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A drop of metal on a semiconductor is like an island in the sea. For an electron to reach that island, it must pay a "toll": overcome a barrier that arises because the island is tiny, and each extra charge significantly changes its energy. This phenomenon is called Coulomb blockade. It underpins single-electron transistors, where current flows one electron at a time.

Scientists studied indium islands on black phosphorus using a scanning tunneling microscope — a device that probes the surface with a needle at distances less than a nanometer. They observed that the barrier for electrons is not uniform across the island: at different points, it is shifted in voltage and curved differently. Comparing with calculations, the researchers realized the cause: the island and the substrate have different "work functions"—the energy needed for an electron to escape from the material. This makes the electric field around the island asymmetric.

So, for precise tuning of future single-electron devices, one must consider not only the size of the islands but also how they pair with the substrate. Otherwise, the device will behave unpredictably, like a turnstile that sometimes works and sometimes doesn't.

Original abstract

as written by the authors arXiv:2603.03783

Coulomb blockade (CB) arises in nanoscale systems with ultra-small capacitance, where discrete charging effects dictate electron transport, enabling wide-ranging applications based on single-electron transistors. Despite established electrostatic control of charge states in quantum dots and nanoislands, a rigorous quantitative link between junction parameters and the CB spectrum remains elusive. Here, using scanning tunneling spectroscopy, we investigate the spatial variation of CB in indium nanoislands on semiconducting black phosphorus. We observe spatially dispersive charging resonances whose trajectories exhibit a finite shift of the symmetry axis in bias as well as a pronounced asymmetric curvature. By comparing the experimental results with calculations based on orthodox theory, we show that these features originate from work function differences in the junctions, underscoring the importance of junction-specific electrostatics in nanoscale charge transport.