Multilength-Scale Chemical Patterning of Self-Assembled Monolayers by Spatially Controlled Plasma Exposure: Nanometer to Centimeter Range

Effect of Ionic Strength, Nanoparticle Surface Charge Density, and Template Diameter on Self-Limiting Single-Particle Placement: A Numerical Study

For the bottom-up approach where functional materials are constructed out of nanoscale building blocks (e.g., nanoparticles), it is essential to have methods that are capable of placing the individual nanoscale building blocks onto exact substrate positions on a large scale and on a large area. One of the promising placement methods is the self-limiting single-particle placement (SPP), in which a single nanoparticle in a colloidal solution is electrostatically guided by electrostatic templates and exactly one single nanoparticle is placed on each target position in a self-limiting way. This paper presents a numerical study on SPP, where the effects of three key parameters, (1) ionic strength (IS), (2) nanoparticle surface charge density (σ_NP), and (3) circular template diameter (d), on SPP are investigated. For 40 different parameter sets of (IS, σ_NP, d), a 30 nm nanoparticle positioned at R⃗ above the substrate was modeled in two configurations (i) without and (ii) with the presence of a 30 nm nanoparticle at the center of a circular template. For each parameter set and each configuration, the electrostatic potentials were calculated by numerically solving the Poisson-Boltzmann equation, from which interaction forces and interaction free energies were subsequently calculated. These have identified realms of parameter sets that enable a successful SPP. A few exemplary parameter sets include (IS, σ_NP, d) = (0.5 mM, -1.5 μC/cm², 100 nm), (0.05 mM, -0.5 μC/cm², 100 nm), (0.5 mM, -1.5 μC/cm², 150 nm), and (0.05 mM, -0.8 μC/cm², 150 nm). This study provides clear guidance toward experimental realizations of large-scale and large-area SPPs, which could lead to bottom-up fabrications of novel electronic, photonic, plasmonic, and spintronic devices and sensors.

One Nanometer Wide Functional Patterns with a Sub-10 Nanometer Pitch Transferred to an Amorphous Elastomeric Material

Decades of work in surface science have established the ability to functionalize clean inorganic surfaces with sub-nm precision, but for many applications, it would be useful to provide similar control over the surface chemistry of amorphous materials such as elastomers. Here, we show that striped monolayers of diyne amphiphiles, assembled on graphite and photopolymerized, can be covalently transferred to polydimethylsiloxane (PDMS), an elastomer common in applications including microfluidics, soft robotics, wearable electronics, and cell culture. This process creates precision polymer films <1 nm thick, with 1 nm wide functional patterns, which control interfacial wetting and reactivity, and template adsorption of flexible, ultranarrow Au nanowires. The polydiacetylenes exhibit polarized fluorescence emission, revealing polymer location, orientation, and environment, and resist engulfment, a common problem in PDMS functionalization. These findings illustrate a route for patterning surface chemistry below the length scale of heterogeneity in an amorphous material.

Vapor Annealing and Colloid Lithography: An Effective Tool To Control Spatial Resolution of Surface Modification

Colloid lithography represents a simple and efficient method for creation of a large-scale template for subsequent surface patterning, deposition of regular metal nanostructures, or periodical surface structures. However, this method is significantly restricted by its ability to create only a limited number of structures with confined geometry and symmetry features. To overcome this limitation, different techniques, such as plasma treatment or tilting angle metal deposition, have been proposed. In this paper, an alternative method based on the vapor annealing of ordered single polystyrene (PS) microspheres layer, followed by the surface grafting with arenediazonium tosylates is proposed. Application of vapor treatment before surface grafting allows effective control of the area screened by PS microspheres. Pristine and vapor-annealed microsphere arrays on the gold substrate were electrochemically modified using ADTs. Subsequent removal of the PS microsphere mask enabled to prepare well-defined nanostructures with controllable surface features. In particular, prepared periodic arrangements were achieved by the grafting of OFGs to the empty interspaces between nanopore arrays. The process of sample preparation was controlled, and the properties of prepared structures were characterized by various techniques, including atomic force microscopy (AFM), conductive AFM, scanning electron microscopy energy-dispersive X-ray spectrometry, Raman spectroscopy, and voltammetry.

