Wuxia Li

Improved performance of electroluminescent devices based on an europium complex

Electroluminescent (EL) devices using an europium complex Eu(DBM)3 bath as the electron-transport emitting layer were fabricated. The quenching effect of the metal cathode and the unstable nature of the Eu complex under EL operation markedly influence the EL efficiency. By keeping the emitting area far from the metal cathode and partly doping the Eu(DBM)3 bath layer with a hole-transport material, the EL performance was significantly improved. Sharp-band red emissions with turn-on voltage of 3 V, brightness of 820 cd/m2, and external quantum efficiency of 1% were achieved.

1.4 μm band electroluminescence from organic light-emitting diodes based on thulium complexes

Near-infrared (NIR) electroluminescence (EL) devices have been fabricated employing thulium complexes as emitting materials. The EL emissions at 1.4 and 0.8 μm were observed from the devices of tris-(dibenzoylmethanato)-mono-(bathophenanthroline or 1,10-phenonthroline) thulium [Tm(DBM)3bath or Tm(DBM)3phen] at room temperature and assigned to 3F4–3H4 and 3F4–3H6 transitions of Tm3+ ions, respectively. By comparison with the NIR emissions of four Tm complexes with different ligands, it was found that the first ligand played a more important role for the Tm3+ ion emissions rather than the second one. In order to meet the requirement of optical communication, both Tm(DBM)3bath and erbium [Er] (DBM)3bath were incorporated into EL devices so that a broadened EL emission band ranging from 1.4 to 1.6 μm was obtained, showing the potential application of Tm complexes for optical communication systems.

Bifunctional photovoltaic and electroluminescent devices using a starburst amine as an electron donor and hole-transporting material

Photovoltaic (PV) devices, using 4,4′,4″-tris(2-methylphenylphenylamino)triphenylamine (m-MTDATA) as an electron donor and tris-(8-hydroxyquinoline) aluminum (Alq) as an electron acceptor, were fabricated. The relationship between photocurrent generation, exciplex formation, and device structure was explored. PV performance was significantly enhanced by inserting a thin mixed layer (5 nm) of m-MTDATA and Alq between the two organic layers of the original m-MTDATA/Alq bilayer device. Both the bilayer and trilayer devices showed PV and electroluminescent (EL) properties, suggesting their potential use as multifunction devices. It was also shown that strong EL emission from exciplex might be used as an indicator of efficient exciton dissociation during the reverse PV process.

Synthesizing nano LaAlO3 powders via co-precipitation method

Abstract Co-preparation method has been applied to synthesize nano LaAlO 3 powders. By using La(NO 3 ) 3 and Al(NO 3 ) 3 as raw materials and keeping the pH≈9, the co-preparation process could be finished at room temperature. Pure LaAlO 3 nano powders with a size of about 50 nm could be obtained after calcining at 700–800 °C. These nano LaAlO 3 powders could be sintered at 1550 °C for 4 h, a high relative density of about 97% could be reached.

Reduced efficiency roll-off in phosphorescent organic light emitting diodes at ultrahigh current densities by suppression of triplet-polaron quenching

High-performance phosphorescent organic light emitting devices with reduced efficiency roll-off at ultrahigh current densities were realized. The devices were Ir(ppy)3-based phosphorescent organic light emitting diodes that employed 1,3-bis[2-(2,2′-bipyridine-6-yl)-1,3,4-oxadiazo-5-yl]benzene as a high mobility electron transfer layer. The device’s brightness was enhanced while the efficiency roll-off was reduced. The device exhibits high current efficiency (21cd∕A) at high brightness (80000cd∕m2), with a maximum luminescence of 136000cd∕m2 at over 1A∕m2 (with an efficiency of 13cd∕A). This reduction in efficiency roll-off is attributed to the suppression of the triplet-polaron quenching rate through balancing the charge carrier ratio in the device.

