Session H31: Transport in Carbon Nanotubes: Theory

First-principles studies of electrical transport in metal-contacted semiconducting carbon nanotubes.

We present first-principles calculations of the transport properties of semiconducting carbon nanotubes (CNT's), coupled to metallic electrodes. Our results indicate that, for realistic end-contact geometries, including atomic relaxation, the Fermi level position within the gap differs between palladium-contacted CNT's and gold-contacted CNT's. More interestingly, the contact resistance for the valence band in the case of Pd is much smaller than in the case of Au, while no significant difference is observed for the conduction band. This could explain experimental results showing that hole conduction is favored in the case of Pd contacts.   

First Principle Study of Electronic Transport in Carbon Nanotubes and Copper Nanowires for Interconnect Applications

We will present our recent first principles calculation modeling work on carbon nanotubes (CNT) and copper wires for Interconnect applications. In particular we have calculated the ballistic transport properties of nanotubes based on their density of states and band structures, and compared with that of copper wires of similar dimension. By using Ohm's law and Landauer Formalism, we computed the resistance of them in mesoscopic sizes. The effect of correlation in the transport properties are discussed in detail. We will present our work on the nanowires and nanotubes packing and their impact on the resistance, while taking into account the surface scattering based on Fuchs-Sondheimer model. The performance of CNT for both local and global Interconnects will be discussed in detail. Our results show that nanotube bundles can outperform copper wires for long intermediate and global interconnects.   

Interwall interactions and electrical conductance in telescoping carbon nanotubes

Telescopically aligned carbon nanotubes, where the inner core shells are pulled out from the house shells with larger diameters in multi-walled nanotubes, are good systems to interwall interactions and their effect on electron conduction. In several tight-binding calculations, there exists some controversy in the quantum conductance of telescoping nanotubes. In this work, using the non-equilibrium matrix Green function approach within the first principles local-density-functional approximation, we study the quantum transport behavior of the (5,5)/(10,10) telescoping nanotube. Varying the hybridized double wall region, we investigate the effect of interwall interactions on the electron transport and compare the results with those obtained from tight-binding calculations. Although individual tubes have two conducting channels at the Fermi level, only one channel gives rise to electrical conduction with antiresonance dips in transmission, while the other channel is suppressed. Thus, the maximum conductance is close to $G_{0}$, in contrast to single $\pi $-orbital tight-binding calculations, which showed the maximum conductance close to 2$ G_{0}$. Our first-principles calculations indicate that the tight-binding model significantly overestimates the interwall coupling between the inner and outer shells.   

Quantum transport in carbon nanotube field effect transistors

We have investigated the transport properties of carbon nanotube field effect transistors using the recently developed source-and-sink method [1]. We report first-principles results on the current-voltage characteristics of semiconducting carbon nanotubes in transverse electric field, highlighting differences with Si-based devices, e.g., band mixing caused by the gate electric field. We also find that the source-drain current exhibits an intrinsic saturation as function of the gate voltage. The calculated results are in good overall agreement with pertinent experiments. \newline \newline [1] K. Varga and S. T. Pantelides, Phys. Rev. Lett. submitted for publication.   

Signature of the electron-electron interaction in the magnetic field dependence of the nonlinear I-V characteristics in non-centrosymmetric conductors

In non-centrosymmetric media, there exists a contribution to the nonlinear I-V characteristics which is linear in magnetic field and quadratic in voltage. This effect is entirely due to electron-electron interaction, and its magnitude is proportional to the electron-electron interaction constant. We present calculations of the magnitude of this effect in mesoscopic samples and in chiral carbon nanotubes as a function of temperature. In the case of a magnetic field oriented parallel to plane of a mesoscopic sample, the effect is proportional to both the electron-electron interaction constant and the spin-orbit scattering amplitude.   

\textit{Ab initio} investigations of formation of the poly-bromine anions encapsulated inside the carbon nanotube.

We have performed \textbf{\textit{ab initio}} density-functional calculations to investigate the electronic and geometric structure of the bromine adsorbates inside the carbon nanotube. It is found that the charged odd-membered molecular species (Br$_{3}$ or Br$_{5}$ ) are energetically favored inside the carbon nanotube rather than common Br$_{2}$ molecule. Vapor phase of bromine molecules (Br$_{2})$ could exothermically adsorb into the nanotubes, and in turn, transform into the Br$_{3}$ or Br$_{5}$ structures without a significant energy barrier. Such a formation of the poly-bromine anions accompanies a strong charge transfer from the nanotube to the adsorbates, rendering the encapsulating nanotube strongly hole-doped. We suggest that an exposure of the tip-opened carbon nanotube samples to a modest Br$_{2 }$partial pressure could result in strong hole-doped, and thus nearly metallic nanotube samples.   

