Size-dependent transition to high-symmetry chiral structures in AgCu, AgCo, AgNi, and AuNi nanoalloys

First Principle Study on Structural, Electronic, Magnetic, and Optical Properties of Co-Doped Middle Size Silver Clusters

The structural, electronic, magnetic, and optical properties of Co-doped 10-20-atom silver clusters are investigated by GGA/PBE via the density functional theory. The Ag-Co clusters form core-shell structures with a Co atom in the center. Co atom doping modulates electronic properties like energy gap, molecular softness, global hardness, electronegativity, and electrophilicity index. For the optical spectra of the Ag-Co clusters, the energy of their spectra overall exhibits little change with increasing numbers of atoms; the strongest peaks are roughly distributed at 3.5 eV, and the intensity of their spectra overall is strengthened. Raman and vibrational spectra reflect structural changes with Co atom addition. The addition of the Co atom alters magnetic moments of specific Ag-Co clusters, while others remain unchanged.

Sudden collective atomic rearrangements trigger the growth of defect-free silver icosahedra

The growth of Ag clusters on amorphous carbon substrates is studied in situ by X-ray scattering experiments, whose final outcome is imaged by electron microscopy. The real-time analysis of the growth process at room temperature shows the formation of a large majority of icosahedral structures by a shell-by-shell growth mode which produces smooth and nearly defect-free structures. Molecular dynamics simulations supported by ab initio calculations reveal that the shell-by-shell mode is possible because of the occurrence of collective displacements which involve the concerted motion of many atoms of the growing shell. These collective processes are a kind of black swan event, as they occur suddenly and rarely, but their occurrence is decisive for the final outcome of the growth. Annealing and ageing experiments show that the as-grown icosahedra are metastable, in agreement with the energetic stability calculations.

Growth pathways of Cu shells on Au and AuCu seeds: interdiffusion, shape transformations, strained shells and patchy surfaces

The growth of AuCu nanoparticles obtained by depositing Cu atoms on starting seeds of pure Au and on mixed AuCu seeds is studied by molecular dynamics simulations. Depending on the shape of the seed, its composition and the growth temperature, different growth pathways are observed, in which several types of structural transformations take place. The final growth structures comprise Au@Cu core@shell arrangements as well as Janus-like structures with patchy surfaces. The results of the growth simulations are rationalized in terms of the activation of different diffusion processes, both on the surface and inside the growing clusters. These diffusion processes regulate structural transitions between different motifs and the occurrence of dewetting phenomena. The simulation results show that depositon of Cu atoms on pure Au or mixed AuCu seed can be an effective tool for producing clusters with uncommon surface atom arrangements of potential interest for catalysis.

Structural transformations in Cu, Ag, and Au metal nanoclusters

Finite-temperature structures of Cu, Ag, and Au metal nanoclusters are calculated in the entire temperature range from 0 K to melting using a computational methodology that we proposed recently [M. Settem et al., Nanoscale 14, 939 (2022)]. In this method, Harmonic Superposition Approximation (HSA) and Parallel Tempering Molecular Dynamics (PTMD) are combined in a complementary manner. HSA is accurate at low temperatures and fails at higher temperatures. PTMD, on the other hand, effectively samples the high temperature region and melts. This method is used to study the size- and system-dependent competition between various structural motifs of Cu, Ag, and Au nanoclusters in the size range 1-2 nm. Results show that there are mainly three types of structural changes in metal nanoclusters, depending on whether a solid-solid transformation occurs. In the first type, the global minimum is the dominant motif in the entire temperature range. In contrast, when a solid-solid transformation occurs, the global minimum transforms either completely to a different motif or partially, resulting in the co-existence of multiple motifs. Finally, nanocluster structures are analyzed to highlight the system-specific differences across the three metals.

Composition-dependent chemical ordering predicted in Pt-Ag nanoalloys

Pt-Ag nanoalloys display an astonishing chemical organization depending on their size and composition. Reversed size-dependent stabilization of ordered nanophases [J. Pirart et al., Nat. Commun., 2019, 10, 1982-1989] has recently been shown around equiconcentration. We extend this study by a theoretical investigation on the whole range of compositions showing a significant composition-dependent chemical ordering in Pt-Ag nanoalloys. At a low silver content, the surface exhibits a strong Ag segregation coupled to a (2 × 1) superstructure on the (100) facets. By increasing the silver concentration, the system displays an L1₁ ordered phase in the core, interrupted in a narrow range of concentrations by a concentric multishell structure characterized by an alternation of Ag-pure/Pt-pure concentric layers starting from the surface shell to the core. Although the L1₁ ordered phase has been observed experimentally, the concentric multishell structure is lacking due to the difficulty of the experimental characterization.