Fabrication of nano Y–TZP materials by superhigh pressure compaction

Abstract Y–TZP nanoceramics have been fabricated by superhigh pressure compacting and pressureless sintering. It was found that, by applying superhigh pressure of about 3GPa, the green compacts with a relative density of about 60% could be obtained, which was 12% higher than that by a normal cold isostatic pressure of 450 MPa. The obvious advantage of the higher relative density of the green compact is that the sintering temperature could be decreased to as low as 1050°C. The reason for the good sintering behavior is believed to be both the increase in contact points between the particles and the decrease in pore size. Nano Y–TZP ceramics with grain size of about 80 nm could be obtained when sintered at 1050°C for 5 h.

ZnO/Zn phosphor thin films prepared by IBED

Abstract ZnO/Zn phosphor thin films were prepared by ion-beam-enhanced deposition (IBED). Post-deposition annealing of these films was performed at temperatures from 100 to 1000°C in a N 2 atmosphere. RBS, XRD and PL spectra were employed to characterize these films. It was detected that there is a large amount of excess Zn in the prepared films. An amorphous structure was found in the films deposited without ion bombardment, and simultaneous ion bombardment could cause the films to contain crystalline phases and even greater excess Zn. The PL spectra showed that UV/violet and blue/green luminescence was excited in ZnO/Zn films. The annealing strongly affected the visible luminescence. Possible reasons may include the recovery of structural defects, homogenization, and evaporation of excess Zn with different contributions at different temperature ranges.

Tunability of the superconductivity of tungsten films grown by focused-ion-beam direct writing

We have grown tungsten-containing films by focused-ion-beam (FIB)-induced chemical vapor deposition. The films lie close to the metal-insulator transition with an electrical conductivity which changes by less than 5% between room temperature and 7 K. The superconducting transition temperature Tc of the films can be controlled between 5.0 and 6.2 K by varying the ion-beam deposition current. The Tc can be correlated with how far the films are from the metal-insulator transition, showing a nonmonotonic dependence, which is well described by the heuristic model of [Osofsky et al., Phys. Rev. Lett. 87, 197004 (2001)]. Our results suggest that FIB direct-writing of W composites might be a potential approach to fabricate mask-free superconducting devices as well as to explore the role of reduced dimensionality on superconductivity.

Observation of 1.5μm photoluminescence and electroluminescence from a holmium organic complex

Electroluminescence (EL) and photoluminescence in both the visible and near-infrared spectral range were observed from a holmium(dibenzoylmethanato)3(bathophenanthroline) [Ho(DBM)3bath]. Five peaks at 580nm, 660nm, 980nm, 1200nm, and 1500nm, respectively, were attributed to the internal 4f electronical transitions of the Ho3+ ions. Except for the emissions of the Ho3+ ions, a broadband exciplex emission from 480nmto670nm appeared in the EL cases. The emission intensity of the exciplex at organic interface showed a tendency to saturation beyond a certain driving voltage, while the emissions of the Ho3+ ions kept increasing. This evolution of visible EL spectra was discussed in terms of the extension of the charge recombination zone. The 1500nm emission corresponding to the F55→I65 transition suggests that the Ho(DBM)3bath is a potential candidate for optical communications.

Emission mechanism in organic light-emitting devices comprising a europium complex as emitter and an electron transporting material as host

The emission mechanism in organic light-emitting devices, where the emission layer is composed of Eu(DBM)3pyzphen (DBM=Dibenzoylmethane, pyzphen=pyrazino-[2,3-f][1,10]-phenanthroline) doped into electron transporting/hole blocking material BPhen (4,7-diphenyl-1, 10-phenanthroline), is investigated. Energy transfer and carrier trapping simultaneously exist in the luminescence process, and carrier trapping is a main process. Direct carrier trapping by Eu(DBM)3pyzphen molecules is confirmed by the difference of electroluminescence and photoluminescence spectra as well as J-V characteristics. Efficient Foster and Dexter energy transfer from BPhen to Eu(DBM)3pyzphen molecules were speculated in terms of analysis of photoluminescence spectra of fixed solutions, triplet energies, and phosphorescent lifetimes. Based on these mechanisms, the overall performances of these devices were improved. High efficiencies were obtained under carrier trapping by Eu(DBM)3pyzphen molecules, and the emission of BPhen was elimina...