Search

This paper presents an ultra-low-profile copper indium gallium selenide (CIGS) based solar cell integrated antenna with a high form factor. A tiny slot was etched from the solar cell to develop an ultra-low-profile solar-cell-integrated antenna. This tiny slot increases the form factor due to the small clearance area from the solar cell. A ground-radiation antenna design method was applied in which lumped elements were employed inside the tiny slot for antenna operation. Another substrate was used under the solar cell for designing the feeding structure with lumped elements connected to the tiny slot using via holes. A prototype was fabricated and measured to verify the operation of a built-in solar-cell antenna and validate the simulated results. The measured results demonstrate that the solar-cell-integrated antenna covers the entire frequency range of the Industrial Scientific Medical band, from 2.4 to 2.5 GHz, with a maximum gain of 2.79 dBi and radiation efficiency higher than 80% within the impedance bandwidth range. Moreover, the proposed design has an ultra-low-profile structure of only 0.0046 λo, where λo represents the free space wavelength at 2.45 GHz, and a high form factor of 99.1% with no optical blockage. The antenna and solar cell were designed to avoid affecting the performance of each other using the radio-frequency decoupler.

In this work, we demonstrated a phonon-polariton in the terahertz (THz) frequency range, generated in a crystallized lead halide perovskite film coated on metamaterials. When the metamaterial resonance was in tune with the phonon resonance of the perovskite film, Rabi splitting occurred due to the strong coupling between the resonances. The Rabi splitting energy was about 1.1 meV, which is larger than the metamaterial and phonon resonance line widths; the interaction potential estimation confirmed that the strong coupling regime was reached successfully. We were able to tune the polaritonic branches by varying the metamaterial resonance, thereby obtaining the dispersion curve with a clear anticrossing behavior. Additionally, we performed

In this study, we developed a rapid nondestructive tool for testing rust spread in a metal covered by a paint layer by using a THz time-domain spectroscopy system at a speed of 100 Hz/pixel. Time-of-flight imaging helps identify rust formation by exclusively obtaining the reflection from the steel below the paint surface. The use of frequency-selective imaging allows us to manipulate the contrast in rust imaging. Higher contrast is generally obtained when monitoring using the higher frequency component. In addition, we monitored the spread of rust in a steel plate under the influence of two different chemical solutions: NaCl and acid. We found that in the early stages, the decrease in THz reflection was governed by the high-frequency components due to the formation of lepidocrocite, whereas the low-frequency component develops as the proportion of hematite increases with time.

Large-scale growth of mostly monolayer molybdenum disulfide (MoS2) on quartz, sapphire, SiO2/Si, and waveguide substrates is demonstrated by chemical vapor deposition with the same growth parameters. Centimeter-scale areas with large flakes and films of MoS2 on all the growth substrates are observed. The atomic force microscopy and Raman measurements indicate the synthesized MoS2 is monolayer with high quality and uniformity. The MoS2 field effect transistors based on the as-grown MoS2 exhibit carrier mobility of 1–2 cm2V−1s−1 and On/Off ratio of ~104 while showing large photoresponse. Our results provide a simple approach to realize MoS2 on various substrates for electronics and optoelectronics applications.

The band alignment in AlGaN/GaN HEMT devices was investigated using scanning photocurrent microscopy (SPCM) with UV and visible light sources. The polarity of the SPCM on the conduction channel exhibited a switching behavior from p-type to n-type response as the gate bias was increased. The flat-band voltage, which was higher than the DC turn-on voltage, indicates that an ohmic metallic contact was formed for electron transport. We obtained the band offset between the conduction band and the metal Fermi level, which yielded a value of 2.1 eV on average.

