Dissertationen zum Thema „Low-energy electron beams“

Thesis (Ph.D.) -- University of Maryland, College Park, 2009.
Thesis research directed by: Dept. of Electrical and Computer Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

Surface-analysis and treatment apparatus have been variously designed, manufactured, developed, and commissioned or re-commissioned, for characterising the surfaces and efficiency of positron moderators based around 3 µm thick polycrystalline-tungsten foil. These include XPS and AES, based around a CLAM2 hemispherical analyser, electron-beam heating, ion bombardment, mass spectroscopy, UHV sample mounting, UHV manipulation, gas-handling lines, and entry-lock apparatus. The CLAM2 electron spectrometer is additionally adapted for operation as a bipolar charged-particle spectrometer. All control software, and much data-analysis software, is implemented in Labview. Apparatus and techniques for safely storing, handling, transferring into vacuum, and manipulating in vacuo, a nominally 1 mCi 22Na, UHV-compatible positron source, are designed, constructed, and implemented. The efficacy of cleaning and surface-analysis apparatus are demonstrated, with some limitations and instrument malfunction identified, and solutions implemented. Methods for passivating positron-trapping states in polycrystalline tungsten are proposed, based on the current understanding of positron moderation and trapping. Improved moderator geometries have been designed and an alternative, simpler, and easier to implement solid-gas moderator proposed.

The observation of graphene’s extraordinary electrical properties has stirred great interest in two dimensional (2D) materials. The rapid pace of discovery for low dimensional materials with exciting properties continue with graphene allotropes, multiple polymorphs of borophene, germanene, and many others. The future of 2D materials goes beyond synthesis and characterization of free standing materials and on to the construction of heterostructures or sophisticated multilayer devices. Knowledge about the resulting local structure and composition of such systems will be key to understanding and optimizing their performance characteristics. 2D materials do not have a repeating crystal structure which can be easily characterized using bulk methods and therefore a localized high resolution method is needed. Electron microscopy is well suited for characterizing 2D materials as a repeating coherent structure is not necessary to produce a measureable signal as may be the case for diffraction methods. A unique opportunity for fine local scale measurements in low dimensional systems exists with a specific class of materials known as ferecrystals, the rotationally disordered relative of misfit layer compounds. Ferecrystals provide an excellent test system to observe effects at heterostructure interfaces as the whole film is composed of interdigitated two dimensional layers. Therefore bulk methods can be used to corroborate local scale measurements. From the qualitative interpretation of high resolution scanning transmission electron microscope (STEM) images to the quantitative application of STEM energy dispersive X-ray spectroscopy (EDX), this thesis uses numerous methods electron microscopy. The culmination of this work is seen at the end of the thesis where atomically resolved STEM-EDX hyperspectral maps could be used to measure element specific atomic distances and the atomically resolved fractional occupancies of a low dimensional alloy. These local scale measurements are corroborated by additional experimental data. The input of multiple techniques leads to improved certainty in local scale measurements and the applicability of these methods to non-ferecrystal low dimensional systems.

Experiments and numerical investigations on trapped electron plasmas and traveling electron bunches are discussed. A Thomson backscattering diagnostics set up was installed in the ELTRAP (Electron TRAP) device, a Penning-Malmberg trap operating at the Department of Physics of the University of Milano since 2001. Here, an infrared (IR) laser pulse collides with nanosecond electron bunches with an energy of 1-20 keV traveling through a longitudinal magnetic field in a dynamical regime where space-charge effects play a significant role. The backscattered radiation is optically filtered and detected by means of a photomultiplier tube. The minimum sensitivity of the backscattering diagnostics has been estimated for the present set-up configuration. Constraints on the number of photons and thus on the information one can obtain with the Thomson backscattering technique are determined by the relatively low density of the electron beam as well as by noise issues. Solutions to increase the signal level and to reduce the noise are briefly discussed. The generation of an electron plasma by stochastic heating was realized in ELTRAP under ultra-high vacuum conditions by means of the application of low power RF (1-20 MHz) drives on one of the azimuthally sectored electrodes of the trap. The relevant experimental results are reviewed. The electron heating mechanism has been studied by means of a two-dimensional (2D) particle-in-cell (PIC) code, starting with a very low electron density, and applying RF drives of various amplitudes in the range 1-15 MHz on different electrodes. The axial kinetic energy of the electrons is in general increasing for all considered cases. Of course, higher temperature increments are obtained by increasing the amplitude of the RF excitation. The simulation results indicate in particular that the heating is initially higher close to the cylindrical wall of the device. These results on the electron heating point in the same direction of the experimental findings, where the plasma formation due to the ionization of the residual gas is found to be localized close to the trap wall. The simulations indicate also major heating effects when the RF drive is applied close to one end of the trap. Similar results are obtained for an electron plasma at higher densities, simulating a situation in which the RF is applied to an already formed plasma. With the aim to extend these RF studies to the microwave range, a bench test analysis has been performed of the transmission efficiency of a microwave injection system up to a few GHz. The test was based on the use of a prototype circular waveguide with the same diameter and length of the ELTRAP electrode stack and of a coupled rectangular waveguide with dimensions suitable for a future installation in the device. Electromagnetic PIC simulations have also been performed of the electron heating effect, again both at very low and relatively high electron densities, applying a microwave drive with a frequency of approximately 3 GHz close to the center and close to one end of the trap. Both the bench test of the injection system and the numerical simulations indicate that the new microwave heating system will allow the extension of the previous RF studies to the GHz range. In particular, the electron cyclotron resonance heating of the electrons will be aimed to increasing the electron temperature, and possibly its density as a consequence of a higher ionization rate of the residual gas. The installation of the new RF system will open up the possibility to study, e.g., the interaction between the confined plasma and traveling electron bunches.

