2009 Vol. 22, No. 1

The electronic transport properties of oligoacenes sandwiched between two Au(111) surfaces with serial and parrallel configurations were investigeted by using a fully self-consistent nonequilibrium Green's function method combined with density functional calculations. This theoretical results show that the conductivity of oligoacenes with both sandwiched configurations at low bias voltage is mainly determined by the tail of the transmission peak from the perturbed highest occupied molecular orbital. When the molecular length increases, the zero-bias voltage conductance G(0) of oligoacenes with serial configuration neither follows Magoga's exponential law nor displays the even-odd oscillation effect, while the G(0) of the oligoacenes sandwiched with parallel configuration monotonically increases. The reduction of energy gaps, the alignment of the Fermi level, and the spatial distribution of the perturbed molecular orbitals are used to self-consistently explore the transport mechanism through oligoacenes.
To study theoretically the relationship between the integral interference angle and the scattering angle in collisional quantum interference, the integral interference angle of atom-2∏[case(a)] diatomic molecules system is described. To simulate the experiment theoretically, the theoretical model on collision-induced rotational energy transfer in an atom-2∏[case(a)]diatom system is presented based on the first order Born approximation taking into account of the long-range interaction potential. For the 2∏ electronic state in the Hund's case(a) diatom, the degree of the interference is discussed. The interference angles of collision-induced rotational energy transfer of CN(A2∏) in Hund's case(a) with He, Ne,and Ar are calculated quantitatively. The key parameters in the determination of integral interference angles are obtained.
The melting points of organic compounds were estimated using a combined method that includes a backpropagation neural network and quantitative structure property relationship (QSPR) parameters in quantum chemistry. Eleven descriptors that reflect the intermolecular forces and molecular symmetry were used as input variables. QSPR parameters were calculated using molecular modeling and PM3 semi-empirical molecular orbital theories. A total of 260 compounds were used to train the network, which was developed using MatLab.Then, the melting points of 73 other compounds were predicted and results were compared to experimental data from the literature. The study shows that the chosen artificial neural network and the quantitative structure property relationships method present an excellentalternative for the estimation of the melting point of an organic compound, with average absolute deviation of 5%.
Zipf's approach in linguistics is utilized to analyze the statistical features of frequency andcorrelation of 16 nearest neighboring nucleotides (AA, AC, AG, … , TT) in 12 human chro-mosomes (Y, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, and 12). It is found that these statisticalfeatures of nearest neighboring nucleotides in human genome: (i) the frequency distributionis a linear function, and (ii) the correlation distribution is an inverse function. The coeffi-cients of the linear function and inverse function depend on the GC content. It proposes thecorrelation distribution of nearest neighboring nucleotides for the first time and extends thedescriptor about nearest neighboring nucleotides.
Highly effective production of hydrogen from bio-oil was achieved by using a low-temperatureelectrochemical catalytic reforming approach over the conventional Ni-based reforming cat-alyst (NiO-Al2O3), where an AC electronic current passed through the catalyst bed. Thepromoting effects of current on the bio-oil reforming were studied. It was found that theperformance of the bio-oil reforming was remarkably enhanced by the current which passedthrough the catalyst. The effects of currents on the microcosmic properties of the catalyst,including the Brunauer-Emmett-Teller (BET) surface area, pore diameter, pore volume, thesize of the crystallites and the reduction level of NiO into Ni, were carefully characterized byBET, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscope.The desorption of the thermal electrons from the electrified catalyst was directly observedby the TOF (time of flight) measurements. The mechanism of the electrochemical catalyticreforming of bio-oil is discussed based on the above investigation.
Liquid mixtures of water and deuterium oxide as the liquid phase, were used to match the density of charged colloidal particles. Kossel diffraction method was used to detect the crystal structures. The experiments under the density-matched (g=0) and unmatched (g=1) conditions are compared to examine the influence of gravity on the crystal structures formed by self-assembly of 110 nm (in diameter) polystyrene microspheres. The result shows that the gravity tends to make the lattice constants of colloidal crystals smaller at lower positions,which indicates that the effect of gravity should be taken into account in the study of thecolloidal crystals.
