Three Contact Modes of Amino Terminal Groups on Gold in Single Molecular Junction of Au/1,4-Diaminobutane/Au

Abstract

The effect of the binding modes of two amino terminal groups present on two gold protrusions on currents passing through a single molecular junction of Au/1,4-diaminobutane/Au was studied via the repeated formation of a break junction using a scanning tunneling microscope (STM) under nitrogen at room temperature. In addition to the two reported (high and low) values of the single molecular conductance of diaminobutane, another lower conductance was found through a careful analysis of histograms in a low-current range (0–0.3 nA). The techniques used here to improve the determination of the values of single molecular conductance were the selection of valid i–s curves with current steps and the use of the robust statistical method to correct for the contribution of the background tunneling currents to the current histograms. These three single molecular conductance values are attributed to three different binding modes of the two terminal amino groups giving the three different contact resistances at room temperature. That is, each amino terminal group is likely to be bound to one or the other gold protrusion through a more or a less conductive contact mode. Thus, a single molecular junction between two gold electrodes has three different contact modes, i.e., i) more–more, ii) more–less (or less–more), and iii) less–less conductive contact modes at the two ends.

The Single-Molecule Conductance and Electrochemical Electron-Transfer Rate Are Related by a Power Law

“…The conductance distribution for the 7 bp TA displays two characteristic regions (see Figure E): a high probability, narrow region with G values between 3 × 10 –4 G 0 and 2 × 10 –3 G 0 and a broad region with G between 2 × 10 –3 G 0 and 15 × 10 –3 G 0 . A possible explanation for this observation is the presence of molecules with two different conformations and/or contact geometries, for which the higher molecular conductance range corresponds to geometries more favorable for charge transport. ,− Each of the two regions includes several conductance peaks that are integral multiples of a fundamental conductance value. These multiples are assigned to more than one molecule trapped in the junction .…”

Influence of Binding Groups on Molecular Junction Formation

“…We also see that, for a given n , mean G values and stretching distances are lower for diamines than for dithiols. We find several reports in the literature discussing the G -distribution for both alkane-dithiols and diamines. − , In this paper, we focus on the stretching length analysis.…”

“…In the case of simple alkane molecules, the resulting conductance, G , distribution spreads over approximately 1 order of magnitude. The origin of this broad distribution is still under debate, but factors which have been considered are possible differences in binding between the molecule and the metal electrode, conformational changes of the alkane, or a large dispersion on the total number of attached molecules. − The chemical group used as anchor for the self-assembly can be crucial in obtaining reproducible molecular junctions. It has been shown, for example, that amines produce narrower G distributions than thiols. , …”

Impact of Junction Formation Method and Surface Roughness on Single Molecule Conductance

“…More recently, other groups have also confirmed the existence of several groups of conductance values using similar methods. − …”

“…More recently, other groups have also confirmed the existence of several groups of conductance values using similar methods. [16][17][18][19] Since both the BJ and I(s) methods have now been adapted in more or less modified forms by a number of groups, 16,[18][19][20][21][22][23][24][25][26][27] the precise origin of the different conduction values remains a subject of interest, as does the finding that different measurement conditions or techniques can favor different conductance values. This is clearly an important issue to address since these differences give rise to apparent inconsistencies in the literature.…”