknesses. Ultrafast dynamics of surface biexcitons in thin films are revealed. It is shown that biexcitons localized near the surface have the shortest radiative lifetime (less than 100 ps) among all bound exciton complexes, which is explained by the large oscillator strength.

Исследование оптических и люминесцентных свойств углеродных наночастиц методом микрофотолюминесценции

The important features of the optical, luminescent, and emission properties of aqueous solutions of carbon nanoparticles (CNPs) of various types in the interaction of particles with electromagnetic radiation have been studied and analyzed. It is shown that the functional groups of CNPs play the dominant role in the spectra of optical absorption and photoluminescence (PL) of particles. Hydrothermal (HT) treatment of CNPs in the presence of ammonia and thermal treatment of particles in a solution of hydrogen peroxide have a strong influence on the absorption spectra, PL, and quantum yield (QY) of emission. It was found that the main PL bands of CNP samples are formed by superposition of several separate PL bands associated with electronic transitions of various types of radiative centers, and their excited states are located in the band gap of the carbon core of the particles. It was established that this circumstance is the reason for the dependence of the position of the PL band peak of most types of CNPs on the excitation wavelength. Linear dependences of the position of the PL band maximum and the emission QY magnitude on temperature and an exponential dependence on the time of HT treatment were revealed. The method of exposure to exciting radiation showed that the change in the PL intensity and the emission QY value under the influence of electromagnetic radiation is due to the photostimulated change in the surface recombination rate and diffuse particle processes in the region of excitation of CNP solutions. The possibility of investigating the stability of PL and QY by exposure of CNPs solutions to excitation radiation was demonstrated.

Photoperiodic induction of pre-migratory phenotype in a migratory songbird: identification of metabolic proteins in flight muscles.

Migratory birds need to undergo physiological changes during their preparation for migration. The current study characterized those changes in photoperiodic migratory black-headed buntings (Emberiza melanocephala), which initiate their northward spring migration in response to increasing day lengths. We measured differences in body mass, testis size and triglycerides levels in buntings between groups exposed to short (8 h light:16 h darkness, 8L:16D; SD) and long (16L:8D; LD) days, and identified proteins that showed significant differences between SD and LD in the flight muscle. To confirm that photostimulated changes were linked with migration, similar measurements were done on photoperiodic non-migratory Indian weaverbirds (Ploceus philippinus), which share the habitat with buntings for almost half-a-year. Buntings were fattened and gained weight and had elevated serum triglyceride levels and recrudesced testes under LD, but not SD. The SDS-polyacrylamide gel electrophoresis revealed differences between SD and LD conditions in the flight muscle protein profiles of buntings, but not of weaverbirds. Two-dimensional gel electrophoresis of flight muscle of bunting separated three proteins, of which two were upregulated under LD condition. Mass spectroscopic analysis and a protein database search identified them as the fatty acid binding protein (FABP), myoglobin and creatine kinase (CK). Further semi-quantitative and quantitative PCR assays revealed that FABP and myoglobin transcript levels in buntings, but not in weaverbirds, were upregulated under LD condition. However, there was no difference in CK mRNA levels between SD and LD in both the species. High FABP is perhaps linked with increased energy demands and high myoglobin with intense physical activity during migration. A difference in the CK protein, but not in mRNA levels between SD and LD may possibly indicate its photoperiodic regulation at the translational level.

Measurement of photostimulated self-diffusion in silicon

Photostimulated diffusion within solid semiconductors has been examined for many years, but its existence above room temperature has not been unambiguously confirmed. Here, diffusion rates for silicon self-diffusion are shown to change by factors of up to 25 in response to optical fluxes on the order of 1 W/cm2. Results depend on doping type; the rates of both interstitial formation and migration are affected in the case of n-type material. A model based on photostimulated changes in defect charge state explains the primary results, and the basic outlines should apply to a wide variety of semiconductors.

Kinetics of photostimulated changes of the refractive index in As-S amorphous films

The kinetics of photostimulated changes in the layers of non-crystalline chalcogenides AsxS1-x with different compositions has been studied be waveguiding techniques. The nature of both dynamic and irreversible components of photoinduced variations in optical parameters has been analyzed. Possible mechanisms of transformation have been proposed by which reversible (temporal) and irreversible components of changes in film parameters are set. The role a defective microstructure of films including the molecular-phase separation for optical effects have been discussed.

