Facile filter cloth brush-coating of large-area uniform silver nanowire conductive films for paper-based heater.

Filter cloth brush-coating (FCBC), using soft filter cloth as a brush-coating medium, in conjunction with viscous silver nanowire (AgNW) conductive solution, is used to prepare AgNW conductive films. The density and uniformity of AgNWs deposited on the substrate are controlled by the interplay between the filter cloth aperture, the conductive solution viscosity, and the brush-coating speed. Further, with appropriate AgNW concentration and flow rate, uniform AgNW transparent conductive film with sheet resistance of 18 Ω sq-1and transmittance of 94% at 550 nm is acquired by FCBC. Due to the precise control of the coating process in FCBC, large-area uniform AgNW conductive film fabricated on printing paper has a low non-uniformity factor of 1.2% at a sheet resistance of 19.0 Ω sq-1. The resultant paper-based AgNW film heater shows sensitive and stable heating performance. FCBC shows great potential in producing large-area uniform AgNW films on various substrates.

Highly sensitive RI and temperature sensor based on an asymmetric fiber coupler.

We propose and demonstrate a highly sensitive refractive index (RI) and temperature sensor based on an asymmetric fiber coupler (AFC). The AFC was fabricated by weak fusion of a pre-stretched single-mode fiber and a few-mode fiber. An ultra-sensitivity RI can be achieved near the dispersion turning point (DTP). The proposed RI sensor achieves a high RI sensitivity of -10,662.4nm/RIU within the RI range of 1.31-1.35. By packaging the AFC into polydimethylsiloxane (PDMS), the temperature sensitivity reaches 11.44 nm/°C. The proposed AFC with high RI and temperature sensitivity can be potentially used in the field of chemical monitoring, biochemical detection, and clinical diagnosis.

Influence of heating temperature on the optical response properties and surface relief patterns of TiO2/GeO2/ormosils composite films containing azobenzene.

With the progress of modern integrated optical technology, organic-inorganic composite materials have been widely used in integrated optoelectronic devices. Because of satisfying optical response properties among azobenzene, it will be an ideal choice to introduce the material into organic-inorganic composite materials. TiO2/GeO2/ormosils composite films containing azobenzene were prepared by combining the solgel technique with the spin-coating process. The optical transmission modes and loss of as-prepared samples at different transmission wavelengths were researched by a prism coupler. The result shows that the composite film is multi-mode transmission at the transmission wavelength of 633 nm and single-mode transmission at 1538 nm. The transmission loss is sufficient for applications in optical elements. The response properties and Fourier transform infrared spectroscopy of as-prepared samples at different heating temperatures were also studied. The composite films obtained at 50°C have the best optical response properties. Furthermore, the banding energy and chemical composition among the films were measured through x-ray photoelectron spectroscopy. Finally, the surface topography of as-prepared samples was observed by atomic force microscopy. The surface of the composite film appears with patterns of relief under the appropriate temperature. The above results show that the as-prepared TiO2/GeO2/ormosils composite films containing azobenzene will be a kind of ideal material in the field of integrated optics applications.

Full-duplex wireless deep ultraviolet light communication.

With recent advancements in deep ultraviolet (DUV) light-emitting diodes (LEDs) and solar-blind photodetectors, wireless DUV light communication is emerging as a novel technique, which can extend transmission ranges and avoid solar interference. Herein, a full-duplex, real-time wireless light communication system using 275 nm DUV LEDs is proposed. We adopted high-power DUV LEDs and designed a high-speed transmitter, a high-sensitivity receiver, and a main processing unit for the system. Furthermore, the DUV communication system, using a Reed-Solomon (RS) encoder and an on-off keying (OOK) modem with frequency control, achieves a 10 Mbit/s bidirectional data transmission rate within 5 m in free space, while a full-duplex video communication link is formed. The encapsulated DUV communication system described in this Letter provides a feasible scheme for confidential and anti-electromagnetic interference communication in Internet of Things (IoT) applications.

Fabrication of microlens arrays with high filling factors by combining a thermal reflow and parylene CVD technique and the applications on OLEDs.