Effect of Size on the Formation of Solid Solutions in Ag-Cu Nanoparticles

Modern technologies stimulate the quest for multicomponent nanosized materials with improved properties, which are ultimately defined by the atomic arrangement and interphase interactions in the nanomaterial. Here, we present the results of the experimental study of the formation of solid solutions in Ag-Cu nanoparticles in a wide size and temperature range using in situ TEM techniques. The Ag-Cu nanoparticles with a eutectic ratio of components were formed on an amorphous carbon film by the physical vapor deposition technique. Electron diffraction, HAADF-STEM imaging, energy-dispersive X-ray spectroscopy, chemical element mapping, and electron energy loss spectral imaging were used for the characterization of mixing patterns and composition of phases in AgCu nanoparticles down to the atomic level. As a result, we constructed the solid-state part of the Ag-Cu phase diagram for nanoparticles with a size down to 5 nm. We found a highly asymmetric behavior of the solvus lines. Thus, the content of Cu in Ag gradually increased with a size reduction and reached the ultimate value for our configuration of 27 wt % Cu at a nanoparticle size below ∼8 nm. At the same time, no Cu-rich solid solution was found in two-phase AgCu nanoparticles, irrespective of the size and temperature. Moreover, a quasi-homogeneous solid solution was revealed in AgCu nanoparticles with a size smaller than 8 nm already at room temperature. A size dependence of the terminal temperature T _term, which limits the existence of AgCu alloy nanoparticles in a vacuum, was constructed. Evaporation of the AgCu phase with the composition of 86 wt % Ag was observed at temperatures above T _term. We show the crucial role of the mutual solubility of components on the type of atomic mixing pattern in AgCu nanoparticles. A gradual transition from a Janus-like to a homogeneous mixing pattern was observed in Ag-Cu nanoparticles (28 wt % Cu) with a decrease in their size.

Growth pathways of exotic Cu@Au core@shell structures: the key role of misfit strain

The CuAu system is characterized by a large lattice mismatch which causes a misfit strain in its core@shell architectures. Here we simulate the formation of Cu@Au core@shell nanoparticles by Au deposition on a preformed seed, and we study the effect of the shape and composition of the starting seed on the growth pathway. Three geometric shapes of the starting seed are considered: truncated octahedra, decahedra and icosahedra. For each shape, we consider two compositions, pure Cu and CuAu, at equicomposition and intermixed chemical ordering. Our results show that the shape and composition of the seed have significant effects on the growth pathways of Cu@Au core@shell nanoparticles. When starting with icosahedral seeds, the growing structure stays in that motif always. When starting with truncated octahedral and decahedral seeds, we have observed that there is a clear difference between the pure and intermixed seeds. For pure seeds, the growth often leads to exotic structures that are obtained after some structural transformations. For mixed seeds, the growth leads to quite regular structures resembling those obtained for pure metals. These growth pathways originate from strain relaxation mechanisms, which are rationalized by calculating the atomic level stress.

Two-Steps Versus One-Step Solidification Pathways of Binary Metallic Nanodroplets

The solidification of AgCo, AgNi, and AgCu nanodroplets is studied by molecular dynamics simulations in the size range of 2-8 nm. All these systems tend to phase separate in the bulk solid with surface segregation of Ag. Despite these similarities, the simulations reveal clear differences in the solidification pathways. AgCo and AgNi already separate in the liquid phase, and they solidify in configurations close to equilibrium. They can show a two-step solidification process in which Co-/Ni-rich parts solidify at higher temperatures than the Ag-rich part. AgCu does not separate in the liquid and solidifies in one step, thereby remaining in a kinetically trapped state down to room temperature. The solidification mechanisms and the size dependence of the solidification temperatures are analyzed, finding qualitatively different behaviors in AgCo/AgNi compared to AgCu. These differences are rationalized by an analytical model.

Machine Learning Assisted Clustering of Nanoparticle Structures

We propose a scheme for the automatic separation (i.e., clustering) of data sets composed of several nanoparticle (NP) structures by means of Machine Learning techniques. These data sets originate from atomistic simulations, such as global optimizations searches and molecular dynamics simulations, which can produce large outputs that are often difficult to inspect by hand. By combining a description of NPs based on their local atomic environment with unsupervised learning algorithms, such as K-Means and Gaussian mixture model, we are able to distinguish between different structural motifs (e.g., icosahedra, decahedra, polyicosahedra, fcc fragments, twins, and so on). We show that this method is able to improve over the results obtained previously thanks to the successful implementation of a more detailed description of NPs, especially for systems showing a large variety of structures, including disordered ones.

Molecular dynamics simulations of the impurity effect on packing structures, local stresses, and thermodynamics of Ih silver clusters

Molecular dynamics simulations are performed to investigate impurity effects on the changes of packing structures, atomic level pressure, and thermodynamics for icosahedral Ag clusters having 55 and 147 atoms. The simulation results indicate that, doping Cu atoms in different cluster shells can significantly affect their thermal stability and thermodynamic values. For the small size clusters, the doping in the interior apparently releases the stress, and improves the stability. The thermodynamic values of entropy and free energy can be used to indicate the order degree of the packing structures and structural transition. The shape changes are related to the local stresses at elevated temperatures.

