2012 Vol. 25, No. 1

Content
2012, 25(1): 0-0. doi: 10.1088/1674-0068/25/1/0-0
The A-band resonance Raman spectra of thiourea were obtained in water and acetonitrile so-lution. B3LYP/6-311++G(3df,3pd) and RCIS/6-311++G(3df,3pd) calculations were done to elucidate the ultraviolet electronic transitions, the distorted geometry structure and the saddle point of thiourea in 21A excited state, respectively. The resonance Raman spectra were assigned. The absorption spectrum and resonance Raman intensities were modeled using Heller's time-dependent wavepacket approach to resonance Raman scattering. The re-sults indicate that largest change in the displacement takes place with the C=S stretch mode ν6 (|△|=0.95) and noticeable changes appear in the H5N3H6+H8N4H7 wag ν5 (|△|=0.19), NCN symmetric stretch+C=S stretch+N3H6+H8N4 wag ν4 (|△|=0.18), while the moderate intensities of 2ν15 and 4ν15 are mostly due to the large excited state frequency changes of ν15, but not due to its significant change in the normal mode displacement. The mechanism of the appearance of even overtones of the S=CN2 out of plane deformation is explored. The results indicate that a Franck-Condon region saddle point is the driving force for the quadric phonon mechanism within the standard A-term of resonance Raman scattering, which leads to the pyramidalization of the carbon center and the geometry distortion of thiourea molecule in 21A excited state.
Photon induced dissociation investigations of neutral tyramine and dopamine are carried out with synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. At low photon energy, only molecular ions are measured by virtue of near-threshold photoionization. While increasing photon energy to 11.7 eV or more, four distinct fragment ions are obtained for tyramine and dopamine, respectively. Besides, the ioniza-tion energies of tyramine and dopamine are determined to be 7.98±0.05 and 7.67±0.05 eV by measuring the photoionization efficiency curves of corresponding molecular ions. With help of density function theory calculations, the detailed fragmentation pathways are es-tablished as well. These two molecular cations have similar aminoethyl group elimination pathways, C7H8O2(m/z=124) and C7H8O(m/z=108) are supposed to be generated by the McLafferty rearrangement via γ-hydrogen (γ-H) shift inducing β-fission. And CH2NH2+ is proposed to derive from the direct fission of C7-C8 bond. Besides, the McLafferty rear-rangement and the C7-C8 bond fission are validated to be dominant dissociation pathways for tyramine and dopamine cations.
Ferromagnetism is induced in pure TiO2 single crystals by oxygen ion irradiation. The ferro-magnetism is observed up to room temperature and is with weak temperature dependence. By combining X-ray diffraction, Rutherford backscattering/channelling, Raman scattering, and electron-spin resonance spectroscopy, supperconducting quantum interference device, displacement per atom, we measured the lattice damage accumulation with increasing flu-ences. A defect complex, i.e., Ti3+ on the substitutional accompanied by oxygen vacancies, has been identified in the irradiated TiO2. This kind of defect complex results in a local (TiO6-x) stretching Raman mode. We elucidate that Ti3+ with one unpaired 3d electron provide the local magnetic moments.
A series of CH2, NH, O, and Se substituted 2,1,3-benzothiadiazole derivatives have been de-signed and investigated computationally to elucidate their potential as organic light-emitting materials for organic light-emitting diodes. Both ab initio Hartree-Fock and hybrid density functional methods are used. It is found that adjusting the central aromatic ring by replacing S by CH2, NH, O, and Se makes it possible to fine-tune the electronic, optical, and charge transport properties of the pristine molecule.
