Transition path sampling simulations of structural phase transformations in nanocrystals under pressure

Diese Arbeit beschreibt eine Computersimulationsstudie von druckinduzierten strukturellen Phasenübergängen in Nanokristallen. Die Mechanismen solcher Übergänge können sich stark von den entsprechenden Mechanismen im ausgedehnten Festkörper unterscheiden; sie bestimmen die Formänderung des Kristalls während der Transformation und beeinflussen die Erreichbarkeit und Stabilität einzelner Kristallstrukturen. Das zeitliche und örtliche Auflösungsvermögen von Experimenten ist jedoch zu gering, um die atomaren Details einer solchen Transformation direkt beobachten zu können. Computersimulationen bieten im Prinzip den notwendige atomaren Blickwinkel, sind aber auf eine Zeitskala von ein paar Nanosekunden eingeschränkt. Um in dieser kurzen Zeit eine Transformation beobachten zu können, muss ein wesentlich höherer Druck verwendet werden als im Experiment --- die Aussagekraft der Simulation wird dadurch erheblich beeinträchtigt. In dieser Arbeit wird deshalb die transition path sampling Methode verwendet, mit der das Zeitskalenproblem umgangen wird und Simulationen unter experimentellen Bedingungen durchgeführt werden können. Zur Simulation von Nanokristallen unter Druck entwickeln wir einen neuen Algorithmus, in dem ein Druckbad aus idealen Gasteilchen verwendet wird. Der Algorithmus erzeugt die korrekte Wahrscheinlichkeitsdichte eines Systems bei konstantem Druck und konstanter Temperatur und wird auf eine Transformation in CdSe Nanokristallen angewandt. Unsere Simulationen identifizieren den dominanten Transformationsmechanismus und ermöglichen darüber hinaus die Bestimmung des kritischen Nukleus der Hochdruckphase durch eine umfassende statistische Analyse der simulierten Transformationspfade. Wir bestimmen Aktivierungsenthalpie und -volumen als Funktion des Drucks und vergleichen diese Größen mit dem Experiment. Die im Experiment beobachtete, annähernd lineare Abhängigkeit der Aktivierungsenthalpie vom Kristalldurchmesser stimmt gut mit unseren Simulationen überein und kann durch die spezielle längliche Geometrie des kritischen Nukleus erklärt werden. Des Weiteren wird ein neuer Algorithmus vorgestellt, der die Effizienz von transition path sampling Simulationen von Systemen mit langen und rauen freien Energiebarrieren entscheidend verbessert.This dissertation describes a computer simulation study of structural transformations in nanocrystals under pressure. The mechanisms of such transformations can be quite different from the bulk, they determine the shape change during the transformation and influence the accessibility and stability of particular crystal structures. Yet the atomistic transformation details elude direct observation by experiment due to limited time and space resolution. Molecular dynamics computer simulations, on the other hand, can provide the necessary atomistic perspective but are restricted to the nanosecond timescale which requires the use of significantly higher pressures. Such deviation from experimental conditions compromises the comparability of simulation and experiment considerably. Here, we use transition path sampling, a simulation method designed to overcome the timescale problem. We introduce a new transition path sampling algorithm for nanoparticles under pressure, which features an efficient barostat of ideal gas particles. We show that the algorithm satisfies detailed balance and apply it to a structural transformation in a model of CdSe nanocrystals. Starting from a previously proposed mechanism, the algorithm quickly finds a more favorable transformation route which is characterized by a consecutive sliding of parallel crystal planes. We subject the pathways obtained with transition path sampling to a comprehensive transition state analysis based on the committor function. These calculations reveal the critical nuclei of the high pressure phase which we quantify by calculating activation enthalpies and volumes. The dependence of these quantities on crystal size shows the same scaling observed experimentally and is consistent with the peculiar shape of the critical nucleus which markedly differs from the bulk. Furthermore, a new algorithm is introduced to the transition path sampling methodology, which exploits the linear dynamics of small phase space displacements and greatly enhances the sampling efficiency of transition pathways in systems involving long and rough free energy barriers

