Reversible Diffusionless Phase Transitions in 3D Nanoparticle Superlattices

Dipole-Dipole Interaction-Induced Direct Self-Assembly of Ag2S Quantum Dots into Supercrystals in Solution

Long-range ordered supercrystals (SCs) built up by colloidal nanocrystals (NCs) represent a class of novel metamaterials with unique collective properties. While great attention has been paid to the ligand-controlled assembly of NCs, the contribution of the inorganic core is considered limited because of the weak core-core interactions. Here, we report the spontaneous assembly of Ag2S quantum dots (QDs) into three-dimensional SCs in solution, driven by pronounced dipole-dipole interactions. Dielectric spectroscopy shows a large permanent dipole moment of 516.7 D in 4.2 nm Ag2S QDs, and multiscale molecular simulation proves the dipole-dipole interaction-driven crystallization of Ag2S QDs. Moreover, we demonstrate that tuning the dipole-dipole interactions facilitates the formation of diverse nanostructures, including SCs, nanochains, and monodisperse nanoparticles. These findings offer a straightforward strategy for SC synthesis and establish the dipole-dipole interactions as a key driving force of NC self-assembly with broad implications for colloidal nanomaterials and their emergent functionalities.

Myelin Surfactant Assemblies as Dynamic Pathways Guiding the Growth of Electrodeposited Copper Dendrites

Self-organization of inorganic matter enables bottom-up construction of materials with target shapes suited to their function. Positioning the building blocks in the growth process involves a well-balanced interplay of the reaction and diffusion. Whereas (supra)molecular structures have been used to template such growth processes, we reasoned that molecular assemblies can be employed to actively create concentration gradients that guide the deposition of solid, wire-like structures. The core of our approach comprises the interaction between myelin assemblies that deliver copper(II) ions to the tips of copper dendrites, which in turn grow along the Cu2+ gradient upon electrodeposition. First, we successfully include Cu2+ ions among amphiphile bilayers in myelin filaments, which grow from tri(ethylene glycol) monododecyl ether (C12E3) source droplets over air-water interfaces. Second, we characterize the growth of dendritic copper structures upon electrodeposition from a negative electrode at the sub-mM Cu2+ concentrations that are anticipated upon release from copper(II)-loaded myelins. Third, we assess the intricate growth of copper dendrites upon electrodeposition, when combined with copper(II)-loaded myelins. The myelins deliver Cu2+ at a negative electrode, feeding copper dendrite growth upon electrodeposition. Intriguingly, the copper dendrites follow the Cu2+ gradient toward the myelins and grow along them toward the source droplet. We demonstrate the growth of dynamic connections among electrodes and surfactant droplets in reconfigurable setups─featuring a unique interplay between molecular assemblies and inorganic, solid structures.

Nanoparticle Superlattices with Nonequilibrium Crystal Shapes

Nanoparticle assembly is a material synthesis strategy that enables precise control of nanoscale structural features. Concepts from traditional crystal growth research have been tremendously useful in predicting and programming the unit cell symmetries of these assemblies, as their thermodynamically favored structures are often identical to atomic crystal analogues. However, these analogies have not yielded similar levels of influence in programming crystallite shapes, which are a consequence of both the thermodynamics and kinetics of crystal growth. Here, we demonstrate kinetic control of the colloidal crystal shape using nanoparticle building blocks that rapidly assemble over a broad range of concentrations, thereby producing well-defined crystal habits with symmetrically oriented dendritic protrusions and providing insight into the crystals' morphological evolution. Counterintuitively, these nonequilibrium crystal shapes actually become more common for colloidal crystals synthesized closer to equilibrium growth conditions. This deviation from typical crystal growth processes observed in atomic or molecular crystals is shown to be a function of the drastically different time scales of atomic and colloidal mass transport. Moreover, the particles are spherical with isotropic ligand grafts, and these kinetic crystal habits are achieved without the need for specifically shaped particle building blocks or external templating or shape-directing agents. Thus, this work provides generalizable design principles to expand the morphological diversity of nanoparticle superlattice crystal habits beyond the anhedral or equilibrium polyhedral shapes synthesized to date. Finally, we use this insight to synthesize crystallite shapes that have never before been observed, demonstrating the ability to both predict and program kinetically controlled superlattice morphologies.

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.