The shear thinning and shear thickening rheological properties of PCC/PEG suspension were investigated with the increase of oscillatory amplitude stress at different constant frequencies.The results show that the complex viscosity was initially independent of stress amplitude and obvious shear thinning occurred, then dramatic shear thickening took place after reaching the minimum viscosity. Typically, in a constant frequency of 5 rad/s, the elastic modulus, viscous modulus, and tanδ (δ is the out-of-phase angle) vs. the stress amplitude was investigated.It is found that the elastic modulus initially appeared to be independent of stress amplitude and then exhibited a rapid decrease, but the viscous modulus was independent of amplitude stress at lower amplitude stress. After reaching the minimum value the viscous modulus showed a rapid increase. On the other hand, tanδ increased from 0.6 to 92, which indicates that the transition from elastic to viscous had taken place and tanδ showed a steep increase when shear thickening occurred. Lissajous plots are shown for the dissipated energy vs.different maximum stress amplitude in the shear thinning and shear thickening regions. The relationship of dissipated energy vs. maximum stress amplitude was determined, which follows a power law. In the shear thinning region the exponent was 1.91, but it steeply increases to 3.97 in the shear thickening region.
Electron paramagnetic resonance and electron-nuclear double resonance methods were used to study the polycyclic aromatic radical cations produced in a Friedel-Crafts alkylating sys-tem, with m-xylene, or p-xylene and alkyl chloride. The results indicate that the observed electron paramagnetic resonance spectra are due to polycyclic aromatic radicals formed from the parent hydrocarbons. It is suggested that benzyl halides produced in the Friedel-Crafts alkylation reactions undergo Scholl self-condensation to give polycyclic aromatic hydrocarbons, which are converted into corresponding polycyclic aromatic radical cations in the presence of AlCl3. The identiˉcation of observed two radicals 2,6-dimethylanthracene and 1,4,5,8-tetramethylanthracene were supported by density functional theory calculations using the B3LYP/6-31G(d,p)//B3LYP/6-31G(d) approach. The theoretical coupling constantssupport the experimental assignment of the observed radicals.
The dihydrogen bonds B–H???H–X (X=F, Cl, Br, C, O, N) in the dimer (NH3BH3)2 and the complexes of NH3BH3 with HF, HCl, HBr, H2CO, H2O, and CH3OH were theoretically studied. The results show that formation of the dihydrogen bond leads to elongation and stretch frequency red shift of the BH and XH bonds, except that in the H2CO system, the CH bond blue shifts. For (NH3BH3)2 and the complexes of the halogenides, red shifts of the XH bonds are caused by the intermolecular hyperconjugation σ(BH)→σ*(XH). For the system of H2CO, a blue shift of the CH bond is caused by a decrease of the intramolecular hyperconjugation n(O) →σ¤(CH). In the other two systems, the red shift of OH bond is a secondary effect of the stronger traditional red-shifted H-bonds N–H???O. In all these systems, red shifts of the BH bonds are caused by two factors: negative repolarization and negative rehybridization of the BH bond, and decrease of occupancy on σ(BH) caused by the intermolecular hyperconjugation σ(BH)→σ¤(XH).
