Arsenic Flux Research Articles

Groundwater nanofiltration process efficiency improvement with additional concentrate membrane treatment

Nanofiltration (NF) of waste water originated from nanofiltration drinking water plant stationed in town of Kikinda (Northern Serbia) was investigated. Experiments on removal characteristics of nanofiltration membranes when exposed to influent rich in arsenic and natural organic matter under different flux and transmembrane pressure (TMP) conditions were conducted in order to obtain drinking water from waste water and reduce total amount of waste water. Applied NF membranes showed remarkable removal characteristic. Also, obtained result for concentrate yield, an indicator of reduced concentrate amount, of 8.89% under optimum flux value presents considerable amount of reclamated drinking water. Calculated empirical and theoretical concentration factors quotients were indicated to probable accumulation of some inlet water components at NF membrane surface. Based on the obtained results that nanofiltration membranes remove natural organic matter, arsenic, ammonia and sodium with high efficiencies of 98%, 96%, 80% and 93% respectively, conceptual design of drinking water plant for City of Kikinda was defined and presented.

Arsenic speciation and diffusion flux in Danshuei Estuary sediments, Northern Taiwan

Sedimentary As, Fe and Mn species in core sediments from the Danshuei Estuary, an anoxic estuary, were studied using a five-sequential extraction method followed by chemical analyses of these elements. The sediment texture, redox potential and dissolved concentrations of these elements in pore water were also determined. Dissolved As diffusion flux was also calculated. Higher concentrations of sedimentary As were found at the middle and lower estuary stations. The sedimentary As species in the samples were dominated by three fractions: amorphous and poorly-crystalline; and well crystallized hydrous oxides of Fe and Al; and residual phases, accounting for 90% of the total As pool. The geochemical behavior of sedimentary As in the Danshuei Estuary is strongly influenced by the redox potential and Fe oxides. The dissolved As concentrations in pore water correlated inversely with the redox potential. The sediment served as a source of dissolved As and the diffusion flux ranged from 0.047–0.35μgcm−2y−1.

Solar-driven flash vaporization membrane distillation for arsenic removal from groundwater: Experimental investigation and analysis of performance parameters

Experimental studies on removal of arsenic from contaminated groundwater were investigated using solar energy for phase change in a new flash vaporization membrane distillation (FVMD) module. The module was fitted with the composite flat-sheet polytetrafluoroethylene (PTFE) membrane. The highest flux (52.94kg/m2h) with more than 99% arsenic rejection was achieved with this cross-flow FVMD module working in the direct contact membrane distillation (DCMD) configuration. In order to evaluate the performance of solar-driven membrane distillation process, it has been analyzed the variation of the flux and the arsenic rejection with the feed and distillate temperatures, the distillate flow rate, the arsenic concentration in the feed, and the operating hours. The study also shows the effects of the performance parameters of the FVMD module and the solar collector with feed temperature. Arsenic removal efficiency and flux at enhanced temperature of around 70°C almost remained constant with the arsenic concentration variation and after 40h of operation, whereas the significant effect on flux was noticed during variation of the distillate flow rate, the feed and distillate temperatures. The present investigation helps one to arrive at a better system configuration that can be effectively implemented in the arsenic affected areas.

Formation of GaAs nanostructures by droplet epitaxy—Monte Carlo simulation

Abstract A kinetic Monte Carlo model of droplet epitaxy is suggested and realized. A basis for the Monte Carlo model is the vapor–liquid–solid mechanism. The proposed model was used for the analysis of GaAs nanostructure formation mechanism during crystallization of Ga drops under the arsenic flux. The dependences of the grown structure morphology on temperature, As 2 flux intensity and GaAs substrate surface orientation were analyzed. Depending on temperature and arsenic flux intensity, different shapes of nanostructures (crystal dots, core–shell compact clusters, nanoholes and nanorings) were achieved. A lack of etching of GaAs substrates with (1 1 1)А and (1 1 1)В surface orientations by liquid gallium was revealed. Nanohole and nanoring formation was observed only on substrates with (0 0 1) surface orientation. The kinetics of nanoring formation was examined.

