Interplay between Short‐ and Long‐Ranged Forces Leading to the Formation of Ag Nanoparticle Superlattice

Nanoparticle (NP) superlattices have attracted increasing attention due to their unique physicochemical properties. However, key questions persist regarding the correlation between short‐ and long‐range driving forces for nanoparticle assembly and resultant capability to predict the transient and final superlattice structure. Here the self‐assembly of Ag NPs in aqueous solutions is investigated by employing in situ liquid cell transmission electron microscopy, combined with atomic force microscopy‐based force measurements, and theoretical calculations. Despite the NPs exhibiting instantaneous Brownian motion, it is found that the dynamic behavior of NPs is correlated with the van der Waals force, sometimes unexpectedly over relatively large particle separations. After the NPs assemble into clusters, a delicate balance between the hydration and van der Waals forces results in a distinct distribution of particle separation, which is ascribed to layers of hydrated ions adsorbed on the NP surface. The study demonstrates pivotal roles of the complicated correlation between interparticle forces; potentially enabling the control of particle separation, which is critical for tailoring the properties of NP superlattices.     

Interparticle Forces Underlying Nanoparticle Self‐Assemblies

Studies on the self‐assembly of nanoparticles have been a hot topic in nanotechnology for decades and still remain relevant for the present and future due to their tunable collective properties as well as their remarkable applications to a wide range of fields. The novel properties of nanoparticle assemblies arise from their internal interactions and assemblies with the desired architecture key to constructing novel nanodevices. Therefore, a comprehensive understanding of the interparticle forces of nanoparticle self‐assemblies is a pre‐requisite to the design and control of the assembly processes, so as to fabricate the ideal nanomaterial and nanoproducts. Here, different categories of interparticle forces are classified and discussed according to their origins, behaviors and functions during the assembly processes, and the induced collective properties of the corresponding nanoparticle assemblies. Common interparticle forces, such as van der Waals forces, electrostatic interactions, electromagnetic dipole‐dipole interactions, hydrogen bonds, solvophonic interactions, and depletion interactions are discussed in detail. In addition, new categories of assembly principles are summarized and introduced. These are termed template‐mediated interactions and shape‐complementary interactions. A deep understanding of the interactions inside self‐assembled nanoparticles, and a broader perspective for the future synthesis and fabrication of these promising nanomaterials is provided.     

Dynamics of Templated Assembly of Nanoparticle Filaments within Nanochannels

Nanoparticles (NPs) can self‐assemble into complex, organized superstructures on patterned surfaces through fluid‐mediated interactions. However, the detailed mechanisms for such NP assemblies are largely unknown. Here, using in situ transmission electron microscopy, the stepwise self‐assembly dynamics of hydrophobic gold NPs into long filaments formed on the surfaces of water‐filled patterned nanochannel templates is observed. First, the formation of a meniscus between the nanochannel walls, during the slow drying of water, causes accumulation of the NPs in the middle of the nanochannels. Second, owing to the strong van der Waals attraction between the NP ligands, the NPs condense into filaments along the centers of the nanochannels. Filaments with highly fluctuating longitudinal NP densities are also observed to fragment into separated structures. Understanding the intermediate stages of fluid‐mediated NP self‐assembly on patterned surfaces will have important implications for the controlled formation of templated NP assemblies with numerous applications.     

Self‐Assembly of CoPt Magnetic Nanoparticle Arrays and its Underlying Forces

Nanoparticles covered with surfactants are often used to study particle motion patterns and self‐assembly processes in solutions. Surfactants have influence on the interparticle interactions and therefore on the particle motion tracks and final patterns. In this study, CoPt nanoparticles are synthesized in aqueous solution without any surfactant. In situ transmission electron microscopy observation is performed to monitor the self‐assemble process. Two types of magnetic nanoparticle superlattice arrays are formed: hexagonal equal distance superlattice arrays when particle size is 3 nm, and tight unequal distance superlattice arrays when particle size is 4.5 nm. It is interesting to observe that two small arrays merge into a large one through rotational and translational movements. A Monte Carlo simulation is carried out which successfully restores the whole process. It is identified that the underlying forces are van der Waals and magnetic dipolar interactions. The latter is responsible for orientation of each particle during the whole process. This investigation leads to a better understanding of the formation mechanism of magnetic nanoparticle superlattice arrays.     

