Bleaching Signal Research Articles

Role of Thermally Occupied Hole States in Room‐Temperature Broadband Gain in CdSe/CdS Giant‐Shell Nanocrystals

AbstractGrowing CdSe/CdS nanocrystals from a large CdSe core, and employing a giant CdS shell, a continuous, broadband gain spectrum, covering the spectral range between the CdSe and the CdS band edge, is induced. As revealed by k·p calculations, this feature is enabled by a set of closely spaced S‐, P‐ and, for larger CdSe cores, D‐state hole levels, which are thermally occupied at room temperature, combined with a sparse density of electron states. This leads to a range of bleach signals in the transient absorption spectra that persist up to a microsecond. By extending a state‐filling model including relevant higher‐energy states and a Fermi–Dirac distribution of holes at finite temperature, it is shown that thermal occupancy can lower the gain threshold for excited states. Inclusion of Gaussian broadening of discrete transitions also leads to a smoothening of the gain threshold spectrum. Next to a direct measurement of the gain threshold, a method is also developed to extract this from the gain lifetime, taking advantage that population inversion is limited by Auger recombination and recombination rates scale with the exciton density as 〈N〉·(〈N〉 − 1). The results should be readily extendable to other systems, such as perovskite or III–V colloidal nanocrystals.

Ultrafast charge carrier dynamics and photoluminescence of CsPbBr3−xIx quantum dots in boro‐germanate glasses

AbstractCsPbBr3−xIx quantum dots were precipitated in boro‐germanate glasses with various Br/I ratios by the melt‐quenching method. When the heat treatment temperature (TH) increased from 480 to 550°C, more I− entered the perovskite crystal lattice, resulting in lattice expansion, the X‐ray diffraction peaks of CsPbBr1.5I1.5 glasses shifted to small angles. The high concentration of I− entered into the perovskite crystal lattice, decreasing the bandgap. The photoluminescence (PL) peaks of CsPbBr1.5I1.5 glasses redshifted from 577 to 620 nm, the PL peaks of CsPbBrI2 glasses shifted from 614 to 636 nm, and the PL peaks of CsPbBr0.5I2.5 glasses shifted from 650 to 672 nm. As the ratio of Br/I reduced from 2.5/0.5 to 0.5/2.5, the PL lifetime of CsPbBr3−xIx glass increased from 2.08 to 74.5 ns. With the delay time from 1 to 1500 ps, the carriers gradually accumulated in the band‐edge state, and the bleaching signal intensity increased. Then more and more carriers returned to the ground state through radiative or nonradiative recombination, the bleaching signal intensity decreased.

Self‐Trapped and Free Exciton Dynamics in Vacuum‐Deposited Cesium Copper Iodide Thin Films

AbstractHighly photoluminescent, lead‐free perovskites are of interest for displays and solid‐state light‐emitting devices. In this report, streak camera‐based time‐resolved emission and transient absorption spanning visible to deep‐ultraviolet (UV) wavelengths are utilized to study self‐trapped and free exciton dynamics in vacuum‐deposited cesium copper halide thin films of CsCu2I3 and Cs3Cu2I5. Self‐trapped exciton emission of CsCu2I3 exhibits more noticeable changes with time in the peak position and width than Cs3Cu2I5. UV‐to‐blue emission is detectable for both compositions, where free exciton emission is distinct for CsCu2I3. Transient absorption shows loss of ground‐state bleach signals at early time delays for both, and the bleach signal shifts toward higher energy as time delay increases, likely due to strains induced by the newly created self‐trapped excitons. Global analysis performed on the transient absorption results yields time constants in these materials that build an overall dynamic scheme. This work aids in building a complete picture regarding light emission in these promising materials.

Vibrational Dynamics of the Intramolecular H-Bond in Acetylacetone Investigated with Transient and 2D IR Spectroscopy.

