1,984 research outputs found
Dissociative Chemisorption of O2 on Al(111)
No full textDissociative chemisorption of O2 on the Al(111) surface represents a frequently studied prototype for understanding the interaction between O2 and metal surfaces. It is well known that the experimentally observed activation barrier for O2 dissociation is not captured by conventional density functional theory. By contrast, recent embedded correlated wavefunction (ECW) calculations naturally yield an adiabatic barrier [1,2]. However, these ECW calculations have been limited to a static analysis of the reaction pathways and not yet tested by dynamics simulations. We present a global six-dimensional potential energy surface (PES) for this system parameterized with about seven hundred ECW data points. This new PES provides a reasonable description of the global topography of the energy landscape, including site-specific and orientation-dependent activation barriers. Quasi-classical trajectory calculations on this PES semi-quantitatively reproduce both the observed translational energy dependence of the sticking probability and steric effects based on the orientation of the O2 molecules.
[1] F. Libisch, C. Huang, P. Liao, M. Pavone, and E. A. Carter, Physical Review Letters 109, 198303 (2012)
[2] J. Cheng, F. Libisch, and E. A. Carter, J. Phys. Chem. Lett. 6, 1661 (2015
Jeunesse: Lecture suivie
No full textI am delighted to have found this little paperback in a favorite bookshop in Paris. I look forward to using it myself. It brings two of his best illustrators to a pupil's book of Florian's fables. The colored illustrations are not the only help for those of us trying to understand a sometimes difficult author. There are vocabulary notes, comments, and -- especially -- a short discussion of each fable's moral. These discussions are pleasantly candid, as in this comment on "La taupe et les lapins": "La fable de Florian est peut-être moins claire qu'elle n'y paraȋt" (55). Great work on rendering the colored illustrations!Language note: FrenchFloria
Quantum-Mechanical Embedding Methods for Surface Catalysis
No full textThe charge transfer reactions upon dissociative absorption of O2 on metal surfaces is critical for many catalytic processes, yet poses a challenging problem for state-of-the-art ab-initio theoretical modeling. For example, the experimentally observed activation barrier for O2 dissociation on Al (111) is not captured by conventional density functional theory. However, accurate wavefunction-based methods that are well suited to handle the multireference character of the charge transfer state are ill suited to describe extended metal surfaces. Embedding methods offer a way forward, by combining highly accurate wave-function based approaches at the absorption site with an environment described by density functional theory. In the case of O2 on Al(111), such an approach naturally yields an adiabatic barrier [1]. I review our recent advances and challenges in solving catalytic problems using embedding methods, including the build-up of six-dimensional potential energy surface (PES) suited for quasi-classical trajectory calculations [2], projection-based approaches that enable freeze-thaw cycles, and the extension to high-level methods such as Quantum Monte Carlo or Coupled Cluster that can treat periodic boundary conditions.
[1] F. Libisch, C. Huang, and E. A. Carter Accounts of Chemical Research, 47, 2768 (2014)
[2] R. Yin, Y. Zhang, F. Libisch, E. A. Carter, H. Guo, and B. Jiang J. Chem. Phys. Lett. 9, 3271 (2018
Defects in two-dimensional crystals
Get PDFZweidimensionale Kristalle, die aus schwach (Van der Waals) gekoppelten, geschichteten Materialien herausgelöst werden, eröffnen die Möglichkeit, 2D-Materialien sowohl theoretisch als auch experimentell zu untersuchen. Ihre im Vergleich zu herkömmlichen “3D-Materialien” unterschiedlichen elektronischen, optischen und mechanischen Eigenschaften, die sie zu erstklassigen Kandidaten für künftige Anwendungen in der Informationstechnologie machen, stoßen auf reges Forschungsinteresse. Aufgrund des maximalen Verhältnisses von Oberfläche zu Volumen dieser Systeme ist es auch möglich, die Eigenschaften dieser Materialien durch externe Maßnahmen zu beeinflussen. Diese äußeren Einflüsse können von elektrostatischen Gates, die eine präzise Ladungslokalisierung und Stromsteuerung ermöglichen, bis hin zu einer weiteren Schicht eines anderen 2D-Materials reichen, die je nach der genauen relativen Ausrichtung zur ersten Schicht zu einer neuartigen Moiréphysik auf verschiedenen Längenskalen führt.Eine weitere Quelle für die Veränderung von Materialeigenschaften - wenn auch eine historisch unerwünschte - sind Gitterfehler und Defekte. Die außerordentliche