Excitation Density Research Articles

Improved Description of Environment and Vibronic Effects with Electrostatically Embedded ML Potentials.

Incorporation of environment and vibronic effects in simulations of optical spectra and excited state dynamics is commonly done by combining molecular dynamics with excited state calculations, which allows to estimate the spectral density describing the frequency-dependent system-bath coupling strength. The need for efficient sampling, however, usually leads to the adoption of classical force fields despite well-known inaccuracies due to the mismatch with the excited state method. Here, we present a multiscale strategy that overcomes this limitation by combining EMLE simulations based on electrostatically embedded ML potentials with the QM/MMPol polarizable embedding model to compute the excited states and spectral density of 3-methyl-indole, the chromophoric moiety of tryptophan that mediates a variety of important biological functions, in the gas phase, in water solution, and in the human serum albumin protein. Our protocol provides highly accurate results that faithfully reproduce their ab initio QM/MM counterparts, thus paving the way for accurate investigations on the interrelation between the time scales of biological motion and the photophysics of tryptophan and other biosystems.

Time-domain signatures of distinct correlated insulators in a moiré superlattice

Among expanding discoveries of quantum phases in moiré superlattices, correlated insulators stand out as both the most stable and most commonly observed. Despite the central importance of these states in moiré physics, little is known about their underlying nature. Here, we use pump-probe spectroscopy to show distinct time-domain signatures of correlated insulators at fillings of one (ν = −1) and two (ν = −2) holes per moiré unit cell in the angle-aligned WSe2/WS2 system. Following photo-doping, we find that the disordering time of the ν = −1 state is independent of excitation density (nex), as expected from the characteristic phonon response time associated with a polaronic state. In contrast, the disordering time of the ν = −2 state scales with 1/nex, in agreement with plasmonic screening from free holons and doublons. These states display disparate reordering behavior dominated either by first order (ν = −1) or second order (ν = −2) recombination, suggesting the presence of Hubbard excitons and free carrier-like holons/doublons, respectively. Our work delineates the roles of electron–phonon (e–ph) versus electron–electron (e–e) interactions in correlated insulators on the moiré landscape and establishes non-equilibrium responses as mechanistic signatures for distinguishing and discovering quantum phases.

Mechanism of Hot-Carrier Photoluminescence in Sn-Based Perovskites.

Metal halide perovskites have shown exceptionally slow hot-carrier cooling, which has been attributed to various physical mechanisms without reaching a consensus. Here, experiment and theory are combined to unveil the carrier cooling process in formamidinium (FA) and caesium (Cs) tin triiodide thin films. Through impulsive vibrational spectroscopy and molecular dynamics, much shorter phonon dephasing times of the hybrid perovskite, which accounts for the larger blueshift in the photoluminescence seen at high excitation density for FASnI3 compared to CsSnI3 is reported. Density functional theory investigations reveal that the largest contribution to the blueshift is accounted by a giant, dynamic band-filling effect in Sn-based perovskites, which in turn can explain the cooling disparity with the Pb-based counterparts. Several years after the first experimental observations, here a deeper understanding of the cooling mechanism of these materials is provided. Design principles for hot-carrier materials, which may be useful for future implementations of hot-carrier solar cells are further provided.

Enhanced upconversion luminescence of UCNPs@CsPbI3 nanocomposites via constructing multiple energy transfer channels.

The application of upconversion nanomaterials relies heavily on the ability to produce bright upconversion luminescence (UCL) or high upconversion quantum yields (UCQYs) at low power density excitation. Herein, we synthesized silica-coated NaYF4:Yb3+@NaGdF4:Tm3+@NaYF4:Tb3+ upconversion nanoparticles (UCNPs) and CsPbI3 perovskites quantum dots (PeQDs) nanocomposites by the slow hydrolysis of (3-aminopropyl)triethoxysilane. The energy transfer (ET) of Gd3+→Tb3+ accelerates the five-photon upconversion process of Yb3+-Tm3+ and the design of the core@shell@shell layer effectively mitigates the energy jumps between Gd3+ ions. Importantly, the involvement of multiple ET channels in the UCNPs@CsPbI3 PeQDs nanocomposites increased the intensity of the UCL on the CsPbI3 PeQDs by about six times. In addition, the stability of PeQDs encapsulated in a silica matrix under air and water conditions was greatly improved.