Cette thèse de doctorat s'inscrit dans le cadre d'une collaboration CIFRE entre Thales-MIS et le Laboratoire d'Optique Appliquée (LOA). L'objectif principal est d'améliorer le courant moyen des accélérateurs laser-plasma à faible énergie, notamment dans la gamme de quelques MeV. Cette avancée revêt un intérêt particulier pour les applications à faible énergie telles que la tomographie industrielle par rayons X, ne nécessitant pas de faisceaux d'électrons monoénergétiques.Des expériences ont été menées au moyen du système laser de 60 TW installé dans la Salle Jaune du LOA, capable de générer des impulsions de 30 fs. À travers une exploration minutieuse des densités de plasma, des énergies laser, des cibles gazeuses et des degrés de focalisation, nous avons identifié les conditions propices à la production de faisceaux d'électrons hautement divergents (i.e., >100 mrad) de quelques MeV, avec des charges variant de 5 à 30 nC. Nous avons également atteint une efficacité maximale de conversion d'énergie laser-électron d'environ 14 %, parmi les plus élevées jamais mesurées. En envisageant les futurs systèmes laser capables d'atteindre des puissances moyennes d'environ 100 W, ces configurations pourraient ouvrir la voie à la réalisation de faisceaux d'électrons accélérés par laser-plasma, avec des courants moyens dépassant 1 microampère, surpassant ainsi l'état de l'art actuel des accélérateurs laser-plasma. Pour mener à bien ces expériences novatrices, nous avons conçu une buse supersonique en verre et des dipôles magnétiques permanents permettant de dévier les électrons vers des écrans scintillants pour effectuer la spectrométrie des faisceaux produits. Parallèlement aux expériences, cette thèse a également approfondi les simulations Particle-In-Cell (PIC) pour étudier les mécanismes d'accélération. Grâce à un outil numérique spécifiquement développé pour traiter les résultats des simulations PIC, nous avons démontré que la force pondéromotrice du laser joue un rôle prépondérant dans l'accélération des électrons. Notamment, la majorité des particules ne sont pas injectées dans les ondes du plasma, mais glissent plutôt sur l'impulsion laser, acquérant ainsi une faible énergie de l'ordre de quelques MeV
This doctoral thesis is part of a CIFRE collaboration between Thales-MIS and the Laboratoire d'Optique Appliquée (LOA). The main objective is to enhance the average current of low-energy laser-plasma accelerators, particularly in the range of a few MeV. This advancement is particularly interesting for low-energy applications such as industrial X-ray tomography, which does not require monoenergetic electron beams.Experiments were conducted using the 60,TW laser system installed in the Salle Jaune at LOA, capable of generating 30 fs pulses. Through meticulous exploration of plasma densities, laser energies, gas targets, and focusing degrees, we identified conditions conducive to producing highly divergent electron beams (i.e., >100 mrad) at energies of a few MeV, with charges ranging from 5 to 30 nC. We also achieved a maximum laser-to-electron energy conversion efficiency of approximately 14 %, one of the highest ever measured. Looking ahead to future laser systems capable of achieving average powers of around 100 W, these configurations could pave the way for generating laser-plasma accelerated electron beams with average currents exceeding 1 microampere, surpassing the current state of the art in laser-plasma accelerators. To facilitate these innovative experiments, we designed a supersonic glass nozzle and permanent magnetic dipoles to deflect electrons towards scintillating screens for beam spectroscopy. Concurrently with the experiments, this thesis also delved into Particle-In-Cell (PIC) simulations to study acceleration mechanisms. Using a dedicated numerical tool for processing PIC simulation results, we demonstrated that the ponderomotive force of the laser plays a predominant role in electron acceleration. Notably, the majority of particles are not injected into plasma waves but rather slide along the laser pulse, thereby gaining low energies on the order of a few MeV

Dans le cadre de cette thèse, la conception, la construction et la mise en service de la plateforme de test LEETECH ont été réalisées. La performance de LEETECH, y compris le mode de fonctionnement à faible multiplicité a été démontrée. En fournissant des paquets d’électrons avec une énergie ajustable jusqu’à 3.5 MeV, une multiplicité ajustable à partir d’électrons simples et une durée des paquets jusqu’à 20ps, LEETECH prend sa place entre les faisceaux tests de hautes énergies et de coûts élevés d’un part et l’utilisation de sources radioactifs d’autre part. Dans la région, qui correspond à la particule d’ionisation minimale, la plateforme offre aux détecteurs de traces les conditions similaires aux celles de faisceaux des hautes énergies. Le mode de fonctionnement à faible multiplicité a été étudié en utilisant un détecteur diamant de grande surface. En plus une capacité d’un capteur diamant de résoudre des paquets à faible nombre des particules a été démontrée. Dans le cadre du développement de la chambre à projection temporelle (TPC) pour le projet ILC, une session de test a été dédiée à un détecteur Micromegas/InGrid de large surface. Pour la première fois les pertes d’énergie par un électron dans un mélange de gaz basée sur Helium ont été mesurées pour une énergie de quelques MeV. La résolution en dE/dx et un algorithme pour la reconstruction de traces ont été développés. Une caractérisation préliminaire du quartz barre lu par MCPPMT – un candidat pour le détecteur temps-de- vol (TOF) avec la mission de l’identification des hadrons chargés dans le futur usine tau-charm HIEPA – a été accomplie. La résolution temporelle de 50 ps obtenue pour le détecteur étudié met cette technologie prometteuse pour les études plus approfondies
Within the present thesis the design, construction and commissioning of a new test beam facility LEETECH have been performed. Performance of the new facility, including low-multiplicity operation mode has been demonstrated. A number of interesting detector tests, including large-area diamond, Micromegas/InGrid and quartz bar detectors have been performed. Development of new detector technologies for future high-energy physics collider experiments calls for selection of versatile test beam facilities, permitting to choose or adjust beam parameters such as particles type, energy and beam intensity, are irreplaceable in characterization and tests of developed instruments. Major applications comprise generic detector R&D, conceptual design and choice of detector technologies, technical design, prototypes and full-scale detector construction and tests, detector calibration and commissioning. A new test beam facility LEETECH (Low Energy Electron TECHnique) was designed, constructed and commissioned in LAL (Orsay) as an extension of existing PHIL accelerator. Providing electron bunches of adjustable energy (up to 3.5 MeV), intensity (starting from a few particles per bunch) and bunch time duration (down to 20 ps), LEETECH fills the gap between high-cost high-energy test beam facilities and use of radioactive sources. Covering a minimum-ionization particles region (electrons of energy above 1.6 MeV), LEETECH provides for tracking detectors similar conditions as high-energy facilities. Using LEETECH as an electron source, several types of detectors were investigated in order to study their performance or applications, also providing a characterization of the LEETECH performance. First studies of the LEETECH facility were performed with a plastic scintillator coupled to the Micro-channel plate photomultiplier. A low-multiplicity mode was investigated using the diamond sensor, at the same time demonstrating its ability to resolve bunches consisting of a few particles. In framework of Time Projection Chamber development for the ILC project, a session dedicated to a large-area Micromegas/InGrid module was performed. For the first time the electron energy losses in Helium-based gas mixtures were measured for the energies of few MeV. The dE/dx resolution was obtained and track reconstruction algorithm was developed. Being a candidate for the time-of- flight detector of the BESIII upgrade and future HIEPA tau-charm factories, a preliminary characterization of the quartz bar performed. The time resolution of the detector module of 50 ps was obtained, giving a promising results for the further detector studies

