Deep blue organic light emitting diode based on anthracene derivative as a dopant

This letter presents a deep blue organic light emitting diode which was fabricated by using 9,10-di(2-naphthyl)anthracene as a dopant and 4,4′-N,N′-dicarbazole-biphenyl as a host. The Commission Internationale de l’Eclairage coordinates of (0.1516, 0.0836) were achieved in the cell, which is very close to the National Television Standards Committee standard of (0.14, 0.08). Meanwhile, maximum luminance over 6500 cd/cm² and maximum current efficiency of 3.5 cd/A were also obtained.     

Surface state control of III–V semiconductors using molecular modification

An attempt to control surface electronics of III–V semiconductor using wet chemical processes has been performed. Here, we report results on the use of self-assembled monolayers (SAMs) of organic molecules on (0 0 1) GaAs surface. Octadecanethiol (ODT) and benzenethiol (BT) have been the choice in the present study.GaAs wafers were modified by thiol molecules on the flat surface after the native oxide layers are removed by chemical etching under optimized conditions. The change in the electronic properties was measured in terms of transport properties via the SAM layer by conductive probe atomic force microscopy. The current–voltage characteristics thus obtained show that ODT functions as a tunnel barrier while BT is conductive due to the presence of π-electrons. As a result, we can control the electronic states of GaAs–molecule interface for realizing novel device structures by the selection of functional molecules.     

Esterification of sodium 4-hydroxybenzoate by ultrasound-assisted solid–liquid phase-transfer catalysis using dual-site phase-transfer catalyst

The catalytic esterification of sodium 4-hydroxybenzoate with benzyl bromide by ultrasound-assisted solid–liquid phase-transfer catalysis (U-SLPTC) was investigated using the novel dual-site phase-transfer catalyst 4,4′-bis(tributylammoniomethyl)-1,1′-biphenyl dichloride (BTBAMBC), which was synthesized from the reaction of 4,4′-bis(chloromethyl)-1,1′-biphenyl and tributylamine. Without catalyst and in the absence of water, the product yield at 60 °C was only 0.36% in 30 min of reaction even under ultrasound irradiation (28 kHz/300 W) and 250 rpm of stirring speed. When 1 cm³ of water and 0.5 mmol of BTBAMBC were added, the yield increased to 84.3%. The catalytic intermediate 4,4′-bis(tributylammoniomethyl)-1,1′-biphenyl di-4-hydroxybenzoate was also synthesized to verify the intrinsic reaction which was mainly conducted in the quasi-aqueous phase locating between solid and organic phases. Pseudo-first-order kinetic equation was used to correlate the overall reaction, and the apparent rate coefficient with ultrasound (28 kHz/300 W) was 0.1057 min⁻¹, with 88% higher than that (0.0563 min⁻¹) without ultrasound. The esterification under ultrasonic irradiation using BTBAMBC by solid–liquid phase-transfer catalysis was developed.     

An organic electroluminescent device made from a gadolinium complex

A gadolinium ternary complex, tris(1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) (phenanthroline) gadolinium [Gd(PMIP)₃(Phen)] was synthesized and used as a light emitting material in the organic electroluminescent (EL) devices. The triple layer device with a structure of indium tin oxide (ITO)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) (20 nm)/Gd(PMIP)₃(Phen) (80 nm)/2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (bathocuproine or BCP) (20 nm)/Mg: Ag(200 nm)/Ag(100 nm) exhibited green emission peaking at 535 nm. A maximum luminance of 230 cd/m² at 17 V and a peak power efficiency of 0.02 lm/w at 9 V were obtained.     

High-efficiency organic light-emitting diodes (OLEDs) with a mixed layer structure

Improved performance of organic light-emitting diodes (OLEDs) as obtained by a mixed layer was investigated. The OLEDs with a mixed layer which were composed of N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1,1′-biphenyl-4,4′-diamine (NPB), tris-(8-hydroxyquinolato) aluminum (Alq₃) and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) showed the highest brightness and efficiency, which reached 19048 cd/m² at 17 V and 4.3 cd/A at 10 mA/cm², respectively. The turn-on voltage of the device is 2.6 V. Its Commission Internationale del’Eclairage (CIE) coordinate is (0.497, 0.456) at 17 V, and the CIE coordinates of the device are largely insensitive to the driving voltages, which depicts stabilized yellow color.     

Low voltage red phosphorescent organic light-emitting devices with triphenylphosphine oxide and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl electron transport layers

We have developed red phosphorescent organic light-emitting devices operating at low voltages by using triphenylphosphine oxide (Ph₃PO) and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) electron transport layers. 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and tris-(1-phenylisoquinolinolato-C²,N) iridium(III) [Ir(piq)₃] were used as host and guest materials, respectively. Small voltage drops across the electron transport layers and direct injection of holes from 4,4′,4″-tris[N-(2-naphthyl)-N-phenyl-amino]-triphenylamine (2-TNATA) hole transport layer into the Ir(piq)₃ guests are responsible for the high current density at low voltage, resulting in a high luminance of 1000 cd/m² at low voltages of 2.8–3.0 V in devices with a structure of ITO/2-TNATA/CBP:Ir(piq)₃/DPVBi/Ph₃PO/LiF/Al.     

