Controlling Crystal Texture in Programmable Atom Equivalent Thin Films

Macroscale-area patterning of three-dimensional DNA-programmable frameworks

DNA, owing to its adaptable structure and sequence-prescribed interactions, provides a versatile molecular tool to program the assembly of organized three-dimensional (3D) nanostructures with precisely incorporated inorganic and biomolecular nanoscale components. While such programmability allows for self-assembly of lattices with diverse symmetries, there is an increasing need to integrate them onto planar substrates for their translation into applications. In this study, we develop an approach for the growth of 3D DNA-programmable frameworks on arbitrarily patterned silicon wafers and metal oxide surfaces, as well as study the leading effects controlling these processes. We achieve the selective growth of DNA origami superlattices into customized surface patterns with feature sizes in the tens of microns across macroscale areas using polymer templates patterned by electron-beam lithography. We uncover the correlation between assembly conditions and superlattice orientations on surfaces, lattice domain sizes, twining, and surface coverage. The demonstrated approach opens possibilities for bridging self-assembly with traditional top-down nanofabrication for creating engineered 3D nanoscale materials over macroscopic areas with nano- and micro-scale controls.

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.

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

Micromirrors are used in integrated photonics to couple extraplanar light into the planar structure of a device by redirecting light via specular reflection. Compared with grating or prism-based couplers, micromirrors allow for coupling of light over a broader range of wavelengths, provided that the micromirror is fabricated with a specific 3D shape to ensure proper reflection angles. In principle, self-assembly methods could enable reliable, parallelizable fabrication of such devices with a high degree of precision by designing self-assembling components that produce the desired microscale geometry as their thermodynamic products. In this work, we use DNA-functionalized nanoparticles to assemble faceted crystallites with predetermined crystal shapes, and demonstrate with microscale retroreflectance measurements that these self-assembled nanoparticle arrays do indeed behave like optically flat mirrors. Furthermore, we show that the tilt angle of the micromirrors can be intentionally controlled by altering the crystallographic symmetry and preferred crystal orientations as a function of the self-assembly process, thereby altering the resulting specular angle in a programmable manner. Measurements of optical coupling from normal incidence into the substrate plane via an optical fiber confirm that the faceted structures can function as optical out-of-plane coupling devices, and coating these structures with reflective materials allows for high efficiency of light reflection in addition to the angular control. Together, these experiments demonstrate how self-assembled nanoparticle materials can be used to generate optically relevant architectures, enabling a significant step in the development of self-assembly as a materials fabrication tool for integrated optical devices.

Programmable Matter: The Nanoparticle Atom and DNA Bond

Colloidal crystal engineering with DNA has led to significant advances in bottom-up materials synthesis and a new way of thinking about fundamental concepts in chemistry. Here, programmable atom equivalents (PAEs), comprised of nanoparticles (the "atoms") functionalized with DNA (the "bonding elements"), are assembled through DNA hybridization into crystalline lattices. Unlike atomic systems, the "atom" (e.g., the nanoparticle shape, size, and composition) and the "bond" (e.g., the DNA length and sequence) can be tuned independently, yielding designer materials with unique catalytic, optical, and biological properties. In this review, nearly three decades of work that have contributed to the evolution of this class of programmable matter is chronicled, starting from the earliest examples based on gold-core PAEs, and then delineating how advances in synthetic capabilities, DNA design, and fundamental understanding of PAE-PAE interactions have led to new classes of functional materials that, in several cases, have no natural equivalent.

Macroscopic materials assembled from nanoparticle superlattices

Nanoparticle assembly has been proposed as an ideal means to program the hierarchical organization of a material by using a selection of nanoscale components to build the entire material from the bottom up. Multiscale structural control is highly desirable because chemical composition, nanoscale ordering, microstructure and macroscopic form all affect physical properties^1,2. However, the chemical interactions that typically dictate nanoparticle ordering^3-5 do not inherently provide any means to manipulate structure at larger length scales^6-9. Nanoparticle-based materials development therefore requires processing strategies to tailor micro- and macrostructure without sacrificing their self-assembled nanoscale arrangements. Here we demonstrate methods to rapidly assemble gram-scale quantities of faceted nanoparticle superlattice crystallites that can be further shaped into macroscopic objects in a manner analogous to the sintering of bulk solids. The key advance of this method is that the chemical interactions that govern nanoparticle assembly remain active during the subsequent processing steps, which enables the local nanoscale ordering of the particles to be preserved as the macroscopic materials are formed. The nano- and microstructure of the bulk solids can be tuned as a function of the size, chemical makeup and crystallographic symmetry of the superlattice crystallites, and the micro- and macrostructures can be controlled via subsequent processing steps. This work therefore provides a versatile method to simultaneously control structural organization across the molecular to macroscopic length scales.

