We utilized a modified reverse-microemulsion method to develop highly emissive and photostable zero-dimensional (0D) Cs4Pb(Br1-xIx)6 perovskite nanocrystals (PNCs). We employed single-particle photoluminescence (PL) spectroscopy to explore blinking statistics and demonstrate single-photon emission from individual PNCs. Low-temperature blinking and photon correlation studies revealed a transition from single- to multiphoton emission with progressively longer "delayed" PL components, reaching ∼70 ns at room temperature and representing a distinctive behavior to previously known iodide PNCs. Such thermally activated PL emission is explained by the existence of defect-related "reservoir" states, feeding back into the PNC's emissive state and providing multiple photons within a single excitation cycle. This work establishes a new member in the 0D class of perovskite materials, studies its photophysical properties, and reveals its potential for future optoelectronic applications.

The 0D cesium lead halide perovskite Cs4 PbBr6 has drawn remarkable interest due to its highly efficient robust green emission compared to its 3D CsPbBr3 counterpart. However, seizing the advantages of the superior photoluminescence properties for practical light-emitting devices remains elusive. To date, Cs4 PbBr6 has been employed only as a higher-bandgap nonluminescent matrix to passivate or provide quantum/dielectric confinement to CsPbBr3 in light-emitting devices and to enhance its photo-/thermal/environmental stability. To resolve this disparity, a novel solvent engineering method to incorporate highly luminescent 0D Cs4 PbBr6 nanocrystals (perovskite nanocrystals (PNCs)) into a 3D CsPbBr3 film, forming the active emissive layer in single-layer perovskite light-emitting electrochemical cells (PeLECs) is designed. A dramatic increase of the maximum external quantum efficiency and luminance from 2.7% and 6050 cd m-2 for a 3D-only PeLEC to 8.3% and 11 200 cd m-2 for a 3D-0D PNC device with only 7% by weight of 0D PNCs is observed. The majority of this increase is driven by the efficient inherent emission of the 0D PNCs, while the concomitant morphology improvement also contributes to reduced leakage current, reduced hysteresis, and enhanced operational lifetime (half-life of 129 h), making this one of the best-performing LECs reported to date.

Photo-catalysis by small-molecules is often limited by catalyst degradation and low electron-transfer efficiency. Herein we report a stable N-phenyl-phenothiazine (PTH)-derived porous coordination cage (PCC) as a highly efficient photocatalyst. By the incorporation of the photocatalytic PTH moiety into a PCC, aggregation-induced quenching (AIQ) was shown to be reduced. An improvement in catalyst stability was discovered, ascribed to the synergistic effects of the PTH moieties. The catalyst, operating through a photolytic single-electron transfer, was utilized for photo-catalyzed dehalogenation and borylation. Evaluation of the catalytic mechanism in the borylation reaction showed that the improved performance results from the more efficient formation of the electron donor-acceptor (EDA) complex with the cage. This discovery provides a potential strategy to improve the photophysical properties and stabilities of small-molecule organic photocatalysts via supramolecular chemistry.

Developments in the field of nanoplasmonics have the potential to advance applications from information processing and telecommunications to light-based sensing. Traditionally, nanoscale noble metals such as gold and silver have been used to achieve the targeted enhancements in light-matter interactions that result from the presence of localized surface plasmons (LSPs). However, interest has recently shifted to intrinsically doped semiconductor nanocrystals (NCs) for their ability to display LSP resonances (LSPRs) over a much broader spectral range, including the infrared (IR). Among semiconducting plasmonic NCs, spinel metal oxides (sp-MOs) are an emerging class of materials with distinct advantages in accessing the telecommunications bands in the IR and affording useful environmental stability. Here, we report the plasmonic properties of Fe3O4 sp-MO NCs, known previously only for their magnetic functionality, and demonstrate their ability to modify the light-emission properties of telecom-emitting quantum dots (QDs). We establish the synthetic conditions for tuning sp-MO NC size, composition and doping characteristics, resulting in unprecedented tunability of electronic behavior and plasmonic response over 450 nm. In particular, with diameter-dependent variations in free-electron concentration across the Fe3O4 NC series, we introduce a strong NC size dependency onto the optical response. In addition, our observation of plasmonics-enhanced decay rates from telecom-emitting QDs reveals Purcell enhancement factors for simple plasmonic-spacer-emitter sandwich structures up to 51-fold, which are comparable to values achieved previously only for emitters in the visible range coupled with conventional noble metal NCs.