Nanoparticle Linker‐Controlled Molecular Wire Devices Based on Double Molecular Monolayers

Highly conductive molecular wires are an important component for realizing molecular electronic devices and have to be explored in terms of interactions between molecules and electrodes in their molecular junctions. Here, new molecular wire junctions are reported to enhance charge transport through gold nanoparticle (AuNP)‐linked double self‐assembled monolayers (SAMs) of cobalt (II) bis‐terpyridine molecules (e.g., Co(II)(tpyphS)₂). Electrical characteristics of the double‐SAM devices are explored in terms of the existence of AuNP. The AuNP linker in the Co(II)(tpyphS)₂–AuNP–Co(II)(tpyphS)₂ junction acts as an electronic contact that is transparent to electrons. The weak temperature dependency of the AuNP‐linked molecular junctions strongly indicates sequential tunneling conduction through the highest occupied molecular orbitals (HOMOs) of Co(II)(tpyphS)₂ molecules. The electrochemical characteristics of the AuNP–Co(II)(tpyphS)₂ SAMs reveal fast electron transfer through molecules linked by AuNP. Density functional theory calculations reveal that the molecular HOMO levels are dominantly affected by the formation of junctions. The intermolecular charge transport, controlled by the AuNP linker, can provide a rational design for molecular connection that achieves a reliable electrical connectivity of molecular electronic components for construction of molecular electronic circuits.     

Pt超晶格组装和对称性转化的原位液体TEM研究（英文）

具有精确控制结构的二维(2D)纳米晶超晶格在光子、等离子体和光电子应用中具有重要意义,并已被广泛研究,但在理解超晶格的形成机理和发展途径方面仍然存在挑战.本文采用液体池透射电镜技术原位观察了铂二维超晶格的形成和六配位到四配位的局部相转化过程.胶体纳米晶在溶液中流动时,通过纳米晶的收缩和重排或者纳米晶的附着形成长程有序的六配位超晶格.当纳米晶的形貌由截角八面体转变为立方体时,六配位超晶格重新排列为四配位立方超晶格.此外,我们的观察和定量分析结果表明,从六配位到四配位的相变主要是由垂直{100}面之间的强范德华相互作用引起的.实时追踪2D立方体超晶格的形成可以为超晶格组装和稳定机制提供独特的见解.     

Shape‐Dependent Interactions of Palladium Nanocrystals with Hydrogen

Elucidation of the nature of hydrogen interactions with palladium nanoparticles is expected to play an important role in the development of new catalysts and hydrogen‐storage nanomaterials. A facile scaled‐up synthesis of uniformly sized single‐crystalline palladium nanoparticles with various shapes, including regular nanocubes, nanocubes with protruded edges, rhombic dodecahedra, and branched nanoparticles, all stabilized with a mesoporous silica shell is developed. Interaction of hydrogen with these nanoparticles is studied by using temperature‐programmed desorption technique and by performing density functional theory modeling. It is found that due to favorable arrangement of Pd atoms on their surface, rhombic dodecahedral palladium nanoparticles enclosed by {110} planes release a larger volume of hydrogen and have a lower desorption energy than palladium nanocubes and branched nanoparticles. These results underline the important role of {110} surfaces in palladium nanoparticles in their interaction with hydrogen. This work provides insight into the mechanism of catalysis of hydrogenation/dehydrogenation reactions by palladium nanoparticles with different shapes.     

