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Vibrational fingerprint of localized excitons in a two-dimensional metal-organic crystal
摘要： Long-lived excitons formed upon visible light absorption play an essential role in photovoltaics, photocatalysis, and even in high-density information storage. Here, we describe a self-assembled two-dimensional metal-organic crystal, composed of graphene-supported macrocycles, each hosting a single FeN4 center, where a single carbon monoxide molecule can adsorb. In this heme-like biomimetic model system, excitons are generated by visible laser light upon a spin transition associated with the layer 2D crystallinity, and are simultaneously detected via the carbon monoxide ligand stretching mode at room temperature and near-ambient pressure. The proposed mechanism is supported by the results of infrared and time-resolved pump-probe spectroscopies, and by ab initio theoretical methods, opening a path towards the handling of exciton dynamics on 2D biomimetic crystals.
关键词： FeN4 center,spin transition,ab initio theoretical methods,excitons,carbon monoxide,infrared spectroscopy,graphene,time-resolved pump-probe spectroscopy,two-dimensional metal-organic crystal
更新于2025-09-23 15:21:21
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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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<i>In situ</i> structural kinetics of picosecond laser-induced heating and fragmentation of colloidal gold spheres
摘要： Fragmentation of colloidal 54 nm gold nanoparticles by picosecond laser pulses is recorded by time-resolved X-ray scattering, giving access to structural dynamics down to a 80 ps resolution. Lattice temperature and energy dissipation have been quantified to verify that the maximum applied fluence of 1800 J m?2 heats up the particles close to boiling. Already within 30 ns, particles with significantly lower particle sizes of 2 to 3 nm are detected, which hints towards an ultrafast process either by a thermal phase explosion or Coulomb instability. An arrested growth is observed on a microsecond time scale resulting in a final particle size of 3–4 nm with high yield. In this context, the fragmentation in a NaCl/NaOH solution seems to limit growth by electrostatic stabilization of fragments, whereas it does not modify the initial product sizes. The laser-induced fragmentation process is identified as a single-step, instantaneous reaction.
关键词： Coulomb instability,fragmentation,thermal phase explosion,time-resolved X-ray scattering,picosecond laser pulses,gold nanoparticles
更新于2025-09-23 15:21:01
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Time-Resolved Impulsive Stimulated Raman Spectroscopy with Synchronized Triple Mode-Locked Lasers
摘要： A complete understanding of a photochemical reaction dynamics begins with real-time measurements of both electronic and vibrational structures of photoexcited molecules. Time-resolved impulsive stimulated Raman spectroscopy (TR-ISRS) with femtosecond actinic pump, Raman pump, and Raman probe pulses is one of the incisive techniques enabling one to investigate the structural changes of photoexcited molecules. Here, we demonstrate that such femtosecond TR-ISRS is feasible with synchronized triple mode-locked lasers without using any time-delay devices. Taking advantage of precise control of the three repetition rates independently, we could achieve automatic scanning of two delay times between the three pulses, which makes both rapid data acquisition and wide dynamic range measurement of the fifth-order TR-ISRS signal achievable. We thus anticipate that the present triple mode-locked laser-based TR-ISRS technique will be of critical use for long-term monitoring of photochemical reaction dynamics in condensed phases and biological systems.
关键词： pump-dump-probe,fifth-order electronic spectroscopy,impulsive stimulated Raman spectroscopy,femtosecond Raman scattering,Time-resolved vibrational spectroscopy
更新于2025-09-23 15:21:01
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Photophysical Properties of Fluorescent Self-Assembled Peptide Nanostructures for Singlet Oxygen Generation
摘要： In this work, a drug delivery system for perillyl alcohol based on the peptide self-assembly containing 3-(2-benzothiazolyl)-7-(diethylamino)coumarin (C6) as a fluorescent additive is obtained, and its photophysical characteristics as well as its release dynamics were studied by steady-state and time-resolved fluorescence spectroscopy. Results proved the dynamics of drug release from the peptide nanostructures and showed that the system formed by the self-assembled peptide and C6, along with perillyl alcohol, presents unique photophysical properties that can be exploited to generate singlet oxygen (1O2) upon irradiation, which is not achieved by the sole components. Through epifluorescence microscopy combined with time-correlated single photon counting fluorescence spectroscopy, the release mechanism was proven to occur upon peptide structure interconversion, which is controlled by environmental changes.
