[show abstract][hide abstract] ABSTRACT: The mechanical behavior of multi-walled carbon nanotubes (MWNTs), being fixed at both ends under uniaxial tensile loading, is investigated via the molecular dynamics (MD) simulation with the Tersoff interatomic potential. It is found that Young's modulus of the MWNTs is in the range between 0.85 and 1.16 TPa via the curvature method based on strain energy density calculations. Anharmonicity in the energy curves is observed, and it may be responsible for the time-dependent properties of the nanotubes. Moreover, the number of atomic layers that is fixed at the boundaries of the MWNTs will affect the critical strain for jumps in strain energy density vs. strain curves. In addition, the boundary conditions may affect “yielding” strength in tension. The van der Waals interaction of the double-walled carbon nanotube (DWNT) is studied to quantify its effects in terms of the chosen potential.
Physica E Low-dimensional Systems and Nanostructures 01/2010; 42(4):775-778. · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: Motivated by recent experimental works on the modifications of endomorphin-2 (EM2, H-Tyr-Pro-Phe-Phe-NH2) to develop better painkiller, we performed structure-activity-relationship (SAR) studies to investigate modified C-terminal ligands by using molecular dynamics (MD) simulations. Specifically, instead of the CONH2 for the unmodified EM2's C-terminus, the analogue 2 with its C-terminus being CONHNH2 and analogue 3 with its C-terminus being COOMe are studied. First, a systematic conformer search was performed via the quantum chemical AM1 calculations. The cis/trans isomers of the lowest energy were hence selected as MD initial structures. We further showed that EM2s in water exhibited similar dihedral angles to those in DMSO, obtained from the NMR experiment. This similarity indicates the reliability of our MD simulations, and enables us to discuss related bioactivity. Our results showed that the interactions of the Tyr(1)-Phe(3) pair for cis-/trans-EM2s played a considerable role for structural stability. Furthermore, we utilized the chi(1) rotamers of individual aromatic side chains to examine the structural bioactivity. It is shown that this criterion to determine the conformational bioactivity toward mu-opioid receptor (MOR) is insufficient. Thus, we have further employed rotamer-combination approaches to examine the characteristics of SAR for cis-/trans-EM2s. Our results suggested that the bioactive chi(1) rotamers for Tyr(1)-Phe(3) pair remained to favor the [trans-trans] status for MOR selectivity. Therefore, based on the analysis of the chi(1) rotamers, it is suggested that the analogue 2 exhibit greater structural bioactivity for MOR than the analogue 3, and both of them be greater than unmodified EM2 for trans isomers.
[show abstract][hide abstract] ABSTRACT: The low-energy electronic properties of a few graphite layers with AA
and ABC stacking under application of the electric field (F),
perpendicular to the layers, are explored through the tight-binding
model. They strongly depend on the interlayer interactions, the stacking
sequences, the layer numbers, and the field strength. In the absence or
presence of F, the AA-stacked N-layer graphites (N=3 and 4) exhibit the
linear bands near the Fermi energy. The interlayer interactions and
electric field chiefly shift the Fermi momenta and the state energies.
The ABC-stacked N-layer graphites are characterized by the complicated
low-energy bands due to the stacking effect, on which F has a great
influence—the change of the state energies and the subband
spacing, the opening of a band gap, the production of the oscillating
bands, and the increase of the band-edge states. As a result, the two
kinds of special structure, whose positions and heights are modulated by
F, are found in the density of states (DOS) in contrast to the
featureless DOS of the AA systems. The comparison with the AB-stacked
few-layer graphites is also made.
Journal of the Physical Society of Japan 02/2007; 76(2):4701-. · 2.09 Impact Factor
[show abstract][hide abstract] ABSTRACT: The π-electronic excitations are studied for the AA- and AB-stacked bilayer graphites within the linear self-consistent-field approach. They are strongly affected by the stacking sequence, the interlayer atomic interactions, the interlayer Coulomb interactions, and the magnitude of the transferred momentum. However, they hardly depend on the direction of the transferred momentum and the temperature. There are three low-frequency plasmon modes in the AA-stacked system but not the AB-stacked system. The AA- and AB-stacked plasmons exhibit the similar π plasmons. The first low-frequency plasmon behaves as an acoustic plasmon, and the others belong to optical plasmons. The bilayer graphites quite differ from the monolayer graphite and the AB-stacked bulk graphite, such as the low-frequency plasmons and the small-momentum π plasmons.
Physical Review B 08/2006; 74(8). · 3.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: In the presence of a perpendicular electric field, the low-energy electronic properties of the AB-stacked N-layer graphites with layer number N = 2, 3, and 4, respectively, are examined through the tight-binding model. The interlayer interactions, the number of layers, and the field strength are closely related to them. The interlayer interactions can significantly change the energy dispersions and produce new band-edge states. Bi-layer and four-layer graphites are two-dimensional semimetals due to a tiny overlap between the valence and conduction bands, while tri-layer graphite is a narrow-gap semiconductor. The electric field affects the low-energy electronic properties: the production of oscillating bands, the cause of subband (anti)crossing, the change in subband spacing, and the increase in band-edge states. Most importantly, the aforementioned effects are revealed completely in the density of states, e.g. the generation of special structures, the shift in peak position, the change in peak height, and the alteration of the band gap.
[show abstract][hide abstract] ABSTRACT: Electronic states of carbon tori in electric and magnetic fields are studied by the tight-binding model with the curvature effect. Electronic properties, such as electronic states, energy gaps, and density of states, are very sensitive to the changes in the direction and the magnitude of the external fields. The electric field can widen the � -electron energy width; furthermore, there are more low- and extreme- energy states. Energy gaps are drastically modulated by E. The zero-gap transition (Eg 6¼ 0 ! Eg ¼ 0) happens more frequently when E deviates from the symmetric axis, or its magnitude is sufficiently large. The electric field could change the state degeneracy. Moreover, the modulation of electronic states is enhanced by the magnetic field.
Journal of the Physical Society of Japan 01/2006; 75(10). · 2.09 Impact Factor