Next-generation electronic and optoelectronic devices require a high-quality channel layer. Graphene is a good candidate because of its high carrier mobility and unique ambipolar transport characteristics. However, the on/off ratio and photoresponsivity of graphene are typically low. Transition metal dichalcogenides (e.g., MoSe2) are semiconductors with high photoresponsivity but lower mobility than that of graphene. Here, we propose a graphene/MoSe2 barristor with a high-k ion-gel gate dielectric. It shows a high on/off ratio (3.3 × 104) and ambipolar behavior that is controlled by an external bias. The barristor exhibits very high external quantum efficiency (EQE, 66.3%) and photoresponsivity (285.0 mA/W). We demonstrate that an electric field applied to the gate electrode substantially modulates the photocurrent of the barristor, resulting in a high gate tuning ratio (1.50 μA/V). Therefore, this barristor shows potential for use as an ambipolar transistor with a high on/off ratio and a gate-tunable photodetector with a high EQE and responsivity. An approach that enhances control over charge carrier movement in graphene-based transistors may be useful in applications such as bendable circuits and light sensors. The difference in current flow when a transistor turns from on to off plays a critical role in determining device performance. Bae Ho Park from Konkuk University in Seoul, South Korea, and colleagues report that stacking graphene on top of another two-dimensional (2D) film of molybdenum selenide can lead to larger on/off current ratios than conventional graphene transistors. Optical and electrical characterizations revealed that the interface between the 2D films created a potential barrier that blocked current flow in the device’s off state without compromising graphene’s mechanical flexibility. Using an external voltage to modulate current flow through the barrier region also allowed the stacked device to serve as a high-efficiency photodetector. we report a field-effect device with a graphene/MoSe2 channel layer and high-k ion-gel gate dielectric. The device shows a high carrier mobility (~247 cm2/V ∙ s), a high on/off ratio (3.3 × 104), and ambipolar behavior that are controlled by an applied gate voltage. The strong gating effect of the device results in higher external quantum efficiency (EQE) (66.3%), photoresponsivity (285.0 mA/W), and gate-tuning ratio (1.50 μA/V) compared to pristine devices. Therefore, our graphene/MoSe2 barristor device can be a suitable candidate for use in ambipolar transistors and gate-tunable broad-area photodetectors.

We investigate the effect of the effective substrate refractive index on the sensitivity of terahertz (THz) metamaterials for dielectric sensing by substrate etching. We found the effective refractive index near the metamaterials can be actively controlled by etching, allowing optimization of the sensitivity. Our experimental findings are in good agreement with finite element method (FEM) simulation results.

This study investigates recent advances in photoelectron emission generated by irradiating ultrashort lasers on metallic nanostructures and low-dimensional carbon materials. Recently, primary focus has been on improving the efficiency of emitters, i.e. increasing the number of field-emitted electrons and their respective kinetic energies. An example of this is the modification of the conventional metal nanotip through adiabatic nanofocusing and various plasmonic metal structures, such as nanorods and bowtie antenna. The coherent emission control with two color irradiation enabled modulation in the emission yield. In addition, THz waves near the metallic nanostructure induced a highly accelerated, monochromatic energy. Alternative to metallic nanotips, carbon nanotubes are emerging as efficient photoelectron emitters, due to the large enhancement factor associated with their high aspect ratio and damage threshold. They particularly allowed the use of femtosecond light sources with a relatively short wavelength, resulting in the generation of photoelectrons with a narrow bandwidth. Additionally, electronic control over the single-walled nanotubes band structure added a degree of freedom for controlling the electron emission yield. Finally, we review the strong-field tunneling emission in graphene edge, with the emission yield showing an anomalous increase of nonlinear order, corresponding to the deep strong tunneling regime.

We demonstrate subcycle electron pulse generation in a nanogap of graphene when irradiated by a femtosecond laser pulse in the near-infrared region (800 nm). A strong photoinduced emission was prod...

Electrical characteristics of ion gels prepared by loading different amounts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) in Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) are investigated and compared with those of ion liquid, [EMIM][TFSI] for possible application as a gate stack for flexible electronic devices. Capacitance and impedance as a function of frequency are measured, which can be well accounted for by a simple circuit model identifying the local device components. The operation of a flexible field effect transistor based on graphene and the ion gel as a top gate stack is also demonstrated.

Nucleation and growth characteristics of HfO2 thin films on graphene are investigated using atomic layer deposition (ALD). Substantial delay (∼70 ALD cycles) in the nucleation of HfO2 films is observed during HfO2 ALD on graphene, which causes large leakage current in Au/HfO2/graphene capacitors at low HfO2 ALD cycles (

We demonstrated imaging of the depletion layer in a MoS2/graphene heterojunction fabricated by chemical vapor deposition and obtained their transport parameters such as diffusion length, lifetime, and mobility by using scanning photocurrent microscopy (SPCM). The device exhibited a n-type operation, which was determined by the MoS2 layer with a lower mobility. The SPCM revealed the presence of the depletion layer at the heterojunction, whereas graphene provided an excellent electrical contact for the MoS2 layer without resulting in a rectifying behavior, even if they were anchored within a very short range. The polarity of the photocurrent signal switched when we applied a drain–source bias voltage, from which we extracted the potential barrier at the junction. More importantly, a bias-dependent SPCM allowed us to simultaneously record the diffusion lengths of both majority and minority carriers for the respective MoS2 and graphene layers. By combining the diffusion lengths with the lifetimes measured by ...

