Tobias Hertel

Single- and multi-wall carbon nanotube field-effect transistors

We fabricated field-effect transistors based on individual single- and multi-wall carbon nanotubes and analyzed their performance. Transport through the nanotubes is dominated by holes and, at room temperature, it appears to be diffusive rather than ballistic. By varying the gate voltage, we successfully modulated the conductance of a single-wall device by more than 5 orders of magnitude. Multi-wall nanotubes show typically no gate effect, but structural deformations—in our case a collapsed tube—can make them operate as field-effect transistors.

Interlayer cohesive energy of graphite from thermal desorption of polyaromatic hydrocarbons

We have studied the interaction of polyaromatic hydrocarbons ~PAHs! with the basal plane of graphite using thermal desorption spectroscopy. Desorption kinetics of benzene, naphthalene, coronene, and ovalene at submonolayer coverages yield activation energies of 0.50 eV, 0.85 eV, 1.40 eV, and 2.1 eV, respectively. Benzene and naphthalene follow simple first order desorption kinetics while coronene and ovalene exhibit fractional order kinetics owing to the stability of two-dimensional adsorbate islands up to the desorption temperature. Preexponential frequency factors are found to be in the range 10 14 ‐10 21 s 21 as obtained from both FalconerMadix ~isothermal desorption! analysis and Antoine’s fit to vapor pressure data. The resulting binding energy per carbon atom of the PAH is 5265 meV and can be identified with the interlayer cohesive energy of graphite. The resulting cleavage energy of graphite is 6165 meV/atom, which is considerably larger than previously reported experimental values.

ATOMIC FORCE MICROSCOPE TIP-INDUCED LOCAL OXIDATION OF SILICON: KINETICS, MECHANISM, AND NANOFABRICATION

Atomic force microscope induced local oxidation of silicon is a process with a strong potential for use in proximal probe nanofabrication. Here we examine its kinetics and mechanism and how such factors as the strength of the electric field, ambient humidity, and thickness of the oxide affect its rate and resolution. Detection of electrochemical currents proves the anodization character of the process. Initial very fast oxidation rates are shown to slow down dramatically as a result of a self-limiting behavior resulting from the build up of stress and a reduction of the electric field strength. The lateral resolution is determined by the defocusing of the electric field in a condensed water film whose extent is a function of ambient humidity.

Carbon nanotubes : nanomechanics, manipulation, and electronic devices

Carbon nanotubes are novel materials with unique electrical and mechanical properties. Here we present results on their atomic structure and mechanical properties in the adsorbed state, on ways to manipulate individual nanotubes, on their electrical properties and, finally, on the fabrication and characteristics of nanotube-based electron devices. Specifically, atomic force microscopy (AFM) and molecular mechanics simulations are used to investigate the effects of van der Waals interactions on the atomic structure of adsorbed nanotubes. Both radial and axial structural deformations are identified and the interaction energy itself is obtained from the observed deformations. The conditions under which the structure of a nanotube will adjust to the topography of the substrate are defined. We show that the strong substrate–nanotube interaction allows the manipulation of both the position and shape of individual nanotubes at inert surfaces using the AFM. AFM manipulation is then utilized to position individual nanotubes on electrical pads so that their electrical characteristics can be evaluated. We demonstrate the operation of a field-effect transistor based on a single semiconducting nanotube and of a single-electron transistor using a nanotube bundle as Coulomb island. Finally, conducting nanotubes are employed as tips for AFM lithography.

Quantitative analysis of optical spectra from individual single-wall carbon nanotubes

We discuss how tight-binding band-structure calculations with a chirality- and diameter-dependent nearest-neighbor hopping integral may be used to relate well resolved features in the UV-VIS-NIR spectra of individual single-wall carbon nanotubes (SWNTs) to electronic excitations in specific tube types. The assignment of (n,m) indices to interband transitions in specific tube types can support a quantitative analysis of absorption spectra which may eventually be used for rapid screening and optimization of sample composition during SWNT synthesis.

Diffusion limited photoluminescence quantum yields in 1-D semiconductors: single-wall carbon nanotubes.

