Panagiotis Grammatikopoulos

Formation Mechanism of Fe Nanocubes by Magnetron Sputtering Inert Gas Condensation

In this work, we study the formation mechanisms of iron nanoparticles (Fe NPs) grown by magnetron sputtering inert gas condensation and emphasize the decisive kinetics effects that give rise specifically to cubic morphologies. Our experimental results, as well as computer simulations carried out by two different methods, indicate that the cubic shape of Fe NPs is explained by basic differences in the kinetic growth modes of {100} and {110} surfaces rather than surface formation energetics. Both our experimental and theoretical investigations show that the final shape is defined by the combination of the condensation temperature and the rate of atomic deposition onto the growing nanocluster. We, thus, construct a comprehensive deposition rate-temperature diagram of Fe NP shapes and develop an analytical model that predicts the temporal evolution of these properties. Combining the shape diagram and the analytical model, morphological control of Fe NPs during formation is feasible; as such, our method proposes a roadmap for experimentalists to engineer NPs of desired shapes for targeted applications.

Coalescence-induced crystallisation wave in Pd nanoparticles

Palladium nanoparticles offer an attractive alternative to bulk palladium for catalysis, hydrogen storage and gas sensing applications. Their performance depends strongly on surface structure; therefore, nanoparticle coalescence can play an important role, as it determines the resultant structure of the active sites where reactions (e.g. catalysis) actually take place, i.e. facets, edges, vertices or protrusions. With this in mind, we performed classical molecular dynamics (MD) simulations and magnetron-sputtering inert gas condensation depositions of palladium nanoparticles, supported by high-resolution transmission electron microscopy (HRTEM), to study the mechanisms that govern their coalescence. Surface energy minimisation drove the interactions initially, leading to the formation of an interface/neck, as expected. Intriguingly, at a later stage, atomic rearrangements triggered a crystallisation wave propagating through the amorphous nanoparticles, leading to mono- or polycrystalline fcc structures. In the case of crystalline nanoparticles, almost-epitaxial alignment occurred and the formation of twins and surface protrusions were observed.

Inoculation of silicon nanoparticles with silver atoms

Silicon (Si) nanoparticles were coated inflight with silver (Ag) atoms using a novel method to prepare multicomponent heterostructured metal-semiconductor nanoparticles. Molecular dynamics (MD) computer simulations were employed, supported by high-resolution bright field (BF) transmission electron microscopy (HRTEM) and aberration-corrected scanning transmission electron microscopy (STEM) with a resolution ≤0.1 nm in high angle annular dark field (HAADF) mode. These studies revealed that the alloying behavior and phase dynamics during the coating process are more complex than when attaching hetero-atoms to preformed nanoparticles. According to the MD simulations, Ag atoms condense, nucleate and diffuse into the liquid Si nanoparticles in a process that we term “inoculation”, and a phase transition begins. Subsequent solidification involves an intermediate alloying stage that enabled us to control the microstructure and crystallinity of the solidified hybrid heterostructured nanoparticles.

Coalescence behaviour of amorphous and crystalline tantalum nanoparticles: a molecular dynamics study

Porous films of tantalum (Ta) and its oxides exhibit numerous properties suitable for high surface area applications, mainly in the semiconductor and bio-implant industries. Such films can be developed by Ta nanoparticle deposition using DC magnetron sputtering with gas aggregation. In order to engineer films of desirable properties, accurate control and in-depth understanding of the processes and parameters of nanoparticle growth, deposition and coalescence are crucial. Of utmost importance is to control the film’s porosity, since it determines many of the other physical properties. To this end, we performed a number of classical Molecular Dynamics simulations to study the coalescence of two or more Ta nanoparticles. Temperature, relative size and crystallographic orientation, defect content, degree of crystallinity and deposition rate effects were taken into account, and a mapping of the sintering processes was acquired. A broad range of possible interaction mechanisms were observed, from simple nanoparticle reorientation in order to achieve epitaxial configuration, to atomic adsorption, neck formation, twinning within the nanoparticles and full consolidation into a single, larger nanoparticle. The parameters studied are directly linked to experimental deposition parameters; therefore, fitting them accordingly can lead to growth of films with bespoke properties.

Nanoparticle design by gas-phase synthesis

Abstract Gas-phase synthesis characterizes a class of bottom-up methods for producing multifunctional nanoparticles (NPs) from individual atoms or molecules. This review aims to summarize recent achievements using this approach, and compare its potential to other physical or chemical NP fabrication techniques. More specifically, emphasis is given to magnetron-sputter gas-phase condensation, since it allows for flexible growth of complex, sophisticated NPs, owing to the fast kinetics and non-equilibrium processes it entails. Nanoparticle synthesis is decomposed into four stages, i.e. aggregation, shell-coating, mass-filtration, and deposition. We present the formation of NPs of various functionalities for different applications, such as magnetic, plasmonic, catalytic and, gas-sensing, emphasizing on the primary dependence of each type on a different stage of the fabrication process, and their resultant physical and chemical properties. Graphical Abstract

Assembly of tantalum porous films with graded oxidation profile from size-selected nanoparticles