The geometry, electronic structure, polarizability and hyperpolarizability of dye sensitizer3,4-bis[1-(carboxymethyl)-3-indolyl]-1H-pyrrole-2,5-dione (BIMCOOH) were studied usingdensity functional theory (DFT) with hybrid functional B3LYP, and the electronic absorp-tion spectra were investigated using semi-empirical quantum chemical method ZINDO-l and time-dependent DFT (TDDFT). The results of natural bond orbital suggest that the natural charges of the dione, indole, and acetic groups are about -0.15e, -0.29e, and 0.44e,respectively. The calculated isotropic polarizability, polarizability anisotropy invariant and hyperpolarizability are 305.4, 188.3, and 1155.4 a.u., respectively. The electronic absorption spectral features in visible and near-UV region were assigned to the ∏→∏*transition due to the qualitative agreement between the experiment and the TDDFT calculations, and the transitions of the excited states 9-11 related to photoinduced intramolecular charge transfer processes. The analysis of electronic structure and UV-Vis absorption indicates that the indole groups primarily contributed sensitization of photo-to-currency conversion processes, and the interfacial electron transfer between semiconductor TiO2 electrode and dye sensitizer BIMCOOH are electron injection processes from excited states of the dyes to the semiconductor conduction band.
Within the framework of the embedded-atom method, we performed molecular-dynamics calculations to investigate the structural transformation during melting of two copper clusters containing 57 and 58 atoms. The simulation results reveal how their different structural changes can strongly influence internal energy and radial distribution functions. The local structural patterns of different regions during the temperature increase, determined by atom density profiles, are identified for the melting of each cluster. The simulations show sensitivities of the structural changes for these two small size clusters with di?erent structures.
Through the theoretical calculation of structural optimization, vibrational frequencies and atomization energies with one method of density functional theory (B3LYP) and two post- Hartree-Fock approaches (MP2, CCSD(T)), several stable isomers for new three pnictogendianionic Sb42-, Bi42-, and (SbBi)22- species were determined. For two homoatomic Sb42- and Bi42- species, there are three stable isomers: square (D4h), roof-shaped (C2v-1), and C2v-2 structure with the square isomer being the ground state. For the heteroatomic dian- ionic (SbBi)22- species, there are also three stable isomers: rhombus (D2h), roof-shaped (C1), and C2v structures with the rhombic isomer being the ground state. The calculated NICS values show that nucleus-independent chemical shifts (NICS) values of roof-shaped isomers for Sb42-, Bi42-, and (SbBi)22- species are all negative, consequently indicating that these roof-shaped isomers possess aromaticities. NICS values for the planar ring isomers are all positive, suggesting that these three planar ring isomers have antiaromatic characters. The aromaticity for the two stable roof-shaped and square isomers are preliminarily explained and discussed with MO analysis.
The effects of magnetic field annealing on the properties of Fe48Co52 alloy nanowire arrayswith various interwire distances (Di=30-60 nm) and wire diameters (Dw=22-46 nm) wereinvestigated in detail. It was found that the array's best annealing temperature and crystalline structure did not show any apparent dependence on the treatment of applying a 3 kOe magnetic field along the wire during the annealing process. For arrays with small Dw or with large Di, the treatment of magnetic field annealing also had no obvious in°uence on their magnetic performances. However, such a magnetic field annealing constrained the shift of the easy magnetization direction and improved the coercivity and the squareness obviously for arrays with large Dw or with small Di. The difference in the intensity of the effective anisotropic field within the arrays was believed to be responsible for this different variationof the array's magnetic properties after magnetic field annealing.
The potential energy surface of O(1D)+C2H5Cl reaction was studied using QCISD(T)/6-311++G(d,p)//MP2/6-31G(d,p) method. The calculations reveal an insertion-elimination mechanism. The insertion reaction of O(1D) and C2H5Cl produces two energy-rich intermediates, IM1 and IM2, which subsequently decompose into various products. The calculations of the branching ratios of various products formed through the two intermediates were carried out using RRKM (Rice-Ramsperger-Kassel-Marcus) theory at the collision energies of 0, 20.9, 41.8, 62.7, 83.6, 104.5, and 125.4 kJ/mol. HCl is the main decomposition product for IM1; CH2OH is the main decomposition product for IM2. Since IM1 is more stable than IM2, HCl is probably the main product of the O(1D)+C2H5Cl reaction.
