In general, the particle shape is determined by two factors, equilibrium and growth forms (Elwell & Scheel, 1975). The equilibrium form is the shape with the minimum surface free energy, and can be derived using the Wulff or γ plot. The Wulff plot is a polar diagram of the specific surface free energy, which is determined by the combination of solid and liquid materials. The equilibrium form is found by drawing all the planes normal to the radius vectors of the surface energy and taking the innermost envelop (Fig. 6(a)). A sharp cusp implies that a certain face has much lower free energy than other faces and the crystal will be facetted. The sharpness of the cusp in the γ plot relates to the roughness of the surfaces at the atomic scale. Figure 6(b) shows a part of the γ plot. Surface A has the lowest surface free energy of γA, and surface B is tilted by an angle θ and has a surface free energy of γB. The difference between γA and γB is the energy of the steps on the A surface (Fig. 6(c)). Therefore, a surface with a sharp cusp has high step energy; the density of steps on the surface with a well-developed facet is low.