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Abstract Microelectromechanical system-based microphones demand high ingress protection levels with regard to their use in harsh environment. Here, we develop environmental protective components comprising polyimide nanofibers combined onto polyether ether ketone fabric meshes and subsequently appraise their impact on the electroacoustic properties of high signal-to-noise-ratio microelectromechanical system-based microphones via industry-standard characterizations and theoretical simulations. Being placed directly on top of the microphone sound port, the nanofiber mesh die-cut parts with an inner diameter of 1.4 mm result in signal-to-noise-ratio and insertion losses of (2.05 ± 0.16) dB(A) and (0.30 ± 0.11) dBFS, respectively, in electroacoustic measurements. Hence, a high signal-to-noise-ratio value of (70.05 ± 0.17) dB(A) can be maintained by the mesh-protected microphone system. Due to their high temperature stability, acoustic performance, environmental robustness, and industry-scale batch production, these nanofibrous meshes reveal high potential to be practically implemented in high-market-volume applications of packaged microelectromechanical system-based microphones.

This study presents a novel composite nanofiber membrane combining polyacrylonitrile/polyvinylidene fluoride (PAN/PVDF) electrospun nanofibers with locally sourced zeolite from Sumatera, Indonesia, for pollutant dye filtration. The Ze-PAN/PVDF composite nanofiber membrane has 100% methylene blue dye rejection even after five filtration cycles with a permeation flux of 185 L m−2 h−1 bar−1. The membrane is highly selective, with 100% dye filtration efficiency for cationic dyes (i.e., methylene blue and crystal violet) vs. 78% and 1% for anionic dyes (i.e., Congo red and methyl orange, respectively). For the cationic dyes, the dye filtration mechanism is mainly governed by electrostatic attraction, while the size exclusion mechanism becomes more dominant for the anionic dyes. Overall, these results demonstrate the potential use of Ze-PAN/PVDF composite nanofiber membrane in wastewater treatment applications.

This work presents the development of hierarchical niobium pentoxide (Nb2O5)-based composite nanofiber membranes for integrated adsorption and photocatalytic degradation of methylene blue (MB) pollutants from aqueous solutions. The Nb2O5 nanorods were vertically grown using a hydrothermal process on a base electrospun nanofibrous membrane made of polyacrylonitrile/polyvinylidene fluoride/ammonium niobate (V) oxalate hydrate (Nb2O5@PAN/PVDF/ANO). They were characterized using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) analysis, and Fourier transform infrared (FTIR) spectroscopy. These composite nanofibers possessed a narrow optical bandgap energy of 3.31 ​eV and demonstrated an MB degradation efficiency of 96 ​% after 480 ​min contact time. The pseudo-first-order kinetic study was also conducted, in which Nb2O5@PAN/PVDF/ANO nanofibers have kinetic constant values of 1.29 ​× ​10−2 ​min−1 and 0.30 ​× ​10−2 ​min−1 for adsorption and photocatalytic degradation of MB aqueous solutions, respectively. These values are 17.7 and 7.8 times greater than those of PAN/PVDF/ANO nanofibers without Nb2O5 nanostructures. Besides their outstanding photocatalytic performance, the developed membrane materials exhibit advantageous characteristics in recycling, which subsequently widen their practical use in environmental remediation applications.

Abstract Since the performance of micro-electro-mechanical system (MEMS)-based microphones is approaching fundamental physical, design, and material limits, it has become challenging to improve them. Several works have demonstrated graphene’s suitability as a microphone diaphragm. The potential for achieving smaller, more sensitive, and scalable on-chip MEMS microphones is yet to be determined. To address large graphene sizes, graphene-polymer heterostructures have been proposed, but they compromise performance due to added polymer mass and stiffness. This work demonstrates the first wafer-scale integrated MEMS condenser microphones with diameters of 2R = 220–320 μm, thickness of 7 nm multi-layer graphene, that is suspended over a back-plate with a residual gap of 5 μm. The microphones are manufactured with MEMS compatible wafer-scale technologies without any transfer steps or polymer layers that are more prone to contaminate and wrinkle the graphene. Different designs, all electrically integrated are fabricated and characterized allowing us to study the effects of the introduction of a back-plate for capacitive read-out. The devices show high mechanical compliances C m = 0.081–1.07 μmPa−1 (10–100 × higher than the silicon reported in the state-of-the-art diaphragms) and pull-in voltages in the range of 2–9.5 V. In addition, to validate the proof of concept, we have electrically characterized the graphene microphone when subjected to sound actuation. An estimated sensitivity of S 1k H z = 24.3–321 mV Pa−1 for a V b i a s = 1.5 V was determined, which is 1.9–25.5 × higher than of state-of-the-art microphone devices while having a ~9 × smaller area.

