Time-resolved Optical Spectroscopy Research Articles

Electrical and optical characteristics of a nanosecond pulsed electrical discharge in air in contact with a water droplet for various electrical conductivities

Abstract Interactions between pulsed electrical discharges and liquid dielectric materials is a growing research field with interests in fundamental discharge physics as well as in applications. Herein, we present an experimental study on the dynamics of nanosecond discharges in air with the presence of a water droplet with various electrical conductivities (EC) and at different applied voltages (Va). The discharges are characterized optically, by employing time-resolved ICCD imaging and optical emission spectroscopy, as well as electrically, by acquiring the current-voltage waveforms for every discharge. The results show that three modes of discharges can be obtained: i) streamer discharge between the cathode and the droplet, ii) streamer discharge between the cathode and the droplet as well as between the anode and the droplet, and iii) spark discharge that connects the two electrodes and propagates over the droplet. We find that the probability to obtain one of the three discharge modes is strongly related to the droplet’s EC as well as to Va. Although the ignition of the streamer is relatively insensitive to EC, its transition to a spark can be finely controlled by the droplet’s EC. The time-resolved ICCD images show that the discharge initiates in the gap between the cathode and the droplet, followed by ignition between the anode/ground electrode and the droplet. Then, an extinction phase is observed before the ignition of a secondary streamer, and depending on the conditions, the discharge may transition to a spark, i.e. a channel with high emission intensity. We find that the duration of each stage of discharge propagation as well as the corresponding emission (path and intensity) are sensitive to droplet’s EC. Finally, emissions from streamers (primary and secondary) as well as from sparks are analyzed using optical spectroscopy. We find that the emission from the streamers is dominated by the second positive system of N2, and that the droplet’s EC does not significantly influence the emission spectra nor the estimated rotational temperature of N2.

The phonon-modulated Jahn–Teller distortion of the nitrogen vacancy center in diamond

The negatively charged nitrogen vacancy (NV) center in diamond is an optically accessible material defect with a unique combination of spin and optical properties that has attracted interest in quantum-information sciences and as a design candidate for nanoscale quantum sensors. Here, we present time-resolved nonlinear optical spectroscopy measurements, conducted with ultrabroadband laser pulses, that reveal strong modulation of the excited-state by the longitudinal optical (LO) phonon of the diamond lattice. The LO phonon and its overtones geometrically distort neighboring NV centers, driving long lived (3.5 ps) excited state relaxation of coupled NV centers after the initial excitation and ultrafast (<150 fs) decay of the Jahn–Teller distortion. These observations elevate the LO phonon to an important tuning mode of the Jahn–Teller conical intersection and help resolve previous spectroscopy experiments that noted longer-lived excited-state dynamics.

Key Role of Oxidising Species for Water Oxidation on Iridium Oxides Revealed By Time-Resolved Optical and X-Ray Spectroscopies

