Time-resolved Decay Research Articles

Unusual Temperature-Dependent Photoluminescence Due to Competing Excited-State Energy Transfer in Rare Earth Double Perovskites.

Lead-free halide double perovskites (DPs) have become a research hotspot in the field of photoelectrons due to their unique optical properties and flexible compositional tuning. However, the luminescence of DPs exhibits thermal quenching at high temperatures, which severely affects their further application. Herein, we synthesized the rare earth Dy3+ and transition metal Mn2+ codoped Cs2NaYCl6 rare earth DPs and characterized the optical properties using temperature-dependent photoluminescence spectra and time-resolved photoluminescence decay profiles at different temperatures. The Mn2+-Dy3+:Cs2NaYCl6 DPs exhibit stronger luminescence with increasing temperature, attributed to antithermal quenching. The successful incorporation of Mn dopants in the DPs lattice results in suppression of nonradiative recombination, more efficient energy transfer (ET) to Dy3+, and a reduced lattice distortion. These kinds of DPs with near-infrared luminescence have promising prospects in applications such as night vision, radiation detection, solid-state lighting, and optical temperature measurement.

Dynamics of Photoinduced Charge Carriers in Metal-Halide Perovskites.

The measurement and description of the charge-carrier lifetime (τc) is crucial for the wide-ranging applications of lead-halide perovskites. We present time-resolved microwave-detected photoconductivity decay (TRMCD) measurements and a detailed analysis of the possible recombination mechanisms including trap-assisted, radiative, and Auger recombination. We prove that performing injection-dependent measurement is crucial in identifying the recombination mechanism. We present temperature and injection level dependent measurements in CsPbBr3, which is the most common inorganic lead-halide perovskite. In this material, we observe the dominance of charge-carrier trapping, which results in ultra-long charge-carrier lifetimes. Although charge trapping can limit the effectiveness of materials in photovoltaic applications, it also offers significant advantages for various alternative uses, including delayed and persistent photodetection, charge-trap memory, afterglow light-emitting diodes, quantum information storage, and photocatalytic activity.

Infrared Fingerprint and Unimolecular Decay Dynamics of the Hydroperoxyalkyl Intermediate (•QOOH) in Cyclopentane Oxidation.

A transient carbon-centered hydroperoxyalkyl intermediate (•QOOH) in the oxidation of cyclopentane is identified by IR action spectroscopy with time-resolved unimolecular decay to hydroxyl (OH) radical products that are detected by UV laser-induced fluorescence. Two nearly degenerate •QOOH isomers, β- and γ-QOOH, are generated by H atom abstraction of the cyclopentyl hydroperoxide precursor. Fundamental and first overtone OH stretch transitions and combination bands of •QOOH are observed and compared with anharmonic frequencies computed by second-order vibrational perturbation theory. An OH stretch transition is also observed for a conformer arising from torsion about a low-energy CCOO barrier. Definitive identification of the β-QOOH isomer relies on its significantly lower transition state (TS) barrier to OH products, which results in rapid unimolecular decay and near unity branching to OH products. A benchmarking approach is utilized to compute high-accuracy stationary point energies, most importantly TS barriers, for cyclopentane oxidation (C5H9O2), building on higher level reference calculations for ethane oxidation (C2H5O2). The experimental OH product appearance rates are compared with computed statistical microcanonical rates using RRKM theory, including heavy-atom tunneling, thereby validating the computed TS barrier. The results are extended to thermal unimolecular decay rate constants at temperatures and pressures relevant to cyclopentane combustion via master-equation modeling. The various torsional and ring puckering states of the wells and transition states are explicitly considered in these calculations.

Control of Hybrid Exciton Lifetime in MoSe2/WS2 Moiré Heterostructures.