Electron transport through molecular-carbon nanotube interfaces

Investigations have focused on electron transport through metal-molecule systems. Less effort has been directed towards semiconductor-molecule systems, and the least attention has been given to electron transport through carbon nanotube-molecule systems. A specific implementation of the latter system consists of two CNTs joined by a molecule, or a CNT-molecule-CNT system. Such a system can provide the electronic functionality of a resonant tunnel diode. The molecular contacts, i.e. the CNTs, are a $\pi $-bond surface and, as such, they are both chemically and geometrically different from metal contacts or sp$^{3}$ semiconductor contacts. A model system is studied to focus solely on the interface geometry of two simple $\pi $-bond systems, CNTs and polyacetylene (CH)$_{n}$. The system is CNT-(CH)$_{n}$-CNT. At the interface, in the relaxed structure, the (CH)$_{n}$ is oriented coplanarly with the tangential plane of the CNT. The transmission, calculated with our DFT (FIREBALL)-NEGF code is, on average, 3 or more orders of magnitude larger than the transmission of an unrelaxed structure in which the (CH)$_{n}$ is perpendicular to the CNT at the point of contact. This is also true when the (CH)$_{n}$ of the relaxed structure undergoes a 180$^{o}$ twist. Interface geometry plays a crucial role in the electron transport.   

Effects of Electron-phonon scattering on Conductance of Carbon nanotubes using Time-dependent wave-packet approach

The application of single-walled carbon nanotubes as the ideal ballistic conductors is expected. However, the electronic current saturates at the high-bias regime due to electron-phonon scattering. In order to improve the conductivity, understanding of the scattering mechanism is highly required. We investigated the electron-phonon coupling effect on the conductance in single-walled carbon nanotubes using the time-dependent wave-packet approach under a tight-binding approximation [1]. The vibrational atomic displacements in real space are introduced through the time-dependent change of the transfer energies. We solve the time-dependent Schr\"odinger equation and obtain the time-dependent diffusion coefficients of the electronic wave packets. From these data, we can extract the coherence length and then the conductance. We found that the optical phonon decreases the conductance of metallic carbon nanotubes, because the propagating speed of electron is reduced by the electron-phonon scattering. Furthermore, we clarify the difference of the scattering effects on the conductivity of the metallic nanotube and the semiconducting one. [1] S. Roche \textit{et al.}, PRL 95 (2005) 076803   

Model Calculation for Molecular Junctions in between Carbon Nanotube Leads.

We present analytical and numerical calculations for several prototypes of molecules in a CNT-molecule-CNT junction. The properties of the transmission function at the Fermi level reveal the influence from the CNT symmetry and the nature of the molecules. In particular, we discuss and compare both one-point contact and two-point contact cases, so as to illustrate how the Fermi level transmission of a multi-molecule junction can be either larger or smaller than that in the single-molecule case, depending on the choice of contact sites.   

Quantum electron transport in toroidal carbon nanotubes with metallic leads.

Carbon nanotubes and carbon nanotori possess all the interesting new electronic features seen in graphene e.g. massless Dirac fermion characteristics, small spin-orbit coupling effects, and quantized conductance, along with interesting curvature and boundary condition effects closing the tube to form a torus. The authors calculate electronic transport properties such as density-of-states and transmissivity for toroidal carbon nanotubes with attached metallic or carbon nanotube leads as functions of the lead positions. A tight-binding Hamiltonian for the nanotorus is applied to a 24-carbon-atom armchair unit cell. The closure of the straight tube to a toroidal geometry introduces an additional off-diagonal coupling term, not encountered for the straight case. The device Green's function is then evaluated in tight-binding approximation using a recursion method to systematically determine its diagonal and off-diagonal matrix elements. {\it References:} 1. M. Encinosa and M. Jack, Phys. Scr. 73 (2006) 439-442. 2. M. Encinosa and M. Jack, Excitation of surface dipole and solenoidal modes on toroidal structures. Photonics and Nanostructures (Elsevier), May 2006. (Submitted) 3. M. Encinosa and M. Jack, Dipole and solenoidal magnetic moments of electronic surface currents on toroidal nanostructures. J. of Computer-Aided Materials Design (Springer), May 2006. (In Press)   

Mechanical and electronic properties at the interface between the Si(100) surface and semiconducting carbon nanotubes

I discuss the \emph{ab initio} mechanical and electronic properties of semiconducting carbon nanotubes adsorbed on the Si(100) surface. After revising results from nanotubes on the fully unpassivated surface[1], the interaction between a semiconducting nanotube and a fully H-passivated Si(100) surface with dopants is examined[2]. As silicon wafers are ordinarily doped, the model closely resembles experimental onditions[2,3,4], allowing for qualitative comparison. The single H-monolayer prevents electronic states in nanotubes from energetically shifting along with those of the doped supporting substrate, permitting the engineering of the relative positions of the slab and nanotube band edges. Finally, and following experimental work, we study adsorption characteristics of nanotubes on partially passivated surfaces. Surface states in the unpassivated regions modify the electronic structure of the interface and provide for the anchoring of nanotubes, deforming them in some cases. Results with and without dopants will be given[2].\\ 1 S. Barraza-Lopez et al. J. Appl. Phys. (in press). 2 Submitted. 3 Appl. Phys. Lett 83, 5029 (2003). 4 P. M. Albrecht and J. W. Lyding, Small (in press).