Form Follows Function: Warming White LEDs Using Metal Cluster

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Form Follows Function: Warming White LEDs using Metal Cluster Loaded Zeolites as Phosphors Wouter Baekelant, Eduardo Coutiño-Gonzalez, Julian A. Steele, Maarten B.J. Roeffaers, and Johan Hofkens ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00765 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 24, 2017

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Form Follows Function: Warming White LEDs using Metal Cluster Loaded Zeolites as Phosphors Wouter Baekelant,δ Eduardo Coutino-Gonzalez,δ,¥ Julian A. Steele,† Maarten B.J. Roeffaers, †,* Johan Hofkens.δ,* δ

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, México. †

Chem&Tech - Centre for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F,

B-3001 Leuven, Belgium. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. Hofkens) *E-mail: [email protected] (M.B.J. Roeffaers)

ACS Paragon Plus Environment

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ABSTRACT The recent development of light emitting diodes (LEDs) using remote phosphors to generate white light has seen this family of devices become well-positioned to replace conventional lighting. They offer higher efficiencies compared to traditional bulbs and do not contain toxic mercury, as with fluorescent lamps. However, the color quality of the light emitted from commercial white LEDs, which is typically bluish, is not yet comparable to that of incandescent lamps. In this perspective, we discuss the potential application of luminescent metal clusters confined inside zeolites as remote phosphors for designing warm white LEDs. Such materials may not only improve the LEDs light quality, but also help maintain device stability and efficiency, compared to other LED phosphors. Moreover, in terms of optical properties and stability of metal loaded zeolites, the necessary improvements and drawbacks are identified and viable solutions – through rational design protocols – are outlined.

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Light emitting diodes (LEDs) are rapidly replacing conventional lighting devices for application in general illumination purposes, due to their high efficiency and low operational costs.1 Nowadays, the efficiency of commercial white LEDs (WLEDs) is surpassing that of fluorescent lamps (FLs) and is up to seven times higher compared to incandescent lamps, with a potential to reach a tenfold improvement by 2020.1 The predicted advances in efficiency for this type of solid-state lighting (SSL) device will significantly reduce both the global energy use for lighting (by 50%) and total electricity use (by 11%) before 2025, when compared to that used in 2000.2,3 Besides efficiency, LEDs further outperform FLs with their longer lifetimes and shorter start-up times, and LEDs do not contain harmful mercury.4,5 In order to compete with existing lighting devices, the efficiency of the WLED is only one aspect to be taken into account. Also important is the quality of the white light emitted. This light quality is generally measured by two parameters: the correlated color temperature (CCT) and color rendering index (CRI). The CCT describes the color warmth of the white light, by relating its chromaticity to that of a black body radiator when heated to a certain temperature, expressed in Kelvin.6 A low CCT (~3000 K and below) indicates a color which is yellow-red in appearance, referred often to as a warm white emission. Cool white light is more bluish in color and has a higher CCT (~6000 K).7 On the other hand, the CRI is a quantitative measure – a unitless scale from 0 to 100 – of the illumination perception of an objects color, in comparison to an ideal reference light source (black body radiator).6 For example, a high CRI value (nearing 100) indicates a good quality white light emission and one resembling the color perceived under illumination by an ideal black body radiator, which is close to that of incandescent lamps or under daylight conditions. Light generated from SSL can be made white in two different ways. The first method is to join multiple single-color LED emissions (usually 3; red, green and blue), which combine to appear


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as white light. Although this is the preferred method in terms of efficiency,8 such WLEDs devices suffer from an unstable driving current and a color quality that shifts over time. Consequently, complex electrical circuits are necessary to correct for these issues, making them less commercially favorable. The second type of WLEDs is based on a single LED emitting in the blue or near-ultraviolet (NUV), in conjunction with suitable phosphors capable of warming the overall emission toward that of a perceived terrestrial solar spectrum. First-generation commercial WLEDs made use of phosphor-converted white light emitting diodes (pc-WLEDs), employing a blue InGaN LED in combination with a yellow cerium-doped yttrium-aluminum garnet (YAG:Ce3+) phosphor. The combination of the blue light of the LED and yellow light of the phosphor is perceived as white, however the cool white light produced by such devices has a high CCT value (large proportion of blue), and low CRI, lacking green and red in its emission spectrum.9 Nowadays, pc-WLEDs devices with high CCT and low CRI values have been improved by adding a second red phosphor to the YAG:Ce3+-blue LED,7,10 or by combining the blue LED with a green and orange phosphor.4 Ultimately, while two-phosphor approaches increase the CRI and decrease the CCT of emissions, further improvements are still necessary to reach the light quality benchmarked by traditional incandescent lamps. In addition, some issues remain problematic for pc-WLED device designs based on a blue exciting LED; such as color temperature instability, temporal color changes due to different deterioration speeds of the primary LED or phosphor(s)9 and the halo-effect.3,11 This halo-effect originates from the directionality of the blue light emitted by the LED, which results in more bluish white light (higher CCT) in the center of the illumination compared to the edges. On the other hand, these issues play less of a role in a NUV-LED device,4,6,7,9 where all of the visible light generated originates from phosphor emissions, i.e. the optical down-conversion of


