Can Xu

Abstract: Hydrated MgO nanotube clusters are constructed and studied by the density functional theory at the B3LYP/6-31G(d) level. A strong exothermicity chemisorption reactivity of MgO nanotube clusters with water, which releases 137.5-171.8 kJ/mol. The averaged charge of Mg ions is steady, and presents a stronger ionic bonding. Mg ions are more sensitive to the coordination number. For the reaction of water onto clusters, electronic properties of hydrated clusters have remarkable change compared with anhydrous clusters. (c) 2008 Elsevier B.V. All rights reserved.

Abstract: Hydrated MgO nanotube clusters are constructed and studied by the density functional theory at the B3LYP/6-31G(d) level. A strong exothermicity chemisorption reactivity of MgO nanotube clusters with water, which releases 137.5-171.8 kJ/mol. The averaged charge of Mg ions is steady, and presents a stronger ionic bonding. Mg ions are more sensitive to the coordination number. For the reaction of water onto clusters, electronic properties of hydrated clusters have remarkable change compared with anhydrous clusters. (c) 2008 Elsevier B.V. All rights reserved.

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Abstract: Finite magnesium oxide nanotubes are investigated. Stacks of squares and hexagons of MgO clusters are constructed and studied by the density-functional theory (OFT). Optimized structures are slightly distorted stacks of polygons. Frequencies of radial breathing modes and radial twisting modes show strong size effect. Raman and infrared activity of the modes present a dependence on parity of layer. The activity can be deduced in the low-dimensional system, based on the symmetry and selection rules. A suggestion for experimental characterization between squares and hexagons cross-sections of nanotubes by Raman and infrared is proposed. (C) 2009 Elsevier B.V. All rights reserved.

Abstract: Pure Au-n (n = 10, 16) and their alloyed clusters Aun-mMm (M = Ag. Cu; m = 1, 2) are theoretically investigated for the structural characteristics, electronic properties, and nonlinear optical properties using density-functional theory (DFT) at B3LYP/LanL2DZ level. Doping exhibits little effect on structural characteristics and energy gaps but enormous augmenting of the second-order nonlinear optical (NLO) coefficients. The primary causation of the enhancement of beta is the lack of centro-symmetry feature in alloyed clusters. Time-dependent density-functional theory (TD-DFT) analysis at the same basis set show doping increases the ground to excited state transition electric dipole moments and transition dipoles, which make important roles in enhancing the second-order NLO coefficients. Increasing number of M atoms increases the NLO coefficients of clusters because the M atoms act as electron donors in alloyed clusters. Increasing the length of clusters can also augment the beta values in our investigation. The reported effect can be utilized to design effective gold-based alloyed Clusters for NLO applications. (C) 2008 Elsevier B.V. All rights reserved.

Abstract: Photonic bands in two-dimensional metallodielectric (MD) periodic systems composed of metal coated cylinders are investigated theoretically based on frequency dependent plane-wave expansion method. For the case of E-polarization, although the thickness of metal coating is less than half of the cylinder�s radius, most of MD photonic bands are the same as photonic bands composed of pure metal cylinders. This property provides us with a way to substitute metal photonic crystals with MD photonic crystals in many applications. In addition, flatbands are discovered in MD photonic band structures, which can be tuned by changing the thickness of metal coating while other photonic bands do not change their positions. For the case of H-polarization, the lowest frequency band gap (between the first and the second bands) can open up when the thickness of metal coating is thick enough. According to approximate calculation based on Maxwell�Garnett type effective medium theory and comparison with recent studies on three-dimensional MD photonic band structures, we predict that the lowest frequency band gap is not because of Bragg scattering but result from the individual metal coated dielectric cylinders, so that the gap is independent on geometry of photonic crystal lattices. Then, numerical calculation validates that our prediction is right.

Abstract: Raman vibration spectra of nanoscaled anhydrous and hydrous (SiO2)(n) (n = 6-20) of single-line, double-line and ring conformations have been calculated using the density functional theory (B3LYP) at 6-31g(d) level. Two specific Raman vibration modes are chosen to demonstrate effect of polar groups. Raman frequency shift law is interpreted successfully using the model of the simple harmonic oscillator combining with the cascade and parallel connection principle. Polarization ratios suggested that polar groups' influence is quite distinct on the different direction due to orientation. (c) 2007 Elsevier B.V. All rights reserved.