Towards van der Waals Epitaxial Growth of GaAs on Si using a Graphene Buffer Layer

Van der Waals growth of GaAs on silicon using a two‐dimensional layered material, graphene, as a lattice mismatch/thermal expansion coefficient mismatch relieving buffer layer is presented. Two‐dimensional growth of GaAs thin films on graphene is a potential route towards heteroepitaxial integration of GaAs on silicon in the developing field of silicon photonics. Hetero‐layered GaAs is deposited by molecular beam epitaxy on graphene/silicon at growth temperatures ranging from 350 °C to 600 °C under a constant arsenic flux. Samples are characterized by plan‐view scanning electron microscopy, atomic force microscopy, Raman microscopy, and X‐ray diffraction. The low energy of the graphene surface and the GaAs/graphene interface is overcome through an optimized growth technique to obtain an atomically smooth low­ temperature GaAs nucleation layer. However, the low adsorption and migration energies of gallium and arsenic atoms on graphene result in cluster‐growth mode during crystallization of GaAs films at an elevated temperature. In this paper, we present the first example of an ultrasmooth morphology for GaAs films with a strong (111) oriented fiber‐texture on graphene/silicon using quasi van der Waals epitaxy, making it a remarkable step towards an eventual demonstration of the epitaxial growth of GaAs by this approach for heterogeneous integration.

Selective growth of InAs quantum dots on GaAs driven by as kinetics

In this paper we examine experimentally and theoretically how the As flux direction and intensity influences the recently observed selective growth of InAs quantum dots on the rippled surface of GaAs, where the dots line up only against well determined slopes of the mounded GaAs surface facing the impinging Arsenic flux. We have previously shown that the observed phenomenon is exclusively related to the As constituent, challenging the widespread belief that As plays only a minor role in the dot formation. The selective growth is obtained by Molecular Beam Epitaxy at high growth temperature and under a high As/In flux ratio. To interpret the experimental results, we use a newly developed kinetic model incorporating new features: (i) anions are explicitly considered; (ii) cations and anions follow a different kinetics; (iii) the dot surface is distinguished by the dot bulk and atoms are transferred back and forth between the two phases depending on the surface interaction with the environment. We find that even a very small difference in the As flux intensity impinging over two substrate regions produces a cation current flow from one region to the other, so that the dots form only on the region exposed to the largest flux.

Fundamental role of arsenic flux in nanohole formation by Ga droplet etching on GaAs(001).

Nanoholes with a depth in the range of tens of nanometers can be formed on GaAs(001) surfaces at a temperature of 500°C by local etching after Ga droplet formation. In this work, we demonstrate that the local etching or nanodrilling process starts when the Ga droplets are exposed to arsenic. The essential role of arsenic in nanohole formation is demonstrated sequentially, from the initial Ga droplets to the final stage consisting of nanoholes surrounded by ringlike structures at the surface and Ga droplets consumed. The kinetics of local etching depends on the arsenic flux intensity, while the ringlike structures are basically the same as those formed underneath the droplets in the absence of arsenic. These structures show motifs with well-defined crystalline facets that correspond to those expected from surface energy minimization. These experimental results are qualitatively analyzed for a better understanding of the nanohole formation underlying processes.

Optical properties of Be-doped GaAs nanowires on Si substrate grown by a catalyst-free molecular beam epitaxy vapor–liquid–solid method

The effect of Be doping on the crystal structure and the optical properties of catalyst-free Be-doped GaAs nanowires (NWs) grown on a (111)Si substrate were investigated by scanning transmission electron microscopy (STEM), X-ray diffraction (XRD) analysis, and photoreflectance (PR) and photoluminescence (PL) techniques. Be-doped NWs sample showed a striped pattern in STEM images. Owing to a high arsenic flux under the growth condition, the sample did not have a wurtzite but a zinc-blend (ZB) structure, and the observed striped pattern in STEM images suggests the presence of twin boundaries and stacking faults. The bandgap energy of the Be-doped NWs was lower than that of the nondoped NWs sample in the entire temperature range. In addition, the deviation from the conventional Varshni curve was found to be large in the low-temperature region. However, this is well explained by considering the strong electron–phonon interaction and high average phonon temperature. Therefore, we concluded that the induced strain increased the linear thermal expansion coefficient. This is because the Be atom has a smaller covalent radius than the Ga and As atoms.

