A layered cake of molybdenum disulfide MoS2 sits on the lab bench. It is prepared in the usual way: adhesive tape peels off the top layers to obtain a thin sample. But beneath the tape remains an invisible cavity where the atomic floors are no longer pressed together. It is there that electrons begin to behave differently than in the crystal bulk.

Electrons in a layered material usually roam freely in all three dimensions. But when the top layers detach from the bulk, a gap opens between them and the rest of the crystal. Electrons find themselves in a narrow slit where their up-and-down motion is restricted. Such a slit acts as a quantum well: only discrete energy levels are allowed, not a continuous spectrum. Physicists observed these levels using ARPES, a technique that kicks electrons out with light and measures their energy and momentum.

The number of levels varied from sample to sample: from one to eight. Their energies lined up like evenly spaced steps of a staircase. This pattern suggests that the bottom of the well is not flat but curved like a parabolic bowl. The width of the well turned out to be tiny: from 0.6 to 4.9 nanometers. For comparison, a DNA molecule is about 2 nanometers in diameter, and here the well holds just a single layer of MoS2.

Previously, it was thought that mechanical exfoliation does not alter the structure of bulk MoS2. Extra states in the spectra were attributed to defects or measurement artifacts. Now it is clear: these states arise from the detachment of top layers, which form a quantum well. This finding changes the interpretation of data for many layered materials. It also offers a simple way to create quantum wells without complex epitaxy: just carefully exfoliate a crystal.

The layered cake is back on the bench. Now it is evident that between its floors hide invisible wells where electrons reside on discrete energy steps. The tape, once considered a simple sample-preparation tool, actually builds quantum structures. And every time a physicist peels off a layer, they inadvertently create a new staircase for electrons.