Abstract Miniaturization of modern microelectronics to accommodate the development of portable and smart devices requires independent energy storage that is compact, lightweight, reliable, and integrable on-chip. Three-dimensional lithium-ion microbatteries are considered as promising candidates to fill the role, owing to their high energy and power density. Combined with silicon as a high-capacity anode material, the performance of the microbatteries can be further enhanced. In this review, the latest developments in three-dimensional silicon-based lithium-ion microbatteries are discussed in terms of material compatibility, cell designs, fabrication methods, and performance in various applications. We highlight the relation between device architecture and performance as well as comparison between different fabrication technologies. Finally, we suggest possible future studies based on the current development status to provide a research direction towards further improved three-dimensional silicon-based lithium-ion microbatteries.

Abstract Silicon has attracted considerable attention for use as high-capacity anodes of lithium-ion microbatteries. However, its extreme volume change upon (de-)lithiation still poses a challenge for adoption as it leads to severe active lithium loss that shortens the cycle life. Here, we fabricate three-dimensional monocrystalline vertical silicon nanowires on a silicon wafer using low-cost metal-assisted chemical etching, then cover them with lithium using thermal evaporation prior to the battery operation as the pre-lithiation step, to investigate its impact on electrochemical performance. To reveal the underlying physical and electrochemical mechanisms, we also process a comparative planar monocrystalline silicon. We find that pre-lithiation results in improved (de-)lithiation behavior, especially in planar silicon-based cells, while silicon nanowire-based cells exhibit low capacity in early cycles. This study sheds light on the surface design and structural modification of monocrystalline silicon nanowires with respect to pre-lithiation by lithium thermal evaporation.

Abstract Authentication of meat floss origin has been highly critical for its consumers due to existing potential risks of having allergic diseases or religion perspective related to pork-containing foods. Herein, we developed and assessed a compact portable electronic nose (e-nose) comprising gas sensor array and supervised machine learning with a window time slicing method to sniff and to classify different meat floss products. We evaluated four different supervised learning methods for data classification (i.e., linear discriminant analysis (LDA), quadratic discriminant analysis (QDA), k-nearest neighbors (k-NN), and random forest (RF)). Among them, an LDA model equipped with five-window-extracted feature yielded the highest accuracy values of >99% for both validation and testing data in discriminating beef, chicken, and pork flosses. The obtained e-nose results were correlated and confirmed with the spectral data from Fourier-transform infrared (FTIR) spectroscopy and gas chromatography–mass spectrometry (GC-MS) measurements. We found that beef and chicken had similar compound groups (i.e., hydrocarbons and alcohol). Meanwhile, aldehyde compounds (e.g., dodecanal and 9-octadecanal) were found to be dominant in pork products. Based on its performance evaluation, the developed e-nose system shows promising results in food authenticity testing, which paves the way for ubiquitously detecting deception and food fraud attempts.

Abstract 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.

Abstract 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.

Abstract Performing long-term cell observations is a non-trivial task for conventional optical microscopy, since it is usually not compatible with environments of an incubator and its temperature and humidity requirements. Lensless holographic microscopy, being entirely based on semiconductor chips without lenses and without any moving parts, has proven to be a very interesting alternative to conventional microscopy. Here, we report on the integration of a computational parfocal feature, which operates based on wave propagation distribution analysis, to perform a fast autofocusing process. This unique non-mechanical focusing approach was implemented to keep the imaged object staying in-focus during continuous long-term and real-time recordings. A light-emitting diode (LED) combined with pinhole setup was used to realize a point light source, leading to a resolution down to 2.76 μm. Our approach delivers not only in-focus sharp images of dynamic cells, but also three-dimensional (3D) information on their (x, y, z)-positions. System reliability tests were conducted inside a sealed incubator to monitor cultures of three different biological living cells (i.e., MIN6, neuroblastoma (SH-SY5Y), and Prorocentrum minimum). Altogether, this autofocusing framework enables new opportunities for highly integrated microscopic imaging and dynamic tracking of moving objects in harsh environments with large sample areas.