Thickness‐Dependent In‐Plane Polarization and Structural Phase Transition in van der Waals Ferroelectric CuInP2S6

van der Waals (vdW) layered materials have rather weaker interlayer bonding than the intralayer bonding, therefore the exfoliation along the stacking direction enables the achievement of monolayer or few layers vdW materials with emerging novel physical properties and functionalities. The ferroelectricity in vdW materials recently attracts renewed interest for the potential use in high‐density storage devices. With the thickness becoming thinner, the competition between the surface energy, depolarization field, and interfacial chemical bonds may give rise to the modification of ferroelectricity and crystalline structure, which has limited investigations. In this work, combining the piezoresponse force microscope scanning, contact resonance imaging, the existence of the intrinsic in‐plane polarization in vdW ferroelectrics CuInP₂S₆ single crystals is reported, whereas below a critical thickness between 90 and 100 nm, the in‐plane polarization disappears. The Young's modulus also shows an abrupt stiffness at the critical thickness. Based on the density functional theory calculations, these behaviors are ascribed to a structural phase transition from monoclinic to trigonal structure, which is further verified by transmission electron microscope technique. These findings demonstrate the foundational importance of structural phase transition for enhancing the rich functionality and broad utility of vdW ferroelectrics.     

Healing of Planar Defects in 2D Materials via Grain Boundary Sliding

Understanding the mechanisms and kinetics of defect annihilations, particularly at the atomic scale, is important for the preparation of high‐quality crystals for realizing the full potential of 2D transition metal dichalcogenides (TMDCs) in electronics and quantum photonics. Herein, by performing in situ annealing experiments in an atomic resolution scanning transmission electron microscope, it is found that stacking faults and rotational disorders in multilayered 2D crystals can be healed by grain boundary (GB) sliding, which works like a “wiper blade” to correct all metastable phases into thermodynamically stable phases along its trace. The driving force for GB sliding is the gain in interlayer binding energy as the more stable phase grows at the expanse of the metastable ones. Density functional theory calculations show that the correction of 2D stacking faults is triggered by the ejection of Mo atoms in mirror twin boundaries, followed by the collective migrations of 1D GB. The study highlights the role of the often‐neglected interlayer interactions for defect repair in 2D materials and shows that exploiting these interactions has significant potential for obtaining large‐scale defect‐free 2D films.     

A Shape‐Induced Orientation Phase within 3D Nanocrystal Solids

When nanocrystals self assemble into ordered superstructures they form functional solids that may inherit the electronical properties of the single nanocrystals. To what extent these properties are enhanced depends on the positional and orientational order of the nanocrystals within the superstructure. Here, the formation of micrometer‐sized free‐standing supercrystals of faceted 20 nm Bi nanocrystals is investigated. The self‐assembly process, induced by nonsolvent into solvent diffusion, is probed in situ by synchrotron X‐ray scattering. The diffusion‐gradient is identified as the critical parameter for controlling the supercrystal‐structure as well as the alignment of the supercrystals with respect to the substrate. Monte Carlo simulations confirm the positional order of the nanocrystals within these superstructures and reveal a unique orientation phase: the nanocrystal shape, determined by the atomic Bi crystal structure, induces a total of 6 global orientations based on facet‐to‐facet alignment. This parallel alignment of facets is a prerequisite for optimized electronic and optical properties within designed nanocrystal solids.     

Dynamics of Antimonene–Graphene Van Der Waals Growth

Van der Waals (vdW) heterostructures have recently been introduced as versatile building blocks for a variety of novel nanoscale and quantum technologies. Harnessing the unique properties of these heterostructures requires a deep understanding of the involved interfacial interactions and a meticulous control of the growth of 2D materials on weakly interacting surfaces. Although several epitaxial vdW heterostructures have been achieved experimentally, the mechanisms governing their synthesis are still nebulous. With this perspective, herein, the growth dynamics of antimonene on graphene are investigated in real time. In situ low‐energy electron microscopy reveals that nucleation predominantly occurs on 3D nuclei followed by a self‐limiting lateral growth with morphology sensitive to the deposition rate. Large 2D layers are observed at high deposition rates, whereas lower growth rates trigger an increased multilayer nucleation at the edges as they become aligned with the Z2 orientation leading to atoll‐like islands with thicker, well‐defined bands. This complexity of the vdW growth is elucidated based on the interplay between the growth rate, surface diffusion, and edges orientation. This understanding lays the groundwork for a better control of the growth of vdW heterostructures, which is critical to their large‐scale integration.     