We employ a variant of optical absorption spectroscopy, namely in situ differential reflectance spectroscopy (DRS), for an analysis of the structure-properties relations of thin epitaxial organic films. Clear correlations between the spectra and the differently intense coupling to the respective substrates are found. While rather broad and almost structureless spectra are obtained for a quaterrylene (QT) monolayer on Au(111), the spectral shape resembles that of isolated molecules when QT is grown on graphite. We even achieve an efficient electronic decoupling from the subjacent Au(111) by inserting an atomically thin organic spacer layer consisting of hexa-peri-hexabenzocoronene (HBC) with a noticeably dissimilar electronic behavior. These observations are further consolidated by a systematic variation of the metal substrate (Au, Ag, and Al), ranging from inert to rather reactive. For this purpose, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) is chosen to ensure comparability of the molecular film structures on the different metals, and also because its electronic alignment on various metal surfaces has previously been studied with great intensity. We present evidence for ionized PTCDA at several interfaces and propose the charge transfer to be related to the electronic level alignment governed by interface dipole formation on the respective metals
Zur Analyse der Struktur-Eigenschafts-Beziehungen dünner, epitaktischer Molekülfilme wird in situ differentielle Reflexionsspektroskopie (DRS) als Variante der optischen Absorptionsspektroskopie verwendet. Klare Zusammenhänge zwischen den Spektren und der unterschiedlich starken Kopplung zum jeweiligen Substrat werden gefunden. Während man breite und beinahe unstrukturierte Spektren für eine Quaterrylen (QT) Monolage auf Au(111) erhält, ist die spektrale Form von auf Graphit abgeschiedenem QT ähnlich der isolierter Moleküle. Durch Einfügen einer atomar dünnen organischen Zwischenschicht bestehend aus Hexa-peri-hexabenzocoronen (HBC) mit einem deutlich unterschiedlichen elektronischen Verhalten gelingt sogar eine effiziente elektronische Entkopplung vom darunter liegenden Au(111). Diese Ergebnisse werden durch systematische Variation der Metallsubstrate (Au, Ag und Al), welche von inert bis sehr reaktiv reichen, untermauert. Zu diesem Zweck wird 3,4,9,10-Perylentetracarbonsäuredianhydrid (PTCDA) gewählt, um Vergleichbarkeit der molekularen Filmstrukturen zu gewährleisten, und weil dessen elektronische Anordnung auf verschiedenen Metalloberflächen bereits eingehend untersucht worden ist. Wir weisen ionisiertes PTCDA an einigen dieser Grenzflächen nach und schlagen vor, dass der Ladungsübergang mit der elektronischen Niveauanpassung zusammenhängt, welche mit der Ausbildung von Grenzflächendipolen auf den entsprechenden Metallen einhergeht

Energiskördning är idag ett växande område och är framstående sett till hållbarhetsaspekterna. Vibrationsbaserad sådan har blivit allt populärare där man kan utnyttja mekanisk energi från olika källor till att generera elektrisk energi. Piezoelektricitet fungerar enligt denna princip och piezoelektrisk energiskördning har varit ett område som fler och fler utnyttjar på grund av dess effektivitet, exempelvis till trådlösa sensornätverk. Ett krav på att piezoelektrisk energiskördning ska fungera optimalt är att vibrationerna sker med en satt frekvens utan större variation, ofta i väldigt höga frekvenser. Syftet med detta projekt är att anpassa denna teknik till mänskliga rörelser vilket kan göra den mer användbar och ett tänkt ändamål kan vara ett demonstrationsexempel för oregelbundna rörelser vid låga frekvenser, precis som mänskliga rörelser. Utmaningen lägger därmed i att utveckla en piezoelektrisk energiskördare som har ett frekvensområde inom mänskliga rörelsers frekvenser på 4 till 7 Hertz, där effektiviteten fortfarande kan vara hög. Detta har beprövats med vibrationsplatta. Vad som observerades var att med flera piezoelektriska material på konsolbalkar i kolfiber av olika dimensioner med olika vikter längst ut, uppstod ett frekvensområde inom mänskliga området med höga spänningar. För att göra det möjligt behövdes vikterna ha en stor massa av upp till hundratals gram så att resonansfrekvenserna kunde vara inom nämnt frekvensområde. Då piezoelektriska material ger en växelspänning, måste spänningen likriktas. Detta gjordes med två olika gränssnitt med ett mönsterkort tillverkat för vardera. Dessa gränssnitt är ett klassiskt som helt enkelt likriktar spänningen, medan den andra, Parallel Synchronized Switch Harvesting on the Inductor (P-SSHI), ska maximera spänningen och effekten. Det visade sig att det inte blev lika lyckat som planerat. Det klassiska gränssnittet gav en likspänning som var nästan lika hög som den inmatade växelspänningen medan det inte gällde för P-SSHI.
Today energy harvesting is an area on the rise and is outstanding in regards to the environmental aspects. Vibration based energy harvesting has become popular where it uses mechanical energy from different sources to produce electrical energy. Piezoelectricity operates according to this principle and piezoelectric energy harvesting has been an area more are using because of its efficiency, with applications such as wireless sensor networks. One demand for piezoelectric energy harvesting to work optimally is that the vibration source must have a well known frequency with minor deviations and this in usually very high frequencies. The purpose of this thesis is to adapt this technology to human motions which could make it even more useful and a proposed usage is a demo product for irregular motions of low frequency, just like human motions. The challenge is hence to create a piezoelectric energy harvester which has a frequency range within the human motions’ frequencies of 4 to 7 Hertz, where the efficiency still could be high. This has been tested using a vibration exciter. What was noticed was that with multiple piezoelectric materials on cantilever beams of carbon fibre with different dimensions and tip masses, a frequency range within human range with high voltages could be created. To make this possible, the masses needed to have a significant mass of up towards hundreds of grams in order for the resonance frequencies to be within the stated frequency range. As the piezoelectric materials provide an AC voltage, the voltage needs to be rectified. This was done with two different interfaces with a PCB created for each. These interfaces are a classic one which simply rectifies the voltage, while the other, Parallel Synchronized Switch Harvesting on the Inductor (PSSHI), is supposed to maximize the voltage and power. This did not turn out to be as successful as predicted. The classical interface delivered a DC voltage almost as much as the provided AC voltage while the P-SSHI interface did not.

