Electrons as probes of chiral materials

Smith, Scott Graeme (2017) Electrons as probes of chiral materials. PhD thesis, University of Glasgow.

Abstract

In this work we present electron beam techniques for probing the chiral structure of materials. The motivation for the work lies in two distinct aspects of chiral materials science. In the first case, chiral plasmonic excitations have recently been proposed for use in a variety of sensing technologies but require structural optimisation, in which modern electron microscopy techniques excel. In the second case, the recent development of intrinsically chiral electron ‘vortex beams’ suggests the prospect of being able to discriminate chiral crystal structures directly within the electron microscope. We were keen to explore this prospect because it could overcome a long-standing and fundamental limitation in TEM techniques.
The introduction section of this thesis provides an overview of the essential background
and theory underpinning the research, specifically considering the theory of surface plasmons, the properties of electron beams and a description of crystal chirality, all of which will be used in our work. We give a description of diffraction of electrons in a crystal and show the wavefunctions of electron vortex beams. Results show electron channeling in a simple cubic crystal, highlighting the impact of the position of the beam on the unit cell. When the beam is focused onto an atomic column the electrons are forward scattered along the column, whereas they are scattered outwards when the beam is centered between columns.
In Chapter 2 we discuss the details of data acquisition and post processing techniques, namely Richardson-Lucy deconvolution and non-negative matrix factorisation, which together extract plasmonic modes from EELS data sets. We give a detailed analysis of the plasmonic excitations which exist on the surfaces of both a nanopatterned gold chiral nanoparticle and a hole in a continuous metallic film that also supports localised surface plasmon resonances. We show, using EELS in a scanning transmission electron microscope that we can map the
resonance modes of the structure with high spatial resolution. We confirm the link between
the modes supported by a plasmonic nanoparticle and a hole of similar shape, though we find that 3D roughness has an effect on the energy of the modes, shifting modes by around 0.3eV in some cases. The modes found are chiral leading to chiral fields, which have applications as sensors of biological molecules.
In Chapter 3 we demonstrate the ability to simulate EELS, with accuracy, of realistic
plasmonic nanoparticles with 3D shapes which extends beyond the usual two-dimensional
or idealised simulations of the literature. We study the effects of inevitable manufacturing
and other structural imperfections on the plasmonic response of real patterned structures
and show that they lead to shifts in energy and spatial intensity of the modes due to these defects. We find that structural defects are enough to make so called dark modes (ie. those
that do not have dipole character and are therefore not usually excited by incident photons)
become bright, and shift the energy of modes between similar structures. We also illustrate
the ability to predict the intensity distribution of plasmonic modes on structures using only
symmetry arguments. This type of calculation gives a simple derivation of the modes which
would exist on a plasmonic nanoparticle without requiring a more complicated eigenmode
analysis. Using symmetry terms and irreducible representations the modes which appear on
a nanoparticle can be grouped by symmetry terms, allowing the breaking of symmetry, via
defects, to be better be visualised.
In the latter part of this work we turn to a completely different experiment to consider
the exploitation of newly-discovered vortex electron beams in assigning chirality to crystals
within a simple electron microscopy experiment. With these beams, which posses a orbital
angular momentum, will show that is possible (in favourable cases) to detect the handedness
of the crystal via a difference in diffraction pattern intensity distribution for beams of opposite OAM, when scattering in opposite handed crystals. Our work demonstrates the ease of assigning chirality using convergent electron vortex diffraction for a crystals with a threefold, fourfold and sixfold screw axis. We present this work using modified multislice simulations of the diffraction of vortex electron beams from chiral potential layers. We demonstrate that the azimuthal phase component of electron vortex beams opens up new opportunities for rapid chiral discrimination and structural studies in electron diffraction. We show that the symmetry of the resulting convergent beam patterns matches the point symmetry of the crystal only when the handedness of both the impinging vortex probe and chirality of the crystal are congruent. This methodology was tested on the real crystal structures of α-quartz and the magnetic crystal Cr1/3NbS2. It was found that effects (due to the matching of the beam and crystal chirality) are most obvious in the overlap of diffraction disks, where interference effects allow the novel phase profile of vortex beams to produce intensity variations. The thickness of the crystal, the convergence angle and the energy of the beam were all parameters which need to be tuned in order to achieve chiral specific scattering. In thinner crystals the effects were less obvious, requiring a deduction in the beam energy in order to see a difference between CBED patterns of enantiomers. In cases where the convergence angle did not lead to overlapping disks in the CBED pattern the effects of chiral specific scattering were not obvious. Future work should include an experimental check of the feasibility of observing such weak diffraction effects in an electron microscope. Should this prove possible, this method could be of use to to discriminate chiral crystal structures directly within the electron microscope using only one set of diffraction patterns.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Electron microscopy, electron vortex beams, electron energy loss spectroscopy, chiral materials, multislice, convergent beam electron diffraction, plasmons.
• Subjects: Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Funder's Name: Engineering and Physical Sciences Research Council (EPSRC)
• Supervisor's Name: Stamps, Professor Robert L. and MacLaren, Dr. Donald A.
• Date of Award: 2017
• Depositing User: Mr Scott Graeme Smith
• Unique ID: glathesis:2017-8580
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 06 Nov 2017 08:30
• Last Modified: 15 Nov 2017 13:59
• URI: https://theses.gla.ac.uk/id/eprint/8580