Phosphor particles (ca. 60 nm in size) of yttrium oxide co-doped with Er 3+ and Yb 3+ ions were prepared by a precipitation method in the presence of EDTA. Their physical properties were compared to much larger particles (ca. 600-800 nm in size) prepared in the absence of EDTA. All of the particles were shown to have crystallized in the cubic phase, and all exhibited blue, blue-green, and green-yellow upconversion emission when excited with laser light of wavelength equal to 632.8 nm. These upconversion emissions were shown to be excited by a two-photon process. The most intense yellow-green upconversion emission occurs when the crystallite size is between 75 and 200 nm and the particle size is 600 to 800 nm. Cross-relaxation processes between Er 3+ ions are suggested to be responsible for the more efficient upconversion in the larger particles. There is evidence from spectra taken in the temperature range 30 to -190°C that there are two different hot bands in the given region of the spectrum. These two emission manifolds are explained as arising from the two different Er 3+ lattice sites in the cubic Y 2 O 3 :Er 3+ structure. When using red excitation (rather than infrared excitation), the presence of Yb 3+ was found to be detrimental, as it diminished the upconversion intensity.

The preparation of spherical phosphor (Y 2 O 3 :Eu) particles that are monosized for a given set of conditions, are described. The nature of the resulting self-assembled and close packed phosphor spherical particles appears to be very promising for both field emission devices (FED) and high definition television (HDTV). The size of the particles can be controlled by careful manipulation of the experimental conditions, the rationale behind this is discussed. The luminescent efficiency of the particles as a function of particle size is also reported. It is demonstrated that good light output is possible from nanocrystals.

The preparation of spherical phosphor europium-doped yttrium oxide (Y2O3:Eu) particles using a copolymer of N-isopropylacrylamide (NIPAM)] and acrylic acid (AAc) is described. The resulting phosphor particles have smaller particle diameters (<0.1 μm) compared to both commercial Y2O3:Eu phosphors and spherical particles synthesized by the urea precipitation method. Cathodoluminescent (CL) measurements demonstrate that exceptional light output is achieved from the smaller particle diameters than 0.1 μm. The luminescent properties of the phosphors synthesized via the copolymer microgel are shown to be at least comparable to commercial products at lower voltages; however, they offer the distinct advantage of a higher packing density and therefore higher resolution for field emissive displays (FED's) and cathode ray tube (CRT) applications.

Aqueous colloidal poly (N-isopropylacrylamide-co-4-vinylpyridine) [poly(NIPAM-co-4-VP)] copolymer microgels have been synthesized using different percentages of 4-vinylpyridine (4-VP). A surfactant-free emulsion polymerization reaction using N-isopropylacrylamide (NIPAM) and 4-vinylpyridine comonomers cross-linked with N,N′-methylenebisacrylamide was utilized. The reaction was initiated using the cationic initiator 2,2′-azobis(2-amidinopropane) dihydrochloride. Transmission electron micrograph data show the copolymer microgels to be monodisperse spheres. The pH and electrolyte sensitivity of the copolymer microgels have been studied, as well as temperature sensitivity, since microgels undergo a reversible volume phase transition in response to heating and cooling. Changes in the hydrodynamic diameters in the system were monitored as a function of temperature (25-60°C), pH (3-8), and ionic strength (10 -3 -10 -1 mol dm -3 NaCl or NaClO4) using photon correlation spectroscopy. The hydrodynamic diameter of poly-(NIPAM-co-4-VP) microgels increases with decreasing pH, as the vinylpyridine units become more protonated. However, the hydrodynamic diameter decreases with increasing ionic strength (over the pH range 3-8) and with increasing temperature (at pH 3 and pH 6). UV-visible spectrophotometry measurements showed a good correlation between the molar absorption and the percentage of vinylpyridine incorporated. Potentiometric titrations were used to determine the pKa values of the copolymer microgels.

Anti-Stokes and Stokes emissions have been observed from cubic Y2O3:Eu3+ under 632.8 nm excitation. All the emission features exhibited a marked thermal dependence, decreasing in intensity as the temperature was lowered. Arrhenius plots of this thermal behavior indicated that the Eu3+ ions were thermally excited to a low-lying level ca. 1300 cm-1 above the ground state; this low-lying excited state was assigned to the 7F2 level. Subsequent absorption of a 632.8 nm photon by the thermally excited Eu3+ ion promoted it to its 5D0 level. Anti-Stokes emission bands at wavelengths longer than 580 nm exhibited a one photon dependence on the 632.8 nm exciting light and are assigned to the 5D0 → 7F0, 5D0 → 7F1, and 5D0 → 7F2 transitions of the Eu3+ ion. Stokes emission bands also showing a one photon dependence were assigned to the 5D0 → 7F3 and 5D0 → 7F4 transitions. Upconversion anti-Stokes emission bands were observed at wavelengths shorter than 580 nm and showed a two photon dependence on the 632.8 nm exciting light. These emission bands are assigned to Eu3+ ions on the C 2 sites of cubic Y2O3 with the only exception being a band at 582.2 nm. Although this emission band showed a thermal behavior similar to that of the other emission bands, it is assigned to the Eu3+ ion on the S 6 site of cubic Y2O3, in keeping with previous assignments of others.