Anomalous Amplitude Mode Dynamics Below the Expected Charge‐Density‐Wave Transition in 1T‐VSe2

AbstractA charge‐density‐wave (CDW) is characterized by a dynamical order parameter consisting of a time‐dependent amplitude and phase, which manifest as optically‐active collective modes of the CDW phase. Studying the behavior of such collective modes in the time‐domain, and their coupling with electronic and lattice order, provides important insight into the underlying mechanisms behind CDW formation. This work reports on femtosecond broadband transient reflectivity experiments of bulk 1T‐VSe2 using near‐infrared excitation. At low temperature, coherent oscillations associated with the CDW amplitude mode and phonons of the distorted lattice are observed. Across the expected transition temperature at 110 K, signatures of a rearrangement of the electronic structure are evident in the quasiparticle dynamics. However, the amplitude mode instead softens to zero frequency at 80 K, possibly indicating an additional phase transition at this temperature. In addition, photoinduced CDW melting, associated with a collapse of the electronic and lattice order, is found to occur at moderate excitation densities, consistent with a dominant electron‐phonon CDW mechanism.

Triboplasma assisted chemical conversion in granular systems: a semi-analytic model

Abstract We present a semi-analytic model for a novel plasma-assisted chemical conversion pathway using triboplasmas generated in granular flows. Triboelectric charge relaxation is a well known phenomena where the potential generated from contact charging of particles exceeds the breakdown voltage of the background gas. In this work, we extend the triboelectric charge relaxation theory to include non equilibrium plasma energy and particle balance equations to predict the formation of dissociated and excited species that act as precursors to chemical conversion, for example in plasma-assisted ammonia synthesis. 
Our example case study with nitrogen background gas and Teflon/aluminum tribomaterial system yielded high excited nitrogen species densities per collision that are comparable to current plasma-assisted conversion pathways. We also present a regime diagram for various gases where Paschen breakdown parameters are used to determine whether triboplasmas can be formed for a given effective work-function difference between two materials. Our sensitivity studies indicate particle velocity, particle radius, solids fraction and space charge effects play a critical role in overall plasma densities and excited species production.

Hot phonon effect in mid-infrared HgTe/CdHgTe quantum wells evaluated by quasi-steady-state photoluminescence

Room-temperature photoluminescence (PL) spectra of intensely pumped HgTe/CdHgTe quantum well (QW) heterostructures emitting at around 5 μm wavelength have been investigated. Based on the model description of the PL spectra using a free-electron recombination band approach, effective electronic temperatures were determined depending on the excitation density. Within the quasi-steady-state approximation, we establish the balance between pump-induced heating of the electron gas in the QWs and phonon-mediated dissipation of this excess energy and deduce hot-phonon lifetime of ∼0.47 ps. Maximum operating temperatures for optically pumped HgTe/CdHgTe QW laser heterostructures emitting at around 5 μm are estimated depending on the excitation wavelength, and lasing at Peltier temperatures appears feasible for the pump wavelength of about 3 μm. Thus, the entire 3∼5 μm atmospheric transparency window can be potentially covered by thermoelectrically cooled HgCdTe-based laser sources.

(Invited) A Statistical Model of Thermal Ionization of Excited Ce Ions in YAG

In this talk we present a statistical model for quantifying the thermal ionization process of an excited impurity ion in a level just below the conduction band of the host while the impurity ion is under constant excitation. YAG:Ce is used as an example. We assume that under constant pumping of the Ce3+ ion into the 5d1 with a blue LED, a condition of thermal and diffusive equilibrium will exist between the conduction electrons and the electrons in the excited 5d1 level of Ce3+ ion. Using the Gibbs sum, we calculate the probability for an excited Ce3+ ion to be thermally ionized, leading to an electron in the conduction band. The density of conduction electrons is then calculated over a wide range of temperatures and excitation densities. The observed quenching of emission at high temperature in Ce-doped YAG with low Ce concentrations is also investigated within this model. It is shown that the experimentally-observed nonradiative losses in this phosphor can be reasonably simulated assuming losses occur via nonradiative decay from the conduction band to the Ce4+ ground state.

