Understanding Symmetry Breaking at the Single-Particle Level via the Growth of Tetrahedron-Shaped Nanocrystals from Higher-Symmetry Precursors

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.

Mesomorphology of clathrate hydrates from molecular ordering

Clathrate hydrates are crystals formed by guest molecules that stabilize cages of hydrogen-bonded water molecules. Whereas thermodynamic equilibrium is well described via the van der Waals and Platteeuw approach, the increasing concerns with global warming and energy transition require extending the knowledge to non-equilibrium conditions in multiphase, sheared systems, in a multiscale framework. Potential macro-applications concern the storage of carbon dioxide in the form of clathrates, and the reduction of hydrate inhibition additives currently required in hydrocarbon production. We evidence porous mesomorphologies as key to bridging the molecular scales to macro-applications of low solubility guests. We discuss the coupling of molecular ordering with the mesoscales, including (i) the emergence of porous patterns as a combined factor from the walk over the free energy landscape and 3D competitive nucleation and growth and (ii) the role of molecular attachment rates in crystallization-diffusion models that allow predicting the timescale of pore sealing. This is a perspective study that discusses the use of discrete models (molecular dynamics) to build continuum models (phase field models, crystallization laws, and transport phenomena) to predict multiscale manifestations at a feasible computational cost. Several advances in correlated fields (ice, polymers, alloys, and nanoparticles) are discussed in the scenario of clathrate hydrates, as well as the challenges and necessary developments to push the field forward.

Experimental High-Resolution Observation of the Truncated Double-Icosahedron Structure: A Stable Twinned Shell in Alloyed Au-Ag Core@Shell Nanoparticles

Given the binary nature of nanoalloy systems, their properties are dependent on their size, shape, structure, composition, and chemical ordering. When energy and entropic factors for shapes and structure variations are considered in nanoparticle growth, the spectra of shapes become so vast that even metastable arrangements have been reported under ambient conditions. Experimental and theoretical variations of multiply twinned particles have been observed, from the Ino and Marks decahedra to polyicosahedra and polydecahedra with comparable energetic stability among them. Herein, we report the experimental production of a stable doubly truncated double-icosahedron structure (TdIh) in Au-Ag nanoparticles, in which a twinned Ag-rich alloyed shell is reconstructed on a Au-Ag alloyed Ino-decahedral core. The structure, chemical composition, and growth pathway are proposed on the basis of high-angle annular dark-field scanning transmission electron microscopy analysis and excess energy calculations, while its structural stability is estimated by large-scale atomic molecular dynamics simulations. This novel nanostructure differs from other structures previously reported.

Unraveling chemical processes during nanoparticle synthesis with liquid phase electron microscopy and correlative techniques

Liquid phase transmission electron microscopy (LPTEM) has enabled unprecedented direct real time imaging of physicochemical processes during solution phase synthesis of metallic nanoparticles. LPTEM primarily provides images of nanometer scale, and sometimes atomic scale, metal nanoparticle crystallization processes, but provides little chemical information about organic surface ligands, metal-ligand complexes and reaction intermediates, and redox reactions. Likewise, complex electron beam-solvent interactions during LPTEM make it challenging to pinpoint the chemical processes, some involving exotic highly reactive radicals, impacting nanoparticle formation. Pairing LPTEM with correlative solution synthesis, ex situ chemical analysis, and theoretical modeling represents a powerful approach to gain a holistic understanding of the chemical processes involved in nanoparticle synthesis. In this feature article, we review recent work by our lab and others that has focused on elucidating chemical processes during nanoparticle synthesis using LPTEM and correlative chemical characterization and modeling, including mass and optical spectrometry, fluorescence microscopy, solution chemistry, and reaction kinetic modeling. In particular, we show how these approaches enable investigating redox chemistry during LPTEM, polymeric and organic capping ligands, metal deposition mechanisms on plasmonic nanoparticles, metal clusters and complexes, and multimetallic nanoparticle formation. Future avenues of research are discussed, including moving beyond electron beam induced nanoparticle formation by using light and thermal stimuli during LPTEM. We discuss prospects for real time LPTEM imaging and online chemical analysis of reaction intermediates using microfluidic flow reactors.

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.

