Silver Clusters in Zeolites: From Self-Assembly to ... - ACS Publications

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Silver Clusters in Zeolites: From Self-Assembly to Ground-Breaking Luminescent Properties Eduardo Coutiño-Gonzalez,†,§,⊥ Wouter Baekelant,†,⊥ Julian A. Steele,‡ Cheol Woong Kim,† Maarten B. J. Roeffaers,‡ and Johan Hofkens*,† †

Chem&Tech - Molecular Imaging and Photonics, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium CONACYT - Centro de Investigación y Desarrollo Tecnológico en Electroquímica, Parque Industrial Querétaro, Sanfandila s/n, Pedro Escobedo, 76703 Querétaro, Mexico ‡ Chem&Tech - Centre for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium §

CONSPECTUS: Interest for functional silver clusters (AgCLs) 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.

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INTRODUCTION At their smallest scale, metal clusters, consisting of only several metal atoms, possess unique properties that strongly differ from nanoparticles and bulk metals. The electronic configuration and discrete energy levels of metal clusters (see Figure 1A) manifest some remarkable electronic and optical properties,1,2 such as molecule-like energy transitions,3,4 strong photoluminescence,5,6 and high catalytic activity.7−9 The synthesis and application of metal clusters with multifunctional properties remain a hot research area. For the pertinent case of luminescent silver clusters (Ag-CLs), interest in the field remains strong, exhibiting truly exponential growth in the past © 2017 American Chemical Society

10 years. Such growth is owed in part to improvements made in Ag-CL synthesis procedures, making these materials more accessible to a wide variety of research laboratories. Ultimately, however, the recent intense scientific interest is due to the fact that insights into these clusters hold strong promise to bridging the gap in our understanding between silver atomic and nanoparticle behavior.10 The synthesis of functional luminescent Ag-CLs is not straightforward; their molecule-like properties and optical Received: June 12, 2017 Published: September 1, 2017 2353

DOI: 10.1021/acs.accounts.7b00295 Acc. Chem. Res. 2017, 50, 2353−2361

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Accounts of Chemical Research

Figure 1. (A) Size dependency of metal properties, showing metal clusters as the link between atoms and nanoparticles, displaying discrete energy levels leading to molecular-like photoluminescence properties. (B) Activation of luminescent Ag-CLs confined inside the sodalite cages of faujasite (FAU) and Linde-type A (LTA) zeolites.

response is defined by the cluster size, geometry, and charge.11 Because of the strong tendency for silver to aggregate, the appealing properties only persist while they are spatially and chemically stabilized into a few silver atoms (Agnδ+) (Figure 1B). To achieve stabilization several strategies have been explored12 with microporous aluminosilicate zeolites13 proving to be an ideal scaffold; zeolite frameworks possess the appropriate molecular-sized pores and cages to contain AgCLs, as well as the cation-exchange capacities to facilitate the uptake of silver ions. Forming the Ag-CLs within zeolites involves a readily scalable bottom-up approach, whereby Ag+ ions self-organize into clusters in a process known as “activation” induced via thermal14,15 or electromagnetic16,17 energy (Figure 1B). Silver−zeolite composites have been employed as catalysts,18 adsorbents,19 molecular sieves,20 and recently phosphors for lighting applications (Figure 1B), with emissions spanning over the whole visible spectrum and external quantum efficiencies (EQEs) up to 97%.15,21,22 The detailed characterization of AgCLs confined in zeolites at the atomic scale is crucial for understanding the size−structure−function relationship and for their rational material optimization. However, physicochemical and structural characterization is challenging due to the size and the sensitivity to perturbations introduced by different experimental probes, which lead to the formation of larger aggregates when examined with high-energy electron beams, Xray irradiation, etc.17,23 In this Account, an up-to-date overview of the characterization techniques utilized in the study of silver−zeolite composites will be given, with a focus toward how experimental insights can lead to their optimization and

which crucial questions remain unanswered. Moreover, we will touch on the emerging applications of luminescent Ag−zeolite systems.