Superaustenitická korozivzdorná ocel typu 22Cr25NiWCoCu určená pro vysokoteplotní aplikace v energetickém průmyslu byla studována za podmínek nízkocyklové únavy při pokojové a zvýšené teplotě. Jednotlivé vzorky byly podrobeny různým zátěžným procedurám, což umožnilo studium materiálové odezvy spolu s mechanismem poškození. Křivky cyklického zpevnění/změkčení, cyklického napětí a Coffin-Mansonovy křivky byly vyhodnoceny. Únavová životnost materiálu byla diskutována s ohledem na uplatňované mechanismy poškození, které se vyvinuly za specifických zátěžných podmínek. Standardní izotermální únavové experimenty byly provedeny při pokojové a zvýšené teplotě. Hysterezní smyčky zaznamenané během cyklického zatěžování byly analyzovány pomocí zobecněné statistické teorie hysterezní smyčky. Pro různé amplitudy napětí byla určena jak distribuce hustoty pravděpodobnosti interních kritických napětí (dále PDF), tak rovněž zjištěn její vývoj během cyklického namáhání. Zjištěné průběhy PDF byly korelovány s vývojem povrchového reliéfu a vnitřního dislokačního uspořádání zdokumentované pro obě teploty pomocí rastrovací elektronové mikroskopie (SEM) vybavené technikou fokusovaného iontového svazku (FIB), která umožnila rovněž efektivní studium nukleace povrchových únavových trhlin. Při cyklickém zatížení při pokojové teplotě byla pozorována lokalizace cyklické plastické deformace do perzistentních skluzových pásů (PSP). V místech, kde tyto PSP vystupují na povrch materiálu byly pozorovány perzistentní skluzové stopy (PSS) tvořené extruzemi a intruzemi. Postupné prohlubování intruzí, zejména na čele nejhlubší intruze, vede k iniciaci únavové trhliny. Odlišný mechanismus tvorby trhlin byl zjištěn při únavové zkoušce při zvýšené teplotě, kde zásadní roli hrál vliv prostředí. Rychlá oxidace hranic zrn a jejich následné popraskání představuje dominantní mechanismus v I. stádiu nukleace trhlin. Aplikace desetiminutové prodlevy v tahové části zátěžného cyklu vedlo k vývoji vnitřního (kavitačního) poškozování. Mechanismy vnitřního poškozování byly studovány na podélných řezech rovnoběžných s napěťovou osou zkušebních vzorků. Trhliny a jejich vztah k hranicím zrn a dvojčat byly studovány pomocí difrakce zpětně odražených elektronů (EBSD). Vliv prodlevy na únavovou životnost byl korelován s vývojem povrchového reliéfu a vnitřního poškození. Vzorky z uvedené oceli byly rovněž podrobeny zkouškám termomechanické únavy (TMF), při nichž se v čase mění jak zátěžná síla tak i teplota. Termomechanické únavové zkoušky v režimu soufázném (in-phase) a protifázném (out-of-phase) byly provedeny jak s prodlevou, tak i bez ní. Ve všech případech bylo pozorováno rychlé cyklické zpevnění bez ohledu na použitou amplitudu deformace, u vzorků testovaných v out-of-phase režimu byla zjištěna tendence k saturaci. Zkoumáním povrchového reliéfu za pomocí technik SEM a FIB byla odhalena přednostní oxidace hranic zrn a následné praskání těchto hranic kolmo k ose zatížení. Prodlevy v cyklech při maximálním napětí vedly ke zvýšení amplitudy plastické deformace a následně ke creepovému poškození ve formě vnitřních kavit a trhlin. Interkrystalické šíření trhlin bylo pozorováno na vzorcích testovaných v režimu in-phase. Vývoj poškození v režimu out-of-phase nebyl principiálně ovlivněn zařazením prodlevy do zátěžného cyklu. Charakteristickým znakem namáhání v režimu out-of-phase je nukleace několika trhlin v homogenní oxidické vrstvě jdoucích napříč zrny kolmo k ose zatěžování.

The topic of this work is the structural characterization and theoretical modeling of organic single and heterolayers. The growth of sub-monolayers and monolayers (ML) of the two polycyclic aromatic hydrocarbons quaterrylene (QT) and hexa-peri-hexabenzocoronene (HBC) on Ag(111) and Au(111) was investigated. A transition from a disordered, isotropic phase to an ordered phase with increasing coverage was found. The lattice of the ordered phase turned out to be coverage dependent. The intermolecular potential was modeled including Coulomb and van der Waals interaction by a force-field approach. The postulated repulsive character of the potential could be connected to the non-uniform intramolecular charge distribution and to a screening of the van der Waals forces. Furthermore, the influence of the variable lattice constant on the epitaxial growth of HBC was studied. The second part of this work deals with a ML of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on a ML of HBC. In dependency on the initial lattice constant of HBC, a total of three line-on-line (LOL) and point-on-line coincident phases of PTCDA (with respect to HBC) was found. Following an analysis of the general properties of LOL coincident systems via force-field calculations, a new method to predict the structure of such systems is introduced
Thema dieser Arbeit ist die strukturelle Charakterisierung von organischen Einfach- und Heterolagen sowie deren theoretische Beschreibung und Modellierung. Es wurden Submonolagen und Monolagen (ML) der polyzyklischen Kohlenwasserstoffe Quaterrylen (QT) und Hexa-peri-hexabenzocoronen (HBC) auf Ag(111) und Au(111) Einkristallen untersucht und ein Übergang von einer ungeordneten, isotropen Phase zu einer geordneten Phase mit steigender Bedeckung beobachtet. Die geordnete Phase wies dabei bedeckungsabhängige Gitterkonstanten auf. Das intermolekulare Potential wurde unter Berücksichtigung von Coulomb und van der Waals Anteilen mittels Kraftfeldmethoden modelliert. Der postulierte repulsive Charakter des Potentials konnte auf die Ladungsverteilung im Molekül und eine Abschwächung des van der Waals Potentials zurückgeführt werden. Weiterhin wurde der Einfluss der variablen HBC Gitterkonstante auf die epitaktische Relation des Gitters zum Metallsubstrat untersucht. Der zweite Teil der Arbeit widmet sich der Untersuchung einer ML 3,4,9,10-Perylenetetracarboxylic dianhydrid (PTCDA) auf einer ML HBC. Dabei wurden, in Abhängigkeit von der HBC Gitterkonstante, insgesamt drei verschiedene Typen von line-on-line bzw. point-on-line Epitaxie nachgewiesen. Im Anschluss an eine Analyse der generellen Eigenschaften solcher epitaktischer Lagen mittels Kraftfeldrechnungen wird eine neue Methode zur Vorhersage der Struktur konkreter Systeme vorgestellt