关键词： photophysical properties,perillyl alcohol,drug delivery system,time-resolved fluorescence spectroscopy,fluorescent additive,peptide self-assembly,epifluorescence microscopy,singlet oxygen generation
更新于2025-09-23 15:21:01
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Spectroscopic Studies of M??ssbauer, Infrared, and Laser-Induced Luminescence for Classifying Rare-Earth Minerals Enriched in Iron-Rich Deposits
摘要： Rare-earth (RE) phosphates often appear as an accessory phase in igneous or metamorphic rocks; however, these rocks are composed of myriad chemical elements and nuclides that interfere with the qualitative or quantitative analyses of the RE phosphates over a range of concentrations in the absence of a pretreatment. In addition, the limit of each analytical methodology constrains the approach as well as the usefulness of the results in geoscience applications. Here, we report the specific mineral characterization of RE-containing ores from Yen Phu mine, Vietnam, using a range of state-of-the-art spectroscopic techniques in conjunction with microscopy: M?ssbauer spectroscopy, infrared microspectroscopy, time-resolved laser-induced fluorescence spectroscopy (TRLFS), and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Because the distribution of each element in the deposit differs, such combinatorial works are necessary and could lead to more plausible answers to questions surrounding the point of origin of RE elements. The results of our M?ssbauer spectroscopic analysis indicate that the three ores sampled at different locations all contain magnetite-like, hematite-like, and iron(III) salts other than hematite. In addition, we confirmed the presence of phosphate around the grain boundary in the magnetite-like mineral phase by infrared microspectroscopic analysis. The present analytical findings of trace amounts of europium(III) using TRLFS suggest that the europium ions generate identical luminescence spectra despite being embedded in three different matrices of iron minerals. This demonstration highlights the benefits of combinatorial spectroscopic analyses to gain insights into the effects of the environment of REs on their solid-state chemistry and shows the potential utility of TRLFS as a resource mining tool. Further applications of this approach in the analytical screening of rocks and minerals are feasible.
关键词： energy-dispersive X-ray spectroscopy,spectroscopic techniques,rare-earth phosphates,Yen Phu mine,infrared microspectroscopy,scanning electron microscopy,time-resolved laser-induced fluorescence spectroscopy,M?ssbauer spectroscopy,Vietnam
更新于2025-09-23 15:21:01
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CdSe/CdMnS Nanoplatelets with Bilayer Core and Magnetically Doped Shell Exhibit Switchable Excitonic Circular Polarization: Implications for Lasers and Light-Emitting Diodes
摘要： We utilized time-resolved photoluminescence (TRPL) spectroscopy to study the excitonic circular polarization (PX) from CdSe/CdMnS core/shell nanoplatelets (NPLs) with a bilayer core. This allows an extensive study of the emission dynamics as a function of magnetic ?eld, temperature, doping concentration, and excitation wavelength. In the presence of an external magnetic ?eld, pulsed excitation below the shell gap results in near-zero excitonic circular polarization PX at all time delays. In contrast, pulsed excitation with photon energy larger than the shell gap results in a rapid (100 ps) buildup of the excitonic circular polarization which subsequently remains constant at a level of up to 40%. We propose a model to describe the dynamics which takes into account the exchange interaction between carrier and magnetic ion (Mn) spins. The studied system exhibits a fast switchable excitonic circular polarization, implying possible applications in lasers and light emitting diodes.
