Session V30: Nanotubes and Nanowires II: Other Properties I

Reversible Charge Induced Separation Between Singlewalled Carbon Nanotubes

We report the observation of local separation between single-walled carbon nanotubes in a bundle using low-energy electron-beam (e-beam) irradiation in scanning electron microscope. The effect of the separation is shown to impact the electrical characteristics of small nanotube bundle devices. By semi-analytical calculation we show that the Coulomb repulsive force due to the electrostatic charging can be stronger than the attractive van der Walls force in order to initiate the separation between the nanotubes in a bundle. In addition, the separated nanotubes are observed to return back to their original packed state on removal of electron exposure. We discuss this reversibility of the separation process in the light of thermal fluctuation and discharging of nanotubes at room temperature in the presence of air.   

Self-Assembly of Multiwall Carbon Nanotubes from Quench-Condensed CNi$_{3}$ Films

Freestanding, vertical, multiwall carbon nanotubes (MWCNT) are formed during the vacuum deposition of thin films of the metastable carbides CT$_{3}$ (T = Ni, Co) onto fire-polished glass substrates. In contrast to widely used chemical and laser vapor deposition techniques, we utilize direct e-beam evaporation of arc-melted CT$_{3}$ targets to produce MWCN's that are self-assembled out of the CT$_{3}$-film matrix. The depositions are made in an ambient vapor pressure that is at least six orders of magnitude lower than the 1-100 Torr typically used in chemical vapor techniques. Furthermore the substrates need not be heated, and, in fact, we observe robust nanotube growth on liquid nitrogen cooled glass and sapphire substrates. High-resolution atomic force microscopy reveals that MWCNT's of heights 1-40 nm are formed in films with nominal thicknesses in the range of 5-60 nm. We show that the growth parameters of the nanotubes are very sensitive to the grain structure of the films. This is consistent with a precipitation mediated root-growth mechanism.   

Self-assembly of Carbon Nanotube Based Devices on Programmable DNA Crystals

The greatest challenges of nanoscience today include the ability to interface the new breed of versatile nanomaterial with the conventional electronics and the ability to arrange multiple nanocomponents into arbitrary pre-designed geometries with high density and nanoscale precision. These factors not only limit the feasibility of using the alternative nanostuctures for technological advances, but also hinder the elucidation of the structures' intrinsic properties. We will present a unique process for self-assembling DNA dispersed single-walled carbon nanotubes (SWNTs) into precise geometric arrangements with orientation control and demonstrate its implementation by self-assembling an all SWNT field effect transistor (FET). Not only does the precision, low cost, reproducibility and parallelism of our new process create novel opportunities for the implementation and investigation of complex nanosystems, but it also enables us to gain insight into the dynamics of molecular interactions between nanoscale objects of mega-daltons size.   

Diameter-dependent conductance oscillations in carbon nanotubes upon torsion.

Torsion-induced conductance oscillations have been recently observed in multi-wall carbon nanotubes$^{1,2}$. These oscillations have been interpreted as metal-semiconductor periodic transitions, while an alternative interpretation attributed the phenomenon to changes in registry between the walls. Here we show$^{3}$ that the period of the oscillations is inversely proportional to the squared diameter of the nanotube (\textit{$\delta \phi \sim $1/d}$^{2})$. This dependence is theoretically predicted from the shifting of the corners of the first Brillouin zone of graphene across different subbands allowed in the nanotube, whereas a change in registry should give rise to a simple inverse dependence (\textit{$\delta \phi \sim $1/d}). Hence, the experimental results validate the interpretation of Fermi level shift across subbands \textit{vs}. that of registry change, as a source of torsion-induced conductance oscillations in carbon nanotubes. \newline \newline [1] T. Cohen-Karni \textit{et al}, Nature Nanotech. \textbf{1}, 36 (2006). \newline [2] E. Joselevich, ChemPhysChem \textbf{7}, 1405 (2006). \newline [3] K. S. Nagapriya \textit{et al}, \textit{in preparation}.   

Nanomechanical properties of carbon nanotubes determined using a scanning laser vibrometer

To better understand the nanomechanical properties of nanotubes and nanowires, reliable nondestructive techniques that measure their Young's modulus, E, under ambient conditions are needed. Using a scanning laser vibrometer, the thermally excited eigenfrequencies of plasma-enhanced carbon vapor deposition (PECVD) multiwalled carbon nanotubes (MWNTs) were measured. Due to the small diameters involved, little light is reflected from a bare MWNT. By carefully attaching a small Au-coated glass bead, the intensity of reflected light is sufficiently increased to allow accurate measurements. The length and diameters of the MWNTs are determined using electron microscopy, allowing E to be inferred from an Euler-Bernoulli analysis of a pinned cantilever beam. A unique aspect of our work is that the attached glass bead exerts a torque on the MWNT. A resonance, attributed to a torsional oscillation, appears for the laden MWNTs, allowing an estimate for the torsional modulus G. Values measured for E and G, along with a description of the experimental procedure, will be presented at the talk.   

