depends on how well you understand
the structure. Complex physics calculations are often needed to make full use
of the potential of electron microscopy. An international research team led by
TU Wien's Prof. Peter Schattschneider set out to analyse the opportunities
offered by EFTEM, that is energy-filtered transmission electron microscopy. The
team demonstrated numerically that under certain conditions, it is possible to
obtain clear images of the orbital of each individual electron within an atom.
Electron microscopy can therefore be used to penetrate down to the subatomic
level – experiments in this area are already planned. The study has now been
published in the physics journal Physical Review Letters.
In search of the electron orbital
We often think of atomic electrons
as little spheres that circle around the nucleus of the atom like tiny planets
around a sun. This image is barely reflected in reality, however. The laws of
quantum physics state that the position of an electron cannot be clearly
defined at any given point in time. The electron is effectively smeared across
an area close to the nucleus. The area that could contain the electron is
called the orbital. Although it has been possible to calculate the shape of
these orbitals for a long time, efforts to image them with electron
microscopes have been unsuccessful to date.
"We have calculated how we
might have a chance of visualising orbitals with an electron microscope",
says Stefan Löffler from the University Service Centre for Transmission
Electron Microscopy (USTEM) at TU Wien. "Graphene, which is made of just
one single layer of carbon atoms, is an excellent candidate for this task. The
electron ray is able to pass easily through the graphene with hardly any
elastic scattering. An image of the graphene structure can be created with
these electrons."
Researchers have been aware of the
principle of "energy-filtered transmission electron microscopy" (EFTEM)
for some time. EFTEM can be used to create quite specific visualisations of
certain kinds of atoms whilst blocking out the others. For this reason, it is
often used today to analyse the chemical composition of microscopic samples.
"The electrons shot through the sample can excite the sample's
atoms", explains Stefan Löffler. "This costs energy, so when the
electrons emerging emerge from the sample, they are slower than when they
entered it. This velocity and energy change is characteristic for certain
excitations of electron orbitals within the sample."
After the electrons have passed
through the sample, a magnetic field sorts the electrons by energy. "A
filter is used to block out electrons that aren't of interest: the recorded
image contains only those electrons that carry the desired information."
Defects can be helpful
The team used simulations to
investigate how this technique could help reach a turning point in the study of
electron
orbitals. While doing so, they discovered something that actually
facilitated the imaging of individual orbitals: "The symmetry of the
graphene has to be broken", says Stefan. "If, for instance, there is
a hole in the graphene structure, the atoms right beside this hole have a
slightly different electronic structure, making it possible to image the
orbitals of these atoms. The same thing can happen if a nitrogen atom rather
than a carbon atom is found somewhere in the graphene. When doing this, it's
important to focus on the electrons found within a narrow and precise energy
window, minimise certain aberrations of the electromagnetic lens and, last but
not least, use a first-rate electron microscope." All of these issues can be
overcome, however, as the research group's calculations show.
The Humboldt-Universität zu Berlin,
the Universität Ulm, and McMaster University in Canada also worked alongside
the TU Wien on the study in a joint FWF-DFG project ("Towards orbital
mapping", I543-N20) and a FWF Erwin-Schrödinger project ("EELS at
interfaces", J3732-N27). Ulm is currently developing a new,
high-performance transmission electron microscope that will be
used to put these ideas into practice in the near future. Initial results have
already exceeded expectations.
depends on how well you
understand the structure. Complex physics calculations are often needed
to make full use of the potential of electron microscopy. An
international research team led by TU Wien's Prof. Peter Schattschneider
set out to analyse the opportunities offered by EFTEM, that is
energy-filtered transmission electron microscopy. The team demonstrated
numerically that under certain conditions, it is possible to obtain
clear images of the orbital of each individual electron within an atom.
Electron microscopy can therefore be used to penetrate down to the
subatomic level – experiments in this area are already planned. The
study has now been published in the physics journal Physical Review Letters.
Read more at: http://phys.org/news/2016-07-glimpse-atom.html#jCp
Read more at: http://phys.org/news/2016-07-glimpse-atom.html#jCp
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