We investigate the dynamics of resonant Raman scattering in the course of the frequency de-tuning. The dephasing in the time domain makes the scattering fast when the photon energy is tuned from the absorption resonance. This makes frequency detuning to act as a cam-era shutter with a regulated scattering duration and provides a practical tool of controlling the scattering time in ordinary stationary measurements. The theory is applied to resonant Raman spectra of a couple of few-mode model systems and to trans-1,3,5-hexatriene and guanine-cytosine (G-C) Watson-Crick base pairs (DNA) molecules. Besides some particular physical effects, the regime of fast scattering leads to a simplification of the spectrum as well as to the scattering theory itself. Strong overtones appear in the Raman spectra when the photon frequency is tuned in the resonant region, while in the mode of fast scattering, the overtones are gradually quenched when the photon frequency is tuned more than one vibra-tional quantum below the first absorption resonance. The detuning from the resonant region thus leads to a strong purification of the Raman spectrum from the contamination by higher overtones and soft modes and purifies the spectrum also in terms of avoidance of dissociationand interfering fluorescence decay of the resonant state. This makes frequency detuning a very useful practical tool in the analysis of the resonant Raman spectra of complex systems and considerably improves the prospects for using the Raman effect for detection of foreign substances at ultra-low concentrations.
The atomic geometries, electronic structures, and formation energies of neutral nitrogen im-purities in ZnO have been investigated by first-principles calculations. The nitrogen impuri-ties are always deep acceptors, thus having no contributions to p-type conductivity. Among all the neutral nitrogen impurities, nitrogen substituting on an oxygen site has the lowest formation energy and the shallowest acceptor level, while nitrogen substituting on a zinc site has the second-lowest formation energy in oxygen-rich conditions. Nitrogen interstitials are unstable at the tetrahedral site and spontaneously relax into a kick-out configuration. Though nitrogen may occupy the octahedral site, the concentrations will be low for the high formation energy. The charge density distributions in various doping cases are discussed, and self-consistent results are obtained.
A detailed analysis of the stability and flipping dynamics of a delayed exclusive toggle switch is performed. We use forward flux sampling method combined with delayed stochastic sim-ulation algorithm to get the stationary distribution function, the switching rate, and path-ways, as well as the transition state ensemble. Interestingly, under the influence of time delay, the stationary distribution corresponding to the stable states become narrower and the population in the transition region is significantly enhanced. In addition, the flipping rate increases monotonically with delay. Such findings demonstrate that time delay could reduce the stability of the bistable genetic switch dramatically. Furthermore, the transition pathways, characterized by the difference in the protein numbers and the state of operator, show larger discrepancy between the forward and backward switching process with increas-ing delay, indicating that transcriptional and translational delay can remarkably affect the flipping dynamics. Specifically, for the transition state, the difference in the probability of finding the operator site bound by the two different protein dimers is enlarged by delay, which further illustrates the crucial role of time delay on the stability and switching dynamics of genetic toggle switches.
The NO2 molecule adsorption on B12N12 nano-cage was investigated using density func-tional theory calculations in terms of adsorption energy, HOMO/LUMO energy gap (Eg) changes, charge transfer, structural deformation, etc. Furthermore, some aspects of stability and properties of B12N12 including calculation of binding electronic and Gibbs free energies, density of states, and molecular electrostatic potential surfaces are investigated. Three pos-sible configurations for NO2 adsorption on the B12N12 nano-cage are energetically found. Interestingly, the results reveals that the Eg of B12N12 cluster is very sensitive to the pres-ence of NO2 molecules as its value reduces from 6.84 eV in free cluster to 3.23 eV in the most stable configuration of NO2/cluster complex. This phenomenon dramatically increases the electrical conductivity of the cluster, suggesting that the B12N12 nano-cluster may be potential sensor for NO2 gaseous molecule detection.
On basis of bond dissociation energies (BDEs) for BH2, B(OH)2, BCl2, and BCl, the diffusion Monte Carlo (DMC) method is applied to explore the BDEs of HB-H, HOB-OH, ClB-Cl, and B-Cl. The effect of the choice of orbitals, as well as the backflow transformation, is studied. The Slater-Jastrow DMC algorithm gives BDEs of 359.1±0.12 kJ/mol for HB?H, 410.5±0.50 kJ/mol for HOB-OH, 357.8±1.46 kJ/mol for ClB-Cl, and 504.5±0.96 kJ/mol for B-Cl using B3PW91 orbitals and similar BDEs when B3LYP orbitals are used. DMC with backflow corrections (BF-DMC) gives a HB-H BDE of 369.9±0.12 kJ/mol which is close to one of the available experimental value (375.8 kJ/mol). In the case of HOB-OH BDE, the BF-DMC calculation is 446.0±1.84 kJ/mol that is closer to the experimental BDE. The BF-DMC BDE for ClB-Cl is 343.2±2.34 kJ/mol and the BF-DMC B-Cl BDE is 523.3±0.33 kJ/mol, which are close to the experimental BDEs, 341.9 and 530.0 kJ/mol, respectively.
