Our Publications

  • Chemisorption of Hydrogen Molecule on Axially Strained (8, 0) Carbon Nanotube. Bin Zhou, Wanlin Guo and Yitao Dai. J. Phys. Chem. C, 2008, 112 (47), 18516–18520

    Bin Zhou, Wanlin Guo and Yitao Dai

    J. Phys. Chem. C, 2008, 112 (47), pp 18516–18520

    The axial strain is shown to have effective influence on the behaviors of hydrogen chemisorption/desorption on the (8, 0) carbon nanotube by density functional simulations. For the chemisorption/desorption pathway with lowest energy barrier in our work, a compressive strain of 7% can lower the energy barrier of the chemisorption (desorption) from 1.98 (1.64) eV to 1.56 (1.20) eV, while the tensile strain increases the energy barrier but makes the chemisorption configuration (C1C4) become more stable compared to the corresponding physisorption configuration. The hydrogen adsorption and release of the most stable chemisorption state (C2C3) are based on the transition between the C2C3 and C1C4 configurations. However, the influence of the axial strain on the hydrogen diffusion behavior which controls the transition between C2C3 and C1C4 configurations is found to be weak.

  • Freestanding (3,0) boron nitride nanotube: Expected to be stable well over room temperature. Zhuhua Zhang, Wanlin Guo, and Yitao Dai. Appl. Phys. Lett., 2008, 93, 223108.

    Zhuhua Zhang, Wanlin Guo, and Yitao Dai

    Appl. Phys. Lett. 93, 223108 (2008); doi:10.1063/1.3040007 (3 pages)

    By semiempirical molecular dynamics simulations and ab initio total energy calculations, the freestanding (3,0) boron nitride nanotube (BNNT) with a diameter of 2.7 Å is expected to be stable well over room temperature with remarkably higher stability than the experimentally reported (2,2) carbon nanotube. We elucidate the underlying physics by examining the variation in surface dipole and charge reordering driven by geometrical relaxation. In addition, the (3,0) BNNT can become globally stable when encapsulated in a larger BNNT.

  • Three-dimensional stress fields near notches and cracks. Chongmin She, Junhua Zhao, and Wanlin Guo. Int. J. Fract., 2008, 151, 151-160.

    Chongmin She, Junhua Zhao, and Wanlin Guo.

    Int. J. Fract., 2008, 151, 151-160.

    Structures with notches have a tendency to develop critical crack growth because of the stress concentration. The strength of the structures with stress gradient usually shows strong three-dimensional (3D) effects even under in-plane loading. The commonly two-dimensional (2D) models for fracture assessments of bodies with notches and cracks may lead to inaccurate predictions when in-plane loading is applied to certain 3D geometries. In the paper, the recent researches on the 3D effects of stress concentrations at notches and 3D stress fields of cracks are summarized. A new concept of equivalent thickness of the point on the crack front line is proposed based on the detailed analyses of the 3D out-of-plane stress fields of cracks, and then new empirical formulae of the 3D out-of-plane stress constraint factor T z of I–II mixed-mode cracks are obtained by use of the equivalent thickness. Combining the T z with the in-plane constraint parameters T or Q, the 3D multi-parameter descriptions of the stress field in front of various types of cracks using K–T z, J–T z, K–T–T z and J–Q–T z combination can be formulated.

    Keywords Three-dimensional effect - Notch - Crack - Constraints - Equivalent thickness - Mixed-mode cracks

  • Stress intensity factors for the inner generative crack induced by the out-of-plane stress in front of the main through-the-thickness crack. Chongmin She, and Wanlin GUo. Acta Mechanica, 2008, 200, 45-57.

    Chongmin She, and Wanlin GUo.

    Acta Mechanica, 2008, 200, 45-57.

    The problem of the inner generative crack induced by the out-of-plane stress in front of the main through-the-thickness crack is discussed. The crack front of the inner generative crack is usually perpendicular to that of the main crack in the high strength pipeline steel. A three-dimensional (3D) semi-analytical method to estimate the stress intensity factor (SIF) of the inner generative crack has been proposed, based on the 3D two-parameter K-T z approach and Bueckner's principle of superposition. Detailed comparison of the 3D semi-analytical, finite element (FE) and the corresponding plane strain solutions has been performed. It was shown that the 3D semi-analytical solutions agree well with the 3D FE results and they both are less conservative than the planar solutions. The influence of the inner generative crack on the SIF of the main crack was also analyzed.