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invisible NUV light into visible wavelengths. In such a scenario, the values of CCT and CRI are only dependent on the phosphor blend used and thus can easily be tuned and optimized.6 Furthermore, LEDs possessing a warmer white emission are more efficient using a NUV-LEDs setup. This is because devices employing blue LEDs can only achieve a higher CCT using a thicker phosphor layer to reduce the blue contribution and, it follows, that thicker layers are detrimental for high brightness (Figure 1). For NUV-LEDs the CCT and thickness of the phosphor layer are not related to each other, thus can be optimized independently.11 However, an application of NUV-WLEDs is stifled by two central drawbacks. First, NUV-LEDs with high efficiencies are not readily available. However, recent reports predict that a market release by Nichia can be expected soon.3,12 The second drawback is that the number of phosphors suitable for NUV-excitation is very limited.6 Therefore research efforts are currently focused on optimizing phosphor materials with desirable properties – phosphors capable of combining high luminous efficiency with good CCT and CRI properties.6 In order to evaluate whether a phosphor meets industrial standards, some criteria should be fulfilled. These criteria can be tracked using six main requirements; (1) suitable excitation and absorption wavelengths; (2) good emission properties; (3) high quantum efficiency; (4) little response to temperature changes; (5) excellent stability; and (6) no saturation effect.7,13 In what follows, we will evaluate the various phosphors currently on offer for white light production, such as rare-earth (RE) based phosphors10,14 quantum dots (QD),7,15–17 and organic dyes,18,19 and compare their properties to a newly emerging family of phosphors,20–23 based on metal clusters confined in zeolite frameworks. These zeolite frameworks possess the appropriate molecular-sized pores and cages to contain silver clusters, as well as the cation-exchange capacities to facilitate the uptake of silver ions. Obtaining luminescent Ag-zeolites involves a


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readily scalable bottom-up approach, whereby silver ions self-assemble into clusters in a process known as “activation” induced via thermal or electromagnetic energy.25 Knowledge on the luminescent properties of metal-containing zeolites is still relatively fresh, only being revealed within the last decade or so. This is particularly the case for Ag-zeolites, where recent progress has led to a better understanding of their luminescent properties, spurring vast interest for their application as phosphors in ligthing.22,24 Nevertheless, further optimization is needed for their potential implementation as LED phosphors. Further, Ag-zeolite-based technology should be developed to exceed the emission and excitation/absorption properties of RE, QD and organicbased phosphors, while possessing similar external quantum efficiencies (EQE), thermal behavior and stability. These issues we discuss in the current perspective. Suitable excitation and absorption wavelengths. The excitation and absorption of a good NUV-LED phosphor should fulfill three important requirements; (1) the excitation wavelength of the phosphor should sufficiently overlap with the emission of the NUV-LED; (2) the reflection of the phosphor should be low at the excitation wavelength; and (3) there should be no significant absorption away from the excitation wavelength, especially in the visible region.7,13 If these requirements are not met, its use as a phosphor will not be effective. For RE-based phosphors, the absorption is optimized by the supporting structure material.10,11,14 The optical absorption of QDs is very broad and possesses a strong size-dependence, typically having an absorption tail which reaches far into the visible. For organic phosphors, the absorption wavelength is dependent on the dye or metal complex, and is often relatively sharp. For the case of Ag-zeolites, it has been observed that the absorption and excitation can be altered through varying the synthesis parameters, such as the type of zeolite framework, choice of counterbalancing extra-framework ions, Ag content and hydration levels.20,24 Yet the excitation


