Machine Learning With Observers Predicts Complex Spatiotemporal Behavior

Chimeras and branching are two archetypical complex phenomena that appear in many physical systems; because of their different intrinsic dynamics, they delineate opposite non-trivial limits in the complexity of wave motion and present severe challenges in predicting chaotic and singular behavior in extended physical systems. We report on the long-term forecasting capability of Long Short-Term Memory (LSTM) and reservoir computing (RC) recurrent neural networks, when they are applied to the spatiotemporal evolution of turbulent chimeras in simulated arrays of coupled superconducting quantum interference devices (SQUIDs) or lasers, and branching in the electronic flow of two-dimensional graphene with random potential. We propose a new method in which we assign one LSTM network to each system node except for “observer” nodes which provide continual “ground truth” measurements as input; we refer to this method as “Observer LSTM” (OLSTM). We demonstrate that even a small number of observers greatly improves the data-driven (model-free) long-term forecasting capability of the LSTM networks and provide the framework for a consistent comparison between the RC and LSTM methods. We find that RC requires smaller training datasets than OLSTMs, but the latter require fewer observers. Both methods are benchmarked against Feed-Forward neural networks (FNNs), also trained to make predictions with observers (OFNNs)

High laser induced damage threshold photoresists for nano-imprint and 3D multi-photon lithography

Optics manufacturing technology is predicted to play a major role in the future production of integrated photonic circuits. One of the major drawbacks in the realization of photonic circuits is the damage of optical materials by intense laser pulses. Here, we report on the preparation of a series of organic-inorganic hybrid photoresists that exhibit enhanced laser-induced damage threshold. These photoresists showed to be candidates for the fabrication of micro-optical elements (MOEs) using three-dimensional multiphoton lithography. Moreover, they demonstrate pattern ability by nanoimprint lithography, making them suitable for future mass production of MOEs

Shape-Dependent Single-Electron Levels for Au Nanoparticles

The shape of metal nanoparticles has a crucial role in their performance in heterogeneous catalysis as well as photocatalysis. We propose a method of determining the shape of nanoparticles based on measurements of single-electron quantum levels. We first consider nanoparticles in two shapes of high symmetry: cube and sphere. We then focus on Au nanoparticles in three characteristic shapes that can be found in metal/inorganic or metal/organic compounds routinely used in catalysis and photocatalysis. We describe the methodology we use to solve the Schrödinger equation for arbitrary nanoparticle shape. The method gives results that agree well with analytical solutions for the high-symmetry shapes. When we apply our method in realistic gold nanoparticle models, which are obtained from Wulff construction based on first principles calculations, the single-electron levels and their density of states exhibit distinct shape-dependent features. Results for clean-surface nanoparticles are closer to those for cubic particles, while CO-covered nanoparticles have energy levels close to those of a sphere. Thiolate-covered nanoparticles with multifaceted polyhedral shape have distinct levels that are in between those for sphere and cube. We discuss how shape-dependent electronic structure features could be identified in experiments and thus guide catalyst design

