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This paper reports a self-actuating and self-sensing gravimetric ethanol sensor, based on 3D ZnO-Nanorods@Si-Nanopillars (ZnO-NRs@Si-NPLs) as the sensing material which, for the first time, are selectively fabricated on a piezoresistive silicon microcantilever (MC, Fig. 1). This novel ZnO-NRs@Si-NPLs functionalized microcantilever (MC) was found to be highly sensitive to ethanol detection at room-temperature, with a sensitivity of hundred times higher than that of a ZnO-nanofilm-covered MC. Furthermore, fast response and recovery time of few seconds to low-concentration ethanol can be expected. In addition, real-time sensing is possible using a low-cost, low-weight, miniaturized phase-locked-loop (PLL) based system, indicating the potential of our proposed sensor for future portable gas-sensing in practical applications.

Based on the recently reported "micro-light-plate" configuration, we have implemented a light activated gas sensor for NO 2 operating at room temperature. With this conductometric device, we achieved detection limits down to a few parts per billion (ppb) with power consumption in the tens of microwatt range (µW). These are record values for conductometric metal-oxides (MOX) sensors activated with light (instead of heat).

This paper reports the fabrication and characterization of GaN nanorods (NRs) with a bottom contact and freestanding top contact. A top-down approach was adopted to produce an ordered array of high surface-to-volume NRs elements of GaN for gas sensing applications [1]. The GaN NRs were then used both as sensitive beams for conductometric gas sensing and as nanosized heating elements to activate their own response to gases (self-heating). GaN NRs with a diameter of 300 nm in arbitrarily large and regular arrays of 1 × 1, 3 × 3, 9 × 10 were fabricated. With power levels in the range of microwatts, these structures reach highly enough temperatures to develop a gas sensing response towards ethanol. In summary, this is one of the first systems showing efficient self-heating that can be produced using conventional microelectronic processing at a wafer scale.

We present a NO 2 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. We achieved these remarkable figures miniaturizing the devices in a novel monolithic-integrated configuration, so-called “micro-light-plate”.

This work presents a fully top-down method to produce ordered arrays of high surface-to-volume elements for gas sensing applications. Specifically, these will be used as highly integrated heating elements for conductometric gas sensing in self-heating operation [1]. We discuss in detail how inductively-coupled-plasma (ICP) and KOH-etching, interspace filling and electron-beam physical vapor deposition (EBPVD) technologies can be adjusted and combined to obtain nanorods as thin as 300 nm in arbitrarily large and regular arrays (e.g. 1×1, 3×3, 10×10). These structures can be heated up with power values in the microwatt range, as demonstrated by the heat-activated response to Ethanol that we present here. Overall, we report a system with efficient self-heating that can be produced at a wafer scale using conventional microelectronic processing.

In this work, we study the optical emission from arrays of InGaN/GaN MQW nanofin and nanorod arrays with sizes ranging from a few micrometers down to sub-wavelength dimensions (i.e., nanometers). Such systems are of interest for developing arrays of single addressable nanoLEDs, which could be used to obtain a visible wavelength super-resolution microscope where the resolution is due to highly localized light spots with sub-wavelength LED-to-LED pitch. We have used commercial full-wave Maxwell solvers (COMSOL, CST) to calculate the optical field emitted from a single nanoLED in a periodic array for a wavelength of 450 nm. Simulations on 11×11 nanoLED arrays with pitches of 200 nm up to 800 nm and diameters of down to 50 nm have been conducted, in which the dependency of the emission pattern on different structural parameters is studied. In case of small nanoLED array with very narrow pitch, a large optical cross-talk between the activated LED and its neighboring pixels was found. Moreover, in presence of cross-talks, test objects smaller than the LED pitch placed on its surface with influence of near field could potentially be resolved by evaluating the varied emission patterns obtained by different pixel activations. Routes to achieve higher localized optical fields and reduce optical cross-talk have been also investigated by modifying the nanoLED array structures (e.g., by introducing filling material among the LED pixels).

A film of gas sensitive ZnO nanoparticles has been coupled with a low-power micro light plate (μLP) to achieve a NO

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 conductometric 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 photoactivation with well controlled, uniform, and high irradiance conditions, 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. Higher irradiance values worsened the sensor response in 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 toward NO2. The effects of humidity on the sensor response are also discussed.

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.

