Solution-Deposited Thin Silver Films on Plastic Surfaces for Low-Cost Applications in Plasmon-Coupled Emission Sensors

We report the deposition of highly uniform thin silver films on plastic materials using a wet-chemistry method, suitable for surface plasmon-coupled emission (SPCE). This approach is reproducible for diverse low-cost applications and versatile to generate silver surfaces on various plastics substrates. An oxygen plasma pretreatment of the plastic provides for rapid silvering, leading to a 47-nm-thick continuous film for SPCE applications. The surface smoothness and thickness of the films have been estimated using atomic force microscope. The higher refractive index of polycarbonate, resulted in an SPCE angle of θ _F = 47⁰ for Rhodamine B, compared to glass (θ _F = 50⁰). The current study presents details on film deposition conditions, appropriate choice of index matching fluids, substrates, and light sources that play a vital role to augment SPCE emission intensity.     

Theoretical Simulation and Focused Ion Beam Fabrication of Gold Nanostructures for Surface-Enhanced Raman Scattering (SERS)

This paper describes the fabrication of gold nanopillar and nanorod arrays and theoretical calculations of electromagnetic fields (EMFs) around ordered arrangements of these nanostructures. The EMFs of both single nanopillars and dimers of nanopillars—having nanoscale gaps between the two adjacent nanopillars forming the dimers—are simulated in this work by employing the finite-difference time-domain method. In the case of simulations for dimers of nanopillars, the nanoscale gaps between the nanopillars are varied between 5 and 20 nm, and calculations of the electromagnetic fields in the vicinity of the nanopillars and in the gaps between the nanopillars were carried out. Fabrication of gold nanopillars in a controlled manner for forming SERS substrates involves focused ion beam (FIB) milling. The nanostructures were fabricated on gold-coated silica, mica, and quartz planar substrates as well as on gold-coated tips of four mode and multimode silica optical fibers.     

Fabrication of Gold Nanostructures and Studies of Their Morphological and Surface Plasmonic Properties

We report fabrication of gold nanostructures on glass and indium tin oxide (ITO)-coated glass substrates using high fluence and highly energetic gold ions generated by hot, dense, and strongly non-equilibrium plasma. Nanodots and nanorods are observed in scanning electron microscopy (SEM) of nanostructures grown on glass substrate with single and double shots of gold ions which is in conformity with the transmission electron microscopy image. SEM images for single and double shots of gold ions on ITO-coated glass substrate show only nanodots. The mean diameter of nanodots obtained on both glass and ITO-coated glass is found to increase with increase in the number of gold ions shot from one to two. The gold nanostructures exhibit red shift in surface plasmon resonance with increased interaction which is in agreement with other reported work.     

Fabrication of 2D and 3D Architectures with Silver Nanostructures Building Blocks and Studying Their Refractive Index Sensitivity

In this research project, a colloidal solution of silver nanocubes was synthesized and using these nanocubes as building blocks, 2D and 3D ordered structures on solid supports were fabricated to study their optical properties and refractive index sensitivities. The silver nanocubes were synthesized by the polyol reduction process while their 2D and 3D ordered structures were fabricated by Langmuir-Blodgett trough (LB). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were employed to investigate the size and shape of the nanostructures as well as the morphologies of 2D and 3D structures. UV-visible absorption spectroscopy was employed to explore their optical properties. Finally, 2D and 3D assemblies of silver nanocubes were employed to investigate their refractive index sensitivity (RIS). The SEM image showed silver nanocubes with nominal edge length of 80 nm. Extinction spectra of 2D and 3D ordered structures are different than those in a colloidal state. Intensity of the plasmon resonance modes is higher for the 3D assembly than that of the 2D assembly. A new band in the low energy region of the spectrum appears for the 3D assembly because of interparticle coupling of the plasmon resonance modes. 3D assembly showed a higher RIS (158.9/ RIU) than of the 2D assembly (150.3/RIU). However, nanocubes are less ordered in 2D substrate than its counterpart 3D. Such 2D and 3D assemblies of silver nanocubes (AgNCs) could be potential candidates for making refractive index-based sensors as well as promising surface-enhanced Raman scattering (SERS) active substrates.     

Direct Electrodeposition of Hollowed Ag Nanostructures on ITO Glass for Reproducible SERS Application

Hollowed Ag nanostructures are, for the first time, electrodeposited on ITO glass without use of surfactant. The hollowed Ag nanostructure was investigated via a collaboration of scanning electron microscopy (SEM), XRD, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), XRD, and UV-vis. Results exhibited that the formation of the hollowed Ag nanostructure can be interpreted as the synergy effect of twin defect and low nucleation driving force. Surface-enhanced Raman scattering (SERS) spectra of rhodamine 6G and adenine molecules adsorbed on the surface of these Ag nanostructures were recorded. The smallest RSD of 1651 cm^?1 Raman bands of rhodamine 6G was 14.7 %, indicating that the hollowed Ag nanostructures can be utilized for reproducible SERS application. Through comparison, it was found the good crystallinity was beneficial for SERS.     

