Growth Mechanism of Five-Fold Twinned Ag Nanowires from Multiscale Theory and Simulations

NiS ultrafine nanorod with translational and rotational symmetry

Anisotropy is a significant and prevalent characteristic of materials, conferring orientation-dependent properties, meaning that the creation of original symmetry enables key functionality that is not found in nature. Even with the advancements in atomic machining, synthesis of separated symmetry in different directions within a single structure remains an extraordinary challenge. Here, we successfully fabricate NiS ultrafine nanorods with separated symmetry along two directions. The atomic structure of the nanorod exhibits rotational symmetry in the radial direction, while its axial direction is characterized by divergent translational symmetry, surpassing the conventional crystalline structures known to date. It does not fit the traditional description of the space group and the point group in three dimensions, so we define it as a new structure in which translational symmetry and rotational symmetry are separated. Further corroborating the atomic symmetric separation in the electronic structure, we observed the combination of stripe and vortex magnetic domains in a single nanorod with different directions, in accordance with the atomic structure. The manipulation of nanostructure at the atomic level introduces a novel approach to regulate new properties finely, leading to the proposal of new nanotechnology mechanisms.

Size and temperature dependent shapes of copper nanocrystals using parallel tempering molecular dynamics

We performed parallel-tempering molecular dynamics simulations to predict the temperature- and size-dependent equilibrium shapes of a series of Cu nanocrystals in the 100- to 200-atom size range. Our study indicates that temperature-dependent, solid-solid shape transitions occur frequently for Cu nanocrystals in this size range. Complementary calculations with electronic density functional theory indicate that vibrational entropy favors nanocrystals with a shape intermediate between a decahedron and an icosahedron. Overall, we find that entropy plays a significant role in determining the shapes Cu nanocrystals, so studies aimed at determining minimum-energy shapes may fail to correctly predict shapes observed at experimental temperatures. We also observe significant shape changes with nanocrystal size - sometimes with changes in a single atom. The information from this study could be useful in efforts to devise processing routes to achieve selective nanocrystal shapes.

Nanocrystal Assemblies: Current Advances and Open Problems

We explore the potential of nanocrystals (a term used equivalently to nanoparticles) as building blocks for nanomaterials, and the current advances and open challenges for fundamental science developments and applications. Nanocrystal assemblies are inherently multiscale, and the generation of revolutionary material properties requires a precise understanding of the relationship between structure and function, the former being determined by classical effects and the latter often by quantum effects. With an emphasis on theory and computation, we discuss challenges that hamper current assembly strategies and to what extent nanocrystal assemblies represent thermodynamic equilibrium or kinetically trapped metastable states. We also examine dynamic effects and optimization of assembly protocols. Finally, we discuss promising material functions and examples of their realization with nanocrystal assemblies.

Minimum Free-Energy Shapes of Ag Nanocrystals: Vacuum vs Solution

We use two variants of replica-exchange molecular dynamics (MD) simulations, parallel tempering MD and partial replica exchange MD, to probe the minimum free-energy shapes of Ag nanocrystals containing 100-200 atoms in a vacuum, ethylene glycol (EG) solvent, and EG solvent with a PVP polymer containing 100 repeat units. Our simulations reveal a shape intermediate between a Dh and an Ih, a Dh-Ih, that has distinct structural signatures and magic sizes. We find several prominent features associated with entropy: pure FCC nanocrystals are less common than FCC crystals containing stacking faults, and crystals with the minimum potential energy are not always preferred over the range of relevant temperatures. The shapes of the nanocrystals in solution are influenced by the chemical identities of the solution-phase molecules. Comparing Ag nanocrystal shapes in EG to those in an EG+PVP solution, we find more icosahedra in EG and more decahedra in EG+PVP across all of the nanocrystal sizes probed in this study. At certain critical sizes, nanocrystal shapes can change dramatically with the addition and removal of a single atom or with a change in temperature at a fixed size. The information in our study could be useful in efforts to devise processing routes to achieve selective nanocrystal shapes.

