Improved optical photoluminescence by charge compensation in the phosphor system CaMoO4: Eu3+

It has been found that charge compensated CaMoO4:Eu3+ phosphors show greatly enhanced red emission under 393 and 467 nm-excitation, compared with CaMoO4:Eu3+ without charge compensation. Two approaches to charge compensation, (a) 2Ca(2+) -> EU3+ + M+, where M+ is a monovalent cation like Li+, Na+ and K+ acting as a charge compensator; (b) 3Ca(2+) -> 2EU(3+) + vacancy, are investigated. The influence of sintering temperature and Eu3+ concentration on the luminescent property of phosphor samples is also discussed.

Spectroscopic studies on charge transfer from Ce3+, to Yb3+ in KMgF3 : Ce, Yb nanocrystals

Nanocrystals of KMgF3 single-doped and codoped with Ce3+ or/and Yb3+ were synthesized separately by the microemulsion method. The X-ray diffraction(XRD) patterns were indexed to show that the KMgF, crystal system was unchanged. The fluorescent spectra of KMgF3:Ce, Yb polycrystal powders were studied and compared with those of the Ce, Yb doped KMgF3 crystals produced using the high-temperature solid phase method. The diffuse reflection spectra and infrared. emission of KMgF3:Ce, Yb were investigated. From the results, the authors could confirm that there were charge transfer processes from Ce3+ to Yb3+ in both KMgF3: Ce,Yb nanocrystals and polycrystal powders.

Investigation on relationship between charge transfer position and dielectric definition of average energy gap in Eu3+-doped compounds

The dielectric definition of average energy gap E-g of the chemical bond has been calculated quantitatively in Eu3+-doped 30 lanthanide compounds based on the dielectric theory of chemical bond for complex structure crystals. The relationship between the experimental charge transfer (CT) energy of Eu3+ and the corresponding average energy gap E-g has been studied. The results show that the CT energy increases linearly with increasing of the average energy gap E-g. The linear model is obtained. It allows us to predict the CT position of Eu3+-doped lanthanide compounds with knowledge of the crystal structure and index of refraction. Applied to the Ca4GdO(BO3)(3):Eu and Li2Lu5O4(BO3)(3):Eu crystals, the predicted results of CT energies are in good agreement with the experimental values, and it can be concluded that the lowest CT energy in Li2Lu5O4(BO3)(3):Eu originates from the site of Lu1.

Effect of doped dye on the charge-carrier transport and electroluminescent performance in molecularly doped polymer light-emitting diodes

The effects of the concentration of 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H, 5H, 11H-(1)-benzopyropyrano(6, 78-i,j)quinolizin-11-one (C545T) as dopant in polyfluorene (PFO) on the charge-carrier transport and electroluminescence (EL) performance were investigated by steady-state and transient EL measurements. A fully green emission from C545T was observed and the EL performance depends strongly on the C545T concentration. The mobility in the C545T-doped PFO film was determined by transient EL. The dopant concentration dependence of the current-voltage relationship indicated clearly the carrier trapping by the C545T molecules. The mobility in C545T:PFO changed significantly with the C545T concentration, and showed a nontrivial dependence on the doping level. The behavior may be understood in terms of the formation of an additional energy disorder due to potential fluctuation caused by the Coulomb interaction of the randomly distributed doping molecules.

Effects of doped dye on the charge carrier injection, transport, and electroluminescent performance in polymeric light-emitting diodes

The effects of doped fluorescent dye 4-(dicyanomethylene)-2-i-propyl-6-(1, 1, 7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTI) on the charge carrier injection, transport and electroluminescence (EL) performance in polyfluorene (PFO)-based polymer light-emitting diodes (PLEDs) were investigated by steady-state current-voltage (I-V) characteristics and transient EL measurements. A red EL from DCJTI was observed and the EL performance depended strongly on the DCJTI concentration. The analysis of the steady-state I-V characteristics at different DCJTI concentrations found that three regions was shown in the I-V characteristics, and each region was controlled by different processes depending on the applied electric field. The effect of the dopant concentration on the potential-barrier height of the interface is estimated using the Fowler-Nordheim model. The dopant concentration dependence of the current-voltage relationship indicated clearly the carrier trapping by the DCJTI molecules. The mobility in DCJTI: PFO changed significantly with the DCJTI concentration, and showed a nontrivial dependence on the doping level. The behavior may be understood in terms of the formation of an additional energy disorder due to potential fluctuation caused by the Coulomb interaction of the randomly distributed doping molecules.