Acetylacetone (AcAc) has proven to be a fruitful but highly challenging model system for the experimental and computational interrogation of strong intramolecular hydrogen bonds. Key questions remain, however, regarding the identity of the minimum-energy structure of AcAc and the dynamics of intramolecular proton transfer. Here, we investigate the OH/OD stretch and bend regions of the enol tautomer of AcAc and its deuterated isotopologue with transient absorption and 2D IR spectroscopy. The OH bend region reveals a single dominant diagonal transition near 1625 cm-1 with intense cross peaks to lower-frequency modes, demonstrating highly mixed fingerprint transitions that contain OH bend character. The anharmonic coupling of the OH bend results in a highly elongated OH bend excited-state absorption transition that indicates a large manifold of OH bend overtone/combination bands in the OH stretch region that leads to strong bend-stretch Fermi resonance interactions. The OH and OD stretch regions consist of broad ground-state bleach signals, but there is no clear evidence of ω21 excited-state absorptions due to rapid population relaxation arising from strong intramolecular coupling to bending, fingerprint, and low-frequency H-bond modes. Orientational relaxation dynamics persist for timescales longer than the vibrational lifetimes, with polarization anisotropy components decaying within approximately 2 and 10 periods of the O-O oscillation for the OH and OD stretch, respectively. The significant isotopic dependence of the orientational dynamics is discussed in the context of intramolecular mode coupling, diffusional processes, and contributions from proton/deuteron transfer dynamics.

Excitation-Energy-Dependent Photoluminescence Quantum Yield is Inherent to Optically Robust Core/Alloy-Shell Quantum Dots in a Vast Energy Landscape.

The importance of alloy-shelling in optically robust Core/Alloy-Shell (CAS) QDs has been described from structural and energetic aspects. Unlike fluorescent dyes, both Core/Shell (CS) and CAS QDs exhibit excitation-energy-dependent photoluminescence quantum yield (PLQY). For both CdSe and InP CAS QDs (with metal- and nonmetal-based alloy-shelling, respectively), with increasing excitation energy, (a) the ultrafast rise-time or relaxation-time to the band-edge increases and (b) the magnitude of the normalized bleach signal decreases. Ultrasensitive single-particle spectroscopic investigation results showed that with decreasing excitation energy, (a) the fraction of ON events increases, (b) the ratio of exciton-detrapping rate/trapping rate increases, and (c) the extent of beneficial hole trapping increases. A relative decrease in PLQY with increasing excitation energy is much less pronounced in CAS QDs than in CS QDs. Unless trap states are removed completely especially in the higher-energy landscape, PLQY will remain inherently dependent on excitation energy for QDs in the vast energy landscape. When reporting the PLQY of QDs, the magnitude of the excitation energy must be mentioned.

Dynamics of Strong Coupling Between Free Charge Carriers in Organometal Halide Perovskites and Aluminum Plasmonic States.

We have investigated a strong coupled system composed of a MAPbIxCl3-x perovskite film and aluminum conical nanopits array. The hybrid states formed by surface plasmons and free carriers, rather than the traditional excitons, is observed in both steady-state reflection measurements and transient absorption spectra. In particular, under near upper band resonant excitation, the bleaching signal from the band edge of uncoupled perovskite was completely separated into two distinctive bleaching signals of the hybrid system, which is clear evidence for the formation of strong coupling states between the free carrier–plasmon state. Besides this, a Rabi splitting up to 260 meV is achieved. The appearance of the lower bands can compensate for the poor absorption of the perovskite in the NIR region. Finally, we found that the lifetime of the free carrier–SP hybrid states is slightly shorter than that of uncoupled perovskite film, which can be caused by the ultrafast damping of the SPs modes. These peculiar features on the strong coupled hybrid states based on free charge carriers can open new perspectives for novel plasmonic perovskite solar cells.

