Ludwig Brehmer

New aromatic poly(1,3,4-oxadiazole)s for light emitting diodes

Abstract New aromatic poly(1,3,4-oxadiazole)s were synthesized having excellent film forming properties due to their solubility in common organic solvents. The investigated new polyoxadiazoles can be used as emission material in single layer LED. The polyoxadiazoles show an emission in the range of blue to yellow light. The external quantum efficiency as well as the turn-on voltage of the devices are influenced when blends of the polyoxadiazole with hole transport materials are used.

SENSITIVITY OF POLYTHIOPHENE PLANAR LIGHT-EMITTING DIODES TO OXYGEN

Surface light emitting diodes (SLEDs) with a polymer-on-top geometry were used to study the sensitivity of light emission to oxygen. In these devices, pre-fabricated electrodes were coated with a conjugated polymer, which was thus directly exposed to the environment. Oxygen caused an immediate ten-to hundred fold decrease in electroluminescence efficiency relative to that in nitrogen or argon. Above the voltage for light emission, there was a sharp increase in current. Removing the oxygen led to recovery of the light intensity over a period of minutes, but the current returned immediately to its lower, original level. The electroluminescence and photoluminescence spectra were identical and were unaltered in shape by oxygen exposure (only decreasing in size). However, photoluminescence was unaffected by oxygen alone. This result indicates that oxygen does not affect excitons directly, but rather influences an intermediate species on the path to exciton formation, one that is significant only in electroluminescence and not in photoluminescence. Under simultaneous exposure to oxygen and UV light, the photoluminescence irreversibly decreased, presumably due to photo-oxidation

Control of the structure of Langmuir-Blodgett films of a discotic liquid crystalline compound via the subphase composition and the adjacent molecular environment

Abstract Changes of the ,olecular arrangement that can be induced by means of the LB technique in the multilayers of a disc-shaped multialkynyl amphiphile are monitored by means of small angle X-ray diffraction. Studies of the monolayers at the air-water interface reveal “edge-on” orientation of the discs. Specific effects of the counter-ions (Na + , Cd 2+ , Pb 2+ and Ba 2+ ) and sub-solution pH on the monolayer collapse pressure, transfer efficiency and molecular order in the multilayers are found. A correlation between the monolayer properties and the ability for formation of periodic discotic structures in the presence of divalent counterions is established. The discotic molecules retain their “edge-on” arrangement in the highly compressed transferred films with slight irregular interdigitation of the flexible wings and inclination to the substrate normal. The tilt and the interdigitation are reduced when the discotic monolayers are deposited in alternating LB films with barium arachidate spacer layers.

Edge-on Langmuir-Blodgett multilayers derived from disc-shaped multiyne mesogens☆

Abstract Langmuir-Blodgett (LB) multilayers were prepared from disc-shaped multiyne mesogens based on amphiphilic alkyl pentakis (aryl-ethynyl) benzene ethers. The two compounds used are characterized by five hydrophobic flexible chains and one hydrophilic substituent at the terminal position of the alkoxy chain. The LB films were analyzed by X-ray scattering and spectroscopic measurements. An edge-on arrangement of the two discotic pentaalkynes within Y -type bilayers with a different packing density proved to be possible for the LB films of both compounds.

Self-assembled monolayers of novel azobenzenes for optically induced switching

Abstract Two novel azobenzenethiols containing either a sulfonamide or an amide group were synthesised and their ultrathin films were prepared by self-assembling (SA) technique. The structure of the SA monolayer (SAM) was characterised by atomic force microscopy (AFM). The switching between trans- and cis-configuration was observed by measuring the surface potential using the Kelvin probe technique while alternating irradiating the SAM with ultraviolet and visible light. A repeated switching of SAMs of 4′-(8-mercapto-octamethyleneoxy)-4-(butyl-amino-sulfonyl)azobenzene on gold with changes of the surface potential by 20 mV was observed.

