Session W29: Focus Session: Carbon Nanotubes and Related Materials XIV: Theory and Sensing

Electron-phonon interaction and excited states relaxation in carbon nanotubes

We will discuss the role of electron-phonon interaction on excited states relaxation and phonon spectra in carbon nanotubes (CNTs). The electron-phonon interaction leads to the polaronic effects of the charge carriers, but it also renormalizes the energy and the lifetime of phonons. We present a theoretical model that predicts the changes induced in the phonon modes of CNTs as a function of the charge carrier doping, i.e. position of the Fermi level. In agreement with the predictions, our experiments show sharpening and blue shifts of the G-phonons of metallic CNTs, but only blue shifts for semiconducting CNTs, making the Raman scattering a useful probe of local doping of CNTs [1]. The non-equilibrium dynamics of charge carriers under external electric field is determined by the electron-phonon scattering. The hot carriers under unipolar transport conditions can be produced, leading to the strong impact excitation and light emission, which intensity is determined by electric field, phonon scattering, and impact excitation cross section [2, 3]. In the reverse process of photoconductivity, light is absorbed creating excited states. We will discuss electronic relaxation of high energy excited states leading to the free carriers, contributing to the photoconductivity, and phonon relaxation, leading to the bound excitons [4]. The later can contribute to the photocurrent only after ionization by the external field [5]. Finally, we will discuss the role of phonons in the long puzzling question regarding the nature of the dominant decay channel of the low energy excited states and the potential of optoelectronic applications of CNTs. [1] J.C. Tsang, M. Freitag, V. Perebeinos, J. Liu, and Ph. Avouris, Nature Nanotechnology 2, 725 (2007); [2] J. Chen, V. Perebeinos, M. Freitag, J. Tsang, Q. Fu, J. Liu, Ph. Avouris, Science 310, 1171 (2005); [3] V. Perebeinos and Ph. Avouris, Phys. Rev. B. 74, 121410(R), (2006); [4] T. Hertel, V. Perebeinos, J. Crochet, K. Arnold, M. Kappes, Ph. Avouris, arXiv:0710.3154; [5] V. Perebeinos and Ph. Avouris, Nano Lett. 7, 609 (2007)   

Study of Butane adsorption on Purified HiPco Carbon Nanotubes

We investigated the adsorption characteristics of butane on purified HiPco single-walled carbon nanotubes for coverages in the first layer. We measured 9 full isotherms between 180 and 260K. The results for butane are compared with those obtained in a previous study of ethane adsorption on the same substrate. Comparable values for the specific surface area of the substrate were found when this quantity was measured using either ethane or butane. This strongly suggests that both of these species have access to essentially the same adsorption sites. We also determined that the strength of binding of the hydrocarbon chains to the SWNT bundles correlates, roughly linearly, with molecular length. An increase in chain length provides a greater number of contacts between sites on the substrate and the adsorbate species, thus, resulting in an increase in the values of the binding energies.   

Adsorption Kinetics of CH$_{4}$ on Purified HiPco Single-Walled Carbon Nanotubes

We present the results of an adsorption kinetics study of CH$_{4}$ on two sets of purified HiPco SWNTs. We monitor the time evolution of the pressure from the instant at which gas is added to the cell containing the nanotubes to the moment at which equilibrium is reached. The two sets of samples were baked, under vacuum, at different temperatures (300 and 400$^{\circ}$C). The difference in baking temperatures resulted in a difference in the specific surface areas; the sample treated at 400$^{\circ}$C has a 15{\%} larger surface area than the sample treated at 300$^{\circ}$C. It also caused a difference in the kinetic behavior of the samples; the equilibration times for the two samples differ by a factor of 3. Moreover, the sample heated at 400$^{\circ}$C exhibits two distinct equilibration times, while the one heated at 300$^{\circ}$C exhibits only one. These changes are probably the result of partially opening the purified SWNTs by baking them at 400$^{\circ}$C, which does not occur when we heat them only to 300$^{\circ}$C.   