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CHARACTERIZATION TECHNIQUES The exact optical absorption and emission centers of different Ag−zeolite assemblies remains elusive, preventing at the moment a deep physical understanding of their unique optical properties. This is partly due to the limited structural details (at the atomistic level) available in the literature, stifling attempts to piece together the exact structure of Ag-CLs. Examining the full extent of information gathered thus far, it is clear that the optical and electronic properties of silver clusters are influenced by their size, composition, morphology, and electronic state.14,15,21 Within this context, a combinatorial approach spanning many materials characterization techniques (overview in Figure 2) has been employed to connect structure with luminescence and overcome the problems associated with material sensitivity.16,17,24 Optical Characterization of Ag−Zeolite Materials

Ag-CLs display molecular-like behavior and discrete energy levels (Figure 1A),25 possessing optical transitions with absorbance and emission bands in the UV−vis range. UV−vis spectroscopy has been successfully applied to characterize different Ag-CLs formed in zeolite matrices. For instance, the origin of the yellow color of silver-containing Linde type A (LTA) zeolites was thoroughly studied by Calzaferri and collaborators.26,27 They showed that upon activation under high vacuum, silver exchanged LTA zeolites turned yellow at 2354


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Figure 2. Characterization techniques employed in the study of luminescent Ag-CLs confined in zeolites highlighting the combinatorial and holistic approaches used nowadays. Adapted from refs 16, 41, and 42. Copyright 2008 Wiley-VCH GmbH & Co. KGaA, Weinheim. Copyright 2016 American Chemical Society. Copyright 2013 The Royal Society of Chemistry.

room temperature and brick-red at 200 °C and that all absorption bands appearing in the UV−vis region were dependent on the hydration of the Ag−zeolite samples. This dynamical change of colors was correlated to different electronic transitions from the lone pairs of oxygen atoms of the zeolite framework to the empty 5s orbital of the Ag ions (ligand-to-metal charge transfer, LMCT). However, UV−vis spectroscopy interrogates at the same time the optical transitions of emissive and nonemissive silver species (AgCLs, silver cations, and in some cases Ag nanoparticles) confined in zeolite scaffolds hindering the specific analysis of luminescent Ag-CLs. Recently, a comprehensive photoluminescence (PL) study on silver exchanged zeolites at room temperature was reported by our group,16 in which individual silver containing zeolite crystals were photoactivated using a focused laser beam in a fluorescence microscope (see Figure 2). The formation of extremely luminescent green emitting species in Ag-LTA was demonstrated, stimulating research in our group and leading to the fluorescence characterization of heat-treated silver exchanged zeolites, at bulk scale, with LTA and faujasite (FAU) topology.14 Emitters with different characteristic

emission colors were reported and shown to be dependent on the nature of the counterbalancing ions (K+, Na+, Ca2+) present in the zeolite framework, the relative silver loading, and the zeolite topology. Green, yellow, and red emission colors were tentatively assigned to partially reduced Ag3m+ and Ag6n+ clusters, respectively. With fluorescence lifetimes in the nanoand microsecond time range,14 they differ from the purely nanosecond lifetimes of luminescent Ag−DNA.12 Thus, current research focuses on deciphering the complex photophysical processes occurring in Ag−zeolite systems. Subsequently, we investigated the influence of heat treatment on the evolution of luminescence in silver exchanged zeolites (FAUY and LTA) under controlled atmosphere by using in situ fluorescence microscopy.28 The results suggested that a stable green/yellow Ag3+ emitter was most efficiently created in a dry zeolite FAUY host, whereas in zeolite LTA such a cluster interacts with neighboring clusters or with water, decreasing the luminescence. While characterization of emission color is straightforward, the precise determination of the external quantum efficiencies (EQEs) is more challenging, not only due to the highly scattering nature but also the UV absorption of the AgCLs. Employing a robust customized methodology based on an 2355


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Figure 3. Combined HAADF-STEM, XRD, and photoluminescence approach followed to decipher the structure of Ag-CLs confined in FAU zeolites. Adapted from ref 41. Copyright 2016 American Chemical Society.

analyzing the interactions between hydroxyl groups and AgCLs.34 IR spectroscopy does provide very useful information, for instance, in analyzing the role of water on the optical properties of silver loaded zeolites. X-ray diffraction (XRD) has been extensively used to determine metal ions and clusters within crystalline matrices,35 providing information that is averaged over the ordered framework structure. As such, XRD is a preferred structural characterization technique in the study of inorganic/organic guests inside natural and synthetic zeolites,36 and in the case of silver exchanged zeolites, several models have been put forward for the common topologies FAU and LTA. However, despite many efforts to use XRD to unravel the exact structure of AgCLs confined in zeolites, a conclusive model is yet to be obtained. For instance, the formation of Ag22+, Ag3+, and Ag32+ silver species in fully exchanged Ag−FAUX zeolites (Si/Al = 1.08) heat-treated under an oxygen rich environment has been reported,37 whereas larger silver species (Ag4n+, Ag8m+) were described in fully exchanged Ag−FAUX zeolites (Si/Al = 1.08) treated under a reductive atmosphere.38 Information regarding the charge of the Ag-CLs is deduced indirectly from the relative atomic locations. Although the above-mentioned techniques are well established, discrepancies among several reports exist most probably due to differing synthesis conditions and environmental parameters, resulting in a wide variety of narrowly specific Ag-CL models. Therefore, there is a need for advanced characterization techniques that could provide detailed structural information, at the atomic level, of luminescent AgCLs confined in zeolites.

