Our Publications

  • Magnetoelectric Effect in Graphene Nanoribbons on Substrates via Electric Bias Control of Exchange Splitting. Zhuhua Zhang, Changfeng Chen, and Wanlin Guo. Phys. Rev. Lett. 2009, 103, 187204.

    Zhuhua Zhang, Changfeng Chen, and Wanlin Guo

    Phys. Rev. Lett. 103, 187204 (2009) [4 pages]

    We predict a magnetoelectric (ME) effect in graphene nanoribbons on silicon substrates by first-principles calculations. It is shown that a bias voltage can produce strong linear ME effect by driving charge transfer between the nanoribbons and substrate, thus tuning the exchange splitting of magnetic edge states; moreover, the bias induced n-to-p-type transition in the ribbon layer can switch the ME coefficient from negative to positive due to the unique symmetry of band structures. This mechanism is proven to be robust against variations in material and physical configurations, thus opening a new avenue for ME coupling in metal-free magnet systems of practical importance.

  • Bias voltage induced n- to p-type transition in epitaxial bilayer graphene on SiC. Yufeng Guo, Wanlin Guo, and Changfeng Chen. Phys. Rev. B, 2009, 80, 085424.

    Yufeng Guo, Wanlin Guo, and Changfeng Chen

    Phys. Rev. B 80, 085424 (2009) [5 pages]

    We show by extensive first-principles calculations that an n- to p-type transition in epitaxial bilayer graphene can be induced by applying bias voltage on C-terminated SiC substrate, but cannot occur on Si-terminated SiC. Bias voltage can cause enough charge transfer between top and bottom graphene layers on C-terminated SiC to shift the Dirac level below or above the Fermi level. On both C- and Si-terminated SiC, change in interlayer spacing of the epitaxial bilayer graphene produces charge redistribution that leads to large increase in the energy gap, but cannot raise the Dirac level efficiently. The surface terminated condition or properties of substrate are of essential importance in possible gate tuning electronic behavior of epitaxial graphene on it.

  • Electronic and Mechanical Coupling in Bent ZnO Nanowires. Xiaobing Han, Liangzhi Kou, Xiaoli Lang, Jianbai Xia, Ning Wang, Rui Qin, Jing Lu, Jun Xu, Zhimin Liao, Xinzheng Zhang, Xudong Shan, Xuefeng Song, Jingyun Gao, Wanlin Guo, Dapeng Yu. Adv. Mater.,20

    Xiaobing Han1, Liangzhi Kou2, Xiaoli Lang3, Jianbai Xia3, Ning Wang4, Rui Qin1, Jing Lu1, Jun Xu1, Zhimin Liao1, Xinzheng Zhang1, Xudong Shan1, Xuefeng Song1, Jingyun Gao1, Wanlin Guo2,*, Dapeng Yu1,*

    Adv. Mater.,2009, 21, 1–5.

    Article first published online: 13 AUG 2009

  • Mixed modes in opening of KcsA potassium channel from a targeted molecular dynamics simulation. Wenyu Zhong, and Wanlin Guo. Biochemical and Biophysical Research Communications, 2009, 388, 86–90.

    Wenyu Zhong, and Wanlin Guo

    Biochemical and Biophysical Research Communications 388 (2009) 86–90

    Potassium channels conduct K+ flow selectively across the membrane through a central pore. During a process called gating, the potassium channels undergo a conformational change that opens or closes the ion-conducting pore. The potassium channel KcsA has been structurally determined in its closed state. However, the dynamic mechanism of the gating transition of the KcsA channel is still being investigated. Here, a targeted molecular dynamics simulation up to 150 ns is performed to investigate the detailed opening process of the KcsA channel with an open Kv1.2 structure serving as the target. The channel arrived at a self-determined quasi-stable state within 60 ns. The rigid-body and hinge-bending modes are observed mixed together in the remaining 90 ns long quasi-stable state. The mixed-mode movement seems come from the competition between the helix rigidity and the biased-applied gating force.