Vielseitigkeit und Beeinflussbarkeit niedrig-dimensionaler Werkstoffe macht sie auch anfälliger für Veränderungen, die durch Fehler in ihrer kristallinen Struktur verursacht werden. Obwohl die Probenvorbereitung und damit die Kontrolle über das Auftreten dieser Defekte seit der Entdeckung dieser niedrig-dimensionalen Systeme erheblich verbessert wurde, ist das Verständnis des Einflusses solcher Defekte nach wie vor ein entscheidender Faktor für die Entwicklung von Materialien mit gewünschten Eigenschaften.Diese Arbeit zielt darauf ab, die theoretische Modellierung solcher Systeme mit Defekten voranzutreiben, und untersucht mehrere Beispiele, bei denen Defekte nicht nur Fehler in der Kristallstruktur sind, sondern auch die Möglichkeit bieten, sowohl optische als auch elektronische Eigenschaften aktiv in wünschenswertere Konfigurationen zu ändern. Ich wende Modelle unterschiedlicher Komplexität an, um experimentelle Daten von Photolumineszenz-, Rastertunnel- und elektronischen Transportmessungen in Systemen zu verstehen und zu erklären, deren Eigenschaften entweder durch Gitterdefekte oder elektrostatische Gates verändert werden. Neben verschiedenen spannenden Möglichkeiten, experimentelle Daten theoretisch zu untermauern, untersuche ich auch einige Systeme von einem rein theoretischen Standpunkt aus und versuche, Anreize für die experimentelle Umsetzung zu schaffen. Die Modellierung dieser Systeme beinhaltet oft Multi-Skalen-Ansätze, bei denen numerisch teure, aber genaue Methoden (z.B. Dichtefunktionaltheorie für Defekt-Superzellen) in ein erfolgreiches Zusammenspiel mit groß-skaligen Methoden (z.B. Tight-Binding für elektronischen Transport) gebracht werden müssen, die die Simulation von Millionen von Atomen (d.h. vergleichbar mit experimentell zugänglichen Längenskalen) ermöglichen.Trotz der Tatsache, dass die Qualität der Übereinstimmung zwischen gemessenen und simulierten Daten nicht immer eine monotone Funktion der Modellierungskomplexität und des Aufwands ist, versuche ich auch die Generierung von ab-inito abgeleiteten Defektparametrisierungen anzugehen. Zu diesem Zweck implementiere ich einen modernen maschinellen Lernalgorithmus, der versucht, die hervorragenden, aber aufwendigen Standardefektmodellierungsmethoden zu umgehen. Darüber hinaus wende ich modernste numerische Algorithmen an, die in Zusammenarbeit mit Florian Libisch und seinen früheren Doktoranden entwickelt wurden, um den elektronischen Transport in Moiré-Strukturen von zweischichtigem Graphen (tBLG) und hexagonalem Bornitrid (hBN) zu untersuchen.Two dimensional crystals — exfoliated from weakly (Van der Waals) coupled layered materials — open the possibility of studying 2D materials both theoretically and experimentally. Their differences in electronic, optical and mechanical properties when compared to conventional “3D materials” makes them prime candidates for future application in information technology, attracting considerable interest. Due to the maximal surface to bulk ratio of these systems it is also possible to tailor the characteristics of these materials via external measures. These can range from electrostatic gates enabling precise charge localization and current control, all the way to a sheet of another 2D material, which depending on the precise relative orientation with respect to the first layer results in novel moiré physics. Another source of altering material properties — albeit a historically undesirable one — are lattice imperfections and defects. In this regard the outstanding versatility and influenceability of low dimensional materials also renders them more susceptible to changes caused by flaws within their crystalline structure. While sample preparation and thus control over the occurrence of these imperfections has undergone vast improvements since the discovery of these low-dimensional systems, understanding the influenceof such defects remains a vital ingredient to tailoring materials with desired properties. This thesis aims at advancing the theoretical modeling of such modified systems and studies several examples where defects are not only flaws in the crystal structure but present opportunities to actively change both optical and electronic properties to more desirable configurations. I apply models of varying complexity to help understand and explain experimental data of photoluminescence, scanning tunneling and electronic transport measurements in systems with characteristics altered by either lattice defects or electrostatic gates. Apart from several exciting opportunities to provide theoretical support to experimental data I also study some systems from a purely theoretical aspect and try to provide incentive for experimental realization. Modeling these systems