Room temperature synthesis of CdSe/CdS triangular nanoemitters and their stabilization in colloidal state and sol-gel glass

Heterostructured cadmium-based core-shell nanoparticles (NPs) are the subject of research because of not only fundamental scientific advances but also a range of technological applications. To increase the range of applications of nanoparticles, it is possible to immobilise them in sol-gel glass that can be easily manufactured and shaped, keeping the properties of the dispersed particles. This allows the creation of new bulk optical materials with tailored properties, opening up opportunities for various technological applications such as lighting or sensing. Herein we report the synthesis of core-shell CdSe/CdS triangular-shaped nanoparticles under an atmosphere of oxygen and at room temperature. A detailed characterisation of the obtained NPs was carried out. The interesting effect of the gelling agent (tetra-n-butylammonium fluoride) on the triangular nanoparticles in solution and the stability of the emission properties over time was investigated. Sol-gel glasses with entrapped triangular NPs were prepared, and their photoluminescence properties were compared with those obtained in colloidal solutions.

Inspired Beyond Nature: Three Decades of Spherical Nucleic Acids and Colloidal Crystal Engineering with DNA

The conception, synthesis, and invention of a nanostructure, now known as the spherical nucleic acid, or SNA, in 1996 marked the advent of a new field of chemistry. Over the past three decades, the SNA and its analogous anisotropic equivalents have provided an avenue for us to think about some of the most fundamental concepts in chemistry in new ways and led to technologies that are significantly impacting fields from medicine to materials science. A prime example is colloidal crystal engineering with DNA, the framework for using SNAs and related structures to synthesize programmable matter. Herein, we document the evolution of this framework, which was initially inspired by nature, and describe how it now allows researchers to chart paths to move beyond it, as programmable matter with real-world significance is envisioned and created.

Controlled Self-Assembly of Gold Nanotetrahedra into Quasicrystals and Complex Periodic Supracrystals

The self-assembly of shape-anisotropic nanocrystals into large-scale structures is a versatile and scalable approach to creating multifunctional materials. The tetrahedral geometry is ubiquitous in natural and manmade materials, yet regular tetrahedra present a formidable challenge in understanding their self-assembly behavior as they do not tile space. Here, we report diverse supracrystals from gold nanotetrahedra including the quasicrystal (QC) and the dimer packing predicted more than a decade ago and hitherto unknown phases. We solve the complex three-dimensional (3D) structure of the QC by a combination of electron microscopy, tomography, and synchrotron X-ray scattering. Nanotetrahedron vertex sharpness, surface ligands, and assembly conditions work in concert to regulate supracrystal structure. We also discover that the surface curvature of supracrystals can induce structural changes of the QC tiling and eventually, for small supracrystals with high curvature, stabilize a hexagonal approximant. Our findings bridge the gap between computational design and experimental realization of soft matter assemblies and demonstrate the importance of accurate control over nanocrystal attributes and the assembly conditions to realize increasingly complex nanopolyhedron supracrystals.

Plate-Like Colloidal Metal Nanoparticles

The pseudo-two-dimensional (2D) morphology of plate-like metal nanoparticles makes them one of the most anisotropic, mechanistically understood, and tunable structures available. Although well-known for their superior plasmonic properties, recent progress in the 2D growth of various other materials has led to an increasingly diverse family of plate-like metal nanoparticles, giving rise to numerous appealing properties and applications. In this review, we summarize recent progress on the solution-phase growth of colloidal plate-like metal nanoparticles, including plasmonic and other metals, with an emphasis on mechanistic insights for different synthetic strategies, the crystallographic habits of different metals, and the use of nanoplates as scaffolds for the synthesis of other derivative structures. We additionally highlight representative self-assembly techniques and provide a brief overview on the attractive properties and unique versatility benefiting from the 2D morphology. Finally, we share our opinions on the existing challenges and future perspectives for plate-like metal nanomaterials.

Biomimetic mineralization based on self-assembling peptides

Biomimetic science has attracted great interest in the fields of chemistry, biology, materials science, and energy. Biomimetic mineralization is the process of synthesizing inorganic minerals under the control of organic molecules or biomolecules under mild conditions. Peptides are the motifs that constitute proteins, and can self-assemble into various hierarchical structures and show a high affinity for inorganic substances. Therefore, peptides can be used as building blocks for the synthesis of functional biomimetic materials. With the participation of peptides, the morphology, size, and composition of mineralized materials can be controlled precisely. Peptides not only provide well-defined templates for the nucleation and growth of inorganic nanomaterials but also have the potential to confer inorganic nanomaterials with high catalytic efficiency, selectivity, and biotherapeutic functions. In this review, we systematically summarize research progress in the formation mechanism, nanostructural manipulation, and applications of peptide-templated mineralized materials. These can further inspire researchers to design structurally complex and functionalized biomimetic materials with great promising applications.

