Characterization of Surface Patterning on Polymer-Grafted Nanocubes Using Atomic Force Microscopy and Force Volume Mapping.

Atomic force microscopy (AFM), in particular force spectroscopy, is a powerful tool for understanding the supramolecular structures associated with polymers grafted to surfaces, especially in regimes of low polymer density where different morphological structures are expected. In this study, we utilize force volume mapping to characterize the nanoscale surfaces of Ag nanocubes (AgNCs) grafted with a monolayer of polyethylene glycol (PEG) chains. Spatially resolved force-distance curves taken for a single AgNC were used to map surface properties, such as adhesion energy and deformation.

Surface-Grafted Biocompatible Polymer Conductors for Stable and Compliant Electrodes for Brain Interfaces.

Durable and conductive interfaces that enable chronic and high-resolution recording of neural activity are essential for understanding and treating neurodegenerative disorders. These chronic implants require long-term stability and small contact areas. Consequently, they are often coated with a blend of conductive polymers and are crosslinked to enhance durability despite the potentially deleterious effect of crosslinking on the mechanical and electrical properties.

Self-assembly of nanocrystal checkerboard patterns via non-specific interactions.

Checkerboard lattices-where the resulting structure is open, porous, and highly symmetric-are difficult to create by self-assembly. Synthetic systems that adopt such structures typically rely on shape complementarity and site-specific chemical interactions that are only available to biomolecular systems (e.g.

Molecular-Scale Visualization of Steric Effects of Ligand Binding to Reconstructed Au(111) Surfaces.

Direct imaging of single molecules at nanostructured interfaces is a grand challenge with potential to enable new, precise material architectures and technologies. Of particular interest are the structural morphology and spectroscopic signatures of the adsorbed molecule, where modern probes are only now being developed with the necessary spatial and energetic resolution to provide detailed information at the molecule-surface interface. Here, we directly characterize the adsorption of individual -terphenyl isocyanide ligands on a reconstructed Au(111) surface through scanning tunneling microscopy and inelastic electron tunneling spectroscopy.

Synthesis of PEDOT:PSS Brushes Grafted from Gold Using ATRP for Increased Electrochemical and Mechanical Stability.

We report PEDOT:PSS brushes grafted from gold using surface-initiated atom-transfer radical polymerization (SI-ATRP) which demonstrate significantly enhanced mechanical stability against sonication and electrochemical cycling compared to spin-coated analogues as well as lower impedances than bare gold at frequencies from 0.1 to 10 Hz. These results suggest SI-ATRP PEDOT:PSS to be a promising candidate for use in microelectrodes for neural activity recording.

Hyperbolic material enhanced scattering nanoscopy for label-free super-resolution imaging.

Fluorescence super-resolution microscopy has, over the last two decades, been extensively developed to access deep-subwavelength nanoscales optically. Label-free super-resolution technologies however have only achieved a slight improvement compared to the diffraction limit. In this context, we demonstrate a label-free imaging method, i.

Curvature-Selective Nanocrystal Surface Ligation Using Sterically-Encumbered Metal-Coordinating Ligands.

Organic ligands are critical in determining the physiochemical properties of inorganic nanocrystals. However, precise nanocrystal surface modification is extremely difficult to achieve. Most research focuses on finding ligands that fully passivate the nanocrystal surface, with an emphasis on the supramolecular structure generated by the ligand shell.

Metasurface-Enhanced Raman Spectroscopy (mSERS) for Oriented Molecular Sensing.

Surface-enhanced Raman spectroscopy (SERS) is a widely used sensing technique for ultrasensitivity chemical sensing, biomedical detection, and environmental analysis. Because SERS signal is proportional to the fourth power of the local electric field, several SERS applications have focused on the design of plasmonic nanogaps to take advantage of the extremely strong near-field enhancement that results from plasmonic coupling, but few designs have focused on how SERS detection is affected by molecular orientation within these nanogaps. Here, we demonstrate a nanoparticle-on-metal metasurface designed for near-perfect optical absorption as a platform for Raman detection of highly oriented molecular analytes, including two-dimensional materials and aromatic molecules.

Highly-efficient electrically-driven localized surface plasmon source enabled by resonant inelastic electron tunneling.

On-chip plasmonic circuitry offers a promising route to meet the ever-increasing requirement for device density and data bandwidth in information processing. As the key building block, electrically-driven nanoscale plasmonic sources such as nanoLEDs, nanolasers, and nanojunctions have attracted intense interest in recent years. Among them, surface plasmon (SP) sources based on inelastic electron tunneling (IET) have been demonstrated as an appealing candidate owing to the ultrafast quantum-mechanical tunneling response and great tunability.

Imaging of Nanoscale Light Confinement in Plasmonic Nanoantennas by Brownian Optical Microscopy.

The strongly enhanced and confined subwavelength optical fields near plasmonic nanoantennas have been extensively studied not only for the fundamental understanding of light-matter interactions at the nanoscale but also for their emerging practical application in enhanced second harmonic generation, improved inelastic electron tunneling, harvesting solar energy, and photocatalysis. However, owing to the deep subwavelength nature of plasmonic field confinement, conventional optical imaging techniques are incapable of characterizing the optical performance of these plasmonic nanoantennas. Here, we demonstrate super-resolution imaging of ∼20 nm optical field confinement by monitoring randomly moving dye molecules near plasmonic nanoantennas.

Large optical nonlinearity enabled by coupled metallic quantum wells.

New materials that exhibit strong second-order optical nonlinearities at a desired operational frequency are of paramount importance for nonlinear optics. Giant second-order susceptibility has been obtained in semiconductor quantum wells (QWs). Unfortunately, the limited confining potential in semiconductor QWs causes formidable challenges in scaling such a scheme to the visible/near-infrared (NIR) frequencies for more vital nonlinear-optic applications.