Abstract: Complex photonic band structures in two-dimensional photonic crystals composed of high-temperature superconductors (HTSCs) for the case of E-polarization are calculated as a function of temperature below T-c, where T-c is the critical temperature of the HTSC. The calculations are based on a temperature dependent complex dielectric function, which includes contributions of both superconducting electrons (SCEs) and normal conducting electrons (NCEs), and a frequency dependent plane wave expansion method. Both temperature independent and temperature dependent damping term in the dielectric function are considered to calculate dispersion relations and the lifetime of the eigenmodes. The results are compared with those obtained by existing calculation methods, which neglect the contribution of NCEs. Our results correspond quite well with those obtained by the existing method in the low-temperature range 0 < T/T-c <= 0.3; however, results obtained with the two methods are quite different in the temperature range 0.3 < T/T-c <= 1. We demonstrate that the contribution of NCEs is non-negligible with increasing temperature.

Abstract: The radial breathing modes (RBMs) of (MgO)(n) and (BeO)(n) rings (n=3-10) were calculated using the density functional theory at B3LYP/6-31G(d) level. It was found that for large rings, the radial breathing mode (RBM) frequency was inversely proportional to the centre diameter, but the variation of bond length may lead to deviations from a linear behavior. The deviations caused by inverse cubic term of diameter and variation of bond length, became dramatic with the decrease of ring diameter. From the point of chemical bond view, using one-dimensional harmonic oscillator and the method of cascade and parallel connection of "springs", the linear relation and deviations were explained. The model can be applied to nanotubes.

Abstract: The IR and Raman spectra, nonlinear optical properties of MgO nanotube Clusters are studied by density-functional theory at B3LYP/6-31G(d) level. The IR spectra are match closely to those in the corresponding MgO cluster and bulk materials. The strongest peaks of the IR spectra are located in the range front 650 to 750 cm(-1). The Raman spectra are very sensitive to structural variations in MgO clusters, and redshift of vibrational frequency is observed in Raman spectra as increasing cluster length. The motion of the strongest peaks in spectra is discussed. The total dipole moment and the first hyperpolarizabilities oscillate between zero and a constant when the layer is grown for the layer dependence of symmetry in MgO nanotube clusters. (C) 2008 Elsevier B.V. All rights reserved.

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Abstract: The vibration spectrum of quasi ID (one dimensional) nanosize lines was calculated using the density function theory (DFT)/B3LYP method at 6-31G(d) level. The results showed that the vibration frequency and intensity of IR and Raman spectra obviously indicated even-odd oscillation with the number of SiO2 units. For different vibration modes, the even-odd oscillation was strong or weak because of the effect of the polar group's orientation relative to silicon-oxygen two-member ring. Moreover, the analyses of the polarization indicated the effect of hydroxyl on even-odd oscillation. ���氧��(SiO2)nO2H4�两����: ��笼�����������, 并��������, ��度���论�B3LYP���6-31G(d)��水平�计������n=2�22(n���)������平�������以�����次差�. ��计�����, 笼������n=4�8���幻���(���已����), �����n=14����类似�幻���; 此�, �(SiO2)n������, (SiO2)nO2H4��������稳���n=4��大�����, ���水���对�氧���稳�������影�.

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Abstract: We have studied silica single and double chain based on two-membered ring and nanotubes based on three-membered ring using the density-functional theory at 6-31G(d) level. The average binding energy, electronic structure and vibration frequency were calculated. Our calculations show that the average binding energy and energy gap monotonically change with increasing SiO2 units. Infrared spectrum indicated that the frequency and intensity of radial and tangential vibration shift obviously. Radial vibration frequency shifts to high frequency while tangential vibration frequency shifts to low frequency showing the existence of quantum size effect and anisotropic property of vibration in SiO2 nanomaterial. ���度��计�, 对�水(SiO2-NTs)�水�(SiO2-WNTs)�SiO2纳米管���������计�, �究��尺寸���平�����红��谱. SiO2-WNTs中���纳米管�����度���平������. SiO2-WNTs�SiO2-NTs����������稳�. 对��为����纳米管红��谱�����, ��峰��������, ���������示�强�尺寸��. SiO2-WNTs��维SiO2纳米管�������模�.