  • Sonochemistry-assisted microwave synthesis and optical study of single-crystalline CdS nanoflowers. Guoan Tai and Wanlin Guo. Ultrasonics Sonochemistry, 2008, 15, 350–356.

    Guoan Tai and Wanlin Guo

    Ultrasonics Sonochemistry 15 (2008) 350–356

    Well-defined flower-like CdS nanostructures have been synthesized by applying ultrasound and microwave simultaneously, which consist of hexagonal nanopyramids and/or nanoplates depending on different sulfur sources. It is shown that the synergistic effect of microwave and sonochemistry is the main mechanism for the formation of the nanoflowers. Optical characterization of the nanoflowers shows a large blue-shift up to 100 nm in comparing with simple low-dimensional CdS nanostructures. This structure induced shift in optical properties may have potential applications in optoelectronics devices, catalysis, and solar cells.

    Keywords: Sonochemistry; Microwave; CdS; Nanoflowers; Synergistic effect

  • Relevance of Timoshenko-beam model to microtubules of low shear modulus. Y.J. Shia, W.L. Guoa, and C.Q. Ru. Physica E, 2008, 41(2), 213-219.

    Y.J. Shi, W.L. Guo, and C.Q. Ru

    Physica E, Volume 41, Issue 2, p. 213-219

    Microtubules are characterized by extremely low shear modulus that is a few orders of magnitude lower than longitudinal modulus. In this paper, the effects of transverse shearing due to low shear modulus of microtubules are investigated using a Timoshenko-beam model, with detailed comparison between the Timoshenko-beam model, classical isotropic Euler Bernoulli beam model and a more accurate 2D orthotropic elastic shell model. It is confirmed that transverse shearing is mainly responsible for the length-dependent flexural rigidity of an isolated microtubule reported in the literature, which cannot be explained by the widely used Euler Bernoulli beam model. Indeed, the length-dependent flexural rigidity predicted by the Timoshenko-beam model is found to be in good quantitative agreement with known experimental data. In particular, the present Timoshenko-beam model predicts that, because of the length dependence of flexural rigidity, microtubules of different lengths could sustain almost equal maximum axial compressive force against column buckling, a conclusion that could have some interesting consequences to the mechanical behavior of cells. These results recommend that the Timoshenko-beam model offers a unified simple 1D model, which can capture the length dependence of flexural rigidity and be applied to various static and dynamic problems of microtubule mechanics.

    Keywords: Filaments, microtubules, their networks, and supramolecular assemblies, Mechanical properties

  • Convex and concave nanodots and lines induced on HOPG surfaces by AFM voltages in ambient air. Yan Jiang and Wanlin Guo. Nanotechnology, 2008, 19, 345302.

    Yan Jiang and Wanlin Guo

    Nanotechnology 19 (2008) 345302 (6pp)

    Convex and concave nanodots were created on highly oriented pyrolytic graphite (HOPG) in ambient air by applying a voltage pulse between a metal-coated atomic force microscope (AFM) tip and the sample surface. Using a linear scan with a positive substrate bias, nanoscale lines were also etched on the HOPG surface. Depending on the amplitude and duration of the voltage pulse, the nanostructures are either convex or concave. The depth of the concave structure sharply increased with the amplitude and duration of the voltage pulse, while the height of the convexity stayed at a low level and varied in a small range with the voltage lower than a threshold value. Under negative substrate bias or in a vacuum, no change occurred on the HOPG surface in the experimental range. The formation of the nanostructures can be ascribed to the primary dissociative adsorption of water and oxygen in air induced by the intensive hole concentration and the subsequent defect-assisted oxidation of graphite.

  • Ultrahigh Frequency Longitudinal Oscillators from Single-Walled Carbon Nanotubes. Yitao Dai, Wanlin Guo, Chun Li, and Chun Tang. Journal of Computational and Theoretical Nanoscience, 2008, 5, 1372–1376.

