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Exciton Coherence Length and Dynamics in Graphene Quantum Dot Assemblies
摘要： Exciton size and dynamics were studied in assemblies of two well-defined graphene quantum dots of varying size: hexabenzocoronene (HBC), where the aromatic core consists of 42 C atoms, and carbon quantum dot (CQD) with 78 C atoms. The synthesis of HBC and CQD were achieved using bottom-up chemical methods, while their assembly was studied using steady-state UV/vis spectroscopy, X-ray scattering, and electron microscopy. While HBC forms long ordered fibers, CQD was found not to assemble well. The exciton size and dynamics were studied using time-resolved laser spectroscopy. At early times (~100 fs), the exciton was found to delocalize over ~1?2 molecular units in both assemblies, which reflects the confined nature of excitons in carbon-based materials and is consistent with the calculated value of ~2 molecular units. Exciton?exciton annihilation measurements provided the exciton diffusion lengths of 16 and 3 nm for HBC and CQD, respectively.
关键词： Time-resolved Laser Spectroscopy,Graphene Quantum Dot,Hexabenzocoronene,Exciton,Carbon Quantum Dot
更新于2025-09-12 10:27:22
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Interferometric time- and energy-resolved photoemission electron microscopy for few-femtosecond nanoplasmonic dynamics
摘要： We report a time-resolved normal-incidence photoemission electron microscope with an imaging time-of-flight detector using ～7-fs near-infrared laser pulses and a phase-stabilized interferometer for studying ultrafast nanoplasmonic dynamics via nonlinear photoemission from metallic nanostructures. The interferometer’s stability (35 ± 6 as root-mean-square from 0.2 Hz to 40 kHz) as well as on-line characterization of the driving laser field, which is a requirement for nanoplasmonic near-field reconstruction, is discussed in detail. We observed strong field enhancement and few-femtosecond localized surface plasmon lifetimes at a monolayer of self-assembled gold nanospheres with ～40 nm diameter and ～2 nm interparticle distance. A wide range of plasmon resonance frequencies could be simultaneously detected in the time domain at different nanospheres, which are distinguishable already within the first optical cycle or as close as about ±1 fs around time-zero. Energy-resolved imaging (microspectroscopy) additionally revealed spectral broadening due to strong-field or space charge effects. These results provide a clear path toward visualizing optically excited nanoplasmonic near-fields at ultimate spatiotemporal resolution.
关键词： photoemission electron microscopy,nanoplasmonic dynamics,few-femtosecond,gold nanospheres,time-resolved
更新于2025-09-12 10:27:22
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Luminescence lifetime encoding in time-domain flow cytometry
摘要： Time-resolved flow cytometry represents an alternative to commonly applied spectral or intensity multiplexing in bioanalytics. At present, the vast majority of the reports on this topic focuses on phase-domain techniques and specific applications. In this report, we present a flow cytometry platform with time-resolved detection based on a compact setup and straightforward time-domain measurements utilizing lifetime-encoded beads with lifetimes in the nanosecond range. We provide general assessment of time-domain flow cytometry and discuss the concept of this platform to address achievable resolution limits, data analysis, and requirements on suitable encoding dyes. Experimental data are complemented by numerical calculations on photon count numbers and impact of noise and measurement time on the obtained lifetime values.
关键词： lifetime-encoded beads,multiplexing,Time-resolved flow cytometry,bioanalytics,time-domain measurements
更新于2025-09-11 14:15:04
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Thiophenylazobenzene – an alternative photoisomerization controlled by lone pair???π interaction
摘要： Azoheteroarene photoswitches have attracted attention due to their unique properties. We present here the stationary photochromism and the ultrafast photoisomerization mechanism of thiophenylazobenzene (TphAB) – an alternative sulfur-based azoheteroarene design. TphAB demonstrates impressive fatigue resistance and photoisomerization efficiency, and shows favorably separated (E)- and (Z)-isomer absorption bands, allowing for highly selective photoconversion. The (Z)-isomer of TphAB adopts an unusual orthogonal geometry where the plane of the thiophene is perfectly perpendicular to that of the benzene. This geometry is stabilized by a rare lone pair???π interaction between the S-atom and the benzene. The photoisomerization of TphAB occurs on the sub-ps to ps timescale and is governed by the lone pair???π interaction. Therefore, the adoption and disruption of the orthogonal geometry requires significant movement along the inversion reaction coordinates (CNN and NNC angles). Our results establish TphAB as an excellent photoswitch with versatile properties that expand the application possibilities of AB derivatives.
