In dieser Arbeit wird die Machbarkeit der Herstellung von Leuchtdioden Strukturen (LEDs) in einzelnen GaN Nanodrähten (ND) und deren Integration mit herkömmlicher Si Technologie untersucht. Hierzu wird zunächst ein generelles Verständnis des Wachstums von GaN ND erarbeitet und dargestellt. Es folgen Untersuchungen zum Einfluss von Dotierstoffen, wie z.B. Mg und Si, auf das Wachstum der ND. Dieses Wissen wird anschließend angewandt um Dotierübergänge in GaN ND herzustellen die nominell n-i-p bzw. p-i-n dotiert sind. Diese Untersuchung brachte die technologisch wichtige Erkenntnis, dass eine p-Dotierung mit Mg am besten erreicht werden kann wenn die ND bereits wohl entwickelt sind. Dies bedeutet, dass der obere Teil der ND LEDs aus p-Typ Material bestehen wird. Eine weitere wichtige Komponente von LEDs ist die aktive Zone in der die Elektron-Loch-Rekombination stattfindet. Im Fall von planaren GaN LEDs wird diese durch Zugabe von In und die Formierung von InGaN hergestellt. Wir untersuchen das Wachstum von InGaN auf Si, GaN NDs und in Form von MQWs, um das Wachstum und insbesondere den In Gehalt unter vielen Bedingungen kontrollieren zu können. Das gesamte Wissen der Voruntersuchungen wird kombiniert und für das Ziel dieser Arbeit nutzbar gemacht: Die Herstellung von GaN ND basierten LEDs. Diese Strukturen werden untersucht und zu einer funktionierenden LED weiter prozessiert. Abschließend wird von den Anstrengungen zur Integration von III-Nitrid LEDs und Si basierter MOSFET Technologie berichtet. Es wird erstmalig erfolgreich die monolithische Integration dieser beiden Bauelemente und ihr gleichzeitiges Funktionieren gezeigt. / This work is concerned with the realization and investigation of a light emitting diode (LED) structure within single GaN nanowires (NWs) and its integration with Si technology. To this end first a general understanding of the GaN NW growth is given. This is followed by investigations of the influence which doping species, such as Mg and Si, have on the growth of the NWs. The experience gathered in these studies set the basis for the synthesis of nominal p-i-n and n-i-p junctions in GaN NWs. Investigations of these structures resulted in the technologically important insight, that p-type doping with Mg is achieved best if it is done in the later NW growth stage. This implies that it is beneficial for a NW LED to place the p-type segment on the NW top. Another important component of an LED is the active zone where electron-hole recombination takes place. In the case of planar GaN LEDs, this is usually achieved by alloying Ga and In to form InGaN. In order to be able to control the growth under a variety of conditions, we investigate the growth of InGaN in the form of extended segments on top of GaN NWs, as well as multi quantum wells (MQWs) in GaN NWs. All the knowledge gained during these preliminary studies is harnessed to reach the overall goal: The realization of a GaN NW LED. Such structures are fabricated, investigated and processed into working LEDs. Finally, a report on the efforts of integrating III-nitride NW LEDs and Si based metal-oxide-semiconductor field effect transistor (MOSFET) technology is given. This demonstrates the feasibility of the monolithic integration of both devices on the same wafer at the same time.

Thermoelektrische Phänomene können in Nanomaterialien im Vergleich zum Volumenmaterial stark modifiziert werden. Die Bestimmung der elektrischen Leitfähigkeit, des absoluten Seebeck-Koeffizienten (S) und der Wärmeleitfähigkeit ist eine wesentliche Herausforderung für die Messtechnik in Hinblick auf Mikro- und Nanostrukturen aufgrund dessen, dass die Transporteigenschaften vom Volumenmaterial sich durch Oberflächen- und Einschränkungseffekte verändern können. Im Rahmen dieser Abschlussarbeit wird der Einfluss von Größeneffekten auf die thermoelektrischen Eigenschaften von dünnen Platinschichten untersucht und mit dem Volumenmaterial verglichen. Dafür wurde eine Messplattform als standardisierte Methode entwickelt, um S einer dünnen Schicht zu bestimmen. Strukturelle Eigenschaften wie Schichtdicke und Korngröße werden variiert. Grenz- und Oberflächenstreuung reduzieren S der dünnen Schichten im Vergleich zum Volumenmaterial. Außerdem wird eine Methode demonstriert um S von einzelnen metallischen Nanodrähten zu bestimmen. Für hochreine und einkristalline Silber-Nanodrähte wird der Einfluss von Nanostrukturierung auf die Temperaturabhängigkeit von S gezeigt. Ein Modell ermöglicht die eindeutige Zerlegung des temperaturabhängigen S von Platin und Silber in einen Thermodiffusions- und Phononen-Drag-Anteil. Des Weiteren werden die thermoelektrischen Transporteigenschaften von einzelnen auf Bismut-basierenden Kern/Hülle-Nanodrähten untersucht. Der Einfluss des Hüllenmaterials (Tellur oder Titandioxid) und der räumlichen Dimension des Nanodrahts auf die Transporteigenschaften wird diskutiert. Streuung an Oberflächen, Einkerbungen und Grenzflächen zwischen dem Kern und der Hülle reduzieren die elektrische und thermische Leitfähigkeit. Eine Druckverformung induziert durch die Hülle kann zu einer Bandöffnung bei Bismut führen, sodass S gesteigert werden kann. Das Kern/Hülle-System zeigt in eine Richtung, um die thermoelektrischen Eigenschaften von Bismut erfolgreich anzupassen. / Thermoelectric phenomena can be strongly modified in nanomaterials compared to the bulk. The determination of the electrical conductivity, the absolute Seebeck coefficient (S) and the thermal