Size control of crystals down to the nanoscale is an established route to manipulating the physical properties of a material without the need to play with structure or composition, famously leading to characteristics such as melting point suppression, to the existence of shape and size dependent localized surface plasmon resonances, and to the ability to tune the semiconducting bandgaps of monodisperse nanocrystals (to name but a few examples). The effect of size confinement on structural phase transformations in particular has drawn interest as a way to stabilize, under atmospheric conditions, otherwise unstable high-temperature phases, opening up a wide structure-composition phase space for exploration under ambient conditions. More recently, nanostructural characteristics have been shown to enable the manipulation of the kinetics of structural phase transformations such that metastable structures and compositions may be trapped, presenting a wealth of unrealized potential, as it becomes clear that the composition of a material does not necessarily uniquely determine its structure, or consequently its physical properties. In our group we employ a combination of complementary synthetic and characterization approaches to understand the characteristics of structural phase transformations for nanoscale crystals, in particular using in situ ultra-high resolution transmission electron microscopy (TEM) to observe key structural transformations in real-time. Here we present two such examples: first, the real-time observation of the solid-state synthesis of an individual iron oxide nanocapsule, whose solid-to-hollow morphology is seen to evolve independently from the thermodynamic and kinetic manipulation of its atomic structure and composition, enabling the creation of hollow nanocapsules of four different iron oxide phases; and second, the real-time atomistic discovery of nucleation and growth kinetics for the monoclinic-to-tetragonal phase transformation of the high-k dielectric material HfO2, resulting in a trapping of the tetragonal phase at over 1000 degrees below its transition temperature.

Real-time observation of nanoscale manipulation of metal oxide phase transformations using in situ transmission electron microscopy

255th National Meeting (2018)

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