Transient Spectra Research Articles

Symmetry Breaking in an Excited Quadrupolar Acridine-Dione Derivative Driven by Hydrogen Bonding.

An acridine-dione derivative (3,3,11,11-tetramethyl-8,16-diphenyl-3,4,8,10,11,12,13,16-octahydroacridino[4,3-c]acridine-1.9(2H,5H)dion) with quadrupolar motif has been synthesized and its stationary and transient spectra have been measured. Stationary absorption and fluorescence spectra as well as nonstationary spectra show no signs of symmetry breaking (SB) in aprotic solvents, even of high polarity. The specific features of SB are revealed in alcohol solvents through a considerable red shift of stationary fluorescence spectra and the appearance of a new excited state absorption band in transient absorption spectra. SB is due to the formation of asymmetric strong hydrogen bonds, mainly on one side of the molecule. An unexpected regularity of symmetry breaking is found in mixtures of aprotic dimethylformamide and protic methanol, where methanol acts as a fluorescence quencher. It is revealed that there is no quenching as long as the methanol concentration is less than the critical value of 9 M. This leads to the conclusion that SB in such mixtures is possible only if the concentration of the protic solvent exceeds a certain threshold value.

Folding and Unfolding of the Tryptophan Zipper in the Presence of Two Thioamide Substitutions.

We studied the stability and folding and unfolding kinetics of the tryptophan zipper, containing different double thioamide subsitutions. Conformation change was triggered by photoisomerization of an integrated AMPP photoswitch in the turn region of the hairpin, and transient spectra were recorded in the deep UV and the mid-IR, covering the time window of the (un)folding transition from picoseconds to tens of microseconds. Thio-substitution of inward-pointing backbone carbonyls was found to strongly destabilize the β-hairpin structures, whereas molecules with two outward pointing thio-carbonyls showed similar or enhanced stability with respect to the unsubstituted sequence, which we attribute to stronger interstrand hydrogen bonding. Thiolation of the two Trp residues closest to the turn can even prevent the opening of the hairpin after cis-trans isomerization of the switch. The circular dichroism due to the two thioamide ππ* transitions is spectrally well-separated from the aromatic tryptophan signal. It changes upon photoswitching, reflecting a local change in coupling and geometry.

Photoexcited carrier and phonon morphology of InSb observed with an ultrafast pump-probe microscope

The ultrafast pump-probe microscope has been utilized to spatially map the ultrafast dynamics of photoexcited carriers and phonons in InSb with varied carrier concentrations. In this study, the negative and positive components of transient reflectivity-change spectra (ΔR/R) are associated with the dynamics of photoexcited holes and phonons, respectively. The mapping of phonon morphology (i.e. positive ΔR/R on a ps-ns timescale) shows the delocalized electron-phonon coupling in InSb, especially for large carrier concentrations. In contrast, the recombination of electrons and holes is rather localized inside the pumped spot. According to morphological analyses of oscillatory signals in ΔR/R, we found that an electric field applied to InSb can further manipulate the phonon strength and lifetime. For the case of large carrier concentrations, the spatial breadth of phonons has been first observed in InSb.

Singlet Fission in Terrylenediimide Single Crystals: Competition between Biexciton Annihilation and Free Triplet Exciton Formation

Understanding singlet fission (SF) is an important strategy for extending the power conversion efficiency of next-generation solar cells beyond the Shockley–Queisser limit. Thus far, most studies of SF have focused on polycrystalline films, whose heterogeneity often makes it difficult to probe the electronic states involved in the formation of the correlated triplet biexciton state, 1(T1T1), and its decorrelation into free triplet excitons. Polarization-resolved, femtosecond transient absorption microscopy of a 1,6,9,14-tetraphenylterrylene-3,4:11,13-bis(dicarboximide) (Ph4TDI) single crystal reveals the formation of transient spectra in <200 fs having features characteristic of 1(T1T1), 1(S1S0), and Ph4TDI•+-Ph4TDI•– charge-transfer (CT) states. This indicates that either an ultrafast equilibrium between these states occurs or that the initially formed biexciton state is a mixture of these states. This spectrum evolves in time to give the Tn ← T1 spectrum of the free triplet excitons. We show further that this initially formed state has its optical transition polarized nearly perpendicular to that of the Tn ← T1 transition in the free triplet excitons. This indicates that the observed free triplet excitons have migrated from the π-stacked Ph4TDI columns where the 1(T1T1) state is produced to neighboring, nearly orthogonal π-stacked Ph4TDI columns, resulting in a weaker electronic interaction between the free triplet excitons essential to extending their lifetimes. However, the mobility of the 1(T1T1) state in the Ph4TDI π-stacking direction results in significant annihilation of biexciton pairs limiting the yield of free triplet excitons to about 60%.

