Self-Assembling Systems for Optical Out-of-Plane Coupling Devices

Controlling the thermally-driven crystallization of DNA-coated nanoparticles with formamide

DNA-coated nanoparticles, also known as programmable atom equivalents (PAEs), facilitate the construction of materials with nanoscopic precision. Thermal annealing plays a pivotal role by controlling DNA hybridization kinetics and thermodynamics, which ensures the formation of intended structures. While various design handles such as particle size, DNA design, and salt concentration influence the stability of the DNA duplexes linking PAEs in a lattice, their influence on the system's melting temperature (Tm) often follows complicated trends that make rational tuning of self-assembly challenging. In this work, the denaturant formamide is used to precisely tune the thermal response of PAEs. Our results reveal a clear and predictable trend in the PAEs' response to formamide, enabling rational control over the Tm of a diverse set of PAE systems. Unlike adjustments made through alterations to PAE design or solution parameters such as ionic strength, formamide achieves its temperature shift without impacting the kinetics of assembly. As a result, PAEs can be rapidly crystallized at ambient temperatures, producing superlattices with similar quality to PAE crystals assembled through standard protocols that use higher temperatures. This study therefore positions formamide as a useful tool for enhancing the synthesis of complex nanostructures under mild conditions.

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.

Dynamic Nanocrystal Superlattices with Thermally Triggerable Lubricating Ligands

The size-dependent and collective physical properties of nanocrystals (NCs) and their self-assembled superlattices (SLs) enable the study of mesoscale phenomena and the design of metamaterials for a broad range of applications. However, the limited mobility of NC building blocks in dried NCSLs often hampers the potential for employing postdeposition methods to produce high-quality NCSLs. In this study, we present tailored promesogenic ligands that exhibit a lubricating property akin to thermotropic liquid crystals. The lubricating ability of ligands is thermally triggerable, allowing the dry solid NC aggregates deposited on the substrates with poor ordering to be transformed into NCSLs with high crystallinity and preferred orientations. The interplay between the dynamic behavior of NCSLs and the molecular structure of the ligands is elucidated through a comprehensive analysis of their lubricating efficacy using both experimental and simulation approaches. Coarse-grained molecular dynamic modeling suggests that a shielding layer from mesogens prevents the interdigitation of ligand tails, facilitating the sliding between outer shells and consequently enhancing the mobility of NC building blocks. The dynamic organization of NCSLs can also be triggered with high spatial resolution by laser illumination. The principles, kinetics, and utility of lubricating ligands could be generalized to unlock stimuli-responsive metamaterials from NCSLs and contribute to the fabrication of NCSLs.

Controlling Colloidal Crystal Nucleation and Growth with Photolithographically Defined Templates

Colloidal crystallization provides a means to synthesize hierarchical nanostructures by design and to use these complex structures for nanodevice fabrication. In particular, DNA provides a means to program interactions between particles with high specificity, thereby enabling the formation of particle superlattice crystallites with tailored unit cell geometries and surface faceting. However, while DNA provides precise control of particle-particle bonding interactions, it does not inherently present a means of controlling higher-level structural features such as the size, shape, position, or orientation of a colloidal crystallite. While altering assembly parameters such as temperature or concentration can enable limited control of crystallite size and geometry, integrating colloidal assemblies into nanodevices requires better tools to manipulate higher-order structuring and improved understanding of how these tools control the fundamental kinetics and mechanisms of colloidal crystal growth. In this work, photolithography is used to produce patterned substrates that can manipulate the placement, size, dispersity, and orientation of colloidal crystals. By adjusting aspects of the pattern, such as feature size and separation, we reveal a diffusion-limited mechanism governing crystal nucleation and growth. Leveraging this insight, patterns are designed that can produce wafer-scale substrates with arrays of nanoparticle superlattices of uniform size and shape. These design principles therefore bridge a gap between a fundamental understanding of nanoparticle assembly and the fabrication of nanostructures compatible with functional devices.

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.

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.