This three-year research project is investigating a promising class of materials for use as lighting and/or laser applications. The materials of interest are oxides containing the rare earth ions praseodymium and terbium, which give off red and green light, respectively. These materials are interesting because there exists a quantum state that allows ultraviolet energy to be put into the system efficiently. Subsequently, that energy can be transformed to red or green light. By changing the composition of the materials, it is possible to alter the characteristics of this quantum state to maximize the energy input and extract light at the desired color. The goals of the project are to: (1) gain insight into the physical and chemical properties of this state, and (2) to optimize the visible light output of these systems for applications to phosphors used in solid-state lighting and in display materials, and for solid-state laser applications.

Technical Abstract

This three-year research project investigates a charge transfer state of d0 transition metal oxides doped with rare-earth ions (praseodymium and terbium) using the techniques of luminescence spectroscopy. The project includes an experimental study into the fundamental properties of the charge transfer state, and into its role in the dynamics of the relaxation processes that lead to emission from the rare earth ions. The energy of the charge transfer state is being tuned to maximize the luminescence efficiency of the specified rare earth ions. This tuning is being accomplished by changing the composition of the materials and by altering the particle size down to the nm-scale. The project provides significant training to undergraduates, giving them hands-on experience in the laboratory, allowing them to present their work at scientific conferences, and by collaborating with scientists at other universities.

Project Report

This RUI grant proposed to conduct a thorough study of the optical properties of d0 transition metal oxides containing Pr and Tb in regards to their potential for use in lighting applications. In addition, we proposed to train several students in the techniques of optical spectroscopy, as well as to expanding research programs into new areas and to increase collaborations with other researchers in the field. The plan also called for the purchase and integration of equipment, including a reflection spectrometer and IR detector, in order to expand the capabilities of the lab so as to be able to fully conduct the necessary experiments. The scientific goals were all met. We studied a number of samples, concentrating on Pr-doped materials. Systems investigated included various titanates and niobates, with samples ranging from single bulk crystals to nanoparticles. The most interesting, from an applications point of view was the calcium niobate system, which showed strong red emission up to about 500K, making it a material of potential interest for lighting applications. Most other systems showed a sharp decrease in the red emission at much lower temperatures, rendering them useless for lighting applications. Much of the work went into understanding the reason(s) why the temperature dependence of red emission differed from one system to another. Our work, presented recently at the International Conference on Luminescence, proposes two possible models for this thermal quenching of the emission. We are now preparing experiments that will help us determine which of the two models best explains the observed results in a wide variety of systems. Under the guidance of the PI, ten undergraduate students were part of the research for this project. Of these students 40% were women and 30% were students of color. The student researchers were involved in all areas of the research process, including the following: optical characterization of materials, advanced spectroscopic techniques to investigate kinetic properties of the systems, the visualization and analysis of data, and the presentation of the results at scientific conferences. Their research work was presented at eight scientific meetings, including local and international conferences. Three of the students have graduated, one is in a Ph.D. program in physics, one works as an engineer, and one plans to attend medical school. Those still in the pipeline are on track to enter graduate school or the workplace in STEM-related fields, such as physics, engineering, and forensics. All students have received training in the Responsible Conduct for Research, and are well equipped to enter research facilities programs either in the graduate programs, industrial laboratories, or medical facilities. This grant has also impacted the research at the college through the establishment of new research collaborations, including two with industry (General Electric and Osram Sylvania) in the area of the optical properties of materials used in lighting, one with a professor from the Chemistry Department at the college on the optical properties of thin films, and one international collaboration with a researcher in Lviv, Ukraine investigating the optical properties of nanostructures silver thin films for sensing applications. Equipment purchased with this grant also used by a local company (NanoLab, of Waltham, MA) to investigate the reflectance properties of carbon nanotubes. In summary, this research investigation shows that the materials investigated in this report are worthy of continued investigation as potential materials as red phosphors for LED-based lighting. The ten students trained under this grant all on track for careers in science and engineering fields. Finally, the grant has contributed to the establishment of several ne collaborations with industry and with other academic institutions, and has raised significantly the quality of the research in the physics department at Wheaton College. Due to the improvement of the capabilities of the lab as a result of this grant, and we anticipate growth in the use of these facilities from physics and chemistry students alike, the continued training of undergraduates in the techniques of spectroscopy, and the further exploration of the optical properties of new materials for applications such as lighting, sensors, and biological applications.

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Lynnette D. Madsen
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Wheaton College
United States
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