  By using an electrochemical gating technique with a new combination of
polymer and electrolyte, we were able to inject surface charge densities n_2D
as high as 3.5 \times 10^15 e/cm^2 in gold films and to observe large relative
variations in the film resistance, DeltaR/R', up to 10% at low temperature.
DeltaR/R' is a linear function of n_2D - as expected within a free-electron
model - if the film is thick enough (> 25 nm), otherwise a tendency to
saturation due to size effects is observed. The application of this technique
to 2D materials will allow extending the field-effect experiments to a range of
charge doping where giant conductance modulations and, in some cases, even the
occurrence of superconductivity are expected.
