Distance Dependence of Electron Tunneling through Self-Assembled Monolayers Measured by Conducting Probe Atomic Force Microscopy:  Unsaturated versus Saturated Molecular Junctions

Wold, David J.; Haag, Rainer; Rampi, Maria Anita; Frisbie, C. Daniel

doi:10.1021/jp013476t

Cited by 464 publications

(616 citation statements)

References 42 publications

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“…The inset shows that the plot of junction conductance versus molecule length fits an exponential form with a decay constant of 1.7 ± 0.1 per phenyl ring. This behaviour is direct evidence for nonresonant tunnelling transport through the amine-terminated molecules [15][16][17][18][19] , with the measured decay constant being close to the estimate of 1.5 per phenyl ring that we obtain [20][21][22] for these molecules (see Supplementary Information). In Table 1, The data in Figures 1-3 also reveal another interesting trend: as the molecule is granted additional rotational degrees of freedom, the conductance peak broadens.…”

supporting

confidence: 83%

Dependence of single-molecule junction conductance on molecular conformation

Venkataraman

Klare

Nuckolls

et al. 2006

Nature

1,298

1,444

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However, the conductance variation from junction to junction has made it difficult to verify even the simplest predictions about how molecules should behave in unimolecular devices. Here, using amine link groups 13 to form single molecule junctions, we show a clear correlation between molecule conformation and junction conductance in a series of seven biphenyl molecules with different ring substitutions that alter the twist angle of the molecules. We find that the conductance for the series decreases with increasing twist angle, consistent with a cosine squared relation predicted theoretically for transport through π-conjugated systems 14 .We recently demonstrated that metal-molecule-metal junctions, formed by breaking Au point contacts in a solution of molecules, exhibit more reliable and reproducible conductance values when amine groups rather than thiols or isonitriles are used to attach the molecules to the junction contacts 13 . Because of this reduced variability, we can 2 determine statistically meaningful average conductance values for specific singlemolecule junctions; this capability in turn allows us to study the impact of molecular properties on junction conductance.We present our experimental results as conductance histograms, where peaks indicate the most prevalent molecular junction conductances while the width of the conductance distributions reflects the microscopic variations from junction to junction.(For details on experimental and data analysis procedures, see SupplementaryInformation.) Figure 1A shows the histograms for 1,4-diaminobenzene (1) and 2,7-diaminofluorene (2), each constructed from over 10000 conductance traces without resorting to any data selection or processing. Many of our conductance traces reveal stepwise changes in conductance not only at conductance values that are multiples of the fundamental quantum of conductance G 0 (2e 2 /h), but also below G 0 (see Fig. 1C and Supplementary Information Figure S1). These steps are due to conduction through a single molecule bridging the gap between the two Au point-contacts. As seen in the histograms of 1 and 2 (Fig. 1A), for each junction type the additional step occurs within a narrow distribution of conductance values. The most prevalent value is determined by a fit to the peak below G 0 in the conductance histograms using a Lorentzian line shape, which we find to fit our peaks more accurately than a Gaussian line shape (see inset of Figure 1A and also Supplementary Information).When the experiment is repeated using a solution containing an equimolar mixture of 1 and 2 (as indicated in Figure 1B), the resulting histogram (red curve in Figure 1A) shows two distinct peaks below G 0 at nearly the same conductance values as those of the peaks seen in the histograms for the individual molecules (green and yellow curves).Individual traces using the solution mixture show conductance plateaus corresponding to either molecule 1 or 2, and sometimes a plateau corresponding to 1 followed by a plateau 3 corresponding to 2 (see Figure 1C). But ...

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supporting

confidence: 83%

Dependence of single-molecule junction conductance on molecular conformation

Venkataraman

Klare

Nuckolls

et al. 2006

Nature

1,298

1,444

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“…9 The agreement between theory and experiment for the exponential decay in the case studied here is similar to that found in other cases. For instance, experimental 43 values of β for another conjugated molecule, oligophenylene, are found to be 0.35-0.5Å −1 . Also, for alkane and peptide chains, β is in the range 0.7-0.9Å −1 experimentally 26,45 and theoretically.…”

Section: Resultsmentioning

confidence: 99%

Electron transport through single conjugated organic molecules: Basis set effects in ab initio calculations

