Yu Xu

The gas sensitivity of a metal-insulator-semiconductor field-effect-transistor based on Langmuir–Blodgett films of a new asymmetrically substituted phthalocyanine

Abstract Langmuir–Blodgett (LB) films of an amino-tri- tert -butyl-phthalocyanine (AmBuPc) were fabricated as the semiconductor thin layer for a metal-insulator-semiconductor field-effect-transistor (MISFET). The gas sensitivity of the low-conducting LB films MISFET was measured in a NO 2 atmosphere at different concentration. The results showed that the gas concentration and the drain-source current of the AmBuPc gas sensor satisfy Langmuir adsorption isotherm.

Schottky diodes fabricated with Langmuir—Blodgett films of C60-doped poly(3-alkylthiophene)s

Abstract By dispersing poly(3-alkylthiophene)s (P3ATs) with surface-active materials such as arachidic acid (AA), three P3AT—AA—C 60 mixed systems were fabricated into Y-type Langmuir—Blodgett (LB) films through the vertical dipping method with a transfer ratio close to unity. The LB films possess a well-layered structure, which was proved by small-angle X-ray diffraction patterns. The LB films were used to prepare model Schottky diodes and the current density—voltage characteristics of the diodes were measured. It was found that P3ATs became semiconductors after doping with C 60 and the model diodes exhibited a rectification effect. The electronic parameters were found to be strongly dependent on the alkyl side chain length of P3AT molecules.

Langmuir-Blodgett films of an asymmetrically substituted metal-free phthalocyanine and the second-order non-linear optical properties

Abstract Langmuir-Blodgett (LB) films of nitro-tri-tert-butylphthalocyanine (NtBuPc) were fabricated. The configuration of the phthalocyanine molecules at the air-water interface is suggested in terms of the limiting area per molecule. A linear relationship between absorbance of the electronic spectrum and the number of layers of the LB films was observed. Second harmonic generation (SHG) from monolayer and multilayers was measured. The SH intensities of the monolayer relative to a Z-cut quartz wedge I 2ω p→p and I 2ω s→p are 4.0×10 −7 and 7.5×10 −8 respectively.

Direct observation of C60 LB film with scanning tunneling microscopy

Abstract We report the direct imaging of monolayer LB film of C 60 . The LB film was prepared onto an Au(100) surface by the vertical dipping method for scanning tunneling microscopy (STM) measurements. Ordered arrays of C 60 molecules and close-packed molecular aggregation in the film are shown on the STM images. The calculated diameters along two mutually perpendicular directions in the arrays are 0.635±0.013 and 0.572±0.024 nm, respectively. Our results demonstrate that the molecular shape of C 60 is easily compressed into an ellipsoid while forming the LB film.

The application of Langmuir-Blodgett films of a new asymmetrically substituted phthalocyanine, amino-tri-tert-butyl-phthalocyanine, in diodes and in all organic field-effect-transistors

Abstract Amino-tri- tert -butyl-phthalocyanine (AmBuPc) was used as the semiconductor thin layers in model Schottky diode and all organic field-effect transistor (OFET). The diode cell showed a rectifying effect with a rectification ratio of 60 at ±3.2 V. The ideality factors were 1.38 in the low voltage region and 3.09 in the high voltage region, respectively. The results of OFET proved that AmBuPc Langmuir–Blodgett (LB) films could be used as the semiconducting layer of OFET, and the OFET could function as a p-channel accumulation device. From the OFET electrical characteristics, the channel carrier mobility is calculated to be about 5.2×10 −6 cm 2 V −1 s −1 , which is higher than that of vacuum-deposited film OFET (7.8×10 −7 cm 2 V −1 s −1 ) utilizing the same polymer. This phenomenon can be explained as the highly ordered structure of AmBuPc molecules in the LB films. This suggests that a wide conduction channel is able to form in this all organic device, which provides a new method to simplify the fabrication process of OFET and improve their properties.

THE GAS SENSITIVITY OF LANGMUIR-BLODGETT FILMS OF A NEW ASYMMETRICALLY SUBSTITUTED PHTHALOCYANINE

Abstract Langmuir–Blodgett (LB) films of amino-tri- tert -butyl-phthalocyanine (AmBuPc) were fabricated. The gas sensitivities of the low-conducting LB films were measured in different concentrations of NO 2 atmosphere. The results showed that a good linear relationship exists between the logarithm of the resistance of AmBuPc gas sensor and the concentration of NO 2 , which could be explained in terms of the gas diffusion theory in the LB films. XPS results proved that the attack centers of NO 2 are the two pyrrole N-linked free H atoms of the Pc macro-ring. Temperature had significant influences to the recovery and cyclic properties of sensor and the conductivity of gas sensor. The values of activation energy are 0.14 and 0.43 eV, respectively, at temperatures lower and higher than 60°C in NO 2 .

Lattice imaging of C60 Langmuir-Blodgett films

Abstract The microstructural features of the LB films of C 60 have been studied by using high resolution electron microscopy. In the ordered regions of the films, lattice fringes of C 60 were observed. The selected area electron diffraction patterns possess hexagonal symmetry and can be indexed as a hexagonal unit cell with parameter a=1.0 nm. Defects, dislocations and even more disordered arrangements of C 60 can also be observed in the different regions of the films. However, the most parts of the films exist in the amorphous state.

Synthesis of an amphiphilic metalloporphyrin and a novel gas sensor fabricated with Langmuir-Blodgett films

Abstract An amphiphilic metalloporphyrin, cobalt (II) 5, 10, 15-tri( p -octadecyloxyphenyl)-20-( p -hydroxyphenyl)porphyrin (CoPPC 18 ) was synthesized and characterized. The molecules of CoPPC 18 alone form moderate stable monolayer at the air-water interface and a clear phase transition was observed in the surface pressure-area isotherm. An optical Mach-Zhender interferometer was prepared as a novel sensor element incorporating with Langmuir-Blodgett films. The preliminary result indicates that the sensor is sensitive and reversible on exposure to NO 2 gas at room temperature.

Second harmonic generation in Langmuir-Blodgett films of an asymmetrically substituted metallophthalocyanine

The formation of Langmuir-Blodgett (LB) films of an asymmetrically substituted metallophthalocyanine, copper nitro-tri-tert-butyl-phthalocyanine, is reported. From the second-harmonic generation (SHG) measurement, macroscopic second-order susceptibility, molecular hyperpolarizability and tilt angle for the molecules on a substrate were determined to be 1.63 × 10−5 esu, 2.11 × 10−27 esu and 26.1°, respectively. The SHG behaviour both in Z-type deposition and in alternating deposition LB films were investigated. A nearly quadratic relation between SH intensity and layer numbers was observed up to seven layers.

Formation and structural characteristic of Langmuir-Blodgett films of C60 and C70

Abstract The fullerenes, C 60 and C 70 , were prepared with high purity by high performance liquid chromatography. These materials could form condensed layers at the air-water interface. Under appropriate experimental conditions, we have fabricated four species of films, consisting of C 60 , C 70 and their mixtures with arachidic acid, onto various substrates by the Langmuir-Blodgett technique. The structural characteristics of these films were studied by UV-visible and Fourier transform IR spectroscopies, scanning tunneling microscopy, transmission electron microscopy and low-angle X-ray diffraction measurements.