Silver nanoparticles protected by monolayer graphene as a stabilized substrate for surface enhanced Raman spectroscopy

Silver nanostructures have been extensively studied as the substrates for surface-enhanced Raman spectroscopy (SERS) due to their strong plasmonic enhancement effect and low loss at optical frequencies. However, unprotected silver nanostructures are chemically unstable under practical conditions, leading frequently to erratic SERS signals and possibly unreliable interpretation of experimental results. Herein, we show that a single layer of graphene grown by chemical vapor deposition can effectively stabilize silver nanoparticles against aerobic oxidation and that the protected nanoparticles can be used as a stable substrate for the production of enhanced SERS signals for up to 28 days under ambient conditions.     

Microfabrication and optical properties of highly ordered silver nanostructures

ABSTRACT: Using thermal evaporation, we fabricated five uniform and regular arrays of Ag nanostructures with different shapes that were based on an anodized aluminum oxide template and analyzed their optical properties. Round-top-shaped structures are obtained readily, whereas to obtain needle-on-round-top-shaped and needle-shaped structures, control of the directionality of evaporation, pore size, length, temperature of the substrate, etc., was required. We then observed optical sensitivity of the nanostructures by using surface-enhanced Raman scattering, and we preliminarily investigated the dependency of Raman signal to the roughness and shape of the nanostructures.     

Surface-Enhanced Raman Scattering from Dye Molecules in Silicon Nanowire Structures Decorated by Gold Nanoparticles

Silicon nanowires (SiNWs) prepared by metal-assisted chemical etching of crystalline silicon wafers followed by deposition of plasmonic gold (Au) nanoparticles (NPs) were explored as templates for surface-enhanced Raman scattering (SERS) from probe molecules of Methylene blue and Rhodamine B. The filling factor by pores (porosity) of SiNW arrays was found to control the SERS efficiency, and the maximal enhancement was observed for the samples with porosity of 55%, which corresponded to dense arrays of SiNWs. The obtained results are discussed in terms of the electromagnetic enhancement of SERS related to the localized surface plasmon resonances in Au-NPs on SiNW’s surfaces accompanied with light scattering in the SiNW arrays. The observed SERS effect combined with the high stability of Au-NPs, scalability, and relatively simple preparation method are promising for the application of SiNW:Au-NP hybrid nanostructures as templates in molecular sensorics.     

Aggregate nanostructures of organic molecular materials

Conjugated organic molecules are interesting materials because of their structures and their electronic, electrical, magnetic, optical, biological, and chemical properties. However, researchers continue to face great challenges in the construction of well-defined organic compounds that aggregate into larger molecular materials such as nanowires, tubes, rods, particles, walls, films, and other structural arrays. Such nanoscale materials could serve as direct device components. In this Account, we describe our recent progress in the construction of nanostructures formed through the aggregation of organic conjugated molecules and in the investigation of the optical, electrical, and electronic properties that depend on the size or morphology of these nanostructures. We have designed and synthesized functional conjugated organic molecules with structural features that favor assembly into aggregate nanostructures via weak intermolecular interactions. These large-area ordered molecular aggregate nanostructures are based on a variety of simpler structures such as fullerenes, perylenes, anthracenes, porphyrins, polydiacetylenes, and their derivatives. We have developed new methods to construct these larger structures including organic vapor-solid phase reaction, natural growth, association via self-polymerization and self-organization, and a combination of self-assembly and electrochemical growth. These methods are both facile and reliable, allowing us to produce ordered and aligned aggregate nanostructures, such as large-area arrays of nanowires, nanorods, and nanotubes. In addition, we can synthesize nanoscale materials with controlled properties. Large-area ordered aggregate nanostructures exhibit interesting electrical, optical, and optoelectronic properties. We also describe the preparation of large-area aggregate nanostructures of charge transfer (CT) complexes using an organic solid-phase reaction technique. By this process, we can finely control the morphologies and sizes of the organic nanostructures on wires, tubes, and rods. Through field emission studies, we demonstrate that the films made from arrays of CT complexes are a new kind of cathode materials, and we systematically investigate the effects of size and morphology on electrical properties. Low-dimension organic/inorganic hybrid nanostructures can be used to produce new classes of organic/inorganic solid materials with properties that are not observed in either the individual nanosize components or the larger bulk materials. We developed the combined self-assembly and templating technique to construct various nanostructured arrays of organic and inorganic semiconductors. The combination of hybrid aggregate nanostructures displays distinct optical and electrical properties compared with their individual components. Such hybrid structures show promise for applications in electronics, optics, photovoltaic cells, and biology. In this Account, we aim to provide an intuition for understanding the structure-function relationships in organic molecular materials. Such principles could lead to new design concepts for the development of new nonhazardous, high-performance molecular materials on aggregate nanostructures.     

