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

Anthony Diaz, Central Washington University

     This research involves the study of the electron 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 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.  This pair migrates through the lattice until it is captured by either a lattice defect, by surface states, or by a dopant.  In luminescent materials, this dopant 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 energy 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 referred to as the transfer efficiency.  Very few reports have been published on attempts 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.  Our goal is to develop general models that relate the crystal and electronic structure of a material to its electron transport properties.

     In our most recent research on this grant we were able to quantitatively assess the transfer efficiency to different activator sites in Eu³⁺-doped SrY₂O₄, and found 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 primary 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 findings are the first of their kind to be reported in the academic literature.  This work was reported in the Journal of Luminescence, and includes an undergraduate co-author whose summer research was partially funded by this grant, and who was a participant in Central Washington University’s Science Honors Program.  This student also presented this research at last year’s Northwest Meeting on Materials Science and Physical Chemistry at Washington State University.  We are now extending the work on this compound 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 are undertaking a study of a fairly large sample set of materials doped with various amounts of Gd³⁺ and Eu³⁺.  Our transfer efficiency data are indeed consistent with this model.  The student working on this project was also partially funded over the summer by this grant, and presented this work at the Northwest Meeting on Materials Science and Physical Chemistry.  A manuscript acknowledging ACS-PRF support is in preparation.

     Finally, we have begun some preliminary investigations of surface losses in nano-scale materials.  The study of nanotechnology has obviously become a focus for the materials science community, as many materials exhibit new and interesting properties at this size scale.  Understanding the loss of energy at a particle’s surface is essential to the development of energy efficiency lighting and displays, and is also critical to work on dye-sensitized solar cells.  We are currently developing synthetic strategies for controlling the particle sizes of compounds of interest, and expect to begin optical measurements shortly.  These data will be used to support the PI’s submission of a grant proposal to the Division of Materials Research at the National Science Foundation this year.

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