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of luminescent Ag-zeolites with high EQE (above 80%) is at present limited to samples excitable with UV-light of 350 nm and shorter.22,23 This is in comparison to NUV-LEDs, which exhibit fairly good luminous efficiencies at wavelengths above 365 nm.26 Shifting the excitation emission of the LED to shorter wavelengths is however not feasible, since the necessary alloying of the AlxGa1-xN-LED to reach the deep UV produces a poor crystalline phase with a lot of defects6 and has a difficult p-type doping12, leading to inefficient LEDs. It is thus necessary to shift the excitation maximum of the Ag-zeolite phosphors to wavelengths above 365 nm. Such Ag-zeolite systems have already been reported in literature (Figure 2),20 however at the moment they possess EQE values below 50%. This defines the optimization path necessary to fabricate NUV-absorbing Ag-zeolites with intense emissions. On the latter two requirements for an efficient NUV-LED phosphor, metal clusters do have high extinction coefficients27 and large Stokes shifts.20 As well, given the formation of large nanometer-sized particles inside the zeolite framework is prevented, absorption of nonluminescent species in the visible region is hindered.22 Thus, light reabsorption of these materials in a phosphor blend should be almost negligible. Good emission properties: The CCT and CRI values of a WLED are ultimately contingent on the emission properties of the device. A warm white LED with low CCT is commonly obtained by blending a blue, a green and a red phosphor, using RE or QDs, or even a combination of both.11 However, phosphor blending is often complex and thus research has focused on decreasing the number of phosphors. For this, systems employing only two or one phosphor components have been explored, by utilizing phosphors which emit at multiple colors4 or a very broad range of wavelengths.28 Broad emissions have been observed for Ag-clusters embedded in glass hosts28 and can also be seen in Ag-zeolites, where the emission bands often have a full


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width at half maximum (FWHM) nearing 100 nm or more. In addition, Ag-zeolites can also exhibit multiple emission peaks simultaneously, at the same excitation wavelength, as depicted in Figure 2.22 Furthermore, Ag-zeolites have the ability to emit at wavelengths spanning the whole visible range.20 Such emission properties clearly establish the potential of using Agzeolites materials as remote phosphors in LEDs. Quantum efficiency: To be attractive for industrial applications,3 the photoluminescence EQE of potential phosphors should ideally achieve values above 90 % to minimize losses7, with an absolute minimum of 70 %. These requirements have been met for the case of QD and organic phosphors, as well as RE phosphors. For Ag-zeolites, an EQE nearing unity was recently reported.22 However, the excitation maximum for this material is 305 nm. Nevertheless, this study22 demonstrates that silver zeolites have the ability to display stunningly high quantum efficiencies, after their optimizing through rational design protocols. Moreover, Figure 3 highlights the fact that the high quantum efficiencies of Ag-zeolites – using deep UV-excitation – are not merely limited to a single emission wavelength, but offer a spectrum of colors, which can be combined to form a white light emitting phosphor blend as can be deduced from the Commission Internationale de l’Éclairage (CIE) color space. Low thermal response: Good phosphor materials should have stable optical features and EQE values from room temperature right up to 150 °C.7 In more detail, the EQE of the phosphor should not drop by more than 10 % at an operational temperature of 150 °C, relative to room temperature. Again, this requirement has been met for RE, QD and organic-based phosphors.7,13 Some reports show that the dehydration experienced by Ag-zeolites at elevated temperatures (ranging from 50 to 450 °C) enact changes to their important optical properties.24,29 Conversely, as displayed in Figure 4, it is also possible to produce luminescent Ag-zeolites that have