AbstractFunctionally graded materials offer a way to improve the physical and chemical properties of thin films and coatings for different applications in the nanotechnology and biomedical fields. In this work, design and assembly of nanoporous tantalum films with a graded oxidation profile perpendicular to the substrate surface are reported. These nanoporous films are composed of size-selected, amorphous tantalum nanoparticles, deposited using a gas-aggregated magnetron sputtering system, and oxidized after coalescence, as samples evolve from mono- to multi-layered structures. Molecular dynamics computer simulations shed light on atomistic mechanisms of nanoparticle coalescence, which govern the films porosity. Aberration-corrected (S) TEM, GIXRD, AFM, SEM, and XPS were employed to study the morphology, phase and oxidation profiles of the tantalum nanoparticles, and the resultant films.Graphical AbstractDesign and assembly of tantalum nanoparticle porous films with a graded oxidation profile perpendicular to the substrate surface were fabricated by magnetron-sputter inert-gas aggregation system. At the top-most layers of the film, the larger free-surface areas of nanoparticles enable the formation of thermodynamically stable Ta2O5.

Nanoscale Heterogeneity of Multilayered Si Anodes with Embedded Nanoparticle Scaffolds for Li-Ion Batteries

Abstract A new approach on the synthesis of Si anodes for Li‐ion batteries is reported, combining advantages of both nanoparticulated and continuous Si films. A multilayered configuration prototype is proposed, comprising amorphous Si arranged in nanostructured, mechanically heterogeneous films, interspersed with Ta nanoparticle scaffolds. Particular structural features such as increased surface roughness, nanogranularity, and porosity are dictated by the nanoparticle scaffolds, boosting the lithiation process due to fast Li diffusion and low electrode polarization. Consequently, a remarkable charge/discharge speed is reached with the proposed anode, in the order of minutes (1200 mAh g−1 at 10 C). Moreover, nanomechanical heterogeneity self‐limits the capacity at intermediate charge/discharge rates; as a consequence, exceptional cycleability is observed at 0.5 C, with 100% retention over 200 cycles with 700 mAh g−1. Higher capacity can be obtained when the first cycles are performed at 0.2 C, due to the formation of microislands, which facilitate the swelling of the active Si. This study indicates a method to tune the mechanical, morphological, and electrochemical properties of Si electrodes via engineering nanoparticle scaffolds, paving the way for a novel design of nanostructured Si electrodes for high‐performance energy storage devices.

Simple analytical model of nanocluster coalescence for porous thin film design

We propose an approach to coalescence studies that encompasses the random nature of nanoparticle deposition, which results in a statistical cancellation of individual sintering mechanisms. We present a rigorous, yet simple and intuitive, analytical method that describes the average coalescence behaviour of nanoparticles, regardless of constituent element or crystallinity, emphasizing only the predominant coalescence dependencies on temperature and size-dependent nanoparticle melting points. We assessed our model using molecular dynamics (MD) computer simulations of dissimilar systems, and found remarkable agreement between its predictions and the MD results. Its simplicity makes our model a suitable starting point for the development of a meso-scale simulation technique that can describe the growth of porous films and allow for their bespoke design.

Endotaxially stabilized B2-FeSi nanodots in Si (100) via ion beam co-sputtering

We report on the formation of embedded B2-FeSi nanodots in [100]-oriented Si substrates, and investigate the crystallographic mechanism underlying the stabilization of this uncommon, bulk-unstable, phase. The nanodots were approximately 10 nm in size, and were formed by iron thin film deposition and subsequent annealing. Cross-sectional transmission electron microscopy, energy loss spectroscopy mapping, and quantitative image simulation and analysis were utilized to identify the phase, strain, and orientational relationship of the nanodots to the host silicon lattice. X-ray photoelectron spectroscopy was utilized to analyze the surface composition and local bonding. Elasticity calculations yielded a nanodot residual strain value of −18%. Geometrical phase analysis graphically pinpointed the positions of misfit dislocations, and clearly showed the presence of pinned (11¯1¯)Si//(100)FeSi, and unpinned (2¯42)Si//(010)FeSi, interfaces. This partial endotaxy in the host silicon lattice was the mechanism that s...

Single-step gas phase synthesis of stable iron aluminide nanoparticles with soft magnetic properties

Soft magnetic alloys at the nanoscale level have long generated a vivid interest as candidate materials for technological and biomedical purposes. Consequently, controlling the structure of bimetallic nanoparticles in order to optimize their magnetic properties, such as high magnetization and low coercivity, can significantly boost their potential for related applications. However, traditional synthesis methods stumble upon the long standing challenge of developing true nanoalloys with effective control over morphology and stability against oxidation. Herein, we report on a single-step approach to the gas phase synthesis of soft magnetic bimetallic iron aluminide nanoparticles, using a versatile co-sputter inert gas condensation technique. This method allowed for precise morphological control of the particles; they consisted of an alloy iron aluminide crystalline core (DO3 phase) and an alumina shell, which reduced inter-particle interactions and also prevented further oxidation and segregation of the bimetallic core. Remarkably, the as-deposited alloy nanoparticles show interesting soft magnetic properties, in that they combine a high saturation magnetization (170 emu/g) and low coercivity (less than 20 Oe) at room temperature. Additional functionality is tenable by modifying the surface of the particles with a polymer, to ensure their good colloidal dispersion in aqueous environments.