Mesoporous BiVO4/2D-g-C3N4 heterostructures for superior visible light-driven photocatalytic reduction of Hg(II) ions

In this contribution, a Z-scheme mesoporous BiVO4/g-C3N4 nanocomposite heterojunction with a considerable surface area and high crystallinity was synthesized by a simple soft and hard template-assisted approach. This material demonstrates superior visible light-driven photocatalysis for the photoreduction of Hg(II) ions. TEM and XRD results show that the mesoporous BiVO4 NPs, with a monoclinic phase and an ellipsoid-like shape, are highly dispersed onto the porous 2D surfaces of g-C3N4 nanosheets with a particle size of 5–10 nm. The obtained BiVO4/g-C3N4 nanocomposites with a p-n heterojunction show significantly enhanced Hg(II) photoreduction efficiency compared to the mesoporous BiVO4 NPs and pristine g-C3N4. Among all synthesized photocatalysts, the 1.2% BiVO4/g-C3N4 nanocomposite indicated the highest photoreduction of Hg(II) performance, reaching ~ 100% within 60 min; this result is 3.9 and 4.5 –fold larger than that of the BiVO4 NPs and pristine g-C3N4. The Hg(II) photoreduction rates highly increase to 208.90, 314.95, 411.23 and 418.68 μmol g−1min−1 for the mesoporous 0.4, 0.8, 1.2 and 1.6% BiVO4/g-C3N4 nanocomposites, respectively. The reduction rate of the mesoporous 1.2% BiVO4/g-C3N4 nanocomposite demonstrated a 5.2 and 3.8 times larger increase than that of the pristine g-C3N4 nanosheets and pure BiVO4 NPs. The superior Hg(II) photoreduction efficiency was ascribed to decreased carrier recombination and the improved utilization of visible light by constructing BiVO4/g-C3N4 nanocomposites with a p-n junction. Transient photocurrent measurement and photoluminescence spectra were employed to confirm the possible Hg(II) photoreduction mechanism over these BiVO4/g-C3N4 photocatalysts. This research provides an accessible route for the nanoengineered design of mesoporous BiVO4/g-C3N4 heterostructures that demonstrated unique photocatalytic performance.

Energy transfer and phase operators for inducing extreme wave events from traditional wave models

A simple nonlinear phase operator was recently introduced which when applied to standard sea wave models was able to generate many of the properties of extreme waves. This technique was not able to describe the largest events observed. The methodology is extended to include a second nonlinear operator which transfers part or all of the energy from the original wave spectrum into far more uniform real valued transient spectra. This in combination with the original phase operator was able to describe all sizes of observed extreme wave phenomena and also provided some insights into the nonlinear mechanisms involved in such events.

More Than 50-Fold Enhanced Nonlinear Optical Response of Porphyrin Molecules in Aqueous Solution Induced by Mixing Base and Organic Solvent

The difference absorption spectrum (DAS) of porphyrin molecules (tetraphenyl-porphyhrin sulfonic acid, TPPS) in aqueous solution induced by continuous wave laser irradiation has been reported previously. It was interpreted that the DAS was caused by the formation of TPPS aggregates induced by laser irradiation. However, transient spectra similar in their shape have already been reported and are attributed to the excited-state absorption and saturable absorption (SA) effects due to the triplet state formation in TPPS. In the present study, we investigated the triplet quenching effect by O2 on the DAS of TPPS aqueous solution and revealed that it originated from the triplet state formation. We also found that mixing the appropriate amount of MeOH and NaOH in TPPS aqueous solution increased its absorbance change by more than 50 times. This may be due to the decrease in dissolved oxygen concentration by mixing them. This result suggests the possibility of controlling the performance of NLO materials by adjusting the solvent mixture ratio and base/acid concentration.