Surface-enhanced Raman scattering in local optical fields of silver and gold nanoaggregates-from single-molecule Raman spectroscopy to ultrasensitive probing in live cells

This Account discusses surface-enhanced Raman scattering at extremely high enhancement levels that can occur for molecules attached to silver and gold nanoclusters. Strongly enhanced and highly confined local optical fields enable surface-enhanced Stokes and anti-Stokes Raman spectroscopy of single molecules even under nonresonant excitation conditions as well as extremely large effective cross sections in two-photon excited Raman spectroscopy. The ability for very sensitive and spatially confined molecular structural probing makes gold and silver nanoclusters very promising tools for studies of small structures in biological materials, such as cellular compartments.     

A surface-enhanced Raman spectroscopy study of thin graphene sheets functionalized with gold and silver nanostructures by seed-mediated growth

We describe a simple method for decorating graphene (1–5 layers) with Au and Ag nanostructures (nanoparticles, nanorods, and nanoplates). We deposit graphene electrostatically from highly-oriented pyrolytic graphite onto Si/SiO₂ surfaces functionalized with (aminopropyl)trimethoxysilane and grow the metal nanostructures by a seed-mediated growth method from hexanethiolate-coated Au monolayer-protected cluster “seeds” that are attached to graphene by hydrophobic interactions. Scanning electron microscopy reveals the selective growth of Au or Ag nanostructures on the graphene surface. In the case of Au, the low pH 2.8 growth solution causes etching of the graphene and formation of scroll-like structures. For Ag, the high pH 9.3 solution does not seem to affect the graphene. Raman spectroscopy is consistent with the graphene morphology and reveals that the presence of Au and Ag nanostructures increases the Raman scattering from the graphene by a factor of about 45 and 150, respectively. This work demonstrates a simple method for decorating graphene with noble metal nanostructures that may have interesting optical, electronic, and chemical properties for applications in nanoelectronics, sensing, and catalysis.     

Localized Surface Plasmon Resonance Biosensing with Large Area of Gold Nanoholes Fabricated by Nanosphere Lithography

Localized surface plasmon resonance (LSPR) has been extensively studied as potential chemical and biological sensing platform due to its high sensitivity to local refractive index change induced by molecule adsorbate. Previous experiments have demonstrated the LSPR generated by gold nanoholes and its biosensing. Here, we realize large uniform area of nanoholes on scale of cm² on glass substrate by nanosphere lithography which is essential for mass production. The morphology of the nanoholes is characterized using scanning electron microscope and atomic force microscope. The LSPR sensitivity of the nanoholes to local refractive index is measured to be 36 nm/RIU. However, the chip has demonstrated high sensitivity and specificity in biosensing: bovine serum albumin adsorption is detected with LSPR peak redshift of 27 nm, and biotin-streptavidin immunoassay renders a LSPR redshift of 11 nm. This work forms a foundation toward the cost-effective, high-throughput, reliable and robust chip-based LSPR biosensor.     

Two-dimensional ultrathin gold film composed of steadily linked dense nanoparticle with surface plasmon resonance

Background

Noble metallic nanoparticles have prominent optical local-field enhancement and light trapping properties in the visible light region resulting from surface plasmon resonances.

Results

We investigate the optical spectral properties and the surface-enhanced Raman spectroscopy of two-dimensional distinctive continuous ultrathin gold nanofilms. Experimental results show that the one- or two-layer nanofilm obviously increases absorbance in PEDOT:PSS and P3HT:PCBM layers and the gold nanofilm acquires high Raman-enhancing capability.

