Romantic Story or Raman Scattering? Rose Petals as Ecofriendly, Low-Cost Substrates for Ultrasensitive Surface-Enhanced Raman Scattering.

In this Article, we present a facile approach for the preparation of ecofriendly substrates, based on common rose petals, for ultrasensitive surface-enhanced Raman scattering (SERS). The hydrophobic concentrating effect of the rose petals allows us to concentrate metal nanoparticle (NP) aggregates and analytes onto their surfaces. From a systematic investigation of the SERS performance when using upper and lower epidermises as substrates, we find that the lower epidermis, with its quasi-three-dimensional (quasi-3D) nanofold structure, is the superior biotemplate for SERS applications. The metal NPs and analytes are both closely packed in the quasi-3D structure of the lower epidermis, thereby enhancing the Raman signals dramatically within the depth of focus (DOF) of the Raman optical system. We have also found the effect of the pigment of the petals on the SERS performance. With the novel petal-based substrate, the SERS measurements reveal a detection limit for rhodamine 6G below the femtomolar regime (10(-15) M), with high reproducibility. Moreover, when we employ an upside-down drying process, the unique effect of the Wenzal state of the hydrophobic petal surface further concentrate the analytes and enhanced the SERS signals. Rose petals are green, natural materials that appear to have great potential for use in biosensors and biophotonics.

Nondestructive characterization of the structural quality and thickness of large-area graphene on various substrates.

We demonstrate an inspection technique, based on only one ellipsometric parameter, Ψ, of spectroscopic ellipsometry (SE), for the rapid, simultaneous identification of both the structural quality and thicknesses of large-area graphene films. The measured Ψ spectra are strongly affected by changes in the out-of-plane absorption coefficients (αTM); they are also correlated to the ratio of the intensities of the D and G bands in Raman spectra of graphene films. In addition, the electronic transition state of graphene within the UV regime assists the characterization of the structural quality. We also demonstrated that the intensities and shifts of the signals in Ψ spectra allow clear identification of the structural qualities and thicknesses, respectively, of graphene films. Moreover, this Ψ-based method can be further applied to graphene films coated on various substrates. In addition, mapping of the values of Ψ is a very convenient and useful means of rapidly characterizing both the structural quality and thickness of 2D materials at local areas. Therefore, this Ψ-based characterization method has great potential for application in the mass production of devices based on large-area graphene.

Plasmonic nanoparticle-film calipers for rapid and ultrasensitive dimensional and refractometric detection.

In this study, we develop an ultrasensitive nanoparticle (NP)-film caliper that functions with high resolution (angstrom scale) in response to both the dimensions and refractive index of the spacer sandwiched between the NPs and the film. The anisotropy of the plasmonic gap mode in the NP-film caliper can be characterized readily using spectroscopic ellipsometry (SE) without the need for further optical modeling. To the best of our knowledge, this paper is the first to report the use of SE to study the plasmonic gap modes in NP-film calipers and to demonstrate that SE is a robust and convenient method for analyzing NP-film calipers. The high sensitivity of this system originates from the plasmonic gap mode in the NP-film caliper, induced by electromagnetic coupling between the NPs and the film. The refractometric sensitivity of this NP-film caliper reaches up to 314 nm per RIU, which is superior to those of other NP-based sensors. The NP-film caliper also provides high dimensional resolution, down to the angstrom scale. In this study, the shift in wavelength in response to the change in gap spacing is approximately 9 nm Å(-1). Taking advantage of the ultrasensitivity of this NP-film caliper, we develop a platform for discriminating among thiol-containing amino acids.

Using optical anisotropy as a quality factor to rapidly characterize structural qualities of large-area graphene films.

In this study, we find that the optical anisotropy of graphene films could be used as an alternative quality factor for the rapid characterization of large-area graphene films prepared through chemical vapor deposition. We develop an angle-variable spectroscopic method to rapidly determine the optical anisotropy of graphene films. Unlike approaches using Raman scattering spectroscopy, this optical anisotropy method allows ready characterization of the structural quality of large-area graphene samples without the application of high-intensity laser irradiation or complicated optical setups. Measurements of optical anisotropy also allow us to distinguish graphene samples with different extents of structural imperfections; the results are consistent with those obtained from using Raman scattering spectroscopy. In addition, we also study the properties of graphene-based transparent conductive films at wide incident angles because of the advantage of the optical anisotropic properties of graphene. The transmittance of graphene is much higher than that of indium tin oxide films, especially at large incident angles.

