Wouter Baekelant

Direct Observation of Luminescent Silver Clusters Confined in Faujasite Zeolites.

One of the ultimate goals in the study of metal clusters is the correlation between the atomic-scale organization and their physicochemical properties. However, direct observation of the atomic organization of such minuscule metal clusters is heavily hindered by radiation damage imposed by the different characterization techniques. We present direct evidence of the structural arrangement, at an atomic level, of luminescent silver species stabilized in faujasite (FAU) zeolites using aberration-corrected scanning transmission electron microscopy. Two different silver clusters were identified in Ag-FAU zeolites, a trinuclear silver species associated with green emission and a tetranuclear silver species related to yellow emission. By combining direct imaging with complementary information obtained from X-ray powder diffraction and Rietveld analysis, we were able to elucidate the main differences at an atomic scale between luminescent (heat-treated) and nonluminescent (cation-exchanged) Ag-FAU zeolites. It is expected that such insights will trigger the directed synthesis of functional metal nanocluster-zeolite composites with tailored luminescent properties.

Thermally activated LTA(Li)–Ag zeolites with water-responsive photoluminescence properties

Silver–zeolite composites are interesting materials with unique optical properties such as high external quantum efficiencies and large Stokes shifts. The selective formation of luminescent silver clusters within zeolite scaffolds can be achieved by varying silver guest and zeolite host conditions. Nevertheless, at present, the controlled synthesis of Ag–zeolite composites with responsive optical properties remains a challenge. In this report, silver–zeolite composites displaying a dynamical emission color change with respect to their water content were synthesized using LTA zeolites containing lithium cations as counter-balancing agents. An intense blue emission was encountered in partially hydrated LTA(Li)–Ag composites, at low silver loadings, whereas a green/yellow emission was observed in their fully hydrated state. The materials synthesized in this report possess high external quantum efficiencies, up to 62%, compared to their close analogues having Na, K, and Ca as counter-balancing ions. Due to the remarkable dynamical change in emission color depending on the hydration level of LTA(Li)–Ag composites, the use of these materials as luminescence-based humidity sensors is suggested.

Silver Clusters in Zeolites: From Self-Assembly to Ground-Breaking Luminescent Properties

Interest for functional silver clusters (Ag-CLs) has rapidly grown over years due to large advances in the field of nanoscale fabrication and materials science. The continuous development of strategies to fabricate small-scale silver clusters, together with their interesting physicochemical properties (molecule-like discrete energy levels, for example), make them very attractive for a wide variety of applied research fields, from biotechnology and the environmental sciences to fundamental chemistry and physics. Apart from useful catalytic properties, silver clusters (Agn, n < 10) were recently shown to also exhibit exceptional optical properties. The optical properties and performance of Ag-CLs offer strong potential for their integration into appealing micro(nano)-optoelectronic devices. To date, however, the rational design and directed synthesis of Ag-CLs with specific functionalities has remained elusive. The inability for rational design stems mainly from a lack of understanding of their novel atomic-scale phenomena. This is because accurately studying silver cluster systems at such a scale is hindered by the perturbations introduced during exposure to various experimental probes. For instance, silver possesses a strong tendency to cluster and form ever-larger Ag aggregates while probed with high-energy electron beams and X-ray irradiation. As well, there exists a need to provide a stabilizing environment for which Agnδ+ clusters can persist, setting up a complex interacting guest-host system, as isolated silver clusters are confined within a suitable hosting medium. Fundamental research into Agnδ+ formation mechanisms and their important optical properties is paramount to establishing truly informed synthesis protocols. Over recent years, we have developed several protocols for the ship-in-a-bottle synthesis of highly luminescent Ag-CLs within the microporous interiors of zeolite frameworks. This approach has yielded materials displaying a wide variety of optical properties, offering a spectrum of possible applications, from nano(micro)photonic devices to smart luminescent labels and sensors. The versatility of the Ag-zeolite multicomponent system is directly related to the intrinsic and complex tunability of the system as a whole. There are several key zeolite parameters that confer properties to the clusters, namely, the framework Si/Al ratio, choice of counterbalancing ions, silver loading, and zeolite topology, and cannot be overlooked. This Account is intended to shed light on the current state-of-the-art of luminescent Ag-CLs confined in zeolitic matrices, emphasizing the use of combinatorial approaches to overcome problems associated with the correct characterization and correlation of their structural, electronic, and photoluminescence properties, all to establish the important design principles for developing functional silver-zeolite-based materials. Additionally, examples of emerging applications and future perspectives for functional luminescent Ag-zeolite materials are addressed in this Account.

Lead silicate glass SiO2–PbF2 doped with luminescent Ag nanoclusters of a fixed site

Bulk SiO2–PbF2 glass doped homogeneously with luminescent Ag nanoclusters has been synthesized using a melt-quenching technique. A broad range of luminescence from 400 nm to 900 nm can be excited with a broad range of UV and some visible excitation wavelengths. The luminescence spectra have been found to be invariant with excitation wavelength at room temperature, indicating a negligible distribution of Ag cluster sites. The UV excitation efficiency of Ag nanoclusters has been determined by shifting the absorption edge further into the UV using an Al2O3 glass-network modifier. The prepared glass is proposed as a low-cost glass phosphor for Hg-free environmentally-friendly white light generation in UV/blue-driven light emitting diodes, in flexible monitors and as down-shifting layers for enhanced solar cells.