Charge carrier transporting, photoluminescent, and electroluminescent properties of zinc(II)-2-(2-hydroxyphenyl)benzothiazolate complex

Zinc(II)-2-(2-hydroxyphenyl)benzothiazolate complex is an excellent white-light-emitting material. Despite some studies devoted to this complex, no information on the real origin of the unusually broad electroluminescent (EL) emission is available. Therefore, we investigate photoluminescent and EL properties of the zinc complex. Orange phosphorescent emission at 580 nm was observed for the complex in thin film at 77 K, whereas only fluorescent emission was obtained at room temperature. Molecular orbitals, excitation energy, and emission energy of the complex were investigated using quantum chemical calculations. We fabricated the device with a structure of ITO/F16CuPc(5.5 nm)/Zn-complex/Al, where F16CuPc is hexadecafluoro copper phthalocyanine. The EL spectra varied strongly with the thickness of the emissive layer. We observed a significant change in the emission spectra with the viewing angles. Optical interference effects and light emission originating both from fluorescence and from phosphorescence can explain all of the observed phenomena, resulting in the broad light emission for the devices based on the Zn complex. We calculated the charge transfer integral and the reorganization energy to explain why the Zn complex is a better electron transporter than a hole transporter.

Charge ordering induced metal-semiconductor transition in Ag2BiO3: A first-principles study

Accurate ab initio density-functional calculations are performed to investigate the relationship of the ground-state crystal structures and electronic properties of Ag2BiO3 compound. The results indicate that Ag2BiO3 in Pnna phase, in which the bismuth atoms occupy the same Wyckoff positions, exhibits metallic conductivity, while in Pnn2 and Pn phases, Ag2BiO3 exhibits semiconducting character, which is in agreement with the experimental results. Charge ordering is indeed induced by the crystal inversion twin in the Pnn2 phase compared with the Pnna phase. In the low temperature phase Pn, the charge ordering is similar to that of Pnn2 phase although it is more distorted in Pn phase. In addition, the calculation indicates that the charge ordering is caused in the 6s electron rearrangement.

Surface charge influence on the surface plasmon absorbance of electroactive thiol-protected gold nanoparticles

Gold nanoparticles (3.1-5.0 nm in size) surface-derivatized with both electroactive and nonelectroactive self-assembled monolayers were synthesized. The surface-derivatized electroactive particles can be easily oxidized/reduced at an electrode surface based on the diffusion-controlled current-voltage curve observed in cyclic voltammetry measurements. Spectroelectrochemical investigation demonstrated that the maximum absorbance of the nanoparticles in their oxidized state red-shifted compared with their reduced state to a different extent according to their size distribution. In the case of the particles surface-derivatized with nonelectroactive monolayers, much less shift was observed. This study showed that surface plasmon absorbance of gold nanoparticles was not only related to core charge states but was also influenced by surface charge states as well.

Conducting probe atomic force microscopy investigation of anisotropic charge transport in solution cast PBD single crystals induced by an external field

2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxdiazole (PBD) is a good electron-transporting material and can form single crystals from solution. In this work, solution cast PBD single crystals with different crystallographic axes (b, c) perpendicular to the Au/S substrates in large area are achieved by controlling the rate of solvent evaporation in the presence and absence of external electrostatic field, respectively. The orientation of these single crystals on Au/S substrate was characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Conducting probe atomic force microscopy (CP-AFM) was used to measure the charge transport characteristics of PBD single crystals grown on Au/S substrates. Transport was measured perpendicular to the substrate between the CP-AFM tip and the Au/S substrate. The electron mobility of 3 x 10(-3) cm(2)/(V s) for PBD single crystal along crystallographic b-axis is determined. And the electron mobility of PBD single crystal along the c-axis is about 2 orders of magnitude higher than that along the b-axis due to the anisotropic charge transport at the low voltage region.

Effect of dye concentration on the charge carrier transport in molecularly doped organic light-emitting diodes

The effect of the concentration of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as dopant in tris(8-quinolinolato) aluminum (Alq(3)) on the charge carrier transport in Alq(3):DCJTB was investigated by measuring the steady current-voltage characteristics and the transient electroluminescence. The dopant concentration dependence of the current-voltage relationship clearly indicates the carrier trapping by the DCJTB molecule. The DCJTB concentration significantly affects the electron mobility in Alq(3):DCJTB. The mobility has a nontrivial dependence on the doping level. For relatively low doping levels, less than 1%, the electron mobility of Alq(3):DCJTB decreases with the doping level. An increasing mobility is then observed if the dopant concentration is further increased, followed by a decrease for doping levels larger than similar to2%. The change of the electron mobility with the DCJTB concentration in Alq(3) is attributed to the additional energetic disorder due to potential fluctuations caused by the dipole-dipole interaction of random distribution dopant at the relatively low doping concentration, and to the phase separation at the high doping concentration.

