Excimers, a portmanteau of “excited dimer”, are transient species that are formed from the electronic interaction of a fluorophore in the excited state with a neighbor in the ground state, which have found extensive use as laser gain media. Although common in molecular fluorophores, this work presents evidence for the formation of excimers in a new class of materials: atomically precise two-dimensional semiconductor nanoplatelets. Colloidal nanoplatelets of CdSe display two-color photoluminescence resolved at low temperatures with one band attributed to band-edge fluorescence and a second, red band attributed to excimer fluorescence. Previously reasonable explanations for two-color fluorescence, such as charging, are shown to be inconsistent with additional evidence. As with excimers in other materials systems, excimer emission is increased by increasing nanoplatelet concentration and the degree of cofacial stacking. Consistent with their promise as low-threshold gain media, amplified spontaneous emission emerges from the excimer emission line.

Formation of uranium polyhydrides UH5–9 is predicted using the evolutionary algorithm USPEX and proved by high-pressure synthesis.

Earth's core is composed of iron (Fe) alloyed with light elements, e.g., silicon (Si). Its thermal conductivity critically affects Earth's thermal structure, evolution, and dynamics, as it controls the magnitude of thermal and compositional sources required to sustain a geodynamo over Earth's history. Here we directly measured thermal conductivities of solid Fe and Fe-Si alloys up to 144 GPa and 3300 K. 15 at% Si alloyed in Fe substantially reduces its conductivity by about 2 folds at 132 GPa and 3000 K. An outer core with 15 at% Si would have a conductivity of about 20 W m −1 K −1 , lower than pure Fe at similar pressure-temperature conditions. This suggests a lower minimum heat flow, around 3 TW, across the core-mantle boundary than previously expected, and thus less thermal energy needed to operate the geodynamo. Our results provide key constraints on inner core age that could be older than two billion-years.