Photoinduced Decrease Research Articles

Effects of Operating Mechanical Conditions and Polymer Networks of Nematic Elastomers on Photo-Induced Mechanical Performances.

Photoresponsive liquid-crystalline elastomers (LCEs) are promising candidates for light-controlled soft actuators. Photoinduced stress/strain originates from the changes in mechanical properties after light irradiation. However, the correlation between the photoinduced mechanical performance and in-use conditions such as stress/strain states and polymer network properties (such as effective crosslink density and dangling chain density) remains unexplored for practical applications. Here, isometric photo-induced stress or isotonic strain is investigated at different operating strains or stresses, respectively, on LCEs with polymer network variations, produced by different amounts of solvent during polymerization. As the solvent volume increases, the moduli and photoinduced stresses decrease. However, the photo-induced strain, fracture strain, fracture stress, and viscosity increase. The optical response performance initially increases with the operating strain/stress, peaks at a higher actuation strain/stress, and then, decreases depending on the polymer network. The maximum work densities, which also depend on the operating stress, are in the range of ≈200-300 kJm-3. These findings, highlighting the significant variations in the mechanical performance with the operating stress/strain ranges and amount of solvent used in the synthesis, are critical for designing LCE-based mechanical devices.

Disentangling Photodoping, Photoconductivity, and Photosuperconductivity in the Cuprates.

The normal-state conductivity and superconducting critical temperature of oxygen-deficient YBa_{2}Cu_{3}O_{7-δ} can be persistently enhanced by illumination. Strongly debated for years, the origin of those effects-termed persistent photoconductivity and photosuperconductivity (PPS)-has remained an unsolved critical problem, whose comprehension may provide key insights to harness the origin of high-temperature superconductivity itself. Here, we make essential steps toward understanding PPS. While the models proposed so far assume that it is caused by a carrier-density increase (photodoping) observed concomitantly, our experiments contradict such conventional belief: we demonstrate that it is instead linked to a photo-induced decrease of the electronic scattering rate. Furthermore, we find that the latter effect and photodoping are completely disconnected and originate from different microscopic mechanisms, since they present different wavelength and oxygen-content dependences as well as strikingly different relaxation dynamics. Besides helping disentangle photodoping, persistent photoconductivity, and PPS, our results provide new evidence for the intimate relation between critical temperature and scattering rate, a key ingredient in modern theories on high-temperature superconductivity.

PHOTOSENSITIVE FIELD-EFFECT TRANSISTOR BASED ON REDUCED GRAPHENE OXIDE FILM

A photosensitive graphene field-effect transistor was created by depositing a reduced graphene oxide (rGO) film on the surface of the SiO2 layer on a silicon substrate, which serves as both a photosensitive medium and the field-effect transistor gate. The electrical and photoelectric properties of the field-effect transistor based on the rGO film were studied in DC and AC modes. Linear sections of the drain current on the gate voltage dependence and significant dependence of the electronic component of the rGO film conductivity on the irradiation with white light were revealed based on the analysis of the switching characteristics of the obtained field-effect transistor. A photoinduced decrease in the internal resistance and an increase in the capacitance of the conducting channel of the graphene field-effect transistor in the range of 102–105 Hz were revealed. It has been established that the photoresponse time of the obtained field-effect transistor to light pulses of IR, visible, and UV radiation is about 1.5 ms. The obtained results can be used to simplify the technology of manufacturing photodetectors based on graphene.

Excitation of phenanthridines in aqueous solution: Comparative theoretical analysis

To elucidate the effect of side groups on the excitation of phenanthridines, six compounds were studied by DFT/TD-DFT: ethidium (E), dimidium (D), desphenylethidium (DE), desphenyldimidium (DD), dideaminoethidium (DDE), and dideaminodesphenylethidium (DDDE). Ten hybrid functionals were used with different contributions to the “exact” Hartree-Fock exchange, including those with long-range correction (B3LYP, APFD, PBE0, M06, M05, BMK, MN15, CAM-B3LYP, MN12SX, and ωB97XD). As a result, the best agreement with the experiment was given by the BMK functional in combination with the 6–311++G(d,p) basis set and the IEFPCM solvent model. The main structural change upon excitation of E, D, and DDE was a significant decrease in the dihedral φ between the phenyl ring and the chromophore. In the φ range from ≈75° to ≈105°, the rotation of the phenyl ring relative to the chromophore is almost unimpeded, but it becomes very difficult at |Δφ|>45°. HOMOs change significantly with the addition/removal of amino groups. At the same time, LUMOs are the same for all studied phenanthridines. HOMOs do not cover the side groups attached to the N5 atom. LUMOs cover these groups slightly. This feature explains the small differences in the absorption maxima λmax between E, D, DE, and DD, as well as between DDE and DDDE. The photoinduced shifts of electron densities have a complex form and quantitatively depend on the side groups. The dipole moments of dyes with phenyl rings increase upon excitation, while the lack of phenyl rings leads to a photoinduced decrease of dipole moments. From this feature, in particular, it follows that the negative solvatochromism of E is determined by hydrogen bonds with the solvent, and not by dipole–dipole interactions.

