Surface Depletion Research Articles

Strength, fatigue strength and toughness of dissimilar Ti17–Ti64 linear friction welded joints: Effect of soft surface contamination and depletion of α precipitates

The effect of microstructure of three dissimilar Ti17–Ti64 linear friction welded joints on the strength, fatigue strength and fracture toughness was studied. A special attention was paid to role of soft contaminants and α precipitate depletion. Three joints were produced: one in the as machined state after electrical discharge machining, one ground prior to welding, and one welded in the as machined state and post-weld heat treated. The microstructure of the weld centre zone (WCZ) exhibited an acicular entangled α' martensite with a Widmanstätten morphology on the Ti64 side and fine equiaxed distorted β grains on the Ti17 side. A depletion of the strengthening α precipitates was observed in the WCZ and the thermo-mechanically affected zone (TMAZ) on the Ti17 side. For the studied welding parameters, the LFW self-cleaning mechanism failed to extrude the contaminants in the weld interface of the joint welded in the as machined state. These contaminants were related to prior to welding machining and were denominated soft contaminants due to their relatively low melting point. A defect layer was formed that led to a pseudo-brittle fracture in at the WCZ during tensile testing, cyclic loading and fracture toughness testing. During tensile testing of the joint that was ground prior to welding, failure occurred after plastic deformation sometimes at the Ti64 parent material (PM) and sometimes at the Ti17 TMAZ. Fatigue cracks initiated at the latter weak zone with a reduced fatigue strength. Compared to the PM, fracture toughness of samples even without fatigue pre-crack was also reduced. Failure at the weak zone highlighted unexpected colonies of elongated dimples that nucleated on transgranular deformation lines.

Analysis of concentrated solar powered conical reactor with coal ash as catalyst

Nowadays there is an increase in the usage of non-renewable products to produce the energy required to sustain in day to day life, which results in the generation of harmful emissions such as greenhouse gases, in turn, leads to an increase in earth’s surface temperature and depletion of the ozone layer. On the other hand, the generation of usable energy requires fossil fuels to satisfy the thermal input. So switching to renewable energy for the thermal input for fossil fuels saves the environment for further damage, mainly solar energy which is abundantly available and free of cost. Hence creating much more interest in researchers to establish and enhance the performance of solar energy coupled reactor. By implementing solar energy as the input for the reactor there will be a decrease in the consumption of fossil fuels to produce heat energy. In this present study solar concentrated power is used as an input fuel supply to the reactor process where the dish type concentrator used of diameter 1.4m and has a focal length of 0.28m which is focused towards the preferred conical shape reactor because of the effective flux distribution inside the reactor than the other shapes. The reactor volume of 13.9m3 is fixed at the focal distance where the receiver absorbs more amount of radiation from the concentrator, leads to an increase in the temperature inside the reactor and the specimen inside the reactor will generate gases. by using the coal ash as a catalyst there is a temperature increase of 10%. That is without any catalyst the reactor inside temperature is 100°C and with coal ash as catalyst 110°C.

Giant enhancement of photoluminescence quantum yield in 2D perovskite thin microplates by graphene encapsulation

The optoelectronic performances of the layered materials are strongly dependent on the thickness of the samples due to the surface effect. As the size of the samples decreases to few nanometers, the surface depletion field and surface defect density are prominent arising from the large surface to volume ratio. For instance, thin two-dimensional (2D) organic-inorganic hybrid perovskite microplates usually exhibit a rather low photoluminescence quantum yield (PLQY), owning to the strong surface effect. Here, we report that the PLQY can be enhanced as large as 28 times in (iso-BA)2Pbl4 (BA = C4H9NH3) 2D perovskite thin microplates encapsulated by graphene, resulting in that the PLQY is more than 18% for the microplate with a thickness of 6.7 nm at 78 K. As the thickness of the 2D perovskite microplate increases, the enhancement is gradually reduced and finally vanishes. This observation is in striking contrast to that in monolayer transition metal dichalcogenides (TMDs), when the PLQY is quenched by covering a layer of graphene due to the efficient charge transfer. The enhancement of PLQY in 2D perovskites can be mainly ascribed to the reduced quantum confined Stark effect (QCSE) due to the reduced surface depletion field after covering graphene flake, resulting in the enhanced radiative recombination efficiency. Our findings provide a cost-effective approach to enhance the luminescence, which may pave the way toward high performance light emitting devices based on 2D perovskites.

