Tunneling Spectroscopy of Metallic Quantum Dots

Abstract

A quantum dot is an isolated piece of conducting material which forms a small enough ”box” that the confinement of electrons within it leads to resolvable discrete quantum energy levels, as opposed to the continuum of energies in a sample of macroscopic size. Here we specifically address metallic quantum dots produced by deposition of a thin, granular film onto an insulating substrate, from which a single selected grain is connected to two electrical leads by high-resistance, low-capacitance tunnel junctions. If these leads have tunneling resistances well in excess of the quantum resistance R Q =h/e2 ≃ 23 kΩ, the number of electrons on the grain is a good quantum number. This number can be changed one at a time by tunneling processes through the junctions, and the equilibrium number also can be controlled by an additional gate electrode which is coupled only electrostatically to the grain under study. In such a system, it is possible to carry out tunneling spectroscopy measurements which directly reveal the structure of the energy eigenvalues of the electrons in a small metallic grain which typically contains a few thousand conduction electrons. Such measurements were first carried out a few years ago by Ralph, Black, and myself on nanograins of Al [1]–[4]. More recent measurements have extended this work to nanoparticles of the heavy metal Au [5, 6] and to alloys of Al and Au [7].