LAUSR

Non–close-packed nanoparticle arrays Films of colloidal nanoparticles usually form dense, close-packed lattices. If binary lattices could be made and one component removed, then a more open array could form, as long as the remaining nanoparticles could be stabilized. Udayabhaskararao et al. formed binary superlattices of gold and magnetite nanoparticles at an air-liquid interface that could then be transferred to carbon-coated surfaces (see the Perspective by Kotov). Selective etching of either of the nanoparticles created non–close-packed arrays with vacancies stabilized by the carbon surface. Science, this issue p. 514; see also p. 448 Binary nanoparticle arrays are transformed into nanoporous materials by selectively etching out one of their components. Self-assembly of inorganic nanoparticles has been used to prepare hundreds of different colloidal crystals, but almost invariably with the restriction that the particles must be densely packed. Here, we show that non–close-packed nanoparticle arrays can be fabricated through the selective removal of one of two components comprising binary nanoparticle superlattices. First, a variety of binary nanoparticle superlattices were prepared at the liquid-air interface, including several arrangements that were previously unknown. Molecular dynamics simulations revealed the particular role of the liquid in templating the formation of superlattices not achievable through self-assembly in bulk solution. Second, upon stabilization, all of these binary superlattices could be transformed into distinct “nanoallotropes”—nanoporous materials having the same chemical composition but differing in their nanoscale architectures.