High-quality RDs were successfully prepared from other mixtures of equally sized Au•MUA and Au•TMA, in the range 3.7-7.4 nm (Fig. ). Oppositely charged NPs whose Au cores were smaller than 3.7 nm assembled into spherical aggregates that lacked crystalline order (Supplementary Fig. ). When the particles were larger than 7.4 nm, the Coulombic forces between them were so strong and they could not be screened in saturated (NH4)2CO3 solution -i.e., in this size regime, the initial amorphous aggregates (1 in Fig. ) could not be converted into crystalline ones.