Bonding of Gold Nanoclusters on Graphene with and without Point Defects

Hybrid nanostructures of size-selected nanoparticles (NPs) and 2D materials exhibit striking physical and chemical properties and are attractive for many technology applications. A major issue for the performance of these applications is device stability. In this work, we investigate the bonding of cuboctahedral, decahedral and icosahedral Au NPs comprising 561 atoms on graphene sheets via 103-atom scale ab initio spin-polarized calculations. Two distinct cases we considered: (i) the Au NPs sit with their (111) facets on graphene and (ii) the NPs are oriented with a vertex on graphene. In both cases, we compare the binding energies with and without a graphene vacancy under the NP. We find that in all cases, the presence of the graphene vacancy enhances the bonding of the NPs. Significantly, in the vertex-on-graphene case, the binding energy is considerably increased by several eVs and becomes similar to the (111) facet-on-graphene case. The strain in the NPs is found to be minimal and the displacement of the carbon atoms in the immediate neighborhood of the vacancy is on the 0.1 Å scale. The work suggests the creation of stable NP-graphene systems for a variety of electronic, chemical and photonic applications

An ab initio study of size-selected Pd nanocluster catalysts for the hydrogenation of 1-pentyne

The hydrogenation of alkynes is an important reaction in the synthesis of both fine and bulk chemicals. Palladium-based catalysts are widely used and therefore size-selected Pd nanoclusters may provide enhanced performance. An investigation of the adsorption and desorption of the molecules involved in the reaction can shed light on the activity and selectivity of the catalysts. We employ ab initio calculations to investigate the binding energies of all the molecules related to the hydrogenation of 1-pentyne (1-pentyne, 1-pentene, cis-2-pentene, trans-2-pentene and pentane) on a comprehensive set of possible binding sites of two Pd147 and Pd561 cuboctahedral nanoclusters. We extract the site and size dependence of these binding energies. We find that the adsorption of 1-pentyne occurs preferably on the (100) facets of the nanoclusters, followed by their (111) facets, their edges and their vertices. The molecule binds more strongly on the larger nanoclusters, which are therefore expected to display higher activity. The binding energies of the pentenes are found to be lower on the smaller nanoclusters. Therefore, these molecules are expected to desorb more easily and the small clusters should display better selectivity, i.e., partial hydrogenation to 1-pentene, compared with large clusters. Our results provide guidelines for the optimal design of Pd nanocatalysts

Molecular dynamics simulation of nanofilament breakage in neuromorphic nanoparticle networks

Neuromorphic computing systems may be the future of computing and cluster-based networks are a promising architecture for the realization of these systems. The creation and dissolution of synapses between the clusters are of great importance for their function. In this work, we model the thermal breakage of a gold nanofilament located between two gold nanoparticles via molecular dynamics simulations to study on the mechanisms of neuromorphic nanoparticle-based devices. We employ simulations of Au nanowires of different lengths ($2-8$ nm), widths ($0.4-0.8$ nm) and shapes connecting two Au$_{1415}$ nanoparticles (NPs) and monitor the evolution of the system via a detailed structural identification analysis. We found that atoms of the nanofilament gradually aggregate towards the clusters, causing the middle of the wire to gradually thin and then break. Most of the system remains crystalline during this process but the center is molten. The terminal NPs increase the melting point of the NWs by fixing the middle wire and act as recrystallization areas. We report a strong dependence on the width of the NWs, but also their length and structure. These results may serve as guidelines for the realization of cluster-based neuromorphic computing systems

Neuromorphic nanocluster networks: Critical role of the substrate in nano-link formation

Atomic cluster-based networks represent a promising architecture for the realization of neuromorphic computing systems, which may overcome some of the limitations of the current computing paradigm. The formation and breakage of synapses between the clusters are of utmost importance for the functioning of these computing systems. This paper reports the results of molecular dynamics simulations of synapse (bridge) formation at elevated temperatures and thermal breaking processes between 2.8 nanometer-sized Au$_{1415}$ clusters deposited on a carbon substrate, a model system. Crucially, we find that the bridge formation process is driven by the diffusion of gold atoms along the substrate, however small the gap between the clusters themselves. The complementary simulations of the bridge-breaking process reveal the existence of a threshold bias voltage to activate bridge rupture via Joule heating. These results provide an atomistic-level understanding of the fundamental dynamical processes occurring in neuromorphic cluster arrays

Δομικά μοντέλα υλικών τεχνολογίας σε ατομικό επίπεδο με υπολογιστικές μεθόδους ανάλυσης

The aim of this thesis was the investigation of the properties of semiconducting materials and nanostructures using computational tools such as ab initio methods, molecular dynamics simulations and structure prediction algorithms, utilizing high performance computers. Initially, the energetics and strain of indium-gallium nitride nanodisks in gallium nitride nanowires and the electronic properties of the nanowires were examined. Furthermore, the structural, electronic and thermal properties of gallium nitride/aluminum nitride core/shell nanowires and the structural properties of gallium arsenide/aluminum-gallium arsenide core/shell nanowires were investigated. Finally, the structures of the ternary alloys of gallium-aluminum nitride, indium-gallium nitride and indium-aluminum nitride and silicon-tin nitride were predicted and afterwards a detailed analysis of their energetics and electronic properties was performed.Σκοπός της διατριβής ήταν η διερεύνηση των ιδιοτήτων ημιαγώγιμων υλικών και νανοδιατάξεων με τη χρήση υπολογιστικών εργαλείων και συγκεκριμένα μεθόδων πρώτων αρχών, μοριακής δυναμικής και αλγορίθμων πρόβλεψης δομής, αξιοποιώντας υπολογιστικές δομές υψηλής ισχύος. Αρχικά, μελετήθηκε η παραμορφωσιακή, εντατική και ενεργειακή κατάσταση νανοδίσκων νιτριδίου του ινδίου-γαλλίου σε νανοσυρμάτα νιτριδίου του γαλλίου και οι ηλεκτρονικές ιδιότητες των νανοσυρμάτων. Στη συνέχεια διερευνήθηκαν οι δομικές, ηλεκτρονικές και θερμικές ιδιότητες νανοσυρμάτων αρχιτεκτονικής πυρήνα/φλοιού νιτριδίου του γαλλίου/νιτριδίου του αλουμινίου και οι δομικές ιδιότητες νανοσυρμάτων αρχιτεκτονικής πυρήνα/φλοιού αρσενιδίου του γαλλίου/αρσενιδίου του αλουμινίου-γαλλίου. Τέλος, πραγματοποιήθηκε πρόβλεψη της δομής των τριμερών νιτριδίων του γαλλίου-ινδίου, του αλουμινίου-γαλλίου, του αλουμινίου-ινδίου και του κασσιτέρου-πυριτίου, την οποία ακολούθησε αναλυτική ενεργειακή μελέτη τους και διερεύνηση των ηλεκτρονικών τους ιδιοτήτων