Ultra-fast responsive colloidal-polymer composite-based volatile organic compounds (VOC) sensor using nanoscale easy tear process

There is an immense need for developing a simple, rapid, and inexpensive detection assay for health-care applications or monitoring environments. To address this need, a photonic crystal (PC)-based sensor has been extensively studied due to its numerous advantages such as colorimetric measurement, high sensitivity, and low cost. However, the response time of a typical PC-based sensor is relatively slow due to the presence of the inevitable upper residual layer in colloidal structures. Hence, we propose an ultra-fast responsive PC-based volatile organic compound (VOC) sensor by using a “nanoscale easy tear (NET) process” inspired by commercially available “easy tear package”. A colloidal crystal-polydimethylsiloxane (PDMS) composite can be successfully realized through nanoscale tear propagation along the interface between the outer surface of crystallized nanoparticles and bulk PDMS. The response time for VOC detection exhibits a significant decrease by allowing the direct contact with VOCs, because of perfect removal of the residual on the colloidal crystals. Moreover, vapor-phase VOCs can be monitored, which had been previously impossible. High-throughput production of the patterned colloidal crystal–polymer composite through the NET process can be applied to other multiplexed selective sensing applications or may be used for nanomolding templates.

Hybrid Sol-Gel-Derived Films That Spontaneously Form Complex Surface Topographies

Surface patterns over multiple length scales are known to influence various biological processes. Here we report the synthesis and characterization of new, two-component xerogel thin films derived from carboxyethylsilanetriol (COE) and tetraethoxysilane (TEOS). Atomic force microscopy (AFM) reveals films surface with branched and hyper branched architectures that are ∼2 to 30 μm in diameter, that extend ∼3 to 1300 nm above the film base plane with surface densities that range from 2 to 77% surface area coverage. Colocalized AFM and Raman spectroscopy show that these branched structures are COE-rich domains, which are slightly stiffer (as shown from phase AFM imaging) and exhibit lower capacitive force in comparison with film base plane. Raman mapping reveals there are also discrete domains (≤300 nm in diameter) that are rich in COE dimers and densified TEOS, which do not appear to correspond with any surface structure seen by AFM.

Nanomanipulation and controlled self-assembly of metal nanoparticles and nanocrystals for plasmonics

Localized surface plasmon resonances (LSPRs) associated with metallic nanostructures offer unique possibilities for light concentration beyond the diffraction limit, which can lead to strong field confinement and enhancement in deep subwavelength regions. In recent years, many transformative plasmonic applications have emerged, taking advantage of the spectral and spatial tunability of LSPRs enabled by near-field coupling between constituent metallic nanostructures in a variety of plasmonic metastructures (dimers, metamolecules, metasurfaces, metamaterials, etc.). For example, the "hot spot" formed at the interstitial site (gap) between two coupled metallic nanostructures in a plasmonic dimer can be spectrally tuned via the gap size. Capitalizing on these capabilities, there have been significant advances in plasmon enhanced or enabled applications in light-based science and technology, including ultrahigh-sensitivity spectroscopies, light energy harvesting, photocatalysis, biomedical imaging and theranostics, optical sensing, nonlinear optics, ultrahigh-density data storage, as well as plasmonic metamaterials and metasurfaces exhibiting unusual linear and nonlinear optical properties. In this review, we present two complementary approaches for fabricating plasmonic metastructures. We discuss how meta-atoms can be assembled into unique plasmonic metastructures using a variety of nanomanipulation methods based on single- or multiple-probes in an atomic force microscope (AFM) or a scanning electron microscope (SEM), optical tweezers, and focused electron-beam nanomanipulation. We also provide a few examples of nanoparticle metamolecules with designed properties realized in such well-controlled plasmonic metastructures. For the spatial controllability on the mesoscopic and macroscopic scales, we show that controlled self-assembly is the method of choice to realize scalable two-dimensional, and three-dimensional plasmonic metastructures. In the section of applications, we discuss some key examples of plasmonic applications based on individual hot spots or ensembles of hot spots with high uniformity and improved controllability.

Surface micropatterning with zirconia and calcium phosphate ceramics by micromoulding in capillaries

Micropatterning of silicon surface with bioinert yttria-stabilised zirconia or bioactive calcium phosphate ceramic by micromoulding in capillaries.

