Session N31: Carbon Nanotubes: Theory

Cyanide Nanotubes

The discovery of carbon nanotubes (CNTs) has given birth to an entire field devoted to the study of these one-dimensional (1D) nano-scale structures with extraordinary properties and tremendous promise for applications. To mention but a few, single wall carbon nanotubes are reported to exhibit Luttinger liquid behavior and proximity-induced superconductivity, and can be efficient hydrogen storage systems. The electronic properties of a carbon nanotube are fully determined by its helicity and range from metallic to semiconducting. However, when growing nanotubes, it is not possible to control the helicity; thus, carbon nanotube properties are not a result of design but luck. To overcome this limitation, Cohen and coworkers predicted the existence of insulating boron-nitride nanotubes (BNTs) and Zettl produced such tubes experimentally; these tubes are semiconducting and their properties vary less with helicity. Here we propose another type of structurally simple and energetically stable nanotubes consisting of transition metals and cyanide units. which are semiconductors with large band gaps ($\sim 2-3$ eV). Using first-principles calculations, we study the properties of these nanotubes and find that their helicity does not significantly affect the electronic band gap. The nature of bonding in these systems singles out a particular helicity as energetically more stable, suggesting that only one type of tube will be predominantly formed with well defined electronic properties.   

Electronic Structure of Core-Shell Semiconductor Nanowires

We investigate the electronic structure of silicon/germanium core-shell nanowires with first-principles calculations using the local density approximation (LDA) with pseudopotentials and plane waves. The atomic configurations of the core-shell nanowires are fully relaxed. By examining the wave functions in real space, the electronic states at the band edge are found to be localized in either the core or the shell part of the nanowire. The band offsets are calculated for different core-shell structures. Given the cylindrical band offsets and the associated confined electronic states, a novel doping mechanism in nanowires is proposed for the manufacturing of high-speed nano-devices.   

Dielectric properties of carbon nanotubes from first principles

We characterize the response of single- (SWNT) and multi-wall (MWNT) carbon nanotubes to static electric fields using first-principles calculations and density-functional theory. The longitudinal polarizability of SWNTs scales as the inverse square of the band gap, while in MWNTs it is given by the sum of the polarizabilities of the constituent tubes. The transverse polarizability of SWNTs is insensitive to band gaps and chiralities and is proportional to the square of the effective radius; in MWNTs the outer few layers dominate the response. The transverse response is intermediate between metallic and insulating, and a simple electrostatic model based on a scale-invariance relation captures accurately the first-principles results. Dielectric response in both directions remains linear up to very high values of the applied field.   

Atomic-scale and electronic structure of double-walled carbon nanotubes.

Ultra-high vacuum scanning tunneling microscopy (STM) and spectroscopy have been used to elucidate the electronic and atomic-scale structure of double-walled carbon nanotubes$^{1}$ (DWNTs) on the Si(100) 2x1:H surface. Atomically clean DWNT-surface interfaces were facilitated by an \textit{in situ} deposition method$^{2}$ which enables simultaneous resolution of the DWNT chirality and surface atomic structure. A key result includes the observation of periodic 2.7 nm spatial modulation superimposed on the nanotube chirality at both positive and negative scanning biases for a 2 nm diameter semiconducting DWNT. The periodic modulation of the DWNT topography suggests the lattices of the inner and outer nanotubes produce an interference pattern depending on the relative alignment of the constituent carbon atoms. Experimental data (diameter, chiral angle, and local density of state measurements) will be supplemented with simulated STM images which illustrate subtle changes in the outer nanotube topography depending on the inner nanotube chirality. 1. DWNTs synthesized by Nanocyl (www.nanocyl.com) 2. P.M. Albrecht and J.W. Lyding. Appl. Phys. Lett. 83, 5029 (2003).   

All-electron electronic properties of carbon nanotubes through multiresolution analysis

Utilizing our latest developments of density functional calculations using multiresolution wavelet-like basis we compute the electronic properties of carbon nanotubes by including all electrons.   

Molecular Dynamics Simulations of DNA-Functionalized Carbon Nanotube Chemical Sensors

We have conducted all-atom classical molecular dynamics simulations onDNA-functionalized carbon nanotube chemical sensors, including the presence of water. Our simulations verify that single stranded DNA (ssDNA) binds to a single-wall carbon nanotube (swCN) via a pi-pi stacking interaction. Preliminary simulations of a partially hydrated system also suggest that the ssDNA conformation about a swCN exhibits nanoscale pockets that can result in additional binding sites for analytes. Molecular dynamics simulations have also been performed to determine binding orientations of analytes adsorbed to the swCN-ssDNA system. To determine possible chemical gating effects of analytes on the swCN, we numerically calculate changes in the electric potential at the surface of the swCN due to the introduction of ssDNA and analytes. Results of further simulations of a fully hydrated system will also be presented   

Fluxional handles for direct control of conductance in functionalized carbon nanotubes

A class of covalent functionalizations for single-wall carbon nanotubes is identified --- from extensive first-principles calculations --- that preserves the conduction channels of metallic nanotubes. Cycloaddition of carbenes or nitrenes can induce bond cleaving between two adjacent sidewall carbons, restoring their original $sp^2$ hybridization and recovering in the process a transparent $\pi$ manifold, radically at variance with the strong scattering permanently induced by other common covalent functionalizations. The chirality and curvature of the nanotube and the chemistry of the addends can force or inhibit this bond cleavage, that in turn controls very distinctly the transport properties of the functionalized conductor. A well-defined range of diameters can be found for which certain addends - such as dicyanocarbene - exhibit a bistable switchable state, where the opening or closing of the sidewall bonds, and the accompanying on/off switch in the conductance, can be directed with chemical, electrochemical or optical means.   