Light-Activated Adhesion and Debonding of Underwater Pressure-Sensitive Adhesives.

Pressure-sensitive adhesives (PSAs) such as sticky notes and labels are a ubiquitous part of modern society. PSAs with a wide range of peel adhesion strength are designed by tailoring the bulk and surface properties of the adhesive. However, designing an adhesive with strong initial adhesion but showing an on-demand decrease in adhesion has been an enduring challenge in the design of PSAs. To address this challenge, we designed alkoxyphenacyl-based polyurethane (APPU) PSAs that show a photoactivated increase and decrease in peel strength. With increasing time of light exposure, the failure mode of our PSAs shifted from cohesive to adhesive failure, providing residue-free removal with up to 83% decrease in peel strength. The APPU-PSAs also adhere to substrates submerged underwater and show a similar photoinduced decrease in adhesion strength.

Direct Optical Lithography of Colloidal Metal Oxide Nanomaterials for Diffractive Optical Elements with 2π Phase Control.

Spatially patterned dielectric materials are ubiquitous in electronic, photonic, and optoelectronic devices. These patterns are typically made by subtractive or additive approaches utilizing vapor-phase reagents. On the other hand, recent advances in solution-phase synthesis of oxide nanomaterials have unlocked a materials library with greater compositional, microstructural, and interfacial tunability. However, methods to pattern and integrate these nanomaterials in real-world devices are less established. In this work, we directly optically pattern oxide nanoparticles (NPs) by mixing them with photosensitive diazo-2-naphthol-4-sulfonic acid and irradiating with widely available 405 nm light. We demonstrate the direct optical lithography of ZrO2, TiO2, HfO2, and ITO NPs and investigate the chemical and physical changes responsible for this photoinduced decrease in solubility. Micron-thick layers of amorphous ZrO2 NPs were patterned with micron resolution and shown to allow 2π phase control of visible light. We also show multilayer patterning and use it to fabricate features with different thicknesses and distinct structural colors. Upon annealing at 400 °C, the deposited ZrO2 structures have excellent optical transparency across a wide wavelength range (0.3-10 μm), a high refractive index (n = 1.84 at 633 nm), and are optically smooth. We then fabricate diffractive optical elements, such as binary phase diffraction gratings, that show efficient diffractive behavior and good thermal stability. Different oxide NPs can also be mixed prior to patterning, providing a high level of material tunability. This work demonstrates a general patterning approach that harnesses the processability and diversity of colloidal oxide nanomaterials for use in photonic applications.

Light-Associated Surface Wrinkling-Based Metrology for the Photosoftening Characterization in Azobenzene-Polymer Supramolecular Complexes.

As an intriguing characteristic of azobenzene-containing materials (azo-materials), photoinduced changes in mechanical properties (e.g., photosoftening) have stimulated many efforts both theoretically and experimentally. Here a simple yet powerful tool (i.e., a light-associated surface wrinkling-based method) to study the photosoftening effect in azobenzene-polymer (azo-polymer) supramolecular complexes is reported. The photo-induced modulus decrease of supramolecular complex films is deduced by analyzing the change of critical wrinkle wavelength of strain-induced surface wrinkling, in the case of varying experiment parameters. In particular, thanks to the facile modular tunability of the supramolecular system, the photosoftening effect has been systematically investigated as a function of azo-moiety content and the molecular weight of the host polymer. Notably, a photosoftening coefficient that is related to the chemical composition/structure of azo-polymers is introduced, and a simple formula that can quantify the connection of the photosoftening with external irradiation conditions and internal chemical factors of azo-polymers is derived for the first time. The obtained results are of great importance not only to enhance understanding of the photosoftening mechanism, but also to thoroughly apply it in diverse smart fields.