Iridium oxide is the state-of-the-art electrocatalyst for water oxidation in polymer electrolyte membrane (PEM) electrolysers, crucial for green hydrogen production. Despite its extensive industrial use, the water oxidation mechanism on this metal oxide and the key factors controlling the reaction rate remain unclear. Understanding these controls at a fundamental level on such benchmark metal oxides is vital for designing more active and stable electrocatalysts for water oxidation in PEM electrolysers.In this talk, I will present our research on probing oxygen evolution reaction (OER) relevant species on iridium-based catalysts. This involves a combination of time-resolved operando optical spectroscopy, soft and hard X-ray absorption spectroscopy (XAS), and electrochemical mass spectrometry. Initially, I will discuss the intermediate and catalytically active states of iridium oxides. This is based on correlating optically detected states with oxygen molecular products identified through on-chip electrochemical mass spectrometry. Subsequently, I will demonstrate how we used optical spectroscopy to quantify these states as a function of potential. The nature of these states, including the Ir oxidation state and surface adsorbates on iridium sites, will be elucidated using a combination of time-resolved Ir-L edge XAS, O-K edge XAS, and supported by Density Functional Theory (DFT).With these quantification results of active species for OER, I will compare the intrinsic kinetics of two state-of-the-art iridium oxide structures - amorphous IrOx versus crystalline rutile IrO2.[1] The effect of the electrolyte on the intrinsic activity of iridium oxides will also be discussed.[2] Finally, based on this molecular-level understanding, I will present a modified volcano model for OER catalyst design. This model considers the impact of adsorbate-adsorbate interactions on binding energetics, demonstrating critical design principles for high intrinsic activity OER catalysts.The authors acknowledge the funding support from the Imperial College-Chinese Scholarship Council Studentship and bp-ICAM 92, which made this research possible.[1] Caiwu Liang, Reshma Rao, Karine Svane et al. Unravelling the effects of active site densities and energetics on the water oxidation activity of iridium oxides, 07 March 2023, PREPRINT (Version 1) available at Research Square [https://doi.org/10.21203/rs.3.rs-2605628/v1][2] Caiwu Liang, Yu Katayama, Yemin Tao, Asuka Morinaga, Benjamin Moss, Verónica Celorrio, et al. Role of electrolyte pH on water oxidation for iridium oxides. ChemRxiv. Cambridge: Cambridge Open Engage; 2023

(Invited) Resolving Catalytic Mechanism at an Electrode Surface: Thermodynamics & Kinetics of Reaction Steps

Computationally, often the energetics of intermediate reaction steps differentiate the efficiency of heterogeneous catalysts for product evolution. Yet, when compared to experiment, kinetic models are applied. For example, a material’s activity measured by one rate of product evolution is plotted as a function of the calculated formation energies of intermediate chemical forms. A critically important reaction for which this dichotomy between experiment and theory exists is the oxygen evolution reaction (OER) from water. In the laboratory group, we employ time-resolved optical and vibrational spectroscopy to deconstruct OER into its individual reaction steps on an electrode surface. In the presentation, I will describe the experimental methodology and the chosen model system, the n-doped SrTiO3/aqueous interface. I’ll show how these experiments identify both the rates and energetics of the first reaction step, the release of a proton and electron from an absorbed water species, denoted by OH* or O*. A recent success was to isolate a Langmuir isotherm of the intermediate population arising within < 2 ps on the SrTiO3 surface. The intermediate population is of trapped holes measured by emission from the conduction band into unoccupied states in the middle of the semi-conductor bandgap. The isotherm is tuned by the pH of the electrolyte. The pH-dependent formation of these intermediates are connected to their pH-dependent decay at microsecond timescales, presumably related to later reaction steps of OER. I'll also present work on the side of vibrational spectroscopy that looks at how the potential energy surface of a particular trapped hole, the terminal oxyl radical, arises in time through the spectral kinetics of its normal mode. A growing area is to apply the particular experimental methodology to a surface of similar electronic structure but diverse crystal geometries, namely polymorphs of TiO2.

Advancing Organic Photoredox Catalysis: Mechanistic Insight through Time-Resolved Spectroscopy.

The rapid development of light-activated organic photoredox catalysts has led to the proliferation of powerful synthetic chemical strategies with industrial and pharmaceutical applications. Despite the advancement in synthetic approaches, a detailed understanding of the mechanisms governing these reactions has lagged. Time-resolved optical spectroscopy provides a method to track organic photoredox catalysis processes and reveal the energy pathways that drive reaction mechanisms. These measurements are sensitive to key processes in organic photoredox catalysis such as charge or energy transfer, lifetimes of singlet or triplet states, and solvation dynamics. The sensitivity and specificity of ultrafast spectroscopic measurements can provide a new perspective on the mechanisms of these reactions, including electron-transfer events, the role of solvent, and the short lifetimes of radical intermediates.

