Depletion Layer Model Research Articles

Enhanced triethylamine sensing properties by fabricating Au@SnO2/α-Fe2O3 core-shell nanoneedles directly on alumina tubes

α-Fe2O3 nanoneedles were directly grown on the surface of Al2O3 tubes by a cost-effective hydrothermal method. In addition, the construction of Au nanoparticle-loaded SnO2/α-Fe2O3 core/shell heterostructure was fabricated by employing pulsed laser deposition (PLD) and DC-sputtering methods Gas sensors were fabricated from Au@SnO2/α-Fe2O3 core-shell nanoneedles on Al2O3 tubes, and their sensing properties have investigated for response to various target gases. The results indicated that the sensor based on Au@SnO2/α-Fe2O3 core-shell nanoneedles showed superior selectivity toward triethylamine (TEA) gas, giving a response of 39–100 ppm, which was higher than that of α-Fe2O3 nanoneedles and SnO2/α-Fe2O3 core-shell nanoneedles. Moreover, the sensor based on Au@SnO2/α-Fe2O3 core-shell nanoneedles showed an obvious better linearity (R = 0.9975) of sensing characteristics than of two other sensors. The enhanced sensing properties are discussed with the semiconductor depletion layer model along with the Schottky contact and N–N heterojunction theory.

Enhanced triethylamine sensing properties by designing Au@SnO2/ZnO nanosheets directly on alumina tubes

A triethylamine (TEA) sensor working temperature of 280 °C with high sensitivity and selectivity for TEA gas has been successfully fabricated by building Au@SnO2/ZnO nanosheets. By introducing a seed layer, ZnO nanosheets have been directly grown on Al2O3 tubes with a facile hydrothermal method. The Au@SnO2/ZnO structure is formed by utilizing DC-sputtering and pulsed laser deposition (PLD) methods. Compared with the pristine ZnO and SnO2/ZnO nanosheets sensors, the Au@SnO2/ZnO nanosheets sensors measured at 280 °C get a higher response of 48.1 toward 50 ppm of TEA gas. Moreover, the Au@SnO2/ZnO nanosheets sensor exhibits excellent selectivity. The enhanced sensing properties of the Au@SnO2/ZnO nanosheets sensor are discussed with a model of semiconductor depletion layer on the basis of n–n heterojunction and Schottky contact.

Low-temperature in-situ growth of SnO2 nanosheets and its high triethylamine sensing response by constructing Au-loaded ZnO/SnO2 heterostructure

In this report, uniform SnO2 nanosheets (NSs) have been in-situ grew on the surface of Al2O3 tubes via a cost-efficient hydrothermal synthesis at low temperature. The nanostructures of ZnO/SnO2 were prepared by employing pulsed laser deposition (PLD) method. Furthermore, Au surface modification with DC-sputtering process is used to improve the sensitivity of ZnO/SnO2 NSs sensor to TEA gas. The Au@ZnO/SnO2 NSs exhibited high response (S = 115) to 100 ppm trimethylamine (TEA) gas at 300 °C, which was about 20 times higher than that of the pristine SnO2 NSs. The enhanced sensing properties of Au@ZnO/SnO2 NSs sensor are discussed from the points of Au@ZnO Schottky contact and ZnO/SnO2 n-n heterojunction on the basic of depletion layer model.

Enhanced triethylamine sensing properties by designing Au@SnO2/MoS2 nanostructure directly on alumina tubes

Two dimensional semiconductors are promising gas sensing materials due to their large surface-to-volume ratio. However, some drawbacks just like low response, long recovery time, and insufficient selectivity limits the development of two dimensional semiconductors gas sensors. In order to optimize the gas sensing properties of MoS2 sensors, tri-material compound nanostructure of sensing materials is designed and synthesized by depositing Au and SnO2 nanoparticles on the surface of MoS2 nanoflowers. Comparing with the pristine MoS2 sensor, the Au@SnO2/MoS2 sensor exhibits better sensing properties to triethylamine, such as higher sensitivity and faster response/recovery speed. The enhanced sensing properties of Au@SnO2/MoS2 sensor are discussed from the points of Au@SnO2 Schottky contact and SnO2/MoS2 N-N heterojunction on the basic of depletion layer model. Moreover, MoS2 nanoflowers are directly grew on Al2O3 tubes via a facile and cost-effective hydrothermal method. And the construction of Au nanoparticles decorated SnO2/MoS2 nanoflowers heterostructure were built by employing the pulsed laser deposition and DC-sputtering methods.