AuCo nanoparticles: ordering, magnetisation, and morphology trends predicted by DFT

The rapid development of applications relying on magnetism at the nanoscale has put a spotlight on nanoparticles with novel morphologies that are associated with enhanced electronic and magnetic properties. In this quest, nanoalloys combining highly magnetic cobalt and weakly reactive gold could offer many application-specific advantages, such as strong magnetic anisotropy. In the present study, we have employed density functional theory (DFT) calculations to provide a systematic overview of the size- and morphology-dependence of the energetic order and magnetic properties of AuCo nanoparticles up to 2.5 nm in diameter. The core-shell icosahedron was captured as the most favourable morphology, showing a small preference over the core-shell decahedron. However, the magnetic properties (total magnetic moments and magnetic anisotropy) were found to be significantly improved within the L1₀ ordered structures, even in comparison to monometallic Co nanoparticles. Atom-resolved charges and orbital moments accessed through the DFT analysis of the electronic level properties permitted insight into the close interrelation between the AuCo nanoparticle morphology and their magnetism. These results are expected to assist in the design of tailored magnetic AuCo nanoalloys for specific applications.

Understanding the strain-dependent structure of Cu nanocrystals in Ag-Cu nanoalloys

The structure of octahedral Ag-Cu nanoalloys is investigated by means of basin hopping Monte Carlo (BHMC) searches involving the optimization of shape and chemical ordering. Due to the significant size mismatch between Ag and Cu, the misfit strain plays a key role in determining the structure of Ag-Cu nanoalloys. At all the compositions, segregated chemical ordering is observed. However, the shape of the Cu nanocrystal and the associated defects are significantly different. At lower amounts of Cu (as little as 2 atom %), defects close to the surface are observed leading to a highly non-compact shape of the Cu nanocrystal which is non-trivial. The number of Cu-Cu bonds is relatively lower in the non-compact shape which is contrary to the preference of bulk Ag-Cu alloys to maximize the homo-atomic bonds. Due to the non-compact shape, {100} Ag-Cu interfaces are observed which are not expected. As the amount of Cu increases, the Cu nanocrystal undergoes a shape transition from non-compact to a compact octahedron. The associated defect structure is also modified. The structural changes due to the strain effects have been explained by calculating the atomic pressure maps and the bond length distributions. The trends relating to the structure have also been verified at larger sizes.

Robustness of the chiral-icosahedral golden shell I-Au60 in multi-shell structures

Motivated by the recent theoretical discovery [S.-M. Mullins et al., Nat. Commun. 9, 3352 (2018)] of a surprisingly contracted 60-atom hollow shell of chiral-icosahedral symmetry (I-Au₆₀) of remarkable rigidity and electronegativity, we have explored, via first-principles density functional theory calculations, its physico-chemical interactions with internal and external shells, enabling conclusions regarding its robustness and identifying composite forms in which an identifiable I-Au₆₀ structure may be realized as a product of natural or laboratory processes. The dimensions and rigidity of I-Au₆₀ suggest a templating approach; e.g., an I_h-C₆₀ fullerene fits nicely within its interior, as a nested cage. In this work, we have focused on its susceptibility, i.e., the extent to which the unique structural and electronic properties of I-Au₆₀ are modified by incorporation into selected multi-shell structures. Our results confirm that the I-Au₆₀ shell is robustly maintained and protected in various bilayer structures: I_h-C₆₀@I-Au₆₀, I_h-Au₃₂@I-Au₆₀ ²⁺, Au₆₀(MgCp)₁₂, and their silver analogs. A detailed analysis of the structural and electronic properties of the selected I-Au₆₀ shell-based nanostructures is presented. We found that the I-Au₆₀ shell structure is quite well retained in several robust forms. In all cases, the I-symmetry is preserved, and the I-Au₆₀ shell is slightly deformed only in the case of the I_h-C₆₀@I-Au₆₀ system. This analysis serves to stimulate and provide guidance toward the identification and isolation of various I-Au₆₀ shell-based nanostructures, with much potential for future applications. We conclude with a critical comparative discussion of these systems and of the implications for continuing theoretical and experimental investigations.