Simplifying asynchronous optical sampling: an experimental approach toward industrial integration exploiting lock-in acquisition

Time-resolved optical spectroscopies are emerging as a go-to technique for non-destructive testing of nanomaterials. Inspecting the thermal and mechanical properties of a mesoscale device requires achieving delay times beyond the ns timescale in a nanoscopy setup, potentially in a vibration polluted environment. These requirements constitute a major challenge for traditional pump-probe techniques based on moving mechanical delay lines and lock-in detection. Asynchronous optical sampling (ASOPS) and electronically controlled optical sampling (ECOPS), avoiding any moving mechanical parts, are good alternatives. However, their detection scheme is based on fast-balanced photodiodes, which, as a technology, are not as widespread, not as developed, and lack the performance of lock-in based detection. In this study, we introduce what we believe is a novel approach that integrates ASOPS/ECOPS and lock-in detection methodologies, eliminating the necessity for a reference signal and streamlining the optical configuration. By leveraging the strengths of each technique, our approach enhances simplicity and efficiency. The scheme is first validated against standard approaches in the frame of a beam-depletion measurement in a sum frequency experiment. It is then tested in a paradigmatic case study to inspect the mechanics of a single gold nanodisk, with dimensions in the 100 nm range, nanopatterned on a sapphire substrate. These results widen the range of applicability of time-resolved optical techniques as a nano-metrology tool to industrial settings.

Unraveling the Role of Particle Size and Nanostructuring on the Oxygen Evolution Activity of Fe-Doped NiO.

Nickel-based oxides and oxyhydroxide catalysts exhibit state-of-the-art activity for the sluggish oxygen evolution reaction (OER) under alkaline conditions. A widely employed strategy to increase the gravimetric activity of the catalyst is to increase the active surface area via nanostructuring or decrease the particle size. However, the fundamental understanding about how tuning these parameters influences the density of oxidized species and their reaction kinetics remains unclear. Here, we use solution combustion synthesis, a low-cost and scalable approach, to synthesize a series of Fe0.1Ni0.9O samples from different precursor salts. Based on the precursor salt, the nanoparticle size can be changed significantly from ∼2.5 to ∼37 nm. The OER activity at pH 13 trends inversely with the particle size. Using operando time-resolved optical spectroscopy, we quantify the density of oxidized species as a function of potential and demonstrate that the OER kinetics exhibits a second-order dependence on the density of these species, suggesting that the OER mechanism relies on O-O coupling between neighboring oxidized species. With the decreasing particle size, the density of species accumulated is found to increase, and their intrinsic reactivity for the OER is found to decrease, attributed to the stronger binding of *O species (i.e., a cathodic shift of species energetics). This signifies that the high apparent OER activity per geometric area of the smaller nanoparticles is driven by their ability to accumulate a larger density of oxidized species. This study not only experimentally disentangles the influence of the density of oxidized species and intrinsic kinetics on the overall rate of the OER but also highlights the importance of tuning these parameters independently to develop more active OER catalysts.

Evaluation of interfacial photophysical processes by time-resolved optical spectroscopy in perovskite solar cells

Evaluation of interfacial photophysical processes by time-resolved optical spectroscopy in perovskite solar cells

Unravelling the effects of active site density and energetics on the water oxidation activity of iridium oxides

Understanding what controls the reaction rate on iridium-based catalysts is central to designing better electrocatalysts for the water oxidation reaction in proton exchange membrane electrolysers. Here we quantify the densities of redox-active centres and probe their binding strengths on amorphous IrOx and rutile IrO2 using operando time-resolved optical spectroscopy. We establish a quantitative experimental correlation between the intrinsic reaction rate and the active-state energetics. We find that adsorbed oxygen species, *O, formed at water oxidation potentials, exhibit repulsive adsorbate–adsorbate interactions. Increasing their coverage weakens their binding, thereby promoting O–O bond formation, which is the rate-determining step. These analyses suggest that although amorphous IrOx exhibits a higher geometric current density, the intrinsic reaction rates per active state on IrOx and IrO2 are comparable at given potentials. Finally, we present a modified volcano plot that elucidates how the intrinsic water oxidation kinetics can be increased by optimizing both the binding energy and the interaction strength between the catalytically active states.