We report an in situ characterization of transition-metal dichalcogenide (TMD) monolayers and twisted bilayers using a high-speed second-harmonic generation (SHG) imaging technique. High-frequency laser modulation and galvano scanning in the SHG imaging enabled a rapid identification of the crystallinity in the TMD, including the orientation and homogeneity with a speed of 1 frame/s. For a twisted bilayer MoS2, we studied the SHG peak intensity and angles as a function of the twist angle under a strong interlayer coupling. In addition, rapid SHG imaging can be used to visualize laser-induced ablation of monolayer and bilayer MoS2 in situ under illumination by a strong femtosecond laser. Importantly, we observed a characteristic threshold behavior, the ablation process occurred for a very short time duration once the preheating condition was reached. We investigated the laser thinning of the bilayer MoS2 with different twist angles. When the twist angle was 0°, the SHG decreased by approximately one-fourth of the initial intensity when one layer was removed. Conversely, when the twist angle was approximately 60° (the SHG intensity was suppressed), the SHG increased abruptly close to that of the nearby monolayer when one layer was removed. Precise layer-by-layer control was possible because of the unique threshold behavior of the laser-induced ablation.

We demonstrate all-optical switching of plasmonic resonance patterned on the carbon nanotube films supported on the silicon substrates. A considerable suppression of terahertz wave was observed in the transmission resonance, when the near-infrared femtosecond laser pulse is irradiated. A Fourier-transformed terahertz spectrum reveals that the resonance feature that was governed by the geometry of the slot antenna are almost completely removed with the laser irradiation having sufficiently high pulse energy reaching 18 μJ. A screening of terahertz field was analyzed in terms of the free carrier generation originating from the photoexcitation in semiconductor film.

We report a 47.1% efficiency improvement resulting from the combination of device-architecture modification and inclusion of poly(3-hexylthiophene-2,5-diyl) nanowires (P3HT NWs) in bulk heterojunction (BHJ) active layers (ALs). Modelling of ellipsometry spectra shows substantial changes in inhomogeneity and optical constants of BHJ ALs with P3HT-NW inclusion. Furthermore, finite-difference time-domain simulation results based on actual device structures with inhomogeneous AL models indicate that enhanced light harvesting is a main contributing factor to efficiency improvement. On the contrary, P3HT-NW inclusion has no significant effect on charge carrier collection, other than suppressing occurrence of cul-de-sac in hole-transport pathways and unfavourable indene-C60 bisadduct domains near top anodes.

We demonstrate a significant enhancement in the sensitivity of split ring resonator terahertz metamaterial dielectric sensors by the introduction of etched trenches into their inductive-capacitive gap area, both through finite element simulations and in experiments performed using terahertz time-domain spectroscopy. The enhanced sensitivity is demonstrated by observation of an increased frequency shift in response to overlaid dielectric material of thicknesses up to 18 μm deposited on to the sensor surface. We show that sensitivity to the dielectric is enhanced by a factor of up to ~2.7 times by the incorporation of locally etched trenches with a depth of ~3.4 μm, for example, and discuss the effect of the etching on the electrical properties of the sensors. Our experimental findings are in good agreement with simulations of the sensors obtained using finite element methods.

In this study, we developed a rapid three-dimensional (3D) time-of-flight imaging tool for inspection of packaged semiconductor chips, using terahertz (THz) time-domain spectroscopy techniques. A high-speed THz system based on the optical sampling by cavity tuning technique is incorporated with a 2-axis galvano scanner to deliver a scanning speed of more than 100 Hz/pixel with a signal-to-noise ratio larger than 20 dB. Through the use of the Hilbert transformation, we reconstruct the 3D structure of the packaged chip in a nondestructive manner. Additionally, the use of frequency-selective imaging allows us to manipulate image resolution, the higher resolution was obtained when monitored using the higher frequency component. Further, using phase information, we were able to detect and identify defects in the packaged chip, such as the delamination area and epoxy-rich regions.