Morphological evolution and optical properties of ZnO microstructures

Three kinds of ZnO microstructures are controlled synthesized on silicon substrates including microcolumns, microfences and hollow microcolumns. The morphological evolution of the prepared ZnO microstructures are systematically analyzed. It is found that the growth parameters play crucial roles on it, such as the growth temperature and the concentration of gaseous source atoms on the growth surface. Moreover, the optical characterizations suggest the structural advantages of ZnO microfence, that is, its excitation density has been enhanced due to its special symmetric hexagonal array, which has also switched on the p-band emission.

Theoretical analysis of argon 2p states' density ratios for nanosecond plasma optical emission spectroscopy

A theoretical analysis of excited argon state densities responsible for optical emission spectra of atmospheric pressure argon plasma is presented for its use in plasma diagnostics. Nanosecond pulsed barrier discharges are simulated using spatially one- and two-dimensional fluid-Poisson models using the reaction kinetics model presented by Stankov et al. [1], which considers all ten argon 2p states (Paschen notation) separately. The very first (single) discharge and repetitive discharges with frequencies from 5 kHz to 100 kHz are considered. A semi-automated procedure is utilized to find appropriate 2p states for electric field determination using an intensity ratio method, which is based on a time-dependent collisional-radiative model. The fluid simulations in combination with the semi-automated procedure are used to quantify the sensitivity of selected 2p-state ratios to given preionization of the gas. A highly sensitive time-correlated single photon counting experiment shows clearly that the selected ratio is sensitive to the electric field variation in the streamer head, yet additional calibration is needed for absolute values determination. Different approaches for effective lifetime determination are tested and applied also to measured data. The influence of radial and axial 2p state density integration on the intensity ratio method is discussed. The above mentioned models and procedures result in a flexible theory-based methodology applicable for development of new diagnostic techniques.

Engineering Photocarrier Redistributions in Graphene/III-V Quantum Dot Mixed-Dimensional Heterostructures for Radiative Recombination Enhancements.

Integration of graphene and quantum dots (QD) is a promising route to improved material and device functionalities. Underlying the improved properties are alterations in carrier dynamics within the graphene/QD heterostructure. In this study, it is shown that graphene functions as a carrier redistribution and supply channel when integrated with InAs QDs. Photoluminescence (PL) spectroscopy provides evidence that graphene modifies the redistribution, escape, and recombination dynamics of carriers in the InAs QD ensemble, which ultimately leads to enhanced radiative recombinations at all temperatures and excitation densities probed. It is also shown that the PL enhancement from the graphene/InAs QD heterostructure is greatest with a thin GaAs cap and at higher temperatures where devices operate. This study advances the understanding of graphene/QD heterostructures and can aid the design of mixed-dimensional optoelectronic devices.

Recombination efficiency in c-plane (In,Ga)N/GaN quantum wells: saturation of localisation sites versus Auger–Meitner recombination

Abstract Light emitting diodes based on c-plane (In,Ga)N/GaN quantum wells (QWs) can have >90% emission efficiency at modest current densities but this drops significantly at higher excitation, an effect known as efficiency droop that limits device efficacy at high brightness. Several explanations for this have been proposed including the saturation of carrier localisation sites at high excitation densities, resulting in a greater exposure of carriers to defects and hence a significant increase in the associated non-radiative recombination processes. Here, power- and temperature-dependent photoluminescence spectroscopy of c-plane (In,Ga)N/GaN QWs is used to investigate the relationship between the saturation of localised states and emission efficiency. For the samples studied, we find that the saturation of localised sites broadly coincides with the onset of efficiency droop. However, it is also found that as the localised states saturate with increasing excitation, the relative contribution of defect-associated non-radiative processes to overall recombination decreases rather than increases. Based on these observations and on modelling of recombination processes in the QW, it is concluded that the saturation of localised states does not significantly contribute to the reduction in emission efficiency at high excitation. Our studies rather suggest that defect-related non-radiative recombination is out-competed by radiative and Auger–Meitner recombination at the carrier densities required for saturation.