Hybrid excitons, characterized by their strong oscillation strength and long lifetimes, hold great potential as information carriers in semiconductors. They offer promising applications in exciton-based devices and circuits. MoSe2/WS2 heterostructures represent an ideal platform for studying hybrid excitons, but how to regulate the exciton lifetime has not yet been explored. In this study, layer hybridization is modulated by applying electric fields parallel or antiparallel to the dipole moment, enabling us to regulate the exciton lifetime from 1.36 to 4.60ns. Furthermore, the time-resolved photoluminescence decay traces are measured at different excitation power. A hybrid exciton annihilation rate of 8.9 × 10-4cm2s-1 is obtained by fitting. This work reveals the effects of electric fields and excitation power on the lifetime of hybrid excitons in MoSe2/WS2 1.5° moiré heterostructures, which play important roles in high photoluminescence quantum yield optoelectronic devices based on transition-metal dichalcogenides heterostructures.

Enhanced Fluorescence Based on Slow Light Effect of ZIF-8 Photonic Crystals for Trace 2,4,6-Trinitrophenol Detection.

In response to growing concerns about public safety and environmental conservation, it is essential to develop a precise identification method for trace explosives. To improve the stability and detection sensitivity of perovskite quantum dots (PQDs) and address the issue of low porosity in traditional polymer-based photonic crystals (PhCs), this study proposed a PQD photoluminescence (PL) enhancement strategy based on the slow light effect of ZIF-8 PhCs for highly sensitive, selective, and convenient detection of 2,4,6-trinitrophenol (TNP). The slow light effect at the photonic band gap edge is the basis of amplifying the PL signal. PhCs were fabricated by the evaporation-induced self-assembly method. The diffraction wavelength overlapping the whole visible region was designed to match the emission wavelength of PQDs. Results showed that PhCs matching the PBG edge with PQDs' emission peak amplified the PL signal 11.3 times, significantly improving sensitivity for trace TNP detection with a limit as low as 2.52 nM. Moreover, there was a 13.3-fold enhancement of PQDs' fluorescence lifetime when the emission wavelength fell in the PBG range. The hydrophobic surface of ZIF-8 PhCs enhanced the PQDs' stability and moisture resistance. Furthermore, the selective quenching mechanism of TNP by the sensor was photoinduced electron transfer (PET) verified by DFT calculations and time-resolved PL decay dynamics measurements. This study demonstrated great potential for manipulating light emission enhancement by PhCs in developing efficient fluorescent sensors for trace environmental pollutant detection.

Enhancing Optical and Thermal Stability of Blue-Emitting Perovskite Nanocrystals through Surface Passivation with Sulfonate or Sulfonic Acid Ligands.

This study aims to enhance the optical and thermal properties of cesium-based perovskite nanocrystals (NCs) through surface passivation with organic sulfonate (or sulfonic acid) ligands. Four different phenylated ligands, including sodium β-styrenesulfonate (SbSS), sodium benzenesulfonate (SBS), sodium p-toluenesulfonate (SPTS), and 4-dodecylbenzenesulfonic acid (DBSA), were employed to modify blue-emitting CsPbBr1.5Cl1.5 perovskite NCs, resulting in improved size uniformity and surface functionalization. Transmission electron microscopy and X-ray photoelectron spectroscopy confirmed the successful anchoring of sulfonate or sulfonic acid ligands on the surface of perovskite NCs. Moreover, the photoluminescence quantum yield increased from 32% of the original perovskite NCs to 63% of the SPTS-modified ones due to effective surface passivation. Time-resolved photoluminescence decay measurements revealed extended PL lifetimes for ligand-modified NCs, indicative of reduced nonradiative recombination. Thermal stability studies demonstrated that the SPTS-modified NCs retained nearly 80% of the initial PL intensity when heated at 60 °C for 10 min, surpassing the performance of the original NCs. These findings emphasize the optical and thermal stability enhancement of cesium-based perovskite NCs through surface passivation with suitable sulfonate ligands.