The remarkable defect-tolerant nature of inorganic cesium halide perovskites, leading to near unity photoluminescence (PL) quantum yield and narrow emission line width across the entire visible spectrum, has provided a tantalizing platform for the development of a plethora of light-emitting applications. Recently, lower-dimensional (2D, 1D, and 0D) perovskites have attracted further attention due to their enhanced thermal, photo, and chemical stability as compared to their three-dimensional (3D) analogues. The combination of external size quantization and internal octahedral organization provides a unique opportunity to study and harness "multi-dimensional" electronic properties engineered on both atomic scale and nanoscale. However, crucial research to understand the elementary charge carrier dynamics in lower-dimensional perovskites lags far behind the enormous effort to incorporate them into optoelectronic devices. In this Perspective, we provide a review of recent developments that focus on studies of the dynamics of excitonic complexes in Cs-based perovskite nanocrystals using single-particle time-resolved PL spectroscopy and photon correlation measurements. Single-photon statistical studies not only offer an unprecedented level of detail to directly assess various recombination pathways, but also provide insights into specifics of the charge carriers' localization. We discuss the underlying physicochemical processes that govern PL emission and draw attention to a number of attributes within this class of the materials, especially lower-dimensional perovskites, that may indicate the common origin of the PL emission as well as provide a route map for the vast unexplored territories where single-particle spectroscopy can be a powerful tool to unravel crucial information.

Deterministic coupling between photonic nodes in a quantum network is an essential step toward implementing various quantum technologies. The omnidirectionality of free-standing emitters, however, makes this coupling highly inefficient, in particular if the distant nodes are coupled via low numerical aperture (NA) channels such as optical fibers. This limitation requires placing quantum emitters in nanoantennas that can direct the photons into the channels with very high efficiency. Moreover, to be able to scale such technologies to a large number of channels, the placing of the emitters should be deterministic. In this work, we present a method for directly locating single free-standing quantum emitters with high spatial accuracy at the center of highly directional bullseye metal–dielectric nanoantennas. We further employ non-blinking, high quantum yield colloidal quantum dots for on-demand single-photon emission that is uncompromised by instabilities or non-radiative exciton recombination processes. Taken together, this approach results in a record-high collection efficiency of 85% of the single photons into a low NA of 0.5, setting the stage for efficient coupling between on-chip, room temperature nanoantenna-emitter devices and a fiber or a remote free-space node without the need for additional optics.

Metal halide perovskites have attracted tremendous attention as promising materials for future-generation optoelectronic devices. Despite their outstanding optical and transport properties, the lack of environmental and operational stability remains a major practical challenge. One of the promising stabilization avenues is metal oxide encapsulation via atomic layer deposition (ALD); however, the unavoidable reaction of metal precursors with the perovskite surface in conventional ALD leads to degradation and restructuring of the perovskites' surfaces. This Perspective highlights the development of a modified gas-phase ALD technique for alumina encapsulation that not only prevents perovskites' degradation but also significantly improves their optical properties and air stability. The correlation between precise atomic interactions at the perovskite-metal oxide interface with the dramatically enhanced optical properties is supported by density functional theory calculations, which also underlines the widespread applicability of this gentle technique for a variety of perovskite nanostructures unbarring potential opportunities offered by combination of these approaches.

