Radiation Measurements 42 (2007) 675 – 678 www.elsevier.com/locate/radmeas Luminescent properties of nanophosphors L.G. Jacobsohn ∗ , B.L. Bennett, R.E. Muenchausen, J.F. Smith, D. Wayne Cooke Materials Science & Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Received 18 December 2006; accepted 31 January 2007 Abstract The characteristics and luminescent properties of two nanophosphors, Y 2 SiO 5 : Ce and Y 2 O 3 : Tb, are reviewed and summarized in this work, with focus on the results obtained in our group. Hydrothermally prepared Y 2 SiO 5 : Ce crystallizes in the P 2 1 /c structure rather than the B2/b structure observed in bulk material. Relative to bulk powder, Y 2 SiO 5 : Ce nanophosphors of particle size ∼ 25–100 nm diameter exhibit red shifts of the photoluminescence excitation and emission spectra, reduced self absorption, enhanced light output, and a medium-dependent radiative lifetime. Perturbation of the Ce ion electric field due to reduced dimensionality of the nanocrystals is responsible for the variable lifetime. Y 2 O 3 : Tb nanophosphors prepared by the solution combustion method exhibit nanocrystals within the 30–70 nm range comprising cubic structure. Photoluminescence emission and excitation measurements show a complex behavior comprised of several sharp lines in the visible spectrum associated with electronic transitions of the Tb +3 ion. The maximum of the nanophosphor photoluminescence quenching curve occurs at Tb concentration three times higher than that in the bulk phosphor. © 2007 Elsevier Ltd. All rights reserved. Keywords: Nanophosphor; Luminescence; Lifetime; Polymorphism; Concentration quenching curve 1. Introduction Nanophosphors are nanostructured, inorganic, insulating solid materials that efficiently emit light under particle and photon excitation. Of particular interest is the synthesis and characterization of rare earth (RE) doped nanophosphors with emphasis on optical properties and how these change with reduced dimensionality. RE ions are particularly effective as luminescent centers in host lattices and are commonly utilized to produce high quantum efficiency bulk phosphors. Unlike semiconductor quantum dots, where the electronic states are spatially extended and thus quantum confined, RE ion 4f elec- tronic states are highly localized and unaffected by reduced di- mensionality. However, nanophosphor phonon levels are delo- calized and the phonon density of states (PDOS) is reduced by particles of nanometric dimensions (Tamura and Ichinokawa, 1983; Chen et al., 2003). Because the low-frequency acoustic phonons strongly contribute to non-radiative relaxation of lu- minescence in nanophosphors, and because these modes are ∗ Corresponding author. E-mail address: lgjacob@lanl.gov (L.G. Jacobsohn). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.01.066 cut off due to spatial confinement, optical properties of nano- phosphors will be different from those observed in bulk phos- phors. Other factors that influence optical properties of RE- doped nanophosphors are crystal field effects, spin–orbit cou- pling of 4f electrons, and large surface-to-volume ratio of the nanostructured material. Collectively these factors provide tools for probing the effects of reduced dimensionality on optical properties of nanophosphors. Since phosphors play an integral role in modern life, with applications ranging from fluorescent lighting and cathode ray tube displays to medical imaging (i.e., positron emission tomography scanners) and nuclear radiation detectors, it is very important to investigate the full potential of nanophosphor materials. In this work, we summarize our re- sults on the investigation of two nanophosphors, Y 2 SiO 5 : Ce (YSO) (Cooke et al., 2006) and Y 2 O 3 : Tb (Muenchausen et al., 2006a). 2. Experimental procedures Nanophosphor YSO was prepared by the hydrothermal method using reagent-grade Y(NO 3 ) 3 6H 2 O, Ce(NO 3 ) 3 · 6H 2 O and SiO 2 of > 99% purity. Powders were mixed with de- ionized water and titrated to pH of 9, and then aged in a