Tm3+/Yb3+:NaGdF4 nanoparticles decorated g-C3N4/BiOBr0.75I0.25 multicomponent heterostructure: Structural, optical properties and UV–Visible-NIR responsive photocatalytic degradation of Rhodamine B and reduction of Cr(VI)

A wide solar spectral range responsive multicomponent heterostructure photocatalyst NGT/g-C3N4/BiOBr0.75I0.25 was successfully synthesized and used for the UV–visible-NIR photodegradation of the dye Rhodamine B and photoreduction of highly toxic hexavalent chromium ions. Our study shows that the heterostructure overcomes the drawbacks of limited visible-light absorption and fast charge recombination of g-C3N4. Herein, the incorporation of BiOBr0.75I0.25 extends the absorption in the visible region and endorses the separation rate of photogenerated charge carriers. Additionally, the upconversion nanoparticles NaGdF4: Tm/Yb (NGT) with its higher infrared light absorption capacity and characteristic emission spectral overlapping with absorption range of g-C3N4/BiOBr0.75I0.25 nanocomposite, further facilitates the photocatalytic activity of the NGT/g-C3N4/BiOBr0.75I0.25 heterostructured nanocomposite. Extensive structural, morphological, and compositional analysis was done using XRD, TEM, SEM, FE-SEM, XPS, and FTIR measurements. The PL emission, time-resolved decay, and electrochemical impedance spectroscopy were used to probe the separation rate of photogenerated electron-hole pairs and efficient energy transfer in NGT/g-C3N4/BiOBr0.75I0.25 heterostructured nanocomposite. The possible photocatalytic degradation mechanism was comprehensively proposed through a combination of UV–Vis absorption, Mott Schottky, and scavenger test analysis. The photodegradation efficiency of the heterojunction was 98 %, which is much higher than the bare g-C3N4 (74 %). Moreover, the heterostructure exhibited excellent long-term reusability, maintaining the photodegradation efficiency of 97.02 % even after four consecutive degradation cycles, establishing it as a preferable material for photocatalytic dye degradation. Additionally, NGT/g-C3N4/BiOBr0.75I0.25 heterostructured nanocomposite also demonstated better performance for the photoreduction of Cr(VI) to Cr(III), achieving photocatalytic efficiency of 98.86 % under acidic condition.

Green synthesis of fluorescent carbon dots from ascorbic acid and their application in sensing and biological imaging

A simple, highly efficient, and environmentally friendly one-step method has been developed to synthesize fluorescent carbon dots (CDs) with the assistance of hydrogen peroxide (H2O2). Here H2O2 acts as a green accelerator to promote the preparation of CDs, while temperature plays a critical role in controlling the entire synthesis process. The prepared CDs demonstrate monodispersion, with an average diameter of ∼6.8 nm, superior stability, and excitation-dependent photoluminescent (PL) behavior. The PL mechanism of the CDs was investigated using time-resolved PL decay, revealing that the PL originates from both the defect state and the intrinsic state of the CDs. Furthermore, the CDs show significant promise as pH sensors and versatile nanothermometry devices, owing to their remarkable sensitivity to both pH and temperature, as demonstrated by the pronounced dependence of their steady-state fluorescence emission spectra. It’s worth noting that temperature and pH sensors present significant reversibility, sensitivity, and linearity. More importantly, the CDs display low cytotoxicity, good biocompatibility, and the ability to enter human breast cancer Bcap-37 cells, making them highly suitable candidates for cellular imaging and labeling.

Luminescent properties and dosimetric behavior of Eu-ion-doped ASO4 (A = Ca, Sr, and Ba)

We investigated the luminescence properties and dosimetric behavior of Eu-ion-doped ASO4 (A = Ca, Sr, or Ba) phosphors synthesized using the coprecipitation method. Structural analysis of the phosphors was performed using X-ray diffraction patterns, which revealed that the prepared samples exhibited a pure phase with diffraction peaks matching those of undoped orthorhombic structures. The luminescence properties of the phosphors were examined by measuring photoluminescence and photoluminescence excitation spectra. We found that both Eu2+ and Eu3+ emissions were detected for A = Ba and Ca, whereas only Eu2+ emission was observed for A = Sr. Further, time-resolved photoluminescence decay curves indicate the presence of Eu2+ and Eu3+ luminescence with different lifetimes. The optically stimulated luminescence (OSL) and thermoluminescence (TL) responses of the samples were investigated. The OSL and TL responses were found to vary linearly with the irradiation dose. This study sheds light on the potential applications of Eu-ion-doped ASO4 phosphors in radiation dosimetry and optoelectronic devices.