often involves multi-scale approaches where numerically expensive but accurate methods (e.g. density functional theory for defect super cells) need to be brought into a successful interplay with large scale methods (e.g tight-binding for electronic transport) that allow for the simulation of millions of atoms (i.e. comparable to experimentally accessible length scales). Although the quality of agreement between measured and simulated datais not always a monotonous function of modeling complexity and effort I also try to address the generation of ab-inito derived defect parametrizations. To this extent I implement a modern machine learning algorithm that tries to circumvent the excellent but cumbersome standard defect modeling methods. Furthermore I also apply state-of-art numerical algorithms developed in a group effort with Florian Libisch and his preceding PhD students to study electronic transport in twisted moiré structures of bilayer graphene (tBLG) and hexagonal boron nitride (hBN)
Fables de Florian, Nouvelle Edition
No full textHere is a small edition, 3¼" x 5¼," of 162 pages. It seems to be identical to Bodemann #274, published by the same people two years earlier. As Bodemann notes in a very short description, the illustrations come as three panels per page. One panel often represents two fables. My favorites among these very small illustrations are "The Blind and the Lame" (18); "Two Bachelors" (48); "Owl and Pigeon" (83); and "Ass and Flute" (111). Do not miss the gigantic crocodile on 120! Illustrations occur facing these pages: 1, 12, 18, 29, 38, 48, 66, 83, 111, and 120. There is an AI at the back.This is a hardbound book (hard cover)Language note: FrenchNo Autho
Size-extensivity-corrected multireference configuration interaction schemes to accurately predict bond dissociation energies of oxygenated hydrocarbons
No full textOxygenated hydrocarbons play important roles in combustion science as renewable fuels and additives, but many details about their combustion chemistry remain poorly understood. Although many methods exist for computing accurate electronic energies of molecules at equilibrium geometries, a consistent description of entire combustion reaction potential energy surfaces (PESs) requires multireference correlated wavefunction theories. Here we use bond dissociation energies (BDEs) as a foundational metric to benchmark methods based on multireference configuration interaction (MRCI) for several classes of oxygenated compounds (alcohols, aldehydes, carboxylic acids, and methyl esters). We compare results from multireference singles and doubles configuration interaction to those utilizing a posteriori and a priori size-extensivity corrections, benchmarked against experiment and coupled cluster theory. We demonstrate that size-extensivity corrections are necessary for chemically accurate BDE predictions even in relatively small molecules and furnish examples of unphysical BDE predictions resulting from using too-small orbital active spaces. We also outline the specific challenges in using MRCI methods for carbonyl-containing compounds. The resulting complete basis set extrapolated, size-extensivity-corrected MRCI scheme produces BDEs generally accurate to within 1 kcal/mol, laying the foundation for this scheme's use on larger molecules and for more complex regions of combustion PESs
Interview with Florian Bieber
No full textInterview with Florian Bieber, lecturer in East European Politics at the University of Kent in Canterbury, England. Interview conducted in Ithaca, NY on March 13, 2009. Dr. Bieber has worked in Belgrade (Serbia) and Sarajevo (Bosnia-Herzegovina) for the European Centre for Minority Issues and has taught at the Central European University, at the University of Sarajevo and at the University of Bologna. He is also the author of a book about Serbian nationalism, entitled Nationalism in Serbia from the Death of Tito to the Fall of Milosevic (Muenster: Lit Verlag, 2005, in German), and another book, Post-War Bosnia: Ethnic Structure, Inequality and Governance of the Public Sector (London: Palgrave, 2006). He?s at Cornell this spring semester of 2009 as the Luigi Einaudi Chair in European and International Studies.Bieber's current book project (01:30)
Bieber's reflections on the 20th anniversary of 1989 (02:00)
On Bieber's background in political science and history (03:19)
When interdisciplinarity works best (5:22)
On the "ghettoization" of the Balkans and its causes/possible solutions (6:30)
Unique contribution of our field to other fields (9:38)
Addressing the ongoing perception of a division of Europe into "East" and "West" (12:04)
On Bieber's interest in the study of nationalism (16:17)
Is there such a thing as "good nationalism"? (19:12)