  • Electronic and magnetic properties of zigzag edge graphene nanoribbons with Stone–Wales defects. Peng Lu, Zhuhua Zhang and Wanlin Guo. Physics Letters A, 2009, 373, 3354-3358.

    Peng Lu, Zhuhua Zhang and Wanlin Guo

    Physics Letters A 373 (2009) 3354–3358

    The electronic and magnetic properties of zigzag graphene nanoribbons (ZGNRs) with Stone–Wales defects are studied by extensive first-principles calculations. It is shown that the asymmetry distribution of the Stone–Wales defects can induce finite magnetic moment in the defective ZGNRs. As the defect near one of the ribbon edges moving to the centre region, the magnetic moment of the defective ZGNRs gradually decreases to zero, following a transition from metal to semi-half-metal and eventually to semiconductor. In addition, by symmetrically placing an additional defect at the opposite side of the defective ZGNRs, the finite magnetic moment vanishes, and the electronic properties depend on the distance between the defect and the closer ribbon edge. These findings are robust within a wide range of defect concentration.

  • Electronic properties of zigzag graphene nanoribbons on Si(001). Zhuhua Zhang and Wanlin Guo. APPLIED PHYSICS LETTERS, 2009, 95, 023107.

    Zhuhua Zhang and Wanlin Guo

    APPLIED PHYSICS LETTERS 95, 023107 2009

    We show by first-principles calculations that the electronic properties of zigzag graphene nanoribbons Z-GNRs adsorbed on Si001 substrate strongly depend on ribbon width and adsorption orientation. Only narrow Z-GNRs with even rows of zigzag chains across their width adsorbed perpendicularly to the Si dimer rows possess an energy gap, while wider Z-GNRs are metallic due to width-dependent interface hybridization. The Z-GNRs can be metastably adsorbed parallel to the Si dimer rows, but show uniform metallic nature independent of ribbon width due to adsorption-induced dangling-bond states on the Si surface. © 2009 American Institute of Physics.

  • Stability and Electronic Properties of a Novel C-BN Heteronanotube from First-Principles Calculations. Zi-Yue Zhang, Zhuhua Zhang, and Wanlin Guo. J. Phys. Chem. C, 2009, 113, 13108-13114.

    Zi-Yue Zhang, Zhuhua Zhang, and Wanlin Guo

    J. Phys. Chem. C 2009, 113, 13108–13114

    We report a family of heteronanotubes circumferentially consisting of a curled carbon ribbon and a curled boron nitride ribbon from first-principles calculations. By molecular dynamics simulations and total energy calculations, the stability of the C-BN heteronanotubes (C-BNNTs) is predicted to be comparable to that of carbon nanotubes, and all the C-BNNTs with diameters larger than 0.4 nm are expected to be stable well over room temperature. Zigzag C-BNNTs are found to be direct gap semiconductors with the band gaps varying depending on the tube diameters and the ratio of carbon dimer lines with respect to BN ones. In contrast, armchair C-BNNTs are metallic except for those with diameters less than 0.6 nm or 1-2 axisoriented zigzag carbon atomic chains around the circumferences, which become semiconducting. The versatile
    electronic properties in these heteronanotubes originate from an intricate interplay between the quantum confinement effect and the local tube curvature.

  • Elasticity of Single-Crystal Calcite by First-Principles Calculations. Junhua Zhao, Bin Zhou, Baiguo Liu, and Wanlin Guo. Journal of Computational and Theoretical Nanoscience, 2009, 6(5), 1181-1188.

    Junhua Zhao, Bin Zhou, Baiguo Liu, and Wanlin Guo.

    Journal of Computational and Theoretical Nanoscience, 2009, 6(5), 1181-1188.