Enhancing Intersystem Crossing by Intermolecular Dimer-Stacking of Cyanine as Photosensitizer for Cancer Therapy

Enhancing Intersystem Crossing by Intermolecular Dimer-Stacking of Cyanine as Photosensitizer for Cancer Therapy

Transient Reflection Spectroscopy on Ultrafast Interlayer Charge Transfer Processes in a MoS2/WSe2van der Waals Heterojunction

Understanding the dynamical process of charge transfer (CT) across van der Waals heterostructures (VdWHs) is vitally important for improving photovoltaic conversion efficiency at the atomically thin interfaces. Here, ultrafast dynamics of VdWHs constructed by vertically stacking monolayer molybdenum disulfide (MoS2) onto tungsten diselenide (WSe2) with a multilayer hexagonal boron nitride (hBN) substrate were investigated by transient reflection measurements. MoS2 and WSe2 were selectively excited to obtain direct evidence of the CT processes. It is interesting to note that the bleaching band of MoS2/WSe2 is composed of two different contributions with different dynamics under excitation at 650 nm: direct excitation-induced bleaching of WSe2 (with a decay time constant of ∼63 ps) and bleaching signals arising from photo-induced hole transfer (with a decay time constant of ∼303 ps). The interlayer CT process was found to occur on the subpicosecond time scale, resulting in long-lived interlayer spatially separated charge carriers (approximately hundreds of picoseconds). These results imply potentially high photovoltaic conversion efficiency of these heterostructures.

Pulse energy and wavelength-dependent ultrafast dynamics of SnSe2 thin film studied by transient absorption

The ultrafast carrier dynamics in SnSe2 film and their influencing factors are studied using femtosecond transient absorption (TA) spectroscopy. Nano-scale SnSe2 sample films are prepared by pulsed laser deposition on the SiO2 substrate. The observed carrier dynamics are influenced by the film thickness and pulse energy. All carrier ultrafast dynamics can be perfectly fitted with a tri-exponential decay model, which means that the carriers have undergone the process of carrier scattering, carrier recombination and diffusion. The influence of the surface reorganization process and the deep defect state in the material makes the carrier lifetime longer in the thicker film. Moreover, the defect state intensifies the state filling effect, leading to the production of bleaching signal in thin thinner film. With the increase of the pump pulse energy, the carrier lifetime becomes longer. This is due to the influence of the pulse energy on the carrier concentration and the temperature difference between the carrier temperature and the lattice temperature. And the influence of pump pulse energy of band gap reconstruction makes the peak in TA spectrum move with the change of pulse energy. The results reveal the carrier dynamics in SnSe2 thin film at different pump pulse energy and identify the critical role of the pump pulse in photo-generated phenomena.

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

Enormous Promotion of Photocatalytic Activity through the Use of Near-Single Layer Covalent Organic Frameworks

Ultrafast Dynamics in Cu-Deficient CuInS2 Quantum Dots: Sub-Bandgap Transitions and Self-Assembled Molecular Catalysts

The photophysical properties of Cu-deficient Cu0.2In1Sx quantum dots synthesized through a facile aqueous-based procedure have been investigated. Transient absorption experiments were carried out probing in the UV–vis, near-IR, and mid-IR regions, with the aim to (i) study the photophysical properties of the quantum dots and (ii) monitor kinetics of electron transfer to a molecular catalyst. When pumping sub-bandgap transitions, negative (bleach) signals were observed that were spectrally and kinetically distinct from those observed with bandgap pump wavelengths. Herein, these distinct contributions are suggested to result from the overlapping bleaching of state filling electrons and trapped holes. Such an interpretation highlights the importance of considering the hole-contributions to the bleach for the proper determination of carrier kinetics in similar systems. A model complex of the [Fe2]-hydrogenase active site was introduced to explore the potential of the quantum dots as photosensitizers for molecular catalysts. The quantum dot photoluminescence was quenched upon catalyst addition, and direct evidence of the singly reduced catalyst was found by transient absorption in the UV–vis and mid-IR. The catalyst accepted reducing equivalents on a subpicosecond time scale upon photoexcitation of the quantum dots, despite no covalent linking chemistry being applied. This implies that charge transfer is not limited by diffusion rates, thus confirming the presence of spontaneous quantum dot and catalyst self-assembly.

Shallow Oxygen Substitution Defect to Deeper Defect Transformation Mechanism in Ta3N5 under Light Irradiation.

