Persistent quantum confinement in a Germanium quantum dot solid

Quantum dot solar cells have the advantage that they can harvest the full spectrum of the sun by layers of quantum dots made out of the same semiconductor but with just a different size. This means that such quantum dots must maintain their quantum confinement and therefore their band gap, upon being in an ensemble (solid) in which all quantum dots are connected. When the quantum dot does not have a protective shell or ligands at the surface, which often hinders charge transport, the preservation of quantum confinement in a highly connected solid remains a question. In this work a germanium quantum dot solid is investigated by probing the quantum dots with scanning tunnelling spectroscopy in which only one or few quantum dots are targeted for each interrogation. Besides the band gap, the discrete energy levels at the edges of the band gap, which accompany quantum confinement, are here used as a key-tool to investigate the quantum confinement. This work forms a next step in assessing how quantum confinement is highly persistent and understanding how it can be utilized in solar-cell architectures based on nanoparticle assemblies.