Hsin Wei Huang

Plasmonic ZnO/Ag Embedded Structures as Collecting Layers for Photogenerating Electrons in Solar Hydrogen Generation Photoelectrodes

A new fabrication strategy in which Ag plasmonics are embedded in the interface between ZnO nanorods and a conducting substrate is experimentally demonstrated using a femtosecond-laser (fs-laser)-induced plasmonic ZnO/Ag photoelectrodes. This fs-laser fabrication technique can be applied to generate patternable plasmonic nanostructures for improving their effectiveness in hydrogen generation. Plasmonic ZnO/Ag nanostructure photoelectrodes show an increase in the photocurrent of a ZnO nanorod photoelectrodes by higher than 85% at 0.5 V. Both localized surface plasmon resonance in metal nanoparticles and plasmon polaritons propagating at the metal/semiconductor interface are available for improving the capture of sunlight and collecting charge carriers. Furthermore, in-situ X-ray absorption spectroscopy is performed to monitor the plasmonic-generating electromagnetic field upon the interface between ZnO/Ag nanostructures. This can reveal induced vacancies on the conduction band of ZnO, which allow effective separation of charge carriers and improves the efficiency of hydrogen generation. Plasmon-induced effects enhance the photoresponse simultaneously, by improving optical absorbance and facilitating the separation of charge carriers.

Fast fabrication of a Ag nanostructure substrate using the femtosecond laser for broad-band and tunable plasmonic enhancement.

Using a femtosecond laser, we have transformed the laser-direct-writing technique into a highly efficient method that can process AgO(x) thin films into Ag nanostructures at a fast scanning rate of 2000 μm(2)/min. The processed AgO(x) thin films exhibit broad-band enhancement of optical absorption and effectively function as active SERS substrates. Probing of the plasmonic hotspots with dyed polymer beads indicates that these hotspots are uniformly distributed over the treated area.

Three‐Dimensional Plasmonic Micro Projector for Light Manipulation

photovoltaics, [ 5 ] super-resolution imaging, [ 6 ] and various twodimensional plasmonic lens. [ 7 ] Besides, using nanostructures to project SPP plane waves into the adjacent free space is also an important issue. The interactions of plasmonic nanostructure on SPP wave involve not only the in-plane behavior, but also out-of-plane scattering which is captured as the far-fi eld radiated light. [ 8 ] A few theoretical approaches to convert the confi ned surface plasmons into radiated waves have been proposed. [ 9 ] It is highly desirable to extend the application range of plasmonic devices into the domain of three-dimensional light manipulation. [ 10 ] Recently, three-dimensional focusing and diverging of SPP waves by a quarter circular structure composed of gold (Au) nanobumps were studied. [ 11 ] The forward and backward scattering from individual Au nanobump are observed above and below Au surface, respectively. Hence, the Au nanobumps confer additional three-dimensional propagating wave vectors ( k x , k y , k z ) on SPP wave for departing from surface. Therefore, it is possible to manipulate the three-dimensional plasmonic scattering into specifi c geometry by arranging the Au nanobumps, which is schematically depicted in Figure 1 a. In this paper, we manipulate the scattering of SPP waves by various plasmonic structures composed of arranged nanobumps on a gold thin fi lm. Upon controlling the geometry of the plasmonic structures, the height, position, and pattern of scattered light can be modifi ed as desired. It provides a simple and effi cient way to project a specifi c light pattern into free space, and demonstrate the capability of three-dimensional light manipulation.

Multi-level surface enhanced Raman scattering using AgOx thin film.

Ag nanostructures with surface-enhanced Raman scattering (SERS) activities have been fabricated by applying laser-direct writing (LDW) technique on silver oxide (AgOx) thin films. By controlling the laser powers, multi-level Raman imaging of organic molecules adsorbed on the nanostructures has been observed. This phenomenon is further investigated by atomic-force microscopy and electromagnetic calculation. The SERS-active nanostructure is also fabricated on transparent and flexible substrate to demonstrate our promising strategy for the development of novel and low-cost sensing chip.

Three-dimensional light manipulation by gold nanobumps

The scattering of surface plasmon polariton (SPP) waves can be manipulated by various plasmonic structures. The plasmonic structure composed of arranged subwavelength nanobumps on a gold thin film is the promising structure to manipulation SPP wave. By controlling the geometric shape of the structures, the height, position, and pattern of scattered light from SPP wave can be modulated as desired. A clear single focusing spot can be reconstructed at a specific altitude by a particular curved structure with appropriate curvature and adjacent interspacing of nanobumps. The designed light patterns reconstructed by the focusing spot from the arranged curved structures at a specific observation plane are clearly demonstrated.

Plasmonic zinc oxide/silver photoelectrode for green hydrogen production

By learning from nature, it may become possible to realize efficient, stable energy conversion by developing photosynthesis technologies that use only earth-abundant materials and operate under mild conditions. In past decades, the photoelectrolysis of water has attracted continual interest as a potential route to sustainable hydrogen production for low-cost green energy.1–4 Water splitting is regarded as artificial photosynthesis in which materials convert the energy of sunlight into chemical energy to produce chemical fuels (hydrogen and oxygen). Our goal is to improve the efficiency of water splitting by photoelectrolysis using plasmonic resonance, that is, the coherent oscillation of free electrons in a noble metal driven by incident electromagnetic waves. In a recent publication,5 we demonstrated the use of direct writing by a femtosecond (fs) laser to fabricate patternable plasmonic silver (Ag) particles from a silver(I,III) oxide (AgO) film. Figure 1(a) shows an artist’s impression of a sample and its operating principle. The embedded Ag nanostructures serve as plasmonic couplers to enhance the photoactivity of a photoelectrode consisting of a zinc oxide (ZnO) nanorod array. In this configuration, plasmon damping can originate in the Ag nanostructures, producing both scattering and absorption behavior. The embedded plasmonics act as subwavelength scattering elements to trap randomly propagating plane waves into an absorbing semiconductor material by multiple high-angle scattering into the ZnO nanorods. This increases the effective optical path length in the photoelectrode. In addition, scattering from the plasmonics on Figure 1. (a) Artist’s impression of zinc oxide/silver (ZnO/Ag) plasmonic photoelectrode. CB: Conduction band. VB: Valence band. e: Electron. h: Hole. H2: Molecular hydrogen. H+: Hydrogen ions. (b) Scanning electron microscopy (SEM) image of sample.

Applications of plasmonic hotspots on laser-treated AgO x thin film

The femto-second-laser-treated AgOx thin film is used as the plasmon-active substrate for the applications of surface-enhanced Raman scattering (SERS), molecule sensing, and photocatalyst. The proposed technique is helpful for the solar energy harvest, green energy source, and biosensing with high efficiency and throughput.

Three-dimensional light manipulation by plasmonic nanostructure

Nanobump structures are fabricated on the gold thin film by femtosecond laser direct writing (fs-LDW) technique. The height and diameter of the gold nanobump are about 30nm, and 400 nm, respectively. The scattering light of surface plasmon wave radiated from a nanobump is observed using a total internal reflection microscopy. A quarter-circle structure composed of nanobumps is designed and produced to manipulate scattering light into specific pattern: The focusing and diverging of the quarter circular structure in three dimensional space are demonstrated. The polarization properties of focusing spot are also examined.