Gallium nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lighting. In the last decade, a huge global market of efficient, long-lasting, and ubiquitous white light sources has developed around the inception of the Nobel-prize-winning blue GaN LEDs. Today, GaN optoelectronics is developing beyond solid-state lighting, leading to new and innovative devices, e.g., for microdisplays, being the core technology for future augmented reality and visualization, as well as point light sources for optical excitation in communications, imaging, and sensing. This explosion of applications is driven by two main directions: the ability to produce very small GaN LEDs (micro-LEDs and nano-LEDs) with high efficiency and across large areas, in combination with the possibility to merge optoelectronic-grade GaN micro-LEDs with silicon microelectronics in a hybrid approach. GaN LED technology is now even spreading into the realm of display technology, which has been occupied by organic LEDs and liquid crystal displays for decades. In this review, the technological transition toward GaN micro- and nanodevices beyond lighting is discussed including an up-to-date overview on the state of the art.

A film of gas sensitive ZnO nanoparticles has been coupled with a low-power micro light plate (µLP) to achieve a NO2-parts-per-billion conductimetric gas sensor operating at room temperature. In this µLP configuration, an InGaN-based LED (emitting at 455 nm) is integrated at a few hundred nanometers distance from the sensor material, leading to sensor photoacti-vation with well controlled, uniform and high irradiance condi-tions, and very low electrical power needs. The response curves to different NO2 concentrations as a function of the irradiance displayed a bell‐like shape. Responses of 20% to 25 ppb of NO2 were already observed at irradiances of 5 mWatts·cm-2 (applying an electrical power as low as 30 µW). In the optimum illumination conditions (around 60 mWatts · cm-2, or 200 µW of electric power), responses of 94% to 25 ppb were achieved, corresponding to a lower detection limit of 1 ppb of NO2. High-er irradiance values worsened the sensor response in all the parts-per-billion range of NO2 concentrations. The responses to other gases such as NH3, CO and CH4 were much smaller, showing a certain selectivity towards NO2. The effects of hu-midity on the sensor response are also discussed.

The reverse transcription-quantitative polymerase chain reaction (RT-qPCR) approach has been widely used to detect the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). However, instead of using it alone, clinicians often prefer to diagnose the coronavirus disease 2019 (COVID-19) by utilizing a combination of clinical signs and symptoms, laboratory test, imaging measurement (e.g., chest computed tomography scan), and multivariable clinical prediction models, including the electronic nose. Here, we report on the development and use of a low cost, noninvasive method to rapidly sniff out COVID-19 based on a portable electronic nose (GeNose C19) integrating an array of metal oxide semiconductor gas sensors, optimized feature extraction, and machine learning models. This approach was evaluated in profiling tests involving a total of 615 breath samples composed of 333 positive and 282 negative samples. The samples were obtained from 43 positive and 40 negative COVID-19 patients, respectively, and confirmed with RT-qPCR at two hospitals located in the Special Region of Yogyakarta, Indonesia. Four different machine learning algorithms (i.e., linear discriminant analysis, support vector machine, stacked multilayer perceptron, and deep neural network) were utilized to identify the top-performing pattern recognition methods and to obtain a high system detection accuracy (88–95%), sensitivity (86–94%), and specificity (88–95%) levels from the testing datasets. Our results suggest that GeNose C19 can be considered a highly potential breathalyzer for fast COVID-19 screening.

With a remarkable theoretical specific capacity of ∼4200 mA h g⁻¹, silicon anodes are at the forefront of enabling lithium-ion batteries (LIBs) with ultra-high energy density. However, we have yet to see the wide utilization of silicon anodes in commercial LIBs due to several challenges, categorized as intrinsic and extrinsic problems. The intrinsic problems originate from the intrinsic properties of silicon, which include its low conductivity and massive volume expansion upon alloying with lithium. The extrinsic problems of silicon anodes are primarily associated with the laborious fabrication processes. Instead of bottom-up approaches, which involve relatively expensive and complicated synthesis routes, the top-down methods provide an affordable, controllable, and simple procedure for large-scale fabrication. The present review aims to collate the recent studies on the fabrication of silicon anodes via top-down approaches that involve etching, metallothermic reduction, and high energy ball milling. Extensive synthesis routes along with their synthesis parameters are reviewed. The morphology and the electrochemical performances of the silicon anodes prepared from various starting materials are also highlighted. Finally, perspectives on the potential future challenges and development of the top-down synthesis of silicon-based anodes are presented.

Wide-range humidity sensing and monitoring applications including instrumentation, agriculture, meteorology, biomedicine, and food processing have attracted long-standing interests, where recently substantial progress is made in both sensing-material science and microfabrication technologies to achieve portable, reliable and low-cost humidity sensing instruments. Due to their high sensitivity, enormous miniaturization potential, and well-developed high-volume microfabrication technologies, microelectromechanical systems (MEMS)-based piezoresistive cantilever devices covered by large-surface-area nanostructures of hygroscopic materials offer an ideal platform for highly sensitive humidity detection. Since resonant gravimetric sensing is the dominant humidity sensing technique in recent research works, in this paper, resonant actuation principles for microcantilevers (i.e. the dynamic operation mode) are addressed and compared with respect to the quality of the amplitude and phase signals, as required for on-line frequency tracking using a phase-locked loop circuit. Parasitic feedthrough effects are considered between the resonance-mode ( f ₀ ) excitation element and the piezoresistive detection circuit, which can lead to a reduction of stop-band attenuation, the generation of a parallel resonance in close vicinity of f ₀ , a hardly detectable 90° phase jump, and a long-term drift of resonance frequency and phase shift. Methods for eliminating these parasitic feedthrough effects have been considered, including de-embedding of the motional signal by later data processing and the integration of a reference cantilever or circuit. Then, different concepts of environmental sensing using microcantilevers are described, including detection of particulate matter and gas molecules/volatile organic compounds. Depending on the condition of the cantilever during sensing operation, two different modes have been used to sense the target analyte (i.e. static and dynamic modes). In a static operation mode, mass change of the cantilever, surface stress, or swelling of a layer on top related to the uptake and binding of particles or molecules on the cantilever are detectable via a deformation of the cantilever (i.e. by deflection or strain), which can be sensed by an integrated piezoresistive strain gauge. Quasistatic bending of the cantilever as well as frequency down-shift of an excited resonance mode are normally used for detection. Humidity adsorption/desorption characteristics of such piezoresistive microcantilevers can be modified using hygroscopic layered materials deposited on the cantilever, among which we address metal oxides, ceramics, organics, or organic/inorganic composites. These thin layers comprise preferentially concave or convex nanostructures (e.g. pores, particles, colloids, rods, or fins), which provide a sensing surface of large surface-to-volume ratio and thus a large number of binding sites for highly efficient adhesion of water molecules. Finally, fabrication processes of integrated piezoresistive microcantilever-based humidity sensors, including micromachining/MEMS technology, integration of nanostructures and their combination with deposited hydrophilic materials are described. Lastly, their humidity sensing performance is compared with competing state-of-the art and advanced MEMS devices, e.g. capacitive micromachined ultrasonic transducers, quartz crystal microbalance, thin-film bulk acoustic resonator, surface acoustic wave resonator and complementary metal oxide semiconductor-MEMS for gravimetric sensing with respect to, e.g. sensitivity, hysteresis, and response time.