Minji Gwon

Resistance state-dependent barrier inhomogeneity and transport mechanisms in resistive-switching Pt/SrTiO3 junctions

We investigated the current–voltage (I−V) and photocurrent characteristics of Pt/Nb-doped SrTiO3 (001) single-crystal junctions that exhibit resistive-switching behaviors. The temperature-dependent I−V data and the photocurrent spectra showed that the barrier height fluctuation depended on the resistance state but the mean barrier height was nearly constant regardless of the junctions’ resistance state. In addition, local barrier height variations allowed transitions from thermionic to tunneling transport for the low-resistance state.

Surface-plasmon-enhanced visible-light emission of ZnO/Ag grating structures.

We investigated the optical properties of ZnO/Ag grating structures fabricated by sputtering and nanoimprint lithography. The grating structures exhibited multiple peak features in broad visible-range photoluminescence (PL) spectra. The PL intensity of the grating was larger than that of a planar thin film by up to two orders of magnitude. The surface plasmon (SP) dispersion relation suggested excitation of SPs with various energies of the grating, explaining the broad PL emission. The spectral dependence of the PL intensity was also well supported by the experimental reflectance spectra and the simulated electric field distribution at the ZnO/Ag interface.

Plasmonic coupling in three-dimensional Au nanoparticle assemblies fabricated by anodic aluminum oxide templates

We investigated optical properties of three-dimensional (3D) assemblies of Au nanoparticles (NPs), which were fabricated by dewetting of thin Au layers on anodic aluminum oxides (AAO). The NP assembly had hexagonal array of repeated multiparticle structures, which consisted of six trimers on the AAO surface and one large NP in the AAO pore (pore-NP). We performed finite difference time-domain simulation to explain the optical response of the NP assemblies and compared the calculation results with experimental data. Such complementary studies clearly revealed how the plasmonic coupling between the constituent NPs influenced the spectral response of our NP assemblies. In particular, comparison of the assemblies with and without pore-NPs suggested that strong plasmonic coupling between trimers and pore-NP significantly affected the spectra and the field distribution of the NP assemblies. Plasmonic multi-NP assemblies could provide us new platforms to realize novel optoelectronic devices.

Beneficial roles of Al back reflectors in optical absorption of Si nanowire array solar cells

We investigate the influence of Al back reflectors on the optical absorption spectra of Si nanowire (NW) arrays by using the finite-difference time-domain simulation method. A flat Al layer enhances the absorption in the NW array due to not only the reflection-induced optical path length enlargement but also reflection of light between NWs and localized surface plasmon induced optical field confinement. An Al underlayer with a grating structure allows grating-coupled surface plasmon polariton excitation and raise the optical absorption in the Si NWs. Interplay among all these factors on the optical absorption and expected solar cell performance of the NW arrays is discussed.

Exciton diffusion in near-infrared absorbing solution-processed organic thin films.

We report on singlet-singlet annihilation and exciton diffusion in as-prepared p-type and annealed n-type thin films of the low-bandgap quinoidal quaterthiophene [QQT(CN)4] using ultrafast transient absorption measurements. The decay dynamics of exciton populations are well described by a one-dimensional diffusion-limited bimolecular recombination, indicating that the singlet excitons migrate preferentially along the stacking direction. Our results show that the exciton diffusion constants in QQT(CN)4 films do not vary significantly upon thermal annealing. Exciton diffusion lengths are measured to be as high as 4 and 5 nm in as-prepared and annealed QQT(CN)4 films, respectively. We also observe an influence of the excitation densities on the singlet exciton diffusion, which is attributed to phonon scattering. Because of the possibility of patterning p-n regions in QQT(CN)4 films by thermal nanolithography techniques, this study provides important insight not only into the photophysical properties of quinoidal oligothiophene derivatives but also for their future integration into high-performance p-n nanostructured near infrared light-sensing devices.

Fabrication of polarization-dependent reflective metamaterial by focused ion beam milling.

By focused ion beam milling, we fabricated near-IR reflective metamaterials consisting of nano-aperture arrays. Optimum parameters of ion beam current and accelerating voltage in the fabrication process are obtained. Nano-apertures constituting reflective metamaterial are successfully milled, and possess a reflective resonance in the near-IR spectral range. With a double-split-ring resonator structure for the nano-aperture, the intensity reflection at resonance is rendered polarization dependent. It is found that the point group symmetry of the nano-aperture array determines the amount of anisotropy in the intensity reflection. Finite-difference time-domain simulation was adopted to identify details of nano-aperture metastructures transferred from nano-aperture patterns by the focused ion beam milling.

Polarization-independent light emission enhancement of ZnO/Ag nanograting via surface plasmon polariton excitation and cavity resonance.

In this study, we observed that the photoluminescence (PL) intensity of ZnO/Ag nanogratings was significantly enhanced compared with that of a planar counterpart under illumination of both transverse magnetic (TM) and transverse electric (TE)-mode light. In the TM mode, angle-resolved reflectance spectra exhibited dispersive dips, indicating cavity resonance as well as grating-coupled surface plasmon polariton (SPP) excitation. In the TE mode, cavity resonance only was allowed, and broad dips appeared in the reflectance spectra. Strong optical field confinement in the ZnO layers, with the help of SPP and cavity modes, facilitated polarization-insensitive PL enhancement. Optical simulation results were in good agreement with the experimental results, supporting the suggested scenario.

Plasmon-Enhanced Surface Photovoltage of ZnO/Ag Nanogratings.

We investigated the surface photovoltage (SPV) behaviors of ZnO/Ag one-dimensional (1D) nanogratings using Kelvin probe force microscopy (KPFM). The grating structure could couple surface plasmon polaritons (SPPs) with photons, giving rise to strong light confinement at the ZnO/Ag interface. The larger field produced more photo-excited carriers and increased the SPV. SPP excitation influenced the spatial distribution of the photo-excited carriers and their recombination processes. As a result, the SPV relaxation time clearly depended on the wavelength and polarization of the incident light. All of these results suggested that SPV measurement using KPFM should be very useful for studying the plasmonic effects in nanoscale metal/semiconductor hybrid structures.

Influence of wetting state on optical reflectance spectra of Si nanopillar arrays

Finite-difference time-domain (FDTD) simulations showed that the reflectance spectra of crystalline Si nanopillar (NP) arrays with diameters of 40, 70, 100, and 130 nm differed depending on wetting state. The observed reflectance dips of the 40-nm-diameter NP array were in good agreement with those estimated from destructive interference conditions at the top and bottom of the NPs: the NP arrays were treated as a homogeneous medium with an effective permittivity according to the effective medium approximation model. In contrast, the dip positions of the FDTD-simulated spectra for 70-, 100-, and 130-nm-diameter NP arrays deviated from the results of interference calculations, particularly for short wavelengths. This suggested that Mie resonances in individual NPs significantly increased the absorption cross-section at the resonant wavelengths, which was sensitive to the refractive index of the surrounding medium (i.e., the wetting state). Optical reflectance measurements provide an easy and efficient means...

SiNx layers on nanostructured Si solar cells: Effective for optical absorption and carrier collection

We compared nanopatterned Si solar cells with and without SiNx layers. The SiNx layer coating significantly improved the internal quantum efficiency of the nanopatterned cells at long wavelengths as well as short wavelengths, whereas the surface passivation helped carrier collection of flat cells mainly at short wavelengths. The surface nanostructured array enhanced the optical absorption and also concentrated incoming light near the surface in broad wavelength range. Resulting high density of the photo-excited carriers near the surface could lead to significant recombination loss and the SiNx layer played a crucial role in the improved carrier collection of the nanostructured solar cells.