Influence of gases on the formation of atomic gold wires. W.H.A. Thijssen, A.M.C. Valkering and J.M. van Ruitenbeek Kamerlingh Onnes Laboratorium, Leiden University, PO Box 9504, 2300 RA Leiden, The Netherlands.
W.H.A. Thijssen, A.M.C. Valkering and J.M. van Ruitenbeek
Kamerlingh Onnes Laboratorium, Leiden University, PO Box 9504, 2300 RA Leiden, The Netherlands
By using a Mechanically Controlled Break Junction (MCBJ) at 4.2 K it is possible to form atomic gold wires of several atoms in length. Here we have investigated the influence of molecular oxygen and hydrogen on the formation and properties of the gold wires. In both cases the dominant conductance peak at one conductance quantum 2e2/h remains, indicating a single open channel. When a small amount of oxygen or hydrogen is admitted, chains are still being formed, but with changing interatomic distance. Phonon spectroscopy sheds further light on the complex structures that can arise.
We create a free standing atomic gold by using the MCBJ technique , as it is depicted in the figure below. In this way we can stretch the contact until it exists of only one gold atom. By stretching further a chain of atoms can be formed .
The conductance trace tells that you have a one atom contact….and quite often a chain
The MCBJ uses a bendable substrate and a piezo to break a notched gold wire in a controlled way
It has been theoretically suggested that experimentally observed large interatomic Au-Au distances , of the order of 3.5 Å are due to the incorporation of impurities in the chain. Two main candidates that have been theoretically studied are oxygen  and hydrogen . In this research we have looked at the possibility of incorporation at low temperatures. In the figure to the right are shown conductance histograms of pure gold, with oxygen and with hydrogen. It is clearly seen that the main peak at 1 G0 stays present.
Conductance histograms taken at 100 mV bias for Au (black); Au + O2 (red) and Au + H2 (blue)
The figure to the left shows length histograms of pure gold (green), gold with ogygen (blue curve) and gold with hydrogen (red curve). Each lengthhistogram consists of around 2000 traces. The differences are quite obvious. In the case of oxygen admission the peaks in the lengthhistograms have changed indicating a different interatomic distance, which would be the case for oxygen incorporation, in order to form a gold-oxygen chain. For hydrogen it is seen that additional peaks have emerged, suggesting hydrogen incorporation. It has to be said that these lengthhistogram are not
standardly observed and deviations are common.
dI/dV spectroscopy is an important tool for determining the structural properties of atomic contacts. Incoming electrons can scatter inelastically with e.g. phonons and then a small decrease in conductance (typically 1 %) is observed.Phonon spectroscopy has successfully been used to verify the presence of molecular hydrogen in platinum contacts . The figure to the right shows two typical dI/dV spectra for gold with H2. The red curve is taken at the position of the second peak in the Au-H2 lengthhistogram and shows possible phonon modes at 40 meV together with a dip at zero bias, while the blue curve taken at the first peak shows are more familiar 15 meV “pure” gold mode.
Shown below are a few regularly appearing spectra for the Au-O2 case. The curves show possible oxygen modes at energies between 50 and 80 meV together with the familiar 15 meV gold mode. Theoretical calculations should give more clarity in this case. The “dip” in dI/dV, seen in the blue curve could be related to the magnetic structure that has been predicted for gold-oxygen chain in the so called “zig-zag” configuration . Dips appear normally at chains with a slightly lower conductance.
The conclusions of this research are not very clear yet. Length histograms give indeed a hint to the possibility of incorporation of atomic oxygen and hydrogen in a gold chain. The dI/dV spectra however are far from clear and give different features that have to be studied further both experimentally and theoretically.
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