Bias-induced transfer of an aluminum atom in the scanning tunneling microscope

Electronic transport properties of PbSi Schottky-clamped transistors with a surrounding metal–insulator gate

Sustaining Moore's law requires the design of new materials and the construction of FET.

Atom-by-atom assembly

Atomic manipulation using a scanning tunneling microscope (STM) tip enables the construction of quantum structures on an atom-by-atom basis, as well as the investigation of the electronic and dynamical properties of individual atoms on a one-atom-at-a-time basis. An STM is not only an instrument that is used to 'see' individual atoms by means of imaging, but is also a tool that is used to 'touch' and 'take' the atoms, or to 'hear' their movements. Therefore, the STM can be considered as the 'eyes', 'hands' and 'ears' of the scientists, connecting our macroscopic world to the exciting atomic world. In this article, various STM atom manipulation schemes and their example applications are described. The future directions of atomic level assembly on surfaces using scanning probe tips are also discussed.

Are current-induced forces conservative?

The expression for the force on an ion in the presence of current can be derived from first principles without any assumption about its conservative character. However, energy functionals have been constructed that indicate that this force can be written as the derivative of a potential. On the other hand, there exist specific arguments that strongly suggest the contrary. We propose physical mechanisms that invalidate such arguments and demonstrate their existence with first-principles calculations. While our results do not constitute a formal resolution to the fundamental question of whether current-induced forces are conservative, they represent a substantial step forward in this direction.

Current-induced forces in molecular wires

We report first-principles calculations of current-induced forces in molecular wires for which experiments are available. We investigate, as an example, the effect of current-induced forces on a benzene molecule connected to two bulk electrodes via sulfur end groups. We find that the molecule twists around an axis perpendicular to its plane and undergoes a "breathing" oscillation at resonant tunneling via antibonding states. However, current-induced forces do not substantially affect the absolute value of the current for biases as high as 5 V, suggesting that molecular wires can operate at very large electric fields without current-induced breakdown.

Current-induced embrittlement of atomic wires

Recent experiments suggest that gold single-atom contacts and atomic chains break at applied voltages of 1 to 2 V. In order to understand why current flow affects these defect-free conductors, we have calculated the current-induced forces on atoms in a Au chain between two Au electrodes. These forces are not by themselves sufficient to rupture the chain. However, the current reduces the work to break the chain, which results in a dramatic increase in the probability of thermally activated spontaneous fracture of the chain. This current-induced embrittlement poses a fundamental limit to the current-carrying capacity of atomic wires.

Temperature effects on the transport properties of molecules

Recent experiments found an unusual temperature-induced large shift in the resonant-tunneling voltage of certain molecules. We report first-principles calculations showing that such behavior can be caused by the excitation of rotational modes of ligands. These modes have classical characteristics, i.e., the maximum excursion is dominant, while at the same time they have a significant effect on the energy levels responsible for resonant tunneling. The proposed mechanism of ligand rotations is unique to molecules and accounts for the fact that the effect is not seen in semiconductor nanostructures.

First-principles calculation of transport properties of a molecular device

We report first-principles calculations of the current-voltage ( I-V) characteristics of a molecular device and compare with experiment. We find that the shape of the I-V curve is largely determined by the electronic structure of the molecule, while the presence of single atoms at the molecule-electrode interface play a key role in determining the absolute value of the current. The results show that such simulations would be useful for the design of future microelectronic devices for which the Boltzmann-equation approach is no longer applicable.