Session X27: Focus Session: Carbon Nanotubes: Mechanical Properties

Structural and electronic characterization of CoMoCat single-walled carbon nanotubes on H-passivated Si(100) surfaces with the UHV-STM

CoMoCat single-walled carbon nanotubes (SWNTs) [1] are deposited onto the Si(100)-2x1:H surface by \textit{in situ} dry contact transfer (DCT) [2], forming a pristine SWNT-Si interface for room temperature UHV-STM studies. Recent spectrofluorimetry data suggests a sample distribution which favors smaller diameter SWNTs, in particular an enhancement of the (7,5) and (6,5) semiconductors [1]. Results to be presented include STM topographic and current images of CoMoCat SWNTs which provide a reasonable estimation of the chiral angle, along with measurements of both the diameter (inferred from STM topographic contours), and the local density of states (via spatially resolved tunneling spectroscopy) for a large sample of CoMoCat SWNTs. Smaller SWNT diameters imply greater curvature of the carbon lattice, yielding an enhanced sp$^{3}$ character [3] which could create an enhanced coupling to the underlying substrate. Experiments integrating H desorption with the adsorbed CoMoCat SWNTs are underway, with the potential for SWNT bandstructure perturbations upon interactions with chemically reactive clean Si. [1] S. M. Bachilo et. al J. Amer. Chem. Soc. 125, 11186 (2003). [2] P. M. Albrecht et. al. Appl. Phys. Lett. 83, 5029 (2003). [3] X. Blase et. al., Phys. Rev. Lett. 72, 1878 (1994).   

UHV-STM of single-walled carbon nanotubes in registration with the atomic lattices of silicon surfaces

A room-temperature UHV-STM is used to elucidate the registration dependence of the electronic and mechanical properties of single-walled carbon nanotubes (SWCNTs) adsorbed onto silicon surfaces. The SWCNTs are deposited onto the Si surface in situ using a dry contact transfer (DCT) technique [1], with the resultant pristine SWCNT-Si interface enabling a joint atomic-resolution topographic and spectroscopic study of individual SWCNTs on both clean and H-passivated Si(100)-2x1 surfaces. Pronounced variations in the I-V and dI/dV-V spectra acquired along an isolated SWCNT were found to correlate with a transition from parallel to perpendicular alignment of the tube with respect to the dimer rows of the clean Si surface. Recent theoretical work [2] suggests that SWCNT-Si alignment is indeed energetically favorable and may give rise to novel nanotube-surface interactions unobserved in previous STM studies of SWCNTs in contact with a metallic substrate. [1] P.M. Albrecht and J.W. Lyding, APL 83, 5029 (2003). [2] W. Orellana, R.H. Miwa, and A. Fazzio, PRL 91, 166802 (2003).   

XSTM Study of SWNTs on GaAs(110) and InAs(110)

In an effort to better understand the interactions between single-walled carbon nanotubes (SWNTs) and supporting III/V substrates, we have used ultra high vacuum (UHV) cross-sectional scanning tunneling microscopy (XSTM) and spectroscopy (STS) to examine SWNTs on the (110) surfaces of GaAs and InAs. Clean, atomically flat surfaces were obtained through in situ cleavage, and SWNTs were deposited using an UHV Dry Contact Transfer (DCT) technique previously demonstrated on H-passivated Si(100).$^{1}$ The deposition resulted in the intact transfer of primarily isolated SWNTs to the III/V surface, with STM images revealing a preferential alignment of individual tubes in the (1\underline {1}0) direction in both cases. STS measurements verify the nanotube/surface band alignments predicted by Kim et. al.$^{2}$: a Type I alignment in the SWNT/GaAs case, with the SWNT gap contained within the GaAs gap, and a Type II alignment in the SWNT/InAs system, with the SWNT valence band edge within the substrate gap and conduction band edge well within the InAs conduction band. \newline \newline $^{1}$P.M. Albrecht and J.W. Lyding, \textit{APL} \textbf{83}, 5029 (2003) \newline $^{2}$Y.-H. Kim, et. al., \textit{PRL} \textbf{92}, 176102-1 (2004) and \textit{unpublished}   