    The elastic constants of single-crystal calcite (CaCO3) have been obtained by extensive first-principles calculations based on the density functional theory (DFT). The corresponding shear, bulk and Young's moduli and Poisson's ratio are evaluated using the Voigt-Reuss-Hill scheme. Based on the present DFT results, the large scatters among nine sets of available experimental data are analyzed. It is found that some of the early experimental data (From Voight (1910) to Reddy et al. (1960)) and some of the recent experimental data obtained since 1963 are in excellent coincidence with the present DFT results. The present results are all among the available experimental data except for a shear modulus C14, while more than three elastic parameters of existed empirical models are out of those of all experimental data. Therefore, the present DFT results provide a sounder theoretical foundation for the elastic properties of crystal calcite, which should be helpful for a better understanding of the exceptional physical mechanical behavior of nature materials with calcite as the main ingredient, such as conches.

  • Temperature dependence of the stability of written bits in a magnetic hard-disk medium investigated by magnetic force microscopy. Yan Jiang, and Wanlin Guo. Journal of Magnetism and Magnetic Materials, 2009, 321, 2963-2965.

    Yan Jiang, and Wanlin Guo

    Journal of Magnetism and Magnetic Materials 321 (2009) 2963–2965

    The thermal stability of written bits in a magnetic hard-disk medium has been investigated with a magnetic force microscope (MFM), which was equipped with an in situ heating system capable of heating the medium up to 300 °C. It is shown that both the annealing temperature and the duration have significant effect on the decay of the MFM signal. No signal decay is observed when annealing for 30 min up to temperatures of 200 °C. The MFM signal decays rapidly with increasing temperature, for temperatures over 200 °C. Repeated annealing at 280 °C with a duration below 10 min does not cause any signal decay.

    Keywords: Thermal stability; Hard disk; Magnetic force microscope; Temperature

  • Tunable Ferromagnetic Spin Ordering in Boron Nitride Nanotubes with Topological Fluorine Adsorption. Zhuhua Zhang and Wanlin Guo. J. Am. Chem. Soc., 2009, 131(19), 6874-6879.

    Zhuhua Zhang and Wanlin Guo

    J. Am. Chem. Soc., 2009, 131 (19), pp 6874-6879

    We find through first-principles calculations that fluorine atoms topologically adsorbed on boron nitride nanotubes induce long-ranged ferromagnetic spin ordering along the tube, offering strong spin polarization around the Fermi level. The spin polarization and magnetic moment increase significantly with decreasing tube radius, even giving rise to half-metal when the tube diameter is reduced to 3.3 Å, while in a flat boron nitride sheet with the same topological fluorine arrangement the magnetic moment nearly disappears. This radius-dependent behavior is then developed into a local curvature modulation procedure to efficiently enhance or quench the ferromagnetic ordering, which enables the F-BNNTs to function as piezomagnetic nanotubes. These findings suggest a new route to facilitate the design of tunable spin devices.

  • Stability and electronic properties of small boron nitride nanotubes. Zhuhua Zhang, Wanlin Guo, and Yitao Dai. J. Appl. Phys., 2009, 105, 084312.

    Zhuhua Zhang, Wanlin Guo, and Yitao Dai

    J. Appl. Phys. 105, 084312 (2009); doi:10.1063/1.3115446 (8 pages)

    We report the stability and electronic structures of the boron nitride nanotubes (BNNTs) with diameters below 4 Å by semiempirical quantum mechanical molecular dynamics simulations and ab initio calculations. Among them (3,0), (3,1), (2,2), (4,0), (4,1), and (3,2) BNNTs can be stable well over room temperature. These small BNNTs become globally stable when encapsulated in a larger BNNT. It is found that the energy gaps and work functions of these small BNNTs are strongly dependent on their chirality and diameters. The small zigzag BNNTs become desirable semiconductors and have peculiar distribution of nearly free electron (NFE) states due to strong hybridization effect. When such a small BNNT is inserted in a larger one, the energy gap of the formed double-walled BNNT can be even much reduced due to the coupled effect of wall buckling difference and interwall NFE-π∗ hybridization.

