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Stark Effect and Environment-Induced Modulation of Emission in Single Halide Perovskite Nanocrystals
摘要： Organic-inorganic halide perovskites have emerged as promising materials for next generation solar cells. In nanostructured form also, these materials are excellent candidates for optoelectronic applications such as lasers and light emitting diodes for displays and lighting. While great progress has been achieved so far in optimizing the intrinsic photophysical properties of perovskite nanocrystals (NCs), in working opto-electronic devices external factors, such as the effects of conducting environment and of the applied electric field on exciton generation and photon emission have been largely unexplored. Here, we use NCs of the all-inorganic perovskite CsPbBr3 dispersed polyvinyl carbazole, a hole-conductor, and in polymethyl methacrylate, an insulator, to examine the effects of applied electric field and conductivity of the matrix on the perovskite photophysics at single-particle level. We found that the conducting environment causes a significant decrease of photoluminescence (PL) brightness of individual NCs due the appearance of intermediate-intensity emitting states with significantly shortened lifetime. Applied electric field has a similar effect and, in addition, causes a non-linear spectral-shift of the PL maxima, a combination of linear and quadratic Stark effect caused by environment-induced polarity and field-related polarizability. The environment and electric field effects are explained by ionization of the NCs through hole transfer and emission of the resulting negatively-charged excitons.
关键词： ionization,halide perovskite nanocrystals,single-particle spectroscopy,Stark effect,blinking
更新于2025-09-23 15:23:52
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Dark Plasmon Modes in Symmetric Gold Nanoparticle Dimers Illuminated by Focused Cylindrical Vector Beams
摘要： The plasmon hybridization model of electromagnetic coupling between plasmonic nanoparticles predicts the formation of lower energy “bonding” and higher energy “antibonding” modes in analogy with the quantum mechanical description of chemical bonding. For a symmetric metallic nanoparticle dimer excited by linearly polarized light, the hybridization picture predicts that in-phase coupling of the dipole moments is optically allowed, creating bright “modes”, whereas the out-of-phase coupling is dark due to the cancellation of the oppositely oriented dipole moments (in the quasistatic approximation). These bright modes are electric dipolar in nature and readily couple to scalar (i.e., linearly or circularly polarized) beams of light. We show that focused cylindrical vector beams, specifically azimuthally and radially polarized beams, directly excite dark plasmon modes in symmetric gold nanoparticle (AuNP) dimers at normal incidence. We use single-particle spectroscopy and electrodynamics simulations to study the resonance scattering of AuNP dimers illuminated by azimuthally and radially polarized light. The electric field distributions of the focused azimuthal or radial beams are locally polarized perpendicular or parallel to the AuNP dimer axis, but with opposite directions at each particle. Therefore, the associated combinations of single-particle dipole moments are out-of-phase, and the excitation (resonance) is of so-called “dark modes”. In addition, multipole expansion of the fields associated with each scattering spectrum shows that the vector beam excitation involves driving multipolar, e.g., magnetic dipolar and electric quadrupolar, modes, and that they even dominate the scattering spectra (vs electric dipole). This work opens new opportunities for investigating dark plasmon modes in nanostructures, which are difficult to selectively excite by conventional polarized light.
关键词： electrodynamics simulations,dark plasmon modes,plasmon hybridization,single-particle spectroscopy,gold nanoparticle dimers,cylindrical vector beams
更新于2025-09-23 15:21:21
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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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Mass Accommodation Coefficients of Water on Organics from Complementary Photoacoustic and Light Scattering Measurements on Laser-Trapped Droplets
摘要： The mass accommodation coefficient, αM, describes the liquid?vapor evaporation and condensation kinetics at interface. In spite of numerous experimental efforts, reliable values of αM are still not available for many substances. Here, we present a novel experimental technique, photothermal single-particle spectroscopy (PSPS), that allows for a robust retrieval of mass accommodation coefficients from three simultaneous independent measurements. PSPS combines resonant photoacoustic absorption spectroscopy with modulated Mie scattering measurements on single particles. We study the mass transport of water on organic aerosol droplets that are optically trapped using counter-propagating tweezers. We find the mass accommodation coefficient of water on a pure model organic that is fully miscible with water to be 0.021 at 296 K and to decrease by more than an order of magnitude when the temperature increases to 309 K. The experimentally observed temperature dependence of αM shows an Arrhenius behavior. Furthermore, the water content of the droplets is found to have a profound effect on αM is observed at low water concentrations, while at elevated water concentrations, we observe a 5-fold increase in αM. The technique presented in this work has the potential to become a reliable method for the retrieval of αM values at liquid?vapor interfaces, which are essential for accurate global climate and pharmaceutical aerosol inhalation modeling, to mention but a few.
