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Temperature-dependent photo-response in multiferroic BiFeO 3 revealed by transmission measurements
摘要： We studied the light-induced effects in BiFeO3 single crystals as a function of temperature by means of optical spectroscopy. Here, we report the observation of several light-induced absorption features, which are discussed in terms of the photostriction effect and are interpreted in terms of excitons. The temperature dependence of their energy position suggests a possible coupling between the excitons and the lattice vibrations. Moreover, there are hints for anomalies in the temperature evolution of the excitonic features, which might be related to the temperature-induced magnetic phase transitions in BiFeO3. Our findings suggest a coupling between light-induced excitons and the lattice and spin degrees of freedom, which might be relevant for the observed ultrafast photostriction effect in multiferroic BiFeO3.
关键词： multiferroic,excitons,optical spectroscopy,BiFeO3,photostriction
更新于2025-11-25 10:30:42
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Probing the light hole / heavy hole switching with correlated magneto-optical spectroscopy and chemical analysis on a single quantum dot
摘要： A whole series of complementary studies have been performed on the same, single nanowire containing a quantum dot: cathodoluminescence spectroscopy and imaging, micro-photoluminescence spectroscopy under magnetic field and as a function of temperature, and energy-dispersive X-ray spectrometry and imaging. The ZnTe nanowire was deposited on a Si3N4 membrane with Ti/Al patterns. The complete set of data shows that the CdTe quantum dot features the heavy-hole state as a ground state, although the compressive mismatch strain promotes a light-hole ground state as soon as the aspect ratio is larger than unity (elongated dot). A numerical calculation of the whole structure shows that the transition from the heavy-hole to the light-hole configuration is pushed toward values of the aspect ratio much larger than unity by the presence of a (Zn,Mg)Te shell, and that the effect is further enhanced by a small valence band offset between the semiconductors in the dot and around it.
关键词： molecular beam epitaxy,optical spectroscopy,EDX,semiconductors,cathodoluminescence,quantum dot,nanowires
更新于2025-11-21 11:20:48
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Anisotropic Circular Dichroism of Light-Harvesting Complex II in Oriented Lipid Bilayers: Theory Meets Experiment
摘要： Anisotropic circular dichroism (ACD) spectroscopy of macroscopically aligned molecules reveals additional information about their excited states that is lost in the CD of randomly-oriented solutions. ACD spectra of light-harvesting complex II (LHCII)—the main peripheral antenna of photosystem II in plants—in oriented lipid bilayers, were recorded from the far-UV to the visible wavelength region. The ACD spectra show a drastically enhanced magnitude and level of detail compared to the isotropic CD spectra, resolving a greater number of bands and weak optical transitions. Exciton calculations show that the spectral features in the chlorophyll Qy region are well reproduced by an existing Hamiltonian for LHCII, providing further evidence for the identity of energy sinks at chlorophylls a603 and a610 in the stromal layer and chlorophylls a604 and a613 in the luminal layer. We propose ACD spectroscopy to be a valuable tool linking the three-dimensional structure and the photophysical properties of pigment–protein complexes.
关键词： optical spectroscopy,photosynthetic complexes,model membranes,oriented CD,excitonic CD,exciton theory
更新于2025-09-23 15:23:52
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Polymers with Exceptional Photoluminescence by Homoconjugation
摘要： Commonly, photoluminescence in organic polymers relies on π-electron delocalization by alternating double and single bonds. Remarkably, however, poly(phenylene methylene) (PPM) and methyl-substituted derivatives, such as those shown in Fig.1a, exhibit fluorescence in the wavelength region of 400–600 nm which cannot be explained by such systems of delocalized electrons. Instead, the results of extended investigations are in line with homoconjugation as the origin of the photoluminescence between 400 nm and 600 nm. Homoconjugation only arises in special chemical structures for which conjugation across individual π-electron systems can occur despite those systems being separated by an electronically insulating group, e.g. a methylene group. Such a conjugation is enabled by the overlap of p-orbitals in PPM (and derivatives), as schematically illustrated in Fig. 1b. DFT calculations for PPM indicate that homoconjugation is indeed energetically favorable. Note that homoconjugation in PPM and its derivatives can only be effective in certain conformations of the polymer