In this report, we study nonlinear electrical behaviors found in vertical-architecture transistors based on wrap-around-gated gallium nitride (GaN) nanowires (NWs) by extending a one-dimensional case of the Landauer-Büttiker formula. Here, the GaN NWs are considered “almost” one-dimensional ideal wires connecting the drain and source terminals, with the gate terminal serving to control the flowing current. Unlike previous models, which require several parameters and complex calculations, our proposed model only needs three parameters and simple calculations to match the experimental data. With this model, we confirm that the maximum current before saturation is a consequence of quasi-ballistic drain current. Thus, electron mobility has no effect in this device. Using a simple formulation, we discuss gating hysteresis in the device that is mediated by the selected oxide layer interface. We show that the memory effect of the device is attributed to time-delay current. The shorter gate length increases the transmission coefficient. As a result, the model can be employed to predict the next-generation NW transistor performance.

In this work, wearable resistive gas sensors based on hybrid graphene/zinc oxide (ZnO) nanocomposites were fabricated on a flexible cotton fabric and employed to monitor odorless and colorless carbon monoxide (CO). Dip-coating and chemical bath deposition (CBD) was used to deposit the graphene layer and grow the ZnO nanorods, respectively. The films were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-Ray diffraction (XRD) to investigate their morphological structures, elemental composition, and crystal phase, respectively. Those characterizations were also confirming the growth of ZnO nanorods on the already-deposited graphene layer on fabrics. From the gas sensor measurements at room temperature, it was revealed that these graphene/ZnO nanocomposites were highly sensitive and selective towards CO gas at low concentration down to 10 ppm. The shortest response and recovery times of the sensors were measured to be 280 s and 45 s, respectively. Moreover, in comparison to bare graphene sensors, the surface modification by ZnO nanorods could obviously enhance the sensing response by up to 40% (i.e., doubled sensitivity). These flexible hybrid sensors are therefore expected to be a promising alternative for the existing rigid CO sensors in the market by offering unique nanostructures, low-cost fabrication, high flexibility, and good sensing performances.

The demand for high-capacity lithium-ion batteries (LIBs) is ever-increasing. Thus, research has been focused on developing silicon-based anodes due to their high theoretical capacity and natural abundance. However, silicon-based anodes still suffer from several drawbacks (e.g., a huge volume expansion during lithiation/delithiation and the low conductivity nature of silicon). In this study, we develop a facile and low-cost synthesis route to create a composite of silicon particles and carbon nanotubes (CNTs) via simple two-step mechanical ball milling with a silicon wafer as the silicon precursor. This method produces a strong interaction between silicon particles and the CNTs, forming Si–C bonds with minimum oxidation of silicon and pulverization of the CNTs. The resulting Si/CNT anode exhibits a first cycle Coulombic efficiency of 98.06%. It retains 71.28% of its first cycle capacity of 2470 mAh g−1 after 100 cycles of charge–discharge at a current density of 400 mA g−1. Furthermore, the Si/CNT anode also shows a good rate capability by retaining 80.15%, and 94.56% of its first cycle capacity at a current density of 1000 mA g−1 and when the current density is reduced back to 200 mA g−1, respectively.

Abstract Background In this work, a nanostructured guided-mode resonance filter matrix with high transmission efficiency and narrow bandwidth is demonstrated. The developed nano-filter arrays have various usages, e.g., combined with the CMOS image sensors to realize compact spectrometers for biomedical sensing applications. Methods In order to optimize the filter performance, the spectral responses of filters with different structural parameters are carefully studied based on the variable-controlling method. A quality factor is carried out for quantitative characterization. Results In this case, a high fill factor of 0.9 can strongly suppress sidebands, while buffer layer thickness can be adjusted to mainly control the bandwidth. The transmission peaks shift from 386 nm to 1060 nm with good linearity when periods vary from 220 nm to 720 nm. The incident angle dependence is simulated to be ~ 1.1 nm/degree in ±30° range. The filters are then fabricated and characterized. The results obtained from both simulations and experiments agree well, where the filters with the period of 352 nm exhibit simulated and measured transmission peaks of 564 nm and 536 nm, the FWHM of 13 nm and 17 nm, respectively. In terms of metal material, besides aluminum, silver is also investigated towards optimization of the transmission efficiency. Conclusion The transmission spectra of designed filters have high transmission and low sideband; its peaks cover the whole visible and near infrared range. These characteristics allow them to have the possibility to be integrated into image sensors for spectrometer applications.

