Single electron transistors made using chemically synthesized metal nanoparticles
By combining electron-beam lithography with chemical self-assembly, we fabricate single-electron-transistor devices incorporating individual chemically synthesized metal nanoparticles, and use the devices to perform low-temperature electron tunneling spectroscopy of the electron-in-a-box states of the nanoparticle. This allows us to determine the discrete electronic spectra of individual nanoparticles which are -- by virtue of chemical synthesis -- well defined in their composition, size and shape
Ferdinand Kuemmeth, Kirill Bolotin, Dan Ralph
Slides from Ferdinand's 2006 APS talk.
Electron tunneling at low temperatures (<100mK) has been used in the past to measure electron-in-a-box quantum states of evaporated metal nanoparticles formed by surface tension on a non-wetting substrate. Although a variety of electronic interactions could be observed, including superconducting pairing, magnetic exchange forces, and spin-orbit interactions, the size and shape of the evaporated particles could not be well-controlled.
In this project, we use chemical techniques to both synthesize and assemble monodisperse gold colloid of nearly spherical shape into a single-electron-transistor geometry. Chemical synthesis enables us to control a particle’s size -- and hence its mean level spacing -- and organic molecules with appropriate end groups allow us to tether the particles to the electrodes with symmetric tunnel barriers. The transistor geometry consists of two lateral gold electrodes (source and drain) bridged by a nanoparticle, on top of a gate electrode, and yields good electrostatic coupling of the nanoparticle to the gate . This coupling is essential to study electron interaction effects as it allows one to change the number of electrons in the particle and see how this modifies the particle’s spectrum.
Source and drain electrodes are fabricated by defining a 16 nm thick gold wire with a 100 nm wide constriction (electron-beam lithography on bilayer PMMA) on top of an oxidized silicon substrate, which serves as a gate electrode in the final device. By running a current through the constriction, a nanometer-sized gap between source and drain is formed via electromigration . To incorporate metallic nanoparticles from a colloidal solution into this gap, we first assemble a monolayer of [(aminoethylamino)propyl]trimethoxysilane on top of all three electrodes, and then immerse the chip into a colloidal solution whose pH has been adjusted to yield an attractive electric force between the negatively charged gold particles and the positively charged amino groups of the organic monolayer. The organic monolayer also serves as a tunnel barrier between the particles and the source/drain electrodes.
After assembly, the chip is rinsed, dried and cooled in a dilution refrigerator to measure the tunnel current I as a function of the source-drain bias V and the gate voltage Vgate. When plotting the differential conductance dI/dV versus V and Vgate for a device where a single particle bridges the gap, we observe diamond shaped areas of zero conductance due to Coulomb blockade and sets of parallel lines of higher conductance due to the particle’s discrete level spectrum. When we apply a magnetic field, each level Zeeman splits into two levels, and different levels avoid each other at higher magnetic fields due to spin-orbit interaction. Performing these measurements on gold particles of various sizes allows us to investigate how g-factors and spin-orbit interaction depend on the mean level spacing.
Figure 1: Chemically-synthesized gold nanoparticles (10nm diameter) assembled into a broken gold wire by means of an organic monolayer, to make a single-electron transistor.
Figure 2: Conductance of a metal nanoparticle transistor as a function of gate voltage and source drain bias, showing one particle’s discrete energy-level spectrum.
Figure 3: A particle’s spectrum as a function of magnetic field. We observe Zeeman splitting and avoided level crossings due to spin and spin-orbit effects.
- K. I. Bolotin et al., Metal-nanoparticle single-electron transistors fabricated using electromigration, Appl. Phys. Lett. 84, 3154 (2004).
- H. Park et al., Fabrication of metallic electrodes with nanometer separation by electromigration, Appl. Phys. Lett. 75, 301 (1999).
Last updated: 11-July-2007