Softness- and Size-Dependent Packing Symmetries of Polymer-Grafted Nanoparticles

Two-Regime Conformation of Grafted Polymer on Nanoparticle Determines Symmetry of Nanoparticle Self-Assembly

One of the key design factors that regulate the properties of grafted nanoparticles (GNPs) and their self-assembly is the conformation of the grafted polymer. On the curved surface of the GNP core, the conformation of the polymer chain is not uniform in the radial direction. The segment is a non-Gaussian chain in the concentrated polymer brush (CPB) regime near the interface between GNP core and grafted polymer, while it is less constrained in the semidilute polymer brush (SDPB) regime near the surface of GNP. Here, the property of polymer conformation showing crossover behavior at the CPB/SDPB threshold through the coarse-grain molecular dynamics simulation of nanoparticles with explicit grafted chains is explored. Moreover, the self-assembly structure depends on the effective softness, which is defined as a function of the threshold of two regimes estimated from the conformation of the polymer.

Icosahedral supracrystal assembly from polymer-grafted nanoparticles via interplay of interfacial energy and confinement effect

Self-assembly of nanoparticles (NPs) in drying emulsion droplets paves the way for intricate three-dimensional (3D) superstructures, given the myriad of control parameters for fine-tuning assembly conditions. With their substantial energetic dynamics that are acutely responsive to emulsion confinements, polymeric ligands incorporated into a system can enrich its structural diversity. Here, we demonstrate the assembly of soft polymer-grafted NPs into Mackay icosahedrons beyond spherical body-centered cubic (BCC) packing structures commonly observed for these soft spheres. This behavior is governed by the free energy minimization within emulsions through the interplay of the oil-water interfacial energy and confinement effect as demonstrated by the experimental observations of structural transitions between icosahedrons and BCC crystals and by corresponding free energy calculations. The anisotropic surface of the icosahedral supracrystals provides the capability of guiding the position of a secondary constituent, creating unique hybrid patchy icosahedrons with the potential to develop into multifunctional 3D clusters that combine the benefits of both polymers and conventional colloids.

Molecular Dynamics Simulation Study on Self-Assembly of Polymer-Grafted Nanocrystals: From Isotropic Cores to Anisotropic Cores

The self-assembly of polymer-grafted nanocrystals (PGNCs) is an important method to manufacture novel nanomaterials. Herein, we focus on the self-assembly of three types of PGNCs with differently shaped cores including sphere, octahedron, and cube by molecular dynamics simulation. By characterizing the positional and orientational order of the assembled superlattices, we construct the phase diagrams as a function of the grafting density and polymer chain length. For PGNCs with spherical cores, we observe the transition from the FCC phase to the BCC phase due to the packing entropy of the ligand polymer chains. For PGNCs with anisotropic cores, the close-packed FCC phase is replaced by the C-BCC phase (octahedral cores) or the C-triclinic phase (cubic cores) due to the directional entropy of core shape. We also study the assembly dynamics by tracking the time evolution of the positional and orientational order. We elucidate the relationship of grafting density and polymer chain length to the packing entropy and directional entropy and reveal their important effects on assembled structures. In general, our simulation results provide useful guidelines for the programmable assembly of PGNCs.

Transformations in crystals of DNA-functionalized nanoparticles by electrolytes

Colloidal crystals have applications in water treatments, including water purification and desalination technologies. It is, therefore, important to understand the interactions between colloids as a function of electrolyte concentration. We study the assembly of DNA-grafted gold nanoparticles immersed in concentrated electrolyte solutions. Increasing the concentration of divalent Ca2+ ions leads to the condensation of nanoparticles into face-centered-cubic (FCC) crystals at low electrolyte concentrations. As the electrolyte concentration increases, the system undergoes a phase change to body-centered-cubic (BCC) crystals. This phase change occurs as the interparticle distance decreases. Molecular dynamics analysis suggests that the interparticle interactions change from strongly repulsive to short-range attractive as the divalent-electrolyte concentration increases. A thermodynamic analysis suggests that increasing the salt concentration leads to significant dehydration of the nanoparticle environment. We conjecture that the intercolloid attractive interactions and dehydrated states favour the BCC structure. Our results gain insight into salting out of colloids such as proteins as the concentration of salt increases in the solution.