Abstract This paper reports on the direct qualitative and quantitative performance comparisons of the field-effect transistors (FETs) based on vertical gallium nitride nanowires (GaN NWs) with different NW numbers (i.e., 1–100) and diameters (i.e., 220–640 nm) fabricated on the same wafer substrate to prove the feasibility of employing the vertical 3D architecture concept towards massively parallel electronic integration, particularly for logic circuitry and metrological applications. A top-down approach combining both inductively coupled plasma dry reactive ion etching (ICP-DRIE) and wet chemical etching is applied in the realization of vertically aligned GaN NWs on metalorganic vapor-phase epitaxy (MOVPE)-based GaN thin films with specific doping profiles. The FETs are fabricated involving a stack of n-p-n GaN layers with embedded inverted p-channel, top drain bridging contact, and wrap-around gating technology. From the electrical characterization of the integrated NWs, a threshold voltage (V th) of (6.6 ± 0.3) V is obtained, which is sufficient for safely operating these devices in an enhancement mode (E-mode). Aluminium oxide (Al2O3) grown by atomic layer deposition (ALD) is used as the gate dielectric material resulting in nearly-zero gate hysteresis (i.e., forward and backward sweep V th shift (ΔV th) of ~0.2 V). Regardless of the required device processing optimization for having better linearity profile, the upscaling capability of the devices from single NW to NW array in terms of the produced currents could already be demonstrated. Thus, the presented concept is expected to bridge the nanoworld into the macroscopic world, and subsequently paves the way to the realization of innovative large-scale vertical GaN nanoelectronics.

The three-dimensionality of 3D GaN field-effect transistors (FETs) provides them with unique advantages compared to their planar counterparts, introducing a promising path towards future FETs beyond Moore's law. Similar to today's Si processor technology, 3D GaN FETs offer multi-gate structures that provide excellent electrostatic control over the channel and enable very low subthreshold swing values close to the theoretical limit. Various concepts have been demonstrated, including both lateral and vertical devices with GaN nanowire (NW) or nanofin (NF) geometries. Outstanding transport properties were achieved with laterally contacted NWs that were grown in a bottom-up approach and transferred onto an insulating substrate. For higher power application, vertical FETs based on regular arrays of GaN nanostructures are particularly promising due to their parallel integration capability and large sidewall surfaces, which can be utilized as channel area. In this paper, we review the current status of 3D GaN FETs and discuss their concepts, fabrication techniques, and performances. In addition to the potential benefits, reliability issues and difficulties that may arise in complex 3D processing are discussed, which need to be tackled to pave the way for future switching applications. Keywords: GaN, 3D architecture, Nanowire, Nanofin, Nanoelectronics, Field-effect transistor (FET), Vertical transistor, Lateral transistor

In this work, nitrogen dioxide (NO2) gas sensors based on zinc oxide nanorods (ZnO NRs) decorated with gold nanoparticles (Au NPs) working under visible-light illumination with different wavelengths at room temperature are presented. The contribution of localized surface plasmon resonant (LSPR) by Au NPs attached to the ZnO NRs is demonstrated. According to our results, the presence of LSPR not only extends the functionality of ZnO NRs towards longer wavelengths (green light) but also increases the response at shorter wavelengths (blue light) by providing new inter-band gap energetic states. Finally, the sensing mechanism based on LSPR Au NPs is proposed.

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.

An electrothermal piezoresistive cantilever (EPC) sensor is a low-cost MEMS resonance sensor that provides self-actuating and self-sensing capabilities. In the platform, which is of MEMS-cantilever shape, the EPC sensor offers several advantages in terms of physical, chemical, and biological sensing, e.g., high sensitivity, low cost, simple procedure, and quick response. However, a crosstalk effect is generated by the coupling of parasitic elements from the actuation part to the sensing part. This study presents a parasitic feedthrough subtraction (PFS) method to mitigate a crosstalk effect in an electrothermal piezoresistive cantilever (EPC) resonance sensor. The PFS method is employed to identify a resonance phase that is, furthermore, deployed to a phase-locked loop (PLL)-based system to track and lock the resonance frequency of the EPC sensor under cigarette smoke exposure. The performance of the EPC sensor is further evaluated and compared to an AFM-microcantilever sensor and a commercial particle counter (DC1100-PRO). The particle mass–concentration measurement result generated from cigarette-smoke puffs shows a good agreement between these three detectors.

