Nanometer-sized zero-dimensional (0D) cavities, 5-10 nm in diameter and 3-4 atomic layers deep, were produced locally on a Au(111) surface in a nitrate electrolyte by applying short voltage pulses (80 ns, - 2.8 V) to a scanning tunnelling microscope (STM) tip. In the system Au(111)/BiO+,NO3-, localised electrochemical underpotential deposition (upd) of 0D bismuth dots was observed in these nano-cavities by in-situ STM. This contrasted to the behaviour of the Au(111)/Ag+,NO3-system in which the 0D cavities remained unfilled in the entire upd range, and only were filled during layer-by-layer growth of Ag on the (111) terraces at overpotentials. The difference between the two systems are discussed in respect to the different and nearly equal atomic radii of the Bi/Au(111) and Ag/Au(111) adsorbate-substrate systems, respectively, and the influence of the anion adsorption on the stability of close-packed 2D ad-layers relative to the formation of 3D phases. 0D deposits can be generated when a sufficiently wide stability region for a dense monolayer existed so that concurrent growth of 2D (terraces) and 3D (bulk) phases were avoided.