2D superlattices via interfacial self-assembly of polymer-grafted Au nanoparticles

Nanoparticle (NP) superlattices are periodic arrays of nanoscale building blocks. Because of the collective effect between functional NPs, NP superlattices can exhibit exciting new properties that are distinct from those of individual NPs or corresponding bulk materials. In particular, two-dimensional (2D) NP superlattices have attracted increasing attention due to their emerging applications in micro/opto-electronics, catalysis, sensing, and other fields. Among various preparation methods, evaporation-induced interfacial self-assembly has become the most popular method for preparing 2D NP superlattices because it is a simple, low-cost, and scalable process that can be widely applied to various NPs. Introducing soft ligands, such as polymers, can not only provide convenience in controlling the self-assembly process and tuning superlattice structures but also improve the properties of 2D NP superlattices. This feature article focuses on the methods of evaporation-induced self-assembly of polymer-grafted Au NPs into free-standing 2D NP superlattice films at air/liquid interfaces and 2D NP superlattice coatings on substrates, followed by studies on in situ tracking of the self-assembly evolution process through small-angle X-ray scattering. Their application in nano-floating gate memory devices is also included. Finally, the challenges and perspectives of this direction are discussed.

Chemical and Physical Properties of Photonic Noble-Metal Nanomaterials

Colloidal noble metal nanoparticles (NPs) are composed of metal cores and organic or inorganic ligand shells. These NPs support size- and shape-dependent plasmonic resonances. They can be assembled from dispersions into artificial metamolecules which have collective plasmonic resonances originating from coupled bright and dark optical electric and magnetic modes that form depending on the size and shape of the constituent NPs and their number, arrangement, and interparticle distance. NPs can also be assembled into extended 2D and 3D metamaterials that are glassy thin films or ordered thin films or crystals, also known as superlattices and supercrystals. The metamaterials have tunable optical properties that depend on the size, shape, and composition of the NPs, and on the number of NP layers and their interparticle distance. Interestingly, strong light-matter interactions in superlattices form plasmon polaritons. Tunable interparticle distances allow designer materials with dielectric functions tailorable from that characteristic of an insulator to that of a metal, and serve as strong optical absorbers or scatterers, respectively. In combination with lithography techniques, these extended assemblies can be patterned to create subwavelength NP superstructures and form large-area 2D and 3D metamaterials that manipulate the amplitude, phase, and polarization of transmitted or reflected light.

Reversible Diffusionless Phase Transitions in 3D Nanoparticle Superlattices

Nanocomposite tectons (NCTs), polymer brush-grafted nanoparticles that use supramolecular interactions to drive their assembly, form ordered nanoparticle superlattices (NPSLs) with well-defined unit cell symmetries when thermally annealed. In this work, we demonstrate that appropriate assembly and processing conditions can also enable control over the microstructure of NCT lattices by balancing the enthalpic and entropic factors associated with ligand packing and supramolecular bonding during crystallization. Unary systems of NCTs are assembled via the addition of a small molecule capable of binding to multiple nanoparticle ligands; these NCTs initially form face-centered-cubic (FCC) structures in solvents that are favorable for the particles' polymer brushes. However, the FCC lattices undergo a reversible, diffusionless phase transition to body-centered-cubic (BCC) lattices when transferred to a solvent that induces polymer brush collapse. The BCC superlattices maintain the same crystal habit as the parent FCC phase but exhibit significant transformation twinning similar to that seen in martensitic alloys. This previously unseen diffusionless phase transformation in NPSLs enables unique microstructural features in the resulting assemblies, suggesting that NPSLs could serve as models for the investigation of microstructural evolution in crystalline systems and extend our understanding of NPSLs as atomic material analogues.