Position- and Orientation-Controlled Growth of Wulff-Shaped Colloidal Crystals Engineered with DNA

Colloidal crystals have emerged as promising candidates for building optical microdevices. Techniques now exist for synthesizing them with control over their nanoscale features (e.g., particle compositions, sizes, shapes, and lattice parameters and symmetry); however, the ability to tune macroscale structural features, such as the relative positions of crystals to one another and lattice orientations, has yet to be realized. Here, inspiration is drawn from epitaxial growth strategies in atomic crystallization, and patterned substrates are prepared that, when used in conjunction with DNA-mediated nanoparticle crystallization, allow for control over individual Wulff-shaped crystal growth, location, and orientation. In addition, the approach allows exquisite control over the patterned substrate/crystal lattice mismatch, something not yet realized for any epitaxy process. This level of structural control is a significant step toward realizing complex, integrated devices with colloidal crystal components, and this approach provides a model system for further exploration in epitaxy systems.

Single-crystal Winterbottom constructions of nanoparticle superlattices

Colloidal nanoparticle assembly methods can serve as ideal models to explore the fundamentals of homogeneous crystallization phenomena, as interparticle interactions can be readily tuned to modify crystal nucleation and growth. However, heterogeneous crystallization at interfaces is often more challenging to control, as it requires that both interparticle and particle-surface interactions be manipulated simultaneously. Here, we demonstrate how programmable DNA hybridization enables the formation of single-crystal Winterbottom constructions of substrate-bound nanoparticle superlattices with defined sizes, shapes, orientations and degrees of anisotropy. Additionally, we show that some crystals exhibit deviations from their predicted Winterbottom structures due to an additional growth pathway that is not typically observed in atomic crystals, providing insight into the differences between this model system and other atomic or molecular crystals. By precisely tailoring both interparticle and particle-surface potentials, we therefore can use this model to both understand and rationally control the complex process of interfacial crystallization.

Optical Processing of DNA-Programmed Nanoparticle Superlattices

Hierarchical structural control across multiple size regimes requires careful consideration of the complex energy- and time-scales which govern the system's morphology at each of these different size ranges. At the nanoscale, synthetic chemistry techniques have been developed to create nanoparticles of well-controlled size and composition. At the macroscale, it is feasible to directly impose material structure via physical manipulation. However, in between these two size regimes at the mesoscale, structural control is more challenging as the physical forces that govern material assembly at larger and smaller scales begin to interfere with one another. In this work, the interplay of structure-directing forces at multiple length-scales is investigated by utilizing optical processing to influence both nanoscale and microscale features of self-assembled, DNA-grafted nanoparticle films. Optical processing is used to generate heat, which causes the self-assembled particles to rearrange from a kinetically trapped, amorphous state into a thermodynamically preferred superlattice structure. The gradient in the heat profile, however, also induces thermophoretic motion within the nanoparticle film, resulting in microscale movement at a comparable time-scale. By utilizing precise exposure times enabled by optical processing, crystallization and thermophoresis occur concurrently in the self-assembling nanoparticle system, enabling a dynamic growth mechanism whereby nucleation and growth occur in separate regions of the material. Furthermore, utilizing sufficiently short processing times allows for the formation of a fluidlike state of the DNA-functionalized nanoparticle materials that is inaccessible via typical thermal processing setups. This unique phase of the material allows for both pathway-dependent and pathway-independent growth phenomena, as appropriately tuning the experimental conditions enables the formation of morphologically equivalent nanoparticle lattices that are generated through different intermediate states (pathway-independent structures), or kinetically preprocessing a material to yield unique thermodynamic arrangements of particles once fully annealed (pathway-dependent structures).