In this work, we present conductometric gas sensors based on p-type calcium iron oxide (CaFe2O4) nanoparticles. CaFe2O4 is a metal oxide (MOx) with a bandgap around 1.9 eV making it a suitable candidate for visible light-activated gas sensors. Our gas sensors were tested under a reducing gas (i.e., ethanol) by illuminating them with different light-emitting diode (LED) wavelengths (i.e., 465−640 nm). Regardless of their inferior response compared to the thermally activated counterparts, the developed sensors have shown their ability to detect ethanol down to 100 ppm in a reversible way and solely with the energy provided by an LED. The highest response was reached using a blue LED (465 nm) activation. Despite some responses found even in dark conditions, it was demonstrated that upon illumination the recovery after the ethanol exposure was improved, showing that the energy provided by the LEDs is sufficient to activate the desorption process between the ethanol and the CaFe2O4 surface.

In this study, we investigate the performance of two piezoresistive micro-electro-mechanical system (MEMS)-based silicon cantilever sensors for measuring target analytes (i.e., ultrafine particulate matters). We use two different types of cantilevers with geometric dimensions of 1000 × 170 × 19.5 µm3 and 300 × 100 × 4 µm3, which refer to the 1st and 2nd types of cantilevers, respectively. For the first case, the cantilever is configured to detect the fundamental in-plane bending mode and is actuated using a resistive heater. Similarly, the second type of cantilever sensor is actuated using a meandering resistive heater (bimorph) and is designed for out-of-plane operation. We have successfully employed these two cantilevers to measure and monitor the changes of mass concentration of carbon nanoparticles in air, provided by atomizing suspensions of these nanoparticles into a sealed chamber, ranging from 0 to several tens of µg/m3 and oversize distributions from ~10 nm to ~350 nm. Here, we deploy both types of cantilever sensors and operate them simultaneously with a standard laboratory system (Fast Mobility Particle Sizer, FMPS, TSI 3091) as a reference.

In this contribution we present a highly miniaturized device that integrates a photoactive material with a highly efficient LED light source. This so-called micro light plate configuration (µLP) allows for maximizing the irradiance impinging on the photoactive material, with a minimum power consumption, excellent uniformity and accurate control of the illumination. We demonstrate that, with the µLP approach, very efficient low power gas sensors can be built, and provide a detailed analysis of the rationales behind such improvement, as well as a quantitative model and a set of design rules to implement it in further integrated applications. As a demonstrator, we will describe a NO2 gas sensor operating in the part per billion range (ppb) with microwatt (µW) power consumption. These are the best figures reported to date in conductometric metal-oxides (MOX) sensors operated with light (instead of heat) at room temperature.

A cantilever is considered the most basic mechanical spring-mass system and has enormous application potential for sensors [...]

Continuous cell culture monitoring as a way of investigating growth, proliferation, and kinetics of biological experiments is in high demand. However, commercially available solutions are typically expensive and large in size. Digital inline-holographic microscopes (DIHM) can provide a cost-effective alternative to conventional microscopes, bridging the gap towards live-cell culture imaging. In this work, a DIHM is built from inexpensive components and applied to different cell cultures. The images are reconstructed by computational methods and the data are analyzed with particle detection and tracking methods. Counting of cells as well as movement tracking of living cells is demonstrated, showing the feasibility of using a field-portable DIHM for basic cell culture investigation and bringing about the potential to deeply understand cell motility.

UV-light emitting diodes (395–278 nm) were used to investigate the gas sensing attributes of planar and nanostructured ZnO/AlN thin films on Si substrate towards NO2 at room temperature. A significant increased sensitivity ((Rg − Ra)/Ra = 65.3 ppm NO2 in air) and a strong reduction in recovery time (Trec = 14 min) were already observed for the planar ZnO/AlN thin films under UV-B (305 nm) irradiation compared to the other UV wavelengths, while the device showed no obvious response in dark. By enlarging the surface-to-volume ratio of the sensors (i.e., creating nanostructured ZnO/AlN thin films), an increased response time is expected to be observed.

In this work, we demonstrate the potential of a piezoelectric resonator for developing a low-cost sensor system to detect microscopic particles in real-time, which can be present in a wide variety of environments and workplaces. The sensor working principle is based on the resonance frequency shift caused by particles collected on the resonator surface. To test the sensor sensitivity obtained from mass-loading effects, an Aluminum Nitride-based piezoelectric resonator was exposed to cigarette particles in a sealed chamber. In order to determine the resonance parameters of interest, an interface circuit was implemented and included within both open-loop and closed-loop schemes for comparison. The system was capable of tracking the resonance frequency with a mass sensitivity of 8.8 Hz/ng. Although the tests shown here were proven by collecting particles from a cigarette, the results obtained in this application may have interest and can be extended towards other applications, such as monitoring of nanoparticles in a workplace environment.