Highly sensitive gold-decorated zinc oxide nanorods sensor for triethylamine working at near room temperature

There is an increasing worldwide demand for chemiresistive sensors for specific gas working at low temperature, in particular for standalone and mobile systems, which call for small and low power devices. In this paper, we successfully assemble highly sensitive triethylamine (TEA) gas sensors working at near-room temperature with gold (Au)-decorated ZnO nanorods. ZnO nanorods grow directly on flat Al2O3 ceramic electrodes by a cost-effective hydrothermal method and Au nanoparticles are deposited onto ZnO nanorods by DC-sputtering. Au-loaded ZnO (Au/ZnO) nanorods sensor at working temperature of 40°C and relative humidity of 30% exhibits high response (22–50ppm TEA), low detection concentration (∼1ppm), and short response/recovery time (11s/15s), which are all much better than the pristine ZnO nanorods sensor. When the relative humidity increases, the sensor response decreases due to the water molecules adsorption. Moreover, the enhanced sensing properties of the Au/ZnO sensors are discussed in detail with the semiconductor depletion layer model introduced by the Au/ZnO Schottky contact and the catalytic effect of noble gold nanoparticles.

Construction of 1D SnO2-coated ZnO nanowire heterojunction for their improved n-butylamine sensing performances.

One-dimensional (1D) SnO2-coated ZnO nanowire (SnO2/ZnO NW) N-N heterojunctions were successfully constructed by an effective solvothermal treatment followed with calcination at 400 °C. The obtained samples were characterized by means of XRD, SEM, TEM, Scanning TEM coupled with EDS and XPS analysis, which confirmed that the outer layers of N-type SnO2 nanoparticles (avg. 4 nm) were uniformly distributed onto our pre-synthesized n-type ZnO nanowire supports (diameter 80~100 nm, length 12~16 μm). Comparisons of the gas sensing performances among pure SnO2, pure ZnO NW and the as-fabricated SnO2/ZnO NW heterojunctions revealed that after modification, SnO2/ZnO NW based sensor exhibited remarkably improved response, fast response and recovery speeds, good selectivity and excellent reproducibility to n-butylamine gas, indicating it can be used as promising candidates for high-performance organic amine sensors. The enhanced gas-sensing behavior should be attributed to the unique 1D wire-like morphology of ZnO support, the small size effect of SnO2 nanoparticles, and the semiconductor depletion layer model induced by the strong interfacial interaction between SnO2 and ZnO of the heterojunctions. The as-prepared SnO2/ZnO NW heterojunctions may also supply other novel applications in the fields like photocatalysis, lithium-ion batteries, waste water purification, and so on.

Near Room Temperature, Fast-Response, and Highly Sensitive Triethylamine Sensor Assembled with Au-Loaded ZnO/SnO₂ Core-Shell Nanorods on Flat Alumina Substrates.

Chemiresistive gas sensors with low power consumption, fast response, and reliable fabrication process for a specific target gas have been now created for many applications. They require both sensitive nanomaterials and an efficient substrate chip for heating and electrical addressing. Herein, a near room working temperature and fast response triethylamine (TEA) gas sensor has been fabricated successfully by designing gold (Au)-loaded ZnO/SnO2 core-shell nanorods. ZnO nanorods grew directly on Al2O3 flat electrodes with a cost-effective hydrothermal process. By employing pulsed laser deposition (PLD) and DC-sputtering methods, the construction of Au nanoparticle-loaded ZnO/SnO2 core/shell nanorod heterostructure is highly controllable and reproducible. In comparison with pristine ZnO, SnO2, and Au-loaded ZnO, SnO2 sensors, Au-ZnO/SnO2 nanorod sensors exhibit a remarkably high and fast response to TEA gas at working temperatures as low as 40 °C. The enhanced sensing property of the Au-ZnO/SnO2 sensor is also discussed with the semiconductor depletion layer model introduced by Au-SnO2 Schottky contact and ZnO/SnO2 N-N heterojunction.