Memory characteristics and tunneling mechanism of Pt nano-crystals embedded in HfAlOx films for nonvolatile flash memory devices

A non-volatile flash memory device based on metal oxide semiconductor (MOS) capacitor structure has been fabricated using platinum nano-crystals(Pt–NCs) as storage units embedded in HfAlO_x high-k tunneling layers. Its memory characteristics and tunneling mechanism are characterized by capacitance–voltage(C–V) and flat-band voltage-time(ΔV_FB-T) measurements. A 6.5 V flat-band voltage (memory window) corresponding to the stored charge density of 2.29 × 10¹³ cm⁻² and about 88% stored electron reserved after apply ±8 V program or erase voltage for 10⁵ s at high frequency of 1 MHz was demonstrated. Investigation of leakage current–voltage(J–V) indicated that defects-enhanced Pool-Frenkel tunneling plays an important role in the tunneling mechanism for the storage charges. Hence, the Pt–NCs and HfAlO_x based MOS structure has a promising application in non-volatile flash memory devices.     

Peculiarities of Al magic cluster self-assembly on Si(1 0 0) surface

Using scanning tunneling microscopy observations and density functional theory calculations, regularities of the Al magic cluster array self-assembly on Si(1 0 0) surface has been elucidated. While a single Al cluster occupies an area of 4a × 3a, an ordered Al-cluster array exhibits a 4 × 5 periodicity, as the clusters in the array are separated by the 4a × 2a “spacers”. The plausible structural model for the “spacer” was proposed in which the “spacer” is arranged as an ordinary 4a × 3a-Al cluster in which the central atomic row with the topmost Si atom is missing. Appearance of the “spacers” in the Al-cluster array was found to reduce formation energy of the array. Ability to incorporate the rows of Al-“spacers” into the completed 4 × 3 In-cluster array was demonstrated.     

Current controlled negative differential resistance behavior in Co2FeO2BO3 and Fe3O2BO3 single crystals

I–V curves showing negative differential resistance (NDR) are reported for single crystals of Co₂FeO₂BO₃ at 315 K and 290 K and for Fe₃O₂BO₃ at 300 K, 260 K and 220 K. Resistivity measurements are presented for both systems, parallel and perpendicular to the c axis, in the range 315–120 K. The high hysteretic behavior of the I–V curves in Co₂FeO₂BO₃ around room temperature is discussed and the heat dissipated is estimated, suggesting an increase in the sample temperature of almost 22 K for the I–V curve at 315 K and a dominant contribution of Joule self-heating for the observed NDR. In contrast, insignificant hysteresis is observed on the I–V curves of Fe₃O₂BO₃ around room temperature. The depinning of charge order domains is suggested as the main contribution to the NDR phenomenon for Fe₃O₂BO₃. The high reproducibility of the NDR in the Fe₃O₂BO₃ single crystal allows its use as a low frequency oscillator, as it is demonstrated.     

Theoretical understanding of adlayer structure,thermal stability and electronic property of graphene molecules

The geometry of hexafluorotribenzo[a,g,m]coronene with n-carbon alkyl chains [FTBC-Cn (n = 4, 6, 8, 12)] and their supramolecule self-assembly on a highly oriented pyrolytic graphite (HOPG) surface has been optimized by molecular dynamics simulations using COMPASS force field at 0 K, 298 K, 333 K and 353 K. Electronic properties and intermolecular interactions in graphene supramolecule assembly have been studied by the first principle methods based on the density functional theory (DFT). It is indicated that the thermal stability and electronic properties of graphene molecules can be tunable by attaching alkyl chains to a triangular graphene sheet, and changing the length of the alkyl chain, and self-assembling on a certain substrate. The main results are as follows. The geometry and energy gap of the FTBC-Cn single molecule and their supramolecule self-assembly on HOPG are both stable with the changes of the temperature from 0 K to 353 K and the number of carbon atoms on the alkyl chain. The simulation results of geometry, energy gap as well as STM images of graphene supramolecule assembly are in good agreement with the corresponding experimental results in room temperature. Furthermore, the electronic properties of graphene supramolecule assembly at the temperatures of 0 K, 333 K and 353 K are also predicted. When a triangular graphene molecule attached with six alkyl chains, the energy gaps are increased and stabilized at the temperature from 0 K to 353 K. After FTBC-Cn molecule self-assembly on a HOPG substrate, the energy gap is reduced but still stable.     