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temperature-independent luminescent properties combined with good stability of the luminescence intensity. In addition, this small thermal dependence can further be retained by completely dehydrating the zeolite framework or by sealing its outer surface with an impermeable layer.30 Excellent stability: In order to maintain the long lifetime of an LED, akin to those reported for RE and QD phosphors,7,13 WLED phosphors should exhibit high thermal, chemical and photostability. On thermal stability, Ag-zeolitic materials have been shown to remain stable up to temperatures of 450 °C.31 Moreover, they can also retain good photo-stability upon irradiation with 1W UV-light,21 although long-term photo-stability still needs further investigation. Unfortunately, in terms of exposure to moisture – in the form of atmospheric humidity – the chemical stability of Ag-zeolite is low.24,29 Yet sealing the zeolite material with an impermeable layer has been suggested to mitigate such effects30 and improve their chemical stability. For organic-based phosphors, realizing high stability is often problematic, due to thermal and photoinduced breakage of the organic bonds. No saturation effect: When the radiation flux emitted by an LED4 is high enough to cause a loss in efficiency for a WLED device using phosphor materials with relatively long decay times (> 1 ms), the system is said to be saturated. The lifetimes reported for Ag clusters32 and Ag-zeolites13 are below this threshold, with some exceptions of outliers pushing up towards values of 900 µs.20 For applying Ag-zeolites as phosphor in WLEDs, it is clear that one path forward might be to eliminate any zeolites with longer lifetimes. Although for Ag-zeolites phosphor materials with longer decay times, which could result in saturation of the system, the use of a remote phosphor configuration is preferred. This is because by distancing the phosphor coating from the LED excitation source, the net flux reaching the phosphor is reduced. A remote phosphor


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configuration is typically employed already for WLEDs based on RE and QD phosphors, to avoid saturation. In addition, this remote configuration is also preferred for providing better color uniformity and lifetime of the LED,8,13 with the latter being especially true for organic-based phosphors. Within the context of the optical features endowed to Ag-zeolites – from their intense absorptions and large Stokes shift, to extremely appealing emissions – it has been shown that confined metal clusters within zeolite frameworks have the potential to surpass the performance of RE and QD phosphors, all while holding comparable efficiencies and stabilities. Stability is often lacking in metal complexes and dyes, because of their respective organic contents.18,19 In order to compete with commercially available phosphors, however, there are still many improvements necessary. In particular, the excitation should be shifted toward the NUV range (from 365 to 400 nm) for warm white light, and optionally to the blue region (~ 450 nm) for WLEDs with higher CCT. Emission peaks should be broad and eventually entail multiple colors covering the whole visible spectrum or a high efficient red emitter should be synthesized. And finally, the stability (mainly towards moisture) should be significantly improved to ensure sufficiently long operating lifetimes of WLED devices, which can be achieved by sealing the pores of the active Ag-zeolite material.30 In order to meet all of the requirements mentioned above for commercially viable LED phosphors, rational design protocols should be developed for the synthesis of these metal zeolite materials (Figure 5). Powerful rational design protocols can be achieved by investigating the structural and electronic properties of the metal clusters, and understanding their influence on important optical and luminescent properties. By using the self-correcting approach outlined in Figure 5, the synthesis parameters can be reviewed empirically and adapted directly, all to create


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metal zeolite phosphors suitable for warm white LED applications.25 Nevertheless, investigation of these type of sub-nanometer clusters is not a straightforward task. For example, the highly energetic beams (synchrotron X-rays, electron beams, etc.) needed to unravel structural and electronic features of Ag-zeolites can lead to radiation-induced changes.33,34 Notably, this can be circumvented by keeping the irradiation doses relatively low. More importantly, employing a holistic approach appears to be the right path forward. The true combining of experimental probes has recently resulted in the directed synthesis of material exhibiting near perfect EQE values.22 Furthermore, current research focuses on unravelling the complex photophysical processes of these luminescent silver zeolites, which have the high potential to enhance our knowledge of these samples and boost the rational design methods.25 Moreover, a fast and more consequential synthesis design can also be realized using single particle studies.35 The diagnosis of single particles can be performed using large zeolite crystal36 and combining confocal fluorescence microscopy with single crystal X-ray diffraction experiments. Finally, at present, the research of luminescent metal zeolites has narrowly focused on Ag-zeolites. Future prospects should also focus on expanding the pallet of metal clusters for inclusion into zeolites (Cu, Zn, etc.), which can lead to varied applicability and/or lower production costs. Ultimately, further work on the luminescent properties of metal cluster inside zeolite scaffolds will help bolster their potential as novel phosphors for warm white NUV-LED-based devices; from enhanced EQEs to improved light qualities, having low CCT and high CRI values.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. Biographical Information Wouter Baekelant is working as a PhD student at KU Leuven (Belgium), investigating the luminescent properties of metal clusters confined inside zeolite scaffolds, under supervision of Prof. Johan Hofkens and Prof. Maarten B.J. Roeffaers. Eduardo Coutino-Gonzalez obtained his PhD at the KU Leuven (Belgium) under the supervision of Prof. Johan Hofkens and Prof. Bert Sels. After a postdoctoral period in Prof. Hofkens group he joined CIDETEQ (Mexico) as researcher working on the development and characterization of functional materials for energy production, catalysis and environmental remediation. Julian A. Steele received his PhD in solid-state physics at The Institute for Superconducting and Electronic Materials, University of Wollongong (Australia), under the supervision of Prof. Roger Lewis. At the start of 2016 he joined the group of Prof. Roeffaers (KU Leuven, Belgium) as postdoctoral researcher, working on nano-scale optoelectronics. Maarten B.J. Roeffaers graduated from the KU Leuven (Belgium) in 2008. Before returning to the KU Leuven to start his own group in 2010. he was postdoctoral researcher with Prof. Xie at


