Bodipy-based quinoline derivative as a highly Hg2+-selective fluorescent chemosensor and its potential applications.

A highly efficient sensor has been successfully developed using quinoline-based BODIPY compounds (8-quinoline-4,4-difluoro-4-boro-3a, 4a-diazaindacene (C1) and 7-hydroxy-8-quinoline-4,4-difluoro-4-boro-3a, 4a-diazindacene (C2) to detect Hg2+ ions. The sensor C1 exhibits remarkable selectivity in detecting Hg2+ with a limit of detection 3.06 × 10-8 mol/L. The developed chemical sensors have shown stability, cost-effectiveness, ease of preparation, and remarkable selectivity towards Hg2+ ions compared to other commonly occurring metal ions. The total recovery of the sensor C1 can be achieved by using a 0.1 mol/L solution of KI. The proposed sensor C1 has been applied to determine Hg2+ in tap and distilled water, yielding excellent results. In addition, the binding mode of C1-Hg2+ and C2-Hg2+ complexes was a 1:1 ratio confirmed by mass spectra, Job's plot, and DFT study. Moreover, the sensor C1 successfully applied for the biological studies results in negligible cytotoxicity, which demonstrates it can be used to determine Hg2+ in HT22 cells.

ZnPSe3 as ultrabright indirect band-gap system with microsecond excitonic lifetimes.

ZnPSe3 was identified as a two-dimensional material wherein valley and spin can be optically controlled in technologically relevant timescales. We report an optical characterization of ZnPSe3 crystals that show indirect band-gap characteristics in combination with unusually strong photoluminescence. We found evidence of interband recombination from photoexcited electron-hole states with lifetimes in a microsecond timescale. Through a comparative analysis of photoluminescence and photoluminescence excitation spectra, we reconstructed the electronic band scheme relevant to fundamental processes of light absorption, carrier relaxation, and radiative recombination through interband pathways and annihilation of defect-bound excitons. The investigation of the radiative processes in the presence of a magnetic field revealed spin splitting of electronic states contributing to the ground excitonic states. Consequently, the magnetic field induces an imbalance in the number of excitons with the opposite angular momentum according to the thermal equilibrium as seen in the photoluminescence decay profiles resolved by circular polarization.

Single-Molecule Time-Resolved Spectroscopy in a Tunable STM Nanocavity.

Spontaneous fluorescence rates of single-molecule emitters are typically on the order of nanoseconds. However, coupling them with plasmonic nanostructures can substantially increase their fluorescence yields. The confinement between a tip and sample in a scanning tunneling microscope creates a tunable nanocavity, an ideal platform for exploring the yields and excitation decay rates of single-molecule emitters, depending on their coupling strength to the nanocavity. With such a setup, we determine the excitation lifetimes from the direct time-resolved measurements of phthalocyanine fluorescence decays, decoupled from the metal substrates by ultrathin NaCl layers. We find that when the tip is approached to single molecules, their lifetimes are reduced to the picosecond range due to the effect of coupling with the tip-sample nanocavity. On the other hand, ensembles of the adsorbed molecules measured without the nanocavity manifest nanosecond-range lifetimes. This approach overcomes the drawbacks associated with the estimation of lifetimes for single molecules from their respective emission line widths.

Metal Fluorides Passivate II-VI and III-V Quantum Dots.

Quantum dots (QDs) with metal fluoride surface ligands were prepared via reaction with anhydrous oleylammonium fluoride. Carboxylate terminated II-VI QDs underwent carboxylate for fluoride exchange, while InP QDs underwent photochemical acidolysis yielding oleylamine, PH3, and InF3. The final photoluminescence quantum yield (PLQY) reached 83% for InP and near unity for core-shell QDs. Core-only CdS QDs showed dramatic improvements in PLQY, but only after exposure to air. Following etching, the InP QDs were bound by oleylamine ligands that were characterized by the frequency and breadth of the corresponding ν(N-H) bands in the infrared absorption spectrum. The fluoride content (1.6-9.2 nm-2) was measured by titration with chlorotrimethylsilane and compared with the oleylamine content (2.3-5.1 nm-2) supporting the formation of densely covered surfaces. The influence of metal fluoride adsorption on the air stability of QDs is discussed.