Fabrication and characterization of a multiwell array SERS chip with biological applications

Uniform, large surface area substrates for surface-enhanced Raman spectroscopy (SERS) are fabricated by oblique angle deposition. The SERS-active substrates are patterned by a polymer-molding technique to provide a uniform array for high throughput biosensing and multiplexing. Using a conventional SERS-active molecule, 1,2-di(4-pyridyl)ethylene (BPE) ≥98%, we show that this device provides a uniform Raman signal enhancement from well to well with a detection limit of at least 10⁻⁸ M of the BPE solution or 10⁻¹⁸ mol of BPE. The SERS intensity is also demonstrated to vary logarithmically with the log of BPE concentration and the apparent sensitivity of the patterned substrate is compared to previous reports from our group on non-patterned substrates. Avian influenza is analyzed to demonstrate the utility of SERS multiwell patterned substrates for biosensing. The spectra acquired from patterned substrates show better reproducibility and less variation compared to the unpatterned substrates according to multivariate analysis. Our results highlight potential advantages of the patterned substrate.     

The Role of Coupled Nanoplasmon Excitation in Growth Mechanism of Laser-Induced Self-Organized Nanostructures in AgCl-Ag Waveguide Thin Films

Formation mechanism of laser-induced spontaneous periodic nanostructures in thin light-sensitive AgCl waveguide films, doped by silver nanoparticles, is studied. It is found that the initial size, geometry, and surface coverage parameters of Ag nanoparticles instate preconditions for the nanostructure formation. These parameters play essential roles in coupled nanoplasmon excitation in silver nanoclusters, which in turn influences scattering of the incident light from the silver clusters. Some parts of the scattered light propagate as waveguide modes in the film and interference with the incident light. Afterward, migration of the Ag nanoparticles into the minima of the interference pattern forms the nanostructure. Simultaneously, excitation of coupled nanoplasmons in the neighboring clusters enhances the scattered light intensity. It is observed that longer exposure results in destruction of the formed nanostructures, because of creation of electrical joint between agglomerated clusters in the interference pattern’s minima, which leads to weakening of the TE mode excitation and, consequently, domination of the Gaussian profile of the incident light. This leads to the deceleration of the self-organized nanostructure development, growth rate, and quality. We have found that competition between the Gaussian profile of the incident laser field and the interference field causes a finally saturating oscillatory behavior of the self-organizing process.     

Fusing Nanowires into Thin Films: Fabrication of Graded‐Heterojunction Perovskite Solar Cells with Enhanced Performance

Perovskite solar cells (PSCs) have recently experienced a rapid rise in power conversion efficiency (PCE), but the prevailing PSCs with conventional mesoscopic or planar device architectures still contain nonideal perovskite/hole‐transporting‐layer (HTL) interfaces, limiting further enhancement in PCE and device stability. In this work, CsPbBr₃ perovskite nanowires are employed for modifying the surface electronic states of bulk perovskite thin films, forming compositionally‐graded heterojunction at the perovskite/HTL interface of PSCs. The nanowire morphology is found to be key to achieving lateral homogeneity in the perovskite film surface states resulting in a near‐ideal graded heterojunction. The hidden role of such lateral homogeneity on the performance of graded‐heterojunction PSCs is revealed for the first time. The resulting PSCs show high PCE up to 21.4%, as well as high operational stability, which is superior to control PSCs fabricated without CsPbBr₃‐nanocrystals modification and with CsPbBr₃‐nanocubes modification. This study demonstrates the promise of controlled hybridization of perovskite nanowires and bulk thin films for more efficient and stable PSCs.     

The Role of Propagating and Localized Surface Plasmons for SERS Enhancement in Periodic Nanostructures

Periodic arrays of plasmonic nanopillars have been shown to provide large, uniform surface-enhanced Raman scattering (SERS) enhancements. We show that these enhancements are the result of the combined impact of localized and propagating surface plasmon modes within the plasmonic architecture. Here, arrays of periodically arranged silicon nanopillars of varying sizes and interpillar gaps were fabricated to enable the exploration of the SERS response from two different structures; one featuring only localized surface plasmon (LSP) modes and the other featuring LSP and propagating (PSP) modes. It is shown that the LSP modes determine the optimal architecture, and thereby determine the optimum diameter for the structures at a given incident. However, the increase in the SERS enhancement factor for a system in which LSP and PSP cooperatively interact was measured to be over an order of magnitude higher and the peak in the diameter dependence was significantly broadened, thus, such structures not only provide larger enhancement factors but are also more forgiving of lithographic variations.     