Probing nanoscale chemical segregation and surface properties of antifouling hybrid xerogel films

Over the past decade there has been significant development in hybrid polymer coatings exhibiting tunable surface morphology, surface charge, and chemical segregation-all believed to be key properties in antifouling (AF) coating performance. While a large body of research exists on these materials, there have yet to be studies on all the aforementioned properties in a colocalized manner with nanoscale spatial resolution. Here, we report colocalized atomic force microscopy, scanning Kelvin probe microscopy, and confocal Raman microscopy on a model AF xerogel film composed of 1:9:9 (mol:mol:mol) 3-aminopropyltriethoxysilane (APTES), n-octyltriethoxysilane (C8), and tetraethoxysilane (TEOS) formed on Al2O3. This AF film is found to consist of three regions that are chemically and physically unique in 2D and 3D across multiple length scales: (i) a 1.5 μm thick base layer derived from all three precursors; (ii) 2-4 μm diameter mesa-like features that are enriched in free amine (from APTES), depleted in the other species and that extend 150-400 nm above the base layer; and (iii) 1-2 μm diameter subsurface inclusions within the base layer that are enriched in hydrogen-bonded amine (from APTES) and depleted in the other species.

Spatial resolution in thin film deposition on silicon surfaces by combining silylation and UV/ozonolysis

A simple procedure has been developed for the processing of silicon wafers in order to facilitate the spatially resolved growth of thin solid films on their surfaces. Specifically, a combination of silylation and UV/ozonolysis was tested as a way to control the concentration of the surface hydroxo groups required for subsequent atomic layer deposition (ALD) of metals or oxides. Water contact angle measurements were used to evaluate the hydrophilicity/hydrophobicity of the surface, a proxy for OH surface coverage, and to optimize the UV/ozonolysis treatment. Silylation with hexamethyldisilazane, trichloro(octadecyl)silane, or trimethylchlorosilane was found to be an efficient way to block the hydroxo sites and to passivate the underlying surface, and UV/O3 treatments were shown to effectively remove the silylation layer and to regain the surface reactivity. Both O3 and 185 nm UV radiation were determined necessary for the removal of the silylation layer, and additional 254 nm radiation was found to enhance the process. Attenuated total reflection-infrared absorption spectroscopy was employed to assess the success of the silylation and UV/O3 removal steps, and atomic force microscopy data provided evidence for the retention of the original smoothness of the surface. Selective growth of HfO2 films via TDMAHf + H2O ALD was seen only on the UV/O3 treated surfaces; total inhibition of the deposition was observed on the untreated silylated surfaces (as determined by x-ray photoelectron spectroscopy and ellipsometry). Residual film growth was still detected on the latter if the ALD was carried out at high temperatures (250 °C), because the silylation layer deteriorates under such harsh conditions and forms surface defects that act as nucleation sites for the growth of oxide grains (as identified by electron microscopy and scanning electron microscopy). We believe that the silylation-UV/O3 procedure advanced here could be easily implemented for the patterning of surfaces in many microelectronic applications.

Tunable surface plasmon resonance frequencies of monodisperse indium tin oxide nanoparticles by controlling composition, size, and morphology

Monodisperse indium tin oxide nanoparticles (ITO NPs) with high crystallinity have been synthesized by the rapid thermal injection method and the seed-mediated growth method. We demonstrate that the surface plasmon resonance (SPR) frequencies of ITO NPs can be manipulated from 1,600 to 1,993 nm in near-infrared band by controlling the composition, size, and morphology. The doping Sn concentration in ITO NPs could be controlled via changing the %Sn in the initial feed from 0% to 30%. The shortest SPR wavelength at 1,600 nm with 10% Sn doping concentration indicates highest free electron carrier concentration in ITO NPs, which has direct relationship with doping Sn(4+) ions. Furthermore, we demonstrate that the SPR peaks can also be tuned by the size of ITO NPs in the case of uniform doping. Besides, compared with the ITO NPs, single crystalline ITO with nanoflower morphology synthesized through the one-pot method exhibit SPR absorption peak features of red-shifting and broadening.

An aptamer based wall-less LSPR array chip for label-free and high throughput detection of biomolecules

Despite recent progress in localized surface plasmon resonance (LSPR) based bio-sensing, it remains challenging to achieve sensitive and high throughput LSPR detection with facilities available in common laboratories. Here we developed a wall-less LSPR array chip for facile, label-free and high throughput detection of biomolecules using a normal microplate reader. The wall-less LSPR array chip was fabricated by immobilizing plasmonic nanoparticles (NPs) on a hydrophilic-hydrophobic patterned glass slide, enabling high throughput detection. The wall-less configuration simplifies chip fabrication and sample processing, and enables miniaturization to significantly reduce sample and reagent consumption. A double-gold NPs enhanced system comprising of 13-nm-gold NPs conjugated to aptamer modified 39-nm-gold NPs on glass substrate was adopted to constitute competitive replacement assay for signal amplification in small molecule (i.e. ATP) detection. Upon enhancement, the detection sensitivity of ATP was augmented by 5 orders of magnitude from 0.01 µM to 100 µM measured by the laboratory microplate reader. The wall-less LSPR sensor chip can be widely applied for miniaturized and high throughput detection of a variety of targets in biomedical applications and environmental monitoring using facilities available in common laboratories.

