Water confined at the nanoscale has been the focus of numerous experimental and theoretical investigations in recent years, yet there is no consensus on such basic properties as diffusion and the nature of hydrogen bonding (HB) under confinement. Unraveling these properties is important to understand fluid flow and transport at the nanoscale, and to shed light on the solvation of biomolecules. Here we report on a first principle, computational study focusing on water confined between prototypical nonpolar substrates, i.e., single-wall carbon nanotubes and graphene sheets, 1-2.5 nm apart. The results of our molecular dynamics simulations show the presence of a thin, interfacial liquid layer (approximately 5 A) whose microscopic structure and thickness are independent of the distance between confining layers. The properties of the HB network are very similar to those of the bulk outside the interfacial region, even in the case of strong confinement. Our findings indicate that the perturbation induced by the presence of confining media is extremely local in liquid water, and we propose that many of the effects attributed to novel phases under confinement are determined by subtle electronic structure rearrangements occurring at the interface with the confining medium.
Although generally ascribed to the presence of defects, an ultimate assignment of the different contributions to the emission spectrum in terms of surface states and deep levels in ZnO nanostructures is still lacking. In this work we unambiguously give first evidence that zinc vacancies at the (1010) nonpolar surfaces are responsible for the green luminescence of ZnO nanostructures. The result is obtained by performing an exhaustive comparison between spatially resolved cathodoluminescence spectroscopy and imaging and ab initio simulations. Our findings are crucial to control undesired recombinations in nanostructured devices.
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