Adsorption kinetics of diatomic molecules on carbon nanotube bundles

A Kinetic Monte Carlo algorithm is used to explore the kinetics of adsorption of diatomic adsorbates on one-dimensional chains of sites. In particular, we monitor the evolution of the orientational configuration of the adsorbate as equilibrium is being reached at different values of temperature and chemical potential. We also analyze the dependence of the orientational evolution of the phases on the interactions between the molecules and on the presence of adsorption sites with different energies.   

Equilibration times of adsorption on external surfaces of carbon nanotube bundles

We investigate the adsorption kinetics of gases on the exterior of a carbon nanotube bundle by monitoring the uptake and exchange of particles in regions of the surface characterized by different binding energies. By using a Kinetic Monte Carlo scheme, we follow the time evolution of the gas uptake for different values of external pressure and temperature. The presence of adsorption sites with different energies gives rise to distinctive features on the equilibration time as function of the coverage. We show that preliminary experimental results for CF$_{4}$, Ar, and CH$_{4 }$on nanotube bundles with closed ends are consistent with our results.   

Adsorption kinetics of binary mixtures on carbon nanotube bundles

We examine kinetic selectivity effects that take place during the adsorption of a binary mixture on one-dimensional chains. After reaching equilibrium at the same chemical potential, the species with the higher binding energy will enjoy the greatest coverage. However, the weaker-binding species has faster adsorption kinetics and is able to reach a coverage higher than its equilibrium value before the stronger species can adsorb significantly. The result of this process is an `overshoot' in the fractional coverage of the weaker species that is reached at a time long before the system equilibrates. We analyze the appearance of this overshoot as a function of the temperature, chemical potentials, and energy parameters of the system.   

Hydrogen sensing properties of palladium-decorated carbon nanotube circuits

Sensitive hydrogen gas sensors can be fabricated from carbon nanotube circuits decorated with palladium metal, and we have investigated the responsible physical mechanisms using isolated, single-walled carbon nanotubes (SWCNTs). Hydrogen sensitivity arises from two active mechanisms, neither of which is the mere adsorption of Pd onto pristine SWCNTs. The first mechanism relies on the chemical sensitivity of Schottky barriers present when semiconducting SWCNTs are contacted by metals. Pd decoration of the barrier region, or the use of pure Pd as the contact metal, produces a modest H$_{2}$ sensitivity, if any. A more sensitive mechanism involves Pd-decorated defect sites, which in both metallic and semiconducting SWNTs results in reversible conductance swings of 100{\%}. This presentation will review the temporal dynamics and pressure dependence of both mechanisms.   

Electrical Measurement of Single Molecule Catalysis using Carbon Nanotubes

We demonstrate single molecule chemical sensors based on single-walled carbon nanotubes (SWNT). The architecture uses a SWNT conductor having a single, reactive species covalently bonded to the sidewall [1]. Dynamics of the molecule are electrically transduced as it interacts with its surrounding environment. As a test case, we investigate the catalytic modification of EDC by a carboxylate. After creating a carboxylate terminus on the SWNT, the circuit is monitored for several hours and through hundreds of individual EDC reactions. Statistical analysis determines the lifetime of the carboxyl-EDC complex, as well as the catalytic turnover rate, from discrete events. Because the carboxylate site can be readily derivatized with proteins, peptides, or other functional molecules, the technique shows promise as a tool for single molecule research independent of optics and scanning probe microscopy. 1. B.R. Goldsmith, et al. \textit{Science} \textbf{315}, 77 (2007).   

