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Researchers at the DIMES institute at Delft University of Technology in Holland have developed a method for trapping a single nanometer-scale particle between two electrodes in a controllable fashion--something that has never been achieved before. The researchers' technique can be applied to any nanoparticle in which electric charge can be separated, enabling for the first time a general method for depositing single nanoparticles onto surfaces. The new trapping technique offers numerous other technological possibilities, such as building single-nanoparticle switches. It also enables one to study the properties of single electrons as they travel across a single nanoparticle between the electrodes.

Figure 2. Technique for trapping single nanoparticles between two electrodes. Pt (red) denotes two platinum electrodes. These electrodes can trap single nanoparticles, such as conducting molecules or nanometer-scale clusters of metal atoms. Applying an electric field polarizes (separates charge in) the palladium cluster (yellow), which is then attracted to the gap between the electrodes. Once the particle is trapped between the electrodes, current flows through the circuit, and the resistor R_s then sharply reduces the electric field, discouraging any additional nanoparticles from entering the gap. Therefore one and only one nanoparticle will be controllably trapped in this arrangement. The electrodes lie on top of two SiN slabs (light green), a silicon dioxide film (blue), and a silicon wafer (black).

Applying electron beams and chemically reactive ions to a SiN thin film, the researchers create a 100-nm slit that narrows to 20 nm at one point. Part of an underlying SiO₂ surface is then etched away, leaving a pair of free-standing SiN protrusions (or "fingers") at the 20-nm constriction. Then platinum is deposited onto each finger, creating two metal electrodes that are separated by as little as 3-4 nm.

To trap the single nanoparticles, the researchers developed a technique which they call electrostatic trapping (ET). It works as follows. They first immerse the electrodes in a solvent containing dissolved nanoparticles. Then, they apply a a voltage to the electrodes. This separates electrical charge in each nanoparticle and attracts one particle to the gap between the electrodes. Once current flows between the electrodes, a resistor in the circuit sharply reduces the electric field, discouraging additional nanoparticles from entering the gap. The ET technique has been demonstrated for nanometer-scale clusters of palladium (Pd) atoms, carbon nanotubes, and a 5nm-long chain of thiophene (a conducting polymer). The researchers believe that this method will work with any nanoparticle whose electric charge can be polarized.

The researchers also explore the transport of single electrons across these electrodes. Studying a palladium cluster, the researchers observed for the first time the co-existence of single-electron tunneling (in which a single electron surmounts the usual energy barrier needed to cross the electrodes) and barrier suppression (a reduction in the energy barrier that the electron normally experiences).

Figure 3. Images of the gap between two platinum (Pt) electrodes. By accumulating platinum metal on top of two tiny SiN "fingers," the gap is reduced from 25 nanometers (billionths of a meter) to 15 nm in (b), and to 4 nm in (c). These images were taken with an electron microscope. Note that the platinum has a very smooth edge even at the nanometer scale.

Figure 4. (a) Platinum electrodes (red) separated by a 14-nanometer gap. (b) A single cluster of palladium atoms (yellow), about 17 nanometers in diameter, now bridges the same electrodes. (c) Another example where three palladium particles are trapped across a gap approximately 26 nanometers in diameter.