Baranger

Yang

2007

The Journal of Chemical Physics

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We investigate electron transport through single conjugated molecules -including benzenedithiol, oligophenylene-ethynylenes of different lengths, and a ferrocene-containing molecule sandwiched between two gold electrodes with different contact structures -by using a single-particle Green function method combined with density functional theory calculation. We focus on the effect of the basis set in the ab initio calculation. It is shown that the position of the Fermi energy in the transport gap is sensitive to the molecule-lead charge transfer which is affected by the size of basis set. This can dramatically change, by orders of magnitude, the conductance for long molecules, though the effect is only minor for short ones. A resonance around the Fermi energy tends to pin the position of the Fermi energy and suppress this effect. The result is discussed in comparison with experimental data.

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“…This latter result is especially noteworthy, because the tunneling length for 2 is ∼0.2 nm larger than that for 1 (because of the presence of two extra C-C bonds), and one would expect the opposite trend. 9 The observed I-V characteristics were linear within the bias range of (0.15 V. Figure 4 shows the averaged single-molecule I-V characteristics in the ohmic range of applied tip biases for 1 (denoted by "×") and 2 (denoted by "b") inserted into a C 6 SAM in bicyclohexyl solvent under a fixed loading force of 5 nN. Each data symbol shown in Figure 4 represents the mean value of current for a series of repeated measurements on the different gold nanoparticles under a particular tip bias (error bars represent the standard deviation of the mean).…”

Section: Resultsmentioning

confidence: 99%

Conjugated Thiol Linker for Enhanced Electrical Conduction of Gold−Molecule Contacts

et al. 2005

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Single-molecule electrical conduction studies are used to evaluate how the molecular linking unit influences the tunneling efficiency in metal-molecule-metal (m-M-m) junctions. This work uses conducting-probe atomic force microscopy (CP-AFM) to compare the molecular conduction of two π-bonded molecules: one with a single thiol linker, and another with a conjugated double thiol linker at both ends of the molecules. The results demonstrate that the molecule with conjugated double thiol linkers displays higher conduction in gold-molecule-gold junctions than nonconjugated single thiol-gold contacts. IntroductionIn recent years, improving the electrical conduction of molecular wires by using conjugated molecules has been a major focus in molecular electronics. However, the conduction of the molecule can become very large, so the metal-moleculemetal (m-M-m) contacts can dominate the response. Recent experimental 1-9 and theoretical [10][11][12][13][14][15] work has demonstrated the importance of metal-molecule contacts to the efficiency of charge transfer through m-M-m junctions. Particularly, for alkanethiol-based junctions, metal-molecule contact resistance decreased up to 2 orders of magnitude when chemically bonded contacts were used instead of nonbonded (mechanical) contacts. 1 Unlike electrical conduction studies on saturated molecules, fewer experimental studies address how the contacts effect conduction through conjugated molecular systems. This work demonstrates that the efficiency of charge transport through π-bonded molecules forming m-M-m junctions with thiol linking units between the metals and the molecule can be improved by using conjugated double thiol linkers at both ends of the molecule. This knowledge should be useful for the design and fabrication of more-efficient electrical and optical devices with potential use in molecular electronics.This work compares electrical conduction measurements for gold-molecule-gold tunnel junctions containing two different types of gold-molecule contacts: single gold-sulfur-Arsulfur-gold and double gold-S 2 C-Ar-CS 2 -gold contacts at both ends of the molecule, where Ar is a biphenyl moiety. The gold-S 2 C-Ar-CS 2 -gold contacts are postulated to provide two points of contact with each gold electrode and better conjugation to the molecule's π-system than the single goldsulfur-Ar-sulfur-gold contact. The junctions are formed using

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Distance Dependence of Electron Tunneling through Self-Assembled Monolayers Measured by Conducting Probe Atomic Force Microscopy: Unsaturated versus Saturated Molecular Junctions

Cited by 464 publications

References 42 publications

Dependence of single-molecule junction conductance on molecular conformation

Dependence of single-molecule junction conductance on molecular conformation

Electron transport through single conjugated organic molecules: Basis set effects in ab initio calculations

Conjugated Thiol Linker for Enhanced Electrical Conduction of Gold−Molecule Contacts

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