Exploring the formation of gold/silver nanoalloys with gas-phase synthesis and machine-learning assisted simulations.

While nanoalloys are of paramount scientific and practical interest, the main processes leading to their formation are still poorly understood. Key structural features in the alloy systems, including the crystal phase, chemical ordering, and morphology, are challenging to control at the nanoscale, making it difficult to extend their use to industrial applications. In this contribution, we focus on the gold/silver system that has two of the most prevalent noble metals and combine experiments with simulations to uncover the formation mechanisms at the atomic level. Nanoparticles were produced using a state-of-the-art inert-gas aggregation source and analyzed using transmission electron microscopy and energy-dispersive X-ray spectroscopy. Machine-learning-assisted molecular dynamics simulations were employed to model the crystallization process from liquid droplets to nanocrystals. Our study finds a preponderance of nanoparticles with five-fold symmetric morphology, including icosahedra and decahedra which is consistent with previous results on mono-metallic nanoparticles. However, we observed that gold atoms, rather than silver atoms, segregate at the surface of the obtained nanoparticles for all the considered alloy compositions. These segregation tendencies are in contrast to previous studies and have consequences on the crystallization dynamics and the subsequent crystal ordering. We finally showed that the underpinning of this surprising segregation dynamics is due to charge transfer and electrostatic interactions rather than surface energy considerations.

Critical comparison of general-purpose collective variables for crystal nucleation.

The nucleation of crystals is a prominent phenomenon in science and technology that still lacks a full atomic-scale understanding. Much work has been devoted to identifying order parameters able to track the process, from the inception of early nuclei to their maturing to critical size until growth of an extended crystal. We critically assess and compare two powerful distance-based collective variables, an effective entropy derived from liquid state theory and the path variable based on permutation invariant vectors using the Kob-Andersen binary mixture and a combination of enhanced-sampling techniques. Our findings reveal a comparable ability to drive nucleation when a bias potential is applied, and comparable free-energy barriers and structural features. Yet, we also found an imperfect correlation with the committor probability on the barrier top which was bypassed by changing the order parameter definition.

Impurity-induced nematic-isotropic transition of liquid crystals.

Complex fluids made of liquid crystals (LCs) and small molecules, surfactants, nanoparticles or 1D/2D nanomaterials show novel and interesting features, making them suitable materials for various applications starting from optoelectronics to biosensing. While these additives (impurities) introduce new features in the complex fluids, they may also alter the phase transition behaviour of LCs depending on the physiochemical properties of the added impurity. This article reports on the phase transition of 4-cyano-4'-alkylbiphenyl (nCB) LCs in the presence of an associative impurity, i.e., water and a non-associative impurity, i.e., hexane employing computational methods and experiments. In particular, all-atom (AA) simulations and coarse-grained (CG) models were designed for two complex systems, i.e., 6CB + water and 6CB + hexane and corresponding spectrophotometry experiments were performed using a homologous LC, i.e., 5CB. Results from the simulations and experiments elucidate that the phase transition of LCs depends on the mixing/demixing phenomenon of the impurity in the LC. While associative liquids like water which do not mix with LCs do not influence the nematic-to-isotropic phase transition of LCs, hexane, being a non-associative liquid, mixes well with LCs and induces a sharp impurity-induced nematic-to-isotropic phase transition. Upon application of both AA and CG simulations, we could reach the conclusion that the mixing/demixing phenomenon in an LC + impurity system influences the entropy of the system and hence the observed phase transitions are entropy-driven.

Out-of-Equilibrium Polymorph Selection in Nanoparticle Freezing.

The ability to design synthesis processes that are out of equilibrium has opened the possibility of creating nanomaterials with remarkable physicochemical properties, choosing from a much richer palette of possible atomic architectures compared to equilibrium processes in extended systems. In this work, we employ atomistic simulations to demonstrate how to control polymorph selection via the cooling rate during nanoparticle freezing in the case of Ni3Al, a material with a rich structural landscape. State-of-the-art free-energy calculations allow us to rationalize the complex nucleation process, discovering a switch between two kinetic pathways, yielding the equilibrium structure at room temperature and an alternative metastable one at higher temperature. Our findings address the key challenge in the synthesis of nanoalloys for technological applications, i.e., rationally exploiting the competition between kinetics and thermodynamics by designing a treatment history that forces the system into desirable metastable states.

