Dhriti Nepal

High‐Yield Assembly of Soluble and Stable Gold Nanorod Pairs for High‐Temperature Plasmonics

Colloidal synthetic approaches to discrete, soluble plasmonic architectures, such as nanorod pairs, offer numerous advantages relative to lithographic techniques, including compositionally asymmetric structures, atomically smooth surfaces, and continuous fabrication. Density-driven colloidal assembly, such as by solvent evaporation, produces some intriguing structures, e.g., particle chains; however, controllability and post-processibility of the final architecture is inadequate. Also the limited quantity of product nominally comprises a broad distribution of assembly size and type. Herein, the high-yield formation of soluble, stable, and compositionally discrete gold nanorod (Au NR) architectures by inducing-then arresting-flocculation is demonstrated using bifunctional nanorods and reversible modulation of solvent quality to deplete and reassemble an electrostatic stabilization layer, thereby eliminating the need for an additional encapsulant. Analogous to dimer formation during step-growth polymerization, the initial yield of Au nanorod side-by-side pairs can be greater than 50%. The high solubility and stability of the assembly enable purification, scale-up of nanomolarity solutions, and subsequent chemical modification of the assembled product. As an example, in situ silica deposition via Stöber synthesis onto the assembled pair produces highly processable nanostructures with a single pair of embedded Au NRs at their center, which exhibit thermal stability at temperatures in excess of 700 °C.

Surface Assembly and Plasmonic Properties in Strongly Coupled Segmented Gold Nanorods

An assembly strategy is reported such that segmented nanorods fabricated through template-assisted methods can be robustly transferred and tethered to a pre-functionalized substrate with excellent uniformity over large surface areas. After embedding the rods, sacrificial nickel segments were selectively etched leaving behind strongly coupled segmented gold nanorods with gaps between rods below 40 nm and as small as 2 nm. Hyper-spectral imaging is utilized to measure Rayleigh scattering spectra from individual and coupled nanorod elements in contrast to common bulk measurements. This approach discerns the effects of not only changing segment and gap size but also the presence of characteristic defects on the plasmonic coupling between closely spaced nanorods. Polarized hyper-spectral measurements are conducted to provide direct observation of the anisotropic plasmonic resonance modes in individual and coupled nanorods, which are close to those predicted by computer simulations for nanorods with ideal shapes. Some common deviations from ideal shape such as non-flat facets and asymmetric tails are demonstrated to result in the appearance of characteristic plasmon resonances, which have not been considered before. The large-scale assembly of coupled noble nanostructures with fine control over geometry and high uniformity provides means to strongly tune the scattering, absorption, and near-field plasmonic properties through the geometric arrangement of precisely controlled nanorod segments.

Orientation Sensing with Color Using Plasmonic Gold Nanorods and Assemblies

Colorimetric analysis of broadband illumination scattered from isolated gold nanorods and reduced symmetry Dolmen structures provide a visible measure of the local nanoscale orientation of the nanostructures relative to the laboratory frame of reference. Polarized dark-field scattering microscopy correlated with scanning electron microscopy of low and high aspect ratio gold nanorods demonstrated accuracies of 2.3 degrees, which is a 5-fold improvement over photothermal and defocused imaging methods. By assigning the three color channels of the imaging detector (red, green, and blue) to the plasmon resonance wavelengths of the nanostructure, the quantitative display of orientation improved by 200%. The reduced symmetry of a gold nanorod Dolmen structure further improved the sensitivity of colorimetric orientation by a factor of 2 due to the comparative intensities of the resonances. Thus the simplicity, high accuracy, and sensitivity of visual colorimetric sensing of local nanoscale orientation holds promise for high throughput, inexpensive structure and dynamics studies in biology and material science.

The use of carbon nanotube yarn as a filter medium to treat nitroaromatic-contaminated water

Abstract Carbon nanotube yarn (CNTY) is a promising material for the removal of organic contaminants from aqueous waste streams owing to its extraordinary mechanical strength, chemical stability, thermal stability and high surface area. CNTY was used to treat water contaminated with a model nitroaromatic compound, 2,4-dinitrotoluene (DNT). The isotherms and kinetics of DNT adsorption onto CNTY were investigated. The adsorption capacities of DNT were compared with the literature values of alternative sorbents. SEM-EDX, HRTEM, Raman spectroscopy and XPS were used to characterize the size, surface morphology and surface chemistry of the CNTY before and after DNT adsorption. Results indicate that adsorption isotherm of DNT onto CNTY could be fitted by the Freundlich isotherm with a Freundlich constant, KF, of 55.0 mg/g (L/mg)1/n and a Freundlich exponent, 1/n, of 0.737. Adsorption kinetics can be formulated by the pseudo-second order kinetic model. This study demonstrates the ability of CNTY to remove organic contaminants from water.

Prediction of Incipient Nano-Scale Rupture for Thermosets in Plane Stress

There is limited experimental evidence that fracture nucleation in polymers includes a small number of covalent bond scissions followed by rapid void growth by chemo-mechanical processes. Generalized criteria for predicting such bond scission, then, would help anticipate fracture in polymer matrix composites. Strain states at incipient bond scission for thermoset resins in plane stress are here predicted by atomistic simulation. Several cured epoxy systems were examined, each having a different chain length. For biaxial extension and a portion of the shearing regime, scission occurs at a critical value of the larger principal strain. This value increases with increasing chain length. The corresponding dilatation is largest for biaxial extension and decreases to nearly zero for pure shear. Results are compared with strain invariants at fracture measured from experiments in which polymer matrix composites having various ply stacking sequences were loaded to rupture.

Coherent plasmonic engineering in self-assembled reduced symmetry nanostructures

Multiple coherent effects including Fano resonances are observed in self-assembled reduced symmetry gold nanorod systems, in particular Dolmen configurations. The bottom-up chemical method provides high quality units and assemblies (single crystal with low surface roughness and sub 5 nm gaps) that reduce radiative losses from the plasmonic structures. Multiple dark and bright plasmonic resonances are observed in optical dark-field scattering measurements and electron energy loss spectroscopy. These high fidelity structures and narrow resonances are promising for future design of high figure of merit sensors, ultrafast switches and slow light devices for optical information processing.

Colorimetric polarization sensing with single plasmonic gold nanorods

The color of scattered light from longitudinal and transverse surface plasmon resonances of individual gold nanorods is used to detect the polarization direction of incident light at the nanoscale. The relative strength of the scattered intensities of the two resonances reflects the relative orientation between the polarization of incident light and the nanorod. The resultant colored spectrum is used as a metric for polarization sensing in a darkfield geometry. This technique is demonstrated in the visible to near infrared region by varying the aspect ratio of the nanorods between 2 and 5 with diameters less than 20 nm. The ability to determine the polarization of light visually at the nanoscale provides an important tool in material science and molecular biology for probing anisotropic material properties at the nanoscale using single nanorods. In contrast to photothermal imaging where laser induced deformation of nanoparticles occur, this bimodal darkfield scattering is non-destructive and internally calibrated. The tunability of the plasmonic bands by varying the aspect ratio is beneficial for the usage of this method over a broad spectral range.