Abstract text (incl. figure legends and references)

Additive manufacturing, often referred to as 3D printing, creates structures typially by depositing material layer-by-layer. However, feature sizes in 3D printing are still not sufficient for the creation of devices on the nanometer-scale. A promising method for building smaller structures is the layer-by-layer assembly of 2D materials. The creation of these so-called van-der-Waals heterostructures can also be regarded as a type of additive manufacturing. While the feature size of each layer in the out-of plane direction is by nature in the order of a few atoms, the common techniques for building van-der-Waals heterostructures can not manipulate and control the in-plane direction on a similar scale.

In this work, we developed a method for the assembly of van-der-Waals heterostructures to overcome these limitations. Free-standing 2D materials are patterned on the nanometer-scale prior to the assembly. This is done by resist-free methods such as water-assisted electron beam etching and by sputtering with high-energy electrons in a scanning transmission electron microscope.

Stacking is performed under observation in a scanning electron microscope and with the help of nanomanipulators, allowing a precise in-plane alignment (Fig. 1).

The assembled structures are analyzed by means of scanning electron microscopy and transmission electron microscopy. Several samples are fabricated to verify the method and investigate the accuracy of the in-plane alignment. Currently, layers can be aligned with an accuracy of about 10 nm. As an example, a ten-layer stack with a parabolic thickness profile is built that serves as phase plate for electrons, with the effect of focussing a plane electron wave (Fig. 2). In principle, phase plates with arbitrary shapes could be made by this approach.

The presented method surpass the limits of previous techniques for the assembly of van-der-Waals heterostructures. Having the individual layers as free-standing membranes, the 2D materials can be patterned with highest resolution prior to the stacking. Accurate alignment during the assembly is ensured by observation in an electron microscope. Together, the method enables the fabrication of 3D structures with highest spatial resolution.

[1] Haas, Jonas, et al. "Aligned Stacking of Nanopatterned 2D Materials for High-Resolution 3D Device Fabrication." ACS Nano 16.2 (2022): 1836-1846.

Fig. 1: (a) Schematic of the stacking process. (1) The free-standing 2D material is patterned, (2-3) brought into contact with a target support, and (4-5) detached from the initial support grid. The process is repeated for subsequent layers (6-10) under SEM observation, thereby enabling a precise lateral alignment. (b) Illustration of the setup used for the process. (c-h) SEM image sequence of placing a single layer. Scale bars are (c) 500 µm, (d,e) 10 µm and (f-h) 5 µm. Adapted from [1].

Fig. 2: (a) Ten-layer stack of 2D materials with increasing hole diameter that can be tailored to form a parabolic electron lens as shown in (b). (c-d) Intensity profile of a plane electron wave focused by a structure as shown in (a). Adapted from [1].