Hints of Growth Mechanism Left in Supercrystals.

Supercrystals of DNA-functionalized nanoparticles are visualized in three dimensions using X-ray ptychographic tomography, and their reciprocal spaces are mapped with small-angle X-ray scattering in order to better understand their internal defect structures. X-ray ptychographic tomography reveals various types of defects in an assembly that otherwise exhibits a single crystalline diffraction pattern. On average, supercrystals composed of smaller nanoparticles are smaller in size than supercrystals composed of larger particles. Additionally, supercrystals composed of small nanoparticles are typically aggregated into larger "necklace-like" structures. Within these larger structures, some but not all pairs of connected domains are coherent in their relative orientations. In contrast, supercrystals composed of larger nanoparticles with longer DNA ligands typically form faceted crystals. The combination of these two complementary X-ray techniques reveals that the crystalline assemblies grow by aggregation of smaller assemblies followed by rearrangement of nanoparticles.

Morphology Engineering in Multicomponent Hollow Metal Chalcogenide Nanoparticles.

Hollow metal chalcogenide nanoparticles are widely applicable in environmental and energy-related processes. Herein, we synthesized such particles with large compositional and morphological diversity by combining scanning probe block copolymer lithography with a Kirkendall effect-based sulfidation process. We explored the influence of temperature-dependent diffusion kinetics, elemental composition and miscibility, and phase boundaries on the resulting particle morphologies. Specifically, CoNi alloys form single-shell sulfides for the synthetic conditions explored because Co and Ni exhibit similar diffusion rates, while CuNi alloys form sulfides with various types of morphologies (yolk-shell, double-shell, and single-shell) because Cu and Ni have different diffusion rates. In contrast, Co-Cu heterodimers form hollow heterostructured sulfides with varying void numbers and locations depending on synthesis temperature and phase boundary. At higher temperatures, the increased miscibility of CoS2 and CuS makes it energetically favorable for the heterostructure to adopt a single alloy shell morphology, which is rationalized using density functional theory-based calculations.

Arrays of Colloidal Single Crystals Engineered with DNA in Lithographically Defined Microwells.

Lithographically defined microwell templates are used to study DNA-guided colloidal crystal assembly parameters, including superlattice position, habit orientation, and size, in an effort to increase our understanding of the crystallization process. In addition to enabling the synthesis of arrays of individual superlattices in arbitrary predefined patterns, the technique allows one to study the growth pathways of the crystals via ex situ scanning electron microscopy. Importantly, a Volmer-Weber (VM) (island formation)-like growth mode is identified, which has been reproduced via simulations. Notably, both experiment and simulation reveal that the crystallites merge and reorient within the microwells that defined the crystal growth to form single-crystalline structures, an observation not common for VM pathways. The control afforded by this platform will facilitate efforts in constructing metamaterials from colloidal crystals as well as their integration into optical devices and applications.

Regioselective Deposition of Metals on Seeds within a Polymer Matrix.

We use scanning probe block copolymer lithography in a two-step sequential manner to explore the deposition of secondary metals on nanoparticle seeds. When single element nanoparticles (Au, Ag, Cu, Co, or Ni) were used as seeds, both heterogeneous and homogeneous growth occurred, as rationalized using the thermodynamic concepts of bond strength and lattice mismatch. Specifically, heterogeneous growth occurs when the heterobond strength between the seed and growth atoms is stronger than the homobond strength between the growth atoms. Moreover, the resulting nanoparticle structure depends on the degree of lattice mismatch between the seed and growth metals. Specifically, a large lattice mismatch (e.g., 13.82% for Au and Ni) typically resulted in heterodimers, whereas a small lattice mismatch (e.g., 0.19% for Au and Ag) resulted in core-shell structures. Interestingly, when heterodimer nanoparticles were used as seeds, the secondary metals deposited asymmetrically on one side of the seed. By programming the deposition conditions of Ag and Cu on AuNi heterodimer seeds, two distinct nanostructures were synthesized with (1) Ag and Cu on the Au domain and (2) Ag on the Au domain and Cu on the Ni domain, illustrating how this technique can be used to predictively synthesize structurally complex, multimetallic nanostructures.

Confined Growth of DNA-Assembled Superlattice Films.

We study the assembly of DNA-functionalized nanocubes under lateral confinement in microscale square trenches on a DNA-functionalized substrate. Microfocus small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) are used to characterize the superlattices (SLs). The results indicate that nanocubes form simple-cubic SLs with square-prism morphology and a (100) out-of-plane orientation to maximize DNA bonding. In-plane, SLs align with the template, exposing their {100} side facets, and the degree of alignment depends on trench size. Interestingly, the distribution of in-plane orientations determined from SAXS and SEM do not agree, indicating that the internal and external structures of the SLs differ. To understand this discrepancy, X-ray ptychography is employed to image the internal structures of the SLs, revealing that SLs which appear to be single-crystalline in SEM may have subsurface grain boundaries, depending on trench size. SEM reveals that the SLs grow via nucleation and growth of randomly oriented domains, which then coalesce; this mechanism explains the observed dependence of alignment and defect structure on size. Interestingly, crystallization occurs via an unusual growth mode, whereby continuous SL layers grow on top of several misoriented islands. Overall, this work elucidates the effect of lateral confinement on the crystallization of DNA-functionalized nanoparticles and shows how X-ray ptychography can be used to gain insight into nanoparticle crystallization.

