Showing papers by "Paul H. Holloway published in 2009"

High efficiency photoluminescence from silica-coated CdSe quantum dots

Abstract: The effects on the luminescent properties of coating CdSe quantum dots with silica are presented. Coating increased the quantum yields from ∼10%–20% to ∼80% at a maximum. The changes in quantum yields and photoluminescence peak at wavelength were discussed in terms of the effects of surface charge. By neutralizing surface charge, the emission from CdSe quantum dots was initially blueshifted followed by redshifting, and the quantum yield increased dramatically.

Functionalized CdS nanospheres and nanorods

Abstract: Functionalized nanoparticles are discussed. Surfaces of CdS:Mn/ZnS core/shell nanospheres (Qdots) were converted from hydrophobic to hydrophilic by growth of a SiO2 shell. The colloidal dispersion was stabilize by adding a surfactant with a negative surface charge, and a cell-penetrating-peptide, TAT, was attached through a primary amine group. The TAT functionalized Qdots were shown to pass the blood–brain-barrier and luminescence in the infused half of the brain. In addition, nanorods of S2− rich CdS were synthesized by reaction of excess S with Cd precursors in the presence of ethylene diamine. The photoluminescence (PL) peak from the S2− rich CdS nanorods was broad with a maximum at ∼710 nm, which was 40 nm longer in wavelength than the PL peak from Cd2+ rich CdS (∼670 nm) nanorods. The influence of surface electron or hole trap states on the luminescent pathway of CdS nanorods were used to explain these shifts in wavelength. Nanocrystals of Au with ∼2 nm diameters were grown on S2− rich surfaces of CdS nanorods. Significant quenching of photoluminescence was observed from Au nanocrystals on CdS nanorods due to interfacial charge separation. Charge separation by Au nanocrystals on CdS resulted in enhanced UV photocatalytic degradation of Procion red mix-5B (PRB) dye in aqueous solution.

Photoluminescence of solution processed poly n‐vinyl carbazole films

Abstract: The photoluminescence (PL) of poly n-vinyl carbazole (PVK) films formed by solution processing was investigated. PVK films were formed by spincasting onto bare glass or glass covered by films of indium tin oxide (ITO) and poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)ethylene (PEDOT : PSS). Some of the spincast films were redissolved in chloroform solvent and redried both in the absence or presence of an electric field (6000 V/cm). The broad aggregate PL peak near 410 nm exhibited both blue and red shifts dependent upon the processing conditions. These shifts in PL were attributed to changes in the excimer populations associated with the molecular conformation of adjacent carbazole groups. The PL data were deconvoluted into two component peaks representing two excimers associated with overlap of only one (partial or P) or full overlap (F) of both aromatic rings on adjacent carbazole groups. It was concluded that the excimer population was dependent on the amount of residual solvent left in the film after processing and upon the type of substrate. The mechanism(s) controlling the molecular configurations was discussed. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Synthesis, characterization, and luminescent properties of ZnO–SiO2:PbS

Abstract: PbS and ZnO nanoparticles prepared separately by a sol-gel process were incorporated in SiO2 by an ex situ method, resulting in a green emitting ZnO–SiO2:PbS powder phosphor. Particle morphology, structure, and chemical composition of the ZnO nanoparticles, PbS nanoparticles, and ZnO–SiO2:PbS composite were analyzed with transmission electron microscopy, x-ray diffraction, and energy dispersive spectroscopy (EDS), respectively. With or without ZnO nanoparticles, green photoluminescence with a peak at 540nm was observed from SiO2:PbS when excited with a 325nm He–Cd laser in air at room temperature. This peak was different from defect-related emission from ZnO nanoparticles at 580nm and red-orange emission from SiO2:Pb2+ with a broad peak at 600–750nm. Again, the 540nm peak was different from the band edge emission from pure PbS nanoparticles at 1200nm. Note that this emission (540nm) was enhanced considerably when ZnO nanoparticles were incorporated. Photoluminescence properties of ZnO–SiO2:PbS is discussed.

Core/Shell Composite of Self-Assembled Hierarchical Bismuth Oxide/Europium Doped Gadolinium Oxide for Scintillating Detection

Abstract: A novel three dimensional (3D) self-assembled hierarchical bismuth oxide was prepared via a sol-gel synthesis with the aid of capping agent of polyethylene glycol-8000 (PEG-8000) at 85 ℃ in 45 min. The morphology evolution was studied versus reaction time and interpreted in terms of growth mechanisms. The as-grown 3D hierarchical flower-like bismuth oxide was crystalline cubic gamma-phase. The morphology and crystal phase of these 3D cubic gamma-phase bismuth oxide flowers were not changed with heating up to 600 ℃. The flower-like morphology was attributed to modification of the growth kinetics by the capping agent from the PEG-OH bond bridging with bismuth ions. Europium doped gadolinium oxide shell were further deposited on the bismuth oxide cores through sol-gel synthesis showing good photoluminescence characteristics at 610 and 622 nm under the excitation at 280 nm.

Structure and Luminescence of Europium-Doped Gadolinia-Based Core/Multi-Shell Scintillation Nanoparticles

Abstract: Scintillating nanoparticles with a SiO 2 core and a Gd 2 O 3 shell doped with Eu 3+ were synthesized with a sol-gel process. Based on transmission electron microscopy (TEM) data, a ∼13 nm Gd 2 O 3 shell was successfully coated onto ∼220 nm mono-dispersed SiO 2 nanocores. Eu 3+ ions at concentrations of nominally 5 at% exhibited photoluminescent (PL) emission from the SiO 2 /Gd 2 O 3 nanoparticles after being calcined at 800 0 C for 2 h. The SiO 2 remained amorphous after calcining, while the Gd 2 O 3 crystallized to a cubic structure. The PL emission was from the 5 D 0 - 7 F 2 transitions of Eu 3+ at 609 and 622 nm. Photoluminescence excitation (PLE) data showed that emission from Eu 3+ could result from direct excitation, but was dominated by the oxygen to europium charge-transfer band (CTB) between 250 and 280 nm for Eu 3+ doped in Gd 2 O 3 . The quantum yield (QY) from thin films drop cast from a mixture of 20 mg of calcined nanoparticles in 500 μL of polymethylmethacrylate (PMMA) and excited in the CTB was 20% for SiO 2 /Gd 2 O 3 :Eu 3+ core/shell scintillation nanoparticles. Finally, the above core/shell nanoparticles were passivated with a shell of SiO 2 to create e.g. SiO 2 /Gd 2 O 3 :Eu 3+ /SiO 2 nanoparticles. The QYs for this nanostructure were lower than unpassivated nanoparticles which was attributed to a weak CTB for the amorphous SiO 2 shell and a higher density of interface quenching sites.