integrating sphere setup, the influence of zeolite topology, purity, silver loading, and choice of counterbalancing ions (K+, Na+, Ca2+) on the EQE was determined.21 Our work established a close connection between the local environment and the Ag-CL optical properties with the highest EQE values (56%) for Ag−FAU samples, whereas Ag−LTA zeolites displayed EQEs of up to 16%. Even relatively straightforward PL spectroscopy has proven to be an extremely useful tool in the study of Ag-CLs. However, arriving at a comprehensive model that effectively connects important parameters of Ag− zeolite photochemistry requires a multifaceted approach; one joining traditional techniques, like PL spectroscopy, with newer and more advanced structural characterization methods. Conventional Structural Characterization Techniques

Traditional structural characterization techniques (examples include electron spin resonance (ESR), infrared (IR) spectroscopy, and X-ray crystallography) have been extensively used in past decades to elucidate the structure of silver-exchanged zeolites, but never in relation to their luminescent state. ESR provides useful information related to the structure and magnetic properties of silver clusters, as well as their location inside the host framework, stability, and reactivity toward different ligands. For example, the stabilization of Ag30 and Ag6+ clusters during γ-irradiation of silver exchanged LTA zeolites was demonstrated.29 ESR experiments further revealed the formation mechanisms of Ag-CLs in zeolites with different silver contents. The formation of trimeric species (Ag3+) was reported in low silver loaded samples, while hexameric (Ag62+), attributed to two interacting trimers, was found at higher silver loadings.14 Nevertheless, ESR analysis is sensitive only toward paramagnetic Ag-CLs, limiting its application toward paramagnetic emissive and nonemissive species. IR spectroscopy directly probes the vibrational properties of Ag−zeolite systems (see Figure 2), revealing interactions between extraframework species (Ag/Na cations, water molecules, adsorbates, and Ag-CLs) and the zeolite T−O−T bonded (T = Si or Al) framework.30−32 Using IR, Baumann and collaborators managed to show the interactions of CO and CO2 with different silver species in reduced Ag-LTA zeolites giving rise to structural changes and kinetics of cluster formation.33 Even further, Wang and collaborators employed in situ Fourier transformed infrared (FT-IR) spectroscopy to determine the location of silver species in silver exchanged LTA zeolites, by

Advanced Structural Characterization Techniques

One of the preferred techniques is extended X-ray absorption fine structure (EXAFS),15 which provides valuable data related to metal-to-metal and metal-to-ligand bonding, allowing an estimate of cluster nuclearity to be made, even in structures lacking long-range order (such as metal clusters). EXAFS experiments can also be applied readily in situ with controlled temperature and atmosphere. Compared to XRD, EXAFS produces more local information such as the nature and coordination of neighboring species with respect to the X-ray absorbing atoms. By using this technique, metallic and cationic silver clusters Ag20, Ag84+, and Ag124+ (the charge of silver species is mostly inferred from the Ag−Ag contact distances 2356


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Figure 4. Compilation of the potential applications in which luminescent silver exchanged zeolites could be employed. Adapted from references 15, 50, and 52. Copyright 2015, The Royal Society of Chemistry. Copyright 2017 and 2010 Wiley-VCH GmbH & Co. KGaA, Weinheim.

membered rings of the sodalite. However, the systematic analysis and correlation between the atomic-scale organization and the photoluminescence properties of such silver clusters were not addressed. By combining HAADF-STEM experiments with powder XRD and subsequent Rietveld analysis, a complete three-dimensional structural characterization of heat-treated luminescent silver-exchanged FAU zeolites possessing different Si/Al ratios was possible.41 In this study, two different silver clusters were identified in Ag-FAU zeolites, a trinuclear species associated with a green emission and a tetranuclear one related to yellow emissions (Figure 3), and the structural differences between samples in their luminescent (heat-treated) and nonluminescent (cation-exchanged) states were also revealed. This report clearly exemplifies the need for several characterization tools to perform the detailed analysis of challenging and dynamic functional nanostructured systems.

