New Insights into Single-Molecule Junctions Using a Robust, Unsupervised Approach to Data Collection and Analysis

Abstract

We have applied a new, robust and unsupervised approach to data collection, sorting and analysis that provides fresh insights into the nature of single-molecule junctions. Automation of tunneling current-distance (I(s)) spectroscopy facilitates the collection of very large data sets (up to 100,000 traces for a single experiment), enabling comprehensive statistical interrogations with respect to underlying tunneling characteristics, noise and junction formation probability (JFP). We frequently observe unusual low-to-high through-molecule conductance features with increasing electrode separation, in addition to numerous other "plateau" shapes, which may be related to changes in interfacial or molecular bridge structure. Furthermore, for the first time we use the JFP to characterize the homogeneity of functionalized surfaces at the nanoscale.

Soft versus hard junction formation for α-terthiophene molecular wires and their charge transfer complexes

“…21,23 More recently (and subsequent to the period during which the data of this paper were collected), a novel experimental approach to fully automated data collection and algorithmbased analysis using the I(s) technique has been developed, and the archetypal 1,8-octanedithiol was chosen as the first exemplar of this new method. 8 A junction conductance of 5 × 10 −5 G 0 was observed, very similar to the "medium" ("B" group) conductance previously determined, but the other conductance values were not distinct in their processed data. This is consistent with the observation of Haiss et al that this conductance group is normally the most prevalent.…”

“…All data shown in the paper were collected using the STM-BJ technique, and no selection was applied to the datasets: we collected thousands of currentdistance traces as junctions were made and broken, and these were compiled in histograms with no further manipulation. As the hit rate (percentage of traces showing molecular bridging) lies between 10% and 30% for such experiments, 8 histograms were compiled including a large number of spurious traces, and this resulted in broad conductance peaks, spanning more than one order of magnitude. We reasoned that a process of rational data selection, removing all the traces that either bear no sign of junction formation or show evident instrumental noise or anomalous current decay, may reveal a finer histogram peak structure, giving more information about J. Chem.…”

“…), and the likely torsional degrees of freedom of the molecule all combine to make a range of metal/molecule/metal junction conductances inevitable. This, together with the fact that, in the tip withdrawal methods, a proportion of I-s traces is simply exponential decays with no evidence of junction formation or is very noisy, perhaps as a result of tip contamination, 8 means that a statistical analysis of a large number of individual experiments has been an indispensible part of all STMbased studies of metal/single molecule/metal junctions. In the pioneering STM-BJ study (involving 4,4 -bipyridine and alkanedithiols HS(CH 2 ) n SH; n = 6, 8, 10), the approach taken was to produce one-dimensional histograms by adding individual I-s traces and plotting current against frequency of observation of such a current.…”

Fast Data Sorting with Modified Principal Component Analysis to Distinguish Unique Single Molecular Break Junction Trajectories

“…Preanalysis methods are often employed to filter the data, with varying degrees of objectivity. Recently, new methods [1][2][3] have analyzed large SMBJ data sets without making a priori assumptions about molecular plateau shape, and without resorting to hand sorting and subjective filtering choices.…”

High-resolution electrochemical STM of redox metalloproteins

“…These experiments are not simple; it is necessary to analyse many I(z) curves and perform careful statistical analysis on them. Early experiments tended to select curves 'by eye', but more robust data selection techniques [15] are undoubtedly the way forward. There are also several variations of these STM type measurements in which the pulling rate between tip and sample is varied or the bias is changed during the I(z) trace [16].…”