Silicon-carbon nano-structures to nano-tubes.

There have been continuing efforts to stabilize silicon cage-type nano-structures or nano-tubes which can be used in similar ways as the carbon-based fullerene structures. This is due to the fact that the current semiconductor industries are based on silicon. Silicon carbide is the focus of scientific research due to its potential use even in extreme conditions, such as extreme high-temperature, high-power capabilities and high radiation conditions. In the present study, a set of novel silicon carbon nanosructures (Si$_{2n}$C$_{n})$ in tubular form have been studied which can be extended to form silicon carbide nano-tubes. Generalized gradient approximation to density functional theory has been used with an all electron basis set to study the stability of these structures. A frequency analysis has been performed to ensure that all the frequencies are real. A slight structural shift has been predicted between the hydrogen saturated and --unsaturated nano-tubes.   

\emph{Ab initio} study of semiconducting carbon nanotubes adsorbed on the Si(100) surface: diameter- and registration-dependent atomic configurations and electronic properties

We present a theoretical study within density functional theory in the local density approximation of semiconducting carbon nanotubes adsorbed on the unpassivated Si(100) surface. We find that the interaction between the nanotube and silicon surface results in significant atomic re-arrangment of the surface atoms. Since the spatial configuration of the surface dimers determines to a great extent the electronic properties of the surface, our first-principles calculations indicate a tendency towards metallicity for the semiconducting tube-Si(100) surface system. We confirm this for nanotubes of different diameters and chiral angles, and find the effect to be independent of the orientation of the nanotubes on the surface. Reference: cond-mat/0510477 and references therein.   

A plasmon absorption model for a super-lattice of single-walled carbon nanotubes

Several years ago one of us (P. F. Williams*) developed a self-consistent dielectric response model for one-dimensional metals at high frequency using a tight-binding approximation. At the time this model was found useful in a study of the single-particle excitations and plasmon dispersion curves of tetrathiofulvalene-tetracyano-quinodimethane (TTF-TCNQ). This paper is a report of work in progress to extend this model to arrays of single-walled carbon nanotubes. First the quasi one-dimensional model is extended to represent free electrons confined to the surfaces of cylindrical shells arranged in a 2-D square array. The collective electronic excitations of this system are characterized by a frequency and wavelength dependent complex dielectric constant obtained using the method of self-consistent fields in the random phase approximation. Progress in extending the model to arrays of shells with surface structure will also be discussed. *P. F. Williams and A. N. Bloch, Phys. Rev. B, \textbf{10, }1097 (1974)   

Numerical Results for SU(4) and SU(2) Kondo Effect in Carbon Nanotubes

New numerical results are presented for the Kondo effect in Carbon Nanotube (CNT) quantum dots (QDs). As recently reported by P. Jarillo-Herrero {\it et al.} (Nature {\bf 434}, 484 (2005)), the Kondo effect in CNTs presents an SU(4) symmetry, which arises from the entanglement of orbital and spin degrees of freedom. As the number of co-tunneling processes increases, thanks to the extra (orbital) degree of freedom, the Kondo temperature reaches a high value of $T_K=7.7K$. Interesting considerations can be drawn regarding the change from SU(4) to SU(2) symmetries depending on the hopping matrix elements between the leads and the CNT QD. Our results will analyze the transition between the SU(4) and the so-called two-level SU(2) (2LSU(2)) Kondo regimes induced by the variation of the coupling of the QD to the leads. The effect of an external magnetic field along the tube direction will also be analyzed. Our results will be compared with available Numerical Renormalization Group (NRG) results by M-S Choi {\it et al.} (Phys. Rev. Lett. {\bf 95}, 067204 (2005)). A comparison with the experimental results will be made to gauge the adequacy of the model and approximations made.   

Deep levels in the band gap of the carbon nanotube with vacancy-related defects

We study the modification in the electronic structure of the carbon nanotube induced by vacancy-related defects using the first-principles calculation. Three defect configurations which are likely to occur in semiconducting carbon nanotubes are considered. A vacancy-adatom complex is found to bring about a pair of localized states deep inside the energy gap. A pentagon- octagon-pentagon topological defect produced by the divacancy is structurally stable and gives rise to an unoccupied localized state in the gap. We also discuss the character of partially- occupied localized state produced by a substitutional impurity plus a monovacancy.   

Transport Properties in Carbon Nanotubes films for Hydrogen Sensor Applications

Thin single-walled carbon nanotue (CNT) films doped with palladium (Pd) on surface are shown to be promising in hydrogen(H2) sensor applications. We study electronic transport properties of CNT/Pd/H2 system by combining first-principles band structure calculations with Boltzmann transport theory. The coupling between Pd atoms and carbon nanotues is described by the Kondo model. An effective scattering potential, which creates the major resistance in nanotubes, is calculated by fitting potential parameters to the calculated band structures. In addition, intersection resistance between the nanotubes is also included in our simulations. We will present the band structure of the CNT/Pd system with and without the hydrogen atoms or hydrogen molecules. The conductance change of the whole nanotube films in the presence of hydrogen gas will be shown.   

Theoretical Study of Iron Filled Carbon Nanotubes

We have investigated, using ab-initio methods, the geometry and magnetic structure of free standing and encapsulated iron nanowires, both in perfect and defective single wall carbon nanotubes. The geometries adopted consist of two layers of iron atoms per unit cell, arranged in hcp(0001) and bcc(011) structures, repeated periodically along the wire axis, When the ratio of the nanowire to nanotube diameter is small there is an attractive interaction among them and the density of states at the Fermi energy corresponds to a single spin orientation, as for the free standing nanowires. These systems are therefore potentially interesting for spintronics. When the same ratio is close to one the systems are less stable and a tendency towards antiferromagnetic ordering is observed due to the confinement.