Analysis of defects dominating carrier recombination in CeO2 single crystal for photocatalytic applications

In photocatalytic water splitting and CO2 reduction, recently, cerium oxide (CeO2) has been widely investigated. Defects that can directly dominate carrier recombination are essential for the photocatalytic performance of CeO2. In this study, several photoluminescence (PL) peaks are observed on the (100) face of an undoped CeO2 single crystal, indicating the presence of defects. Moreover, we characterize carrier recombination using time-resolved photoluminescence (TR-PL) and microwave photoconductivity decay (μ-PCD) measurements. The temperature dependence of decay curves is the result of carrier trapping and emission at deep levels. These decay curves are observed separately using a 565 nm band-pass filter (BPF) based on the PL spectra. The trap energy level ( ET ) and majority carrier capture cross-section ( σT ) of each defect are also analyzed using rate equations to fit the experimental results. The temperature-dependent time constants are well reproduced by a recombination model using hole traps HTinfrared and HTvisible at ET of 0.76 and 0.55 eV from the conduction or valence band, with estimated σT of the order of 10−21 and 10−22 cm2 for without and with the BPF, respectively. These findings regarding the presence of multiple defects in a single crystal indicate the necessity to focus on defect control.

Effects of Nanobubbles on Photochemical Processes of Levofloxacin Photosensitizer.

Photodynamic therapy (PDT) stands as an efficacious modality for the treatment of cancer and various diseases, in which optimization of the electron transfer and augmentation of the production of lethal reactive oxygen species (ROS) represent pivotal challenges to enhance its therapeutic efficacy. Empirical investigations have established that the spontaneous initiation of redox reactions associated with electron transfer is feasible and is located in the gas-liquid interfaces. Meanwhile, nanobubbles (NBs) are emerging as entities capable of furnishing a plethora of such interfaces, attributed to their stability and large surface/volume ratio in bulk water. Thus, NBs provide a chance to expedite the electron-transfer kinetics within the context of PDT in an ambient environment. In this paper, we present a pioneering exploration into the impact of nitrogen nanobubbles (N2-NBs) on the electron transfer of the photosensitizer levofloxacin (LEV). Transient absorption spectra and time-resolved decay spectra, as determined through laser flash photolysis, unequivocally reveal that N2-NBs exhibit a mitigating effect on the decay of the LEV excitation triplet state, thereby facilitating subsequent processes. Of paramount significance is the observation that the presence of N2-NBs markedly accelerates the electron transfer of LEV, albeit with a marginal inhibitory influence on its energy-transfer reaction. This observation is corroborated through absorbance measurements and offers compelling evidence substantiating the role of NBs in expediting electron transfer within the ambit of PDT. The mechanism elucidated herein sheds light on how N2-NBs intricately influence both electron-transfer and energy-transfer reactions in the photosensitizer LEV. These findings not only contribute to a nuanced understanding of the underlying processes but also furnish novel insights that may inform the application of NBs in the realm of photodynamic therapy.

Ag-CsPbI3 nanocomposite structure in glasses boosts photoluminescence for efficient light emitting diodes

