  Two-dimensional materials can be combined by placing individual layers on top
of each other, so that they are bound only by their van der Waals interaction.
The sequence of layers can be chosen arbitrarily, enabling an essentially
atomic-level control of the material and thereby a wide choice of properties
along one dimension. However, simultaneous control over the structure in the
in-plane directions is so far still rather limited. Here, 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. A novel assembly process makes it possible to structure
each layer with an arbitrary pattern prior to the assembly into the
heterostructure. Moreover, it enables a stacking of the layers with accurate
lateral alignment, with an accuracy of currently 10nm, under observation in an
electron microscope. Together, this enables the fabrication of almost arbitrary
3D structures with highest spatial resolution.