Ligand-Ligand-Interaction-Dominated Self-Assembly of Gold Nanoparticles at the Oil/Water Interface: An Atomic-Scale Simulation

The self-assembly of nanoparticles (NPs) into ordered superlattices is a powerful strategy to fabricate functional nanomaterials. Subtle variations in the interactions between NPs will influence the self-assembled superlattices. Using all-atom molecular dynamics simulations, we explore the self-assembly of 16 gold NPs, 4 nm in diameter, capped with ligands at the oil-water interface, and quantify the interactions between NPs at the atomic scale. We demonstrate that the interaction between capping ligands rather than that between NPs is dominant during the assembly process. For dodecanethiol (DDT)-capped Au NPs, the assembled superlattice is highly ordered in a close-packed configuration at a slow evaporation rate, while it is disordered at a fast evaporation rate. When replacing the capping ligands with stronger polarization than DDT molecules, the NPs form a robust ordered configuration at different evaporation rates due to the stronger electrostatic attraction between capping ligands from different NPs. Moreover, Au-Ag binary clusters exhibit similar assembly behavior with Au NPs. Our work uncovers the nonequilibrium nature of NP assembly at the atomic scale and would be helpful in rationally controlling NPs superlattice by changing passivating ligands, solvent evaporation rate, or both.

In Situ Self-Assembly of Nanoscale Particles into Macroscale Ordered Monolayers with Enhanced Memory Performance

In situ fabrication of macroscale ordered monolayers of nanoparticles (NPs) on targeted substrates is highly desirable for precision electronic and optical devices, while it remains a great challenge. In this study, a solution is provided to address this challenge by developing a colloidal ink formulation and employing the direct-ink-writing (DIW) technique, where on-demand delivery of ink at a targeted location and directional evaporation with controllable rate are leveraged to precisely guide the deposition of polystyrene-grafted gold NPs (Au@PS NPs) into a macroscale monolayer with an ordered Au NP array embedded in a PS thin film. A 2D steady-state diffusion-controlled evaporation model, which explains the parameter dependence of the experimental results and gives semiquantitative agreement with the experimental evaporation kinetics is proposed. The ordered monolayer is used as both nanocrystal floating gates and the tunneling layer for nonvolatile memory devices. It shows significantly enhanced performance compared with a disordered NP film prepared by spin coating. This approach allows for fine control of NP self-assembly to print macroscaleordered monolayers directly onto substrates, which has great promise for application in broad fields, including microelectronic and photoelectronic devices, sensors, and functional coatings.

Surface functionalization of inorganic nanoparticles with ligands: a necessary step for their utility

The importance of protecting inorganic nanoparticles with organic ligands and thus imparting the needed stabilization as colloidal dispersions for their potential applications is highlighted in this review.

Strengthening Engineered Nanocrystal Three-Dimensional Superlattices via Ligand Conformation and Reactivity

Nanocrystal assembly into ordered structures provides mesostructural functional materials with a precise control that starts at the atomic scale. However, the lack of understanding on the self-assembly itself plus the poor structural integrity of the resulting supercrystalline materials still limits their application into engineered materials and devices. Surface functionalization of the nanobuilding blocks with organic ligands can be used not only as a means to control the interparticle interactions during self-assembly but also as a reactive platform to further strengthen the final material via ligand cross-linking. Here, we explore the influence of the ligands on superlattice formation and during cross-linking via thermal annealing. We elucidate the effect of the surface functionalization on the nanostructure during self-assembly and show how the ligand-promoted superlattice changes subsequently alter the cross-linking behavior. By gaining further insights on the chemical species derived from the thermally activated cross-linking and its effect in the overall mechanical response, we identify an oxidative radical polymerization as the main mechanism responsible for the ligand cross-linking. In the cascade of reactions occurring during the surface-ligands polymerization, the nanocrystal core material plays a catalytic role, being strongly affected by the anchoring group of the surface ligands. Ultimately, we demonstrate how the found mechanistic insights can be used to adjust the mechanical and nanostructural properties of the obtained nanocomposites. These results enable engineering supercrystalline nanocomposites with improved cohesion while preserving their characteristic nanostructure, which is required to achieve the collective properties for broad functional applications.

