Dou-Yol Kang

Synthesis and electroluminescent properties of bis(3-N-ethylcarbazolyl)cyanoterephthalidene

A new blue electroluminescent material analog to poly(3,6-N-2-ethylhexylcarbazolyl cyanotere-phthalidene) (PECCP) was synthesized by a facile reaction of 9-ethyl–3-carbazole carboxy aldehyde with 1,4-phenylenediacetonitrile. The resulting bis(3-N-ethylcarbazolyl)cyanoterephthalidene (BECCP) having cyano (electron acceptor part) and carbazole moiety (electron donor part) was characterized by NMR, IR, UV and was vacuum-deposited into thin film of smooth surface. BECCP shows blue PL and EL spectra at 480 nm corresponding to 2.58 eV and typical rectifying diode characteristics in ITO/BECCP/Al devices in both forward and reverse bias. The turn on voltage are +8 V and 4 V when the BECCP layer are 600 and 300 A thick, respectively.

Study of Langmuir-Blodgett films of a conjugated poly(3,6-N-2-ethylhexyl carbazolyl cyanotere-phthalidene)

Green light-emitting material, poly(3,6-N-2-ethylhexyl carbazolyl cyanotere-phthalidene)(PECCP), was synthesized which has an electron-donor and electron-acceptor moiety in a repeated unit of polymer. We have made PECCP Langmuir-Blodgett(LB) films and they exhibit fluorescence. Organic light-emitting diodes with a single-layer structure (ITO/emitting layer/Al) have been fabricated with the PECCP LB films as an emitting material. We have observed green photoluminescent and electroluminescent spectra at λ max = 536nm corresponding to 2.34eV.

The preparation and characterization of poly(p-phenylene vinylene) Langmuire-Blodgett films formed via precursor method

Abstract Recent advances on π-conjugated conducting polymers for electroluminescence(EL) have shown a strong possibility for various display applications. In this paper, we will describe the preparation of ultra thin films of oil-soluble poly(p-phenylene vinylene) (PPV) precursor polymer by the Langmuir-Blodgett technique and by conversion into PPV films via a thermal treatment. The characterization of these films was also carried out by FT-IR spectroscopy, UV-VIS absorption spectroscopy, photoluminescence (PL), and Scanning Tunneling Microscopy (STM). FT-IR, and UV-VIS results indicated that multilayer films of PPV precursor polymer were formed well and the elimination of methoxy group in PPV precursor polymer was partially achieved by the thermal treatment. Red shift was observed in PL analysis after thermal treatment. An STM image of a PPV monolayer on graphite showing the polymer chains was observed.

Temperature-Dependent I-V Characteristics and Thermal Annealing Effects of C22-Quinolium(TCNQ) Langmuir-Blodgett Films

Abstract Electrical properties and thermal annealing effects of C22-quinoliuni(TCNQ) Langmuir-Blodgett(LB) films were studied. Typical current-voltage(I-V) characteristics along the perpendicular direction show an anomalous behavior of breakdown near the electric-field strength of 106 V/cm. From the study of I-V characteristics in the high-electric field region, it is found that there is a contribution of joule heating in addition to the electrical effect. To see the thermal influence of the specimen, current was measured as a function of temperature(20 ∼ 180°C). It shows that the current increases about 4 orders of magnitude near 60 ∼ 70°C, and remains constant for a while up to ∼ 150 °C and then suddenly drops. Such increase of current near 60 ∼ 70 °Cseems to be related to a softness of alkyl chains. Besides the electrical measurements, UV/visible absorption(300 ∼ 800nm) of the thermally annealed sample was measured to see the internal-structure change. It is found there are four characteristic peaks. A...

Diffusion of Electron in Langmuir-Blodgett Ultra-Thin Films

Abstract Diffusion of electron in polyimide LB films of A1 / polyimide LB films / Au (MIM) structures is reported. Time dependences of the generated voltage in this MIM structures were proportional to the square root of time. Such a relation could be explained by the diffusion of electron from the Au electrode to the A1 electrode through polyimide LB films. From the experimental results, the diffusion constant of electron was estimated as about 2.5×10−17(cm2/sec). Furthermore, the diffusion current calculated by the diffusion constant coincided with the current measured in the experiments. It is considered that the diffusion of electron is one of the main causes of the voltage generation in the MIM structures of polyimide LB films.

