Session L7: Carbon Nanotube Hybrid Structures - Properties, Synthesis, Characterization

Self-assembly and electrostatic interactions of carbon nanotubes in nonpolar dielectric liquids

We report on the self-assembly of carbon nanotubes (CNTs) in nonpolar dielectric liquids under the influence of DC-generated electric fields. This process gives rise to electrostatic and hydrodynamic interactions of the CNTs in insulating nonpolar solvents. While some studies of the self-assembly of carbon nanotubes in response to a DC-field have been carried out in conductive solvents, the analysis of the self-assembly process is complicated by the current flow temporally affecting the particle charge and an assembly timescale of tenths to hundredths of seconds. In contrast, experiments in insulating liquids allow for the investigation of self-assembly processes where: the particle charge is not expected to change as a function of time and at a timescale of seconds this allows for an investigation of the transient states of the assembly process. In the presence of an electric-field, CNTs present in the solution experience an electrophoretic force due to their surface charges. When a DC field is applied across the electrodes, CNT bundles move according to their electrophoretic mobility. We find that the threshold voltage, above which the insulator-to-conductor transition occurs, varies sensitively as a function of zeta potential and hydrodynamic particle size. In addition, a percolation power law supports the observed threshold voltage as a function of CNT concentration and zeta potential.   

A Group Theoretic Approach to Nonlinear and Gradient Elastic Terms for Graphene and Carbon Nanotubes

Nonlinear and gradient corrections to the elastic theory of graphene and carbon nanotubes are important for determining the thermal conductivity, mechanical properties under large deformations, dissipation in nanotube oscillators and other mechanical properties. By applying group theory, we can systematically and efficiently work out all nonlinear elastic terms that can appear in a Ginzburg-Landau formulation of the elastic free energy in terms of the material displacement gradient tensor. Graphene serves as a perfect example material, having an interesting hexagonal symmetry and being a material in which nonlinear elastic terms are important. This approach allows us to demonstrate the usual cubic corrections. Additionally, we find higher order strain corrections which correspond to bending and rolling the graphene sheet, as well as gradient terms which are important for describing the long wavelength phonon dispersion. We will enumerate the most important contributions to the free energy, and measure the corresponding elastic constants with simulation.   

A Signature of Spatial Correlations between rare earth ions and single-wall nanotubes wrapped with DNA in their mixed solution

We propose that the fluorescence resonance energy transfer (FRET) between the rare earth ions (REI) and single-wall nanotubes (SWNT) can be used to measure their Coulomb correlation in solution. As a calibration experiment the FRET between two different REIs, being the energy donor and the acceptor, in their mixed solution has been used. From the photoluminescence decay time we were able to extract the characteristic distance between unlike REIs. Our study revealed negative correlation (the repulsion) for Tb-Eu solution. In the case of the solution containing the REI and the SWNTs wrapped with DNA we observed a significant positive correlation (the attraction and the complex formation). The data is in a good agreement with the theoretical estimates and allows to propose REIs and their FRET as a sensitive tool for detecting kinetics of interaction of SWNTs in aqueous solutions.   

Computational Modeling of Cancer Treatment Using Carbon Nanotubes

Laser thermal therapy selectively kills cancer cells without harming surrounding cells due to the high-energy absorbance of the functionalized carbon nanotubes (CNTs). However, the technique application is still very limited due to lack of experimental and computational works to understand how to kill cancer cells effectively and how the CNTs are heated by the external laser. The goal of this work is to present integrated computer models to capture changes in CNT heat transfer characteristics due to variations in the properties of CNTs and tissues during laser surgery. Numerical results show that the models are able to characterize variations of tissue properties for laser surgical procedures (by three dimensional finite element models) and predict anisotropic temperature fields within the CNTs (by finite different models). The current model is made more realistic and accurate than previous models by taking into account the anisotropic properties of CNTs and the energy loss from the CNT surface. The effects of the CNT concentration, morphology, orientations, and the power and heating duration of the laser are also investigated. The developed models help experimentalists to understand cancer treatment mechanisms and optimize operating conditions of the laser thermal therapy.   