and coordination numbers, giving their neutral or charged nature) have been reported for silver exchanged zeolite LTA and FAUX.39,40 Even more, a direct correlation between Ag-CL nuclearity, the hydration level, and their luminescent properties was made for Ag-exchanged Li-LTA zeolites.15 Nevertheless, the EXAFS data collection and interpretation on silver exchanged zeolites should be performed with caution, as we have recently demonstrated that the X-ray beam has great influence on the dynamics of formation and destruction of luminescent silver clusters in LTA and FAU zeolites.17 The avenue of advanced aberration correction has made it possible to precisely locate individual atoms using transmission electron microscopy (TEM). However, zeolites containing silver clusters are known to be prone to radiation damage;23,24 this is particularly the case for topologies having a relatively high aluminum content, such as LTA and FAU zeolites, which are two of the most employed zeolite scaffolds for the stabilization of Ag-CLs (Figure 1C). High angle annular dark field scanning transmission electron microscopy (HAADFSTEM) was recently used to unravel the structure of subnanometer silver species in fully exchanged Ag−LTA zeolites,23 which had been a matter of debate for over four decades. An octahedral Ag6 structure was visualized within the sodalite cages of heat-treated Ag−LTA zeolites, surrounded by a cubic arrangement containing eight Ag atoms located at six-

Emerging Computational Methods

Theoretical modeling has recently matured to a level that model guided design has become within reach, and recent increases in computational power are likely the catalyst for the rise in investigations into relatively large systems, like zeolites. A close synergy between experiments and theoreticians has led to a deep understanding of complex systems; computational modeling is able to elucidate the structural, electronic, and 2357


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advances in the field of luminescent metal-containing zeolites together with the recent technological progress in the lighting industry might act to accelerate this process. The near future might bear witness to the first generation of light emitting devices using Ag−zeolite phosphors, providing a much-needed break-through toward the replacement of lanthanide-based phosphors.

compositional information on Ag-CLs stabilized in zeolites. However, at present, due to the complexity and size of the silver−zeolite systems, only a small volume of studies have simulated their important optical properties.42−44 A representative example is the work of Nguyen and collaborators42 where the absorption bands of two emitting silver species, Ag80 and Ag146+ clusters, embedded in the sodalite cavity of fully silver exchanged LTA zeolites were predicted, although the latter cluster (Ag146+) is unlikely to fit within the sodalite cages of LTA due to size constraints. In a subsequent study, the stability of Ag-CLs, Agn with n = 3, 4, 6, and 8, embedded in the sodalite cages of EMT zeolites (Si/Al = 1) was demonstrated.43 In a very recent study, the electronic properties of Ag-CLs confined in ZSM-5 zeolites were theoretically calculated using superatom orbital ideas,44 demonstrating that S → P electronic transitions in the UV-region from a S2 electronic configuration in the ground state are responsible for the absorption properties of the clusters. This is in line with the luminescent mechanism proposed by Cuong and co-workers, where the optical spectra of hydrated doubly charged Ag42+ and hydrated multiply charged Ag6p+ Ag-CLs encapsulated into the sodalite cavities of partially and fully silver exchanged zeolites were systematically predicted using density functional theory methods (DFT, TDDFT and CASSCF/CASPT2).45 These theoretical results could thus explain the mechanism involved in the luminescent properties of the Ag−zeolite systems; however, experimental proof of this mechanism is yet under investigation.

Silver−Zeolites As Smart Fluorescent Tags

Inspired by the pioneering work of Dickson and collaborators,51 we recently demonstrated the photoactivation of AgCLs in single large zeolite crystals (see Figure 4) and a new type of optically encoded microcarrier using a two-photon activation process with near-infrared light.52 It is believed the formation of bright internal Ag-CLs was accomplished through photochemical reduction of silver ions (serving as counterbalancing ions in the framework) present in nonirradiated samples. Very high writing resolutions down to the diffraction limit were obtained, as well as an excellent readability due to the positive contrast imaging and brightness stability of the emissive clusters. This strategy yielded several advanced matrix codes in different layers of individual zeolite microcarriers and offered excellent 3D resolution. Therefore, these materials open new opportunities for safe and quality labels to prevent counterfeiting, for instance, in bank notes. Moreover, this strategy has also been extended to other confinement hosts (using Ag-CLs as guests) such as polymers53,54 and glass matrices.55 Moreover, recent advances in the field of zeolite synthesis have demonstrated the possibility to fabricate nanosized zeolites56 that could be potentially used, in combination with the two-photon activation strategy, as smart labels for bioimaging.