Direct Imaging of the Kinetic Crystallization Pathway: Simulation and Liquid-Phase Transmission Electron Microscopy Observations

The crystallization of materials from a suspension determines the structure and function of the final product, and numerous pieces of evidence have pointed out that the classical crystallization pathway may not capture the whole picture of the crystallization pathways. However, visualizing the initial nucleation and further growth of a crystal at the nanoscale has been challenging due to the difficulties of imaging individual atoms or nanoparticles during the crystallization process in solution. Recent progress in nanoscale microscopy had tackled this problem by monitoring the dynamic structural evolution of crystallization in a liquid environment. In this review, we summarized several crystallization pathways captured by the liquid-phase transmission electron microscopy technique and compared the observations with computer simulation. Apart from the classical nucleation pathway, we highlight three nonclassical pathways that are both observed in experiments and computer simulations: formation of an amorphous cluster below the critical nucleus size, nucleation of the crystalline phase from an amorphous intermediate, and transition between multiple crystalline structures before achieving the final product. Among these pathways, we also highlight the similarities and differences between the experimental results of the crystallization of single nanocrystals from atoms and the assembly of a colloidal superlattice from a large number of colloidal nanoparticles. By comparing the experimental results with computer simulations, we point out the importance of theory and simulation in developing a mechanistic approach to facilitate the understanding of the crystallization pathway in experimental systems. We also discuss the challenges and future perspectives for investigating the crystallization pathways at the nanoscale with the development of in situ nanoscale imaging techniques and potential applications to the understanding of biomineralization and protein self-assembly.

How a Facet of a Nanocrystal Is Formed: The Concept of the Symmetry Based Kinematic Theory

Conventional nanocrystal (NC) growth mechanisms have overwhelmingly focused on the final exposed facets to explain shape evolution. However, how the final facets are formed from the initial nuclei or seeds, has not been specifically interrogated. In this concept paper, we would like to concentrate on this specific topic, and introduce the symmetry based kinematic theory (SBKT) to explain the formation and evolution of crystal facets. It is a crystallographic theory based on the classical crystal growth concepts developed to illustrate the shape evolution during the NC growth. The most important principles connecting the basic NC growth processes and morphology evolution are the preferential growth directions and the properties of kinematic waves. On the contrary, the final facets are just indications of how the crystal growth terminates, and their formation and evolution rely on the NC growth processes: surface nucleation and layer advancement. Accordingly, the SBKT could even be applied to situations where non-faceted NCs such as spheres are formed.

Colloidal Synthesis of Metal Nanocrystals: From Asymmetrical Growth to Symmetry Breaking

Nanocrystals offer a unique platform for tailoring the physicochemical properties of solid materials to enhance their performances in various applications. While most work on controlling their shapes revolves around symmetrical growth, the introduction of asymmetrical growth and thus symmetry breaking has also emerged as a powerful route to enrich metal nanocrystals with new shapes and complex morphologies as well as unprecedented properties and functionalities. The success of this route critically relies on our ability to lift the confinement on symmetry by the underlying unit cell of the crystal structure and/or the initial seed in a systematic manner. This Review aims to provide an account of recent progress in understanding and controlling asymmetrical growth and symmetry breaking in a colloidal synthesis of noble-metal nanocrystals. With a touch on both the nucleation and growth steps, we discuss a number of methods capable of generating seeds with diverse symmetry while achieving asymmetrical growth for mono-, bi-, and multimetallic systems. We then showcase a variety of symmetry-broken nanocrystals that have been reported, together with insights into their growth mechanisms. We also highlight their properties and applications and conclude with perspectives on future directions in developing this class of nanomaterials. It is hoped that the concepts and existing challenges outlined in this Review will drive further research into understanding and controlling the symmetry breaking process.

