This thesis investigates the application of low energy electron irradiation to 1,3,5-trimethylbenzene films to test whether site-selective C–H bond cleavage can be achieved in a molecule presenting both aliphatic and aromatic target sites. IRRAS was used to characterize the orientation of vapour-deposited 1,3,5-trimethylbenzene on Au(111) and alkanethiolated gold under UHV and cryogenic conditions. On both substrates, the disordered as-deposited films were transformed by thermal treatment, producing two film structures – within the first 4-monolayers, aromatic rings lie nearly parallel to the metal surface, while molecules in additional layers are more upright. Low energy electron irradiation (0-10 eV) produced no dissociations in 1,3,5-trimethylbenzene; however, low energy electron transmission spectra indicate charge accumulates at interstitial sites in the mesitylene film, decelerating subsequent incident electrons. These decelerated electrons traverse the charged film and are reaccelerated, inducing dissociations in the underlying SAM. Contrary to literature claims, 1,3,5-trimethylbezene is a poor molecule for adsorption studies such as BET as the initial adsorption is disordered and thermodynamically unstable.

We investigate the conditions under which electron irradiation of single-walled carbon nanotube (SWCNT) bundles with 2 keV electrons produces an increase in the Raman D peak. We find that an increase in the D peak does not occur when SWCNTs are preheated in situ at 600 C for 1 h in ultrahigh vacuum (UHV) before irradiation is performed. Exposing SWCNTs to air or other gases after preheating in UHV and before irradiation results in an increase in the D peak. Small diameter SWCNTs that are not preheated or preheated and exposed to air show a significant increase in the D and G bands after irradiation. X-ray photoelectron spectroscopy shows no chemical shifts in the C1s peak of SWCNTs that have been irradiated versus SWCNTs that have not been irradiated, suggesting that the increase in the D peak is not due to chemisorption of adsorbates on the nanotubes.

Ion irradiation of carbon nanotubes is a tool that can be used to achieve modification of the structure. Irradiation stability of carbon nanotubes was studied by ion and electron bombardment of the samples. Different ion species at various energies were used in experiments, and several defect characterization techniques were applied to characterize the damage. Development of dimensional changes of carbon nanotubes in microscopes operated at accelerating voltages of 30 keV revealed that binding energy of carbon atoms in CNs is much lower than in bulk materials. Resistivity measurements during irradiation demonstrated existence of a quasi state of defect creation. Linear relationship between ID/IG ratio and increasing irradiation fluence was revealed by Raman spectroscopy study of irradiated carbon buckypapers. The deviations from linear relationship were observed for the samples irradiated to very high fluence values. Annealing of irradiated samples was able to reduce the value of ID/IG ratio and remove defects. However, annealing could not affect ID/IG ratio and remove defects in amorphized samples. The extracted value of activation energy for irradiated sample was 0.36 ±0.05 eV. The value of activation energy was in good agreement with theoretical studies.

NV centers in a diamond are proving themselves to be good building blocks for quantum information, electron spin resonance (ESR) imaging, and sensor applications. The key feature of the NV is that it has an electron spin that can be polarized and read out at room temperature. The readout is optical, thus the magnetic field imaging can also be done easily. Magnetic field variation with feature sizes below 0.3 microns cannot be directly resolved, and so in this region magnetic resonance imaging must be employed. To realize the full sensitivity of NV diamond, the spin transition linewidth must be as narrow as possible. Additionally, in the case of NV ensembles for micron-sized magnetometers, there must be a high concentration of NV. To this end three techniques are explored: (1) Electron paramagnetic resonance (EPR) imaging with microwave field gradients, (2) Magic angle rotation of magnetic field, and (3) TEM irradiation to optimize the yield of NV in a diamond. For the EPR imaging demonstration a resonant microwave field gradient is used in place of the usual DC magnetic gradient to obtain enough spatial resolution to resolve two very close "double NV" centers in a type Ib bulk diamond. Microfabrication technology enabled the micron-size wire structure to sit directly on the surface of millimeter-scale diamond plate. In contrast to conventional magnetic resonance imaging pulsed ESR was used to measure the Rabi oscillations. From the beating of Rabi oscillations from a "double NV," the pair was resolved using the one-dimension EPR imaging (EPRI) and the spatial distance was obtained. To achieve high sensitivity in nitrogen-doped diamond, the dipole-dipole coupling between the electron spin of the NV center and the substitutional nitrogen (14N) electron must be suppressed because it causes linewidth broadening. Magic angle spinning is an accepted technique to push T2 and T2 * down toward the T1 limit. An experiment was performed using the HPHT diamond with a high concentration of nitrogen, and a rotating field was applied with a microfabricated wire structure to reduce line broadening. In this experiment, ~50% suppression of the linewidth was observed and the effective time constant T2* improved from 114 ns to 227 ns. To achieve the highest possible sensitivity for micro-scale magnetic sensors the concentration of NV should be large. Since the unconverted N are magnetic impurities they shorten T2 and T2*, giving a tradeoff between NV (and therefore N) concentration and sensitivity. To construct a damage monitor, a type Ib HPHT sample was irradiated with electrons from a transmission electron microscope (TEM) and the effects on the ESR transition were seen well before physical damage appeared on the diamond and thus this proved to be a sensitive metric for irradiation damage.

