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dc.contributor.author
Bertolini, Gabriele
dc.contributor.supervisor
Pescia, Danilo
dc.contributor.supervisor
Gürlü, Oğuzhan
dc.contributor.supervisor
Xanthakis, J.P.
dc.contributor.supervisor
Vaterlaus, Andreas
dc.date.accessioned
2021-03-29T07:04:48Z
dc.date.available
2021-03-28T12:07:15Z
dc.date.available
2021-03-29T07:04:48Z
dc.date.issued
2020
dc.identifier.uri
http://hdl.handle.net/20.500.11850/476660
dc.identifier.doi
10.3929/ethz-b-000476660
dc.description.abstract
In the Fowler-Nordheim (FN or field-emission) regime of Scanning Tunneling Microscopy (STM), the tip-target distance is from a few to tens of nanometers. In this situation, the quantum mechanical tunneling of electrons between tip and target is entirely suppressed. Instead, electrons can be field-emitted out of the tip to the vacuum by a process first described at the dawn of quantum mechanics by Fowler and Nordheim. Their energy is typically in the range of a few tens of electronvolts. They build the primary beam in this imaging technology that we call SFEM – standing for Scanning Field Emission Microscopy – to distinguish it from STM. The primary beam’s impact on the target leads to the excitation of electrons off the surface. Under favorable electrostatic circumstances, the excited electrons escape the tip-target junction and build a new electronic system of secondary electrons (SE), absent in the tunneling regime of STM. A dissertation by L. De Pietro [1] discovered that secondary electrons are spin-polarized [2]. D. Zanin [3] recorded secondary electrons’ energy spectra characterized mainly by two peaks. The former corresponding to the elastically scattered electrons. The latter corresponding to the ”true” secondary electrons, i.e., those with kinetic energy just above the vacuum level at the sample, resulting from the cascade of inelastically scattering processes. A clear signature for the appearance of characteristic energy losses or energy gains was missing in Zanin’s dissertation. In SFEM, not only scattering processes such as those leading to the production of elastic and inelastic electrons are relevant, but also the behavior of the electrons excited off from the surface in the presence of the strong electric field existing in the vicinity of the tip apex must be taken into account. As a consequence of this complexity, the mechanism of contrast formation and spatial resolution in images acquired by SFEM has yet to be deciphered. This Dissertation reports experimental results that shed light on the physical processes involved in primary electrons and secondary SFEM imaging, performed with and without energy resolution. In particular The standard view of the current-voltage characteristic measured in field emission foresees a diode-like law that reflects the existence of a potential barrier surrounding the tip through which electrons can iv tunnel. Such a barrier depends on the work function of the tip and the electric field at the tip apex. Here we observe a yet undetected dependence of the field emitted (primary) current from the material residing on the target side separated several nanometers from the tip. The contrast, observed upon imaging dual systems such as domains of W and Carbidic-W (WC) on a W(110)-surface, is about one hundred times larger than the contrast one expects from the vertical surface corrugation and about ten times larger than the contrast expected from the work function differences between W and WC. Other dual systems (such as W versus Fe/W and W versus Fe/WC or W versus Au/W) show a similar contrast in the primary electron channel of detection. This dissertation records similar figures of contrast and lateral spatial resolution along the various channels available in SFEM-imaging, i.e., absorbed current imaging, emitted current imaging, total electron imaging, and energy-resolved SFEM images, as demonstrated by imaging of surfaces with dual systems, including semiconducting Ge versus Au-covered Ge. These systematic observations suggest SFEM as a suitable spectro-microscopy technology not only for metallic surface investigation but also for semiconductor surfaces. We observe a clear signature for characteristic energy gains within the ”sea” of inelastically scattered electrons residing between the ”true” secondary and the elastically scattered electrons. We are able to assign one specific energy gain (at about 22 eV) to those electrons that are excited upon the decay of a W-plasmon. In virtue of this result, SFEM is established as a technology that can be used to detect energy-resolved features with nanoscale spatial resolution. Almost all electrons field-emitted from the tip are absorbed by the target, and only less than 1% typically escape the tip-sample junction. This dissertation detects for the first time minute but significant difference between field-emitted current and absorbed current at selected voltages, opening the possibility to performing spectro-microscopy directly in the primary beam channel, thus avoiding the necessity of detecting any SE.
en_US
dc.format
application/pdf
en_US
dc.language.iso
en
en_US
dc.publisher
ETH Zurich
en_US
dc.rights.uri
http://rightsstatements.org/page/InC-NC/1.0/
dc.subject
Microscopy
en_US
dc.subject
Spectroscopy
en_US
dc.subject
Field emission
en_US
dc.subject
Scanning tunneling microscopy (STM)
en_US
dc.subject
Low Energy Electron microscopy
en_US
dc.title
Spectro-microscopy in the field emission regime of scanning tunneling microscopy
en_US
dc.type
Doctoral Thesis
dc.rights.license
In Copyright - Non-Commercial Use Permitted
dc.date.published
2021-03-29
ethz.size
140 p.
en_US
ethz.code.ddc
DDC - DDC::5 - Science::530 - Physics
en_US
ethz.identifier.diss
27240
en_US
ethz.publication.place
Zurich
en_US
ethz.publication.status
published
en_US
ethz.leitzahl
ETH Zürich::00002 - ETH Zürich::00012 - Lehre und Forschung::00007 - Departemente::02010 - Dep. Physik / Dep. of Physics::02505 - Laboratorium für Festkörperphysik / Laboratory for Solid State Physics::03351 - Pescia, Danilo (emeritus) / Pescia, Danilo (emeritus)
en_US
ethz.date.deposited
2021-03-28T12:07:24Z
ethz.source
FORM
ethz.eth
yes
en_US
ethz.availability
Open access
en_US
ethz.rosetta.installDate
2021-03-29T07:05:00Z
ethz.rosetta.lastUpdated
2022-03-29T06:05:06Z
ethz.rosetta.versionExported
true
ethz.COinS
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