Analyzing and assembling layered materials, atom by atom and layer by layer, in 2D and 3D

Abstract text (incl. figure legends and references)

Aberration-corrected electron microscopy is emerging as a versatile tool not only for analyzing, but also for manipulating materials down to the level of single atoms. I will discuss both of these aspects and present some of our recent developments in the context of layered, two-dimensional materials: On the analysis side, we developed means to identify the 3D atomic configuration of defects, grain boundaries or impurities, from only two exposures of the same structures [1,2]. The same approach, combined with the simple dependence of the intensity in annular dark field scanning transmission electron microscopy images on the atomic number provides (to some extent) chemical information about the sample, and hence allows an elemental identification in the case of light-element single-layer samples [3]. However, the intensity of individual atoms and atomic columns is affected by residual aberrations and the confidence of an identification is limited by the available signal to noise. As shown in Fig. 1, already in presence of small non-round aberrations, the histogram of intensities is significantly broadened if these aberrations are not taken into account. Here, we show that matching a simulation to an experimental image by iterative optimization provides a reliable analysis of atomic intensities even in presence of residual non-round aberrations [4]. We compare this method with other approaches demonstrating its high reliability at limited dose and with different aberrations. This is of particular relevance for analyzing moderately beam-sensitive materials, such as most 2D materials, where the limited sample stability makes it difficult to obtain spectroscopic information at atomic resolution.

Towards an atomic-level manipulation, I will present a recent development where we combine spatially controlled modifications of 2D materials, using focused electron irradiation or electron beam induced etching, with the layer-by-layer assembly of van der Waals heterostructures [5] (Fig. 2). A new transfer and assembly process makes it possible to stack the layers under observation in an electron microscope, such that pre-patterned features can be aligned to each other. The aligned stacking of individually patterned 2D materials layers can be considered as a form of 3D printing, where each layer is only one or a few atoms thick, and features within each layer can be defined with a nm-scale resolution. Moreover, as each layer can be chosen from the large zoo of 2D materials, it should be possible to directly generate a wide variety of functional devices.

Fig. 1. Intensity histograms from a simulated image in presence of small non-round aberrations (C_1,0 = 2 nm, C_1,2 = 1 nm, C_2,1=50 nm, C_2,3=50 nm). (a) Part of the simulated image, and (b) the simulated probe. (c) Intensity histogram using the optimization method, Gaussian fits, local maxima and the Voronoi cell integration (from left to right) [4].

Fig. 2. (a-c) Schematic of cutting a pattern into individual graphene layers (a,b) followed by aligned stacking (c). (d-f) Experimental realization, (d,e) SEM images of crosshair structures cut into graphene, (f) Dark-field TEM image showing the assembled structure, set for highlighting one of the two layers. Scale bars are (d,e) 500 nm and (f) 200 nm [5].

References
[1] 2D Materials 5, 045029 (2018)
[2] Appl. Phys. Lett. 114, 053102 (2019)
[3] Nature Commun. 10, 4570, (2019)
[4] Ultramicroscopy 227, 113292 (2021)
[5] ACS Nano 16, 1836-46 (2022)