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Broadband vibrational sum frequency spectroscopy (VSFS) of modified graphene and polymeric thin filmsHolroyd, Chloe January 2017 (has links)
The surface-specific technique of vibrational sum frequency spectroscopy (VSFS) can provide vibrational information about chemical bonds at surfaces and interfaces. Two photons, of visible and infrared frequency, are spatially and temporally overlapped at a surface/interface to produce a photon at the sum frequency (SF) of the two input photons. As well as this process only being allowed in non-centrosymmetric media (i.e. VSFS is surface/interface specific), the SF process is enhanced when the IR beam is resonant with vibrational resonances. Broadband VSFS has been used in this project to study surfaces of two distinct classes of materials, namely graphene and polymers. Firstly, broadband VSFS was used to investigate the heating polymeric thin films using a home-built heated sample cell. The cell was tested using self-assembled monolayers (SAMs) of 1-octadecanethiol (ODT) grown on gold substrates. It was subsequently used to investigate thin films of poly(methyl methacrylate) (PMMA) of four different thicknesses and two different molecular weights that were spin-coated onto gold substrates. It was shown that the monolayers of ODT become disordered upon heating and solidified to incorporate the disorder introduced by the heating process. The PMMA films were also shown to become more disordered as a function of temperature. Secondly, broadband VSFS was used to investigate modified graphene, motivated by the fact that modifications to pristine graphene, be it intentional (i.e. functionalisation) or unintentional (i.e. contamination), cause the properties of graphene to change. This project focused on studying hydrogenated graphene, N-methylbenzamide functionalised graphene and contamination on commercial graphene. A method for calculating the number of hydrogen atoms in a hydrogen island was developed. VSF spectra of CH stretches in N-methylbenzamide functionalised graphene were obtained. Residues on commercially bought graphene were detected using VSFS and RAIRS. These residues were assigned to PMMA that remained on the CVD graphene by the process of transferring the CVD graphene from the copper foil on which it was grown onto the gold substrates.
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Vodíkem modifikované grafenové struktury pro polem řízené tranzistory / The hydrogen modification of the graphene structures for field effect transistorsKurfürstová, Markéta January 2016 (has links)
This master’s thesis is focused on the subject of graphene modified with atomic hydrogen and its electronic transport properties. Structural and electronic properties of graphene and hydrogenated graphene are compared in the theoretical part of the thesis. The Raman spectroscopy technique is described, including characterization of typical Raman spectra of both unmodified and modified graphene. Samples used during experimental part of the thesis are prepared via laser and electron lithography, and are set to be measured in a vacuum chamber. Subsequently, electronic transport properties are measured before and after hydrogen modification of graphene. Finally, hydrogenated graphene is irradiated using electron beam and changes in its structure are analyzed with Raman spectroscopy techniques.
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Étude par imagerie Raman du dopage d’échantillons de graphène hydrogénéGodbout, Émile 08 1900 (has links)
Depuis une vingtaine d’années, le graphène est étudié à travers le monde pour ses propriétés
opto-électroniques remarquables. Malgré tous ces efforts et la simplicité apparente de ce feuillet
monoatomique de carbone, sa physique subtile continue de surprendre et reste à découvrir.
Cette étude exploratoire vise à évaluer l’effet du dopage et de l’hydrogénation sur le spectre
Raman du graphène afin de mieux comprendre les propriétés électroniques sous-jacentes. Pour ce
faire, on utilise le RIMA, un imageur Raman hyperspectral qui se distingue des montages Raman
traditionnels par sa capacité à produire rapidement des cartes Raman d’une centaine de microns
de côté, ce qui permet de résoudre spatialement les propriétés de l’échantillon en plus d’avoir un
nombre statistique de spectres.
Les échantillons sont produits intégralement dans nos laboratoires et chaque procédé est
contrôlé et détaillé dans ce mémoire. Le graphène est synthétisé par dépôt chimique en phase
vapeur (Chemical Vapor Deposition, CVD) puis exposé à un faisceau d’hydrogène atomique à
haute température pour former des liens C-H sur la surface. Le dopage est généré et contrôlé
en immergeant simplement l’échantillon dans une solution de pH variable en ayant préalablement
déposé des nanoparticules de platine à sa surface. L’équilibre chimique impliquant le couple rédox
Pt/PtO permet de fixer son énergie à un pH donné et d’effectuer un transfert de charge efficace
avec le graphène. On obtient ainsi un dopage ajustable, allant d’un fort dopage p à un faible dopage
n.
Nos résultats révèlent la présence d’un mécanisme de dégradation inattendu relié à l’exposition
continue au laser qui suggère une migration de l’hydrogène à la surface pour se concentrer dans la
région irradiée. L’évolution des propriétés optiques laisse croire qu’on atteint une densité suffisante
d’hydrogène pour modifier la structure de bandes du graphène et le rendre significativement
semi-conducteur. Les cartes Raman ont aussi révélé que l’hydrogénation ne semble pas homogène à
l’échelle de nos mesures. La densité de défauts a été quantifiée avec deux méthodes différentes qui
sont généralement en accord.