Mechanical Sensing with Flexible Metallic Nanowires

A calibrated method of force sensing is demonstrated in which the buckled shape of a long flexible metallic nanowire is interpreted to determine the applied force. Using a nanomanipulator the nanowire is buckled in the chamber of a scanning electron microscope (SEM) and the buckled shapes are recorded in SEM images. Force is determined as a function of deflection for an assumed elastic modulus by fitting the shapes using the generalized elastica model. In this calibration the elastic modulus was determined using an auxiliary AFM measurement, with the needle in the same orientation as in the SEM. Following this calibration the needle was used as a sensor in a different orientation than the AFM coordinates to deflect a suspended PLLA polymer fiber from which the elastic modulus (2.96 GPa) was determined. In this study the same needle remained rigidly secured to the AFM cantilever throughout the entire SEM/AFM calibration procedure and the characterization of the nanofiber.   

c-axis GaN nanowires for high-quality-factor mechanical oscillators

We report on the electromechanical properties of c-axis GaN nanowires in high-quality-factor mechanical resonators and oscillators. The nanowires are grown by catalyst-free molecular beam epitaxy, are single crystal, hexagonal in cross section, from 50 - 500 nm diameter, and 5 - 15 microns in length depending upon growth time. As-grown nanowires display singly-clamped cantilever mechanical resonances above 1 MHz, with typical resonance full width at half maximum power of less than 100 Hz i.e., a mechanical quality factor, Q, well above 10$^{4}$. We are attempting to obtain similar high-Q for processed nanowires, utilizing dielectrophoresis to position the nanowires within lithographic test structures, and nanowire metallization of two types: First, titanium/aluminum over the ends of the nanowires allows for ohmic contact formation and direct measurement of nanowire resistance versus strain. Second, a layer of aluminum over the entire length of the wires leads to a metallic backbone with a resistance on the order of 50 $\Omega $. This is useful for simple magnetomotive measurements. We will discuss the processing steps and observed behavior.   

Self-detecting mechanical resonators made from suspended carbon nanotubes

We study the flexural and torsional mechanical properties of suspended carbon nanotubes. We have used a suspended carbon nanotube as a frequency mixer to detect its own mechanical motion. A single gate-dependent resonance is observed, which we attribute to the fundamental bending mode vibration of the suspended carbon nanotubes. Using a continuum model fit to the measurements, we show that the nanotubes in our devices have no slack and that, by applying a gate voltage, the nanotube can be tuned from a regime without strain to a regime where it behaves as a vibrating string under tension. We are currently investigating the low-temperature properties of these devices. We are also investigating the torsional mechanical properties of nanotubes.   

Characterization and wear resistance of carbon nanotube-based tips for AFM local anodic oxidation nanolithography

The AFM local anodic oxidation (LAO) lithography is a powerful scanning-probe-based patterning technique for fabrication and post-fabrication tuning of nanoscale structures and devices. The conventional tips suffer from rapid wear which degrades the quality of imaging and LAO lithography. Much research has been devoted to seek for wear-resistant AFM tips, and carbon nanotubes (CNT) are viable candidates. Apart from featuring small diameter, high aspect ratio, and mechanical flexibility, the CNT tips has been shown to last longer during LAO. We investigate the wear of CNT-based tips during LAO lithography by imaging the tips before and after the lithography using scanning electron microscope. CNTs show small contamination on the tip apex while preserve their original length and diameter after more than 200 micron of the lithography. We also analyze tip-surface interaction in order to optimize the quality of lithography and correlate energy dissipation during tapping mode to the line width and line thickness during LAO.   

Ultrahigh Vacuum Scanning Tunneling Microscopy and Spectroscopy of Single-Walled Carbon Nanotubes Interfaced with Silicon Surfaces

The UHV-STM was used to examine SWNTs directly interfaced with hydrogen-passivated Si(100). Dry contact transfer (DCT) [1] enabled the UHV deposition of SWNTs with minimal disruption of the atomically flat Si(100)-2x1:H surface. Isolated, rather than bundled, SWNTs could be routinely located for atomically resolved imaging, tunneling I-V spectroscopy [2], lateral manipulation [3], and proximal substrate modification. Weakly adsorbed SWNTs initially unstable in the presence of the rastered STM tip could be stabilized by depassivating the underlying H-Si(100) surface via UHV-STM electron-stimulated H desorption [4], which in the case of one chiral semiconducting SWNT also promoted the local alignment of the zigzag symmetry direction on the underside of the tube with the clean Si dimer rows [5]. The growing body of first-principles simulations of the SWNT/Si(100) system [6] was drawn upon in our interpretation of such local perturbations. [1] APL 83, 5029 (2003). [2] Nanotechnology 18, 095204 (2007). [3] Small 3, 146 (2007). [4] Nanotechnology 18, 125302 (2007). [5] Small 3, 1402 (2007). [6] JAP 100, 124304 (2006).   

