Binary nanocrystal superlattice membranes self-assembled at the liquid-air interface

The spontaneous organization of multicomponent micrometresized colloids(1) or nanocrystals(2) into superlattices is of scientific importance for understanding the assembly process on the nanometre scale and is of great interest for bottom-up fabrication of functional devices. In particular, co-assembly of two types of nanocrystal into binary nanocrystal superlattices (BNSLs) has recently attracted significant attention(2-8), as this provides a lowcost, programmable way to design metamaterials(4) with precisely controlled properties that arise from the organization and interactions of the constituent nanocrystal components(9). Although challenging, the ability to grow and manipulate large-scale BNSLs is critical for extensive exploration of this new class of material. Here we report a general method of growing centimetre-scale, uniform membranes of BNSLs that can readily be transferred to arbitrary substrates. Our method is based on the liquid-air interfacial assembly of multicomponent nanocrystals and circumvents the limitations associated with the current assembly strategies, allowing integration of BNSLs on any substrate for the fabrication of nanocrystal-based devices(10). We demonstrate the construction of magnetoresistive devices by incorporating large-area (1.5 mm x 32.5 mm) BNSL membranes; their magnetotransport measurements clearly show that device magnetoresistance is dependent onthe structure (stoichiometry) of the BNSLs. The ability to transfer BNSLs also allows the construction of free-standing membranes and other complex architectures that have not been accessible previously.