Nanopores in GaN by electrochemical anodization in hydrofluoric acid: Formation and mechanism

We report the use of hydrofluoric acid (HF) as an electrolyte in etching and porosifying GaN. HF is found to be effective in rendering a wide range of nanoporous morphology, from curved branches to highly parallel straight pores. Under suitable conditions, the porosification proceeds at a rate greater than 100 μm/min. To elucidate the etching mechanism, cyclic voltammetry is performed, together with a parametric mapping of electrolysis variables such as the doping of GaN, the concentration of HF electrolyte, and the anodization voltage. We demonstrate that the formation of nanoporous structures is largely due to the local breakdown of the reverse-biased semiconductor junction. A quantitative agreement between the estimated width of space-charge region and the observed variation in morphology lends support to a depletion layer model developed previously in the etching of porous-Si.

Size-dependent response of single-nanowire gas sensors

Monocrystalline tin oxide nanowires (NWs) are grown from Sn powder through thermal chemical vapour deposition. They are then dispersed onto a Si/SiO2 substrate and singularly contacted to fabricate single nanowire (NW)-resistive gas sensors. NWs with different diameter sizes are characterized as nitrogen dioxide sensors. The gas sensor response of the devices is investigated as a function of working temperature, gas concentration, and NW diameter. All the devices show optimum working performances at temperatures around 250–350°C. Their responses were linearly related to gas concentration of up to 500ppm, and then started to saturate up to a maximum response of 18.9 to 1000ppm NO2 for the best device (thinnest nanowire). The limit of detection was 7.2ppm for the same device. The gas sensing properties (response, response time, and recovery time) were studied as a function of NW diameter to investigate the depletion layer model, which is used to explain the sensing mechanism of monocrystalline metal-oxide NWs. A confirmation of such model was found, with a depletion layer depth of around 14nm.

WO 3 nanocrystals: Synthesis and application in highly sensitive detection of acetone

WO 3 nanocrystals have been prepared by a sol–gel route and characterized by X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. The experimental results show that WO 3 nanocrystals have a high crystallographic quality and a good dispersivity. The particles’ sizes are in the range of 25–100 nm. The fabricated WO 3 nanocrystal-based sensors have an excellent sensitivity and selectivity to acetone, and display a rapid response and recovery characteristics. The developed sensors exhibit a detection limit down to 0.05 ppm at 300 °C, rendering a promising application in noninvasive diagnosis of diabetes. The response mechanism of the WO 3 nanocrystal sensor to low concentration of acetone has been discussed based on the depletion layer model.

Surfactant-Induced Modulation of Light Emission in Porous Silicon Produced by Metal-Assisted Electroless Etching

Photoluminescent porous silicon (PSi) was produced by Pt-assisted electroless chemical etching of p(-)-Si in a 1:1:2 (v/v/v) solution of HF, methanol, and H2O2. Upon irradiation with ultraviolet light PSi produced under these conditions luminesces with a peak emission near 590 nm that is sufficiently intense to be visible by eye. Because PSi light emission is an attractive modality for chemical sensing, the effect of charged surfactant adsorbates on the photoluminescence (PL) intensity was investigated. PSi was exposed to aqueous solutions of cationic, cetyltrimethylammonium bromide (CTAB), and anionic, sodium dodecyl sulfate (SDS), surfactants as a function of solution concentration and pH. Adsorption produces both chemical and physical changes at the PSi-solution interface, which were followed by a combination of PL and infrared absorption spectroscopy. Luminescence is quenched in the presence of CTAB and enhanced in the presence of SDS, both in a pH-dependent manner, the behavior being explained by a depletion layer model. PSi crystallites generated from p-Si exhibit a hole-depletion layer at the Si-solution interface, and the depletion layer expands in the presence of cationic surfactant and contracts in the presence of anionic surfactant. Because the surface depletion region is nonemissive (dead layer), surfactant adsorbate-induced modulation of the depletion layer width determines the luminescence intensity of PSi. At very basic pH, PL quenching was observed independent of surfactant identity or concentration, an observation likely tied to the dissolution of the PSi nanocrystallites in strong base.