Managing Edge States in Reduced-Dimensional Perovskites for Highly Efficient Deep-Blue LEDs.

Pure-halide reduced-dimensional perovskites, featuring large exciton binding energy and tunable bandgap, show great potential for high-efficiency deep-blue perovskite light-emitting diodes (PeLEDs). However, their efficiency, particularly in the low n-value phase domain ("n" represents the number of octahedral sheets), lags behind analogous perovskite emitters. Herein, it is demonstrated that the vibration of edge-dangling octahedra in the low n-value phase activates notorious exciton-phonon (EP) coupling, thereby deteriorating efficiency. To address this issue, an approach is reported to manage edge-state lattices by introducing tris(4-fluorophenyl) phosphine (TFP) ligands. Attributed to the large steric hindrance of TFP ligands and their strong binding affinity for edge-dangling octahedra, the edged-octahedral tilting reconstruction can effectively suppress lattice vibration and inhibit EP coupling. This strategy yields deep-blue emitting film with a spectral linewidth of 21nm and a photoluminescence quantum yield of 85% at low excitation densities. The resulting PeLEDs achieve deep-blue emission at 469nm, with a maximum luminance of 2,428cdm-2 and a maximum external quantum efficiency of 10.4%, marking them among the most efficient deep-blue PeLEDs reported. The strategy also showcases universality for higher n-value reduced-dimensional perovskites. It is believed that the observation, along with the edge-state management strategy, lays the groundwork for further advancements in reduced-dimensional perovskite optoelectronic devices.

Transient Optical Modulation in Vanadium and Selenium Doped MoS2 by Carrier–Carrier and Carrier–Phonon Interactions

AbstractDoping and alloying induce defect states in atomically thin transition metal dichalcogenides (TMDCs), leading to strong carrier–phonon interactions. The robust excitonic behavior of these layered materials can be modified by injecting a high density of charge carriers. However, comprehending the influence of carrier–phonon and carrier–carrier interactions on the optical properties of 2D materials is crucial for their optoelectronic and photonic applications. Here, transient absorption (TA) spectroscopy is employed to demonstrate the modulation of the transient optical behavior of TMDCs through doping and excitation near Mott density. The TA spectra reveal broadening attributed to carrier–carrier and carrier–phonon interactions, with the broadening being particularly pronounced in vanadium (V) doped TMDCs due to the hybridization of defect and exciton transitions. Analysis of TA kinetics suggests the involvement of various carrier species in the carrier dynamics of TMDCs, with the influence of mid‐gap carriers dominating at higher excitation densities. Nonetheless, the presence of strong carrier–phonon coupling in V‐doped TMDCs is demonstrated by temperature‐dependent Raman and photoluminescence spectroscopy. The results reveal that the enhanced coupling between acoustic phonons and carriers can lead to multiphonon emission. The findings of this study hold promise for controlling the optical response of TMDCs in ultrafast optoelectronic applications.

A many-body quantum model is proposed as the mechanism responsible for accelerating rates of heat uptake by oceans as anthropogenic heat inputs rise

