1. IBM, T. J. Watson Research Center, Nanoscale Materials and Devices, 1101 Kitchawan Road, Route 134, Yorktown Heights, New York 10598, USA
2. Department of Applied Physics & Applied Mathematics, Columbia University, 200 SW Mudd Building, 500 West 120th Street, New York, New York 10027, USA
3. Advanced Materials Research Institute (AMRI), University of New Orleans, New Orleans, Louisiana, 70148, USA

Correspondence to: S. O'Brien² Correspondence and requests for materials should be addressed to C.B.M. (Email: cbmurray@us.ibm.com).

Recent advances in strategies for synthesizing nanoparticles—such as semiconductor quantum dots¹, magnets and noble-metal clusters²—have enabled the precise control of composition, size, shape³, crystal structure⁴, and surface chemistry. The distinct properties of the resulting nanometre-scale building blocks can be harnessed in assemblies with new collective properties^{2, 5, 6}, which can be further engineered by controlling interparticle spacing and by material processing. Our study is motivated by the emerging concept of metamaterials⁷—materials with properties arising from the controlled interaction of the different nanocrystals in an assembly. Previous multi-component nanocrystal assemblies have usually resulted in amorphous or short-range-ordered materials^{8, 9} because of non-directional forces or insufficient mobility during assembly^{10, 11, 12, 13, 14}. Here we report the self-assembly of PbSe semiconductor quantum dots and Fe₂O₃ magnetic nanocrystals into precisely ordered three-dimensional superlattices. The use of specific size ratios directs the assembly of the magnetic and semiconducting nanoparticles into AB₁₃ or AB₂ superlattices with potentially tunable optical and magnetic properties. This synthesis concept could ultimately enable the fine-tuning of material responses to magnetic, electrical, optical and mechanical stimuli⁶.