Defects are ubiquitous in semiconductors and critical to photo(electro)chemical performance, but the change of defect properties under light irradiation remains poorly understood. Herein, we studied defect change properties of Ta3N5 with transient absorption (TA) spectroscopy. A broad transient absorption (>650 nm) was observed and attributed to trapped electrons in oxygen impurities (substitution oxygen at nitrogen sites, ON), and two bleach signals at 510 and 580 nm were obtained and ascribed to free holes of Ta3N5. The charge recombination between the trapped electrons and the free holes is sensitively related to ON defects. The trap-detrapping recombination is retarded by increase of excitation intensity, which is contrary to the normal dependence of charge dynamics on excitation intensity. This abnormal dependence indicates that shallow ON• (singly positive charge states) defects of Ta3N5 transform to deeper ON× (neutral charge states) defects under strong light irradiation. The defect transformation results in long-lived free holes in Ta3N5 for photo(electro)catalysis.

Singlet fission in nanoaggregate of bis(phenylethynyl) derivative of benzene (BPEB): High energy triplet exciton generation with >100 % yield

1,4-bis(phenylethynyl)benzene (BPEB) is observed to show a prominent ∼0.17 eV red shift of absorption band due to monomer-aggregate conversion. Along with the absorption band shift it also causes nearly one order reduction in both emission yield and singlet lifetime. The ultrafast transient absorption measurements show annihilation free decay of singlet excitons with ∼40 ps lifetime for this nanoaggregate. Concomitant growth of triplet excitonic absorption and ground state bleach signal with similar time constant ensures presence of singlet fission (SF) process in the present system. Flash photolysis study indicates separated triplet exciton yield of ∼139 % and its decay with ∼4.4 ms lifetime. Observation of triplet-triplet annihilation (TTA) and resultant delayed fluorescence process in this NA also confirms the near-isoergic or endoergic nature of SF process. It also indicates the presence of high energy triplet excitonic state which can be utilized to boost the silicon solar cell efficiency or sensitize other materials having application in photodynamic therapy or near-IR phosphorescence based bio-imaging.

Observation of quantum-confined exciton states in monolayer WS2 quantum dots by ultrafast spectroscopy.

Monolayer transition metal dichalcogenide quantum dots (TMDC QDs) could exhibit unique photophysical properties, because of both lateral quantum confinement effect and edge effect. However, there is little fundamental study on the quantum-confined exciton dynamics in monolayer TMDC QDs, to date. Here, by selective excitations of monolayer WS2 QDs in broadband transient absorption (TA) spectroscopy experiments, the excitation-wavelength-dependent ground state bleaching signals corresponding to the quantum-confined exciton states are directly observed. Compared to the time-resolved photophysical properties of WS2 nanosheets, the selected monolayer WS2 QDs only show one ground state bleaching peak with larger initial values for the linear polarization anisotropy of band-edge excitons, probably due to the expired spin-orbit coupling. This suggests a complete change of the band structure for monolayer WS2 QDs. In the femtosecond time-resolved circular polarization anisotropy experiments, a valley depolarization time of ∼100 fs is observed for WS2 nanosheets at room temperature, which is not observed for monolayer WS2 QDs. Our findings suggest a strong state-mixing of band-edge valley excitons responsible for the large linear polarization in monolayer WS2 QDs, which could be helpful for understanding the exciton relaxation mechanisms in colloidal monolayer TMDC QDs.