Large-Area, Highly Crystalline DNA-Assembled Metasurfaces Exhibiting Widely Tunable Epsilon-Near-Zero Behavior.

Metasurfaces prepared via bottom-up nanoparticle assembly enable the deliberate manipulation of light in the optical regime, resulting in media with various engineered optical responses. Here, we report a scalable method to grow highly crystalline 2D metasurfaces composed of colloidal gold nanocubes, over macroscopic areas, using DNA-mediated assembly under equilibrium conditions. Using an effective medium description, we predict that these plasmonic metasurfaces behave as dielectric media with high refractive indices that can be dynamically tuned by tuning DNA length. Furthermore, we predict that, when coupled with an underlying thin gold film, the real permittivity of these metasurfaces exhibits a crossover region between positive and negative values, known as the epsilon-near-zero (ENZ) condition, which can be tuned between 1.5 and 2.6 µm by changing DNA length. Optical characterization performed on the DNA-assembled metasurfaces reveals that the predicted optical properties agree well with the measured response. Overall, we propose an efficient method for realizing large-area plasmonic metasurfaces that enable dynamic control over optical characteristics. High-index and ENZ metasurfaces operating in the telecommunications regime could have significant implications in high-speed optical computing, optical communications, optical imaging, and other areas.

Low-Density 2D Superlattices Assembled via Directional DNA Bonding.

It is critical to assemble nanoparticles (NPs) into superlattices with controlled symmetries and spacings on substrates for metamaterials applications, where such structural parameters dictate their properties. Here, we use DNA to assemble anisotropic NPs of three shapes-cubes, octahedra, and rhombic dodecahedra-on substrates and investigate their thermally induced reorganization into two-dimensional (2D) crystalline films. We report two new low-density 2D structures, including a honeycomb lattice based on octahedral NPs. The low-density lattices favored here are not usually seen when particles are crystallized via other bottom-up assembly techniques. Furthermore, we show that, consistent with the complementary contact model, a primary driving force for crystallization is the formation of directional, face-to-face DNA bonds between neighboring NPs and between NPs and the substrate. Our results can be used to deliberately prepare crystalline NP films with novel morphologies.

Crystal structure engineering in multimetallic high-index facet nanocatalysts.

In the context of metal particle catalysts, composition, shape, exposed facets, crystal structure, and atom distribution dictate activity. While techniques have been developed to control each of these parameters, there is no general method that allows one to optimize all parameters in the context of polyelemental systems. Herein, by combining a solid-state, Bi-influenced, high-index facet shape regulation strategy with thermal annealing, we achieve control over crystal structure and atom distribution on the exposed high-index facets, resulting in an unprecedentedly diverse library of chemically disordered and ordered multimetallic (Pt, Co, Ni, Cu, Fe, and Mn) tetrahexahedral (THH) nanoparticles. Density functional theory calculations show that surface Bi modification stabilizes the {210} high-index facets of the nanoparticles, regardless of their internal atomic ordering. Moreover, we find that the ordering transition temperatures for the nanoparticles are dependent on their composition, and, in the case of Pt3Fe1 THH nanoparticles, increasing Ni substitution leads to an order-to-disorder transition at 900 °C. Finally, we have discovered that ordered intermetallic THH Pt1Co1 nanocatalysts exhibit a catalytic performance superior to disordered THH Pt1Co1 nanoparticles and commercial Pt/C catalysts toward methanol electrooxidation, highlighting the importance of crystal structure and atom distribution control on high-index facets in nanoscale catalysts.

Mie-Resonant Three-Dimensional Metacrystals.

Optical metamaterials, engineered to exhibit electromagnetic properties not found in natural materials, may enable new light-based applications including cloaking and optical computing. While there have been significant advances in the fabrication of two-dimensional metasurfaces, planar structures create nontrivial angular and polarization sensitivities, making omnidirectional operation impossible. Although three-dimensional (3D) metamaterials have been proposed, their fabrication remains challenging. Here, we use colloidal crystal engineering with DNA to prepare isotropic 3D metacrystals from Au nanocubes. We show that such structures can exhibit refractive indices as large as ∼8 in the mid-infrared, far greater than that of common high-index dielectrics. Additionally, we report the first observation of multipolar Mie resonances in metacrystals with well-formed habits, occurring in the mid-infrared for submicrometer metacrystals, which we measured using synchrotron infrared microspectroscopy. Finally, we predict that arrays of metacrystals could exhibit negative refraction. The results present a promising platform for engineering devices with unnatural optical properties.

