Ammonia sensing characteristics of a Pt/AlGaN/GaN Schottky diode

The interesting ammonia sensing current-voltage (I-V) characteristics of a Pt/AlGaN/GaN Schottky diode are firstly studied and demonstrated. It is found that the ammonia sensitivity is increased by increasing the temperature. Yet, the sensitivity is decreased when the temperature is higher than 423 K. Experimentally, the studied device exhibits a good sensitivity of 13.1 under exposing to a relatively low concentration ammonia gas of 35 ppm NH₃/air. In addition, the good sensing performance of the studied device is demonstrated over a wide operating temperature regime from 298 K to 473 K. A highest ammonia sensing response of 182.7 is found at 423 K while a 10,000 ppm NH₃/air gas is introduced.     

Hydrogen sensing of N-polar and Ga-polar GaN Schottky diodes

N-polar and Ga-polar GaN grown on c-plane sapphire were used to fabricate platinum deposited Schottky contacts for hydrogen sensing at room temperature. After exposure to hydrogen, Ga-polar GaN Schottky barrier reduced by 3–4 meV, while the N-polar GaN Schottky contacts became fully Ohmic. The N-polar GaN Schottky diodes showed stronger and faster response to 4% hydrogen than that of Ga-polar Schottky diodes. The abrupt current increase from N-polar GaN Schottky exposure to hydrogen was attributed to the high reactivity of the N-face surface termination. The surface termination dominates the sensitivity and response time of the hydrogen sensors made of GaN Schottky diodes.     

Hydrogen sensors with double dipole layers using a Pd-mixture-Pd triple-layer sensing structure

This paper reports on new GaN sensors using a Pd-mixture-Pd triple-layer sensing structure to enhance their sensitivity to hydrogen at the tens of ppm level. The proposed hydrogen sensor biased with a constant voltage produced relatively high sensing responses of 4.84 × 10⁵% at 10,100 ppm and 8.7 × 10⁴% at 49.1 ppm H₂ in N₂. The corresponding barrier height variations are calculated to be 220 and 168 mV. When the sensor is biased by a constant current with maximum power consumption of 0.4 mW, a sensing voltage as an output signal showed a voltage shift of more than 17 V (the highest value ever reported) at 49.1 ppm H₂ in N₂. By comparison to Pd-deposited GaN sensors, the improvement in static-state performance is likely attributed to double dipole layers formed individually at the Pd–GaN interface and inside the mixture. Moreover, voltage transient response and current transient response to various hydrogen-containing gases were experimentally studied. The new finding is that the former response time is shorter than the latter one.     

Ordered mesoporous Pd/SnO2 synthesized by a nanocasting route for high hydrogen sensing performance

Ordered mesoporous SnO₂ and mesoporous Pd/SnO₂ have been successfully synthesized via nanocasting method using the hexagonal mesoporous SBA-15 as template. Two different procedures, impregnation technique and direct synthesis, were utilized for the doping of Pd in the mesoporous SnO₂. The results of small angle X-ray diffraction (SAXD), nitrogen adsorption–desorption and transmission electron microscopy (TEM) demonstrate that the SnO₂ and Pd/SnO₂ display ordered mesoporous structures and high surface areas. Wide angle X-ray diffraction (WAXD) and X-ray photoelectron spectroscopy (XPS) reveal tetragonal structure of SnO₂ and the existence of Pd element. The sensing properties of mesoporous SnO₂ and mesoporous Pd/SnO₂ for H₂ were detected. The sensor utilizing mesoporous Pd/SnO₂ via direct synthesis method exhibits excellent response and recovery behavior and much higher sensitivity to H₂, compared to those using mesoporous SnO₂ and mesoporous Pd/SnO₂ via impregnation technique. It is believed that its high gas sensing performance is derived from the large surface area, high activity and well dispersion of Pd additive, as well as high porosity, which lead to highly effective surface interaction between the target gas molecules and the surface active sites.     

Development of a sensor for surface state measurements using experimental and numerical modal analysis

The characterization of the surface state becomes an increasing necessity for many industries. This paper presents a patented measurement method [M.A. Bueno, S. Fontaine, M. Renner, Dispositif Pour Évaluer L’état de Surface d’un Matériau et Procédé de Mise en œuvre Dudit Dispositif, Patent No. PCT/FR01/01770, 2000], which is able to evaluate the surface state of materials. This method gives roughness–friction criteria and is based on the principle of a tribometer of type “blade-disc”, where the analysed surface is the disc. The sensor is an active element, which vibrates according to its vibrating modes. This work shows how experimental and numerical analysis has allowed optimizing its gage instrumentation and its geometry. Moreover, the modal analysis and the building of a numerical model have set up the coupling between mechanical solicitations of the tested surface and mechanical responses of the sensor in each mode. Finally, an example of measurement of known surfaces is illustrated to explain the results of this study and the efficiency of this measurement method.     