Suppressed Magnitude of Spectral Diffusion in Cube-Shaped CdSe/CdS Core/Shell Nanocrystals with Exceedingly Stable Photoluminescence.

Colloidal semiconductor nanocrystals are promising candidates for quantum light sources, yet their application has been impeded by photoluminescence instability due to blinking and spectral diffusion. This study introduces a new category of cube-shaped CdSe/CdS core/shell nanocrystals with exceptionally stable photoluminescence characteristics. Under continuous excitation, the emissive quantum state remained consistent without alterations of the charge state for 4000 s, and the average photon energy variation stayed within the bounds of spectral resolution throughout this extended duration. Systematic examination of single-nanocrystal photoluminescence, upon variation of the core and shell dimensions, revealed that a thicker CdS shell and increased core edge length significantly curtail spectral diffusion, considering that the nanocrystals possess well-controlled CdSe-CdS and facet-ligand interfaces. This study advances the optimization of colloidal semiconductor nanocrystals as high-performance quantum light sources.

Optical Cooling of Dot-in-Crystal Halide Perovskites: Challenges of Nonlinear Exciton Recombination.

Highly efficient anti-Stokes (AS) photoluminescence (PL) is observed from halide perovskite quantum dots (QDs) due to their strong electron-phonon interactions. The AS PL is particularly intriguing, as it suggests the potential for semiconductor optical cooling if the external quantum efficiency approaches 100%. However, the PL quantum efficiency in QDs is primarily dominated by multiparticle nonradiative Auger recombination processes under intense photoexcitation, which impose limits on the optical cooling gain. Here, we investigate the Auger recombination of dot-in-crystal perovskites. We quantitatively estimate the maximum optical cooling gain and the corresponding excitation intensity. We further conducted optical cooling experiments and demonstrate a maximum photocooling of approximately 9 K from room temperature. Additionally, we confirmed that increasing the excitation intensity leads to a transition from photocooling to photoheating. These observations are consistent with our time-resolved measurements, offering insights into the potential and limitations of optical cooling in semiconductor QDs.

Deterministic Assembly of Single-Emitter Plasmonic Antenna for Ultrahigh Photoluminescence Enhancement.

Single-emitter nanoantennas play a crucial role in the fabrication of nanosensors and integrated sources. Since the coupling of single emitter to nanoantennas is largely based on stochastic methods, low qualified rate still hinders a massive deployment. Here, we proposed a deterministic, optical-force-driven method to achieve gap-plasmonic photoluminescence enhancement. Two deterministic steps are carried out in sequence: a composite nanoemitter is first synthesized by linking quantum dots to a silica-rapped gold nanoparticle, followed by an optical delivery of the nanoparticle into a nanoaperture in a gold film. We reason that the nanoparticle-in-nanoaperture (NPiNA) structure efficiently couples out-of-plane excitation light into a gap-plasmon via a transverse electromagnetic mode (TEM)-like transmission mode. An in situ photoluminescence measurement demonstrates a 3× brightness as compared to the nanoparticle-on-mirror (NPoM). This approach paves the way toward deterministic positioning of individual nanoparticles for a wide range of applications on nanophotonics structures on-a-chip.

Phonon Directionality Impacts Electron-Phonon Coupling and Polarization of the Band-Edge Emission in Two-Dimensional Metal Halide Perovskites.

Two-dimensional metal halide perovskites are highly versatile for light-driven applications due to their exceptional variety in material composition, which can be exploited for the tunability of mechanical and optoelectronic properties. The band-edge emission is defined by the structure and composition of both organic and inorganic layers, and electron-phonon coupling plays a crucial role in the recombination dynamics. However, the nature of the electron-phonon coupling and what kind of phonons are involved are still under debate. Here we investigate the emission, reflectance, and phonon response from single two-dimensional lead iodide microcrystals with angle-resolved polarized spectroscopy. We find an intricate dependence of the emission polarization with the vibrational directionality in the materials, which reveals that several bands of low-frequency phonons with nonorthogonal directionality contribute to the band-edge emission. Such complex electron-phonon coupling requires adequate models to predict the thermal broadening of the emission and provides opportunities to design polarization properties.