  • Molecular dynamics simulation of tensile elongation of carbon nanotubes: Temperature and size effects. Chun Tang, Wanlin Guo, and Changfeng Chen. Phys. Rev. B, 2009, 79, 155436.

    Chun Tang, Wanlin Guo, and Changfeng Chen

    Phys. Rev. B 79, 155436 (2009) [9 pages]

    We report molecular dynamics simulations of tensile elongation of carbon nanotubes (CNTs) over a wide temperature range. In particular, we examine temperature and size effects on tensile ductility of CNTs and compare our results with recent experimental observation on superplastic deformation of CNTs at high temperatures. Our simulations produce substantial tensile ductility in CNTs with large diameters at high temperatures and reveal that similar behavior can be realized over a surprisingly large temperature range between 500 and 2400 K that is yet to be fully explored by experiments. At lower temperatures, tensile deformation modes become brittle due to defect localization attributed to insufficient thermal energy for wide distribution of defect nucleation. For CNTs with smaller diameters, our simulations produce strong defect localization which leads to brittle behavior even at high temperatures. Sensitive dependence on the distribution of incipient defects on thermal energy results in a significant decrease in the elastic limit with increasing temperature. We propose an effective tensile ductility enhancement via temperature reduction beyond the elastic limit. The results offer insights for understanding intriguing temperature effects on tensile deformation modes of CNTs.

  • Electric-field-induced deformation in boron nitride nanotubes. Yitao Dai, Wanlin Guo, Zhuhua Zhang, Bin Zhou and Chun Tang. J. Phys. D: Appl. Phys., 2009, 42, 085403.

    Yitao Dai, Wanlin Guo, Zhuhua Zhang, Bin Zhou and Chun Tang

    J. Phys. D: Appl. Phys. 42 (2009) 085403 (4pp)

    Axial deformation of boron nitride nanotubes (BNNTs) induced by an external electric field is studied by density functional theory. It is found that the field-induced deformation can reach 1% at a field strength of around 10 V nm−1 due to both the converse piezoelectric effect and the electrostrictive effect. This deformation is about nine times larger than that of traditional piezoceramics and can be enhanced with increasing length or decreasing diameter of the BNNTs. The corresponding volumetric work capacity is calculated to be two orders of magnitude higher than those of traditional piezoceramics.

  • Numerical studies on the effective shear modulus of particle reinforced composites with an inhomogeneous inter-phase. Yunpeng Jiang, Wanlin Guo, and Hui Yang. Computational Materials Science, 2009, 43, 724-731.

    Yunpeng Jiang, Wanlin Guo, and Hui Yang

    Computational Materials Science, 2009, 43, 724–731.

    Numerical studies on the effective shear modulus of particle reinforced composites with an inhomogeneous inter-phase are performed. The influences of many parameters to the equivalent shear modulus of composites are carefully analyzed, including the inter-phase thickness, variation of interfacial properties, boundary conditions and volume fraction of particles, etc. Numerical results show that the Poisson ratio can be assumed as a constant across the whole inter-phase zone in the computation. The form of property variation across the inter-phase also greatly affects the effective shear modulus of composite. Numerical results predicted by the rigid boundary conditions are remarkably higher than those by the free boundary conditions and the exact solutions. The reasonability and exactness of the available models for predicting the effective shear modulus of composites are accessed by the numerical results in the present work.

    Keywords: Inter-phase; Finite element method (FEM); Particle reinforced composites; Effective shear modulus

  • Surface Concavity−Convexity Sensitive Oxidation Dynamics of Carbon Nanotubes. Bin Zhou, Wanlin Guo, and Changfeng Chen. J. Phys. Chem. C, 2009, 113(9), 3569–3573.

