Non-invasive electron microscopy reveals structure of proton-irradiated graphene

June 21, 2014 - A science team from Ulm University, Manchester Centre for Mesoscience & Nanotechnology and University of Helsinki has succeeded to study the defects of proton-irradiated graphene in TEM with a seemingly simple technique that includes the encapsulation within two monolayers of graphene (as a sandwich structure, see for example Fig. 1) and imaging at an optimized defocus [1]. Previous experiments with ion radiation have shown that it has a strong impact on graphenes transport, and its magnetic and mechanical characteristics [2-5]. To systematically modify graphene's properties for future applications in engineering science it is very important to understand the effect that irradiation has at the atomic scale.

The authors write in their article [1]: "To use ion irradiation as an engineering tool requires understanding of the type and detailed characteristics of the produced defects which is still lacking, as the use of high-resolution transmission electron microscopy (HRTEM) – the only technique allowing direct imaging of atomic-scale defects – often modifies or even creates defects during imaging, thus making it impossible to determine the intrinsic atomic structure."

Modern high-resolution transmission electron microscopy is a technique with which individual (including light) atoms can be made visible even at relatively low accelerating voltages [6-10]. However, even at voltages below the knock-on damage of graphene (≤ 80 keV), defects inside graphene can be modified strongly during the image process and their pristine structure may remain invisible. .

The researchers have shown that with the additional encapsulation of graphene between two other (protective) graphene layers, it is possible to almost completely prevent the influencing of existing defects by the electron beam; that is this new method allows researchers now non-invasive imaging of graphene, including the analysis of pristine atomic defects for the first time. The non-invasive technique allows any high electron doses, and thus leads to a significant improvement of the signal-to-noise ratio in the TEM image.

To visualise the defects in the intermediate graphene layer and to distinguish them from defects in the protective layers is rather challenging. Fourier-filtering isn’t sensitive enough to the vertical position of the defect and thus cannot be used, especially when the stacking of the graphene layers is symmetric. While under standard focus conditions (Scherzer focus) defects are rather imperceptible, they can be made visible at higher underdefocus conditions because this adjustment "filters out" the regular honeycomb lattice and thus highlights the non-regular structure of the defects. The focus adjustments used here are similar to those in Figure 2 (right).

As an example of this new technique, The researchers were able to show that proton irradiation of graphene generates reconstructed monovacancies, predicted by theoretists. This important finding could explain the magnetic behavior of proton-irradiated graphene. Thereby these results clearly improve the general understanding of the behavior of graphene devices under irradiation.

In brief

It has been shown that non-invasive transmission electron microscopy of the e-beam sensitive defect created in proton-irradiated graphene is possible by encapsulation the irradiated graphene between two additional graphene layers and using underfocus imaging conditions. This particular defect shows very important as it has interesting magnetic and mechanical properties.