The mobility of two-dimensional electron gas in AlGaN/GaN heterostructures with varied Al content

The mobility of the two-dimensional electron gas （2DEG） in AIGaN/GaN hetero-structures changes significantly with AI content in the AIGaN barrier layer, while few mechanism analyses focus on it. Theoretical calculation and analysis of the 2DEG mobility in AIGaN/GaN heterostructures with varied AI content are carried out based on the recently reported experimental data. The 2DEG mobility is modeled analytically as the total effects of the scattering mechanisms including acoustic deformation-potential, piezoelectric, polar optic phonon, alloy disorder, interface roughness, dislocation and remote modulation doping scattering. We show that the increase of the 2DEG density, caused by the ascension of the AI content in the barrier layer, is a dominant factor that leads to the changes of the individual scat- tering processes. The change of the 2DEG mobility with AI content are mainly determined by the interface roughness scattering and the alloy disorder scattering at 77 K, and the polar optic phonon scattering and the interface roughness scattering at the room temperature. The calculated function of the interface roughness pa- rameters on the AI content shows that the stress caused AIGaN/GaN interface degradation at higher AI content is an important factor in the limitation of the in- terface roughness scattering on the 2DEG mobility in AIGaN/GaN heterostructures with high AI content.     

Sensitivity enhancement of a surface plasmon resonance based biomolecules sensor using graphene and silicon layers

A surface plasmon resonance based biomolecules sensor using silicon and graphene layers coated over the base of the high index prism sputtered with gold has been analyzed. The graphene layer has been used to enhance the adsorption of biomolecules while the addition of silicon layer between gold and graphene increases the sensitivity. The thicknesses of gold and silicon layers along with the number of graphene layers have been optimized to achieve the best performance of the sensor in terms of sensitivity and Full Width at Half Maximum (FWHM). To see the effect of wavelength of the light source, simulations have been carried out for three different wavelengths. The best performance is obtained for 633 nm wavelength with optimized thicknesses of gold and silicon layers as 40 nm and 7 nm respectively while the optimum number of graphene layers is 2. The sensitivity obtained with optimized parameters and additional silicon layer, is more than twice the value reported in the literature.     

Comprehensive study of hydrogen sensing characteristics of Pd metal–oxide–semiconductor (MOS) transistors with Al0.24Ga0.76As and In0.49Ga0.51P Schottky contact layers

The interesting hydrogen sensing characteristics of two transistors with an Al_0.24Ga_0.76As (device A) and In_0.49Ga_0.51P (device B) Schottky layer are demonstrated and studied. Experimentally, device A shows a lower hydrogen detection limit of 4.3 ppm H₂/air, a higher current variation of 7.79 mA and a shorter adsorption time of 10.95 s in a 9970 ppm H₂/air at room temperature. On the other hand, device B exhibits more stable hydrogen-sensing characteristics at high temperatures. Even at a low concentration of 14 ppm H₂/air the hydrogen sensing properties of device B can be obtained as the temperature increases from 30 to 160 °C. Because the Al_0.24Ga_0.76As and In_0.49Ga_0.51P materials are lattice-matched to the GaAs substrate, the studied devices can be integrated as sensor arrays to obtain superior hydrogen sensing characteristics including higher sensing signals, lower detection limit, shorter response time, and widespread detection and temperature regimes.     

Improving the performance of organic light-emitting devices by employing oxygen plasma treatment on indium-tin-oxide surfaces

ITO was modified by an oxygen plasma treatment, and its surface characteristics were evaluated by ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, and contact angle technologies. The plasma discharge boosted the ITO work function without inducing surface damage, attributed to eliminating carbon contaminants instead of decreasing oxygen vacancy, and improved the surface morphology, confirmed by the smaller root-mean-square. Besides, the plasma treatment increased the surface wettability evaluated by the surface energy, which improved the adhesion between organic molecular and ITO surfaces and was beneficial for realizing high-efficiency devices. The prepared white device based on the oxygen-plasma-treated ITO performed better than the reference white device with a pristine ITO anode, demonstrating higher current efficiencies and power efficiency over a wide range of current densities. The improved efficiency is ascribed to alleviating leakage current density and reducing operating voltages.     

Parametric representation of a surface pencil with a common spatial geodesic

In this paper, we study the problem of constructing a family of surfaces from a given spatial geodesic curve. We derive a parametric representation for a surface pencil whose members share the same geodesic curve as an isoparametric curve. By utilizing the Frenet trihedron frame along the given geodesic, we express the surface pencil as a linear combination of the components of this local coordinate frame, and derive the necessary and sufficient conditions for the coefficients to satisfy both the geodesic and the isoparametric requirements. We illustrate and verify the method by finding exact surface pencil formulations for some simple surfaces, such as surfaces of revolution and ruled surfaces. Finally, we demonstrate the use of this method in a garment design application.     