Light-Responsive Proton Conductor: Record High Gain of Proton Conductivity Achieved by Photoinduced Electron-Transfer Strategy

Light-Responsive Proton Conductor: Record High Gain of Proton Conductivity Achieved by Photoinduced Electron-Transfer Strategy

Bias voltage-dependent photoinduced current and photoluminescence in organometal perovskite layers on silicon substrates

Thin organometal perovskite layers on crystalline silicon substrates under excitation with nanosecond laser pulses were prepared and investigated to reveal an effect of the bias voltage and parameters of the perovskite/silicon structure on photoinduced current and photoluminescence. Numerical simulations of the diffusion, separation and recombination of photoinduced charge carriers in perovskite/silicon structures were carried out to describe both the photoluminescence transient and time-dependent photocurrent at different bias voltage. While the nonradiative surface recombination does not significantly influence the photoluminescence transients for times shorter than 100 ns, the photoinduced current decreases strongly with increasing the surface recombination velocity. The simulation results were compared with experimental ones obtained for organometal perovskite deposited on substrates of optically polished crystalline silicon wafers of p-type conductivity and those covered with silicon nanowires. The obtained results are discussed in view of possible applications of the investigated structures in photovoltaics and light emitting optoelectronics.

Long-living nonlinear behavior in CsPbBr3 carrier recombination dynamics

Abstract By means of time-resolved photoluminescence (TR-PL) spectroscopy, we present a detailed investigation of the carrier relaxation dynamics in a CsPbBr3 bulk sample and microcrystal ensemble at cryogenic temperature on a picosecond time scale. We provide evidence of a long temperature-dependent cooling rate for the excitons and free carriers population, with an initial cooling time constant of a few tens of picoseconds. A relaxation bottleneck in the thermalization process was found that cannot be explained by the Auger effect or hot phonon population, since we address a very low excitation regime, not commonly investigated in literature, where such processes are not effective. Adding a continuous wave optical bias to the picosecond excitation, we probed the photoinduced PL decrease of the localized states and the photoinduced PL increase of the population in the high energy states. A long recovery time from the photoinduced PL decrease was found for localized states and quite significant differences were detected, depending on the resonance/off resonance bias used in the experiment.

Dye Encapsulation in Fluorescent Core-Shell Silica Nanoparticles as Probed by Fluorescence Correlation Spectroscopy.

Synthetic advances in the formation of ultrasmall (<10 nm) fluorescent poly(ethylene glycol)-coated (PEGylated) core-shell silica nanoparticles (SNPs), enabling improved particle size and surface chemical property control have led to successful clinical translation of SNPs as diagnostic probes in oncology. Despite the success of such probes, details of the dye incorporation and resulting silica architecture are still poorly understood. Here, we employ afterpulse-corrected fluorescence correlation spectroscopy (FCS) to monitor fast fluorescence fluctuations (lag times <10-5 s) of the negatively charged cyanine dye Cy5 as a probe to study such details for dye encapsulation in 5 nm silica cores of PEGylated core-shell SNPs (C dots). Upon deposition of additional silica shells over the silica core we find that the amplitude of photo-induced cis-trans isomerization decreases, suggesting that the Cy5 dyes are located near or on the surface of the original SNP cores. In combination with time correlated fluorescence decay measurements we deduce radiative and non-radiative rates of the Cy5 dye in these particles. Results demonstrate that FCS is a well-suited tool to investigate aspects of the photophysics of fluorescent nanoparticles, and that conformational changes of cyanine dyes like Cy5 are excellent indicators for the local dye environment within ultrasmall SNPs.

Photoconductance of ITO/Conductive Polymer Junctions in the UV and Visible Ranges

Controlling charge transfer at indium-doped tin oxide (ITO)/conductive polymer junctions is of special importance for organic photovoltaic (OPV) devices and organic light emitting diodes (OLEDs), where ITO is often the transparent electrode of choice. Light induced conductance enhancement, i.e., photoconductance, can allow such control. ITO/conductive polymer junctions are shown herein to exhibit photoconductance under UV illumination mostly due to photoinduced decrease of an electron barrier at the ITO–polymer interface by discharging of ITO extrinsic surface states, related to the adsorption of oxygen species. Furthermore, we show that ITO surface modification by photoactive porphyrin adsorption can sensitize the ITO/conductive polymer junctions, extending the photoconductance to the visible range, to which ITO is transparent. This process is ascribed mostly to discharging of ITO adsorbate states by recombination with photogenerated holes in the photoexcited molecules. Such sensitization is highly relevant for organic optoelectronic devices utilizing ITO interfaced with photoactive organic species and operating in the visible range, such as OPV and OLED devices, and might be applicable also to other UV-photoconductive metal oxide electrodes.