Optically controlled spectral-band selective terahertz (THz) modulators based on metal-organic hybrid metamaterials were investigated. An artificially structured material, which consists of two single split-ring resonators put together on the split gap side, was patterned on a silicon substrate to generate frequency-selective properties. An active layer of an organic thin film (fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester, also called PCBM) was deposited on the metamaterial-silicon structure for modulating the transmission of incident THz radiation. The metal-organic hybrid metamaterials enabled active control of spectral bands present in the transmission spectra of THz waves. In addition, the changes in the photo-excited carrier density due to the transfer of charges between the layers were quantitatively analyzed by simulation results.

We studied experimentally and theoretically the vertical range of the confined electric field in the gap area of metamaterials, which was analyzed for various gap widths using terahertz time-domain spectroscopy. We measured the resonant frequency as a function of the thickness of poly(methyl methacrylate) in the range 0 to 3.2 µm to quantify the effective detection volumes. We found that the effective vertical range of the metamaterial is determined by the size of the gap width. The vertical range was found to decrease as the gap width of the metamaterial decreases, whereas the sensitivity is enhanced as the gap width decreases due to the highly concentrated electric field. Our experimental findings are in good agreement with the finite-difference time-domain simulation results. Finally, a numerical expression was obtained for the vertical range as a function of the gap width. This expression is expected to be very useful for optimizing the sensing efficiency.

In this paper, we demonstrate that terahertz (THz) metamaterials are powerful tools for determination of dielectric constants of polymer films and polar liquids. As we deposit a dielectric film on a metamaterial, the resonant frequency shifts, but saturates at a specific thickness due to the limited sensing volume of the metamaterial. From the saturated value, we can extract the dielectric constants of various polymers that are transparent to the THz frequency range. In addition, we fabricated a microfluidic channel that contains the metamaterials to address the real dielectric constants for a polar liquid solution. This was possible due to an extremely confined electric field near the gap area of the metamaterials, enabling us to employ very thin liquid layers. We found that the resonance shifts do not depend critically on the imaginary dielectric constants, proving that our approach can be universal in terms of various materials, including absorptive materials. As an example, the dielectric constants of sodium chloride and potassium chloride solutions have been determined with various concentrations. Our experimental findings were successfully confirmed by finite-difference time-domain simulations.

We used a negative-tone e-beam resist (N-ER) to perform direct laser writing lithography based on a single-photon absorption process with a 405-nm laser source. The linewidth of the N-ER reached 150 nm, which is over three times thinner than that of a conventional photoresist. To optimize the process, the linewidth, lithographic contrast, and aspect ratio of the N-ER were investigated with respect to the dose and baking temperature. We were able to achieve a lithographic contrast of 4.8 and a maximum aspect ratio of 1.43, thereby confirming the superior resolution of the N-ER.

We developed hybrid slot antenna structures for microbial sensing in the THz frequency range, where silver nanowires (AgNWs) were employed to increase the sensitivity. In order to fabricate the hybrid devices, we partially etched the AgNW in the slot antenna region, where we can expect the field enhancement effect at the AgNW tip. We measured the resonant-frequency shift observed upon the deposition of a polymer layer, and observed that the sensitivity increased upon the introduction of AgNWs, with an enhancement factor of more than four times (approximately six times in terms of figure-of-merit). The sensitivity increased with the AgNW density until saturation. In addition, we tested devices with PRD1 viruses, and obtained an enhancement factor of 3.4 for a slot antenna width of 3 μm. Furthermore, we performed finite-difference time-domain simulations, which confirmed the experimental results. The sensitivity enhancement factor decreased with the decrease of the slot width, consistent with the experimental findings. Two-dimensional mapping of the electric field confirmed the strong field localization and enhancement at the AgNW tips.

We demonstrate substrate electron-beam-induced current (s-EBIC) measurements of individual single-walled carbon nanotubes (SWNTs) by measuring the current collected by the substrate electrode, which penetrates through the insulating oxide layer. We found that s-EBIC provided better image contrast than ordinary secondary electron imaging methods for locating SWNTs that are in contact with metal electrodes. The s-EBIC has been measured for different acceleration voltages and probe currents. We found that s-EBIC did not depend critically on the acceleration voltage when the e-beam irradiated an insulating layer as compared to the case when it irradiated metal electrodes. Importantly, s-EBIC signals were increased by more than 10%, when the SWNT part was irradiated, and this makes s-EBIC imaging a very useful tool for locating individual SWNTs efficiently.