Revealing the Optical Transition Properties of Interlayer Excitons in Defective WS2/WSe2 Heterobilayers.

Manipulation of physical properties in multidimensional tunable moiré superlattice systems is a key focus in nanophotonics, especially for interlayer excitons (IXs) in two-dimensional materials. However, the impact of defects on IXs remains unclear. Here, we thoroughly study the optical properties of WS2/WSe2 heterobilayers with varying defect densities. Low-temperature photoluminescence (PL) characterizations reveal that the low-energy IXs are more susceptible to defects compared to the high-energy IXs. The low-energy IXs also show much faster PL quenching rate with temperature, faster peak width broadening rate with laser power, shorter lifetime, and lower circular polarization compared to the low-energy IXs in the region with fewer defects. These effects are attributed to the combined effects of increased electron scattering, exciton-phonon interactions, and nonradiative channels introduced by the defects. Our findings aid in optimizing moiré superlattice structures.

Giant Photoluminescence Enhancement of Monolayer WSe2 Using a Plasmonic Nanocavity with On-Demand Resonance.

Monolayer transition metal dichalcogenides (TMDs) are considered promising building blocks for next-generation photonic and optoelectronic devices, owing to their fascinating optical properties. However, their inherent weak light absorption and low quantum yield severely hinder their practical applications. Here, we report up to 18000-fold photoluminescence (PL) enhancement in a monolayer WSe2-coupled plasmonic nanocavity. A spectroscopy-assisted nanomanipulation technique enables the assembly of a nanocavity with customizable resonances to simultaneously enhance the excitation and emission processes. In particular, precise control over the magnetic cavity mode facilitates spectral and spatial overlap with the exciton, resulting in plasmon-exciton intermediate coupling that approaches the maximum emission rate in the hybrid system. Meanwhile, the cavity mode exhibits high radiation directivity, which overwhelmingly directs surface-normal PL emission and leads to a 17-fold increase in the collection efficiency. Our approach opens up a new avenue to enhance the PL intensity of monolayer TMDs, facilitating their implementation in highly efficient optoelectronic devices.

Printable Light-Emitting Metasurfaces with Enhanced Directional Photoluminescence.

Nanoimprint lithography is gaining popularity as a cost-efficient way to reproduce nanostructures in large quantities. Recent advances in nanoimprinting lithography using high-index nanoparticles have demonstrated replication of photonic devices, but it is difficult to confer special properties on nanostructures beyond general metasurfaces. Here, we introduce a novel method for fabricating light-emitting metasurfaces using nanoimprinting lithography. By utilizing quantum dots embedded in resin, we successfully imprint dielectric metasurfaces that function simultaneously as both emitters and resonators. This approach to incorporating quantum dots into metasurfaces demonstrates an improvement in photoluminescence characteristics compared to the situation where quantum dots and metasurfaces are independently incorporated. Design of the metasurface is specifically tailored to support photonic modes within the emission band of quantum dots with a large enhancement of photoluminescence. This study indicates that nanoimprinting lithography has the capability to construct nanostructures using functionalized nanoparticles and could be used in various fields of nanophotonic applications.

Large-Area Near-Infrared Emission Enhancement on Single Upconversion Nanoparticles by Metal Nanohole Array.