Molecular dynamics simulation of nanofilament breakage in neuromorphic nanoparticle networks

Neuromorphic computing systems may be the future of computing and cluster-based networks are a promising architecture for the realization of these systems. The creation and dissolution of synapses between the clusters are of great importance for their function. In this work, we model the thermal breakage of a gold nanofilament located between two gold nanoparticles via molecular dynamics simulations to study on the mechanisms of neuromorphic nanoparticle-based devices. We employ simulations of Au nanowires of different lengths (20–80 Å), widths (4–8 Å) and shapes connecting two Au1415 nanoparticles (NPs) and monitor the evolution of the system via a detailed structural identification analysis. We found that atoms of the nanofilament gradually aggregate towards the clusters, causing the middle of wire to gradually thin and then break. Most of the system remains crystalline during this process but the center is molten. The terminal NPs increase the melting point of the NWs by fixing the middle wire and act as recrystallization areas. We report a strong dependence on the width of the NWs, but also their length and structure. These results may serve as guidelines for the realization of cluster-based neuromorphic computing systems

Bonding of Gold Nanoclusters on Graphene with and without Point Defects

Hybrid nanostructures of size-selected nanoparticles (NPs) and 2D materials exhibit striking physical and chemical properties and are attractive for many technology applications. A major issue for the performance of these applications is device stability. In this work, we investigate the bonding of cuboctahedral, decahedral and icosahedral Au NPs comprising 561 atoms on graphene sheets via 103-atom scale ab initio spin-polarized calculations. Two distinct cases we considered: (i) the Au NPs sit with their (111) facets on graphene and (ii) the NPs are oriented with a vertex on graphene. In both cases, we compare the binding energies with and without a graphene vacancy under the NP. We find that in all cases, the presence of the graphene vacancy enhances the bonding of the NPs. Significantly, in the vertex-on-graphene case, the binding energy is considerably increased by several eVs and becomes similar to the (111) facet-on-graphene case. The strain in the NPs is found to be minimal and the displacement of the carbon atoms in the immediate neighborhood of the vacancy is on the 0.1 Å scale. The work suggests the creation of stable NP-graphene systems for a variety of electronic, chemical and photonic applications

p-Type Iodine-Doping of Cu₃N and Its Conversion to γ-CuI for the Fabrication of γ-CuI/Cu₃N p-n Heterojunctions

Cu3N with a cubic crystal structure is obtained in this paper by the sputtering of Cu under N2 followed by annealing under NH3: H2 at 400 °C, after which it was doped with iodine at room temperature resulting into p-type Cu3N with hole densities between 1016 and 1017 cm−3. The Cu3N exhibited distinct maxima in differential transmission at ~2.01 eV and 1.87 eV as shown by ultrafast pump-probe spectroscopy, corresponding to the M and R direct energy band gaps in excellent agreement with density functional theory calculations, suggesting that the band gap is clean and free of mid-gap states. The Cu3N was gradually converted into optically transparent γ-CuI that had a hole density of 4 × 1017 cm−3, mobility of 12 cm2/Vs and room temperature photoluminescence at 3.1 eV corresponding to its direct energy band gap. We describe the fabrication and properties of γ-CuI/TiO2/Cu3N and γ-CuI/Cu3N p-n heterojunctions that exhibited rectifying current-voltage characteristics, but no photogenerated current attributed to indirect recombination via shallow states in Cu3N and/or deep states in the γ-CuI consistent with the short (ps) lifetimes of the photoexcited electrons-holes determined from transient absorption–transmission spectroscopy

Stabilization of 2D raft structures of Au nanoclusters with up to 60 atoms by a carbon support

Herein, the stabilization of 2D single‐atom high gold rafts containing up to ≈60 Au atoms on amorphous carbon, fabricated by sputtering of atoms and imaged by aberration‐corrected scanning transmission electron microscopy, is demonstrated. These rafts deviate from the established cluster transition from 2D to 3D Au structural motifs in free clusters, which occurs in the region of 10–14 atoms. The experimental findings by performing explicit ab initio calculations of Au n (n = 3–147) clusters on graphene are supported and the role of cluster–surface interactions in the stabilization of the 2D single‐atom high Au cluster rafts on graphene is revealed. The transition from equilibrium 2D–3D structures is delayed to n = 19, while metastable 2D single‐atom high rafts compete with 3D structures up to about n = 60 atoms. The catalytic activity of supported nanoclusters depends strongly on their structure (and carbon‐based supports are used for a number of reactions); therefore these results are relevant to the catalytic performance of nanocluster‐based catalysts