Ultrafast Optical Generation of Coherent Bright and Dark Surface Phonon Polaritons in Nanowires

The coherence of polaritons plays a fundamental role in numerous recent experimental observations, from strong coupling to Bose-Einstein condensation, but accessing the coherence of polaritons is a difficult task due to the high energy of plasmons and excitons. However, surface phonon polaritons offer similar promises at much lower energy. Here, we demonstrate the possibility to use visible ultrafast lasers to generate and time resolve coherent surface phonon polaritons in nanowires. We show that these modes are generated through the ultrafast screening of the surface depletion field. By comparing experiments and FDTD simulations, we show that this method allows the simultaneous generation of dark and bright mode. Our observations open the way to deeper investigations of the role of coherence in the rich polariton physics.

Leaching of monolithic geopolymer mortars

The environmental compatibility of geopolymers was investigated in this work in terms of the leaching properties of major components, heavy metals and trace elements from monolithic mortars. Fly ash geopolymer, metakaolin geopolymer and a commercial geopolymer mortar were used. Leaching mechanism analysis demonstrates that the leaching of chemical components is mainly controlled by diffusion, while surface wash-off, solubility and depletion also take place. The results were compared with cement-based materials and the regulatory limits in Germany and in the Netherlands for environmental compatibility assessment. Leaching of studied heavy metals and trace elements (except B, V and Mo) from geopolymer are in a similar range as cementitious materials. Only the cumulative release of V is higher than the threshold in Germany, but it still meets the requirement in the Netherlands. Strength decrease after leaching was detected, which was 11.8%, 9.4% and 14.9% for fly ash geopolymer, metakaolin geopolymer and the commercial geopolymer, respectively. In addition, no difference in terms of phase composition in the surface layer was detected after leaching.

Drug Release from a Spherical Matrix: Theoretical Analysis for a Finite Dissolution Rate Affected by Geometric Shape of Dispersed Drugs

Amending the neglect of finite dissolution in traditional release models, this study proposed a more generalized drug release model considering the simultaneous dissolution and diffusion procedure from a drug-loaded spherical matrix. How the shape factor (n = 0, 1/2, and 2/3 for the planar, cylindrical, and spherical geometry, respectively) of dispersed drug particles affected the release from the matrix was examined for the first time. Numerical solutions of this generalized model were validated by consensus with a short-time analytical solution for planar drugs and by the approach of the diffusion-controlled limits with Higuchi’s model. The drug release rate increases with the ratio of dissolution/diffusion rate (G) and the ratio of solubility/drug loading (K) but decreases with the shape factor of drug particles. A zero-order release profile is identified for planar drugs before starting the surface depletion layer, and also found for cylindrical and spherical dispersed drugs when K and G are small, i.e. the loaded drug is mainly un-dissolved and the drug release rate is dissolution-controlled. It is also shown that for the case of a small G value, the variation of drug release profile, due to the drug particle geometry, becomes prominent. Detailed comparison with the results of the traditional Higuchi’s model indicates that Higuchi’s model can be applied only when G is large because of the assumption of an instantaneous dissolution. For K = 1/101–1/2, the present analysis suggests an error of 33–85% for drug release predicted by Higuchi’s model for G = 100, 14–44% error for G = 101, while a less than 5% error for G ≧ 103.