Dynamic Transformation of High-Architectural Nanocrystal Superlattices upon Solvent Molecule Exposure

The cluster-based body-centered-cubic superlattice (cBCC SL) represents one of the most complicated structures among reported nanocrystal assemblies, comprised of 72 truncated tetrahedral quantum dots per unit cell. Our previous report revealed that truncated tetrahedral quantum dots within cBCC SLs possessed highly controlled translational and orientational order owing to an unusual energetic landscape based on the balancing of entropic and enthalpic contributions during the assembly process. However, the cBCC SL's structural transformability and mechanical properties, uniquely originating from such complicated nanostructures, have yet to be investigated. Herein, we report that cBCC SLs can undergo dynamic transformation to face-centered-cubic SLs in response to post-assembly molecular exposure. We monitored the dynamic transformation process using in situ synchrotron-based small-angle X-ray scattering, revealing a dynamic transformation involving multiple steps underpinned by interactions between incoming molecules and TTQDs' surface ligands. Furthermore, our mechanistic study demonstrated that the precise configuration of TTQDs' ligand molecules in cBCC SLs was key to their high structural transformability and unique jelly-like soft mechanical properties. While ligand molecular configurations in nanocrystal SLs are often considered minor features, our findings emphasize their significance in controlling weak van der Waals interactions between nanocrystals within assembled SLs, leading to previously unremarked superstructural transformability and unique mechanical properties. Our findings promote a facile route toward further creation of soft materials, nanorobotics, and out-of-equilibrium assemblies based on nanocrystal building blocks.

Hierarchically Responsive Alternating Nano-Copolymers with Tailored Interparticle Bonds

Self-assembly of inorganic nanoparticles (NPs) is an essential tool for constructing structured materials with a wide range of applications. However, achieving ordered assembly structures with externally programmable properties in binary NP systems remains challenging. In this work, we assemble binary inorganic NPs into hierarchically pH-responsive alternating copolymer-like nanostructures in an aqueous medium by engineering the interparticle electrostatic interactions. The polymer-grafted NPs bearing opposite charges are viewed as nanoscale monomers ("nanomers"), and copolymerized into alternating nano-copolymers (ANCPs) driven by the formation of interparticle "bonds" between nanomers. The resulting ANCPs exhibit reversibly responsive "bond" length (i.e., the distance between nanomers) in response to the variation of pH in a range of ~7-10, allowing precise control over the surface plasmon resonance of ANCPs. Moreover, specific interparticle "bonds" can break up at pH≥11, leading to the dis-assembly of ANCPs into molecule-like dimers and trimers. These dimeric and trimeric structures can reassemble to form ANCPs owing to the resuming of interparticle "bonds", when the pH value of the solution changes from 11 to 7. The hierarchically responsive nanostructures may find applications in such as biosensing, optical waveguide, and electronic devices.

Rationally Designing the Supramolecular Interfaces of Nanoparticle Superlattices with Multivalent Polymers

In supramolecular materials, multiple weak binding groups can act as a single collective unit when confined to a localized volume, thereby producing strong but dynamic bonds between material building blocks. This principle of multivalency provides a versatile means of controlling material assembly, as both the number and the type of supramolecular moieties become design handles to modulate the strength of intermolecular interactions. However, in materials with building blocks significantly larger than individual supramolecular moieties (e.g., polymer or nanoparticle scaffolds), the degree of multivalency is difficult to predict or control, as sufficiently large scaffolds inherently preclude separated supramolecular moieties from interacting. Because molecular models commonly used to examine supramolecular interactions are intrinsically unable to examine any trends or emergent behaviors that arise due to nanoscale scaffold geometry, our understanding of the thermodynamics of these massively multivalent systems remains limited. Here we address this challenge via the coassembly of polymer-grafted nanoparticles and multivalent polymers, systematically examining how multivalent scaffold size, shape, and spacing affect their collective thermodynamics. Investigating the interplay of polymer structure and supramolecular group stoichiometry reveals complicated but rationally describable trends that demonstrate how the supramolecular scaffold design can modulate the strength of multivalent interactions. This approach to self-assembled supramolecular materials thus allows for the manipulation of polymer-nanoparticle composites with controlled thermal stability, nanoparticle organization, and tailored meso- to microscopic structures. The sophisticated control of multivalent thermodynamics through precise modulation of the nanoscale scaffold geometry represents a significant advance in the ability to rationally design complex hierarchically structured materials via self-assembly.

Growth of Nanocrystal Superlattices from Liquid Crystals

The growth of superlattices (SLs) made from self-assembled nanocrystals (NCs) is a powerful method for creating new materials and gaining insight into fundamental molecular dynamics. Previous explorations of NCSL syntheses have mostly compared them to crystallization. However, NCSL synthesis has not broadly shown cooling crystallization from saturated solutions as a reversible crystallization-dissolution process. We demonstrate the reversible growth of NCSLs by dispersing NCs in liquid crystal (LC) "smart solvents," and harnessing the transitions between the isotropic and nematic phases of the LCs. The growth mode and morphology can be tuned. This process is a model platform for studying crystallization and demonstrates great potential in manufacturing NCSLs as colloidal crystals through liquid-phase epitaxy or colloidal synthesis.

