Electrical conductance and structure of copper atomic junctions in the presence of water molecules

Do quantum interference effects manifest in acyclic aliphatic molecules with anchoring groups?

The ability to control single molecule electronic conductance is imperative for achieving functional molecular electronics applications such as insulation, switching, and energy conversion. Quantum interference (QI) effects are generally used to control electronic transmission through single molecular junctions by tuning the molecular structure or the position of the anchoring group(s) in the molecule. While previous studies focussed on the QI between σ and/or π channels of the molecular backbone, here, we show that single molecule electronic devices can be designed based on QI effects originating from the interactions of anchoring groups. Furthermore, while previous studies have concentrated on the QI mostly in conjugated/cyclic systems, our study showcases that QI effects can be harnessed even in the simplest acyclic aliphatic systems-alkanedithiols, alkanediamines, and alkanediselenols. We identify band gap state resonances in the transmission spectrum of these molecules whose positions and intensities depend on the chain length, and anchoring group sensitive QI between the nearly degenerate molecular orbitals localized on the anchoring groups. We predict that these QI features can be harnessed through an external mechanical stimulus to tune the charge transport properties of single molecules in the break-junction experiments.

Systematic study of low energy geometries of copper nano-junctions exposed to water and to species that can result from dissociation of water

A detailed computational analysis has been performed, considering copper atomic contacts that are exposed directly to water molecules, hydroxyl groups, and monatomic as well as molecular hydrogen and oxygen species. The optimized physical bonding structure, electrical conductance and inelastic tunneling spectra (IETS) have been determined theoretically for moderately large structures by performing appropriateab initioand semi-empirical calculations. By considering the aforementioned properties, it has been possible to determine that some of the molecular bridging structures may be regarded as being highly-probable outcomes, resulting from the exposure of copper electrodes to the atomic/molecular contaminants. We specifically identify the conductance properties of a variety of configurations including examples with very high and very low conductance values. This is done in order to identify junction geometries that may be realized experimentally and their conductance and IETS signatures. By reporting geometries with very high and very low conductance values here, we intend to provide a wider perspective view than previous studies of copper-molecular junctions that have focused on high conductance structures. In addition, we explore the properties of metal junctions with multiple molecules, a class of systems for which little theoretical work has been available in the molecular electronics literature. We find that water molecules surrounding the junction can influence the bonding geometry of the molecules within the junction and consequently can affect strongly the calculated conductances of such junctions.

Copper atomic contacts exposed to water molecules

Monatomic and molecular hydrogen and also oxygen, as well as water molecules and OH that are exposed to atomic copper in intimate contact, have been studied theoretically using computational methods. The authors optimized moderately large structures of Cu/H/Cu, Cu/HCuH/Cu, Cu/H₂/Cu, Cu/H₂O/Cu, Cu/OH/Cu, Cu/O/Cu, and Cu/O₂/Cu and calculated appropriate values for conductance and inelastic tunneling spectroscopy (IETS) properties of the contact junctions, elucidating them as being a possible outcome resulting from the exposure of copper electrodes to the atomic/molecular contaminant species. Here we also demonstrate the IETS properties, by means of ab initio calculations, which can determine the form of the junction geometries. Furthermore, we identify the bonding geometries at the interfaces of the copper electrodes that directly give rise to the specific IETS signatures that have been observed in recent experiments. Based on low-bias conductance and IETS calculations, for the specific case of water exposure of copper electrodes, it was concluded that a single hydrogen or a single oxygen atom bridging the copper electrodes is not responsible for the high conductance peak measurements. Regarding Model 4, where an individual water molecule is considered to be the bridging constituent, our computational results suggest that it has a relatively low probability of being an appropriate candidate. Based upon current computational results, the two hydrogens in Model 3 appear to be in molecular form, although they still form a bond with the adjacent copper atoms. Comparing computational with experimental results indicates that Model 3 is in acceptable agreement with available data.

Detecting Electron Transport of Amino Acids by Using Conductance Measurement

The single molecular conductance of amino acids was measured by a scanning tunneling microscope (STM) break junction. Conductance measurement of alanine gives out two conductance values at 10^−1.85 G₀ (1095 nS) and 10^−3.7 G₀ (15.5 nS), while similar conductance values are also observed for aspartic acid and glutamic acid, which have one more carboxylic acid group compared with alanine. This may show that the backbone of NH₂–C–COOH is the primary means of electron transport in the molecular junction of aspartic acid and glutamic acid. However, NH₂–C–COOH is not the primary means of electron transport in the methionine junction, which may be caused by the strong interaction of the Au–SMe (methyl sulfide) bond for the methionine junction. The current work reveals the important role of the anchoring group in the electron transport in different amino acids junctions.

Atomic structure of water/Au, Ag, Cu and Pt atomic junctions

Much progress has been made in understanding the transport properties of atomic-scale conductors. We prepared atomic-scale metal contacts of Cu, Ag, Au and Pt using a mechanically controllable break junction method at 10 K in a cryogenic vacuum. Water molecules were exposed to the metal atomic contacts and the effect of molecular adsorption was investigated by electronic conductance measurements. Statistical analysis of the electronic conductance showed that the water molecule(s) interacted with the surface of the inert Au contact and the reactive Cu ant Pt contacts, where molecular adsorption decreased the electronic conductance. A clear conductance signature of water adsorption was not apparent at the Ag contact. Detailed analysis of the conductance behaviour during a contact-stretching process indicated that metal atomic wires were formed for the Au and Pt contacts. The formation of an Au atomic wire consisting of low coordination number atoms leads to increased reactivity of the inert Au surface towards the adsorption of water.