Underpotential deposition (UPD) is a term given to the phenomenon when you can electrochemically deposit a single layer of atoms (referred to as a monolayer) onto a surface. This technique is often used to add a thin layer of a more noble, expensive metal, or as a means to measure the surface area.
Another option we have explored further is using UPD to understand the early stages of metal adsorption, in particular the deposition of silver onto gold. Using UPD and studying the kinetics of the system, one can determine the activation energy for the spontaneous alloying of silver on to gold.