Σχήμα και ιδιότητες νανοσωματιδίων χρυσού σε διάφορα περιβάλλοντα

Στην παρούσα Διδακτορική Διατριβή παρουσιάζεται μια υπολογιστικη μελετη που συνδυαζει εκτενείς πολογισμους κβαντικης μηχανικης βασισμενους στην Θεωρία του Συναρτησιακου της Πυκνοτητας [Density Functional Theory (DFT)], με την κατασκευη Wulff, προκειμενου να προβλεψει την εξαρτηση του σχηματος ισορροπίας και των ιδιοτητων μεγαλων νανοσωματιδίων χρυσου απο το περιβαλλον τους. Αρχικα παρουσιαζουμε την μεθοδολογία που χρησιμοποιηθηκε για να βρεθεί το σχημα ισορροπίας, συμπεριλαμβανομενων των κυριων πτυχων της Θεωρία του Συναρτησιακου της Πυκνοτητας και της μεθοδου κατασκευης Wulff. Συνεχίζουμε παρουσιαζοντας τον κωδικα που αναπτυχθηκε για την κατασκευη οποιασδηποτε καθαρης η με καποιο μοριο προσροφημενο σε δεδομενη θεση Au(hkl) επιφανειας. Στην συνεχεια, μελεταμε την προσροφηση μονοξειδίου του ανθρακα και την διασπαστικη προσροφηση του διμεθυλου δισουλφιδίου (CH3S-SCH3) για καθε πιθανη Au(hkl) με h,k,l ≤ 3 συν την Au(421) και συζηταμε τασεις στις ενεργειες προσροφησης, στα μηκη και τις γωνίες των δεσμων με την αλλαγη στην επιφανειακη δομη. Υπολογίζουμε την επιφανειακη ενεργεια ανα μοναδα επιφανειας για καθαρες επιφανειες και την χρησιμοποιουμε μαζί με την μεθοδο κατασκευης Wulff για να προβλεψουμε το σχημα ισορροπίας καθαρων νανοσωματιδίων χρυσου με διαμετρο εως μερικες δεκαδες νανομετρα, απροσιτα με απευθείας ατομιστικες προσομοιωσεις. Επίσης παρουσιαζουμε τους κωδικες που αναπτυχθηκαν για την κατασκευη ατομιστικων μοντελων των νανοσωματιδίων χρησιμοποιωντας την μεθοδο κατασκευης Wulff και για την αναλυση των γεωμετρικων χαρακτηριστικων των νανοσωματιδίων. Επισημαίνουμε την απροσδοκητη συμφωνία μεταξυ της κατασκευης Wulff, πειραματων και ατομιστικων προσομοιωσεων σε μικρες διαστασεις. Νανοσωματίδια χρυσου με διαμετρο μικροτερη των 16.3 nm εχουν σχημα κολοβου οκταεδρου, αποτελουμενο μονο απο (111) και (100) επιφανειες. Μεγαλυτερα νανοσωματίδια εμφανίζουν επιφανειες με μεγαλυτερους δείκτες, κυρίως την (332). Στην συνεχεια χρησιμοποιουμε τις επιφανειακες ενεργειες ανα μοναδα επιφανειας και τα αποτελεσματα για την ενεργεια προσροφησης για να βρουμε την διεπιφανειακη ενεργεια ανα μοναδα επιφανειας μεταξυ Au και CO η Au και θειολικων ενωσεων (CH3S-) σε χαμηλη πίεση και θερμοκρασία. Τα αποτελεσματα αυτα χρησιμοποιουνται μετα για να βρουμε το σχημα ισορροπίας νανοσωματιδίων χρυσου σε περιβαλλον CO και CH3S. Σε συμφωνία με πειραματικα δεδομενα, τα νανοσωματίδια χρυσου σε περιβαλλον μονοξειδίου του ανθρακα βρεθηκαν να είναι πιο σφαιρικα και περισσοτερο ενεργα σε συγκριση με αυτα σε μη αλληλεπιδρων περιβαλλον. Τα νανοσωματίδια χρυσου αλλαζουν το σχημα τους κατα την προσροφηση θειολικων ενωσεων προς περισσοτερο σφαιρικα σχηματα με μεγαλυτερη συγκεντρωση σε ατομα σε ακμες. Τελος, ελεγχουμε την σταθεροτητα των νανοσωματιδίων μεσω προσομοιωσεων Μοριακης Δυναμικης και παρουσιαζουμε μια θεωρητικη μελετη που παρεχει φασματοσκοπικες ιδιοτητες εξαρτωμενες απο το σχημα των νανοσωματιδίων χρυσου.In the present Thesis a computational study that links extensive quantum- mechanical calculations, based on density functional theory (DFT), to Wulff constructions in order to predict the environment-dependent equilibrium shape and properties of large gold nanoparticles is presented. We firstly present the methodology used to obtain the equilibrium shape, including the main aspects of the density functional theory and the Wulff construction method. We continue by presenting the code developed to construct any clean or having a molecule adsorbed at a given site Au(hkl) surface. Then, we study the adsorption of CO and the dissociative adsorption of dimethyl disulfide (CH3S-SCH3) on every possible Au(hkl) with h,k,l ≤ 3 plus the kinked Au(421) and we dis- cuss trends on adsorption energies, bond lengths and bond angles as the surface structure changes. We calculate the surface energy per unit area for clean surfaces and use it together with the Wulff construction method to predict the equilibrium shape of clean gold nanoparticles with diameters up to several tenths of a nanometer, in-accessible by direct atomistic simulations. We also present the codes developed to construct atomistic models of nanoparticles using the Wulff construction method and analyse the geometrical features of them. We point out a surprising agreement between Wulff construction, experiments, and atomistic simulations at small sizes. Au nanoparticles smaller than 16.3 nm in diameter have truncated octahedral shape, exposing only (111) and (100) faces. Larger nanoparticles also expose higher-index faces, mostly (332). Then we use the surface energies per unit area and the results for the adsorption energy to obtain the interface energy per unit area between Au and CO or Au and thiolates (CH3S-) at low pressure and temperature. These results are then used to obtain the equilibrium shape of CO- or thiolate-covered Au nanoparticles. In agreement with experimental data, Au nanoparticles in CO are found to be more spherical and more reactive compared to Au nanoparticles in noninteracting environments. Gold nanoparticles change their shape upon adsorption of thiolates towards shapes of higher sphericity and higher concentration of step-edge atoms. Finally, we check the stability of these nanoparticles through Molecular Dynamics simulations and we present a theoretical method providing shape-dependent spectroscopic properties of gold nanoparticles