Shell Thickness-Dependent Plasmon Coupling and Creation of SERS Hot Spots in Au@Ag Core-Shell Nanostructures

The main objective of the present study is to investigate the shell thickness-dependent Raman enhancement activity of silver-coated gold nanoparticles (Au@Ag NPs) when bound to a model analyte 2-mercaptobenzoic acid (2-MBA). With an optimized Ag:Au ratio, dimeric and trimeric Au@Ag nanostructures were prepared in the presence of 2-MBA and are characterized by spectroscopic and microscopic techniques. These dimeric junctions act as hot spots and the molecules trapped at these junctions showed higher Raman signal enhancements due to the presence of amplified electric field.     

Combination of Nanoholes with Metal Nanoparticles–Fabrication and Characterization of Novel Plasmonic Nanostructures

Small metal nanostructures, especially gold and silver nanoparticles, are known for their interesting optical properties caused by plasmonic effects. Molecular plasmonics, a combination of these optically active nanostructures with the molecular world, opens new possibilities for bioanalytics and (bio-) nanophotonics. Isotropic or anisotropic, homogeneous or heterogeneous metal nanoparticles provide a platform for different, highly defined functional units with interesting optical properties such as plasmon waveguides or molecular beacons. Nanohole arrays in metal layers are another promising component for nanophotonics. New photonic materials were realized from combinations of single metal nanoparticles with individual nanoholes in metals. Atomic force microscopic imaging was used to determine the particle location as well as the lateral dimensions and the topography of the resulting structures. Besides ultramicroscopic characterization of the nanoarrangements, such as nanoparticles positioned in nanoholes, far-field optical methods were also applied to investigate their optical properties.     

The Suitable Condition of Using LSPR Model in SERS: LSPR Effect Versus Chemical Effect on Microparticles Surface-Modified with Nanostructures

The conditions under which the localized surface plasmon resonance (LSPR) model can be applied to the calculation of surface-enhanced Raman scattering (SERS) enhancement factors have been questioned because the chemical effect presents simultaneously with LSPR effect, resulting in calculations that are not always consistent with the measured data. The SERS spectra of crystal violet (CV) molecules on single, dimer, trimer, and aggregates of silver microparticles surface-modified with nanostructures (MSMN) were obtained. It is found that the chemical effect is determined by the chemical adsorption behavior of CV molecules on single particle. As more particles are introduced as amplifiers, to assemble dimer, trimer, and aggregates, no new SERS signals related to the chemical effect can be observed, except for the further enhancement to the original signals. The further enhancement is attributed to the LSPR effect from the electromagnetic coupling with introduced particles. This is also demonstrated by dark field scattering. The LSPR theoretical values of single, dimer, trimer, and aggregates of MSMNs should fit the measured enhancement factor (G _LSPR) after correcting the SERS enhancement factor (G _SERS) with the chemical enhancement factor on the single particle (G _Chem-Sgl), i.e., G _LSPR?=?G _SERS/G _Chem-Sgl. Tip-enhanced Raman spectroscopy with a gold nanoparticle further implies that this could be extended to nanoparticle systems. This work provides an effective and simple route, whereby only the chemical effect from a single particle needs to be considered when studying the fit between the LSPR model and the measured LSPR enhancement factor.     

Solvent‐Mediated Dimension Tuning of Semiconducting Oxide Nanostructures as Efficient Charge Extraction Thin Films for Perovskite Solar Cells with Efficiency Exceeding 16%

The recent surge in efficiency and progress of organohalide perovskite solar cells (PSCs) has been significant. The PSC performance is significantly influenced by nanostructuring as this varies the intrinsic optical, electrical, and electrochemical properties. Diverse TiO₂ electron transport layers (ETLs) are solvothermally grown on the transparent conducting oxide substrate with different dimensionalities, 0D nanoparticles (TNP), 1D nanowires (TNW) to 2D nanosheets (TNS), by varying the organic solvent used. These layers feature enhanced optical transparency (≈2%–5% transmittance improvement compared to pristine fluorine doped tin oxide, FTO, glass) minimizing light absorption losses. PSCs constructed using 1D TNW or 2D TNS yield enhanced photovoltaic performance compared to the 0D TNP counterparts. This is a result of i) improved infiltration of the perovskite in the porous TNW or TNS network and ii) facilitated electron transport and charge extraction at the TNW/perovskite or TNS/perovskite interfaces, thus reduced interfacial recombination loss. Employing a bilayered ETL film consisting of a self‐assembled TiO₂ blocking layer and a subsequent TNW active layer, produces PSC devices with an efficiency exceeding 16%. This bilayered ETL film can simultaneously block the photogenerated holes and enhance electron ­extraction, therefore improving PSC performance.     

