Hierarchical assembly of nanoparticles has been attracting wide interest, as advanced functionalities can be achieved. However, the ability to manipulate structural evolution of artificial nanoparticles into assemblies with atomic precision has been largely unsuccessful. Here we report the evolution from monomeric Au 24 Au 20 into dimeric Au 43 Ag 38 nanoclusters: Au 43 Ag 38 inherits the kernel frameworks from parent Au 24 Ag 20 but exhibits distinct surface motifs; Au 24 Ag 20 is racemic, while Au 43 Ag 38 is mesomeric. Importantly, the evolution from monomers to dimers opens up exciting opportunities exploring currently unknown properties of monomeric and dimeric alloy nanoclusters. The Au 24 Ag 20 clusters show superatomic electronic configurations, while Au 43 Ag 38 clusters have molecular-like characteristics. Furthermore, monomeric Au 24 Ag 20 catalysts readily outperform dimeric Au 43 Ag 38 catalysts in the catalytic reduction of CO 2 .