Molecular Electronics -- STM Characterization of Molecular Switches
My investigations of single molecules behaving as molecular electronic devices have been mostly centered around one family of molecular switches. These molecules were synthesized by the Tour group at Rice University, and are shown in Figure 2. The molecules have a fully conjugated pi electron system, allowing electron conduction through their length. The molecules are terminated on one end with a thiol functional group, so that they will readily chemisorb to gold surfaces. Additionally, the bonding structure of the molecules causes them to be rigid; this is ideal for STM imaging.
Figure 2. Candidate molecular devices, a wire (top) and two different switches (middle and bottom).
In order for us to study conduction along the length of conjugated molecules, we require a technique that will fix the molecule upright on the gold substrate. We use pure alkanethiol self assembled monolayers (SAMs) that are exposed to solutions containing the conjugated oligomers. The devices insert themselves into the film at domain boundaries and defect sites. The crystalline nature of the SAM forces the molecules to stand upright on the gold surface.
Figure 3 is STM images of a SAM with a molecular device protruding from the alkanethiol film. Periodic changes in height, as shown in Figure 3, are an indication of changing conductance of the devices. We believe that this molecule is behaving as a molecular switch.
Figure 3. Images of a switch molecules inserted into a dodecanethiol SAM. The left image shows the molecule in the high conductance state, while the right image shows the molecule in the low conductance state. Inset are close-up images of the same molecule in the two different states.
We have also observed the molecules switching reversibly between the on and off states. The molecules were studied in a series of images over several hours. A STM movie recorded over 16 hours shows several images extracted from the series at different times. The movie shows two molecules switching reversibly between high and low conductance states. This reversible switching behavior indicates that the molecules are staying in the same location while changing conductance, and that the switching behavior is not the destruction of the molecules or their migration out of the field of view.
If you have any questions regarding this ongoing work, please contact me: zjd101@psu.edu.