Session V31: Focus Session: Carbon Nanotubes: Thermal, Mechanical, Adsorption

Thermoelectric power of Single Walled Carbon Nanotubes at the Ballistic Conduction Limit

Thermoelectric power (TEP) measurements of single walled carbon nanotubes(SWNTs) with low electrical contact resistance are reported. TEP was measured in-situ using a microfabricated heater and thermometers. High quality Ohmic contact to the SWNT was achieved with Pd electrodes. TEP measurements are sensitive to the change in conductivity, and therefore provide a complementary method for probing the electronic band structure of SWNTs. Deviations of the low temperature TEP gate dependence from the semiclassical Mott relation allows us to gain insight into the quantum transport regime in this one dimensional conductor. Modulation of TEP as a function of applied gate voltage will be discussed in connection with the shell filling effects and Fabri-Perot oscillations observed in electrical conductance.   

Phonon-Phonon Interaction In Carbon Nanotube Assemblies.

We present the comparative study of the anisotropic 1D thermal conductivity and the thermal diffusivity of assemblies of carbon nanotubes (CNTs) comprising an increasing number of aligned free standing carbon nanotubes (SWNT and MWNT) using two techniques: laser flash and self-heating 3$\omega $ methods. The concept of mode quenching is considered for alignment of few individual CNTs. The length dependence of thermal conductivity is studied for CNT with different number of intrinsic defects (HiPCO, Laser ablation, Arc-Charge). The extremely high surface area of CNT assemblies like highly aligned MWNT sheet [1] leads to the excessive radial radiation of the heat and dose not allow to transfer the heat energy by means of phonons to distances more than 2 mm. [1]. M. Zhang, S. Fang, A. A. Zakhidov, S. B. Lee, A. E. Aliev, C. D. Williams, K. R. Atkinson, R. H. Baughman, \textit{Science } \textbf{309}, 1215 (2005).   

Electromechanical Resonators from Atomically Thin Graphite

We fabricate nanoelectromechanical systems (NEMS) from atomically thin graphite by mechanically exfoliating thin sheets over trenches in SiO$_{2}$. Vibrations with fundamental resonant frequencies in the MHz range are actuated either optically or electrically and detected optically by interferometry. We make a detailed study of the mechanical properties of these resonators including resonance frequency, spring constant, built in tension, and quality factor. The thinnest resonator consists of a single suspended layer of atoms and represents the ultimate limit of a two dimensional NEMS.   

Microwave Nano-abacus Electro-mechanical Oscillator

We describe nanoscale electromechanical oscillators capable of operating in ambient-pressure air at room temperature with unprecedented fundamental resonance frequency of $\sim $4 GHz. The devices, created from suspended carbon nanotubes loaded abacus-style with inertial metal clamps yielding short effective beam lengths, open windows for immediate practical microwave frequency nanoelectromechanical systems (NEMS) applications.   

Metal-carbon nanotube composite nanoelectromechanical torsional resonators

Metallic based nanoelectromechanical systems (NEMS) resonator structures are of interest due to higher optical reflectivity, ductility, and conductivity compared to insulator- and semiconductor- based NEMS structures. We present NEMS torsional resonator structures fabricated from aluminum-carbon nanotube (CNT) and palladium-CNT composites. Metal and metal-CNT NEMS structures are released from III-V based substrates. The resonators are electrostatically driven and are detected at room temperatures under moderate vacuum conditions using optical modulation techniques. We note significant differences in the resonant frequencies (f$_{0})$ and the quality factors (Q) between metal and metal-CNT NEMS torsional resonators. Aluminum based structures with paddle dimensions of $\sim $5 micron x $\sim $5 micron, with support beams of $\sim $1 micron x $\sim $3 micron, show a fundamental resonant frequency corresponding to translational mode of 1.7 MHz with Q of 20, while Al-CNT based structures of same dimensions show f$_{0}$ of 3 MHz and Q of 50, as a typical example. We will further discuss the effects on the mechanical properties of metallic NEMS torsional resonators due to addition of CNT. {\dag}parkyd@phya.snu.ac.kr   