Polycyclic aromatic hydrocarbons such as perylene and pyrene and their derivatives are highly emissive fluorophores in solution. However, the practical applications of these materials in the field of molecular electronic and light-emitting devices are often hindered by self-quenching effects because of the formation of nonfluorescent aggregates in concentrated solutions or in the solid state. Herein, we demonstrate that aggregation-caused quenching of perylenes can be minimalized by molecular incorporation into metal-organic frameworks (MOFs). This study utilized a stable Zr6 cluster-based MOF, UiO-67, as a matrix. Linear linkers containing photoresponsive moieties were designed and incorporated into the parent UiO-67 scaffold through the partial replacement of the nonfluorescent linkers of a similar length, forming mixed-linker MOFs. The average distance between perylene moieties was tuned by changing the linker ratios, thus controlling the fluorescence intensity, emission wavelength, and quantum yield. Molecular modeling was further adopted to correlate the number of isolated perylene linkers within the framework with the ratio between the two linkers, thereby rationalizing the change in the observed fluorescent properties. Taking advantage of the tunable fluorescence, inherent porosity, and high chemical stability of this MOF platform, it was applied as a fluorescent sensor for oxygen detection in the gas phase, a model reaction, showing fast response and good recyclability.

Cesium lead halide perovskite nanocrystals (PNCs), while possessing facile chemical synthesis and high photoluminescence (PL) emission properties, are still challenged by issues of instability and degradation. Although atomic layer deposition (ALD) of metal oxides has been one of the common encapsulation approaches for longer-term stability, its application inevitably resulted in severe loss of emission efficiency, and at times partial loss of structural integrity of perovskites, creating a bottleneck in its practical viability. We demonstrate a non-destructive modified gas-phase technique with co-deposition of both precursors trimethylaluminum (TMA) and water, to dramatically enhance the PL emission in zero-dimensional (0D) Cs4PbBr6 PNCs via alumina encapsulation. XPS surface analysis of Cs4PbBr6 films reveals the alumina deposition to be accompanied by the elemental composition changes, particularly by the reduction of the excessive cesium content. Ab-initio DFT simulations further unfold that the presence of excess Cs on the surface of PNCs leads to decomposition of structural [PbBr6] octahedra in 0D perovskite lattice, which can be prevented in presence of added hydroxyl groups. Our study thus unveils the pivotal role of the PNC surface composition and treatment in the process of its interaction with metal oxide precursors to control the PL properties as well as the stability of PNCs, providing the unprecedented way to use conventional ALD technique for their successful integration into optoelectronic and photonic devices with improved properties.

Cesium-based perovskite nanocrystals (PNCs) possess alluring optical and electronic properties via compositional and structural versatility, tunable bandgap, high photoluminescence quantum yield and facile chemical synthesis. Despite the recent progress, origins of the photoluminescence emission in various types of PNCs remains unclear. Here, we study the photon emission from individual three-dimensional and zero-dimensional cesium lead bromide PNCs. Using photon antibunching and lifetime measurements, we demonstrate that emission statistics of both type of PNCs are akin to individual molecular fluorophores, rather than traditional semiconductor quantum dots. Aided by density functional modelling, we provide compelling evidence that green emission in zero-dimensional PNCs stems from exciton recombination at bromide vacancy centres within lead-halide octahedra, unrelated to external confinement. These findings provide key information about the nature of defect formation and the origin of emission in cesium lead halide perovskite materials, which foster their utilization in the emerging optoelectronic applications.

We demonstrate enhancement of the photoluminescence (PL) properties of individual zero-dimensional (0D) Cs4PbBr6 perovskite nanocrystals (PNCs) upon encapsulation by alumina using an appropriately modified atomic layer deposition method. In addition to the increased PL intensity and improved long-term stability of encapsulated PNCs, our single-particle studies reveal substantial changes in the PL blinking statistics and the persistent appearance of the long-lived, "delayed" PL components. The blinking patterns exhibit a modification from the fast switching between fluorescent ON and OFF states found in bare PNCs to a behavior with longer ON states and more isolated OFF states in alumina-encapsulated PNCs. Controlled exposure of 0D nanocrystals to moisture suggests that the observed PL lifetime changes may be related to water-induced "reservoir" states that allow for longer-lived charge storage with subsequent back-feeding into the emissive states. Viable encapsulation of PNCs with metal oxides that can preserve and even enhance their PL properties can be utilized in the fabrication of extended structures on their basis for optoelectronic and photonic applications.