Facile Morphology-Controlled Synthesis of Organolead Iodide Perovskite Nanocrystals Using Binary Capping Agents

Abstract Controlling the morphology of organolead halide perovskite crystals is crucial to a fundamental understanding of the materials and to tune their properties for device applications. Here, we report a facile solution‐based method for morphology‐controlled synthesis of rod‐like and plate‐like organolead halide perovskite nanocrystals using binary capping agents. The morphology control is likely due to an interplay between surface binding kinetics of the two capping agents at different crystal facets. By high‐resolution scanning transmission electron microscopy, we show that the obtained nanocrystals are monocrystalline. Moreover, long photoluminescence decay times of the nanocrystals indicate long charge diffusion lengths and low trap/defect densities. Our results pave the way for large‐scale solution synthesis of organolead halide perovskite nanocrystals with controlled morphology for future device applications.

'Just Clean Enough': Wet Cleaning for Solar Cell Manufacturing Applications

The cumulative installed solar power generation has been rising exponentially over the past decade. This has lead to a concomitant rise in production capabilities, leading eventually to excess production capabilities and rapid price declines per unit. In order to compete with the standard electricity generation the cost of solar panel production and installation needs to decrease even further. At the same time the solar panel and cell makers need to be able to keep a healthy margin. A crucial element in this exercise is a close control on the Cost of Ownership (CoO) of a solar cell / panel fabrication site.

Origin of the bright photoluminescence of few-atom silver clusters confined in LTA zeolites

Unmasking the glow of silver clusters Small silver clusters stabilized by organic materials or inorganic surfaces can exhibit bright photoluminescence, but the origin of this effect has been difficult to establish, in part because the materials are heterogeneous and contain many larger but inactive clusters. Grandjean et al. studied silver clusters in zeolites, using x-ray excited optical luminescence to monitor only the emissive structures (see the Perspective by Quintanilla and Liz-Marzán). Aided by theoretical calculations, they identified the electronic states of four-atom silver clusters bound with water molecules that produce bright green emission—thus identifying candidate materials for application in lighting, imaging, and therapeutics. Science, this issue p. 686; see also p. 645 The bright luminescence of Ag-LTA zeolites originates from long-lived triplet states in Ag4(H2O)2 and Ag4(H2O)4 clusters. 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 Ag4(H2O)x2+ (x = 2 and x = 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.

Luminescence of fixed site Ag nanoclusters in a simple oxyfluoride glass host and plasmon absorption of amorphous Ag nanoparticles in a complex oxyfluoride glass host

Ag nanocluster-doped glasses have been prepared by a conventional melt-quenching method. The effect of melt temperature and dwell time on the formation of Ag nanoclusters and Ag nanoparticles in simple host oxyfluoride glasses has been studied. The increase of melt temperature and dwell time results in the dissolution of Ag nanoparticles and substantial red-shift of absorption and photoluminescence spectra of the prepared glasses. The quantum yield of the glasses is ~ 5% and does not depend on melt temperature and dwell time. The prepared glasses may be used as red phosphors or down-conversion layers for solar-cells.

Photophysical Pathways in Highly Sensitive Cs2AgBiBr6 Double‐Perovskite Single‐Crystal X‐Ray Detectors

The sensitive detection of X-rays embodies an important research area, being motivated by a common desire to minimize the radiation doses required for detection. Among metal halide perovskites, the double-perovskite Cs2 AgBiBr6 system has emerged as a promising candidate for the detection of X-rays, capable of high X-ray stability and sensitivity (105 μC Gy-1 cm-2 ). Herein, the important photophysical pathways in single-crystal Cs2 AgBiBr6 are detailed at both room (RT) and liquid-nitrogen (LN2 T) temperatures, with emphasis made toward understanding the carrier dynamics that influence X-ray sensitivity. This study draws upon several optical probes and an RT excitation model is developed which is far from optimal, being plagued by a large trap density and fast free-carrier recombination pathways. Substantially improved operating conditions are revealed at 77 K, with a long fundamental carrier lifetime (>1.5 µs) and a marked depopulation of parasitic recombination pathways. The temperature dependence of a single-crystal Cs2 AgBiBr6 X-ray detecting device is characterized and a strong and monotonic enhancement to the X-ray sensitivity upon cooling is demonstrated, moving from 316 μC Gy-1 cm-2 at RT to 988 μC Gy-1 cm-2 near LN2 T. It is concluded that even modest cooling-via a Peltier device-will facilitate a substantial enhancement in device performance, ultimately lowering the radiation doses required.

Nanostructured Ag-zeolite Composites as Luminescence-based Humidity Sensors.

Small silver clusters confined inside zeolite matrices have recently emerged as a novel type of highly luminescent materials. Their emission has high external quantum efficiencies (EQE) and spans the whole visible spectrum. It has been recently reported that the UV excited luminescence of partially Li-exchanged sodium Linde type A zeolites [LTA(Na)] containing luminescent silver clusters can be controlled by adjusting the water content of the zeolite. These samples showed a dynamic change in their emission color from blue to green and yellow upon an increase of the hydration level of the zeolite, showing the great potential that these materials can have as luminescence-based humidity sensors at the macro and micro scale. Here, we describe the detailed procedure to fabricate a humidity sensor prototype using silver-exchanged zeolite composites. The sensor is produced by suspending the luminescent Ag-zeolites in an aqueous solution of polyethylenimine (PEI) to subsequently deposit a film of the material onto a quartz plate. The coated plate is subjected to several hydration/dehydration cycles to show the functionality of the sensing film.