Acid-Gated Nanoscale Transistors Based on Gold Nanoparticles
We have identified an electrochemical process by which closely spaced gold electrodes in acidic solution can exhibit transistor action. Gold nanoparticles etched from the electrodes during measurement can shuttle charge between electrodes. By sweeping a gate voltage through the redox potential of the gold particles, the electrical conductance can be turned from off to on to off again.
Jacob E. Grose, Burak Ülgüt, Abhay Pasupathy, Héctor Abruña, Dan Ralph
The ultimate aim of this project is to make a transistor based on a few strands of
the conducting polymer, polyaniline (PANI). Previous work on PANI-film transistors
with lateral dimensions on the order of a few microns has shown that currents on the
order of tens of microamps can be turned from off to on to off again by sweeping a
voltage applied to an acidic solution . Since we attempted to measure a few
PANI as opposed to a macroscopic film, we expected qualitatively similar behavior
except with currents on the order of nanoamperes. However, in test samples using bare
gold electrodes with no bridging PANI, we already observed transistor characteristics.
We have been able to determine that this artifact is caused by gold nanoparticles
shuttle charges through the acid electrolyte.
We fabricated small gold wires 150 nm wide by 30 nm high on a silicon substrate using electron-beam lithography. These wires were broken into separate contacts (source and drain) using electromigration , leaving a gap a few nanometers wide. A macroscopic gold wire (gate electrode) was placed in a glass micropipette with a tip diameter of around 10 microns filled with an aqueous solution of 0.5 M HClO4. A chip supporting the nanoscale wires was pre-cleaned with an oxygen plasma and then brought up to the pipette tip, forming a small drop of solution over the electrodes. The drop remained connected at all times to the reservoir of solution in the pipette and thus was in contact with the macroscopic gate electrode that controlled its potential.
The data consisted of current measurements gathered during source voltage sweeps (drain is always grounded) from -600 mV to 600 mV at 50 mV/s at various constant gate voltages. The drain current was observed to turn from off to on to off again as the gate voltage was swept (a typical range is from -1.4V to -0.4V).
SEM images of the wires before and after the measurement in acidic solution show clear evidence of etching. In previous STM measurements on Au(111) surfaces undergoing electrochemical cycling in 1 M sulfuric acid, Nieto et al.  inferred the formation of gold clusters with diameters on the order of a few nanometers. We also observed gold nanoparticles in SEM images of the residue taken from an acidic solution that was used to etch macroscopic gold wires and then dried on a silicon surface.
These nanoparticles can carry charge through the acid between the electrodes, thereby explaining the currents that we measure. If the potential of the solution is tuned using the gate voltage to a value such that a small bias between the source and drain allows the surfaces of gold nanoparticles to be oxidized at one electrode and reduced at the other, then the nanoparticles will shuttle charge between the electrodes to generate a steady-state current. Current can flow whenever the applied gate voltage causes the redox potential to lie between the source and drain potentials.
Figure 1: Drain current as a function of source, gate bias. Note the turn on and turn off of current as a function of gate bias.
Figure 2: SEM images of breakjunctions before (a) and after (b) measurement in acidic solution.
Figure 3: SEM images of gold nanoparticles etched from macroscopic wires during a similar measurement process.
- E. W. Paul, A. J. Ricco, and M. S. J. Wrighton, Phys. Chem. 89, 1441 (1985).
- H. Park et al., Appl. Phys. Lett. 75, 301 (1999).
- F. J. R. Nieto et al., Phys. Chem. B 107, 11452 (2003).
Last updated: 6/18/04