Probing the local density of states in carbon nanotubes with electrostatic forces

We discuss a simple yet powerful technique based on electrostatic force microscopy (EFM) that can probe the compressibility of the electron gas in single-walled carbon nanotubes. By varying the tip voltage, we are able to populate additional one-dimensional (1D) subbands. The resulting variation in the compressibility of the electron gas modulates the tip-sample capacitance and enables the van Hove singularities in the density of states to be resolved. This demonstrates the effects of quantum confinement on a nanotube's capacitance, which may have important implications for the high-frequency operation of nanotube devices. We have exploited this capability to measure the local band gap of an intratube quantum-well structure, created by the application of a non-uniform uniaxial strain. From the local band gap versus the local strain, we infer the nanotube chiral angle by comparison to theoretical models. The technique is applicable to other materials and should find wide applicability in investigating the properties of nano-scaled systems.   

Construction of a Atomic Force Microscope for low temperature measurements of nanostructures

We have constructed an Atomic Force Microscope for low temperature studies of nanostructures. The microscope fits inside a cryostat of 1.5'' bore. The scanning probe is attached to a quartz tuning fork, and the amplitude of the tuning fork oscillations is registered as the feedback signal. The scanning tip is separately contacted, which allows us to carry out scanning gating and tunneling spectroscopy measurements of nanostructures. We demonstrate room temperature scanning gating microscopy measurements of single walled carbon nanotubes. Low temperatures measurements are underway.   

Stretchable nanotube electronics

We have fabricated single-walled carbon nanotube devices on elastomeric polydimethylsiloxane substrates consisting of a number of individual tubes bridging gold electrodes. The electrodes are capable of sustaining $>$10{\%} strain without breaking. Upon stretching and releasing the substrate, we observe reproducible modulations in the device conductance, typically giving $\sim $1{\%} conductance modulation per {\%} of strain. These devices may thus act as nano-scaled strain sensors for lab-on-a-chip or biosensing applications. While stretching the substrate elongates the nanotubes, compressing the nanotubes leads to a buckling instability, creating nanometer scale quasi-periodic undulations in the nanotubes. Through the effect of strain on nanotubes' band structure, these may act as a strain-induced superlattice structure. Our latest results will be discussed.   

Growing suspended carbon nanotubes: Uncovering the importance of thermal vibrations.

Nanotubes are grown by chemical vapor deposition into suspended structures~over etched trenches. When the length of the nanotube is short (L$<$500nm) the entire length of the nanotube remains suspended. In contrast, for long tubes with length L$>$ 2 microns the ends of the nanotubes remain pinned to the ridge tops, but for many nanotubes the central body of the tube drops $\sim $80nm~and sticks to the substrate. For nanotubes with lengths between (500nm-2 microns) the probability that the nanotube is stuck to the substrate increases with tube length. We propose that thermally driven oscillations of the nanotube during the CVD growth cause the nanotube to oscillate with amplitudes large enough ($\sim $80nm) to touch the substrate, then stick. Using the length of the nanotubes, and the diameter distribution from the CVD growth we are able to non-invasively place limits on the range of Young's modulus (Y = 200 GPa -- 2 TPa). This work is supported by the National Science Foundation under Grant No. DMR-0079983, DMR-0094063, and the Research Corporation.   

Mechanical and electrical properties of carbon nanotube nanosprings

We have studied the mechanical properties of carbon nanotube (CNT) nanosprings with a nanomanipulator operated inside of a scanning electron microscope and compared the results with a previous measurement of amorphous carbon nanosprings. The results of tensile loading experiments show that the CNT nanosprings have a force constant of 20-50 N/m per unit coil and the amorphous carbon nanosprings have a force constant of 1.1N/m. We have also performed compression experiments, and in this talk, we will discuss the buckling of a CNT nanospring. Two clamping methods have been used in this work, and the results show that both are reliable methods for mechanical measurements. With atomic force microscopy, electrical measurements have also been performed. \newline\newline \textit{We gratefully acknowledge the grant support from the NASA Langley Research Center for Computational Materials: Nanotechnology Modeling and Simulation Program, the NASA University Research, Engineering and Technology Institute on Bio Inspired Materials (BIMat) under award No. NCC-1-02037, and the Office of Naval Research ``Mechanics of Nanostructures'' under award No. N000140210870.}   