Introducing dopants into InGaN NWs is known to significantly improve their device performances through a variety of mechanisms. However, to further optimize device operation under the influence of large specific surfaces, thorough knowledge of ultrafast dynamical processes at the surface and interface of these NWs is imperative. Here, we describe the development of four-dimensional scanning ultrafast electron microscopy (4D S-UEM) as an extremely surface-sensitive method to directly visualize in space and time the enormous impact of silicon doping on the surface-carrier dynamics of InGaN NWs. Two time regimes of surface dynamics are identified for the first time in a 4D S-UEM experiment: an early time behavior (within 200 ps) associated with the deferred evolution of secondary electrons due to the presence of localized trap states that decrease the electron escape rate and a longer time scale behavior (several ns) marked by accelerated charge carrier recombination. The results are further corroborated by conductivity studies carried out in the dark and under illumination.

Insertion of alkali metal ions, especially Na, is a well-established method to significantly increase the power conversion efficiency of copper indium gallium selenide (CIGSe)-based photovoltaic devices. However, although it is known that Na ions mostly reside on the surface of CIGSe layer following diffusion, the exact mechanism of how Na affects the carrier dynamics of CIGSe still remains ambiguous. This is mainly due to the unavailability of suitable surface sensitive techniques. Herein, we employ four-dimensional scanning ultrafast electron microscopy (4D S-UEM), which has the unique capability of mapping the charge carrier dynamics in real time and space selectively on the materials surfaces, to directly observe the effect of Na insertion on the carrier dynamics of shelled CIGSe film. It is found that an additional layer of NaF to the thin film of ZnS-shelled CIGSe nanocrystals not only increases the grain size and improves the texture of the film but, more importantly, reduces fast electron trap channels on the surface of the material, as observed from the secondary electron dynamics in 4D S-UEM. Our density functional theory calculations further confirm that Na ions can occupy Cu vacancies and reduce the interfacial charge carrier-defect scatterings. Removal of such undesirable electron trapping channels results in increased photoconductivity of the material, thereby serving as one of the critical parameters that lead to enhancement of the efficiency of CIGSe for light harvesting purposes.

Managing deposition of multilayered nanocrystal quantum dot (NQD) thin films is crucial for future photonic devices to maximize solar energy extraction efficiency. Solution based NQD deposition methods require additional protection to achieve a discrete layered structure and to prevent optical degradation during processing. An attractive method to passivate and protect NQD films is overcoating with metal oxides, usually grown using atomic layer deposition (ALD). However, a significant quenching of NQD photoluminescence (PL) is typically observed after encapsulation, hindering performance and applicability. Here, we demonstrate a modified gas-phase deposition technique that fully passivates NQD assemblies and, in contrast to standard ALD, maintains PL properties. Combined in situ FTIR and ex situ XPS measurements reveal that upon Al2O3 deposition by ALD, the metal precursor trimethylaluminum (TMA) interacts with oleic acid-capped CdSe-CdS-ZnS core-shell-shell NQDs by reorganizing the ligands and replacing Zn atoms with Al. This modification leads to PL quenching, particularly severe at elevated temperatures (similar to 100 degrees C). In contrast, simultaneous exposures of both precursors (TMA and water) lead to metal oxide deposition from gas-phase reactions taking place in the immediate vicinity of the NQD surface, without affecting the chemical nature of the NQDs. Contrary to ALD, this technique retains and even improves NQDs' photoluminescence, observed as increased PL intensities and longer lifetimes.