Dynamic imaging of interfacial electrochemistry on single Ag nanowires by azimuth-modulated plasmonic scattering interferometry

Direct visualization of surface chemical dynamics in solution is essential for understanding the mechanisms involved in nanocatalysis and electrochemistry; however, it is challenging to achieve high spatial and temporal resolution. Here, we present an azimuth-modulated plasmonic imaging technique capable of imaging dynamic interfacial changes. The method avoids strong interference from reflected light and consequently eliminates the parabolic-like interferometric patterns in the images, allowing for a 67-fold increase in the spatial resolution of plasmonic imaging. We demonstrate that this optical imaging approach enables comprehensive analyses of surface chemical dynamics and identification of previously unknown surface reaction heterogeneity by investigating electrochemical redox reactions over single silver nanowires as an example. This work provides a general strategy for high-resolution plasmonic imaging of surface electrochemical dynamics and other interfacial chemical reactions, complementing existing surface characterization methods.

Theory of Anisotropic Metal Nanostructures

A significant challenge in the development of functional materials is understanding the growth and transformations of anisotropic colloidal metal nanocrystals. Theory and simulations can aid in the development and understanding of anisotropic nanocrystal syntheses. The focus of this review is on how results from first-principles calculations and classical techniques, such as Monte Carlo and molecular dynamics simulations, have been integrated into multiscale theoretical predictions useful in understanding shape-selective nanocrystal syntheses. Also, examples are discussed in which machine learning has been useful in this field. There are many areas at the frontier in condensed matter theory and simulation that are or could be beneficial in this area and these prospects for future progress are discussed.

Asymmetric Interfacet Adatom Migration as a Mode of Anisotropic Nanocrystal Growth

Crystals are known to grow nonclassically or via four classical modes (the layer-by-layer, dislocation-driven, dendritic, and normal modes, which generally involve minimal interfacet surface diffusion). The field of nanoscience considers this framework to interpret how nanocrystals grow; yet, the growth of many anisotropic nanocrystals remains enigmatic, suggesting that the framework may be incomplete. Here, we study the solution-phase growth of pentatwinned Au nanorods without Br, Ag, or surfactants. Lower supersaturation conditions favored anisotropic growth, which appeared at variance with the known modes. Temporal electron microscopy revealed kinetically limited adatom funneling, as adatoms diffused asymmetrically along the vicinal facets (situated inbetween the {100} side-facets and {111} end-facets) of our nanorods. These vicinal facets were perpetuated throughout the synthesis and, especially at lower supersaturation, facilitated {100}-to-vicinal-to-{111} adatom diffusion. We derived a growth model from classical theory in view of our findings, which showed that our experimental growth kinetics were consistent with nanorods growing via two modes simultaneously: radial growth occurred via the layer-by-layer mode on {100} side-facets, whereas the asymmetric interfacet diffusion of adatoms to {111} end-facets mediated longitudinal growth. Thus, shape anisotropy was not driven by modulating the relative rates of monomer deposition on different facets, as conventionally thought, but rather by modulating the relative rates of monomer integration via interfacet diffusion. This work shows how controlling supersaturation, a thermodynamic parameter, can uncover distinct kinetic phenomena on nanocrystals, such as asymmetric interfacet surface diffusion and a fundamental growth mode for which monomer deposition and integration occur on different facets.

An Ultrahigh Rate and Stable Zinc Anode by Facet-Matching-Induced Dendrite Regulation

Resource-abundant metal (e.g., zinc) batteries feature intrinsic advantages of safety and sustainability. Their practical feasibility, however, is impeded by the poor reversibility of metal anodes, typically caused by the uncontrollable dendrite enlargement. Significant effort is exerted to completely prevent dendrites from forming, but this seems less effective at high current densities. Herein, this work presents an alternative dendrite regulation strategy of forming tiny, homogeneously distributed, and identical zinc dendrites by facet matching, which effectively avoids undesirable dendrite enlargement. Confirmed by multiscale theoretical screening and characterization, the regularly exposed Cu(111) facets at the ridges of a copper nanowire are capable of such dendrite regulation by forming a low-mismatched Zn(002)/Cu(111) interface. Consequently, reversible zinc electroplating/stripping is achieved at an unprecedentedly high rate of 100 mA cm^-2 for over 30 000 cycles, corresponding to an accumulative areal capacity up to 30 Ah cm^-2 . A full cell using this anode shows a high capacity of 308.3 mAh g^-1 and a high capacity retention of 91.4% after 800 cycles. This strategy is also viable for magnesium and aluminum anodes, thus opening up a promising and universal avenue toward long-life and high-rate metal anodes.