We report a miniaturized device integrating a photoactive material with a highly efficient Light Emitting Diode light source. This so-called micro light plate configuration 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 these advantages with an example application: photoactivated gas sensors with a power consumption as low as 30 μW (this is 1000 times lower than the best figures reported to date). The letter also presents a quantitative model and a set of design rules to implement it in further integrated applications.

This work presents a first prototype for a new approach to microscopy: a system basing its resolving power on the light emitters instead of the sensors, without using lenses. This new approach builds on the possibility of making LEDs smaller than current technology sensors, offering a new approach to microscopy we plan on developing towards superresolution. The microscope consists on a SPAD based camera, a 8x8 LED array with 5x5 μm LEDs distributed with a pitch of 10 μm, and discrete driving electronics to control them. We present simulations of the system, as well as the first microscope prototype implementing the method, and the results obtained through it.

Nonlinear absorption of GaN and AlxGa1-xN thin films were studied by the Z-scan technique in the range of 0.53~0.63 Ephoton/Eg, with values ranging from 0.8 to 2.2 cm/GW depending of sample composition, indicating its influences on the nonlinearity.

Plasma profiling time of flight mass spectrometry (PP-TOFMS) has recently gained interest, as it enables the elemental profiling of semiconductor structures with high depth resolution in short acquisition times. As recently shown by Tempez et al., PP-TOFMS can be used to obtain the composition in the structures for modern field effect transistors [1]. There, the results were compared to conventional SIMS measurements. In the present study, we compare PP-TOFMS measurements of an Al-/In-/GaN quantum well multi stack to established micro- and nano-analysis techniques like cathodoluminescence (CL), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). We show that PP-TOFMS is able to resolve the layer structure of the sample even more than 500 nm deep into the sample and allows the determination of a relative elemental composition with an accuracy of about 10 rel. %. Therefore, it is an extremely rapid alternative method to obtain semiconductor elemental depth profiles without expensive and time consuming sample preparation as it is needed for TEM. Besides, PP-TOFMS offers better depth resolution and more elemental information than for example electrochemical capacitance-voltage (ECV), as the acquisition of all elements occurs in parallel and not only electrically (ECV) or optically (CL) active elements are observed.

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-price-winning blue GaN LEDs. Today GaN optoelectronics is developing beyond lighting, leading to new and innovative devices, e.g. for micro-displays, 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 (microLEDs and nanoLEDs) with high efficiency and across large areas, in combination with the possibility to merge optoelectronic-grade GaN microLEDs with silicon microelectronics in a fully hybrid approach. GaN LED technology today is even spreading into the realm of display technology, which has been occupied by organic LED (OLED) and liquid crystal display (LCD) for decades. In this review, the technological transition towards GaN micro- and nanodevices beyond lighting is discussed including an up-to-date overview on the state of the art.

GaN fins on GaN-on-sapphire templates are fabricated by continuous mode selective area metalorganic vapor phase epitaxy. The fins exhibit high aspect ratios and smooth nonpolar a-plane sidewalls with an ultra-low threading dislocation density of a few 105 cm^-2 making them ideally suited for optoelectronic to electronic applications. A detailed analysis of the inner structure of GaN fins is provided by the help of marker layer experiments and correlation of results from fins fabricated under different growth conditions, leading to the development of a growth model to explain the final geometry and optical as well as electrical properties of these high aspect ratio fins. Distinctly different material properties for the central and outer parts of the fins are detected. Whereas the outer sidewalls represent high quality GaN surfaces with very low defect densities, a strong quenching of near band edge emission (NBE) in the central part of the fins is accompanied by heavy compensation of free electrons. A possible explanation is the incorporation of excessive point defects, like intrinsic defects or carbon impurity. The sidewall regions, however, prove to be highly suitable for device applications due to their strong NBE emission, low dislocation density and high free carrier concentration.

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.

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 to use visible light for sensor activation. LEDs with different wavelengths (i.e., 465–590 nm) were employed. The highest sensitivity (3.7%) was reached using a green LED activation that corresponds to the bandgap of CaFe2O4.

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.

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.

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.

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.

ZnO nanorods (NRs) arrays with good vertical alignment were selectively grown on microscale patterned surfaces by a MEMS-compatible, low-temperature chemical-bath deposition method (CBD). The direct-current (DC) sputtered and subsequently annealed ZnO seed-layer was found to have a crucial effect on the ZnO NRs growth. Depending on the pre-annealing temperature between 200 °C and 700 °C, which is compatible with our microcantilever fabrication process, diameters and area densities of the NRs of 60–99 nm and 17–27 µm−2 were observed, respectively, with the best alignment at 600 °C. A surface-area enlargement factor of 48 was achieved with respect to a ZnO layer indicating the potential of ZnO NRs arrays for MEMS applications, such as gas sensing.