Self-assembly of anisotropy gold nanocubes into large area two-dimensional monolayer superlattices

The spontaneous self-assembly of metal nanocrystals into two-dimensional (2D) monolayer superlattices with highly ordered symmetry and configuration paves the way towards the fabrication of functional materials. However, there remains great challenge for anisotropic nanocrystals to self-assembly into high quality superlattice because of the orientation and configuration consistency. Here, a facile yet universal solvent annealing driven 2D interfacial assembly of synthetic dried metal nanocrystals is firstly developed to realize the construction of the non-close-packing 2D monolayer gold nanocube (AuNC) superlattice with tunable interparticle distance and internal configurations (i.e. face-to-face and hexagonally-packed arrangement), which is achieved by precisely controlling molecular weight of polymer ligands tethered on AuNCs and the van der Waals forces between the adjacent AuNCs. In addition, the scale of the generated 2D monolayer AuNC superlattice with highly ordered internal arrangement and orientation can reach up to hundreds of micrometers, thus acquiring significant surface-enhanced Raman scattering performance of the large scale superlattice due to the strong plasma coupling effect. This strategy not only provides a robust route to fabricate nanocrystal superlattice structures but also offers a promising platform for preparing diverse functional materials with potential applications in electronics, photonics, detections, and others.

Self-Assembly of Colloidal Nanocrystals into 3D Binary Mesocrystals

ConspectusBiominerals are unique materials found in many living organisms that often display outstanding functionalities attributed to their mesocrystalline structure. Mesocrystals are nanocrystal superstructures with mutual crystallographic alignment of the building units. One could thus imagine these optimized evolutionary systems as archetypes to fabricate advanced materials. The main advantage of such systems relies on their ability to combine the features of the nanocrystals with those of single crystalline microscopic structures, yielding assemblies with directional, enhanced, and potentially emergent properties. Moreover, fueled by the promises of multifunctional materials with unprecedented and tunable properties, the rational design of mesocrystals assembled from two distinct colloidal nanocrystal ensembles has become a recent focus of research. However, the combination of dissimilar nanocrystals into ordered binary superstructures is still a major scientific challenge due to the nature of the coassembly process.We focus this Account on the growth of tridimensional (3D) binary mesocrystals and the understanding of the self-assembly of two colloidal nanocrystal ensembles with the ultimate goal to serve as a basis for more rational mesocrystal syntheses in the future. The formation of mesocrystals demands nanocrystals with defined surface faceting, the primary factor influencing their oriented self-assembly. Notably, such a process cannot be successfully afforded without functionalized nanocrystals with high and, in many cases, tunable colloidal stability. Besides, the nature and solvation degree of the surface ligand shell influences the effective shape of the nanocrystals and the kinetics of self-assembly. If the assembly is triggered by reducing the colloidal stability with nonsolvents, 3D single-component mesocrystals are often grown. Here, the different magnitude of the van der Waals attraction forces between nanocrystals with differing compositions, dimensions, and morphologies generally favors the segregation and growth of single component mesocrystals. This phenomenon was illustrated during the successful preparation of 3D binary mesocrystals composed of iron oxide and platinum nanocubes. Although the building blocks possessed comparable sizes and were stabilized by similar ligands, the amount of the second component could only be arbitrarily tuned up to some extent, even when the assembly conditions were rationally optimized to achieve 3D binary mesocrystals. Only a small amount of it was effectively incorporated into the matrix of the initial mesocrystal. The 3D binary mesocrystal growth process demands a delicate control over the size, shape, and surface chemistry of the nanocrystals, the solvent nature, and the self-assembly process. Hence, the improvement of our ability to control the synthesis of 3D binary mesocrystalline materials is critical to exploit their potential toward technological applications in catalysis, energy storage, or structural materials.

