N. D. Lang

Measurement of the conductance of a hydrogen molecule

Recent years have shown steady progress towards molecular electronics, in which molecules form basic components such as switches, diodes and electronic mixers. Often, a scanning tunnelling microscope is used to address an individual molecule, although this arrangement does not provide long-term stability. Therefore, metal–molecule–metal links using break-junction devices have also been explored; however, it is difficult to establish unambiguously that a single molecule forms the contact. Here we show that a single hydrogen molecule can form a stable bridge between platinum electrodes. In contrast to results for organic molecules, the bridge has a nearly perfect conductance of one quantum unit, carried by a single channel. The hydrogen bridge represents a simple test system in which to understand fundamental transport properties of single-molecule devices.

The Density-Functional Formalism and the Electronic Structure of Metal Surfaces

Publisher Summary A prerequisite to analysis of the electronic structure of metal surfaces is a procedure for studying large, strongly inhomogeneous systems of electrons. Two such procedures that have been widely used in the past are the Thomas–Fermi method and the Hartree method. Not long ago, a general theory of inhomogeneous electron gases in their ground state, which we shall refer to as the density-functional formalism, was introduced by Hohenberg, Kohn, and Sham. The central quantity in this theory is the electron density, whose basic role is established by the theorem that the properties of the system, in particular the groundstate energy, are functionals only of this density. With the density as the varied function, a variational principle is established for the energy. The associated Euler equation is formulated in two ways—both in principle exact—that are particularly convenient for the study of strongly inhomogeneous systems. One formulation is similar to the Thomas–Fermi method, the other to the Hartree method; in their various approximate versions, they represent systematic ways of extending the classic methods which they resemble. This chapter outlines the density-functional formalisms and discusses its application to two of the most basic static properties of a surface: the work function and the surface energy.

The benzene molecule as a molecular resonant-tunneling transistor

Experiments and theory have so far demonstrated that single molecules can form the core of a two-terminal device. Here we report first-principles calculations of transport through a benzene-1, 4-dithiolate molecule with a third capacitive terminal (gate). We find that the resistance of the molecule rises from its zero-gate-bias value to a value roughly equal to the quantum of resistance (12.9 kΩ) when resonant tunneling through the π* antibonding orbitals occurs.

Mechanisms of atomic ion emission during sputtering

Abstract Several major experimental and theoretical findings made in the last few years on the mechanisms of secondary ion emission are summarized. There is a strong indication that the phenomena can be divided into two categories: Ion emission from the surfaces of metals and semiconductors tends to have a strong correlation with the work function and can be described quite well with an electron tunneling model. Ion emission from systems which show large chemical enhancement has only a weak correlation with global surface properties like the work function or the bandgap. Instead it seems to be related more to local chemical bonds and coordination numbers. A localized bond breaking picture may be more appropriate in these circumstances.

Electronic transport in single molecules

Abstract We present a review of recent results on the non-linear transport properties of single molecules using density-functional theory. In particular, we investigate the role of contact chemistry and geometry, current-induced forces, and polarization effects induced by a gate field on the current–voltage characteristics of molecules for which experiments are available. The results show that single molecules, if appropriately tailored, have physical properties that could be useful in electronic applications.

Ion beam alignment for liquid crystal display fabrication

The ability to align liquid crystals to a substrate is a critical step in the liquid crystal display (LCD) manufacturing process with the industry standard technique employing a mechanical rubbing technique to accomplish this function. However, mechanical rubbing can result in debris generation contaminating not only the substrate being processed but also the clean room housing the equipment. As such, post-cleaning of the display panels is required to remove the debris from the surface in addition to the physical isolation of the mechanical rubbing equipment within the clean room environment introducing considerable time and expense. In addition, uneven wear of the mechanical roller during the rubbing process may result in localized defects that will not be observed until final inspection of a completely assembled display. We have developed and introduced into LCD manufacturing a non-contact alignment technique utilizing both diamond-like carbon (DLC) and a low energy ion beam (IB). The replacement of the polyimide alignment layer with DLC results in a completely dry processing technique for both the thin film deposition and alignment steps.

Molecular electronics by the numbers

Abstract The paper gives an overview of recent work by the authors on first-principles, parameter-free calculations of electronic transport in molecules in the context of experimental measurements of current–voltage ( I – V ) characteristics of several molecules by Reed et al. The results show that the shape of I – V characteristics is determined by the electronic structure of the molecule in the presence of the external voltage whereas the absolute magnitude of the current is determined by the chemistry of individual atoms at the contacts. A three-terminal device has been modeled, showing gain. Finally, recent data that show large negative differential resistance and a peak that shifts substantially as a function of temperature have been accounted for.

Effects of geometry and doping on the operation of molecular transistors

We report first-principles calculations of current versus gate voltage characteristics of a molecular transistor with a phenyldithiolate molecule as active element. We show that (i) when the molecule is placed in proximity to the gate electrode, current modulation and resonant tunneling can occur at very small gate voltages. This is due to the first-order perturbation of the electronic states induced by the electrostatic potential of the gate in the molecular region. Such perturbation is present even if the molecule does not have an intrinsic dipole moment. (ii) The molecular transistor can be converted from n-type to p-type by the simple co-adsorption of a single oxygen atom placed near the molecule. While the latter finding suggests that the character of molecular transistors can be easily changed by doping the electrode surfaces, it also puts severe constraints on the experimental control of such structures for molecular electronics applications.

First-principles simulations of molecular electronics.

Abstract: This paper gives an overview of recent work by the authors in first‐principles, parameter‐free calculations of electronic transport in molecules in the context of experimental measurements of current‐voltage (I‐V) characteristics of several molecules by Reed et al. The results show that the shape of I‐V characteristics is determined by the electronic structure of the molecule in the presence of the external voltage, whereas the absolute magnitude of the current is determined by the chemistry of individual atoms at the contacts. A three‐terminal device has been simulated, showing gain. Finally, recent data that show large negative differential resistance and a peak that shifts substantially as a function of temperature have been accounted for in terms of rotations of ligands attached to the main molecule, a phenomenon that is not present in semiconductor nanostructures.

Gating of a three-leg molecule.

We study the use of a simple three-leg molecule, triphenylene, as a transistor. This configuration allows increased voltage gain to be achieved. We analyze control of the transport between electrodes attached to two of the legs by a gate closely coupled electrostatically to the third leg, using self-consistent density functional calculations. In spite of the close coupling, the maximum voltage gain was less than unity, and this was attributed to efficient screening of the internal potential due to polarization of the molecular states. The transistor current vs. voltage characteristics were able to be reproduced using a simple electrostatic model.