Cardillo-Zallo, Ian T. B.
(2025)
Imaging and analysis of single atoms and molecules by transmission electron microscopy.
PhD thesis, University of Nottingham.
Abstract
The transmission electron microscope (TEM) is a powerful tool that can resolve the positions of individual atoms within a material. Measurement of the position and motion of single atoms via TEM imaging and spectroscopy provides understanding of how matter behaves at the fundamental level, without relying on bulk measurements averaged over many particles. Further, the high energy electron beam can initiate chemical transformations within the microscope, which can be imaged in real time with high spatial resolution.
In this thesis, the TEM is used to study the behaviour of individual atoms of the noble gases (Ng) krypton, argon and neon. Ng atoms were inserted into the cavities of fullerene cages, which were in turn encapsulated by single-walled carbon nanotubes (SWCNT), to form electron transparent materials that are sufficiently stable under electron irradiation, and hence suitable for single atom studies within the TEM. The resultant linear “endohedral peapods” were thoroughly characterised by TEM imaging and spectroscopy, including the properties of individual Ng atoms such as scattering strength and motion within the fullerene cage. Controlled energy transfer from the electron beam was applied to selectively coalesce fullerene molecules within SWCNT to form 2Ng@C120 species, within which the bonding states of Ng2 dimers were probed via time-resolved TEM imaging. Application of Lennard-Jones empirical potential models to these systems was used to rationalise observed atomic behaviour, including van der Waals bonding. The behaviour of different Ng atoms could be discriminated by their relative atomic sizes, masses and polarisabilities. Application of heat to noble gas endohedral peapods led to the formation of “nested nanotube” containers. Within these containers, guest Ng atoms became able to translate along a single axis, and hence transitioned to a state similar to a one-dimensional gas. Cooling of this gas in the TEM revealed a reversible transition between gaseous and atomic chain states, akin to a 1D thermal phase transition.
A variety of analysis methods are utilised for imaging and chemical identification, including bright field high-resolution TEM (BF-HRTEM) imaging, annular dark field scanning TEM (ADF-STEM) imaging, energy dispersive X-ray
spectroscopy (EDS), STEM electron energy loss spectroscopic (STEM-EELS) mapping, and cryogenic TEM (cryo-TEM) imaging. Bulk analysis by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) also provided valuable information on the structure of the host SWCNT and guest Ng atoms, respectively.
This work advances our understanding of single atom behaviour and of electron beam-matter interactions, including dispersion interactions, bond formation and breaking, and the behaviour of dimensionally constrained matter.
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