On Bieber's "European" upbringing and early education (21:34)
Bieber on experiences/opportunities all Europeans should ideally have (24:56)
On the future of Southeastern Europe (26:17)1_ruh0kx6
Fables de Florian
No full textNouvelle édition, ornée de figures. This diminutive but complete edition (3¼ x 5¼) is not in Bodemann. The British Library seems to have an edition of Florian published by Genets Jeune in 1808. There are four illustrations here: The Cat and the Binoculars (56); The Children and the Partridges (122); The Savant and the Farmer (138); The Countryman and the River (182). There is an AI at the back. The first title-page lists 1821 as the publication date, but the second title-page has 1820.This is a hardbound book (hard cover)Language note: FrenchJean-Pierre Claris de Floria
Embedding approaches for bulk systems using projector-augmented waves
No full textDensity-functional embedding allows for combining different levels of theory in one
calculation (for a review see [1]). Consider, for example, dissociative adsorption of gas
molecules on metal surfaces: the extended metal surfaces can be accurately treated by
periodic density functional theory approaches, while charge transfer processes at the
adsorption site require more accurate correlated wave function (CW) approaches. The
latter, in turn, scale unfavourably with system size, and thus do not allow for treating
more than approximately fifteen heavy atoms. Instead, we separate the problem into a
cluster of interest and the surrounding metal surface. Their interaction is mediated by a
scalar embedding potential determined at the DFT level. The cluster can now efficiently
be treated by CW techniques, in the presence of the embedding potential.
We have recently extended our approach [2] to the popular VASP software package,
including the projector-augmented wave (PAW) formalism [3]. This allows for treating
much larger unit cells at higher accuracy, and to exploit the full framework of VASP: the
PAW formalism (i) offers an exact transformation between the original, oscillating wave
function and the pseudized, slowly varying pseudo wave function used in the
computation and (ii) explicitly treats the all-electron wave functions, albeit within the
frozen-core approximation. We show that PAW-based density-functional embedding
yields robust, accurate embedding potentials, and discuss application cases and future
extensions based on a set of mutually orthogonal orbitals [4].
[1] F. Libisch, C. Huang, and E. A. Carter, Acc. Chem. Research, 47, 2768 (2014).
[2] K. Yu, F. Libisch, and E. A. Carter, J. Chem. Phys. 143, 102806 (2015).
[3] G. Kresse, and D. Joubert, Phys. Rev. B, 59, 1758, (1999)
[4] F. Manby, M. Stella, J. D. Goodpaster, and T. F. Miller Chem. Theory Comput., 8,
2564-2568 (2012)
Photonic effects in the non-equilibrium optical response of two-dimensional semiconductors
Get PDFTransient absorption spectroscopy is a powerful tool to monitor the out-of-equilibrium optical response of photoexcited semiconductors. When this method is applied to two-dimensional semiconductors deposited on different substrates, the excited state optical properties are inferred from the pump-induced changes in the transmission/reflection of the probe, i.e., ∆T/T or ∆R/R. Transient optical spectra are often interpreted as the manifestation of the intrinsic optical response of the monolayer, including effects such as the reduction of the exciton oscillator strength, electron-phonon coupling or many-body interactions like bandgap renormalization, trion or biexciton formation. Here we scrutinize the assumption that one can determine the non-equilibrium optical response of the TMD without accounting for the substrate used in the experiment. We systematically investigate the effect of the substrate on the broadband transient optical response of monolayer MoS2 (1L-MoS2) by measuring ∆T/T and ∆R/R with different excitation photon energies. Employing the boundary conditions given by the Fresnel equations, we analyze the transient transmission/reflection spectra across the main excitonic resonances of 1L-MoS2. We show that pure interference effects induced by the different substrates explain the substantial differences (i.e., intensity, peak energy and exciton linewidth) observed in the transient spectra of the same monolayer. We thus demonstrate that the substrate strongly affects the magnitude of the exciton energy shift and the change of the oscillator strength in the transient optical spectra. By highlighting the key role played by the substrate, our results set the stage for a unified interpretation of the transient response of optoelectronic devices based on a broad class of TMDs
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