conductivity is a major challenge for metrology with respect to micro- and nanostructures because the transport properties of the bulk may change due to surface and confinement effects. Within the scope of this thesis, the influence of size effects on the thermoelectric properties of thin platinum films is investigated and compared to the bulk. For this reason, a measurement platform was developed as a standardized method to determine S of a thin film. Structural properties, like film thickness and grain size, are varied. Boundary and surface scattering reduce S of the thin films compared to the bulk. In addition, a method is demonstrated to determine S of individual metallic nanowires. For highly pure and single crystalline silver nanowires, the influence of nanopatterning on the temperature dependence of S is shown. A model allows the distinct decomposition of the temperature-dependent S of platinum and silver into a thermodiffusion and phonon drag contribution. Furthermore, the thermoelectric transport properties of individual bismuth-based core/shell nanowires are investigated. The influence of the shell material (tellurium or titanium dioxide) and spatial dimension of the nanowire on the transport properties are discussed. Scattering at surfaces, indentations and interfaces between the core and the shell reduces the electrical and the thermal conductivity. A compressive strain induced by the shell can lead to a band opening of bismuth increasing S. The core/shell system points towards a route to successfully tailor the thermoelectric properties of bismuth.

Diese Arbeit beschäftigt sich mit dem Wachstumsmechanismus von Silberjodid Nanodrähten. Das Wachstum wurde über einen Zeitraum von Minuten bis hin zu Tagen untersucht. Im frühen Stadium bilden sich Silbernanopartikel innerhalb der Farbstoffröhren, welche als Keime für das weitere Wachstum von isolierten Drahtstücken dienen. Der Durchmesser dieser Drähte wird durch den Innendurchmesser der Röhren definiert. Im letzten Stadium wachsen diese Drahtstücke zusammen bis sie das gesamte Aggregat füllen. Dieser Wachstumsprozess impliziert einen Transport von Silber Ionen durch die Wand der Röhre. Das Wachstum der Drähte setzt sich weiter fort nachdem das Template gleichmäßig mit Drähten gefüllt ist und zerstört die Röhren in der Folge. Die Kristallstruktur der Drähte wurde sowohl mit hochauflösender Elektronenmikroskopie als auch Elektronenbeugung untersucht. Das Silberjodid konnte aufgrund seiner charakteristischen Wurtzite Struktur in der beta-Phase identifiziert werden. Da der Lösung nur Silbernitrat beigesetzt wurde, konnte die Quelle der Jod-Ionen als Verunreinigung im Farbstoffpulver ausgemacht werden. Das fragmentierte Wachstum der Drähte von verschiedenen Startpunkten aus führt zu Kristallen mit einkristallinen Domänen von mehr als 100 nm Länge. Eine bevorzugte Orientierung der Kristallstruktur relativ zur Aggregatachse wurde gefunden und durch die Molekülstruktur der Aggregate erklärt. Basierend auf diesen Ergebnissen wurde ein Model zum Wachstum von Silberjodid Nanodrähten im Inneren eines röhrenförmigen Molekülaggregats entwickelt. Es wurde angenommen, dass das Wachstum an Silberkeimen beginnt, die durch Photooxidation der bereits vorhandenen Jod Ionen mit Silber Ionen während der Belichtung der Probe gebildet werden. Diese Silberkeime ermöglichen die Bildung von stabilen Silberjodid Kristalliten und das nachfolgende Wachstum zu Drähten. Die Ergebnisse zeigen einen möglichen Weg zur Synthese von Metall-Halogenid Strukturen innerhalb von Farbstoffröhren. / This thesis reports on the growth mechanism of silver iodide nanowires as revealed by conventional as well as cryogenic transmission electron microscopy. The growth, initiated by short illumination with UV light, has been observed over time scales ranging from minutes to days. In an early stage, within the tubular aggregates nanoparticles are formed which act as seeds for continuous growth of separate pieces of wires. The diameter of the wires is determined by the inner diameter of the tubes. In the final state, the pieces of wire totally fill the aggregate. The growth process indicates transport of at least silver ions through the tubular wall membrane. After homogeneously filling the template the wires grow onwards over the diameter of the nanotubes, destroying it in the process. The crystal structure of the wires was investigated by means of high resolution transmission electron microscopy and selected area electron diffraction. The silver iodide could be clearly identified in its beta-phase by its typical wurtzite structure. Since only silver nitrate was added to the solutions, the source of the iodide ions could be attributed to impurities within the dye powder itself. The fragmented growth of the wires from separate seeds leads to nanowires consisting of single crystalline domains exceeding 100 nm in length. A preferential orientation of the crystal lattice planes with respect to the aggregate axis was observed which is explained by the molecular structure of the aggregates. Based on these findings a model for the growth of silver iodide nanowires within the inner space of the tubular molecular aggregate is presented. The growth is assumed to start at silver seeds that are formed due to photo-oxidation of the already present iodide ions by the silver ions during the illumination of the sample. These silver seeds facilitate nucleation of silver iodide and subsequent growth into wires.