Natural wetland emissions of methylated trace elements

Natural wetlands are well known for their significant methane emissions. However, trace element emissions via biomethylation and subsequent volatilization from pristine wetlands are virtually unstudied, even though wetlands constitute large reservoirs for trace elements. Here we show that the average volatile fluxes of selenium (<0.12 μg m(-2) day(-1)), sulphur (<37 μg m(-2) day(-1)) and arsenic (<0.54 μg m(-2) day(-1)) from a pristine peatland are considerable and consistent over two summers. We compare these fluxes with the total concentrations in the peat and show that selenium is up to 40 times more efficiently volatilized than arsenic, and over 100 times more efficiently volatilized than sulphur. We further show that the volatilization of selenium and arsenic increases with temperature, implying that emissions of these health-relevant trace elements will increase with global warming. We suggest that biomethylation and volatilization in wetlands play a crucial role in the mobilization and global biogeochemical cycling of trace elements.

Kinetically driven selective growth of InAs quantum dots on GaAs

Abstract

Effect of an arsenic flux on the molecular-beam epitaxy of self-catalytic (Ga,Mn)As nanowire crystals

The effect of an arsenic flux on the growth rate of self-catalytic (Ga,Mn)As nanowire crystals is studied. It is shown that, at low arsenic fluxes, nanowire-crystal growth is limited by the crystallization rate of the material below the droplet. However, at high arsenic fluxes, the growth kinetics are controlled by gallium transport into the droplet. It is experimentally demonstrated that, at low arsenic fluxes, the dependence of the nanowire length on the nanowire diameter is a steadily increasing function adequately described by Givargizov-Chernov’s model. At the same time, a steadily decreasing diffusion dependence is observed at high arsenic fluxes.

Crystalline and Glassy Phases in the Cs/Bi/As/S System

The quaternary Cs2S/Bi/As/S system was studied in an attempt to introduce two different asymmetric but isoelectronic building units, namely, [Bi(III)(x)S(y)] and [As(III)(x)S(y)], in a single structure. Reactions with a comparatively lower equivalent of arsenic in the Cs2S/Bi/As/S mixture led to the crystalline compound Cs3Bi(AsS4)2. The structure features tetrahedral [As(V)S4](3-) connected to Bi(III) centers to give infinite (1/∞)[Bi(AsS4)2(3-)] chains. When the basicity was raised in these low arsenic fluxes by increasing the Cs2S fraction, the crystalline compound Cs9Bi(AsS4)4, also featuring [As(V)S4](3-) anions, was formed. On the other hand, arsenic-rich mixtures of Cs2S/Bi/As/S led to the formation of the glassy phase Cs2BiAs3S7, which contains As(III) species. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and pair distribution function (PDF) analysis indicate the presence of As(III)-containing [AsnS2n+1] fragments in the glass structure. Several glasses in the series Cs(n-1)BiAs(n)S(2n+1) were also prepared using solid-state fusion reactions. The band gaps of the Cs(n-1)BiAs(n)S(2n+1) glasses are in the range of 1.51-1.81 eV, while that of the crystalline compound Cs3Bi(AsS4)2 is ~2.33 eV. The thermal and optical behaviors of these compounds are correlated with their structures and building units.

