Chirality Determination of Nanocrystals by Electron Crystallography

Abstract

Chirality is a common phenomenon in nature and plays an important role in the properties of matter. The rational synthesis of chiral compounds and exploration of their applications in various fields require an unambiguous determination of their handedness. However, in many cases, determinations of the chiral crystal structure and chiral morphology have been a challenging task due to the lack of proper characterization methods, especially for nanosized crystals. Therefore, it is crucial to develop novel and efficient characterization methods. Owing to the strong interactions between matter and electrons, electron crystallography has become a powerful tool for structural analysis of nanomaterials. In recent years, methods based on electron crystallography, such as high-resolution electron microscopy imaging and electron diffraction, have been developed to unravel the chirality of nanomaterials. This brings new opportunities to the design, synthesis, and applications of versatile chiral nanomaterials. In this perspective, we summarize the recent methodology developments and ongoing research of electron crystallography for chiral structure and morphology determination of nanocrystals, including inorganic and organic materials, as well as highlight the potential and further improvement of these methods in the future.C hirality is a geometric property of a rigid object being nonsuperposable on its mirror image. 1 In nature, chirality is ubiquitous at multiple levels, from subatomic particles, organic molecules such as amino acids, macromolecules including proteins and DNA, macroscopic entities like sea shells, to even spiral galaxies. 2 Chirality has attracted great interest from multidisciplinary fields, including chemistry, biology, pharmacy, and materials science. 3 The Nobel Prize in Chemistry has recognized outstanding research in chiral synthesis and asymmetric catalysis twice in the past two decades. 4,5 In the pharmaceutical industry, more than half of clinical drugs belong to chiral molecules. 6 Moreover, chiral materials exhibit a wide range of exploitable properties, including optical behavior, enantioselectivity, ferroelectricity, electrochemical properties, and biological activity. 7−10 These cumulative factors highlight the significance of studying chirality in various scientific fields.With the rapid development of synthesis and applications of chiral compounds, one important and challenging task becomes prominent: how to effectively characterize the handedness of chiral materials. One of the difficulties is that two enantiomers usually exhibit similar physical and chemical properties, which makes it not easy to distinguish them using common characterization methods. Chirality of nanocrystals can be divided into two categories: chiral morphology and chiral crystal structure (Figure 1). Chiral morphology is a macroscopic property of a material. For example, some chiral materials show a special morphology (such as a helical shape). However, when we talk about chiral crystal structure, it means that the spa...