Dynamic Diffuse Double-Layer Model for the Electrochemistry of Nanometer-Sized Electrodes

A dynamic diffuse double-layer model is developed for describing the electrode/electrolyte interface bearing a redox reaction. It overcomes the dilemma of the traditional voltammetric theories based on the depletion layer and Frumkin's model for double-layer effects in predicating the voltammetric behavior of nanometer-sized electrodes. Starting from the Nernst-Planck equation, a dynamic interfacial concentration distribution is derived, which has a similar form to the Boltzmann distribution equation but contains the influence of current density. Incorporation of the dynamic concentration distribution into the Poisson and Butler-Volmer equations, respectively, produces a dynamic potential distribution equation containing the influence of current and a voltammetric equation containing the double-layer effects. Computation based on these two equations gives both the interfacial structure (potential and concentration profiles) and voltammetric behavior. The results show that the electrochemical interface at electrodes of nanometer scales is more like an electric-double-layer, whereas the interface at electrodes larger than 100 nm can be treated as a concentration depletion layer. The double-layer nature of the electrode/electrolyte interface of nanometer scale causes the voltammetric responses to vary with electrode size, reactant charge, the value of formal redox potential, and the dielectric properties of the compact double-layer. These voltammetric features are novel in comparison to the traditional voltammetric theory based on the transport of redox molecules in the depletion layer.

Observed and simulated depletion layers with southward IMF

Abstract. We present observations from the Polar satellite that confirm the existence of two types of depletion layers predicted under southward interplanetary magnetic field (IMF) conditions in magnetohydrodynamic simulations. The first depletion type occurs along the stagnation line when IMF BX and/or dipole tilt are/is present. Magnetic merging occurred away from the equator (Maynard et al., 2003) and flux pile-ups developed while the field lines drape to the high-latitude merging sites. This high-shear type of depletion is consistent with the depletion layer model suggested by Zwan and Wolf (1976) for low-shear northward IMF conditions. Expected sites for depletion layers are associated with places where IMF tubes of force first impinge upon the magnetopause. The second depletion type develops poleward of the cusp. Under strongly driven conditions, magnetic fields from Region 1 current closure over the lobes (Siscoe et al., 2002c) cause the high-latitude magnetopause to bulge outward, creating a shoulder above the cusp. These shoulders present the initial obstacle with which the IMF interacts. Flow is impeded, causing local flux pile-ups and low-shear depletion layers to form poleward of the cusps. Merging at the high-shear dayside magnetopause is consequently delayed. In both low- and high-shear cases, we show that the depletion layer structure is part of a slow mode wave standing in front of the magnetopause. As suggested by Southwood and Kivelson (1995), the depletions are rarefactions on the magnetopause side of slow-mode density compressions. While highly sheared magnetic fields are often used as proxies for ongoing local magnetic merging, depletion layers are prohibited at merging locations. Therefore, the existence of a depletion layer is evidence that the location of merging must be remote relative to the observation.

Analysis of conductivity and PTCR effect in Er-doped BaTiO 3 ceramics

Barium titanate ceramics with different Er additions in the range from 0.25 to 8 at.% were prepared by a solid state route. Several values of the nominal Ba/Ti ratio were considered according to different potential incorporation mechanisms of the dopant. PTCR effect was detected also for high Er content and for different nominal incorporation mechanisms. Impedance Spectroscopy and I–V measurements in the temperature range from 20 to 200 °C were used to verify the Double Depletion Layer model over the whole temperature range. First results of Differential Analysis of Impedance data are presented.

The inner magnetosheath of Venus: An analogue for Earth?

The unmagnetized planets provide examples of solar wind interactions that are free from the complications associated with magnetopause reconnection and with sensitive obstacle response to incident solar wind pressure changes. Using the Venus magnetosheath as a testbed, we search for evidence of standing slow mode “transitions” in the inner subsolar region as reported for Earth by Song et al. [1990a; 1992a; b]. Although the system at Venus is much smaller in scale, the Pioneer Venus Orbiter magnetometer data indicate that for perpendicular interplanetary magnetic field conditions the general behavior of the plasma in the magnetosheath is as expected from the simple depletion layer model. In examples of magnetic field measurements chosen for the apparently steady interplanetary conditions during the spacecraft pass, there is no clear evidence for a slow mode structure near the ionopause as might be expected on the basis of the Song et al. study. The implication is that some aspect of the Venus magnetosheath (such as its small size or the presence of local planetary ion production) makes it physically different from Earth's, that the conditions of the magnetosheath during Song's study differed significantly from those in the Venus study, or that the observations of Song et al. do not represent a steady state.

