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In this chapter, recent advances in scanning/transmission electron microscopic techniques and their applications to nanomaterials are highlighted. Aberration corrected electron microscopy is now routinely employed to study carbon nanomaterials, nanoparticles, 2D materials, alloys and heterostructures, etc. However, there have been several significant achievements in employing this technique towards the study of the various nanostructures. Aberration corrected TEM/STEM imaging tends to be very successful in identifying new nanoalloys, enables understanding of the growth mechanisms as well as the behaviour of supported atoms/clusters towards catalytic reactions, etc. HAADF-STEM imaging in combination with spectroscopy is an essential tool for the characterization of new 2D materials, such as borophenes and phosphorenes. At the same time, it provides insights into the structure of hetero-interfaces as well as enables identifying defects and new phases associated with these materials. It is now possible to go down in accelerating voltages upto 15 kV in both TEM/STEM modes and image specimens with minimal damage. It is foreseen, that these new developments in advanced electron microscopic techniques will provide significant possibilities for unravelling new phenomena, as well as explore novel properties of nanomaterials enabling new applications.

CdSe nanowires (CdSe-NWs) at large scale were obtained through a simple and clean method. The reaction was carried out without complexing agents which often facilitate the preferential nanostructure growth. In this work, CdSe-NWs with lineal and zigzag shape around 40 and 130 nm in diameter with a wurtzite-type structure were synthesized. Quantitative EDX analysis indicated the stoichiometric formation of CdSe, while both HRTEM and HAADF-STEM analysis ruled out the presence of other phases such as CdO or nanotubes formation, respectively. Diameter and length of the nanowires varied with the reaction time and temperature. This synthesis method makes the nanostructures purification process easier and also is non-toxic and its high conversion make it an efficient method for obtaining CdSe-NWs.

In the present manuscript we present the feasibility of two different routes to obtain metal nanoparticles: (1) inert gas deposition technique (IGC) and (2) chemical routes. These two routes can enable the production of metal nanoparticles with a well-controlled size and shape distribution. In addition, Co/Au and Pd/Au nanoparticles have been deposited on three different grids with the aim of studying the coalescence of the different metals. The as-synthesized materials, that initially were forming isolated, core-shell, were sintered by thermal treatment (Co/Au) or by strong electron beam irradiation and the subsequent characterization was carried out in situ in an aberration corrected STEM.

Bimetallic nanoparticles of Au–Pd find important applications in catalysis. Their catalytic performance is directly related to the structure, alloy formation and variation of composition in the structure. A standard idea is that bimetallic nanoparticles can be either an alloy or a core shell structure. Our group has investigated the structure and composition of Pd–Au nanoparticles by using aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF–STEM). We reported previously that the nanoparticles are composed of an evenly alloyed inner core, an Au-rich intermediate layer, and a Pd-rich outer shell. The structure is more complicated than what simple models can predict. In this paper, we report additional studies of this system wherein by carrying out spectral and chemical analysis (STEM–EDS and STEM–EELS) the interface structure can now be better identified and understood. Apart from the three-layered core-shell structures, we have also been able to observe in some cases a four-layered core-shell structure as well. The entire core-shell structure is not rigid and there is indeed intercalation of Au–Pd into the other layers as well. In addition we have been able to locate stacking faults present in the nanoparticles. We also address the problem of the interface structure between the layers. By using nanodiffraction we have found that the whole structure of the nanoparticles becomes hcp in contrast to the bulk structure of Au or Pd.

The first results from our group that have been obtained using the newly installed aberration-corrected STEM microscope (JEOL JEMARM200F) are reported. Studies were carried out on noble metal nanoparticles and their corresponding alloys and/or core-shell structures. In this paper we focus on some of the exciting areas of research that have been under investigation in our group. These include studies on Au nanoparticles and core-shell Au-Pd nanoparticles. In addition, studies that were carried out on very small clusters namely Pd-Ir and other similar systems have also been high-lighted in this report.