Spin-polarized electron beam has not yet been produced from an unpolarized electron beam using Stern-Gerlach type spin filter, because of the Lorentz force and Heisenberg uncertainty principle. At present, electron spin detectors and filters work on the basis of spin-dependent scattering of an electron beam from crystal surfaces. Single channel efficiencies of all the spin detectors for electrons are orders of magnitudes lower than the ideal one. Specular reflection type spin-polarized low energy electron diffraction (SPLEED)-spin detectors are having higher single channel efficiencies compared to the conventional Mott detectors. Moreover, multichannel detection can be realized from specular reflection type SPLEED-spin detectors. They have higher effective efficiency than the ideal spin detector. In order to develop specular reflection type spin filter, it is important to develop a spin-polarized low energy electron diffraction system. In addition, SPLEED system allows us to study the spin-orbit and exchange scattering at crystal surfaces. The general direction of the thesis has been the development of spin-polarized low energy electron diffraction (SPLEED) system. This system has been used to investigate the spin-orbit interactions on Ir(100) surface and exchange interactions of Fe grown on Ir(100). The thesis is organized into chapters as follows. Chapter 1 introduce the reader to some of the basic concepts of polarized electrons and the evolution of spin-polarized electron sources and detectors. Sources of polarized electrons are discussed with emphasis on photocathodes such as GaAs and strained GaAs. Widely used spin detector is the Mott detector which works in the higher energy range. The working principle of the Mott detector is discussed. Commonly used spin detector in the lower energy range is the LEED detector. The concept of the LEED detector is also discussed. Working principle and recent developments of specular reflection type SPLEED spin filters are introduced. Evolution of electron spin detector is discussed towards the end of the chapter. Chapter 2 discusses about the two instruments designed and developed during the course of the thesis. The first one is a spin-polarized low energy electron diffraction system working in the reflected electron pulse counting mode in UHV. This system is capable of measuring spin asymmetries due to spin-orbit interaction and exchange interaction. This instrument is useful in understanding structure and magnetism at surfaces as well as helps to develop new spin polarimeter based on SPLEED by evaluating spin asymmetries from different surfaces. All instruments connected to SPLEED system, measurement protocol and controlling software are discussed with some details. Along with this, standard characterization tools such as X-ray diffraction and magneto-optic Kerr effect measurements are discussed. The second instrument is a novel quadratic magneto-optic Kerr effect measurement system using permanent magnets, which is simple, compact and cost-effective. We have used rotating field method to extract QMOKE component in saturation. So there is no need for precise real-time measurement of magnitude and direction of the magnetic field as in the case of vector magnet. This instrument can easily quantify QMOKE coefficients for ferrimagnetic and ferromagnetic thin films and single crystals. Chapter 3 discusses SPLEED experiments carried out on Ir(100)-(1×5)-Hex and Ir(100)-(1×2+2×1)-O surfaces. The surface structure and surface preparation techniques are discussed. The stability of the Ir(100)-(1×5)-Hex surface is evaluated by monitoring the spin asymmetry as the function of time. Within 25 hours after the surface preparation, the profile of the spin asymmetry and the reflected electron count for Ir(100)-(1×5)-Hex surface resembles that of hydrogen adsorbed Ir(100)-(1×5)-H surface. The electron energy-angle of incidence landscape of reflectivity, spin asymmetry and figure of merit are recorded for Ir(100)-(1×2+2×1)-O surface. Many wide regions with a large figure of merit are identified in the E- landscape. Chapter 4 reports SPLEED experiments carried out on Ir(100)-(1×5)-H surface. The comparison between asymmetries evaluated for the Ir(100)-(1×5)-Hex surface after 25 hours and Ir(100)-(1×5)-H surface suggests that Ir(100)-(1×5)-Hex surface is transforming to Ir(100)-(1×5)-H surface, in 25 hours. This can be due to the adsorption of more than four Langmuir of residual hydrogen during this time. The energy-angle landscape of reflectivity, asymmetry and figure of merit are recorded for Ir(100)-(1×5)-H surface in an energy range 20 eV to 100 eV and angle range 10 to 60 . Many regions are identified as the working point for specular reflection type spin filter based on SPLEED. The surface structure and surface preparation techniques are discussed. The stability of the surface is also evaluated. Chapter 5 investigates the growth and magnetic properties of Fe(100) film on Ir(100)-(1×1), Ir(100)-(1×5)-Hex and Ir(100)-(1×2+2×1)-O surfaces. LEED, MEED, LMOKE and QMOKE studies were presented. The growth is found to be layer-by-layer at least up to 20 monolayers (ML) at room temperature. At higher deposition temperature, the MEED oscillations disappear around 3-5 ML. Magnetic anisotropy of the Fe(100) film grown on Ir(100)-(1×2+2×1)-O surfaces is evaluated using LMOKE measurement using Kerr microscope. Simultaneous in-situ LMOKE and MEED measurements were carried out during the deposition. Ferromagnetic ordering with an in-plane easy axis starts above 4.5 ML at room temperature. The Kerr rotation normalized by thickness is evaluated in the pseudomorphic regime and strain relaxed regime. The probing depth of the MOKE is found to be around 14 nm in Fe(100)/Ir(100). An antisymmetric component is observed in the re-magnetization loop measured using MOKE. This antisymmetric loop arises due to the quadratic magneto-optic coupling which is separated by symmetrization and antisymmetrization procedure. The observed quadratic magneto-optic coupling suggests that the analysis based on the assumption that the magneto-optic coupling is linear in magnetization has to be modified. In order to quantify the quadratic magneto-optic coupling parameters, a QMOKE measurement system is developed and measurements were carried out. Chapter 6 discusses SPLEED experiments carried out on various thicknesses of Fe(100) film. Fe(100) films grown on Ir(100) substrate with the thickness less than or equal to 4 ML is not ferromagnetic with in-plane easy axis at room temperature. The non-zero exchange asymmetry observed for 5 ML and above indicates the presence of ferromagnetic ordering. A difference in the profile of exchange asymmetry is observed between pseudomorphic and strain relaxed regime. Large spin-orbit asymmetry is observed for 1 ML and 2 ML Fe(100) which is unexpected from a low atomic number (Z) material. The reason for large spin-orbit asymmetry is still unknown. The energy-angle landscape of reflectivity, exchange asymmetry, spin-orbit asymmetry and figure of merit were evaluated for 21 ML of Fe(100). Many working points were identified for different types multichannel spin filter based on exchange interaction Finally, the various results are summarized and a broad outlook is given.