Thiolate Adsorption on Au(hkl) and Equilibrium Shape of Large Thiolate-covered Gold Nanoparticles

The adsorption of thiolates on Au surfaces employing density-functional-theory calculations has been studied. The dissociative chemisorption of dimethyl disulfide (CH3S−SCH3) on 14 different Au(hkl) is used as a model system. We discuss trends on adsorption energies, bond lengths, and bond angles as the surface structure changes, considering every possible Au(hkl) with h, k, l ≤ 3 plus the kinked Au(421). Methanethiolate (CH3S-) prefers adsorption on bridge sites on all surfaces considered; hollow and on top sites are highly unfavourable. The interface tensions for Au(hkl)-thiolate interfaces is determined at low coverage. Using the interface tensions in a Wulff construction method, we construct atomistic models for the equilibrium shape of large thiolate-covered gold nanoparticles. Gold atoms in a nanoparticle change their equilibrium positions upon adsorption of thiolates towards shapes of higher sphericity and higher concentration of step-edge atoms.peerReviewe

Nanoparticle shapes by using Wulff constructions and first-principles calculations

The majority of complex and advanced materials contain nanoparticles. The properties of these materials depend crucially on the size and shape of these nanoparticles. Wulff construction offers a simple method of predicting the equilibrium shape of nanoparticles given the surface energies of the material

Nanoparticle shapes by using Wulff constructions and first-principles calculations

Background: The majority of complex and advanced materials contain nanoparticles. The properties of these materials depend crucially on the size and shape of these nanoparticles. Wulff construction offers a simple method of predicting the equilibrium shape of nanoparticles given the surface energies of the material.Results: We review the mathematical formulation and the main applications of Wulff construction during the last two decades. We then focus to three recent extensions: active sites of metal nanoparticles for heterogeneous catalysis, ligand-protected nanoparticles generated as colloidal suspensions and nanoparticles of complex metal hydrides for hydrogen storage.Conclusion: Wulff construction, in particular when linked to first-principles calculations, is a powerful tool for the analysis and prediction of the shapes of nanoparticles and tailor the properties of shape-inducing species