  Electric charge detection by atomic force microscopy (AFM) with single-
electron resolution (e-EFM) is a promising way to investigate the electronic
level structure of individual quantum dots (QD). The oscillating AFM tip
modulates the energy of the QDs, causing single electrons to tunnel between QDs
and an electrode. The resulting oscillating electrostatic force changes the
resonant frequency and damping of the AFM cantilever, enabling electrometry
with a single-electron sensitivity. Quantitative electronic level spectroscopy
is possible by sweeping the bias voltage. Charge stability diagram can be
obtained by scanning the AFM tip around the QD. e-EFM technique enables to
investigate individual colloidal nanoparticles and self- assembled QDs without
nanoscale electrodes. e-EFM is a quantum electromechanical system where the
back-action of a tunneling electron is detected by AFM; it can also be
considered as a mechanical analog of admittance spectroscopy with a radio
frequency resonator, which is emerging as a promising tool for quantum state
readout for quantum computing. In combination with the topography imaging
capability of the AFM, e-EFM is a powerful tool for investigating new nanoscale
material systems which can be used as quantum bits.
