Maximum Photoluminescence Emission Research Articles

Synthesis, Photophysics, and Electroluminescence of Conjugated Poly(p-phenylenevinylene) Derivatives with 1,3,4-Oxadiazoles in the Backbone

Starting from 4-bromobenzaldehyde or 1,4-benzenedicarboxaldehyde, two new poly(p-phenylenevinylene) derivatives P1 and P2 were synthesized by a five-step synthetic route. These fully conjugated polymers contain 1 or 2 oxadiazole rings and 3 or 4 vinylene bonds per repeat unit for P1 and P2, respectively, and were amorphous and soluble in common organic solvents. The Tg values were 28 °C for P1 and 57 °C for P2. The polymers emitted greenish-blue light in solution with photoluminescence (PL) emission maximum at 487−511 nm and yellowish-green light with PL emission maximum at 515−558 nm in thin films. Electroluminescence (EL) was achieved from single-layer LEDs of polymer P1 with the configuration ITO/PEDOT/P1/Al with voltage-tunable EL colors from 558 nm (9 V) to 527 nm (16 V). The observed EL spectral blue shift with increasing voltage is a result of conformational changes of the polymer backbone with increase in temperature, as evidenced from the absorption and PL spectra changes upon annealing of P1 thi...

Electroplex emission from bilayer LED based on 1-phenyl-3-(dimethyl-amino) styryl-5-( p-(dimethylamino) phenyl) pyrazoline and 2,2′-(2,5-thiophenediyl) bis-(5-phenyl)-1,3,4-oxadiazole

The photoluminescence (PL) and electroluminescence (EL) properties of a novel organic compound, 2,2′-(2,5-thiophenediyl) bis-(5-phenyl)-1,3,4-oxadiazole (TOXD), were studied as electron transporting material. The PL emission maximum peak is at 408 nm of TOXD in chloroform and is at 472 nm in solid film, respectively. The EL emission maximum peaks of the bilayer ITO/PDP/TOXD/Al LED located between emission maximum peaks of TOXD (470 nm) and PDP (500 nm) at lower driving voltage, 4.5–5.4 V, and shifted to 508 nm at 6.0 V and 518 nm at 7.0 V with increase of applied voltage. The new EL emission peak at 518 nm from the bilayer LED is ascribed to electroplex formation at the interface of TOXD and PDP film in bilayer LED under external electric field. The electroplex emission is close to the energy levels of PDP and TOXD.

Annealing effects on the crystal structure of GaInNAs quantum wells with large In and N content grown by molecular beam epitaxy

The impact of rapid thermal annealing on the optical emission of GaInNAs/GaAs quantum wells (QWs) grown by molecular beam epitaxy with high In and N content is shown to be highly dependent on the crystal structure of the QWs, as determined by transmission electron microscopy. Due to the presence of higher concentrations of nonradiative recombination centers, the annealing temperature required to obtain maximum photoluminescence emission is higher for the QW with strong structural modulation of the upper interface [at the onset of three-dimensional (3D) growth], intermediate for the two-dimensional (2D) grown QW with compositional fluctuations, and lower for the homogeneous 2D grown QW. Moreover, the transition from homogeneous 2D growth, to 2D growth with compositional fluctuations, and finally 3D growth, leads to progressively deeper carrier localization states below the conduction-band edge. Increasing annealing temperatures gradually shifts the localization states closer to the conduction-band edge, predominantly when compositional fluctuations are present. These results suggest a link between the formation of carrier localization centers and the presence of alloy fluctuations along the QW.

EL properties of a new green light-emitting PPV derivative

A germyl substituted new PPV derivative, poly(2-triethylgermyl-1,4-phenylenevinylene) (TEG-PPV), was synthesized through a soluble precursor route. The synthesized polymers were characterized using thermogravimetric analysis (TGA), photoluminescence (PL) and electroluminescence (EL) spectroscopy. The TEG-PPV showed a maximum UV-visible absorption and peak PL emission at 400 nm and 511 nm, respectively. The ionization potential of TEG-PPV was determined to be 5.7 eV. Electroluminescent device was fabricated in an ITO/TEG-PPV/LiF/Al configuration. The EL spectra showed similar emission to the PL results, and device showed a green light-emission from 13 V. The EL device showed maximum brightness of 600 cd/m 2 and luminous efficiency of 1.5 x 10 -2 lm/W.

