Stepped single-crystal surfaces are viewed as models of real catalysts, which consist of small metal particles exposing a large number of low-coordination sites. We found that stepped platinum (Pt) surfaces can undergo extensive and reversible restructuring when exposed to carbon monoxide (CO) at pressures above 0.1 torr. Scanning tunneling microscopy and photoelectron spectroscopy studies under gaseous environments near ambient pressure at room temperature revealed that as the CO surface coverage approaches 100%, the originally flat terraces of (557) and (332) oriented Pt crystals break up into nanometer-sized clusters and revert to the initial morphology after pumping out the CO gas. Density functional theory calculations provide a rationale for the observations whereby the creation of increased concentrations of low-coordination Pt edge sites in the formed nanoclusters relieves the strong CO-CO repulsion in the highly compressed adsorbate film. This restructuring phenomenon has important implications for heterogeneous catalytic reactions.

We studied the adsorption of single atoms on a semiconducting and metallic single-wall carbon nanotube from first principles for a large number of foreign atoms. The stable adsorption sites, binding energy, and the resulting electronic properties are analyzed. The character of the bonding and associated physical properties exhibit dramatic variations depending on the type of the adsorbed atom. While the atoms of good conducting metals, such as Cu and Au, form very weak bonding, atoms such as Ti, Sc, Nb, and Ta are adsorbed with relatively high binding energy. Most of the adsorbed transition-metal atoms excluding Ni, Pd, and Pt have a magnetic ground state with a significant magnetic moment. Our results suggest that carbon nanotubes can be functionalized in different ways by their coverage with different atoms, showing interesting applications such as one-dimensional nanomagnets or nanoconductors and conducting connects, etc. DOI: 10.1103/PhysRevB.67.201401 PACS number͑s͒: 73.22.Ϫf, 68.43.Bc, 73.20.Hb, 68.43.Fg Single-wall carbon nanotubes ͑SWNT's͒ can serve as templates to produce reproducible, very thin metallic wires with controllable sizes.1 These metallic nanowires can be used as conducting connects and hence are important in nanodevices based on molecular electronics. Recently, Zhang et al.2 have shown that a continuous Ti coating of varying thickness and a quasicontinuous coating of Ni and Pd can be obtained by using electron-beam evaporation techniques. Metal atoms such as Au, Al, Fe, Pb were able to form only isolated discrete particles or clusters instead of a continuous coating of SWNT's. Low-resistance contacts to metallic and semiconducting SWNT's were achieved by Ti and Ni ohmic contacts.3 Most recently, ab initio density-functional calculations 4 have indicated that stable rings and tubes of Al atoms can form around a semiconducting SWNT. It is argued that either persistent currents through these conducting nanorings, or conversely very high magnetic fields can be induced at their center. 4 It is expected that novel molecular nanomagnets and electromagnetic devices can be generated from these metallic nanostructures formed by adatom adsorption on SWNT's. As an example, one can contemplate to generate a nanodevice by the modulating adsorption of adatoms on a bare ͑8,0͒ SWNT, which is a semiconductor 5 with an energy gap of ϳ0.64 eV. This band gap can increase to 2 eV by the adsorption of a hydrogen atom.6 Then, a quantum well ͑or dot͒ can form between two barriers at the hydrogen covered sections of the ͑8,0͒ tube. This structure is connected to the metallic reservoirs through metal coated ends of SWNT's. This way a resonant tunneling device with metal reservoirs and connects at both ends can be fabricated on a single SWNT.Clearly, the study of adsorption of atoms on nanotube surfaces is essential to achieve low-resistance ohmic contacts to nanotubes, to produce nanowires with controllable size, and to fabricate functional nanodevices. In particular, it is important to know the following: ͑i͒ Ho...

We investigated interaction between hydrogen molecules and bare as well as functionalized singlewall carbon nanotubes (SWNT) using first-principles plane wave method. We found that the binding energy of the H2 physisorbed on the bare SWNT is very weak, and can be enhanced neither by increasing the curvature of the surface through radial deformation, nor by the coadsorption of Li atom that makes the semiconducting tube metallic. Though the bonding is strengthened upon adsorption directly to Li atom, yet its nature continues to be physisorption. However, the character of the bonding changes dramatically when SWNT is functionalized by the adsorption of Pt atom. Single H2 is chemisorbed to Pt atom on the SWNT either dissociatively or molecularly. If Pt-SWNT bond is weakened either by displacing Pt from bridge site to a specific position or by increasing number of the adsorbed H2, the dissociative adsorption of H2 is favored. For example, out of two adsorbed H2, first one can be adsorbed dissociatively, second one is chemisorbed molecularly. The nature of bonding is weak physisorption for the third adsorbed H2. Palladium also promotes the chemisorption of H2 with relatively smaller binding energy. Present results reveal the important effect of transition metal atom adsorbed on SWNT and advance our understanding of the molecular and dissociative adsorption of hydrogen for efficient hydrogen storage.

We show extensive theoretical studies related to the generation and characterization of 2D and 3D ordered networks using 1D units that are connected covalently. We experimentally created multi-terminal junctions containing 1D carbon blocks in order to study the most common morphologies and branched structures that could be used in the theoretical design of network models. We found that the mechanical and electronic characteristics of ordered networks based on carbon nanotubes (ON-CNTs) are dominated by their specific super-architecture (hexagonal, cubic, square, and diamond-type). We show that charges follow specific paths through the nodes of the multi-terminal systems, which could result in complex integrated nanoelectronic circuits. The 3D architectures reveal their ability to support extremely high unidirectional stress when their mechanical properties are studied. In addition, these networks are shown to perform better than standard carbon aerogels because of their low mass densities, continuous porosities, and high surface areas.

First-principles calculations show that monatomic strings of carbon have high cohesive energy and axial strength, and exhibit stability even at high temperatures. Due to their flexibility and reactivity, carbon chains are suitable for structural and chemical functionalizations; they form also stable ring, helix, grid and network structures. Analysis of electronic conductance of various infinite, finite and doped string structures reveal fundamental and technologically interesting features. Changes in doping and geometry give rise to dramatic variations in conductance. In even-numbered linear chains strain induces substantial decrease of conductance. The double covalent bonding of carbon atoms underlies their unusual chemical, mechanical and transport properties.