  To date, high-resolution (< 1 nm) imaging of extended objects in
three-dimensions (3D) has not been possible. A restriction known as the
Crowther criterion forces a tradeoff between object size and resolution for 3D
reconstructions by tomography. Further, the sub-Angstrom resolution of
aberration-corrected electron microscopes is accompanied by a greatly
diminished depth of field, causing regions of larger specimens (> 6 nm) to
appear blurred or missing. Here we demonstrate a three-dimensional imaging
method that overcomes both these limits by combining through-focal depth
sectioning and traditional tilt-series tomography to reconstruct extended
objects, with high-resolution, in all three dimensions. The large convergence
angle in aberration corrected instruments now becomes a benefit and not a
hindrance to higher quality reconstructions. A through-focal reconstruction
over a 390 nm 3D carbon support containing over one hundred dealloyed and
nanoporous PtCu catalyst particles revealed with sub-nanometer detail the
extensive and connected interior pore structure that is created by the
dealloying instability.
