Optical Studies of Single Molecule Transistors

We are studying the effect of applied light on electron transport in single molecule transistors. Our three-terminal devices are fabricated using electron-beam lithography followed by electromigration. The experimental setup for illuminating the devices consists of a tunable Ar/Kr laser coupled into an optical cryostat. Light-induced changes in molecular conductance can be studied as a function of the gate and bias voltages.

People Involved

Jacob Grose, Eugenia Tam, Samuel Flores-Torres, Geoff Hutchison, Hector Abruņa, and Dan Ralph

Summary

We are working to extend the study of single molecule transistors [1,2] to include light-induced effects. Electronic excitations in an optically active molecule can be probed using tunneling electrons provided the flow of electrons through the molecule is comparable to the relaxation time of the excitation. We will be looking for features in the electron tunneling spectra which, in addition to being a function of bias voltage and gate voltage, should depend on the wavelength and power of the incident light. Because the nature of the response should be molecule-specific, light studies should also provide clear confirmation that the device contains the molecule of interest, rather than some other species which might produce artifacts.

To make a single molecule transistor, a molecule must be connected within a nanometer-scale gap between two conducting electrodes. Gaps of this size cannot be made using conventional lithography techniques, so we fabricate a continuous wire and then create the gap using electromigration [3]. The fabrication is done by starting with an insulating SiO2 layer on Si. A thin Al gate electrode is deposited where the devices will be, and a native oxide is allowed to form. We then deposit Au or Pt wires (150 nm wide, 500 nm long, and 10 nm thick) on top of the gate electrode with e-beam lithography and liftoff. These wires are contacted by larger Au leads.

We clean each chip in an oxygen plasma and then deposit molecules from dilute solution. The sample is loaded into an optical cryostat which enables studies at liquid helium temperatures (4.2 K) for improved energy resolution in electron-tunneling spectroscopy. We break the wires in situ using electromigration with low-resistance contacts-ramping the voltage bias until the wire breaks, to leave a gap of order 1 nm between the resulting electrodes. In some of the devices, we find that a molecule then bridges this gap. The presence of a molecule can be detected by Coulomb blockade, in which current turns on above a gate dependent threshold bias voltage.

The molecule we have chosen for initial experiments is Ru2(tppz)3, synthesized by the Abruņa group at Cornell. This molecule was chosen because it is redox active at low potentials, stable in both air and acetonitrile, and has a large optical cross section. In addition to this ruthenium dimer, we may also try optical experiments involving semi-conducting quantum dots.


Figure 1

Figure 1: SEM images of (a) an unbroken wire and (b) a wire broken by electromigration. In some devices a molecule deposited on the surface can be trapped in the gap, creating a SMT.

Figure 2

Figure 2: Schematic of experimental setup. (Inset) Photograph of optical cryostat.

References

  1. H. Park et al., Nanomechanical oscillations in a single-C60 transistor. Nature 407, 57 (2000).
  2. J. Park et al., Coulomb blockade and the Kondo effect in single-atom transistors. Nature 417, 722 (2002).
  3. H. Park et al., Fabrication of metallic electrodes with nanometer separation by electromigration. Applied Physics Letters 75, 301 (1999).


Last updated: 11-July-2007

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