With poly(3-hexylthiophene) (P3HT) nanowire (NW) inclusion in active layers (ALs), organic solar cells (OSCs) based on P3HT donor and indene-C60 bisadduct (ICBA) acceptor showed power conversion efficiency (PCE) improvements for both bulk heterojunction (BHJ)- and bilayer (BL)-structure AL devices. The PCE increase was approximately 14 % for both types of P3HT:ICBA OSCs. However, improvements in short-circuit current density (Jsc ) were about 4.4 and 6.4 % for BHJ- and BL-type AL devices, respectively. A systematic study showed that the addition of P3HT NWs did not result in enhanced internal quantum efficiencies for either type of device. However, the difference in light-harvesting efficiency was important in accounting for Jsc variations. Interestingly, there was no correlation between Jsc and PCE variations, whereas the open-circuit voltage (Voc ) and fill factor (FF) showed correlations with the PCE. The variation in FF is discussed in terms of Voc and equivalent-circuit parameters based on a nonideal diode model.

We demonstrate dielectric substrate effects on the resonance shift of terahertz metamaterials with various metal thicknesses by using finite-difference time-domain simulations. We found a small red shift in the metamaterial resonance with increasing metal thickness for the free-standing case. Conversely, when the metamaterial pattern was supported by a substrate with a high dielectric constant, the resonant frequency exhibited a large blue shift because the relative contribution of the substrate’s refractive index to the resonant frequency decreased drastically as we increased the metal thickness. We determined the substrate’s refractive index, 1.26, at which the metamaterial resonance was independent of the metal thickness. We extracted the effective refractive index as a function of the substrate’s refractive index explicitly, which was noticeably different for different film thicknesses.

Most semiconductors have surface dynamics radically different from its bulk counterpart due to surface defect, doping level, and symmetry breaking. Because of the technical challenge of direct observation of the surface carrier dynamics, however, experimental studies have been allowed in severely shrunk structures including nanowires, thin films, or quantum wells where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nanoprobing system to investigate the surface dynamics of bulk semiconductors, using metallic nanogap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically decrease close to the limit of THz time resolution (∼1 ps) as the gap size decreases down to nanoscale and that they return to their original values once the nanogap patterns are removed. Our THz nanoprobing system will open up pathways toward direct and nondestructive measurements of surface dynamics of bulk semiconductors.

We demonstrate THz metamaterials as efficient label-free detectors for microorganisms such as fungi, bacteria, and viruses. A shift in the resonant frequency is observed following the deposition of various living and viable microorganisms, which arises due to the change in the effective dielectric constant in a capacitive gap of the metamaterials. The metamaterial sensing is a universal method, while a selective detection is also possible by functionalizing the substrates with antibodies specific to the target substances. Dielectric constants of the various microorganisms have been recorded using the metamaterial sensors, which will help to achieve selective and sensitive identification of the pathogens without the help of receptors.

Most semiconductors have surface dynamics radically different from its bulk counterpart due to the surface state. However, direct observation of the surface carrier dynamics has been limited to nano structures where the surface-to-volume ratio is very high. Here, we develop a new type of terahertz (THz) nanoscopy to investigate surface dynamics of bulk semiconductors, using metallic nano gap accompanying strong THz field confinement. We observed that carrier lifetimes of InP and GaAs dramatically as the gap size decreases, and that they return to their bulk values once the nano patterns are removed.

We demonstrate highly sensitive detection of viruses using terahertz split-ring resonators with various capacitive gap widths. Two types of viruses, with sizes ranging from 60 nm (PRD1) to 30 nm (MS2), were detected at low densities on the metamaterial surface. The dielectric constants of the virus layers in the THz frequency range were first measured using thick films, and the large values found identified them as efficient target substances for dielectric sensing. We observed the resonance-frequency shift of the THz metamaterial following deposition of the viruses on the surface at low-density. The resonance shift was higher for the MS2 virus, which has a relatively large dielectric constant. The frequency shift increases with surface density until saturation and the sensitivity is then obtained from the initial slope. Significantly, the sensitivity increases by about 13 times as the gap width in the metamaterials is decreased from 3 µm to 200 nm. This results from a combination of size-related factors, leading to field enhancement accompanying strong field localization.