A comparative study on the spectral characteristics of nanosecond pulsed discharges in atmospheric He and a He+2.3%H2O mixture

Nanosecond pulsed discharges at atmospheric pressure in a pin-to-pin electrode configuration are well reproducible in time and space, which is beneficial to the fundamentals and applications of low-temperature plasmas. In this experiment, the discharges in helium (He) and He with 2.3% water vapor (H2O) are driven by a series of 10 ns overvoltage pulses (~13 kV). Special attention is paid to the spectral characteristics obtained in the center of discharges by time-resolved optical emission spectroscopy. It is found that in helium, the emission of atomic and molecular helium during the afterglow is more intense than that in the active discharge, while in the He+2.3%H2O mixture, helium emission is only observed during the discharge pulse and the molecular helium emission disappears. In addition, the emissions of OH(A-X) and Hα present similar behavior that increases sharply during the falling edge of the voltage pulse as the electrons cool down rapidly. The gas temperature is set to remain low at 540 K by fitting the OH(A-X) band. A comparative study on the emission of radiative species (He, He2, OH and H) is performed between these two discharge cases to derive their main production mechanisms. In both cases, the dominant primary ion is He+ at the onset of discharges, but their He+ charge transfer processes are quite different. Based on these experimental data and a qualitative discussion on the discharge kinetics, with regard to the present discharge conditions, it is shown that the electron-assisted three-body recombination processes appear to be the significant sources of radiative OH and H species in high-density plasmas.

Photochemical Formation and Electronic Structure of an Alkane σ-Complex from Time-Resolved Optical and X-ray Absorption Spectroscopy.

C-H bond activation reactions with transition metals typically proceed via the formation of alkane σ-complexes, where an alkane C-H σ-bond binds to the metal. Due to the weak nature of metal-alkane bonds, σ-complexes are challenging to characterize experimentally. Here, we establish the complete pathways of photochemical formation of the model σ-complex Cr(CO)5-alkane from Cr(CO)6 in octane solution and characterize the nature of its metal-ligand bonding interactions. Using femtosecond optical absorption spectroscopy, we find photoinduced CO dissociation from Cr(CO)6 to occur within the 100 fs time resolution of the experiment. Rapid geminate recombination by a fraction of molecules is found to occur with a time constant of 150 fs. The formation of bare Cr(CO)5 in its singlet ground state is followed by complexation of an octane molecule from solution with a time constant of 8.2 ps. Picosecond X-ray absorption spectroscopy at the Cr L-edge and O K-edge provides unique information on the electronic structure of the Cr(CO)5-alkane σ-complex from both the metal and ligand perspectives. Based on clear experimental observables, we find substantial destabilization of the lowest unoccupied molecular orbital upon coordination of the C-H bond to the undercoordinated Cr center in the Cr(CO)5-alkane σ-complex, and we define this as a general, orbital-based descriptor of the metal-alkane bond. Our study demonstrates the value of combining optical and X-ray spectroscopic methods as complementary tools to study the stability and reactivity of alkane σ-complexes in their role as the decisive intermediates in C-H bond activation reactions.

Influence of the RF voltage amplitude on the space- and time-resolved properties of RF–LF dielectric barrier discharges in α–γ mode

This work reports the results of an experimental and modeling study on dual-frequency Ar–NH3 dielectric barrier discharges (DBDs) exhibiting the α–γ transition. A combination of space- and time-resolved optical absorption and emission spectroscopy is used to record spatio-temporal mappings of the Ar metastable number density, Ar 750.4 nm line emission intensity, and electron–Ar bremsstrahlung continuum emission intensity. With the increase of the radio frequency (RF) voltage amplitude in a 50 kHz–5 MHz DBD, maximum populations of Ar excited species (1s and 2p states, linked to the population of high-energy electrons) observed in the γ mode decrease and appear earlier in the low-frequency cycle. On the other hand, the density of the bulk electrons, monitored from the continuum emission intensity, increases, with a more prominent rise in the RF-α mode than in the γ regime. Such behaviors are consistent with the predictions of 1D fluid model and results from a decrease of the gas voltage required for self-maintenance of the cathode sheath in the γ breakdown.

