The triptycene molecule looks like a three-pointed star. When such stars are scattered on a lead surface, they self-assemble into a pattern reminiscent of a woven basket: triangles and hexagons alternate, and each node hosts a pair of stars. Theorists have long argued that special electrons should live at the edges of such a basket, but no one had managed to see them.

Scientists from Japan and Germany used large molecules instead of atoms. Each molecule is a ready-made node, and electrons can hop from one node to another. On the lead surface, the molecules arranged into neat islands with zigzag edges. A scanning tunneling microscope, which probes the surface with a fine tip, found a bright peak at a voltage of 1.42 volts along these edges.

Electrons that live only at the edge are a hallmark of topological materials. Inside such a material, electrons behave one way; at the edge, they behave differently, and this difference is insensitive to minor defects. In graphene, such edge states arise on zigzag edges, and now their counterpart has been found in a molecular lattice. Calculations show that these electrons are protected by the crystal's geometry, just as a fortress is protected by the shape of its walls.

Previously, such protection could only be computed on paper. Now it has manifested as a chain of bright spots on a conductivity map. The molecular construction kit proved more precise than the atomic one: it allows assembling a lattice with the desired geometry. The same approach could be applied to other molecules to create materials with predetermined electronic properties.