Photoplasticization Effect of an Azobenzene-doped Liquid Crystal Depending on Phase Structures

A photoresponsive plasticizer for a photo-inactive polymer was developed using an azobenzene-doped liquid crystal (LC). The azo-doped LC was miscible with poly(methyl methacrylate) up to a critical concentration, above which it phase-separated from the polymer. A photoinduced decrease in the glass transition temperature (Tg) of the polymer was accomplished by photochemically changing the phase structures of the phase-separated LC; the large decrease in Tg (ΔTg = −23 °C) was obtained with the addition of 70 wt % azo-doped LC.

Ultraviolet- and infrared-induced 11 beta-hydroxysteroid dehydrogenase type 1 activating skin photoaging is inhibited by red ginseng extract containing high concentration of ginsenoside Rg3(S).

Sun irradiation is one of major extrinsic stressors responsible for premature skin aging through activation and expression of 11 beta-hydroxysteroid dehydrogenase type 1 (11β-HSD1), which converts inactive cortisone to active cortisol. The aim of this study was to evaluate the inhibitory effects of red ginseng extract containing high concentrations of ginsenoside Rg3 (S) (GERg3) on 11β-HSD1-induced skin photoaging. To evaluate the inhibitory effects of GERg3 on ultraviolet- (UV) or infrared (IR)-induced skin photoaging, human dermal fibroblasts or a normal human 3D skin model was exposed to UV or an IR. RT-PCR, ELISA, Western blot, and H&E staining were used for evaluations. GERg3 was isolated from crude red ginseng. GERg3 inhibited the increased expressions of 11β-HSD1, interleukin (IL)-6, and matrix metalloproteinase-1 (MMP-1) in UVB- or IR-exposed Hs68 cells. Additionally, the increased cortisol, IL-6, and MMP-1 expressions were effectively reduced by GERg3 in UVA-exposed 3D skin models. The photoinduced decrease in type 1 procollagen also recovered as a result of GERg3 treatment in Hs68 cells and the 3D skin model. In addition, the UVA-exposed dermal thickness was decreased in comparison with the UVA-protected 3D skin model, recovered with GERg3 treatment. GERg3 had antiphotoaging effects in UV- or IR-exposed human dermal fibroblasts and normal human 3D skin model.

Photoinduced processes in hybrid structures on the basis of ТіO2 nanoparticles and CdSe/ZnS quantum dots

The morphology of and photoinduced changes in the luminescent properties of hybrid structures on the basis of TiO2 nanoparticles and CdSe/ZnS quantum dots are studied. It is established that the morphology of the structures depends on the method of their formation and the type of stabilizer of the CdSe/ZnS surface. It is shown that a photoinduced decrease in the efficiency of nonradiative relaxation of the excitation in the quantum dots leads to an increase in the quantum yield of their luminescence and an increase in the efficiency of photoinduced charge transfer in hybrid structures.

Photoinduced transformation of waste-derived soluble bio-based substances

Waste-derived, soluble bio-based substances (SBO), are effective low-cost photosensitizers that could find application in pollutant photodegradation. For this reason, it is important to understand if and to what extent irradiation could modify their properties. The exposure of SBO to simulated sunlight induced important spectral and structural modifications. Both the whole material and its acid-soluble fraction were characterized, highlighting several properties in common with humic and fulvic substances, including absorption spectra, specific absorbance and fluorescence behavior. The latter was described with a three-component model using PARAFAC analysis. Irradiation induced SBO photobleaching, but the absorbance of the acid-soluble fraction increased with irradiation. This finding suggests a progressive photochemical solubilization of SBO, which is confirmed by the increase of the carboxylic groups. In addition to absorbance, the fluorescence of whole SBO was also decreased by irradiation, thereby suggesting that both chromophores and fluorophores were photodegraded. The increasingly hydrophilic character given to SBO by irradiation also accounted for the photoinduced decrease of the surfactant properties of the material.