关键词： photochromism,time-resolved spectroscopy,thiophenylazobenzene,isomerization mechanism,photoswitch
更新于2025-09-11 14:15:04
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Temporal evolution of photo-induced thermal strain in InSb probed by ultra-short laser produced Cu Kα x-rays
摘要： Development and characterization of a wire target based kHz rep rate Cu Kα x-ray source using a Ti:sapphire laser system and its use in time resolved x-ray diffraction (TXRD) of the InSb (111) sample are presented. The observed Kα x-ray photon flux is ～3.2 × 109 photons sr?1 s?1 at a laser intensity of ～3.5 × 1016 W cm?2. TXRD signal from the InSb (111) crystal pumped by an ultrashort Ti:sapphire laser pulse (fluence ～ 13 mJ cm?2) shows a lattice expansion due to heating on a multipicosecond time scale. The crystal gradually cools down and recovers at ～1.5 ns after the laser excitation. The observed strain variation in the crystal matches well with the simulated results. The study of full recovery of the sample will be helpful for the development of InSb based devices.
关键词： Cu Kα x-ray source,time resolved x-ray diffraction,ultrashort laser pulses,thermal strain,InSb
更新于2025-09-11 14:15:04
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Present status of photoemission electron microscope newly installed in SPring-8 for time-resolved nanospectroscopy
摘要： A photoemission electron microscope (PEEM) system has been newly installed at the soft X-ray undulator beamline (BL17SU) of SPring-8 to realize time-resolved nanospectroscopy for the local transient electronic structures of advanced materials. This PEEM is a versatile machine composed of an electrostatic lens system and is intended for use in specific experiments such as time-resolved measurements. Pump–probe measurements in tandem with a femtosecond pulsed-laser system and an X-ray chopper are now readily available.
关键词： photoemission electron microscope,time-resolved nanospectroscopy,SPring-8,soft X-ray undulator beamline,PEEM
更新于2025-09-11 14:15:04
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Defect Density-Dependent Electron Injection From Excited-State Ru(II) Tris-Diimine Complexes Into Defect- Controlled Oxide Semiconductors
摘要： Dye-sensitized solar cells and photocatalysts that consist of a light absorbing dye and a wide gap oxide semiconductor substrate have been studied extensively as a means of solar energy conversion. Although defects existing at an oxide surface have a significant impact on the electron injection efficiency from the excited state dye-molecule into the oxide, the effects of defects on the electron injection process has not been fully understood in any dye-sensitized system. In this study, we present a systematic evaluation of electron injection into defects using emissive Ru(II) complexes adsorbed on oxide substrates (HCa2Nb3O10 nanosheets and nonstoichiometric SrTiO3—"), which had different defect densities. Using these oxides, electron injection from adsorbed Ru(II) complexes was observed by time-resolved emission spectroscopy. It was shown that electron injection from the excited state Ru(II) complex into an oxide was influenced by the defect density of the oxide as well as by the excited state oxidation potential (Eox*) of the Ru(II) complex. Electron injection was clearly accelerated with increasing defect density of the oxide, and was inhibited with increasing electron density of the oxide because of a trap-filling effect. Even though the Eox* of the Ru(II) complex was more positive than the conduction band edge potential of the oxide, electron injection into defects could be identified when a defective oxide was employed. The electron injection event is discussed in detail, on the basis of the defect density and the energy levels of oxides as well as the Eox* values of Ru(II) complexes. Overall the results suggest that it is possible to estimate the potential of surface defect states in an oxide by changing Eox* of an emissive complex dye.