    Yitao Dai, Wanlin Guo, Chun Li, and Chun Tang

    Journal of Computational and Theoretical Nanoscience Vol.5, 1372–1376, 2008

    The promising application of single-walled carbon nanotubes as ultrahigh frequency longitudinal oscillators is presented in this paper. A short (3, 3) carbon nanotube is considered as an example and its axial oscillation is investigated by the ab initio molecular dynamics simulations. The effect of the electromechanical coupling on the frequency-domain characteristic is studied. The variation of the electronic structure induced by the geometry deformation produces the anharmonic behavior of the oscillation, while a strong axial electric field hardly affects the eigenfrequencies. A discrete model is also demonstrated to be efficient to describe this oscillation, though the electromechanical coupling effect can not be taken into account, it can predict the fundamental frequency with an error of 0.8% compared with the result of the ab initio molecular dynamics simulation. Additionally, both the discrete model and a continuum hollow rod model are used to predict the fundamental frequencies of the single-walled carbon nanotubes with different lengths and diameters. For a tube shorter than 5 nm, the fundamental frequency is over 1 terahertz.

    Keywords: Ultrahigh Frequency Longitudinal Oscillator, Electromechanical Coupling, Ab initio Molecular Dynamics, Single-Walled Carbon Nanotube

  • Interlayer energy-optimum stacking registry for two curved graphene sheets of nanometer dimensions. Yufeng Guo, and Wanlin Guo. Molecular Simulation, 2008, 34(8), 813-819

    Yufeng Guo; Wanlin Guo

    Molecular Simulation, Volume 34, Number 8, July 2008 , pp. 813-819(7)

    The interlayer energy between two circular graphene sheets of nanometer scale, which is curved into cylindrical shape with different curvature radius, is investigated by a molecular force field based on a registry-dependent interlayer interaction potential. It is found that there is a special interlayer stacking angle near which the interlayer energy is significantly lower. This interlayer energy-minimum stacking registry angle shifts away from the original Bernal (AB) stacking orientation when the curvature radius of curved graphene sheets is less than 20 nm, and increases with decreasing curvature radius beyond this point. The stability of interlayer energy-minimum stacking of the curved graphene sheets decreases with decreasing curvature radius.

    Keywords: curved graphene; interlayer stacking registry; energy-optimum; molecular force field; static calculation

  • Simulation studies of a “nanogun” based on carbon nanotubes. Yitao Dai, Chun Tang and Wanlin Guo. NANO RESEARCH, 2008, 1(2), 176-183.

    Yitao Dai, Chun Tang and Wanlin Guo

    NANO RESEARCH Volume 1, Number 2, 176-183

    Quantum mechanical molecular dynamics simulations show that electrically neutral carbon nanotubes or fullerene balls housed in an outer carbon nanotube can be driven into motion by charging the outer tube uniformly. Positively and negatively charged outer tube are found to have quite different actions on the initially neutral nanotubes or fullerene balls. A positively charged tube can drive out the molecule inside it out at speeds over 1 km/s, just like a “nanogun”, while a negatively charged tube can drive the molecule into oscillation inside it and can absorb inwards a neutral molecule in the vicinity of its open end, like a “nanomanipulator”. The results demonstrate that changing the charge environment in specific ways may open the door to conceptually new nano/molecular electromechanical devices.

    Keywords: Energy conversion - carbon nanotube - neutral molecule - driving mechanisms

  • Tuning field-induced energy gap of bilayer graphene via interlayer spacing. Yufeng Guo, Wanlin Guo, and Changfeng Chen. Appl. Phys. Lett., 2008, 92, 243101.

    Yufeng Guo, Wanlin Guo, and Changfeng Chen

    Appl. Phys. Lett. 92, 243101 (2008); doi:10.1063/1.2943414 (3 pages)

    Our first-principles calculations reveal surprisingly high sensitivity of the field-induced energy gap of bilayer graphene to changes in its interlayer spacing. Small adjustments in the interlayer spacing near its equilibrium value produce large modulations in the gap over a wide range of field strength. We elucidate the mechanism for the extremely effective gap tuning by examining the interlayer charge redistribution driven by the coupled electric field and nanomechanical effect.

  • Structural Characterization and Thermoelectric Transport Properties of Uniform Single-Crystalline Lead Telluride Nanowires. Guo’an Tai, Bin Zhou and Wanlin Guo. J. Phys. Chem. C, 2008, 112(30), 11314-11318.