Electron Donor-Specific Surface Interactions Promote the Photocatalytic Activity of Metal-Semiconductor Nanohybrids.

In the past two decades, the application of colloidal semiconductor-metal nanoparticles (NPs) as photocatalysts for the hydrogen generation from water has been extensively studied. The present body of literature studies agrees that the photocatalytic yield strongly depends on the electron donating agent (EDA) added for scavenging the photogenerated holes. The highest reported hydrogen production rates are obtained in the presence of ionic EDAs and at high pH. The large hydrogen production rates are attributed to fast hole transfer from the NP onto the EDAs. However, the present discussions do not treat the influence of EDA-specific surface interactions. This systematic study focuses on that aspect by combining steady-state hydrogen production measurements with time-resolved and static optical spectroscopy, employing 11-mercaptoundecanoic acid-capped, Pt-tipped CdSe/CdS dot-in-rods in the presence of a large set of EDAs. Based on the experimental results, two distinct EDA groups are identified: surface-active and diffusion-limited EDAs. The largest photocatalytic efficiencies are obtained in the presence of surface-active EDAs that induce an agglomeration of the NPs. This demonstrates that the introduction of surface-active EDAs can significantly enhance the photocatalytic activity of the NPs, despite reducing their colloidal stability and inducing the formation of NP networks.

Dual-frequency sheath oscillations and consequences on the ion and electron transport in dielectric barrier discharges at atmospheric pressure

Current–voltage characteristics, space- and time-resolved optical emission spectroscopy, and 1D fluid modeling are used to examine the effect of dual-frequency sheath oscillations on the ion and electron transport in dielectric barrier discharges sustained by a combination of low frequency (LF, 50 kHz, 650 V) and radiofrequency (RF, 5.3 MHz, 195 V) voltages, exhibiting the α-to-γ mode transition. On one hand, when polarities of the LF and RF voltages are opposite, an electric field near the LF cathode (due to LF cathode sheath) drives the secondary electrons to the plasma bulk and an opposite electric field between the sheath edge and the LF anode attracts the electrons toward the LF cathode (to maintain quasi-neutrality in the plasma bulk). At the sheath edge, electrons become trapped and ions drift toward the cathode and the anode simultaneously according to their position in the gap. On the other hand, when the RF voltage has the same polarity as the LF voltage, the total applied voltage increases and this yields to enhanced production of electrons and ions in the sheath. To maintain quasi-neutrality in the bulk, the electric field along the gap exhibits the same polarity as the one in the sheath, allowing electrons created in the sheath to be evacuated toward the LF anode. The behavior of the LF cathode is, therefore, controlled by the LF sheath, and, thus, by the LF voltage amplitude, while the behavior in the bulk and at the anode alternates on the time scale of the RF voltage.

Layer-by-layer structured nanocomposite deposits from plasma-synthesized organosilicon nanoparticles and organosilicon nanoparticles decorated with Ag nanoparticles by taking advantage of cyclic nanoparticle formation in Ar/HMDSO reactive plasmas