关键词： temperature dependence,photoacoustic absorption spectroscopy,optical trapping,water transport,organic aerosol droplets,modulated Mie scattering,mass accommodation coefficient,Arrhenius behavior,PSPS,photothermal single-particle spectroscopy
更新于2025-09-23 15:19:57
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Extent of Shallow/Deep Trap States beyond the Conduction Band Minimum in Defect-Tolerant CsPbBr <sub/>3</sub> Perovskite Quantum Dot: Control over the Degree of Charge Carrier Recombination
摘要： Perovskite quantum dots (PQDs) are known to be defect tolerant possessing clean band-gap with optically inactive benign defect states. However, we show that there exists significant deep trap states beyond conduction band minimum, although the extent of shallow trap states is observed to be minimal. Extent of deep trap states beyond conduction band minimum seem to significant in PQD, however the extent is less than even optically robust CdSe and InP based core/alloy-shell QDs. In-depth analyses based on ultrafast transient absorption and ultrasensitive single particle spectroscopic investigations decode the underlying degree of charge carrier recombination in CsPbBr3 PQD which are quite important for energy applications.
关键词： Perovskite quantum dots,ultrafast transient absorption,defect tolerant,single particle spectroscopy,charge carrier recombination,trap states
更新于2025-09-19 17:13:59
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Absolute Scattering Cross-Sections of Titanium Nitride Nanoparticles Determined by Single-Particle Spectroscopy: Implications for Plasmonic Nanoantennas
摘要： Titanium nitride (TiN) nanostructures are promising building blocks for photothermal applications because of the surface plasmon-induced light absorption. However, compared to noble metal nanostructures, detailed analyses of the plasmonic responses of TiN nanostructures have been limited. In this work, by combining transmission electron microscopy (TEM) and single-particle spectroscopy, we determine absolute scattering cross-sections of individual TiN nanocubes (NCs). The key aspect of the developed methodology is placing a TiN NC on an ultrathin SiO2 membrane and performing the precise structural analysis and the scattering spectrum measurement on identical TiN NCs. Furthermore, by employing Au nanospheres as references, we quantitatively compare the scattering spectra with those obtained for TiN NCs and determine the absolute scattering cross-section as a function of NC size.
关键词： titanium nitride,nanoantenna,scattering,single particle spectroscopy,Plasmon
更新于2025-09-11 14:15:04
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Determination of Handedness in a Single Chiral Nanocrystal via Circularly Polarized Luminescence
摘要： The occurrence of biological homochirality is attributed to symmetry breaking mechanisms which are still debatable. Studies of symmetry breaking require tools for monitoring the population ratios of individual chiral nano-objects, such as molecules, polymers or nanocrystals. Moreover, mapping their spatial distributions may elucidate on their symmetry breaking mechanism. While luminescence is preferred for detecting single particle chirality due to its high signal to noise ratio, the typical low optical activity of chromophores limits its applicability. Here, we report on handedness determination of single chiral lanthanide based luminescent nanocrystals with a total photon count of 2×104. Due to the large emission dissymmetry we could determine the handedness of individual particles using only a single circular polarization component of the emission spectrum, without polarization modulation. A machine learning algorithm, trained to several spectral lineshape features, enabled us to determine and spatially map the handedness of individual nanocrystals with high accuracy and speed. This technique may become invaluable in studies of symmetry breaking in chiral materials.
关键词： chiroptical activity,circularly polarized luminescence,lanthanide nanocrystals,single-particle spectroscopy
更新于2025-09-09 09:28:46