In this paper, a self-out-readable, miniaturized cantilever resonator for highly sensitive airborne nanoparticle (NP) detection is presented. The cantilever, which is operated in the fundamental in-plane resonance mode, is used as a microbalance with femtogram resolution. To maximize sensitivity and read-out signal amplitude of the piezo-resistive Wheatstone half bridge, the geometric parameters of the sensor design are optimized by finite element modelling (FEM). The electrical read-out of the cantilever movement is realized by piezo-resistive struts at the sides of the cantilever resonator that enable real-time tracking using a phase-locked loop (PLL) circuit. Cantilevers with minimum resonator mass of 1.72 ng and resonance frequency of ~440 kHz were fabricated, providing a theoretical sensitivity of 7.8 fg/Hz. In addition, for electrostatic NP collection, the cantilever has a negative-biased electrode located at its free end. Moreover, the counter-electrode surrounding the cantilever and a µ-channel, guiding the particle-laden air flow towards the cantilever, are integrated with the sensor chip. µ-channels and varying sampling voltages will also be used to accomplish particle separation for size-selective NP detection. To sum up, the presented airborne NP sensor is expected to demonstrate significant improvements in the field of handheld, micro-/nanoelectromechanical systems (M/NEMS)-based NP monitoring devices.

The asymmetric resonance response in electro-thermal piezoresistive cantilever resonators causes a need of an optimization treatment for taking parasitic actuation-sensing effects into account. An electronic reference circuit for signal subtraction, integrated with the cantilever resonator has the capability to reduce the effect of parasitic coupling. Measurement results demonstrated that a symmetric amplitude shape (Lorentzian) and an optimized phase characteristic (i.e., monotonically decreasing) were successfully extracted from an asymmetric resonance response. With the monotonic phase response, real-time frequency tracking can be easier to implement using a phase-locked loop (PLL) system. In this work, an electro-thermal piezoresistive cantilever resonator functionalized with self-assembled monolayers of chitosan-covered ZnO nanorod arrays as sensitive layers has been investigated under different relative humidity (rH) levels. Enhancement of resonance phase response has been demonstrated by implementing the reference signal subtraction. Subsequently, a lock-in amplifier integrated with PLL system (MFLI, Zurich Instruments, Zurich, Switzerland) was then employed for continuously tracking the resonant frequency. As a result, we find a good correlation of frequency shift (∆f0) with change in rH monitored using a commercial reference sensor.

Guidelines for the fabrication of nanoscale light-emitting diode arrays (i.e., nanoLED arrays) based on patterned gallium nitride (GaN) with very small dimensions and pitches have been derived in this work. Several challenges during top-down LED array processing have been tackled involving hybrid etching and polymer-based planarization to yield completely insulated highaspect-ratio LED fin structures and support the creation of p-GaN crossing line contacts, respectively. Furthermore, simulations of the light emission patterns were also performed providing hints for enhancing the device designs. As a result, regardless of the required device processing optimization, the developed nanoLED arrays are expected to offer high potential as novel illumination sources in biomedical imaging and sensing applications (e.g., mini compact microscopes and wearable biological/chemical nanoparticle counters)

Gas sensors based on CaFe2O4 nanopowders, which are p⁻type metal oxide semiconductor (MOX), have been fabricated and assessed for ethanol gas monitoring under visible light activation at room temperature. Regardless of their inferior sensitivity compared to thermally activated counterparts, the developed sensors have shown responsive sensing behavior towards ethanol vapors confirming the ability of using visible light for sensor activation. LEDs with different wavelengths (i.e., 465⁻590 nm) were employed. The highest sensitivity (3.7%) was reached using green LED activation that corresponds to the band gap of CaFe2O4.

In this paper, a self-out-readable, miniaturized cantilever resonator for highly sensitiveairborne nanoparticle (NP) detection is presented. The cantilever, which is operated in thefundamental in-plane resonance mode, is used as a microbalance with femtogram resolution. Toachieve a maximum measurement signal of the piezo resistive Wheatstone half-bridge, thegeometric parameters of the sensor design were optimized by finite element modelling (FEM).Struts at the sides of the cantilever resonator act as piezo resistors and enable an electrical read-outof the phase information of the cantilever movement whereby they do not contribute to theresonators rest mass. For the optimized design, a resonator mass of 0.93 ng, a resonance frequencyof ~440 kHz, and thus a theoretical sensitivity of 4.23 fg/Hz can be achieved. A μ-channel guiding aparticle-laden air flow towards the cantilever is integrated into the sensor chip. Electrically chargedNPs will be collected by an electrostatic field between the cantilever and a counter-electrode at theedges of the μ-channel. Such μ-channels will also be used to accomplish particle separation for sizeselectiveNP detection. Throughout, the presented airborne NP sensor is expected to demonstratesignificant improvements in the field of handheld, MEMS-based NP monitoring devices.

