Impact of Screw and Edge Dislocation on the Thermal Conductivity of Nanowires and Bulk GaN

We report on thermal transport properties of wurtzite GaN in the presence of dislocations, by using molecular dynamics simulations. A variety of isolated dislocations in a nanowire configuration were analyzed and found to reduce considerably the thermal conductivity while impacting its temperature dependence in a different manner. We demonstrate that isolated screw dislocations reduce the thermal conductivity by a factor of two, while the influence of edge dislocations is less pronounced. The relative reduction of thermal conductivity is correlated with the strain energy of each of the five studied types of dislocations and the nature of the bonds around the dislocation core. The temperature dependence of the thermal conductivity follows a physical law described by a T$^{-1}$ variation in combination with an exponent factor which depends on the material's nature, the type and the structural characteristics of the dislocation's core. Furthermore, the impact of the dislocations density on the thermal conductivity of bulk GaN is examined. The variation and even the absolute values of the total thermal conductivity as a function of the dislocation density is similar for both types of dislocations. The thermal conductivity tensors along the parallel and perpendicular directions to the dislocation lines are analyzed. The discrepancy of the anisotropy of the thermal conductivity grows in increasing the density of dislocations and it is more pronounced for the systems with edge dislocations

On the melting point depression, coalescence, and chemical ordering of bimetallic nanoparticles: the miscible Ni–Pt system

Among the properties that distinguish nanoparticles (NPs) from their bulk counterparts is their lower melting points. It is also common knowledge that relatively low melting points enhance the coalescence of (usually) nascent nanoclusters toward larger NPs. Finally, it is well established that the chemical ordering of bi- (or multi-) metallic NPs can have a profound effect on their physical and chemical properties, dictating their potential applications. With these three considerations in mind, we investigated the coalescence mechanisms for Ni and Pt NPs of various configurations using classical molecular dynamics (MD) computer simulations. Benchmarking the coalescence process, we identified a steeper melting point depression for Pt than for Ni, which indicates a reversal in the order of melting for same-size NPs of the two elements. This reversal, also evident in the nano-phase diagram thermodynamically constructed using the regular solution model, may be useful for utilising NP coalescence as a means to design and engineer non-equilibrium NPs via gas-phase synthesis. Indeed, our MD simulations revealed different coalescence mechanisms at play depending on the conditions, leading to segregated chemical orderings such as quasi-Janus core-satellite, or core–(partial) shell NPs, despite the expected theoretical tendency for elemental mixing

Data-driven simulation and characterisation of gold nanoparticle melting

The simulation and analysis of the thermal stability of nanoparticles, a stepping stone towards their application in technological devices, require fast and accurate force fields, in conjunction with effective characterisation methods.In this work, we develop efficient, transferable, and interpretable machine learning force fields for gold nanoparticles based on data gathered from Density Functional Theory calculations.We use them to investigate the thermodynamic stability of gold nanoparticles of different sizes (1 to 6 nm), containing up to 6266 atoms, concerning a solid-liquid phase change through molecular dynamics simulations.We predict nanoparticle melting temperatures in good agreement with available experimental data.Furthermore, we characterize the solid-liquid phase change mechanism employing an unsupervised learning scheme to categorize local atomic environments.We thus provide a data-driven definition of liquid atomic arrangements in the inner and surface regions of a nanoparticle and employ it to show that melting initiates at the outer layers

Tuning the onset of ferromagnetism in heterogeneous bimetallic nanoparticles by gas phase doping