Fabrication of Oriented Colloidal Crystals from Capillary Assembly of Polymer-Tethered Gold Nanoparticles

Self-assembled colloidal crystals (CCs) or nanoparticle (NPs) superlattices have attracted significant attention due to their potential applications in many fields. However, due to the complex interactions that govern the self-assembly, it is difficult to predict and control the superstructure organization of CCs. Herein, a facile yet effective way is demonstrated to fabricate oriented CCs from capillary assembly of polymer-tethered gold NPs (AuNPs). Assembly mechanism of polymer-tethered AuNPs and their superlattice structures are systematically studied by in situ small-angle X-ray scattering (SAXS) technology. The results show that the oriented CCs of polymer-tethered AuNPs can be obtained upon solvent evaporation in a capillary tube and the oriented structure is mainly determined by the chain length of polymer ligands and size of AuNPs. Assembly of AuNPs tethered by short-chain ligand can result in oriented face-centered cubic (fcc) superlattice, whereas AuNPs tethered by long-chain ligand can assemble into an oriented body-centered tetragonal (bct) superlattice structure. Interestingly, in situ SAXS study shows that for the sample of bct superlattice structure, a transformation from fcc to bct superlattice upon solvent evaporation is observed, which strongly depends on chain length of ligands. This work provides a useful guide for polymer-tethered AuNPs to prepare orientation colloidal crystals.

Macromolecular Ligand Engineering for Programmable Nanoprism Assembly

Ligands play a central role for the energetics and kinetics of nanocrystal assembly. Yet, the precise and simultaneous manipulation of ligands to dictate assembly outcome has proven difficult. Here, we present macromolecular ligand-engineering strategies to control, characterize, and model four molecular parameters of grafted polymer chains: chain length, chain dispersity, grafting density, and chain distribution. Direct ligand-exchange between nanoprisms and polymers functionalizes facets selectively and produces patchy nanocrystals. We develop a generalizable two-step ligand-exchange approach for the independent control of the two emergent brush parameters, brush thickness and brush softness. The resultant polymer-grafted prismatic nanocrystals with programmable ligand brushes self-assemble into thin-film superstructures of different wallpaper symmetries and faceted supracrystals. Our experiments are complemented by coarse-grained computer simulations of nanoprisms with directional, facet-specific interactions. This work paves the way for the precision synthesis of polymer-nanocrystal hybrid materials and enables the further refinement of theoretical models for particle brush materials.

Hierarchical superstructures assembled from pH-responsive gold nanoparticles in deformable emulsion droplets

Integrating stimuli-responsive nanoparticles (NPs) into shape-tunable hierarchical superstructures has emerged as a powerful way to fabricate smart materials. Here, we propose a robust strategy for the fabrication of sheet-like superlattices and hollow capsules by precisely tuning the interfacial tension of emulsion droplets containing pH-responsive gold nanoparticle building blocks, providing an efficient route to construct hierarchical superstructures with different applications.

Body centered tetragonal nanoparticle superlattices: why and when they form?

Body centered tetragonal (BCT) phases are structural intermediates between body centered cubic (BCC) and face centered cubic (FCC) structures. However, BCC ↔ FCC transitions may or may not involve a stable BCT intermediate. Interestingly, nanoparticle superlattices usually crystallize in BCT structures, but this phase is much less frequent for colloidal crystals of micrometer-sized particles. Two origins have been proposed for the formation of BCT NPSLs: (i) the influence of the substrate on which the nanoparticle superlattice is deposited, and (ii) non-spherical nanoparticle shapes, combined with the fact that different crystal facets have different ligand organizations. Notably, none of these two mechanisms alone is able to explain the set of available experimental observations. In this work, these two hypotheses were independently tested using a recently developed molecular theory for nanoparticle superlattices that explicitly captures the degrees of freedom associated with the ligands on the nanoparticle surface and the crystallization solvent. We show that the presence of a substrate can stabilize the BCT structure for spherical nanoparticles, but only for very specific combinations of parameters. On the other hand, a truncated-octahedron nanoparticle shape strongly stabilizes BCT structures in a wide region of the phase diagram. In the latter case, we show that the stabilization of BCT results from the geometry of the system and it does not require different crystal facets to have different ligand properties, as previously proposed. These results shed light on the mechanisms of BCT stabilization in nanoparticle superlattices and provide guidelines to control its formation.