Properties of transient spectrum and field purity for a qubit system in squeezed states

The present study investigates the transient spectrum (TS) of a two-level system that interacts with a generalized squeezed state without using the rotating wave approximation (RWA). We consider the purity of the optical field, which is developed in a generalized squeezed state, utilizing the linear entropy. The analytic expressions of the TS and field purity (FP) of the bipartite system are evaluated. Moreover, the study exhibits the influence of the squeezed parameter and some photons transition on the TS and FP. We obtain that the control of the FP may be generated based on an adequate choice of the photons transition and squeezed parameter. Such results can be utilized in the understanding and development of various tasks of quantum physics and optics.

Enhancing photocatalytic performance by direct photo-excited electron transfer from organic pollutants to low-polymerized graphitic carbon nitride with more C-NH/NH2 exposure

A partially low-polymerized g-C3N4 (LCN) has been successfully prepared via a two-step treatment, in which the basic skeleton of g-C3N4 is preserved with more CNH/NH2 exposure. Density functional theory (DFT) calculations indicate that the exposure of CNH/NH2 groups not only introduces an acceptor impurity level, but also strengthens the adsorption interaction between pollutants and LCN by fostering stronger π-π interactions between the aromatic ring of BPA and the N atom in CNC of LCN. Based on the results of UV–vis diffuse reflectance spectra, electron paramagnetic resonance spectra and transient photocurrent response spectra, the adsorbed organic pollutant creates a donor impurity level in the energy band of LCN, endowing LCN with efficient light-harvesting properties. Therefore, in addition to pollutant degradation by HO2/O2−, the direct photo-excited electron transfer from pollutant molecules to conduction band/acceptor impurity level of LCN under visible-light irradiation further enhances the photocatalytic pollutant degradation performance compared with bulk g-C3N4.

The Role of Si Self‐interstitial Atoms in the Formation of Electrically Active Defects in Reverse‐Biased Silicon n+–p Diodes upon Irradiation with Alpha Particles

Results of a study of changes in electrical characteristics of n+–p diodes on boron‐doped epi‐silicon induced by irradiation with alpha particles under applied reverse bias voltages are presented. It is found that the irradiation results in a significantly lower introduction rate of carrier‐compensating radiation‐induced defects (RIDs) in the space charge region of the reverse‐biased structures compared with those in the neutral base region and in similar diodes irradiated without bias. The dominant hole and electron emission signals in the capacitance transient spectra of the irradiated diodes are characterized and identified with energy levels of some known RIDs in moderately doped p‐type Si:B. Changes in concentration of the defects are monitored after postirradiation minority carrier injection and thermal treatments. It is argued that the observed effect of the reduced concentration of RIDs in the space charge regions of the diodes is related mainly to some specific features of the silicon self‐interstitials (ISi): a very strong dependence of their thermal stability on the charge state and a highly enhanced mobility in p‐type Si under minority carrier injection conditions. The activation energy of electron emission from the doubly positively charged state of ISi is determined.

Exploring Photogenerated Molecular Quartet States as Spin Qubits and Qudits.

Photogenerated molecular spin systems hold great promise for applications in quantum information science because they can be prepared in well-defined spin states at modest temperatures, they often exhibit long coherence times, and their properties can be tuned by chemical synthesis. Here, we investigate a molecular spin system composed of a 1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (PDI) chromophore covalently linked to a stable nitroxide radical (TEMPO) by optical and electron paramagnetic resonance (EPR) techniques. Upon photoexcitation of the spin system, a quartet state is formed as confirmed by transient nutation experiments. This quartet state has spin polarization lifetimes longer than 0.1 ms and is characterized by relatively long coherence times of ∼1.8 μs even at 80 K. Rabi oscillation experiments reveal that more than 60 single-qubit logic operations can be performed with this system at 80 K. The large magnitude of the nitroxide 14N hyperfine coupling in the quartet state of PDI-TEMPO is resolved in the transient EPR spectra and leads to a further splitting of the quartet state electron spin sublevels. We discuss the properties of this photogenerated multilevel system, comprising 12 electron-nuclear spin states, in the context of its viability as a qubit for applications in quantum information science.

