Identification of Semiconductor Nanocrystals with Bright Ground-State Excitons

While semiconductor nanocrystals provide versatile fluorescent
materials for light-emitting devices, their brightness suffers from
the “dark exciton”an optically inactive electronic
state into which nanocrystals relax before emitting. Recently, a theoretical
mechanism, the Rashba effect, was discovered that can overcome this
limitation by inverting the lowest-lying levels and creating a bright
excitonic ground state. However, no methodology is available to systematically
identify materials that exhibit this inversion, hindering the development
of superbright nanocrystals and their devices. Here, based on a detailed
understanding of the Rashba mechanism, we demonstrate a procedure
that reveals previously unknown “bright-exciton” nanocrystals.
We first define physical criteria to reduce over 500,000 known solids
to 173 targets. Higher-level first-principles calculations then refine
this list to 28 candidates. From these, we select five with high oscillator
strength and develop effective-mass models to determine the nature
of their lowest excitonic state. We confirm that four of the five
solids yield bright ground-state excitons in nanocrystals. Thus, our
results provide a badly needed roadmap for experimental investigation
of bright-exciton nanomaterials.