Skin-Attached Arrayed Piezoelectric Sensors for Continuous and Safe Monitoring of Oculomotor Movements.

Abnormal oculomotor movements are known to be linked to various types of brain disorders, physical/mental shocks to the brain, and other neurological disorders, hence its monitoring can be developed into a simple but effective diagnostic tool. To overcome the limitations in the current eye-tracking system and electrooculography, a piezoelectric arrayed sensor system is developed using single-crystalline III-N thin-film transducers, which offers advantages of mechanical flexibility, biocompatibility, and high electromechanical conversion, for continuous monitoring of oculomotor movements by skin-attachable, safe, and highly sensitive sensors. The flexible piezoelectric eye movement sensor array (F-PEMSA), consisting of three transducers, is attached on the the face temple area where it can be comfortably wearable and can detect the muscles' activity associated with the eye motions. Output voltages from upper, mid, and lower sensors (transducers) on different temple areas generate discernable patterns of output voltage signals with different combinations of positive/negative signs and their relative magnitudes for the various movements of eyeballs including 8 directional (lateral, vertical, and diagonal) and two rotational movements, which enable various types of saccade and pursuit tests. The F-PEMSA can be used in clinical studies on the brain-eye relationship to evaluate the functional integrity of multiple brain systems and cognitive processes. This article is protected by copyright. All rights reserved.

Transferred monolayer MoS2 onto GaN for heterostructure photoanode: Toward stable and efficient photoelectrochemical water splitting.

Solar-driven photoelectrochemical water splitting (PEC-WS) using semiconductor photoelectrodes is considered a promising solution for sustainable, renewable, clean, safe and alternative energy sources such as hydrogen. Here, we report the synthesis and characterization of a novel heterostructure MoS2/GaN to be used as a photoanode for PEC-WS. The heterostructure was synthesized by metal-organic chemical vapor deposition of single crystalline GaN onto a c-plane sapphire substrate, followed by the deposition of a visible light responding MoS2 monolayer (Eg = 1.9 eV) formed by a Mo-sulfurization technique. Our experimental results reveal that MoS2/GaN photoanode achieved efficient light harvesting with photocurrent density of 5.2 mA cm-2 at 0 V vs Ag/AgCl, which is 2.6 times higher than pristine GaN. Interestingly, MoS2/GaN exhibited a significantly enhanced applied-bias-photon-to-current conversion efficiency of 0.91%, whereas reference GaN yielded an efficiency of 0.32%. The superior PEC performance of the MoS2/GaN photoelectrode is mainly related to the enhanced light absorption due to excellent photocatalytic behavior of MoS2, which reduces charge transfer resistance between the semiconductor and electrolyte interface, and the improvement of charge separation and transport. This result gives a new perspective on the importance of MoS2 as a cocatalyst coated onto GaN to synthesize photoelectrodes for efficient solar energy conversion devices.

Improvement in Light Extraction Efficiency of InGaN/GaN Blue Light Emitting Diodes Using Sidewall Texturing.

Herein, we reported the effects of the geometric morphology of the sidewall on the extraction efficiency of GaN-based light-emitting diodes (LEDs). We performed numerical analysis based on the ray-tracing method. We found that the extraction efficiency of the LEDs increased with the texturing of the sidewall. The light output intensity of the LEDs (at an injection current of 100 mA) increased by 13.8% after sidewall texturing. These results confirmed that the geometric morphology of the sidewall plays an important role in improving the extraction efficiency of LEDs.

Augmented Quantum Yield of a 2D Monolayer Photodetector by Surface Plasmon Coupling.

Monolayer (1L) transition metal dichalcogenides (TMDCs) are promising materials for nanoscale optoelectronic devices because of their direct band gap and wide absorption range (ultraviolet to infrared). However, 1L-TMDCs cannot be easily utilized for practical optoelectronic device applications (e.g., photodetectors, solar cells, and light-emitting diodes) because of their extremely low optical quantum yields (QYs). In this investigation, a high-gain 1L-MoS2 photodetector was successfully realized, based on the surface plasmon (SP) of the Ag nanowire (NW) network. Through systematic optical characterization of the hybrid structure consisting of a 1L-MoS2 and the Ag NW network, it was determined that a strong SP and strain relaxation effect influenced a greatly enhanced optical QY. The photoluminescence (PL) emission was drastically increased by a factor of 560, and the main peak was shifted to the neutral exciton of 1L-MoS2. Consequently, the overall photocurrent of the hybrid 1L-MoS2 photodetector was observed to be 250 times better than that of the pristine 1L-MoS2 photodetector. In addition, the photoresponsivity and photodetectivity of the hybrid photodetector were effectively improved by a factor of ∼1000. This study provides a new approach for realizing highly efficient optoelectronic devices based on TMDCs.

Role of the Growth Temperature in MoS2 Growth by Chemical Vapor Deposition.

