Song-Ho Byeon

Morphology and cathodoluminescence of Li-doped Gd2O3:Eu3+, a red phosphor operating at low voltages

A red phosphor, Li-doped Gd2O3:Eu3+ with high cathodoluminescent brightness was developed. In contrast to Gd2O3:Eu3+ which showed an irregular shape of agglomerated fine particles (the mean particle size <0.5 μm), the morphology of Li doped Gd2O3:Eu3+ crystals was quite regular and pseudospherical (the mean particle size ≈2 μm). In particular, the incorporation of Li+ ions into Gd2O3 lattice could induce a remarkable increase of cathodoluminescence efficiency at low voltages (500 V–1 kV). The highest emission intensity was observed with Gd1.84Li0.08Eu0.08O3−δ, whose brightness at 500 V was increased by a factor of 1.8 in comparison with that of commercial Y2O3:Eu3+. This phosphor may be an application to the field emission display operating at low voltages.

Synthesis of colloidal aqueous suspensions of a layered gadolinium hydroxide: a potential MRI contrast agent

A layered gadolinium hydroxychloride (LGdH), [Gd2(OH)5(H2O)x]Cl, was synthesized from an aqueous solution of GdCl3.6H2O. The X-ray diffraction (XRD) and the selected area electron diffraction (SAED) studies showed that this compound crystallizes in the orthorhombic structure (a = 12.88(4) A, b = 7.30(2) A, and c = 8.46(3) A) which is isostructural with [Eu2(OH)5(H2O)x]Cl. Interestingly, this layered material was readily dispersed and led to a stable colloidal nanosheet in aqueous medium. The obtained colloidal solutions were characterized by the evaluation of their stability in acidic solution, their in vitro cytotoxicity, and their magnetic resonance imaging (MRI) relaxation properties. It is reported that the relaxometry analysis of LGdH suspensions exhibits a sufficient contrast effect for T1 weighted magnetic resonance imaging.

Fabrication of a silica sphere with fluorescent and MR contrasting GdPO4 nanoparticles from layered gadolinium hydroxide

The delaminated gadolinium hydroxide layers doped with Eu(3+) ions were assembled on the surface of silica spheres and annealed at high temperatures, resulting in the formation of fluorescent and MR active GdPO(4) : Eu nanoparticles at the surface.

Highly enhanced photoluminescence of SrTiO3:Pr by substitution of (Li0.5, La0.5) pair for Sr

Photoluminescence (PL) of SrTiO3‐(Li0.5La0.5)TiO3:Pr system was investigated. This system was characterized by a very intense red emission at 611 nm corresponding to the 1 D2! 3 H4 inner transition of Pr 31 ions. A weak PL intensity of SrTiO3:Pr was remarkably increased when the Sr atom was replaced by the (Li, La) pair. Through the elemental analysis for the Li atom, the dependence of PL intensity on the composition and heating temperature is correlated with the amount of defect sites for the Li atoms. Based on the experimental data, it is proposed that the substitution of the (Li, La) pair for Sr of SrTiO 3 likely produces the holes trapped near Li 1 . The increased recombination probability of electrons and trapped holes would then cause the highly enhanced emission intensity of Pr 31 by the resultant energy transfer. q 2000 Published by Elsevier Science Ltd. All rights reserved.

Unexplored thermal transformation behavior of two-dimensionally bound gadolinium hydroxide layers: fabrication of oriented crystalline films of gadolinium oxychloride nanosheets suitable for the multicolor luminescence with color tunability.

Metal oxide and hydroxide nanosheets, which are derived from their layered host compounds, are some of the most important building blocks for constructing various functional fi lms using sequential layer-by-layer (LbL) self-assembly and LangmuirBlodgett (LB) deposition processes. [ 1 − 3 ] The use of nanosheets with high two-dimensional anisotropy and molecular thickness has enabled the recent production of well oriented crystallite fi lms or rare crystalline phases that cannot be achieved using three dimensional bulk precursors. [ 4 ] Nevertheless, there is still little knowledge about the crystalline transformations behavior occurring in two-dimensionally bound systems. Recently, a new series of layered rare earth hydroxides (LRHs) with the general composition, RE 2 (OH) 5 X ⋅ n H 2 O (RE = rare earths, X = anions), were synthesized. [ 5–7 ] These materials have generated a great deal of interest on account of their unique ability to combine the useful properties of rare earth ions with the host-guest chemistry of layered double hydroxides (LDHs). [ 5a , 6b,c , 7 , 8 ] One of the most attractive aspects of LRHs is that they can incorporate activator ions and generate a photoluminescent fi lm on a range of substrates using previously established deposition techniques. [ 9 ] In this context, our research efforts were initially intended to fabricate a photoluminescent multilayer fi lm based on gadolinium oxide (Gd 2 O 3 ) by employing activator-doped layered gadolinium hydroxide ( LGdH ) as a precursor. In the course of annealing Eu 3 + or Tb 3 + -doped LGdH multilayers, we observed the unexpected growth of gadolinium oxychloride (GdOCl) crystallites through a confi ned crystallization process. This is a unique and interesting phenomenon that was not expected with any bulk compounds or randomly aggregated nanosheets and, to the best of our knowledge, has never been reported. Along with providing fundamental knowledge, this fi nding also offers a simple and novel method of fabricating multicolor photoluminescent inorganic thin fi lms, which will fi nd considerable use in the development of display and

Grafting of dodecylsulfate groups on gadolinium hydroxocation nanosheets for self-construction of a lamellar structure

The self-stacking of gadolinium hydroxocation nanosheets to form a layered structure was achieved by using the dodecylsulfate (DS) group as an anionic surfactant at room temperature. DS-assisted construction behavior of layered gadolinium hydroxocations (LGdH) differed significantly depending on the pH of the solutions. At pH 7 showed an interlayer spacing of 24∼25 A, which can result from an interdigitated monolayer packing of the DS groups. The products obtained at this pH range were directly transformed to Gd2O2SO4 without formation of Gd2(SO4)3 with increasing temperature and showed no shift of the S–O asymmetric stretching band in the IR spectra. Thus, no strong interaction between the sulfate headgroups and the LGdH layers suggested that typical restacking of the exfoliated nanosheets and DS groups leads to the lamellar structure at pH > 7.

