Abstract
Hole-free phase plates (HFPPs), also known as Volta phase plates, were already demonstrated to be well suited for in-focus transmission electron microscopy imaging of organic objects. However, the underlying physical processes have not been fully understood yet. To further elucidate the imaging properties of HFPPs, phase shift measurements were carried out under different experimental conditions. Both positive and negative phase shifts occur depending on the diameter of the zero-order electron beam and the HFPP film temperature. The analysis of Thon ring patterns of an amorphous carbon test sample reveals that the phase-shifting patch can be significantly larger than the size of the zero-order beam on the HFPP film. An HFPP was used for in-focus phase contrast imaging of carbon nanotube (CNT) bundles under positive and negative phase-shifting conditions. The comparison of experimental and simulated images of CNT bundles gives detailed information on the phase shift profile, which depends on the spatial frequency in the vicinity of the zero-order beam. The shape of the phase shift profile also explains halo-like image artifacts that surround the imaged objects.
Highlights
Organic materials are highly transparent for high-energy electrons in transmission electron microscopy (TEM), which leads to weak or even vanishing contrast in conventional in-focus TEM images
Phase shift determination and hole-free phase plate (HFPP) TEM imaging We present phase shift measurements conducted at room temperature with a small and a large condenser aperture (50 μm and 150 μm) resulting in a small (~ 100 nm) and large (~ 250 nm) diameter of the zero-order beam (ZOB) on the HFPP film and in addition at an HFPP film temperature of 200 °C with a small ZOB diameter
The phase-shifting behavior of HFPPs was studied under different operation conditions, namely, for two different ZOB diameters on the HFPP film (100 nm and 250 nm) and HFPP temperatures of 25 °C and 200 °C
Summary
Organic materials are highly transparent for high-energy electrons in transmission electron microscopy (TEM), which leads to weak or even vanishing contrast in conventional in-focus TEM images. Such objects cause a negligible modulation of the amplitude of the exit wave function and only weakly influence the phase of the incoming electron wave. For a long time, defocusing of the objective lens has been commonly used to visualize the structure of weakly scattering samples by means of phase contrast [1]. A PP is a device installed in the back focal plane (BFP) of the objective lens, where it introduces a phase shift difference φPP between the unscattered and the scattered electrons.
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