Abstract
Chiral features in metal nanoparticles (NP) result in chiroptical properties, of interest to many applications, such as enantioselective catalysis or separation, chiral sensing or drug delivery. These applications arise from the different interactions of chiral plasmonic NPs with left- and right-handed circularly polarized light (CPL). Therefore, much effort has been put into the development of NPs with complex structures and morphologies. The properties of such nanomaterials are usually characterized by measuring their circular dichroism (CD), which quantifies the interaction of an ensemble of particles with CPL. Multiple factors can be at the origin of the recorded CD signal, such as a helical morphology, chiral features in the crystalline structure, or the presence of chiral molecules at or near the surface of the chiral NPs. Unfortunately, there is no consensus on the relative importance of these different aspects. To obtain NPs with tailored chiroptical properties, it is thus important to understand the connection between the CD signal and the NP morphology.
Transmission electron microscopy (TEM) is an excellent technique to investigate the structure of nanomaterials. Several approaches, within the field of expertise of EMAT, can be used to investigate the morphology, structure or properties of chiral nanoparticles. Examples include 3D structural characterization by scanning electron microscopy (SEM), electron tomography in real and reciprocal space and electron beam induced current measurements (SEEBIC). Moreover, there is still debate in the field on the application of electron energy loss spectroscopy (EELS) to measure chiral features of nanoparticles. Although these techniques are already being developed at EMAT in the framework of other projects, their combined application to chiral nanoparticles will require further development. For example, identification of chiral surface facets is currently impossible due to a lack of 3D resolution by electron tomography or SEEBIC. Visualisation of chiral molecules and micelles that lie at the origin of the growth of chiral nanoparticles is another challenge that will require the development of low-dose imaging techniques, ideally combined with in situ TEM to generate a relevant, liquid, environment. Moreover, quantification procedures, e.g. to define the degree of helicity will need to be developed. Such computational techniques, eventually based on the use of training a neural network as well as modeling of the connection between structure and properties are challenging aspects but the knowhow to help talented postdocs to overcome current limitations is available at EMAT.
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