An augmented (multi-descriptor) grouping algorithm to optimize chemical ordering in nanoalloys

We propose the Augmented Grouping Approach (AugGA) and its deployment in the Augmented Grouping GO (AugGGO) scheme, for an efficient exploration of the chemical ordering (or compositional structure) of multi-component (alloyed) nanoparticles. The approach is based on a 'grouping' strategy (previously proposed for high-symmetry structures) by which the number of compositional degrees of freedom of the system is decreased by defining sets of atoms (groups, or orbits, or shells) that are constrained to be populated by the same element. Three fundamental advances are here included with respect to previous proposals: (i) groups are defined on the basis of descriptors (no point-group symmetry is assumed), (ii) bulk groups can exploit general chemical ordering patterns taken from databases, and (iii) sub-grouping is realized via a multi-descriptor strategy (here using two basic descriptors: the atomic energy and a few types of geometry patterns). The AugGGO approach is applied to two prototypical examples of binary nanoalloys: Pd-Pt and Ag-Cu, with a size between ≈500 and ≈1300 atoms, in different configurations, and the convex hull of the mixing energy as a function of composition is derived. It is shown how the three advances here proposed decisively extend the power and scope of the grouping approach: (i) making it applicable to any generic structural framework, (ii) achieving a thorough sampling of the core regions of nanoparticles, and (iii) catching exotic/unexpected chemical ordering arrangements, at a computational cost which is 1-2 orders of magnitude smaller than that of traditional Monte Carlo single-exchange techniques.

Regression and clustering algorithms for AgCu nanoalloys: from mixing energy predictions to structure recognition

The lowest-energy structures of AgCu nanoalloys are searched for by global optimization algorithms for sizes 100 and 200 atoms depending on composition. Even though the AgCu system is very weakly miscible in macroscopic samples, the mixing energy for these nanoalloys turns out to be clearly negative for both sizes, a result which is attributed to the stabilization of non-crystalline Cu@Ag core-shell structures at the nanoscale. The mixing energy is a quantity nowadays unknown in its functional form, so that its prediction may take advantage of machine learning techniques. A support vector regressor is then implemented to successfully predict the mixing energy of AgCu nanoalloys of both sizes. Moreover, with the help of unsupervised learning algorithms, it is shown that the automatic classification of such nanoalloys into different physically meaningful structural families is indeed possible. Finally, thanks to the harmonic superposition approximation, the temperature-dependent probabilities of such structural families are calculated.

Impurity diffusion in magic-size icosahedral clusters

Atomic diffusion is at the basis of chemical ordering transformations in nanoalloys. Understanding the diffusion mechanisms at the atomic level is therefore a key issue in the study of the thermodynamic behavior of these systems and, in particular, of their evolution from out-of-equilibrium chemical ordering types often obtained in the experiments. Here, the diffusion is studied in the case of a single-atom impurity of Ag or Au moving within otherwise pure magic-size icosahedral clusters of Cu or Co by means of two different computational techniques, i.e., molecular dynamics and metadynamics. Our simulations reveal unexpected diffusion pathways, in which the displacement of the impurity is coupled with the creation of vacancies in the central part of the cluster. We show that the observed mechanism is quite different from the vacancy-mediated diffusion processes identified so far, and we demonstrate that it can be related to the presence of non-homogeneous compressive stress in the inner part of the icosahedral structure.

Melting Behavior of Bimetallic and Trimetallic Nanoparticles: A Review of MD Simulation Studies

In recent years, bimetallic and trimetallic nanoparticles (NPs) have become attractive materials for many researchers especially in the field of catalysis due to their interesting physical and chemical properties. These unique properties arise mainly from simultaneous effects of two different metal atoms in their structure. In this review, recent theoretical studies on these NPs using molecular dynamics simulation are presented. Since investigation of thermodynamic stabilities of metallic NPs is a critical factor in their construction for catalytic applications, our focus in this review is on the thermal stability of bimetallic and trimetallic NPs. The melting behavior of these materials with different atomic arrangements including core-shell, three-shell, crown-jewel, ordered and disordered alloy, and Janus materials are discussed. Other factors including stress, strain, atomic radius, thermal expansion coefficient, cohesive energy, surface energy, size, composition, and morphology are described in detail, because these properties lead to complexity in the melting behavior of bimetallic and trimetallic NPs.

Kinetically Stable Nonequilibrium Gold-Cobalt Alloy Nanoparticles with Magnetic and Plasmonic Properties Obtained by Laser Ablation in Liquid

Nonequilibrium nanoalloys are metastable solids obtained at the nanoscale under nonequilibrium conditions that allow the study of kinetically frozen atoms and the discovery of new physical and chemical properties. However, the stabilization of metastable phases in the nanometric size regime is challenging and the synthetic route should be easy and sustainable, for the nonequilibrium nanoalloys to be practically available. Here we report on the one-step laser ablation synthesis in solution (LASiS) of nonequilibrium Au-Co alloy nanoparticles (NPs) and their characterization on ensembles and at the single nanoparticle level. The NPs are obtained as a polycrystalline solid solution stable in air and water, although surface cobalt atoms undergo oxidation to Co(II). Since gold is a renowned plasmonic material and metallic cobalt is ferromagnetic at room temperature, these properties are both found in the NPs. Besides, surface conjugation with thiolated molecules is possible and it was exploited to obtain colloidally stable solutions in water. Taking advantage of these features, an array of magnetic-plasmonic dots was obtained and used for surface-enhanced Raman scattering experiments. Overall, this study confirms that LASiS is an effective method for the formation of kinetically stable nonequilibrium nanoalloys and shows that Au-Co alloy NPs are appealing magnetically responsive plasmonic building blocks for several nanotechnological applications.

