G. Han

Chemically synthesized nanowire TiO2/ZnO core-shell p-n junction array for high sensitivity ultraviolet photodetector

A sol-gel-based ultrathin TiO2 lamination coating was adapted to a hydrothermally grown ZnO nanowire array to realize an all-oxide ultra-sensitive p-n photodiode. The core-shell heterojunction—the key component of the device—is composed of a 5–10 nm thick p-type Cr-doped TiO2 nanoshell and n-type single-crystalline ZnO nanowires (50 nm radius). Owing to the enhanced light scattering and carrier separation in the core-shell architecture, this device exhibits the highest performance among the ZnO nanowire-based photodetectors. At a moderate reverse bias of −5 V and under ultraviolet light illumination at 104 μW, it shows a switch current ratio of 140 and a responsivity as large as 250 A/W, while it shows nearly no response to the infrared and visible light.

Plasmons in nanoscale and atomic-scale systems

Abstract Plasmons in metallic nanomaterials exhibit very strong size and shape effects, and thus have recently gained considerable attention in nanotechnology, information technology, and life science. In this review, we overview the fundamental properties of plasmons in materials with various dimensionalities and discuss the optical functional properties of localized plasmon polaritons in nanometer-scale to atomic-scale objects. First, the pioneering works on plasmons by electron energy loss spectroscopy are briefly surveyed. Then, we discuss the effects of atomistic charge dynamics on the dispersion relation of propagating plasmon modes, such as those for planar crystal surface, atomic sheets and straight atomic wires. Finally, standing-wave plasmons, or antenna resonances of plasmon polariton, of some widely used nanometer-scale structures and atomic-scale wires (the smallest possible plasmonic building blocks) are exemplified along with their applications.

Infrared spectroscopic and electron microscopic characterization of gold nanogap structure fabricated by focused ion beam

Using infrared spectroscopy of plasmonic resonances and mapping of elemental composition and structure, we investigated the correlation between optical and structural properties of nanometre-scale gaps in gold nanorod dimers fabricated by electron beam lithography (EBL) and focused ion beam (FIB) milling. In spite of their very similar scanning electron microscopy (SEM) images, a fully cut nanogap and a shallower cut with slight imperfection near the gap region were clearly distinguished by their strongly different infrared plasmonic resonance behaviour. The differences in the infrared spectra are related to different structural and chemical results from elaborated cross-sectional transmission electron micrographs and energy dispersive x-ray spectrometry (EDX) mapping of the gap region.

Optical detection of plasmonic and interband excitations in 1-nm-wide indium atomic wires

Infrared spectroscopy is demonstrated to sensitively detect electronic excitations in 1-nm-wide wires made of indium. The polarization-dependent spectra measured at room temperature show a strong broadband plasmonic absorption feature in the direction parallel to the wires, while in the perpendicular direction the wires stay nearly transparent in the same spectral range. At 88 K the wires do not show this broadband absorption anymore, but instead, several interband-transition features arise for both polarizations, in agreement to the gap opening of the metal-to-insulator transition as known for this one-dimensional structure.

Surface metallic states in ultrathin Bi(001) films studied with terahertz time-domain spectroscopy

Dynamical response of surface metallic states in single crystalline ultrathin Bi(001) films on Si(111) 7 × 7 surface was investigated at a spectral range of 0.1–12 THz by broadband terahertz time-domain spectroscopy. The observed transmittance increased with a decrease in the thickness, without showing a gap structure. The measured complex dielectric dispersion was analyzed using a Drude model, and the plasma frequency (ωp) and damping constant (γ) were found to be inversely proportional to the thickness. The results strongly indicate the existence of surface metallic states, whose carrier density and damping constant are estimated to be 3.08 × 1019 cm−3 and 4.83 × 102 THz, respectively.

Nanoantennas for surface enhanced infrared spectroscopy: Effects of interaction and higher order resonant excitations

The sensitivity in surface enhanced infrared spectroscopy (SEIRS) strongly depends on where the resonant excitation is spectrally located compared to the molecular vibration that is to be enhanced. In this contribution, we study the effect of coupling in the electromagnetic properties of 2D gold nanorod arrays in the IR. We also study the SEIRS activity of higher order resonant excitations in long nanoantennas to identify polaritonic signals of a supporting SiO 2 layer with nanometer thickness (3 nm) on a silicon substrate.