Molecular ConductorsBy: Adam KrausePhysics 6724/17/07Introduction to Molecular ConductorsExperimental Measurements of Molecular ConductanceMechanically Controlled Break-Junction Method: SAM of benzene-1,4-dithiol on Gold ElectrodesCrossed-Wire Method: SAM of OPE on Gold WireSTM Break-Junction Method: BipyridineCurrent Experimental Applications of Molecular ConductorsRectifying DiodeMolecular MemoryReferencesMolecular ConductorsBy: Adam KrausePhysics 6724/17/07Introduction to Molecular ConductorsIn the continuing search for smaller electronic devices, this paper briefly describes the extreme case of molecular conductors and their natural application:molecular electronics. Generally speaking molecular conductors are molecules that carry current. However, as we shall see, these molecules can be functionalized to perform in various ways. Currently there are two types of molecular conductors being studied for the purpose of electrical conductance andmolecular electronics application: carbon nanotubes and polyphenylene-based molecules [1]. The latter will be studied in the experiments described below.This paper will not only show how molecular conductors behave, but also the experimental methods used to measure conductance and current (I(V)) plots.It will also explain the proposed application of polyphenylene-based conductors to create a rectifying diode: one of the building blocks of modern electronics. Finally the application of molecular conductor molecules to produce an electronicmemory device is described.Experimental Measurements of Molecular ConductanceMechanically Controlled Break-Junction Method: SAM of benzene-1,4-dithiol on Gold ElectrodesThe first experimental method to be considered is the mechanically controlled break-junction method. One group used this method to explore the conductance (dI/dV) of the molecule: benzene-1,4-dithiol [2]. In this experiment, a gold wire is immersed in a 1mM THF solution of the benzene-1,4-dithiol molecule. The gold wire is mechanically stretched until it breaks which is wherethis method gets the name (Fig. 1). The control mechanism is a piezo element which can be precisely controlled. The resulting electrode tips are atomically sharp and covered in a self assembled monolayer (SAM) of the molecule. A voltage, V, is put across the coated electrodes which are then brought close together until an electric current is produced. Fig. 2 shows a close-up drawing of the configuration.Figure 1: The method of mechanically controlled break-junction is illustrated. A.) The method starts with anarrow gold wire onto which B) a SAM is deposited. C) While still immersed in the SAM solution, usingcontrolled mechanical means the wire is stretched until broken. This produces two SAM coated electrodesD) These electrodes are then brought close enough that a molecular bridge is formed between them. [2]Figure 2: An illustration of benzene-1,4-dithiolate forming a connection between the two electrodes. [2]Fig. 3 below shows the results of this experiment. Fig. 3A shows the typical current and conductance data versus bias voltage. Notice how there is a 0.7V gap centered on 0V. According to this group the gap behavior was seen for all cases. Fig. 3B shows three offset conductance traces for the purpose of illustrating the reproducibility of the data. The conductance shows a step behavior as the voltage is increased for both positive and negative bias. For the three traces shown in Fig. 3B the first step has a resistance of 22.2, 22.2, and 22.7 megohms respectively. The higher step has lower resistances of 12.5, 13.3, and 14.3 megohms. For one particular measurement the resistance was measured to be approximately half of the typical value (Fig. 3C). This suggests that there were actually two molecules bridging the electrodes instead of one.Figure 3: The results of the Reed’s experiment. A) This is a plot of the current and conductance vs. voltagemeasured during the experiment. B) This is three measurements of the conductance. They are offset forclarity. C) This shows one particular measurement which is about half the maximum resistance measuredduring the experiments suggesting there are two molecular bridges connecting the electrodes. [2]Crossed-Wire Method: SAM of OPE on Gold WireKushmerick’s group used a crossed-wire method to measure the electricalproperties of a SAM of oligo(phenylene ethnylene) otherwise known as OPE. In this experiment the group aimed to explore the metal-molecule contact and how it affects the metal-molecule-metal electrical properties. Fig. 4A shows the crossed-wire setup. One wire is coated with a SAM of the molecule in question while the uncoated wire is set perpendicular to the other. A magnetic field is applied as shown and a current is sent through the coated wire to induce a Lorentz force which moves the wires closer. To see what kind of connection the molecules are making as a function of Lorentz force, the resistance is measured. Fig 4B shows this data. At low force, the resistance is constant but as the forceincreases the molecules become distorted and the resistance drops [3]. Also shown in Fig. 4B are the two molecules used in this experiment. One has a thioacetyl group on both terminating ends where the gold will bond, but the other has this group on only one end. In this manner the two different metal-molecule contacts can be compared.Figure 4: A) Crossed-Wire Method setup. B) Electrical resistance vs. Lorentz Force. After a certain value,the resistance drops corresponding to a deformation of the monolayer molecules. C) Both moleculesstudied in this experiment are shown. One is symmetric, the other is not. [3]Fig. 5 shows the results of this experiment. In Fig. 5A the data for the symmetrical molecule (thioacetyl groups on both sides) is shown. The open symbols are for a positive voltage and the closed symbols are for negative voltage. The absolute value of the voltage is shown for comparison between the positive voltage and the negative voltage. Clearly in Fig. 5A the current varies thesame for the positive voltage as for the negative voltage. In Fig. 5B the data for the asymmetrical molecule is shown. It is interesting to note that while the currentlooks as expected for a positive voltage, it is considerably lower for the higher negative voltage levers. This means that the asymmetrical molecule acts as a rectifier allowing current to flow predominantly in one direction. To be specific, theelectrons flow more readily from