Effect of Hydrogen on Radiation-Induced Displacement Damage in AlGaN/GaN HEMTs

To explore the role of hydrogen in radiation degradation of AlGaN/GaN high-electron-mobility transistors (HEMTs), the performance degradation of the hydrogen untreated and pretreated devices is compared under the exposure of carbon ions. The energy of carbon ions is chosen as 7.6 MeV, and the maximum fluence reaches 4 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . By electrical character measuring, it is found that the positive shift of the threshold voltage and the decrease of the transconductance occur after to the irradiation. Hydrogen pretreatment accelerates the shift of threshold voltage and the decrease of the transconductance. The threshold voltage of the device is almost unchanged under 1-MeV electron irradiations. This means that the displacement damage leads to the shift of the threshold voltage. The height of Schottky barrier hardly changes in the hydrogen pretreated devices, showing that the ionization defects such as the formation of interface states under the gate are not the main cause of device degradation. Deep level transient spectrum (DLTS) results show that the irradiations cause the increase of gallium vacancies, and the hydrogen pretreatment inhibits its formation. According to the first principle calculation, the existence of hydrogen in GaN layer can reduce the formation energy of and the number of charges of the defects. It is speculated that hydrogen atoms participate in the evolution of radiation defects, which changes the types and number of defects in the devices.

Passivation of thermally-induced defects with hydrogen in float-zone silicon

In this study, passivation of thermally-activated recombination centers with hydrogen in n-type float zone (FZ) Si containing nitrogen has been investigated. Prior to hydrogenation samples were heated to 550 °C using rapid thermal annealing and conventional furnaces. A large decrease in minority carrier lifetime occurred upon the heat-treatments confirming previous reports. A sequence of electron traps created in this process have been detected in the deep level transient spectra and characterized. Significant changes in the spectra have occurred after treatments in remote hydrogen plasma and subsequent annealing of the hydrogenated samples in the temperature range 100 °C–400 °C. A total elimination of electrical activity of the thermally induced defects has been observed in the hydrogenated samples subjected to annealing in the temperature range 150 °C–300 °C. The results obtained suggest a simple way for an effective cure of the degraded FZ-Si-based solar cells. Possible defect reactions occurring in the FZ-Si crystals and the role of nitrogen and carbon upon the performed treatments are discussed.

Light absorption, photocarrier dynamic properties of hierarchical SnS2 microspheres and their performances on photodegradation of high concentration Rhodamine B

Heterogeneous photocatalysis has attracted considerable attention for environmental remediations, artificial photosynthesis, and the performance mainly depends on the light absorption, photocarrier dynamics and surface reactions of photocatalysts. Herein, the photocarrier dynamics of hierarchical SnS2 microspheres were studied by surface photovoltage and transient photovoltage spectra, in which the chemical composition, crystallinity, primary building blocks and assembling structures are affected by the light absorption and photocarrier transfer characteristics of SnS2. Further studies revealed that the enhanced visible light absorption and photocatalytic activity of Red-Embroidery-Ball-like microspheres were derived from the synergistic effects of S self-doping, the exciton transitions of surface states and the light trapping effects of porous hierarchical structure.

Wavelet Spectra of a Chirped Gaussian Pulsed-Driven Harmonic Oscillator

We investigate the nature of spectral lines of emitted radiation due to of a non-dissipative single-mode quantized harmonic oscillator (HO) with train of n-chirped Gaussian pulses. Specifically, we analyze the transient emitted spectrum through two window wavelet functions of the radiation detector, namely the Haar wavelet and Morlet wavelet. Computational display of the exact analytical results shows how the driving pulse parameters (strength and number of pulses, chirping, repetition time) act as control knobs to shape the detected emitted spectrum as desired.

Real-time observation of pulsating period-doubled vector solitons in a passively mode-locked fiber laser.

Dissipative solitons (DSs) are self-organized localized structures in non-conservative systems, which require a continuous energy exchange with external sources. In addition to parameter-invariant stationary DSs, there exists a variety of dynamical ones manifesting breathing behaviors. Such intriguing phenomena, termed as soliton pulsations, have been widely studied in recent years under the impetus of advances in real-time spectroscopy. Here, we experimentally investigate various pulsating period-doubled solitons (PDSs) in a fiber laser mode-locked by single-wall carbon nanotubes. Both single- and double-periodic PDS pulsations are found in the cavity. Thanks to the emerging dispersive Fourier transform technique, the polarization-resolved transient spectra of these pulsating PDSs are measured. It is shown that their polarization ellipses rotate with a period of two cavity roundtrips. Moreover, the intensity-modulation behaviors of the two orthogonal polarization components in the odd (even) roundtrips are always asynchronous, which confirms additional slower polarization modulations. Especially, we demonstrate that three combined intensity-modulation periods are involved in the double-periodic PDS pulsation process for the first time, to the best of our knowledge. Our results would stimulate further research on the vector features of multiple-period pulsating solitons in mode-locked fiber lasers.