From metastability to equilibrium during the sequential growth of Co-Ag supported clusters: a real-time investigation

Atomic motions and morphological evolution of growing Co-Ag nanoparticles are followed in situ and in real time, by wide and small angle X-ray scattering obtained simultaneously in grazing incidence geometry (GISAXS and GIWAXS), in single or multi-wavelength anomalous modes. The structural analysis of the experimental data is performed with the aid of equilibrium Monte Carlo simulations and of molecular-dynamics simulations of nanoparticle growth. Growth is performed by depositing Co atoms above preformed Ag nanoparticles. This growth procedure is strongly out of equilibrium, because Ag tends to surface segregation, and generates complex growth sequences. The real time analysis of the growth allows to follow the nanoparticle evolution pathways almost atom-by-atom, determining the key mechanisms during Co deposition: starting with the incorporation of Co atoms in sub-surface positions, to the off-center Co domain formation, then by which the nanoparticles finally approach their equilibrium quasi-Janus then core-shell structures.

Rapid Prediction of Bimetallic Mixing Behavior at the Nanoscale

The nanoparticle (NP) design space allows for variations in size, shape, composition, and chemical ordering. In the search for low-energy structures, this results in an extremely large search space which cannot be screened by brute force methods. In this work, we develop a genetic algorithm to predict stable bimetallic NPs of any size, shape, and metal composition. Our method predicts nanostructures in agreement with experimental trends and it captures the detailed chemical ordering of an experimental 23,196-atom FePt NP with nearly atom-by-atom accuracy. Our developed screening process is extremely fast, allowing us to generate and analyze a database of 5454 low-energy bimetallic NPs. By identifying thermodynamically stable NPs, we rationalize bimetallic mixing at the nanoscale and reveal metal-, size-, and temperature-dependent mixing behavior. Importantly, our method is applicable to any bimetallic NP size, bridging the materials gap in nanoscale simulations, and guides experimentation in the lab by elucidating stability, mixing, and detailed chemical ordering behavior of bimetallic NPs.

Towards a morphology of cobalt nanoparticles: size and strain effects

Cobalt nanoparticles with diameters of 8 nm have recently shown promising performance for biomedical applications. However, it is still unclear how the shape of cobalt clusters changes with size when reaching the nanoparticle range. In the present work, density functional theory calculations have been employed to compare the stabilities of two non-crystalline (icosahedron and decahedron) shapes, and three crystalline motifs (hcp, fcc, and bcc) for magic numbered cobalt clusters with up to 1500 atoms, based on the changes in the cohesive energies, coordination numbers, and nearest-neighbour distances arising from varying geometries. Obtained trends were extrapolated to a 10⁴ size range, and an icosahedral shape was predicted for clusters up to 5500 atoms. Larger sized clusters adopt hcp stacking, in correspondence with the bulk phase. To explain the crystalline/non-crystalline crossovers, the contributions of the elastic strain density and twin boundary from the specimen surfaces to the cohesive energy of different motifs were evaluated. These results are expected to aid the design and synthesis of cobalt nanoparticles for applications ranging from catalysis to biomedical treatments.

Role of core-shell energetics on anti-Mackay, chiral stacking in AgCu nanoalloys and thermally induced transition to chiral stacking

In AgCu nanoalloys a size-dependent transition to the chiral stacking from the anti-Mackay stacking has been predicted previously. This trend is explained by considering the interplay between the core-shell energetics. Results indicate that the energy changes in the Ag shell alone is not sufficient to explain the stability of the chiral stacking and the energy changes in the Cu core also need to be considered. In addition to this, thermally induced transition to chiral stacking was observed at sizes where anti-Mackay stacking is energetically favourable. On transition to the chiral stacking, the Ag-Ag, Ag-Cu and Cu-Cu bond lengths change significantly. These observations are also applicable for AgCu nanoalloys with incomplete Ag shells.

Structural surface and thermodynamics analysis of nanoparticles with defects

In this work, we analyze the surface structure and thermodynamics regarding the decoration of nanoparticles with defects, using statistical calculations and Monte Carlo simulations in a complementary way.

Chemically directed structure evolution for crystal structure prediction

The chemically directed structure evolution method uses chemical models to quantify the environment of atoms and vacancy sites in a crystal structure with that information used to inform how to modify the structure for crystal structure prediction.

