Stoichiometric control of DNA-grafted colloid self-assembly

There has been considerable interest in understanding the self-assembly of DNA-grafted nanoparticles into different crystal structures, e.g., CsCl, AlB<inf>2</inf>, and Cr<inf>3</inf>Si. Although there are important exceptions, a generally accepted view is that the right stoichiometry of the two building block colloids needs to be mixed to form the desired crystal structure. To incisively probe this issue, we combine experiments and theory on a series of DNA-grafted nanoparticles at varying stoichiometries, including noninteger values. We show that stoichiometry can couple with the geometries of the building blocks to tune the resulting equilibrium crystal morphology. As a concrete example, a stoichiometric ratio of 3:1 typically results in the Cr<inf>3</inf>Si structure. However, AlB<inf>2</inf> can formwhen appropriate building blocks are used so that the AlB<inf>2</inf> standard-state free energy is low enough to overcome the entropic preference for Cr<inf>3</inf>Si. These situations can also lead to an undesirable phase coexistence between crystal polymorphs. Thus, whereas stoichiometry can be a powerful handle for direct control of lattice formation, care must be taken in its design and selection to avoid polymorph coexistence. colloidal interactions functional particle superlattice engineering molecular design modeling.