Advanced electron tomography of nanoparticle assemblies

Nanoparticle assemblies have attracted enormous scientific interest during the last years, due to their unique properties compared to those of their building blocks. To understand the origin of these properties and to establish the connection with their structure, a detailed and quantitative structural characterization is essential. Transmission electron microscopy has been widely used to investigate nano-assemblies. However, TEM images only correspond to a twodimensional projection of a three-dimensional object. Therefore, in order to obtain the necessary 3D structural information electron tomography has to be applied. By means of advanced electron tomography, both qualitative and quantitative information can be obtained, which can be used for detailed theoretical studies. focus article Copyright c © EPLA, 2017 Introduction. – Assemblies of nanoparticles have recently attracted increasing interest due to their improved properties compared to those of their building blocks. By varying the size and the shape of the nanoparticles as well as the synthesis parameters, assemblies with unique configurations can be obtained, yielding applications in different scientific fields including plasmonics [1], signal enhancement [2,3], sensoric [4], catalysis [5,6] and data storage [7,8]. Although the behaviour of such assemblies is empirically understood, a thorough insight into the structure-property connection is often still lacking. A detailed structural characterization is therefore of utmost importance. Transmission electron microscopy (TEM) is a wellknown technique to characterize materials at the nanometre scale and below. However, it conventionally only allows for the acquisition of 2D projections of 3D objects, which is not sufficient for a quantitative characterization of complex 3D nanostructures. To overcome this limitation, electron tomography, a technique during which 2D projections are acquired over a large tilt range and combined through the use of a mathematical reconstruction algorithm, has been developed [9]. Over the last decades, electron tomography has developed (a)Contribution to the Focus Issue Self-assemblies of Inorganic and Organic Nanomaterials edited by Marie-Paule Pileni. (b)E-mail: [email protected] into a powerful characterization tool that has been widely used in the field of materials science. Mostly, electron tomography is based on high-angle annulardark-field scanning transmission electron microscopy (HAADF-STEM) [10]. Using HAADF-STEM, the image intensity scales with the thickness of the samples and with the atomic number Z of the elements that are present [11]. In this manner, the morphology of a broad range of nanomaterials has been investigated. Tomography has furthermore been combined with spectroscopic techniques such as electron energy loss spectroscopy (EELS) [12–14] and energy dispersive X-ray spectroscopy (EDS) [15–18], which enabled the 3D investigation of chemical composition, bonding nature and surface plasmons of nanomaterials. In addition, great effort was made to develop advanced reconstruction algorithms, enabling quantification of the 3D results and pushing the resolution of the technique to the atomic scale [19–22]. Electron tomography is nowadays also a standard technique for the characterization of nano-assemblies, yielding a description of the morphology and inner structure (fig. 1) [23–27]. In this perspective, we will provide an overview of the latest progress and the future challenges in the field of the 3D characterization of nano-assemblies using electron tomography. Indeed, one of the current goals is to investigate more complex or larger assemblies of nanoparticles in a quantitative manner. A quantitative description of the assemblies is required to determine