Single lanthanide (Ln) ion doped upconversion nanoparticles (UCNPs) exhibit great potential for biomolecule sensing and counting. Plasmonic structures can improve the emission efficiency of single UCNPs by modulating the energy transferring process. Yet, achieving robust and large-area single UCNP emission modulation remains a challenge, which obstructs investigation and application of single UCNPs. Here, we present a strategy using metal nanohole arrays (NHAs) to achieve energy-transfer modulation on single UCNPs simultaneously within large-area plasmonic structures. By coupling surface plasmon polaritons (SPPs) with higher-intermediate state (1D2 → 3F3, 1D2 → 3H4) transitions, we achieved a remarkable up to 10-fold enhancement in 800 nm emission, surpassing the conventional approach of coupling SPPs with an intermediate ground state (3H4 → 3H6). We numerically simulate the electrical field distribution and reveal that luminescent enhancement is robust and insensitive to the exact location of particles. It is anticipated that the strategy provides a platform for widely exploring applications in single-particle quantitative biosensing.

Room Temperature Dynamics of an Optically Addressable Single Spin in Hexagonal Boron Nitride.

Hexagonal boron nitride (h-BN) hosts pure single-photon emitters that have shown evidence of optically detected electronic spin dynamics. However, the electrical and chemical structures of these optically addressable spins are unknown, and the nature of their spin-optical interactions remains mysterious. Here, we use time-domain optical and microwave experiments to characterize a single emitter in h-BN exhibiting room temperature optically detected magnetic resonance. Using dynamical simulations, we constrain and quantify transition rates in the model, and we design optical control protocols that optimize the signal-to-noise ratio for spin readout. This constitutes a necessary step toward quantum control of spin states in h-BN.

Photo Luminescence and Radiative Carrier Losses in Monolayer Transition Metal Dichalcogenides.

The carrier losses due to radiative recombination in monolayer transition metal dichalcogenides are studied using fully microscopic many-body models. The density- and temperature-dependent losses in various Mo- and W-based materials are shown to be dominated by Coulomb correlations beyond the Hartree-Fock level. Despite the much stronger Coulomb interaction in 2D materials, the radiative losses are comparable-if not weaker-than in conventional III-V materials. A strong dependence on the dielectric environment is found in agreement with experimental results.

Optical Absorption and Photoluminescence of Single-Layer Boron Nitride from a First-Principles Cumulant Approach.

The photoluminescence spectrum of a single-layer boron nitride remains elusive, marked by enigmatic satellites that hint at significant but unidentified exciton-phonon coupling. Here, by employing a first-principles approach based on the many-body cumulant expansion of the charge response, we calculate the optical absorption and photoluminescence of a single-layer boron nitride. We identify the specific exciton-phonon scattering channels and unravel their impact on the optical absorption and photoluminescence spectra, thereby providing an interpretation of the experimental features. Finally, we show that, even in a strongly polar material such as h-BN monolayer, the electron-hole interaction responsible for the excitonic effect results in the cancellation of the Frölich interaction at small phonon momenta. This effect is captured only if the invariance of the exciton-phonon matrix elements under unitary transformations in the Bloch function manifold is preserved in the calculation.

Thermally Activated Photoluminescence Induced by Tunable Interlayer Interactions in Naturally Occurring van der Waals Superlattice SnS/TiS2.

van der Waals (vdW) superlattices, comprising different 2D materials aligned alternately by weak interlayer interactions, offer versatile structures for the fabrication of novel semiconductor devices. Despite their potential, the precise control of optoelectronic properties with interlayer interactions remains challenging. Here, we investigate the discrepancies between the SnS/TiS2 superlattice (SnTiS3) and its subsystems by comprehensive characterization and DFT calculations. The disappearance of certain Raman modes suggests that the interactions alter the SnS subsystem structure. Specifically, such structural changes transform the band structure from indirect to direct band gap, causing a strong PL emission (∼2.18 eV) in SnTiS3. In addition, the modulation of the optoelectronic properties ultimately leads to the unique phenomenon of thermally activated photoluminescence. This phenomenon is attributed to the inhibition of charge transfer induced by tunable intralayer strains. Our findings extend the understanding of the mechanism of interlayer interactions in van der Waals superlattices and provide insights into the design of high-temperature optoelectronic devices.

Excitonic Quantum Coherence in Light Emission from CsPbBr3 Metal-Halide Perovskite Nanocrystals.