How interface properties control the equilibrium shape of core-shell Fe-Au and Fe-Ag nanoparticles.

While combining two metals in the same nanoparticle can lead to remarkable novel applications, the resulting structure in terms of crystallinity and shape remains difficult to predict. It is thus essential to provide a detailed atomistic picture of the underlying growth processes. In the present work we address the case of core-shell Fe-Au and Fe-Ag nanoparticles. Interface properties between Fe and the noble metals Au and Ag, computed using DFT, were used to parameterize Fe-Au and Fe-Ag pairwise interactions in combination with available many-body potentials for the pure elements. The growth of Au or Ag shells on nanometric Fe cores with prescribed shapes was then modelled by means of Monte Carlo simulations. The shape of the obtained Fe-Au nanoparticles is found to strongly evolve with the amount of metal deposited on the Fe core, a transition from the polyhedral Wulff shape of bare iron to a cubic shape taking place as the amount of deposited gold exceeds two monolayers. In striking contrast, the growth of silver proceeds in a much more anisotropic, Janus-like way and with a lesser dependence on the iron core shape. In both cases, the predicted morphologies are found to be in good agreement with experimental observations in which the nanoparticles are grown by physical deposition methods. Understanding the origin of these differences, which can be traced back to subtle variations in the electronic structure of the Au/Fe and Ag/Fe interfaces, should further contribute to the better design of core-shell bimetallic nanoparticles.

Long-wavelength density fluctuations as nucleation precursors.

Recent theories of nucleation that go beyond classical nucleation theory predict that diffusion-limited nucleation of both liquid droplets and of crystals from a low-density vapor (or weak solution) begins with long-wavelength density fluctuations. This means that in the early stages of nucleation, "clusters" can have low density but large spatial extent, which is at odds with the classical picture of arbitrarily small clusters of the condensed phase. We present the results of kinetic Monte Carlo simulations using forward flux sampling to show that these predictions are confirmed, namely, that on average, nucleation begins in the presence of low-amplitude, but spatially extended density fluctuations thus confirming a significant prediction of the nonclassical theory.

Combining Quantum Mechanics and Machine-Learning Calculations for Anharmonic Corrections to Vibrational Frequencies.

Several methods are available to compute the anharmonicity in semirigid molecules. However, such methods are not yet routinely employed because of their high computational cost, especially for large molecules. The potential energy surface is required and generally approximated by a quartic force field potential based on ab initio calculation, thus limiting this approach to medium-sized molecules. We developed a new, fast, and accurate hybrid quantum mechanics/machine learning (QM/ML) approach to reduce the computational time for large systems. With this novel approach, we evaluated anharmonic frequencies of 37 molecules, thus covering a broad range of vibrational modes and chemical environments. The obtained fundamental frequencies reproduce results obtained using B2PLYP/def2tzvpp with a root-mean-square deviation (RMSD) of 21 cm-1 and experimental results with a RMSD of 23 cm-1. Along with this very good accuracy, the computational time with our hybrid QM/ML approach scales linearly with N, while the traditional full ab initio method scales as N2, where N is the number of atoms.

Alloy, Janus and core-shell nanoparticles: numerical modeling of their nucleation and growth in physical synthesis.

While alloy, core-shell and Janus binary nanoclusters are found in more and more technological applications, their formation mechanisms are still poorly understood, especially during synthesis methods involving physical approaches. In this work, we employ a very simple model of such complex systems using Lennard-Jones interactions and inert gas quenching. After demonstrating the ability of the model to well reproduce the formation of alloy, core-shell or Janus nanoparticles, we studied their temporal evolution from the gas via droplets to nanocrystalline particles. In particular, we showed that the growth mechanisms exhibit qualitative differences between these three chemical orderings. Then, we determined how the quenching rate can be used to finely tune structural characteristics of the final nanoparticles, including size, shape and crystallinity.

