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Efficient product separation is an essential requirement for the safe operation and implementation of solar water splitting devices. Ion exchange membranes are typically used, but for a device that requires sunlight illumination to travel from one electrode to the other, these membranes may introduce parasitic absorption and reduce the achievable device efficiency. Here, we investigate the feasibility of utilizing transparent polymer sheets as separators in solar water splitting devices. Porosity is introduced into the polymer sheets by femtosecond laser patterned micropore arrays with various diameters and pitch distances. Our experimental data and numerical simulation results show that these polymer sheets not only have low parasitic absorption, but also lower resistances as compared to a commercial anion exchange membrane. Finally, product crossover through the transparent porous separators is evaluated using a particle tracing model under the realistic operation conditions of solar water splitting devices. Minimal crossover can be obtained by optimizing the separator as well as the operational parameters

The integration of gallium nitride (GaN) nanowire light-emitting diodes (nanoLEDs) on flexible substrates offers opportunities for applications beyond rigid solid-state lighting (e.g., for wearable optoelectronics and bendable inorganic displays). Here, we report on a fast physical transfer route based on femtosecond laser lift-off (fs-LLO) to realize wafer-scale top–down GaN nanoLED arrays on unconventional platforms. Combined with photolithography and hybrid etching processes, we successfully transferred GaN blue nanoLEDs from a full two-inch sapphire substrate onto a flexible copper (Cu) foil with a high nanowire density (~107 wires/cm2), transfer yield (~99.5%), and reproducibility. Various nanoanalytical measurements were conducted to evaluate the performance and limitations of the fs-LLO technique as well as to gain insights into physical material properties such as strain relaxation and assess the maturity of the transfer process. This work could enable the easy recycling of native growth substrates and inspire the development of large-scale hybrid GaN nanowire optoelectronic devices by solely employing standard epitaxial LED wafers (i.e., customized LED wafers with additional embedded sacrificial materials and a complicated growth process are not required).

We fabricated highly sensitive and selective ammonia gas sensors based on quartz crystal microbalance (QCM) platforms that were functionalized with electrospun polyvinyl acetate (PVAc) nanofibers a...

Generation of amplified stimulated emission inside mammalian cells has paved the way for a novel bioimaging and cell sensing approach. Single cells carrying gain media (e.g., fluorescent molecules) are placed inside an optical cavity, allowing the production of intracellular laser emission upon sufficient optical pumping. Here, we investigate the possibility to trigger another amplified emission phenomenon (i.e., amplified spontaneous emission or ASE) inside two different cell types, namely macrophage and epithelial cells from different species and tissues, in the presence of a poorly reflecting cavity. Furthermore, the resulting ASE properties can be enhanced by introducing plasmonic nanoparticles. The presence of gold nanoparticles (AuNPs) in rhodamine 6G-labeled A549 epithelial cells results in higher intensity and lowered ASE threshold in comparison to cells without nanoparticles, due to the effect of plasmonic field enhancement. An increase in intracellular concentration of AuNPs in rhodamine 6G-labeled macrophages is, however, responsible for the twofold increase in the ASE threshold and a reduction in the ASE intensity, dominantly due to a suppressed in and out-coupling of light at high nanoparticle concentrations.

Due to its high theoretical specific capacity, a silicon anode is one of the candidates for realizing high energy density lithium-ion batteries (LIBs). However, problems related to bulk silicon (e.g., low intrinsic conductivity and massive volume expansion) limit the performance of silicon anodes. In this work, to improve the performance of silicon anodes, a vertically aligned n-type silicon nanowire array (n-SiNW) was fabricated using a well-controlled, top-down nano-machining technique by combining photolithography and inductively coupled plasma reactive ion etching (ICP-RIE) at a cryogenic temperature. The array of nanowires ~1 µm in diameter and with the aspect ratio of ~10 was successfully prepared from commercial n-type silicon wafer. The half-cell LIB with free-standing n-SiNW electrode exhibited an initial Coulombic efficiency of 91.1%, which was higher than the battery with a blank n-silicon wafer electrode (i.e., 67.5%). Upon 100 cycles of stability testing at 0.06 mA cm−2, the battery with the n-SiNW electrode retained 85.9% of its 0.50 mAh cm−2 capacity after the pre-lithiation step, whereas its counterpart, the blank n-silicon wafer electrode, only maintained 61.4% of 0.21 mAh cm−2 capacity. Furthermore, 76.7% capacity retention can be obtained at a current density of 0.2 mA cm−2, showing the potential of n-SiNW anodes for high current density applications. This work presents an alternative method for facile, high precision, and high throughput patterning on a wafer-scale to obtain a high aspect ratio n-SiNW, and its application in LIBs.