In the nanoregime, chemical species can reorganize in ways not predicted by their equilibrium bulk behavior. Here, we engineer Ni-Cr nanoalloys at the magnetic end of their compositional range (i.e., 0–15 at. % Cr), and we investigate the effect of Cr incorporation on their structural stability and resultant magnetic ordering. To ensure their stoichiometric compositions, the nanoalloys are grown by cluster beam deposition, a method that allows one-step, chemical-free fabrication of bimetallic nanoparticles. While full Cr segregation toward nanoparticle surfaces is thermodynamically expected for low Cr concentrations, metastability occurs as the Cr dopant level increases in the form of residual Cr in the core region, yielding desirable magnetic properties in a compensatory manner. Using nudged elastic band calculations, residual Cr in the core is explained based on modifications in the local environment of individual Cr atoms. The resultant competition between ferromagnetic and antiferromagnetic ordering gives rise to a wide assortment of interesting phenomena, such as a cluster-glass ground state at very low temperatures and an increase in Curie temperature values. We emphasize the importance of obtaining the commonly elusive magnetic nanophase diagram for M-Cr (M=Fe, Co, and Ni) nanoalloys, and we propose an efficient single-parameter method of tuning the Curie temperature for various technological applications.Peer reviewe

Nanoassemblies of ultrasmall clusters with remarkable activity in carbon dioxide conversion into C1 fuels

Cu nanoassemblies formed transiently during reaction from size-selected subnanometer Cu4 clusters supported on amorphous OH-terminated alumina convert CO2 into methanol and hydrocarbons under near-atmospheric pressure at rates considerably higher than those of individually standing Cu4 clusters. An in situ characterization reveals that the clusters self-assemble into 2D nanoassemblies at higher temperatures which then disintegrate upon cooling down to room temperature. DFT calculations postulate a formation mechanism of these nanoassemblies by hydrogen-bond bridges between the clusters and H2O molecules, which keep the building blocks together while preventing their coalescence

Hydrogen Flux through Size Selected Pd Nanoparticles into Underlying Mg Nanofilms

The application of Mg for hydrogen storage is hindered due to the slow absorption of hydrogen in Mg films. Herein, the hydrogenation process is explored theoretically using density functional theory calculations, and energy barriers are compared for hydrogen diffusion through Pd nanoparticle/Mg film interfaces and their variations, i.e., Pd(H)/Mg(O). Decomposing the mechanism into basic steps, it is shown that Pd undergoes a strain‐induced crystallographic phase transformation near the interface, and indicated that hydrogen saturation of Pd nanoparticles enhances their efficiency as nanoportals. Using energetic arguments, it is explained why hydrogen diffusion is practically prohibited through native Mg oxide and seriously suppressed through existing hydride domains. Hydrogen flux is experimentally investigated through the nanoportals in Pd‐nanoparticle decorated Mg films by pressure‐composition isotherm measurements. An r ≈ t1/3 relationship is theoretically calculated for the radial growth of hemispherical hydride domains, and this relationship is confirmed by atomic force microscopy. The diffusion constant of hydrogen in Mg films is estimated as DHfilm ≈ 8 × 10−18 m2 s−1, based on transmission electron microscopy characterization. The unique nanoportal configuration allows direct measurement of hydride domain sizes, thus forming a model system for the experimental investigation of hydrogenation in any material