The monolithic integration of III-V semiconductors on Si substrates is a part of a long-term technological roadmap for the semiconductor industry towards More-than-Moore technologies. Despite of the different lattice constants and thermal expansion coefficients, research efforts over the last two decades have shown that III-V crystals with a high structural quality can be grown epitaxially in the form of nanowires directly on Si using CMOS-compatible (Au-free) methods. Among other III-V compounds, InxGa1-xAs is of the special interest for the use in infrared photonics and high-speed electronics due to its tunable direct bandgap and low electron effective mass, respectively. For comparison, InxGa1-xAs thin films are typically grown on lattice-matched InP substrates with a limited range of compositions at around x=0.52. The realization of InxGa1-xAs nanowires on Si, though, has been proved challenging owing to the limited In-content when the nanowires are grown Ga-catalyzed or the high density of stacking faults when the nanowires are grown catalyst-free. In this work, the use of highly lattice-mismatched GaAs/InxGa1-xAs and GaAs/InxAl1-xAs core/shell nanowires on Si(111) substrates have been studied as an alternative to InxGa1-xAs nanowires. The core/shell mismatch strain and its accommodation within the nanowires plays an important role in the growth, the structural, and the electronic properties of the nanowires. A key parameter in this work was the unusually small diameter of 20 – 25 nm of the GaAs core. First, the strain-induced bending of the nanowires during the growth of the shell by molecular beam epitaxy was investigated. It was apparent that the nanowires bend as a result of a preferential incorporation of In adatoms on one side of the nanowires. To obtain straight nanowires with symmetric shell composition and thickness around the core, it was necessary to choose relatively low growth temperatures and high growth rates that limited the surface diffusivity of In adatoms. Second, the strain accommodation in straight nanowires was investigated as a function of the shell thickness and composition using a combination of Raman scattering spectroscopy and X-ray diffraction. For a fixed shell composition of x=0.20 and small enough shell thicknesses, the strain in the shell is compressive and decreases progressively as the shell grows thicker. On the other hand, the strain in the core is tensile with hydrostatic character and increases with shell thickness. Finally, for shell thicknesses larger than 40 nm, the shell becomes strain-free, whereas the strain in the core saturates at 3.2% without any dislocations. For a fixed shell thickness of 80 nm, the strain in the core was further increased by increasing the In-content in the shell, reaching values as high as 7% for x=0.54. A plastic relaxation via misfit dislocations was observed only for the next highest In-content of x=0.70. In agreement to theoretical predictions, the tensile strain in the core resulted in a large reduction of the GaAs bandgap (as measured by photoluminescence spectroscopy), up to approximately 40% of the strain-free value. A similar reduction in electron effective mass is also expected. The transport properties of electrons inside the strained GaAs core were assessed by optical-pump terahertz-probe spectroscopy. Quite high mobility values of approximately 6100 cm2/Vs at 300 K for a carrier concentration of 9×1017 cm−3 were measured, which are the highest reported in the literature for GaAs nanowires, but also higher than the values for unstrained bulk GaAs. The importance of the results in this work is two-fold. On the one hand, strain-free InxGa1-xAs nanowire shells were grown on Si substrates with x up to 0.54 and thicknesses well beyond the critical thickness of their thin film counterparts. Such shells could potentially be employed as conduction channels in high electron mobility transistors (HEMTs) integrated in Si platforms. On the other hand, highly tensile-strained GaAs cores with electronic properties like those of InxGa1-xAs thin films were obtained. In this case, the results demonstrate, that GaAs nanowires can be suitable for photonic devices across the near-infrared range, including telecom photonics at 1.3 and potentially 1.55 μm, as well as for high-speed electronics. GaAs as a binary material is expected to be advantageous compared to InxGa1-xAs due to the absence of structural imperfections typically present in ternary alloys. Finally, to explore the potential of the core/shell nanowires as HEMTs, self-consistent Schrödinger-Poisson calculations of