High-Index-Facet Metal-Alloy Nanoparticles as Fuel Cell Electrocatalysts.

A method to introduce high-index facets into colloidally synthesized nanoparticles is used to produce compositionally uniform Pt-M (M = Ni, Co, and Cu) and Rh-M (M = Ni and Co) tetrahexahedral nanoparticles. The realization of this method allows for a systematic study of catalyst activity as a function of particle composition for various electrooxidation reactions of liquid fuels (formic acid, methanol, and ethanol). The individual contributions of their high-index facets, internal alloying of transition metals, and surface Bi modification to their electrocatalytic properties are experimentally explored, resulting in three key findings. First, the presence of high-index facets is favorable for improving the catalytic activity for all three classes of reactions studied. Second, the effect of transition metal alloying on catalytic activity differs from reaction to reaction. For methanol electrooxidation in an acid electrolyte, due to the contribution from surface Bi modification being negligible, transition metal alloying can significantly the improve overall catalytic efficiency. However, for the other studied reactions, where the surface Bi is highly favorable for improving catalytic activity, there is little influence from transition metal alloying. Finally, multimetallic tetrahexahedral particles have improved stabilities during prolonged operation compared to their monometallic counterparts due to the presence of the alloyed transition metal atoms.

Multimetallic High-Index Faceted Heterostructured Nanoparticles.

Multimetallic heterostructured nanoparticles with high-index facets potentially represent an important class of highly efficient catalysts. However, due to their complexity, they are often difficult to synthesize. Herein, a library of heterostructured, multimetallic (Pt, Pd, Rh, and Au) tetrahexahedral nanoparticles was synthesized through alloying/dealloying with Bi in a tube furnace at 900-1000 °C. Electron microscopy and selected area diffraction measurements show that the domains of the heterostructured nanoparticles are epitaxially aligned. Although nanoparticles formed from Au alone exhibit low-index facets, Pt and Au form PtAu heterostructured nanoparticles with high-index facets, including domains that are primarily made of Au. Furthermore, the alloying/dealloying of Bi occurs at different rates and under different conditions within the heterostructured nanoparticles. This influences the types of architectures observed en route to the final high-index state, a phenomenon clearly observable in the case of PdRhAu nanoparticles. Finally, scanning probe block copolymer lithography was used in combination with this synthetic strategy to control nanoparticle composition in the context of PtAu nanoparticles (1:4 to 4:1 ratio range) and size (15 to 45 nm range).

Tunable Fluorescence from Dye-Modified DNA-Assembled Plasmonic Nanocube Arrays.

Colloidal crystal engineering with DNA on template-confined surfaces is used to prepare arrays of nanocube-based plasmonic antennas and deliberately place dyes with sub-nm precision into their hotspots, on the DNA bonds that confine the cubes to the underlying gold substrate. This combined top-down and bottom-up approach provides independent control over both the plasmonic gap and photonic lattice modes of the surface-confined particle assemblies and allows for the tuning of the interactions between the excited dyes and plasmonically active antennas. Furthermore, the gap mode of the antennas can be modified in situ by utilizing the solvent-dependent structure of the DNA bonds. This is studied by placing two dyes, with different emission wavelengths, under the nanocubes and recording their solvent-dependent emission. It is shown that dye emission not only depends upon the in-plane structure of the antennas but also the size of the gap, which is regulated with solvent. Importantly, this approach allows for the systematic understanding of the relationship between nanoscale architecture and plasmonically coupled dye emission, and points toward the use of colloidal crystal engineering with DNA to create stimuli responsive architectures, which can find use in chemical sensing and tunable light sources.

A Cross-Linking Approach to Stabilizing Stimuli-Responsive Colloidal Crystals Engineered with DNA.

Two DNA-cross-linking reagents, bis-chloroethylnitrosourea and 8-methoxypsoralen, are used to covalently cross-link interstrand base pairs in DNA bonds that, in part, define colloidal crystals engineered with DNA. The irreversible linkages formed increase the chemical and thermal stability of the crystals and do not significantly affect their long-range order, as evidenced by small-angle X-ray scattering data. The post-modified crystals are stable in environments that the pre-modified structures are not, including solvents that encompass a broad range of polarities from ethanol to hexanes, and in aqueous media at pH 0 and 14. Interestingly, the cross-linked DNA bonds within these crystals still retain their flexibility, which is reflected by a solvent-dependent reversible change in lattice parameter. Since these organic cross-linking reagents, in comparison with inorganic approaches (use of silver ions or SiO2), have marginal effects on the composition and properties of the crystals, they provide an attractive alternative for stabilizing colloidal crystals engineered with DNA and make them potentially useful in a broader range of media.