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Luminescent metal cluster confined in zeolites. Towards LED phosphor applications

The ever-growing demand for energy has led to an increasing pressure on the energy production, which in its turn has motivated research for more energy efficient products. An example of such products are light emitting diodes (LEDs). The implementation of LEDs as replacement for conventional lighting products, such as light bulbs and fluorescent lamps, is predicted to lead to a reduction of the global energy consumption for lighting by 50 % and the total electricity use by 11 % by 2025, when compared to the use in 2000. Although, the usage of LEDs is growing rapidly, there are some aspect which are hindering their complete breakthrough.

A first important aspect is their higher purchase price compared to that of conventional products; this is resulting from the higher raw material cost for LEDs. Secondly, the light color of current commercial white LEDs does not reach the same quality as that of light bulbs, having a low color rendering index (CRI) combined with a high correlated color temperatures (CCT). These aspects have led to a quest for more cost-effective LED with an improved light quality. The research for better light quality is mainly focused on a quest for better phosphor materials.

Within this PhD dissertation, the potential use of luminescent oligoatomic metal clusters confined inside zeolite scaffolds as phosphor materials has been investigated. Luminescent metal clusters have shown to display remarkable luminescent properties, having strong light absorption at the wavelength of primary light sources combined with large Stokes shifts, resulting in visible light emission and high quantum efficiencies, all resulting from their atomic-like discrete energy levels in their energy diagram. Nevertheless, these metal clusters are prone to aggregation to larger particles, which lead to a loss of their luminescence. In order to prevent this aggregation, metal clusters need to be stabilized. One of the most successful techniques for long term stabilization of those cluster is the use of zeolite scaffolds. Zeolites are crystalline microporous aluminosilicates, having a negatively charged framework which forms a network of molecular-sized pores and cages. These latter have shown to stabilize silver clusters (AgCLs) via a ship-in-a-bottle confinement process. The luminescent properties of these AgCLs are largely influenced by the local environment of cluster, showing a relation between the luminescent properties and the zeolite topology, silver loading, type of counterbalancing ions and hydration level of the zeolites. To improve the luminescent properties of these metal-zeolite for their application as NUV-LED phosphors, rational design protocols should be developed based on a full understanding of the relationship between the structure-to-luminescent properties, as introduced in Chapter 1 and 2. These rational design protocols can be implemented for the development of new, improved LED phosphors

In Chapter 3, it is shown that emission properties of AgCL confined inside hydrated LTA zeolites can be altered through guest-host-guest interactions upon the gradual replacement of Na+ counterbalancing cations by Li+, while maintaining their excitation wavelengths and EQE. These observations were related to a Li-induced shrinkage of the framework which in its part leads to a reduced Agc-Agc distance of the atoms in the cluster in its excited state. These guest-host-guest interactions were determined using two hereto not exploited characterization techniques for luminescent Ag-zeolites, namely XEOL-detected EXAFS and UV-Raman.

These gained insights led in Chapter 4 to the synthesis of a green luminescent phosphor with an EQE of around 83 % upon excitation between 300 and 400 nm, which is by far the highest EQE observed so far for Ag-exchanged LTA zeolites. The luminescent properties of this dehydrated Ag1-exchanged Li-LTA zeolite sample is attributed to a Ag4CL surrounded by non-framework oxygen ligands, with a very short Agc-Agc interatomic distance, by a combination of XEOL- and transmission-detected EXAFS. Next to the aforementioned guest-host-guest interactions, the Li-ions also act as a shield to prevent the direct interaction between the AgCL and the framework. This interaction between the cluster and LTA framework normally leads to a quenching of the luminescent properties of dehydrated Ag-LTA zeolite.

In Chapter 5 it is shown that, next to single color phosphors Ag-exchanged zeolites can also display single-phase broad white light emitting phosphors. Spectral fine-tuning of the white emitting materials is achieved by incorporating Ag+ inside luminescent S-SOD zeolites. This incorporation of the Ag+ consumes the orange luminescent S-species and results in the formation of a blue-white S-Ag species and an orange-red emitting AgCLs as determined by a full photophysical characterization of the materials. The relative ratio between the S-Ag species and AgCL determines the warmth of the white emission of the AgS-SOD materials.

To lower the cost of this type of materials, other metals then silver were investigated. In Chapter 6, the potential to form highly luminescent Pb clusters confined inside LTA zeolites is shown, displaying an EQE of up to 69 % at 250 nm excitation. This high luminescence was reached for a low Pb-loaded LTA zeolite which is dehydrated at 200 °C. EXAFS analysis was used to determine that a Pb4(OH)2-4 is responsible for the observed luminescence. Even though, the potential application of these Pb-LTA composites as LED phosphor is highly unlikely, because of its deep-UV excitation and the toxicity of Pb. Nevertheless, these materials are very useful to generalize the rational design protocols for luminescent metal clusters confined inside zeolite frameworks based on other metals.

Throughout this dissertation the potential of Ag-exchanged zeolite for their application as near-ultraviolet LED phosphors is shown, either as a mixture of different single phosphor or as a single-phase white emitting phosphor. Furthermore, it is shown that the luminescent properties can be tuned as a result of changes to the zeolite framework upon guest-host-guest interactions. Finally, it also has been demonstrated that the concept is amenable to other metal clusters, with as a first example the generation of luminescent clusters in Pb-exchanged LTA zeolites.

Date:1 Oct 2013 →  3 May 2019
Keywords:Photoluminescence, Metal Clusters, Zeolite, Nanoporous materials, LED phosphor
Disciplines:Sustainable chemistry, Physical chemistry, Inorganic chemistry, Organic chemistry, Theoretical and computational chemistry, Other chemical sciences
Project type:PhD project