Rational engineering of thin nanocomposite layers, deposited in reactive plasmas, requires knowledge on the plasma behavior in order to produce multifunctional deposits with tailored properties (structural, optical, electrical, etc.) This work presents an experimental study of nanoparticles synthesized in the plasma gas-phase and their subsequent use as building-blocks to form layer-by-layer nanostructures. The experiment is performed in a plasma process that successfully combines plasma polymerization of an organosilicon molecular precursor (hexamethyldisiloxane, HMDSO) and sputtering of a metallic (silver) target. Pulsed injection of the precursor is found to promote cyclic nanoparticle formation in Ar/HMDSO reactive plasmas. The plasma electron temperature is found to vary in the range 1.6—2.2 eV as derived from time-resolved optical emission spectroscopy of the plasma energetic conditions. This diagnostic method is also shown to provide a reliable tool for online monitoring of the nanoparticle synthesis process. Two types of layer-by-layer structured nanocomposites can be obtained depending on the type of nanoparticles synthesized: (i) organosilicon nanoparticles of size less than 100 nm in all studied plasma conditions for a large quantity of injected HMDSO and (ii) raspberry-like nanoparticles of size less than 150 nm when the quantity of injected HMDSO is reduced. The organosilicon nanoparticle growth follows a polydimethylsiloxane (PDMS)-like oligomerization scheme in which the R2-Si(-O)2 silicon bond tends towards the formation of polymeric structure in a R3-Si(-O)1 silicon chemical environment, containing Si-(CH2)-Si type bridges that are involved in cross-linking. The elemental composition of the raspberry-like nanoparticles is similar to that of the organosilicon nanoparticles, supplemented by the Ag component. The decorating silver nanoparticles are ∼15 nm of size, round in shape and polycrystalline. There is no evidence for silver oxides in the nanostructures. The Si-O-Ag bridges, revealed by infrared spectroscopy, suggest the presence of junction sites between the metallic and the organosilicon parts of the raspberry-like nanoparticles. The silver nanoparticles are found to decorate the organosilicon nanoparticles to form the raspberry-like nanoparticles in the plasma gas-phase, before being deposited. This reveals a very interesting phenomenon of simultaneous growth of the silver- and organosilicon-parts in the plasma without mixing during the nucleation phase.

Study on microscopic origin of spin excitation in TmFeO3 by time-resolved multicolor optical pump-probe spectroscopy

Spin dynamics of TmFeO3, mainly induced by two different mechanisms, were investigated by time-resolved multicolor pump-probe spectroscopy using femtosecond laser pulses. The spin dynamics, caused either by spin-reorientation phase transition or optical polarization-dependent effect, were distinguished by investigating their dependencies on external magnetic field and optical polarization. Spectral-dependent magneto-optic studies of the spin phase transition-induced spin dynamics revealed that laser-induced nonthermal exchange interaction between the electron states of iron (Fe3+) and thulium (Tm3+) ions is crucial for subsequent spin dynamics. This exchange interaction is influenced by the population change of the 4f electron states of Tm3+ ions, making the spin wave generation highly dependent on crystal temperature. However, spectroscopic studies of the polarization-dependent spin dynamics show that they are only related to the 3d electrons of Fe3+ ions, therefore, the spin dynamics appear to be a universal effect over a wide temperature range. Our observation can be further applied to study extended orthoferrite systems with other rare earth atoms.

Reversible CsPbBr3 ↔ CsPb2Br5 Transformation via Reverse Micellar Aqueous Solution.

Lead halide perovskites suffer from water and moisture instability due to the highly ionic nature of the crystal structures, though a few groups took advantage of it for chemical transformation via water-assisted strategy. However, direct exposure of the perovskite to bulk water leads to uncontrolled chemical transformation. Here, we report a controlled chemical transformation of CsPbBr3 to CsPb2Br5 triggered by nanoconfined water by placing CsPbBr3 in the nonpolar phase within a reverse micelle. The chemical transformation reaction is probed by using steady-state and time-resolved optical spectroscopy. We observe absorption and photoluminescence in the UV region stemming clearly from the CsPb2Br5 phase upon interaction with the reverse micellar aqueous solution. Transmission electron microscopy and X-ray diffraction measurements further provided the structure and morphology. Our results direct the formation of CsPbBr3-CsPb2Br5 nanocomposite under dry conditions while the chemically transformed CsPb2Br5 phase exists only in moist conditions, which we explain via the CsBr-stripping mechanism.