Extended defects in semiconductors

The study reported in this thesis is related to Gallium Nitride based semiconducting layers whose industrial development needs a strong support from fundamental approaches. III-V materials are compound semiconductor with large direct band-gap are nowadays used for the production of optoelectronic devices in blue and ultraviolet wavelengths and in high frequency and high temperature transistors. GaN epitaxial layers with the wurtzite crystal structure are usually grown on cplane sapphire substrate using different deposition techniques. They commonly contain misfit and threading dislocations, basal and prismatic stacking faults, nanopipes, and inversion domain boundaries. It is challenging that devices comprising this material exhibit satisfactory performance despite the large density and multiplicity of microstructural defects. The aim of this thesis is to define and analyse the structural and energetic characteristics of extended defects in GaN layers. Experimental observations, using high resolution electron microscopy (HRTEM), provide the microstructural investigation at the atomic level. Morphological and structural analysis of extended defects in GaN films is performed using the topological theory of interfacial defects, the geometric phase method and computer simulation of HRTEM images. In particular, the initial atomic configurations were established a priori using crystallography and topological theory or by employing displacements expected from elasticity theory. Empirical potential and ab-initio calculations are employed to define the energetic relaxation of the defect configurations and the energetic favourable models, which are encounter in GaN films. Although the energies of many of the isolated planar defects observed in epitaxial GaN layers have been determined using ab initio calculations, the total energy of large scale atomic configurations involving different planar defects cannot be easily calculated, for supercells exceed 10,000 atoms. For this purpose, we modified the Stillinger-Weber empirical potential in order to achieve a satisfactory description of the microstructure and the energetics of extended defects. The formulation is based on the adjustment of the parameters in order to represent the Ga-Ga, N-N and Ga-N bonds. The input data are taken for different crystalline phases of gallium, nitrogen and GaN. A satisfactory agreement on the values of the energy versus atomic volume per atom are obtained compared to those derived by ab-initio calculations and experimental data for all the cases studied. By employing the modified Stillinger-Weber potential the energy of inversion domain boundaries (IDBs), basal stacking faults (SFs) and translation domain boundaries (TDBs), which have been observed experimentally in GaN thin films, are calculated providing results comparable with ab-initio calculations.Στην παρούσα διατριβή μελετώνται ημιαγωγικά υμένια που βασίζονται στο Νιτρίδιο του Γαλλίου, τα οποία εμφανίζουν εξαιρετικές εφαρμογές στα κυανά και υπεριώδη μήκη κύματος σε οπτοηλεκτρονικές διατάξεις καθώς και σε διατάξεις τρανζίστορ υψηλών συχνοτήτων και υψηλών θερμοκρασιών. Τα υπό μελέτη υλικά είναι III-V ημιαγωγοί με ευρύ άμεσο ενεργειακό χάσμα. Τα επιταξιακά υμένια των σύνθετων III-V ημιαγωγών με βάση το νιτρίδιο του γαλλίου αναπτύσσονται, συνήθως, με δομή του βουρτσίτη, πάνω στο c-επίπεδο του σάπφειρου με τη χρήση των πλέον σύγχρονων τεχνικών ανάπτυξης όπως η επιταξία μοριακής δέσμης και η μεταλλοργανική χημική εναπόπθεση ατμών. Τέτοιου τύπου λεπτά υμένια συνήθως περιέχουν μεγάλη πυκνότητα ατελειών δομής όπως είναι οι εξαρμόσεις κακής συναρμογής και οι νηματοειδής, τα σφάλματα επιστοίβασης, τα όρια ανάστροφης πολικότητας και οι νανοσωλήνες. Προκαλεί ενδιαφέρων ότι διατάξεις με βάση τις παραπάνω ημιαγωγικές ενώσεις παρουσιάζουν ικανοποιητικές αποδόσεις παρά το μεγάλο πλήθος και την πολλαπλότητα των ατελειών που συναντώνται. Ο στόχος της παρούσας διατριβής είναι η ανάλυση των δομικών ιδιοτήτων και η μελέτη της ενέργειας και των δομών ηρεμίας των εκτεταμένων ατελειών των υμενίων GaN. Η μελέτη της μικροδομής των υμενίων γίνεται σε ατομικό επίπεδο από πειραματικές παρατηρήσεις ηλεκτρονικής μικροσκοπίας διερχόμενης δέσμης υψηλής διακριτικής ικανότητας (HRTEM). Η μορφολογική και δομική ανάλυση των εκτεταμένων ατελειών γίνεται με την βοήθεια της τοπολογικής θεωρίας, της απεικόνισης και ανάλυσης φάσης των HRTEM παρατηρήσεων και των εικόνων προσομοίωσης. Ειδικότερα, οι αρχικές ατομικές δομές προσδιορίζονται είτε με τη βοήθεια της κρυσταλλογραφίας και της τοπολογικής θεωρίας είτε με τον υπολογισμό του πεδίου μετατοπίσεων βάσει της ελαστικής θεωρίας. Ενεργειακοί υπολογισμοί με την χρήση εμπειρικού δυναμικού και ab initio μεθόδων χρησιμοποιούνται για την εύρεση των ατομικών δομών χαμηλότερης ενέργειας και άλλων χαρακτηριστικών των εκτεταμένων ατελειών. Παρόλο που οι ενέργειες των μεμονωμένων επίπεδων ατελειώ