Cavity-controlled, electrically-induced infrared emission from a single single-wall carbon nanotube

Single-wall carbon nanotube (SWCNT) is attracting significant attention from the photonics community recently due to its unique optical properties and potential applications in optical communications, optical interconnects, and, in particular, nanophotonics. Tunable and truly nanometer-scale (diameters of 0.5 to 2nm) light emitters can possibly be realized using individual semiconducting nanotubes. However, optical cavities are needed to control the emission wavelength, direction, and emission rate of these broadband, nanoscale emitters. Here, we report the monolithic integration of an electrically-excited light emitter based on a single (SWCNT) with a planar photonic $\lambda $/2-cavity. Emission properties were dominated by the cavity characteristics almost regardless of the free-space emission. The broad, free-space emission spectrum of a single SWCNT with a FWHM of 300 to 500nm was transformed to cavity controlled emission with a FWHM of $\sim $40nm. We also modeled the device as a broadband emitter in a planar cavity. Good agreement between the simulated and experimental results was achieved. In our configuration, the maximum enhancement in emission rate is estimated to be 4.4. We consider this result to be an important first step in the development of truly nanometer-scale photonic devices based on SWCNTs.   

Fluorination and Defluorination of Double-wall Carbon Nanotubes

Due to the unique physical structure double-wall carbon nanotubes (DWNTs), the outer tube can be chemically functionalized while the inner tube is left in pristine condition. Using a fluorinating agent, DWNTs are fluorinated and X-ray photoelectron spectroscopy data indicates that the resulting product composition is equivalent to C3F. The diameter of bare DWNTs is around 2-3 nm as measured by scanning tunneling microscopy, but fluorinated DWNTs possess much larger diameters in a range from 3-10 nm due to a stronger electronic interaction. In addition to imaging the as prepared material, the material was imaged after annealing at temperatures up to 650 K. Due to defluorination, the diameter is decreased down to that of the initial bare DWNTs and atomic resolution of the lattice was recovered. In addition, it was possible to observe that the initial and final structures on the same nanotubes and the evolution of their associated defect structures. Lastly, Raman spectroscopy was employed to confirm the defluoration by revealing the recovery of the RBM peak which disappeared upon fluorination.   

Magnetic studies of multi-walled carbon nanotube mats: Ultra-high temperature ferromagnetism or superconductivity?

We report magnetic measurements up to 1200 K on multi-walled carbon nanotube mats using a Quantum Design vibrating sample magnetometer. Extensive magnetic data consistently show two ferrromagnetic-like transitions at about 1000 K and 1275 K, respectively. The lower transition at about 1000 K is associated with an Fe impurity and its saturation magnetization is in quantitative agreement with the Fe concentration measured from an inductively coupled plasma mass spectrometer. On the other hand, the saturation magnetization for the higher transition phase corresponds to about 0.6$\%$ Co impurity concentration, which is about four orders of magnitude larger than that measured from the mass spectrometer. We show that this transition at about 1275 K is not consistent with ferromagnetism of any carbon-based phases or magnetic impurities but with the paramagnetic Meissner effect due to the existence of $\pi$ Josephson junctions in a granular superconductor.   

A quadruply twinned core for the growth of nanotetrapods and related structures.

Nanotetrapods and related complex nanoarchitectures, such as multi-armed and tricrystal structures, are key functional elements and interconnections in future ``bottom-up'' approach to nanotechnology. The growth of these special nanostructures is believed to proceed from a core, . several models of have been suggested. However, the occurrence of some tetrapod related nanostructures, observed by us and also reported by others, can not all be explained by these models. We have obtained aligned ZnCdSe tetrapod related nanostructures using metalorganic chemical vapor deposition on GaAs substrates. Based on high resolution transmission electron microscopy and crystallographic analyses, we propose a new quadruply-twinned model for their core, from which nanotetrapods, nanoswords and related nanoarchitectures can grow. In this model, the core contains four wurtzite structured heptahedrons connected by six {\{}0-113{\}} twins. Different from the other existing models, no polarity of surface is needed to explain the growth of the branching arms of the nanotetrapods   

Individual carbon nanotubes on trench vs. substrate: A Raman study

Raman excitation spectroscopy is a powerful tool for characterizing the electronic and phononic structure of single-wall carbon nanotubes (SWNTs). The transitions associated with Raman-active phonon modes in SWNTs, e.g. the radial breathing mode (RBM), the defect-induced mode (D) and the tangential modes (G), carry unique information regarding both electron-phonon interactions in SWNTs and phonon-specific non-radiative coupling to their nano-environment. We study individual, spatially isolated SWNTs grown by chemical vapor deposition across micron-wide trenches etched in a silicon dioxide substrate. Using an optical microscope, we spatially address parts of the same SWNT that are either supported by the substrate or freely suspended across a trench. In a first step, we probe a specific electronic level of a SWNT by monitoring the intensities of different Raman bands, i. e. RBM, D and G, as a function of the laser excitation wavelength. In a second step, taking advantage of the resonance Raman condition, we demonstrate how the positions, widths and shapes of the spectral bands associated with individual Raman transitions of the same SWNT are modified if brought into contact with the substrate. We discuss the results in the context of performance limits of optoelectronic devices based on SWNT.