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, C(60) 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.

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 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.

We have fabricated high performance field-effect transistors made from semiconducting single-walled carbon nanotubes (SWNTs). Using chemical vapor deposition to grow the tubes, annealing to improve the contacts, and an electrolyte as a gate, we obtain very high device mobilities and transconductances. These measurements demonstrate that SWNTs are attractive for both electronic applications and for chemical and biological sensing.

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 bandgap in a metallic NT and modify the bandgap in a semiconducting NT. Theoretical work predicts that bandgap changes can range between ± 100 meV per 1% stretch, depending on NT chirality, and our measurements are consistent with this predicted range. PACS numbers: 62.25.+g, 71.20.Tx, 73.63.Fg, 81.07.De, 85.35.Kt The electronic and mechanical properties of carbon NTs make them interesting for both technological applications and basic science. A NT can be either metallic or semiconducting depending on the orientation between the atomic lattice and the tube axis [1,2]. NTs can accommodate very large mechanical strains [3] and have an extremely high Young's modulus [4]. Both theory and experiment indicate that NTs also have interesting electromechanical properties [5][6][7][8][9][10][11][12]. A pioneering experiment [10] showed that the conductance of a metallic NT could decrease by orders of magnitude when strained by an atomic force microscope (AFM) tip. The authors suggest that a local distortion of the sp 2 bonding where the NT is touched by the AFM tip causes the drop in conductance. In Ref.[12], however, it is argued that the observed drop in conductance is due to a bandgap induced in the NT as it is axially stretched [5,8,11] as illustrated in Fig. 1(a). Evidence for the effect of strain on NT bandgap also comes from recent STM work on semiconducting NTs containing encapsulated metallofullerenes [13]. The authors found a bandgap reduction of 60% at the expected positions of the metallofullerenes and postulated that strain could account for this change.Here we present measurements to demonstrate conclusively that strain modulates the band structure of NTs. We employ an AFM tip to simultaneously vary the NT strain and to electrostatically gate the tube. We find that, under strain, the conductance of the NT can increase or decrease, depending on the tube. By using the tip as a gate, we show that this is related to the increase or decrease in the bandgap of a NT under strain. The magnitude of the effect and its dependence on strain are consistent with theoretical expectations.The samples consist of NTs suspended over a trench and clamped at both ends by electrical contacts [10,[14][15][16][17]. CVD growth is utilized to grow NTs with diameters between 1 and 10 nm at lithographically defined catalyst sites [18] on a Si substrate with a 500nm oxide. Metal contacts (5nm Cr and 50-80nm gold) are made using photolithography, as described previously [19]. An ashing step (400°C for 10 minutes in Ar atmosphere) removes photoresist residue and improves contact resistances. An HF etch (3 minutes in 6:1 BHF, etch rate 80 nm/min) followed by critical point drying is used to suspend the NTs [16]. Device conductances are not changed significantly by the etching/drying p...

The electronic structure of a NT is elegantly described by the quantization of wave states around a graphene cylinder 1 . Graphene is a zero band-gap semiconductor in which the valence and conduction states meet at two points in k-space, K 1 and K 2 (Fig. 1a). The dispersion around each of these points is a cone (Fig 1b). When graphene is wrapped into a cylinder the electron wave number perpendicular to the NT axis, , is quantized, satisfying the boundary condition πD = 2πj where D is the NT diameter and j is an integer. The resulting allowed k's correspond to the horizontal lines in

We perform low-temperature electrical transport measurements on gated, quasi-2D graphite quantum dots. In devices with low contact resistances, we use longitudinal and Hall resistances to extract carrier densities of 9.2-13 x 10(12) cm(-2) and mobilities of 200-1900 cm(2)/V.s. In devices with high resistance contacts, we observe Coulomb blockade phenomena and infer the charging energies and capacitive couplings. These experiments demonstrate that electrons in mesoscopic graphite pieces are delocalized over nearly the whole graphite piece down to low temperatures.

A novel 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 disorder enhances the low temperature susceptibility up to 7.5 folds compared with the Pauli susceptibility of non-interacting 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.