Argonne researchers using the XSD 8-ID-E beamline at the APS are gaining insight into Janus-like nanoparticle membranes in the pursuit to open new avenues for designing nanoparticle superstructures. The research has been published in the journal Nature Materials.
Solids self-assembled from nanoparticles form a highly versatile class of hybrid materials in which both the inorganic core and the organic ligand shell can be tuned independently with unlimited possibility. The ultrathin limit of such solids, nanoparticle membranes comprised of just a single layer of nanoparticles, is particularly interesting because it exemplifies the special electronic and mechanical properties of these materials and because it can be viewed as a basic unit for building up more complex structures in a controlled sequential deposition process.
Self-assembly of nanoparticles at fluid interfaces has emerged as a simple yet efficient way to create two-dimensional membranes with tunable properties. In these membranes, inorganic nanoparticles are coated with a shell of organic ligands that interlock as spacers and provide tensile strength. Although curvature due to gradients in lipid-bilayer composition and protein scaffolding7, 8 is a key feature of many biological membranes, creating gradients in nanoparticle membranes has been difficult.
Here, we show by X-ray scattering that nanoparticle membranes formed at air/water interfaces exhibit a small but significant ~6 Å difference in average ligand-shell thickness between their two sides. This affects surface-enhanced Raman scattering and can be used to fold detached free-standing membranes into tubes by exposure to electron beams. Molecular dynamics simulations elucidate the roles of ligand coverage and mobility in producing and maintaining this asymmetry. Understanding this Janus-like membrane asymmetry opens up new avenues for designing nanoparticle superstructures.
Zhang Jiang, Jinbo He, Sanket A. Deshmukh, Pongsakorn Kanjanaboos, Ganesh Kamath, Yifan Wang, Subramanian K. R. S. Sankaranarayanan, Jin Wang, Heinrich M. Jaeger and Xiao-Min Lin, “Subnanometre Ligand-shell Asymmetry Leads to Janus-like Nanoparticle Membranes,” Nature Materials, Letter. DOI: 10.1038/nmat4321, Published Online June 8, 2015.