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Harvard University (USA). His group (www.roeffaers-lab.org) focuses on the development of optical microscopy tools to study heterogeneous catalysts and materials for sustainable chemistry. Amongst others, he received an ERC starting grant (2012). Johan Hofkens received his PhD 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 Laboratory. His research interests are (single molecule) spectroscopy and fluorescence microscopy.

ACKNOWLEDGMENT The authors gratefully acknowledge financial support 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 agreements no. 310651 SACS and no. 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, and the Fund for Scientific Research Flanders (FWO) grant G.0B39.15. W.B. gratefully acknowledges the chemistry department of the KU Leuven for a FLOF-scholarship. J.A.S. acknowledges the Fund for Scientific Research Flanders (FWO) for a postdoctoral fellowship. E.C.G. gratefully acknowledges the support provided by Cátedras-CONACYT. The authors thank UOP Antwerp for the kind donation of zeolite samples and Mr. B. Dieu for the preparation of graphical material.


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De Cremer, G.; Antoku, Y.; Roeffaers, M. B. J.; Sliwa, M.; Van Noyen, J.; Smout, S.; Hofkens, J.; De Vos, D. E.; Sels, B. F.; Vosch, T. Photoactivation of Silver-Exchanged Zeolite A. Angew. Chemie - Int. Ed. 2008, 47, 2813–2816.

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Fenwick, O.; Coutiño-Gonzalez, E.; Grandjean, D.; Baekelant, W.; Richard, F.; Bonacchi, S.; De Vos, D.; Lievens, P.; Roeffaers, M.; Hofkens, J.; et al. Tuning the Energetics and Tailoring the Optical Properties of Silver Clusters Confined in Zeolites. Nat. Mater. 2016, 15, 1017–1022.

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Coutino-Gonzalez, E.; Roeffaers, M. B. J.; Dieu, B.; De Cremer, G.; Leyre, S.; Hanselaer, P.; Fyen, W.; Sels, B.; Hofkens, J. Determination and Optimization of the Luminescence External Quantum Efficiency of Silver-Clusters Zeolite Composites. J. Phys. Chem. C 2013, 117, 6998–7004.

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Coutino-Gonzalez, E.; Baekelant, W.; Grandjean, D.; Roeffaers, M. B. J.; Fron, E.; Aghakhani, M. S.; Bovet, N.; Van der Auweraer, M.; Lievens, P.; Vosch, T.; et al. Thermally Activated LTA(Li)–Ag Zeolites with Water-Responsive Photoluminescence Properties. J. Mater. Chem. C 2015, 3, 11857–11867.

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Coutiño-Gonzalez, E.; Baekelant, W.; Steele, J. A.; Kim, C. W.; Roeffaers, M. B. J.; Hofkens, J. Silver Clusters in Zeolites: From Self-Assembly to Ground-Breaking Luminescent Properties. Acc. Chem. Res. 2017, DOI: 10.1021/acs.accounts.7b00295.

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Krames, M. R.; Shchekin, O. B.; Mueller-Mach, R.; Mueller, G. O.; Zhou, L.; Harbers, G.; Craford, M. G. Status and Future of High-Power Light-Emitting Diodes for SolidState Lighting. J. Disp. Technol. 2007, 3, 160–175.

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Vosch, T.; Antoku, Y.; Hsiang, J.-C.; Richards, C. I.; Gonzalez, J. I.; Dickson, R. M. Strongly Emissive Individual DNA-Encapsulated Ag Nanoclusters as Single-Molecule Fluorophores. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 12616–12621.

