Plasmonic photocatalyst has attracted much attention since plasmonic nanostructures were demonstrated to increase the visible and/or infrared light activity of conventional semiconductor and further to improve the performance of the photoelectrochemical (PEC) water splitting. Here we utilized highly conductive reduced graphene oxide (RGO) nanosheets and gold nanotriangles (NTs) with remarkable localized surface plasmon resonance (LSPR) in the visible region to improve the photoresponse of TiO<sub>2</sub> branched nanorods (NRs), which were fabricated by a two-step hydrothermal grown method. Upon the concurrent addition of Au NTs and RGO, the photocurrent, which was measured by three-electrode PEC reactor under illumination of simulated solar light, showed a pronounced ~37% improvement compared to TiO<sub>2</sub> branched NRs and ~450 % enhancement compared to TO<sub>2</sub> NRs. It iss believed that not only the photon scattering effect and LSPR response in visible region (~675 and ~530 nm) of Au NTs but also the high conductivity and large surface area of RGO assisted in harvesting visible light, accelerated charge carrier transportation, and reduced the charge recombination rate to improve the PEC water splitting performance of TiO<sub>2</sub>.
By using both linear and nonlinear terahertz spectroscopy on epitaxial Bi and Bi<sub>1-x</sub>Sb<sub>x</sub> thin films, we systematically investigated the linear and nonlinear terahertz dynamics of Dirac electrons. The linear terahertz transmittance was analyzed by the Drude model up to 50 THz, and then the plasma frequency and the damping constant were evaluated as functions of the film thickness and Sb-concentration. We found surface metallic state for Bi ultra-thin films, while semimetal to semiconductor crossover for Bi<sub>1-x</sub>Sb<sub>x</sub> thin films. In the nonlinear terahertz spectroscopy, the terahertz transmittance increases with increasing the field strength, which could be assigned to the carrier acceleration along the Dirac-like band dispersion at the L point in the Brillouin zone. In addition, we observed the terahertz-induced absorption in terahertz-pump and terahertz-probe spectroscopy, which could be assigned to carrier generation due to Zener tunneling in Dirac band structure. The results demonstrate that Bi-related materials are promising candidates for future nonlinear terahertz devices.
We experimentally demonstrate photocurrent generation from a titanium nitride thin film forming an interface to a zinc
oxide thin film by the illumination of visible light up to 800 nm in wavelength. The photocurrent is attributed to hot
electrons excited in titanium nitride whose excitation is not limited by the bandgap of zinc oxide. Our result paves the
way to use titanium nitride instead of metals for phototectors and solar photocatalysis.
Plasmon-enhanced vibrational spectroscopy, including surface-enhanced infrared absorption spectroscopy (SEIRA) and surface-enhanced Raman scattering (SERS), has attracted great attention in molecular sensing and nano-spectroscopy. In this work, we present a facile <i>in situ</i>-controlled method for the chemical synthesis of patchy SiO<sub>2</sub>@Au core-shell nanoparticles with multiple plasmonic nanogaps. The multiple sizes and shapes of Au nano-islands on patchy Au nanoshells and their plasmonic coupling exhibit broadband resonances ranging from the near infrared (NIR) region to the middle infrared (MIR) region, making patchy Au particles ensemble suitable for both SEIRA and SERS applications. In the SEIRA application, we demonstrate <i>in situ</i> and real-time monitoring of monolayer of reduced glutathione molecules (GSH) adsorbed on the plasmonic Au surface. By using GSH as the molecular linker, we also demonstrate <i>in situ </i>detection of trace amount of mercuric ions in water at nanomolar level. In the SERS application, we show the applicability of patchy Au nanoparticles for SERS at 785 nm excitation.
We demonstrate the development of colloidal lithography technique to fabricate large-area plasmonic perfect absorbers using Al, which is an earth abundant low-cost plasmonic material in contrast to Au and Ag. Using numerical electromagnetic simulations, we optimize the geometrical parameters of Al perfect absorbers (AlPAs) with resonances at desired wavelengths depending on the applications. The fabricated AlPAs exhibit narrowband absorptions with high efficiency up to 98 %. By tuning AlPAs parameters, the resonance of AlPAs can be tuned from the visible to the middle infrared region. The AlPAs can be applied for spectrally selective infrared devices such as selective thermal emitters, selective surface-enhanced vibrational spectroscopy (SEIRA) for molecular sensing and selective IR detectors. In this report, we demonstrate applications of AlPAs for selective thermal emitters and SEIRA. The results obtained here reveal a simple technique to fabricate scalable plasmonic perfect absorbers as well as their potential applications in optoelectronic and photonic devices.
Nanosphere lithography (NSL) uses self-assembled layers of monodisperse micro-/nano-spheres as masks to fabricate
plasmonic metal nanoparticles. Different variants of NSL have been proposed with the combination with dry etching
and/or angled-deposition. These techniques have employed to fabricate a wide variety of plasmonic nanoparticles or
nanostructures. Here we report another promising extension - moiré nanosphere lithography (MNSL), which
incorporates in-plane twisting between neighboring monolayers, to extend the patterning capability of conventional
NSL. In conventional NSL, the masks, either a monolayer or bilayer, are formed by spontaneous self-assembly.
Therefore, the resulted colloidal crystal configurations are limited. In this work we used sequential stacking of
polystyrene nanosphere monolayers to form a bilayer crystal at the air/water interfaces. During this layer-by-layer
stacking process, a crystal domain in the top layer gains the freedom to positon itself in a relative angle to that in the
bottom layer allowing for the formation of moiré patterns. Subsequent O<sub>2</sub> plasma etching results in a variety of complex
nanostructures that have not been reported before. Using etched moiré patterns as masks, we further fabricated the
corresponding gold nanostructures and characterized their scattering optical properties. We believe this facile technique
provides a new strategy to fabricate novel and complex plasmonic nanostructures or metasurfaces.
Examples of plasmonic excitations in atomically thin metal films and wires are presented. The low-energy electron
energy loss spectroscopy with high momentum and energy resolutions allows us to determine the strongly dispersing
low-energy collective excitation from mid- to near-infrared frequency range with momentum range up to ~0.1 Å<sup>-1</sup>. The dispersion relation is far apart from the light line and strongly reflects the shape and size effects in nanometer to
Ångstrom scale. The two-dimensional type plasmon is observed in metallic atom sheets and the one-dimensional type
plasmons are also measured from some metallic atom chains. From the direct measurement of their plasmonic band
dispersion, we are able to detect the carrier doping effect, electronic correlation effect, and Rashba-spin-orbit splitting
effect in these ultimately tiny systems.
We report on the direct measurement of dispersion relations of plasmons confined in atomically thin metal films and
wires by electron energy loss spectroscopy in wide energy-momentum range. Ultrathin Ag films are prepared on single
crystal Si surfaces by molecular beam epitaxy, and its crystallinity is checked by electron diffraction. For the case of
multi-atomic-layer Ag films, two plasmon modes are observed at around 3.9 eV and 1.8 eV which are localized at the
top and the bottom surfaces of the films, respectively. For the case of Ag monoatomic layer, a single mode is observed
that steeply disperses in the mid-infrared range. Nonlocal and quantum effects are found to be essential in understanding
its full plasmon dispersion curve up to the critical wave number of Landau damping. For the case of Au atom chains, an
anisotropic sound-wave-like plasmon dispersion is found that clearly shows 1D plasmon confinement in each atom chain.