Abstract: Two new structures of cage (terminal cage) and ring for silica clusters (SiO2)(n)O2H4 (n=2 similar to 22, n is even) are presented and compared with line structure. Geometric structures, average binding energies, energy gaps and second order difference of energy are systematically studied by density function theory (DFT) B3LYP with basis set 6-31G(d). The results indicate that for cage structures of (SiO2)(n)O2H4 (n=2 similar to 22, n is even) magic number clusters exist not only at n=4, 8, but also at n=14. Ring structures of (SiO2)(n)O2H4 clusters are different from that of (SiO2)(n). For the latter they are more stable than line structures from n=11. However, for the former it is from n=4 ring structures begin to be more stable than line structures. It means that the addition of water has an important effect on the stabilities of silica clusters. ���氧��(SiO2)nO2H4�两����: ��笼�����������, 并��������, ��度���论�B3LYP���6-31G(d)��水平�计������n=2�22(n���)������平�������以�����次差�. ��计�����, 笼������n=4�8���幻���(���已����), �����n=14����类似�幻���; 此�, �(SiO2)n������, (SiO2)nO2H4��������稳���n=4��大�����, ���水���对�氧���稳�������影�.

Abstract: The magic number silica clusters [(SiO2)(n)O2H3](-) with n = 4 and 8 have been observed in the XeCl excimer laser (308 nm) ablation of various porous siliceous materials. The structural origin of the magic number clusters has been studied by the density functional theoretical calculation at the B3LYP/6-31G** level, with a genetic algorithm as a supplementary tool for global structure searching. The DFT results of the first magic number cluster are parallel to the corresponding Hartree-Fock results previously reported with only small differences in the structural parameters. Theoretical calculation predicts that the first magic number cluster (SiO2)(4)O2H4 and its anion [(SiO2)(4)O2H3](-) will most probably take pseudotetrahedral cage-like structures. To study the structural properties of the second magic number cluster, geometries of the bare cluster (SiO2)(8), the neutral complex cluster (SiO2)(8)O2H4, and the anionic cluster [(SiO2)(8)O2H3](-) are fully optimized at the B3LYP/6-31G** level, and the corresponding vibrational frequencies are calculated. The DFT calculations predict that the ground state of the bare silica octamer (SiO2)(8) has a linear chain structure, whereas the second magic number complex cluster (SiO2)(8)O2H4 and its anion [(SiO2)(8)O2H3](-) are most probably a mixture of cubic cage-like structural isomers with an O atom inside the cage and several quasi-bicage isomers with high intercage interactions. The stabilization of these structures can also be attributed to the active participation of the group of atoms 2O and 4H (3H for the anion) in chemical bonding during cluster formation. Our theoretical calculation gives preliminary structural interpretation of the presence of the first and second magic number clusters and the absence of higher magic numbers. (c) 2005 Wiley Periodicals, Inc.

Abstract: With selective excitation around BChl-B800 and BChl-B850 absorption bands, we observed the evolution of excited-state dynamics in LH2 from Rhodobacter sphaeroides 601. The dynamical traces demonstrate a dominant excited-state absorption (ESA) followed concomitantly by an ultrafast transmission increase and decay with pulse-width limited time scale at 818 nm and 828 nm excitation. The ESA occurring prior to excitonic thermalization or ground-state bleach was observed at 840 nm as well. These experimental results indicate the competition between the transition from excitonic states to higher-lying excited states and interexciton relaxation, which are of physical significance for understanding excitation transfer and related mechanisms in LH2. (C) 2002 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.

