Photon management with lanthanides

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Abstract

Efficient conversion of photons from high energy radiation (e.g. ultraviolet or X-rays) to lower energies (visible) has been optimized by using luminescent materials based on the optical properties of lanthanide ions. Presently, luminescent materials with efficiencies close to the theoretical maximum are applied in e.g. fluorescent tubes, X-ray imaging and color television. Contrary to the mature status of luminescent materials in these fields, areas requiring new luminescent materials are emerging. There is great challenge in research on up- and downconversion materials and lanthanide ions are the prime candidates to achieve efficient materials. Here downconversion processes will be discussed for VUV phosphors with Pr3+. The efficiency of resonant energy transfer of the 1S01I6 energy from Pr3+ to Eu3+and Mn2+ is investigated. The aim is to convert the 405 nm photon of the first step of the well-known cascade emission of Pr3+ into a more useful visible photon. For co-doping with Eu3+ it is observed that the Pr3+ emission is quenched, most probably through a metal-to-metal charge-transfer state. The energy transfer from Pr3+ to Mn2+ is found to be inefficient.

An alternative for downconversion through resonant energy transfer is the non-resonant process of cooperative sensitization. For the Tb–Yb couple efficient cooperative energy transfer from the 5D4 level of Tb3+ to two Yb3+ neighbors is observed with a transfer rate of 0.26 ms−1. This corresponds to an upper limit of 188% for the conversion efficiency of visible (490 nm) photons to infrared (∼1000 nm) photons.

Introduction

The lanthanides occupy a special place in the periodic table of elements. They are situated at the bottom of the periodic table, one row above the actinides. The word lanthanide has a Greek origin (“λανθανειν”) which means “to lie hidden”. This may seem appropriate in view of the position of the lanthanides in the periodic table and the fact that it took more than a century to separate and discover all the lanthanides. Nevertheless, at present lanthanides are prominently present in a wide range of products related to e.g. the catalytic, magnetic and optical properties of the lanthanide ions. Especially in optical applications the lanthanides have become (literally) visible in the past decades. The applications of lanthanide ions in the field of optical materials is related to the unique energy level diagrams of the lanthanides which are known as the Dieke diagram. The rich energy level structure make that lanthanide ions are perfect “photon managers” that can be used to efficiently convert radiation into light of any desired wavelength [1]. In the past four decades the use of lanthanide ions as photon managers has rapidly increased.

The first commercial luminescent material based on lanthanide luminescence was YVO4:Eu3+. This material was discovered in the early 1960s and found to efficiently convert the energy of high energy electrons into visible (red) light in a color television. The high luminescence efficiency triggered the application of lanthanide luminescence in other areas, for example in luminescent materials for fluorescent tubes and X-ray imaging. The fluorescence tube is based on a mercury discharge and research has resulted in luminescent materials (phosphors) for the conversion of 254 nm ultraviolet (UV) radiation into visible light. With the introduction of lanthanide-based luminescent materials, the quantum efficiency for the conversion of 254 nm UV radiation into red, green or blue light has reached the limit of close to 100%. Further research on lamp phosphors is mainly aimed at cost price reduction and marginal improvements of the efficiency or stability. Due to the mature status of the product, research on luminescent materials for mercury discharge fluorescent tubes has strongly decreased in the past decades. The same is true for luminescent materials used in cathode ray tubes and X-ray imaging. The efficiency of these materials is close to the theoretical maximum and research on new luminescent materials for these applications has decreased due to the ideal photon management qualities of lanthanides.

On the other hand, new areas of research have emerged in the past decades. Extensive research is conducted for e.g. (upconversion) lasers based on lanthanides, lanthanide doped fiber amplifiers for telecommunication, 3-D television with lanthanide doped crystals or glasses, scintillators and optical switches [1], [2]. Lanthanide doped photonic materials research has become a prominent area of research. Two specific areas in the field that have recently emerged are the search for more efficient and stable luminescent materials for the conversion of high energy UV (vacuum ultraviolet, VUV) radiation into visible light (for xenon-based fluorescent tubes and plasma display panels) and the conversion of low energy UV or blue light (for white light GaN diodes). The search for new luminescent materials for the conversion of vacuum ultraviolet radiation from a xenon discharge (around 172 nm) into visible light involves research on finding ways for the generation of two visible photons for a single VUV photon [3], [4]. This so-called quantum cutting process is possible using the energy level structure of lanthanide ions. After an introduction into the VUV spectroscopy of lanthanides, this paper will discuss recent developments in the field of quantum cutting. Subsequently, efficient quantum cutting by cooperative energy transfer is demonstrated for the Tb–Yb couple converting one visible photon into two infrared photons. The visible to infrared downconversion with quantum yields close to 200% may be used to increase the efficiency of solar cells.

Section snippets

Sample preparation

In all experiments measurements are performed on microcrystalline powders synthesized by common solid state synthesis. Fluorides were prepared by mixing starting fluorides in stoichiometric ratios, adding ∼2 g of ammoniumfluoride and firing the intimate mixture in a nitrogen atmosphere at typically 600 °C. Powder samples of YbxY0.99−xTb0.01PO4 were prepared from stock solutions of Yb3+ and Y3+, both containing 1% of Tb3+, prepared by dissolving rare earth (RE) oxides in concentrated hydrochloric

Vacuum ultraviolet spectroscopy and quantum cutting

The vacuum ultraviolet region of the electromagnetic spectrum covers the region between 50 and 200 nm (200 000–50 000 cm−1). Research on the VUV energy levels of lanthanides has been very limited until recently. Extensive research in this area was triggered by the need of new phosphors for VUV excitation for mercury free fluorescent tubes and plasma display panels. In these devices the VUV radiation generated in a Xe discharge (around 172 nm) needs to be converted into visible light. For the

Cooperative sensitization

The idea for the two quantum cutting systems discussed in Section 3 is based on resonant energy transfer. Splitting of the energy is achieved by population of an intermediate energy level of the donor. If overlap between donor emission and acceptor absorption is absent, second-order downconversion may become the dominant relaxation process (competing with spontaneous emission). In this process a donor excites two acceptors simultaneously. The resonance condition is fulfilled if the sum of the

Conclusion

The possibility of efficient visible quantum cutting has been investigated for materials in which Pr3+ shows a cascade emission from the 1S0 level. In order to convert the 405 nm photon emitted by Pr3+ in the first step into a more useful visible wavelength, two types of co-activators were added, viz. Eu3+ or Mn2+. In the case of Eu3+ a quenching of the 1S0 emission from Pr3+ is observed. The quenching is ascribed to relaxation through a metal-to-metal charge-transfer state. Upon adding Mn2+ as

Acknowledgements

The authors are grateful to H.W. de Wijn and M. Giesselbach for helpful discussions. The work described here was supported by the Council for Chemical Sciences (CW), with financial aid from the Netherlands Foundation for Technical Research (STW).

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