Nanocomposite structures based on perovskite nanocrystals (NCs) and metal nanostructures within transparent monolithic media hold promise for a wide range of applications in optoelectronics. However, the microstructure of such nanocomposites and the reason for enhanced emissions remain incompletely understood. In this work, we present the successful creation of Ag-CsPbI3 nanocomposite within a highly transparent borosilicate glass (Ag-CsPbI3 NCs@glass). We reveal that Ag and CsPbI3 NCs form after thermal treatment of the glass, either in the form of Ag-CsPbI3 heterostructures or of separate Ag and CsPbI3 NCs. Based on combined characterization of Mott-Schottky and steady-state photoluminescence (PL) of the products, we attribute the enhanced PL in the nanocomposite of Ag-CsPbI3 to the regulation of CsPbI3 lattice by Ag ions and/or the plasmonic effect of Ag NCs. Temperature-dependent time-resolved PL decay measurements suggest that Ag NCs could play an important role in the relaxation process of excited carriers by increasing the radiative recombination. Moreover, the Ag-CsPbI3 nanocomposite structure within the glass matrix curtails PL quenching caused by water molecules and high temperature. Using the Ag-CsPbI3 nanocomposite, we achieve high-performance white light-emitting diodes with a color rendering index of 83.9 and luminous efficiency of 58.07 lmW−1. This work elucidates the underlying mechanism on the enhanced PL in nanocomposites containing perovskite NCs and metal nanostructures, and aids in the rational design of glass-based perovskite nanocomposites for a diversity of optoelectronic applications.

Luminescence of nanocrystalline BaFCl codoped with Eu2+/3+ and Tb3.

BaFCl:Eu,Tb nanocrystals were synthesized via hot-injection thermolysis of metal chlorodifluoroacetates and their luminescence response characterized between 80 and 430 K. Though unintended, partial reduction of Eu3+ to Eu2+ occurred during the synthesis stage, leading to luminescent nanocrystals coactivated by Eu2+, Eu3+, and Tb3+. Their emission was dominated by Eu2+, specifically by radiative transitions 4f7 (6P7/2) → 4f7 (8S7/2) and 4f65d1 (lowest level) → 4f7 (8S7/2). Owing to the thermal coupling between 4f7 (6P7/2) and 4f65d1 (lowest) levels, line-like emission from this manifold at 80 K evolved into band-like emission at 430 K. The change in the shape of the emission profile of Eu2+ was used to demonstrate that BaFCl:Eu,Tb may serve to realize bandshape luminescence thermometry as a more straightforward alternative to the usual ratiometric approach. The energy gap between 4f7 (6P7/2) and 4f65d1 (lowest) levels and their radiative transition probabilities were estimated from time-resolved variable-temperature decays.

Solute dynamics of a hydrophobic molecule in a menthol-thymol based type-V deep eutectic solvent: effect of composition of the components.

Type-V deep eutectic solvents (DESs) are a newly emerging unique class of solvents obtained by physical mixing and heating of non-ionic components. These solvents show deviation from the thermodynamic ideality. Compared to type-I to IV DESs, type-V DESs are less explored and their physical chemistry is in its nascent stage. In this work, we have chosen a type-V DES based on menthol-thymol (MT) for our working media. Solvent and rotational dynamics were studied with varying temperature using a well-known solvatochromic probe, Coumarin 153 (C153). We prepared the MT-based DES using a reported procedure at three molar ratios: 1 : 1 (M1T1), 1 : 1.5 (M1T1.5), and 2 : 1 (M2T1) of menthol (M) and thymol (T). Time-resolved emission spectra (TRES) were constructed with varying temperature. Utilizing TRES, the decay of the solvent correlation function (C(t)) was plotted. We have correlated the solvent relaxation time in these DESs as a function of viscosity. The time-resolved anisotropy decays were also collected to perceive the rotational relaxation dynamics of C153 as a function of temperature. The decay of solvent relaxation was found to be bi-exponential, and the average solvation time (〈τs〉) in M2T1 was found to be longer than those of M1T1.5 and M1T1. The rotational reorientation times (〈τrot〉) also follow the same trend. We have analysed the rotational dynamics of C153 in type-V DESs employing the Stokes-Einstein-Debye (SED) hydrodynamic model. The rotational dynamics in DESs demonstrate a good correlation with the SED model with a little deviation. In MT-based DESs, the solute's rotational relaxation times approach hydrodynamic stick boundary condition at low viscosity (or at high temperatures) for all molar compositions. Using the Arrhenius-type equations, we have correlated the activation energies for the rotational motion of C153, along with the viscous flow and non-radiative pathways for all the DESs.