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Kuznetsov, A. S.; Tikhomirov, V. K.; Shestakov, M. V; Moshchalkov, V. V. Ag Nanocluster Functionalized Glasses for Efficient Photonic Conversion in Light Sources,


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Gabrus, E.; Nastaj, J.; Tabero, P.; Aleksandrzak, T. Experimental Studies on 3A and 4A Zeolite Molecular Sieves Regeneration in TSA Process: Aliphatic Alcohols DewateringWater Desorption. Chem. Eng. J. 2015, 259, 232–242.

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Antoku, Y.; Hotta, J.; Mizuno, H.; Dickson, R. M.; Hofkens, J.; Vosch, T. Transfection of Living HeLa Cells with Fluorescent Poly-Cytosine Encapsulated Ag Nanoclusters. Photochem. Photobiol. Sci. 2010, 9, 716.

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Coutino-Gonzalez, E.; Grandjean, D.; Roeffaers, M.; Kvashnina, K.; Fron, E.; Dieu, B.; De Cremer, G.; Lievens, P.; Sels, B.; Hofkens, J. X-Ray Irradiation-Induced Formation of Luminescent Silver Clusters in Nanoporous Matrices. Chem. Commun. 2014, 50, 1350– 1352.

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ACS Energy Letters

Figure 1. Schematic representation of the gradient of the total LED efficiency for blue and NUV-LEDs compared to the CCT of the LED. Adapted from Reference 11.

Figure 2. Two dimensional plot of Linde type A (LTA) zeolite samples with a K+/Ag+-ratio of 1 (28 wt% Ag), showing multiple emission bands at 425 nm excitation, which can reduce the number of phosphors used in pc-WLED devices.


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Figure 3. Picture of different Ag-zeolite samples exhibiting a range of emission colors, while all possessing relatively high EQE when excited at 254 nm. Exact EQE values are indicated below each phosphor. Note that the relative sensitivity of the used color camera gives an incorrect impression of the relative intensity. Below the position of the different samples on a 1976 CIE color space are depicted.


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ACS Energy Letters

Figure 4. Thermal response of the emission intensity for a dehydrated Ag-zeolite phosphor when excited with 335 nm UV-light and measured at an emission wavelength of 525 to 530 nm. The Ag-zeolite sample displays low thermal behavior, with a decrease below 10% at 150 °C. The inset shows that both maximal excitation and emission wavelengths are stable across temperature range.


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Figure 5. Schematic representation of the rational design flowchart for optimizing the luminescent properties of metal loaded zeolites to NUV-LED phosphors. By understanding the relations between the different synthesis parameters (metal and metal loading; zeolite topology and co-cations; and post-synthesis treatment) and the properties (structural, optical and electronics), a rational design can be developed. Using this rational design we can reformulate the synthesis parameters to obtain the necessary properties for a LED phosphor, which can be implemented to produce a warm white emitting pc-WLED.


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ACS Energy Letters

Figure 1. Schematic representation of the gradient of the total LED efficiency for blue and NUV-LEDs compared to the CCT of the LED. Adapted from Reference 11. 82x75mm (300 x 300 DPI)


ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Two dimensional plot of Linde type A (LTA) zeolite samples with a K+/Ag+-ratio of 1 (28 wt% Ag), showing multiple emission bands at 425 nm excitation, which can reduce the number of phosphors used in pc-WLED devices. 82x76mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Figure 3. Picture of different Ag-zeolite samples exhibiting a range of emission colors, while all possessing relatively high EQE when excited at 254 nm. Exact EQE values are indicated below each phosphor. Note that the relative sensitivity of the used color camera gives an incorrect impression of the relative intensity. Below the position of the different samples on a 1976 CIE color space are depicted.


ACS Energy Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Thermal response of the emission intensity for a dehydrated Ag-zeolite phosphor when excited with 335 nm UV-light and measured at an emission wavelength of 525 to 530 nm. The Ag-zeolite sample displays low thermal behavior, with a decrease below 10% at 150 °C. The inset shows that both maximal excitation and emission wavelengths are stable across temperature range.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Figure 5. Schematic representation of the rational design flowchart for optimizing the luminescent properties of metal loaded zeolites to NUV-LED phosphors. By understanding the relations between the different synthesis parameters (metal and metal loading; zeolite topology and co-cations; and post-synthesis treatment) and the properties (structural, optical and electronics), a rational design can be developed. Using this rational design we can reformulate the synthesis parameters to obtain the necessary properties for a LED phosphor, which can be implemented to produce a warm white emitting pc-WLED.


Form Follows Function: Warming White LEDs Using Metal Cluster

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