Abstract
The reduced dimensionality in two-dimensional materials leads a wealth of unusual properties, which are currently explored for both fundamental and applied sciences. In order to study the crystal structure, edge states, the formation of defects and grain boundaries, or the impact of adsorbates, high resolution microscopy techniques are indispensible. Here we report on the development of an electron holography (EH) transmission electron microscopy (TEM) technique, which facilitates high spatial resolution by an automatic correction of geometric aberrations. Distinguished features of EH beyond conventional TEM imaging are the gap-free spatial information signal transfer and higher dose efficiency for certain spatial frequency bands as well as direct access to the projected electrostatic potential of the 2D material. We demonstrate these features at the example of h-BN, at which we measure the electrostatic potential as a function of layer number down to the monolayer limit and obtain evidence for a systematic increase of the potential at the zig-zag edges.
Highlights
The discovery of graphene and its intriguing properties more than ten years ago [1,2] has sparked large and ongoing research efforts into two-dimensional materials (2DMs)
We report on the development of an electron holography (EH) transmission electron microscopy (TEM) technique, which facilitates high spatial resolution by an automatic correction of geometric aberrations
Distinguished features of EH beyond conventional TEM imaging are gap-free spatial information signal transfer and higher dose efficiency for certain spatial frequency bands as well as direct access to the projected electrostatic potential of the two-dimensional material. We demonstrate these features with the example of hexagonal boron nitride (h-BN), for which we measure the electrostatic potential as a function of layer number down to the monolayer limit and obtain evidence for a systematic increase of the potential at the zig-zag edges
Summary
The discovery of graphene and its intriguing properties more than ten years ago [1,2] has sparked large and ongoing research efforts into two-dimensional materials (2DMs). In the following we address the phase problem in weak scatterers (for the example of 2DMs) by advancing off-axis electron holography, an interferometric technique allowing one to reconstruct the phase shift of the electron wave over the whole spatial frequency band, up to the information limit These advantages have triggered a small number of previous studies on WPOs, notably for biological materials [29,30] and 2DMs [31,32,33,34,35,36]. Amongst others we reconstruct the number of layers, the mean inner potential (MIP) of individual layers, and the structure of the monolayer as well as the edges, and correlate this to material properties such as the charge delocalization or the stability and electronic properties of the edge states
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