Abstract: The generation of silicon dioxide-aluminum oxide compound clusters under 308 nm XeCl excimer laser ablation of ZSM-5 zeolites with different SiO2/Al2O3 ratios was studied by time-of-flight mass spectrometry. Two Al-containing cluster sequences [(SiO2)(n-1)(AlO2)] and [(SiO2)(n)OAl](-) are observed in the negative ion channel. Through the relationship between the abundance distribution of cluster sequences and the silicon aluminum ratio, the growth mechanism of [(SiO2)(n-1)(AlO2)]. is discussed. Owing to the high electron affinity of AlO2 compared to small (SiO2)(n) clusters, AlO2 is considered to be the core of growth of the compound cluster [(SiO2)(n-1)(AlO2)](-). (C) 2000 Elsevier Science B.V. All rights reserved.

Abstract: Desorption of neutral oxygen atoms induced by 266 nm laser pulses from pressure pressed as-prepared (AP) sample and from high temperature (873 K, 40 h) pretreated (HT) sample is investigated by time-of-flight (TOF) mass spectrometric analysis. The obtained TOF data are analyzed by Maxwell-Boltzmann energy distributions. It is concluded that desorption of neutral oxygen atoms occurs from oxygen defect sites via photochemical processes, followed by some subsequent thermal quenching processes. Kinetic energies of desorbed species are reflected by the sub-band structure in the band gap of TiO2. Band gap energies of the AP sample and the HT sample are estimated to be 2.02 and 2.37 eV, respectively, in accordance with values obtained from previous theoretical calculations. The surface of the HT sample was studied with ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) to elucidate laser irradiation effects.

Abstract: The generation of magic number silica clusters [(SiO2)(n)O2H3](-) with n = 4 and 8 by XeCl laser (308 nm) ablation of porous siliceous materials is reported. The production of magic cluster [(SiO2)(4)O2H3](-) can be enhanced by sample selection and experimental optimization so that it becomes the most prominent species in silica clusters. To study the structure of the magic cluster [(SiO2)(4)O2H3](-), we performed structural optimization for the neutral bare cluster (SiO2)(4), the neutral complex cluster (SiO2)(4)O2H4, and the anionic cluster [(SiO2)(4)O2H3](-) at the HF/6-31G** level. It was found that the ground state of the bare silica tetramer has a linear chain structure whereas a pseudotetrahedral cage-like structural isomer of Sq symmetry is most stable for the complex cluster (SiO2)(4)O2H4 The stabilization of the three-dimensional (3D) structure can be attributed to the active participation of the O2H4 group in chemical bonding during cluster formation. Our theoretical calculation and bonding analysis indicate that the magic number anionic cluster [(SiO2)(4)O2H3](-) might also take a pseudotetrahedral structure similar to (but with a different symmetry) that of the neutral precursor (SiO2)(4)O2H4 as the ground state in which the valence, coordination, and bonding characteristics of all the constituent atoms are nearly fully satisfied.

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Abstract: -based materials of different porosity and surface states have been studied by time-of-flight mass spectrometry (TOF-MS) under 308 nm excimer laser ablation. Sequences of SiO2-based negative ion clusters [(SiO2)nX]- are observed and variations of their abundance distributions with sample properties are studied. The unique roles of surface states in cluster formation are discussed.

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Abstract: The clustering dynamics of SiO2-containing negative ion clusters generated from ultrafine amorphous SiO2 powders with high specific surface area is investigated by XeCl excimer laser ablation time-of-flight mass spectrometry (TOF-MS). From the evolution of the mass spectra with the delay time and the number of laser irradiation shots, the key roles played by OH and OH- fragments from the surface silanol groups in the formation of SiO2-containing negative ion clusters are discussed.

Abstract: We report photoluminescence (PL) and time-resolved PL spectroscopic measurements of PVK/C60 combination film fabricated by a new physical jet deposition technique by which PVK and C60 are deposited as separate or mixed molecular layers resulting in an effective charge transfer between these two kinds of molecules. The decay time of PVK PL emission at 630 nm decreases and that of C60 PL emission around 730 nm increases in both the mixed and multilayer PVK/C60 film compared with those in pure PVK and C60. These results could be explained by the effective charge transfer and excitation transfer processes happening in PVK/C60 combination films.