Characterisation of doped graphene with focal-series reconstruction

Abstract text (incl. figure legends and references)

Introduction

Phase recovery techniques such as focal series reconstruction have been shown to allow characterization of materials in terms of electrostatic potential, electric and magnetic field, as well as charge density down to the atomic level. It has the advantage that, without additional physical modification to a standard microscope the exit wave"s amplitude and phase can be recovered with high spatial resolution and with relatively low noise [1,2,3]. Being very flexible regarding beam setting protocols, this technique is therefore well suited for the characterization of complex systems or materials where, for example, the contrast of different atoms in the structure is indiscernible and as such a HRTEM image is not directly interpretable or when radiation damage of a material is a limitation [4].

Objectives

The main goal of this experiment is to characterize N, O and Si doped graphene using focal series reconstruction. It is known that each of these dopant atoms occupies specific positions in the graphene structure, e. g. substitutional nitrogen, or oxygen forming an epoxy group bridging between two carbon atoms [5]. Accordingly, each dopant atom in each configuration will affect the structure and the properties of graphene differently and in a specific manner.

Materials & Methods

The fabrication of the graphene samples for TEM follows the general CVD graphene transferring method to grids [6]. The doping of graphene with nitrogen and oxygen was made by plasma generation. While silicon doping was just made with the intrinsic Si contamination on the sample, which in this case was exacerbated by removing hydrocarbon contamination on graphene [6] and the subsequently plasma generation or the electron beam irradiation. The samples were characterised in the SALVE TEM microscope [7] operated at 60 kV. The series of defocused images were taken with linear focus increments and the reconstruction was performed using the full-resolution wave-reconstruction algorithm [1]. Structure calculations and image simulations were carried out using the abTEM package [8].

Results & Conclusions

With the phase-reconstructed images it is possible to resolve the atomic position, distinguish if the doping atoms are substitutional or superficial, and ascertain the chemical composition of the dopants by phase analysis with complementary theoretical calculations and image simulations. In our experiments all three types of dopants in their most common configurations [5, 9, 10] could be characterised within the same field of view (e. g. approx. 40 nm x 40 nm). This was only possible by sequentially treating and doping the graphene. The latter experiment also highlights the robustness of graphene to external manipulation and the possible extent of graphene functionalization.

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[7] https://www.salve-project.de/tools/salve3/

[8] J. Madsen et al. Open Res. Europe 1:24 (2021)

[9] W. Zhou et al. Phys. Rev. Lett. 109 (2012) 206803

[10] H. Terrones et al. Rep. Prog. Phys. 75 (2012) 062501

* Project PReGrAM funded by the DFG with Project number: 451037016