The Structure and Texture of Y 2 O 3 :Tb Nanocrystals
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395 Mat. Res. Soc. Symp. Proc. Vol. 452 0 1997 Materials Research Society
EXPERIMENTAL PROCEDURE Three different samples of doped nanocrystalline (DNC) Y20 3 :Tb phosphors were prepared by room-temperature organometallic synthesis [8]. Specimens for TEM imaging and diffraction were prepared from aqueous solutions of colloidal phosphors. Each solution was sonicated for 10-30 minutes. A drop of solution was placed on a holey carbon TEM grid and allowed to dry. Both high-resolution and bright-field image data were collected using a 300 keV Philips CM30 SuperTwin TEM with a LaB6 source. Its point resolution is 0.19 nm. Selected-area diffraction (SAD) patterns were collected from specimen areas -200-300 nm in diameter. Approximately 15 patterns were collected from each DNC specimen. These were digitized by scanning negatives or enlarged prints and analyzed using Gatan's Digital Micrograph software running on a Power Macintosh. The patterns and the analysis procedure were calibrated using reflections from nanocrystalline gold evaporated onto the surfaces of the DNC TEM specimens. Experimental electron-diffraction data were compared to JCPDS cardfile data [9] and to electrondiffraction patterns calculated using the EMS software system [10]. Focused-probe electron energy-loss spectroscopy (EELS) was done using a 200 keV Philips CM20 TEM/STEM. This instrument has a Schottky field-emission electron source and a Gatan 666 parallel EELS spectrometer. RESULTS AND DISCUSSION The individual DNC particles are almost always found as clusters ranging in size from -100 nm to -50 urm in diameter. Figure 1 shows a bright-field image from a portion of one such cluster. The light and dark contrast corresponds to individual crystals satisfying different Bragg conditions. Near the cluster edges the nanocrystals are arranged almost in monolayer fashion, but the
lOOnm
0-K edge
12K" 400
6 K
500
Figure 1 - Bright-field TEM image of
560
kpart
of a Y20 3 :Tb nanocrystal cluster. Figure 2 - Electron energy-loss spectra showing oxygen K-edge and yttrium Ledge peaks at 532eV and 2080eV, respectively.
Y-L edge
I 1800
2000
2200
Energy Loss (eV)
396
Figure 3 - High-resolution image of a typical Y20 3 :Tb nanocrystal showing
thickness rapidly increases away from the edge. Interpretation of image data from areas with more than one or two DNC layers is difficult. Increased sonication tends to break the clusters into smaller sizes and enhances the effectiveness of TEM imaging and diffraction. Electron energy-loss spectroscopy confirmed that the principal elemental species in these nanocrystals are in fact yttrium and oxygen. Figure 2 presents is a typical result. There is an oxygen K-edge at 532 eV on a strong carbon K-edge (284 eV) background due to the support film. The yttrium L-edge begins at 2080 eV. No effort was made to quantify the composition. Figure 3 shows a high-resolution image of a typical single nanocrystal lying on the carbon support film. This crystal shows facets typical of these and other nanocrystalline materials [1
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