The above analysis of interparticle interactions (Fig. ) suggests that the assembly process is driven predominantly by short-range electrostatic interactions between NP-adsorbed ligands, with negligible contributions from the van der Waals interactions between the NP cores. To verify this hypothesis, we synthesized positively charged palladium NPs (Pd•TMA) and studied their interactions with similarly sized Au•MUA. The bimetallic Au/Pd RDs (Fig. ) were formed similarly to the all-Au RDs, and the two kinds of RDs were indistinguishable by SEM; however, elemental analysis revealed the expected 1:1 ratio of Au and Pd (Supplementary Fig. ). By controllably removing the monolayers of organic ligands from the NPs, 3 such mixed crystals could be transformed into new nanostructured materials, in which the nanoscopic domains of two metals alternate. Next, we worked with mixtures of differently sized Au•MUA and Au•TMA, i.e., outside the diagonal line in Fig. . We found that NP combinations that afforded well-defined RDs span a relatively large region of the phase diagram (see the green markers in Fig. ); for example, a mixture of 5.78 nm Au•TMA and 4.19 nm Au•MUA afforded RDs in a near-quantitative yield. This is an interesting finding given that these RDs are built from equal numbers of differently sized Au•MUA and Au•TMA, while the electroneutrality condition implies that the overall number of positive and negative charges within the crystals is the same. Assuming that both MUA and TMA occupy the same surface area on the NP, and that the positive charge on Au•TMA is diluted by an electrically neutral ligand (~34 mol%), it follows from simple geometric considerations that the TMA/MUA molar ratio within these RDs is as high as ~1.25.