Eco-friendly plasmonic sensors: using the photothermal effect to prepare metal nanoparticle-containing test papers for highly sensitive colorimetric detection.

Convenient, rapid, and accurate detection of chemical and biomolecules would be a great benefit to medical, pharmaceutical, and environmental sciences. Many chemical and biosensors based on metal nanoparticles (NPs) have been developed. However, as a result of the inconvenience and complexity of most of the current preparation techniques, surface plasmon-based test papers are not as common as, for example, litmus paper, which finds daily use. In this paper, we propose a convenient and practical technique, based on the photothermal effect, to fabricate the plasmonic test paper. This technique is superior to other reported methods for its rapid fabrication time (a few seconds), large-area throughput, selectivity in the positioning of the NPs, and the capability of preparing NP arrays in high density on various paper substrates. In addition to their low cost, portability, flexibility, and biodegradability, plasmonic test paper can be burned after detecting contagious biomolecules, making them safe and eco-friendly.

A permanent optical storage medium exhibiting ultrahigh contrast, superior stability, and a broad working wavelength regime.

In this paper we demonstrate an optical storage medium having advantages of ultrahigh contrast, superior stability, and broadband working wavelengths. Combining a single shot of deep-ultraviolet (UV) laser illumination with a Au particle-assisted etching process, we formed broadband antireflective, one-dimensional silicon nanowire arrays (SiNWs) with selectively at specific positions. Optical measurements and three-dimensional finite-difference time domain (3D-FDTD) simulations revealed ultrahigh reflection contrast between the Au and the SiNWs for both far- and near-field regimes. Relative to typical organic-based storage media, Au films and SiNWs are more stable, both chemically and thermally; therefore, we suspect that this new storage medium would exhibit high stability toward moisture, sunshine, and elevated temperatures.

Astronomical liquid mirrors as highly ultrasensitive, broadband-operational surface-enhanced Raman scattering-active substrates.

In this study, we found that an astronomical liquid mirror can be prepared as a highly ultrasensitive, low-cost, highly reproducible, broadband-operational surface-enhanced Raman scattering (SERS)-active substrate. Astronomical liquid mirrors are highly specularly reflective because of their perfectly dense-packed silver nanoparticles; they possess a large number and high density of hot spots that experience a very high intensity electric field, resulting in excellent SERS performance. When using the liquid mirror-based SERS-active substrate to detect 4-aminothiophenol (4-ATP), we obtained measured analytical enhancement factors (AEFs) of up to 2.7×10(12) and detection limits as low as 10(-15) M. We also found that the same liquid mirror could exhibit superior SERS capability at several distinct wavelengths (532, 632.8, and 785 nm). The presence of hot spots everywhere in the liquid mirror provided highly repeatable Raman signals from low concentrations of analytes. In addition, the astronomical liquid mirrors could be transferred readily onto cheap, flexible, and biodegradable substrates and still retain their excellent SERS performance, suggesting that they might find widespread applicability in various (bio)chemical detection fields.

Short-range plasmonic nanofocusing within submicron regimes facilitates in situ probing and promoting of interfacial reactions.

In this study, a simple configuration, based on high-index dielectric nanoparticles (NPs) and plasmonic nanostructures, is employed for the nanofocusing of submicron-short-range surface plasmon polaritons (SPPs). The excited SPPs are locally bound and focused at the interface between the dielectric NPs and the underlying metallic nanostructures, thereby greatly enhancing the local electromagnetic field. Taking advantage of the surface properties of the dielectric NPs, this system performs various functions. For example, the nanofocusing of submicron-short-range SPPs is used to enhance the Raman signals of gas molecules adsorbed on the dielectric NPs. In addition, the presence of the local strong electromagnetic field accelerates the rates of interfacial reactions on the surfaces of the dielectric NPs. Therefore, the proposed nanofocusing configuration can both promote and probe interfacial reactions simultaneously. Herein, the promotion and probing of the desorption of EtOH vapor are described, as well as the photodegradation of methylene blue. Moreover, the nanofocusing of SPPs is demonstrated on an aluminum surface in both the visible and UV regimes, a process that has not been achieved using conventional tapered waveguide nanofocusing structures. Therefore, the nanofocusing of submicron-short-range SPPs by dielectric NPs on plasmonic nanostructures is not limited to low-loss noble metals. Accordingly, this system has potential for use in light management and on-chip green devices and sensors.

White-Light-Induced Collective Heating of Gold Nanocomposite/Bombyx mori Silk Thin Films with Ultrahigh Broadband Absorbance.

