Indium hydroxide, In(OH)3, nano-microstructures with two kinds of morphology, nanorod bundles (around 500 nm in length and 200 nm in diameter) and caddice spherelike agglomerates (around 750-1000 nm in diameter), were successfully prepared by the cetyltrimethylammonium bromide (CTAB)/water/cyclohexane/n-pentanol microemulsion-mediated hydrothermal process. Calcination of the In(OH)3 crystals with different morphologies (nanorod bundles and spheres) at 600 degrees C in air yielded In2O3 crystals with the same morphology. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and photoluminescence (PL) spectra as well as kinetic decays were used to characterize the samples. The pH values of microemulsion play an important role in the morphological control of the as-formed In(OH)3 nano-microstructures from the hydrothermal process. The formation mechanisms for the In(OH)3 nano-microstructures have been proposed on an aggregation mechanism. In2O3 nanorod bundles and spheres show a similar blue emission peaking around 416 and 439 nm under the 383-nm UV excitation, which is mainly attributed to the oxygen vacancies in the In2O3 nano-microstructures.

This feature article highlights work from the authors' laboratories on the various kinds of oxide optical materials, mainly luminescence and pigment materials with different forms (powder, core-shell structures, thin film and patterning) prepared by the Pechini-type sol-gel (PSG) process. The PSG process, which uses the common metal salts (nitrates, acetates, chlorides, etc.) as precursors and citric acid (CA) as chelating ligands of metal ions and polyhydroxy alcohol (such as ethylene glycol or poly ethylene glycol) as a cross-linking agent to form a polymeric resin on molecular level, reduces segregation of particular metal ions and ensures compositional homogeneity. This process can overcome most of the difficulties and disadvantages that frequently occur in the alkoxides based sol-gel process. Using the PSG process, we are able to prepare luminescent powder materials that cannot be well synthesized by the solid-state reaction method, environmentally friendly and highly efficient phosphors that lack metal activator ions, core-shell structured monodisperse and spherical optical materials with tunable physical chemical properties, and thin film phosphors and their patterning combined with soft lithography techniques. The extensive applicability of this process and potential material applications are demonstrated.

Ca In 2 O 4 : x Eu 3 +  (x=0.5%,1.0%,1.5%) phosphors were prepared by the Pechini sol-gel process [U.S. Patent No. 3,330,697 (1967)] and characterized by x-ray diffraction and photoluminescence and cathodoluminescence spectra as well as lifetimes. Under the excitation of 397nm ultraviolet light and low voltage electron beams, these phosphors show the emission lines of Eu3+ corresponding to D0,1,2,35-FJ7 (J=0,1,2,3,4) transitions from 400to700nm (whole visible spectral region) with comparable intensity, resulting in a white light emission with a quantum efficiency near 10%. The luminescence mechanism for Eu3+ in CaIn2O4 has been elucidated.

There has been an increasing interest in the past few years in the synthesis of ferroelectric nanocrystals because of their scientific importance and widespread applications in electronics, sensing, catalysis, and nonlinear optics. 1 BaTiO 3 is a perovskite-type ceramic with unique mechanical, ferroelectric, electro-optic, pyroelectric, dielectric, and elastic properties that find use in multilayered capacitors, random access memories, thermistors, photonic crystals, pressure transducers, and waveguide modulators. 2 It is well-known that perovskite nanocrystals possess structural and physical properties that are strongly dependent on their size, shape, crystallinity, and surface composition. For example, in nanoscale BaTiO 3 , the transition temperature from the ferroelectric (tetragonal) to the paraelectric (cubic) phase decreases progressively with the size of the particles as a result of a less distorted coordination environment of the Ti 4þ ions within the TiO 6 octahedra. However, there is no clear consensus as to the critical size at which ferroelectricity is suppressed, and consequently, the values reported extend over a wide range, typically from 120 to 4.2 nm. 3 Despite the increasing number

Nanocrystalline ZrO2 fine powders were prepared via the Pechini-type sol−gel process followed by annealing from 500 to 1000 °C. The obtained ZrO2 samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR), and photoluminescence spectra (PL), respectively. The phase transition process from tetragonal (T) to monoclinic (M) was observed for the nanocrystalline ZrO2 powders in the annealing process, accompanied by the change of their photoluminescence properties. The 500 °C annealed ZrO2 powder with tetragonal structure shows an intense whitish blue emission (λmax = 425 nm) with a wide range of excitation (230−400 nm). This emission decreased in intensity after being annealed at 600 °C (T + M-ZrO2) and disappeared at 700 (T + M-ZrO2), 800 (T + M-ZrO2), and 900 °C (M-ZrO2). After further annealing at 1000 °C (M-ZrO2), a strong blue-green emission appeared again (λmax = 470 nm). Based on spectral analysis and EPR results, the whitish blue emission (425 nm) and blue-green emission (470 nm) can be ascribed to interstitial carbon impurities (Ci) in the tetragonal ZrO2 and oxygen vacancies (VO) in the monoclinic ZrO2, respectively.

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An engineered plasmonic gold surface, specifically designed to couple with 980 nm radiation, is shown to enhance near-infrared-to-visible upconversion luminescence from a monolayer of β-NaYF4: 17%Yb, 3%Er nanocrystals in poly(methyl methacrylate) on that gold surface. Confocal imaging of upconversion luminescence from the surface is used to characterize the nature of the enhancement. It is shown that the luminescence data were acquired below the so-called “high power limit” for excitation, but some saturation was evident, as the observed power dependence was less than quadratic. Over the range of excitation power densities used, the intrinsic enhancement factor for upconversion from the patterned surface was greater than a factor of 3 but decreased slowly with increasing excitation power. The red and green upconversion were enhanced by similar factors, which would support the intensification of the excitation field by the plasmonic surface as being the mechanism of enhancement. In the absence of other enhancement or quenching mechanisms, the data imply an approximate 2-fold magnification of the excitation field intensity relative to smooth gold.

A method is described for producing highly luminescent composite NIR-to-visible upconversion thin films, made from β-NaYF4:3%Er,17%Yb nanocrystals in a polymethyl methacrylate (PMMA) matrix, which require no postdeposition heat treatment. Nanocrystals are synthesized via a single-phase, high-boiling-point solvent method, which requires neither metal-trifluoroacetate precursors nor the use of autoclaves. Highly luminescent films are produced that can be varied in thickness down to dimensions approaching those of the nanocrystals themselves. The physical properties of the films are characterized by AFM and TEM, whereas the spectroscopic properties are characterized by NIR-to-visible confocal microscopy and by the time-dependence of upconversion luminescence following pulsed NIR excitation. It is shown that dispersal of β-NaYF4:3%Er,17%Yb nanocrystals in PMMA has no adverse effect on the intrinsic quantum efficiency of upconversion. By focusing the NIR pump beam (980 nm, cw) in the film, linear intensity response and constant color balance are achieved at pump powers down to 40 μW. It is also demonstrated that the thin-film method can be modified to produce large NIR-to-visible upconversion monoliths of high optical quality. This study supports an earlier assertion that the upconversion properties of β-NaYF4:Er,Yb nanocrystals approach those of the bulk material when nanocrystal size is greater than ∼70 nm.