Advances in flexible optoelectronic devices have led to an increasing need for developing highly efficient, low-cost, flexible transparent conducting electrodes. Copper-based electrodes have been unattainable due to the relatively low optical transmission and poor oxidation resistance of copper. Here, we report the synthesis of a completely continuous, smooth copper ultra-thin film via limited copper oxidation with a trace amount of oxygen. The weakly oxidized copper thin film sandwiched between zinc oxide films exhibits good optoelectrical performance (an average transmittance of 83% over the visible spectral range of 400–800 nm and a sheet resistance of 9 Ω sq−1) and strong oxidation resistance. These values surpass those previously reported for copper-based electrodes; further, the record power conversion efficiency of 7.5% makes it clear that the use of an oxidized copper-based transparent electrode on a polymer substrate can provide an effective solution for the fabrication of flexible organic solar cells.

Positioning probe molecules at electromagnetic hot spots with nanometer precision is required to achieve highly sensitive and reproducible surfaceenhanced Raman spectroscopy (SERS) analysis. In this article, molecular positioning at plasmonic nanogaps is reported using a high aspect ratio (HAR) plasmonic nanopillar array with a controlled surface energy. A largearea HAR plasmonic nanopillar array is generated using a nanolithographyfree simple process involving Ar plasma treatment applied to a smooth polymer surface and the subsequent evaporation of metal onto the polymer nanopillars. The surface energy can be precisely controlled through the selective removal of an adsorbed self-assembled monolayer of low surfaceenergy molecules prepared on the plasmonic nanopillars. This process can be used to tune the surface energy and provide a superhydrophobic surface with a water contact angle of 165.8° on the one hand or a hydrophilic surface with a water contact angle of 40.0° on the other. The highly tunable surface wettability is employed to systematically investigate the effects of the surface energy on the capillary-force-induced clustering among the HAR plasmonic nanopillars as well as on molecular concentration at the collapsed nanogaps present at the tops of the clustered nanopillars.

Plasmonic biosensors have demonstrated superior performance in detecting various biomolecules with high sensitivity through simple assays. Scaled-up, reproducible chip production with a high density of hotspots in a large area has been technically challenging, limiting the commercialization and clinical translation of these biosensors. A new fabrication method for 3D plasmonic nanostructures with a high density, large volume of hotspots and therefore inherently improved detection capabilities is developed. Specifically, Au nanoparticle-spiked Au nanopillar arrays are prepared by utilizing enhanced surface diffusion of adsorbed Au atoms on a slippery Au nanopillar arrays through a simple vacuum process. This process enables the direct formation of a high density of spherical Au nanoparticles on the 1 nm-thick dielectric coated Au nanopillar arrays without high-temperature annealing, which results in multiple plasmonic coupling, and thereby large effective volume of hotspots in 3D spaces. The plasmonic nanostructures show signal enhancements over 8.3 × 10 8 -fold for surface-enhanced Raman spectroscopy and over 2.7 × 10 2 -fold for plasmon-enhanced fluorescence. The 3D plasmonic chip is used to detect avian influenza-associated antibodies at 100 times higher sensitivity compared with unstructured Au substrates for plasmon-enhanced fluorescence detection. Such a simple and scalable fabrication of highly sensitive 3D plasmonic nanostructures provides new opportunities to broaden plasmon-enhanced sensing applications.so-called "plasmonic hotspots" are the fundamental basis of numerous promising technologies in the fields of plasmonenhanced spectroscopy, [1][2][3][4][5][6][7][8][9][10] plasmonic biosensing, [11][12][13][14] photocatalysis, [15][16][17][18][19] and nanophotonics. [20,21] One major challenge in expanding the use of plasmonenhanced applications lies in reproducible fabrication of high-density plasmonic hotspots over large areas in a low-cost, highthroughput manner. Various methods have been explored, including aggregations of metallic nanoparticles, nanolithographic patterning, thin-film processing, and hybrid nanostructures. Despite extensive efforts, the development of a reproducible, commercially ready fabrication method that achieves both high quality (i.e., high sensitivity and good reproducibility) and high throughput (i.e., lowcost and wafer-scale fabrication) remains elusive.Among various plasmonic configurations, a nanoparticle-on-mirror (NPOM) geometry, where a nanoparticle is separated from a plain metal film by an ultrathin dielectric spacer layer, has been reported to be a highly efficient plasmonic substrate. [1][2][3][4][5] In the 2D NPOM configuration, however, electromagnetic hotspots are formed around the nanoparticle in a limited area; the effective hotspot volume accounts for a small fraction of the total

3D hybrid plasmonic nanomaterials are composed of 3D-stacked Ag nanowires and nanoparticles separated by a nanoscale-thick alumina interlayer. The 3D hybrid plasmonic nanostructures exhibit strong plasmonic coupling between the ultrahigh populations of plasmonic nanomaterials, overcoming the physical limitation of inefficient plasmonic coupling of the Ag nanowire stacks.

