EL Mechanism Research Articles

Enhanced Electroluminescence from a Silicon Nanocrystal/Silicon Carbide Multilayer Light-Emitting Diode

Developing high-performance Si-based light-emitting devices is the key step to realizing all-Si-based optical telecommunication. Usually, silica (SiO2) as the host matrix is used to passivate silicon nanocrystals, and a strong quantum confinement effect can be observed due to the large band offset between Si and SiO2 (~8.9 eV). Here, for further development of device properties, we fabricate Si nanocrystals (NCs)/SiC multilayers and study the changes in photoelectric properties of the LEDs induced by P dopants. PL peaks centered at 500 nm, 650 nm and 800 nm can be detected, which are attributed to surface states between SiC and Si NCs, amorphous SiC and Si NCs, respectively. PL intensities are first enhanced and then decreased after introducing P dopants. It is believed that the enhancement is due to passivation of the Si dangling bonds at the surface of Si NCs, while the suppression is ascribed to enhanced Auger recombination and new defects induced by excessive P dopants. Un-doped and P-doped LEDs based on Si NCs/SiC multilayers are fabricated and the performance is enhanced greatly after doping. As fitted, emission peaks near 500 nm and 750 nm can be detected. The current density-voltage properties indicate that the carrier transport process is dominated by FN tunneling mechanisms, while the linear relationship between the integrated EL intensity and injection current illustrates that the EL mechanism is attributed to recombination of electron-hole pairs at Si NCs induced by bipolar injection. After doping, the integrated EL intensities are enhanced by about an order of magnitude, indicating that EQE is greatly improved.

Enhanced electroluminescence from SiN-based multilayer structure by laser crystallization of ultrathin amorphous Si-rich SiN layers

Luminescent SiN-based multilayers were prepared in a plasma enhanced chemical vapor deposition system followed by subsequently laser crystallization of ultrathin amorphous Si-rich SiN sublayers. The cross-sectional TEM analysis reveals that grain size of Si nanocrystals embedded in the Si-rich SiN sublayers is independent of the laser fluence, while the grain density can be well controlled by the laser fluence. The devices containing the laser crystallized multilayers show a low turn-on voltage of 5 V and exhibit strong green light emission under both optical and electrical excitations. Moreover, the device after laser-irradiated at 554 mJ/cm(2) shows a significantly enhanced EL intensity as well as external quantum efficiency compared with the device without laser irradiation. The EL mechanism is suggested from the bipolar recombination of electron-hole pairs at Si nanocrystals. The improved performance of the devices was discussed.

Electroluminescence from ZnO/n<sup>+</sup>-Si Heterojunction

The ZnO/n+-Si heterojunction has been fabricated via depositing nominally undoped ZnO film by reactive sputtering on a heavily arsenic-doped (n+) silicon substrate. The sputtered ZnO film was n-type in conductivity with an electron concentration of 1.0×1018 cm-3. The current-voltage characteristics indicate that the ZnO/n+-Si heterojunction does not possess rectifying function. Under the forward bias with the negative voltage applied on the n+-Si substrate, the heterojunction emits ultraviolet and broad visible lights characteristics of near-band-edge and defect-related emissions of ZnO, respectively. The EL mechanism has been tentatively explained in terms of the energy-band diagram.

Electroluminescent Properties of Organic Light-Emitting Diodes with Blue-Emitting Alq

Aluminum(III) bis(2-methyl-8-quinolinate)4-phenylphenolate (BAlq) is well-known to be a hole blocking material against phosphorescent light-emitting diodes. The authors investigated the EL mechanism of BAlq. The PL intensity of BAlq was stronger than that of Alq3, but the EL performance of BAlq LED was poorer than that of Alq3 LED because of lower electron injection. The introduction of Alq3 electron injection layer improved EL performance, but EL decreased with decreasing BAlq thickness beyond the optimal Alq3 thickeness. The authors clarified by doping method that the recombination zone of BAlq/Alq3 LED is located near the interface between BAlq and Alq3 and Alq3 may act as quencher to BAlq.

