Organo-metal halide perovskites are promising solution-processed semiconductors, however, they possess diverse and largely not understood non-radiative mechanisms. Here, we resolve contributions of ...individual non-radiative recombination centers (quenchers) in nanocrystals of methylammonium lead iodide by studying their photoluminescence blinking caused by random switching of quenchers between active and passive states. We propose a model to describe the observed reduction of blinking upon cooling and determine energetic barriers of 0.2 to 0.8 eV for enabling the switching process, which points to ion migration as the underlying mechanism. Moreover, due to the strong influence of individual quenchers, the crystals show very individually-shaped photoluminescence enhancement upon cooling, suggesting that the high variety of activation energies of the PL enhancement reported in literature is not related to intrinsic properties but rather to the defect chemistry. Stabilizing the fluctuating quenchers in their passive states thus appears to be a promising strategy for improving the material quality.
Silver (Ag) clusters confined in matrices possess remarkable luminescence properties, but little is known about their structural and electronic properties. We characterized the bright green ...luminescence of Ag clusters confined in partially exchanged Ag-Linde Type A (LTA) zeolites by means of a combination of x-ray excited optical luminescence-extended x-ray absorption fine structure, time-dependent-density functional theory calculations, and time-resolved spectroscopy. A mixture of tetrahedral Ag
(H
O)
(
= 2 and
= 4) clusters occupies the center of a fraction of the sodalite cages. Their optical properties originate from a confined two-electron superatom quantum system with hybridized Ag and water O orbitals delocalized over the cluster. Upon excitation, one electron of the s-type highest occupied molecular orbital is promoted to the p-type lowest unoccupied molecular orbitals and relaxes through enhanced intersystem crossing into long-lived triplet states.
Halide perovskites possess enormous potential for various optoelectronic applications. Presently, a clear understanding of the interplay between the lattice and electronic effects is still elusive. ...Specifically, the weakly absorbing tail states and dual emission from perovskites are not satisfactorily described by existing theories based on the Urbach tail and reabsorption effect. Herein, through temperature-dependent and time-resolved spectroscopy on metal halide perovskite single crystals with organic or inorganic A-site cations, we confirm the existence of indirect tail states below the direct transition edge to arise from a dynamical Rashba splitting effect, caused by the PbBr
octahedral thermal polar distortions at elevated temperatures. This dynamic effect is distinct from the static Rashba splitting effect, caused by non-spherical A-site cations or surface induced lattice distortions. Our findings shed fresh perspectives on the electronic-lattice relations paramount for the design and optimization of emergent perovskites, revealing broad implications for light harvesting/photo-detection and light emission/lasing applications.
This Review summarizes the latest advances in the field of rylene dyes and rylene nanoemitters for applications in photonics, and describes the influence of the dye design on the optical properties, ...the self‐assembly, the molecular interactions, as well as the labeling specificity of the compounds. The interplay between tailored (macro)molecular design and bulk/single‐molecule spectroscopy enables complex processes to be explained, for example, the kinetics of energy‐transfer processes or (bio)catalysis. Such investigations are essential for the ultimate design of optimized nanoemitters, and require a close cooperation between spectroscopists and preparative organic chemists.
The electrochemical reduction of CO2 to multi-carbon products has attracted much attention because it provides an avenue to the synthesis of value-added carbon-based fuels and feedstocks using ...renewable electricity. Unfortunately, the efficiency of CO2 conversion to C2 products remains below that necessary for its implementation at scale. Modifying the local electronic structure of copper with positive valence sites has been predicted to boost conversion to C2 products. Here, we use boron to tune the ratio of Cuδ+ to Cu0 active sites and improve both stability and C2-product generation. Simulations show that the ability to tune the average oxidation state of copper enables control over CO adsorption and dimerization, and makes it possible to implement a preference for the electrosynthesis of C2 products. We report experimentally a C2 Faradaic efficiency of 79 ± 2% on boron-doped copper catalysts and further show that boron doping leads to catalysts that are stable for in excess of ~40 hours while electrochemically reducing CO2 to multi-carbon hydrocarbons.
The deposition of material at the edge of evaporating droplets, known as the 'coffee ring effect', is caused by a radially outward capillary flow. This phenomenon is common to a wide array of systems ...including colloidal and bacterial systems. The role of surfactants in counteracting these coffee ring depositions is related to the occurrence of local vortices known as Marangoni eddies. Here we show that these swirling flows are universal, and not only lead to a uniform deposition of colloids but also occur in living bacterial systems. Experiments on Pseudomonas aeruginosa suggest that the auto-production of biosurfactants has an essential role in creating a homogeneous deposition of the bacteria upon drying. Moreover, at biologically relevant conditions, intricate time-dependent flows are observed in addition to the vortex regime, which are also effective in reversing the coffee ring effect at even lower surfactant concentrations.