Design of High-Performance Co-Based Alloy Nanocatalysts for the Oxygen Reduction Reaction

Co-based nanoalloys show potential applications as nanocatalysts for the oxygen reduction reaction (ORR), but improving their activity is still a great challenge. In this paper, a strategy is proposed to design efficient Co-M (M=Au, Ag, Pd, Pt, Ir, and Rh) nanoalloys as ORR catalysts by using density functional theory (DFT) calculations. Through the Sabatier analysis, the overpotential as a function of ΔG_OH * is identified as a quantitative descriptor for analyzing the effect of dopants and atomic structures on the activity of the Co-based nanoalloys. By adopting the suitable dopants and atomic structures, ΔG_OH * accompanied by overpotential could be adjusted to the optimal range to enhance the activity of the Co-based nanoalloys. With this strategy, the core-shell structured Ag₄₂ Co₁₃ nanoalloy is predicted to have the highest catalytic activity for ORR among these Co-based nanoalloys. To give a deeper insight into the properties of Ag-Co nanoalloys, the structure, thermal stability, and reaction mechanism of Ag-Co nanoalloys with different compositions are also studied by using molecular simulations and DFT calculations. It is found that core-shell Ag₄₂ Co₁₃ exhibits the highest structural and thermal stability among these Ag-Co nanoalloys. In addition, the core-shell Ag₄₂ Co₁₃ shows the lowest ORR reaction energy barriers among these Ag-Co nanoalloys. It is expected that this kind of strategy could provide a viable way to design highly efficient heterogeneous catalysts in extensive applications.

Core-shell vs. multi-shell formation in nanoalloy evolution from disordered configurations

The evolution towards equilibrium of AuCo, AgNi and AgCu nanoparticles is studied by molecular dynamics simulations. Nanoparticle sizes of about 2.5 nm are considered, in the temperature range from 300 to 700 K. The simulations reveal complex equilibration pathways, in which geometric structure and chemical ordering change with time. These nanoparticles present the same type of strong tendency to phase separation and to surface segregation of either Au or Ag, which lead to the same type of core@shell equilibrium structures. In spite of these similarities, the equilibration pathways of these nanoparticles from chemically disordered configurations present both quantitative and qualitative differences. Quantitative differences are found in the equilibration time scale, which is much longer in AgCu than in AgNi and AuCo. Qualitative differences are found in the presence or absence of geometric structure transformations, and in the formation of different types of three-shell metastable chemical ordering during evolution. It is also shown that surface segregation depends on the geometric structure, being faster in icosahedra than in fcc nanoparticles.

Universal description of heating-induced reshaping preference of core-shell bimetallic nanoparticles

To achieve universal description of the reshaping process of core-shell bimetallic nanoparticles, we combined the tight-binding Ising Hamiltonian model with molecular dynamic simulations to propose a general theoretical model at the atomic scale while considering the temperature, bond energy, atomic size, and surface energy effects. Based on this model, we can quantitatively analyze the tendency of core-shell structured bimetallic nanoparticles toward the reshaping phenomenon upon heating. By rapidly screening 196 types of bimetallic nanoparticles (containing transition metal elements from VIII to IIB groups in the fourth, fifth, and sixth rows of the periodic table), we identified forty-four kinds of bimetallic nanoparticles with reshaping tendency upon heating, which was validated by molecular dynamic simulations and available experimental results. With increasing temperature, the bimetallic nanoparticles with reshaping preference were transformed from an icosahedron to a star-like shape. In contrast, the structure of bimetallic nanoparticles without reshaping preference was transformed from an icosahedron to a sphere shape, which is usually considered to be the normal pre-melting phenomenon. Further structural analysis indicated that the reshaping of bimetallic nanoparticles could be ascribed to different diffusion mechanisms, where a dominant unidirectional mechanism leads to reshaped bimetallic nanoparticles and a bidirectional diffusion mechanism results in no-reshaped bimetallic nanoparticles. This study provides a deep insight into the origin of reshaping in bimetallic nanoparticles, and it may stimulate extensive studies on engineering bimetallic nanoparticles to switch on/off reshaping upon heating, for example, by modifying the structures, atomic arrangement or composites of bimetallic systems in future.

Towards operando computational modeling in heterogeneous catalysis

An increased synergy between experimental and theoretical investigations in heterogeneous catalysis has become apparent during the last decade. Experimental work has extended from ultra-high vacuum and low temperature towards operando conditions. These developments have motivated the computational community to move from standard descriptive computational models, based on inspection of the potential energy surface at 0 K and low reactant concentrations (0 K/UHV model), to more realistic conditions. The transition from 0 K/UHV to operando models has been backed by significant developments in computer hardware and software over the past few decades. New methodological developments, designed to overcome part of the gap between 0 K/UHV and operando conditions, include (i) global optimization techniques, (ii) ab initio constrained thermodynamics, (iii) biased molecular dynamics, (iv) microkinetic models of reaction networks and (v) machine learning approaches. The importance of the transition is highlighted by discussing how the molecular level picture of catalytic sites and the associated reaction mechanisms changes when the chemical environment, pressure and temperature effects are correctly accounted for in molecular simulations. It is the purpose of this review to discuss each method on an equal footing, and to draw connections between methods, particularly where they may be applied in combination.

