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@PHDTHESIS{Wirth:1009726,
author = {Wirth, Konstantin Georg},
othercontributors = {Taubner, Thomas and Dittmann, Regina},
title = {{N}ear-field optical characterization of few-layer graphene
and memristive {T}a$_2${O}$_5$thin film devices},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2025-03644},
pages = {1 Online-Ressource : Illustrationen},
year = {2025},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2025},
abstract = {Classical light microscopy is fundamentally limited by the
diffraction limit, which restricts the minimal resolvable
distance between two objects to roughly half of the
wavelength of light. For visible light (around 500~nm
wavelength), this limits resolution to approximately 250~nm.
These limitations are even more pronounced in the infrared
regime, with wavelengths typically exceeding $1~\textmu$ m.
Scattering-type scanning near-field optical microscopy
(s-SNOM) overcomes this limit, achieving
wavelength-independent resolution down to 20~nm through
optical near-field interactions between a sharp tip and a
sample. This technique provides access to optical contrast
mechanisms at the nanoscale, making it ideal for
investigating structures larger than atomic scales but far
below the diffraction limit.In this thesis, sub-diffraction
limit structures are investigated with s-SNOM, focusing on
two key areas in the basic research for novel materials for
advanced electronic devices.The first part is focused on the
characterization of few-layer graphene (FLG) in
collaboration with partners from the Aachen Graphene $\\&$
2D Materials Center. The second part of optical
investigations of resistive switching devices was part of
the Sonderforschungsbereich 917 "Nanoswitches", whose goal
was to understand resistive switching phenomena in oxides
and chalcogenides. The number of graphene layers and their
orientation to each other greatly influences the electronic
and optical properties of FLG. Characterizing the stacking
order with infrared light between 0.2 and 0.9~eV gives
access to characteristic absorption peaks around interband
transitions, which are directly linked to the electronic
structure. So far, the lack of tunable laser sources in the
spectral range of the interband transitions inhibited the
characterization of the stacking order on the nanoscale with
s-SNOM. Here, a new tunable light source for s-SNOM is
utilized to investigate the electronic properties and
stacking order of FLG. A proof-of-principle experiment for
s-SNOM spectroscopy at the interband transitions is
performed in the spectral range between 0.28 and 0.54~eV to
study the interband transitions in bilayer graphene (BLG).By
tuning the laser system in the regime of interband
transitions of BLG, we successfully extract the amplitude
and phase of scattered light, enabling the reconstruction of
the complex optical conductivity resonance around
0.39~eV.Subsequently, s-SNOM spectroscopy at the interband
transitions is extended to characterize the stacking order
in FLG by applying it to tetralayer graphene (4LG).By
analyzing the characteristic interband transition
contributions in the optical conductivity, we distinguish
between stacking sequences, including rhombohedral (ABCA)
and Bernal (ABAB) configurations. The approach enables us to
identify and characterize domains of ABCB stacked 4LG, a
configuration previously considered unstable, for the first
time. The observation of ABCB stacking is verified by Raman
and infrared spectroscopy. S-SNOM spectroscopy at the
interband transitions paves the way for nanoscopic
non-contact measurements of the electronic properties in
complex hybrid 2D- and van-der-Waals material systems. Our
results establish s-SNOM spectroscopy at the interband
transitions as a semi-quantitative tool to assign stacking
orders in FLG, even of previously unobserved ones. In the
second part, s-SNOM is used to investigate oxide-based
resistive switching devices. These devices rely on the
formation of nanoscale conductive paths, known as filaments,
which are crucial for device performance characteristics
such as cycle-to-cycle variability,
$R\textsubscript{off}/R\textsubscript{on}$ ratio, and
endurance. Traditional techniques like conductive AFM or TEM
require delaminating the metal top electrode, aggravating
in-operando investigations. In contrast, s-SNOM allows for
the non-invasive characterization of individual filaments in
$Ta\textsubscript{2}O\textsubscript{5}$ thin films by
integrating a transparent graphene top electrode.By
selecting appropriate illumination frequencies, we can
simultaneously trace filaments' evolution and device
morphology changes over several switching cycles.
Investigating filaments in oxides in the infrared regime
promises a deeper understanding of resistive switching
devices' microscopic behavior. S-SNOM applies to a wide
range of resistive switching oxides, such as
$HfO\textsubscript{2},$ $SrTiO\textsubscript{3},$ and
$SiO\textsubscript{2}.$ Spectroscopy at the interband
transitions in FLG and the in-situ characterization of
memristive device with a transparent top electrode highlight
the power of s-SNOM in the infrared regime to resolve and
characterize structures well below the diffraction limit.
The two parts demonstrate s-SNOM's capability for
nanoscopic, non-contact measurements of electronic
properties in complex layered material systems. The
developed approaches pave the way for future research into
graphene-based materials and resistive switching devices,
potentially facilitating the easier characterization of
these material systems on the nanoscale.},
cin = {131110 / 136720 / 130000},
ddc = {530},
cid = {$I:(DE-82)131110_20140620$ / $I:(DE-82)136720_20140620$ /
$I:(DE-82)130000_20140620$},
pnm = {SFB 917 B05 - Untersuchung des Einflusses von Defekten auf
die Ladungsträgereigenschaften beim resistiven Schalten mit
Nahfeld-Mikroskopie und -Spektroskopie (B05) (202267494) /
SFB 917: Resistiv schaltende Chalkogenide für zukünftige
Elektronikanwendungen: Struktur, Kinetik und
Bauelementskalierung "Nanoswitches" (167917811)},
pid = {G:(GEPRIS)202267494 / G:(GEPRIS)167917811},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2025-03644},
url = {https://publications.rwth-aachen.de/record/1009726},
}
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