Atomic-Scale Study of Cation Ordering in Potassium Tungsten Bronze Nanosheets.

It has long been accepted that the formation of superlattices in hexagonal-based potassium tungsten bronzes is attributed to K vacancies only, together with small displacements of W cations. Here, the superlattices within potassium tungsten bronze nanosheets both structurally and spectroscopically at atomic resolution using comprehensive transmission electron microscopy techniques are studied. The multidimensional chemical analyses are realized by energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy, and X-ray photoelectron spectroscopy, the atomic-scale structures are characterized using aberration-corrected scanning transmission electron microscopy with high-angle annular-dark-field detector. The observed superstructures are mainly attributed to small amount of W vacancies within single atomic layer, which would recover to more uniform distributions of W vacancies with lower concentrations at higher temperature. The band regions of different orientation from the matrix tend to regulate the superstructures to be pinned along the same direction, forming domains of highly ordered structures. The characterization of cation ordering and recovery processes of nanostructures from chemical and structural point of view at atomic resolution enables rational design of optoelectronic devices with controlled physical properties.