A compact lensless microscope comprising a custom-made LED engine and a CMOS imaging sensor has been developed for live-cell culture imaging inside a cell incubator environment. The imaging technique is based on digital inline-holographic microscopy, while the image reconstruction is carried out by angular spectrum approach with a custom written software. The system was tested with various biological samples including immortalized mouse astrocyte cells inside a petri dish. Besides the imaging possibility, the capability of automated cell counting and tracking could be demonstrated. By using image sensors capable of video frame rate, time series of cell movement can be captured.

Arrays of 1D-vertically arranged gallium nitride (GaN) nanorods (NRs) are fabricated on sapphire and connected to both bottom and freestanding top contacts. This shows a fully validated top-down method to obtain ordered arrays of high-surface-to-volume elements that can be electrically interrogated and used, e.g., for sensing applications. Specifically, these will be used as highly integrated heating elements for conductometric gas sensors in self-heating operation. Detailed fabrication and processing steps involving inductively coupled plasma reactive ion etching (ICP-RIE), KOH-etching, interspace filling, and electron-beam physical vapor deposition technologies are discussed, in which they can be well adjusted and combined to obtain vertical GaN NRs as thin as 300 nm in arbitrarily large and regular arrays (e.g., 1 × 1, 3 × 3, 9 × 10 elements). These developed devices are proposed as a novel sensor platform for temperature-activated measurements that can be produced at a large scale offering low-power, and very stable temperature control.

Pinhole‐shaped light‐emitting diode (LED) arrays with dimension ranging from 100 μm down to 5 μm have been developed as point illumination sources. The proposed microLED arrays, which are based on gallium nitride (GaN) technology and emitting in the blue spectral region (λ = 465 nm), are integrated into a compact lensless holographic microscope for a non‐invasive, label‐free cell sensing and imaging. From the experimental results using single pinhole LEDs having a diameter of 90 μm, the reconstructed images display better resolution and enhanced image quality compared to those captured using a commercial surface‐mount device (SMD)‐based LED.

A laser lift-off (LLO) process has been developed for detaching thin InGaN/GaN lightemitting diodes (LED) from their original sapphire substrates by applying an ultrafast laser. LLO is usually based on intense UV irradiation, which is transmitted through the sapphire substrate and subsequently absorbed at the interface to the epitaxially grown GaN stack. Here, we present a successful implementation of a two-step LLO process with 350 fs short pulses in the green spectral range (520 nm) based on a two-photon absorption mechanism. Cathodo- and electroluminescence experiments have proven the functionality of the LLO-based chips. The impact of radiation on the material quality was analysed with scanning (SEM) and transmission electron microscopy (TEM), revealing structural modifications inside the GaN layer in some cases.

Transferable substrate-less InGaN/GaN light-emitting diode (LED) chips have successfully been fabricated in a laser lift-off (LLO) process employing high power ultrashort laser pulses with a wavelength of 520 nm. The irradiation of the sample was conducted in two sequential steps involving high and low pulse energies from the backside of the sapphire substrate, which led to self-detachment of the GaN stack layer without any additional tape release procedure. To guarantee their optoelectrical function and surface quality, the lifted LED chips were assessed in scanning electron microscopy (SEM) and electroluminescence (EL) measurements. Moreover, surface characterizations were done using atomic force microscopy (AFM) and Auger Electron Spectroscopy (AES).

In this work, a novel multispectral sensing system consisting of nanostructured filter matrix and a charge-coupled device (CCD)-based image sensor has been developed to overcome the limitation of the conventional pigment filtered sensors, which are difficult to be fabricated at a microscale and usually showing a pronounced degradation. By designing the filters in guided-mode resonance (GMR) architecture, light transmission efficiencies of ~90% with low sidebands and sharp peaks can be obtained, which are critical characteristics for realizing precise optical measurement systems. To optimize the transmission functions, various materials and structural parameters have been simulated. Electron beam nanolithography is employed in the device fabrication to fabricate pixel-wise independent filter functions. After being characterized in terms of their wavelength filtering capability, the developed GMR filters are then combined with image sensors, particularly for addressing biological applications.

