Yougen Yi

Ordered array of Ag semishells on different diameter monolayer polystyrene colloidal crystals: An ultrasensitive and reproducible SERS substrate

Ag semishells (AgSS) ordered arrays for surface-enhanced Raman scattering (SERS) spectroscopy have been prepared by depositing Ag film onto polystyrene colloidal particle (PSCP) monolayer templates array. The diversified activity for SERS activity with the ordered AgSS arrays mainly depends on the PSCP diameter and Ag film thickness. The high SERS sensitivity and reproducibility are proved by the detection of rhodamine 6G (R6G) and 4-aminothiophenol (4-ATP) molecules. The prominent enhancements of SERS are mainly from the “V”-shaped or “U”-shaped nanogaps on AgSS, which are experimentally and theoretically investigated. The higher SERS activity, stability and reproducibility make the ordered AgSS a promising choice for practical SERS low concentration detection applications.

Plasmonic Absorption Enhancement in Elliptical Graphene Arrays

In this paper, we come up with a wavelength tunable absorber which is made up of periodically elliptical graphene arrays in the far-infrared and terahertz regions. Through simulation, we find that we can increase the length of long axis of the ellipse, raise the incidence angles of TM- and TE-polarization (TM- and TE-polarization indicate the direction of the incident electric field along the direction of the x and the y axis, respectively.) within certain limits, and increase the chemical potential of graphene, so as to enhance the absorption of light in the elliptical graphene arrays. We also compare the absorption spectra of the original structure and the complementary structure, and find that the absorption of the original structure is higher than that of the complementary structure. In the end, we study the changes in the absorption rate of the double layer structure of the elliptical array with the increase in the thickness of SiO2. The elliptical array structure can be applied to tunable spectral detectors, filters and sensors at far-infrared and terahertz wavelengths.

Mesoporous gold sponges: electric charge-assisted seed mediated synthesis and application as surface-enhanced Raman scattering substrates

Mesoporous gold sponges were prepared using 4-dimethylaminopyridine (DMAP)-stabilized Au seeds. This is a general process, which involves a simple template-free method, room temperature reduction of HAuCl4·4H2O with hydroxylamine. The formation process of mesoporous gold sponges could be accounted for the electrostatic interaction (the small Au nanoparticles (~3 nm) and the positively charged DMAP-stabilized Au seeds) and Ostwald ripening process. The mesoporous gold sponges had appeared to undergo electrostatic adsorption initially, sequentially linear aggregation, welding and Ostwald ripening, then, they randomly cross link into self-supporting, three-dimensional networks with time. The mesoporous gold sponges exhibit higher surface area than the literature. In addition, application of the spongelike networks as an active material for surface-enhanced Raman scattering has been investigated by employing 4-aminothiophenol (4-ATP) molecules as a probe.

Nanoparticle attachment on Ag nanorings and nanoantenna for large increases of surface-enhanced Raman scattering

A simple and inexpensive approach based on the heat-treatment of Ag+/PVA/PVP composite film on quartz glass has been developed for fabricating large-area Ag nanorings attached small nanoparticles. The explosive decomposition of AgNO3, PVA and PVP by calcination could explain their formation. A maximum enhancement factor of 1.9 × 1010 can be obtained with the self-organized Ag nanorings attached small nanoparticles. Moreover, using the three-dimensional finite-difference time-domain (3D-FDTD) simulations, we stipulate that the EF can be obviously improved via some small Ag particle attachment on these nanorings because of the strong coupling between the discrete plasmon states of the small nanoparticles and the term of propagating plasmons of the Ag nanorings. Understanding and realization of the enhancing mechanism of nanostructured surface attachment small nanoparticles could have potential to effectively improve the SERS property of the SERS substrates.

Self-Organized Ag Nanorings Antenna Substrates for Surface-Enhanced Raman Spectroscopy

We investigate the surface-enhanced Raman spectroscopy of Ag nanorings antenna in both experiment and simulation. Self-organized Ag nanorings antenna were formed on quartz glass wafers by a simple chemistry reaction without any template. The three-dimensional finite-difference time-domain simulation calculations indicate that the electric field enhancement of Ag nanoring antenna is strongly dependent on the gap distance. A very strong surface plasmon coupling in the gap region of Ag nanoring antenna is observed, whose field intensity is enhanced four times compared to that for Ag nanodomes antenna with the same gap distance. Surface-enhanced Raman scattering (SERS) measurements have shown that the SERS intensity acquired from the Ag nanoring antenna is about 16 times stronger than that obtained from Ag nanodomes antenna. These results pave the way to design plasmonic nanostructures for practical applications that require coupled metallic nanoparticles with enhanced electric fields.