Spin-polarized electron beam has not yet been produced from an unpolarized electron beam using Stern-Gerlach type spin filter, because of the Lorentz force and Heisenberg uncertainty principle. At present, electron spin detectors and filters work on the basis of spin-dependent scattering of an electron beam from crystal surfaces. Single channel efficiencies of all the spin detectors for electrons are orders of magnitudes lower than the ideal one. Specular reflection type spin-polarized low energy electron diffraction (SPLEED)-spin detectors are having higher single channel efficiencies compared to the conventional Mott detectors. Moreover, multichannel detection can be realized from specular reflection type SPLEED-spin detectors. They have higher effective efficiency than the ideal spin detector. In order to develop specular reflection type spin filter, it is important to develop a spin-polarized low energy electron diffraction system. In addition, SPLEED system allows us to study the spin-orbit and exchange scattering at crystal surfaces. The general direction of the thesis has been the development of spin-polarized low energy electron diffraction (SPLEED) system. This system has been used to investigate the spin-orbit interactions on Ir(100) surface and exchange interactions of Fe grown on Ir(100). The thesis is organized into chapters as follows. Chapter 1 introduce the reader to some of the basic concepts of polarized electrons and the evolution of spin-polarized electron sources and detectors. Sources of polarized electrons are discussed with emphasis on photocathodes such as GaAs and strained GaAs. Widely used spin detector is the Mott detector which works in the higher energy range. The working principle of the Mott detector is discussed. Commonly used spin detector in the lower energy range is the LEED detector. The concept of the LEED detector is also discussed. Working principle and recent developments of specular reflection type SPLEED spin filters are introduced. Evolution of electron spin detector is discussed towards the end of the chapter. Chapter 2 discusses about the two instruments designed and developed during the course of the thesis. The first one is a spin-polarized low energy electron diffraction system working in the reflected electron pulse counting mode in UHV. This system is capable of measuring spin asymmetries due to spin-orbit interaction and exchange interaction. This instrument is useful in understanding structure and magnetism at surfaces as well as helps to develop new spin polarimeter based on SPLEED by evaluating spin asymmetries from different surfaces. All instruments connected to SPLEED system, measurement protocol and controlling software are discussed with some details. Along with this, standard characterization tools such as X-ray diffraction and magneto-optic Kerr effect measurements are discussed. The second instrument is a novel quadratic magneto-optic Kerr effect measurement system using permanent magnets, which is simple, compact and cost-effective. We have used rotating field method to extract QMOKE component in saturation. So there is no need for precise real-time measurement of magnitude and direction of the magnetic field as in the case of vector magnet. This instrument can easily quantify QMOKE coefficients for ferrimagnetic and ferromagnetic thin films and single crystals. Chapter 3 discusses SPLEED experiments carried out on Ir(100)-(1×5)-Hex and Ir(100)-(1×2+2×1)-O surfaces. The surface structure and surface preparation techniques are discussed. The stability of the Ir(100)-(1×5)-Hex surface is evaluated by monitoring the spin asymmetry as the function of time. Within 25 hours after the surface preparation, the profile of the spin asymmetry and the reflected electron count for Ir(100)-(1×5)-Hex surface resembles that of hydrogen adsorbed Ir(100)-(1×5)-H surface. The electron energy-angle of incidence landscape of reflectivity, spin asymmetry and figure of merit are recorded for Ir(100)-(1×2+2×1)-O surface. Many wide regions with a large figure of merit are identified in the E- landscape. Chapter 4 reports SPLEED experiments carried out on Ir(100)-(1×5)-H surface. The comparison between asymmetries evaluated for the Ir(100)-(1×5)-Hex surface after 25 hours and Ir(100)-(1×5)-H surface suggests that Ir(100)-(1×5)-Hex surface is transforming to Ir(100)-(1×5)-H surface, in 25 hours. This can be due to the adsorption of more than four Langmuir of residual hydrogen during this time. The energy-angle landscape of reflectivity, asymmetry and figure of merit are recorded for Ir(100)-(1×5)-H surface in an energy range 20 eV to 100 eV and angle range 10 to 60 . Many regions are identified as the working point for specular reflection type spin filter based on SPLEED. The surface structure and surface preparation techniques are discussed. The stability of the surface is also evaluated. Chapter 5 investigates the growth and magnetic properties of Fe(100) film on Ir(100)-(1×1), Ir(100)-(1×5)-Hex and Ir(100)-(1×2+2×1)-O surfaces. LEED, MEED, LMOKE and QMOKE studies were presented. The growth is found to be layer-by-layer at least up to 20 monolayers (ML) at room temperature. At higher deposition temperature, the MEED oscillations disappear around 3-5 ML. Magnetic anisotropy of the Fe(100) film grown on Ir(100)-(1×2+2×1)-O surfaces is evaluated using LMOKE measurement using Kerr microscope. Simultaneous in-situ LMOKE and MEED measurements were carried out during the deposition. Ferromagnetic ordering with an in-plane easy axis starts above 4.5 ML at room temperature. The Kerr rotation normalized by thickness is evaluated in the pseudomorphic regime and strain relaxed regime. The probing depth of the MOKE is found to be around 14 nm in Fe(100)/Ir(100). An antisymmetric component is observed in the re-magnetization loop measured using MOKE. This antisymmetric loop arises due to the quadratic magneto-optic coupling which is separated by symmetrization and antisymmetrization procedure. The observed quadratic magneto-optic coupling suggests that the analysis based on the assumption that the magneto-optic coupling is linear in magnetization has to be modified. In order to quantify the quadratic magneto-optic coupling parameters, a QMOKE measurement system is developed and measurements were carried out. Chapter 6 discusses SPLEED experiments carried out on various thicknesses of Fe(100) film. Fe(100) films grown on Ir(100) substrate with the thickness less than or equal to 4 ML is not ferromagnetic with in-plane easy axis at room temperature. The non-zero exchange asymmetry observed for 5 ML and above indicates the presence of ferromagnetic ordering. A difference in the profile of exchange asymmetry is observed between pseudomorphic and strain relaxed regime. Large spin-orbit asymmetry is observed for 1 ML and 2 ML Fe(100) which is unexpected from a low atomic number (Z) material. The reason for large spin-orbit asymmetry is still unknown. The energy-angle landscape of reflectivity, exchange asymmetry, spin-orbit asymmetry and figure of merit were evaluated for 21 ML of Fe(100). Many working points were identified for different types multichannel spin filter based on exchange interaction Finally, the various results are summarized and a broad outlook is given.

