Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Chainmail for catalysts: a catalyst with iron nanoparticles confined inside pea-pod-like carbon nanotubes exhibits a high activity and remarkable stability as a cathode catalyst in polymer electrolyte membrane fuel cells (PEMFC), even in presence of SO(2). The approach offers a new route to electro- and heterogeneous catalysts for harsh conditions.

Iron Encapsulated within Pod‐like Carbon Nanotubes for Oxygen Reduction Reaction

In this critical review we survey non-covalent interactions of carbon nanotubes with molecular species from a chemical perspective, particularly emphasising the relationship between the structure and dynamics of these structures and their functional properties. We demonstrate the synergistic character of the nanotube–molecule interactions, as molecules that affect nanotube properties are also altered by the presence of the nanotube. The diversity of mechanisms of molecule–nanotube interactions and the range of experimental techniques employed for their characterisation are illustrated by examples from recent reports. Some practical applications for carbon nanotubes involved in non-covalent interactions with molecules are discussed.

Noncovalent interactions of molecules with single walled carbon nanotubes

Filling single-wall carbon nanotubes

Preparation and modification of carbon nanotubes: Review of recent advances and applications in catalysis and sensing

Improved field emission properties of double-walled carbon nanotubes decorated with Ru nanoparticles

We investigate the electronic structure of the fullerenes encapsulated inside carbon nanotubes, the socalled nanopeapods, using the first-principles study. The orbital hybridization of LUMO 1 (the state above the lowest unoccupied molecular orbital) of C60, rather than LUMO as previously proposed, with the nanotube states explains the peak at 1e Vin recent scanning-tunneling-spectroscopy (STS) data. For the endohedral metallofullerenes nested in the strained nanotube, the charge transfer shifts the relative energy levels of the different states and produces a spatial modulation of the energy gap in agreement with another STS experiment. The encapsulation of various kinds of molecules inside the carbon nanotube (CNT) by chemical or physical insertion methods [1] opened a new way to engineering the physical properties of the CNT. The acquired novel electronic and mechanical properties would be useful for practical applications. Recently, several groups have reported the observation of the single-walled nanotube with a chain of fullerenes inside, so-called carbon nanopeapods, through the high-resolution transmission electron microscopy or the electron diffraction pattern [2]. The encapsulation process is exothermic if the separation between the inserted fullerene and the wall of the nanotube is around the van der Waals distance ( 3:3 � A). The fullerene bigger than this size can still be incorporated depending on environment, albeit endothermically [3]. In the recent scanning-tunneling-microscopy (STM) experiment on the nanopeapod [4,5], it was found that the fullerenes or metallofullerenes encapsulated in the single-walled semiconducting nanotube significantly affect the local electronic structure of the tube. For example, the STM image on C60@CNT showed the modulation of the density of states (DOS) in the conduction band with the period of the underlying C60 array [4]. When C82 incorporating a gadolinium (Gd) atom is encapsulated [5], the band gap is found to decrease significantly at the location of C82. In this Letter, we have carried out first-principles pseudopotential calculations for the carbon nanopeapods to understand the above experiments. Based on the simulated STM and scanningtunneling-spectroscopy (STS) data, we show that the modification of the electronic structure of nanopeapods is explained in terms of the coupling strength between the fullerene and the CNT, charge transfer among constituents of the peapod, and the intermolecular distance between inserted fullerenes inside the nanotube. As the model systems for theoretical investigation, we choose C60@17; 0, C82@17; 0 ,a ndLa@C82@17; 0 peapods. The semiconducting 17; 0 nanotube has the same diameter and the band gap as those used in experiment [4,5], and its relatively small cell size is computationally manageable. Because of the computational difficulties in dealing with f-electrons in Gd with which the experiment in Ref. [5] was done, we replace Gd with an isovalent lanthanum (La) atom which would behave similarly to Gd in the present situation. The first-principles pseudopotential calculations are carried out within the local density approximation using a plane-wave basis set [6]. The ultrasoft pseudopotentials [7] are employed with the cutoff energy of 25 Ry. For La, we treat all 11 valence electrons (5s 2 5p 6 6s 2 5d 1 ) explicitly

Orbital Hybridization and Charge Transfer in Carbon Nanopeapods

Effects of various metal coating (Co, Ti, Pd, W, and Ru) on electronic structures of carbon nanotubes are systematically studied by both ab initio calculations and field-emission experiments. The theoretical results indicate that the adsorption of metal atoms leads to substantial changes in the band structures and work functions of nanotubes. In particular, titanium is found to be the most effective coating material for the application of nanotubes to the field emission display, by lowering the work function and increasing the local density of states near the Fermi level. This is confirmed by the field-emission experiments using Ti-coated nanotubes, which shows enhanced emission performances. In addition, it is found that the Ti coating extends the lifetime of the nanotube substantially. Through the thermogravimetric analysis and theoretical modeling, we propose that this is related to the role of metal coating as a protection layer against residual gases such as oxygen, which cause the degradation of nanotubes. The applications of metal-coated nanotubes to other types of electronic devices are also discussed.

Electronic structure tailoring and selective adsorption mechanism of metal-coated nanotubes.

Molecular orientation in vapor-deposited organic semiconductor films is known to improve the optical and electrical efficiencies of organic light-emitting diodes, but atomistic understanding is still incomplete. In this study, using all-atom simulation of vapor deposition, we theoretically investigate how the molecular orientation depends on various factors such as the substrate temperature, molecular shape, and material composition. The simulation results are in good agreement with experiment, indicating that the all-atom simulation can predict the molecular orientation reliably. From the detailed analysis of the dynamics of molecules, we suggest that the kinetics of molecules near the surface mainly determines the orientation of the deposited film. In addition, the oriented films have higher density and thermal stability than randomly oriented films. We also show that higher mobility of laterally oriented films can be explained in terms of the site-energy correlation.

All-atom simulation of molecular orientation in vapor-deposited organic light-emitting diodes

We investigate the vertical ionization potential (IP) and electron affinity (EA) of organic semiconductors in the solid state that govern the optoelectrical property of organic devices using a fully ab initio way. The present method combines the density functional theory and many-body perturbation theory based on $GW$ approximations. To demonstrate the accuracy of this approach, we carry out calculations on several prototypical organic molecules. Since IP and EA depend on the molecular orientation at the surface, the molecular geometry of the surface is explicitly considered through the slab model. The computed IP and EA are in reasonable and consistent agreements with spectroscopic data on organic surfaces with various molecular arrangements. However, the transport gaps are slightly underestimated in calculations, which can be explained by different screening effects between surface and bulk regions.

Youngmi Cho

Papers

Improved field emission properties of double-walled carbon nanotubes decorated with Ru nanoparticles

Orbital Hybridization and Charge Transfer in Carbon Nanopeapods

Electronic structure tailoring and selective adsorption mechanism of metal-coated nanotubes.

All-atom simulation of molecular orientation in vapor-deposited organic light-emitting diodes

Ab initio calculation of ionization potential and electron affinity in solid-state organic semiconductors