Measurements of discrete electronic states in a gold nanoparticle using tunnel junctions formed from self-assembled monolayers

We present results from nanometer-scale tunnel junctions fabricated using organic self-assembled monolayers (SAMs) as tunnel barriers. Single junctions have resistances consistent with tunneling through a single layer of molecules. In several devices, a gold nanoparticle nucleated within the SAM barrier. Such samples allow a sensitive test of mechanical stability—some are sufficiently stable to permit the observation of electron tunneling through individual electron-in-a-box states at low voltages and mK temperatures. We also observe anomalous transport characteristics at larger voltages, which may be due to the motion of the gold nanoparticle within the monolayer.

People Involved

Jason Petta, David Salinas, Dan Ralph

Motivation

We are interested in measuring the energy level spectrum of nanoparticles. Currently, all of our devices are fabricated with tunnel barriers made from either oxidized aluminum or e-beam evaporated aluminum oxide. These devices have proven to be very useful, but we are limited to studying materials that can be evaporated or possibly even sputtered. In the future we would like to study chemically synthesized nanoparticles. Chemically synthesized nanoparticles are ideal because there composition can be tailored (Au, CdSe, FePt, etc.) and the reaction rates can be adjusted to produce anything from spherical nanoparticles to nanocubes to nanorods. One drawback of using chemically synthesized nanoparticles in our devices is that they are covered with ligands to prevent oxidation and to keep the particles in solution. The results that follow are from our efforts to determine if the springy capping molecules are stable enough to allow us to access the discrete "electrons-in-a-box" energy levels of gold nanoparticles.

Figure 1. Histogram of 1,8 Octanedithiol tunnel junction resistances at 4.2K. A device schematic is shown in the inset.

Figure 2. (a) I-V curve for a gold nanoparticle device made with 1,8-Octanedithiol tunnel barriers. (b) dI/dV-V curve obtained by numerically differentiating the I-V curve. The regularly spaced peaks in dI/dV-V indicate tunneling through a single particle. The background conductance is likely due to conduction paths that bypass the gold nanoparticle. Inset: Device schematic.

Figure 3. Low voltage transport characteristics for a Coulomb-
blockade device fabricated with 3-Mercaptopropionic-acid barriers. Well resolved tunneling resonances are observed. The lines indicate a Zeeman doublet. The conductance curves are offset for clarity.

Figure 4. Transport characteristics for a Coulomb-blockade device fabricated with 1,8-Octanedithiol tunnel barriers. Three resonances, marked with lines, give anomalous g-factors.

For more information see: APL 77, 4419 (2000).

Last updated: 2001-11-30