Micellization and reversible pH-sensitive phase transfer of the hyperbranched multiarm PEI-PBLG copolymer

A novel, hyperbranched, amphiphilic multiarm biodegradable polyethylenimine-poly(gamma-benZyl-L-gluta- mate) (PEI-PBLG) copolymer was prepared by the ring-opening polymerization of gamma-benzyl-L-glutamate-N-car-boxyanhydride (BLG-NCA) with hyperbranched PEI as a macroinitiator. The copolymer could self-assemble into core-shell micelles in aqueous solution with highly hydrophobic micelle cores. As the PBLG content was increased, the size of the micelles increased and the critical micelle concentration (CMC) decreased. The surface of the micelles had a positive potential. The cationic micelles were capable of complexing with plasmid DNA (pDNA), which could be released subsequently by treatment with polyanions. The PEI-PBLG copolymer formed unimolecular micelles in chloroform solution. ne pH-sensitive phase-transfer behavior exhibited two critical pH points for triggering the encapsulation and release of guest molecules. Both the encapsulation and release processes were rapid and reversible. Under strong acidic or alkaline conditions, the release process became partially or completely irreversible.

Dependence of charge transfer energy on crystal structure and composition in Eu3+-doped compounds

We report a method for estimating the positions of charge transfer (CT) bands in Eu3+-doped complex crystals. The environmental factor ( he) influencing the CT energy is presented. he consists of four chemical bond parameters: the covalency, the bond volume polarization, the presented charge of the ligand in the chemical bond, and the coordination number of the central ion. These parameters are calculated with the dielectric theory of complex crystals. The relationship between the experimental CT energies and calculated environmental factors was established by an empirical formula. The calculated values are in good agreement with the experimental results. Such a relationship was confirmed by detailed analysis. In addition, our method is also useful to predict the charge-transfer position of any other rare earth ion.

Synthesis of PtNPs/AQ/Ru(bpy)(3)(2+) colloid and its application as a sensitive solid-state electrochemiluminescence sensor material

The facile synthesis of the novel platinum nanoparticles/Eastman AQ55D/ruthenium(II) tris( bipyridine) (PtNPs/ AQ/Ru(bpy)(3)(2+)) colloidal material for ultrasensitive ECL solid-state sensors was reported for the first time. The cation ion-exchanger AQ was used not only to immobilize ECL active species Ru(bpy)(3)(2+) but also as the dispersant of PtNPs. Colloidal characterization was accomplished by transmission electron microscopy (TEM), X-ray photoelectron spectrum (XPS), and UV-vis spectroscopy. Directly coating the as-prepared colloid on the surface of a glassy carbon electrode produces an electrochemiluminescence (ECL) sensor. The electronic conductivity and electroactivity of PtNPs in composite film made the sensor exhibit faster electron transfer, higher ECL intensity of Ru(bpy)(3)(2+), and a shorter equilibration time than Ru(bpy)(3)(2+) immobilized in pure AQ film. Furthermore, it was demonstrated that the combination of PtNPs and permselective cation exchanger made the sensor exhibite excellent ECL behavior and stability and a very low limit of detection (1 x 10(-15) M) of tripropylamine with application prospects in bioanalysis. This method was very simple, effective, and low cost.

Effect of DNA flanking sequence on charge transport in short DNA duplexes

Several factors can influence charge transport (CT)-mediated DNA, such as sequence, distance, base stacking, base pair mismatch, conformation, tether length, etc. However, the DNA context effect or how flanking sequences influence redox active drugs in the DNA CT reaction and later in DNA enzymatic repair and synthesis is still not well understood. The set of seven DNA molecules in this study have been characterized well for the study of flanking sequence effects. These DNA duplexes are formed from self-complementary strands and contain the common central four-base sequence 5'-A-G-C-T-3', flanked on both sides by either (AT)(n) or (AA)(n) (n = 2, 3, or 4) or AA(AT)(2). UV-vis, fluorescence, UV melting, circular dichroism, and cyclic voltammetry experiments were used to study the flanking sequence effect on CT-mediated DNA by using daunomycin or adriamycin cross-linked with these seven DNA molecules. Our results showed that charge transport was related to the flanking sequence, DNA melting free energy, and ionic strength. For (AA)(n) or (AT)(n) species of the same length, (AA)(n) series were more stable and more efficient CT was observed through the (AA)(n) series. The same trend was observed for (AA)(n) and (AT)(n) series at different ionic strengths, further supporting the idea that flanking sequence can result in different base stacking and modulate charge transport through these seven DNA molecules.