Anisotropically Fused Clusters Form a 2D Superatomic Sheet Exhibiting Polarized Light Emission

Polarized light emission is desirable for a number of optoelectronic applications and is difficult to realize in conventional semiconductors. Here we take advantage of the tunability in structure, symmetry, and functionality of superatomic semiconductors, i.e., crystalline arrays of atomically precise polyatomic clusters. We report the in-plane anisotropy in electronic and optical properties in the van der Waals two-dimensional (2D) superatomic semiconductor CsRe6Se8I3. Consistent with structural anisotropy within the 2D plane, CsRe6Se8I3 shows linearly polarized photoluminescence (PL) emission at room temperature. By lowering the temperature, we observe a new PL peak with nearly orthogonal linear polarization without an accompanying structural phase transition. Theoretical calculations together with spectroscopic measurements suggest that the high- and low-temperature PL peaks originate, respectively, from direct and indirect excitons. These results demonstrate rich optical properties of superatomic solids, which may be precisely programmed with the hierarchical design of the constituent clusters and the intercluster bonding.