Vera Sazonova

A tunable carbon nanotube electromechanical oscillator

Nanoelectromechanical systems (NEMS) hold promise for a number of scientific and technological applications. In particular, NEMS oscillators have been proposed for use in ultrasensitive mass detection, radio-frequency signal processing, and as a model system for exploring quantum phenomena in macroscopic systems. Perhaps the ultimate material for these applications is a carbon nanotube. 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 may be able to sense its own motion. In spite of this great promise, a room-temperature, self-detecting nanotube oscillator has not been realized, although some progress has been made. Here we report the electrical actuation and detection of the guitar-string-like oscillation modes of doubly clamped nanotube oscillators. We show that the resonance frequency can be widely tuned and that the devices can be used to transduce very small forces.

Electron−Phonon Scattering in Metallic Single-Walled Carbon Nanotubes

Electron scattering rates in metallic single-walled carbon nanotubes are studied using an atomic force microscope as an electrical probe. From the scaling of the resistance of the same nanotube with length in the low- and high-bias regimes, the mean-free paths for both regimes are inferred. The observed scattering rates are consistent with calculations for acoustic-phonon scattering at low biases and zone boundary/optical phonon scattering at high biases.

Tuning carbon nanotube band gaps with strain.

We show that the band structure of a carbon nanotube (NT) can be dramatically altered by mechanical strain. We employ an atomic force microscope tip to simultaneously vary the NT strain and to electrostatically gate the tube. We show that strain can open a band gap in a metallic NT and modify the band gap in a semiconducting NT. Theoretical work predicts that band gap changes can range between +/-100 meV per 1% stretch, depending on NT chirality, and our measurements are consistent with this predicted range.

Determination of electron orbital magnetic moments in carbon nanotubes

The remarkable transport properties of carbon nanotubes (CNTs) are determined by their unusual electronic structure. The electronic states of a carbon nanotube form one-dimensional electron and hole sub-bands, which, in general, are separated by an energy gap. States near the energy gap are predicted to have an orbital magnetic moment, µorb, that is much larger than the Bohr magneton (the magnetic moment of an electron due to its spin). This large moment is due to the motion of electrons around the circumference of the nanotube, and is thought to play a role in the magnetic susceptibility of CNTs and the magnetoresistance observed in large multiwalled CNTs. But the coupling between magnetic field and the electronic states of individual nanotubes remains to be quantified experimentally. Here we report electrical measurements of relatively small diameter (2–5 nm) individual CNTs in the presence of an axial magnetic field. We observe field-induced energy shifts of electronic states and the associated changes in sub-band structure, which enable us to confirm quantitatively the predicted values for µorb.

Mixing at 50GHz using a single-walled carbon nanotube transistor

We have probed the electrical properties of top-gated single-walled carbon nanotube transistors at frequencies up to 50GHz by using the device as a microwave mixer. We find that the amplitude of the mixing signal decays as a function of frequency with a characteristic time constant that is limited by the setup. Despite the setup-limited cutoff frequency of ∼10GHz, we show that the devices still operate faster than 50GHz.

Electrical Nanoprobing of Semiconducting Carbon Nanotubes Using an Atomic Force Microscope

We use an atomic force microscope (AFM) tip to locally probe the electronic properties of semiconducting carbon nanotube transistors. A gold-coated AFM tip serves as a voltage or current probe in three-probe measurement setup. Using the tip as a movable current probe, we investigate the scaling of the device properties with channel length. Using the tip as a voltage probe, we study the properties of the contacts. We find that Au makes an excellent contact in the p region, with no Schottky barrier. In the n region, large contact resistances were found which dominate the transport properties.

Fluctuation broadening in carbon nanotube resonators

We simulated the behavior of suspended carbon nanotube resonators over a broad range of temperatures to explore the physics of semiflexible polymers in underdamped environments. We find that thermal fluctuations induce strong coupling between resonance modes. This effect leads to spectral fluctuations that readily account for the experimentally observed quality factors Q ∼ 100 at 300 K. Using a mean-field approach to describe fluctuations, we analytically calculate Q and frequency shifts in tensioned and buckled carbon nanotubes and find excellent agreement with simulations.