We developed an integrated platform to build up conductometric sensors with controlled illumination. Our device contains a miniaturized indium gallium nitride (InGaN) LED as a light source, and a set of interdigitated electrodes (IDEs) in close contact with the LED. The sensor material is later deposited on top of the IDE, to monitor its resistance. In this configuration, all the light emitted by the LED is collected by the sensor material, leading to a very efficient photoexcitation. We demonstrate the effectiveness of the approach building a photoactivated gas sensor based on ZnO operating with as little as 100 μW.

In this work we studied the influence of light emitting diode (LED) generated light on living cells which were cultivated in common cell culture microtiter plates. In detail we investigated signaling side effects including apoptosis by the use of a cell permeable peroxide activatable fluorescent dye (5,6-Chloromethyl-2′,7′-dichlorodihydrofluorescein diacetate). A high level production of peroxides in UV and blue light exposed cells was measured while the light of longer wavelengths caused only minor effects on the cells.

Cell registration by artificial neural networks (ANNs) in combination with principal component analysis (PCA) has been demonstrated for cell images acquired by light emitting diode (LED)-based compact holographic microscopy. In this approach, principal component analysis was used to find the feature values from cells and background, which would be subsequently employed as neural inputs into the artificial neural networks. Image datasets were acquired from multiple cell cultures using a lensless microscope, where the reference data was generated by a manually analyzed recording. To evaluate the developed automatic cell counter, the trained system was assessed on different data sets to detect immortalized mouse astrocytes, exhibiting a detection accuracy of ~81% compared with manual analysis. The results show that the feature values from principal component analysis and feature learning by artificial neural networks are able to provide an automatic approach on the cell detection and registration in lensless holographic imaging.

A piezo resistive, phase locked loop (PLL) controlled micro tactile measurement system for contact resonance spectroscopy (CRS) at high-aspect-ratio geometries was developed and characterised. Therefore, a piezo resistive silicon cantilever with a silicon tip at its free end was brought into contact with a sample surface and excited into resonance by a piezo actuator. The resonance frequency of the contacted cantilever was tracked by a homemade closed-loop PLL circuit. Different materials and layer thicknesses of photo resist (PR) on silicon were used to validate the system. To optimise the sensitivity and efficiency of the measurement system, amplitude and phase of the cantilever in surface contact were analysed under different contact forces and excitation amplitudes.

Silicon and germanium pillar structures (i.e., micro- and nanowires) were fabricated by a top-down approach including nanoimprint lithography and cryogenic dry etching. Various etching parameters were tested to ensure a reliable fabrication process. The impression of nanomechanical properties of such 3-D structures were extracted experimentally by nanoindentation showing promising and comparative results to utilize such nanostructures as small force artefacts.

Vertically aligned 3D gallium nitride (GaN) nanowire arrays with sub-50 nm feature sizes were fabricated using a nanosphere lift-off lithography (NSLL) technique combined with hybrid top-down etching steps (i.e., inductively coupled plasma dry reactive ion etching (ICP-DRIE) and wet chemical etching). Owing to the well-controlled chemical surface treatment prior to the nanobead deposition and etching process, vertical GaN nanowire arrays with diameter of ~35 nm, pitch of ~350 nm, and aspect ratio of >10 could be realized using 500 nm polystyrene nanobead (PN) masks. This work has demonstrated a feasibility of using NSLL as an alternative for other sophisticated but expensive nanolithography methods to manufacture low-cost but highly ordered 3D GaN nanostructures.

A portable lensless imaging device combining light-emitting diodes (LEDs) and a CMOS image sensor was developed and its suitability for non-invasive live-cell in vitro monitoring of pancreatic islets was demonstrated. A microfluidic lab-on-a-chip platform containing micro wells with various depths was also fabricated and integrated into the optical sensor system, which allows for immobilization of the single islets and continuous recording of their behavior. This promising technique may provide further insight into the structure and function of pancreatic islets and their deficiencies in type 2 diabetes.

An integrated nano-optoelectronic biological sensor system is developed to obtain insights of the biochemical and physical processes of Geobacter sulfurreducens-based biofilm growth inside a miniaturized microbial fuel cell (MFC) reactor. Gallium nitride (GaN), which was used as a novel electrode material, has been investigated in terms of its biocompatibility and performance to transport the electrons delivered by the microorganisms. Moreover, in order to enhance the produced current density, vertical 3D GaN nanoarchitectures (i.e., arrays of nanowires and nanofins) with larger surface-to-volume ratios were fabricated using a top-down nanomachining method involving nanolithography and hybrid etching technique.