Xu Xiangdong.
"December 2001."
Thesis (Ph.D.)--Chinese University of Hong Kong, 2001.
Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web.
Mode of access: World Wide Web.
Abstracts in English and Chinese.

We employ a variant of optical absorption spectroscopy, namely in situ differential reflectance spectroscopy (DRS), for an analysis of the structure-properties relations of thin epitaxial organic films. Clear correlations between the spectra and the differently intense coupling to the respective substrates are found. While rather broad and almost structureless spectra are obtained for a quaterrylene (QT) monolayer on Au(111), the spectral shape resembles that of isolated molecules when QT is grown on graphite. We even achieve an efficient electronic decoupling from the subjacent Au(111) by inserting an atomically thin organic spacer layer consisting of hexa-peri-hexabenzocoronene (HBC) with a noticeably dissimilar electronic behavior. These observations are further consolidated by a systematic variation of the metal substrate (Au, Ag, and Al), ranging from inert to rather reactive. For this purpose, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) is chosen to ensure comparability of the molecular film structures on the different metals, and also because its electronic alignment on various metal surfaces has previously been studied with great intensity. We present evidence for ionized PTCDA at several interfaces and propose the charge transfer to be related to the electronic level alignment governed by interface dipole formation on the respective metals.
Zur Analyse der Struktur-Eigenschafts-Beziehungen dünner, epitaktischer Molekülfilme wird in situ differentielle Reflexionsspektroskopie (DRS) als Variante der optischen Absorptionsspektroskopie verwendet. Klare Zusammenhänge zwischen den Spektren und der unterschiedlich starken Kopplung zum jeweiligen Substrat werden gefunden. Während man breite und beinahe unstrukturierte Spektren für eine Quaterrylen (QT) Monolage auf Au(111) erhält, ist die spektrale Form von auf Graphit abgeschiedenem QT ähnlich der isolierter Moleküle. Durch Einfügen einer atomar dünnen organischen Zwischenschicht bestehend aus Hexa-peri-hexabenzocoronen (HBC) mit einem deutlich unterschiedlichen elektronischen Verhalten gelingt sogar eine effiziente elektronische Entkopplung vom darunter liegenden Au(111). Diese Ergebnisse werden durch systematische Variation der Metallsubstrate (Au, Ag und Al), welche von inert bis sehr reaktiv reichen, untermauert. Zu diesem Zweck wird 3,4,9,10-Perylentetracarbonsäuredianhydrid (PTCDA) gewählt, um Vergleichbarkeit der molekularen Filmstrukturen zu gewährleisten, und weil dessen elektronische Anordnung auf verschiedenen Metalloberflächen bereits eingehend untersucht worden ist. Wir weisen ionisiertes PTCDA an einigen dieser Grenzflächen nach und schlagen vor, dass der Ladungsübergang mit der elektronischen Niveauanpassung zusammenhängt, welche mit der Ausbildung von Grenzflächendipolen auf den entsprechenden Metallen einhergeht.

Thin metallic films with magnetic properties like magnetite are an interesting material in current technological applications. In the presented work the iron oxide films are grown by molecular beam epitaxy on MgO(001) substrates at temperatures between room temperature and 600K. The film and surface structure are investigated by x-ray reflectometry (XRR), x-ray diffraction (XRD) and low energy electron diffraction (LEED). The chemical properties are investigated by x-ray photoelectron spectroscopy (XPS). Furthermore, charge transfer multiplet (CTM) calculations are performed as a means to gain additional information from photoemission spectra. It is shown that only for temperatures higher than 500K the oxide film forms a spinel structure. A previously unobserved (2x1) surface reconstruction in two orthogonal domains is found for various preparation conditions. The application of CTMs results in good quantitative and qualitative agreement to other methods for the determination of the film stoichiometry. In addition CTMs can well describe the segregation of Mg atoms into the oxide film either during film growth or during film annealing. It is found that initially Mg substitutes Fe on all possible lattice sites, only for prolonged treatment at high temperature do Mg atoms favour the octahedral lattice sites of divalent Fe.

