We review recent experimental and theoretical work on ultrasmall metallic grains, i.e. grains sufficiently small that the conduction electron energy spectrum becomes discrete. The discrete excitation spectrum of an individual grain can be measured by the technique of single-electron tunneling spectroscopy: the spectrum is extracted from the current-voltage characteristics of a single-electron transistor containing the grain as central island. We review experiments studying the influence on the discrete spectrum of superconductivity, nonequilibrium excitations, spin-orbit scattering and ferromagnetism. We also review the theoretical descriptions of these phenomena in ultrasmall grains, which require modifications or extensions of the standard bulk theories to include the effects of level discreteness.

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We have investigated the spectrum of discrete electronic states in single, nm-scale Al particles incorporated into new tunneling transistors, complete with a gate electrode. The addition of the gate has allowed (a) measurements of the electronic spectra for different numbers of electrons in the same particle, (b) greatly improved resolution and qualitatively new results for spectra within superconducting particles, and (c) detailed studies of the gate-voltage dependence of the resonance level widths, which have directly demonstrated the effects of non-equilibrium excitations.Recently it has become possible to measure the discrete spectrum of quantum energy levels for the interacting electrons within single semiconductor quantum dots [1] and nm-scale metal particles [2][3][4], and thereby to investigate the forces governing electronic structure. Our earlier experiments on Al particles were performed with simple tunneling devices, lacking a gate with which the electric potential of the particle could be adjusted. In this Letter, we describe the fabrication and study of nanoparticle transistors, complete with a gate electrode. This greatly expands the accessible physics. We have used the gate to tune the number of electrons in the particle, so as to measure excitation spectra for different numbers of electrons in the same grain and to confirm even-odd effects. The gate has also allowed significantly improved spectroscopic resolution, providing new understanding about the destruction of superconductivity in a nm-scale metal particle by an applied magnetic field. Studies of the gatevoltage dependence of tunneling resonance widths have shown that non-equilibrium excitations in the nanoparticle are a primary source of resonance broadening.A schematic cross-section of our device geometry is shown in Fig. 1(a). The gate electrode forms a ring around the Al nanoparticle. The devices are fabricated by first using electron beam lithography and reactive ion etching to make a bowl-shaped hole in a suspended silicon nitride membrane, with an orifice between 5 and 10 nm in diameter [5]. The gate electrode is formed by evaporating 12 nm of Al onto the flat (lower in Fig. 1(a)) side of the membrane. Plasma anodization and deposition of insulating SiO are then used to provide electrical isolation for the gate. We next form an aluminum electrode which fills the bowl-shaped side (top in Fig. 1(a)) of the nitride membrane by evaporation of 100 nm of Al, followed by oxidation in 50 mtorr O 2 for 45 sec to form a tunnel barrier near the lower opening of the bowl-shaped hole. We create a layer of nanoparticles by depositing 2.5 nm of Al onto the lower side of the device; due to surface tension the metal beads up into separate grains less than 10 nm in diameter [6]. In approximately 25% of the samples (determined as those showing "Coulombstaircase" structure as described below), a single particle forms under the nm-scale tunnel junction to contact the top Al electrode. Finally, after a second oxidation step to form a tunnel junction on t...

We report single-molecule-transistor measurements on devices incorporating magnetic molecules. By studying the electron-tunneling spectrum as a function of magnetic field, we are able to identify signatures of magnetic states and their associated magnetic anisotropy. A comparison of the data to simulations also suggests that sequential electron tunneling may enhance the magnetic relaxation of the magnetic molecule.