Initial non-invasive in vivo sensing of the lung using time domain diffuse optics

The in vivo diagnosis and monitoring of pulmonary disorders (caused for example by emphysema, Covid-19, immature lung tissue in infants) could be effectively supported by the non-invasive sensing of the lung through light. With this purpose, we investigated the feasibility of probing the lung by means of time-resolved diffuse optics, leveraging the increased depth (a few centimeters) attained by photons collected after prolonged propagation time (a few nanoseconds). We present an initial study that includes measurements performed on 5 healthy volunteers during a breathing protocol, using a time-resolved broadband diffuse optical spectroscopy system. Those measurements were carried out across the spectral range of 600–1100 nm at a source-detector distance of 3 cm, and at 820 nm over a longer distance (7–9 cm). The preliminary analysis of the in vivo data with a simplified homogeneous model revealed a maximum probing depth of 2.6–3.9 cm, suitable for reaching the lung. Furthermore, we observed variations in signal associated with respiration, particularly evident at long photon propagation times. However, challenges stemming from both intra- and inter-subject variability, along with inconsistencies potentially arising from conflicting scattering and absorption effects on the collected signal, hindered a clear interpretation. Aspects that require further investigation for a more comprehensive understanding are outlined.

Ligand-Directed Self-Assembly of Organic-Semiconductor/Quantum-Dot Blend Films Enables Efficient Triplet Exciton-Photon Conversion.

Blends comprising organic semiconductors and inorganic quantum dots (QDs) are relevant for many optoelectronic applications and devices. However, the individual components in organic-QD blends have a strong tendency to aggregate and phase-separate during film processing, compromising both their structural and electronic properties. Here, we demonstrate a QD surface engineering approach using electronically active, highly soluble semiconductor ligands that are matched to the organic semiconductor host material to achieve well-dispersed inorganic-organic blend films, as characterized by X-ray and neutron scattering, and electron microscopies. This approach preserves the electronic properties of the organic and QD phases and also creates an optimized interface between them. We exemplify this in two emerging applications, singlet-fission-based photon multiplication (SF-PM) and triplet-triplet annihilation-based photon upconversion (TTA-UC). Steady-state and time-resolved optical spectroscopy shows that triplet excitons can be transferred with near unity efficiently across the organic-inorganic interface, while the organic films maintain efficient SF (190% yield) in the organic phase. By changing the relative energy between organic and inorganic components, yellow upconverted emission is observed upon 790 nm NIR excitation. Overall, we provide a highly versatile approach to overcome longstanding challenges in the blending of organic semiconductors with QDs that have relevance for many optical and optoelectronic applications.

Spectroscopic investigation of early light emission from anode-initiated surface flashover in vacuum

Time-resolved optical emission spectroscopy is used to explore the early development of anode-initiated flashover in vacuum. An experimental apparatus for collecting flashover self-luminosity is introduced, which enables spatially resolved light collection from regions adjacent to the anode and cathode triple junctions. The emitted light is spectrally and temporally resolved utilizing an imaging spectrograph and a fast electronic shutter camera. The first light detectable from flashovers across both Rexolite and polytetrafluoroethylene (PTFE) insulators begins less than 10 ns prior to the flashover gap's impedance collapse and consists of faint broadband feature in primarily visible wavelengths. This suggests luminosity due to excitation in the solid, possibly occurring as a result of field emitted electrons. In the few nanoseconds leading up to and including the impedance collapse, the broadband spectrum grows in intensity and extends into the UV, eventually accompanied by a few emission lines of the insulator material. This is strong evidence that the early stages of anode-initiated flashover include surface layer breakdown of the insulator, as a contrast from cathode-initiated flashover, which is predominately an above-surface process. Spectra accumulated over longer exposures of PTFE flashovers indicate that, in the first few hundred nanoseconds after the impedance collapse, ions from the insulator material constitute the majority of the emission lines. Later, neutral and ionic metal species from the electrode contribute with similar prominence as well as molecular bands from diatomic carbon. Finally, a comparison is provided of the results of these studies to other spectroscopic investigations of vacuum flashover from the literature.