    Guo’an Tai, Bin Zhou and Wanlin Guo

    J. Phys. Chem. C, 2008, 112 (30), pp 11314–11318

    Uniform single-crystalline PbTe nanowires with average diameter of about 30 nm, below its average excitonic Bohr radius of 46 nm, were successfully synthesized in large quantity by a two-step hydrothermal process using tellurium nanowires as templates and Pb(NO 3) 2 as a precursor. It is shown that the reaction temperature, duration, and concentration of Pb(NO 3) 2 play important roles in the formation of the PbTe nanowires. An in situ diffusion and growth mechanism together with a nontopotactic transformation process was proposed to explain the formation of the PbTe nanowires. The thermoelectric transport measurement indicates a high Seebeck coefficient of about 628 μV/K in the thin film sample composed of the obtained PbTe nanowires, about 137% exceeding that of the state-of-the-art bulk PbTe. The calculated local density of states (LDOS) by the density functional theory shows a strong increase with decreasing nanowire diameter, which is consistent with the measured large enhancement in Seebeck coefficient in the PbTe nanowire sample.

  • Size-dependent polarizabilities of finite-length single-walled carbon nanotubes. Shaojie Ma, and Wanlin Guo. Physics Letters A, 2008, 372, 4835-4838.

    Shaojie Ma, and Wanlin Guo

    Physics Letters A, 2008, 372, 4835–4838.

    Polarizabilities of single-walled carbon nanotubes with length up to 13.5 nm are studied by systematic semi-empirical PM3 calculations. Longitudinal polarizabilities increase exponentially with increasing length and about 10% higher in zigzag tubes than in armchair tubes, at least in the calculated range. The chirality-dependence of the longitudinal polarizability in finite-length nanotubes is much weaker than that in infinite-length tubes. The transverse polarizabilities of nanotubes always increase linearly with increasing length, but they are independent of chiralities and energy gaps.

  • Transversely isotropic elastic properties of single-walled carbon nanotubes by a rectangular beam model for the C-C bonds. Haijun Li and Wanlin Guo. J. Appl. Phys., 2008, 103, 103501.

    Haijun Li and Wanlin Guo

    J. Appl. Phys. 103, 103501 (2008); doi:10.1063/1.2930999 (11 page)

    Continuum mechanics modeling of carbon nanotubes has long been an attractive issue, but how to reflect exactly the physics essential of the atomic bonds still remains to be a challenging problem. To capture the distinguishing in-plane σ-σ and out-of-plane σ-π bond angle bending rigidities of CC bonds in carbon nanotubes, an equivalent beam element with rectangular section is proposed and a corresponding frame structure model for a single-walled carbon nanotube (SWNT) is developed. By using the model, the five independent elastic moduli of SWNTs with arbitrary chirality and diameter are evaluated systematically. It is found that the elastic properties of the SWNTs are transversely isotropic when the tube diameter is small. The smaller the tube diameter is, the stronger the dependence of the elastic properties on the tube size and chirality is, while when the tube diameter is large enough, the SWNTs degenerate from transversely isotropic to isotropic and the elastic moduli tend to that of a graphite sheet. The present model can be incorporated into any standard finite element software directly, providing an extremely versatile and powerful tool for the study of nanostructures that beyond the computational capability of current atomistic approaches.

  • Three-parameter approach for elastic–plastic fracture of the semi-elliptical surface crack under tension. Junhua Zhao, Wanlin Guo, and Chongmin She. International Journal of Mechanical Sciences, 2008, 50, 1168-1182.

    Junhua Zhao, Wanlin Guo, and Chongmin She

    International Journal of Mechanical Sciences 50 (2008) 1168– 1182

    Systematic three-dimensional elastic–plastic finite element analyses are carried out for a semi-elliptical surface crack in plates under tension. Various aspect ratios (a/c) of three-dimensional fields are analyzed near the semi-elliptical surface crack front. It is shown that the developed J–Q annulus can effectively describe the influence of the in-plane stress parameters as the radial distances (r/(J/σ0)) are relatively small, while the approach can hardly characterize it very well with the increase of r/(J/σ0) and strain hardening exponent n. In order to characterize the important stress parameters well, such as the equivalent stress σe, the hydrostatic stress σm and the stress triaxiality Rσ, the three-parameter J–QT–Tz approach is proposed based on the numerical analysis as well as a critical discussion on the previous studies. By introducing the out-of-plane stress constraint factor Tz and the QT term, which is determined by matching the finite element analysis results, the J–QT–Tz solution can predict the corresponding three-dimensional stress state parameters and the equivalent strain effectively in the whole plastic zone. Furthermore, it is exciting to find that the values of J-integral are independent of n under small-scale yielding condition when the stress-free boundary conditions at the side and back surfaces of the plate have negligible effect on the stress state along the crack front, and the normalized J tends to a same value when φ equals about 31.5° for different a/c and n. Finally, the empirical formula of Tz and the stress components are provided to predict the stress state parameters effectively.