High-rate deposition of plasma polymerized thin films using PECVD method and characterization of their optical properties

Plasma polymerized organic thin films were deposited at temperature in the range of room temperature to 693 K by plasma enhanced chemical vapor deposition method using the thiophene (C 4H 4S) precursor. Radio frequency (RF) with 13.56 MHz was applied and the RF power was changed in the range from 30 to 100 W with a gas ratio of Ar:H 2=1:1. The as-grown organic thin films were basically characterized with ellipsometry, UV–Vis spectroscopy, and photoluminescence (PL) measurements. In order to compare the difference of optical properties of the plasma polymerized organic thin films, the effects of the RF power and deposition temperature on their structural properties were mainly studied in this work. UV–Vis spectra showed an energy band gap shift from 3.78 to 4.02 eV with increasing RF power and quite high optical transmittance up to 95%. During CVD, moreover, the plasma diagnostics were in situ carried out by using optical emission spectroscopy that showed a strong dependency of intensity on RF powers. To check a possibility of optical device application, PL measurements were also carried out. From the PL data, we could obtain the maximum PL emission intensity at approximately 550 nm wavelength from a plasma polymerized organic thin film grown at room temperature and RF power of 70 W with gas ratio of Ar:H 2=1:1. The maximum growth rate is obtained to be 115 nm/min.

Band gap tuning of new light emitting conjugated polymers

Abstract A series of copolymers, poly{9,9-bis(2 ′ -ethylhexyl)fluorene-2,7-diyl- co -2,5-bis(2-thienyl-1-cyanovinyl)-1-(2 ′ -ethylhexyloxy)-4-methoxybenzene-5 ″ ,5 ‴ -diyl} has been synthesized from the monomers, 2,7-dibromo-9,9-bis(2 ′ -ethylhexyl)fluorene and 2,5-bis(2-(5 ′ -bromothienyl)-1-cyanovinyl)-1-(2 ″ -ethylhexyloxy)-4-methoxybenzene (BTCVB) using the Ni(0) mediated polymerization. The synthesized copolymers showed the absorption maxima at about 380 nm and the absorption between 425 and 600 nm increased as the fraction of the thiophene-containing monomer (BTCVB) increased. In photoluminescence (PL), the emission maxima of the copolymers were red-shifted as the fraction of BTCVB increased, despite the similar absorption characteristics were shown in the UV–vis spectra. The copolymer containing 15 mol% of BTCVB showed a maximum PL emission at 620 nm.

Photoluminescence in amorphous YNiO 3 and La 0.5Nd 0.5NiO 3 systems

Photoluminescence (PL) at room temperature has been achieved in amorphous powders of the YNiO 3 and La 0.5Nd 0.5NiO 3 systems. They were prepared by the polymeric precursor method (PPM). The energy position of maximum PL emission and the intensity showed dependence on the rare earth element (RE) size. The RE small size suggests increase of non-bridging oxygens (NBO) in the amorphous NiO 6 network. These results support the relationship between photoluminescence and structure in Ni 3+/Ni 2+-based amorphous materials.

Photoluminescence in amorphous TiO 2 -PbO systems

Photoluminescence (PL) at room temperature has been achieved in amorphous thin films and powders of the TiO2–PbO system. They were prepared by the polymeric precursor method with [PbO]/[TiO2] molar ratios ranging from 0.0 to 1.0. The energy position of maximum PL emission and the PL intensity showed dependence on Pb concentration. The Pb addition suggests an increase in the number of non-bridging oxygens (NBO) in the amorphous TiO2 network. These results support the relationship between photoluminescence and structure in TiO2-based amorphous materials.

Raman and photoluminescence characterization of Ge nanocrystals in co-sputtered Ge+SiO 2 system

Raman and photoluminescence (PL) results of Ge nanocrystals prepared from co-sputtered Ge+SiO 2 samples under rapid-thermal annealling (RTA) with different annealing conditions are presented. The Raman results showed a transition from amorphous to nanocrystalline Ge when the samples were annealed at a temperature higher than 700 °C for longer than 300 s. The Raman spectrum of sample annealed at 1000 °C is very similar to that of the as-deposited amorphous sample. However, when the annealing duration is reduced to less than 50 s, clear Raman signals can be seen for samples annealed at 1000 °C. Transmission electron micrographs showed that uniform Ge nanocrystals were obtained for samples annealed at 800 °C for 300 s or 1000 °C for 50 s, and nanocrystals with multiple twinned structure (of diameter ∼200 Å) were observed near the Si–SiO 2 interface (1000 °C for 300 s). PL results showed that maximum intensity of the 1.9 and 3.0 eV peaks was obtained from samples annealed at 800 °C for 20 s or transiently to 1000 °C at 30 °C/s. These results suggest that a critical thermal budget is required for the formation of uniform nanocrystals and for maximum PL emission.

