Understanding the structure of the first atomic contact in gold
© Sabater et al.; licensee Springer. 2013
Received: 17 November 2012
Accepted: 15 April 2013
Published: 29 May 2013
We have studied experimentally jump-to-contact (JC) and jump-out-of-contact (JOC) phenomena in gold electrodes. JC can be observed at first contact when two metals approach each other, while JOC occurs in the last contact before breaking. When the indentation depth between the electrodes is limited to a certain value of conductance, a highly reproducible behaviour in the evolution of the conductance can be obtained for hundreds of cycles of formation and rupture. Molecular dynamics simulations of this process show how the two metallic electrodes are shaped into tips of a well-defined crystallographic structure formed through a mechanical annealing mechanism. We report a detailed analysis of the atomic configurations obtained before contact and rupture of these stable structures and obtained their conductance using first-principles quantum transport calculations. These results help us understand the values of conductance obtained experimentally in the JC and JOC phenomena and improve our understanding of atomic-sized contacts and the evolution of their structural characteristics.
KeywordsElectronic transport Atomic size contacts Mechanical annealing Jump-to-contact phenomena Jump-out-of-contact phenomena Molecular dynamics simulations Ab initio DFT
Metallic atomic-sized contacts can be created by scanning tunneling microscopy (STM) [1, 2] or by mechanically controlled break junctions [1, 3]. In such nanocontacts, the electrical conductance is closely related to their minimum cross section. Therefore, by recording the conductance while the electrodes are displaced with respect to each other (traces of conductance), one can infer the atomic structure of these contacts. However, to understand the structures formed at the contact, it is necessary to make use of theoretical models. Landman et al.  pioneered the use of molecular dynamics (MD) simulations to follow the variation of the minimum cross section during the process of stretching a nanocontact. Later, Untiedt et al. , by experimentally studying the jump-to-contact (JC) phenomena in gold and combining MD and electronic transport calculations, were able to identify the formation of three basic structures before contact between the two electrodes, although a limited analysis on the conductance values was presented there.
Trouwborst et al.  have also studied the phenomena of JC and JOC using indentation loops where the maximum conductance was limited to 1G0, where (quantum of conductance). These experiments showed that the elasticity of the two electrodes is one of the relevant parameters to explain these phenomena. Despite these, presently, there is not a unique picture that correlates the experiments with the MD and transport calculations regarding the different atomic structures that can be found at the contact.
On the other hand, experiments, together with molecular dynamics and electronic transport calculations based on density functional theory, show how very stable structures can be obtained by repeated indentation. This has been described as a mechanical annealing phenomenon . Limiting the maximum conductance value (5G0 for gold) in the process of formation and rupture of a nanocontact leads to reproducible and atomically sharp pyramidal electrodes. This technique has recently been used by other authors  to prepare tips in situ for low-temperature STM.
In this paper, we show experimental results of the JC and JOC phenomena for gold that are analyzed simultaneously. We study the most probable configurations before the formation and breaking of nanocontacts with pyramidal form obtained from MD simulations emulating the process of mechanical annealing. As found earlier , the contacts can be classified into monomer, dimer and double contact. In order to correlate with the experimentally obtained conductance values, we calculated the conductance of these structures using first-principles quantum transport models.
To emulate the movement of the STM and simulate the tip and surface that are annealed mechanically, we used MD simulations with embedded atom potentials. Density function theory (DFT)-based calculations are performed to obtain the electronic transport in the simulated structures . For the MD simulations, we have selected an embedded atom potential  because elasticity of the electrodes seems to be one of the key parameters in the processes to be studied , and these empirical potentials are fitted to reproduce the experimental elastic properties of bulk materials.
Furthermore, the computational cost with this simulation method is low, which makes it an appealing tool since we need to simulate tens of these cycles of breaking and formation of the nanocontact.
Results and discussion
Experimental values of conductance that appear more frequently in the case JC and JOC
Pairs of values obtained in the density plots in Figure1
(G a ,G b )G0
(G a ,G b )G0
(G a ,G b )G0
As mentioned, we make use of molecular dynamics simulations and DFT calculations of conductance to understand these experimental measurements and observations. Figure 2 shows the two structures studied using MD, as described earlier. In Figure 3, we show some snapshots of the configurations found just after the contact between the two tips and just before breaking a nanocontact. Three basic atomic structures are found: a monomer (Figure 3A), a dimer (Figure 3B) and a double contact (D.C.) (Figure 3C,D,E). For the case of a double contact, we have identified different geometries, three of which are shown in this figure. We introduce, for the first time, the concept of a double dimeric (Figure 3C,D) and monomeric (Figure 3E) contact. We define a double dimeric contact as the one where the contact is between two atoms facing two other atoms, while we define a double monomeric contact as a contact where two atoms are contacting each other. Another interesting point is that for the double dimeric contact, we have identified two possible structures: one where two atoms are perpendicular to the other two (Figure 3C), which we call transversal configuration (D.C. Dimeric T), and one where two atoms are parallel to the other two (Figure 3D), which we call parallel configuration (D.C. Dimeric P).
MD results of first or last contact (JC/JOC) type in structures A and B annealed mechanically
Percentage of cases of type monomer, dimer and D.C.
JC structure A 15 inden
JOC structure A 15 inden
JC structure B 15 inden
JOC structure B 15 inden
JC structure B 25 inden
JOC structure B 25 inden
Electronic conductance calculated by DFT on typical contacts obtained from MD structures
Structure and value of conductanceG0
0.92 ± 0.07
0.97 ± 0.15
1.73 ± 0.02
Experiments of JC and JOC show that certain structures are more likely to occur than others. This depends on the metal and on the process of breaking/formation and the type of structure at the electrodes. Simulations and calculations (MD and DFT) of these experiments show that three basic atomic structures are formed at the contact: monomers, dimers and double contacts. We have identified within the double contact structure several different atomic arrangements that we named double dimeric contact (parallel and perpendicular), and double monomeric contact. According to DFT electronic transport calculations, double contacts have an average value of conductance of 1.73G0, which correlates very well with one of the peaks observed experimentally both for JC and for JOC. This configuration is also obtained in JC and JOC from the MD simulations and, for some very stable tips, is the dominant configuration. Monomers and dimers, however, are difficult to distinguish from the simulations since their average conductance values are very similar (0.97G0 and 0.92G0, respectively). In the case of JOC, these two peaks cannot be resolved. Interestingly, the conductance values are somehow lower than in the case of JC, which could indicate the most likely formation of stretched contacts.
This work was supported by the Spanish government through grants FIS2010-21883, CONSOLIDER CSD2007-0010, Generalitat Valenciana through PROMETEO/2012/011, ACOMP/2012/127 and Feder funds from E.U.
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