NOVELTY - A film is formed on a substrate by immersing substrate in electrolyte comprising predetermined concentration of ions of mediating element and material to be deposited as the film; initiating a forward potential sweep at a first potential which is greater than a bulk deposition potential of the mediating element; sweeping the potential applied to the substrate in a positive direction; stopping the positive sweep; sweeping the potential applied to the substrate in a negative direction; stopping the negative sweep; and repeating positive and negative sweeps.    USE - Method of forming film on a substrate for use in energy conversion device (claimed) including fuel cells and material batteries.    ADVANTAGE - The method provides an atomic-level control on the deposition of uniform and conformal ultrathin films on three-dimensional particles. It also allows layer-by-layer deposition of films from submonolayer to multilayer thickness. The method is simple and cost-effective.    DETAILED DESCRIPTION - A film is formed on a substrate by immersing a substrate in an electrolyte comprising a predetermined concentration of ions of a mediating element and a predetermined concentration of ions of a material to be deposited as the film; initiating a forward potential sweep at a first potential which is greater than a bulk deposition potential of the mediating element; sweeping the potential applied to the substrate in a positive direction at a first sweep; stopping the positive sweep at a second potential at which the mediating element is removed and the material to be deposited as the film remains; sweeping the potential applied to the substrate in a negative direction at a second sweep rate; stopping the negative sweep at the first potential; and repeating the positive and negative sweeps between the first and second potentials. An INDEPENDENT CLAIM is included for an energy conversion device comprising an electrode with nanoparticles having a platinum film formed on the surface of the nanoparticles.    DESCRIPTION OF DRAWING(S) - The drawing shows a graph that illustrates changes in voltammetry features after underpotential deposition-mediated layer-by-layer growth of platinum on carbon-supported palladium and platinum.