two different modulation-doped heterostructures were performed. In the case of a strained GaAs core overgrown by an unstrained InxGa1-xAs shell and an additional unstrained Si-doped InxAl1-xAs shell, the possibility to form a cylindrical-like two-dimensional electron gas inside the InxGa1-xAs shell was found. In the alternative case of a strained GaAs core overgrown by an unstrained Si-doped InxAl1-xAs shell, it was found that it is possible to form a quasi-one-dimensional electron gas at the center of the core. Both structures are the subject of ongoing research.:1 Introduction 1 2 Fundamentals and state-of-the-art 7 2.1 Electronic and structural properties of III-V semiconductors 7 2.2 Growth of III-V nanowires on Si 20 2.3 Core/shell heterostructure nanowires 29 2.4 Strain in epilayers and core/shell nanowires 36 2.5 Strain engineering in core/shell nanowires and its effect on band parameters 46 2.6 Modulation-doped III-V semiconductor heterostructures 56 3 Methods 61 3.1 Optical and electron microscopes 61 3.2 X-ray diffraction 64 3.3 Raman scattering spectroscopy 65 3.4 Photoluminescence spectroscopy 75 3.5 Optical-pump terahertz-probe spectroscopy and photoconductivity in semiconductors 77 3.6 Device processing 82 3.7 Semiconductor nanodevice software “nextnano” 85 3.8 MBE for crystal growth and core/shell nanowire growth 86 4 Results and discussions 91 4.1 Structural, compositional analyses of straight nanowires and coherent growth limit 91 4.2 Bent nanowires 95 4.3 Strain analyses in core/shell nanowires 97 4.3.1 Dependence of strain on shell thickness 97 4.3.2 Dependence of strain on the shell chemical composition 102 4.3.3 Dependence of strain on the core diameter 105 4.4 Strain-induced modification of electronic properties 106 4.5 Strain-enhanced electron mobility of GaAs nanowires higher than the bulk limit 114 4.6 Towards high electron mobility transistors 123 5 Conclusion and outlook 129 Bibliography 131 List of abbreviations I List of Symbols III List of publications VII List of conference contributions VIII Acknowledgements X

GaAs-based nanowires are attractive building blocks for the development of future (opto)electronic devices owing to their excellent intrinsic material properties, such as the direct band gap and high electron mobility. A pre-requisite for the implementation of novel functionalities on a single Si chip is the monolithic integration of the nanowires on the well-established Si complementary-metal-oxide-semiconductor (CMOS) platform with precise control of the nanowire growth process. The self-catalyzed (Ga-assisted) growth of GaAs nanowires on Si(111) substrates using molecular beam epitaxy has offered the possibility to obtain vertical nanowires with predominant zinc blende structure, while potential contamination by external catalysts like Au is eliminated. Although the growth mechanism is fairly well understood, control of the nucleation stage, the nanowire number density and the crystal structure has been proven rather challenging. Moreover, conventional growth processes are typically performed at relatively high substrate temperatures in the range of 560-630 °C, which limit their application to the industrial Si platform. This thesis provides two original methods in order to tackle the aforementioned challenges in the conventional growth processes. In the first part of this thesis, a simple surface modification procedure (SMP) for the in situ preparation of native-SiOx/Si(111) substrates has been developed. Using a pre-growth treatment of the substrates with Ga droplets and two annealing cycles, the SMP enables highly synchronized nucleation of all nanowires on their substrate and thus, the growth of exceptionally uniform GaAs nanowire ensembles with sub-Poissonian length distributions. Moreover, the nanowire number density can be tuned within three orders of magnitude and independent of the nanowire dimensions without prior ex situ patterning of the substrate. This work delivers a fundamental understanding of the nucleation kinetics of Ga droplets on native-SiOx and their interaction with SiOx, and confirms theoretical predictions about the so-called nucleation antibunching, the temporal anti-correlation of consecutive nucleation events. In the second part of this thesis, an alternative method called droplet-confined alternate-pulsed epitaxy (DCAPE) for the self-catalyzed growth of GaAs nanowires and GaAs/AlxGa1-xAs axial nanowire heterostructures has been developed. DCAPE enables nanowire growth at unconventional, low temperatures in the range of 450-550 °C and is compatible with the standard