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Conductance of a Molecular Junction

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Science  10 Oct 1997:
Vol. 278, Issue 5336, pp. 252-254
DOI: 10.1126/science.278.5336.252

Abstract

Molecules of benzene-1,4-dithiol were self-assembled onto the two facing gold electrodes of a mechanically controllable break junction to form a statically stable gold-sulfur-aryl-sulfur-gold system, allowing for direct observation of charge transport through the molecules. Current-voltage measurements at room temperature demonstrated a highly reproducible apparent gap at about 0.7 volt, and the conductance-voltage curve showed two steps in both bias directions. This study provides a quantative measure of the conductance of a junction containing a single molecule, which is a fundamental step in the emerging area of molecular-scale electronics.

The measurement of charge transport in single organic molecules and the determination of their conductance are long-sought goals. Such measurements are experimentally challenging and intriguing because one can test the validity of transport approximations at the molecular level. A conceptually simple configuration would be to connect a single molecule between metallic contacts. Such a metal-molecule-metal configuration would present the molecular embodiment of a system analogous to a quantum dot (1-9), with the potential barriers of the semiconductor system being replaced by any existing contact barrier of the molecule-metal interface.

Previous measurements on atomic and molecular systems have been made with scanning tunneling microscopes (STMs) (10-12) and can yield conductivity information (13-15). Experiments with an evaporated-metal-top-contact/molecules/metallic-bottom-contact configuration, which has ten of thousands of parallel active molecules, have also been demonstrated (16, 17). One experiment on an organic system (18) reported evidence for Coulomb charging.

We have performed measurements in the configuration of a single molecule between metallic contacts; specifically, on benzene-1,4-dithiolate connected between stable, proximal, metallic gold contacts at room temperature. This approach complements previous approaches by presenting statically stable contacts and concurrently restricts the number of active molecules to as few as one.

Experiments were conducted at room temperature with a mechanically controllable break junction (MCB) (19) (Fig.1). In this approach, a notched metal wire is glued onto a flexible substrate and is fractured by bending of the substrate, after which an adjustable tunneling gap can be established. A large reduction factor between the piezo elongation and the electrode separation ensures an inherently stable contact or tunnel junction. The wire contacts are atomically sharp when broken, which is demonstrated in the conductance quantization as previously reported (20). In the experiments reported here, benzene-1,4-dithiol was adsorbed from a 1 mM solution in tetrahydrofuran (THF) onto the two facing gold electrodes of the break junction, which were broken in solution under an Ar atmosphere (21), resulting in formation of a self-assembled monolayer (SAM) on the gold electrodes that was nearly perpendicular to the surface (21). The THF solvent was allowed to evaporate in the ambient Ar atmosphere before the conductance measurements, and there was no further surface preparation or cleaning. The removal of the THF led to thermal gradients that disturbed the picometer static dimensional stability of the MCB, requiring the tips to be withdrawn and then returned to measure the electrical properties of the molecule or molecules adsorbed on the surfaces (Fig. 2). The configuration shown in Fig. 3 is probable because the displacement of thiols has been shown (10) and the formation of a disulfide bridge would require oxygen (21).

Figure 1

A schematic of the MCB junction with (a) the bending beam, (b) the counter supports, (c) the notched gold wire, (d) the glue contacts, (e) the pizeo element, and (f) the glass tube containing the solution.

Figure 2

Schematic of the measurement process. (A) The gold wire of the break junction before breaking and tip formation. (B) After addition of benzene-1,4-dithiol, SAMs form on the gold wire surfaces. (C) Mechanical breakage of the wire in solution produces two opposing gold contacts that are SAM-covered. (D) After the solvent is evaporated, the gold contacts are slowly moved together until the onset of conductance is achieved. Steps (C) and (D) (without solution) can be repeated numerous times to test for reprodicibility.

Figure 3

A schematic of a benzene-1,4-dithiolate SAM between proximal gold electrodes formed in an MCB. The thiolate is normally H-terminated after deposition; end groups denoted as X can be either H or Au, with the Au potentially arising from a previous contact/retraction event. These molecules remain nearly perpendicular to the Au surface, making other molecular orientations unlikely (21).

