Quantum structures are semiconductor structures that confine electrons in one, two or three directions. Structures in which the electrons are confined in all three directions are known as quantum dots. In addition to being of fundamental interest, quantum structures could also have applications in semiconductor lasers, data storage devices and quantum computers.

An atomic force microscope (AFM) works by measuring how the force between the sample and a tiny "tip" on an oscillating cantilever changes as the microscope is moved over the surface of the sample. AFMs have been used to probe single electron events before, but it has always been necessary to attach electrical leads to the system under study, which is not a trivial task for nanosized structures.

Peter Grütter and co-workers at McGill University in Montreal and the National Research Council of Canada in Ottawa exploited the electrostatic interaction between the quantum dot and the microscope tip. Grütter compared this system to a capacitor: "If the quantum dot is one plate of the capacitor and the AFM tip the other, changing the charging state of the quantum dot will lead to a change in the Coulomb interaction between the capacitor plates. This can be detected as a change in resonant frequency of the AFM cantilever."

The quantum dots studied by the Canadian team were made of indium arsenide and were formed by self-assembly on the surface of a 20 nanometre thick layer of indium phosphide, below which was a 10 nanometre thick layer of indium gallium arsenide. This layer of indium gallium arsenide acted as a quantum well, confining a two-dimensional electron gas (2DEG), which acted as the "back electrode" in the system (figure 1). The experiment had to be carried out at a temperature of 4.5 Kelvin.

Grütter and co-workers placed the AFM tip between 5 and 20 nanometres above the quantum dot and measured how the resonance frequency of the cantilever changed as the voltage between the tip and the back electrode was increased. They observed several distinct jumps in the electrostatic force spectra that they attributed to single electrons tunnelling between the dot and quantum well (figure 2).