A crystal one atom thick is like a sheet of paper you can write on with electricity. Such a material promises to become the foundation for tiny, fast microchips. But before putting it into production, you need to know exactly what it's made of. And that's where the usual formula falls apart.
Researchers took a two-dimensional titanium oxide — a compound that insulates electricity well and could be useful in transistors. Using an atom probe, they plucked atoms from the crystal one by one and identified them. It turned out that the material contains less oxygen than the ideal formula requires. Some of its spots in the crystal lattice are empty, like empty seats in an auditorium.
In addition, the material still contained lithium atoms: these were supposed to be completely washed out during synthesis. But they remained in small amounts. These "extra" atoms also change the electrical properties. So the real material is not the perfect lattice drawn in textbooks, but a slightly more complex structure.
This means that to control the properties of two-dimensional materials, you need not only to create them but also to carefully check their composition. Then you can tune them for specific tasks — make better insulators, faster conductors, and smaller microchips.
An atom probe is a device that evaporates atoms from a sample's surface with a strong electric field and catches them one by one. This lets you know the material's composition literally piece by piece, as if you were counting candies in a box.