Combinatorial Characterization Approach

Although a range of methods have been employed to decipher the physicochemical properties and formation mechanisms of Ag-CLs in zeolites, there is no universal technique to deal with this task, and a combinatorial approach is needed (see Figure 3). Implementing a particular technique above others is determined by the physical approach and the information required (Figure 2). A clear example resides in our recent work22 where an in-depth characterization of luminescent silver-exchanged zeolites with various advanced spectroscopy techniques provided unambiguous evidence of a strong influence of the zeolite host and degree of silver uptake on the structural, electronic, and optical properties of Ag-CLs in zeolites. The deep understanding acquired allowed the development of rational design rules for assembling metal clusters in zeolites. Further, by applying such design principles, the synthesis of materials with luminescence efficiencies close to 100% was possible.

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Silver−Zeolites As Responsive Luminescence-Based Sensors

The ability of Ag-CLs in zeolites to optically respond to external stimuli can be utilized in sensing applications. Several reports have proposed Ag−zeolites as vapor sensors exploiting their hydration level dependent absorption and luminescence.13,28,40,57−59 For instance, the exposure of partially exchanged heat-treated Ag−LTA zeolites to high humidity environments led to a decrease of their luminescence intensity.28 This process was demonstrated to be fully reversible and a subsequent drying step by mild heating showed the reestablishment of their original luminescent properties. Up to now, the sensing characteristics of Ag-CLs in zeolites were based on a “positive or negative” response using the depletion of their luminescence properties. However, in recent studies, only “positive” response systems have been demonstrated. For instance, Lin and collaborators reported on the reversible emission evolution of Ag−zeolites upon dehydration/hydration cycles.57 Moreover, the dynamic emission color change of AgCLs stabilized in LTA (containing lithium as counter balancing ion) zeolites with respect to their water content was shown by our group (Figure 4).15 This major improvement in the stimulito-response process could have direct implications in the use of these materials as luminescence-based humidity sensors at macro- and microscale.

APPLICATIONS

Silver−Zeolite Materials for Lighting Applications

Luminescent silver−zeolite composites display interesting emissive properties, such as large Stokes shifts, high external quantum efficiencies, and large photostability, among others.14−16,21,22 Depending on the zeolite topology, the presence of specific counterbalancing ions, and the silver loading, different emissive Ag-CLs can be created upon thermal treatment, with spectral properties ranging from blue to red. Therefore, this new class of photostable luminescent materials with tunable emission colors offer interesting possibilities as wavelength converters in fluorescent lamps, LEDs, and hybrid OLEDs (Figure 4). To date, several publications and patents have demonstrated the high applicability of these materials to be implemented in light emitting devices.14,21,22,46−50 Yet, no working prototypes have been developed. However, the

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CONCLUSIONS In this Account, we have addressed recent progress in the exciting field of luminescent metal clusters self-assembled in confined spaces, and particularly the case of highly luminescent Ag-CLs in zeolite matrices. Also several aspects dealing with 2358


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Accounts of Chemical Research characterization approaches and potential applications for such materials were thoroughly discussed. Due to the recent advances in the intricate host−guest chemistry of zeolites, it has been possible to design and synthesize functional metal clusters with specific size, geometry, and charge within their porous framework. For the case of silver−zeolite composites, this development has resulted in the fabrication of a wide palette of emitting clusters spanning the whole visible spectrum displaying high external quantum efficiencies (almost unity), great chemical and photostability, and responsive optical properties. Altogether, these useful optical properties could be applied for the development of smart sensing devices and more efficient (and cost-effective) phosphors for lighting applications. Looking beyond these optoelectronic applications, Ag-CLs have been shown to display exceptional catalytic properties.60 Though to date, the relationship between the electronic and structural properties of such metal nanostructures is still not fully understood. This is mainly due to the challenges associated with the experimental and theoretical characterization of such clusters at the atomic level. To overcome this problem, complementary and holistic approaches have been developed and applied to understand the different phenomena that govern the electronic and optical properties of luminescent Ag-CLs confined in zeolites at the atomic scale. Ultimately, the deep understanding obtained will allow the development of rational design rules for assembling silver and other metal clusters in zeolites. There is still a long way ahead to arrive at a clear image of how the electronic and structural features of Ag-CLs stabilized in zeolites influence their optical properties; however, innovative studies have recently paved the way to achieve such goals. Finally, it is expected that the scientific community will get inspired by the approach followed to self-assemble luminescent metal clusters in zeolites, which will ultimately trigger the development of related novel nanostructured materials, such as the case of luminescent metal doped MOFs, for which the first examples have been recently reported.61,62