OptiBoost: A Method for Choosing a Safe and Efficient Boost for the Bond-Boost Method in Accelerated Molecular Dynamics Simulations with Hyperdynamics

Accelerated molecular-dynamics (MD) simulations based on hyperdynamics (HD) can significantly improve the efficiency of MD simulations of condensed-phase systems that evolve via rare events. However, such simulations are not generally easy to apply since appropriate boosts are usually unknown. In this work, we developed a method called OptiBoost to adjust the value of the boost in HD simulations based on the bond-boost method. We demonstrated the OptiBoost method in simulations on a cosine potential and applied it in three different systems involving Ag diffusion on Ag(100) in vacuum and in ethylene-glycol solvent. In all cases, OptiBoost was able to predict safe and effective values of the boost, indicating the OptiBoost protocol is an effective way to advance the applicability of HD simulations.

The influence of iodide on the solution-phase growth of Cu microplates: a multi-scale theoretical analysis from first principles

We use first-principles density functional theory (DFT) to quantify the role of iodide in the solution-phase growth of Cu microplates. Our calculations show that a Cu adatom binds more strongly to hcp hollow sites than fcc hollow sites on iodine-covered Cu(111) - the basal facet of two-dimensional (2D) Cu plates. This feature promotes the formation of stacking faults during seed and plate which, in turn, promotes 2D growth. We also found that iodine adsorption leads to strong Cu atom binding and prohibitively slow diffusion of Cu atoms on Cu(100) - a feature that promotes Cu atom accumulation on the {100} site facets of a growing 2D plate. Incorporating these insights into analog experiments, in which we initiated the growth of Cu plates from small seeds consisting of magnetic spheres, we confirmed that two or more stacking faults are required for lateral plate growth, consistent with prior studies. Moreover, plates can take on a variety of shapes during growth: from triangular and truncated triangular to round and hexagonal - consistent with experiment. Using absorbing Markov chain calculations, we assessed the propensity for 2D vs. 3D kinetic growth of the plates. At experimental temperatures, we predict plates can grow to achieve lateral dimensions in the 1-10 micron range, as observed in experiments.

Molecular Dynamics Study on Mechanical Properties of Nanopolycrystalline Cu-Sn Alloy

Molecular dynamics simulation is one kinds of important methods to research the nanocrystalline materials which is difficult to be studied through experimental characterization. In order to study the effects of Sn content and strain rate on the mechanical properties of nanopolycrystalline Cu-Sn alloy, the tensile simulation of nanopolycrystalline Cu-Sn alloy was carried out by molecular dynamics in the present study. The results demonstrate that the addition of Sn reduces the ductility of Cu-Sn alloy. However, the elastic modulus and tensile strength of Cu-Sn alloy are improved with increasing the Sn content initially, but they will be reduced when the Sn content exceeds 4% and 8%, respectively. Then, strain rate ranges from 1 × 10⁹ s^-1 to 5 × 10⁹ s^-1 were applied to the Cu-7Sn alloy, the results show that the strain rate influence elastic modulus of nanopolycrystalline Cu-7Sn alloy weakly, but the tensile strength and ductility enhance obviously with increasing the strain rate. Finally, the microstructure evolution of nanopolycrystalline Cu-Sn alloy during the whole tensile process was studied. It is found that the dislocation density in the Cu-Sn alloy reduces with increasing the Sn content. However, high strain rate leads to stacking faults more easily to generate and high dislocation density in the Cu-7Sn alloy.