    Keywords: Out-of-plane stress constraint factor Tz; J-integral; Semi-elliptical surface crack; Three-dimensional elastic–plastic finite element

  • Mechanism for Superelongation of Carbon Nanotubes at High Temperatures. Chun Tang, Wanlin Guo, and Changfeng Chen. Phys. Rev. Lett., 2008, 100, 175501.

    Chun Tang, Wanlin Guo, and Changfeng Chen

    Phys. Rev. Lett. 100, 175501 (2008) [4 pages]

    We report molecular dynamics simulations of the recently discovered superelongation of carbon nanotubes (CNTs) at high temperatures. The nearly simultaneous activation and wide distribution of a large number of defects near the elastic limit play a key role in impeding the formation of localized predominant instability and facilitating large tensile elongation. It suggests new and more complex mechanisms for CNT superelongation in contrast with the previously proposed ideal defect glide and pseudoclimb. Defect interaction and evolution generate multistage necking and kinking and new types of larger defects that dominate the tensile elongation and breaking process. Intricate interplay between CNT sizes and defect nucleation and motion determine the overall deformation pattern.

  • Hydrothermal Synthesis and Thermoelectric Transport Properties of Uniform Single-Crystalline Pearl-Necklace-Shaped PbTe Nanowires. Guoan Tai, Wanlin Guo and Zhuhua Zhang. Cryst. Growth Des., 2008, 8(10), 3878.

    Guoan Tai, Wanlin Guo and Zhuhua Zhang

    Cryst. Growth Des., 2008, 8 (10), p 3878

    Uniform single-crystalline pearl-necklace-shaped PbTe nanowires with an average diameter of about 30 nm, smaller than its average excitonic Bohr radius of 46 nm, were successfully synthesized by a hydrothermal process using tellurium nanowires as templates and Pb(NO3)2 as a precursor at a molar ratio of 1:1. It is shown that the reaction temperature, duration, and concentration of Pb(NO3)2 play important roles in the formation of the PbTe nanowires. The growth process of the pearl-necklace-shaped PbTe nanowires can be reasonably explained by an oriented attachment mechanism. Thermoelectric transport measurement indicates that the film composed of the obtained PbTe nanowires has a Seebeck coefficient of about 307 μV/K, up to about 16% higher than that of the state-of-the-art bulk PbTe at room temperature. The synthetic route can be applied to obtain other low-dimensional semiconducting telluride nanostructures and optimization of the thermoelectric properties through nanowire alignment and doping may lead to practical applications.

  • Numerical study of the influence of particle-cracking to the damage of MMC by the incremental damage theory. Yunpeng Jiang, Hui Yang, and Wanlin Guo. Materials Science and Engineering A, 2008, 492, 370-376.

    Yunpeng Jiang, Hui Yang, and Wanlin Guo

    Materials Science and Engineering A, 2008, 492, 370–376.

    Based on the incremental damage theory, the influences of particle-cracking damage and its residue strengthening capacity on the stress–strain response of particle reinforced (metal matrix composite) MMC under uniaxial tension are carefully investigated in this paper. Two kinds of models are adopted in the numerical calculation to predict the damage evolution of MMC, one is modeling the broken particles as voids and the other is considering the remaining load carrying capacity of the damaged particles. Special emphasis is placed on the detailed comparison between the results predicted by the two models under different parameters such as the aspect ratio, volume fraction of particle and the elastic–plasticity properties of matrix. The damage process of MMC and the development of stress in the particles are predicted by two models and carefully analyzed.

    Keywords: Metal matrix composite (MMC); Incremental damage theory; Particle-cracking; Aspect ratio

  • Group velocity of wave propagation in carbon nanotubes. Lifeng Wang, Wanlin Guo and Haiyan Hu. Proc. R. Soc. A, 2008, 464, 1423–1438.