Les phénomènes de charge des isolants ont été étudiés à l'aide d'un MEB (Microscope Electronique à Balayage). L'effet de la température sur ces phénomènes a été également discuté. Cette étude concernera, plus particulièrement, la mesure des courants induits sous irradiation électronique (courants de conduction et de déplacement) et la détermination du rendement d'émission électronique secondaire. Notre travail est porté en premier lieu sur des échantillons de verre à base de silice. Nous avons prouvé que l'augmentation de la température fait diminuer la capacité de charge du verre. Les résultats sont expliqués par l'augmentation de la conductivité activée par la température qui tend à réduire la formation de la charge d'espace négative grâce à l'augmentation de la mobilité de porteurs de charge. Cette explication est confirmée par une augmentation dans le courant de fuite mesuré sur le verre en fonction de la température.La seconde partie de ce travail a été consacrée à l'étude de l'effet de la température sur le comportement du polyéthylène téréphtalate (PET) irradié avec les électrons. Nous avons montré que la température joue un rôle important dans la grande rétention de la charge accumulée à cause du un phénomène de repiégeage rapide qui se produit dans le volume de l'échantillon dans le cas de la relaxation de la charge. Nos résultats indiquent que le phénomène de «flashover» a lieu aussi bien en volume qu'au niveau de la surface de l'échantillon. L'émission électronique secondaire de ce polymère devient de moins en moins faible avec l'augmentation de la température. / The charging phenomena of insulators have been studied using a SEM (Scanning Electron Microscope). The temperature effect on these phenomena was also discussed. The study will cover, in particular, the measurement of the induced currents under electron irradiation (conduction current and displacement current) and determining the secondary electron emission yield. Our work is focused primarily on silica glass samples. We have shown that increasing of the temperature decreases the capacity of the glass. The results are explained by the increase in conductivity activated by temperature, which tends to reduce the formation of the negative space charge due to the increased mobility of charge carriers. This explanation is confirmed by an increase in the conduction current measured on the glass as a function of temperature.The second part of this work was devoted to the study of the temperature effect on the behavior of polyethylene terephthalate (PET) irradiated with electrons. We have shown that temperature plays an important role in the high retention of accumulated charge due to a rapid trapping phenomenon that occurs in the bulk sample in the case of the charges relaxation. Our results indicate that the "flashover" phenomenon occurs both in bulk and to the surface of sample. The secondary electron emission of the polymer becomes less weak with increasing temperature.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第17579号 / 工博第3738号 / 新制||工||1570(附属図書館) / 30345 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 髙岡 義寛, 准教授 船戸 充 / 学位規則第4条第1項該当

This thesis focuses on electron irradiated gelatin hydrogel composites for the development of a magnetically-controllable material. Smart materials comprised of magnetic nanoparticles embedded in hydrogels are known as ferrogels. Deformation, swelling and viscoelasticity of ferrogels can be controlled by external magnetic fields, with potential applications in drug delivery, tissue engineering, actuation and sensing. High energy electron irradiation was used to create stable gelatin hydrogels. Geometry, swelling, solubility and viscoelasticity were experimentally quantified for the irradiated gelatin. The degree of crosslinking and mesh size were calculated by theories of rubber elasticity and Flory-Rehner. Fourier transform infrared spectroscopy was used to confirm minimal chemical changes occurred as a result of crosslinking. The micro- and nanostructure of the hydrogels were investigated using small-angle X-ray scattering to supplement macroscopic investigations, allowing for comparison of experimental data with additional semiflexible polymer models. The cytotoxicity of the irradiated hydrogels and liquid byproduct were analyzed using NIH 3T3 mouse embryonic fibroblasts and human umbilical vein endothelial cells. The influence of the degree of crosslinking on cellular morphologies was also explored. Additionally, surface wettability and hydrogel degradation times were quantified with respect to the irradiation dose. Preliminary experiments examined the potential of irradiated gelatin hydrogels as components of vascular scaffolds. Potential surface modification strategies to enhance and direct cellular interactions were briefly explored, such as surface coating and patterning. After integration of magnetic nanoparticles into the gelatin, the magnetic response of the ferrogels was investigated using magnetic particle spectroscopy and magnetorelaxometry. These techniques were highly sensitive to the changing matrix viscoelasticity around the sol-gel transition. Irradiated ferrogels exhibited thermal stability across the sol-gel transition, although some local softening was observed. This research highlights the potential of electron irradiated gelatin hydrogels and ferrogels, while providing fundamental insights into the physical processes influencing the network structure, mechanics and resulting cellular interactions.