One-step synthesis of orange-red emissive carbon dots: photophysical insight into their excitation wavelength-independent and dependent luminescence.

A low-temperature method was developed to synthesize orange-red luminescence phosphor-doped carbon dots (CDs) without complicated purification procedures. These CDs showed excitation wavelength-independent narrow emission (photo-luminescence quantum yield, Φf ∼ 12 to 22%) with single exponential time-resolved decay in weakly polar/non-polar solvents, indicating the presence of one kind of chromophore. In contrast, the same CDs showed excitation wavelength-dependent broad emission (Φf ∼ 1 to 8%) with multi-exponential fluorescence decay in polar solvents. These CDs exhibited poor solubility in polar solvents, resulting in CD aggregates contributed by excitation wavelength-dependent weak luminescence. The CDs embedded in polymethyl methacrylate (PMMA) polymer film displayed bright orange-red fluorescence under UV 365 nm illumination, indicating their potential application in solid-state luminescence. Further, an analytical method was developed for the naked-eye detection of trifluoracetic acid (red emission) and triethylamine (green emission) under UV 365 nm illumination with reversible two switch-mode luminescence. Additionally, this efficient orange-red luminescence of CDs was utilized for possible bioimaging applications with negligible cytotoxicity in 3T3 mouse fibroblast cells.

(Poster Award - 2nd Place) Layer-Number Engineered Momentum-Indirect Interlayer Excitons with Large Spectral Tunability

The diversity of two-dimensional (2D) materials provides van der Waals heterostructures with abundant degrees of freedom and makes the family a promising candidate to realize novel optoelectronic devices and an attractive platform for exploring physical phenomena. Interlayer excitons in type-II van der Waals heterostructures are equipped with an oriented permanent dipole moment and long lifetime and thus would allow promising applications in excitonic and optoelectronic devices. However, in the widely studied van der Waals heterostructures constructed by transition metal dichalcogenides, the formation of interlayer excitons is limited by the lattice mismatch, rotational and translational alignment between the constituent layers, increasing the complexity of the device fabrication. Here, I will report on the robust momentum-indirect interlayer exciton emission with widely tunable emission energy via layer engineering in transition metal dichalcogenide/2D perovskite heterostructures. The transition metal dichalcogenides with different thicknesses and 2D perovskites with different layer numbers of the inorganic octahedral slabs are stacked to form type-II van der Waals heterostructures without considering special orientation arrangement and momentum mismatch. The presence of interlayer excitons is supported by the excitation-power- and temperature-dependent photoluminescence studies, photoluminescence excitation spectroscopy, time-resolved photoluminescence decay studies and electric field-dependent photoluminescence studies. Taking advantage of the thickness- or layer-number-dependence of the electronic band structure of the constituent materials, the emission energy of the interlayer excitons in our heterostructures can be tuned from 1.3 eV to 1.6 eV, providing strong evidence for the applications of van der Waals interface engineering to broad-spectrum optoelectronics. In addition, the diffusion coefficient of interlayer excitons in our heterostructures can be estimated to be ~10 cm2 s−1, which is at least one order of magnitude larger than that in the van der Waals heterostructures with Moiré superlattice, suggesting a non-localized nature that would be beneficial to the excitonic devices. Our study would offer new insights into the nature of interlayer excitons and provoke further research into their dynamics.

