Session K30: Carbon Nanotubes: Applications and Novel Phenomena

Weighing Molecules with Carbon Nanotubes

We have constructed a sensitive mass balance by coating a thin quartz transducer with debundled carbon nanotubes.~ The transducer operates in theshear mode at 29 MHz.~~ Application of the nanotube film to the transducer downshifts the frequency as expected (mass loading), but we observe an unexpected increase in the Q-factor, i.e., the nanotube loading reduces the mechanical losses in the resonator. This nanomechanical resonator is also sensitive to mass loading of the internal pores of the nanotubes. By exposing the nanotube-coated resonator to various gases (e.g., inert gases, N2, SF6, etc.), we~ are able to easily observe the adsorption/desorption of the gases.~ At constant temperature and pressure, we observe an interesting M\^{}\^{}0.45 shift in the resonance frequency, where M is the molecular mass.~ The mass exponent we observe in these mass-balance experiments results is in good agreement with that published recently [1] on the collision-induced changes in the resistance and thermolectric power of thin nanotube films. The theory behind exponential mass dependence will be discussed and relates to the deformation of the nanotube wall and the residence time of the molecule on the tube wall.   

Probing Mechanical Properties of Carbon Nanotubes by Light Scattering

An important capability for studying and exploiting the remarkable mechanical properties of carbon nanotubes is the detection of their physical motion. Here we present a scheme based on light scattering by a nanotube placed slightly off-center of a tightly focused laser beam. The approach permits measurement of the movement of individual single-walled carbon nanotubes with nanometer sensitivity and high detection bandwidth. The method can be readily combined with optical characterization by Rayleigh scattering spectroscopy for studies of well-defined nanotube structures [1]. The technique is demonstrated for suspended pristine nanotubes, as well as for nanotubes modified by mass loading. By passing a current through the nanotube in the presence of a magnetic field, we can apply static and oscillating Lorentz forces. In this fashion, we have produced both static deflection and vibration excitation of nanotubes. Sharply defined nanotube vibrational modes are observed by sweeping the frequency of the driving force. We discuss the nature of these resonances and how to extract fundamental nanotube properties from the measurements. [1] M. Y. Sfeir, Science \textbf{306}, 1540 (2004).   

Continuum Theory for Nanotube Piezoelectricity

We develop and solve a continuum theory of the piezoelectric response in 1-D nanotubes and nanowires. We find that the piezoelectric response depends on the aspect ratio, the chiral angle, and two dimensionless parameters giving the relative strengths of the two elastic constants to the piezoelectric constant. Solutions for several limiting cases in the parameter space are discussed. The low dimensionality of the model system gives rise to several interesting effects not seen in conventional 3-D systems. We find that a uniform axial stress will induce a spatially non-uniform polarization and a non- linear variation of the electrostatic potential along the tube. The model predicts a strong coupling between longitudinal strain and torsional strain in chiral nanotubes and nanowires. The theory is applied to estimate the piezoelectric response of boron-nitride nanotubes.   

Analysis of band-gap formation in squashed armchair CNTs

The electronic properties of squashed arm-chair CNTs are modeled using constraint free density functional tight binding molecular dynamics simulations. Independent from CNT diameter, squashing path can be divided into {\it three} regimes. In the first regime, the CNT deforms with negligible force. In the second one, there is significantly more resistance to squashing with the force being $\sim 40-100$ nN/per CNT unit cell. In the last regime, the CNT looses its hexagonal structure resulting in force drop-off followed by substantial force enhancement upon squashing. We compute the change in band-gap ($E_g$) as a function of squashing and our main results are: (i) $E_g$ initially opens due to interaction between atoms at the top and bottom sides of CNT. The $\pi-$orbital approximation is successful in modeling the E$_g$ opening at this stage. (ii) In the second regime of squashing, large $\pi-\sigma$ interaction at the edges becomes important, which can lead to $E_g$ oscillation. (iii) Contrary to a common perception, nanotubes with broken mirror symmetry can have {\it zero} $E_g$. (iv) All armchair nanotubes become metallic in the third regime of squashing.[Phys. Rev. B 71, 155421 (2005)]   

Conductivity, Rheology and Processing of Carbon Nanotube Composites

The primary application for CNT composites is the enhancement of their electrical properties relative to pure polymers. The conductivity is controlled by the alignment and the dispersion of the nanotubes in the polymer matrix during processing. Understanding the interplay between conductivity, processing, alignment and rheology is key their efficient use. We present simultaneous measurements of the Rheology and conductivity of molten polymer nanotube composites over a range of concentrations near the percolation threshold. We find that simple shear fields can change the conductivity by orders of magnitude. Surprisingly, upon cessation of shear, the conductivity returns to its quiescent value.   

Focused electron beam from open-tip single-wall carbon nanotubes

Open-tip single-wall carbon nanotubes can produce focused electron spot in the field emission. We calculate direct evolution of the nanotube wavefunction under applied electric field by solving the time dependent Shr\"{o}dinger equation in the first principles scheme. (5,5), (10,10), and (12,12) carbon nanotubes are investigated and We obtain the focused spot size of a few angstroms. The spot size of electron beam from the (10,10) tube is smaller than that of the (5,5) or (12,12) tube. We also find that s-like state near the Fermi level contributes most to the field emission current.   

Controlling the diameter of carbon nanotubes

We report a method to control the diameter of an individual carbon nanotube. Electronic transport measurements performed in situ reveal a striking dependence of conductance on nanotube geometry. As the diameter of the nanotube is reduced to near zero, we observe negative differential resistance.