We investigate the impact of coupling on the reliability of the logic system as well as the logical stochastic resonance (LSR) phenomenon in the coupled logic gates system. It is found that compared with single logic gate, the coupled system could yield reliable logic outputs in a much wider noise region, which means coupling can obviously improve the reliability of the logic system and thus enhance the LSR effect. Moreover, we find that the enhancement is larger for larger system size, whereas for large enough size the enhancement seems to be saturated. Finally, we also examine the effect of coupling strength, it can be observed that the noise region where reliable logic outputs can be obtained evolves non-monotonically as the coupling strength increases, displaying a resonance-like effect.
Systematic search of the potential energy surface of tetrapeptide glycine-phenylalanine-glycine-glycine (GFGG) in gas phase is conducted by a combination of PM3, HF and BHandHLYP methods. The conformational search method is described in detail. The rela-tive electronic energies, zero point vibrational energies, dipole moments, rotational constants, vertical ionization energies and the temperature dependent conformational distributions for a number of important conformers are obtained. The structural characteristics of these conformers are analyzed and it is found that the entropic effect is a dominating factor in determining the relative stabilities of the conformers. The measurements of dipole moments and some characteristic IR mode are shown to be effective approaches to verify the theoreti-cal prediction. The structures of the low energy GFGG conformers are also analyzed in their connection with the secondary structures of proteins. Similarity between the local structures of low energy GFGG conformers and the α-helix is discussed and many β- and γ-turn local structures in GFGG conformers are found.
Hydrogenation and ammoniation of SrTiO3 (STO), a normal ultraviolet photocatalyst, were performed by annealing STO〈100〉in H2:N2=5%:95% and NH3, respectively, at various tem-peratures T. It was found that hydrogenation at T≥900 oC remarkably enhanced the UV photocatalytic ability of STO, but the visible-light photocatalysis was still unavailable, while ammoniation at T≥800 oC introduced the N doping, resulting in visible-light photocat-alytic activity. Furthermore, when a hydrogenated STO was subjected to ammoniation, the visible-light photocatalytic ability was nearly the same as that of the ammoniated one; but the hydrogenation of an ammoniated one significantly enhanced visible-light photocatalysis, indicating a synergetic effect of hydrogenation and ammoniation. Discussions and identifi-cations have been made to analyze these results.
A novel titanium dioxide (TiO2) film comprising both nanotubes and nanopaticles was fab-ricated by an anodization process of the modified titanium. The local electric field at the anodized surface was simulated and its influence on the morphology of the TiO2 film was discussed. The results show that the electric field strength is enhanced by the covering. The growth rate of TiO2 increases with the assist of the local electric field. However, TiO2 dis-solution is hindered since the local electric field prevents [TiF6]6- from diffusing. It means that the balance condition for the formation of nanotubes is broken, and TiO2 nanoparticles are formed. Moreover, the crystal structure of the TiO2 film was confirmed using X-ray diffraction and Raman analysis. The anatase is a main phase for the proposed film.
Lanthanum doped mesoporous titanium dioxide photocatalysts with different La content were synthesized by template method using tetrabutyltitanate (Ti(OC4H9)4) as precursor and Pluronic P123 as template. The catalysts were characterized by thermogravimetric dif-ferential thermal analysis, N2 adsorption-desorption measurements, X-ray diffraction, and UV-Vis adsorption spectroscopy. The effect of La3+ doping concentration from 0.1% to 1% on the photocatalytic activity of mesoporous TiO2 was investigated. The characterizations indicated that the photocatalysts possessed a homogeneous pore diameter of about 10 nm with high surface area of 165 m2/g. X-ray photoelectron spectroscopy measurements in-dicated the presence of C in the doped samples in addition to La. Compared with pure mesoporous TiO2, the La-doped samples extended the photoabsorption edge into the visible light region. The results of phenol photodecomposition showed that La-doped mesoporous TiO2 exhibited higher photocatalytic activities than pure mesoporous TiO2 under UV and visible light irradiation.