Ocean carbon pump decomposition and its application to CMIP5 earth system model simulations

The ocean stores 60 times as much carbon as the atmosphere, and the ocean carbon cycle has a critical role in controlling atmospheric CO2 concentration. The concept of ocean carbon pump is widely used for describing the ocean carbon cycle, but the term “ocean carbon pump” is not necessarily strictly defined and has been differently referred in previous studies. Here, using three dimensional distributions of dissolved inorganic carbon concentration, total alkalinity, phosphate, and salinity, four types of ocean carbon pump (organic matter, calcium carbonate, gas exchange, and freshwater flux pumps) were formulated. Although previously proposed decomposition emphasizes the enrichment in the deep ocean, my decomposition focuses on surface depletion which directly affects air-sea CO2 exchanges. Based on this decomposition, vector diagram for quantifying the individual roles of the pumps in controlling the ocean surface pCO2, which is in balance with atmospheric CO2 concentration, was demonstrated in this study. The method was applied to the observational climatology, and the contributions of the four carbon pump components to atmospheric CO2 were visualized in a single figure (the vector diagram); each carbon pump component was represented by one vector, and its contribution to CO2 concentration was measured from the difference in the contour values between the beginning and end of the vector. The same analysis was also applied to the CMIP5 earth system model simulations. All the models reproduced the same level of atmospheric CO2 concentration as the observation; however, the contributions from the four carbon pumps varied. The vector diagram was shown to quantify the differences in the contributions from the pumps between the models and against the observation. This study demonstrated that the proposed vector diagram analysis is a useful tool for quantifying the individual contributions of the ocean carbon pumps to atmospheric CO2 concentration and is helpful for evaluating the reproducibility of ocean carbon cycle models.

Insight into the spontaneous breakdown of ‘Toughened glass’: From nano- to macroscale

Instances of sudden breakdown of ‘toughened glass’ (TG) without apparent causes have grave consequences. Here we present results of investigations on two types of samples – pieces of a TG door that broke down spontaneously (STG) and pieces of a TG sheet broken down intentionally (ITG). In both cases, the broken pieces have the same morphology and were studied through X-ray Diffraction (XRD), X-ray Fluorescence Spectroscopy (XRF), X-ray Absorption Near-edge Spectroscopy (XANES) at Na K-edge, Fourier Transform Infrared Spectroscopy (FTIR), Twin Mercury Porosimetry (TMP), Atomic Pair Distribution Function (PDF) and Positron Annihilation Spectroscopy (PAS). XRD and PAS showed the same structure and high defect concentration for both the samples indicating no major difference in microstructure due to aging in STG. No trace of Ni or S could be observed from XRF, casting doubt on the presence of NiS inclusions. PAS studies indicated very large concentration of free volume cavities in the STG sample, implying that mechanical instability leading to the erosion of the structure and loss of rigidity is a result of atomic motions within the glass and the associated weakening of the molecular bonds. From XRF, a small depletion in Na content was seen in STG, which became much more apparent from low angle XANES, pointing to a surface depletion. From FTIR, a reduction in the Si–O–Si bonds and an enhancement in the O–H bonds could be identified indicating a conversion of the former to Si(OH)4. The bond distances of different pair of atoms were calculated from atomic Pair Distribution Function experiments. An increase in total porosity especially in larger pores (diameter ≥100–200 µm, from TMP) were observed in STG over ITG. Based on these results, we propose a simple model in which the Si–O network is visualized as suffering from destabilization due to the leaching out of the Na-ions from the STG over time, leading to the accumulation of large pores, increase of overall porosity and the final collapse of the structure.

Interface confinement on the exciton recombination in thin CdS/ZnO shell/core nanowires

The modulation of exciton recombination is one of the core issues for the electronic/optoelectronic devices. Here, we investigate the exciton recombination and surface depletion of ZnO nanowires under various interface conditions in terms of continuum medium mechanics and bond relaxation method. We find that the enhancement of donor concentration, nanowire dimension and external temperature can modify the carrier recombination, surface barrier height and surface depletion depth. Moreover, coated a thin epitaxial layer on the nanowires is of great benefit to reduce the surface state for suppression of recombination rate. The related mechanism on the interface confinement has also been clarified. Our predictions agree well with the experimental measurements, which provide an effective guidance for the organization of electronic device design.

Characterization of agonist-dependent somatostatin receptor subtype 2 trafficking in neuroendocrine cells.