The Unexpected Role of Arsenic in Driving the Selective Growth of InAs Quantum Dots on GaAs

Here we show a new effect due to the arsenic flux in the molecular beam epitaxy growth of InAs quantum dots on GaAs(001) at temperatures higher than 500 °C and high As/In flux ratio. We show that, by changing and tuning the direction of the As flux on a rippled substrate, a selective growth can be obtained where the dots form only on some appropriately orientated slopes of a sequence of mounds elongated along the [110] surface direction. Since the relative As flux intensity difference over the two opposite mound slopes is very small (2-5%), the observed large effect cannot be explained simply as a pure shadowing effect and reveals instead that As, whose contribution to the modeling of growth has often been ignored or underestimated, probably for a lack of knowledge, plays a fundamental role at these growth conditions. To explain our experiment, we have developed a kinetic model that explicitly takes into account the coupling between cations (In) and anions (As) and found that the very small surface gradient in the anion flux, due to the oblique evaporation on the mounded surface, is responsible for a massive drain of cations toward the surface anion-rich areas, thus generating the selective growth of quantum dots. We expect a comparable behavior for the anions of other III-V and II-VI compound semiconductors.

Changes in catchment conditions lead to enhanced remobilization of arsenic in a water reservoir

Increasing arsenic concentrations in freshwater ecosystems is of global concern. Processes affecting arsenic fluxes in catchments are known. These processes are in turn controlled by the underlying geology and air pollution history. In contrast to the knowledge on catchment processes less is known about the hydrochemical processes controlling the fixation/remobilization of arsenic within lakes and artificial reservoirs. Consequently, we examined a reservoir system in the Ore Mts. (Germany) regarding its sink and source potentials affecting arsenic fluxes. This area was faced with heavy deposition inputs from coal burning based acid rain until the beginning of the 1990s. Hereafter concentrations of sulfate and nitrate in runoff waters decreased, whereas dissolved organic carbon (DOC) concentrations are still increasing. Along with this, arsenic concentrations in the water discharge from the catchments increase. Our results reveal that the sediments of the investigated reservoir system contain high inventories of arsenic in association with ferric and organic phases. A nitrate deficit dependent arsenic release is suggested. It is indicated that arsenic release from the reservoir sediments may be controlled by water nitrate concentration, which in turn is dependent on the nitrate concentration in the runoff water from the catchment.

Long-Wavelength Photoluminescence from Stacked Layers of High-Quality Type-II GaSb/GaAs Quantum Rings

We report the successful molecular-beam epitaxial growth of 10 stacked layers of GaSb/GaAs quantum rings using a new procedure. Exact control of the arsenic flux during capping helps to reduce the strong group-V As–Sb exchange reactions, enabling the rings to be capped at the same growth temperature (480 °C) without dissolution. X-ray diffraction and transmission electron microscopy indicate excellent structural quality and uniformity with no threading dislocations. This is due to the reduction in the average strain through the quantum-ring formation. The total ring density in the stacks is 1 × 1011 cm–2. An unusually long-wavelength quantum-ring photoluminescence peak of 1.3 μm is observed at low temperature, which is attributed to a reduction in the quantum-ring charging due to lower unintentional p-doping in the GaAs cap layer. The impact that this effect will have on future device designs in solar cells and lasers is also discussed.

Structural properties of ultra-low density nanoholes for the generation of well-separated GaAs quantum dots

We apply the local droplet etching (LDE) technique in a conventional molecular beam epitaxy system for self-organized nanohole formation on AlGaAs surfaces. By optimizing the arsenic flux during the Al droplet deposition the hole density can be reduced down to the 106cm−2 regime. The nanohole shape and density are tunable in a wide range by the amount of droplet material deposited and the initial holes are customized by overgrowth with AlGaAs of varied thickness. Nanoholes with controlled depths from 30 down to a few nanometers are fabricated, while the ultra-low density is sustained. These nanoholes are filled with GaAs to form well separated GaAs QDs, with size precisely controlled by the filling level. Studies of the optical emission of single dots exhibit sharp excitonic lines at low and a clear shell structure at high excitation power. In particular, the quantization energies in dependence of the QD size are measured.

