Shape Anisotropy Influencing Functional Properties: Trigonal Prismatic ZnO Nanoparticles as an Example

The shape of crystalline particles is recognized as one important parameter for the adjustment of functional properties of inorganic materials. The surfaces of a thermodynamically stable crystal correspond to a set of lattice planes determined by the minimum interface energy. Thus, a morphology deviating from the most stable state correlates to either a change of the proportion of those surfaces to each other or ultimately a new set of surfaces emerges. At the nanoscale, when the surface-to-volume ratio is large, it is expected that a change in morphology implies a measurable alteration of properties. Here, the synthesis of nanocrystalline ZnO nanoparticles possessing a new non-equilibrium shape is presented. The reaction of special organometallic precursors at the interface of a water-in-oil emulsion facilitates the synthesis of fairly monodisperse prismatic ZnO nanocrystallites with an adjustable aspect ratio in gram amounts. It is found that the special morphology influences the bulk properties of the ZnO materials. Contrary to the well-known quantum size effect (smaller particles produce a blue-shift), a shortening of the ZnO nanoprisms induces a decrease in the bandgap (red-shift). This effect is due to the influence of an electric field inside the particles caused by the polarity of the surfaces terminating the nanoprisms (the quantum-confined Stark effect).