Solution-processed white organic light-emitting diodes with mixed-host structures

Efficient white light-emitting diodes (WOLEDs) were fabricated with a solution-processed single emission layer composed of a molecular and polymeric material mixed-host (MH). The main host used was a blue-emitting molecular material of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) and the assisting host used was a hole-transport-type polymer of poly(9-vinylcarbazole) (PVK). By co-doping 4,4′-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl and 5,6,11,12-tetraphenylnaphacene into the MH, the performances of the fabricated devices made with different mixing ratio of host materials were investigated, and were to depend on the mixing ratios. Under the optimal PVK:DPVBi ratio (3:7), we achieved a maximum luminance of 14 110 cd/m² and a maximum current efficiency of 9.5 cd/A. These improvements were attributed to the MH structure, which effectively improved the thermal stability of spin-coated film and enhanced the hole-injection/transporting properties of WOLEDs.     

Electrical characteristics and efficiency of organic light-emitting diodes depending on hole-injection layer

In a device structure of ITO/hole-injection layer/N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphthyl]4,4′-diamine(NPB)/tris(8-hydroxyquinoline)aluminum(Alq₃)/Al, we investigated the effect of the hole-injection layer on the electrical characteristics and external quantum efficiency of organic light-emitting diodes. Thermal evaporation was performed to make a thickness of NPB layer with a rate of 0.5–1.0 Å/s at a base pressure of 5 × 10⁻⁶ Torr. We measured current–voltage characteristics and external quantum efficiency with a thickness variation of the hole-injection layer. CuPc and PVK buffer layers improve the performance of the device in several aspects, such as good mechanical junction, reducing the operating voltage, and energy band adjustment. Compared with devices without a hole-injection layer, we found that the optimal thickness of NPB was 20 nm in the device structure of ITO/NPB/Alq₃/Al. By using a CuPc or PVK buffer layer, the external quantum efficiencies of the devices were improved by 28.9% and 51.3%, respectively.     

The thermopower of transition-metal double-perovskites: A sensitive probe of their electronic structures around the Fermi level

We report on a comparative study of S(T) for a series of transition-metal double-perovskites A₂BB′O₆ (A – Ca, Sr, Ba, and B, B′ = transition metal ions), some of them known to have half-metallic ground states. For Sr₂BB′O₆ with BB′ = CrMo, CrW, CrRe, FeMo, and FeRe (ferrimagnetic with high Curie temperatures), S(T) is metallic, for B′ = Mo and W it is n-type and for B′ =  Re, p-type. For A₂FeMoO₆ (A = Ca, Sr, Ba), the crystallographic differences (monoclinic, tetragonal and cubic space-groups, respectively) are accompanied by prominent differences in their (metallic) S(T). For the insulating Sr₂MnReO₆ and Ba₂MnReO₆, the onset of ferromagnetic order below T_c ∼ 120 K is marked by a steep drop of S(T) accompanied by only a slight change in the slope of ln ρ versus 1/T^1/2. Significant conclusions were drawn from the experimental results without the need for elaborate models.     

Investigation of a graded channel InGaAs/GaAs heterostructure transistor

An InGaAS/GaAs heterostructure transistor utilizing a gradedIn_xGa_1 − xAs channel grown by low-pressure metal-olorganic chemical vapor deposition has been demonstrated. A negative differential resistance (NDR) phenomenon is observed. Electron mobilities are significantly improved by using the graded InGaAs channel. For the In composition varying fromx =  0.25 (at the buffer–channel interface) to x =  0.1 (at the spacer–channel interface) structure, a peak extrinsic transconductance of 24.6 S mm ^− 1(atV_DS =  6.5 V,V_GSstep =  − 0.5 mV) and a saturation current density as high as 555 mA mm ^− 1for a gate length of 1.5 μ m are obtained.     

AC impedance spectroscopic studies of transport properties in metal oxide doped α-NPD

The potential application of molecular organic semiconductor needs the control adjustment of conductivity. The hole transport materials (HTL) α-NPD (N,N′-di(1-naphthyl)-N,N′-diphenylbenzidin) has been doped with metal oxide (molybdenum oxide) for the study of transport phenomena. Impedance spectroscopic measurements were performed with the applied external electric filed. The activation energy calculated (0.55, 0.48 and 0.50 eV) decreases with doping of metal oxide. There is a clear indication of space charge limited (SCL) conduction in the doped and undoped thin films. The cole–cole plot indicates the device can be represented by a parallel resistance and capacitance network in series with a series resistance (of 70 Ω). At high frequency; the conduction follows the universal power law for both doped–undoped α-NPD with MoO₃ and the onset frequency increases with increasing bias voltages. The conduction mechanism can be explained by classical hopping barriers (CHB) mechanism for the system.     