The band alignment at an Al2O3/SrTiO3 heterointerface forming a two-dimensional electron gas (2DEG) was investigated using scanning photocurrent microscopy (SPCM) in an electrolyte-gated environment. We used a focused UV laser source for above-the-bandgap illumination on the SrTiO3 layer, creating electron-hole pairs that contributed to the photocurrent through migration towards the metal electrodes. The polarity of the SPCM signals of a bare SrTiO3 device shows typical p-type behavior at zero gate bias, in which the photogenerated electrons are collected by the electrodes. In contrast, the SPCM polarity of 2DEG device indicates that the hole carriers were collected by the metal electrodes. Careful transport measurements revealed that the gate-dependent conductance of the 2DEG devices exhibits n-type switching behavior. More importantly, the SPCM signals in 2DEG devices demonstrated very unique gate-responses that cannot be found in conventional semiconducting devices, based on which we were able to perform detailed investigation into the electronic band alignment of the 2DEG devices and obtain the valence band offset at the heterointerface.

In the past few years, perovskite film has been considered as a promising materials for solar cell devices due to its outstanding performance. To maximize the perovskite solar cell performance, it is necessary to understand the crystallization mechanism of perovskite film. In this study, we monitored the crystallization and decrystallization of the lead halide perovskite (MAPbI3-xClx) film under thermal annealing and UV-laser exposure processes by using in-situ terahertz time-domain spectroscopy. The strength of vibrational resonances in THz frequency range is found to be a good indicator of perovskite crystallinity. We measured the THz spectra as we annealed the perovskite film at various temperatures in order to achieve the degree of crystallization, i.e., the transition of perovskite structure from the intermediate phase to the tetragonal phase. In addition, we investigated the UV-laser-induced phase transition of the perovskite film. Because it is widely known that UV light illumination on perovskite film tends degrade the perovskite cell efficiency, its influence on the crystallization is our primary concern. Surprisingly, the crystallization phase increases for 10 min, until it starts to degrade over a couple of hours. We also studied the transient transport properties of the films under UV illumination. The correlation between the degree of crystallization (obtained from THz transmission) and the transport parameters exhibited the electric percolation threshold behaviors in the perovskite films. These information are expected to be crucial for optimizing the fabrication method of perovskite solar cell.

We fabricated plasmonic devices operating in the terahertz (THz) frequency range using silver nanowire (AgNW) network films. AgNW films exhibit high conductivity and good transparency in the visible range, with a figure of merit comparable to that of conventional transparent conducting oxide films. The THz conductivity of AgNW films can be improved by post-treatment procedures such as welding using graphene oxide flakes. Using photolithography, we fabricated the slot antenna arrays whose resonance behaviors are determined by geometric parameters such as the length of individual elements. The plasmonic resonance varied with the sheet resistances of the film, enabling us to manipulate the quality factors and the peak position of the resonance, in particular, by controlling the films thickness and by the post-procedures such as the chemical vapor treatment. (C) 2017 Optical Society of America

In the present study, we visualize ultrafast carrier dynamics in one-dimensional nanoscale devices, such as Si nanowire and carbon nanotube transistors using femtosecond photocurrent microscopy. We investigate transit times of ultrashort carriers that are generated near one metallic electrode and subsequently transported toward the opposite electrode based on drift and diffusion motions. Conversely, pure diffusion motion is observed when the pump pulse is located in the middle of the nanowires. Carrier dynamics have been addressed for various working conditions, in which we found that the carrier velocity and pulse width can be manipulated by the external electrodes. In particular, the carrier velocities extracted from transit times increase for a larger negative gate bias because of the increased field strength at the Schottky barrier.

We fabricated hybrid light emitting devices based on colloidal CdSe/ZnS core/shell quantum dots and a solution-processed NiO layer. The use of a sol-gel NiO layer as a hole injection layer (HIL) resulted in overall improvement in device operation compared to a control device with a more conventional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) HIL. In particular, luminous efficiency increased substantially because of the suppression of excessive currents and became as large as 2.45 cd/A. To manifest the origin of current reduction, temperature- and electric field-dependent variations of currents with respect to bias voltages were investigated. In a low bias voltage range below the threshold for luminance turn-on, the Poole-Frenkel (PF) emission mechanism was responsible for the current-density variation. However, the space-charge-limited current modified with PF-type mobility ruled the current-density variation in high bias voltage range above the threshold.

We report on the effect of dielectric substrate on the resonance shift of metallic metamaterials operating in the terahertz frequency region. The resonance frequencies for various metamaterials obtained by time-domain spectroscopy agree well with calculations based on the finite-difference time-domain method. The dependencies of the resonance frequency to substrate index are studied, and are systematically determined by introducing an effective substrate index. The relative contributions of the substrate index for various metamaterials are obtained by fitting the numerically obtained effective refractive indices as a function of substrate index, which is found to be ∼0.63 regardless of the metamaterial geometry.

