Zhiling Xu

Enhancement of electron injection in organic light-emitting devices using an Ag/LiF cathode

A LiF-buffered silver cathode has been used in organic light-emitting devices (OLEDs) with structure indium–tin–oxide/N,N′-bis-(1-naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine (50 nm)/Alq3 (100 nm)/cathode. The efficiency of electron injection from the cathode is strongly dependent on the thickness of the LiF buffer layer. While a LiF layer thinner than 1.0 nm leads to higher turn-on voltage and decreased electroluminescent (EL) efficiency, a LiF layer of 3.0 nm significantly enhances the electron injection and results in lower turn-on voltage and increased EL efficiency. A brightness of 16 000 cd/m2 and EL efficiency of 4.8 cd/A can be achieved with an Ag/LiF cathode. This dependence of electron injection on the LiF thickness is quite different from that reported for OLEDs with a Al/LiF cathode, but can be well understood using the tunneling model.

Sodium stearate, an effective amphiphilic molecule buffer material between organic and metal layers in organic light-emitting devices

Tris (8-hydroxyquinoline) aluminum (Alq3)-based organic light-emitting devices using an amphiphilic molecule sodium stearate (NaSt) layer between aluminum (Al) cathode and Alq3 have been fabricated. By comparing the devices with those containing a LiF buffer layer, the results demonstrate that both have almost the same high electroluminescent (EL) brightness but the former is more stable. The amphiphilic property of NaSt is considered as the main reason for this enhancement.

Aggregation and permeation of 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran molecules in Alq

The morphologic and luminescent behaviors of various 4-(dicyanomethylene)-2methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) containing films have been investigated. This involves pure DCM layers deposited on top of a tris-(8-hydroxyquinoline) aluminum (Alq) layer or positioned between two Alq layer, DCM-doped Alq films, and periodically arranged Alq/DCM multilayer structures. The occurrence over a period of time of aggregation and permeation of DCM molecules at room temperature and at temperatures as low as ∼260 K is found in all the cases studied. Such a phenomenon will result in degradation of related organic light-emitting devices and is closely related to the electric polarity of the DCM molecule.

Nonuniform bulk second-order optical nonlinearity in PbO/B2O3 glass

Second-order nonlinearity (SON) of thermally poled PbO/B2O3 glass was investigated. The SON distribution within the glass was obtained by repeatedly grinding the poled glass and measuring its Maker fringes. After grinding away up to one third of the thickness of the poled sample, i.e., about 300 μm, clear fringes with the interference minimum not equal to zero could still be observed, indicating that the area of SON was not confined within thin layers near the two surfaces. Cathode side grinding and anode side grinding had quite different influences on the SON of the poled sample. A model of the anode-charge-layer dependent bulk electric field distribution was proposed and this electric field was regarded as playing a key role in the generation of a nonuniform bulk SON distribution in PbO/B2O3 glass.

Photoinduced magnetic softening of perpendicularly magnetized L10-FePt granular films

Ultrafast spin dynamics has for the first time been studied in perpendicular magnetized granular films. For FePt continuous films and FePt–MgO and FePt–Ag granular films with femtosecond laser excitations, the coercivity HC and the saturation Kerr rotation θKS are dramatically reduced, accompanied by a sharp increase in the reflectivity R. Afterward, these physical quantities are slowly recovered. The changes in HC, θKS, and R are all different among FePt, FePt–MgO, and FePt–Ag films. The difference is caused by different film thicknesses and in particular by the surface plasmon resonance of metallic nanoparticles.

Second-order optical nonlinearity in bulk PbO/B2O3 glass

Abstract By investigating the second-harmonic generation (SHG) of the bulk PbO/B 2 O 3 glass samples with different compositions after thermal poling, it was found that there was an optimal poling temperature for each sample with different compositions and there was also a relation between optimal poling temperature and glass transition temperature. At their own optimal poling temperatures, the samples had different frequency doubling efficiencies with the same applied voltage. We also found that the frequency doubling efficiency of PbO/B 2 O 3 glass increased with the increase of poling voltage. An induced dipole model was proposed to explain the super-quadratic relation between the SHG intensity and the poling voltage.

Influence of different poling methods on the second-order nonlinearity in fused silica glasses

Fused silica glass samples poled by different methods showed large differences in their second-order optical nonlinearities. Large second-harmonic signals were obtained only when a depletion layer was formed near the anode surface. By comparing the influences of plate poling and corona poling, the necessary conditions to form a depletion layer in fused silica glass are discussed.

Optical nonlinearity and structural phase-transition observation of organic dye-doped polymer silica hybrid material.

The optical nonlinearity of organic dye-doped poly(methyl methacrylate) (PMMA)-silica-gel hybrid material was investigated by second-harmonic-generation measurement. We found that incorporation of in situ polymerized solgel precursors into the organic dye-doped PMMA significantly improved the nonlinear optical stability of the system. However, improvement of thermal stability occurred only when a sufficient amount of silica gel was incorporated. A structural phase transition from pure polymer to a hybrid system was found near a 10-mol.% silica-gel concentration. The optimum polymer/tetraethoxysilane molar ratio is 2:1 to 1:1.

In situ SHG investigation on the gelation process of organic doped silica film

Abstract The gelation process of organic doped silica film fabricated by the sol–gel technique was investigated by in-situ second harmonic generation (SHG) measurements. The SH signal came from the doped hemicyanine molecules which automatically oriented in the film at or near the film–substrate interface. We found that film shrinkage during gelation resulted in disordering of the doped hemicyanine molecules, so the SH intensity decreased during annealing. Thus, the SHG measurement can be used as a semi-quantitative probe of the gelation process in that particular region, which was hard to detect using conventional methods. We found that the time for gelation depends sensitively on the annealing temperatures. The activation energy of the gelling process was obtained from the SHG relaxation measurement. In addition, aggregated hemicyanine molecules dissociated with increasing annealing temperature.

Investigation on the super-quadratic relation between second-harmonic generation intensity and poling voltage in SiO 2 -GeO 2 thin films

Sol-gel method was used to prepare SiO2-GeO2 films. By investigating the second-harmonic generation (SHG) signal of the films after plate poling and corona poling, it was found that the SHG intensity of the samples increases with the plate poling voltage. A model of effectivedipole- inducing was proposed to explain the super-quadratic relation between the SHG intensity and the plate poling voltage.