Hierarchical Structure Control and Unconventional Thermal Response of Excitonic Emission in a Superatomic Semiconductor

Electron–phonon interactions play a pivotal role in determining the electronic and optical properties of quantum materials. Generally, electron–phonon coupling increases as the chemical bonds shorten because those closer atoms normally have larger orbital overlap. Unlike conventional crystals, superatomic hierarchical crystals, assembled from 0D clusters (“superatomic”) with strong intra-cluster bonding and relatively-weak inter-cluster bonding (“hierarchical”), exhibit unconventional lattice responses to the external stimuli such as pressure or heating, thus providing an interesting material platform to tune electron–phonon interactions. Herein, a highly-tunable excitonic renormalization is demonstrated with temperature coefficients transitioning from negative to positive in superatomic crystal Re₆Se₈Cl₂ under pressure. Note that such an anomalous positive temperature coefficient, in sharp contrast with the negative one in pristine Re₆Se₈Cl₂, is seldom realized in conventional semiconductors. First-principles calculations reveal that, the inter-cluster bonds stiffness under pressure, which effectively weakens electron–phonon coupling and reduces its contribution to the bandgap evolution behavior with heating, results in the positive temperature coefficient in exciton emission under pressure. This work demonstrates that pressurized superatomic crystals, with tunable hierarchical structures and insensitivity of bandgap on heating, offers a new material platform for exploration of emergent phonon-mediated phenomena toward optoelectronic and energy-harvesting devices.