  We report 3D coherent diffractive imaging of Au/Pd core-shell nanoparticles
with 6 nm resolution on 5-6 femtosecond timescales. We measured single-shot
diffraction patterns of core-shell nanoparticles using very intense and short
x-ray free electron laser pulses. By taking advantage of the curvature of the
Ewald sphere and the symmetry of the nanoparticle, we reconstructed the 3D
electron density of 34 core-shell structures from single-shot diffraction
patterns. We determined the size of the Au core and the thickness of the Pd
shell to be 65.0 +/- 1.0 nm and 4.0 +/- 0.5 nm, respectively, and identified
the 3D elemental distribution inside the nanoparticles with an accuracy better
than 2%. We anticipate this method can be used for quantitative 3D imaging of
symmetrical nanostructures and virus particles.