Surface‐treatment effects of a sapphire substrate by He+‐, Ar+‐, and Xe+‐ion implantations for GaN epitaxial layers

Abstract— The structural, electrical, and optical properties of GaN epilayers grown on various ion‐implanted sapphire(0001) substrates by MOCVD were investigated. GaN or AlN buffer layers and pre‐treatment were indispensably introduced before GaN‐epilayer growth. The ion‐implanted substrate's surface had decreased internal free energies during the growth of the ion‐implanted sapphire(0001) substrates. The crystal and optical properties of the GaN epilayers grown in ion‐implanted sapphire(0001) substrates were improved. Also, an excessively roughened and modified surface caused by ions degraded the GaN epilayers. Not only the ionic radius but also the chemical species of implanted sapphire(0001) substrates improved the properties of the GaN epilayers grown by MOCVD. It is obvious that the ion‐implanted pre‐treatment of sapphire(0001) substrates can be an alternative pre‐treatment procedure for GaN deposition and has the potential to improve the properties of the GaN epilayers on sapphire(0001) substrates.     

Effect of surface defects on biosensing properties of TiO2 nanotube arrays

In this paper, highly ordered titania nanotube (TNT) arrays fabricated by anodization were annealed at different temperatures in CO to create different concentrations of surface defects. The samples were characterized by SEM, XRD and XPS. The results showed different concentrations of Ti³⁺ defects were doped in TNT arrays successfully. Furthermore, after co-immobilized with horseradish peroxidase (HRP) and thionine chloride (Th), TNT arrays was employed as a biosensor to detect hydrogen peroxide (H₂O₂) using an amperometric method. Cyclic voltammetry results and UV-Vis absorption spectra presented that with an increase of Ti³⁺ defects concentration, the electron transfer rate and enzyme adsorption amount of TNT arrays were improved largely, which could be ascribed to the creation of hydroxyl groups on TNT surface due to dissociative adsorption of water by Ti³⁺ defects. Annealing in CO at 500 °C appeared to be the most favorable condition to achieve desirable nanotube array structure and surface defects density (0.27%), thus the TNT arrays showed the largest adsorption amount of enzyme (9.16 μg/cm²), faster electron transfer rate (1.34 × 10⁻³ cm/s) and the best response sensitivity (88.5 μA/mM l⁻¹).     

Characterization of porous cubic silicon carbide deposited with Pd and Pt nanoparticles as a hydrogen sensor

Porous 3C-SiC (pSiC) samples with different pore diameters were prepared from polycrystalline n-type 3C-SiC films by electrochemical anodization using a mixture of HF and ethanol as the electrolyte solution. The pSiC surface was chemically modified via the sputtering of Pd and Pt nanoparticles as a hydrogen catalyst. Changes in the resistance were monitored with respect to hydrogen concentrations in the range of 110-410 ppm. The variations in the electrical resistance at the presence of nitrogen diluted with hydrogen demonstrated that Pd and Pt-deposited pSiC samples have the ability to detect hydrogen. Regardless of the catalyst, the 25 nm pore diameter samples exhibited better response and recovery properties than the 60 nm pore diameter samples. It was found that the pore size affects the catalyst reaction and results in changes in the response factor to hydrogen. A large change in resistivity was observed with the Pd catalyst and the hydrogen sensing performance improved at high temperatures.     

Strategies for improving performance of IEEE 802.15.4/ZigBee WSNs with path-constrained mobile sink(s)

Most of the existing works on the topic of sink mobility in wireless sensor networks (WSNs) are of purely theoretical nature. The aim of this paper is to discuss the challenges as well as potential benefits associated with the use of mobile sinks in WSNs that operate in space-constrained environments and employ real-world technology. Specifically, we examine the pros and cons of deploying path-constrained sink mobility in the framework of IEEE 802.15.4/ZigBee enabled sensor networks.The main contributions of this paper are as follows: First, we demonstrate that the advantages of deploying path-constrained sink mobility, as identified in one of our earlier works [4], are not fully applicable to ZigBee WSNs. Specifically, our OPNET-based simulation study shows that in ZigBee WSNs the findings from [4] hold only conceptually, at the highest level of user-data routing. However, once all of the mobility-related overhead is accounted for, no actual benefit of deploying a mobile-over deploying a static-sink can be observed. Subsequently, we propose the use of three mechanisms for control of mobility-related overhead in ZigBee WSNs: Suppressed Route Discover, Node Association Based on Residual Energy, and Footprint Chaining. The most complex of the three mechanisms (Footprint Chaining) is studied in detail, and conditions under which this technique achieves optimal performance are precisely identified. The presented simulation results prove that with the three proposed mechanisms in place the benefits of mobile-over static-sink deployment can be regained, almost to the same extent as theoretically identified in [4].To our knowledge, this paper is one of the first attempts to bring the topics of path-constrained sink mobility and ZigBee standard together. It is also the first published work to propose improvements to the current ZigBee standard specifically targeted for WSNs that involve the use of mobile sinks.     

