Title: Exploring two-dimensional superatomic semiconductors
Description: Two-dimensional (2D) van der Waals materials have received widespread attention due to their novel 2D properties that are distinct from their bulk counterparts. These unique properties offer new possibilities for fundamental research and for diverse applications in electronics, optoelectronics, and valleytronics. It is therefore of great interest to design 2D materials from complex, hierarchical and/or tunable building blocks. Atomic and molecular clusters are attractive target due to their atomic precision, structural and compositional diversity and synthetic flexibility. In this thesis, we report two novel quasi-2D superatomic semiconductors: Re6Se8Cl2 and Mo6S3Br6, whose building blocks are atomic clusters rather than simple atoms. In Chapter 3, we determine the electronic bandgap (1.58 eV), optical bandgap (indirect, 1.48 eV), and exciton binding energy (100 meV) of Re6Se8Cl2 crystals by using scanning tunneling spectroscopy, photoluminescence and ultraviolet photoelectron spectroscopy, and first principles calculations.

The exciton binding energy is consistent with the partially 2D nature of the exciton. In Chapter 4, the layered van der Waals material Mo6S3Br6 possesses a robust 2D character with a direct gap of 1.64 eV, as determined by scanning tunneling spectroscopy. By using polarization dependent Raman spectroscopy and DFT calculations, we determine its strong in-plane electronic anisotropy. The complex, hierarchical structures with 2D characters of these two materials thus suggest an effective strategy to expand the design space for 2D materials research with multi-functionality and novel physical properties.