Si-CMOS platform. The novel growth approach allows one to precisely control the crystal structure of the nanowires and, thus, to produce defect-free pure zinc blende GaAs-based nanowires. The strength of DCAPE is further highlighted by the controlled growth of GaAs/AlxGa1-xAs axial quantum well nanowires with abrupt interfaces and tunable thickness and Al-content of the AlxGa1-xAs sections. The GaAs/AlxGa1-xAs axial nanowire heterostructures are interesting for applications as single photon emitters with tunable emission wavelength, when they are overgrown with thick lattice-mismatched InxAl1-xAs layers in a core-shell fashion. All results presented in this thesis contribute to paving the way for a successful monolithic integration of highly uniform GaAs-based nanowires with controlled number density, dimensions and crystal structure on the mature Si platform. / GaAs-basierte Nanodrähte sind attraktive Bausteine für die Entwicklung von zukünftigen (opto)elektronischen Bauelementen dank ihrer exzellenten intrinsischen Materialeigenschaften wie zum Beispiel die direkte Bandlücke und die hohe Elektronenbeweglichkeit. Eine Voraussetzung für die Realisierung neuer Funktionalitäten auf einem einzelnen Si Chip ist die monolithische Integration der Nanodrähte auf der etablierten Si-Metall-Oxid-Halbleiter-Plattform (CMOS) mit präziser Kontrolle des Wachstumsprozesses der Nanodrähte. Das selbstkatalytische (Ga-unterstützte) Wachstum von GaAs Nanodrähten auf Si(111)-Substrat mittels Molekularstrahlepitaxie bietet die Möglichkeit vertikale Nanodrähte mit vorwiegend Zinkblende-Struktur herzustellen, während die potentielle Verunreinigung der Nanodrähte und des Substrats durch externe Katalysatoren wie Au vermieden wird. Obwohl der Wachstumsmechanismus gut verstanden ist, erweist sich die Kontrolle der Nukleationsphase, Anzahldichte und Kristallstruktur der Nanodrähte als sehr schwierig. Darüber hinaus sind relativ hohe Temperaturen im Bereich von 560-630 °C in konventionellen Wachstumsprozessen notwendig, die deren Anwendung auf der industriellen Si Plattform begrenzen. Die vorliegende Arbeit liefert zwei originelle Methoden um die bestehenden Herausforderungen in konventionellen Wachstumsprozessen zu bewältigen. Im ersten Teil dieser Arbeit wurde eine einfache Prozedur, bezeichnet als surface modification procedure (SMP), für die in situ Vorbehandlung von nativem-SiOx/Si(111)-Substrat entwickelt. Die Substratvorbehandlung mit Ga-Tröpfchen und zwei Hochtemperaturschritten vor dem Wachstumsprozess ermöglicht eine synchronisierte Nukleation aller Nanodrähte auf ihrem Substrat und folglich das Wachstum von sehr gleichförmigen GaAs Nanodraht-Ensembles mit einer sub-Poisson Verteilung der Nanodrahtlängen. Des Weiteren kann die Anzahldichte der Nanodrähte unabhängig von deren Abmessungen und ohne ex situ Vorstrukturierung des Substrats über drei Größenordnungen eingestellt werden. Diese Arbeit liefert außerdem ein grundlegendes Verständnis zur Nukleationskinetik von Ga-Tröpfchen auf nativem-SiOx und deren Wechselwirkung mit SiOx und bestätigt theoretische Voraussagen zum sogenannten Nukleations-Antibunching, dem Auftreten einer zeitlichen Anti-Korrelation aufeinanderfolgender Nukleationsereignisse. Im zweiten Teil dieser Arbeit wurde eine alternative Methode, bezeichnet als droplet-confined alternate-pulsed epitaxy (DCAPE), für das selbstkatalytische Wachstum von GaAs Nanodrähten und GaAs/AlxGa1-xAs axialen Nanodraht-Heterostrukturen entwickelt. DCAPE ermöglicht das Nanodrahtwachstum bei unkonventionell geringeren Temperaturen im Bereich von 450-550 °C und ist vollständig kompatibel mit der Standard-Si-CMOS-Plattform. Der neue Wachstumsansatz erlaubt eine präzise Kontrolle der Kristallstruktur der Nanodrähte und folglich das Wachstum von defektfreien Nanodrähten mit phasenreiner Zinkblende-Struktur. Die Stärke der DCAPE Methode wird des Weiteren durch das kontrollierte Wachstum von GaAs/AlxGa1-xAs axialen Quantentopf-Nanodrähten mit abrupten Grenzflächen und einstellbarer Dicke und Al-Anteil der AlxGa1-xAs-Segmente aufgezeigt. Die GaAs/AlxGa1-xAs axialen Nanodraht-Heterostrukturen sind interessant für den Einsatz als Einzelphotonen-Emitter mit einstellbarer Emissionswellenlänge, wenn diese mit gitterfehlangepassten InxAl1-xAs-Schichten in einer Kern-Hülle-Konfiguration überwachsen werden. Alle Ergebnisse dieser Arbeit tragen dazu bei, den Weg für eine erfolgreiche monolithische Integration von sehr gleichförmigen GaAs-basierten Nanodrähten mit kontrollierbarer Anzahldichte, Abmessungen und Kristallstruktur auf der industriell etablierten Si-Plattform zu ebnen.

