Yuval Yaish

Coulomb blockade and the Kondo effect in single-atom transistors

Using molecules as electronic components is a powerful new direction in the science and technology of nanometre-scale systems. Experiments to date have examined a multitude of molecules conducting in parallel, or, in some cases, transport through single molecules. The latter includes molecules probed in a two-terminal geometry using mechanically controlled break junctions or scanning probes as well as three-terminal single-molecule transistors made from carbon nanotubes, C60 molecules, and conjugated molecules diluted in a less-conducting molecular layer. The ultimate limit would be a device where electrons hop on to, and off from, a single atom between two contacts. Here we describe transistors incorporating a transition-metal complex designed so that electron transport occurs through well-defined charge states of a single atom. We examine two related molecules containing a Co ion bonded to polypyridyl ligands, attached to insulating tethers of different lengths. Changing the length of the insulating tether alters the coupling of the ion to the electrodes, enabling the fabrication of devices that exhibit either single-electron phenomena, such as Coulomb blockade, or the Kondo effect.

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.

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.

Electrical cutting and nicking of carbon nanotubes using an atomic force microscope

In this letter, we demonstrate that voltage pulses from a metal-coated AFM tip can be used to permanently modify the electrical properties of NT devices. By adjusting the properties of the voltage pulses, we can either electrically break ~‘‘cut’’ ! NTs or create tunneling barriers ~‘‘nick’’ ! at any point along them. We demonstrate the utility of these techniques by creating single NT devices through the cutting of unwanted extra NTs, and making ultrasmall NT quantum dots can be created by nicking a NT at two places along its length. The NT devices used in this work were prepared following an approach similar to that of Kong et al. 21 First, catalyst islands containing Fe(NO 3 ) 3 i9H 2 O, MoO 2 (acac) 2 and alumina nanoparticles were defined on a degenerately doped silicon wafer with 200-nm-thick thermally grown oxide. Photolithography and etching were used to pattern a poly~methylmethacrylate! layer, which was subsequently used as a lift-off mask for the catalyst. NTs were then grown by chemical vapor deposition. 21 Metal electrodes consisting of Cr~ 5n m! and Au~50 nm! were patterned over the catalyst islands using photolithography and a lift-off process, with a spacing between source and drain electrodes between 1 and 3 mm. This process allowed the parallel production of hundreds of NT devices using only optical lithography. The samples were annealed at 600 °C for 45 min in an Ar environment to decrease the contact resistance between the NTs and the electrodes. Typical NT conductances obtained were 0.2‐ 2e 2 /h.

Thermodynamic spin magnetization of strongly correlated two-dimensional electrons in a silicon inversion layer

A method invented to measure the minute thermodynamic magnetization of dilute two-dimensional fermions is applied to electrons in a silicon inversion layer. The interplay between the ferromagnetic interaction and disorderenhances the low temperature susceptibility up to 7.5 folds compared with the Pauli susceptibility of noninteracting electrons. The magnetization peaks in the vicinity of the density, where transition to strong localization takes place. At the same density, the susceptibility approaches the free spins value (Curie susceptibility), indicating an almost perfect compensation of the kinetic energy toll associated with spin polarization by the energy gained from the Coulomb correlation. Yet, the balance favors a paramagnetic phase over spontaneous magnetization in the whole density range.

Wiring up single molecules

The possibility of using single molecules as active elements of electronic devices offers a variety of scientific and technological opportunities. In this article, we discuss transistors, where electrons flow through discrete quantum states of a single molecule. First, we will describe molecules, where current flows through one cobalt atom surrounded by two insulating terpyridyl ligands. Depending on the length of the insulating part of the molecules, two different behaviors are observed: Coulomb blockade for a longer molecule and the Kondo effect for a shorter molecule. We will also discuss measurements of the C70 fullerene and its dimer (C140). In C140 devices, the transport measurements are affected by an intercage vibrational mode that has an energy of 11 meV. We observe a large current increase when this mode is excited, indicating a strong coupling between the electronic and mechanical degrees of freedom in C140 molecules.