As-deposited CuP 2+x films did not exhibit any x-ray diffraction (XRD) peaks, so we crystallized them in an RTA furnace at atmospheric pressure under a N 2 flow. Loss of phosphorus at moderate temperatures is a wellknown phenomenon in many P-rich phosphides. We also observed P losses in all our post-annealed CuP 2 films (Figs. )). However, the dependence of these P losses on the initial composition of the as-deposited films is not trivial. In Fig. (a) we compare the P/Cu ratio beforeand after annealing at 400 • C for 5 min for various initial compositions between CuP 1.3 and CuP 4.5 . The P/Cu ratio is measured by x-ray fluorescence (XRF) so it represents an average through the depth of the film. Several interesting trends can be identified. First, thicker films generally experience milder P losses, since P located deeper in the film requires a longer time to diffuse out. Second, sufficiently thick films with initial composition in the CuP 2.2 -CuP 2.7 range can be "locked" into the desired CuP 2 stoichiometry by annealing (Fig. ). Third, films with severe P losses tend to approach the Cu 3 P composition after annealing. Cu 3 P is the most commonly reported binary stoichiometry in the Cu-P system. Last, and most surprisingly, we find that highly Prich initial compositions do not help achieve a higher P content in the post-annealed films. In fact the opposite is true. When the initial composition is in the (P-rich) CuP 4.1 -CuP 4.5 range, the post-annealed composition is around CuP 0.5 (Fig. ). When the initial composition is much poorer in P (CuP 1.3 -CuP 1.4 range) the post-annealed composition is similar, around CuP 0.4 (Fig. ). On the other hand, when the initial composition is in an intermediate CuP 2.2 -CuP 2.7 range closer to the desired CuP 2 stoichiometry, P losses upon annealing are much slower in films of comparable thickness. Using these atomically dispersed precursors with moderate P excess with respect to the target CuP 2 stoichiometry, the necessary species for forming crystalline CuP 2 are readily available within a short distance of their ideal crystallographic site. This enables fast crystallization of monoclinic CuP 2 by essentially bypassing diffusion processes. We assume that the lower total energy achieved by crystallizing the originally amorphous CuP 2 film helps delay P losses. On the other hand, precursor films that are too P-rich require solid-state diffusion to form a crys-talline CuP 2 phase because Cu atoms are too far apart in the initial amorphous phase. This Cu diffusion process competes against the tendency of P to evaporate due to the weakness of P-P bonds. As a result, P losses are much faster. These findings are summarized in the qualitative diagram shown in Fig. .