Mit Fe-Pd-Legierungen nahe der Zusammensetzung Fe70Pd30 kann man aufgrund des thermischen oder magnetischen Formgedächtniseffekts große Dehnungen erzeugen. Daher sind sie für Mikro- und Nanoaktoren sowie Sensoren von großem wissenschaftlichen und technologischen Interesse. Im Vergleich zu Massivmaterial und dünnen Schichten erwartet man für eindimensionale Geometrien wie Nanodrähte deutlich höhere Arbeitsfrequenzen und Dehnungen. Zur Herstellung von Nanodrähten eignet sich die elektrochemische Abscheidung in selbstordnende nanoporöse Membranen als effizienteste Methode gegenüber lithographischen oder physikalischen Methoden. Um den Formgedächtniseffekt auch in Fe-Pd-Nanodrähten mit ca. 30 at.% Pd zu nutzen, werden in dieser Arbeit entsprechende Herstellungsbedingungen wie Elektrolytsystem, Abscheideparameter und Nachbehandlung herausgearbeitet. Die Zusammenhänge zwischen Abscheidebedingungen und Morphologie, lokaler Mikrostruktur, Struktur sowie magnetischen Eigenschaften werden untersucht und bewertet. Es wird gezeigt, dass Fe-Pd-Nanodrähte trotz der Kombination aus edlem und unedlem Metall elektrochemisch hergestellt werden können. Ein komplexierter Fe-Pd-Elektrolyt in Kombination mit optimierten alternierenden Abscheidepotentialen führt reproduzierbar zu durchgehenden, nahezu defektfreien Nanodrähten nahe der Zusammensetzung Fe70Pd30. Mit einer nachträglichen Wärmebehandlung erreicht man eine vollständige Umwandlung der Fe-Pd-Legierung von der kubisch raumzentrierten zur kubisch flächenzentrierten Struktur. Die erfolgreiche Herstellung dieser Nanodrähte stellt eine Schlüsselposition auf dem Weg zu formgedächtnisbasierten Nanoaktoren dar. In dieser Arbeit konnten wichtige Ansatzpunkte zur Strukturkontrolle während der elektrochemischen Abscheidung und somit zur Aktivierung des Formgedächtniseffekts identifiziert werden. / Fe-Pd alloys at about 30 at.% Pd allow obtaining high length changes or strains in the percent range due to thermal or magnetic shape memory effect. They are especially promising candidates for smart and intelligent materials in micro- and nanoactuators as well as sensors. In comparison to bulk materials and thin films the utilization of nanowires promises higher actuation frequencies and strains, which further heighten the scientific and technological interest. Electrodeposition within self-organized nanoporous templates is a very time efficient method to prepare even large arrays of Fe-Pd nanowires of different length and diameter compared to lithographic or physical methods. The aim of this work is to exhibit the preparation conditions such as electrolyte system, deposition parameter and post treatment for shape memory active Fe-Pd nanowires at about 30 at.% Pd. Correlations between morphology, local microstructure, structure and magnetic properties are investigated and evaluated. Fe-Pd nanowires are successfully prepared by electrodeposition despite the combination of noble Pd and less noble Fe metals. The usage of an electrolyte with complexed Fe and Pd ions and an optimized alternating potential deposition regime leads to continuous and almost defect free nanowires close to the composition Fe70Pd30. The complete transition from the bcc to fcc structure of the Fe-Pd alloy is achieved by an additional heat treatment. However, the successful preparation of these nanowires represents a key element towards nanoactuators based on shape memory alloys. Fundamental knowledge about electrochemical preparation of Fe-Pd nanowires is gained. Important starting points towards structure control during deposition and activation of the shape memory effect are identified.

Pursuing the development and implementation of novel synthesis techniques to produce nanostructures with an interesting set of properties is a goal that advances the frontiers of nanotechnology. Also of fundamental importance is to revisit well-established synthesis techniques employing a new set of materials as precursors, substrates and catalysts. Fundamental breakthroughs in the field of nanotechnology can be achieved by developing new synthesis procedures as well as by adapting known procedures to new materials. This thesis focuses on both kinds of experiments. A variant of chemical vapor deposition (CVD) has been used to produce Al5BO9 nanowires out of sapphire wafers without the need of a catalyst material. The novelty of the work relies on the formation mechanism of the Al5BO9 nanowires. Essentially, the process can be described as a large-scale topological transformation taking place on the substrate’s surface as its chemical composition changes due to the arrival of precursor molecules. Dense mats of Al5BO9 nanowires cover large areas of the substrate that were previously relatively flat. The process is enhanced by a high temperature and the presence of pre-existing superficial defects (cracks, terraces, etc.) on the substrates. Al5BO9 nanowires as well as B/BOX nanowires and BOX nanotubes were also produced via a novel in-situ electron beam-induced synthesis technique. The process was carried out at room temperature and inside a transmission electron microscope. Au nanoparticles were used as catalyst for the case of B/BOX nanowires and BOX nanotubes, while the Al5BO9 nanowires were synthesized without the need of a catalyst material. The formation and growth of the nanostructures is solely driven by the electron beam. The growth mechanism of the B/BOX nanowires and BOX nanotubes relies on interplay between electrostatic charging of the precursor material (to produce and transport feedstock material) and electron stimulated desorption of oxygen which is able to activate the catalytic properties of the Au nanoparticles. For the case Al5BO9 nanowires a nucleation process based on massive atomic rearrangement in the precursor is instigated by the e-beam, afterwards, the length of some of the nanowires can be extended by a mechanism analogous to that of the growth of the B/BOX nanowires.