chains, such as that displayed in Fig. 1b. For example, it appears that the conformations in the as-synthesized materials, which were finally obtained by rapid precipitation, are frozen in non-optimal geometries for homoconjugation because absorption and fluorescence intensity for such samples markedly increase following annealing above the glass transition temperature (66 °C for PPM; onset of thermal decomposition by TGA at 470 °C). Fluorescence of PPM and its derivatives is observed in the solid (Fig. 1c) as well as in the dissolved states. The polymers were extensively studied by UV-vis absorption spectroscopy, photoluminescence excitation spectroscopy and cyclovoltammetry. The HOMO-LUMO gap of 3.2–3.3 eV corresponds to an onset of optical absorption at approximately 375–390 nm, i.e. at higher wavelengths than expected for isolated phenylene groups. This is in agreement with homoconjugation for which electron delocalization occurs over several phenylene units. Further to this, the observed peak shifts of photoluminescence spectra towards higher wavelengths with increasing degree of polymerization (DP), up to DP ≈ 10, are fully consistent with electron delocalization along the polymer backbone. The photoluminescence lifetime recorded for thin PPM films is remarkably high, namely 8.55 ns. The photoluminescence quantum efficiency in solutions amounts to 69% and in films to 41%; notably the latter value is comparable to those for high-performance π-conjugated polymers. Interestingly, however, the absorption coefficient of PPM, measured for thick films, amounted to 80 cm–1 which, for comparison, is about 2600 times lower than the value of poly(dioctylfluorene). Notably, π-stacking, aggregation/crystallization and impurities were excluded as the origins of fluorescence in the course of the extended investigations. We expect that poly(phenylene methylene) and its derivatives will serve as examples of a new class of fluorescent polymers characterized by homoconjugation along the main chain.
关键词： Optical spectroscopy,Fluorescence,Homoconjugation,Poly(phenylene methylene)s,Light emission
更新于2025-09-23 15:22:29
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Effects of selenisation temperature on photoluminescence and photoluminescence excitation spectra of ZnO/CdS/Cu2ZnSnSe4/Mo/glass
摘要： The effect of solar cell processing (including etching in KCN along with deposition of CdS and ZnO) on photoluminescence (PL) spectra and bandgap Eg (measured at 4.2 K by photoluminescence excitation) of Cu2ZnSnSe4 films, produced by selenising metallic precursors at 450°C, 500°C and 550°C, was studied. Temperature and excitation intensity analysis of the P1 dominant band in the PL spectra of solar cells suggests that after processing this band still can be assigned to the free-to-bound recombination of free electrons with holes bound at deep acceptor levels influenced by valence band-tails. However processing increased the intensity of P1 and blue shifted it. The strongest effect was observed for the film selenised at 500°C. For the film selenised at 450°C the blue shift and increase in the intensity were smaller and only a slight intensity rise was found for the film selenised at 550°C. The intensity increase we assign to a reduction in the concentration of non-radiative recombination centers on the surface because of the etching and changes in doping due to inter-diffusion of Cd, S, Se and Zn after the deposition of CdS. Such an inter-diffusion depends on the elemental composition of the films defining the chemistry of defects and influencing Eg which increased in the film selenised at 500°C but decreased in the other films. Processing increased the P1 shift rate (j-shift) with excitation power change in all the films demonstrating a higher compensation degree in the solar cells which is consistent with the formation of an interface layer containing new donors CdCu.
关键词： Optical spectroscopy,Copper zinc tin selenide,Photoluminescence,Solar cells,Selenisation
更新于2025-09-23 15:22:29
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Insights into the Synthesis Mechanism of Ag <sub/>29</sub> Nanoclusters
摘要： The current understanding of the synthesis mechanisms of noble metal clusters is limited, in particular for Ag clusters. Here, we present a detailed investigation into the synthesis process of atomically monodisperse Ag29 clusters, prepared via reduction of AgNO3 in the presence of dithiolate ligands. Using optical spectroscopy, mass spectrometry and X-ray spectroscopy, it was determined that the synthesis involves a rapid nucleation and growth to species with up to a few hundred Ag atoms. From these larger species, Ag29 clusters are formed and their concentration increases steadily over time. Oxygen plays an important role in the etching of large particles to Ag29. No other stable Ag cluster species are observed at any point during the synthesis.