We report the fabrication of efficient indium-tin-oxide-free organic solar cells based on poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). All layers of the devices from the lowermost silver nanowire cathode to the uppermost conducting polymer anode are deposited from solution and processed at plastic-compatible temperatures

The electronic control of an ultrafast tunneling electron emission was demonstrated in the nanogap of a single-walled nanotube (SWNT) when irradiated by a femtosecond laser pulse. The SWNT apex possesses a nanoscale morphology with a large damage threshold and thus enabled the achievement of a large emission rate. More importantly, the DC field-emission characteristics varied when the gate bias was changed. This was analyzed in terms of the change in the effective barrier height and enhancement factors. Photoinduced electron emission was observed when the gap area was illuminated with a femtosecond laser centered at a wavelength of 800 nm. As the laser power was increased, a saturated tunneling current was observed, reaching more than 10 electrons per pulse. Finally, the photoelectron emission yield was tuned with the help of gate-induced variations in the electronic band structures of the SWNTs.The electronic control of an ultrafast tunneling electron emission was demonstrated in the nanogap of a single-walled nanotube (SWNT) when irradiated by a femtosecond laser pulse. The SWNT apex possesses a nanoscale morphology with a large damage threshold and thus enabled the achievement of a large emission rate. More importantly, the DC field-emission characteristics varied when the gate bias was changed. This was analyzed in terms of the change in the effective barrier height and enhancement factors. Photoinduced electron emission was observed when the gap area was illuminated with a femtosecond laser centered at a wavelength of 800 nm. As the laser power was increased, a saturated tunneling current was observed, reaching more than 10 electrons per pulse. Finally, the photoelectron emission yield was tuned with the help of gate-induced variations in the electronic band structures of the SWNTs.

In this study, we developed hybrid humidity sensing methods by incorporating silk fibroin protein onto metamaterials, operating in the terahertz (THz) frequencies; the resonant frequency shifted but saturated at a specific thickness due to the limited sensing volume of the metamaterial. From the saturated value, we extracted the dielectric constant for the silk films. We also observed additional resonance shifts when we applied humid air to silk-coated metamaterials, due to the increased water molecule numbers on the film. Frequency shifts depend linearly on relative humidity. Also, in situ THz spectroscopy measurements reveal that the time response is instantaneous within our detection limit, especially upon exposure to humid air, whereas the small slowly decaying component appeared when we applied dry air. The time taken by the slow component in the drying process was 10-50 s, depending on film thickness. This could optimize humidity sensors as a fast and efficient detection tool to measure air humidity.

We report the characterization of individual carbon nanotube and Si nanowire field-effect transistors through high-speed scanning photocurrent microscopy with a scanning speed of 1 frame/s and a photocurrent sensitivity of less than 1 pA. This enables us to record photocurrent images that are free from hysteresis effects that modify the field configurations applied by the gate bias voltage. We can clearly resolve the photocurrent signals with polarity inversion near the metallic contacts under gate bias conditions which cause severe hysteresis effects in the nanowire devices. We also studied the dynamics of the hysteresis effects for different gate bias configurations. This high-speed photocurrent imaging technique is particularly useful for obtaining two-dimensional, localized optoelectronic characteristics and their correlation with overall device performance without encountering undesired dynamic responses.

We report on terahertz electromagnetic field transmission through metallic slot antennas supported on various dielectric substrates. The substrate effect strongly modifies the transmission resonance compared to the free-standing case, while the resonance can be determined systematically by introducing an effective refractive index. The relative contribution of the substrate index to the effective index has been measured for various substrates, and shows a good agreement with theoretical calculations based on a coupled mode method. In addition, the slot antenna arrays embedded in a flexible substrate show a transmission resonance governed by a substrate refractive index.

We determined the carrier diffusion lengths in nanoporous layers of dye-sensitized solar cells by using scanning photocurrent microscopy. The diffusion lengths were found to be 60-100 μm for the conventional cells. In addition, we found a correlation between the carrier diffusion lengths and the cell efficiency, which proved that improvement in the diffusion length is one of the crucial factors for optimizing device performance. The diffusion length was measured for various operating conditions by varying parameters such as solar light intensity and applied electrical voltage. In particular, we observed electric-field-driven, carrier transport phenomena (i.e., drift current) in modified cells. Fitting with the drift-diffusion model enabled us to extract the electric field strengths present in the TiO2 nanoporous layer.

