Three-dimensional nanoparticle transformations captured by an electron microscope

Three-dimensional (3D) morphology and composition govern the properties of nanopartides (NPs). However, due to their high surface-to-volume ratio, the morphology and composition of nanomaterials are not as static as those for their bulk counterparts. One major influence is the increase in relative contribution of surface diffusion, which underlines rapid reshaping of NPs in response to changes in their environment. If not accounted for, these effects might affect the robustness of prospective NPs in practically relevant conditions, such as elevated temperatures, intense light illumination, or changing chemical environments. In situ techniques are promising tools to study NP transformations under relevant conditions. Among those tools, in situ transmission electron microscopy (TEM) provides an elegant platform to directly visualize NP changes down to the atomic scale. By the use of specialized holders or microscopes, external stimuli, such as heat, or environments, such as gas and liquids, can be controllably introduced inside the TEM. In addition, TEM is also a valuable tool to determine NP transformations upon ex situ stimuli such as laser excitation. However, standard TEM yields two-dimensional (2D) projection images of 3D objects. With the growing complexity of NP shapes and compositions, the information that is obtained in this manner is often insufficient to understand intricate diffusion dynamics. In this Account, we describe recent progress on measuring NP transformations in 3D inside the electron microscope. First, we discuss existing possibilities to obtain 3D information using either tomographic methods or the so-called atom counting technique, which utilizes single projection images. Next, we show how these techniques can be combined with in situ holders to quantify diffusion processes on a single nanoparticle level. Specifically, we focus on anisotropic metal NPs at elevated temperatures and in varying gas environments. Anisotropic metal NPs are important for plasmonic applications, because sharp tips and edges result in strong electromagnetic field enhancements. By electron tomography, surface diffusion as well as elemental diffusion can be tracked in monometallic and bimetallic NPs, which can then be directly related to changes in plasmonic properties of these systems. By atom counting, it has furthermore become possible to monitor the evolution of crystalline facets of metal NPs under gas and heat treatments, a change that influences catalytic properties. Next to in situ processes, we also demonstrate the value of electron tomography to assess external laser-induced NP transformations, making it viable to detect structural changes with atomic resolution. The application of the proposed methodologies is by far not limited to metal nanopartides. In the final section, we therefore outline future material research that can benefit from tracking NP transformations from 3D techniques.

Publication year:2021

Keywords:A1 Journal article

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Accessibility:Open