Somatostatin (SOM) receptor subtype 2 (SSTR2) is the major receptor subtype mediating SOM effects throughout the neuraxis. We previously demonstrated that the non-selective agonist [D-Trp8]-SOM induces intracellular sequestration of SSTR2, whereas this receptor is maintained at the cell surface after treatment with the SSTR2-selective agonist L-779,976 in cells co-expressing SSTR2 and SSTR5. In this study, we knocked-out SSTR5 in AtT20 cells endogenously expressing both SSTR2 and SSTR5 and used immuno-labeling and confocal microscopy to investigate the effect of SSTR5 on regulation of SSTR2 trafficking. Our results indicate that unlike [D-Trp8]-SOM-induced intracellular sequestration, L-779,976 stimulation results in the maintenance of SSTR2 at the cell surface regardless of whether SSTR5 is present or not. We then examined the trafficking pathways of SSTR2 upon stimulation by either agonist. We found that both [D-Trp8]-SOM and L-779,976 induce SSTR2 internalization via transferrin-positive vesicles. However, SSTR2 internalized upon L-779,976 treatment undergoes rapid recycling to the plasma membrane, whereas receptors internalized by [D-Trp8]-SOM recycle slowly after washout of the agonist. Furthermore, [D-Trp8]-SOM stimulation induces degradation of a fraction of internalized SSTR2 whereas L-779,976-dependent, rapid SSTR2 recycling appears to protect internalized SSTR2 from degradation. In addition, Octreotide which has preferential SSTR2 affinity, induced differential effects on both SSTR2 trafficking and degradation. Our results indicate that the biased agonistic property of L-779,976 protects against SSTR2 surface depletion by rapidly initiating SSTR2 recycling while SSTR5 does not regulate L-779-976-dependent SSTR2 trafficking.

Effect of Heterojunction Interface (SnO2\\ZnO) on Gas Sensing Properties of Core-Shell Nanostructures

Introduction Metal oxide-based core-shell (C-S) nanostructures have been studied for gas sensing applications to eliminate poor selectivity [1]. When designing core-shell nanostructures with n-n, p-n and n-p junctions, an equalization of the Fermi level induces the formation of accumulation and depletion layers at the interface through charge redistribution, modifying the conduction channel. The selection of the core and the shell material is important as it would affect the properties of the heterojunction interface. Parameters such as the thickness of the shell layer, type of sensing material and working temperature have been studied for C-S nanostructure-based gas sensors. However, less attention has been given to factors affecting the conduction mechanisms in C-S nanostructures. Gas Sensing Mechanism In C-S nanostructures optimizing the shell layer thickness to the Debye length range of the material has proven to enhance the sensor performance [2]. Mechanisms such as potential barrier carrier transport and surface depletion layer formation play important roles in enhancing the gas sensing performance of C-S nanostructures [3]. Understanding the gas sensing mechanism is not straightforward due to the presence of the heterojunction interface. Therefore, there is a need to understand the sensing mechanism in C-S nanostructures to determine the role of heterojunction. One way to determine the role of the heterojunction is by exchanging the core and shell layers in C-S nanostructures and analyzing the gas sensing properties. Method Metal oxides such as Tin dioxide (SnO2) and Zinc oxide (ZnO) are chosen to synthesize C-S nanostructures. SnO2 nanowires were synthesized by using the VLS (Vapor-liquid- Solid) process with Au as the catalyst. The ZnO layer was deposited by using a sputtering technique. The thickness of the ZnO shell layer can be varied by changing the deposition time. To study the effect of exchanging the core and shell layer, ZnO nanorods were synthesized on a Si substrate by using the hydrothermal process. The Sn layer was deposited on ZnO nanorods by using a sputtering technique. Then the samples were annealed in air to form the SnO2 layer. X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) characterizations are used to confirm the phase, morphology, and dimensions of the synthesized C-S nanostructures. Kelvin probe force microscopy is used to calculate the work function of C-S nanostructures to determine the formation of the heterojunction. Gas sensors are fabricated by pipetting the C-S nanostructures solution on to the interdigitated Pt electrodes deposited on Si substrate. Response curves are measured in a custom built probe station towards 1000 ppm of H2 gas. The effect of shell layer thickness is analyzed towards maximum gas sensing response. Impedance spectroscopy (AC) measurements are performed to determine the contribution from different parts of C-S nanostructures towards the gas sensing response. This study that is ongoing will help in the understanding of the gas sensing mechanism and the role of heterojunction in C-S nanostructures. Results and Conclusions ZnO was sputtered on SnO2 nanowires grown through the VLS process for 30 seconds. The sputtering resulted in synthesizing ZnO decorated SnO2 nanowires which can be confirmed from the SEM image in Figure 1a. To study the effect of the heterojunction on gas sensing, SnO2 was grown on ZnO nanorods by sputtering Sn for 30 seconds and annealing in air at 400°C which is confirmed in Figure 1b. Figure 1c shows the response curve towards 1000 ppm hydrogen gas at 300°C for ZnO decorated SnO2 nanowires. The inset figure in Figure 1c shows the response of pristine SnO2 nanowires towards hydrogen gas. Response (Ra/Rg) is calculated as the ratio between resistance in air (Ra) and resistance in hydrogen gas (Rg). The response of ZnO decorated SnO2 nanowires (11) is enhanced when compared to the pristine SnO2 nanowires (7.39). The enhanced response proves the formation of a heterojunction between sputtered ZnO and SnO2 nanowires. Figure 1d shows the impedance curves for ZnO decorated SnO2 nanowires in nitrogen gas at different temperatures. It is evident from the plots that as the temperature is increased the resistance of the decorated nanostructures is decreasing which is the expected behavior for semiconductors. The ability to measure impedance curves also proves the stability of the hetero-nanostructures. Future work involves measuring response and impedance curves for SnO2 decorated ZnO nanorods towards 1000 ppm hydrogen gas. The effect on heterojunction (SnO2/ZnO) will be analyzed by fitting the impedance curves.