Depletion layers, plasmon dispersion, and the effects of temperature in degenerate InSb(100): A study by electron-energy-loss spectroscopy.

High-resolution electron-energy-loss spectroscopy has been used to observe the effects of temperature on the excitation of the conduction-band electron plasmon in degenerate n-type InSb(100). Measurements of the intensity and energy of the plasmon loss at both 290 and 435 K were made over a wide range of incident electron energies. The data have been analysed using semiclassical dielectric theory and a depletion-layer model which incorporates a q-dependent surface loss function and spatial dispersion. As the temperature is increased the depletion layer is reduced from 100 \AA{} at 290 K to 90 \AA{} at 435 K. Accounting for the nonparabolic nature of the conduction band, the highly degenerate nature of InSb at these bulk doping levels, and a temperature-dependent bulk band gap, this decrease reflects the change in band bending which accompanies the lowering of the Fermi level as the temperature is raised. The changes in plasmon energy over this temperature range are not a consequence of any change in the free-carrier concentration.

Impedance of porous Si

The impedance of photoelectrochemically etched n-Si was measured in a liquid junction made of a methanolic solution of oxidized and reduced dimethylferrocene. The results show that both the equivalent circuit and each of the individual elements are almost identical with those of the smooth Si that was used as a substrate. These results were interpreted in terms of a depletion layer model in which the space-charge layer is present only at the bottom of the macropores and at the interface between the electrolyte and the smooth part of the substrate. The poles that separate the pores are completely depleted of majority carriers. The nanoporous layer that is deposited on top of the macropores and can be removed by KOH is completely transparent to the impedance measurements.

Neutron degradation of the I-V characteristics of AlGaAs/GaAs modulation-doped field-effect transistors

AlGaAs/GaAs MODFETs have been neutron-irradiated to fluences approaching 10/sup 15/ cm/sup -2/. The source-drain current and the transconductance degrade about 30% over this range of fluences. The source-drain resistance increases by 60% and the end resistance increases by 30% at fluences near 10/sup 15/ cm/sup -2/. A triangular-well one-subband depletion layer model which applies over the range of I-V characteristics from subthreshold to saturation has been developed. The model has been extended to describe neutron degradation of source-drain current and transconductance. Both the source-drain current and the transconductance exhibit a nonlinear dependence on neutron fluence, which is reflected in the theoretical results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The role of acceptor density on the high channel carrier density I– V characteristics of AlGaAs/GaAs MODFETs

A triangular-well, one-subband, depletion layer model has been developed for the high density region of a MODFET. The high density operation is defined as when the channel carrier density, in the entire channel, is equal to or greater than m 1 kT/ πh 2. This high density model has been used to describe the effects of the depletion layer charge on the I– V characteristics. An approximation for the experimentally determined threshold voltage is derived. It is shown that for small acceptor densities, ∼10 13 cm −3, the experimentally determined threshold voltage may differ from the strong inversion threshold voltage by ∼0.25 V. We show that this discrepancy is due to the effect of the depletion layer in the device capacitance and the AlGaAs layer capacitance. The effective layer thickness, Δd, is shown to decrease from ∼90 Å at an acceptor density of 10 13 cm −3 to 75 Å at 10 17 cm −3.

The role of unintentional acceptor concentration on the threshold voltage of modulation-doped field-effect transistors

A strong-inversion, depletion-layer model of the threshold voltage for MODFETs has been used to assess the role of unintentional acceptor doping. For concentrations less than 10/sup 13/ cm/sup -3/, acceptor doping has little effect on the threshold voltage. As acceptor density increases, the GaAs depletion-layer charge contribution will dominate, causing the threshold voltage to exceed the low-density value by as much as 1.4 V at acceptor densities approaching 10/sup 17/ cm/sup -3/. >