A surface-enhanced Raman scattering (SERS) sensor comprising silver nanowires (AgNWs) and genetically engineered M13 bacteriophages expressing a tryptophan-histidine-tryptophan (WHW) peptide sequence (BPWHW) was fabricated by simple mixing of BPWHW and AgNW solutions, followed by vacuum filtration onto a glass-fiber filter paper (GFFP) membrane. The AgNWs stacked on the GFFP formed a high density of SERS-active hot spots at the points of nanowire intersections, and the surface-coated BPWHW functioned as a bioreceptor for selective pesticide detection. The BPWHW-functionalized AgNW (BPWHW/AgNW) sensor was characterized by scanning electron microscopy, confocal scanning fluorescence microscopy, atomic force microscopy, and Fourier transform infrared spectroscopy. The Raman signal enhancement and the selective pesticide SERS detection properties of the BPWHW/AgNW sensor were investigated in the presence of control substrates such as wild-type M13 bacteriophage-decorated AgNWs (BPWT/AgNW) and undecorated AgNWs (AgNW). The BPWHW/AgNW sensor exhibited a significantly higher capture capability for pesticides, especially paraquat (PQ), than the control SERS substrates, and it also showed a relatively higher selectivity for PQ than for other bipyridylium pesticides such as diquat and difenzoquat. Furthermore, as a field application test, PQ was detected on the surface of PQ-pretreated apple peels, and the results demonstrated the feasibility of using a paper-based SERS substrate for on-site residual pesticide detection. The developed M13 bacteriophage-functionalized AgNW SERS sensor might be applicable for the detection of various pesticides and chemicals through modification of the M13 bacteriophage surface peptide sequence.

We utilized a fast Raman spectral mapping technique for fast detection of bacterial pathogens. Three-dimensional (3D) plasmonic nanopillar arrays were fabricated using the nanolithography-free process consisting of maskless Ar plasma treatment of a polyethylene terephthalate substrate and subsequent metal deposition. Bacterial pathogens were immobilized on the positively charged poly(l-lysine)-coated 3D plasmonic substrate through electrostatic interactions. Then, the bacterial surfaces were selectively labeled with antibody-conjugated surface-enhanced Raman scattering (SERS) nanotags, and Raman mapping images were collected and statistically analyzed for quantitative analysis of bacteria. Salmonella typhimurium was selected as a model pathogen bacterium to confirm the efficacy of our SERS imaging technique. Minimum number of Raman mapping points with statistical reliability was determined to reduce assay time. It was possible to get a statistically reliable standard calibration curve for 529 pixels (laser spot with 60 μm interval), which required a total mapping time of 45 min to get a standard calibration curve for five different concentrations of bacteria in the 0 to 10 CFU/mL range. No amplification step was necessary for quantification because low-abundance target bacteria could be measured using the Raman spectral mapping technique. Therefore, this approach allows accurate quantification of bacterial pathogens without any culturing or enrichment process.

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Enhancing light-matter interactions is essential to improving nanophotonic and optoelectronic device performance. In the present work, we developed a new design for 3D plasmonic nanostructures with enhanced near-field interactions among the plasmonic nanomaterials. The 3D plasmonic nanostructures consisted of multilayered bottom Ag/polydimethylsiloxane (PDMS) nanostructures, an alumina middle layer, and top Ag nanoparticles (NPs). High areal density PDMS nanoprotrusions were self-organized by a simple maskless plasma etching process. The conformal deposition of alumina using atomic layer deposition and Ag deposition produced 3D plasmonic nanostructures. These structures induced multiple near-field interactions between the ultrahigh-areal-density (1400 mm À2 ) top Ag NPs and the underlying Ag nanostructures, and among the top Ag NPs themselves. The high density of hot spots across the 3D space yielded highly efficient and widely tunable plasmonic responses across the entire visible range. The SERS signal enhancement measured at the 3D plasmonic nanostructures was 3.9 times the signal measured at the 2D multilayered structures and 48.0 times the signal measured at a Ag NP layer deposited onto a Si substrate. Finally, the 3D plasmonic nanostructures exhibited excellent uniformity with a variation of 6.8%, based on a microscale Raman mapping analysis. The excellent Raman signal uniformity can be attributed to the ultrahigh areal density of the Ag NPs and the uniform thickness of the alumina spacing layer.