Co3O4/SiO2 catalysts for CO oxidation were prepared by conventional incipient wetness impregnation followed by calcination at various temperatures. Their structures were char-acterized with X-ray diffraction (XRD), laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR) and X-ray absorption fine structure (XAFS) spectroscopy. Both XRD and Raman spectroscopy only detect the ex-istence of Co3O4 crystallites in all catalysts. However, XPS results indicate that excess Co2+ ions are present on the surface of Co3O4 in Co3O4(200)/SiO2 as compared with bulk Co3O4. Meanwhile, TPR results suggest the presence of surface oxygen vacancies on Co3O4 in Co3O4(200)/SiO2, and XAFS results demonstrate that Co3O4 in Co3O4(200)/SiO2 con-tains excess Co2+. Increasing calcination temperature results in oxidation of excess Co2+ and the decrease of the concentration of surface oxygen vacancies, consequently the for-mation of stoichiometric Co3O4 on supported catalysts. Among all Co3O4/SiO2 catalysts,Co3O4(200)/SiO2 exhibits the best catalytic performance towards CO oxidation, demon-strating that excess Co2+ and surface oxygen vacancies can enhance the catalytic activity of Co3O4 towards CO oxidation. These results nicely demonstrate the effect of calcination temperature on the structure and catalytic performance towards CO oxidation of silica-supported Co3O4 catalysts and highlight the important role of surface oxygen vacancies on Co3O4.
Hydrogenated silicon nitride films as an effective antireflection and passivation coating of silicon solar cell were prepared on p-type polished silicon substrate (1.0 Ωcm) by direct LF-PECVD (low frequency plasma enhanced chemical vapor deposition) of Centrotherm. The preferable passivation effect was obtained and the refractive index was in the range of 2.017-2.082. The refractive index of the hydrogenated silicon nitride films became larger with the increase of the pressure. Fourier transform infrared spectroscopy was used to study the pressure influence on the film structural properties. The results highlighted highhydrogen bond and high Si-N bonds density in the film, which were greatly influenced by the pressure. The passivation effect of the films was influenced by the Si dangling bonds density. Finally the effective minority liftetime degradation with time was shown and discussed by considering the relationship between the structural properties and passivation.
CoFe2O4/Pb(Zr0.53Ti0.47)O3 (CFO/PZT) magnetoelectric composite thin films of 2-2 type structure had been prepared onto Pt/Ti/SiO2/Si substrate by a sol-gel process and spin coat-ing technique. The structure of the prepared thin film is substrate/PZT/CFO/PZT/CFO. Two CFO ferromagnetic layers are separated from each other by a thin PZT layer. The upper CFO layer is magnetostatically coupled with the lower CFO layer. Subsequent scan-ning electron microscopy (SEM) investigations show that the prepared thin films exhibit good morphologies and compact structure, and cross-sectional micrographs clearly display a multilayered nanostructure of multilayered thin films. The composite thin films exhibit both good magnetic and ferroelectric properties. The spacing between ferromagnetic layers can be varied by adjusting the thickness of intermediate PZT layer. It is found that the strength of magnetostatic coupling has a great impact on magnetoelectric properties of composite thin films, i.e., the magnetoelectric voltage coefficient of composite thin film tends to increase with the decreasing of pacing between two neighboring CFO ferromagnetic layers as a result of magnetostatic coupling effect.
Core-shell structured SiO2/poly(N-isopropylacrylamide) (SiO2/PNIPAM) microspheres were successfully fabricated through hydrolysis and condensation reaction of tertraethyl or-thosilicate (TEOS) on the surface of PNIPAM template at 50 oC. The PNIPAM template can be easily removed by water at room temperature so that SiO2 hollow microspheres were finally obtained. The transmission electron microscope and scanning electron microscope observations indicated that SiO2 hollow microspheres with an average diameter of 150 nm can be formed only if there are enough concentration of PNIPAM and TEOS, and the hy-drolysis time of TEOS. FTIR analysis showed that part of PNIPAM remained on the wall of SiO2 because of the strong interaction between PNIPAM and silica. This work provides a clean and efficient way to prepare hollow microspheres.
Chinese abstract