Chiral symmetry breaking yields the I-Au60 perfect golden shell of singular rigidity

The combination of profound chirality and high symmetry on the nm-scale is unusual and would open exciting avenues, both fundamental and applied. Here we show how the unique electronic structure and bonding of quasi-2D gold makes this possible. We report a chiral symmetry breaking, i.e., the spontaneous formation of a chiral-icosahedral shell (I-Au60) from achiral (Ih) precursor forms, accompanied by a contraction in the Au-Au bonding and hence the radius of this perfect golden sphere, in which all 60 sites are chemically equivalent. This structure, which resembles the most complex of semi-regular (Archimedean) polyhedra (34.5*), may be viewed as an optimal solution to the topological problem: how to close a 60-vertex 2D (triangular) net in 3D. The singular rigidity of the I-Au60 manifests in uniquely discrete structural, vibrational, electronic, and optical signatures, which we report herein as a guide to its experimental detection and ultimately its isolation in material forms.

Band Edge Optical Excitation of Pyridine-Adsorbed CuAg Nanoparticles

Understanding the structure-property relationship of multielement nanoparticles is vital for developing novel nanodevices. In the present paper, via a combination of a basin hopping global sampling method, a symmetry-orbit shell optimization technique, and density functional theory reoptimizations, we determine the energetically most stable CuAg face-centered cubic nanoparticles. The calculated structures show a clear tendency toward Cu_coreAg_shell chemical ordering by populating the more cohesive Cu in the core region and of Ag in the shell region. Further, using time-dependent density functional theory (TDDFT) calculations, we analyze the band edge optical excitations of the nanoparticles with pyridine molecule on top. With the help of charge difference density plots, we found dramatic modifications in the electron density distribution of the nanoparticles. We believe that the present theoretical findings will be useful for the development of novel nanosensors.

Global structure search for molecules on surfaces: Efficient sampling with curvilinear coordinates

Efficient structure search is a major challenge in computational materials science. We present a modification of the basin hopping global geometry optimization approach that uses a curvilinear coordinate system to describe global trial moves. This approach has recently been shown to be efficient in structure determination of clusters [C. Panosetti et al., Nano Lett. 15, 8044-8048 (2015)] and is here extended for its application to covalent, complex molecules and large adsorbates on surfaces. The employed automatically constructed delocalized internal coordinates are similar to molecular vibrations, which enhances the generation of chemically meaningful trial structures. By introducing flexible constraints and local translation and rotation of independent geometrical subunits, we enable the use of this method for molecules adsorbed on surfaces and interfaces. For two test systems, trans-β-ionylideneacetic acid adsorbed on a Au(111) surface and methane adsorbed on a Ag(111) surface, we obtain superior performance of the method compared to standard optimization moves based on Cartesian coordinates.

Kinetic trapping through coalescence and the formation of patterned Ag-Cu nanoparticles

In recent years, due to its inherent flexibility, magnetron-sputtering has been widely used to synthesise bi-metallic nanoparticles (NPs) via subsequent inert-gas cooling and gas-phase condensation of the sputtered atomic vapour. Utilising two separate sputter targets allows for good control over composition. Simultaneously, it involves fast kinetics and non-equilibrium processes, which can trap the nascent NPs into metastable configurations. In this study, we observed such configurations in immiscible, bi-metallic Ag-Cu NPs by scanning transmission electron microscopy (S/TEM) and electron energy-loss spectroscopy (EELS), and noticed a marked difference in the shape of NPs belonging to Ag- and Cu-rich samples. We explained the formation of Janus or Ag@Cu core/shell metastable structures on the grounds of in-flight mixed NP coalescence. We utilised molecular dynamics (MD) and Monte Carlo (MC) computer simulations to demonstrate that such configurations cannot occur as a result of nanoalloy segregation. Instead, sintering at relatively low temperatures can give rise to metastable structures, which eventually can be stabilised by subsequent quenching. Furthermore, we compared the heteroepitaxial diffusivities along various surfaces of both Ag and Cu NPs, and emphasised the differences between the sintering mechanisms of Ag- and Cu-rich NP compositions: small Cu NPs deform as coherent objects on large Ag NPs, whereas small Ag NPs dissolve into large Cu NPs, with their atoms diffusing along specific directions. Taking advantage of this observation, we propose controlled NP coalescence as a method to engineer mixed NPs of a unique, patterned core@partial-shell structure, which we refer to as a "glass-float" (ukidama) structure.

Phase diagram and segregation of Ag-Co nanoalloys: insights from theory and simulation

Understanding the phase diagram is the first step to identifying the structure-performance relationship of a material at the nanoscale. In this work, a modified nanothermodynamical model has been developed to predict the phase diagrams of Ag-Co nanoalloys with the size of 1 ∼ 100 nm, which also overcomes the difference in the predicted results between theory and simulation for the first time. Based on this modified model, the phase diagrams of Ag-Co nanoalloys with various polyhedral morphologies (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) have been predicted, showing good agreement with molecular dynamics simulations at the nanoscale of 1 ∼ 4 nm. In addition, the surface segregation of Ag-Co nanoalloys has been predicted with a Co-rich core/Ag-rich surface, which is also consistent with the simulation results. Our results highlight a useful roadmap for bridging the difference between theory and simulation in the prediction of the phase diagram at the nanoscale, which will help both theorists and experimentalists.