Doping nanoparticles using pulsed laser ablation in a liquid containing the doping agent.

While doping of semiconductors or oxides is crucial for numerous technological applications, its control remains difficult especially when the material is reduced down to the nanometric scale. In this paper, we show that pulsed laser ablation of an undoped solid target in an aqueous solution containing activator ions offers a new way to synthesise doped-nanoparticles. The doping efficiency is evaluated for laser ablation of an undoped Gd2O3 target in aqueous solutions of EuCl3 with molar concentration from 10-5 mol L-1 to 10-3 mol L-1. Thanks to luminescence experiments, we show that the europium ions penetrate the core of the synthesised monoclinic Gd2O3 nanoparticles. We also show that the concentration of the activators in the nanoparticles is proportional to the initial concentration in europium ions in the aqueous solution, and a doping of about 1% ([Eu]/[Gd] atomic ratio) is reached. On the one hand, this work could open new ways for the synthesis of doped nanomaterials. On the other hand, it also raises the question of undesired penetration of impurities in laser-generated nanoparticles in liquids.

Solvent-mediated interactions between nanostructures: From water to Lennard-Jones liquid.

Solvent-mediated interactions emerge from complex mechanisms that depend on the solute structure, its wetting properties, and the nature of the liquid. While numerous studies have focused on the first two influences, here, we compare the results from water and Lennard-Jones liquid in order to reveal to what extent solvent-mediated interactions are universal with respect to the nature of the liquid. Besides the influence of the liquid, the results were obtained with classical density functional theory and brute-force molecular dynamics simulations which allow us to contrast these two numerical techniques.

Classical density functional theory, unconstrained crystallization, and polymorphic behavior.

While in principle, classical density functional theory (cDFT) should be a powerful tool for the study of crystallization, in practice this has not so far been the case. Progress has been hampered by technical problems which have plagued the study of the crystalline systems using the most sophisticated fundamental measure theory models. In this paper, the reasons for the difficulties are examined and it is proposed that the tensor functionals currently favored are in fact numerically unstable. By reverting to an older, more heuristic model it is shown that all of the technical difficulties are eliminated. Application to a Lennard-Jones fluid results in a demonstration of power of cDFT to describe crystallization in a highly inhomogeneous system. First, we show that droplets attached to a slightly hydrophobic wall crystallize spontaneously upon being quenched. The resulting crystallites are clearly faceted structures and are predominantly HCP structures. In contrast, droplets in a fully periodic calculational cell remain stable to lower temperatures and eventually show the same spontaneous localization of the density into "atoms" but in an amorphous structure having many of the structural characteristics of a glass. A small change of the protocol leads, at the same temperature, to the formation of crystals, this time with the fcc structure typical of bulk Lennard-Jones solids. The fcc crystals have lower free energy than the amorphous structures which in turn are more stable than the liquid droplets. It is demonstrated that as the temperature is raised, the free energy differences between the structures decrease until the solid clusters become less stable than the liquid droplets and spontaneously melt. The presence of energy barriers separating the various structures is therefore clearly demonstrated.

Lattice induced crystallization of nanodroplets: the role of finite-size effects and substrate properties in controlling polymorphism.

Targeting specific technological applications requires the control of nanoparticle properties, especially the crystalline polymorph. Freezing a nanodroplet deposited on a solid substrate leads to the formation of crystalline structures. We study the inherent mechanisms underlying this general phenomenon by means of molecular dynamics simulations. Our work shows that different crystal structures can be selected by finely tuning the solid substrate lattice parameter. Indeed, while for our system, face-centered cubic is usually the most preponderant structure, the growth of two distinct polymorphs, hexagonal centered packing and body-centered cubic, was also observed even when the solid substrate was face-centered cubic. Finally, we also demonstrated that the growth of hexagonal centered packing is conditioned by the appearance of large enough body-centered cubic clusters thus suggesting the presence of a cross-nucleation pathway. Our results provide insights into the impact of nanoscale effects and solid substrate properties towards the growth of polymorphic nanomaterials.