Abstract A recent many-body quantum approach to thermal radiation (Smith GB, et al 2022 J. Phys. Commun. 6, 065004) reveals that the energy capacitance of heated water is about twice its heat capacitance, due to energy stored as local hybrid pairs of photons coupled resonantly to local matter excitations, with pair density weighted by mass density. Energy exchange between photons and localised dipole excitations inside water at steady temperature T, occurs at sites where a hybridisation potential within the Hamiltonian couples local oscillation modes to ‘free’ photon modes. Photon modes in both directions are phase retarded at exit sides of a hybrid site. Optically sharp variations with frequency in the index of refraction n(f) and photon density of states follow. Exit spectral intensities at equilibrium temperature T carry quantum information on the energy density of matter excitations coupled to photons, while occupied hybrid sites are the source of all ‘free’ photons and internal spectral intensities. The matter excitations coupled locally to photons are distinct from those that define specific heat, but in water both arise from molecular oscillations. The energy capacitance CEX(T) of hybrid pairs depends on mass density and is independent of the heat capacitance CHT(T), which sets temperature T. CEX(T) is sensitive to T and doubles as a capacitance of energy and of ‘quantum information’. Expressions for CEX(T) are derived from first principles and applied to water. The density of occupied hybrids is amplified near surfaces by photons reflected off the surface, which create new hybrid pairs, but not extra heat. Energy capacitance CEX(T)+CHT(T) sets dT(t)/dt prior to a new equilibrium state emerging, after input heating rate dQ/dt is switched. Rising atmospheric intensities then raise T, dQ/dt, CEX(T) and CHT(T). The times available to cool overnight are fixed, so that a rising capacitance, adds to energy stored at an accelerating rate.

Electron Properties of Baicalein and its Derivatives via Quantum Chemistry Calculation: The Effect of Hydroxyl-substitution at A and C Rings

Abstract: The electron properties of baicalein-family are of great importance in influencing its properties and corresponding bioactivities. In this work, we conducted comprehensive quantum chemistry calculations on pristine baicalein, and its two hydroxyl-substituted derivatives where the hydroxylsubstitution respectively occur at A and C rings. By contrasting with each other, the effects of the hydroxyl-substitution on the electron properties were studied from the aspects of the density of states, molecular orbital, electronic excitation, electrostatic potential, and electron delocalization. According to our computation, the hydroxyl-substitution results in variations in geometry and the consequent electron properties among the discussed molecules. Certainly, this research can contribute to the development of the research on the electron involved properties and the structure-property-activity relationship for the baicalein-family.

GA-NIFS: JWST/NIRSpec IFS view of the z ∼ 3.5 galaxy GS5001 and its close environment at the core of a large-scale overdensity

We present JWST Near-Infrared Spectrograph (NIRSpec) observations in integral field spectroscopic (IFS) mode of the galaxy GS5001 at redshift z = 3.47, the central member of a candidate protocluster in the GOODS-S field. The data cover a field of view (FoV) of 4″ × 4″ (∼30 × 30 kpc2) and were obtained as part of the Galaxy Assembly with NIRSpec IFS (GA-NIFS) GTO programme. The observations include both high (R ∼ 2700) and low (R ∼ 100) spectral resolution data, spanning the rest-frame wavelength ranges 3700–6780 Å and 1300–11850 Å, respectively. These observations enable the detection and mapping of the main optical emission lines from [O II]λλ3726, 29 to [S III]λ9531. We analysed the spatially resolved ionised gas kinematics and interstellar medium properties, including obscuration, gas metallicity, excitation, ionisation parameter, and electron density. In addition to the main galaxy (GS5001), the NIRSpec FoV covers a close companion in the south, with three sub-structures with velocities blueshifted by ∼ − 150 km s−1 with respect to GS5001, and another source in the north redshifted by ∼200 km s−1. Optical line ratio diagnostics indicate star formation ionisation and electron densities of ∼500 cm−3 across all sources in the FoV. The gas-phase metallicity in the main galaxy is 12+log(O/H) = 8.45 ± 0.04, and slightly lower in the companions (12+log(O/H) = 8.34 − 8.42), consistent with the mass-metallicity relation at z ∼ 3. We find peculiar line ratios (high log[N II]/Hα = [−0.45, −0.3], low log[O III]/Hβ = [0.06, 0.10]) in the northern part of GS5001. These could be attributed to either higher metallicity, or to shocks resulting from the interaction of the main galaxy with the northern source. We identify a spatially resolved outflow in the main galaxy, traced by a broad symmetric component in Hα and [O III], with an extension of about 3 kpc. We find maximum outflow velocities of ∼400 km s−1, an outflow mass of (1.7 ± 0.4)×108 M⊙, a mass outflow rate of 23 ± 5 M⊙ yr−1, and a mass loading factor of 0.23. These properties are compatible with star formation being the driver of the outflow. Our analysis of these JWST NIRSpec IFS data therefore provides valuable, unprecedented insights into the interplay between star formation, galactic outflows, and interactions in the core of a z ∼ 3.5 candidate protocluster.