In this paper, microlens array (MLA) templates with high filling factors were prepared by combining a thermal reflow method and parylene chemical vapor deposition (CVD). Then photoresist MLAs were replicated from the MLA templates by using ultraviolet nanoimprint technology. The surface morphology of the replicated photoresist MLAs was characterized by scanning an electron microscope and optical microscope. Results show that the photoresist MLAs have a relatively smooth surface, and the filling factor has been improved obviously. Also, the surface profiles of the MLAs were measured. The optical imaging properties of the MLAs were also characterized, and they had a relatively good imaging performance. Finally, the photoresist MLAs were applied on organic LEDs (OLEDs), and their luminance and current efficiencies were measured. Results show that the current efficiency of the OLEDs increased by about 42.41%, 29.01%, and 35.51%, respectively, for OLEDs with circular, hexagonal, and square MLAs. All the results above indicate that it is a simple and effective process to prepare MLA templates with high filling factors by combining thermal reflow and CVD techniques, and the prepared photoresist MLAs have great application potential in OLED areas.

Switching-behavior improvement in HfO2/ZnO bilayer memory devices by tailoring of interfacial and microstructural characteristics.

We investigated the effect of top contact interface and microstructural characteristics of the insulating layers on resistive switching behaviors by fabricating and characterizing the HfO2/ZnO bilayer heterostructures. Different thickness of ZnO underlying layer and different deposition temperatures of the upper HfO2layer were designed to analyze the intrinsic contribution of the crystalline microstructure of the insulating bilayer. Pt and Ti top electrodes were used to demonstrate the extrinsic contribution of the interface configuration. It was observed that all devices show bipolar RS characteristics. Unlike the device composed of Pt/HfO2/ZnO/Pt that exhibit an abrupt switching, a gradually continuous switching in the reset process was identified in the device composed of Ti/HfO2/ZnO/Pt. Interfacial charge migration process/characteristic plays a key role in the RS process as well as its conduction mechanism. The RS performance of the former is significantly better than that of the latter, including much lower reset voltage, two orders of magnitude larger OFF/ON ratio and HRS resistance. In addition, as compared to the intrinsic contribution arising from the microstructure of the HfO2/ZnO bilayer to the RS performances and current transport mechanism, the extrinsic effect contributed from the electrode characteristics (and its interface) is dominant.

PMMA-doped composite films microlens arrays on a flexible substrate for improving extraction of an organic light-emitting diode.

Poly (methyl methacrylate) (PMMA)-doped organic-inorganic composite films are prepared on flexible substrates through a combination of the sol-gel technique and the spin-coating method. Circular and hexagonal microlens arrays (MLAs) are then built into the composite films by using ultraviolet nanoimprint technology. Atomic force microscope and ultraviolet spectrophotometer characterization results of the films show that the films have low surface roughness and good optical transmittance. A scanning electron microscope is used to observe the surface morphology of the MLAs, and the results show that the prepared MLAs are regular and neat. The surface profiles of the MLAs are also measured by using a surface profiler. Optical microscopy results show that the prepared MLAs have good optical imaging properties. Finally, the MLAs are applied on the green organic light-emitting diodes (OLEDs), and the influence of the shape and diameter of the MLAs on the luminance and current efficiency of the OLEDs is discussed. Results indicate that there is a relatively high enhancement of the current efficiency and luminance for the OLEDs with hexagonal MLAs and a single microlens height of 9 µm, where the luminance can reach 10611cd/cm2, and the current efficiency can be enhanced by about 20.1%. Furthermore, there is a higher enhancement of the luminance and current efficiency for the OLEDs with PMMA-doped MLAs than that of the OLEDs with no PMMA-doped MLAs. Based on these results, we believe that the obtained PMMA-doped composite film MLAs on flexible substrates have important applications in the flexible OLEDs displays areas.

Titanium content influence on the optical response characteristics of TiO2/ormosils composite films doped with azobenzene.

TiO2-based organic-inorganic composite films doped with azobenzene and photosensitive groups were prepared by combining a low-temperature solgel technique and a spin-coating method. The influence of TiO2 content on the optical and structural properties of the composite films including the film thickness, the refractive index, the transmission loss, the thermal gravity analysis, and Fourier transform infrared spectroscopy spectra was studied. Photoisomerization and optical switching characteristics of the composite film were investigated under the irradiation of 365 nm ultraviolet light and 450 nm visible light. Results indicate that several micrometer thick films can be easily obtained at room temperature and there is a proportional relationship between the refractive index value and the TiO2 content. In addition, the composite films have a low optical propagation loss of about 0.1 dB/cm. The composite films with 0.2 M TiO2 content have an obvious photoisomerization and good optical switching properties. Finally, the hexagonal microlens array was fabricated in the composite films by using an ultraviolet nanoimprint technology. All these results above indicate that the as-prepared TiO2-based organic-inorganic composite film has potential applications in optical switching devices and photonic elements.

