Mifeng Li

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.

Enhanced dielectric responses in Mg-doped CaCu3Ti4O12

We report the effects of the Mg doping on the dielectric properties of CaCu3Ti4O12 in the frequency range of 200 Hz–200 kHz and in the temperature range of 58–473 K. It is found that the incorporations of Mg2+ result in increases in the dielectric constant by 12%–20% and decreases in the dielectric loss by 41%–64% (with the minimum of 0.042 for CaCu2.7Mg0.3Ti4O12) at room temperature and at 1 kHz. The x-ray photoemission spectroscopy measurements reveal that the content of Cu3+ increases with the increasing concentration of Mg2+. The enhanced dielectric response may be related to the influence of Cu3+ and/or Mg2+. In other words, Mg2+ is an effective ion to optimize the dielectric properties of CaCu3Ti4O12.

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.

Improved electroluminescent efficiency of organic light emitting devices by co-doping N, N-'-dimethyl-quinacridone and Coumarin6 in tris-(8-hydroxyquinoline) aluminum

Electroluminescent (EL) efficiency of organic light emitting devices was improved by co-doping N, N′-dimethyl-quinacridone (DMQA) and Coumarin6 (C6) in tris-(8-hydroxyquinoline) aluminum (Alq3). At 20mA∕cm2, superior EL efficiency of 9.33cd∕A was obtained from 1.6% co-doped (0.8% DMQA and 0.8% C6) device, much higher than those of single-doped devices based on 1.6% DMQA (∼5.0cd∕A) or 1.6% C6 (∼6.0cd∕A). EL efficiency of the single-doped devices with optimal dopant concentration, 0.8% in both cases, are merely 7.13cd∕A for DMQA and 6.43cd∕A for 0.8% C6, respectively. The significant improvement could be attributed to effective utilization of the host energy and depression of concentration quenching. The co-doping technique provides an effective way to overcome the notorious concentration quenching and hence to improve the EL efficiency.

Single InAs Quantum Dot Grown at the Junction of Branched Gold- Free GaAs Nanowire

We report a new type of single InAs quantum dot (QD) embedded at the junction of gold-free branched GaAs/AlGaAs nanowire (NW) grown on silicon substrate. The photoluminescence intensity of such QD is ~20 times stronger than that from randomly distributed QD grown on the facet of straight NW. Sharp excitonic emission is observed at 4.2 K with a line width of 101 μeV and a vanishing two-photon emission probability of g(2)(0) = 0.031(2). This new nanostructure may open new ways for designing novel quantum optoelectronic devices.

Single InAs quantum dot coupled to different “environments” in one wafer for quantum photonics

Self assembled small InAs quantum dots (SQDs) were formed in various densities and environments using gradient InAs deposition on a non-rotating GaAs substrate. Two SQD environments (SQD I and SQD II) were characterized. SQD I featured SQDs surrounded by large QDs, and SQD II featured individual SQDs in the wetting layer (WL). Micro-photoluminescence of single QDs embedded in a cavity under various excitation powers and electric fields gave insight into carrier transport processes. Potential fluctuations of the WL in SQD II, induced by charge redistribution, show promise for charge-tunable QD devices; SQD I shows higher luminescence intensity as a single-photon source.

Effect of exciplex formation on organic light emitting diodes based on rare-earth complex

An exciplex can be formed due to the charge transfer between the lowest unoccupied molecular orbital (LUMO) of the acceptor and the highest occupied molecular orbital (HOMO) of the donor. By introducing a mixing layer composed of [N,N′-diphenyl-N,N′bis (3-methylphenyl)-1,1′-diphenyl-4,4′-diamine] (TPD) and europium(dibenzoylmethanato)3(bathophenanthroline) [Eu(DBM)3bath] and a graded interface, elimination of light emission from the exciplex and significant luminescence enhancement of trivalent europium ions (Eu3+) in organic light emitting devices have been achieved. The elimination mechanism of exciplex emission based on the concept that an exciplex can be formed between LUMO of the acceptor (Eu complex) and HOMO of donor (TPD) was investigated. To comprehensively understand the mechanism, devices consisting of a Eu(DBM)3bath as the emitting material and the devices using other rare-earth (RE) complex [RE(DBM)3bath] as the emitting material were fabricated with the same device configuration. As a refere...

In situ accurate control of 2D-3D transition parameters for growth of low-density InAs/GaAs self-assembled quantum dots

A method to improve the growth repeatability of low-density InAs/GaAs self-assembled quantum dots by molecular beam epitaxy is reported. A sacrificed InAs layer was deposited firstly to determine in situ the accurate parameters of two- to three-dimensional transitions by observation of reflection high-energy electron diffraction patterns, and then the InAs layer annealed immediately before the growth of the low-density InAs quantum dots (QDs). It is confirmed by micro-photoluminescence that control repeatability of low-density QD growth is improved averagely to about 80% which is much higher than that of the QD samples without using a sacrificed InAs layer.

Strain-driven synthesis of self-catalyzed branched GaAs nanowires

We report the strain-driven synthesis of self-catalyzed branched GaAs nanowires (NWs). Decoration of facets with branches is achieved as NWs elongate or with the insertion of InAs. The hemisphere tip shaped branches on the backbone implies identical Vapor-Liquid-Solid growth mechanism. We present the homogeneous gallium-droplets (GDs) nucleation on the GaAs {110} side facets in the form of GaAs quantum-rings, specifying the role of GDs in branching. Structural characterization revealed strain defects at the crotch between the backbones and branches of the NWs. The evolution mechanism of self-catalyzed branched NWs is discussed and finally nano-trees with hyper-branches are demonstrated.

Optimization of the GaAs-on-Si Substrate for Microelectromechanical Systems (MEMS) Sensor Application

Resonant Tunneling Diodes (RTD) and High Electron Mobility Transistor (HEMT) based on GaAs, as the piezoresistive sensing element, exhibit extremely high sensitivity in the MEMS sensors based on GaAs. To further expand their applications to the fields of MEMS sensors based on Si, we have studied the optimization of the GaAs epitaxy layers on Si wafers. Matching superlattice and strain superlattice were used, and the surface defect density can be improved by two orders of magnitude. Combing with the Raman spectrum, the residual stress was characterized, and it can be concluded from the experimental results that the residual stress can be reduced by 50%, in comparison with the original substrate. This method gives us a solution to optimize the epitaxy GaAs layers on Si substrate, which will also optimize our future process of integration RTD and HEMT based on GaAs on Si substrate for the MEMS sensor applications.