Synthesis and Electronic Properties of Silicon-Nitrogen Hetero-doped Single Walled Carbon Nanotubes

We investigated the stability and electronic properties of hetero-doped carbon nanotubes using first-principles density functional theory. Silicon, silicon-nitrogen, and silicon-oxygen were incorporated within the lattice of different types of single-walled carbon nanotubes. The structural stability, electronic density of states, doping energy, band structure, HOMO and LUMO were analyzed. When silicon and nitrogen are bonded and inserted in the nanotube lattice, non-dispersive bands appear around the Fermi level. The Nitrogen arranged in a pyridine-like fashion together with a silicon atom placed inside the vacancy was also studied. The latter configuration becomes more stable than the substitutional nitrogen embedded in the (9,0) and (5,5) nanotubes. We have also succeeded in the synthesis of Si and SiN-doped single-walled carbon nanotubes (CSixNy -SWNTs) by chemical vapor deposition. We carried out Raman spectroscopy, high-resolution electron microscopy, electron energy loss spectroscopy, energy-dispersive X-Ray spectroscopy, Auger spectroscopy and X-Ray photoelectron spectroscopy, in order to identify the presence of both dopants within the nanotube lattice.   

Magneto-photoluminescence in lanthanide-bearing endohedral metallofullerenes with various cage symmetries

Taken as a family, endohedral metallofullerenes (EMF) nanomaterials provide opportunities for exquisite functional tunability at the nanoscale, enabling a wide range of synthetic nanoparticles with diverse sizes, symmetries, electronic, optical and, especially, magnetic properties. In particular, metallofullerenes incarcerating lanthanide ions will permit endohedral luminescence due to the 4f optically-active electrons being uninvolved in the stabilizing charge transfer between the endohedral guest and cage. In addition, if those lanthanide ions possess optical transitions beyond the absorption onset of the cage, a well-defined optical spectrum may be observed for the metallofullerene system. In this talk, several magneto-optical and time-resolved studies at high magnetic fields on lanthanide-based EMFs with different cage symmetries will be presented, where the residual magnetic degeneracies in the lanthanide ion energy levels are lifted and observed in the optical spectrum with magnetic field strengths in excess of 10 T.   

Exploring the Fundamental Properties of Carbon Nanotubes using Electromechanical Resonators

By monitoring the nanoelectromechanical (NEM) response of suspended individual carbon nanotubes (CNT), we measure the expansion coefficient of individual suspended single-walled CNTs. Here, the temperature dependence of the mechanical resonance frequency of NEM CNT resonator is used to determine the change in tension on the device caused by the expansion on the nanotube. We also measured the effects of gas adsorption on the CNT surface, by measuring change in conductivity and its response to gate doping. By monitoring the mass loading on the surface of the resonator we calculate the adsorption energy of the specific gasses on the surface of the nanotube.   

Fano Resonances in Mid-Infrared Spectra of Single-Walled Carbon Nanotubes

We show that optical phonon modes in single-walled carbon nanotubes (SWNTs) become observable in mid-infrared (MIR) spectroscopy by the means of Fano resonances. The scattering of a low energy electronic continuum over phonon discrete states yields anti-resonances that are recognizable in the spectra by their characteristic asymmetric line shape. Experimentally, we control the charge carrier density in SWNTs by \emph{p}~doping with different molecular oxidizers at saturation and compare the spectra of doped and intrinsic samples. The only measurable feature in the intrinsic state is a kink at $\sim865$~cm$^{-1}$. Kinks at $\sim1600$ and $\sim1250$~cm$^{-1}$ appear upon doping. We find no significant differences between the dopants; hence the bands belong to the SWNTs. Fitting of the band at $\sim1600$~cm$^{-1}$ yields good agreement with a phenomenological Fano resonance model. Finally, SWNTs mats are functionalized with bromophenyls, which are known to increase the number of defects. We find that upon \emph{p}~doping, the Fano resonances' cross sections of damaged SWNTs increase compared to that of \emph{p}~doped pristine SWNTs. Hence, we conclude that defects lower the symmetry of the lattice and activate optical phonon modes in MIR spectroscopy.   

