Thin films: Folded in hierarchy.

907 news & views Molecular J-aggregates provide a classic example of GOST; indeed, it is their exceptionally sharp absorption spectrum and absorption strength that cemented these somewhat complex structures as premium dyes for photographic media. It has been found that their radiative lifetime is short, and exhibits a characteristic temperature dependence 10. As in the case of nanoplatelets, that unusual temperature dependence is related to the structures' dimensionality. The nanoplatelets are formed by the aggregation of magic-sized nanocrystals. Is that a clue to the physical origin of the GOST? Can dielectric screening be defeated by coherent superpositions of excitation throughout an array of intimately connected nanocrystals? Such coupling of nanocrystals might also provide a way to reverse the ordering of bright and dark exciton states, which is hinted at by the observation that photoluminescence decay time decreases with temperature. Mirafzal and Kelley proposed such a bright–dark exciton reversal for coupled gallium selenide nanoparticle aggregates 11. The essential idea is that if long-range exchange from the coupling between nanoparticles is larger than the exchange interaction, then the bright state can lie below the dark state. Long-range exchange is the same kind of electronic coupling that gives molecular exciton splitting or promotes Förster energy transfer. It can have a magnitude of a few tens of millielectron volts (meV). The exchange interaction is half the singlet– triplet splitting in molecules and has a value of a few meV in nanocrystals. Reversal of the bright–dark splitting would have substantial consequences for the photoluminescence properties and it may well be at play in the nanoplatelets. As one example, a key difference between poly(phenylenevinylene)-based conjugated polymers, which are excellent fluorophores, and non-fluorescent polyenes is the ordering of their lowest, bright and dark singlet excited states. Size and shape are well recognized ways to tune the optical properties of nanoscale excitons. However, evidently, there is a richer variety of tuning knobs for adjustment of optical properties connected to the dimensionality, structure and organization of the building blocks. W rinkles in ageing skin, drying fruit and wetted fingertips have a common origin: they emerge as a result of the compression of a stiff thin film (the skin of the fingers and fruits) that is attached to a soft foundation (the subcutaneous tissue and the fruits' pulp). Wrinkles are realizations of buckling — a mechanical instability whereby, above a critical applied load, a flat film develops out-of-plane undulations — …