Conclusions

The fabricated novel structure of the continuous ultrathin gold nanofilms possesses high surface plasmon resonance properties and boasts a high surface-enhanced Raman scattering (SERS) enhancement factor, which can be a robust and cost-efficient SERS substrate. Interestingly, owing to the distinctive morphology and high light transmittance, the peculiar nanofilm can be used in multilayer photovoltaic devices to trap light without affecting the physical thickness of solar photovoltaic absorber layers and yielding new options for solar cell design.     

Nanoplasmonic sensing of metal-halide complex formation and the electric double layer capacitor

Many nanotechnological devices are based on implementing electrochemistry with plasmonic nanostructures, but these systems are challenging to understand. We present a detailed study of the influence of electrochemical potentials on plasmon resonances, in the absence of surface coatings and redox active molecules, by synchronized voltammetry and spectroscopy. The experiments are performed on gold nanodisks and nanohole arrays in thin gold films, which are fabricated by improved methods. New insights are provided by high resolution spectroscopy and variable scan rates. Furthermore, we introduce new analytical models in order to understand the spectral changes quantitatively. In contrast to most previous literature, we find that the plasmonic signal is caused almost entirely by the formation of ionic complexes on the metal surface, most likely gold chloride in this study. The refractometric sensing effect from the ions in the electric double layer can be fully neglected, and the charging of the metal gives a surprisingly small effect for these systems. Our conclusions are consistent for both localized nanoparticle plasmons and propagating surface plasmons. We consider the results in this work especially important in the context of combined electrochemical and optical sensors.     

Solid-state thermal dewetted silver nanoparticles onto electrochemically grown self-standing vertically aligned ZnO nanorods for three-dimensional plasmonic nanostructures

Three dimensional (3D) plasmonic nanostructures composed of silver nanoparticles decorated ZnO NRs arrays, have been fabricated by a process combining the electrochemical growth of ZnO NRs and further formation of Ag nanoparticles by the solid-state thermal dewetting (SSD) process. The effect of SSD parameters on the morphological, structural and optical properties of the Ag NPs decorated ZnO NRs arrays has been investigated. It is possible to tune the bandgap of the Ag NPs@ZnO nanorods array 3D plasmonic nanostructure by tailoring the Ag nanoparticle sizes, allowing light manipulation at the nanoscale. The silver nanoparticles attached to the ZnO NRs arrays experienced surface plasmonic coupling effect, causing enhancement in the room temperature photoluminescence (PL) UV emission and quenching the corresponding visible light one. An enhancement in the near band edge emission PL intensity of ZnO to the deep level emission PL intensity ratio after Ag NPs decoration of the ZnO nanostructures corresponding to ca. 11 folds has been observed, indicating that the defect emission is obviously suppressed.     

Conjugated polymer mediated synthesis of nanoparticle clusters and core/shell nanoparticles

Two water soluble conjugated polymers, poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and ammonium ion stabilized poly(phenylene vinylene) (P2), are found to be able to reduce noble metal ions to zero-valent metals via a direct chemical deposition technique. Au nanoparticle clusters can be obtained through reduction of Au³⁺ ions by PEDOT:PSS and the electronic coupling between them can be controlled by HAuCl₄ concentration. Core/shell Ag/polymer nanostructures are prepared from reduction of Ag⁺ ions by P2, which have a ppb detection limit for 4-MBA using surface-enhanced Raman spectroscopy (SERS). This conjugated polymer mediated synthesis of metal nanoparticles may open a new avenue for fabricating nanomaterials and nanocomposites with tunable optical properties that are dominated by their structure and electronic coupling between nanoparticles.     