Large area resist-free soft lithographic patterning of graphene

Large area low-cost patterning is a challenging problem in graphene research. A resist-free, single-step, large area and cost effective soft lithographic patterning strategy is presented for graphene. The technique is applicable on any arbitrary substrate that needs to be covered with a graphene film and provides a viable route to large-area patterning of graphene for device applications.

Solution-based adaptive parallel patterning by laser-induced local plasmonic surface defunctionalization

Adaptive mass fabrication method based on laser-induced plasmonic local surface defunctionalization was suggested to realize solution-based high resolution self-patterning on transparent substrate in parallel. After non-patterned functional monolayer was locally deactivated by laser-induced metallic plasma species, various micro/nano metal structures could be simultaneously fabricated by the parallel self-selective deposition of metal nanoparticles on a specific region. This method makes the eco-friendly and cost-effective production of high resolution pattern possible. Moreover, it can respond to design change actively due to the broad controllable range and easy change of key patterning specifications such as a resolution (subwavelength~100 μm), thickness (100 nm~6 μm), type (dot and line), and shape.

Secrets revealed — Spatially selective wetting of plasma-patterned periodic mesoporous organosilica

We report a simple method to pattern wetting properties on thin films of periodic mesoporous organosilica (PMO). A hydrophobic methane PMO thin film was covered by masks and exposed to oxygen plasma to make the unmasked area hydrophilic. The wettability patterns could be revealed only when the films were immersed in water or exposed to moisture. We expect that our method would extend the utility of PMO to such areas as sensing and information security.

Bottom-up assembly of colloidal gold and silver nanostructures for designable plasmonic structures and metamaterials

We report on bottom-up assembly routes for fabricating plasmonic structures and metamaterials composed of colloidal gold and silver nanostructures, such as nanoparticles ("metatoms") and shape-controlled nanocrystals. Owing to their well-controlled sizes/shapes, facile surface functionalization, and excellent plasmonic properties in the visible and near-infrared regions, these nanoparticles and nanocrystals are excellent building blocks of plasmonic structures and metamaterials for optical applications. Recently, we have utilized two kinds of bottom-up techniques (i.e., multiple-probe-based nanomanipulation and layer-by-layer self-assembly) to fabricate strongly coupled plasmonic dimers, one-dimensional (1D) chains, and large-scale two-dimensional/three-dimensional (2D/3D) nanoparticle supercrystals. These coupled nanoparticle/nanocrystal assemblies exhibit unique and tunable plasmonic properties, depending on the material composition, size/shape, intergap distance, the number of composing nanoparticles/nanocrystals (1D chains), and the nanoparticle layer number in the case of 3D nanoparticle supercrystals. By studying these coupled nanoparticle/nanocrystal assemblies, the fundamental plasmonic metamaterial effects could be investigated in detail under well-prepared and previously unexplored experimental settings.

Polyelectrolyte multilayers as a platform for luminescent nanocrystal patterned assemblies

The fabrication of uniform and patterned nanocrystal (NC) assemblies has been investigated by exploiting the possibility of carefully tailoring colloidal NC surface chemistry and the ability of polyelectrolyte (PE) to functionalize substrates through an electrostatic layer-by-layer (LbL) strategy. Appropriate deposition conditions, substrate functionalization, and post-preparative treatments were selected to tailor the substrate surface chemistry to effectively direct the homogeneous electrostatic-induced assembly of NCs. Water-dispersible luminescent NCs, namely, (CdSe)ZnS and CdS, were differently functionalized by (1) ligand-exchange reaction, (2) growth of a hydrophilic silica shell, and (3) formation of a hydrophilic inclusion complex, thus providing functional NCs stable in a defined pH range. The electrostatically charged functional NCs represent a comprehensive selection of examples of surface-functionalized NCs, which enables the systematic investigation of experimental parameters in NC assembly processes carried out by combining LbL procedures with microcontact printing and also exploiting NC emission, relevant for potential applications, as a prompt and effective probe for evaluating assembly quality. Thus, an ample showcase of combinations has been investigated, and the spectroscopic and morphological features of the resulting NC-based structures have been discussed.

