To visualize the P loss process, we image a film with final composition CuP 1.3 by scanning electron microscopy (SEM, Fig. ). Two phases can be clearly distinguished on the micrometer scale: a porous polycrystalline matrix with grain size around 30 nm and islands of more compact morphology. The intensity ratio between the Cu and the P peaks in energy-dispersive x-ray spectroscopy (EDX) increases by a factor ∼ 5.5 when moving from the matrix to the islands (Fig. , Supporting Information). Thus, we conclude that the matrix consists of CuP 2 and the islands consist of Cu 3 P. The mechanism of conversion from CuP 2 to Cu 3 P appears to be diffusion of Cu in the plane of the substrate, contributing to the enlargement of seed Cu 3 P islands. At the same time, P gradually evaporates elsewhere.