In this paper, we discuss the effect of synthesis temperature on the lateral growth of MoS2 thin films in chemical vapor deposition. With increasing temperature, surface coverage with MoS2 triangular islands is significantly improved due to an increase in the density of nuclei and fully continuous MoS2 thin film is grown when the growth temperature reached 800 °C. The MoS2 triangular islands grown at the temperature from 650 to 750 °C are monolayer and highly crystalline, whereas the large-area continuous film grown at the temperature of 800 °C is composed of double-layer or overlapping MoS2 nanosheets. Our research provides that synthesis temperature is the key to growth large area and high quality single crystal MoS2 films.

Improvement of the Light Extraction Efficiency of InGaN/GaN Blue Light Emitting Diodes Using ZnO Nanostructures.

Herein, we report the effects of geometric morphology of ZnO nanostructures on the extraction efficiency of GaN-based light emitting diodes (LEDs). We performed numerical analysis based on the two-dimensional (2D) finite difference of time domain (FDTD) method that was utilized to calculate the light extraction efficiency of the LEDs. We found that the extraction efficiency of the LED increased upon changing the shape of ZnO nanostructure from nanorods to pencil-likenanorods. The current-voltage characteristics of the LED with ZnO nanorods or pencil-like nanorods were similar to those of the LED that did not contain any ZnO nanostructures. The light output power of the LEDs containing ZnO nanorods or pencil-like nanorods at 100 mA increased additionally to 28% and 39%, respectively, relative to that of the LED that did not contain any ZnO nanostructures. These results confirm that the geometric morphology of the ZnO nanostructure plays an important role in improving the extraction efficiency of the LEDs.

Indium Tin Oxide-Free Transparent Conductive Electrode for GaN-Based Ultraviolet Light-Emitting Diodes.

Transparent conducting electrodes are important components of highly efficient ultraviolet light-emitting diodes (UV LEDs). Indium tin oxide (ITO) is commonly used to form a current spreading layer, but its UV-range optical transparency is limited with a low sheet resistance. We demonstrate a simple solution-based coating technique to obtain large-area, highly uniform, and conductive silver-nanowire-based electrodes that exhibit UV-range optical transparency better than that of ITO for the same sheet resistance. The UV LEDs fabricated using this current spreading layer showed improved optical power emission as well as improvement in electrical properties.

Light extraction enhancement of GaN based light emitting diodes by ZnO nanorod arrays.

This study examined the effects of the alignment of ZnO nanorod arrays (NRAs) on the light extraction enhancement of GaN-based light emitting diodes (LEDs), where ZnO NRAs were synthesized hydrothermally. The shape of the ZnO NRAs was controlled using seed layers for flower and vertical structures. Numerical analysis based on the two-dimensional (2D) finite difference of time domain (FDTD) method showed that the extraction efficiency of LED with bare (without ZnO NRA), vertical ZnO NRA and flower shaped ZnO NRA was 37%, 60% and 49%, respectively. The optical output power of the LEDs with vertical ZnO NRA and flower ZnO NRA was improved by 50% and 30% compared to that of the LED without ZnO NRA at an input current of 100 mA. These results suggest that the vertical alignment of ZnO NRA is important for enhancing the light extraction of GaN based LEDs.

High-efficiency light-emitting diode with air voids embedded in lateral epitaxially overgrown GaN using a metal mask.

We report high-efficiency blue light-emitting diodes (LEDs) with air voids embedded in GaN. The air void structures were created by the lateral epitaxial overgrowth (LEO) of GaN using a tungsten mask. The optical output power was increased by 60% at an injection current of 20 mA compared with that of conventional LEDs without air voids. The enhancement is attributed to improved internal quantum efficiency because the air voids reduce the threading dislocation and strain in the LEO GaN epilayer. A ray-tracing simulation revealed that the path length of light escaping from the LED with air voids is much shorter because the air voids efficiently change the light path toward the top direction to improve the light extraction of the LED.

Surface plasmon-enhanced light-emitting diodes using silver nanoparticles embedded in p-GaN.

We demonstrate the surface plasmon-enhanced blue light-emitting diodes (LEDs) using Ag nanoparticles embedded in p-GaN. A large increase in optical output power of 38% is achieved at an injection current of 20 mA due to an improved internal quantum efficiency of the LEDs. The enhancement of optical output power is dependent on the density of the Ag nanoparticles. This improvement can be attributed to an increase in the spontaneous emission rate through resonance coupling between the excitons in multiple quantum wells and localized surface plasmons in Ag nanoparticles embedded in p-GaN.

Comparison of Si doping effect on GaN nanowires and films synthesized by metal-organic chemical vapor deposition.

We investigated Si doping effect on GaN nanowires and GaN films grown by metal-organic chemical vapor deposition (MOCVD). Si as n-type dopant is incorporated to GaN nanowires and GaN films controlled by SiH4 flow rate (0, 1, 5, 8, and 10 sccm). The charge concentration and mobility of GaN films increased and decreased, respectively, as increasing the SiH4 flow rate, whereas those for GaN nanowires were not influenced by the SiH4 flow rate. Significant vacancies and impurities resulted in the intense yellow band in GaN nanowires as compared with GaN films, which leads to the large device-to-device variation and negligible dependence of Si doping and the SiH4 flux rate on the electrical properties of GaN nanowires.