Nanoparticle Brushes: Macromolecular Ligands for Materials Synthesis

ConspectusColloidal nanoparticles have unique attributes that can be used to synthesize materials with exotic properties, but leveraging these properties requires fine control over the particles' interactions with one another and their surrounding environment. Small molecules adsorbed on a nanoparticle's surface have traditionally served as ligands to govern these interactions, providing a means of ensuring colloidal stability and dictating the particles' assembly behavior. Alternatively, nanoscience is increasingly interested in instead using macromolecular ligands that form well-defined polymer brushes, as these brushes provide a much more tailorable surface ligand with significantly greater versatility in both composition and ligand size. While initial research in this area is promising, synthesizing macromolecules that can appropriately form brush architectures remains a barrier to their more widespread use and limits understanding of the fundamental chemical and physical principles that influence brush-grafted particles' ability to form functional materials. Therefore, enhancing the capabilities of polymer-grafted nanoparticles as tools for materials synthesis requires a multidisciplinary effort, with specific focus on both developing new synthetic routes to polymer-brush-coated nanoparticles and investigating the structure-property relationships the brush enables.In this Account, we describe our recent work in developing polymer brush coatings for nanoparticles, which we use to modulate particle behavior on demand, select specific nanoscopic architectures to form, and bolster traditional bulk polymers to form stronger materials by design. Distinguished by the polymer type and capabilities, three classes of nanoparticles are discussed here: nanocomposite tectons (NCTs), which use synthetic polymers end-functionalized with supramolecular recognition groups capable of directing their assembly; programmable atom equivalents (PAEs) containing brushes of synthetic DNA that employ Watson-Crick base pairing to encode particle binding interactions; and cross-linkable nanoparticles (XNPs) that can both stabilize nanoparticles in solution and polymer matrices and subsequently form multivalent cross-links to strengthen polymer composites. We describe the formation of these brushes through "grafting-from" and "grafting-to" strategies and illustrate aspects that are important for future advancement. We also examine the new capabilities brushes provide, looking closely at dynamic polymer processes that provide control over the assembly state of particles. Finally, we provide a brief overview of the technological applications of nanoparticles with polymer brushes, focusing on the integration of nanoparticles into traditional materials and the processing of nanoparticles into bulk solids.

Tunable Mechanical Response of Self-Assembled Nanoparticle Superlattices

Self-assembled nanoparticle superlattices (NPSLs) are an emergent class of self-architected nanocomposite materials that possess promising properties arising from precise nanoparticle ordering. Their multiple coupled properties make them desirable as functional components in devices where mechanical robustness is critical. However, questions remain about NPSL mechanical properties and how shaping them affects their mechanical response. Here, we perform in situ nanomechanical experiments that evidence up to an 11-fold increase in stiffness (∼1.49 to 16.9 GPa) and a 5-fold increase in strength (∼88 to 426 MPa) because of surface stiffening/strengthening from shaping these nanomaterials via focused-ion-beam milling. To predict the mechanical properties of shaped NPSLs, we present discrete element method (DEM) simulations and an analytical core-shell model that capture the FIB-induced stiffening response. This work presents a route for tunable mechanical responses of self-architected NPSLs and provides two frameworks to predict their mechanical response and guide the design of future NPSL-containing devices.

Temperature-Controlled Reversible Formation and Phase Transformation of 3D Nanocrystal Superlattices Through In Situ Small-Angle X-ray Scattering

For decades, the spontaneous organization of nanocrystals into superlattices has captivated the scientific community. However, achieving direct control over the formation of the superlattice and its phase transformations has proven to be a grand challenge, often resulting in the generation of multiple symmetries under the same experimental conditions. Here, we achieve direct control over the formation of the superlattice and its phase transformations by modulating the thermal energy of a nanocrystal dispersion without relying on solvent evaporation. We follow the temperature-dependent dynamics of the self-assembly process using synchrotron-based small-angle X-ray scattering. When cooled below -24.5 °C, lead sulfide nanocrystals form micrometer-sized three-dimensional phase-pure body-centered cubic superlattices. When cooled below -35.1 °C, these superlattices undergo a collective diffusionless phase transformation that yields denser body-centered tetragonal phases. These structural changes can be reversed by increasing the temperature of the dispersion and may lead to the direct modulation of the optical properties of these artificial solids.