Assembling Biocompatible Polymers on Gold Nanoparticles: Toward a Rational Design of Particle Shape by Molecular Dynamics

Gold nanoparticles (AuNPs) have received great attention in a number of fields ranging from the energy sector to biomedical applications. As far as the latter is concerned, due to rapid renal clearance and a short lifetime in blood, AuNPs are often encapsulated in a poly(lactic-co-glycolic acid) (PLGA) matrix owing to its biocompatibility and biodegradability. A better understanding of the PLGA polymers on the AuNP surface is crucial to improve and optimize the above encapsulation process. In this study, we combine a number of computational approaches to explore the adsorption mechanisms of PLGA oligomers on a Au crystalline NP and to rationalize the PLGA coating process toward a more efficient design of the NP shape. Atomistic simulations supported by a recently developed unsupervised machine learning scheme show the temporal evolution and behavior of PLGA clusterization by tuning the oligomer concentration in aqueous solutions. Then, a detailed surface coverage analysis coupled with free energy landscape calculations sheds light on the anisotropic nature of PLGA adsorption onto the AuNP. Our results prove that the NP shape and topology may address and privilege specific sites of adsorption, such as the Au {1 1 1} crystal planes in selected NP samples. The modeling-based investigation suggested in this article offers a solid platform to guide the design of coated NPs.

Atomic-Scale Structure Dynamics of Nanocrystals Revealed By In Situ and Environmental Transmission Electron Microscopy

Nanocrystals are of great importance in material sciences and industry. Engineering nanocrystals with desired structures and properties is no doubt one of the most important challenges in the field, which requires deep insight into atomic-scale dynamics of nanocrystals during the process. The rapid developments of in situ transmission electron microscopy (TEM), especially environmental TEM, reveal insights into nanocrystals to digest. According to the considerable progress based on in situ electron microscopy, a comprehensive review on nanocrystal dynamics from three aspects: nucleation and growth, structure evolution, and dynamics in reaction conditions are given. In the nucleation and growth part, existing nucleation theories and growth pathways are organized based on liquid and gas-solid phases. In the structure evolution part, the focus is on in-depth mechanistic understanding of the evolution, including defects, phase, and disorder/order transitions. In the part of dynamics in reaction conditions, solid-solid and gas-solid interfaces of nanocrystals in atmosphere are discussed and the structure-property relationship is correlated. Even though impressive progress is made, additional efforts are required to develop the integrated and operando TEM methodologies for unveiling nanocrystal dynamics with high spatial, energy, and temporal resolutions.

Assembly of planar chiral superlattices from achiral building blocks

The spontaneous assembly of chiral structures from building blocks that lack chirality is fundamentally important for colloidal chemistry and has implications for the formation of advanced optical materials. Here, we find that purified achiral gold tetrahedron-shaped nanoparticles assemble into two-dimensional superlattices that exhibit planar chirality under a balance of repulsive electrostatic and attractive van der Waals and depletion forces. A model accounting for these interactions shows that the growth of planar structures is kinetically preferred over similar three-dimensional products, explaining their selective formation. Exploration and mapping of different packing symmetries demonstrates that the hexagonal chiral phase forms exclusively because of geometric constraints imposed by the presence of constituent tetrahedra with sharp tips. A formation mechanism is proposed in which the chiral phase nucleates from within a related 2D achiral phase by clockwise or counterclockwise rotation of tetrahedra about their central axis. These results lay the scientific foundation for the high-throughput assembly of planar chiral metamaterials.

The formation of nanostructures with chiral symmetry often requires chiral directing agents at a smaller length scale. Here, the authors report the self-assembly of 2D chiral superlattices from achiral tetrahedron-shaped building blocks.

Interparticle gap geometry effects on chiroptical properties of plasmonic nanoparticle assemblies

Chiral linear assemblies of plasmonic nanoparticles with chiral optical activity often show low asymmetry factors. Systematic understanding of the structure-property relationship in these systems must be improved to facilitate rational design of their chiroptical response. Here we study the effect of large area interparticle gaps in chiral linear nanoparticle assemblies on their chiroptical properties using a tetrahelix structure formed by a linear face-to-face assembly of nanoscale Au tetrahedra. Using finite-difference time-domain and finite element methods, we performed in-depth evaluation of the extinction spectra and electric field distribution in the tetrahelix structure and its dependence on various geometric parameters. The reported structure supports various plasmonic modes, one of which shows a strong incident light handedness selectivity that is associated with large face-to-face junctions. This works highlights the importance of gap engineering in chiral plasmonic assemblies to achieveg-factors greater than 1 and produce structures with a handedness-selective optical response.