Photoluminescence and electroluminescence of squarylium cyanine dyes

A series of squarylium cyanine dyes (Sq1, Sq2 and Sq3) were synthesized to explore their applications in organic electroluminescence devices (ELDs) with the aim of achieving highly efficient red emission. The absorption and fluorescence spectra of squarylium cyanine dyes (Sqs) in organic solvents, as well as their interaction with 8-hydroxyquinoline aluminum (Alq) in solution and in film, were investigated to help understand the EL mechanism. The basic device structure consists of a holetransport layer (HTL) of N,N′-diphenyl- N,N′-bis (3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD) and an emission layer (EML) of Sq-doped Alq. By the doping method, the EL color can be readily tuned from green (EL of Alq) to red (EL of Sq) upon varying application of voltage. The EL mechanism is suggested to be associated with the energy transfer from the excited Alq to the dopant Sq.

交互積層型有機発光素子の発光機構の検討と発光モデルの提案

交互積層型有機発光素子の発光機構の検討と発光モデルの提案

Increased ether lipid cytotoxicity by reducing membrane cholesterol content

Ether-linked glycerophospholipids (ether lipids, EL) are selectively toxic and anti-proliferative agents against cancer cells in vitro. The reason for such selectivity is not completely clear. Their mechanism of action is mediated through an interaction with the plasma membrane and the membrane lipid composition may modulate it. As a continuation of previous reports, we now present data showing that cholesterol concentration modulates EL toxicity in the K562, U937 and MOLT4 leukemic cell lines in vitro. Cells become sensitive to otherwise ineffective doses of EL when their cholesterol content is lowered. Cell cholesterol levels were reduced by exposure to an egg lipid mixture (neutral glycerides, phosphatidylcholine and phosphatidylethanolamine, AL721). The data contribute to an understanding of the EL mechanism of action on membranes and suggest that the cellular cholesterol concentration must be considered a major factor in modulating the cytotoxic effects of EL.

Electroluminescence of SiOx-LnF3Thin Films)

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Heavy atom isotope effect studies of elimination reaction mechanisms. 1. A kinetic and carbon-14 kinetic isotope effect study of the base-promoted dehydrochlorination of substituted 1-phenylethyl-2-14C chlorides

Upon treatment with the solvent conjugate base, the primary reaction of substituted 1-phenylethyl chlorides is elimination to substituted styrenes in tert-butanol (t-BuOH)-10% v/v dimethyl sulfoxide (Me/sub 2/SO) at 60/sup 0/C and in bis(2-hydroxyethyl) ether-10% v/v Me/sub 2/SO at 45/sup 0/C. Kinetic studies using eight substituted chlorides show that these reactions are strongly accelerated by both electron-donating and electron-withdrawing substituents, probably indicating a fairly reactant-like E2 transition state with the mechanism changing from El-like E2 to ElcB-like E2 as the substituents change from electron donating group (EDG) to electron withdrawing (EWG). In this first carbon isotope effect study of an elimination reaction of an alkyl chloride, carbon-14 kinetic isotope effects have been measured in the alkoxide/bis(2-hydroxyethyl) ether-Me/sub 2/SO system for 1-(4-methylphenyl)ethyl-2-/sup 14/C chloride, k/sup ..beta../k = 1.038, 1-phenylethyl-2-/sup 14/C chloride, k/sup ..beta../k = 1.058, and 1-(4-chlorophenyl)ethyl-2-/sup 14/C chloride, k/sup ..beta../k = 1.068. Clearly none of these compounds reacts by an irreversible El mechanism, for which no ..beta..-carbon isotope effect should be observed. The trend in the results is what would be expected from increased C/sub ..beta../-H bond rupture as the E2 transition state shifts from El-like to ElcB-like. Theoretical calculations involving El-like E2 transition-state models show that in this region onlymore » for a reaction coordinate motion tht strongly couples C/sub ..beta../-H bond rupture, C/sub ..cap alpha../-C/sub ..beta../ double bond formation, and C/sub ..cap alpha../-Cl bond rupture does the calculated value for k/sup ..beta../k come into the experimentally observed range, and then only for reactant-like models. 2 figures, 5 tables.« less

The reaction of 2-substituted-9-( [formula omitted]-substituted penylmethyl) fluoren-9-yltrimethylammonium ions in chloroform. Alkene formation via a carbocation.

The reaction of several quaternary ammonium salts in chloroform is found to give exclusively the alkene product by the El mechanism.