京都大学 / 0048 / 新制・論文博士 / 博士(理学) / 乙第12996号 / 論理博第1552号 / 新制||理||1604(附属図書館) / 32924 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 松田 祐司, 教授 前野 悦輝, 教授 石田 憲二 / 学位規則第4条第2項該当 / Doctor of Science / Kyoto University / DGAM

Yes / Osmium nanocrystals can be fabricated by electron (3–50 nm, formed by atom migration), 785–815 nm laser (20–50 nm, in micelle islands), and microwave (ca. 1 nm in arrays, >100 mg scale) irradiation of a polymer-encapsulated OsII carborane; microfocus X-ray absorption studies at the Os LIII-edge show differences between the three preparation methods, suggesting that the electron-beam irradiated materials have a significant support interaction and/or surface oxidation, while the laser and microwave samples are more like metallic osmium. / Royal Society (University Research Fellowship No. UF150295 to NPEB), the Leverhulme Trust (Early Career Fellowship No. ECF-2013-414 to NPEB), the ERC (Grant No. 247450 to PJS), EPSRC (Grant No. EP/F034210/1 to PJS and EP/ J007153/1 to VGS), Diamond Light Source (Beam-time grant number SP11314).

Since their discovery at the beginning of the 21st century, two-dimensional (2D) materials have emerged as one of the most exciting material groups offering unique properties which promise a plethora of potential applications in nanoelectronics, quantum computing, and surface science. The progress in the study of 2D materials has advanced rapidly stimulated by the ever-growing interest in their behavior and the fact that they are the ideal specimen for transmission electron microscopy (TEM), as their geometry allows to identify every single atom. Their morphology – 2D materials consist of “surface” only – at the same time makes them sensitive to beam damage, since high-energy electrons easily sputter atoms and introduce defects. While this is in general not desirable – as non-destructive imaging is aimed at – it allows to precisely quantify the damage in TEM and even pattern the 2D material with atomic resolution using the electron beam. Alternatively, patterning of 2D materials can be achieved using focused ion irradiation, which makes studying its effect on 2D materials relevant and essential. In this thesis, we theoretically study the effects of electron and ion irradiation on 2D materials, exemplarily on 2D MoS2 . Specifically, we address the combined effect of electronic excitations and direct momentum transfer by high-energy electrons (knock-on damage) in 2D MoS2 using advanced first-principles simulation techniques, such as Ehrenfest dynamics based on time-dependent density functional theory (DFT). Here, we stress the importance of the combined effect of ionization damage and knock-on damage as neither of these alone can account for experimentally-observed defect production below the displacement threshold – the minimum energy required for the displacement of an atom from the pristine system. A mechanism of defect production relying on the localization of the electronic excitation at the emerging vacancy site is presented. The localized excitation eventually leads to a significant drop in the displacement threshold. The combination of electronic excitation and knock-on damage may in addition to beam-induced chemical etching explain the observed sub-threshold damage in low voltage TEM experiments. Apart from non-destructive imaging, electrons may be used to modify the 2D material intentionally. In this light, we consider the electron-beam driven phase transformation in 2D MoS2 , where the semiconducting polymorph transforms into its metallic counterpart. The phase energetics and a possible transformation mechanism under electron irradiation are investigated using DFT based first-principles calculations. The detailed understanding of the interaction of the electron beam with the 2D material promises to improve the patterning resolution enabling circuit design on the nanoscale. Ion irradiation employed in focussed ion beams (FIB), e.g., the helium ion microscope (HIM) constitutes another tool widely used to pattern and even image 2D materials. Ion bombardment experiment usually carried out for the 2D material placed on a substrate are frequently rationalized using simulations for free-standing systems neglecting the effect of the substrate. Combining Monte Carlo with analytical potential molecular dynamics simulations, we demonstrate that the substrate plays a crucial role in damage production under ion irradiation and cannot be neglected. Especially for light ions such as He and Ne, which are usually used in the HIM, the effect of the substrate needs to