Profile Simulation and Fabrication of Gold Nanostructures by Separated Nanospheres with Oblique Deposition and Perpendicular Etching

This paper investigates in detail the profiles of the nanostructures fabricated by nanosphere lithography through oblique deposition and perpendicular etching. 2D or 3D nanostructures can be achieved by this cost-effective method. Because the optical response of a particular nanoparticle depends on its size and shape, this angle deposition method can produce various shapes of nanostructures, which are suitable for localized surface plasmon resonance biosensor applications. The nanostructure profiles under various deposition and etching conditions are simulated in our work. The calculated 3D profiles are verified by the 3D nanostructures fabricated in our experiments, and the calculated 2D profiles are in good agreement with the fabricated nanocrescents reported by another research group. This paper gives a full theoretical solution of the obtainable nanostructure shapes by nanosphere lithography utilizing oblique deposition and perpendicular etching.     

Method for Staining and Washing Thin Sections on Support Films

A procedure is described for staining large numbers of thin sections on support films for use with one-hole grids. The film is picked up, carried and protected using easily made plastic blocks. Loop-tipped forceps are then used to transfer tissue ribbons from the knife boat to the support film. A large number of tissue sections can then be stained and washed simultaneously in a modified Pyrex dish without damaging the film. After staining, the slot in the one-hole grid is centered over the tissue ribbon, and the grid is attached to the film. The method is suitable for serial reconstruction and the unobstructed viewing of large thin sections in the TEM.     

Fabrication and Optical Properties of Au-Coated Polystyrene Nanosphere Arrays with Controlled Gaps

Large area monolayer of self-assembled polystyrene sphere (PS) arrays were formed by an interface self-assembly method and the gap of PS was adjusted by O₂ plasma treatment (OPT). After different duration of OPT, the spacing between the PSs varied from 5 to 88 nm. Then, 20 nm Au film was deposited on the PS arrays by electron beam deposition. The absorption peaks of Au-coated PS array red-shifted obviously by changing the gap of PS. The new absorption peaks emerged when the gap of the PS decreased to about 20 nm. The surface plasmon resonance (SPR) was employed to explain the absorption proportion of this Au-PS structure. These micro-nano structures exhibit tunable SPR bands, which may be useful to the applications in some research fields, such as biosensing, single molecule detection, and novel optoelectronic devices.     

In Situ Preparation of Biomimetic Thin Films and Their Surface-Shielding Effect for Organisms in High Vacuum

Self-standing biocompatible films have yet to be prepared by physical or chemical vapor deposition assisted by plasma polymerization because gaseous monomers have thus far been used to create only polymer membranes. Using a nongaseous monomer, we previously found a simple fabrication method for a free-standing thin film prepared from solution by plasma polymerization, and a nano-suit made by polyoxyethylene (20) sorbitan monolaurate can render multicellular organisms highly tolerant to high vacuum. Here we report thin films prepared by plasma polymerization from various monomer solutions. The films had a flat surface at the irradiated site and were similar to films produced by vapor deposition of gaseous monomers. However, they also exhibited unique characteristics, such as a pinhole-free surface, transparency, solvent stability, flexibility, and a unique out-of-plane molecular density gradient from the irradiated to the unirradiated surface of the film. Additionally, covering mosquito larvae with the films protected the shape of the organism and kept them alive under the high vacuum conditions in a field emission-scanning electron microscope. Our method will be useful for numerous applications, particularly in the biological sciences.     

Fabrication of Polymeric Coatings with Controlled Microtopographies Using an Electrospraying Technique

Surface topography of medical implants provides an important biophysical cue on guiding cellular functions at the cell-implant interface. However, few techniques are available to produce polymeric coatings with controlled microtopographies onto surgical implants, especially onto implant devices of small dimension and with complex structures such as drug-eluting stents. Therefore, the main objective of this study was to develop a new strategy to fabricate polymeric coatings using an electrospraying technique based on the uniqueness of this technique in that it can be used to produce a mist of charged droplets with a precise control of their shape and dimension. We hypothesized that this technique would allow facile manipulation of coating morphology by controlling the shape and dimension of electrosprayed droplets. More specifically, we employed the electrospraying technique to coat a layer of biodegradable polyurethane with tailored microtopographies onto commercial coronary stents. The topography of such stent coatings was modulated by controlling the ratio of round to stretched droplets or the ratio of round to crumped droplets under high electric field before deposition. The shape of electrosprayed droplets was governed by the stability of these charged droplets right after ejection or during their flight in the air. Using the electrospraying technique, we achieved conformal polymeric coatings with tailored microtopographies onto conductive surgical implants. The approach offers potential for controlling the surface topography of surgical implant devices to modulate their integration with surrounding tissues.