This paper describes a systematic investigation of the phenomenon of white-light-induced heating in silk fibroin films embedded with gold nanoparticles (Au NPs). The Au NPs functioned to develop an ultrahigh broadband absorber, allowing white light to be used as a source for photothermal generation. With an increase of the Au content in the composite films, the absorbance was enhanced significantly around the localized surface plasmon resonance (LSPR) wavelength, while non-LSPR wavelengths were also increased dramatically. The greater amount of absorbed light increased the rate of photoheating. The optimized composite film exhibited ultrahigh absorbances of approximately 95% over the spectral range from 350 to 750 nm, with moderate absorbances (>60%) at longer wavelengths (750-1000 nm). As a result, the composite film absorbed almost all of the incident light and, accordingly, converted this optical energy to local heat. Therefore, significant temperature increases (ca. 100 °C) were readily obtained when we irradiated the composite film under a light-emitting diode or halogen lamp. Moreover, such composite films displayed linear light-to-heat responses with respect to the light intensity, as well as great photothermal stability. A broadband absorptive film coated on a simple Al/Si Schottky diode displayed a linear, significant, stable photo-thermo-electronic effect in response to varying the light intensity.

Single-shot laser treatment provides quasi-three-dimensional paper-based substrates for SERS with attomolar sensitivity.

In this study, an eco-friendly and ultrasensitive paper substrate is developed for surface-enhanced Raman scattering (SERS) with performance approaching single molecule detection. By exploiting the laser-induced photothermal effect, paper fibrils with hybrid micro- and nanostructures can facilitate the formation of highly dense metal nanoparticles (NPs) after a single shot of laser illumination. Metal films deposited on the paper substrates feature discontinuous morphologies, with the fragments acting as multiple nucleation sites. Because thermal conductivity is low on the broken films and the underlying paper fibrils, the incident energy is absorbed efficiently. Moreover, the quasi-three-dimensional distribution of NPs on the SERS paper greatly enhances the SERS signals within the effective collection volume of a Raman microscope. As a result of the large number of highly effective hot spots and the condensation effect, the hydrophobic SERS paper provides SERS signals with stable and uniform reproducibility throughout the detection area. The limits of detection when using the paper substrates reach the attomolar (10(-18) M) level, thereby approaching single molecule detection.

Incident angle-tuned, broadband, ultrahigh-sensitivity plasmonic antennas prepared from nanoparticles on imprinted mirrors.

We have used a direct imprint-in-metal method that is cheap and rapid to prepare incident angle-tuned, broadband, ultrahigh-sensitivity plasmonic antennas from nanoparticles (NPs) and imprinted metal mirrors. By changing the angle of incidence, the nanoparticle-imprinted mirror antennas (NIMAs) exhibited broadband electromagnetic enhancement from the visible to the near-infrared (NIR) regime, making them suitable for use as surface-enhanced Raman scattering (SERS)-active substrates. Unlike other SERS-active substrates that feature various structures with different periods or morphologies, the NIMAs achieved broadband electromagnetic enhancement from single configurations. The enhancement of the electric field intensity in the NIMAs originated from coupling between the localized surface plasmon resonance of the NPs and the periodic structure-excited surface plasmon resonance (SPR) of the imprinted mirror. Moreover, the coupling wavelengths could be modulated because the SPR wavelength was readily tuned by changing the angle of the incident light. Herein, we demonstrate that such NIMAs are robust substrates for visible and NIR surface-enhanced resonance Raman scattering under multiple laser lines (532, 633, and 785 nm) of excitation. In addition, we have found that NIMAs are ultrasensitive SERS-active substrates that can detect analytes (e.g., rhodamine 6G) at concentrations as low as 10(-15) M.

Transparent, broadband, flexible, and bifacial-operable photodetectors containing a large-area graphene-gold oxide heterojunction.

In this study, we combine graphene with gold oxide (AuOx), a transparent and high-work-function electrode material, to achieve a high-efficient, low-bias, large-area, flexible, transparent, broadband, and bifacial-operable photodetector. The photodetector operates through hot electrons being generated in the graphene and charge separation occurring at the AuOx-graphene heterojunction. The large-area graphene covering the AuOx electrode efficiently prevented reduction of its surface; it also acted as a square-centimeter-scale active area for light harvesting and photodetection. Our graphene/AuOx photodetector displays high responsivity under low-intensity light illumination, demonstrating picowatt sensitivity in the ultraviolet regime and nanowatt sensitivity in the infrared regime for optical telecommunication. In addition, this photodetector not only exhibited broadband (from UV to IR) high responsivity-3300 A W(-1) at 310 nm (UV), 58 A W(-1) at 500 nm (visible), and 9 A W(-1) at 1550 nm (IR)-but also required only a low applied bias (0.1 V). The hot-carrier-assisted photoresponse was excellent, especially in the short-wavelength regime. In addition, the graphene/AuOx photodetector exhibited great flexibility and stability. Moreover, such vertical heterojunction-based graphene/AuOx photodetectors should be compatible with other transparent optoelectronic devices, suggesting applications in flexible and wearable optoelectronic technologies.