Surface-Plasmon-Enhanced Band Emission and Enhanced Photocatalytic Activity of Au Nanoparticles-Decorated ZnO Nanorods

We reported a simple hydrothermal method to fabricate Au nanoparticles-decorated ZnO nanorods (ZnO-NRs) using aligned ZnO-NRs arrays as a template. Via changing the concentration of HAuCl4 aqueous solution, the size and density of Au nanoparticles (NPs) on the surface of ZnO-NRs could be readily tuned. The photoluminescence of Au-NPs decorated ZnO-NRs were investigated, in order to optimize the configuration of the Au-NPs decorated ZnO-NRs system realizing the maximum band emission. Due to a synergistic effect of the adjacent Au NPs and ZnO-NRs and efficiently coupled localized surface plasmon resonance (SPR) excitation, an optimized sample employing Au NPs with 15-nm size showed best catalytic efficiency. We have proposed a mechanism that is the electron transfer from surface-plasmon-stimulated \Au NPs to the conduction band of ZnO-NRs. These results demonstrate that Au NPs can significantly enhance the charge separation by extracting electrons from the photoexcited ZnO and consequently improve the photocatalytic activity of the composites.

Study of strong dipole and quadrupole plasmon resonance in Ag nanorings antenna

Self-organized Ag nanorings antenna were formed on quartz glass wafers by a simple chemistry reaction without any template. By using absorption measurements and three-dimensional finite-difference time-domain (3D-FDTD) calculations, the dipole and quadrupole plasmon resonances of Ag nanorings antenna were investigated experimentally and theoretically. Calculations have shown that large electric fields are confined at the quadrupole of the Ag nanoring, leading to quadrupole plasmon resonances. Compared the electric enhancement factor of the exterior surfaces of Ag nanoring, the electric enhancement factor of the interior surface is about six times excited by an incident light with 514.5 nm wavelength. Furthermore, the highest electric-field intensity of Ag nanorings is around four times larger than that for Ag nanodome with the same condition. These results pave the way to design plasmonic nanostructures for practical applications that require metallic nanoparticles with enhanced electric fields.

Plasmonic absorption characteristics based on dumbbell-shaped graphene metamaterial arrays

Abstract In this paper, we proposed a theoretical model in the far-infrared and terahertz (THz) bands, which are dumbbell-shaped graphene metamaterial arrays with a combination of graphene nanobelt and two semisphere-suspended heads. We report a detailed theoretical investigation on how to enhance localized electric field and the absorption in the dumbbell-shaped graphene metamaterial arrays. The simulation results show that absorption characteristics can be changed by changing the geometrical parameters of the structure and the Fermi level of graphene . Furthermore, we have discovered that the resonant wavelength is insensitive to TM polarization. In addition, we also find that the double-layer graphene arrays have better absorption characteristics than single-layer graphene arrays. This work allows us to achieve tunable terahertz absorber and may also provide potential applications in optical filter and biochemical sensing.

Experimental and simulative study on surface enhanced Raman scattering of rhodamine 6G adsorbed on big bulk-nanocrystalline metal substrates

Big bulk-nanocrystalline metal materials of silver (Ag) and aluminum (Al) for surface-enhanced Raman scattering (SERS) spectroscopy have been synthesized in a mold under different pressures using vacuum-warm-compaction technology. It was discovered that pressure could control the SERS activity of the bulk-nanocrystalline material. SERS properties of the bulk-nanocrystalline material in the presence of adsorbed rhodamine (R6G) could be obtained through selecting a proper pressure. Compared with the Ag nanoparticles (Al nanoparticles), the SERS peak intensity of R6G adsorbed on the bulk-nanocrystalline material is about 1000 times (100 times) stronger. The electric field enhancement of the bulk-nanocrystalline material has been described to be a systematic investigation by using three-dimensional finite-difference time-domain (3D-FDTD) simulation. The FDTD calculations have shown that the electric field enhancement of the bulk-nanocrystalline material is strongly dependent on the gap distance. In summary, SERS active bulk-nanocrystalline materials have been synthesized simply, greenly and cost effectively by the method reported here, and this method is expected to be utilized in the development of SERS-based analytical devices.

Nanoscale Energy Confinement and Hybridization of Surface Plasmons Based on Skin Depth in Au/Ag Core-Shell Nanostructures

The electric field tends to become distributed within a conductor due to skin effect such that the field density is largest near the surface of the conductor and decreases with greater depths in the conductor. The electric field mainly distributes at the skin of the conductor, between the outer surface and a level called the skin depth. For a plasmonic nanosystem smaller than the skin depth, oscillations of the metal electrons will be driven by the optical electric field penetrating the entire system. This effect will induce confinement of optical energy inside the whole systems and influence the performance of surface plasmon. A theoretical model that a gold core is embedded within Ag nanoshell is constructed to simulate hybridization of surface plasmon and energy confinement in Au/Ag core/shell nanostructures based on the skin depth. Indeed, nanoshells in the core/shell system greatly influence the surface plasmon resonance, and the shell frequency is tuned efficiently through hybridization of surface plasmon. The plasmon resonances in core/shell particles can be understood in terms of hybridization between the plasmon modes of the surfaces and interfaces supported by cavity of the metallic shells. The hybridized plasmons induced by interaction of these plasmon modes can result in energy alteration over a wide range in wavelength. Combining with extinction spectra and field distributions, it can be seen that skin depth plays an important role in energy confinement and hybridization of surface plasmom.