Solution-Phase Growth of Cu Nanowires with Aspect Ratios Greater Than 1000: Multiscale Theory

Penta-twinned metal nanowires are finding widespread application in existing and emerging technologies. However, little is known about their growth mechanisms. We probe the origins of chloride- and alkylamine-mediated, solution-phase growth of penta-twinned Cu nanowires from first-principles using multiscale theory. Using quantum density functional theory (DFT) calculations, we characterize the binding and surface diffusion of Cu atoms on chlorine-covered Cu(100) and Cu(111) surfaces. We find stronger binding and slower diffusion of Cu atoms on chlorinated Cu(111) than on chlorinated Cu(100), which is a reversal of the trend for bare Cu surfaces. We also probe interfacet diffusion and find that this proceeds faster from Cu(100) to Cu(111) than the reverse. Using the DFT rates for hopping between individual sites at Ångstrom scales, we calculate coarse-grained, interfacet rates for nanowires of various lengths─up to hundreds of micrometers─and diameters in the 10 nm range. We predict nanowires with aspect ratios of ∼100, based on surface diffusion alone. We also account for the influence of a self-assembled alkylamine layer that covers most of the {100} facets, but is absent or thin and disordered on the {111} facets and in an "end zone" near the {100}/{111} boundary. With an end zone, we predict a wide range of nanowire aspect ratios in the experimental ranges. Our work reveals the mechanisms by which a halide─chloride─promotes the growth of high-aspect-ratio nanowires.

Self-Assembly of a Linear Alkylamine Bilayer around a Cu Nanocrystal: Molecular Dynamics

Copper nanocrystals are often grown with the help of alkylamine capping agents, which direct the nanocrystal shape. However, the role of these molecules is still unclear. We characterized the assembly of aqueous tetradecylamine (TDA) around a Cu nanocrystal and found that TDA exhibits a temperature-dependent bilayer structure. The bilayer involves an inner layer, in which TDA binds to Cu via the amine group and tends to orient the alkyl tail perpendicular to the surface, and an outer layer whose structure depends on temperature. At low temperatures, alkylamines in the inner layer form bundles with no apparent relation to the crystal facets. Alkylamines in the outer layer tend to orient their long axes perpendicular to the Cu surfaces, with interdigitation into the inner layer. At high temperatures, alkylamines in the inner layer lose their bundle structure, and outer-layer alkylamines tend to orient themselves tangential to the Cu surfaces, forming a "web" above inner-layer TDA. TDA exhibits a rapid interlayer exchange at typical synthesis temperatures, consistent with experiment. The variety in the assemblies seen here and in other studies of alkanethiols around gold nanocrystals indicates a richness in the assemblies that can be achieved by modulating the interaction between the strongly binding end group and the surface.

Understanding the Solution-Phase Growth of Cu and Ag Nanowires and Nanocubes from First Principles

In this feature article, we provide an account of the Langmuir Lecture delivered by Kristen Fichthorn at the Fall 2020 Virtual Meeting of the American Chemical Society. We discuss how multiscale theory and simulations based on first-principles DFT were useful in uncovering the intertwined influences of kinetics and thermodynamics on the shapes of Ag and Cu cubes and nanowires grown in solution. We discuss how Ag nanocubes can form through PVP-modified deposition kinetics and how the addition of chloride to the synthesis can promote thermodynamic cubic shapes for both Ag and Cu. We discuss kinetic factors contributing to nanowire growth: in the case of Ag, we show that high-aspect-ratio nanowires can form as a consequence of Ag atom surface diffusion on the strained surfaces of Marks-like decahedral seeds. On the other hand, solution-phase chloride enhances Cu nanowire growth due to a synergistic interaction between adsorbed chloride and hexadecylamine (HDA), which leaves the {111} nanowire ends virtually bare while the {100} sides are fully covered with HDA. For each of these topics, a synergy between theory and experiment led to significant progress.

Controlling plasmon propagation and enhancement via reducing agent in wet chemistry synthesized silver nanowires

Silver nanowires with varying diameters and submillimeter lengths were obtained by changing a reducing agent used during hydrothermal synthesis. The control over the nanowire diameter turns out to play a critical role in determining their plasmonic properties, including fluorescence enhancement and surface plasmon polariton propagation. Advanced fluorescence imaging of hybrid nanostructures assembled of silver nanowires and photoactive proteins indicates longer propagation lengths for nanowires featuring larger diameters. At the same time, with increasing diameter of the nanowires, we measure a substantial reduction of fluorescence enhancement. The results point at possible ways to control the influence of plasmon excitations in silver nanowires by tuning their morphology.