Silicon-based broadband antenna for high responsivity and polarization-insensitive photodetection at telecommunication wavelengths.

Although the concept of using local surface plasmon resonance based nanoantenna for photodetection well below the semiconductor band edge has been demonstrated previously, the nature of local surface plasmon resonance based devices cannot meet many requirements of photodetection applications. Here we propose the concept of deep-trench/thin-metal (DTTM) active antenna that take advantage of surface plasmon resonance phenomena, three-dimensional cavity effects, and large-area metal/semiconductor junctions to effectively generate and collect hot electrons arising from plasmon decay and, thereby, increase photocurrent. The DTTM-based devices exhibited superior photoelectron conversion ability and high degrees of detection linearity under infrared light of both low and high intensity. Therefore, these DTTM-based devices have the attractive properties of high responsivity, extremely low power consumption, and polarization-insensitive detection over a broad bandwidth, suggesting great potential for use in photodetection and on-chip Si photonics in many applications of telecommunication fields.

Highly reflective liquid mirrors: exploring the effects of localized surface plasmon resonance and the arrangement of nanoparticles on metal liquid-like films.

In this paper, we describe a high-reflectance liquid mirror prepared from densely packed silver nanoparticles (AgNPs) of two different sizes. We controlled the particle size during the synthetic process by controlling the temperature. Varying the concentration of the ligand also allowed us to optimize the arrangement of the AgNPs to achieve liquid mirrors exhibiting high specular reflectance. Scanning electron microscopy and atomic force microscopy confirmed that the particles of the liquid mirror were well-packed with an interparticle distance of merely 2 nm; thus, the interstices and surface roughness of the NPs were effectively minimized. As a result of decreased scattering loss, the reflectance in the shorter wavelength regime was increased effectively. The AgNP film was also sufficiently thick to reflect the light in the longer wavelength regime. In addition, we used three-dimensional finite-difference time domain simulations and experimental measurements to investigate the relationship between the localized surface plasmon resonance (LSPR) and the specular reflection of the liquid mirrors. By changing the packing density of the AgNPs, we found that the LSPR effect could yield either a specular reflection peak or dip at the LSPR wavelengths in the reflection spectra of the liquid mirrors. Relative to previously reported liquid mirrors, the reflectance of our system is obviously much greater, especially in the shorter wavelength regime. The average reflectance in the range from 400 to 1000 nm could reach 77%, comparable with that of mercury-based liquid mirrors.

Ultralow reflection from a-Si nanograss/Si nanofrustum double layers.

A double-layer nanostructure comprising amorphous Si nanograss on top of Si nanofrustums (NFs) with a total height of 680 nm exhibits ultralow reflection. Almost near-unity absorption and near-zero reflectance result in this layered nanostructure, over a broad range of wavelengths and a wide range of angles of incidence, due to the low packing density of a-Si and the smooth transition of the refractive index from the air to the Si substrate across both the nanograss and NF layers.

Interactions between fluorescence of atomically layered graphene oxide and metallic nanoparticles.

Graphene oxide (GO) demonstrates interesting photoluminescence (PL) because of its unique heterogeneous atomic structure, which consists of variable sp(2)- and sp(3)-bonded carbons. In this study, we report the interaction between the luminescence of GO ranging from a single atomic layer to few-layered thin films and localized surface plasmon resonance (LSPR) from silver nanoparticles (Ag NPs). The photoluminescence of GO in the vicinity of the Ag NPs is enhanced significantly due to the near-field plasmonic effect by coupling electron-hole pairs of GO with oscillating electrons in Ag NPs, leading to an increased PL intensity and a decreased PL decay lifetime. The maxima 30-fold enhancement in PL intensity is obtained with an optimized film thickness of GO, and the luminescence image from a single atomic layer GO sheet is successfully observed with the assistance of the LSPR effect. The results provide an ideal platform for exploring the interactions between the fluorescence of two-dimensional layered materials and the LSPR effect.

