We studied the wurtzite to rock salt phase transition in gallium nitride ( GaN ). Using the mapping algorithm of COMSUBS we found 435 possible mechanisms for this transition. We then used FIREBALL to do density functional theory calculations and found enthalpy barrier heights for the transition pathway. We used this to determine the mechanisms that are the most favorable for GaN. The most favorable mechanisms for GaN are those that break no bonds during the phase transition. The bond-preserving mechanisms involve bilayer sliding of (010) hexagonal plane in the plus or minus [100] hexagonal direction.

We compare the transition barrier that accompanies a first-order phase transition in the canonical and microcanonical ensemble. This is directly encoded in the probability distributions of standard Metropolis Monte Carlo simulations and a proper microcanonical sampling technique. For the example of droplet formation, we find that in both ensembles the transition barrier scales as expected but that the barrier is much smaller in the microcanonical ensemble. In addition its growth with system size is weaker which will enhance this difference for larger systems. We provide an intuitive physical explanation for this observation

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We examine the effect of lane preference on a quasi one-dimensional three-state driven lattice gas, consisting of holes and positive and negative particles, and periodic boundary conditions in the longitudinal direction. Particles move via particle-hole and, with a lesser rate, particle-particle exchanges; the species are driven in opposite directions along the lattice, each preferring one of the lanes with a given probability, <I>p</I>. The model can be interpreted as traffic flow on a two-lane beltway, with fast cars preferring the left lane and slow cars preferring the right, viewed in a comoving frame. In steady-sate, the system typically exhibits a macroscopic cluster containing a majority of the particles. At very high values of <I>p</I>, a first order transition takes the system to a spatially disordered state. Using Monte Carlo simulations to analyze the system, we find that the size of the cluster increases with lane preference. We also observe a region of negative response, where increasing the lane preference <I>decreases</I> the number of particles in their favored lane, against all expectations. In addition, simulations show an intriguing sequence of density profiles for the two species. We apply mean-field theory, continuity equations, and symmetries to derive relationships between observables to make a number of predictions verified by the Monte Carlo data. / Master of Science

Elastin-like polypeptides (ELP) are artificially designed protein biopolymers that can be produced by living organisms. These proteins have the unique ability to undergo reversible inverse phase transition, in response to changes in temperature and/or addition of chaotropic salts. Below the transition temperature (T1) , ELP is soluble in water. Increasing the temperature above Ti, ELP coacervates into an aqeous ELP-rich phase. In this thesis, this unique feature of ELP was used in for recombinant protein purification and for the formation of aqueous multiple-phase systems. For protein purification, ELP was fused with an intein and a model protein (thioredoxin), to demonstrate a simple and inexpensive approach for recombinant protein purification. The ELP tags replace the chromatographic media and the intein replaces the use of the protease in conventional methods. Using ELP tags was found to be consistent with large -scale recombinant protein production/purification by purifying an ELP tagged protein using a stirred cell equipped with a microfiltration membrane. When the temperature and/or salt concer.tration is increased for mixtures containing free ELP and ELP tagged proteins, simultaneous phase transition takes place. This served as the basis for the development of a method suitable for selectively recovering molecules from complex mixtures with high specificity, full reversibility, and virtually unlimited affinity. The second parts of this thesis focus on the ability of ELP to form aqueous twophase systems (A TPS) in vitro and most importantly, in vivo- with the formation of aqueous microcompartments in living cells. These compartments exclude the protein making machinery of the cell, acting as depots for newly expressed protein. It is also shown (in vitro) that ELP bastd droplets exclude proteases, protecting proteins from degradation. These observations are important for high-level production of recombinant proteins. Also described, is the formation of protein based aqueous multiphasic systems, with tunable morphologies. / Thesis / Doctor of Philosophy (PhD)

In dieser Arbeit wurde in situ TEM genutzt, um Phasendiffusionsprozesse in Echtzeit mit hoher räumlicher Auflösung während struktureller Übergangsphänomene in verschiedenen Systemen zu untersuchen, die durch eine zunehmende Anzahl von Einflussparametern wie Kristallorientierung oder Dehnung charakterisiert sind. Zur Entwicklung und Erprobung der Methode wurde die Interdiffusion an planaren Grenzflächen zwischen (Al,Ga)As-Schichten unterschiedlicher Zusammensetzung während des Glühens untersucht. Ein neuer hybrider Probenpräparationsansatz wurde verwendet, um die Interdiffusion in der Heterostruktur bei Temperaturen bis zu 800 Grad Celsius mit der in situ Weitwinkel-Dunkelfeld-Rastertransmissionselektronenmikroskopie (HAADF STEM) zu untersuchen. Die beobachtete Grenzflächenverbreiterung zeigte eine starke Abhängigkeit der Diffusionskoeffizienten von der lokalen Zusammensetzung von Al und Ga. Als nächstes wurde HAADF STEM verwendet, um die Phasenseparationsbildung von Bi-reichen Clustern in einem Ga(Sb,Bi)-Film direkt zu beobachten. Die Ergebnisse zeigten, dass sie sich durch spinodale Zersetzung bilden. Der komplexeste strukturelle Übergang, der in dieser Arbeit untersucht wurde, ist die Festphasenepitaxie (SPE) von Ge auf Fe3Si, die zur Bildung einer neuartigen epitaktisch stabilisierten FeGe2-Phase führt. Mittels in situ hochauflösendem (HR)TEM konnten die verschiedenen Schritte dieses Phasenübergangs alle in Echtzeit beobachtet werden. Die Ergebnisse zeigten, dass eine intermediäre CsCl-ähnliche Phase von FeGe2 zunächst durch einen diffusionsbegrenzten Prozess Schicht für Schicht von der Ge/Fe3Si-Grenzfläche aus wächst. Nach einer bestimmten Filmdicke wandelt eine zweite Umwandlung den Film in eine tetragonale Schichtstruktur von FeGe2 um. Dieser Prozess beginnt ebenfalls an der Grenzfläche zum FeGe2 und kann auf Gitterdehnung zurückgeführt werden. / In this work, in situ TEM was utilized to investigate phase diffusional processes in real time with high spatial resolution during structural transition phenomena in various systems which are characterized by an increasing number of impact parameters such as crystal orientation or strain. In order to develop and evaluate the experimental method interdiffusion at planar interfaces between (Al,Ga)As layers of different composition during annealing was investigated. A new hybrid sample preparation approach was used to investigate the interdiffusion in the heterostructure at temperatures up to 800 _C with in situ high angle annular dark field scanning transmission electron microscopy (HAADF STEM). The observed interface broadening revealed a strong dependence of the diffusion coefficients on the local composition of Al and Ga. Next in situ HAADF STEM was used to directly observe the phase separation formation of Bi-rich clusters in a Ga(Sb,Bi) film. The results showed that they form by spinodal decomposition. The most complex structural transition investigated in this work is the solid phase epitaxy (SPE) of Ge on Fe3Si resulting in the formation of a novel epitaxially stabilized FeGe2 phase. By using in situ high resolution (HR)TEM the different steps of this phase transition could all be observed in real time. The results showed that an intermediate CsCl-like phase of FeGe2 grows first by a diffusion limited process layer-by-layer from the Ge/Fe3Si interface. After a certain film thickness, a second transformation transforms the film into a tetragonal layered structure of FeGe2. This process also initiates at the interface to the FeGe2 and can be attributed to strain.

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