As the tips were brought together, current-voltageI(V) and conductance G(V) (= dI/dV) measurements showed characteristic features (Fig. 4A) that proved to be highly reproducible (Fig. 4B). The spacing between the electrodes was ∼8 Å, set by the pizeo voltage as determined by previous calibration of the spacing–to–pizeo voltage conversion factor established by the expotential dependence of the current with distance in the tunneling regime (22). However, calibration shift due to solvent evaporation cannot be eliminated. By comparison, an approximate molecule length of 8.46 Å was calculated with the use of an MM2 force field and by measuring to the center of the two gold radii minus the covalent radii of both gold atoms. An apparent gap of ∼0.7 V was observed in all cases. The first derivative ofI(V) shows two steps in both bias directions with the lower step ∼22.2 megohm (0.045 μS) and the higher step ∼13.3 megohm (0.075 μS), possibly indicative of a Coulomb staircase. It is noted that the high Fermi energy of the gold contacts (∼2 eV) as compared with the low energies of semiconductor quantum dot systems (<100 meV) precludes the observation of negative differential resistance in the present system (23), which is often seen in semiconductor systems (1-7). A control experiment with unevaporated THF solvent alone (that is, without the benzene-1,4-dithiolate) exhibited a resistance of 1 to 2 gigohm (linear up to 10 V) independent of electrode spacing, implying ionic conduction through the solvent. When the solvent was evaporated, regular vacuum tunneling with a much higher resistance was observed (19), with expotential dependence of the current with applied voltage implying the absence of deleterious effects on the MCB due to the solvent.

Figure 4

(A) TypicalI(V) characteristics, which illustrate a gap of 0.7 V; and the first derivativeG(V), which shows a steplike structure. (B) Three independent G(V) measurements, offset for clarity, illustrating the reproducibility of the conductance values. The measurements were made with the same MCB but for different retractions/contacts and thus different contact configurations. Offsets of 0.01 μS for the middle curve and 0.02 μS for the top curve are used for clarity. The first step for these three measurements gives values of 22.2, 22.2, and 22.7 megohm (top to bottom); the next step gives values of 12.5, 13.3, and 14.3 megohm. The middle curve is the same data as in (A). (C) AnI(V) andG(V) measurement illustrating conductance values approximately twice the observed minimum conductance values. Resistances of ∼14 megohm for the first step and 7.1 megohm (negative bias) and 5 megohm (positive bias) for the second step were measured.

The first step for these three measurements gives resistance values of 22.2, 22.2, and 22.7 megohm (top to bottom); the next step gives resistance values of 12.5, 13.3, and 14.3 megohm. This is compared to a resistance of ∼9 megohm (11) and 18 ± 12 megohm (12) deduced from measurements of an ensemble of similar molecules contacted to a gold nanocrystal, and a calculated resistance of this system of 100 kilohm (24). A resistance greater than ∼22 megohm was not observed in our measurements; however, resistances less than this maximum were occasionally observed. Figure 4C shows I(V) andG(V) measurements of one singular observation that gave resistances that were approximately half (that is, 0.5) the value of the maximum resistances (using averages, 0.63, and 0.45, respectively). This suggests a configuration of two noninteracting self-assembled molecules in parallel, substantiating the idea that the threshold resistance of a single molecule is ∼22 megohm, and compares with the previously deduced value of 18 ± 12 megohm of a similar system.

One interpretation of the observed gap around zero voltage is that it is a Coulomb gap. Using the “apparent” Coulomb gap, an experimental capacitance of 1.1 × 10–19 F is obtained. Although the charge transport through the molecule is in principle a many-body effect, as a first step one can estimate the capacitance of the aryl group with a crude model: A 4.5 Å metallic sphere bound 2.0 Å from proximal metallic planes with intervening vacuum barriers gives a capacitance of 0.4 × 10–19 F, as compared with the experimentally derived 1.1 × 10–19 F. However, a definitive demonstration of Coulomb blockade would require a third gate electrode, which is problematic in the present configuration because a third proximal probe cannot be placed near the molecule. A second interpretation of the observed gap is that it is due to the mismatch between the contact Fermi level and the lowest unoccupied molecular orbital (LUMO). Preliminary calculations using this interpretation give characteristics similar to those of the experimentally observed data (25).

The reproducibility of the minimum conductance at a consistent value implies that the number of active molecules could be as few as one. A better theoretical understanding of the threshold resistance of this system, either the apparent Coulomb gap derived from the capacitance of a single molecule configuration or the determination of the contact Fermi level–LUMO gap alignment, is needed to compare to the experimental values of ∼22 megohm and ∼0.7 V, respectively.

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