The assembly of nanostructures into “multi”-dimensional materials is one of the main topics occurring in nanoscience today. It is now possible to produce high quality nanostructures reproducibly but for their further application larger structures that are easier to handle are required. Nevertheless during their assembly their nanometer size and accompanying properties must be maintained. This challenge was addressed in this work. Lead chalcogenides have been chosen as an example system because they are expected to offer great opportunities as thermoelectric materials. Three different ways to achieve assemblies of lead chalcogenide nanostructures were used and the resulting structures characterized with respect to their potential application as thermoelectric material. The first means by which a “multi”-dimensional assembly of lead chalcogenide quantum dots can be produced is the formation of porous structures such as aerogels and xerogels. A procedure, where the addition of an initiator such as oxidizers or incident radiation is unnecessary, is introduced and the formation process studied by absorption spectroscopy. The time-consuming aggregation step could be significantly reduced by employing a slightly elevated temperature during gelation that does not lead to any observable differences within the resulting gel structures. After either supercritical or subcritical drying, highly porous monolithic gel structures can be achieved. During the gel formation the size and the shape of the particles changed and they were directly linked together. Nevertheless the resulting porous structures remain crystalline and size dependent effects of the optical properties could be shown. Gels produced from a mixture of PbS and PbSe QDs show a homogenous distribution of both materials but it is not clear to what extent they form an alloy. Although the particles are directly linked together the resulting porous structures possess a very high resistivity and so it was not possible to characterize the semiconductor aerogels with regard to their thermoelectric properties. To achieve an enhanced conductivity porous structures containing PbS and Au nanoparticles have been produced. As has been seen for the pure semiconductor gels the size of the PbS quantum dots has increased and elongated particles were formed. In contrast to the PbS QDs the Au nanoparticles did not change their size and shape and are unevenly distributed within the PbS network. Through the use of the gold nanoparticles the conductivity could be increased and although the conductivity is still quite small, it was possible to determine Seebeck coefficients near room temperature for a mixed semiconductor-metal gel. The second means by which QD solids could be formed was by the compaction of the QD building blocks into a material that is still nanostructured. Therefore the synthesis of PbS was optimized to achieve sufficient amounts of PbS quantum dots. The ligands used in the synthesis of the QDs unfortunately act as an insulating layer resulting in QD solids with resistivities as high as 2 Gigaohm. For this reason different surface modification strategies were introduced to minimize the interparticle distance and to increase the coupling between the QDs so as to increase the conductivity of the resulting quantum dot solids. One very promising method was the exchange of the initial ligands by shorter ones that can be destroyed at lower temperatures. By such heat treatments the resistivity could be decreased by up to six orders of magnitude. For the pressing of the quantum dots two different compaction methods (SPS and hydraulic pressing) were compared. While the grain growth within the SPS pressed samples is significantly higher the same densification can be achieved by a cold hydraulic pressing as well as by SPS. The densification could be further increased through the use of preheated PbS QDs due to the destruction of the ligands. Samples which had been surface modified with MPA and subsequently thermally treated show the best results with respect to their thermopower and resistivities. Nevertheless the conductivity of the QD solids is still too high for them to be used as efficient thermoelectric materials. The final assembly method does not involve QDs but instead with one dimensional nanowires. Therefore a synthesis was developed that enables the formation of PbS nanowires of different diameters and one that is easy up-scalable. By the use of a less reactive sulfur precursor and an additional surfactant the formation of nuclei is significantly retarded and within an annealing time of two hours nanowires can be formed presumably by an oriented attachment mechanism. Single crystalline nanowires with a diameter of 65-105 nm could be achieved with the longest axes of the nanowires being parallel to [100]. The resulting nanowires were used as building blocks for film formation on glass substrates by an easily implemented method that requires no special equipment. To characterize the films with a view to their possible application as a thermoelectric material, surface modifications of the films were performed to improve the charge transfer in the films and the Seebeck coefficients of the resulting films measured. Therefore the previous approach of using MPA was applied and a subsequent thermal treatment demonstrated very promising results. In addition an crosslinking ligand was used for surface treatment that leads to similar results as was observed for the thermally treated MPA approach. Both approaches lead to an order of magnitude decrease in the resistivity and due to the fewer grain boundaries present in the films composed of nanowires as compared to the QD assemblies the conductivity is significantly higher. The Seebeck coefficient measurements show that the thermal treatment only slightly affects the Seebeck coefficients. Therefore a significantly higher power factor could be achieved for the nanowire films than for the QD solids.