New soluble light-emitting diode polymer containing oxadiazole unit

A new light emitting (LED) polymer (POOXPV) with oxadiazole unit has been prepared and characterized by UV-visible and photoluminescence (PL) spectroscopy. This polymer is soluble in common organic solvents and has good thermal stability up to ca. 400 °C. The UV absorption and PL emission maxima were 360 nm and about 500 nm, respectively. In order to investigate electron transporting property, the polymer was blended with poly[2-methoxy-5-(2-ethylhexyloxy)-l,4-phenylenevinylene] (MEH-PPV). The device showed bright orange-red color and enhanced EL power relative to that of MEH-PPV.

Blue Electroluminescent Polymers: Control of Conjugation Length by Kink Linkages and Substituents in the Poly(p-phenylenevinylene)-Related Copolymers

Poly[o(m,p)-phenylenevinylene-alt-2-methoxy-5-(2-ethylhexyloxy)-p-phenylenevinylene], o(m,p)-PMEH-PPV, and poly[o(m,p)-phenylenevinylene-alt-2,5-bis(trimethylsilyl)-p-phenylenevinylene], o(m,p)-PBTMS-PPV, of varying effective conjugation lengths were synthesized by the well-known Wittig condensation polymerization between the appropriate diphosphonium salts and the dialdehyde monomers such as terephthaldicarboxaldehyde, isophthalaldehyde, and phthalicdicarboxaldehyde. The conjugation lengths of the polymers were controlled by substituents and kink (ortho and meta) linkages. The resulting polymers were highly soluble in common organic solvents. The synthesized polymers showed UV−visible absorbance and photoluminescence (PL) in the ranges of 330−430 nm and 440−550 nm, respectively. The maximum emission peak of p-PMEH-PPV was blueshifted about 30 nm compared to that of MEH-PPV, which is due to an unsubstituted phenylene unit. In addition, o-PMEH-PPV and m-PMEH-PPV showed PL emission maximum peaks at 500 and 490 nm, respectively, because ortho and meta linkage of the o(m)-PMEH-PPV reduced π-conjugation lengths. The trimethylsilyl substituent has no electrondonating effect; therefore, the PL maximum of p-PBTMS-PPV was far more blueshifted (to 485 nm). Consequently, maximum PL wavelengths for o-PBTMS-PPV and m-PBTMS-PPV appeared around 470 and 440 nm, respectively. A single-layer light-emitting diode device was fabricated which has a simple ITO (indium−tin oxide)/polymer/Al configuration. The threshold bias of trimethylsilyl-substituted o(m,p)-PBTMS-PPV was in the range of 8−9 V. As in the photoluminescence spectra, the dramatic change of emission color was also shown in electroluminescence spectra of p-PMEH-PPV, p-PBTMS-PPV, and o-PBTMS-PPV when the operating voltage was about 8−9 V.

Synthesis and Properties of Light-Emitting Polymers Containing High Electron Attracting Groups

Two light-emitting polymers containing high electron attracting groups have been synthesized and their luminescent properties have been investigated. The synthesized polymers showed their absorption maxima at about 310 nm and the photoluminescence emission maxima at about 470 nm and 488 nm depending on the substituents. Single layer electroluminescent (EL) devices using the polymers showed blue and blue-green light emission.

Etching and electrochemistry of silicon in acidic bromine solutions

An electrochemical study of p-type and n-type (100) Si in acidic fluoride solutions containing bromine is described. It is shown that the electrochemical reduction of bromine takes place via the conduction band of the semiconductor. Etch rate measurements in combination with electrochemical experiments reveal that Si etching in bromine solutions involves a chemical mechanism. The etch rates of (100) and (111) surfaces are both diffusion-controlled. It is shown that bromine can be used for open-circuit photoselective etching of both p- and n-type Si. In addition, bromine etching of porous Si samples causes a shift of the photoluminescence emission maximum to shorter wavelengths.