Photoinduced perturbations of the magnetic superexchange in core@shell Prussian blue analogues

Cubic heterostructured core@shell particles of Prussian blue analogues, composed of a shell (∼80nm thick) of ferromagnetic K0.3Ni[Cr(CN)6]0.8·1.3H2O (A), Tc∼70K, surrounding a bulk (∼350 nm) core of photoactive ferrimagnetic Rb0.4Co[Fe(CN)6]0.8·1.2H2O (B), Tc∼20K, have been studied. Below nominally 70K, these CoFe@NiCr samples exhibit a persistent photoinduced decrease in low-field magnetization, and these results resemble data from other BA core@shell particles and analogous ABA heterostructured films. This net decrease suggests that the photoinduced lattice expansion in the B layer generates a strain-induced decrease in the magnetization of the NiCr layer, similar to a pressure-induced decrease observed by others in a similar, pure NiCr material and by us in our CoFe@NiCr cubes. Upon further examination, the data also reveal a significant portion of the NiCr shell whose magnetic superexchange, J, is perturbed by the photoinduced strain from the CoFe constituent.

Demagnetization in photomagnetic films

We present a model for demagnetization in photomagnetic films, and investigate different regimes for the magnetizing process using finite element analysis. It is found that the demagnetizing factor may depend strongly upon the high-spin fraction of the film, and the specifics of the dependence are dictated by the microscopic morphology of the photomagnetic domains. This picture allows for facile interpretation of existing data on photomagnetic films, and can even explain an observed photoinduced decrease in low-field magnetization concurrent with increase in high-spin fraction. As a whole, these results reiterate the need to consider demagnetizing effects in photomagnetic films.

Simultaneous Analysis of Optical and Mechanical Properties of Cross-Linked Azobenzene-Containing Liquid-Crystalline Polymer Films

The photomechanical behavior of cross-linked azobenzene-containing liquid-crystalline polymer films was investigated by means of simultaneous measurement of their optical and mechanical properties. The connection between photoisomerization of the azobenzene moieties, photoinduced change in molecular alignment, photoinduced stress generation, and macroscopic bending was analyzed. Upon UV irradiation, the films exhibited bending due to gradient in cis-azobenzene content, and subsequent unbending when cis-azobenzene content became uniform throughout the film. The maximum photoinduced stress was generated in the same time scale as the time required to reach photostationary state in the cis-azobenzene concentration. The maximum values of photogenerated stress strongly depended on the crosslinker concentration, even if the azobenzene concentration and the cis-azobenzene content in the photostationary state were similar for all the polymer films. The stress is connected to the initial Young's modulus and also to the photoinduced change in birefringence of the polymer films. In addition, a significant photoinduced decrease in Young's modulus was for the first time observed in cross-linked azobenzene-containing liquid-crystalline polymers, which is likely to be an important factor in dictating their photomechanical behavior.

10.1007/s11447-008-2011-2

New “configuration” modes, which were predicted by us for CdF2:In crystals, have been revealed at the frequencies ν1 ≈ 32.4 cm−1 and ν2 ≈ 96.3 cm−1 for deep and shallow impurity states, respectively. The frequencies of these oscillations exactly correspond to the potential-energy curves calculated by us for shallow and deep states of In with regard to the reduced mass M = 2m 1 m 2/(m 1 + 2m 2) of the In ion (m 1) and two F ions (2m 2) per primitive fluorite cell. This correspondence confirms the correct choice of the height of the potential barrier between the impurity states of In in CdF2 (0.02 eV), which was used in the calculations. The dielectric contributions of the noted modes were determined, which made it possible to calculate the concentrations of In impurity ions in the deep (N 1) and shallow (N 2) states. The obtained ratio N 2/N 1 ≈ 2 directly indicates that photoionization of deep In centers leads to the formation of a doubled number of shallow centers and that two electrons are localized in the deep state of the In ion; such behavior is characteristic of DX centers. A photoinduced increase in the real (ε′) and imaginary (ε″) parts of the dielectric constant has been found (at a frequency of 25 cm−1, Δε′ ≈ 0.2 and Δε″ ≈ 0.06). These changes correspond to the changes in the dielectric contributions of the configuration modes under illumination. A photoinduced decrease in the lattice reflection of CdF2:In, related to the impurity lattice modes, has also been revealed.