Superhydrophilic and antireflective La(OH)3/SiO2-nanorod/nanosphere films

La(OH)(3) nanorods were self-stacked on the glass slide substrates using an aqueous suspension obtained from the hydrolysis of LaOCl. The key for producing a high optical quality film of La(OH)(3) lies in the preparation of an aqueous suspension in which La(OH)(3) nanorods are well dispersed. These thin-film coatings of La(OH)(3) nanorods led to a significantly reduced reflective losses in the visible region, exhibiting an attractive and potentially useful single-layer antireflection property. Furthermore, La(OH)(3) nanorod layer provides a sufficiently porous and rough surface required to achieve superhydrophilicity. Thus, when SiO(2) nanoparticles of ca. 20nm in diameter were deposited onto La(OH)(3) layer of high roughness, the resulting La(OH)(3)/SiO(2) film demonstrated an interesting nanoporosity-derived superhydrophilicity and antifogging property with no significant loss of antireflective property.

XANES study on Ruddlesden‐Popper phase, Lan+1NinO3n+1 (n = 1, 2, and ∞)

Ruddlesden-Popper phase, La(n+1)Ni(n)O(3n+1 (n=1, 2, and infinity) compounds were prepared by citrate sol-gel method. We revealed the origin of the variation of the electrical conductivities in La(n+1)Ni(n)O(3n+1 (n=1, 2, and infinity) using resistivity measurements, Rietveld analysis, and X-ray absorption spectroscopy. According to the XANES spectra, it is found that the degree of 4p(pi) - 4psigma energy splitting between 8345 eV and 8350 eV is qualitatively proportional to the elongation of the out-of-plane Ni-O bond length. With the decrease of 4p(pi) - 4psigma splitting, the strong hybridization of the sigma-bonding between Ni-3d and O-2p orbitals creates narrow antibonding sigma* bands, which finally results in the lower electrical resistivity.

Relationship between interlayer anions and photoluminescence of layered rare earth hydroxides

The effect of interlayer anions on the photoluminescence of layered rare earth hydroxides was investigated with the rare earth (RE)-doped layered gadolinium hydroxynitrate as a representative base matrix for efficient and stable anion-exchange reactions. Eu3+, Tb3+, and Ce3+ were employed as RE activator ions for red, green, and blue emissions, respectively. The excitation and emission behaviors of Gd1.80RE0.20(OH)5X·nH2O (LGdH:RE) were systematically compared for various interlayer inorganic and organic anions, where X = F−, Cl−, I−, OH−, ClO3−, S2−, CO32−, SO42−, terephthalate, 2-naphthoate, and dodecylsulfate members were obtained by the exchange reaction of corresponding X = NO3− members. Interestingly, a close relationship was found between the UV-Vis absorption spectra of aqueous solutions containing X anions and the excitation behavior of LGdH:RE. Thus, NO3−, I−, and S2− anions showing high absorbance in aqueous solution consistently shielded the excitation light for the 8S7/2 → 6IJ transition of Gd3+ and the 4f → 5d interconfigurational transitions of Tb3+ and Ce3+ to turn off the corresponding emissions from LGdH:Eu, LGdH:Tb, and LGdH:Ce. In contrast, the effect of terephthalate and 2-naphthoate, despite high absorbance in aqueous solutions, was significantly different depending on the RE activator ion. It is proposed that an identical interlayer anion in the gallery of LRHs can filter or sensitize the UV energy for excitation of RE3+-doped LRHs and its role (whether as a filter, a sensitizer, or just a spacer) is determined by the nature of activator ions.

Antenna Effect on the Organic Spacer-Modified Eu-Doped Layered Gadolinium Hydroxide for the Detection of Vanadate Ions over a Wide pH Range

The excitation of the adsorbed vanadate group led to the red emission arising from the efficient energy transfer to Eu-doped layered gadolinium hydroxide (LGdH:Eu). This light-harvesting antenna effect allowed LGdH:Eu to detect selectively a vanadate in aqueous solution at different pHs. Because vanadate exists in various forms by extensive oligomerization and protonation reactions in aqueous solution depending on pH, it is important to detect a vanadate regardless of its form over a wide pH range. In particular, spacer molecules with long alkyl chains greatly facilitated access of a vanadate antenna into the interlayer surface of LGdH:Eu. The concomitant increase in adsorption capacity of LGdH:Eu achieved a strong antenna effect of vanadate on the red emission from Eu(3+). When a suspension containing LGdH:Eu nanosheets (1.0 g/L) was used, the vanadate concentration down to 1 × 10(-5) M could even be visually monitored, and the detection limit based on the (5)D0 → (7)F2 emission intensity could reach 4.5 × 10(-8) M.