Alternative Measures of Core Inflation in Chile

Inflation targeting central banks usually base their objectives on headline inflation, which is often affected by price movements of specific components whose behavior makes it difficult to extract clear signals about the inflationary outlook relevant for the monetary policy horizon. For this reason, several measures of underlying (core) inflation have been proposed with the purpose of removing this “noise”. This paper presents some alternatives calculated for the Chilean economy: three different measures that trim CPI components according to criteria of price changes and volatility, and two measures that re-weight the components according to historical volatility. In general terms, we find that the measures trimming away items with extreme price changes or high volatility are informative about the inflationary outlook given that they are unbiased with respect to the seasonally adjusted CPI and superior to other measures in tracking the headline inflation trend.

Transferable Pair Potentials for CdS and ZnS Crystals

A set of interatomic pair potentials is developed for CdS and ZnS crystals. We show that a simple energy function, which has been used to describe the properties of CdSe [J. Chem. Phys. 116, 258 (2002)], can be parametrized to accurately describe the lattice and elastic constants, and phonon dispersion relations of bulk CdS and ZnS in the wurtzite and rocksalt crystal structures. The predicted coexistence pressure of the wurtzite and rocksalt structures, as well as the equation of state are in good agreement with experimental observations. These new pair potentials enable the study of a wide range of processes in bulk and nanocrystalline II-VI semiconductor materials

Metastability in pressure-induced structural transformations of CdSe/ZnS core/shell nanocrystals

The kinetics and thermodynamics of structural transformations under pressure depend strongly on particle size due to the influence of surface free energy. By suitable design of surface structure, composition, and passivation it is possible, in principle, to prepare nanocrystals in structures inaccessible to bulk materials. However, few realizations of such extreme size-dependent behavior exist. Here we show with molecular dynamics computer simulation that in a model of CdSe/ZnS core/shell nanocrystals the core high pressure structure can be made metastable under ambient conditions by tuning the thickness of the shell. In nanocrystals with thick shells, we furthermore observe a wurtzite to NiAs transformation, which does not occur in the pure bulk materials. These phenomena are linked to a fundamental change in the atomistic transformation mechanism from heterogenous nucleation at the surface to homogenous nucleation in the crystal core. Our results suggest a new route towards expanding the range of available nanoscale materials

Nutzung von Resistenzmechanismen verschiedener Rebarten als Alternative zum Einsatz von Kupfer im Ökoweinbau

Extrakte aus Wildreben bzw. Hybriden mit hoher Resistenz: Es war zu prüfen, ob durch die Applikation von Blattextrakten von Nicht-Vitis-vinifera-Rebsorten auf Qualitätsrebsorten Plasmopara viticola und andere Pathogene bekämpft, unterdrückt oder pflanzeneigene Abwehrmechanismen bei Qualitätsrebsorten durch in den Extrakten enthaltene Elicitoren aktiviert werden können resp. ob mit den Pflanzenextrakten eine direkte Bekämpfung dieser Problemschaderreger möglich ist. Kupferreduktionspotential bei Anbau neuer Vitis vinifera PIWI - Sorten: Der Anbau von PIWI’s ermöglicht einen weitgehenden Verzicht auf Pflanzenschutz und ist ökologisch und ökonomisch die nachhaltigste Form des Weinbaus überhaupt. Das Kupfereinsparungspotential durch den Anbau von PIWI’s ist vermutlich abhängig von der Resistenz der Sorte und von den klimatischen Bedingungen am Standort. Das für die jeweilige Sorte notwendige Maß an Pflanzenschutz soll in diesem Projekt erstmals ermittelt werden. Daraus lässt sich ein durchschnittliches Einsparpotential an Kupfer bestimmen. Orientierungsversuche mit geringen Stockzahlen im Freiland an Zuchtstämmen mit bereits pyramidisierten Plasmopara Resistenzen wurden in einer 2009 erstellten Prüfanlage mit Überkronenberegnung durchgeführt, in der beliebig hohe Befallsbedingungen für die Rebenperonospora geschaffen werden können

