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.

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 properties1,2. However, the chemical interactions that typically dictate nanoparticle ordering3-5 do not inherently provide any means to manipulate structure at larger length scales6-9. 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.

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).

Programmable Atom Equivalents: Atomic Crystallization as a Framework for Synthesizing Nanoparticle Superlattices.

Decades of research efforts into atomic crystallization phenomenon have led to a comprehensive understanding of the pathways through which atoms form different crystal structures. With the onset of nanotechnology, methods that use colloidal nanoparticles (NPs) as nanoscale "artificial atoms" to generate hierarchically ordered materials are being developed as an alternative strategy for materials synthesis. However, the assembly mechanisms of NP-based crystals are not always as well-understood as their atomic counterparts. The creation of a tunable nanoscale synthon whose assembly can be explained using the context of extensively examined atomic crystallization will therefore provide significant advancement in nanomaterials synthesis. DNA-grafted NPs have emerged as a strong candidate for such a "programmable atom equivalent" (PAE), because the predictable nature of DNA base-pairing allows for complex yet easily controlled assembly. This Review highlights the characteristics of these PAEs that enable controlled assembly behaviors analogous to atomic phenomena, which allows for rational material design well beyond what can be achieved with other crystallization techniques.