Supramolecular Assembly of Single-Source Metal-Chalcogenide Nanocrystal Precursors.

In this feature article, we discuss our recent work in the synthesis of novel supramolecular precursors for semiconductor nanocrystals. Metal chalcogenolates that adopt liquid-crystalline phases are employed as single-source precursors that template the growth of shaped solid-state nanocrystals. Supramolecular assembly is programmed by both precursor chemical composition and molecular parameters such as the alkyl chain length, steric bulk, and the intercalation of halide ions.

Colloidal Plasmonic Nanocomposites: From Fabrication to Optical Function.

Plasmonic nanostructures are extensively used building blocks for engineering optical materials and device architectures. Plasmonic nanocomposites (pNCs) are an emerging class of materials that integrate these nanostructures into hierarchical and often multifunctional systems. These pNCs can be highly customizable by modifying both the plasmonic and matrix components, as well as by controlling the nano- to macroscale morphology of the composite as a whole.

Photoemission-based microelectronic devices.

The vast majority of modern microelectronic devices rely on carriers within semiconductors due to their integrability. Therefore, the performance of these devices is limited due to natural semiconductor properties such as band gap and electron velocity. Replacing the semiconductor channel in conventional microelectronic devices with a gas or vacuum channel may scale their speed, wavelength and power beyond what is available today.

Digenite Nanosheets Synthesized by Thermolysis of Layered Copper-Alkanethiolate Frameworks.

Copper sulfide nanocrystals support localized surface plasmon resonances in the near-infrared wavelengths and have significant potential as active plasmonic nanomaterials due to the tunability of this optical response. While numerous strategies exist for synthesizing copper sulfide nanocrystals, few methods result in nanocrystals with both controlled morphological shapes and crystallinity. Here, we synthesize and characterize ultrathin (<5 nm) CuS nanosheets that are formed by solventless thermolysis, utilizing Cu alkanethiolates as single-source precursors.

Colloidal Nanoantennas for Hyperspectral Chemical Mapping.

Tip-enhanced Raman spectroscopy enables access to chemical information with nanoscale spatial resolution and single-molecule sensitivities by utilizing optical probes that are capable of confining light to subwavelength dimensions. Because the probes themselves possess nanoscale features, they are notoriously difficult to fabricate, and more critically, can result in poor reproducibility. Here, we demonstrate high-performance, predictable, and readily tunable nanospectroscopy probes that are fabricated by self-assembly.

Investigating the effect of Ag nanocube polydispersity on gap-mode SERS enhancement factors.

High Raman enhancement factors (EFs) have been observed for surface-enhanced Raman spectroscopy (SERS) substrates fabricated from colloidal metal nanoparticles. Electrodynamic models of single nanoparticles often do not accurately predict the Raman EFs measured experimentally for such colloidal substrates, which often consist of nanoparticles that exhibit heterogeneity in both size and shape. Here, we investigate the size and shape dispersity of colloidal Ag nanocube samples and their effect on Raman EF.

Dynamics of nanoparticle assembly from disjointed images of nanoparticle-polymer composites.

Understanding how nanoparticles (NPs) diffuse, stick, and assemble into larger structures within polymers is key to the design and fabrication of NP-polymer composites. Here we describe an approach for inferring the dynamic parameters of NP assembly from spatially and temporally disjointed images of composites. The approach involves iterative adjustment of the parameters of a kinetic model of assembly until the computed size statistics of NP clusters match those obtained from high-throughput analysis of the experimental images.

Modular, polymer-directed nanoparticle assembly for fabricating metamaterials.

We achieve the fabrication of plasmonic meta-atoms by utilizing a novel, modular approach to nanoparticle self-assembly that utilizes polymer templating to control meta-atom size and geometry. Ag nanocubes are deposited and embedded into a polymer thin-film, where the polymer embedding depth is used to dictate which nanocube faces are available for further nanocrystal binding. Horizontal and vertical nanocube dimers were successfully fabricated with remarkably high yield using a bifunctional molecular linker to bind a second nanocube.

Computationally Guided Assembly of Oriented Nanocubes by Modulating Grafted Polymer-Surface Interactions.

The bottom-up fabrication of ordered and oriented colloidal nanoparticle assemblies is critical for engineering functional nanomaterials beyond conventional polymer-particle composites. Here, we probe the influence of polymer surface ligands on the self-orientation of shaped metal nanoparticles for the formation of nanojunctions. We examine how polymer graft-surface interactions dictate Ag nanocube orientation into either edge-edge or face-face nanojunctions.

Colloidal metasurfaces displaying near-ideal and tunable light absorbance in the infrared.

Metasurfaces are ultrathin, two-dimensional arrays of subwavelength resonators that have been demonstrated to control the flow of light in ways that are otherwise unattainable with natural materials. These arrays are typically composed of metallic Ag or Au nanostructures shaped like split rings, nanowire pairs or nanorods (commonly referred to as meta-atoms) that are arranged to produce a collective optical response spanning an impressive range of properties, from the perfect absorption of incident light to superresolution imaging. However, metasurfaces pose major challenges in their fabrication over large areas, which can be prohibitively expensive and time consuming using conventional nanolithography techniques.

Automated quantitative image analysis of nanoparticle assembly.

The ability to characterize higher-order structures formed by nanoparticle (NP) assembly is critical for predicting and engineering the properties of advanced nanocomposite materials. Here we develop a quantitative image analysis software to characterize key structural properties of NP clusters from experimental images of nanocomposites. This analysis can be carried out on images captured at intermittent times during assembly to monitor the time evolution of NP clusters in a highly automated manner.