Hydration-induced coupling of the excitonic state of Y 2 O 3 with its phonon: Negative effect on the luminescence effici
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Luminescence properties of a series of samples of Y2O3:Eu+3 red phosphor of particle sizes ranging from 50 to 300 nm were investigated as a function of time to decipher the long-standing mystery of the effect of reduction of particle size on the luminescence efficiency of the phosphor. The samples were found to lose luminescence efficiency and suffer a change in the excitation profile with time. Infrared studies showed that although the samples at their freshly prepared stage were almost free from contaminated water, on aging in air at room temperature, they absorbed the latter. The phenomenon of hydration-induced coupling of the excitonic state of Y2O3 with one of its Fu modes was detected in the case of the aged sample, which was shown to be instrumental in introducing newer nonradiative channels in the system. Because of larger surface-to-volume ratio, the effect was most pronounced in the case of nanocrystalline samples.
I. INTRODUCTION
Europium-ion-activated cubic Y2O3 is one of the most important commercial red phosphors used in lighting, plasma display panels (PDP), field emission displays (FED), thermal calibration, etc. In the recent years, there has been great interest in preparing nanocrystalline phosphors1–22 with the objective of developing materials of higher luminescence efficiency and providing better resolution to the images5 of a phosphor-based display panel; in that endeavor, Y2O3:Eu+3 red phosphor is of no exception.1–17 However, earlier reports1–17 on the luminescence efficiency of nanocrystalline Y2O3:Eu+3 are, in many cases, contradictory to each other and confusing. Some groups reported enhancement of efficiency of the 5 D0 → 7F2 red luminescence with the reduction of particle size3,4 while others observed a significant loss of the same.6–10 There have also been reports to suggest that an optimum efficiency is obtained only when the particle size of the phosphor lies in the range of 40–100 nm.5,15 Surface defects and surface dangling bonds are commonly mentioned as the main cause of luminescence efficiency loss with the lowering of the particle size, but that cannot explain the opposite results reported by others.3,4 Moreover, because the diameter of the electronic wave function of a rare-earth ion is on the order of 0.1 nm, which is much smaller than the diameter of even a 5-nm a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0027 J. Mater. Res., Vol. 22, No. 1, Jan 2007
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crystallite, the size confinement effect is not expected to disturb the energy levels of an incorporated rare-earth ion. It is therefore important to determine the reason such confusing results have been observed and resolve whether reduction in the crystallite size of Y2O3:Eu+3 phosphor to the nanometer level is at all helpful to our endeavor for enhancement of the luminescence efficiency of the material. Here we report the results of our study on the luminescence properties of a series of samples of Y2O3:Eu+3 red phos
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