Assessment of Mechanical Properties of Nanocomposites Via Nanoindentation and Atomic Force Microscopy

Considerable effort is being directed towards the development of novel composite materials to meet future challenges. The introduction of nanoparticles into composite systems holds promise for next generation composites and raises the question of the impact of scale and particle dispersion upon such systems. Nanoindentation and Atomic Force Microscopy have been used to interrogate carbon nanotubes composites and nanoclay composites with the ultimate goal of providing data to build multi-scale models of these systems. Morphology, local material response, and constitutive behavior at the nano- and submicroscales were examined. The results of these experiments, and the degree of correlation between them, will be discussed.   

A Tunable Carbon Nanotube Oscillator

Nanoelectromechanical systems (NEMS) hold promise for a number of scientific and technological applications. Carbon nanotubes (NT) are perhaps the ultimate material for realizing a NEMS device as they are the stiffest material known, have low density, ultrasmall cross sections and can be defect-free. Equally important, a nanotube can act as a transistor and thus is able to sense its own motion. Here, we report the electrical actuation and detection of the guitar-string oscillation modes of doubly-clamped NT oscillators. We observed resonance frequencies in the 5MHz to 150MHz range with quality factors in the 50 to 100 range. We showed that the resonance frequencies can be widely tuned by a gate voltage. We also report on the temperature dependence of the quality factor and present a discussion of possible loss mechanisms.   

Ideal Torsional Strength of (n,0) Carbon Nanotubes

The torsional stiffness and ideal torsional strengths of zig-zag carbon nanotubes are computed using an {\em ab initio} electronic structure total energy technique coupled with a scaling form. Predicted stiffnesses are in good agreement with experimentally measured stiffnesses taken from the literature. The ideal torsional strengths of multiwall nanotubes are predicted to exceed those of equivalently sized iron rods. This research was supported by the Department of Energy, Basic Energy Sciences under the Office of Science under contract DE-AC03- 76SF00098, and, in part, by the National Science Foundation under grant DMR-0304629.   

Bending Modulus of CVD Grown Multi-walled Carbon Nanotubes (MWNTs)

We have measured the bending modulus of several different CVD grown MWNTs using a vibrating reed technique. The MWNTs are produced from a thermal decomposition of three different precursors: (i) xylene/ferrocene, (ii) xylene/ferrocene/melamine (nitrogen-doped), and (iii) trimethylamine (TMA)/ferrocene. The first two precursors are used to compare the mechanical properties of typical CVD-grown to bamboo-type MWNTs. Nanotubes prepared using the third precursor shows relatively fewer walls ($\sim $ 4-20 compared to $\sim $15-40) and defects compared to those prepared from the xylene/ferrocene mixture. The resonant frequencies of these nanotubes were measured optically and electronically in air using a dark field light microscope. The diameters of these nanotubes range from 50 -- 160 nm as determined from TEM and the average length is $\sim $ 10 microns. For the xylene/ferrocene and trimethylamine/ferrocene tubes, the average bending modulus is estimated to be 0.1 and 0.3 TPa, respectively. However, the bending modulus for the nitrogen-doped tubes is $\sim $9 GPa which is significantly lower compared to regular MWNTs implying that the bending modulus decreases with an increase in wall defects.   

Stress-strain measurements of individual SWNTs using Lorentz force and optical detection

We describe a novel method for performing direct stress-strain measurements on individual single-walled carbon nanotubes. Long freely- suspended nanotubes are grown over a wide slit in a Si wafer, and contacted at either end with metal electrodes. Portions of the freely suspended section are marked with metal to render them visible in an optical microscope. Passing a current through the nanotube in a moderate magnetic field causes the nanotube to deflect. Because of the length of the structure, the deflection is detectable using optical microscopy. The deflection can then be correlated with the applied current to yield a stress-strain curve.