The physics of electrons, photons, and their plasmonic interactions changes greatly when one or more dimensions are reduced down to the nanometer scale 1 . For example, graphene shows unique electrical, optical, and plasmonic properties, which are tunable through gating or chemical doping 2-5 . Similarly, ultrathin metal films (UTMFs) down to atomic thickness can possess new quantum optical effects 6,7 , peculiar dielectric properties 8 , and predicted strong plasmons 9,10 . However, truly two-dimensional plasmonics in metals has so far elusive because of the difficulty in producing large areas of sufficiently thin continuous films. Thanks to a deposition technique that allows percolation even at 1 nm thickness, we demonstrate plasmons in few-nanometer gold UTMFs, with clear evidence of new dispersion regimes and large electrical tunability. Resonance peaks at 1.5-5 m wavelengths are shifted by hundreds of nanometers and amplitude-modulated by tens of per cent through gating using relatively low voltages. The results suggest ways to use metals in plasmonic applications, such as electrooptic modulation, bio-sensing, and smart windows. Main text:Since ancient times, plasmons in nanoparticles of noble metals such as silver and gold have been used to color glass, culminating during the last two decades with a remarkable broadening of the use of plasmon excitations triggered by an improved understanding of their origin and behaviour, as well as by the availability of more sophisticated means to synthesize and pattern the metals [11][12][13] . New applications promise to have an impact on the optical industry: for example, super lenses allowing unprecedented sub-diffraction-limited optical imaging 14 , metasurfaces providing on-chip functionality in ultrathin form factor 15 , light modulation 16 , compact biosensors 17 and electrochemical effects that can be used in smart windows 18 . All

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AFM images are always affected by artifacts arising from tip convolution effects, resulting in a decrease in the lateral resolution of this technique. The magnitude of such effects is described by means of geometrical considerations, thereby providing better understanding of the convolution phenomenon. We demonstrate that for a constant tip radius, the convolution error is increased with the object height, mainly for the narrowest motifs. Certain influence of the object shape is observed between rectangular and elliptical objects with the same height. Such moderate differences are essentially expected among elongated objects; in contrast they are reduced as the object aspect ratio is increased. Finally, we propose an algorithm to study the influence of the size, shape and aspect ratio of different nanometric motifs on a flat substrate. Indeed, with this algorithm, convolution artifacts can be extended to any kind of motif including real surface roughness. From the simulation results we demonstrate that in most cases the real motif's width can be estimated from AFM images without knowing its shape in detail.

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