Defects and Surface Chemistry of Novel PH-Tunable NiO-Mn_3O_4 ± Mn_xNi_1-xO Heterostructured Nanocrystals as Determined Using X-ray Photoemission Spectroscopy

Transition-metal oxides represent exclusive solid-state material platforms across which a broad diversity of optoelectronic, magnetic chemical, and catalytic properties can be allowed to coexist, interact, and be manipulated. This is an opportunity that holds significance for both fundamental and applied research in optoelectronics, energy storage/conversion, and biomedicine. Among the various synthetic approaches, colloidal techniques stand out as powerful routes to freestanding, solution-processable nanocrystals with a programmable crystal structure, geometry, composition, and surface functionality. Knowledge of growth thermodynamics and kinetics underpinning nanocrystal evolution in liquid media has triggered significant advances in these fabrication techniques, paving the way for complex hybrid nanoarchitectures in which sections of different materials are welded together as freestanding, easily processable multifunctional nanoheterostructures. In this chapter, we will illustrate recent progress in the wet-chemical development and characterization of last-generation breeds of colloidal heterostructured nanocrystals (HNCs), in which distinct material modules are interconnected via direct bonding (heteroepitaxial) interfaces in elaborate onion-like or oligomer-type topologies. The focus will be on HNCs that incorporate at least one transition-metal oxide material component, in association with semiconductors and/or plasmonic metals as a means of generating enhanced, unconventional and/or diversified properties and functionalities. Various synthetic strategies, all based on variants of the seeded-growth technique, will be illustrated and rationalized within the framework of the relevant mechanisms of heteroepitaxial deposition and topology selection.

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Chapter 13 - Colloidal oxide-based heterostructured nanocrystals