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.

This paper demonstrates the first vertical field-effect transistor based on gallium nitride (GaN) fin structures with an inverted p-doped channel layer. A top-down hybrid etching approach combining inductively coupled plasma reactive ion etching and KOH-based wet etching was applied to fabricate regular fields of GaN fins with smooth a-plane sidewalls. The obtained morphologies are explained using a cavity step-flow model. A 3D processing scheme has been developed and evaluated via focussed ion beam cross-sections. The top-down approach allows the introduction of arbitrary doping profiles along the channel without regrowth, enabling the modulation of the channel properties and thus increasing the flexibility of the device concept. Here, a vertical npn-doping profile was used to achieve normally-off operation with an increased threshold voltage as high as 2.65 V. The p-doped region and the 3D gate wrapped around the sidewalls create a very narrow vertical electron channel close to the interface between dielectric and semiconductor, resulting in good electrostatic gate control, low leakage currents through the inner fin core and high sensitivity to the interface between GaN and gate oxide. Hydrodynamic transport simulations were carried out and show good agreement with the performed current–voltage and capacitance–voltage measurements. The simulation indicates a reduced channel mobility which we attribute to interface scattering being particularly relevant in narrow channels. We also demonstrate the existence of oxide and interface traps with an estimated sheet density of 3.2 × 1012 cm−2 related to the Al2O3 gate dielectric causing an increased subthreshold swing. Thus, improving the interface quality is essential to reach the full potential of the presented vertical 3D transistor concept.

We present an extensive investigation of the charge-trapping processes in vertical GaN nanowire FETs with a gate-all-around structure. Two sets of devices were investigated: Gen1 samples have unipolar (n-type) epitaxy, whereas Gen2 samples have a p-doped channel and an n-p-n gate stack. From experimental results, we demonstrate the superior performance of the transistor structure with a p-GaN channel/Al2O3 gate insulator in terms of dc performance. In addition, we demonstrate that Gen2 devices have highly stable threshold voltage, thus representing ideal devices for power electronic applications. Insight into the trapping processes in the two generations of devices was obtained by modeling the threshold voltage variations via differential rate equations.

Inorganic conductometric gas sensors struggle to overcome limitations in high power consumption and poor selectivi-ty. Herein, recent advances in developing self-powered gas sensors with tunable selectivity are introduced. Alternative general approaches for powering gas sensors were realized via proper integration of complementary functionalities (namely; powering and sensing) in a singular heterostructure. These solar light driven gas sensors operating at room temperature without applying any additional external powering sources are comparatively discussed. The TYPE-1 gas sensor based on integration of pure inorganic interfaces (e.g. CdS/n-ZnO/p-Si) is capable of delivering a self-sustained sensing response, while it shows a non-selective interaction towards oxidizing and reducing gases. The structural and the optical merits of TYPE-1 sensor are investigated giving more insights into the role of light activation on the modu-lation of the self-powered sensing response. In the TYPE-2 sensor, the selectivity of inorganic materials is tailored through surface functionalization with self-assembled organic monolayers (SAMs). Such hybrid interfaces (e.g. SAMs/ZnO/p-Si) have specific surface interactions with target gases compared to the non-specific oxidation-reduction interactions governing the sensing mechanism of simple inorganic sensors. The theoretical modeling using density functional theory (DFT) has been used to simulate the sensing behavior of inorganic/organic/gas interfaces, revealing that the alignment of organic/gas frontier molecular orbitals with respect to the inorganic Fermi level is the key factor for tuning selectivity. These platforms open new avenues for developing advanced energy-neutral gas sensing devices and concepts.

A uniform array of gallium nitride core-shell microrod (MR) light-emitting diode (LED) structures was grown by metalorganic vapor phase epitaxy. Defects and the quantum well (QW) luminescence in an individual rod were investigated by scanning tunneling electron microscopy (STEM) and STEM cathodoluminescence. Luminescence with different wavelength was detected from the quantum wells on the semipolar tip facets and the nonpolar sidewalls of the MRs. Furthermore, the MR array is processed into LED chips. The electro-optical characteristics of the devices are analyzed. Two separate emission bands are distinguished, which are attributed to the QWs on the semipolar tip facets and the nonpolar sidewalls, respectively. To obtain white LEDs, micrograin phosphors were developed which fit in between individual MRs. By using electrophoretic particle deposition, these phosphors are deposited onto the MR LED chips. Color coordinates, color temperature, and device efficiency are evaluated. Blue (top) and phosphor-converted white (bottom) microrod LEDs on 4″ wafer.

