Showing papers by "Roberto Car published in 1999"

Two-Dimensional Self-Assembly of Supramolecular Clusters and Chains

Abstract: Two-dimensional supramolecular clusters and chains are observed upon submonolayer deposition of 1-nitronaphthalene (NN) onto reconstructed Au(111) The molecules become pseudochiral upon adsorption Their handedness is determined from high-resolution scanning tunneling microscope images and local-density calculations Modeling shows that hydrogen bonds cause the observed self-assembly Clusters and chains mutually interact via electrostatic repulsion

Structural and electronic properties of composite BxCyNz nanotubes and heterojunctions

Abstract: The structure, stability and electronic properties of composite BxCyNz nanotubes and related heterojunctions have been studied using both ab initio and semi-empirical approaches. Pure BN nanotubes present a very stable quasi particle band gap around 5.5-6.0 eV independent of the tube radius and helicity. The bottom of the conduction bands is controlled by a nearly-free-electronn state localized inside the nanotube, suggesting interesting properties under doping. In the case of nanotubes with BC2N stoichiometry, we show that in the thermodynamic limit the system is driven towards segregation of pure C and BN sections. This demixing significantly affects the electronic properties of such materials. The same process of segregation into BC3 islands is evidenced in the case of B-doped carbon nanotubes. These spontaneous segregation processes lead to the formation of quantum dots or nanotube heterojunctions. In particular, C/BN superlattices or isolated junctions have been investigated as specific examples of the wide variety of electronic devices that can be realized using such nanotubes.

A comparison of methods for the calculation of NMR chemical shifts

Abstract: A theory (MPL) to compute the NMR chemical shifts in condensed matter systems using periodic boundary conditions was presented by F. Mauri, B. Pfrommer, and S. G. Louie [Phys. Rev. Lett. 77, 5300 (1996)]. The MPL method has been implemented so far within a pseudopotential formulation in which the wave functions are expanded in plane waves. In this paper, we compare analytically the MPL approach within the density functional theory to existing methods for the calculation of the chemical shifts such as GIAO (gauge-including atomic orbitals), CSGT (continuous set of gauge transformations), and IGAIM (individual gauges for atoms in molecules). To this end we apply the MPL approach to molecules since the latter methods are conceived only for finite systems. We show theoretically the equivalence between a variant of the CSGT and the MPL method applied to finite systems. Moreover, we analyze numerically the efficiency of the different methods when atomic orbital basis sets are employed, by comparing the basis-se...

Boron-mediated growth of long helicity-selected carbon nanotubes

Abstract: We investigate the growth of B-doped carbon nanotubes combining experimental and theoretical techniques. Electron microscopy observations and electron diffraction patterns reveal that B doping considerably increases the length of carbon tubes and leads to a remarkable preferred zigzag chirality. These findings are corroborated by first-principles static add dynamical simulations which indicate that, in the zigzag geometry, B atoms act as a surfactant during growth, preventing tube closure. This mechanism does not extend to armchair tubes, suggesting a helicity selection during growth.

Electronic structure at carbon nanotube tips

Abstract: We discuss a number of recent observations on carbon nanotubes tips. Evidence from scanning tunneling microscopy (STM) and spectroscopy (STS) experiments [1] indicates that the topology of the carbon bond network at the tip, and in particular the relative location of pentagons in the curved regions of the caps, is ultimately responsible for the space-resolved electronic properties of the tubes. The theoretical analysis of the local density of (electronic) states (LDOS) is shown to be a powerful tool to rationalize the experimental data. Images of carbon nanotube tips acquired in field emission microscopy (FEM) experiments [2] can be related to the STM/STS observations.

Microscopic growth mechanisms for carbon and boron-nitride nanotubes

Abstract: The growth of carbon (C) and boron nitride (BN) nanotubes cannot be directly observed and the underlying microscopic mechanism is a controversial subject. Transition metal catalysts are necessary to produce single-walled nanotubes (SWNT) of carbon, but they are not needed in the multi-walled (MWNT) case, suggesting different growth mechanisms. Here we report on the results of first-principles dynamical simulations of both single- and double-walled carbon nanotube edges. We find that the open end of carbon SWNTs spontaneously closes by forming a graphitic dome in the 2500-3000 K temperature range of synthesis experiments. On the other hand, "lip-lip" interactions, consisting of chemical bonding between the edges of adjacent coaxial tubes, trap the end of the double-walled carbon nanotube into a metastable energy minimum, preventing dome closure. The resulting end geometry is highly chemically active, and can easily accommodate incoming carbon fragments? thus allowing growth by chemisorption from the vapour phase. The growth mechanisms of boron nitride SWNTs is studied as well, and is compared to the case of pure carbon tubes. In the experimental temperature conditions, the behavior of growing BN nanotubes strongly depends on the nanotube network helicity. In particular, we find that open-ended "zigzag" SWNTs close rapidly into an amorphous like tip? preventing further growth. In the case of "armchair" SWNTs, the formation of sro squares traps the tip into a flat cap presenting a large central even-member ring. This structure is metastable and able to revert to a growing hexagonal framework by incorporation of incoming atoms. These findings are directly related to frustration effects, namely that B-N bonds are energetically favored over B-B and N-N bonds.

Stability of two-dimensional nanostructures

Abstract: We investigate atomic and molecular nanostructures on metal surfaces by variable low-temperature scanning tunnelling microscopy. In combination with molecular dynamics calculations we achieve a detailed understanding of the stability of these structures.¶Atomic nanostructures in homoepitaxial metallic systems are thermodynamically only metastable. Two-dimensional islands on Ag(110) decay above a threshold temperature of Tl=175 K. Caused by the anisotropy of the surface, distinct decay behaviours exist above and below a critical temperature of Tc=220 K. Calculations based on effective medium potentials of the underlying rate limiting atomic processes allow us to identify the one-dimensional decay below Tc as well as the two-dimensional decay above Tc.¶In contrast to atoms, the intermolecular electrostatic interaction of polar molecules leads to thermodynamically stable structures. On the reconstructed Au(111) surface, the pseudo-chiral 1-nitronaphthalin forms two-dimensional supermolecular clusters consisting predominantly of ten molecules. Comparison of images with submolecular resolution to local density calculations elucidates the thermodynamical stability as well as the internal structure of the decamers.