Structures and segregation patterns of Ag-Cu and Ag-Ni nanoalloys adsorbed on MgO(0 0 1)

Low-energy geometric structures and segregation patterns of Ag-Cu and Ag-Ni nanoparticles adsorbed on MgO(0 0 1) are searched for by global optimisation methods within an atomistic potential model. Sizes betwen 100 and 300 atoms are considered for several compositions. In all cases, Ag segregates to the nanoparticle surface, so that Cu@Ag and Ni@Ag core-shell arrangements are found, with off-centre cores for Ag-rich compositions. The behaviours of Ag-Cu and Ag-Ni differ at the interface with the MgO substrate. For Ag-Cu, some Cu atoms are at the interface even for compositions that are very rich in Ag, where Ag-Ni nanoparticles present an interface completely made of Ag atoms. Ag-Ni and Ag-Cu also differ concerning their geometric structures. With increasing Ag content, in Ag-Cu we find the structural sequence faulted fcc [Formula: see text] icosahedral [Formula: see text] fcc, while in Ag-Ni we find the sequence hcp [Formula: see text] faulted fcc-faulted hcp [Formula: see text] icosahedral [Formula: see text] fcc.

Decoupling strain and ligand effects in ternary nanoparticles for improved ORR electrocatalysis

Ternary Pt–Au–M (M = 3d transition metal) nanoparticles show reduced OH adsorption energies and improved activity for the oxygen reduction reaction (ORR) compared to pure Pt nanoparticles, as obtained by density functional theory.

Global Materials Structure Search with Chemically Motivated Coordinates

Identification of relevant reaction pathways in ever more complex composite materials and nanostructures poses a central challenge to computational materials discovery. Efficient global structure search, tailored to identify chemically relevant intermediates, could provide the necessary first-principles atomistic insight to enable a rational process design. In this work we modify a common feature of global geometry optimization schemes by employing automatically generated collective curvilinear coordinates. The similarity of these coordinates to molecular vibrations enhances the generation of chemically meaningful trial structures for covalently bound systems. In the application to hydrogenated Si clusters, we concomitantly observe a significantly increased efficiency in identifying low-energy structures and exploit it for an extensive sampling of potential products of silicon-cluster soft landing on Si(001) surfaces.

Magnetic and noble metal nanocomposites for separation and optical detection of biological species

Nanoalloys and nanocomposites are widely studied classes of nanomaterials within the context of biological systems. They are of immense interest because of the possibility of tuning the optical, magnetic, electronic and chemical properties through particle composition and internal architecture. In principle these properties can therefore be optimized for application in biological detections such as of DNA sequences, bacteria, viruses, antibodies, antigens, and cancer cells. This article presents an overview of methods currently used for nanoalloy and nanocomposite synthesis and characterisation, focusing on Au-Ag and FexOy@Au structures as primary components in detection platforms for plasmonic and magnetically enabled plasmonic bio-sensing.

How to determine accurate chemical ordering in several nanometer large bimetallic crystallites from electronic structure calculations

The proposed method allows to efficiently determine the atomic arrangement in bimetallic nanoparticles based on electronic structure calculations and unravels the relationship between structural preferences of atoms and binding in nanoalloys.

Chemical and physical properties of binary metallic nanoparticles (nanoalloys) are to a great extent defined by their chemical ordering, i.e. the pattern in which atoms of the two elements are located in a given crystal lattice. The reliable determination of the lowest-energy chemical ordering is a challenge that impedes in-depth studies of several-nm large bimetallic particles. We propose a method to efficiently optimize the chemical ordering based solely on results of electronic structure (density functional) calculations. We show that the accuracy of this method is practically the same as the accuracy of the underlying quantum mechanical approach. This method, due to its simplicity, immediately reveals why one or another chemical ordering is preferred and unravels the nature of the binding within the nanoparticles. For instance, our results provide very intuitive understanding of why gold and silver segregate on low-coordinated sites in Pd₇₀Au₇₀ and Pd₇₀Ag₇₀ particles, while Pd₇₀Cu₇₀ exhibits matryoshka-like structure and Pd₇₀Zn₇₀ features Zn and Pd atoms arranged in layers. To illustrate the power of the new method we optimized the chemical ordering in much larger Pd₇₃₂Au₇₃₁, Pd₇₃₂Ag₇₃₁, Pd₇₃₂Cu₇₃₁, and Pd₇₃₂Zn₇₃₁ nanocrystals, whose size ∼4.4 nm is common for catalytic applications.