Softness-Enhanced Self-Assembly of Pyrochlore- and Perovskite-like Colloidal Photonic Crystals from Triblock Janus Particles

It remains extremely challenging to build three-dimensional photonic crystals with complete photonic bandgaps by simple and experimentally realizable colloidal building blocks. Here, we demonstrate that particle softness can enhance both the self-assembly of pyrochlore- and perovskite-like lattice structures from simple deformable triblock Janus colloids and their photonic bandgap performances. Dynamics simulation results show that the region of stability of pyrochlore lattices can be greatly expanded by appropriately increasing softness, and the perovskite lattices are unexpectedly obtained at enough high softness. Photonic calculations show that the direct pyrochlore lattices formed from overlapping soft triblock Janus particles exhibit even larger photonic bandgaps than the ideal nonoverlapping pyrochlore lattice, and proper overlap arising from softness can also dramatically improve the photonic properties of the inverse pyrochlore and perovskite lattices. Our study offers a new and feasible self-assembly path toward three-dimensional photonic crystals with large and robust photonic bandgaps.

Fluorescence Switchable Block Copolymer Particles with Doubly Alternate-Layered Nanoparticle Arrays

The precise self-assembly of block copolymers (BCPs) and inorganic nanoparticles (NPs) under 3D confinement offers microparticles with programmable nanostructures and functionalities. Here, fluorescence-switchable hybrid microspheres are developed by forming doubly alternating arrays of Au NPs and CdSe/ZnS quantum dots (QDs) within polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) BCP domains. These doubly alternating arrays afford controlled nonradiative energy transfer (NRET) between the QDs and Au NPs that is dependent on the layer-to-layer distance. Solvent-selective swelling of the hybrid particles tunes the distance between layers, modulating their NRET behavior and affording switchable fluorescence. The particle fluorescence is "OFF" in water through strong NRET from the QDs to Au NPs, but is "ON" in alcohols due to the increased distance between the Au NP and QD arrays in the swollen P4VP domains. The experimentally observed NRET intensity as a function of interparticle distance shows larger quenching efficiencies than those theoretically predicted due to the enhanced quenching within a 3D-confined system. Finally, the robust and reversible fluorescence switching of the hybrid particles in different solvents is demonstrated, highlighting their potentials for bioimaging, sensing, and diagnostic applications.

Kinetically Controlled Self-Assembly of Binary Polymer-Grafted Nanocrystals into Ordered Superstructures via Solvent Vapor Annealing

Polymer-inorganic nanocomposites based on polymer-grafted nanocrystals (PGNCs) are enabling technologically relevant applications owing to their unique physical, chemical, and mechanical properties. While diverse PGNC superstructures have been realized through evaporation-driven self-assembly, this approach presents multifaceted challenges in experimentally probing and controlling assembly kinetics. Here, we report a kinetically controlled assembly of binary superstructures from a homogeneous disordered PGNC mixture utilizing solvent vapor annealing (SVA). Using a NaZn₁₃-type superstructure as a model system, we demonstrate that varying the solvent vapor pressure during SVA allows for exquisite control of the rate and extent of PGNC assembly, providing access to nearly complete kinetic pathways of binary PGNC crystallization. Characterization of kinetically arrested intermediates reveals that assembly follows a multistep crystallization pathway involving spinodal-like preordering of PGNCs prior to NaZn₁₃ nucleation. Our work opens up new avenues for the synthesis of multicomponent PGNC superstructures exhibiting multifunctionalities and emergent properties through a thorough understanding of kinetic pathways.

Engineered nanoparticle network models for autonomous computing

Materials that exhibit synaptic properties are a key target for our effort to develop computing devices that mimic the brain intrinsically. If successful, they could lead to high performance, low energy consumption, and huge data storage. A 2D square array of engineered nanoparticles (ENPs) interconnected by an emergent polymer network is a possible candidate. Its behavior has been observed and characterized using coarse-grained molecular dynamics (CGMD) simulations and analytical lattice network models. Both models are consistent in predicting network links at varying temperatures, free volumes, and E-field (E⃗) strengths. Hysteretic behavior, synaptic short-term plasticity and long-term plasticity-necessary for brain-like data storage and computing-have been observed in CGMD simulations of the ENP networks in response to E-fields. Non-volatility properties of the ENP networks were also confirmed to be robust to perturbations in the dielectric constant, temperature, and affine geometry.