Driven Qubit by Train of Gaussian-Pulses

The simplest non-dissipative 2-level atom system, a qubit, excited by a train of resonant n-Gaussian laser pulses is investigated. This concerns examination of the averaged atomic variables, the intensity-intensity correlation function, and the transient fluorescent radiation. Analytical formulas for the above expressions are obtained. Computational results show that the transient spectra with the initial ground and coherent atomic states exhibit asymmetric Mollow structure with dip structure, dense oscillation, and narrowing, and depends on the pulse number (n), the repetition time (τR), and the observed time.

Investigating HOMO Energy Levels of Terminal Emitters for Realizing High‐Brightness and Stable TADF‐Assisted Fluorescence Organic Light‐Emitting Diodes

AbstractBy simple modification of the functional groups on the boron–nitrogen‐containing skeleton, the energy level of the highest occupied molecular orbital (EHOMO) of emitters can be easily adjusted. Blue‐emission derivatives are developed, which are capable of showing small full width at half maximums and high photoluminescence quantum yields. Blue thermally activated delayed fluorescence (TADF)‐assisted fluorescence organic light‐emitting diodes (TAF‐OLEDs) based on two new emitters as the terminal emitter are fabricated, resulting in high external quantum efficiency (EQE) of up to 21.9%, high color purity, and high brightness (Lmax = 63 777 cd m−2). By analyzing the transient electroluminescence spectra of the TAF‐OLEDs, it is found that a smaller EHOMO difference between TADF‐assistant dopant (TADF‐AD) and terminal emitter efficiently helps to decrease hole trapping inside the emitting layer, hence resulting in a lower efficiency rolloff and a longer operational device lifetime. TAF‐OLEDs based on CzBNCz as a terminal emitter having the closest EHOMO to that of TADF‐AD show a maximum EQE of 21.9% together with a reduced efficiency rolloff (EQEs of 21.2% and 19.8% at 100 and 1000 cd m−2, respectively). This research provides a designing principle for a terminal emitter in TAF‐OLEDs with well‐matched energy levels towards reaching the requirements of commercial displays.

Photoinduced Dynamics of (CH3NH3)4Cu2Br6 Thin Films Indicating Efficient Triplet Photoluminescence.

Hybrid organic-inorganic halogenidocuprates based on copper(I) represent materials with rich structural diversity and high photoluminescence (PL) quantum yield, yet the mechanism responsible for their efficient, strongly Stokes-shifted emission is still unclear. Here we report the successful preparation of (CH3NH3)4Cu2Br6 thin films with a zero-dimensional molecular salt structure featuring "isolated" [Cu2Br6]4- ions. Time-resolved broadband PL measurements provide an excited-state lifetime of 114 μs at 298 K. Results from femto- to microsecond UV-vis-NIR transient absorption experiments combined with DFT/TDDFT calculations suggest the formation of a long-lived structurally relaxed triplet species through intersystem crossing (61 ps), which almost exclusively decays by phosphorescence. In addition, time scales for structural relaxation and cooling processes are extracted from a global kinetic analysis of the transient spectra. Calculations for the isolated [Cu2Br6]4- anion and the (CH3NH3)4Cu2Br6 crystal suggest a strong impact of the crystal environment on the structure of the anion.

Surface engineering of hematite nanorods by 2D Ti3C2-MXene: Suppressing the electron-hole recombination for enhanced photoelectrochemical performance

Photoelectrochemical (PEC) water splitting is a promising approach to improve solar energy conversion, but still suffers from poor efficiency due to intrinsic high charge recombination and low conductivity of semiconductor photocatalysts. In this work, we applied Ti3C2-MXene to construct hematite/MXene nanorods (α-Fe2O3/MXene NRs) with enhanced PEC performance. The as-formed MXene-derived Ti-rich layer could not only serve as a passivation layer to significantly suppress surface electron-hole recombination, but also act as Ti source to promote bulk electron transfer. The optimum α-Fe2O3/MXene5/1 NRs resulted in a 7 times PEC enhancement compared with pristine α-Fe2O3 NRs while maintaining high stability. Mechanism studies by in-situ ultrafast transient absorbance spectra (TAS) directly proved that the duration of the photo-induced holes for optimum α-Fe2O3/MXene NRs was 10 times longer than pristine α-Fe2O3 NRs, proving notably enhanced surface charge separation. This study gives a clue to construct series of MXene based anode materials with promoted performance.