The low cost electrodeposition method was used to grow Cu2O thin films and experimentally determine the optimal absorber layer thickness. Raman scattering studies indicate the presence of solely crystalline Cu2O and SEM images show that the thin films consist of grains with a pyramidal shape. The influence of the thickness of the light absorbing Cu2O layer on the basic characteristic of the heterojunction and their properties have been investigated using reflectivity, current–voltage (J–V), capacitance–voltage (C–V) and the external quantum efficiency (EQE) measurements. The depletion layer, the charge collection length of the minority carrier, and reflectivity are the main factors describing the device properties. The efficiency of 1.09% with an open circuit voltage of Voc=0.35 V, a short circuit current density Jsc=6.21 mA cm−2 and a filling factor of FF=50% was obtained as the highest value for our solar cells. A Gaussian fit shows that the film thickness of around 3 µm is an optimum with respect to a high efficiency of the solar cells. C–V measurements shows that the estimated value of the built-in potential is Vb=0.35 V and the acceptor concentration at the junction is NA=4.375×1016 cm−3 for solar cell with the highest efficiency.

We present here for the first time the fabracition of a p-Cu 2 O/ZnO/AZO flexible heterojunction solar cell by electrodeposition of Cu 2 O thin film on a plastic substrate and sputtering of ZnO:Al layer. The Atomic Layer Deposition (ALD) has been employed to insert 5 nm ZnO as buffer layer. The heterojunction solar cell was characterized by Raman spectroscopy and scanning electron microscopy that show pyramidal shape and phonon modes for Cu 2 O thin film. Current–voltage ( J – V ), capacitance–voltage ( C – V ) and the external quantum efficiency (EQE) measurements were performed to understand the heterojunction properties. The solar cell device exhibits a power conversion efficiency of 0.897 ± 0.005% with an open circuit voltage of V oc = 300 mV, a short circuit current density of J sc = 6.819 ± 0.048 mA cm −2 and a fill factor of FF = 0.439 ± 0.006. The values of the built-in potential and the acceptor concentration at the junction were estimated from the reverse bias C – V measurement to be 0.37 V and 6.67 × 10 16 cm −3 , respectively.

Herein, the optical properties of aluminum nitride (AlN) epitaxial layers grown on sapphire substrates by metal-organic chemical vapor deposition (MOCVD) are reported. The structures investigated in this study are grown at highly different degrees of supersaturation in the MOCVD process. In addition, both pulsed and continuous growth conditions are employed and AlN is deposited on nucleation layers favoring different polarities. The samples are investigated by photoluminescence (PL), photoluminescence excitation (PLE), and absorption spectroscopy and are found to vary significantly in absorption and emission characteristics. Two distinct absorption bands in the UV-C spectral range are observed and examined in greater detail, with either giving rise to a significant absorption coefficient of around 1000 cm−1. The corresponding defect transitions are identified by PL spectroscopy. Combined with secondary-ion mass spectrometry (SIMS) measurements, these absorption bands are allocated to the incorporation of carbon and oxygen impurities, depending on the applied growth conditions. Furthermore, similarities with other epitaxial growth techniques serving as basis for UV-C applications are highlighted. These results are highly relevant for a better understanding of absorption issues in AlN templates grown by various deposition techniques. In addition, consequences for the growth of efficient UV-C devices by MOCVD on sapphire substrates are outlined.

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.

We report on the surface functionalization of ZnO nanowire (NW) arrays by attachment of carbon nanodots (C-dots) stabilized by polyethylenimine. The photoconductive properties of the ZnO NWs/C-dots devices were investigated under photoexcitation with photon energies below and above the ZnO band gap. The results indicate an increased photoresponse of the functionalized devices in the visible spectral range, as well as enhanced UV photoconductivity. This is attributed to the fast injection of photoexcited electrons from the C-dots into the conduction band of the ZnO NWs, and the subsequent slower desorption of molecular species from the NW surface, which reduces the surface depletion region in the NWs. The surface functionalization of the ZnO NWs with carbon nanodots also impacts the dynamics of the photocurrent decay, inducing a slower relaxation of the photogenerated charge carriers.

