Spontaneous Formations of Superlattices and Supracrystals from Various Forms of Mn3O4 Nanocrystals

In this work, we demonstrate a one-step approach for synthesis and construction of self-assembled superlattices (SLs) or supracrystals (SCs) using Mn3O4 nanocrystal building blocks. The Mn3O4 crystals have been prepared from simple starting chemicals; only oleylamine and manganese acetate are involved in the synthesis. Surprisingly, shape-controlled Mn3O4 nanocrystals (e.g., spheres, cubes, plates, or rice) have been obtained by just tuning precursor concentration, temperature, and reaction time. A process scheme has been devised as a guide for morphological control. Furthermore, due to surface modification with oleylamine molecules and the presence of strong interactive forces (such as dipole–dipole and van der Waals interactions) among the Mn3O4 nanocrystals, stringent requirements for formation of SLs and SCs have been relaxed in terms of shape uniformity and size monodispersity. The self-organization of nanocrystals is a fast spontaneous process requiring no additional postgrowth treatments. The projected domain size of SCs in the range of several tens to hundreds of square micrometers has been achieved in the solution phase. Apart from the metal oxides, we also extend this synthesis-cum-assembly route to metal sulfides. Our preliminary results on synthesis and self-assembly of ZnS nanocrystals are also presented.