Simultaneous transmission, detection, and energy harvesting.

Due to the electro-optic property of InGaN multiple quantum wells, a III-nitride diode can provide light transmission, photo detection, and energy harvesting under different bias conditions. Made of III-nitride diodes arrayed in a single chip, the combination allows the diodes to transmit, detect, and harvest visible light at the same time. Here, we monolithically integrate a III-nitride transmitter, receiver, and energy harvester using a compatible foundry process. By adopting a bottom SiO2/TiO2 distributed Bragg reflector, we present a III-nitride diode with a peak external quantum efficiency of 50.65% at a forward voltage of 2.6 V for light emission, a power conversion efficiency of 6.68% for energy harvesting, and a peak external quantum efficiency of 50.9% at a wavelength of 388 nm for photon detection. The energy harvester generates electricity from ambient light to directly turn the transmitter on. By integrating a circuit, the electrical signals generated by the receiver pulse the emitted light to relay information. The multifunctioning system can continuously operate without an external power supply. Our work opens up a promising approach to develop multicomponent systems with new interactive functions and multitasking devices, due to III-nitride diode arrays that can simultaneously transmit, detect, and harvest light.

Photoinduced semiconductor-metal transition in ultrathin troilite FeS nanosheets to trigger efficient hydrogen evolution.

The exploitation of the stable and earth-abundant electrocatalyst with high catalytic activity remains a significant challenge for hydrogen evolution reaction. Being different from complex nanostructuring, this work focuses on a simple and feasible way to improve hydrogen evolution reaction performance via manipulation of intrinsic physical properties of the material. Herein, we present an interesting semiconductor-metal transition in ultrathin troilite FeS nanosheets triggered by near infrared radiation at near room temperature for the first time. The photogenerated metal-phase FeS nanosheets demonstrate intrinsically high catalytic activity and fast carrier transfer for hydrogen evolution reaction, leading to an overpotential of 142 mV at 10 mA cm-2 and a lower Tafel slope of 36.9 mV per decade. Our findings provide new inspirations for the steering of electron transfer and designing new-type catalysts.

Optofluidic variable optical attenuator controlled by electricity.

An optofluidic variable optical attenuator (VOA) is proposed in this paper, where the microfluidic driving technology adopts the electrically controlled way. The proposed driving technology solves some problems of existing microfluidic driving technologies and introduces a simple structure, a small volume, high precision, and a quick response for the VOA. This VOA has some advantages over other VOAs, such as a wide wavelength band (from visible light to the near infrared), a wide adjustable attenuation range, a low wavelength-dependent loss, and a quick response. The experiment results indicate that the attenuation range of this VOA is more than 80 dB and the wavelength-dependent loss is 0.09 dB at an attenuation of 20 dB in the C-band. Most VOAs have millisecond-scale response times, whereas the response time here is about 155-180 µs. Our work shows a new way to design miniaturized VOAs with good performance and can also promote optofluidics.

InGaN/GaN micro mirror with electrostatic comb drive actuation integrated on a patterned silicon-on-insulator wafer.

A double-sided mask process is used to define the micro actuators on a silicon-on-insulator (SOI) wafer. Then, an InGaN/GaN multilayer film was deposited on the surface of the above patterned SOI substrate by employed MBE technology. Thus, the final device consisting of comb-drive actuators, springs and micro-mirror is implemented to obtain the two-dimensional (2D) tuning effect of the mirror. The displacement response is characterized by applying voltage to the micro actuators. It shows an approximation linear relationship between the displacement and the square of applied voltage. And the influences of the cross-axis coupling effect are also measured and evaluated. Moreover, the luminous performances of the InGaN/GaN multiquantum well films were also studied by analyzing temperature-dependent and laser intensity-dependent PL spectra. The authors observed the localized state effect and/or band-gap narrowing effect which results in an S-shaped behavior with increasing temperature. The PL intensity and wavelength are also significantly affected by the excitation energy of laser.

III-Nitride grating grown on freestanding HfO2 gratings.