    Bin Zhou†, Wanlin Guo*‡ and Changfeng Chen‡

    Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China, and Department of Physics and High Pressure Science and Engineering Center, University of Nevada, Las Vegas, Nevada 89154

    J. Phys. Chem. C, 2009, 113 (9), pp 3569–3573

    Publication Date (Web): February 10, 2009

    Carbon nanotubes (CNTs) exhibit remarkable oxygen-exposure sensitive behavior with their hollow interiors providing an environment for enhanced chemical reactions, but the mechanisms remain elusive. Here we show by first-principles calculations that oxidation dynamics of CNTs are highly sensitive to surface concavity−convexity. Careful kinetic analyses indicate that oxygen chemisorption is much easier to occur on a convex surface, while diffusion of dissociated oxygen atoms is more favorable on a concave surface. Motivated by these findings, we demonstrate that turning the concavity−convexity of the tube wall via reversible nanomechanical deformation can achieve simultaneous reduction of the diffusion barrier on the outer surface and the chemisorption barrier on the inner surface of CNTs. It greatly facilitates the access for oxygen into the CNTs and leads to significantly enhanced oxidation of CNTs on both outside and interior walls. These results suggest a new route to efficient oxidation of CNTs and may open a new avenue for biasing reaction dynamics via surface curvature in tubular nanostructures in general.

  • Four Intrinsic Aqueduct Orifices Outstretched from the Central Cavity Facilitate Potassium Channels Gating. Wanlin Guo, and Wenyu Zhong. Biophysical Journal, 2009, 96(3), 671a-672a.

    Wanlin Guo and Wenyu Zhong

    Nanjing University of Aeronautics and Astronautics, Nanjing, China

    Biophysical Journal, 2009, 96(3), 671a-672a.

    Available online 10 February 2009

    Potassium channels enable K+ ions to flow selectively across cell membrane through a central pore. The mechanisms of ion selectivity and channel gating have long been the attractive secrets. The breakthrough in determination of the structure of the KcsA potassium channel (Science 280, 69-77, 1998) has raised a high tide in structure and function study, but channel gating still remains a long secret. The core structure of K+ channels was found to be highly conserved, and constructed of an inverted teepee with a large water-filled cavity at center and the well studied selectivity filter at its wide end. Here we find four aqueduct orifices outstretched from the cavity and perpendicular to the central pore, leading to shape of a swastika or Greek Fleurée Cross, having subtle gating function. We demonstrated by systematical molecular dynamics simulations that water molecules flowing in the orifices can harmonize the space changing in the cavity to reduce the opening resistance significantly, and blocking the aqueduct orifices makes the intracellular entryway difficult to be opened. This is strongly supported by existed mutation experiments. Homology analyses of all available pore structures and amino acid sequences of K+ channels show that the aqueduct orifices are intrinsic structure feature to the whole potassium channel genre, but their size and conformation are less conserved among different subfamilies, shedding light on their functional diversity.

    Potassium and water channels have been the wining channels for 2003 Nobel prize. The finding of the intrinsic aqueduct orifices and their vital functions in channel gating shows water flowing merges with ion activity. The orifices even exist in the newly determined atomic structure of NaK channel (Nature 440, 570-574, 2006), which belongs to another large ion channel family (cyclic nucleotide-gated channels).

  • Water Dominated Ions Stability and Conduction in NaK Channel. Wanlin Guo, and Rong Shen. Biophysical Journal, 2009, 96, 671.

    Wanlin Guo and Rong Shen

    Nanjing University of Aeronautics and Astronautics, Nanjing, China

    Biophysical Journal, 2009, 96, 671.

    Available online 10 February 2009.

