Abstract text (incl. figure legends and references)

Transmission electron off-axis holography is a phase retrieval technique which enables access to the full complex-valued exit-wave of thin samples. At atomic resolution, the reconstructed phase allows direct determination of the projected potential of the lattice arrangement. In the field of catalyst research electrostatic surface fields are of particular interest, since they can be assumed to play a major role in reaction mechanisms such as the oxygen evolution reaction¹.

Conventional off-axis holography reconstruction operates via the Fourier domain. As a consequence, the accessible range of spatial frequencies is band-limited by the carrier frequency of the hologram. Various studies have demonstrated high-resolution recording of lattice images by Fourier reconstruction². However, a trade-off is always necessary in order to optimize fringe frequency, visibility and phase-contrast transfer strongly depending on the instrument stability³.

In phase-shifting holography⁴ a series of tilts of the incident wave shifts the hologram fringes over the sample. The resulting series of holograms after careful correction for sample and biprism drift is then used to carry out a linear fit of the local phase and amplitude of the fringe modulation due to the object potential. This obviates the use of a reconstruction aperture, and the upper bound of the spatial resolution is thus only limited by the performance characteristics of the instrument, while the low-frequency information is also retained.

We demonstrate the implementation of phase-shifting holography at sub-nm resolution on a single-biprism setup, which is combined with the in-situ capabilities of a third-order aberration-corrected environmental TEM in order to investigate catalytic platinum samples under low-pressure gaseous environments. The resulting high-resolution exit-waves are successfully matched to multislice image simulations. Lens aberrations up to second order are corrected via minimization of the amplitude contrast in propagated exit-waves. The method enables in-situ investigation of atomic structure and surface effects in polycrystalline platinum under the influence of up to 5e-2 mbar of O₂ and H₂O.

Figure 1: Principle of phase-shifting holography. The pixel projection of an aligned holographic tilt series yields the fit of local phase θ, centerband amplitude a and sideband amplitude modulation b. The reference phase shift φ_n is corrected from a vacuum reference region.

Figure 2: Reconstructed exit-wave of a Pt[110] surface, supercell-averaged amplitude and phase before and after 2nd-order correction, at n-6nm numerical defocus.

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