Photoluminescence quantum yields and nonradiative decay of the excitonic S(1) state in length fractionated (6,5) single-wall carbon nanotubes (SWNTs) are studied by continuous wave and time-resolved fluorescence spectroscopy. The experimental data are modeled by diffusion limited contact quenching of excitons at stationary quenching sites including tube ends. A combined analysis of the time-resolved photoluminescence decay and the length dependence of photoluminescence quantum yields (PL QYs) from SWNTs in sodium cholate suspensions allows to determine the exciton diffusion coefficient D = 10.7 ± 0.4 cm(2)s(-1) and lifetime τ(PL) for long tubes of 20 ± 1 ps. PL quantum yields Φ(PL) are found to scale with the inverse diffusion coefficient and the square of the mean quenching site distance, here l(d) = 120 ± 25 nm. The results suggest that low PL QYs of SWNTs are due to the combination of high-diffusive exciton mobility with the presence of only a few quenching sites.

UV photostimulated desorption of ammonia from Cu(111)

Upon irradiation with 193 and 308 nm laser light photoinduced desorption of ammonia from Cu(111) was studied at three coverages less than one monolayer (ML). The linear power dependence of the desorption yield and angle‐resolved translational energy distributions of desorbed molecules indicate that desorption occurs due to an electronic excitation rather than a thermal process. Polarization measurements indicate an excitation process which is mediated by hot substrate electrons. The isotope effect, i.e., the ratio of the cross sections for photostimulated desorption (at 193 nm) of NH3 and ND3, respectively, decreases from 4.1 ± 1.2 to 1.9 ± 0.5 when the coverage—with respect to the substrate atom density—was raised from ≊0.02 to ≊0.14 ML. The magnitude of this isotope effect suggests that the energy which is required to break the molecule–surface bond is acquired in an intramolecular coordinate during a short‐lived electronic excitation. We propose that for high vibrational excitation on the ground‐state ...

Dynamics of photoexcited electrons in metals studied with time-resolved two-photon photoemission

Femtosecond time-resolved two-photon photoemission (2PPE) is used to investigate the hot electron dynamics of Cu(111), Ag(110) and Ta(poly). The experimentally derived relaxation rates of photoexcited electrons scale with the available phase space for scattering with electrons below the Fermi level and are compared with predictions from Fermi liquid theory. A simulation employing rate equations to account for hot electron lifetimes, transport effects and the creation of secondary electrons demonstrates that the relaxation dynamics is strongly influenced by secondary electron cascades. With Ag(110) a pronounced temperature dependence of the photoemission yield is observed, indicating the importance of electron-phonon scattering for momentum conservation in the 2PPE process.

Ultrafast Excitation Energy Transfer in Small Semiconducting Carbon Nanotube Aggregates

We study excitation energy transfer in small aggregates of chirality enriched carbon nanotubes by transient absorption spectroscopy. Ground state photobleaching is used to monitor exciton population dynamics with sub-10 fs time resolution. Upon resonant excitation of the first exciton transition in (6,5) tubes, we find evidence for energy transfer to (7,5) tubes within our time resolution (<10 fs). Excitation in the visible spectral range, where the second excitonic transitions occur, is followed by fast intratube relaxation and subsequent energy transfer, in particular from the (8,4) tube toward other tubes, the latter process occurring in less than 10 fs.

Interaction of molecular oxygen with single-wall carbon nanotube bundles and graphite

The adsorption of oxygen on highly oriented pyrolytic graphite (HOPG) and bundles of single-wall carbon nanotubes (SWNTs) at 28 K is studied using thermal desorption spectroscopy and by a measurement of sticking probabilities. The low-coverage binding energy of oxygen adsorbed on SWNT bundles, 18.5 kJ/mol, is 55% higher than the low-coverage binding energy on HOPG, 12.0 kJ/mol. Molecular mechanics calculations reveal that such an increase can be attributed to the higher effective coordination of binding sites on SWNT bundles. The character of the oxygen-SWNT interaction should therefore be van der Waals type which suggests that the observed oxygen species is physisorbed and does not facilitate bulk doping of SWNT samples.