Ambipolar electrical transport in semiconducting single-wall carbon nanotubes.

Ambipolar electrical transport is reported in single-wall carbon nanotube (SWNT) field-effect transistors. In particular, the properties of SWNT junctions to TiC are discussed in detail. The carbide-nanotube junctions are abrupt and robust. In contrast to planar junctions, these contacts present low resistance for the injection of both p- and n-type carriers--the apparent barrier height of the junction is modified by the gate field. Thus SWNTs offer the novel possibility of ambipolar Ohmic contacts.

Current saturation and electrical breakdown in multiwalled carbon nanotubes.

We investigate the limits of high energy transport in multiwalled carbon nanotubes (MWNTs). In contrast to metal wires, MWNTs do not fail in the continuous, accelerating manner typical of electromigration. Instead, they fail via a series of sharp, equally sized current steps. We assign these steps to the sequential destruction of individual nanotube shells, consistent with the MWNT's concentric-shell geometry. Furthermore, the initiation of this failure is very sensitive to air exposure. In air failure is initiated by oxidation at a particular power, whereas in vacuum MWNTs can withstand much higher power densities and reach their full current carrying capacities.

Engineering carbon nanotubes and nanotube circuits using electrical breakdown.

Carbon nanotubes display either metallic or semiconducting properties. Both large, multiwalled nanotubes (MWNTs), with many concentric carbon shells, and bundles or "ropes" of aligned single-walled nanotubes (SWNTs), are complex composite conductors that incorporate many weakly coupled nanotubes that each have a different electronic structure. Here we demonstrate a simple and reliable method for selectively removing single carbon shells from MWNTs and SWNT ropes to tailor the properties of these composite nanotubes. We can remove shells of MWNTs stepwise and individually characterize the different shells. By choosing among the shells, we can convert a MWNT into either a metallic or a semiconducting conductor, as well as directly address the issue of multiple-shell transport. With SWNT ropes, similar selectivity allows us to generate entire arrays of nanoscale field-effect transistors based solely on the fraction of semiconducting SWNTs.

Intertube coupling in ropes of single-wall carbon nanotubes

We investigate the coupling between individual tubes in a rope of single-wall carbon nanotubes using four probe resistance measurements. By introducing defects through the controlled sputtering of the rope we generate a strong nonmonotonic temperature dependence of the four terminal resistance. This behavior reflects the interplay between localization in the intentionally damaged tubes and coupling to undamaged tubes in the same rope. Using a simple model we obtain the coherence length and the coupling resistance. The coupling mechanism is argued to involve direct tunneling between tubes.

Carbon-atom wires: charge-transfer doping, voltage drop, and the effect of distortions

We present first-principles calculations on electrical conduction through carbon atomic wires. The changes in charge distribution induced by a large bias exhibit the primary involvement of the wire's pi states. A significant fraction ( approximately 40%) of the voltage drops across the atomic wire itself. At zero bias, there is a large transfer of charge from the electrodes to the wire, effectively providing doping without introducing scattering centers. This transfer leads, however, to potential barriers at the wire-electrode junctions. Bending the wire reduces its conductance.

Electrical transport in rings of single-wall nanotubes: one-dimensional localization

We report low-temperature magnetoresistance (MR) measurements on rings of single-wall carbon nanotubes. Negative MR characteristic of weak one-dimensional localization is clearly observed from 3.0 to 60 K, and the coherence length L(varphi) is obtained as a function of temperature. The dominant dephasing mechanism is identified as electron-electron scattering. Below 1 K, we observe a transition from weak to strong localization, and below 0.7 K a weak antilocalization is induced by spin-orbit scattering.

Metal-semiconductor nanocontacts: silicon nanowires

Silicon nanowires assembled from clusters or etched from the bulk, connected to aluminum electrodes and passivated, are studied with large-scale local-density-functional simulations. Short ( approximately 0.6 nm) wires are fully metallized by metal-induced gap states resulting in finite conductance ( approximately e(2)/h). For longer wires ( approximately 2.5 nm) nanoscale Schottky barriers develop with heights larger than the corresponding bulk value by 40% to 90%. Electric transport requires doping dependent gate voltages with the conductance spectra exhibiting interference resonances due to scattering of ballistic channels by the contacts.

Atomic-scale desorption through electronic and vibrational excitation mechanisms.

The scanning tunneling microscope has been used to desorb hydrogen from hydrogen-terminated silicon (100) surfaces. As a result of control of the dose of incident electrons, a countable number of desorption sites can be created and the yield and cross section are thereby obtained. Two distinct desorption mechanisms are observed: (i) direct electronic excitation of the Si-H bond by field-emitted electrons and (ii) an atomic resolution mechanism that involves multiple-vibrational excitation by tunneling electrons at low applied voltages. This vibrational heating effect offers significant potential for controlling surface reactions involving adsorbed individual atoms and molecules.

Observation of Quantum-Size Effects at Room Temperature on Metal Surfaces With STM.

Surface steps act as confining barriers for electrons in metal-surface states. Thus, narrow terraces and small single-atom-high metal islands act as low-dimensional, electron-confining structures. In sufficiently small structures, quantum-size effects are observable even at room temperature. Scanning tunneling spectroscopy is used to image the probability amplitude distributions and discrete spectra of the confined states. Examination of the electronic structure of the steps provides evidence for electron-density smoothing and the formation of step-edge states. Estimates of the electron-confining barriers are obtained.

Manipulation of the Reconstruction of the Au(111) Surface with the STM.

Modification of the reconstruction of an Au(111) surface with a scanning tunneling microscope (STM) is demonstrated. This modification is accomplished by transferring a number of surface atoms to the STM tip to generate a surface multivacancy (hole), which modifies the stress distribution at the surface. The structural changes that follow the tip-induced surface perturbation are imaged in a time-resolved manner. The structural modification is the result of both short-range interactions, which lead to local atomic relaxation, and long-range elastic interactions, which produce large-scale rearrangements.

Dissociation of individual molecules with electrons from the tip of a scanning tunneling microscope.

The scanning tunneling microscope (STM) can be used to select a particular adsorbed molecule, probe its electronic structure, dissociate the molecule by using electrons from the STM tip, and then examine the dissociation products. These capabilities are demonstrated for decaborane(14) (B(10)H(14)) molecules adsorbed on a silicon(111)-(7 x 7) surface. In addition to basic studies, such selective dissociation processes can be used in a variety of applications to control surface chemistry on the molecular scale.

Field-Induced Nanometer- to Atomic-Scale Manipulation of Silicon Surfaces with the STM.

The controlled manipulation of silicon at the nanometer scale will facilitate the fabrication of new types of electronic devices. The scanning tunneling microscope (STM) can be used to manipulate strongly bound silicon atoms or clusters at room temperature. Specifically, by using a combination of electrostatic and chemical forces, surface atoms can be removed and deposited on the STM tip. The tip can then move to a predetermined surface site, and the atom or cluster can be redeposited. The magnitude of such forces and the amount of material removed can be controlled by applying voltage pulses at different tip-surface separations.

Negative differential resistance on the atomic scale: implications for atomic scale devices.

Negative differential resistance (NDR) is the essential property that allows fast switching in certain types of electronic devices. With scanning tunneling microscopy (STM) and scanning tunneling spectroscopy, it is shown that the current-voltage characteristics of a diode configuration consisting of an STM tip over specific sites of a boron-exposed silicon(111) surface exhibit NDR. These NDR-active sites are of atomic dimensions ( approximately 1 nanometer). NDR in this case is the result of tunneling through localized, atomic-like states. Thus, desirable device characteristics can be obtained even on the atomic scale.