Unusual performances of metamaterials such as negative index of refraction, memory effect, and cloaking originate from the resonance features of the metallic composite atom(1-6). Indeed, control of metamaterial properties by changing dielectric environments of thin films below the metallic resonators has been demonstrated(7-11). However, the dynamic control ranges are still limited to less than a factor of 10,(7-11) with the applicable bandwidth defined by the sharp resonance features. Here, we present ultra-broad-band metamaterial thin film with colossal dynamic control range, fulfilling present day research demands. Hybridized with thin VO(2) (vanadium dioxide) (12-18) films, nanoresonator supercell arrays designed for one decade of spectral width in terahertz frequency region show an unprecedented extinction ratio of over 10000 when the underlying thin film experiences a phase transition. Our nanoresonator approach realizes the full potential of the thin film technology for long wavelength applications.

본 연구에서는 전도성이 매우 높은 탄소나노튜브 박막을 제작하고 테라파 영역에서의 광학상수를 측정하였다 . 탄소나노튜브박막은 스핀코팅법 또는 진공여과법을 이용하여 연성플라스틱 기판 위에 증착되었다. 테라파 영역의 전자파 투과율은 박막의 두께 조절을 통해 가능하며, 산처리 등의 후속공정을 통해서도 조절이 가능하다. 시분해 테라파 투과파의 진폭과 위상 측정을 통해유전상수를 포함한 광학상수의 스펙트럼을 측정하였다. 이를 통해 탄소나노튜브 박막이 Drude 자유전자 모델에 잘 부합하며, 높은 플라즈마 진동수를 가지는 등, 우수한 금속의 특성을 가지고 있음이 밝혀졌다 . 또한 산처리 전후 유전상수가 변화하는 것을 직접 확인할 수 있었다. 마지막으로 셀룰로즈 멤브레인에 증착된 CNT 필름의 경우엔 기판의 효과가 제거된 광학계수 측정이 가능함을 보여준다.

Vertically aligned ZnO nanorods have been grown on silicon substrates pre-coated with thin, less than 10 nm, textured ZnO seeding layers via a vapor-solid mechanism. The ZnO seeding layers, which were essential for vertical alignment of ZnO nanorods without using any metal catalyst, were prepared by decomposing zinc acetate. The structure and the luminescence properties of the ZnO nanorods synthesized onto ZnO seeding layers were investigated and their morphologies were compared with those of single-crystalline GaN substrates and silicon substrates covered with sputtered ZnO flms. Patterning of ZnO seed layers using photolithography allowed the fabrication of patterned ZnO-nanorod arrays.

Cathodoluminescence (CL) emissions from individual ZnO nanostructures with diameters of 30−100 nm are investigated to correlate their optical and electrical properties. Two types of ZnO nanostructu...

We use scanning photocurrent microscopy (SPCM) to investigate the properties of internal p-n junctions in ambipolar carbon nanotube (CNT) transistors. Our SPCM images show strong signals near metal contacts whose polarity and positions change depending on the gate bias. SPCM images analyzed in conjunction with the overall conductance also indicate the existence and gate-dependent evolution of internal p-n junctions near contacts in the n-type operation regime. To determine the p-n junction position and the depletion width with a nanometer scale resolution, a Gaussian fit was used. We also measure the electric potential profile of partially suspended CNT devices at different gate biases, which shows that induced local fields can be imaged using the SPCM technique. Our experiment clearly demonstrates that SPCM is a valuable tool for imaging and optimizing electrical and optoelectronic properties of CNT based devices.

In this paper, we demonstrate the fabrication of liquid flow sensors employing partially suspended single-walled carbon nanotubes (SWNTs). We have found that the sign of the conductance change in SWNT flow sensors is not influenced by the direction of water flow for both supported and suspended devices. Therefore, the streaming potential is not the principal mechanism of the SWNT sensor response. Instead, the conductance change is more likely due to a reduction in the cation density in the electrical double layer, whose equilibrium conditions are determined by the liquid flow rate. More importantly, we have found that the sensitivity of suspended SWNT devices is more than 10 times greater than that of supported SWNT devices. A reduced screening effect and an increase in effective sensing volume are responsible for the enhanced sensitivity, which is consistent with the ion depletion model. We also have measured conductance as a function of gate bias at different flow rates and have determined the flow-rate dependent effective charge density, which influences the electrostatic configuration around SWNT devices.