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.

We present a simple and inexpensive method to implement a Griess-Saltzman-type reaction that combines the advantages of the liquid phase method (high specificity and fast response time) with the benefits of a solid implementation (easy to handle). We demonstrate that the measurements can be carried out using conventional RGB sensors; circumventing all the limitations around the measurement of the samples with spectrometers. We also present a method to optimize the measurement protocol and target a specific range of NO

In this paper, we present the structural, optical, and electrical characterization of InGaN/GaN core-shell micro-LED structures selectively grown by metal organic vapor phase epitaxy (MOVPE) in arranged arrays. In particular we analyze the core-shell geometry of the pillars, consisting of a fivefold InGaN/GaN multiple quantum well and a p-type layer on all sidewall and top facets. Additionally we analyze the optical properties of the structure and the active region with high spatial resolution by cathodoluminescence. Electron beam induced current measurements (EBIC) are performed using an SEM based manipulator setup, giving proof of the presence of a depletion region as well as the intended doping polarity of the n-type core and a p-type shell wrapped around the whole structure. Using a p–n–p configuration also current crowding is discussed by EBIC and electroluminescence is demonstrated by measuring emission patterns from single core–shell structures. Color overlay of SE image (grey, FOV = 12.7 μm) and EBIC image (red) of a cleaved core–shell LED pillar contacted by two probe tips at a substrate tilt of 30°.

Display Omitted We develop a handheld cantilever-based airborne nanoparticle detector (CANTOR-2).Main components of CANTOR-2 are described in details.CANTOR-2 is evaluated using carbon aerosols in typical workplace conditions.CANTOR-2 is calibrated with a fast mobility particle sizer (FMPS, TSI 3091).Comparison with other particle monitoring instruments is demonstrated. The development and real-time performance test of a fully integrated low-cost handheld cantilever-based airborne nanoparticle (NP) detector (CANTOR-2) are described in this paper. The device is the enhancement of the previously developed cylindrical electrophoretic NP sampler (CANTOR-1), which is used for direct-reading of exposure to airborne carbon engineered nanoparticles (ENPs) in indoor workplaces. All components of the proposed detector can be divided into two main units depending on their packaging placements (i.e., the NP sampler head and the electronics mounted in a handy-format housing). For the NP sampler, a miniaturized electrophoretic aerosol sampler created in a cubical shape and an electrothermal piezoresistive resonant silicon cantilever mass sensor are employed for collecting the ENPs from the air stream to the cantilever surfaces and measuring their mass concentration, respectively. To realize a real-time measurement, a frequency tracking system based on phase-locked loop (PLL) is built and integrated to the device. From the device calibration, a good correlation of the CANTOR-2 data is found with the fast mobility particle sizer (FMPS, TSI 3091) reference at a precision of 8-14%. By having a total device volume of 540cm3, weight of 375g, and power consumption of 1.25W in the current version, this developed CANTOR-2 provides a very good portability for being used as a personal airborne NP monitoring device, which can be easily held or worn by workers during their activities.

The three dimensional growth of GaN structures as a basis for the fabrication of 3D GaN core-shell LEDs has attracted substantial attention in the past years. GaN nanorods or microrods with high aspect ratios can be grown by selective area epitaxy on a GaN buffer through a SiOx mask. It has been found earlier that silane substantially initiates vertical growth, with the exact underlying mechanisms being still unclear. Here, the influence of silane on the 3D GaN column growth was investigated by performing detailed growth experiments in combination with a thorough surface analysis in order to get insight into these mechanisms. The vertical growth rate is significantly enhanced by high silane fluxes, whereas the saturation of growth rate with the time is reduced. Thus, homogenous GaN columns with an aspect ratio of more than 35 could be achieved. A thin Si-rich layer on the non-polar m-plane facets of the columns has been detected using a combination of transmission electron microscopy, energy dispersive X-ray spectroscopy and Auger electron spectroscopy. This layer is suggested to be the reason for the increase in growth rate, modifying the effective collection range of the species along the sidewalls, and preventing the lateral growth.