Production of high-aspect-ratio silicon (Si) nanowire-based anode for lithium ion batteries is challenging particularly in terms of controlling wire property and geometry to improve the battery performance. This report demonstrates tunable optimization of inductively coupled plasma reactive ion etching (ICP-RIE) at cryogenic temperature to fabricate vertically-aligned silicon nanowire array anodes with high verticality, controllable morphology, and good homogeneity. Three different materials [i.e., photoresist, chromium (Cr), and silicon dioxide (SiO2)] were employed as masks during the subsequent photolithography and cryogenic ICP-RIE processes to investigate their effects on the resulting nanowire structures. Silicon nanowire arrays with a high aspect ratio of up to 22 can be achieved by tuning several etching parameters [i.e., temperature, oxygen/sulfur hexafluoride (O2/SF6) gas mixture ratio, chamber pressure, plasma density, and ion energy]. Higher compressive stress was revealed for longer Si wires by means of Raman spectroscopy. Moreover, an anisotropy of lattice stress was found at the top and sidewall of Si nanowire, indicating compressive and tensile stresses, respectively. From electrochemical characterization, half-cell battery integrating ICP-RIE-based silicon nanowire anode exhibits a capacity of 0.25 mAh cm−2 with 16.67% capacity fading until 20 cycles, which has to be improved for application in future energy storage devices.

For various lighting and monolithic sensor systems application, vertically aligned three-dimensional (3D) gallium nitride (GaN)- and indium gallium nitride (InGaN)/GaN-based LED nanowire arrays with sub-200 nm feature sizes (down to 35 nm) were fabricated using a nanosphere lift-off lithography (NSLL) technique combined with hybrid top-down etching (i.e., inductively coupled plasma dry reactive ion etching (ICP-DRIE) and wet chemical etching). Owing to the lithographic opening and well-controlled surface functionalization prior to the polystyrene nanosphere (PN) deposition, vertical GaN nanowire arrays with an area density of 9.74 × 108 cm–2 and an aspect ratio of >10 could be realized in a specified large area of 1.5 × 1.5 mm2. Optoelectrical characteristics of the nanoLEDs were further investigated in cathodoluminescence (CL) measurements, in which multiquantum well (MQW) shows a clear CL-emission at a wavelength of 465 nm. Thus, using NSLL to manufacture low-cost but highly ordered 3D GaN-based nanowir...

Gallium nitride GaN film delamination is an important process during the fabrication of GaN light emitting diodes LEDs and laser diodes. Here, we utilize 520 nm femtosecond laser pulses, exploiting nonlinear absorption rather than single photon absorption such as in conventional laser lift off LLO employing excimer or Q switched laser sources. The focus of this study is to investigate the influence of laser scanning speed and integrated fluence corresponding to laser energy per area during the LLO processing of GaN LED chips and their resulting structural properties. Because both the sapphire substrate and InGaN GaN heterostructures are fully transparent to the emission of the laser system, a key question is related to the impact of laser pulses on the quality of a thin film structure. Therefore, several characterization methods i.e., scanning electron microscopy, atomic force microscopy, X ray diffraction, Raman spectroscopy, and electroluminescence spectroscopy were employed to understand the material modifications made by femtosecond LLO fs LLO . We demonstrated that by adjusting the laser scanning speed, smooth GaN surfaces and good crystal quality could be obtained regardless of the existing delamination of metal contact, which then slightly downgraded the LED performance. Here, the integrated fluence level was set in the range of 2.6 4.4 J cm2 to enable the fs LLO process. Moreover, two mitigation strategies were developed and proven to improve the optoelectrical characteristics of the lifted off LEDs i.e., modification of the processing step related to the metal creation and reduction of laser energy

Laser lift‐off (LLO) is commonly applied to separate functional thin films from the underlying substrate, in particular light‐emitting diodes (LEDs) on a gallium nitride (GaN) basis from sapphire. By transferring the LED layer stack to foreign carriers with tailored characteristics, for example, highly reflective surfaces, the performance of optoelectronic devices can be drastically improved. Conventionally, LLO is conducted with UV laser pulses in the nanosecond regime. When directed to the sapphire side of the wafer, absorption of the pulses in the first GaN layers at the sapphire/GaN interface leads to detachment. In this work, a novel approach towards LLO based on femtosecond pulses at 520 nm wavelength is demonstrated for the first time. Despite relying on two‐photon absorption with sub‐bandgap excitation, the ultrashort pulse widths may reduce structural damage in comparison to conventional LLO. Based on a detailed study of the laser impact as a function of process parameters, a two‐step process scheme is developed to create freestanding InGaN/GaN LED chips with up to 1.2 mm edge length and ≈5 μm thickness. The detached chips are assessed by scanning electron microscopy and cathodoluminescence, revealing similar emission properties before and after LLO.