Polycrystalline ZnS films were prepared by pulsed laser deposition (PLD) on quartz glass substrates under different growth conditions at different substrate temperatures of 20, 200,400, and 600 oC, which is a suitable alternative to chemical bath deposited (CBD) CdS as a buffer layer in Cu(In,Ga)Se2 (CIGS) solar cells. X-ray diffraction studies indicate the films are polycrystalline with zinc-blende structure and they exhibit preferential orientation along the cubic phase β-ZnS (111) direction, which conflicts with the conclusion of wurtzite structure by Murali that the ZnS films deposited by pulse plating technique was polycrystalline with wurtzite structure. The Raman spectra of grown films show A1 mode at approximately 350 cm-1, generally observed in the cubic phase β-ZnS compounds. The planar and the cross-sectional morphology were observed by scanning electron microscopic. The dense,smooth, uniform grains are formed on the quartz glass substrates through PLD technique.The grain size of ZnS deposited by PLD is much smaller than that of CdS by conventional CBD method, which is analyzed as the main reason of detrimental cell performance. The composition of the ZnS films was also measured by X-ray fluorescence. The typical ZnS films obtained in this work are near stoichiometric and only a small amount of S-rich. The energy band gaps at different temperatures were obtained by absorption spectroscopy measurement,which increases from 3.2 eV to 3.7 eV with the increasing of the deposition temperature.ZnS has a wider energy band gap than CdS (2.4 eV), which can enhance the blue response of the photovoltaic cells. These results show the high-quality of these substitute buffer layer materials are prepared through an all-dry technology, which can be used in the manufacture of CIGS thin film solar cells.
SiC films were prepared by heating polystyrene/Si(111) in normal pressure argon atmosphere at different temperatures. The films were investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and Fourier transform infrared absorption measurements. The thicknesses of SiC films were calculated from FTIR spectra. The growth kinetics of the growth process of SiC films were investigated as well. The thicknesses of the SiC films grown for 1 h with increasing growth temperatures have different trends in the three temperature ranges: increasing slowly (1200-1250 oC), increasing quickly (1250-1270 oC), and decreasing (1270-1300 oC). The apparent activation energies of the growth process of SiC films in the three ranges were calculated to be 122.5, 522.5, and -127.5 J/mol respectively. Mechanisms of the different growth processes were discussed. The relation between film thicknesses and growth temperatures indicated that the growth process was a 2D mechanism in the first range and 3D mechanism in the second range. In the third range, the thicknesses of SiC films were decreased by the volatility of Si and C atoms.
A novel ambient negative corona discharge ion source with mini line-cylinder electrodes is designed. The diameters of inner and outer electrode are 0.16 and 4 mm respectively. With a special assembly method, a perfect coaxiality of the two electrodes is obtained. An injection system utilizing a temperature control technique, achieves a constant and stable concentration of the sample, which is critical to the experiment. The formulas of the corona onset voltage of line-cylinder electrodes are also introduced. The experiment results show that negative substances such as formic acid and acetic acid can be ionized under ambient conditions. When combined with micro electrical mechanical system fabrication process, the volume of the ion source can be reduced dramatically, but there is an undesirable surface discharge. To solve the surface discharge problem, an improved structure was designed and tested. The simplicity of the interface of the ion source makes it suitable for mass spectrometer, micro mass spectrometer, ion mobility spectrometer, and high-field asymmetric waveform ion mobility spectrometer applications
Fragmentations of N2 in linearly polarized femtosecond 410 and 820 nm intense laser fields were studied by using the velocity mapping technique. Different behaviors of N2 at 410 and 820 nm were observed. Both the kinetic energy distributions and angular distributions of fragment ions in 410 nm field show weak dependency on laser intensities in the non-saturation regime, in contrast to the case in 820 nm. Different excited electronic states, i.e.,non-Coulombic potentials populated via vertical excitation, are suggested to play crucial roles in fragmentations at short wavelength.