关键词： Dye-sensitized solar cells,defect density,electron injection,Ru(II) complexes,time-resolved emission spectroscopy,photocatalysts
更新于2025-09-11 14:15:04
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Efficient fluorescence quenching of CdSe quantum dots on epitaxial GaAs nanostructures
摘要： Interaction of CdSe quantum dots (QDs) with epitaxially grown GaAs nanostructures has been studied using photoluminescence (PL) technique. Highly fluorescent CdSe QDs of size 3.9 nm were synthesized by colloidal method and coated over GaAs nanostructures grown on GaAs (111)B substrate by metal organic vapor phase epitaxy (MOVPE) using self-assembled Ga droplets as catalyst. Effect of conditions like catalyst growth time and temperature on the growth of GaAs nanostructures has also been studied. Highly uniform tapered hexagonal nanostructures of height around 500 nm were obtained in nearly 100% yield at 420 °C using Ga droplets grown for 10 s. The fluorescence emission of the CdSe QDs on sample bearing the GaAs nanostructures was measured by steady state and time-resolved photoluminescence (TRPL) techniques and compared with the one obtained on bare substrate. Enhanced quenching of the fluorescence of QDs has been witnessed on the sample in which GaAs nanostructures were present. It has been attributed to more efficient defect-related non-radiative relaxation of excited QDs on the surface of six {110} side facets of the GaAs nanostructures that were not present in the bare substrate. The detailed analysis of the TRPL characteristics of the samples has suggested F?rster-like resonance energy transfer (FRET)-based relaxation of CdSe QDs on GaAs nanostructures through shallow traps with significantly reduced average lifetime and the high quenching efficiency.
关键词： Nanostructures,MOVPE,GaAs,CdSe,Time-resolved photoluminescence,Quantum dots
更新于2025-09-11 14:15:04
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Charge transfer dynamics in chlorophyll-based biosolar cells
摘要： We fabricated a chlorophyll (Chl)-based biosolar cell with H2Chl-sensitized TiO2 as an acceptor and (ZnChl)n as a donor. This solar cell gives a relatively high quantum yield from the absorption spectral contribution from both the donor and acceptor species. We employed subpicosecond time-resolved absorption spectroscopy (TAS) to study the excited state dynamics at the Chl interface. A charge transfer (CT) state between TiO2–H2Chl and (ZnChl)n was observed at 640 nm after excitation at the Qy peaks, 680 nm and 720 nm. This CT state is entirely different from the CT states observed for either TiO2–H2Chl (TiO2–H2Chl/spiro-OMeTAD) or TiO2–(ZnChl)n systems. Due to the slower charge transfer process from H2Chl+ to TiO2 as compared to that from (ZnChl)n+ to H2Chl, the CT lifetimes of H2Chl–(ZnChl)n+ (t1 = 0.1 ps, t2 = 1.4 ps) excited at 720 nm are slightly shorter than that excited at 680 nm (t1 = 0.2 ps, t2 = 5.6 ps). The TAS results suggest that the interface of TiO2–H2Chl and (ZnChl)n not only transfers holes as spiro-OMeTAD does, but also provides a built-in field for charge dissociation between the two Chl species.
关键词： TiO2,charge transfer,biosolar cell,chlorophyll,time-resolved absorption spectroscopy
更新于2025-09-11 14:15:04
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Magneto-fluorescent microbeads for bacteria detection constructed from superparamagnetic Fe <sub/>3</sub> O <sub/>4</sub> nanoparticles and AIS/ZnS quantum dots
摘要： The efficient and sensitive detection of pathogenic microorganisms in aqueous environments such as water used in medical applications, drinking water, and cooling water of industrial plants requires simple and fast methods suitable for multiplexed detection such as flow cytometry (FCM) with optically encoded carrier beads. For this purpose, we combine fluorescent Cd-free Ag-In-S ternary quantum dots (t-QDs) with fluorescence lifetimes (LTs) of several hundred nanoseconds and superparamagnetic Fe3O4 nanoparticles (SPIONs) with mesoporous CaCO3 microbeads to a magneto-fluorescent bead platform that can be surface-functionalized with bioligands such as antibodies. This inorganic bead platform enables immuno-magnetic separation, target enrichment, and target quantification with optical readout. The beads can be detected with steady-state and time-resolved fluorescence microscopy and flow cytometry (FCM). Moreover, they are suited for readout by time gated emission. In the following, the preparation of these magneto-fluorescent CaCO3 beads, their spectroscopic and analytic characterization, and their conjugation with bacteria-specific antibodies are presented as well as proof-of-concept measurements with Legionella pneumophila including cell cultivation and plating experiments for bacteria quantification. Additionally, the possibility to discriminate between the long-lived emission of the LT-encoded capture and carrier CaCO3 beads and the short-lived emission of the dye-stained bacteria with time-resolved fluorescence techniques and single wavelength excitation is demonstrated.
关键词： AIS/ZnS quantum dots,flow cytometry,magneto-fluorescent microbeads,time-resolved fluorescence,superparamagnetic Fe3O4 nanoparticles,bacteria detection
更新于2025-09-11 14:15:04