In this work, we devised hybrid humidity sensors by integrating quartz crystal microbalance (QCM) platforms with polyethyleneimine (PEI)-grafted polyacrylonitrile (PAN) nanofibrous mats (i.e., PAN/PEI nanofibers) as active layers. The produced devices were characterized using a scanning electron microscope (SEM), an atomic force microscope (AFM), and a Fourier transform infrared (FTIR) spectrometer to investigate their morphologies, roughness, and molecular compositions, respectively. To understand the humidity sensing mechanism, an exponential kinetic model was developed for the adsorption and desorption processes of water molecules on nanofibers. The viscoelasticity of the active layer started to increase when the sensor was exposed to relative humidity (RH) of > 60 %rH. The PAN/PEI nanofiber-functionalized QCM sensors exhibited a sensitivity of up to 164 Hz/%rH with the response and recovery times of 13 and 7 s, respectively, during humidity exposure assessments performed at room temperature of (30 ± 1) °C. Moreover, they also provided high selectivity, long-term stability, and excellent repeatability with low error rates. Among the other existing QCM-based humidity sensors that were coated with various materials (e.g., metal oxides, carbon, and polymers), our unique electrospun PAN/PEI nanofibrous mat-functionalized QCM sensing devices have demonstrated the most superior performances in terms of their sensitivity and responsivity, which could then offer an alternative for high-sensing-performance yet low-cost humidity sensors.

Laser lift-off (LLO) has become a viable technique to detach epitaxially grown light-emitting diodes (LEDs) from their original sapphire substrates. In conventional LLO, intense UV irradiation from e.g. excimer lasers with pulse widths in the nanosecond regime is applied. It is directed to the backside of the wafer, transmitted through the sapphire substrate and then linearly absorbed at the interface to the epitaxially grown GaN stack, resulting in thermal decomposition of the first layers. As an alternative novel approach, we present a successful implementation of LLO process with 350 fs short pulses in the green spectral range (520 nm) based on two-photon absorption mechanism. By means of electroluminescence measurements and electron microscopy (i.e., SEM and TEM), the impact of the irradiation on the GaN crystal quality has been investigated and compared with that from conventional lift-off processes.

Chirped surface acoustic wave (SAW) resonators based on aluminum nitride (AlN) thin films have been designed and fabricated to comprehend the wave propagation characteristics induced by interdigitated transducers (IDTs) deposited on their surfaces. From the simulation results, design and geometry of the metal fingers including their width and pitch play critical roles on the wavelength of the acoustic wave and the mechanical displacement, which subsequently set the device resonant frequency. A single-step metal lift-off process involving photolithography and electron beam metal evaporation has been used to pattern and deposit Cr/Au IDT on AlN-on-Si wafers.

A microwave generator system has been built using the mixing technique of two DFB lasers. DFB lasers used in this experiment are operated on C-band frequency and have 2 nm wavelength ranges which can be adjusted into a specific wavelength value. The microwave signal as the result of lasers frequency mixing can be controlled with tunability factor of 10.35 GHz/°C. Also, the current drive of DFB laser system has contributed on the changing value of DFB laser operational temperature. This means the current drive has indirectly controlled the frequency of the generated microwave signal. In this case, the microwave frequency has changed about 0.37 GHz as we the temperature was shifted by 0.006 °C for 1 mA current drive.

We successfully developed a method utilizing colloidal CdSe nanocrystalline quantum dot for anti-counterfeiting label on a piece of glossy paper. We deposited numbers and lines patterns of toluene soluble CdSe quantum dot using rubber stamper on a glossy paper. The width of line pattern was about 1-2 mm with 1-2 mm separation between lines. It required less than one minute for deposited CdSe quantum dot on glossy paper to dry and become invisible by naked eyes. However, patterned quantum dot become visible using long-pass filter glasses upon excitation of UV lamp or blue laser. We characterized photoluminescence of line patterns of quantum dot, and we found that emission boundaries of line patterns were clearly observed. The error of line size and shape were mainly due to defect of the original stamper. The emission peak wavelength of CdSe quantum dot was 629 nm. The emission spectrum of deposited quantum dot has full width at half maximum (FWHM) of 30-40 nm. The spectra similarity between deposited quant...

We reported the fabrication of 1D photonic crystal structure and investigated its application for tuning colloidal CdSe quantum dot emission. The photonic crystals was made by using sol-gel method and consists of TiO 2 and SiO 2 multilayers on glass substrate. The colloidal quantum dot is CdSe quantum dot with emission wavelength around 630 nm, which is similar to photonic band gap of fabricated photonic crystals. The quantum dot was dropped and dried on the opposite side of photonic crystals. We measured angle dependence of transmission of photonic crystal and angle dependence photoluminescence of quantum dot emission. We found that quantum dot emission was influenced by excitation angle ; therefore photonic crystals can be useful for tuning the emission of quantum dot. We have confirmed that the angle dependence photoluminescence of quantum dot is significantly related to photonic band gap of photonic crystal. This work leads to future application of photonic crystal on quantum dot emission such as quantum dot lasing. Keywords: Photonic crystal, colloidal quantum dot, photoluminescence, photonic band gap