Strain sensitivity in the photocurrent of single wall nanotubes

The energy spectrum of carbon nanotubes is highly sensitive to strain and mechanical deformation. Calculations predict a shift in the bandgap of single wall nanotubes (SWNT) with axial strain, which in turn affects the conductance\footnote{ Liu Yang et.al, Phys. Rev. B 60, 13874 (1999) }. We have measured the influence of strain on the photocurrent spectrum of SWNT's and observe as much as 600meV shift in the band gap energy of semiconducting nanotubes for 300 micro strains. The experiments were performed on SWNT's CVD grown on a 300$\mu $m thick quartz cantilever; the SWNT's are strained by pushing down on the free end of the cantilever. We use a capacitive photocurrent technique for detecting the photocurrent as a function of incident photon energy. The peak corresponding to the band- to- band free electron transition in the semiconducting nanotubes is observed to shift to lower energies with increasing strain. Further measurements using a fixed wavelength (488nm) Ar ion laser show as much as an order of magnitude change in the photocurrent with strain, implying a gauge factor of more than a 1000. Our measurements provide a direct probe of the influence of strain on the bandgap of SWNT's and open up the possibility of using SWNT's as optical strain sensors.   

Nanotubes stretched in Liquid-Metal-Ion-Sources:their influence on the cluster emission and on the isotopic anomalies.

The present contribution argues that an intense electric field (few V/A) provides an alternative method to stretch matter and to form nanotubes locally. The very high electric field is supplied by a Liquid Metal Ion Source (LMIS). Intriguing aspects are displayed by the LMIS mass spectra of some pure elements. The periodicity of pure Ge or Sn LMIS i.e. series of equidistant peaks such Ge$_{6n+1}^{3+}$ with n=3 to 8 or Ge$_{6n+4}^{3+}$ with n=7 to 14 or the formation of unexplained Au$_{8}^{3+}$ and Au$_{16}^{3+}$ ions for the pure Au LMIS are attributed to the existence of Ge, Sn or Au nanotubes in operating LMIS. LMIS results on eutectic Au$_{0.73}$Ge$_{0.27}$ alloy show the formation of a gold nanotube associated with the strong Au$_{8}^{3+}$ emission. The Ge$_{2}^{+}$ emitted near the gold nanotube interact with a larger electric field than in the pure Ge LMIS provoking a bond break in heteroisotope dimers and therefore isotope anomalies in dimer emission. Finally we analyse the results from Au-Si eutectic   

Organic-vapor-induced repeatable movements of C$_{60}$ in/from single-wall carbon nanohorns at room temperature

Incorporation of functional materials inside carbon nanotubes (CNTs) has been actively investigated with the goal of improving the physical and chemical properties of CNTs. We found intriguing phenomena of repeatable movement of materials from inside to outside of CNTs and vice versa, which resulted simply by exposure to different organic vapors at room temperature. For example, C$_{60}$ entered inside single-wall carbon nanohorn (SWNHs), a type of single-wall CNTs, when exposed to toluene vapor at room temperature, and the incorporated-C$_{60}$ exited when exposed to ethanol vapor. Here the entrance and exit were evidenced by X-ray diffraction and N$_{2}$ adsorption measurements, and microscopic observations. The back-and-forth transportations of C$_{60 }$could be repeated. We think that the C$_{60}$ movements were mediated by the layers of toluene or ethanol adsorbed on the SWNH surfaces. The present findings will help in the fabrication of various nanometer-scale hybrid systems from CNTs.   

Resonance Raman study of polyynes inside single-walled carbon nanotubes.

We report a resonance Raman study of polyynes C$n$H2 molecules ($n $= 10 and 12) composed of linearly bonded sp-carbon atoms aligned inside the single-walled carbon nanotubes (SWNTs), using many laser lines in the range 1.9 to 2.7 eV. The C10H2@SWNT hybrid material exhibits a Raman peak at 2066 cm-1 related with the stretching vibration of the C10H2 molecules interacting with SWNTs, while the C12H2@SWNT exhibits a peak at a lower frequency, around 2020 cm-1. The intensities of these peaks are strongly dependent on the laser energy, and exhibit maxima around 2.15 and 2.10 eV for the C10H2@SWNT and C12H2@SWNT materials, respectively. However, the optical absorption spectrum of the purified C10H2 and C12H2 dispersed in n-hexane exhibits peaks in the UV, around 4.9 and 4.5 eV, respectively.This result can be associated with a two-photon Raman resonance enhancement or can reflect important changes in the electronic structure of the polyynes when they are encaged in a carbon nanotube. New experimental and theoretical works are needed to understand this result.   