Tuning the carrier density in SrTiO3/LaTiO3/SrTiO3 quantum wells

We discuss methods of built-in carrier density control in SrTiO3/LaTiO3/SrTiO3 heterostructures that exhibit quasi-two-dimensional carrier confinement in an interfacial quantum well. Unlike the electronically similar LaAlO3/SrTiO3 heterostructures, where the polar discontinuity at the interface defines the accumulated carrier density, the LaTiO3 heterostructures offer two means of carrier density control—changing the La doping level and utilizing the effect of surface depletion through the change in the SrTiO3 capping layer thickness. Dynamic carrier tuning over a limited range is possible by the application of a back-gate bias, which primarily affects the depth distribution of carriers. We find that small changes in the pre-annealing conditions of a SrTiO3 substrate can have a dramatic effect on the low-temperature sheet resistance of the heterostructures.

An Insight into the Spontaneous Breakdown of 'Toughened Glass' from Nano- to Macroscale

Sudden breakdown of 'toughened glass' (TG) without apparent causes have grave consequences. We present results of investigations on two samples - pieces of a TG door that broke down spontaneously (STG) and pieces of a TG sheet broken down intentionally (ITG). Both samples have same morphology and X-ray Diffraction showed the same structure for both samples indicating no major difference in microstructure. No trace of Ni or S could be observed from X-ray Fluorescence Spectroscopy whereas a small depletion in Na content was seen in STG, which became much more apparent from low angle X-ray Absorption Near-edge Spectroscopy at Na K-edge, pointing to a surface depletion. Positron Annihilation Spectroscopy studies indicated very large concentration of free volume cavities in STG, implying that the erosion of the structure and loss of rigidity is a result of atomic motions within the glass and the associated weakening of the molecular bonds. From Fourier Transform Infrared Spectroscopy, a reduction in the Si-O-Si bonds and an enhancement in the O-H bonds could be identified indicating a conversion of the former to Si(OH)_4. The bond distances of different pair of atoms were calculated from Atomic Pair Distribution Function experiments. An increase in total porosity especially in larger pores were observed in STG over ITG, from Twin Mercury Porosimetry. We propose a simple model in which the Si-O network is destabilized due to leaching out of Na-ions from STG over time, leading to accumulation of large pores, increase of overall porosity and final collapse of the structure.

