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TEM images for (a) silica spheres with an average 20 nm diameter, (b) monometallic silver shells on the silica spheres, (c) monometallic gold shells on the silica spheres, (d) heterogeneous bimetallic shells of silica/Ag/Au, (e) heterogeneous bimetallic shells silica/Au/Ag, (f) homogeneous bimetallic Ag-Au alloy shells on the silica spheres, and gold shells after the silica cores were removed by hydrofluoric acid.