Localization of inserted species inside single walled carbon nanotubes bundles: experimental and \textit{ab initio} study.

The aim of this work is to understand the structural organization of inserted alkali atoms inside single walled carbon nanotube bundles. First of all, we present X-rays and neutrons diffraction results obtained on rubidium inserted carbon nanotubes ($n$-doping). The results of X-rays and neutrons diffraction experiments are surprising and in apparent contradiction, and will be discussed in connection with the debated question of the lattice expansion of the hexagonal tubes framework under insertion. The possible insertion sites of the rubidium atoms in the nanotube bundle will be discussed in terms of their effects on the diffraction spectra. The experimental results will be compared to diffraction simulations and ab initio DFT calculations. The main outcome of our combined experimental and theoretical study is that: i) up to saturation, the spectra show no lattice expansion; ii) the extinction of the (10) peak is only compatible with \textit{Rb} insertion inside the tubes; iii) DFT calculations show that at constant lattice parameter the insertion within the tubes is energetically favored with respect to insertion between the tubes.   

Experimental Evidence for Water Intercalation into Graphite

Using different experimental methods we follow the uptake and release of water from highly oriented pyrolytic graphite sample. We found that water can intercalate into graphite following transient binding to defect sites and accumulate in the subsurface regions with concentrations amounting up to 10{\%} of the monolayer. The process is thermally activated and could be manipulated by changing the water vapor pressure or amount of water (ice) on the surface. Photoelectron and vibration spectroscopy data indicate strong perturbance of the intercalated water molecules and lowered barrier for dissociation.   

Dependence of single-walled carbon nanotubes' adsorption kinetics on temperature and binding energy

We present adsorption kinetics results for hydrogen, freon and ethane on single walled carbon nanotubes. We measured the decrease of the pressure as a function of time as equilibrium is approached. Our results indicate that the equilibration time is a function of$\frac{\varepsilon }{T}$; where$\varepsilon $ is the binding energy of adsorbate and T is the isotherm temperature. We also compare the dependence of the equilibration time on the shape of the adsorbate. We found that for linear molecules the equilibration times decrease with increasing coverage at a much slower rate than those times for spherical molecules.   

Temperature and Pressure Dependence of Hydrogen Coverage on Single-Walled Carbon Nanotubes

The safe and compact storage of hydrogen is of great interest in theoretical and experimental research. Carbon nanotubes are reported to be highly efficient for gas and alkali atom storage. The process of hydrogen adsorption on the carbon nanotubes is changed under various circumstances. it is important to know how the hydrogen coverage depends on the tube diameters, temperature and pressure. We have investigated the stability of various hydrogenated single-walled carbon nanotubes. We find the storage capacity of hydrogen depends significantly on the diameters of carbon nanotubes. The full hydrogen coverage can be reached for the nanotubes with small size, while for the nanotubes with large size, the saturation coverage is lower than 1. We have calculated the variation of the hydrogen coverage with the change of temperature and pressure. In particular, we find that the nanotubes with diameters of about 1 nm can achieve the coverage of 80{\%} at ambient temperature and low pressure, which is in agreement with the experimental results.   

Transport characteristics of a single multiwall carbon nanotube by bending in SEM and STM

Multiwall carbon nanotubes(MWCNTs) were grown on a W wire by chemical vapor deposition(CVD). Two homebuilt xyz-walkers were employed to manipulate individual CNTs in our scanning electron microscope (SEM). To improve the electrical and mechanical contact to a second electrode, we welded the CNT by delivering gas to the welding point while focusing the SEM beam on the same spot. The bending dependent I-V characteristics were observed in situ in the SEM at room temperature. We will measure the transport properties by bending the same MWCNT (already measured in SEM) inside our ultrahigh vacuum low temperature scanning tunneling microscope (UHV-LTSTM). We will also compare the bending properties of MWCNTs at different temperatures.