Ordering of liquid water at metal surfaces in tunnel junction devices

Abstract

A tunnel junction device was made by immersing mercury electrodes in an aqueous nitrate solution. The junction conductance was measured at zero bias as the two mercury surfaces were brought together in the solution. Changes in separation between the mercury surfaces were calculated from changes in the junction conductance using a simple model of elastic electron tunneling, due to Simmons. An absolute distance scale was established using the estimated hard-sphere diameter of water as an internal standard. Discrete changes in junction conductance were observed when the metal surfaces were separated by less than about 1 nm. We interpret this behavior to be due to the presence of quasi-equilibrium junction geometries which are themselves due to time-averaged structuring of liquid water near the metal surfaces. The longitudinal structuring in the water was found to decay normal to the metal surface with a characteristic length on the order of the molecular diameter. The time-averaged structures of the liquid water domains appear to be similar to the structure of hexagonal ice Ih and do not resemble hard-sphere packing. At zero bias, there appears to be no strong preference for one type of ordered water structure over another, suggesting that hydrogen bonding is the dominant factor determining structure in the liquid water near the metal surface and not metal-water bonding in this case. Our experimental data are in substantial agreement with recent molecular dynamics and Monte Carlo simulations and with analytic theory. There are significant differences between our results for ordering of liquid water at metal surfaces and the results reported previously for local ordering of liquid water in the mica/water/mica surface force-balance apparatus.

Structure of hydrated oligonucleotides studied by in situ scanning tunneling microscopy.

“…Current-vs.-distance measurements imply barriers between 1.7 and 4 eV (depending upon the adsorbed species). For example, on clean gold, we obtain 2.1 ± 0.2 eV, in reasonable agreement with estimates of the tunnel barrier in a metal-liquid-metal junction (31,32). Furthermore, measurements of the contrast as a function of the set-point tunnel current are consistent with a Tersoff-Hamann model of STM contrast using these measured values of barrier height (30,34).…”

Molecular Dynamic Simulations in Interfacial Electrochemistry

“…However, the density profile of water near liquid mercury 85 is significantly less pronounced than that of water near the Pt surface or the solid mercury surface, 51 reflecting the fluidity of the metal, which smears out the profile. The oscillatory density profile of the mercury atoms is consistent with many theoretical 9,46,48,50,86,87 and experimental 81,88 studies of liquid metals and their surfaces.…”

Anomalous electrical conductivity of aqueous solutions in submicron cracks and gaps

“…As was noted above, the ionic electrical conductivity of solutions cannot be measured under our experimental conditions (direct current at a low, 0.1-1.0 mV, voltage drop). For nonionic conductivity, we note that, as the distance between microcontacts (tip of the scanning tunneling microscope [14], crossed cylindrical [15] or spherical [16] atomically smooth surfaces) in electrolytes was changed, only the electron tunnel currents at distances of up to 1 nm were observed. This distance is three orders of magnitude smaller than the distances dealt with in our work; that is, electron tunneling cannot account for the phenomenon observed in this work.…”

“…This distance is three orders of magnitude smaller than the distances dealt with in our work; that is, electron tunneling cannot account for the phenomenon observed in this work. However, we should stress that the contacts studied in works [14][15][16] are essentially point contacts and do not adequately represent a three-dimensional electrolyte layer in actual super narrow gaps and cracks.…”

“…Therefore, during the alignment of electrodes under heightened compression, these spots are elastically deformed. With a gradual increase in DL and the loss of mechanical contact, each spot transforms into a point contact, for which the role of conductivity of an electrolyte layer with a thickness more than 1 nm is insubstantial [14][15][16]. By contrast, the density of contact spots on ground surfaces is much higher, while the area of each spot is smaller.…”