Xibo Li

In situ organic coating of metal nanoparticles

A physical approach has been developed to in situ coat metal nanoparticles with a nanolayer of organic hydrocarbon film. Copper nanoparticles produced by the flow-levitation method were in situ coated with hydrocarbon films via a dielectric barrier discharge. The hydrocarbon-coated Cu nanoparticles have been analyzed by transmission electron microscopy and infrared absorption to determine the size distribution and dispersion of nanoparticles as well as the chemical components and thickness of the coatings.

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.

Nanodisk-Induced Modification of Plasmon Coupling and Appearance of Fano Resonance Without Symmetry Breaking in Concentric Ag Nanoring-Nanodisk

Simulation based on the finite-difference time-domain (FDTD) has been performed, and theoretical models comprising concentric Ag nanoring-nanodisk of different parameters have been constructed to research their plasmon properties. According to unique electronic properties of concentric nanoring-nanodisk, abundant plasmon properties could be obtained at their interfaces, including dipole, quadrupole, and octupole plasmon resonance modes. Complex field distributions which are induced by concentric nanoring-nanodisk support the possibility to create dark resonance mode. This can lead to Fano-like resonance combining with bright resonance mode by predominantly dipole resonance. A concentric nanoring-nanodisk system has been proved to support Fano-like resonance without symmetry breaking. At the frequency of the Fano resonance, strong localized optical fields can be obtained. Narrow spectral features with high local fields of Fano resonances make it possible to achieve many applications based on surface plasmon property.

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.

Surface-initiated phase transition in solid hydrogen under the high-pressure compression

The large-scale molecular dynamics simulations have been performed to understand the microscopic mechanism governing the phase transition of solid hydrogen under the high-pressure compression. These results demonstrate that the face-centered-cubic-to-hexagonal close-packed phase transition is initiated first at the surfaces at a much lower pressure than in the volume and then extends gradually from the surface to volume in the solid hydrogen. The infrared spectra from the surface are revealed to exhibit a different pressure-dependent feature from those of the volume during the high-pressure compression. It is thus deduced that the weakening intramolecular H-H bonds are always accompanied by hardening surface phonons through strengthening the intermolecular H2-H2 coupling at the surfaces with respect to the counterparts in the volume at high pressures. This is just opposite to the conventional atomic crystals, in which the surface phonons are softening. The high-pressure compression has further been predic...