    Lifeng Wang, Wanlin Guo and Haiyan Hu

    Proc. R. Soc. A (2008) 464, 1423–1438

    The group velocities of longitudinal and flexural wave propagations in single- and multi-walled carbon nanotubes are studied in the frame of continuum mechanics. The dispersion relations between the group velocity and the wavenumber for flexural and longitudinal waves, described by a beam model and a cylindrical shell model, are established for both single- and multi-walled carbon nanotubes. The effect of microstructures in carbon nanotubes on the wave dispersion is revealed through the non-local elastic models of a beam and a cylindrical shell, including the second-order gradient of strain and a parameter of microstructure. It is shown that the microstructures in the carbon nanotubes play an important role in the dispersion of both longitudinal and flexural waves. In addition, the non-local elastic models predict that the cut-off wavenumber of the dispersion relation between the group velocity and the wavenumber is approximately 2×1010 m−1 for the longitudinal and flexural wave propagations in both single- and multi-walled carbon nanotubes. This may explain why the direct molecular dynamics simulation cannot give a proper dispersion relation between the phase velocity and the wavenumber when the wavenumber approaches approximately 2×1010 m−1, much lower than the cut-off wavenumber for the dispersion relation between the phase velocity and the wavenumber predicted by continuum mechanics.

    Keywords: carbon nanotube; non-local elasticity; group velocity; Timoshenko beam; cylindrical shell

  • The influence of Poisson’s ratio on thickness-dependent stress concentration at elliptic holes in elastic plates. Peishi Yu, Wanlin Guo, Chongmin She, and Junhua Zhao. International Journal of Fatigue, 2008, 30, 165-171.

    Peishi Yu, Wanlin Guo, Chongmin She, and Junhua Zhao

    International Journal of Fatigue 30 (2008) 165–171

    The influence of Poisson’s ratio (ν) on the thickness-dependent stress concentration factor (SCF) along the root of elliptic holes in elastic plates subjected to tension is systematically investigated by use of three-dimensional finite element method. It is found that the thickness-dependent maximum of SCF, (Kt)max, increases significantly with increasing ν. As the thickness to root radius ratio B/ρ grows from 0.1 to 1000, the (Kt)max undergo a peak value, which can be increased 9% for a circular hole and 23% for an elliptic hole with length of short to long axial aspect ratio t = 0.1 when ν increases from 0.1 to 0.49. It is also found that the peak value occurs in a narrow range of the thickness to elliptic short axis ratio B/b (2–3) with different t and ν. When B/ρ is high enough, an increase of ν from 0.1 to 0.49 leads to decreasing in the SCFs on the free surface (Kt)surf about 24% and 61% and increasing in the ratio of (Kt)max/(Kt)surf about 38% and 195% for circular hole and elliptic hole with t = 0.1. The ν-dependent empirical formulae of the relationships among (Kt)max, (Kt)surf and the corresponding planar solution (Kt)p−σ have been obtained by fitting the numerical results with satisfied accuracy, which will be useful for strength and fatigue designs of engineering structures with notches and holes.

    Keywords: Stress concentration; Elliptic hole; Poisson’s ratio; Thickness; Three-dimensional finite element method

  • Piezoelectricity of ZnO and its Nanostructures. Liangzhi Kou, Wanlin Guo, and Chun Li. Proceedings of the 2008 Symposium on iezoelectricity, Acoustic Waves and Device Applications, 2008, 354-359.

    Liangzhi Kou, Wanlin Guo, and Chun Li.

    Proceedings of the 2008 Symposium on iezoelectricity, Acoustic Waves and Device Applications, 2008, 354-359.

    The wurtzite ZnO material exhibits excellent piezoelectric property along the [0001]-direction because of the noncentrosymmetric structure. As a typical II-VI wide band gap compound, it has been long studied as a piezoelectric material. Here we review the previous theoretical and experimental researches on the piezoelectric properties of ZnO and its nanostructures. Some practical applications in nanodevices are also exhibited. The present review could serve as a good reference for future investigations in the relative fields, and also indicates potential applications in nanoscale devices. Our contributions on this review topic are focused on the theoretical investigations of piezoelectricity of ZnO nanostructures, including nanowires and nanofilms, by using first-principles calculations. For the nanowires, size-dependent axial piezoelectricity in [0001]-oriented ZnO nanowires with diameters ranging from 0.4 to 3.0 nm is investigated. It is shown that the effective piezoelectric constant e33 of the nanowires increases with increasing diameter, and is approximately one order smaller than the bulk value due to the structural change on the boundary of the nanowires. The absolute value of the axial piezoelectricity induced by the radial strain e31 is around half of the effective e33, which is similar to the bulk case. For the ZnO nanofilms, we find that the effective piezoelectric constant e33 of ZnO nanofilms is also size dependent, and increases with increasing thickness in the nanoscale simulated in our work. When the film thickness is larger than 2.4 nm, the corresponding piezoelectric coefficient becomes higher than that of bulk ZnO. The enhancement over the bulk value reaches 11% when the film thickness is 2.9 nm.