be considered to account for the increased number of defects and their broadened spatial distribution which limits the patterning resolution for typical HIM energies. / Seit ihrer Entdeckung Anfang des 21. Jahrhunderts haben sich zwei-dimensionale (2D) Materialien zu einer der spannendsten Materialklassen im Forschungsfeld aus Materialwissenschaft, Physik und Chemie entwickelt. Ihre einzigartigen Eigenschaften versprechen eine Vielzahl potentieller Anwendungen in der Nanoelektronik, für Quantencomputer und in der Oberflächenwissenschaft. Beflügelt durch das wachsende Interesse an ihrem Verhalten und der Tatsache, dass sie die idealen Proben für die Transmissions-Elektronen-Mikroskopie (TEM) darstellen – ihre Geometrie erlaubt es, jedes einzelne Atom zu identifizieren – sind die Forschungen an 2D-Materialien rapide vorangeschritten. Ihre Morphologie – 2D-Materialien bestehen nur aus “Oberfläche” – bedingt zugleich ihre Sensitivität bezüglich Strahlschäden. Hochenergetische Elektronen lösen sehr leicht Atome aus dem 2D-Material und induzieren Defekte. Obwohl dies im Allgemeinen unerwünscht ist – Ziel ist eine nicht-destruktive Bildgebung – erlaubt es doch präzise Einblicke in die Schadensentstehung im TEM. Überdies können 2D-Materialien mit Hilfe des Elektronenstrahls mit atomarer Auflösung strukturiert werden. Alternativ kann die Strukturierung des 2D-Materials über fokussierte Ionenstrahlung erfolgen, weshalb es lohnenswert erscheint, auch deren Effekt auf 2D-Materialien zu untersuchen. In dieser Arbeit werden die Effekte von Elektronen- und Ionenstrahlung auf 2D-Materialien aus theoretischer Sicht exemplarisch an 2D-MoS2 untersucht. Besonderes Augenmerk liegt dabei auf dem kombinierten Effekt von elektronischer Anregung und dem direkten Impulsübertrag durch hochenergetische Elektronen (Kollisionsschaden) in 2D-MoS2 , der durch die Anwendung von Ab-Initio-Simulationstechniken wie der Ehrenfest-Molekulardynamik, basierend auf zeitabhängiger Dichtefunktionaltheorie (DFT), studiert wird. Dabei liegt die Betonung auf der Kombination beider Effekte, da weder Ionisierungs- noch Kollisionsschäden allein die experimentell beobachtete Defekterzeugung unterhalb der Displacement Threshold – der notwendigen Mindestenergie, um ein Atom aus dem reinen Material herauszulösen – erklären. Ein möglicher Mechanismus der Defekterzeugung, basierend auf der Lokalisierung der elektronischen Anregung an der entstehenden Vakanzstelle, wird vorgeschlagen. Die lokalisierte Anregung führt dabei schließlich zu einem signifikanten Absinken der Displacement Threshold. Die Kombination von elektronischer Anregung und Kollisionsschaden trägt neben strahlinduzierten chemischen Reaktionen zur Erklärung der beobachteten Schäden unterhalb der Displacement Threshold in Niederspannungs-TEM-Experimenten bei. Neben nicht-destruktiver Bildgebung können Elektronenstrahlen auch dafür benutzt werden, 2D-Materialien gezielt zu modifizieren. In diesem Sinne wird der elektronenstrahl-induzierte Phasenübergang in 2D-MoS2 , bei dem sich das Material von einem halbleitenden in einen metallischen Zustand transformiert, betrachtet. Die Phasenenergetik und ein möglicher Transformationsmechanismus werden unter Zuhilfenahme von DFT-basierten Ab-Initio-Simulationen untersucht. Das detaillierte Verständnis der Interaktion des Elektronenstrahls mit dem 2D-Material verspricht dabei die Strukturierungsauflösung zu verbessern und ermöglicht Schaltkreisdesign auf der Nanoskala. Fokussierte Ionenstrahlen, wie sie in Ionenstrahlinstrumenten – wie dem Helium-Ionen-Mikroskop (HIM) zum Einsatz kommen – stellen ein weiteres häufig verwendetes Werkzeug zur Modifikation sowie zur Bildgebung von 2D-Materialien dar. Ionenstrahlexperimente – üblicherweise mit dem auf einem Substrat platzierten 2D-Material durchgeführt – werden hingegen oft mit Simulationen für freistehende 2D-Materialien rationalisiert, wobei jegliche Einwirkung des Substrats vernachlässigt wird. Die Kombination von Monte-Carlo-Simulationen mit Molekulardynamik-Simulationen (auf der Basis analytischer Potentiale) in dieser Arbeit verdeutlicht, dass das Substrat eine wichtige Rolle in der Defekterzeugung spielt und nicht vernachlässigt werden kann. Besonders für leichte Ionen, wie He und Ne, wie sie typischerweise im HIM zum Einsatz kommen, sollte der Effekt des Substrats berücksichtigt werden. Dieses führt für typische Ionenenergien im HIM – im Vergleich zum freistehenden 2D-Material – zu einer ansteigenden Anzahl an Defekten und einer breiteren räumlichen Defektverteilung, welche die Strukturierungsauflösung begrenzt.