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43841-B10
Energy Flow and Trapping Processes in Luminescent Materials under Vacuum Ultraviolet Excitation

Anthony Diaz, Central Washington University

This project involves the study of the electron trapping and transport properties of insulating oxide materials. A detailed understanding of these properties is important to a wide variety of fields within materials science, including the development of more efficient mercury-free lighting, improved materials for dye-sensitized solar cells, and the creation of high-performance transparent conducting oxides. Of particular interest to this PI are processes associated with vacuum ultraviolet (VUV) excitation of luminescent materials, such as are found in plasma display panels (PDP) and Xe-based mercury-free lamps. These technologies have created the need for new state-of-the-art materials, but at present few suitable materials are known. More importantly, many fundamental questions relating to the materials physics and chemistry of VUV excitation have yet to be answered.

In some of our previous work, we developed spectroscopic methods for quantitatively assessing the efficiency with which an electron-hole pair created in a solid is trapped by a doped impurity. VUV (120 – 200 nm) radiation incident on an oxide insulator creates an electron-hole pair in the solid. The trapped electron-hole pair is sometimes referred to as a self-trapped exciton, or STE. This pair migrates through the lattice until it is captured by either a lattice defect, by surface states, or by a doped impurity. In luminescent materials, this doped impurity is called the activator and is responsible for the emission of visible light in display and lighting technologies. YBO₃:Eu³⁺, for example, is an yttrium borate host with a small amount of Eu³⁺ activator doped into the lattice. Under VUV excitation the incident photon is absorbed by the YBO₃ and then transferred to the Eu³⁺ activator to produce visible emission. The fraction of electron hole pairs that are captured by the activator is generally called the transfer efficiency. Very few published reports exist that attempt to quantify the transfer efficiency – most groups focus on the overall efficiency of a material. However, understanding the transfer efficiency properties also allows one to assess other interesting quantities, such as the relative electron mobility in a given host. In our previous research we were able to show that the electron mobility in YBO₃ is much lower than it is in the related material Y₂O₃. We are now extending this work to include other hosts, with the goal of developing general models that relate the crystal and electronic structure of a material to its electron transport properties.

The first project of interest is the study of doped SrY₂O₄. This material was chosen because we wanted to compare its electron transport properties to those of Y₂O₃. In addition, SrY₂O₄ contains two unique crystallographic sites for Eu³⁺, and we wanted to see if we could successfully measure energy transfer efficiency to two sites in the same host. We were, in fact, able to show that energy transfer is about 30% more efficient to one of the two sites. We also discovered that, by modeling the transfer efficiency data with equations developed by several research groups, we could quantify the degree of energy loss to surface states in this host. This is important because the surface of a material can be a primarily loss pathway in many materials, and can be the limiting factor in developing better conductors or better luminescent materials. The surface becomes even more important as technologies move toward materials with smaller and smaller particle sizes. To our knowledge, both of these finding are the first of their kind to be reported in the academic literature. A manuscript has been submitted to the Journal of Luminescence, and is currently under review. This paper includes an undergraduate co-author whose summer research was partially funded by this grant. We are now extending the work to other activators.

A second project involves the study of YBO₃ co-doped with Gd³⁺. It has been known for some time that Gd³⁺ increases the VUV efficiency of some materials, and it is usually suggested that this is because Gd³⁺ increases the electron mobility in oxides. However, to our knowledge, no direct measurements of the relationship between Gd and electron mobility have been conducted. Thus we have undertaken a study of a fairly large sample set of materials doped with various amounts of Gd³⁺ and Eu³⁺, and found that our transfer efficiency data are indeed consistent with this model. More surprising is that we appear to have some evidence for a phenomenon known as quantum cutting – the creation of two visible photons from one absorbed VUV photon. This would be an extremely exciting finding, particularly as it would allow us to extend our spectroscopic methods to the study of two-photon systems. However, it is still too early to determine if this finding is correct. The student working on this project was also partially funded over the summer by this grant.

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