Furthermore, if inter-layer diffusion is needed to maintain inertness as NPs slowly lose carbon, that might explain why many carbon NPs alternated between periods of inertness and fast etching as they evolved (Fig. ). During inert periods, the NPs must have been capped by a fully-coordinated surface layer, but if inter-layer diffusion in partially evolved NPs were slow, then carbon loss from the surface layer would eventually overwhelm the defect healing possible from intra-layer processes, triggering a wave of fast etching.