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of functional materials for clean energy production, catalysis, and environmental remediation. Wouter Baekelant is working as a Ph.D. student at KU Leuven, investigating the luminescent properties of metal clusters confined inside zeolite scaffolds, under the supervision of Prof. Hofkens and Prof. Roeffaers. Julian A. Steele received his Ph.D. in solid-state physics at The Institute for Superconducting and Electronic Materials, University of Wollongong (Australia), under the supervision of Prof. Roger Lewis, working in the structural and optical characterization of bismuthcontaining optoelectronic materials. At the start of 2016, he joined the group of Prof. Roeffaers (KU Leuven, Belgium) as postdoctoral researcher where he is mainly working on nanoscale optical materials, whereby luminescent metal clusters confined inside zeolites feature heavily. Cheol Woong Kim received his Ph.D. in applied chemistry at the Kyungpook National University (Korea) under the supervision of Prof. Nam Ho Heo, working in the crystallographic characterization of zeolite single-crystals containing functional metals. After a postdoctoral period in the same group, he joined the group of Prof. Hofkens (KU Leuven, Belgium), as postdoctoral researcher where he is mainly working on the preparation and characterization of luminescent metal clusters confined inside zeolites. Maarten B. J. Roeffaers graduated from the Centre for Surface Chemistry and Catalysis, KU Leuven (Belgium), in 2008 studying zeolite catalysis with fluorescence microscopy. After a postdoctoral stay (2009−2010) with Prof. Xie at Harvard University (USA) on the development and use of coherent Raman microscopy, he returned to the KU Leuven. In 2010, he started his own research group (www. roeffaers-lab.org) focusing on the development of optical microscopy tools to study heterogeneous catalysis and materials for sustainable chemistry. Amongst others, he was awarded a prestigious ERC starting grant (2012) and received the biennial ExxonMobil Chemical European Science and Engineering Award (2015). Johan Hofkens received his M.Sc. and Ph.D. degrees in Chemistry from the KU Leuven. After postdoctoral research with Prof. Masuhara at Osaka University and Prof. Barbara at the University of Minneapolis, he rejoined the KU Leuven where he started the Single Molecule Unit. In 2005, he was appointed Research Professor at the KU Leuven, and in 2008 he was promoted to full professor. His research interests are single molecule spectroscopy, fluorescence and nonlinear microscopy, and the application of these techniques in timely topics including materials science and biosciences. With respect to these topics, he received an ERC advanced grant in 2012.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J. Hofkens). ORCID

Eduardo Coutiño-Gonzalez: 0000-0001-8296-0168 Wouter Baekelant: 0000-0001-7541-2171 Julian A. Steele: 0000-0001-7982-4413 Cheol Woong Kim: 0000-0002-7053-9744 Maarten B. J. Roeffaers: 0000-0001-6582-6514 Johan Hofkens: 0000-0002-9101-0567

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support over the past decade to support the research into luminescent Ag-CLs from the Belgian Federal government (Belspo through the IAP VI/27 and IAP-7/05 programs), the European Union’s Seventh Framework Programme (FP7/2007-2013 under grant agreement nos. 310651 SACS and 307523 ERC-Stg LIGTH to M.B.J.R.), the Flemish government in the form of long-term structural funding “Methusalem” grant METH/15/04 CASAS2, the Hercules foundation (HER/11/14), the “Strategisch Initiatief Materialen” SoPPoM program, KU Leuven Research Fund (C14/15/053, OT/12/059), and the Fund for Scientific Research Flanders (FWO) Grants G096213N and G.0B39.15 and a postdoctoral fellowship to J.A.S. We also thank Mr. B. Dieu for the preparation of graphical material. E. Coutino-

Author Contributions ⊥

E.C.-G. and W.B. contributed equally.

Notes

The authors declare no competing financial interest. Biographies Eduardo Coutino-Gonzalez obtained his Ph.D. at the KU Leuven (Belgium) under the supervision of Prof. Johan Hofkens and Prof. Bert Sels, working in the development of novel nanostructured materials for applications in optoelectronics. After a postdoctoral period in the group of Prof. Hofkens, he joined CIDETEQ (Mexico) as researcher where he is mainly working on the development and characterization 2359


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Gonzalez gratefully acknowledges the support provided by Cátedras CONACYT.

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REFERENCES

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