Hydrophobic,Electrostatic, and Dynamic Polymer Forces at Silicone Surfaces Modified with Long‐Chain Bolaform Surfactants

Surfactant self‐assembly on surfaces is an effective way to tailor the complex forces at and between hydrophobic‐water interfaces. Here, the range of structures and forces that are possible at surfactant‐adsorbed hydrophobic surfaces are demonstrated: certain long‐chain bolaform surfactants—containing a polydimethylsiloxane (PDMS) mid‐block domain and two cationic α, ω‐quarternary ammonium end‐groups—readily adsorb onto thin PDMS films and form dynamically fluctuating nanostructures. Through measurements with the surface forces apparatus (SFA), it is found that these soft protruding nanostructures display polymer‐like exploration behavior at the PDMS surface and give rise to a long‐ranged, temperature‐ and rate‐dependent attractive bridging force (not due to viscous forces) on approach to a hydrophilic bare mica surface. Coulombic interactions between the cationic surfactant end‐groups and negatively‐charged mica result in a rate‐dependent polymer bridging force during separation as the hydrophobic surfactant mid‐blocks are pulled out from the PDMS interface, yielding strong adhesion energies. Thus, (i) the versatile array of surfactant structures that may form at hydrophobic surfaces is highlighted, (ii) the need to consider the interaction dynamics of such self‐assembled polymer layers is emphasized, and (iii) it is shown that long‐chain surfactants can promote robust adhesion in aqueous solutions.     

Real‐Time Tracking of Superparamagnetic Nanoparticle Self‐Assembly

The spontaneous self‐assembly process of superparamagnetic nanoparticles in a fast‐drying colloidal drop is observed in real time. The grazing‐incidence small‐angle X‐ray scattering (GISAXS) technique is employed for an in situ tracking of the reciprocal space, with a 3 ms delay time between subsequent frames delivered by a new generation of X‐ray cameras. A focused synchrotron beam and sophisticated sample oscillations make it possible to relate the dynamic reciprocal to direct space features and to localize the self‐assembly. In particular, no nanoparticle ordering is found inside the evaporating drop and near‐surface region down to a drop thickness of 90 µm. Scanning through the shrinking drop‐contact line indicates the start of self‐assembly near the drop three‐phase interface, in accord with theoretical predictions. The results obtained have direct implications for establishing the self‐assembly process as a routine technological step in the preparation of new nanostructures.     

Misorientation‐Angle‐Dependent Phase Transformation in van der Waals Multilayers via Electron‐Beam Irradiation

Misorientation‐angle dependence on layer thickness is an intriguing feature of van der Waals materials, which causes stark optical gain and electrical transport modulation. However, the influence of misorientation angle on phase transformation is not determined yet. Herein, this phenomenon in a MoS₂ multilayer via in situ electron‐beam irradiation is reported. An AA′‐stacked MoS₂ bilayer undergoes structural transformation from the 2H semiconducting phase to the 1T′ metallic phase, similar to a MoS₂ monolayer, which is confirmed via in situ transmission electron microscopy. Moreover, non‐AA′ stacking, which has no local AA′ stacking order in the Moiré pattern, does not reveal such a phase transformation. While a collective sliding motion of chalcogen atoms easily occurs during the transformation in AA′ stacking, in non‐AA′ stacking it is suppressed by the weak van der Waals strength and by the chalcogen atoms interlocked at different orientations, which disfavor their kinetics by the increased entropy of mixing.