The domain of mesoscale science, where the discrete granularity of atoms and quantization of energy give way to apparently continuous and infinitely divisible matter and energy, presents a new frontier of scientific opportunity and yields new complex architectures, phenomena, and functionalities. In this article, we describe some hallmarks of mesoscale science and highlight research directions that are described in greater detail in subsequent articles in this issue of MRS Bulletin. The exciting progress of the past several years and the rich unexplored opportunities at the mesoscale offer extraordinary prospects for future advances.

The regime of mesoscale science, where the granularity of atoms and quantization of energy gives way to apparently continuous and infinitely divisible matter and energy, yields strikingly complex architectures, phenomena, and functionalities that control macroscopic material behavior. Research in mesoscale materials and chemical science is an opportunity space for next-generation discovery, science, technology, and innovation, with promise of new solutions for societal problems such as energy, environment, climate, advanced manufacturing, and economic growth.

The electronic structure of Pu materials is examined using photoelectron spectroscopy. For delta-phase Pu metal as well as PuCoGa5 and PuIn3, the 5f electrons appear to be at the threshold between localized and itinerant character. A mixed level model computational scheme is used which results in non-magnetic solutions for the electronic structure and agrees well with the photoemission measurements. Several other computational schemes are assessed against photoemission results for delta Pu. Additional insight is provided by O2 and H2 dosing of the delta Pu samples and consideration of surface effects. The experimental and computational results are consistent with the 5f electrons in Pu materials exhibiting a dual nature with some fraction of the 5f levels localized and not participating in the bonding while the other fraction of 5f character is involved in bonding and hybridization with the conduction electrons.