Self-assembly of C60 fullerenes on quasi-one-dimensional Si(111)4 × 1-In surface

Using scanning tunneling microscopy observations, self-assembly of C₆₀ fullerenes in the course of room-temperature adsorption onto Si(111)4 × 1-In reconstruction and after subsequent annealing at temperatures ranging from 150 to 450 °C has been studied. Adsorbed C₆₀ fullerenes have been found to occupy off-centered positions on In-atom rows forming linear chains with a maximal length of eight C₆₀ molecules. Intermolecular spacing within the regular chains equals three lattice constants of Si(111) surface. Two energetically different adsorption states of C₆₀ have been detected, one of which is occupied preferentially at room temperature, while occupation of the second (more tight) state dominates at temperature above ~ 150 °C. In the first state, C₆₀ fullerene resides plausibly in a continuous rotation, while in the second state a C₆₀ molecule is fixed tightly in a single orientation with a C₆₀ hexagon pointing upward. Transition of C₆₀ fullerenes to the more stable state is accompanied by expelling In atoms from the Si(111)4 × 1-In reconstruction.     

Propargylation of indene-1,3-dione under a new phase-transfer catalyst combined with ultrasonication – A kinetic study

In the present study, kinetics of synthesis of 2,2-di(prop-2-ynyl)-1H-indene-1,3(2H)-dione was successfully carried out by propargylation of indene-1,3-dione with propargyl bromide using aqueous potassium hydroxide and catalyzed by a newly synthesized phase-transfer catalyst viz., N-benzyl-N-ethyl-N-isopropylpropan-2-ammonium bromide, PTC under ultrasonic (40 kHz, 300 W) assisted organic solvent condition. The pseudo first-order kinetic equation was applied to describe the overall reaction. Under ultrasound irradiation (40 kHz, 300 W) in a batch reactor, it shows that the overall reaction rate can be greatly enhanced with ultrasound irradiation than without ultrasound.     

Comparison of the higher diffraction order images in methyl-red-doped liquid crystal films

We investigate the change of higher diffraction order images in holographic image storage and reconstruction process. In experiments, an s-polarized Ar⁺ laser (488.0 nm) was used to record permanent grating in the dye-doped liquid crystal, 4,4′-n-entylcyanobiphenyl (5CB) doped with 1 wt% methyl-red (MR), at a small incident angle. Higher diffraction order images were observed when the signal beam was focused in front of and behind the film. Then the film was illuminated by an s-polarized He–Ne laser (632.8 nm) in the direction perpendicular to the surface. The higher diffraction order images were reconstructed. A theory about the change of higher diffraction order images is developed, which is in good agreement with experimental results. The results show that the higher diffraction order provides a useful method for optical information processing.     

Preparation,thermal expansion,chemical compatibility,electrical conductivity and polarization of A2−αA′αMO4 (A = Pr,Sm; A′ = Sr; M = Mn,Ni; α = 0.3, 0.6) as a new cathode for SOFC

A_2−αA′_αMO₄ (A = Pr, Sm, A′ = Sr, M = Ni, Mn) with K₂NiF₄-type structure were synthesized by solid reaction. Their chemical stability, electrical conductivity and thermal expansion behavior as well as cathodic polarization were investigated in relation to the cathode of SOFC. The results showed that A_2−αA′_αMO₄ exhibited a low reactivity with yttria stabilized zirconia (YSZ) electrolyte. The thermal expansion coefficient (TEC) values were changed with the ionic radius of A. The specific conductivities of the nickelates were higher than those of manganites. While the nickelates showed a lower cathodic polarization in comparison with manganites.     

STM spectroscopy of an organic superconductor

Electron tunneling spectroscopy of the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂using low temperature scanning tunneling microscope (STM) is reported. The tunneling differential conductance in the superconducting phase was obtained in theb–cplane of a single crystal, by varying the tip position on the sample surface. The differential conductance is reduced near zero bias voltage and enhanced at the gap edge, associated with the superconducting gap structure below[formula] K. The gap width differs slightly from sample to sample, while the overall functional shape of the conductance is sample-independent. The tunneling conductance is reduced to almost zero near zero bias voltage, while it is finite inside the gap edge. The curve obtained cannot be fit to the BCS density of states withs-wave pairing symmetry, even if the life-time broadening of one-electron levels is taken into account. Finite conductance inside the gap edge suggests anisotropy of the gap. However, the conductance curve obtained is not explained by a simpled-wave symmetry for Δ(k). The reduced conductance near zero bias voltage suggests a finite gap. An anisotropic model with a finite gap, in which Δ(k) varies depending on the direction ink-space, is examined. The tunneling conductance in the low-energy region is almost fit by the model with Δ_min = 2 meV and Δ_max = 6 meV. The finite conductance is explained by introducing a small effect of life time broadening. We conclude that the gap is anisotropic and is finite (at least Δ_min = 2 meV) on the entire Fermi surface.