Atmospheric Arsenic Deposition in Chiayi County in Southern Taiwan

Although arsenic contamination of underground water in southern Taiwan is well known, few studies examine atmospheric arsenic deposition in this area, which might be the major source of such pollution to the soil, water, and even underground water. This research focused on the atmospheric arsenic concentration, dry and wet depositions, and the As distribution around Chiayi County, located in the south of Taiwan. Eight sampling sites are used, both upwind and downwind of an area with heavy industrial and human activities. All samples were collected by a PS-1 high volume sampler at each site, pretreated by a digestion process, and further analyzed with an ICP-MS. The results show that the arsenic deposition flux ranged from 65.0 to 473 μg/m2-month in Chiayi County. This deposition flux has no significant seasonal variation, based on the multiple trend results obtained from pooling the dry and wet deposition data. The average dry deposition flux of As ranged from 34 to 161 μg/m2-month during the sampling year, and had the opposite trend to the wind speed. Additionally, the correlations between atmospheric arsenic concentration and PM levels were significant, supported by the low p-value (< 0.01) at a 99% confidence level. Meanwhile, the highly linear correlation between As and PM concentrations was also evaluated. The amount of precipitation was statistically correlated to the wet deposition flux (11.3–414 μg/m2-month with a p-value = 2 × 10–7 and r-value = 0.9306. Furthermore, the composition of the As mass concentrations in ambient air were dominated by As (V) in both fall (66.4%) and winter (68.9%). According to the GIS spatial analysis of the As concentration in the dry season, from October to March, the As contribution of local emissions were not significant. Consequently, this study indicates that the atmospheric As around Chiayi County might be sourced from the upwind northern area, and mainly deposited through wet scavenging.

Arsenic Pathways in Self-Catalyzed Growth of GaAs Nanowires

Self-catalyzed growth of GaAs nanowires by molecular beam epitaxy on (111)Si substrates is investigated by introducing AlxGa1–xAs time markers. The nanowire elongation rate is found to be radius-independent, constant at substrate temperatures below 650 °C and linearly increasing with the incoming arsenic flux. The basic question of which pathways are followed by the arsenic species contributing to nanowire growth is clarified. The flow rate of As atoms directly impinging on the Ga catalyst drop is significantly smaller than the As consumption by nanowire growth. Thus, supplementary As atoms are necessary to explain the actual elongation rate. We show that surface diffusion of adsorbed Asx species toward the catalyst cannot account for the missing atoms. On the other hand, the reevaporation of Asx species from the substrate and from nanowire sidewall surfaces can act as an efficient secondary arsenic source. We argue that a sufficient amount of these species can be intercepted by the Ga drop and add up with the direct As impingement to explain the actual elongation rate.

Probability of twin formation on self-catalyzed GaAs nanowires on Si substrate

We attempted to control the incorporation of twin boundaries in self-catalyzed GaAs nanowires (NWs). Self-catalyzed GaAs NWs were grown on a Si substrate under various arsenic pressures using molecular beam epitaxy and the vapor-liquid-solid method. When the arsenic flux is low, wurtzite structures are dominant in the GaAs NWs. On the other hand, zinc blende structures become dominant as the arsenic flux rises. We discussed this phenomenon on the basis of thermodynamics and examined the probability of twin-boundary formation in detail.

Highly versatile ultra-low density GaAs quantum dots fabricated by filling of self-assembled nanoholes

GaAs quantum dots (QDs) of ultra-low density (ULD) are fabricated by filling of nanoholes in AlGaAs surfaces. The holes are drilled using self-assembled local droplet etching with Al droplets. By precisely controlling the arsenic flux and the substrate temperature, the QD density is reduced down to the 106 cm−2 range uniformly over the whole wafer. The QD size is precisely adjustable via the hole filling level. By this, the optical emission energy of the QDs can be adjusted over a wide energy range of at least 120 meV. The surface visibility of ULD QDs allows their simple integration into lithographic processing.