关键词： spd exchange interaction,quasi-2D,time-resolved photoluminescence,nanoplatelets,magneto-optical switch
更新于2025-09-23 15:21:01
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Spatial range of the plasmonic Dicke effect in an InGaN/GaN multiple quantum well structure
摘要： The plasmonic Dicke effect means a cooperative emission mechanism of multiple light emitters when they are simultaneously coupled with the same surface plasmon (SP) mode of a metal nanostructure to achieve a higher collective emission efficiency. Here, we compare the enhancements of emission efficiency among a series of SP-coupled InGaN/GaN quantum-well (QW) structures of different QW period numbers to show an emission behavior consistent with the plasmonic Dicke effect. The relative enhancement of overall emission efficiency increases with QW period number until it reaches a critical value, beyond which the enhancement starts to decrease. This critical QW period number corresponds to the effective depth range of the plasmonic Dicke effect in a multiple-QW system. It also represents an optimized QW structure for maximizing the SP coupling effect. Internal quantum efficiency and time-resolved photoluminescence are measured for comparing the enhanced emission efficiencies of blue and green QW structures with different QW period numbers through SP coupling induced by surface Ag nanoparticles.
关键词： multiple quantum well,internal quantum efficiency,Ag nanoparticle,surface plasmon coupling,plasmonic Dicke effect,time-resolved photoluminescence
更新于2025-09-23 15:21:01
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Spectroscopic Study of Ensemble and Individual Graphene Quantum Dots
摘要： Graphene quantum dots (GQDs) have attract broad interest during the last several years. One of the mysteries for GQDs is the excitation-dependent emission spectrum. In this work, we tried to reveal the fundamental understanding of this issue by investigating the spectroscopy of individual GQDs. We measured emission spectra on individual GQDs and white-light N-doped GQDs (IGQDs) prepared by electron beam irradiation, with different excitation lights at 488 nm, 532 nm, and 633 nm. We found narrow spectra for both individual GQDs and IGQDs compare to the ensemble spectra. The spectrum by summing up spectra of individual dots is almost consistent with ensemble spectrum in solution. More importantly, different dots were observed in the same sample with different excitation wavelengths indicating the heterogeneity of the GQDs structure. Together with the crystal structure characterization of the IGQDs, we can conclude that the luminescence is not from the band-to-band transition but from defect states. These results directly proved the heterogeneity of the GQDs materials revealing the mechanism of the spectroscopic mystery. The observed narrow emission spectrum of individual dots also shows their potential application for light emitting devices after purification.
关键词： Graphene Quantum Dots,Photoluminescence,Time-Resolved,Single-Particle Spectroscopy
更新于2025-09-23 15:21:01
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How the Mixed Cations (Guanidium, Formamidinium, and Phenylethylamine) in Tin Iodide Perovskites Affect Their Charge Carrier Dynamics and Solar Cell Characteristics
摘要： Despite recent interest in lead-free Sn iodide perovskite (ASnI3) solar cells, the role of mixed A-site cations is yet to be fully understood. Here, we report the effect of the ternary mixing of organic A-site cations (guanidium, GA; formamidinium, FA; and phenylethylamine, PEA) on the solar cell performance and charge carrier dynamics that are evaluated using time-resolved microwave conductivity (TRMC). (GAxFA1?x)0.9PEA0.1SnI3 exhibits the maximum power conversion efficiency (PCE) of 7.90% at x = 0.15 and a drastic decrease with increasing GA content. Notably, our TRMC measurements of ASnI3 with/without a hole transport layer reveal the same trend with the devices. From the analyses, we suggest that a variation of electron mobility affected by the location of the GA cation in the grains significantly impacts the PCE. Our work sheds light on the role of mixed A-site cations and directs a route toward the further development of Sn perovskite solar cells.
关键词： time-resolved microwave conductivity,mixed A-site cations,solar cell performance,Sn iodide perovskite,charge carrier dynamics,lead-free
更新于2025-09-23 15:21:01