不同核心尺寸及壳层厚度CdTe/CdS量子点非线性光学及超快动力学特性

CdTe core-shell semiconductor quantum dots are being widely explored due to their special nonlinear optics and ultrafast dynamics characteristics that include solar cells, optoelectronic devices, biological labeling, and optics fiber sensing fields. In this work, the six kinds of CdTe/CdS core-shell quantum dots were researched in the different core sizes and shell thicknesses for the nonlinear optics and ultrafast dynamics characteristics. The nonlinear absorption and refraction coefficients of the samples were measured by using Z-scan technology under the action of 400 nm wavelength and 130 fs laser pulse width. The experimental results show that the shell thickness of CdTe/CdS core-shell quantum dots affects the nonlinear absorption and refraction characteristics, in which the nonlinear absorption and refraction coefficients increase with the shell thickness. And the core size mainly affects the nonlinear absorption characteristics, while the nonlinear absorption coefficient decreases with the increase of the core size. At the same time, femtosecond time-resolved transient absorption spectroscopy technology was used to measure the ultrafast dynamics characteristics of the samples under the conditions of 400 nm wavelength, 130 fs pulse width, 1 kHz frequency, and 400 nJ single-pulse energy. The transient absorption spectra and ultrafast dynamics curves were obtained. The results suggest that the rising time of bleaching signal increases with the shell thickness. The decay time of the fast process increases with the shell thickness and core size. The decay time of the slow process increases with the shell thickness. The research reveals the influence of the core size and shell thickness of CdTe core-shell quantum dots on the nonlinear optics and ultrafast dynamics, providing a theoretical basis for the preparation of core-shell quantum dots and the research of the photophysical properties.

Probing the primary processes of a triazido-cobalt(III) complex with femtosecond vibrational and electronic spectroscopies. Photochemical selectivity and multi-state reactivity.

The elementary dynamics following 355 nm-excitation of the complex, mer-[Co(dien)(N3)3], were studied in liquid dimethyl sulfoxide (DMSO) solution using femtosecond-ultraviolet-pump/mid-infrared-to-near-ultraviolet probe spectroscopy in conjunction with electronic structure calculations based on density functional theory. Following the initial N3--to-Co charge transfer excitation, the parent complex undergoes an ultrafast metal-to-ligand back electron transfer (BET) within 2 ps thereby populating a metal-centered singlet excited state, 1MC, which can either repopulate the electronic ground state or cleave an azido ligand from the ligand sphere surrounding the metal center. From the asymptotic ground-state bleaching signal after 1 ns, a primary quantum yield for ligand loss of ca. 13% is estimated. The IR-spectrum of the product demonstrates that the photodissociation occurs selectively from the equatorial binding site thereby leading exclusively to the solvolysis product, mer-trans-[Co(dien)(N3)2(DMSO)]+, which features the solvent ligand in the equatorial coordination plane and the azides in the two axial positions. The remarkable photochemical selectivity is traced back to the initial BET and the nature of the intermediate state, 1MC, whose electronic structure entails occupancy of the σ-antibonding d(x2-y2)-orbital. A stereochemical scrambling at the stage of the primary penta-coordinated diazido product is kinetically inhibited on the singlet surface by an energy barrier of roughly 27 kJ mol-1. Primary penta-coordinated products that may be born on the triplet surface are funneled to their singlet ground-state preferentially from geometries with trans-oriented azido ligands thereby also preventing a stereochemical isomerization that could possibly arise from an intersystem crossing.

Structural, Spectroscopic, and Excitonic Dynamic Characterization in Atomically Thin Yb3+‐Doped MoS2, Fabricated by Femtosecond Pulsed Laser Deposition

AbstractThe large area deposition and synthesis of 10 mm × 10 mm atomically thin Yb3+‐doped MoS2 films by femtosecond pulsed laser deposition on a silica glass optical platform for device applications are demonstrated for the first time. The presence of Yb3+‐ion doping is confirmed using photoluminescence (PL), X‐ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The Yb3+‐doped MoS2 films, when excited with a 976 nm laser, exhibit room temperature PL with a peak at 1002 nm. The XPS and Raman spectroscopic analyses of the Yb3+‐doped and undoped films show that the deposited films are a mixture of 2H‐ and 1T‐MoS2 after postdeposition annealing at 500 °C. The density functional theory analysis shows that the 1T phase is metastable by +77 kJ (≈0.8 eV) mol‐1, when compared with the 2H state at 0 K. Ultrafast transient nonlinear optical spectroscopic measurements prove that the saturable absorption of undoped MoS2 is significantly modified after Yb3+‐ion doping, by displaying dopant‐host structure charge transfer. The complex transient absorption line shape shows a combination of bleach (negative) signals at the A (670 nm) and B (630 nm) exciton energies, and a strong induced absorption below the A exciton level. The results presented herein provide critical insight in designing novel rare‐earth‐ion doped 2D materials and devices.

