Imaging the potential distribution of individual charged impurities on graphene by low-energy electron holography

Abstract

While imaging individual atoms can routinely be achieved in high resolution transmission electron microscopy, visualizing the potential distribution of individually charged adsorbates leading to a phase shift of the probing electron wave is still a challenging task. Low-energy electrons (30 - 250 eV) are sensitive to localized potential gradients. We employed low-energy electron holography to acquire in-line holograms of individual charged impurities on free-standing graphene. By applying an iterative phase retrieval reconstruction routine we recover the potential distribution of the localized charged impurities present on free-standing graphene.

Direct visualization of charge transport in suspended (or free-standing) DNA strands by low-energy electron microscopy

“…The high sensitivity of low-energy electrons to local potentials allows detecting smallest charges 28,29 . The effect of electron trajectories deflections due to charges and their redistribution in biological samples is often discussed in biological single particle transmission electron microscopy imaging 35,36 .…”

Holography and Coherent Diffraction Imaging with Low-(30–250 eV) and High-(80–300 keV) Energy Electrons: History, Principles, and Recent Trends

“…The object distribution can be also reconstructed from a single in-line hologram (defocus image) by applying iterative reconstruction, as explained in detail in the literature [53,[72][73][74][75][76]. We provide only a brief summary of the reconstruction steps here.…”

“…To ensure an undisturbed reference wave, the sample needs to be placed onto an equipotential surface, for example graphene [92,93], in in-line low-energy electron holography. On the other hand, their high sensitivity to local potentials makes low-energy electrons the perfect type of radiation for studying 2D materials such as graphene and van der Waals structures [9,74,94,132]. Moreover, low-energy electrons can be employed for mapping unoccupied band structure of freestanding 2D materials, such as graphene by angle-resolved low-energy electron transmission measurements realized in in-line holography mode [133].…”

“…Iterative reconstruction of in-line holograms of individual charges provides the amplitude (associated with absorption) and phase (associated with the potential) distributions [ 74 ] ( Figure 23 ). The reconstructed absorption distributions ( Figure 23 d,e) appear to be narrower than the reconstructed phase distributions ( Figure 23 f,g).…”

“…( e , g ) corresponding angular-averaged radial profiles. Reprinted from [ 74 ], with permission from Elsevier.…”