Computational modeling of electron transfer in hydrogenase and carbon material complexes

In biohybrid and biomimetic devices for energy conversion, the electron transfer between the enzyme and the electrode plays a central role. We use hydrogenase and carbon material as model systems and investigate the binding and electron transfer configurations between hydrogenase and carbon materials, including single-wall carbon nanotubes and graphene surfaces. We use Brownian dynamics simulations to sample the hydrogenase/carbon material phase-space. The results provide an atomistic picture of how enzyme interacts with the electrode materials. We find that the optimal enzyme/electrode binding configurations are not optimal for electronic tranfer.   

Theory of coherent phonons in carbon nanoribbons

We have developed a microscopic theory for the generation and detection of coherent phonons in armchair and zigzag carbon nanoribbons using an extended tight-binding model for electrons and a valence force field model for the phonons. Coherent phonons are generated through the electron-phonon deformation potential interaction and we use Heisenberg's equation to obtain a driven oscillator equation for the coherent phonon amplitudes. We find that the driving function depends explicitly on the time-dependent photoexcited carrier distribution functions. We simulate the generation and detection of coherent phonons in coherent phonon spectroscopy experiments. We consider coherent phonon oscillations of the lowest lying radial breathing like mode (RBLM) in zigzag and armchair nanoribbons as a function of ribbon width and pump/probe polarization angle.   

Biomimetic Carbon Nanotube for Catalytic Hydrolysis of CO$_{2}$: First Principles Investigation of Role of Oxidation State and Metal Substitution

Reducing the amount of carbon dioxide (CO$_{2})$ in the atmosphere is one of the most important challenges we face in this century. Metallo-enzyme, carbonic anhydraze (CA), is known for its catalytic activity of CO2 hydrolysis, and a number of research groups have been experimentally working to mimic this activity in small molecules for the CO2 collection processes. Using accurate first principles electronic structure calculations, we investigate how the catalytic hydrolysis reaction of CA can be mimicked in a metal-porphyrin carbon nanotube system. Our work shows that the two-step catalytic process can be improved remarkably by controlling the oxidation state and also through the metal substitution in the porphyrin unit. Our work shows the feasibility of CO$_{2}$ hydrolysis in the metal-porphyrin carbon nanotube and also how the catalytic activity could be improved. This work is Prepared by LLNL under Contract DE-AC52-07NA27344.   

Coupling of carbon nanotubes and graphene nanoribbons by the titanium and vanadium nanowires: First-principles study

We investigate the combined structure of a carbon nanotube (CNT) and graphene nanoribbon (GNR) through the adsorption of a titanium or vanadium nanowire (NW), using first-principles calculations. The binding energy depends upon the stacked configuration and is much larger than that between the two subsystems without the nanowire. The band structure reveals strong hybridization between {\$}d{\$} orbitals of the transition metal and {\$}p{\$} orbitals of the carbon atoms. Furthermore, if the CNT is deposited near the border of GNR, structural stability is enhanced and magnetic moments of the edge atoms are reduced. The result points to possible application for synthesizing nanowires in nanoelectronic devices.   

Synthesis and phase behavior of AgI atomicwires in carbon nanotubes

$\alpha -$phase silver iodide($\alpha $-AgI) has been well-known as a solid state ionic conductor due to its superionic conductivity and is one of the promising candidates for solid-state electrolytes for various electrochemical devices. Below 420 K, $\alpha $-AgI undergoes a phase transition into the poorly conducting $\beta $- and $\gamma $-polymorphs, thereby limiting their applications. Recently, we have found that AgI nanowires with a diameter of 10 nm can retain $\alpha $-phase even at 313 K where size and morphology of AgI presumably plays a great role in this $\alpha $-AgI stabilization [1]. To investigate the effect further, we have focused on low dimensional nanostructure of AgI with a diameter of 5 - 10 nm. For this purpose, one-dimensional (1D) nanospace of carbon nanotubes (CNTs) has been utilized. CNTs have unique 1D nanospace ranging in diameter from 0.4 to 50 nm, which can stabilize otherwise unstable nanomaterials. We have synthesized AgI@CNTs by the sublimation method already reported [2]. In the presentation, we will discuss detailed characterization of structure and properties based on electron beam diffraction and HR-TEM. \\[4pt] [1] R. Makiura et al., Nature.Mater.8, 476 (2009). \\[0pt] [2] R. Kitaura, et al., Nano Res.1, 152 (2008).