Detection of protein aggregation with a Bloch surface wave based sensor

We present the innovative application of a Bloch surface wave based sensor to the detection of protein aggregation. In Hen Egg White Lysozyme (HEWL) solutions, aggregates are discriminated from the monomeric forms in a label-free detection scheme.     

Improved performance of GaAs-based micro-solar cell with novel polyimide/SiO_2/TiAu/SiO_2 structure

A novel interconnected structure consisting of a conventional polyimide layer(PM) and an additional SiO2/TiAu/SiO2 sandwich multilayer has been firstly proposed for improving the performance of GaAs micro-solar cell array.The specific experimental investigations on the array with 108 unit cells have demonstrated that the novel structure of PM/SiO2/TiAu/SiO2 can effectively enhance the open-circuit voltage from 75 to 84 V and the fill factor from 35% to 57%.The performance improvement of our devices can be a...     

污水处理过程溶解氧浓度的自抗扰控制

溶解氧是活性污泥法处理污水的一个关键变量,它关系到污水中有机物的生物降解、微生物生长和出水水质.多数污水处理过程选择溶解氧为被控量.然而,溶解氧浓度受进水流量,进水组分、浓度波动等诸多因素影响,较难控制.本文根据曝气量变化确定溶解氧浓度设定值,以污水进水变化率为控制量,设计线性自抗扰控制实现对溶解氧浓度的跟踪,进而获得对污水出水底物浓度的间接控制.设计两组仿真实验,分别模拟进水底物浓度固定和变化时,线性自抗扰控制对溶解氧浓度和出水底物浓度的控制;同时,设计仿真实验验证线性自抗扰控制对总扰动的估计和补偿效果.仿真结果表明,线性自抗扰控制可获得良好的溶解氧浓度跟踪和出水底物浓度控制效果;在进水水质波动时,线性自抗扰控制亦具有很强的干扰补偿能力,可保证出水水质.     

A detection problem: Sensitivity and uncertainty analysis of a land surface temperature approach to detecting dynamics of water use by groundwater-dependent vegetation

Sustainable management of groundwater-dependent vegetation (GDV) requires the accurate identification of GDVs, characterisation of their water use dynamics and an understanding of associated errors. This paper presents sensitivity and uncertainty analyses of one GDV mapping method which uses temperature differences between time-series of modelled and observed land surface temperature (LST) to detect groundwater use by vegetation in a subtropical woodland. Uncertainty in modelled LST was quantified using the Jacobian method with error variances obtained from literature. Groundwater use was inferred where modelled and observed LST were significantly different using a Student's t-test. Modelled LST was most sensitive to low-range wind speeds (<1.5 m s⁻¹), low-range vegetation height (<=0.5 m), and low-range leaf area index (<=0.5 m² m⁻²), limiting the detectability of groundwater use by vegetation under such conditions. The model-data approach was well-suited to detection of GDV because model-data errors were lowest for climatic conditions conducive to groundwater use.     

Estimation of orientation of a textured planar surface using projective equations and separable analysis with M-channel wavelet decomposition

Estimation of the orientation of a textured planar surface is one of the basic tasks in the area of “shape from texture”. For the solution of this task, many successful approaches were proposed. In this paper, we have examined a few unaddressed questions: First, is there a mathematical formulation that relates the spectral characteristics of the texture pattern and the orientation of an inclined planar surface in a polar-coordinate system? Second, is there a good wavelet-based approach that produces an accurate estimate of the orientation angle of the textured planar surface by analyzing the spectral behavior of one single uncalibrated image?To answer these questions at first we present the formulation of a “texture projective equation”, which relates the depth and orientation of an inclined planar surface in a polar coordinate system with the spectral properties of its image texture. A suitable imaging geometry has been considered to enable separable analysis of the effect of inclination of the texture surface. Next, a method for shape from texture is presented based on discrete wavelet analysis to estimate the orientation of the planar surface. This approach although designed mainly for M-channel wavelets, is also applicable for dyadic wavelet analysis. Texture characteristics in the subbands of wavelet decomposition are analyzed using scalograms, and quantitatively evaluated based on texture projective equations. The proposed method of estimation of the orientation of a planar texture surface is evaluated using a set of simulated and real world textured images.