The decay of excited states via radiative and nonradiative paths is well understood in molecules and bulk semiconductors but less so in nanocrystals. Here, we perform time-resolved photoluminescence (t-PL) experiments on CsPbBr3 metal-halide perovskite nanocrystals, with a time resolution of 3 ps, sufficient to observe the decay of both excitons and biexcitons as a function of temperature. The striking result is that the radiative rate constant of the single exciton increases at low temperatures with an exponential functional form, suggesting quantum coherent effects with dephasing at high temperatures. The opposing directions of the radiative and nonradiative decay rate constants enable enhanced brightening of PL from excitons to biexcitons due to quantum effects, promoting a faster approach to the quantum theoretical limits of light emission. Ab initio quantum dynamics simulations reproduce the experimental observations of radiation controlled by quantum spatial coherence enhanced at low temperatures.

Diffusion-Dominated Luminescence Dynamics of CsPbBr3 Studied Using Cathodoluminescence and Microphotoluminescence Spectroscopy.

Time-resolved or time-correlation measurements using cathodoluminescence (CL) reveal the electronic and optical properties of semiconductors, such as their carrier lifetimes, at the nanoscale. However, halide perovskites, which are promising optoelectronic materials, exhibit significantly different decay dynamics in their CL and photoluminescence (PL). We conducted time-correlation CL measurements of CsPbBr3 using Hanbury Brown-Twiss interferometry and compared them with time-resolved PL. The measured CL decay time was on the order of subnanoseconds and was faster than PL decay at an excited carrier density of 2.1 × 1018 cm-3. Our experiment and analytical model revealed the CL dynamics induced by individual electron incidences, which are characterized by highly localized carrier generation followed by a rapid decrease in carrier density due to diffusion. This carrier diffusion can play a dominant role in the CL decay time for undoped semiconductors, in general, when the diffusion dynamics are faster than the carrier recombination.

Trap-Mediated Sensitization Governs Near-Infrared Emission from Yb3+-Doped Mixed-Halide CsPbClxBr3-x Perovskite Nanocrystals.

Understanding the photosensitization mechanisms in Yb3+-doped perovskite nanocrystals is crucial for developing their anticipated photonic applications. Here, we address this question by investigating near-infrared photoluminescence of Yb3+-doped mixed-halide CsPbClxBr3-x nanocrystals as a function of temperature and revealing its strong dependence on the stoichiometry of the host perovskite matrix. To explain the observed experimental trends, we developed a theoretical model in which energy transfer from the perovskite matrix to Yb3+ ions occurs through intermediate trap states situated beneath the conduction band of the host. The developed model provides an excellent agreement with experimental results and is further validated through the measurements of emission saturation at high excitation powers and near-infrared photoluminescence quantum yield as a function of the anion composition. Our findings establish trap-mediated energy transfer as a dominant photosensitization mechanism in Yb3+-doped CsPbClxBr3-x nanocrystals and open up new ways of engineering their optical properties for light-emitting and light-harvesting applications.

Nanoscale Infrared Spectroscopic Characterization of Extended Defects in 4H-Silicon Carbide.

Extended defects in wide-bandgap semiconductors have been widely investigated using techniques providing either spectroscopic or microscopic information. Nano-Fourier transform infrared spectroscopy (nano-FTIR) is a nondestructive characterization method combining FTIR with nanoscale spatial resolution (∼20 nm) and topographic information. Here, we demonstrate the capability of nano-FTIR for the characterization of extended defects in semiconductors by investigating an in-grown stacking fault (IGSF) present in a 4H-SiC epitaxial layer. We observe a local spectral shift of the mid-infrared near-field response, consistent with the identification of the defect stacking order as 3C-SiC (cubic) from comparative simulations based on the finite dipole model (FDM). This 3C-SiC IGSF contrasts with the more typical 8H-SiC IGSFs reported previously and is exemplary in showing that nanoscale spectroscopy with nano-FTIR can provide new insights into the properties of extended defects, the understanding of which is crucial for mitigating deleterious effects of such defects in alternative semiconductor materials and devices.