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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 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.  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.

Our most recent research on this grant 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 and represent the first quantitative study of these types of phenomena.  Preliminary data from this work were presented at the 2009 Society for Information Display Conference in June.  This talk (and associated manuscript for the conference proceedings) included two student co-authors along with acknowledgment of ACS/PRF.  We are almost finished with this study, and have begun preparation of a manuscript for Chemistry of Materials.

A second project involves the study of materials of more complex composition and structure, such as CaYBO₄, Ca₃Y₂(BO₃)₄ and Ca₄YO(BO₃)₃.  Our primary goal here is to relate differences in structure to differences in the degree of relative electron mobility. In the long term such information will aid in the development of more efficient materials for opto-electronic applications.  This group of materials is somewhat more challenging to synthesize, and though we are close we have not developed a reliable synthetic procedure.  Thus, optical studies of these materials have not yet been undertaken.

Finally, we have also conducted some preliminary investigations of surface losses in nano-scale luminescent 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 have developed synthetic strategies for controlling the particle sizes of compounds of interest, and have begun optical measurements.  Preliminary data obtained last year was used to support the PI’s submission of a grant proposal to the Division of Materials Research at the National Science Foundation.  The P.I. is pleased to note that this grant was funded for three years beginning in summer 2009.