Single electron transistors fabricated with individual chemically synthesized metal nanoparticles
We fabricate single-electron transistor devices (SETs) by connecting individual metal nanoparticles to electrodes through molecular tunnel barriers. We use these devices to perform detailed tunneling spectroscopy of the electron-in-a-box states inside the nanoparticles at milliKelvin temperatures. We are exploring nanoparticles made from a variety of different metals to probe how different strengths of the magnetic exchange interaction affect the energy-level spectrum.
Sufei Shi, Wan Li, Ferdinand Kuemmeth, Kirill Bolotin
When metal particles are made smaller than a few nanometers in diameter, the quantum-mechanical electron-in-a-box energy levels inside the particle become spaced sufficiently far apart in energy that the level spectrum can be measured by electron tunneling at low temperatures. The first experiments to probe these spectra in detail examined nanoparticles that were formed by random self-assembly of atoms during the evaporation of metal onto an insulating surface. Our group has developed techniques to contact individual chemically-synthesized metal nanoparticles, which have well-controlled size, shape, and composition, into single-electron transistor devices capable of measuring the electron-in-a-box level spectra . We have completed a study of gold nanoparticles, and are now pursuing measurements on platinum and palladium. These two metals are interesting because the strength of their exchange interaction between electrons is strong, but not quite strong enough to induce bulk ferromagnetism. We hope to explore predictions that small Pt and Pd particles will exhibit a previously undiscovered form of mesoscopic magnetism , whose strength depends on details of the energy-level spacing within a given nanoparticle.
We fabricate our devices by first using e-beam lithography to define a gold wire 16 nm thick with a width narrowed at one point to make a 100 nm wide constriction, on top of an oxidized n-doped silicon wafer that operates as a back gate. We then break the wire by running a current through the constriction until the wire fails by electromigration, resulting in a nanometer-scale gap. We chemically synthesize metal nanoparticles in solution, and protect their surfaces by tri-sodium citrate ligands. To help incorporate the particles into the nanometer-scale gap with good yield, we take advantage of Coulomb attraction. We first assemble a monolayer of Aminopropyltriethoxysilane (APTES) molecules on the electrodes, which become positively charged when the wafers are immersed in a solution with an appropriate pH. We then incubate the chip with a solution containing the metal particles, which are naturally negatively charged. The Coulomb attraction between the particles and the surface induces a large yield of particles attached to the surface, while Coulomb repulsion between like-charge particles keeps them from attaching too close together, allowing us to make contact to an individual nanoparticle (Fig. 1). The molecules on the surfaces of the particles and the electrodes function as a thin tunnel barrier in our electrical measurements.
After assembly, each chip is tested with a probe station at room temperature, and devices showing a resistance decrease from GΩ to MΩ upon particle attachment are chosen to be cooled down in a dilution refrigerator to measure the energy-level spectrum in detail. We have completed a study of gold nanoparticles, observing the effects of strong spin-orbit coupling and finding good agreement with random matrix theory for the statistics of the level spectra . We are currently pursuing measurements on Pt and Pd nanoparticles. We have produced SETs from single Pt nanoparticles, as evidenced by a well-defined “Coulomb diamond” pattern in measurements of the differential conductance as a function of source-drain voltage and gate voltage (Fig. 2). We have also observed the signature of discrete energy levels from devices with two Pt particles conducting in series.
Figure 1: Chemically synthesized Pt particles assembled in a nanometer-scale gap between gold electrodes to make a single electron transistor.
Figure 2: Differential conductance of a Pt nanoparticle transistor as a function of gate and bias, showing Coulomb blockade diamonds indicative of a one-particle device.
Figure 3: Conductance resonances due to discrete quantum energy levels in a device with two Pt particles in series.
- F. K. Ferdinand et al., “Measurement of Discrete Energy-level spectra in individual Chemically Synthesized Gold Nanoparticles”, Nano Lett. 8, 4506 (2008).
- I. L. Aleiner, P. W. Brouwer, and L. I. Glazman, “Quantum Effects in Coulomb Blockade,” Phys. Rep. 358, 309 (2002).
Last updated: 13-Aug-2010