Thermally stable Gd4(P2O7)3: Tb3+ phosphor synthesized via co-precipitation: A study on pH-dependent formation and luminescence

A uniformly doped Gd4(P2O7)3: Tb3+ phosphor was synthesized via the co-precipitation method. The influences of pH and annealing temperature on the formation, phase composition, and morphology of the phosphors were thoroughly investigated. The phase structure, morphology, thermal properties, and luminescence properties of the phosphors were characterized using XRD, SEM, FT-IR, TG-DSC, Raman, and photoluminescence (PL) spectroscopy. The Gd4(P2O7)3: Tb3+ phosphors exhibited optimal crystallinity and phase purity after adjusting the pH to 0.86 and annealing at 850 °C. The excitation-emission spectra demonstrated intense green light emission at 543 nm under 274 nm UV excitation, attributed to the 5D4-7F5 transition of Tb3+. The internal quantum efficiency (IQE) of the Gd4(P2O7)3: 2 % Tb3+ sample is 39.10 %. The luminescent intensity of the Gd₄(P₂O₇)₃: 2 % Tb³⁺ phosphor at 491 K retains 94 % of its intensity at 298 K, demonstrating remarkable luminescent thermal stability. The luminescent lifetime of the sample shows negligible change below 441K, remaining approximately 35.6 ± 0.2 ms. These finding confirm the outstanding luminescent thermal stability of Gd₄(P₂O₇)₃: Tb³⁺ phosphors, positioning them as promising candidates for high power density excitation and high temperature lighting applications.

Evolution from a guided-streamer mode to a continuous-discharge mode in an atmospheric pressure argon plasma jet

Atmospheric pressure plasma jet (APPJ) has attracted much attention due to its versatile applications in various fields, which normally operates in either a guided-streamer mode or a continuous-discharge mode. In this paper, temporal evolution is observed from the guided-streamer mode to the continuous-discharge one in an argon APPJ excited by a sinusoidal voltage. Waveforms of voltage and current indicate that the voltage applied to the electrode is distorted severely from the ideal sine. For the guided-streamer mode, there is only one discharge pulse per half voltage cycle. For the mode transition or the continuous-discharge mode, there is only one discharge composed of a pulse and a hump per half voltage cycle. Fast photography implemented for the mode transition reveals that both the discharges in the positive and the negative voltage polarities evolve from the guided-streamer mode to the continuous-discharge one. In the guided-streamer mode of the mode transition, the discharge behaves as a positive streamer in the positive voltage polarity, while a negative streamer in the negative voltage polarity. In addition, the mode transition only occurs in a range of dissipated power and gap width. Based on optical emission spectroscopy, temporal evolutions of the mode transition are investigated for excited electron temperature, electron density, and field strength. Moreover, their spatial distributions along the argon stream are studied at different time instants. These results are of great importance for the discharge dynamics of APPJ.

Using ground state and excited state density functional theory to decipher 3d dopant defects in GaN

Using ground state density functional theory (DFT) and implementing an occupation-constrained DFT (occ-DFT) for self-consistent excited state calculations, we decipher the electronic structure of the Mn dopant and other 3ddefects in GaN across the band gap. Our analysis, validated with broad agreement with defect levels (ground-state calculations) and photoluminescence data (excited-state calculations), mandates reinterpretation and reassignment of 3ddefect data in GaN. The MnGadefect is determined to span stable charge states from (1-) inn-type GaN through (2+) inp-type GaN. The Mn(2+) is predicted to be ad2ground state spin triplet defect with a singlet excited state, isoelectronic with the defect associated with the 1.19 eV photoluminescence inn-type GaN. The combined analysis of defect levels and excited states invites reassessment of alld2-capable dopants in GaN. We demonstrate that the 1.19 eV defect, a candidate defect for optically controlled quantum applications, cannot be the Cr(1+) assumed in literature and instead must be the V(0). The combined ground-state/excited-state DFT analysis is shown to be able to chemically fingerprint defects.