The topic of this work is the structural characterization and theoretical modeling of organic single and heterolayers. The growth of sub-monolayers and monolayers (ML) of the two polycyclic aromatic hydrocarbons quaterrylene (QT) and hexa-peri-hexabenzocoronene (HBC) on Ag(111) and Au(111) was investigated. A transition from a disordered, isotropic phase to an ordered phase with increasing coverage was found. The lattice of the ordered phase turned out to be coverage dependent. The intermolecular potential was modeled including Coulomb and van der Waals interaction by a force-field approach. The postulated repulsive character of the potential could be connected to the non-uniform intramolecular charge distribution and to a screening of the van der Waals forces. Furthermore, the influence of the variable lattice constant on the epitaxial growth of HBC was studied. The second part of this work deals with a ML of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on a ML of HBC. In dependency on the initial lattice constant of HBC, a total of three line-on-line (LOL) and point-on-line coincident phases of PTCDA (with respect to HBC) was found. Following an analysis of the general properties of LOL coincident systems via force-field calculations, a new method to predict the structure of such systems is introduced.:1 Introduction 2 Experimental Methods 2.1 Organic molecular beam epitaxy 2.2 Scanning tunneling microscopy (STM) 2.3 Low-energy electron diffraction (LEED) 2.4 Molecules and substrates: Basic properties and literature review 2.4.1 3,4,9,10-Perylenetetracarboxylic dianhydride 2.4.2 Hexa-peri-hexabenzocoronene 2.4.3 Quaterrylene 2.4.4 Metal substrates: Au(111) and Ag(111) 3 Theory and Modeling 3.1 Reciprocal space and LEED 3.1.1 Fourier transform and geometrical LEED theory 3.1.2 Kinematic and dynamic LEED theory 3.1.3 Further applications of the Fourier transform 3.2 Computational chemistry 3.2.1 Calculating molecular properties 3.2.2 The atomic force-field method 3.2.3 Potential energy calculations in extended systems 4 Epitaxy in terms of potential energy 5 Interaction of QT and HBC at Sub-ML and ML Coverage 5.1 Experimental results 5.2 Modeling technique 5.3 Results of the model calculation 5.4 Discussion of results 5.5 Conclusion 6 The Ordered Phases of HBC on Ag(111) and Au(111 6.1 Geometrical analysis of epitaxy 6.2 Energetic gain of epitaxial structures 6.3 Comparison to experiment 6.4 Influence of the Au(111) surface reconstruction 6.5 Conclusion 7 Organic Heterosystems of PTCDA and HBC on Au(111) 7.1 PTCDA on Au(111) revisited 7.2 LEED and STM on PTCDA/HBC/Au(111) samples 7.2.1 A “compact” HBC layer substrate 7.2.2 A “loosely packed” HBC layer substrate 7.2.3 Summary of LEED results 7.2.4 STM results 7.3 Epitaxial relations in the system PTCDA/HBC/Au(111) 7.3.1 Geometrical analysis of epitaxy 7.3.2 Energetic gain of epitaxial structures 7.3.3 Mutual alignment of lattices 7.4 Heterosystems of PTCDA and HBC with inverted stacking sequence 8 General Properties of POL and LOL Epitaxy 8.1 A new coordinate system 8.2 Specific properties of the substrate-adsorbate potential 8.3 The “natural order” of the lattice lines 8.4 Prediction of epitaxial growth - a “LOL predictor” 8.4.1 Method 8.4.2 Results 9 General Conclusions and Future Perspectives 9.1 Conclusion 9.2 Outlook Appendix A.1 Conductance in a STM: The 1D WKB model A.2 Extraction of the DOS from STS measurements by means of the 1D WKB model A.3 Practical application of the 1D WKB model A.4 The normalized differential conductivity A.5 A new normalization method
Thema dieser Arbeit ist die strukturelle Charakterisierung von organischen Einfach- und Heterolagen sowie deren theoretische Beschreibung und Modellierung. Es wurden Submonolagen und Monolagen (ML) der polyzyklischen Kohlenwasserstoffe Quaterrylen (QT) und Hexa-peri-hexabenzocoronen (HBC) auf Ag(111) und Au(111) Einkristallen untersucht und ein Übergang von einer ungeordneten, isotropen Phase zu einer geordneten Phase mit steigender Bedeckung beobachtet. Die geordnete Phase wies dabei bedeckungsabhängige Gitterkonstanten auf. Das intermolekulare Potential wurde unter Berücksichtigung von Coulomb und van der Waals Anteilen mittels Kraftfeldmethoden modelliert. Der postulierte repulsive Charakter des Potentials konnte auf die Ladungsverteilung im Molekül und eine Abschwächung des van der Waals Potentials zurückgeführt werden. Weiterhin wurde der Einfluss der variablen HBC Gitterkonstante auf die epitaktische Relation des Gitters zum Metallsubstrat untersucht. Der zweite Teil der Arbeit widmet sich der Untersuchung einer ML 3,4,9,10-Perylenetetracarboxylic dianhydrid (PTCDA) auf einer ML HBC. Dabei wurden, in Abhängigkeit von der HBC Gitterkonstante, insgesamt drei verschiedene Typen von line-on-line bzw. point-on-line Epitaxie nachgewiesen. Im Anschluss an eine Analyse der generellen Eigenschaften solcher epitaktischer Lagen mittels Kraftfeldrechnungen wird eine neue Methode zur Vorhersage der Struktur konkreter Systeme vorgestellt.:1 Introduction 2 Experimental Methods 2.1 Organic molecular beam epitaxy 2.2 Scanning tunneling microscopy (STM) 2.3 Low-energy electron diffraction (LEED) 2.4 Molecules and substrates: Basic properties and literature review 2.4.1 3,4,9,10-Perylenetetracarboxylic dianhydride 2.4.2 Hexa-peri-hexabenzocoronene 2.4.3 Quaterrylene 2.4.4 Metal substrates: Au(111) and Ag(111) 3 Theory and Modeling 3.1 Reciprocal space and LEED 3.1.1 Fourier transform and geometrical LEED theory 3.1.2 Kinematic and dynamic LEED theory 3.1.3 Further applications of the Fourier transform 3.2 Computational chemistry 3.2.1 Calculating molecular properties 3.2.2 The atomic force-field method 3.2.3 Potential energy calculations in extended systems 4 Epitaxy in terms of potential energy 5 Interaction of QT and HBC at Sub-ML and ML Coverage 5.1 Experimental results 5.2 Modeling technique 5.3 Results of the model calculation 5.4 Discussion of results 5.5 Conclusion 6 The Ordered Phases of HBC on Ag(111) and Au(111 6.1 Geometrical analysis of epitaxy 6.2 Energetic gain of epitaxial structures 6.3 Comparison to experiment 6.4 Influence of the Au(111) surface reconstruction 6.5 Conclusion 7 Organic Heterosystems of PTCDA and HBC on Au(111) 7.1 PTCDA on Au(111) revisited 7.2 LEED and STM on PTCDA/HBC/Au(111) samples 7.2.1 A “compact” HBC layer substrate 7.2.2 A “loosely packed” HBC layer substrate 7.2.3 Summary of LEED results 7.2.4 STM results 7.3 Epitaxial relations in the system PTCDA/HBC/Au(111) 7.3.1 Geometrical analysis of epitaxy 7.3.2 Energetic gain of epitaxial structures 7.3.3 Mutual alignment of lattices 7.4 Heterosystems of PTCDA and HBC with inverted stacking sequence 8 General Properties of POL and LOL Epitaxy 8.1 A new coordinate system 8.2 Specific properties of the substrate-adsorbate potential 8.3 The “natural order” of the lattice lines 8.4 Prediction of epitaxial growth - a “LOL predictor” 8.4.1 Method 8.4.2 Results 9 General Conclusions and Future Perspectives 9.1 Conclusion 9.2 Outlook Appendix A.1 Conductance in a STM: The 1D WKB model A.2 Extraction of the DOS from STS measurements by means of the 1D WKB model A.3 Practical application of the 1D WKB model A.4 The normalized differential conductivity A.5 A new normalization method