Unconventional-Phase Crystalline Materials Constructed from Multiscale Building Blocks

Crystal phase, an intrinsic characteristic of crystalline materials, is one of the key parameters to determine their physicochemical properties. Recently, great progress has been made in the synthesis of nanomaterials with unconventional phases that are different from their thermodynamically stable bulk counterparts via various synthetic methods. A nanocrystalline material can also be viewed as an assembly of atoms with long-range order. When larger entities, such as nanoclusters, nanoparticles, and microparticles, are used as building blocks, supercrystalline materials with rich phases are obtained, some of which even have no analogues in the atomic and molecular crystals. The unconventional phases of nanocrystalline and supercrystalline materials endow them with distinctive properties as compared to their conventional counterparts. This Review highlights the state-of-the-art progress of nanocrystalline and supercrystalline materials with unconventional phases constructed from multiscale building blocks, including atoms, nanoclusters, spherical and anisotropic nanoparticles, and microparticles. Emerging strategies for engineering their crystal phases are introduced, with highlights on the governing parameters that are essential for the formation of unconventional phases. Phase-dependent properties and applications of nanocrystalline and supercrystalline materials are summarized. Finally, major challenges and opportunities in future research directions are proposed.

Polymer Coupling via Hetero-Disulfide Exchange and Its Applications to Rewritable Polymer Brushes

An iodide-terminated polymer (Polymer-I) is converted to a thiol-terminated polymer (Polymer-SH) using HSCH₂CH₂SH in a remarkably short time (10 min). Polymer-SH is further converted to a pyridyl disulfide-terminated polymer (Polymer-SS-Py). The hetero-coupling of Polymer-SH and Polymer-SS-Py is successfully achieved to quantitatively generate a polymer disulfide (Polymer-SS-Polymer). Exploiting this efficient hetero-coupling technique, Polymer-SH is attached (grafted) on a Py-SS-immobilized surface to generate a polymer brush via a disulfide (-SS-) linkage (writing process). The -SS- linkage is cleaved by the treatment with dithiothreitol (DTT) to detach the polymer from the surface (erasing process). Subsequently, another Polymer-SH is attached on the surface to generate another polymer brush (rewriting process). Thus, a writable, erasable, and rewritable polymer brush surface is achieved. Hydrophilic, hydrophobic, and super-hydrophobic polymers (Polymer-SH) are attached on the surface, tailoring the surface wettability in the writing-erasing-rewriting cycles. Polymer-SH is also attached on a chain-end Py-SS-functionalized polymer brush surface, generating a rewritable block copolymer brush surface. A patterned block copolymer brush surface is also obtained using photo-irradiation and a photo-mask in the erasing process. The metal-free synthetic procedure, accessibility to patterned brushes, and switchable surface properties via the writing-erasing-rewriting process are attractive features of the present approach.

Nanoengineering with RAFT polymers: from nanocomposite design to applications

Reversible addition–fragmentation chain-transfer (RAFT) polymerization is a powerful tool for the precise formation of macromolecular building blocks that can be used for the construction of well-defined nanocomposites.

Solvent Quality-Mediated Regioselective Modification of Gold Nanorods with Thiol-Terminated Polymers

Modification of nanorods (NRs) with functional polymer ligands is of great significance to enhance their surface chemistry and prompt their applications in many fields (e.g., photothermal therapy, bioimaging, and catalysis). However, the regioselective modification of AuNRs still remains a great challenge. Herein, we introduce a facile yet versatile strategy to achieve the regioselective modification of AuNRs through a solvent quality-mediated strategy. By employing a poor solvent of the original ligand cetyltrimethylammonium bromide (CTAB) as the medium in the modification, polymer ligands would selectively graft onto the two ends of AuNRs, while polymer ligands would graft onto the entire surface when employing a good solvent. This strategy demonstrates good reproducibility and is applicable to both hydrophilic and hydrophobic polymer ligand modifications. Moreover, by combing our strategy with the preoccupation route, the two ends and sidewall of AuNRs modified by two different polymers form an "ABA"-type building block, which can further self-assemble into well-ordered superstructures. Our finding provides a new opportunity for multifunctionalization of NRs.