The spontaneous organization of multicomponent micrometre-sized colloids1 or nanocrystals2 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 attention2, 3, 4, 5, 6, 7, 8, as this provides a low-cost, programmable way to design metamaterials4 with precisely controlled properties that arise from the organization and interactions of the constituent nanocrystal components9. 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 devices10. We demonstrate the construction of magnetoresistive devices by incorporating large-area (1.5 mm × 2.5 mm) BNSL membranes; their magnetotransport measurements clearly show that device magnetoresistance is dependent on the 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.
Figures at a glance
Figure 1: Large-scale BNSL membranes self-assembled at the liquid–air interface. 
a, Schematic of the BNSL membrane growth and transfer processes. The photograph shows a typical BNSL membrane transferred to a SiO2–Si wafer. Mechanical damage from tweezers in the membrane’s upper right corner (photo) helps visualize the scale. b–g, AlB2-type BNSL membranes self-assembled from 15-nm Fe3O4 and 6-nm FePt nanocrystals. b, TEM image of the (001) lattice projection (upper inset, magnified view; lower inset, small-angle electron diffraction pattern). c, d, Crystallographic model (c) and high-resolution scanning electron microscopy (HRSEM; d) image of the (001) projection. e, TEM image of the (100) lattice projection (upper inset, magnified view; lower inset, small-angle electron diffraction pattern). f, g, Crystallographic model (f) and HRSEM image of the (100) projection (g).
Figure 2: BNSL membrane one unit cell thick. 
a, AFM height image (scan size, 1 μm × 1 μm) of an AlB2-type BNSL membrane consisting of 15-nm Fe3O4 and 6-nm FePt nanocrystals. Inset, height analysis of the profile indicated in the AFM image. b, AFM phase image of the same membrane, showing a (100) projection in plane view. c, Side view of a crystallographic model of the membrane, showing that the membrane is one unit cell thick.
Figure 3: Magnetotransport of large-area (1.5 mm × 2.5 mm) BNSL membranes. 
a, Schematic of the device. b, TEM image of the thermally annealed AlB2-type BNSL membrane. c, TEM image of the thermally annealed ico-AB13-type BNSL membrane. d, Temperature dependence of zero-bias-voltage conductance (G0) for the AlB2-type (red) and ico-AB13-type (blue) BNSL membranes. G0 is plotted on a logarithmic scale. The black lines are the corresponding linear fits to the experimental data. e, Magnetoresistance of AlB2-type (red) and ico-AB13-type (blue) BNSL membranes at various temperatures. f, Temperature dependence of magnetoresistance for the AlB2-type (red) and ico-AB13-type (blue) BNSL membranes at H = 1 T.
Figure 4: Free-standing BNSL membranes consisting of 11-nm Fe3O4 and 4.5-nm FePt nanocrystals. 
a, TEM overview shows that most free-standing membranes suspended in the holes remained intact through transfer and drying. One fractured membrane in the upper right corner helps visualize the membrane. b, Magnified view of the region indicated in a, showing the membrane edge attached to the copper grid as well as the AlB2-type structure of the membrane. c, TEM image of a fractured free-standing membrane, showing the sharp crack along the fracture. d, Magnified view of the region indicated in c.
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