The Critical Role of AlInP Window Design in III–V Rear-Emitter Solar Cells

This article highlights the critical role of window design on short circuit carrier collection in rear-emitter solar cells, as demonstrated through modeling and experiment using metamorphic GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-y</sub> . Ultimately, if the window design is not carefully considered, surface depletion caused by Fermi level pinning at the window/air interface can extend into regions of active collection resulting in a large increase in effective window/base interface recombination velocity. This was experimentally shown here to result in a potential AM1.5G photocurrent loss of 8.1 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , or nearly 50%, based on integrated internal quantum efficiency (IQE). Associated IQE modeling and curve fitting indicate that the effective IRV at the GaAs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.64</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.36</sub> P interface is increased by multiple orders of magnitude when the window is not sufficiently thick or doped to fully contain the surface depletion to within the window layer. Calculations of the surface depletion depth as a function of doping and the surface Fermi pinning energy level provides insight into the fundamental limits of window thickness. This allows the estimation of the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.64</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.36</sub> P surface pinning level to be at least 1.25 eV below the conduction band edge, or in the bottom half of the bandgap.

Explicit Gain Equations for Single Crystalline Photoconductors.

Photoconductors based on semiconducting thin films, nanowires, and two-dimensional atomic layers have been extensively investigated in the past decades. However, there is no explicit photogain equation that allows for fitting and designing photoresponses of these devices. In this work, we managed to derive explicit photogain equations for silicon nanowire photoconductors based on experimental observations. The silicon nanowires were fabricated by patterning the device layer of silicon-on-insulator wafers by standard lithography that were doped with boron at a concentration of ∼8.6 × 1017 cm-3. It was found that the as-fabricated silicon nanowires have a surface depletion region ∼32 nm wide. This depletion region protects charge carriers in the channel from surface scatterings, resulting in the independence of charge carrier mobilities on nanowire size. Under light illumination, the depletion region logarithmically narrows down, and the nanowire channel widens accordingly. Photo Hall effect measurements show that the nanowire photoconductance is not contributed by the increase of carrier concentrations but by the widening of the nanowire channel. As a result, a nanowire photoconductor can be modeled as a resistor in connection with floating Schottky junctions near the nanowire surfaces. Based on the photoresponses of a Schottky junction, we derived explicit photogain equations for nanowire photoconductors that are a function of light intensity and device physical parameters. The gain equations fit well with the experimental data, from which we extracted the minority carrier lifetimes as tens of nanoseconds, consistent with the minority carrier lifetime in nanowires reported in literature.

Development of dual bias modulation electrostatic force microscopy for variable frequency measurements of capacitance.

We propose dual bias modulation electrostatic force microscopy (DEFM) for variable frequency measurements of surface depletion capacitance on a semiconductor. In DEFM, dual alternating current bias voltages at angular frequencies of ω1 and ω2 are applied to generate an electrostatic force, and we detect the high order term at an angular frequency of ω2 - 2ω1 in the electrostatic force from which a derivative of surface depletion capacitance by voltage (∂C/∂V) can be evaluated. Even with a fixed value of ω2 - 2ω1 at a specific resonant frequency of the cantilever, to ensure sufficient sensitivity of the electrostatic force, a pair of ω1 and ω2 can be varied; this enables variable frequency measurements of ∂C/∂V by DEFM. The validity of the quantitation and spatial resolution of DEFM were assessed through the analysis of metal-oxide-silicon and Zn(O, S)/Cu(In,Ga)(Se,S)2 samples.

