X-Ray-Induced Growth Dynamics of Luminescent Silver Clusters in Zeolites.

Herein, AlKα X-rays are used to drive the growth of luminescent silver clusters in zeolites. The growth of the silver species is tracked using Auger spectroscopy and fluorescence microscopy, by monitoring the evolution from their ions to luminescent clusters and then metallic, dark nanoparticles. It is shown that the growth rate in different zeolites is determined by the mobility of the silver ions in the framework and that the growth dynamics in calcined samples obeys the Hill-Langmuir equation for noncooperative binding. Comparison of the optical properties of X-ray-grown silver clusters with silver clusters formed by standard heat treatment indicates that the latter have a higher specificity toward the formation of luminescent clusters of a specific (small) nuclearity, whereas the former produce a wide distribution of cluster species as well as larger nanoparticles.

Tunable white emission of silver-sulfur-zeolites as single-phase LED phosphors.

Metal clusters confined inside zeolite materials display remarkable luminescent properties, making them very suitable as potential alternative phosphors in white LED applications. However, up to date, only single-color emitters have been reported for luminescent metal-exchanged zeolites. In this study, we synthesized and characterized white emitting silver-sulfur zeolites, which show a remarkable color tunability upon the incorporation of silver species in highly luminescent sulfur-zeolites. Via a combined steady-state and time-resolved photoluminescence spectroscopy characterization, we suggest that the observed luminescence and tunability arise from the presence of two different species. The first associated to an orange-red emitting silver cluster (Ag-CL), whereas the second is related to a blue-white emitting S-Ag-species. The relative contribution of both luminescent species depends on the synthesis procedure. It was shown that the formation of the blue-white emitting S-Ag-species is favored upon a heat-treatment of the samples.

Silver Zeolite Composite-Based LEDs: Origin of Electroluminescence and Charge Transport.

In this contribution, we report on the first time use of silver-exchanged zeolites embedded in the nonconductive polystyrene (PS) and their use as hybrid emitters in light-emitting diodes (ZEOLEDs). The turn on voltage and EL intensity are strongly dependent on the concentration of metal clusters. It is shown that the key to optimize this technology is improving the zeolite anode contact. Such an optimized device based on cheap abundant materials could provide an alternative for the commercial phosphor converted LEDs. A ZEOLED with a voltage polarity dependent color is demonstrated.

Photophysical Pathways in Highly Sensitive Cs2 AgBiBr6 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.

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

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.

Shaping the Optical Properties of Silver Clusters Inside Zeolite A via Guest-Host-Guest Interactions.

The appealing luminescent properties of Ag-zeolites have been shown to be dependent on the local environment of the confined silver clusters. Herein, we shed light on the properties of Ag clusters inside hydrated Linde-type A (LTA) zeolites and relate them to the nature of the host framework when expanded and compressed by the incorporation of Li+ cations and the Ag+ loading. Within this scenario, we measure a strong emission color shift in these materials, which we directly correlate with the fine structure details derived by optical luminescence-detected X-ray absorption in combination with deep UV-Raman spectroscopy and X-ray diffraction. Strong guest-host-guest interactions are revealed to underpin the variations in the optical properties; a modification in the zeolite lattice parameter results in changing bond lengths of the silver cluster. This interplay between the host zeolite and its confined guests can thus be harnessed to easily tune the Ag-zeolites' emission properties.

Atomic scale reversible opto-structural switching of few atom luminescent silver clusters confined in LTA zeolites.

Luminescent silver clusters (AgCLs) stabilized inside partially Ag exchanged Na LTA zeolites show a remarkable reversible on-off switching of their green-yellowish luminescence that is easily tuned by a hydration and dehydration cycle, making them very promising materials for sensing applications. We have used a unique combination of photoluminescence (PL), UV-visible-NIR Diffuse Reflectance (DRS), X-ray absorption fine structure (XAFS), Fourier Transform-Infrared (FTIR) and electron spin resonance (ESR) spectroscopies to unravel the atomic-scale structural changes responsible for the reversible optical behavior of the confined AgCLs in LTA zeolites. Water coordinated, diamagnetic, tetrahedral AgCLs [Ag4(H2O)4]2+ with Ag atoms positioned along the axis of the sodalite six-membered rings are at the origin of the broad and intense green-yellowish luminescence in the hydrated sample. Upon dehydration, luminescent [Ag4(H2O)4]2+ clusters are transformed into non-luminescent (dark), diamagnetic, octahedral AgCLs [Ag6(OF)14]2+ with Ag atoms interacting strongly with zeolite framework oxygen (OF) of the sodalite four-membered rings. This highly responsive on-off switching reveals that besides quantum confinement and molecular-size, coordinated water and framework oxygen ligands strongly affect the organization of AgCLs valence electrons and play a crucial role in the opto-structural properties of AgCLs.

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.

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

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.

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.

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.

Tuning the energetics and tailoring the optical properties of silver clusters confined in zeolites.

The integration of metal atoms and clusters in well-defined dielectric cavities is a powerful strategy to impart new properties to them that depend on the size and geometry of the confined space as well as on metal-host electrostatic interactions. Here, we unravel the dependence of the electronic properties of metal clusters on space confinement by studying the ionization potential of silver clusters embedded in four different zeolite environments over a range of silver concentrations. Extensive characterization reveals a strong influence of silver loading and host environment on the cluster ionization potential, which is also correlated to the cluster's optical and structural properties. Through fine-tuning of the zeolite host environment, we demonstrate photoluminescence quantum yields approaching unity. This work extends our understanding of structure-property relationships of small metal clusters and applies this understanding to develop highly photoluminescent materials with potential applications in optoelectronics and bioimaging.

Silver-induced reconstruction of an adeninate-based metal-organic framework for encapsulation of luminescent adenine-stabilized silver clusters.

Bright luminescent silver-adenine species were successfully stabilized in the pores of the MOF-69A (zinc biphenyldicarboxylate) metal-organic framework, starting from the intrinsically blue luminescent bio-MOF-1 (zinc adeninate 4,4'-biphenyldicarboxylate). Bio-MOF-1 is transformed to the MOF-69A framework by selectively leaching structural adenine linkers from the original framework using silver nitrate solutions in aqueous ethanol. Simultaneously, bright blue-green luminescent silver-adenine clusters are formed inside the pores of the recrystallized MOF-69A matrix in high local concentrations. The structural transition and concurrent changes in optical properties were characterized using a range of structural, physicochemical and spectroscopic techniques (steady-state and time-resolved luminescence, quantum yield determination, fluorescence microscopy). The presented results open new avenues for exploring the use of MOFs containing luminescent silver clusters for solid-state lighting and sensor applications.

Fabrication of silver nanoparticles with limited size distribution on TiO2 containing zeolites.

Here we present a simple route to produce well-defined photo-reduced silver nanoparticles on TiO2 containing zeolites. We used natural and artificial irradiation sources to study their effect on the particle size distribution. The samples were investigated by electron microscopy, X-ray diffraction, fluorescence microscopy and UV-Vis diffuse reflectance spectroscopy.