Exploiting non-equilibrium phase separation for self-assembly

Demixing can occur in systems of two or more particle species that experience different driving forces, e.g., mixtures of self-propelled active particles or of oppositely charged colloids subject to an electric field. Here we show with macroscopic experiments and computer simulations that the forces underlying such non-equilibrium segregation can be used to control the self-assembly of particles that lack attractive interactions. We demonstrate that, depending on the direction, amplitude and frequency of a periodic external force acting on one article species, the structures formed by a second, undriven species can range from compact clusters to elongated, string-like patterns.Chemistry and Chemical Biolog

Everolimus after failure of one prior VEGF-targeted therapy in metastatic renal cell carcinoma : Final results of the MARC-2 trial

MARC-2, a prospective, multicenter phase IV trial, aimed to investigate clinical outcomes in patients with metastatic renal cell carcinoma (mRCC) treated with everolimus after failure of one initial vascular endothelial growth factor receptor tyrosine kinase inhibitor (VEGFR-TKI) therapy and to identify subgroups benefiting most, based on clinical characteristics and biomarkers. Patients with clear cell mRCC failing one initial VEGFR-TKI received everolimus until progression or unacceptable toxicity. Primary endpoint was 6-month progression-free survival rate (6moPFS). Secondary endpoints were overall response rate (ORR), PFS, overall survival (OS), and safety. Between 2011 and 2015, 63 patients were enrolled. Median age was 65.4 years (range 43.3-81.1). 6moPFS was 39.3% (95% confidence interval [CI], 27.0-51.3) overall, 54.4% (95% CI, 35.2-70.1) vs 23.7% (95% CI, 10.5-39.9) for patients aged ≥65 vs 25 vs ≤25 kg/m2. A Cox proportional hazards model confirmed a longer PFS for patients aged ≥65 years (hazard ratio [HR] 0.46; 95% CI, 0.26-0.80) and a longer OS for patients with BMI >25 kg/m2 (HR 0.36; 95% CI, 0.18-0.71). Median PFS and median OS were 3.8 months (95% CI, 3.2-6.2) and 16.8 months (95% CI, 14.3-24.3). ORR was 7.9% and disease control rate was 60.3%. No new safety signals emerged. Most common adverse events were stomatitis (31.7%), fatigue (31.7%), and anemia (30.2%). One patient died from treatment-related upper gastrointestinal hemorrhage. Everolimus remains a safe and effective treatment option for mRCC patients after one prior VEGFR-TKI therapy. Patients aged ≥65 years and patients with BMI >25 kg/m2 benefited most

Thrombospondin-2 and LDH Are Putative Predictive Biomarkers for Treatment with Everolimus in Second-Line Metastatic Clear Cell Renal Cell Carcinoma (MARC-2 Study)

There is an unmet need for predictive biomarkers in metastatic renal cell carcinoma (mRCC) therapy. The phase IV MARC-2 trial searched for predictive blood biomarkers in patients with predominant clear cell mRCC who benefit from second-line treatment with everolimus. In an exploratory approach, potential biomarkers were assessed employing proteomics, ELISA, and polymorphism analyses. Lower levels of angiogenesis-related protein thrombospondin-2 (TSP-2) at baseline (≤665 parts per billion, ppb) identified therapy responders with longer median progressionfree survival (PFS; ≤665 ppb at baseline: 6.9 months vs. 1.8, p = 0.005). Responders had higher lactate dehydrogenase (LDH) levels in serum two weeks after therapy initiation (>27.14 nmol/L), associated with a longer median PFS (3.8 months vs. 2.2, p = 0.013) and improved overall survival (OS; 31.0 months vs. 14.0 months, p < 0.001). Baseline TSP-2 levels had a stronger relation to PFS (HR 0.36, p = 0.008) than baseline patient parameters, including IMDC score. Increased serum LDH levels two weeks after therapy initiation were the best predictor for OS (HR 0.21, p < 0.001). mTOR polymorphisms appeared to be associated with therapy response but were not significant. Hence, we identified TSP-2 and LDH as promising predictive biomarkers for therapy response on everolimus after failure of one VEGF-targeted therapy in patients with clear cell mRCC