This work reports on the growth of (In, Ga)N core–shell micro pillars by plasma-assisted molecular beam epitaxy using an ordered array of GaN cores grown by metal organic vapor phase epitaxy as a template. Upon (In, Ga)N growth, core–shell structures with emission at around 3.0 eV are formed. With spatially resolved cathodoluminescence, an increasing In content toward the pillar top is found to be present in the (In, Ga)N shell, as indicated by a shift of CL peak position from 3.2 eV at the shell bottom to 3.0 eV at the shell top. Further, the fabrication of a core–shell pin structure is demonstrated. Spatially resolved electroluminescence measurements performed on individual pillars confirm electroluminescence from the (In, Ga)N shell (lateral diode) at around 3.0 eV, as well as from the pillar top facet (axial diode) at around 2.3 eV.

GaN columns are grown by metal organic vapor phase epitaxy (MOVPE) with a high silane flow, which often leads to rough sidewalls and related problems during a subsequent shell overgrowth by e.g., InGaN/GaN quantum wells. In order to get more information on the surface of GaN columns before overgrowth, surface photovoltage (SPV) measurements have been performed by photo-assisted Kelvin probe force microscopy (KPFM) before and after chemical etching of the surface. Its electronic properties are demonstrated to be influenced by the silane injection during the samples' growth. Distinct differences are shown in the SPV behavior of GaN columns before and after etching with phosphoric acid. SPV decreases before etching, whereas after etching it increases like that on GaN bulk material. Silicon related surface states introduced during the growth are considered as the origin of the SPV behavior of the as grown samples. The change of this behavior after etching is attributed to the removal of the Si containing surface coating. Our investigations not only show the important role of silane injection during the growth on the SPV behavior of GaN columns, but it also reveals that the photo-assisted KPFM technique is an easy and quick method to analyze surfaces.

The growth mechanisms of selective area growth of GaN microcolumns by metal organic vapor phase epitaxy on patterned SiOx/sapphire templates, are investigated. Both Ga- and N-polar domains within a GaN column are detected. The growth system of mixed polar GaN columns is near reaction limitation, which is quantitatively determined by the Damkohler number. The major part of the mixed polar GaN columns is N-polar. However, the Ga-polar domains increase the vertical growth rate of the whole column. The strain status of the columns is almost totally relaxed. Core–shell LED structures were realized on the mixed polar GaN columns. The optical properties of the core–shell LEDs were characterized.

Operation of thermoelectric generator (TEG) modules based on bismuth telluride alloys at temperatures higher than 250°C is mostly limited by the melting point of the assembly solder. Although the thermoelectric parameters of bismuth telluride materials degrade for temperatures >130°C, the power output of the module can be enhanced with an increase in the temperature difference. For this, a temperature-stable joining technique, especially for the hot side of the modules, is required. Fabrication and process parameters of TEG modules consisting of bismuth telluride legs, alumina ceramics and copper interconnects using a joining technique based on pressure-assisted silver powder sintering are described. Measurements of the thermal force, electrical resistance, and output power are presented that were performed for hot side module temperatures up to 350°C and temperature differences higher than 300°C. Temperature cycling and results measured during extended high-temperature operation are addressed.

Selectivity and low power consumption are major challenges in the development of sophisticated gas sensor devices. A sensor system is presented that unifies selective sensor-gas interactions and energy-harvesting properties, using defined organic-inorganic hybrid materials. Simulations of chemical-binding interactions and the consequent electronic surface modulation give more insight into the complex sensing mechanism of selective gas detection.

The fabrication and use of silicon nanowire (SiNW) array-patterned microcantilever sensors for enhancing aerosol mass detection are described. Surface modification of the cantilever is performed selectively by combining the processes of nanoimprint lithography, photolithography and inductively coupled plasma cryogenic reactive ion etching. Cylindrical wire structures of 300 nm diameter with aspect ratios of 3-7 can be realised for the current SiNWs, which can be altered depending on the nanoimprint stamp size and etching recipe. Owing to the rise in the collection surface area of the sensor provided by vertical SiNWs, an increase of aerosol sampling efficiency can be obtained during cigarette smoke exposure, which is a factor of 1.5 higher than that of a corresponding plain cantilever. This proposed structure is intended to be used as a sensor module of a personal aerosol mass detector.

We present a simple an inexpensive method to implement a Griess-Saltzman-type reaction that combines the advantages of the liquid phase method (high specificity, fast response time) with the benefits of a solid implementation (easy to handle). We demonstrate that the measurements can be carried out using conventional RGB sensors; circumventing all the limitations around the measurement of the samples with spectrometers. We also present a method to optimize the measurement protocol and target a specific range of NO2 concentrations. We demonstrate that it is possible to measure the concentration of NO2 from 50 ppb to 300 ppm with high specificity and without modifying the sensing elements.

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.