  • Energy-gap modulation of boron nitride nanoribbons by transverse electric fields: First-principles calculations. Zhuhua Zhang and Wanlin Guo. Phys. Rev. B, 2008, 77, 075403.

    Energy-gap modulation of boron nitride nanoribbons by transverse electric fields: First-principles calculations

    Zhuhua Zhang and Wanlin Guo

    Phys. Rev. B 77, 075403 (2008) [5 pages]

    Systematic ab initio calculations show that the energy gap of BN nanoribbons (BNNRs) with zigzag or armchair edges can be significantly reduced by a transverse electric field and be completely closed at a critical field which decreases with increasing ribbon width. In addition, a distinct gap modulation in the ribbons with zigzag edges is presented when a reversed electric field is applied. In a weak field, the gap reduction of the BNNRs with zigzag edges originates from the field-induced energy level shifts of the spatially separated edge states, while the gap reduction of the BNNRs with armchair edges arises from the Stark effect. As the field gets stronger, the energy gaps of both types of the BNNRs gradually close due to the field-induced motion of nearly free electron states. Without the applied fields, the energy gap modulation by varying ribbon width is rather limited.

  • Chemisorption of hydrogen molecules on carbon nanotubes: charging effect from first-principles calculations. Bin Zhou, Wanlin Guo and Chun Tang. Nanotechnology, 2008, 19, 075707.

    Bin Zhou, Wanlin Guo and Chun Tang

    Nanotechnology 19 (2008) 075707 (4pp)

    We report a systematic investigation of the charging effect on hydrogen molecule chemisorption on (3, 3), (5, 5), (5, 0), and (8, 0) carbon nanotubes by first-principles calculations. The influence of injected charge on the chemisorption energy barriers is found to be sensitive to the nanotube diameter and chirality. The calculated results also indicate that electron injection is more effective in lowering the energy barrier for armchair carbon nanotubes while hole injection is more effective for zigzag nanotubes. The origin of these interesting trends and systematics can be understood by a close examination of the underlying electronic structure and the electron transfer between the hydrogen molecules and the nanotubes.

  • Mechanism of Carbon Nanotubes Aligning along Applied Electric Field. Shaojie Ma, and Wanlin Guo. Chinese Phys. Lett., 2008, 25, 270.

    Shaojie Ma, and Wanlin Guo

    Chinese Phys. Lett., 2008, 25, 270.

    The mechanism of single-walled carbon nanotubes (SWCNTs) aligning in the direction of external electric field is studied by quantum mechanics calculations. The rotational torque on the carbon nanotubes is proportional to the difference between the longitudinal and transverse polarizabilities and varies with the angle of SWCNTs to the external electric field. The longitudinal polarizability increases with second power of length, while the transverse polarizability increases linearly with length. A zigzag SWCNT has larger longitudinal and transverse polarizabilities than an armchair SWCNT with the same diameter and the discrepancy becomes larger for longer tubes.

  • Robustness of the residue conservation score reflecting both frequencies and physicochemistries. Xinsheng Liu, and Wanlin Guo. Amino Acids, 2008, 34, 643-652.

    Xinsheng Liu, and Wanlin Guo

    Amino Acids (2008) 34:643–652

    Measuring residue conservation at aligned positions has many applications in biology. Recently, a new conservation score has been defined. Unlike the previous methods, the new approach considers both residue frequencies and physicochemistries. Specifically, it measures physicochemistries based on BLOSUM matrices disregarding the meaning of the entries in such matrices, which may involve the problem of log–log probability. In this paper we present a conservation measure that also reflects both frequencies and physicochemistries while considering the fact that the entries of BLOSUM matrices are already interpreted as log probability. When the supposed score is applied to 14 protein examples, the results show that these two conservation scores are equivalent aside from the different score ranges. The method is also used to score the functional sites of three protein families. Compared with the widely used entropy-based methods, the resulting scores are more robust and consistent in the sense that the functional sites are much more conserved because of functional constraints.

    Keywords: Conservation score - Physicochemistry - Multiple sequence alignment - Functional site - Protein folding nucleus