Engineering the surface energy through careful manipulation of the surface chemistry is a convenient approach to control quantum confinement and structure dimensionality during nanocrystal growth. Here, we demonstrate that the introduction of pyridine during the synthesis of methylammonium lead bromide (MAPbBr(3)) perovskite nanocrystals can transform three-dimensional (3D) cubes into two-dimensional (2D) nanostructures. Density functional theory (DFT) calculations show that pyridine preferentially binds to Pb atoms terminating the surface, driving the selective 2D growth of the nanostructures. These 2D nanostructures exhibit strong quantum confinement effects, high photoluminescence quantum yields in the visible spectral range, and efficient charge transfer to molecular acceptors. These qualities indicate the suitability of the synthesized 2D nanostructures for a wide range of optoelectronic applications.

Four-dimensional ultrafast electron microscopy (4D-UEM) is a novel analytical technique that aims to fulfill the long-held dream of researchers to investigate materials at extremely short spatial and temporal resolutions by integrating the excellent spatial resolution of electron microscopes with the temporal resolution of ultrafast femtosecond laser-based spectroscopy. The ingenious use of pulsed photoelectrons to probe surfaces and volumes of materials enables time-resolved snapshots of the dynamics to be captured in a way hitherto impossible by other conventional techniques. The flexibility of 4D-UEM lies in the fact that it can be used in both the scanning (S-UEM) and transmission (UEM) modes depending upon the type of electron microscope involved. While UEM can be employed to monitor elementary structural changes and phase transitions in samples using real-space mapping, diffraction, electron energy-loss spectroscopy, and tomography, S-UEM is well suited to map ultrafast dynamical events on materials surfaces in space and time. This review provides an overview of the unique features that distinguish these techniques and also illustrates the applications of both S-UEM and UEM to a multitude of problems relevant to materials science and chemistry.

Managing trap states and understanding their role in ultrafast charge-carrier dynamics, particularly at surface and interfaces, remains a major bottleneck preventing further advancements and commercial exploitation of nanowire (NW)-based devices. A key challenge is to selectively map such ultrafast dynamical processes on the surfaces of NWs, a capability so far out of reach of time-resolved laser techniques. Selective mapping of surface dynamics in real space and time can only be achieved by applying four-dimensional scanning ultrafast electron microscopy (4D S-UEM). Charge carrier dynamics are spatially and temporally visualized on the surface of InGaN NW arrays before and after surface passivation with octadecylthiol (ODT). The time-resolved secondary electron images clearly demonstrate that carrier recombination on the NW surface is significantly slowed down after ODT treatment. This observation is fully supported by enhancement of the performance of the light emitting device. Direct observation of surface dynamics provides a profound understanding of the photophysical mechanisms on materials' surfaces and enables the formulation of effective surface trap state management strategies for the next generation of high-performance NW-based optoelectronic devices.

Surface trap states in copper indium gallium selenide semiconductor nanocrystals (NCs), which serve as undesirable channels for nonradiative carrier recombination, remain a great challenge impeding the development of solar and optoelectronics devices based on these NCs. In order to design efficient passivation techniques to minimize these trap states, a precise knowledge about the charge carrier dynamics on the NCs surface is essential. However, selective mapping of surface traps requires capabilities beyond the reach of conventional laser spectroscopy and static electron microscopy; it can only be accessed by using a one-of-a-kind, second-generation four-dimensional scanning ultrafast electron microscope (4D S-UEM) with subpicosecond temporal and nanometer spatial resolutions. Here, we precisely map the collective surface charge carrier dynamics of copper indium gallium selenide NCs as a function of the surface trap states before and after surface passivation in real space and time using S-UEM. The time-resolved snapshots clearly demonstrate that the density of the trap states is significantly reduced after zinc sulfide (ZnS) shelling. Furthermore, the removal of trap states and elongation of carrier lifetime are confirmed by the increased photocurrent of the self-biased photodetector fabricated using the shelled NCs.