Critical Undercooling in DNA-Mediated Nanoparticle Crystallization

The nucleation of DNA-functionalized nanoparticle superlattices is observed to exhibit a temperature hysteresis between melting (superlattice dissociation) and freezing (particle association) transitions that allows for the study of nucleation thermodynamics. Through detailed study of the assembly of these particles, which can be considered programmable atom equivalents (PAEs), we identify this hysteresis as critical undercoolinga phase transition phenomenon related to a thermodynamic barrier to nucleation. The separable nature of the DNA bonding elements and nanoparticle core enables the PAE platform to pose unique questions about the microscopic dependencies of critical undercooling and, ultimately, to control the nucleation pathway. Specifically, we find that the undercooling required to initiate nucleation increases as the nanoparticle coordination number increases (number of particles to which a single particle can bind)

High-Throughput, Algorithmic Determination of Nanoparticle Structure from Electron Microscopy Images

Electron microscopy (EM) represents the most powerful tool to directly characterize the structure of individual nanoparticles. Accurate descriptions of nanoparticle populations with EM, however, are currently limited by the lack of tools to quantitatively analyze populations in a high-throughput manner. Herein, we report a computational method to algorithmically analyze EM images that allows for the first automated structural quantification of heterogeneous nanostructure populations, with species that differ in both size and shape. This allows one to accurately describe nanoscale structure at the bulk level, analogous to ensemble measurements with individual particle resolution. With our described EM protocol and our inclusion of freely available code for our algorithmic analysis, we aim to standardize EM characterization of nanostructure populations to increase reproducibility, objectivity, and throughput in measurements. We believe this work will have significant implications in diverse research areas involving nanomaterials, including, but not limited to, fundamental studies of structural control in nanoparticle synthesis, nanomaterial-based therapeutics and diagnostics, optoelectronics, and catalysis

Universal Noble Metal Nanoparticle Seeds Realized Through Iterative Reductive Growth and Oxidative Dissolution Reactions

Control over nanoparticle shape and size is commonly achieved via a seed-mediated approach, where nanoparticle precursors, or seeds, are hypothesized to act as templates for the heterogeneous nucleation of anisotropic products. Despite the wide variety of shapes that have been produced via this approach, high yield and uniformity have been more difficult to achieve. These shortcomings are attributed to limited structural control and characterization of the initial distribution of seeds. Herein, we report how iterative reductive growth and oxidative dissolution reactions can be used to systematically control seed structural uniformity. Using these reactions, we verify that seed structure dictates anisotropic nanoparticle uniformity and show that iterative seed refinement leads to unprecedented noble metal nanoparticle uniformities and purities for eight different shapes produced from a single seed source. Because of this uniformity, the first nanoparticle optical extinction coefficients for these eight shapes were analytically determined

Locally Altering the Electronic Properties of Graphene by Nanoscopically Doping It with Rhodamine 6G

We show that Rhodamine 6G (R6G), patterned by dip-pen nanolithography on graphene, can be used to locally n-dope it in a controlled fashion. In addition, we study the transport and assembly properties of R6G on graphene and show that in general the π–π stacking between the aromatic components of R6G and the underlying graphene drives the assembly of these molecules onto the underlying substrate. However, two distinct transport and assembly behaviors, dependent upon the presence or absence of R6G dimers, have been identified. In particular, at high concentrations of R6G on the tip, dimers are transferred to the substrate and form contiguous and stable lines, while at low concentrations, the R6G is transferred as monomers and forms patchy, unstable, and relatively ill-defined features. Finally, Kelvin probe force microscopy experiments show that the local electrostatic potential of the graphene changes as function of modification with R6G; this behavior is consistent with local molecular doping, highlighting a path for controlling the electronic properties of graphene with nanoscale resolution

Modular and Chemically Responsive Oligonucleotide “Bonds” in Nanoparticle Superlattices

Chemical bonds are a key determinant of the structure and properties of a material. Thus, rationally designing arbitrary materials requires complete control over the bond. While atomic bonding is dictated by the identity of the atoms, nanoparticle superlattice engineering, where nanoparticle “atoms” are held together by DNA “bonds”, offers a route to design crystal lattices in a way that nature cannot: through altering the oligonucleotide bond. Herein, the use of RNA, as opposed to DNA, is explored by synthesizing superlattices in which nanoparticles are bonded by DNA/DNA, RNA/RNA, and DNA/RNA duplexes. By moving beyond nanoparticle superlattices assembled only with DNA, a new degree of freedom is introduced, providing programmed responsiveness to enzymes and greater bond versatility. Therefore, the oligonucleotide bond can have programmable function beyond dictating the structure of the material and moves nanoparticle superlattices closer to naturally occurring biomaterials, where the line between structural and functional elements is blurred

The Significance of Multivalent Bonding Motifs and “Bond Order” in DNA-Directed Nanoparticle Crystallization

Multivalent oligonucleotide-based bonding elements have been synthesized and studied for the assembly and crystallization of gold nanoparticles. Through the use of organic branching points, divalent and trivalent DNA linkers were readily incorporated into the oligonucleotide shells that define DNA-nanoparticles and compared to monovalent linker systems. These multivalent bonding motifs enable the change of “bond strength” between particles and therefore modulate the effective “bond order.” In addition, the improved accessibility of strands between neighboring particles, either due to multivalency or modifications to increase strand flexibility, gives rise to superlattices with less strain in the crystallites compared to traditional designs. Furthermore, the increased availability and number of binding modes also provide a new variable that allows previously unobserved crystal structures to be synthesized, as evidenced by the formation of a thorium phosphide superlattice

OWL-Based Nanomasks for Preparing Graphene Ribbons with Sub-10 nm Gaps

We report a simple and highly efficient method for creating graphene nanostructures with gaps that can be controlled on the sub-10 nm length scale by utilizing etch masks comprised of electrochemically synthesized multisegmented metal nanowires. This method involves depositing striped nanowires with Au and Ni segments on a graphene-coated substrate, chemically etching the Ni segments, and using a reactive ion etch to remove the graphene not protected by the remaining Au segments. Graphene nanoribbons with gaps as small as 6 nm are fabricated and characterized with atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. The high level of control afforded by electrochemical synthesis of the nanowires allows us to specify the dimensions of the nanoribbon, as well as the number, location, and size of nanogaps within the nanoribbon. In addition, the generality of this technique is demonstrated by creating silicon nanostructures with nanogaps