Solute particle near a nanopore: influence of size and surface properties on the solvent-mediated forces.

Nanoscopic pores are used in various systems to attract nanoparticles. In general the behaviour is a result of two types of interactions: the material specific affinity and the solvent-mediated influence also called the depletion force. The latter is more universal but also much more complex to understand since it requires modeling both the nanoparticle and the solvent. Here, we employed classical density functional theory to determine the forces acting on a nanoparticle near a nanoscopic pore as a function of its hydrophobicity and its size. A simple capillary model is constructed to predict those depletion forces for various surface properties. For a nanoscopic pore, complexity arises from both the specific geometry and the fact that hydrophobic pores are not necessarily filled with liquid. Taking all of these effects into account and including electrostatic effects, we establish a phase diagram describing the entrance and the rejection of the nanoparticle from the pore.

Origin of the nano-carbon allotropes in pulsed laser ablation in liquids synthesis.

Carbon-based materials are of great technological and scientific interest in materials science. Pulsed laser ablation in liquids (PLAL) is extensively used as a method to produce nanoparticles including nanodiamond and related materials. In this feature article, we will review the use of PLAL to tackle the challenges of synthesizing carbon-based nanostructures. Surprisingly, reported results have shown very poor reproducibility despite the use of similar experimental conditions. We use plasma spectroscopy and shadowgraph imaging to provide a picture of the thermodynamic properties, and then to better understand this apparent contradiction. Our study was carried out under traditional conditions which involve nanosecond laser, and radiant exposures from tens to thousands J/cm2. Prompted by these results, the different scenarios reported in the literature are discussed including shockwave induced phase transition, growth in high temperatures-high pressures like conditions, and vapor phase chemistry.

Atomistic Mechanisms for the Nucleation of Aluminum Oxide Nanoparticles.

A predictive model for nanoparticle nucleation has not yet been successfully achieved. Classical nucleation theory fails because the atomistic nature of the seed has to be considered. Indeed, geometrical structure as well as stoichiometry do not always match the bulk values. We present a fully microscopic approach based on a first-principle study of aluminum oxide clusters. We calculated stable structures of AlxOy and their associated thermodynamic properties. From these data, the chemical composition of a gas composed of aluminum and oxygen atoms can be calculated as a function of temperature, pressure, and aluminum to oxygen ratio. We demonstrate the accuracy of this approach in reproducing experimental results obtained with time-resolved spectroscopy of a laser-induced plasma from an Al2O3 target. We thus extended the calculation to lower temperatures, i.e., longer time scales, to propose a scenario of composition gas evolution leading to the first alumina seeds.

γ-Al2O3 nanoparticles synthesised by pulsed laser ablation in liquids: a plasma analysis.

Pulsed laser ablation has proved its reliability for the synthesis of nano-particles and nano-structured materials, including metastable phases and complex stoichiometries. The possible nucleation of the nanoparticles in the gas phase and their growth has been little investigated, due to the difficulty of following the gas composition as well as the thermodynamic parameters. We show that such information can be obtained from the optically active plasma during its short lifetime, only a few microseconds for each laser pulse, as a result of a quick quenching due to the liquid environment. For this purpose, we follow the laser ablation of an α-Al2O3 target (corindon) in water, which leads to the synthesis of nanoparticles of γ-Al2O3. The AlO blue-green emission and the Al(I) (2)P(0)-(2)S doublet emission provide the electron density, the density ratio between the Al atoms and AlO molecules, and the rotational and vibrational temperatures of the AlO molecules. These diagnostic considerations are discussed in the framework of theoretical studies from the literature (density functional theory). We have found that starting from a hot atomized gas, the nucleation cannot occur in the first microseconds. We also raise the question of the influence of water on the control of the stoichiometry.