Photo-excitation dynamics of N, N-dimethyl-p-nitroaniline in ionic liquids: Effect of cation alkyl-chain length

Transient absorption spectroscopy has been applied to study the photo-excitation dynamics of N, N-dimethyl-p-nitroaniline in twelve imidazolium/phosphonium cation-based ionic liquids (ILs) with alkyl chains of different lengths. The transient absorption spectra showed an initial bleach signal around the absorption center (400 nm) and a stimulated emission band around 500 nm. The bleach signal was recovered mostly within a few picoseconds, and a hot band absorption appeared around 450 nm. The time profiles of the band integrals for the bleach and absorption spectral regions were simulated by a sum of four exponential decays. The internal conversion rate corresponding to the back electron transfer (b-ET) rate from the charge-separated excited state, was determined by the time profile of the band integral of the bleach region. The vibrational energy relaxation (VER) rate in the ground state was determined from the time profile of the hot band absorption decay band integral. The b-ET rate decreased with increasing number of alkyl carbons in the cation (#C). The solvent dependence was discussed in terms of the free energy change at the Marcus inverted region, which was indicated by the blue-shift of the ground state absorption band. The change of the free energy is ascribed to the change of the charge concentration of ILs. The VER rate decreased slightly with increasing #C. The reason was discussed in terms of the interaction of DMpNA with the polar region of ILs. The intersystem crossing yield also increased with increasing #C, reflecting an increase in the non-polar region of the ILs.

Spectroscopic Evidence for the Contribution of Holes to the Bleach of Cd-Chalcogenide Quantum Dots.

In transient absorption (TA) measurements on Cd-chalcogenide quantum dots (QDs), the presence of a band-edge (BE) bleach signal is commonly attributed entirely to conduction-band electrons in the 1S(e) state, neglecting contributions from BE holes. While this has been the accepted view for more than 20 years, and has often been used to distinguish electron and hole kinetics, the reason for the absence of a hole contribution to the BE-bleach has remained unclear. Here, we show with three independent experiments that holes do in fact have a significant impact on the BE-bleach of well-passivated Cd-chalcogenide QD samples. Transient absorption experiments on high photoluminescence quantum yield CdSe/CdS/ZnS core–shell–shell QDs clearly show an increase of the band-edge bleach as holes cool down to the band edge. The relative contribution of electron-to-hole bleach is 2:1, as predicted by theory. The same measurements on core-only CdSe QDs with a lower quantum yield do not show a contribution of holes to the band-edge bleach. We assign the lack of hole bleach to the presence of ultrafast hole trapping in samples with insufficient passivation of the QD surface. In addition, we show measurements of optical gain in core–shell–shell QD solutions, providing clear evidence of a significant hole contribution to the BE transient absorption signal. Finally, we present spectroelectrochemical measurements on CdTe QDs films, showing the presence of a BE-bleach for both electron and hole injections. The presence of a contribution of holes to the bleach in passivated Cd-chalcogenides QDs bears important implications for quantitative studies on optical gain as well as for TA determinations of carrier dynamics.

Insights into the Femtosecond to Nanosecond Charge Carrier Kinetics in Perovskite Materials for Solar Cells

In this work, we analyze a few simple methods that permit getting insight into the charge dynamics in methylammonium lead iodide (MAPbI3) perovskites, on the basis of femtosecond transient absorption and fluorescence studies. We show that proper determination of the charge population decay kinetics requires broad spectral integration of the long-wavelength bleach signal (∼760 nm) assigned to the band-filling effect, which can be realized, e.g., by band integral analysis. On the contrary, the kinetics related to a shorter-wavelength bleach band (∼480 nm) behave differently: a single-wavelength decay is sufficient to probe charge population. Moreover, both the shorter-wavelength excitation and higher pump pulse intensities result in significantly longer charge cooling times, which might be important in efficient hot carrier extraction. The validity of our previously proposed easy approach for the elimination of higher-order recombination rates is also discussed and confirmed in terms of proper estimation of...