We report here the epitaxial growth of III-nitride material on freestanding HfO2 gratings by molecular beam epitaxy. Freestanding HfO2 gratings are fabricated by combining film evaporation, electron beam lithography, and fast atom beam etching of an HfO2 film by a front-side silicon process. The 60-µm long HfO2 grating beam can sustain the stress change during the epitaxial growth of a III-nitride material. Grating structures locally change the growth condition and vary indium composition in the InGaN/GaN quantum wells and thus, the photoluminescence spectra of epitaxial III-nitride grating are tuned. Guided mode resonances are experimentally demonstrated in fabricated III-nitride gratings, opening the possibility to achieve the interaction between the excited light and the grating structure through guided mode resonance.PACS: 78.55.Cr; 81.65.Cf; 81.15.Hi.

Patterned growth of InGaN/GaN quantum wells on freestanding GaN grating by molecular beam epitaxy.

We report here the epitaxial growth of InGaN/GaN quantum wells on freestanding GaN gratings by molecular beam epitaxy (MBE). Various GaN gratings are defined by electron beam lithography and realized on GaN-on-silicon substrate by fast atom beam etching. Silicon substrate beneath GaN grating region is removed from the backside to form freestanding GaN gratings, and the patterned growth is subsequently performed on the prepared GaN template by MBE. The selective growth takes place with the assistance of nanoscale GaN gratings and depends on the grating period P and the grating width W. Importantly, coalescences between two side facets are realized to generate epitaxial gratings with triangular section. Thin epitaxial gratings produce the promising photoluminescence performance. This work provides a feasible way for further GaN-based integrated optics devices by a combination of GaN micromachining and epitaxial growth on a GaN-on-silicon substrate.PACS81.05.Ea; 81.65.Cf; 81.15.Hi.

Large area, freestanding GaN nanocolumn membrane with bottom subwavelength nanostructure.

We propose, fabricate and characterize the freestanding GaN nanocolumn membrane with bottom subwavelength nanostructures. The GaN nanocolumns are epitaxially grown on freestanding nanostructured silicon substrate that is achieved by a combination of self-assemble technique and silicon-on-insulator (SOI) technology. Optical reflection is greatly suppressed in the visible range due to the graded refractive index effect of subwavelength nanostructures. The freestanding GaN nanocolumn membrane is realized by removing silicon substrate from the backside, eliminating the silicon absorption of the emitted light and leading to a strong blue emission from the bottom side. The obtained structures also demonstrate the potential application for anti-reflective (AR) coating and GaN-Si hybrid microelectromechanical system (MEMS).

Fabrication and characterization of subwavelength nanostructures on freestanding GaN slab.

We develop a novel way to fabricate subwavelength nanostructures on the freestanding GaN slab using a GaN-on-silicon system by combining self-assemble technique and backside thinning method. Silicon substrate beneath the GaN slab is removed by bulk silicon micromachining, generating the freestanding GaN slab and eliminating silicon absorption of the emitted light. Fast atom beam (FAB) etching is conducted to thin the freestanding GaN slab from the backside, reducing the number of confined modes inside the GaN slab. With self-assembled silica nanospheres acting as an etching mask, subwavelength nanostructures are realized on the GaN surface by FAB etching. The reflection losses at the GaN interfaces are thus suppressed. When the InGaN/GaN multiple quantum wells (MQWs) active layers are excited, the light extraction efficiency is significantly improved for the freestanding nanostructured GaN slab. This work provides a very practical approach to fabricate freestanding nanostructures on the GaN-on-silicon system for further improving the light extraction efficiency.

Fabrication and characterization of freestanding circular GaN gratings.

It's of significant interest to combine freestanding nanostructure with active gallium nitride (GaN) material for surface-emitting optoelectronic application. By utilizing bulk micromachining of silicon, we demonstrate here a promising way to fabricate freestanding GaN nanostructures using a GaN-on-silicon system. The well-defined nanoscale circular GaN gratings are realized by fast-atom beam (FAB) etching, and the freestanding GaN gratings are obtained by removing silicon substrate using deep reactive ion etching (DRIE). The freestanding GaN slab is thinned from the backside by FAB etching to reduce the confined modes inside the GaN slab. The measured microphotoluminescence (micro-PL) spectra experimentally demonstrate significant enhancements in peak intensity and integrated intensity by introducing freestanding circular grating. This work represents an important step in combining GaN-based active material with freestanding nanostructures for further increasing light-extraction efficiency.