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Atom manipulation with the scanning tunneling microscope (STM) makes it possible to create ultimately small structures at surfaces. We extended this technique to III-V semiconductors [1,2] and found that an atomically precise electrostatic surface-potential landscape can be designed by the controlled positioning of charged adatoms. In this way, quantum dots with identical, deterministic sizes can be created one atom at a time. By using the lattice of the InAs(111)A surface to define the allowed atomic positions, the shape and location of the dots is controlled with effectively zero error. The dots are assembled from -1 charged indium adatoms, leading to the confinement of intrinsic surface-state electrons [3]. This approach enables one to construct quantum dot assemblies whose quantum coupling has no intrinsic variation but can nonetheless be tuned over a wide range.
In a related experiment, we found that the charge state and tunneling conductance of a single organic molecule adsorbed on InAs(111)A can be controlled by the adatom-induced gating potential [4]. Depending on the potential, the charge state can be tuned from neutral to -1. Moreover, the molecule changes its orientational conformation upon charging. This coupling between charge and conformation induces a conductance gap more than one order of magnitude larger than reported previously. The observed behavior can be understood as charge transport through a gated molecular quantum dot with coupled charge and orientational degrees of freedom.
Atom manipulation in combination with STM-based spectroscopy provides detailed insight into the quantum-physical properties of artificial surface structures at the smallest size scales. Understanding and controlling these properties – and the new kinds of behavior to which they can lead – will be crucial for integrating atomic-scale devices with existing semiconductor technologies.

[1] S. Fölsch et al., Phys. Rev. Lett. 103, 096104 (2009).
[2] J. Yang et al., J. Phys. Condens. Mater. 24, 354008 (2012).
[3] S. Fölsch et al., Nature Nanotech. 9, 505 (2014).
[4] J. Martínez-Blanco et al., Nature Phys. 11, 640 (2015).

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