A density functional theory based thermodynamic model of hydrogen coverage on the W(110) surface

Tungsten will be used as a plasma facing material in the next generation of fusion reactors. To aid in understanding atomic scale H–W interactions, we investigated hydrogen coverage on the tungsten (110) surface via periodic density functional theory, providing the most stable configurations that hydrogen forms on the surface at coverage ratios of interval 0.25, step-wise, up to a full mono-layer of hydrogen. We then calculate the Gibbs free energy for the stable configurations in the presence of hydrogen gas at specified temperature and pressure. It follows that the configuration, and corresponding coverage ratio, which yields the lowest Gibbs free energy is used to estimate the macroscopic surface state. Our findings based on the model compare well to low energy electron diffraction measurements, primarily the presence of well-ordered phases with coverage ratios 0.5, 0.75, and 1.0, respectively, and that no ordered phases are expected as temperature increases and surface depletion occurs.

Improved sub-ppm acetone sensing properties of SnO2 nanowire-based sensor by attachment of Co3O4 nanoparticles

Co3O4 nanoparticle-attached SnO2 nanowires are synthesized to fabricate highly sensitive acetone gas sensor by vapor-liquid-solid (VLS), sol-gel, and thermal annealing processes. To analyze enhanced acetone gas sensing responses, Co3O4 nanoparticles are attached SnO2 nanowires, and several samples are synthesized followed by the cycles of Co3O4 nanoparticle attachment process. The sensing response of Co3O4 nanoparticle-attached SnO2 nanowires, which are one time performed Co3O4 nanoparticle attachment process, is improved by 7 times compared with as-synthesized SnO2 nanowires when exposed to 50 ppm acetone gas. In particular, when exposed to 0.5 ppm acetone gas, as-synthesized SnO2 nanowires present an extremely low response — close to negligible. However, when Co3O4 nanoparticles are attached, the response is improved drastically. Furthermore, the sensing selectivity toward acetone gas is improved compared with its counterpart. This improved sensing property is derived from the increasing variation in the surface depletion area located in the p-n heterojunction.

Oxidative dehydrogenation of ethane over M1 MoVNbTeO catalysts modified by the addition of Nd, Mn, Ga or Ge

Modification by Nd, Mn, Ga, and Ge substantially affect both physicochemical and catalytic properties of MoVNbTeO oxides in the oxidative dehydrogenation of ethane (ODE). The introduction of modifying additives changes the content of the M1 phase, surface acidity, and surface composition. It does not have an influence on the state of Mo, V, Nb, and Te on catalyst surface, but it induces surface enrichment (Ga, Mn) or depletion (Ge, Nd) by the modifying element. All of these additives have positive effects on catalytic properties at the M/Mo (M = modifying element) ratio of 0.0075 ÷ 0.025. They exhibit an increase of activity and prevent ethylene over-oxidation. The relationship between the rate constant of total ethane transformation and the M1 content confirms that this phase is active in ODE. The correlation between ethylene selectivity and the relative number of intermediate acid sites is revealed, indicating the role of surface acidity on ethylene over-oxidation.

Size effect of electronic properties in highly arsenic-doped silicon nanowires

The unique electrostatic properties of semiconductor nanowires enable the realization of novel transistor types by the possibility to use surround gate architectures resembling ideal gate electrostatic control. Nevertheless one fundamental issue of semiconducting nanowire channels is the reliable control of doping to adjust the charge carrier concentration. Indeed, as dimensions scale down the surrounding media and the interfaces become more important. In this study we experimentally investigate the role of surface depletion and dielectric mismatch on the electronic charge transport of highly arsenic doped and bottom-up grown silicon nanowires. Electrical characterization of silicon nanowires (SiNWs) synthesized by Au catalyzed vapour-liquid-solid (VLS) growth and in-situ arsine (AsH3) doping is reported for the first time. We demonstrate that high n-type doping is possible by adjusting the dopant precursor flow ratio during growth. Based on electrical measurements of individual nanowires, reproducible donor concentrations of up to 5.2 × 1019 cm−3 could be revealed. By measuring the electrical characteristics for individual nanowires in dependence of their radius, we show that the electrically active carrier density drastically reduces for small nanowires at radii much larger than those at which quantization or dopant surface segregation effects are expected to occur. Furthermore, enhancement of the contact transparency for small radii nanowires is demonstrated through dopant segregation upon metal silicidation. Size dependent measurement of electrical characteristics revealed improved contact resistivities as low as 1.4 × 10−11 Ωm2.