Formamidinium-lead-iodide (FAPbI
)-based perovskites with bandgap below 1.55 eV are of interest for photovoltaics in view of their close-to-ideal bandgap. Record-performance FAPbI
-based solar cells ...have relied on fabrication via the sequential-deposition method; however, these devices exhibit unstable output under illumination due to the difficulty of incorporating cesium cations (stabilizer) in sequentially deposited films. Here we devise a perovskite seeding method that efficiently incorporates cesium and beneficially modulates perovskite crystallization. First, perovskite seed crystals are embedded in the PbI
film. The perovskite seeds serve as cesium sources and act as nuclei to facilitate crystallization during the formation of perovskite. Perovskite films with perovskite seeding growth exhibit a lowered trap density, and the resulting planar solar cells achieve stabilized efficiency of 21.5% with a high open-circuit voltage of 1.13 V and a fill factor that exceeds 80%. The Cs-containing FAPbI
-based devices show a striking improvement in operational stability and retain 60% of their initial efficiency after 140 h operation under one sun illumination.
The room-temperature charge carrier mobility and excitation–emission properties of metal halide perovskites are governed by their electronic band structures and intrinsic lattice phonon scattering ...mechanisms. Establishing how charge carriers interact within this scenario will have far-reaching consequences for developing high-efficiency materials for optoelectronic applications. Herein we evaluate the charge carrier scattering properties and conduction band environment of the double perovskite Cs2AgBiBr6 via a combinatorial approach; single crystal X-ray diffraction, optical excitation and temperature-dependent emission spectroscopy, resonant and nonresonant Raman scattering, further supported by first-principles calculations. We identify deep conduction band energy levels and that scattering from longitudinal optical phononsvia the Fröhlich interactiondominates electron scattering at room temperature, manifesting within the nominally nonresonant Raman spectrum as multiphonon processes up to the fourth order. A Fröhlich coupling constant nearing 230 meV is inferred from a temperature-dependent emission line width analysis and is found to be extremely large compared to popular lead halide perovskites (between 40 and 60 meV), highlighting the fundamentally different nature of the two “single” and “double” perovskite materials branches.
The construction of advanced micro‐supercapacitors (MSCs) with both wide working‐voltage and high energy density is promising but still challenging. In this work, a series of nitrogen‐doped, ...cross‐coupled micro‐mesoporous carbon–metal networks (N‐STC/MxOy) is developed as robust additives to 3D printing inks for MSCs fabrication. Taking the N‐STC/Fe2O3 nanocomposite as an example, both experimental results and theoretical simulations reveal that the well‐developed hierarchical networks with abundantly decorated ultrafine Fe2O3 nanoparticles not only significantly facilitate the ion adsorption at its three‐phase boundaries (Fe2O3, N‐STC, and electrolyte), but also greatly favor ionic diffusion/transport with shortened pathways. Consequently, the as‐prepared N‐STC/Fe2O3 electrode delivers a high gravimetric capacitance (267 F g−1 at 2 mV s−1) and outstanding stability in a liquid‐electrolyte‐based symmetric device, as well as a record‐high energy density of 114 Wh kg−1 for an asymmetric supercapacitor. Particularly, the gravimetric capacitance of the ionogel‐based quasi‐solid‐state MSCs by 3D printing reaches 377 F g−1 and the device can operate under a wide temperature range (−10 to 60 °C).
A series of nitrogen‐doped, cross‐coupled micro‐mesoporous carbon–metal networks is developed as robust additives to 3D printing inks for the fabrication of micro‐supercapacitors (MSCs). Attributed to the abundant three‐phase boundaries, the ionogel‐based quasi‐solid‐state MSC exhibits an outstanding energy storage performance and a wide temperature range resistance for practical usage.
Defect engineering in photocatalysts represents a fundamental method toward tailoring their solar-to-chemical energy conversion performance, although determining the nature and impact of subsurface ...defects remains challenging. Single-unit-cell Bi2WO6 monolayers, forming a sandwich-like structure, BiO+–WO42––BiO+, exhibit promising photocatalytic performance and are an ideal system for isolating subsurface defects. We report the single-step synthesis of Bi2WO6 monolayers rich in stable interior W vacancies and characterize their influence on the physical properties necessary for effective photocatalytic surface reactions. Defect-rich monolayers benefit from enhanced visible-light absorption and photocarrier transport, boosting the solar photocatalytic oxidation of benzylic alcohols by 140% at no cost to selectivity or stability. This work highlights the importance of subsurface defects within surface-driven photocatalytic applications and prescribes a general strategy for their isolated study via 2D compounds exhibiting symmetric surface termination.
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