Mit Fe-Pd-Legierungen nahe der Zusammensetzung Fe70Pd30 kann man aufgrund des thermischen oder magnetischen Formgedächtniseffekts große Dehnungen erzeugen. Daher sind sie für Mikro- und Nanoaktoren sowie Sensoren von großem wissenschaftlichen und technologischen Interesse. Im Vergleich zu Massivmaterial und dünnen Schichten erwartet man für eindimensionale Geometrien wie Nanodrähte deutlich höhere Arbeitsfrequenzen und Dehnungen. Zur Herstellung von Nanodrähten eignet sich die elektrochemische Abscheidung in selbstordnende nanoporöse Membranen als effizienteste Methode gegenüber lithographischen oder physikalischen Methoden. Um den Formgedächtniseffekt auch in Fe-Pd-Nanodrähten mit ca. 30 at.% Pd zu nutzen, werden in dieser Arbeit entsprechende Herstellungsbedingungen wie Elektrolytsystem, Abscheideparameter und Nachbehandlung herausgearbeitet. Die Zusammenhänge zwischen Abscheidebedingungen und Morphologie, lokaler Mikrostruktur, Struktur sowie magnetischen Eigenschaften werden untersucht und bewertet. Es wird gezeigt, dass Fe-Pd-Nanodrähte trotz der Kombination aus edlem und unedlem Metall elektrochemisch hergestellt werden können. Ein komplexierter Fe-Pd-Elektrolyt in Kombination mit optimierten alternierenden Abscheidepotentialen führt reproduzierbar zu durchgehenden, nahezu defektfreien Nanodrähten nahe der Zusammensetzung Fe70Pd30. Mit einer nachträglichen Wärmebehandlung erreicht man eine vollständige Umwandlung der Fe-Pd-Legierung von der kubisch raumzentrierten zur kubisch flächenzentrierten Struktur. Die erfolgreiche Herstellung dieser Nanodrähte stellt eine Schlüsselposition auf dem Weg zu formgedächtnisbasierten Nanoaktoren dar. In dieser Arbeit konnten wichtige Ansatzpunkte zur Strukturkontrolle während der elektrochemischen Abscheidung und somit zur Aktivierung des Formgedächtniseffekts identifiziert werden. / Fe-Pd alloys at about 30 at.% Pd allow obtaining high length changes or strains in the percent range due to thermal or magnetic shape memory effect. They are especially promising candidates for smart and intelligent materials in micro- and nanoactuators as well as sensors. In comparison to bulk materials and thin films the utilization of nanowires promises higher actuation frequencies and strains, which further heighten the scientific and technological interest. Electrodeposition within self-organized nanoporous templates is a very time efficient method to prepare even large arrays of Fe-Pd nanowires of different length and diameter compared to lithographic or physical methods. The aim of this work is to exhibit the preparation conditions such as electrolyte system, deposition parameter and post treatment for shape memory active Fe-Pd nanowires at about 30 at.% Pd. Correlations between morphology, local microstructure, structure and magnetic properties are investigated and evaluated. Fe-Pd nanowires are successfully prepared by electrodeposition despite the combination of noble Pd and less noble Fe metals. The usage of an electrolyte with complexed Fe and Pd ions and an optimized alternating potential deposition regime leads to continuous and almost defect free nanowires close to the composition Fe70Pd30. The complete transition from the bcc to fcc structure of the Fe-Pd alloy is achieved by an additional heat treatment. However, the successful preparation of these nanowires represents a key element towards nanoactuators based on shape memory alloys. Fundamental knowledge about electrochemical preparation of Fe-Pd nanowires is gained. Important starting points towards structure control during deposition and activation of the shape memory effect are identified.

Pursuing the development and implementation of novel synthesis techniques to produce nanostructures with an interesting set of properties is a goal that advances the frontiers of nanotechnology. Also of fundamental importance is to revisit well-established synthesis techniques employing a new set of materials as precursors, substrates and catalysts. Fundamental breakthroughs in the field of nanotechnology can be achieved by developing new synthesis procedures as well as by adapting known procedures to new materials. This thesis focuses on both kinds of experiments. A variant of chemical vapor deposition (CVD) has been used to produce Al5BO9 nanowires out of sapphire wafers without the need of a catalyst material. The novelty of the work relies on the formation mechanism of the Al5BO9 nanowires. Essentially, the process can be described as a large-scale topological transformation taking place on the substrate’s surface as its chemical composition changes due to the arrival of precursor molecules. Dense mats of Al5BO9 nanowires cover large areas of the substrate that were previously relatively flat. The process is enhanced by a high temperature and the presence of pre-existing superficial defects (cracks, terraces, etc.) on the substrates. Al5BO9 nanowires as well as B/BOX nanowires and BOX nanotubes were also produced via a novel in-situ electron beam-induced synthesis technique. The process was carried out at room temperature and inside a transmission electron microscope. Au nanoparticles were used as catalyst for the case of B/BOX nanowires and BOX nanotubes, while the Al5BO9 nanowires were synthesized without the need of a catalyst material. The formation and growth of the nanostructures is solely driven by the electron beam. The growth mechanism of the B/BOX nanowires and BOX nanotubes relies on interplay between electrostatic charging of the precursor material (to produce and transport feedstock material) and electron stimulated desorption of oxygen which is able to activate the catalytic properties of the Au nanoparticles. For the case Al5BO9 nanowires a nucleation process based on massive atomic rearrangement in the precursor is instigated by the e-beam, afterwards, the length of some of the nanowires can be extended by a mechanism analogous to that of the growth of the B/BOX nanowires.