关键词： synthesis mechanism,Ag29 nanoclusters,optical spectroscopy,X-ray spectroscopy,mass spectrometry
更新于2025-09-23 15:21:01
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Characterization of a Time-Resolved Diffuse Optical Spectroscopy Prototype Using Low-Cost, Compact Single Photon Avalanche Detectors for Tissue Optics Applications
摘要： Time-resolved diffuse optical spectroscopy (TR-DOS) is an increasingly used method to determine the optical properties of diffusive media, particularly for medical applications including functional brain, breast and muscle measurements. For medical imaging applications, important features of new generation TR-DOS systems are low-cost, small size and efficient inverse modeling. To address the issues of low-cost, compact size and high integration capabilities, we have developed free-running (FR) single-photon avalanche diodes (SPADs) using 130 nm silicon complementary metal-oxide-semiconductor (CMOS) technology and used it in a TR-DOS prototype. This prototype was validated using assessments from two known protocols for evaluating TR-DOS systems for tissue optics applications. Following the basic instrumental performance protocol, our prototype had sub-nanosecond total instrument response function and low differential non-linearity of a few percent. Also, using light with optical power lower than the maximum permissible exposure for human skin, this prototype can acquire raw data in reflectance geometry for phantoms with optical properties similar to human tissues. Following the MEDPHOT protocol, the absolute values of the optical properties for several homogeneous phantoms were retrieved with good accuracy and linearity using a best-fitting model based on the Levenberg-Marquardt method. Overall, the results of this study show that our silicon CMOS-based SPAD detectors can be used to build a multichannel TR-DOS prototype. Also, real-time functional monitoring of human tissue such as muscles, breasts and newborn heads will be possible by integrating this detector with a time-to-digital converter (TDC).
关键词： diffuse optical spectroscopy,time-resolved spectroscopy,tissue optics,single-photon avalanche diode,silicon photodetectors,time-correlated single-photon counting
更新于2025-09-23 15:21:01
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Effect of Concentration on the Spectrala??Luminescent Properties of Quantum Dots in Colloidal Solutions
摘要： The effect the concentration of double-shell CdSe/CdS/ZnS semiconductor colloidal quantum dots has on the measured parameters of the absorption and luminescence spectra of a solutions of these quantum dots in toluene is studied via optical spectroscopy. Possible mechanisms of the effect the concentration of quantum dots in the solutions has on the characteristics of the luminescence spectra are analyzed.
关键词： optical spectroscopy,colloidal solutions,concentration,quantum dots,spectral-luminescent properties
更新于2025-09-23 15:21:01
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Morphological, spectroscopic and photocatalytic properties of Eu3+:TiO2 synthesized by solid-state and hydrothermal-assisted sol-gel processes
摘要： Eu3+ doped TiO2 powders were synthesized by solid-state reaction and hydrothermal-assisted sol-gel process. They were characterized by X-ray diffraction, scanning electron microscopy and photoluminescence techniques. Both synthetic procedures yield the pure anatase phase of titania. The excitation and emission spectra and the decay profiles were measured for different thermal treatments of the samples, in order to investigate the effects of the synthesis conditions on the doping mechanism. The photocatalytic activity was investigated by measuring the degradation of methylene blue under UV light irradiation and a correlation between its efficiency and the luminescence performance has been proposed.
关键词： TiO2:Eu,Solid-state,Optical spectroscopy,Anatase,Photocatalysis,Hydrothermal-assisted sol-gel
更新于2025-09-23 15:21:01
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Evaluation of molecular spectroscopy for predicting oxidative degradation of biodiesel and vegetable oil: Correlation analysis between acid value and UV–Vis absorbance and fluorescence
摘要： The present study evaluated the applicability of UV–Vis absorption and fluorescence spectroscopy for monitoring oxidation level of oil extracted from white sesame seeds (Sesamum indicum L.) and its respective biodiesel degraded at 110 °C for various times. Correlations between spectroscopic data and acid value of the samples demonstrated that both optical techniques could be used to monitor the oxidative degradation. Additionally, a quantitative model to predict the acid value of oil and biodiesel using the UV–Vis absorption data was proposed. The prediction model was tested using a cross-validation (leave-one-out) analysis and demonstrated, for the first time, that the acid value of both biodiesel and vegetable oil can be predicted by applying the same protocol for UV–Vis spectra analysis. An average error of about 3 and 8% was achieved for the predicted acid value of the oil and biodiesel samples with respect to the measured acid values, respectively. Therefore, UV–Vis absorption technique could be applied to quantitatively monitor the oxidative degradation of vegetable oil and biodiesel in a rapid and practical way.
关键词： Oxidative stability,Optical spectroscopy,Biodiesel,Acid value,Vegetable oil
更新于2025-09-23 15:21:01