Effects of Strain and Kinetics on the H2O2-Assisted Reconstruction of Ag-Au-Ag Nanorods

Morphology of Ag nanocrystals (NCs) is essential to the NC application in catalysis, optics, and as antibacterial agents. Therefore, it is important to develop synthetic methods and understand the evaluation of NC morphology in different chemical environments. In this study, we report interesting findings of the morphological change of fivefold-twinned Ag-Au-Ag nanorods (NRs) under the effect of H2O2 both as an oxidant (etchant) and a reductant. At low H2O2 concentration, the reconstruction of Ag-Au-Ag NRs was dominated by the growth along the longitudinal direction of NRs. With the increase of H2O2 concentration, the reconstruction also occurs in the transverse direction, and a clear change in particle morphology was observed. We further systematically studied the mechanism of the reaction. The results showed that the transition of the morphology was a two-step process: (1) the etching of Ag on the seeds and (2) the reduction of Ag2O. In the second step, the reaction kinetics was highly affected by H2O2 concentration. At low H2O2 concentration, the growth mainly occurs along ⟨110⟩. However, at high H2O2 concentration, the reduction of Ag was not facet-selective. Using the developed method, we can prepare various bimetallic NCs (high aspect ratio NRs with abundant pinholes, nanoplates, and other NCs). The effect of the reconstruction process on the surface-enhanced Raman scattering (SERS) performance of NCs was investigated.

Atomically resolved Au52Cu72(SR)55 nanoalloy reveals Marks decahedron truncation and Penrose tiling surface

Gold-copper alloys have rich forms. Here we report an atomically resolved [Au₅₂Cu₇₂(p-MBT)₅₅]⁺Cl^- nanoalloy (p-MBT = SPh-p-CH₃). This nanoalloy exhibits unusual structural patterns. First, two Cu atoms are located in the inner 7-atom decahedral kernel (M₇, M = Au/Cu). The M₇ kernel is then enclosed by a second shell of homogold (Au₄₇), giving rise to a two-shelled M₅₄ (i.e. Au₅₂Cu₂) full decahedron. A comparison of the non-truncated M₅₄ decahedron with the truncated homogold Au₄₉ kernel in similar-sized gold nanoparticles provides for the first time an explanation for Marks decahedron truncation. Second, a Cu₇₀(SR)₅₅ exterior cage resembling a 3D Penrose tiling protects the M₅₄ decahedral kernel. Compared to the discrete staple motifs in gold:thiolate nanoparticles, the Cu-thiolate surface of Au₅₂Cu₇₂ forms an extended cage. The Cu-SR Penrose tiling retains the M₅₄ kernel's high symmetry (D_5h). Third, interparticle interactions in the assembly are closely related to the symmetry of the particle, and a "quadruple-gear-like" interlocking pattern is observed.

Nanowire Genome: A Magic Toolbox for 1D Nanostructures

1D nanomaterials with high aspect ratio, i.e., nanowires and nanotubes, have inspired considerable research interest thanks to the fact that exotic physical and chemical properties emerge as their diameters approach or fall into certain length scales, such as the wavelength of light, the mean free path of phonons, the exciton Bohr radius, the critical size of magnetic domains, and the exciton diffusion length. On the basis of their components, aspect ratio, and properties, there may be imperceptible connections among hundreds of nanowires prepared by different strategies. Inspired by the heredity system in life, a new concept termed the "nanowire genome" is introduced here to clarify the relationships between hundreds of nanowires reported previously. As such, this approach will not only improve the tools incorporating the prior nanowires but also help to precisely synthesize new nanowires and even assist in the prediction on the properties of nanowires. Although the road from start-ups to maturity is long and fraught with challenges, the genetical syntheses of more than 200 kinds of nanostructures stemming from three mother nanowires (Te, Ag, and Cu) are summarized here to demonstrate the nanowire genome as a versatile toolbox. A summary and outlook on future challenges in this field are also presented.