I–V and π−A characteristics of ultrathin Langmuir-Blodgett films

Abstract The deposition conditions and electrical properties of C 22 -quinolium(TCNQ) Langmuir-Blodgett (LB) films have been studied. The optimum conditions of film deposition were examined by a study of surface pressure-area (π− A ) isotherms with variations of pH, temperature, barrier moving speed, dipping speed, spreading amount of solution etc. The LB films were deposited on a slide glass substrate and tested with several tools. The π− A isotherm shows that there might be two phase transitions near surface pressures of 30 and 45 mN m −1 . It is thought that the phase just below 30 mN m −1 is a liquid phase and that above 30 mN m −1 is a solid phase. The LB films were made under the following conditions; surface pressure, 45 mN m −1 ; 25°C; pH 5.6; spreading amount 60–70 A 2 per molecule; and dipping speed, 4 mm min −1 . The electrical conductivities and conduction mechanism were investigated by measuring current-voltage ( I–V ) characteristics along the vertical and horizontal direction of the film. We observed an anisotropy in conductivity between the vertical and horizontal direction of the film. The conductivity along the film surface direction is about 10 −7 S cm −1 and that of the perpendicular direction is about 10 −14 S cm −1 at room temperature. We saw non-ohmic conducting behaviour in the presence of high electric field along the vertical direction. When the electric field is high in this direction, a breakdown occurs in the I–V characteristics. We analysed the I–V characteristics in detail near the breakdown region by changing the type of applied voltage (step or pulse) as well as the temperature.

The electromotive force generation in metal-insulator-metal structures of Langmuir-Blodgett films

Abstract It was found that several hundred millivolts of d.c. voltage is generated in a metal-insulator-metal structured device of Langmuir-Blodgett (LB) films. The voltage generation is caused by LB films only or by a chemical reaction between LB films and the electrode. In the work reported in this paper, the experiments are processed for a sample which has been left under atmospheric conditions since its fabrication in 1986. We measured the same voltage as at the time of fabricating the LB films. The thickness of the LB films and the evaporated electrode used for the experiment is about 20∼30 A and 2000 A respectively. Therefore, we cannot think that a chemical reaction which is generating several hundred millivolts takes place continuously for 6∼7 years. It is proper that the chemical reaction has terminate. In our experimental results, the cause is some kind of e.m.f. which has occured at elements of the film.

Anisotropic DC Electrical Conductivity and AC Response in C22-Quinolium (TCNQ) LB Films at Room Temperature

Abstract Ultra-thin organic films of C22-Quinolium (TCNQ) were deposited onto ordinary microscope slide-glass substrates with a Langmuir-Blodgett technique. Surface pressure(π) - area(A) isotherms were studied to find optimum conditions of deposition by varying the environments. Several tools were employed to confirm the deposition such as the electrical capacitance and the direct thickness measurement. Electrical conductivities were studied in two different directions of the film. It has turned out that this film is highly anisotropic. The obtained conductivities are ∼ 1 × 10−7 S/cm along the film surface direction, and ∼ 1 ∼ 10−14 S/cm in the perpendicular direction. We have also studied ac response by applying a step voltage (1 Hz. 1 V) to the film. From this study, we were able to correlate the electrical capacitance to the number of film layers.

Electrical properties of C22-quinolinium(TCNQ) and polyamic acid alkylamine salts Langmuir–Blodgett films

Abstract Electrical properties of C22-quinolinium(TCNQ) and polyamic acid alkylamine salts Langmuir–Blodgett (LB) films have been investigated through a study of surface pressure (π)–area (A) isotherms, current–voltage(I–V) characteristics, frequency-dependent dielectric responses, and thermally-stimulated currents (TSC). Through-plane I–V characteristics of C22-quinolinium(TCNQ) LB films show an anomalous behavior of breakdown near the electric-field strength of 106 V/cm. Frequency-dependent dielectric constants of the thermally annealed C22-quinolinium(TCNQ) LB films show that there are two characteristic dispersions in a frequency range of 10–13 Hz; one is a dispersion occurring near 1 MHz coming from the orientational polarization of the molecules and the other is probably due to an interfacial polarization below 1 kHz when the annealing temperature is above 80°C. The annealing temperature-dependent dielectric constants of the C22-quinolinium(TCNQ) LB films show that there are two maximum; one at near 80°C and the other at near 180°C. The first peak seems to be related to a disorder of alkyl chains. In the I–V characteristics of Z-type polyamic acid alkylamine salts (PAAS) LB films measured along the through-plane at room temperature, ohmic conduction has been observed below around 105 V/cm, which is similar to that of C22-quinolinium(TCNQ) LB films. Non-ohmic conduction has been observed above around 105 V/cm and the conduction mechanism can be explained by Schottky effect. Frequency-dependent dielectric responses of the PAAS LB film show that there is a dispersion at 104 Hz when the annealing temperature reaches about 100°C. Measurements of thermally-stimulated currents of those LB films were performed from room temperature to about 250°C and the temperature was increased at a rate of 0.2°C/s linearly. TSC peaks are observed at about 70°C in the arachidic acid LB films, and at about 70 and 160°C in the PAAS LB films. They indicate that the peak at 70°C is resulted from the alkyl group, and the other peak at 160°C is probably due to C O group in PAAS molecules.

Characterization of octadecylviologen-TCNQ Langmuir-Blodgett films

N-octadecylviologen-TCNQ−2 salt (OV-(TCNQ)2) was synthesized and identified by its characteristic NMR peaks. Charge transfer absorption of the TCNQ anion radical was observed at 600