Quenching mechanism and luminescence enhancement strategy of Eu2+-activator in fluoroborate host Ba5B4O10F2

Fluoroborates have garnered significant research interest due to their intricate structure and unique nonlinear optical properties. In this work, pure and Eu-activated Ba5B4O10F2 were prepared via the high-temperature solid-state ceramic method. Ba5B4O10F2 is a wide band semiconductor (Eg = 4.51 eV) with an indirect transition characteristic. The optical absorption, photoluminescence, and dynamic spectra of Eu2+/3+ activators were investigated. Luminescence intensities of Eu2+ and Eu3+ had distinctly different responses to doping content. The residual Eu3+ centers in Ba5-5xEu5xB4O10F2 (x > 0.03) cannot be avoided even it was sintered in a reducing atmosphere. Time-resolved spectra and decay curves also confirm the presence of Eu3+ with Eu2+. Under near-UV light excitation, a strong emission band from the 5d→4f transition of Eu2+ was detected overlapping with sharp peaks from (5D0→7F0,1,2,3,4) of Eu3+ in Ba5-5xEu5xB4O10F2 (x > 0.03). As the Eu-doping level increases, the intensity ratio of Eu2+ and Eu3+ changes regularly, and the luminescent intensity of Eu3+ obviously and systematically becomes stronger. As the temperature changes from 25 °C to 150 °C, the luminescent intensity of Eu3+ increases, while that of Eu2+ gradually decreases. The results confirm the energy transfer (ET) from Eu2+ to Eu3+ in Ba5B4O10F2. Eu3+ can act as killers, leading to the luminescence quenching of Eu2+ activators. The heterovalent substitution of Eu and NaF in Ba5B4O10F2 not only reduces the content of Eu3+ but also significantly enhances the efficiency of Eu2+. The experiments demonstrate that to improve the luminescence efficiency of Eu2+, it is important to focus on reducing the residual Eu3+ in the lattice through enhancing the reduction from Eu3+ to Eu2+.

Study of time-resolved photoluminescence decay curves in Al-doped ZnO and Eu-doped Cd1−xZnxS nanophosphors

Study of time-resolved photoluminescence decay curves in Al-doped ZnO and Eu-doped Cd1−xZnxS nanophosphors

A composite electrodynamic mechanism to reconcile spatiotemporally resolved exciton transport in quantum dot superlattices.

Quantum dot (QD) solids are promising optoelectronic materials; further advancing their device functionality requires understanding their energy transport mechanisms. The commonly invoked near-field Förster resonance energy transfer (FRET) theory often underestimates the exciton hopping rate in QD solids, yet no consensus exists on the underlying cause. In response, we use time-resolved ultrafast stimulated emission depletion (STED) microscopy, an ultrafast transformation of STED to spatiotemporally resolve exciton diffusion in tellurium-doped cadmium selenide-core/cadmium sulfide-shell QD superlattices. We measure the concomitant time-resolved exciton energy decay due to excitons sampling a heterogeneous energetic landscape within the superlattice. The heterogeneity is quantified by single-particle emission spectroscopy. This powerful multimodal set of observables provides sufficient constraints on a kinetic Monte Carlo simulation of exciton transport to elucidate a composite transport mechanism that includes both near-field FRET and previously neglected far-field emission/reabsorption contributions. Uncovering this mechanism offers a much-needed unified framework in which to characterize transport in QD solids and additional principles for device design.

Electrochemical control of surface passivation and deformation in InP nanotetrapods

In this study, we investigate the influence of electrochemical doping on the optical properties of single-crystalline InP nanotetrapods. Our focus is on examining the variations in photoluminescence (PL) intensity exhibited by these InP tetrapods in response to applied electrical potential. We observe that the PL intensity undergoes significant changes depending on the nature of the charge transfer occurring during electrochemical doping. Specifically, electron transfer from the electrode to the InP tetrapods leads to a noticeable quenching of the PL intensity, which can be attributed to the removal of surface ligands. However, when additional holes are injected, we observe a remarkable increase in the PL intensity exceeding 60%. Through an analysis of steady-state PL spectra and time-resolved PL decay dynamics, we propose that the injected holes occupy surface hole trap states, resulting in surface passivation and thus enhancing the PL intensity. Our findings highlight the potential of these InP nanotetrapods as a novel nanostructure for future advancements in nanocrystal applications. By elucidating the impact of electrochemical doping on the optical properties of these materials, we contribute to the understanding and exploration of their potential applications in optoelectronic devices and related fields.