Hydrophobic ligands of a nano-particle sheet redistribute unevenly when the sheet is formed on a water surface. This leads to a small but significant ~6 Å difference in average ligand-shell thickness between the two faces of the sheet, as measured by Grazing-incidence X-ray small-angle scattering (GISAXS) at beamline Sector 8-ID-E, the Advanced Photon Source (see the paper by Jiang et al, for details about the beamline). This tiny structural asymmetry is preserved when the underneath water is removed; hence it causes a stress asymmetry, if an electron beam hits the sheet and bonds the ligand molecules. The sheet therefore curves up but always to one side, the originally water-facing side.
Z. Jiang, J. He, S. A. Deshmukh, P. Kanjanaboos, G Kamath., Y. Wang, S. Sankaranarayanan, J. Wang, H. M. Jaeger, and X.-M. Lin, Subnanometre ligand-shell asymmetry leads to Janus-like nanoparticle membranes. Nature Mater. 14, 912-917 (2015). DOI:10.1038/nmat4321
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