Laser-induced jets of nanoparticles: exploiting air drag forces to select the particle size of nanoparticle arrays.

In this study, we developed a new method-based on laser-induced jets of nanoparticles (NPs) and air drag forces-to select the particle size of NP arrays. First, the incident wavelength of an excimer laser was varied to ensure good photo-to-thermal energy conversion efficiency. We then exploited air drag forces to select NPs with sizes ranging from 5 to 50 nm at different captured distances. Controlling the jet distances allowed us to finely tune the localized surface plasmon resonance (LSPR) wavelength. The shifting range of the LSPR wavelengths of the corresponding NP arrays prepared using the laser-induced jet was wider than that of a single NP or an NP dimer. We further calculated the relationship between the air drag force and the diameter of the NPs to provide good control over the mean NP size (capture size ≧ 300 µm) by varying the capture distance. Laser-induced jets of NPs could also be used to fabricate NP arrays on a variety of substrates, including Si, glass, plastic, and paper. This method has the attractive features of rapid, large-area preparation in an ambient environment, no need for further thermal annealing treatment, ready control over mean particle size, and high selectivity in the positioning of NP arrays. Finally, we used this method to prepare large NP arrays for acting hot spots on surface-enhanced Raman scattering-active substrates, and 10(-12) M R6G can be detected. Besides, we also prepare small NP arrays to act as metal catalysts for constructing low-reflection, broadband light trapping nanostructures on Si substrates.

Using the nanoimprint-in-metal method to prepare corrugated metal structures for plasmonic biosensors through both surface plasmon resonance and index-matching effects.

In this study, we prepared metallic corrugated structures for use as highly sensitive plasmonic sensors. Relying on the direct nanoimprint-in-metal method, fabrication of this metallic corrugated structure was readily achieved in a single step. The metallic corrugated structures were capable of sensing both surface plasmon resonance (SPR) wavelengths and index-matching effects. The corrugated Au films exhibited high sensitivity (ca. 800 nm/RIU), comparable with or even higher than those of other reported SPR-based sensors. Because of the unique index-matching effect, refractometric sensing could also be performed by measuring the transmission intensity of the Au/substrate SPR mode-conveniently, without a spectrometer. In the last, we demonstrated the corrugated Au film was capable of sensing biomolecules, revealing the ability of the structure to be a highly sensitive biosensor.

Top laminated graphene electrode in a semitransparent polymer solar cell by simultaneous thermal annealing/releasing method.

In this article, we demonstrate a semitransparent inverted-type polymer solar cell using a top laminated graphene electrode without damaging the underlying organic photoactive layer. The lamination process involves the simultaneous thermal releasing deposition of the graphene top electrode during thermal annealing of the photoactive layer. The resulting semitransparent polymer solar cell exhibits a promising power conversion efficiency of approximately 76% of that of the standard opaque device using an Ag metal electrode. The asymmetric photovoltaic performances of the semitransparent solar cell while illuminated from two respective sides were further analyzed using optical simulation and photocarrier recombination measurement. The devices consisting of the top laminated transparent graphene electrode enable the feasible roll-to-roll manufacturing of low-cost semitransparent polymer solar cells and can be utilized in new applications such as power-generated windows or multijunction or bifacial photovoltaic devices.

Nanoscale of biomimetic moth eye structures exhibiting inverse polarization phenomena at the Brewster angle.

The unique functionalities of nanoscale structures in the natural world are an inspiration to the development of new nano-manufacturing techniques. For example, the cornea of the moth's eye features a sub-wavelength natural antireflective architecture. To date, almost all optical research into moth eye structures has been focused on their antireflective properties. No studies of inverse polarization phenomena at the Brewster angle have been reported, especially in biomimetic structures. For the first time, we discovered a unique inverse polarization phenomenon on moth eye structures that arises from TM-polarized light having a higher reflectance than TE-polarized light on moth eye structures at angles of incidence near the Brewster angle, unlike the behavior of polarized light on flat interfaces. Herein, we report a one-step colloidal lithography process that allows the fabrication of several kinds of moth eye structures. We characterized these moth eye structures experimentally and through rigorous coupled-wave analysis to understand the mechanism underlying this inverse polarization phenomenon in both visible and near infrared ray (NIR) regimes. Controlling the structural height and degree of non-close-packing of the moth eye structures had a dramatic effect on the extent of inverse polarization. This study is potentially important for various polarization-dependent devices and measurements.