Sensor activity of thin polymer films containing lead nanoparticles

Thin poly(p-xylylene) films containing lead nanoparticles were prepared by vacuum deposition technique. The vapors of p-xylylene monomer and lead were condensed onto the surfaces cooled down to 80 K. Films thus obtained were characterized by electric conductivity measurements during film samples formation. Such metallopolymer films exhibit sensor activity in the presence of ammonia in the atmospheric air. The influence of air humidity and the co-operative effect of ammonia together with water vapors on film resistance were studied.

Characteristics of polythiophene surface light emitting diodes

Abstract Surface light emitting diodes (SLEDs), in which previously microfabricated electrodes were coated with a conjugated polymer, were made with greatly different electrode spacings (250 nm and 10 or 20 μm) and with different electrode material combinations. The fabrication process allowed us to compare several electrode materials. The SLED structures also enabled imaging of the light emission zone with fluorescence video microscopy. Conventional sandwich structures were also made for comparison (electrode separation 50 nm). In this study, the emitting layer was poly[3-(2′,5′-bis(1″,4″,7″trioxaoctyl)phenyl)-2,2′-bithiophene] (EO–PT), a conjugated polymer based on polythiophene with oligo(ethyleneoxide) side chains. The current–voltage ( I ( V )) and light–voltage ( L ( V )) characteristics of the SLEDs were largely insensitive to electrode separation except at high voltages, at which the current in the devices with the largest separations was limited. Sandwich structures had the same light output at a given current. Light could be obtained in forward and reverse bias from indium tin oxide (ITO)–aluminum, gold silicide–aluminum, and gold silicide–gold SLEDs, but the turn-on voltages were lowest with the ITO–aluminum devices, and these were also the brightest and most reliable. Adding salt to the EO–PT increased the current and brightness, decreased the turn-on voltages, and made the I ( V ) characteristics symmetric; thus, a device with an electrode separation of 10 μm had the extraordinarily low turn-on voltage of 6 V. The location of the light emission was at the electron-injecting contact.

Dispersive surface plasmon microscopy for the characterization of ultrathin organic films

We report on experiments, applying a novel microscopy method for ultrathin‐film characterization which uses the dispersion properties of plasmon surface polaritons (PSP) as a contrast giving mechanism. In contrast to prior methods, we apply white light instead of a laser for resonant PSP excitation. For a given incident angle and an area of given thickness, a narrow spectral band of the incident light is at resonance for PSP excitation. Therefore, we obtain images with areas of different thickness appearing in different color. As an example, we present microscopic images taken from a thin Cu–phthalocyanine film, evaporated onto solid support through a mask. In addition, we compare the obtained images to theoretical considerations on the contrast mechanism.

Planar microfabricated polymer light-emitting diodes

Conjugated polymers are organic semiconducting materials that can emit light. These polymers have the advantages of being light, cheap and easy to process, and in addition the band gap can be tailored. We report the microfabrication of surface light-emitting diodes (SLEDs) on silicon substrates in which the electrodes are underneath the organic electroluminescent layer. Patterned electrodes are separated by a 2500 ? thick insulating layer of silicon oxide or are interdigitated with a separation of 10 or 20 m; the luminescent polymer is spin coated or solvent cast on top of the electrodes. This fabrication method is completely compatible with conventional silicon processing because the polymer is deposited last and the light is emitted from the upper surface of the diodes. Despite the large spacing between electrodes, and despite the absence of an evaporated top contact, the voltages required for light emission were not much greater than those used in conventional sandwich-type structures.

Scanning near-field ellipsometric microscope-imaging ellipsometry with a lateral resolution in nanometer range

An apertureless optical near-field scanning microscope system has been created by combining a commercially available atomic force microscope and an ellipsometer without any prior changes in design of the respective devices. In preliminary experiments, an optical resolution of about 20 nm (λ/32) has been achieved using the combined microscope. The intensity of the measured optical signal has been found to be a periodic function of the thickness of the sample. Moreover, the period of this function is dependent upon the local optical properties of the sample material.