Artificial introduction of defects in carbon nanotubes through Argon and Hydrogen ion irradaition, and application to chemical sensors

The goal of this study is to quantify the effect of defect density on the electrochemical properties of multi-walled CNTs. Consequently, ion irradiation, with argon (Ar) and hydrogen (H), individually, has been performed to systematically incorporate defects into vertically aligned MWCNTs. Raman spectroscopy was used to characterize the amount of disorder within the nanotube samples. The electrochemical behavior of the irradiated MWCNT samples was then characterized through cyclic voltammetry (CV) measurements. Raman spectroscopy revealed an increase in the disorder in MWCNTs with the argon and hydrogen irradiation, as evidenced by an increase in the I$_{D}$/I$_{G}$ peak intensity ratio. However, Ar is intercalated into the CNTs, and charges the nanotubes (forming dangling bonds), while H treatment terminates residual CNT dangling bonds. In CV, we have seen that only the Ar treated samples exhibit \textit{perfect} reversible Nernstian behavior characteristic of ideal electrodes. Hydrogen treated CNT electrode ensembles seem to exhibit quicker response, as glucose sensors, with exquisite ($\sim $ 1 $\mu $M) sensitivity   

Structure and thermodynamics of a Neon monolayer adsorbed on carbon nanotube bundles

We report results from neutron diffraction measurements of five submonolayer coverages of Ne adsorbed on single-wall, closed-end carbon nanotube bundles (SWCNB). Our recent thermodynamic study of Ne adsorbed on SWCNB, Phys. Rev. B, 76, 075404 (2007), showed one-dimensional (1d) solid behavior below 4K at doses less than 0.08 monolayer, which crossed over to 3d-like behavior above 16K without signature of a phase transition. Above 0.18 monolayer there is a 2d-like solid behavior below 8K, with Debye temperatures in the 45 to 53K range, similar to those found for 2d Ne/graphite, but there is no melting transition at 13.5K as seen in 2d. Our structure measurements on Ne adsorbed on SWCNB similar to those used for the thermodynamic study were performed at 2K using beam line D20 at ILL. Results show the shift from a 1d to a 2d solid structure with increasing coverage. Lattice parameters, the relationship between the thermodynamic and structural measurements, and theoretical expectations will be shown and discussed.   

Structure of a DNA-carbon nanotube hybrid using replica exchange molecular dynamics

DNA-carbon nanotube hybrids (DNA-CN) are novel nanoscale materials that consist of single-wall carbon nanotubes coated with a self-assembled monolayer of single stranded DNA (ssDNA). Many recent experiments have demonstrated that this nanomaterial is an ideal candidate for a variety of nanotechnological applications. Despite the importance of this material, a complete understanding of its structural and physical properties is lacking. Recent molecular dynamics (MD) simulations of this nanomaterial have provided information about the self-assembly mechanisms and possible ssDNA conformations that characterize DNA-CN. However, MD simulations of biopolymers at low temperatures (T $\sim $ 300 K) result in kinetic trapping and limits sampling of ssDNA configurational space. Here, we present the results of large scale replica exchange molecular dynamics simulations that provide robust sampling of the multitude of ssDNA conformations about SWCN.   

Photon sensing with carbon nanotubes: low-temperature photothermal effect on carbon nanotube transistors

While thermal effects on the electrical properties of carbon nanotubes (CNTs) are an area of great interest, the effect of single photons on the low temperature conductance of CNTs has not been carefully studied. We recently developed a low-temperature fiber-based laser confocal microscope with simultaneous electrical measurement capability and diffraction-limited laser illumination and detection. In our experiment, CNT devices functionalized with gold nanoparticles were illuminated with a focused laser beam while their conductance was measured at temperatures as low as 300 mK. Photon absorption by either CNTs or gold nanoparticles results in local heating and provides spatially-resolved information about the thermal effects on transport in the CNT as the laser is scanned across the device. Our technique can be easily extended to the study of single-photon effects in other nanostructures including nanowires and graphene in the near future.   

Transport and Charge Sensing in $^{12}$C and $^{13}$C Carbon Nanotube Double Quantum Dots

We report measurements of gate-defined carbon nanotube double quantum dot devices with a charge sensor fabricated on the same nanotube. The methane used during growth controls the $^{13}$C content of the nanotubes. $^{12}$C nuclei have zero nuclear spin, and $^{13}$C nuclei have spin 1/2. We compare samples with natural abundance (1\%) and enriched (99\%) $^{13}$C content. A strong isotope effect is observed in the magnetic field dependence of transport at finite bias. Fast control of these devices is demonstrated using a pulsed-gate technique.