Gold nanonetwork film on the ITO surface exhibiting one-dimensional optical properties

A network of gold nanostructures exhibiting one-dimensional gold nanostructure properties may become a prospective novel structure for optical, electrical and catalytic applications benefited by its unusual characteristics resulting from the collective properties of individual nanostructures in the network. In this paper, we demonstrate a facile method for the formation of high-density gold nanonetwork film on the substrate surface composed of quasi-1D nanoparticles (typically fusiform) with length ca. 10 nm - via reduction of gold ions in the presence of nanoseeds attached surface, binary surfactants of cetyltrimethylammonium bromide and hexamethyleneteramine and Ag⁺ ions. The length of the nanonetworks can be up to ca. 100 nm, which corresponds to the aspect ratio of ca. 10. The quasi-1D gold nanostructures as well as the nanonetworks were found to be sensitive to the binary surfactants system and the Ag⁺ ions as they can only be formed if all the chemicals are available in the reaction. The nanonetworks exhibit unique 1D optical properties with the presence of transverse and longitudinal surface plasmon resonance absorption. Owing to their peculiar structures that are composed of small quasi-1D nanoparticles, the nanonetworks may produce unusual optical and catalytic properties, which are potentially used in surface-enhanced Raman scattering, catalysis and optical and non-linear optical applications.     

Super-SERS-active and highly effective antimicrobial Ag nanodendrites

We have developed simple and green electrochemistry to synthesize Ag nanostructures with high purity, good crystallinity and smooth surface for applications as super-SERS (surface-enhanced Raman scattering), SERS-active substrates and with highly effective antimicrobial activities. This synthesis takes place in a clean and slow reaction environment without any chemical additives, which ensures an ultrahigh active surface of the as-synthesized Ag nanostructures owing to their purity, good crystallinity and smooth morphology. Using this method, we synthesized nearly perfect Ag nanodendrites (NDs), which exhibit super-SERS sensitivity when they are used to detect the SERS spectra of rhodamine 6G at concentrations as low as 5 × 10(-16) M, and have an ultrahigh electromagnetic (EM) enhancement factor of the order of 10(13), breaking through the theoretical limit of EM enhancement. Meanwhile, the as-synthesized Ag NDs possess highly effective antimicrobial activities for Escherichia coli, Candida albicans and Staphylococcus aureus, which are over 10 times that of silver nanoparticles. Additionally, the basic physics and chemistry involved in the fabrication of Ag nanostructures are pursued. These investigations show that silver nanostructures with highly active surfaces can make the most of Ag nanostructures functioning as super-SERS-active substrates and multiple antibiotics.     

Synthesis and field emission properties of different ZnO nanostructure arrays

In this article, zinc oxide (ZnO) nanostructures of different shapes were fabricated on silicon substrate. Well-aligned and long ZnO nanowire (NW) arrays, as well as leaf-like ZnO nanostructures (which consist of modulated and single-phase structures), were fabricated by a chemical vapor deposition (CVD) method without the assistance of a catalyst. On the other hand, needle-like ZnO NW arrays were first fabricated with the CVD process followed by chemical etching of the NW arrays. The use of chemical etching provides a low-cost and convenient method of obtaining the needle-like arrays. In addition, the field emission properties of the different ZnO NW arrays were also investigated where some differences in the turn-on field and the field-enhancement factors were observed for the ZnO nanostructures of different lengths and shapes. It was experimentally observed that the leaf-like ZnO nanostructure is most suitable for field emission due to its lowest turn-on and threshold field as well as its high field-enhancement factor among the different synthesized nanostructures.     

Fabrication of Large Area Periodic Nanostructures Using Nanosphere Photolithography

Large area periodic nanostructures exhibit unique optical and electronic properties and have found many applications, such as photonic band-gap materials, high dense data storage, and photonic devices. We have developed a maskless photolithography method—Nanosphere Photolithography (NSP)—to produce a large area of uniform nanopatterns in the photoresist utilizing the silica micro-spheres to focus UV light. Here, we will extend the idea to fabricate metallic nanostructures using the NSP method. We produced large areas of periodic uniform nanohole array perforated in different metallic films, such as gold and aluminum. The diameters of these nanoholes are much smaller than the wavelength of UV light used and they are very uniformly distributed. The method introduced here inherently has both the advantages of photolithography and self-assembled methods. Besides, it also generates very uniform repetitive nanopatterns because the focused beam waist is almost unchanged with different sphere sizes.     