Au niveau du dopage, notre méthode ne semble pas produire un transfert de charges aussi
important que prévu par la loi de Nernst, ce qui pourrait être expliqué par un mauvais contact
entre le platine et le graphène. Par contre, on observe en général les tendances prévues dans la
littérature, mais avec un décalage en énergie qui pourrait être expliqué par une augmentation du
travail de sortie du graphène de 100-200 meV après hydrogénation. / For the past twenty years, graphene has been studied worldwide for its remarkable optoelectronic
properties. Despite all these efforts and the apparent simplicity of this monoatomic sheet of
carbon, its subtle physics continues to surprise and remains to be discovered.
The aim of this exploratory study is to assess the effect of doping and hydrogenation on the
Raman spectrum of graphene, in order to better understand the underlying electronic properties.
To do this, we are using RIMA, a hyperspectral Raman imager that differs from traditional Raman
setups in its ability to rapidly produce Raman maps of around 100 microns on a side, enabling us
to spatially resolve the properties of the sample in addition to having a statistical number of spectra.
The samples are produced entirely in our laboratories, and each process is controlled and detailed
in this thesis. Graphene is synthesized by chemical vapor deposition (CVD), then exposed to a
high-temperature atomic hydrogen beam to form C-H bonds on the surface. Doping is generated
and controlled simply by immersing the sample in a solution of variable pH, having previously deposited
platinum nanoparticles on its surface. The chemical equilibrium involving the redox couple
Pt/PtO enables its energy to be fixed for a given pH and an efficient charge transfer to take place
with the graphene. This results in an adjustable doping, ranging from high p-doping to low n-doping.
Our results reveal the presence of an unexpected degradation mechanism linked to continuous
laser exposure, suggesting hydrogen migration across the surface to concentrate in the irradiated
region. The evolution of optical properties suggests that a sufficient density of hydrogen is reached
to modify the band structure of graphene and render it significantly semiconducting. Raman
maps also revealed that hydrogenation does not appear to be homogeneous at the scale of our
measurements. Defect density was quantified using two different methods which are in general
agreement.
In terms of doping, our method does not seem to produce as much charge transfer as predicted
by Nernst’s law, which could be explained by poor contact between platinum and graphene. On the
other hand, we generally observe the trends predicted in the literature, but with an energy shift that
could be explained by an increase in graphene work function of 100-200 meV after hydrogenation.
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Electron energy loss spectroscopy of graphene and boron nitride with impurities or defects in the transmission electron microscopePan, Cheng-Ta January 2014 (has links)
The two-dimensional material graphene possesses many impressive properties such asextraordinary carrier mobility, mechanical stiffness and optical transmittance. However,the properties of pristine graphene do not always complement the requirements of applications. Of particular interest, a band gap is needed for electronic logic devices. Recent research shows that using few-layer hexagonal boron nitride as a substrate for graphene-based electronic devices can open a band gap in graphene. Also, introducing impurities such as hydrogen atoms, transition metals or silicon atoms on or within graphene can control the electronic properties according to recent studies. Furthermore, ion irradiation is an alternative option to tailor the properties of graphene by introducing defects. In this thesis, pristine, impure or defective graphene and few-layer boron nitride were characterised by scanning transmission electron microscopy (STEM) and electron energy loss (EEL) spectroscopy. Through STEM and EEL spectroscopy, lattice structures and electronic properties of these two-dimensional materials could be investigated at the atomic scale. This thesis focuses on the frontier studies of theoretical and experimental EEL spectroscopy of graphene and few-layer boron nitride with impurities. In the EEL spectra of pristine graphene and boron nitride two prominent peaks were observed, which are attributed to the plasmon excitations of π- and π+σ-electrons. By introducing impurities such as hydrogen adatoms on graphene and substitutional oxygen and carbon atoms within single-layer boron nitride, our experimental and simulated EEL spectra show that their π-plasmon peaks are modified. The concentrations of these impurities were then evaluated via EEL spectra and atomic-resolution images. If other impurities, such as various transition metals and silicon atoms, are introduced on or within single-layer graphene, our simulated EEL spectra demonstrate that the geometry of these impurity clusters affects the π-plasmon peak in graphene and some impurities even enhance it. Finally, experiments on in-situ transmission electron microscopy and ex-situ STEM and Raman spectroscopy were conducted to investigate ion irradiated graphene. Many topological defects were, for the first time, observed in atomic-resolution STEM images of ion irradiated graphene. Simulated EEL spectra of defective graphene are also reported, which suggests that corrugations and dangling bonds in defects can modify out-of-plane EEL spectra and introduce intraband transitions, respectively.
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