Photostimulated changes in WO3 nanosized films

During the irradiation of WO3 films d = 7–160 nm thick by light at λ = 320 nm (I = (1.5–7) × 1015 quantum cm−2 s−1), absorption band at λ = 850 nm appeared along with absorption band edge shift to shorter waves. The subsequent irradiation of samples at λ = 850 nm caused the disappearance of the longwave absorption band. The intrinsic absorption edge of WO3 films was determined (λ = 320 nm). The degree of transformations of WO3 films increased under atmospheric conditions as the intensity of incident light and the time of irradiation (1–140 min) grew and as film thickness decreased. A mechanism of photochemical transformations of WO3 films was suggested. This mechanism included the generation of electron-hole pairs, the recombination of part of nonequilibrium charge carriers, the formation of [eV a 2+ e] centers, and the isolation of photolysis products.

CURRENT-VOLTAGE CHARACTERISTICS AND PHOTOSTIMULATED CONDUCTIVITY IN FERROELECTRIC HETEROGENEOUS STRUCTURE Al/Sn2P2S6/Al

ABSTRACT The mechanisms of conductivity of ferroelectric semiconductor films Sn2P2S6 and its photostimulated change are investigated. Leakage current studies on Al/Sn2P2S6/Al capacitors show the asymmetry of the I-V characteristics which is ascribed to an effect of the internal electric field varying the height of potential barriers at the film interfaces. The visible light illumination increases conductivity of the films especially for the wavelength λ0 = 520 nm corresponding to the fundamental absorption edge of Sn2P2S6, although appreciable growth of the conductivity also occurs in the long-wave range due to the photoionization transitions.

The effects of fluorescent probes on model membrane organization: photo-induced lipid sorting and soft structure formation

Liquid crystalline phase behaviour in model cell membranes prepared in-vitro has generated an enormous amount of interest in recent years and the phase diagrams of various bio-mimetic mixtures explored. Most recently photo-induced oxidation in fluorescently labelled membranes has been shown to influence these phase diagrams and can be used to generate interesting membrane-based geometries based on differences in membrane composition. In this article the different phases observed in model membranes will be discussed. Methods used to study these systems and recent developments in photo-stimulated phase changes are described along with areas for future investigation.

Photostimulated changes of properties of CdTe films

AbstractThe effect of illumination during the close‐spaced sublimation (CSS) growth on composition, structural, electrical, optical and photovoltaic properties of CdTe films and CdTe/CdS solar cells were investigated. Data on comparative study by using X‐ray diffraction (XRD), scanning electron microscopy (SEM), absorption spectra and conductivity‐temperature measurements of CdTe films prepared by CSS method in dark (CSSD) and under illumination (CSSI) were presented. It is shown that the growth rate and the grain size of CdTe films grown under illumination is higher (by factor about of 1.5 and 3 respectively) than those for films prepared without illumination. The energy band gap of CdTe films fabricated by both technology, determined from absorption spectra, is same (about of 1.50 eV), however conductivity of the CdTe films produced by CSSI is considerably greater (by factor of 107) than that of films prepared by CSSD. The photovoltaic parameters of pCdTe/nCdS solar cells fabricated by photostimulated CSSI technology (Jsc = 28 mA/cm2, Voc =0.63 V) are considerably larger than those for cells prepared by CSSD method (Jsc = 22 mA/cm2, Voc = 0.52 V). A mechanism of photostimulated changes of properties of CdTe films and improvement of photovoltaic parameters of CdTe/CdS solar cells is suggested. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The nature of photostimulated processes in As2Se3 layers used in fabricating the elements of diffraction optics

This paper presents the results of investigations of the manifestations of photostimulated structural changes in thermally deposited layers of As2Se3 from the absorption spectra in the far-IR region, using the methods of Fourier spectroscopy. It is shown that photostructural changes in the layers used for fabricating the holograms show up in an intensity redistribution of the bands of the IR absorption spectrum associated with the valence vibrations in moleculelike pyramidal structures that form part of the unit cells of the crystalline phase of As2Se3. © 2005 Optical Society of America

Photostimulated structural changes in yellow arsenic

DTA and ESR methods were applied for studying photostimulated changes in molecular arsenic (y-As). It is shown that under the effect of illumination/irradiation the stability of y-As decreases, which is due to the formation of stable fragments of the polymeric phase (a-As). Using the principles of orbital symmetry conservation, it is shown that polymerization takes place most favourably from a staggered 'face to face' configuration of As4 molecules. Photopolymerization may proceed without activation. As a result we get a formation of an intermediate molecular cluster As8 of 'chairlike' symmetry D2h.