9 research outputs found
An ab initio study of size-selected Pd nanocluster catalysts for the hydrogenation of 1-pentyne
No full textThe 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
Bonding of Gold Nanoclusters on Graphene with and without Point Defects
No full textHybrid 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
Gold Nanoparticle Melting: Effects of Size, Support Interaction, and Orientation
No full textAn understanding of nanoparticle (NP) melting is essential for both fundamental nanoscience and the design of high-temperature catalytic systems. We investigate the melting behavior of truncated octahedral gold NPs, ranging in size from 2 to 4 nm, supported on their edges, (100) or (111) facets, using molecular dynamics simulations, with a machine-learning force field trained on density functional theory data. We systematically examine the effects of NP size, support interactions, and orientational dependence by applying spring-like constraints to specific facets or edges. Our results show that NP melting follows the liquid nucleation and growth model, with surface disorder preceding rapid melting at a critical temperature. Constraining the atoms to simulate contact with a support consistently raises the melting temperature, with stronger effects for smaller clusters, and for (100) facets compared with (111) facets, that is, there is an orientational effect. Importantly, the extent of the offset in melting temperature is quite independent of the interaction strength, implying that all support interactions can significantly stabilize small NPs. These findings provide a framework for more accurate predictions of nanoscale melting in practical catalytic environments
Core atoms escape from the shell: reverse segregation of Pb–Al core–shell nanoclusters via nanoscale melting
No full textMelting is a phase transition that profoundly affects the fabrication and diverse applications of metal nanoclusters. Core–shell clusters offer distinctive properties and thus opportunities compared with other classes of nano-alloys. Molecular dynamics simulations have been employed to investigate the melting behaviour of Pb–Al core–shell clusters containing a fixed Pb147 core and varying shell thickness. Our results show that the core and shell melt separately. Surprisingly, core melting always drives the core Pb atoms to break out the shell and coat the nanoclusters in a reversed segregation process at the nanoscale. The melting point of the core increases with the shell thickness to exceed that of the bare core cluster, but the thinnest shell always supresses the core melting point. These results can be a reference for the future fabrication, manipulation, and exploitation of the core–shell nanoalloys chosen. The system chosen is ideally suited for experimental observations
Thermal stability of L-cysteine-protected Au25 clusters: interplay between melting and ligand desorption
No full textWe investigate the thermal stability and melting of a monolayer-protected Au25Cys18 cluster using classical reactive molecular dynamics simulations. While the enhanced thermal stability of thiol ligand-protected gold clusters compared to corresponding unprotected gold clusters is well known, the mechanism of melting of the protected clusters has not yet been studied in detail. Our results demonstrate that the covalent bonding of the thiol ligands in a Au25Cys18 cluster stabilises the gold core against thermally induced isomerisation and melting. The Au25Cys18 cluster undergoes a melting phase transition at temperatures of B580–760 K, which exceeds by approximately 400 K the melting temperature of a bare Au25 cluster. The loss of thermal stability of the ligand-protected cluster occurs through an interplay of the metal core melting and the cascade evaporation of cysteine (Cys) ligands on the nanosecond timescale. The simulation results are validated by comparison with the results of ab initio calculations and relevant experimental data
Neuromorphic nanocluster networks: Critical role of the substrate in nano-link formation
No full textAtomic 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 links 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 temperature and thermal breaking processes between 2.8 nm-sized Au1415 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, no matter how 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
Molecular dynamics simulation of nanofilament breakage in neuromorphic nanoparticle networks
No full textNeuromorphic 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
Atomistic modelling of electron beam induced structural transformations in deposited metal clusters
No full textStructural transformations in gold clusters deposited on a graphite substrate induced by the focused electron beam of a scanning transmission electron microscope are investigated using the classical molecular dynamics (MD) approach. The particular case study concerns Au309 clusters softly deposited on few-layer graphite and exposed to a 300 keV electron beam. Two mechanisms of energy transfer to the cluster during the irradiation are considered: (i) through the relaxation of collective electronic excitations and (ii) through the momentum transfer by the energetic primary electrons. A relativistic MD approach implemented in the MBN Explorer software package is used to simulate the collisions of energetic primary electrons with cluster atoms and to evaluate the amount of energy transferred to the cluster for different collision geometries. Characteristic times for the occurrence of these energy deposition events are estimated for realistic experimental irradiation conditions. The MD simulations of the cluster dynamics after irradiation show that the cluster temperature decreases rapidly during the first few tens of picoseconds, and the cluster cools down to a temperature close to its initial temperature within several hundred picoseconds. This time period is comparable to the characteristic time between two successive energy transfer events induced by plasmon excitations in the deposited cluster. A large number of successive energy transfer events (on the order of ∼103–104) during irradiation can cumulatively lead to substantial heating of the deposited cluster and induce its structural transformations
Atomic-resolution imaging of surface and core melting in individual size-selected Au nanoclusters on carbon
No full text<p>Experimental and theoretical data sets for the Nature Communications paper "Atomic-resolution imaging of surface and core melting in individual size-selected Au nanoclusters on carbon". Data structure and formats are described in the README.docx file.</p>
