Advanced Materials Research Vols. 306-307

Abstract: Based on our earlier theoretical investigation, p-type C:Si codoped AlN crystals were grown on SiC substrates by a sublimation method in a improved growth reactor. Hall-effect measurement shows that the AlN crystals have a high hole density of 1.4×10¹⁴ cm^-3 and mobility of 52 cm²V^-1s^-1 in spite of the high resistivity (896 Ω•cm). In the AlN samples, Si dopants act as donors due to substituting Al atoms, and most of C dopants act as acceptors for replacing N atoms. It is also observed that the activation energy of C acceptors in C:Si codoped AlN is reduced by codoping Si donors, which agrees with the computational results.

Abstract: Zn₃Ta₂O₈ is a promising host for low voltage cathodoluminescence (CL) applications. Surface chemical stability during low voltage electron beam excitation is a prime concern for phosphors to be used in various new generation information displays. Photoluminescence (PL) and CL characteristics of the Zn₃Ta₂O₈ host doped with Pr³⁺ are presented. The phosphors were synthesized via solid-state reaction route at 1100°C. Red CL or PL with a maximum at 611 nm, attributed to the ¹D₂-³H₄ transition of the Pr³⁺ ion, was observed at room temperature under high energy electron (2 keV, 12 μA) or a monochromatic xenon lamp (257 nm) irradiation. Electron stimulated chemical changes on the surface of the Zn₃Ta₂O₈:Pr³⁺ phosphor during an electron beam exposure from 0-350 C/cm² was monitored using Auger electron spectroscopy. The CL exhibited only a 20% loss in the original intensity during the continuous electron beam exposure. X-ray photoelectron spectroscopy (XPS) was used to estimate the redox states of the chemical constituents and a comparison of binding energies was made with the standard Ta₂O₅ and ZnO compounds. A correlation between the structural configuration of Zn₃Ta₂O₈ and the XPS data is also established.

Abstract: 3-D composite photonic crystals were fabricated on ITO substrates and slide glass substrates by two different methods, modified dip-coating and coating first method. It was found that there were lots of line defects existing in the composite films fabricated by dip-coating method but few existing in the composite films fabricated by coating first method. The reflectance spectra were measured by UV-vis spectrometry and revealed that all the fabricated photonic crystals exhibit a photonic band gap in the normal direction. The optical properties of the core-shell structure strongly depended on the intrinsic defects in the films.

Abstract: Conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) has been synthesized by solid-state polymerization of 2,5-diiodo-3,4-ethylenedioxythiophene under normal conditions. The resulting iodine-doped PEDOT is completely insoluble in common organic solvents and retained partial crystal structure of monomer as indicated by scanning electron microscopy. Electrical conductivity measurement showed that PEDOT exhibited conductivity in the range of 0.16–0.23 S/cm (pressed powder pellet) with temperature dependence. Thermogravimetric and differential thermal gravimetric analysis of PEDOT indicated that its main chain was stable up to ca. 200 °C.

Abstract: Cadmium sulfide (CdS) thin films with (111) preferential orientation were grown on glass substrates at room temperature by radio frequency (R.F.) magnetron sputtering. The structural and optical properties of CdS films have been investigated by X-ray diffraction, Scanning Electron Microscope micrographs, PL spectra and transmittance spectra. The grain sizes have been evaluated. The transmission spectra of the obtained films reveal a relatively high transmission coefficient (80%) in the visible range. All these results show that the grain sizes increased while the optical band gap decreased with increasing the thickness of CdS films.

Abstract: Based on the first-principles calculations with density functional theory, the formation energy and electronic structure of (S, Cu) co-doped ZnO has been investigated, where the doping cases including related defects for Cu mono-doped, S-Cu co-doped, and S-2Cu co-doped ZnO are studied. The calculated results show that the formation energy of S-2Cu complex is lower than that of S-Cu complex under the O-rich condition. From the electronic structure, S-2Cu complex forms a peak of impurity state at the top of valence band. It was further found that heavy doping of Cu, not only enhances the acceptor concentration, but also leads to shallower acceptor energy level. Therefore, we concluded that S-2Cu complex is suitable for yielding better p-type conductivity in ZnO. The results are in good agreement with the experiment results.

Abstract: AlN and TiN thin films are widely used in electronic devices and acoustic material and other fields because of its unique merit, the preparation of nitride thin films by using the arc ion plating has not been a systematic and deep study. The article presents our research procedure which the AlN and TiN thin films are deposited on stainless steel substrate by arc ion plating (AIP). The characteristics of thin films, for example microstructure, morphology, composition analysis and hardness, are examined and analyzed. The results showed that: Droplet-like particles appear in the microstructure of nitride thin films, and the grain size of droplet-like particles in AlN thin films is greater than in TiN thin films. The micro-hardness of nitride films preparation in experiment has improved significantly, and establish firmly basic for extending the application field of nitride film.

Abstract: A series of donor-acceptor (D-A) substituted with triphenylamine-core fluorescent derivatives have been synthesized via nucleophilic addition of Grignard reagent to 2,7-dibromo-9-fluorenone, CF₃SO₃H-promoted Friedel-Crafts reaction, Ullmann coupling reaction, and Pd-catalyzed Suzuki cross-coupling reaction in turn. Aromatic imide (N-phthalimide) group was firstly introduced as electron acceptor in the donor-π-acceptor (D-π-A) opto-electronic materials. Hole-injection capability of the fluorene derivatives were promoted by introducing 4-methyl-triphenylamine (MTPA) group at the 9-position. All these compounds present excellent solubility and thermal stability (with glass transition temperature of 191-279 °C), thermo gravimetric analysis (TGA) indicated that 5% or 10% weight losses occurred at about 415-463 °C or 427-498 °C respectively. The relationship of the solvent polarity or the conjugation length of the D-A structure with the luminescent properties has been studied by the UV-vis spectra and the fluorescent spectra. The CV data revealed that these fluorescent derivatives are possessed low-lying HOMO energy levels ranging from -5.19 to -5.37 eV and are exhibited strong blue-green luminance around 448-546 nm.

Abstract: In this paper, a novel method to measure the luminescence time interval between the initial and terminal of light emission in frequency domain is proposed. Under the condition that the time of excitation pulse is equal to the luminescence time interval between the initial and terminal of light emission, the energy of excitation is complete dynamically balanced by the light emission and the maximum luminescence intensity can be achieved. So the luminescence time interval between the initial and terminal of light emission can be expressed as (is the driving frequency which is corresponding to the maximum luminescence intensity). The luminescence time interval between the initial and terminal of light emission () of Alq₃ (Tris-(8-quinolinolato)aluminum) is [s] measured using the novel method.

Abstract: A cement based piezoelectric composite sensor using 1-3 cement based piezoelectric composite as sensing element was fabricated. The basic properties of the sensor were mainly investigated. The results indicate that in the frequency range from 0.1 to 40 Hz, the output voltage amplitudes of the sensor increase nonlinearly with increasing frequency of input load under 10 Hz. When the frequency of input load is larger than about 10 Hz, the output voltage amplitudes of the sensor is nearly independent of frequency. There exists an obviously linear relationship between the output voltage amplitude of the sensor and input load amplitude. The output voltage of the sensor is correspond to the complex load very well. The phase difference between the output voltage of the sensor and input load is near zero. Therefore such sensors have a good potential to be used in civil engineering structural health monitoring.