Wetting in color: colorimetric differentiation of organic liquids with high selectivity

Colorimetric litmus tests such as pH paper have enjoyed wide commercial success due to their inexpensive production and exceptional ease of use. Expansion of colorimetry to new sensing paradigms is challenging because macroscopic color changes are seldom coupled to arbitrary differences in the physical/chemical properties of a system. Here we present in detail the design of a "Wetting In Color Kit" (WICK), an inexpensive and highly selective colorimetric indicator for organic liquids that exploits chemically encoded inverse-opal photonic crystals to project minute differences in liquids' wettability to macroscopically distinct, easy-to-visualize structural color patterns. We show experimentally and corroborate with theoretical modeling using percolation theory that the highly symmetric structure of our large-area, defect-free SiO(2) inverse-opal films leads to sharply defined threshold wettability for liquid infiltration, occurring at intrinsic contact angles near 20° with an estimated resolution smaller than 5°. The regular structure also produces a bright iridescent color, which disappears when infiltrated with liquid, naturally coupling the optical and fluidic responses. To deterministically design a WICK that differentiates a broad range of liquids, we introduced a nondestructive quality control procedure to regulate the pore structure and developed two new surface modification protocols, both requiring only silanization and selective oxidation. The resulting tunable, built-in horizontal and vertical chemistry gradients let us tailor the wettability threshold to specific liquids across a continuous range. With patterned oxidation as a final step, we control the shape of the liquid-specific patterns displayed, making WICK easier to read. Using these techniques, we demonstrate the applicability of WICKs in several exemplary systems that colorimetrically distinguish (i) ethanol-water mixtures varying by only 2.5% in concentration; (ii) methanol, ethanol, and isopropyl alcohol; (iii) hexane, heptane, octane, nonane, and decane; and (iv) samples of gasoline (regular unleaded) and diesel. As wetting is a generic fluidic phenomenon, we envision that WICK could be suitable for applications in authentication or identification of unknown liquids across a broad range of industries.

Nanoscale patterning of organosilane molecular thin films from the gas phase and its applications: fabrication of multifunctional surfaces and large area molecular templates for site-selective material deposition

A simple methodology to fabricate micrometer- and nanometer-scale patterned surfaces with multiple chemical functionalities is presented. Patterns with lateral dimensions down to 110 nm were made. The fabrication process involves multistep gas-phase patterning of amine, thiol, alkyl, and fluorinated alkyl-functional organosilane molecules using PDMS molds as shadow masks. Also, a combination process of channel diffused plasma etching of organosilane molecular thin films in combination with masked gas-phase deposition to fabricate multilength scale, multifunctional surfaces is demonstrated.

Patterning functional materials using channel diffused plasma-etched self-assembled monolayer templates

A simple and cost-effective methodology for large-area micrometer-scale patterning of a wide range of metallic and oxidic functional materials is presented. Self-assembled monolayers (SAM) of alkyl thiols on Au were micropatterned by channel-diffused oxygen plasma etching, a method in which selected areas of SAM were protected from plasma oxidation via a soft lithographic stamp. The patterned SAMs were used as templates for site-selective electrodeposition, electroless deposition and solution-phase deposition of functional materials such as ZnO, Ni, Ag thin films, and ZnO nanowires. The patterned SAMs and functional materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and tunneling AFM (TUNA).

Building upon patterned organic monolayers produced via catalytic stamp lithography

Soft lithographic sub-100 nm chemical patterning was demonstrated on organic monolayer surfaces using poly(dimethylsiloxane)-based stamps decorated with Pd nanostructures, structures termed "catalytic stamps". Chemically reactive azide or alkene functionalities were incorporated on oxide-capped silicon surfaces and utilized for patterning via Pd-catalyzed hydrogenation or Heck reactions. The catalytic stamps were soft lithographic stamps based on PDMS with embedded nanoscale palladium catalysts, prepared via block copolymer-based templating. Nanoscale chemical patterns were readily generated on the azide or alkene precursor surfaces simply by applying the Pd catalytic stamps and the reactive molecule, the molecular ink, to the surface, thanks to the highly localized catalytic transformations induced by the patterned, immobilized solid Pd catalysts. A series of successful postfunctionalization reactions on the resulting patterned surfaces further demonstrated the utility of this approach to construct novel designs of nanoarchitectures, with potentially unique and innovative properties.

