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, an invisible cavity remains, where the atomic floors are no longer pressed against each other. It is there that electrons begin to behave differently than in the bulk of the crystal.

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 in a series resembling stair steps with equal spacing. This pattern indicates 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, the diameter of a DNA molecule is about 2 nanometers, and here the well accommodates just a single layer of MoS2.

Previously, it was thought that mechanical exfoliation does not alter the structure of bulk MoS2. Extra states in spectra were attributed to defects or measurement errors. Now it is clear: these states arise from the detachment of the top layers, which form a quantum well. This discovery 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 the crystal.

The layered cake is back on the table. Now it is visible that between its floors hide invisible wells where electrons live on discrete energy steps. The tape, which seemed like 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.