    Water plays an important role in ion channels. It stabilizes ions in the central cavity and accompanies them to permeate through the channel, and it also participates in processes of ion selection. Here we find four water grottos connecting with the vestibule of the NaK selectivity filter, and they form a vestibule-grotto (V-G) complex in a plane perpendicular to the ion conducting pore. Molecular dynamics (MD) simulations show that water can penetrate and escape the grottos from the extracellular water pits above the grottos around the extracellular entrance, and two aromatic residues Tyr55 and Phe56 serve as a gate between the grottos and water pits. In the rest state, water molecules are confined in the vestibule and grottos and seldom exchange between them, and they have little impact on the K+ ion binding states in the selectivity filter. While in the active state, the water molecules in the V-G complex become highly activated and they can flow easily between the vestibule and grottos. MD and free energy calculations show that the water molecules moving in the V-G complex hydrate and stabilize ions in the filter and serve as a valve in conveying ions through the vestibule for controllable ion permeating. The existence of the grottos and the simple and beautiful structure-function correlation of the hydration valve can be expected in the whole family of CNG channels, which function in our photoreceptors and olfactory cells.

  • Ion binding properties and structure stability of the NaK channel. Rong Shen, Wanlin Guo. Biochimica et Biophysica Acta, 2009, 1788 1024–1032.

    Rong Shen, Wanlin Guo

    Biochimica et Biophysica Acta 1788 (2009) 1024–1032

    Ion distribution in the selectivity filter and ion–water and ion–protein interactions of NaK channel are systematically investigated by all-atom molecular dynamics simulations, with the tetramer channel protein being embedded in a solvated phospholipid bilayer. Analysis of the simulation results indicates that K+ ions prefer to bind within the sites formed by two adjacent planes of oxygen atoms from the selectivity filter, while Na+ ions are inclined to bind to a single plane of four oxygen atoms. At the same time, both K+ and Na+ ions can diffuse in the vestibule, accompanying with movements of the water molecules confined in a complex formed by the vestibule together with four small grottos connecting to it. As a result, K+ ions show a wide range of coordination numbers (6–8), while Na+ ions display a constant coordination number of 6 in the selectivity filter, which may result in the loss of selectivity of NaK. It is also found that a Ca2+ can bind at the extracellular site as reported in the crystal structure in a partially hydrated state, or at a higher site in a full hydration state. Furthermore, the carbonyl group of Asp66 can reorient to point towards the center pore when an ion exists in the vestibule, while that of Gly65 always aligns tangentially to the channel axis, as in the crystallographic structures.

    Keywords: NaK channel; Molecular dynamics; Ion distribution; Ion solvation

  • First-principles investigation of technetium carbides and nitrides. Yongcheng Liang, Chun Li, Wanlin Guo, and Wenqing Zhang. Phys. Rev. B, 2009, 79, 024111.

    Yongcheng Liang1,2, Chun Li3, Wanlin Guo4, and Wenqing Zhang2

    1. College of Engineering Science and Technology, Shanghai Ocean University, Shanghai 201306, China
    2. State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
    3. Department of Physics and Research Center OPTIMAS, Kaiserslautern University of Technology, 67653 Kaiserslautern, Germany
    4. Institute of Nanoscience, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

    Phys. Rev. B 79, 024111 (2009) [5 pages]

    Received 29 July 2008; revised 3 December 2008; published 29 January 2009

    Phase stabilities and mechanical properties of ideal stoichiometric technetium monocarbide (TcC) and technetium mononitride (TcN) in the tungsten carbide (WC), nickel arsenide (NiAs), rocksalt (NaCl), and zinc-blende (ZnS) structures, respectively, have been studied systematically by first-principles calculations. It is found that both TcC and TcN in two hexagonal phases (WC and NiAs) are not only elastically stable but also hard and ultrastiff materials. Remarkably, for the two hexagonal TcC phases, both bulk moduli and linear incompressibilities along the c axis exceed that of c BN and even rival with diamond. Their hardness can also match the known hard materials such as WC. The combination of good metallicity, strong stiffness, and high hardness suggests that the materials may find applications as hard conductors and cutting tools.