Wettability control and water droplet dynamics on SiC-SiO2 core-shell nanowires

We present a simple method for fabricating superhydrophobic SiC-SiO(2) core-shell nanowire surfaces via the facile dip-coating of alkyltrichlorosilanes. Water droplets displayed a variety of shapes with varying surface energies on the nanowire surfaces, which could be modified through chemisorption of alkyltrichlorosilanes with variable carbon chain length. The effects of UV irradiation on the superhydrophobic nanowire arrays were also investigated. UV light efficiently decomposed the chemisorbed molecules, and the superhydrophobic surface gradually converted into a hydrophilic surface with increasing UV exposure. The water droplet impact behavior on the modified surfaces was studied to test the stability of the superhydrophobicity under dynamic conditions.

Orthogonal functionalization of silicon substrates using self-assembled monolayers

A fabrication process for multifunctional surfaces is designed leading to five different functional moieties (amine, thiol, carboxylic acid, fluoro, and methyl) being present on a single structured surface. The multifunctional surface is created by combining UV-ozone patterning, electro-oxidative lithography, the local deposition of self-assembled monolayers (SAMs), and surface modification schemes. Besides the characterization with conventional surface-sensitive techniques, the nature of the locally functionalized regions is demonstrated by self-assembly of three different probe nanomaterials (Si nanoparticles, Au nanoparticles, and hydroxyl functionalized micelles). A versatile fabrication approach for complex surfaces with addressable functionalities can be created, and it was possible to integrate five different functionalized areas on one substrate.

Site-selective biofunctionalization of aluminum nitride surfaces using patterned organosilane self-assembled monolayers

Surface biochemical functionalization of group-III nitride semiconductors has recently attracted much interest because of their biocompatibility, nontoxicity, and long-term chemical stability under demanding physiochemical conditions for chemical and biological sensing. Among III-nitrides, aluminum nitride (AlN) and aluminum gallium nitride (AlGaN) are particularly important because they are often used as the sensing surfaces for sensors based on field-effect transistor or surface acoustic wave (SAW) sensor structures. To demonstrate the possibility of site-selective biofunctionalization on AlN surfaces, we have fabricated two-dimensional antibody micropatterns on AlN surfaces by using patterned self-assembled monolayer (SAM) templates. Patterned SAM templates are composed of two types of organosilane molecules terminated with different functional groups (amino and methyl), which were fabricated on AlN/sapphire substrates by combining photolithography, lift-off process, and self-assembly technique. Because the patterned SAM templates have different surface properties on the same surface, clear imaging contrast of SAM micropatterns can be observed by field-emission scanning electron microscopy (FE-SEM) operating at a low accelerating voltage in the range of 0.5-1.5 kV. Furthermore, the contrast in surface potential of the binary SAM microstructures was confirmed by selective adsorption of negatively charged colloidal gold nanoparticles (AuNPs). The immobilization of AuNPs was limited on the positively charged amino-terminated regions, while they were scarcely found on the surface regions terminated by methyl groups. In this work, selective immobilization of green fluorescent protein (GFP) antibodies was demonstrated by the specific protein binding of enhanced GFP (EGFP) labeling. The observed strong fluorescent signal from antibody functionalized regions on the SAM-patterned AlN surface indicates the retained biological activity of specific molecular recognition resulting from the antibody-EGFP interaction. The results reported here show that micropatterning of organosilane SAMs by the combination of photolithographic process and lift-off technique is a practical approach for the fabrication of reaction regions on AlN-based bioanalytical microdevices.

Plasmon hybridization in individual gold nanocrystal dimers: direct observation of bright and dark modes

We apply a nanomanipulation technique to assemble pairs of monodispersed octahedral gold nanocrystals (side length, 150 nm) along their major axes with a varying tip-to-tip separation (25-125 nm). These pairs are immobilized onto indium tin oxide coated silica substrates and studied as plasmonic dimers by polarization-selective total internal reflection (TIR) microscopy and spectroscopy. We confirm that the plasmon coupling modes with the scattering polarization along the incident light direction result from the transverse-magnetic-polarized incident light, which induces two near-field-coupled dipole moments oriented normal to the air-substrate interface. In such cases, both in-phase (antibonding) and antiphase (bonding) plasmon coupling modes can be directly observed with the incident light wave vector perpendicular and parallel to the dimer axis, respectively. The observation of antiphase plasmon coupling modes ("dark" plasmons) is made possible by the unique polarization nature of the TIR-generated evanescent field. Furthermore, with decreasing nanocrystal separation, the plasmon coupling modes shift to shorter wavelengths for the incident light perpendicular to the dimer axis, whereas relatively large red shifts of the plasmonic coupling modes are found for the parallel incident light.