Fabrication and spectroscopic investigation of branched silver nanowires and nanomeshworks

Wide wavelength ranges of light localization and scattering characteristics can be attributed to shape-dependent longitude surface plasmon resonance in complicated nanostructures. We have studied this phenomenon by spectroscopic measurement and a three-dimensional numerical simulation, for the first time, on the high-density branched silver nanowires and nanomeshworks at room temperature. These nanostructures were fabricated with simple light-induced colloidal method. In the range from the visible to the near-infrared wavelengths, light has been found effectively trapped in those trapping sites which were randomly distributed at the corners, the branches, and the junctions of the nanostructures in those nanostructures in three dimensions. The broadened bandwidth electromagnetic field enhancement property makes these branched nanostructures useful in optical processing and photovoltaic applications.     

Effects of annealing on bandgap and surface plasmon resonance enhancement in Au/SnO2 quantum dots

Au/SnO₂ quantum dots (AuSQDs) were synthesized, and the effects of annealing on their structural and optical properties were examined. Significant changes were observed in the bandgap and surface plasmon resonance (SPR) of the AuSQDs after thermal treatment at different temperatures (400, 500, and 600 °C). The properties of the as-prepared and annealed samples were characterized via X-ray diffraction analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy, and diffuse reflectance spectroscopy. Annealing reduced the bandgap from 3.03 to 2.33 eV and increased the crystallinity while maintaining an average crystallite size below 10 nm. XPS valence band (VB) profiles provided information regarding the VB edge potentials, which helped to determine the conduction band edge potentials. An enhancement in the SPR of the Au nanoparticles was observed for AuSQD-500, which had the smallest bandgap among the samples investigated.     

Nanoscale chemical and structural study of Co-based FEBID structures by STEM-EELS and HRTEM

Nanolithography techniques in a scanning electron microscope/focused ion beam are very attractive tools for a number of synthetic processes, including the fabrication of ferromagnetic nano-objects, with potential applications in magnetic storage or magnetic sensing. One of the most versatile techniques is the focused electron beam induced deposition, an efficient method for the production of magnetic structures highly resolved at the nanometric scale. In this work, this method has been applied to the controlled growth of magnetic nanostructures using Co2(CO)8. The chemical and structural properties of these deposits have been studied by electron energy loss spectroscopy and high-resolution transmission electron microscopy at the nanometric scale. The obtained results allow us to correlate the chemical and structural properties with the functionality of these magnetic nanostructures.     

Guided self-assembly of lateral InAs/GaAs quantum-dot molecules for single molecule spectroscopy

We report on the growth and characterization of lateral InAs/GaAs (001) quantum-dot molecules (QDMs) suitable for single QDM optical spectroscopy. The QDMs, forming by depositing InAs on GaAs surfaces with self-assembled nanoholes, are aligned along the [ ] direction. The relative number of isolated single quantum dots (QDs) is substantially reduced by performing the growth on GaAs surfaces containing stepped mounds. Surface morphology and X-ray measurements suggest that the strain produced by InGaAs-filled nanoholes superimposed to the strain relaxation at the step edges are responsible for the improved QDM properties. QDMs are Ga-richer compared to single QDs, consistent with strain- enhanced intermixing. The high optical quality of single QDMs is probed by micro-photoluminescence spectroscopy in samples with QDM densities lower than 10⁸ cm⁻².     

Tunable plasmon resonances in a metallic nanotip-film system

Tip-enhanced Raman spectroscopy (TERS) has emerged as a powerful tool for optical imaging at nanoscale spatial resolution, and for investigating the vibrational properties of molecules adsorbed on a substrate. Plasmonic enhancement of the electromagnetic fields near a metallic nanostructure plays a very important role in TERS, where resonant excitation of plasmons is crucial. When two metallic nanostructures are placed at a gap of nanometric distance, their plasmons can interact with one other and result in hybridized shifted plasmon modes. Here, we apply this idea to TERS and demonstrate a significant tunability of the plasmon resonance enabling large electric field enhancement at a desired excitation wavelength. This finding paves the way for efficient optimization of TERS in imaging and spectroscopy applications.