Si Desorption Research Articles

Benthic silicon cycling in the Arctic Barents Sea: a reaction–transport model study

Abstract. Over recent decades the highest rates of water column warming and sea ice loss across the Arctic Ocean have been observed in the Barents Sea. These physical changes have resulted in rapid ecosystem adjustments, manifesting as a northward migration of temperate phytoplankton species at the expense of silica-based diatoms. These changes will potentially alter the composition of phytodetritus deposited at the seafloor, which acts as a biogeochemical reactor and is pivotal in the recycling of key nutrients, such as silicon (Si). To appreciate the sensitivity of the Barents Sea benthic system to the observed changes in surface primary production, there is a need to better understand this benthic–pelagic coupling. Stable Si isotopic compositions of sediment pore waters and the solid phase from three stations in the Barents Sea reveal a coupling of the iron (Fe) and Si cycles, the contemporaneous dissolution of lithogenic silicate minerals (LSi) alongside biogenic silica (BSi), and the potential for the reprecipitation of dissolved silicic acid (DSi) as authigenic clay minerals (AuSi). However, as reaction rates cannot be quantified from observational data alone, a mechanistic understanding of which factors control these processes is missing. Here, we employ reaction–transport modelling together with observational data to disentangle the reaction pathways controlling the cycling of Si within the seafloor. Processes such as the dissolution of BSi are active on multiple timescales, ranging from weeks to hundreds of years, which we are able to examine through steady state and transient model runs. Steady state simulations show that 60 % to 98 % of the sediment pore water DSi pool may be sourced from the dissolution of LSi, while the isotopic composition is also strongly influenced by the desorption of Si from metal oxides, most likely Fe (oxyhydr)oxides (FeSi), as they reductively dissolve. Further, our model simulations indicate that between 2.9 % and 37 % of the DSi released into sediment pore waters is subsequently removed by a process that has a fractionation factor of approximately −2 ‰, most likely representing reprecipitation as AuSi. These observations are significant as the dissolution of LSi represents a source of new Si to the ocean DSi pool and precipitation of AuSi an additional sink, which could address imbalances in the current regional ocean Si budget. Lastly, transient modelling suggests that at least one-third of the total annual benthic DSi flux could be sourced from the dissolution of more reactive, diatom-derived BSi deposited after the surface water bloom at the marginal ice zone. This benthic–pelagic coupling will be subject to change with the continued northward migration of Atlantic phytoplankton species, the northward retreat of the marginal ice zone and the observed decline in the DSi inventory of the subpolar North Atlantic Ocean over the last 3 decades.

Competitive and noncompetitive adsorption of silicate and phosphate by two acid Si-deficient soils and their effects on P and Si extractability

ABSTRACTThe effects of pH on the adsorption of silicate and phosphate, either singly or in competition, by two acid soils were investigated. Both soils adsorbed two to three times more P than Si and adsorption isotherms at pH 5.0, 5.5, 6.0 and 6.5 showed that increasing pH greatly increased Si adsorption but decreased that of P. Silicate adsorption was very low below pH 5.0, increased rapidly up to pH 9–10 before decreasing again. Adsorption of P was at a maximum at pH 2.0, decreased slowly up to pH 7.0 and then more rapidly above pH 7.0. When Si and P were added at equimolar concentrations, the presence of P decreased Si adsorption between pH 6.0 and 8.0 while the presence of Si decreased P adsorption in the pH region 6.0 and 11. Addition of calcium silicate at rates equivalent to 300, 600 and 1200 kg Si ha−1 resulted in a progressive increase in soil pH. Separate samples of soil were treated with Ca(OH)2 to give the same pH values so that the effect of Si could be identified. The highest rate of Si (1200 kg ha−1 which gave a pH of 6.5) caused a significant decrease in P adsorption (as determined by adsorption isotherms) and an increase in resin-extractable P but the lower rates had little effect. Addition of P to the soil as calcium phosphate at rates equivalent to 30, 60 and 100 kg P ha−1 all caused a decrease in Si adsorption capacity and an increase in CaCl2-extractable Si. It was concluded that the strategy of adding Si to lower P requirements in acid soils is not likely to be effective while addition of fertilizer P may well lower Si adsorption and promote Si desorption and its increased mobility.

Groundwater Arsenic and Silicate Adsorption on TiO2 and the Regeneration of TiO2

Titanium dioxide (TiO2) was widely used to remove arsenic (As) from groundwater due to its excellent properties. Previous studies show that the coexisting silicate ions (Si) could occupy the available surface sites of TiO2 and further inhibit As adsorption and TiO2 regeneration. To investigate the effect of Si adsorption on the As molecular surface structure, an extended X-ray absorption fine structure (EXAFS) analysis was conducted in this work. The results indicated that the presence of Si exhibited no impact on the As adsorption configuration on TiO2. In situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy results demonstrated that the polymerization of Si that formed on the TiO2 surface compete with As adsorption sites, increasing the difficulty for TiO2 regeneration. To effectively regenerate TiO2, the removal efficiency of Si polymers on TiO2 via sodium fluoride (NaF) was studied. The results showed that NaF could remove Si monomer and polymer from TiO2, and the regenerated TiO2 could be reused with a stable adsorption performance. In situ ATR-FTIR spectroscopy suggested that NaF desorbed the Si monomer and polymer effectively. When spent TiO2 was regenerated with NaOH and NaF in three treatment cycles, As and Si desorption rates were 86.8%-100.3% and 67.9%-82.0%, respectively. The present study provides a new insight into regenerating absorbents with coadsorbed As and Si in groundwater.

Selenidation of epitaxial silicene on ZrB2

Abstract The deposition of elemental Se on epitaxial silicene on ZrB 2 thin films was investigated with synchrotron-based core-level photoelectron spectroscopy and low-energy electron diffraction. The deposition of Se at room temperature caused the appearance of Si 2 p peaks with chemical shifts of n × 0.51 ± 0.04 eV (n = 1-4), suggesting the formation of SiSe 2 . This shows that capping the silicene monolayer, without affecting its structural and electronic properties, is not possible with Se. The annealing treatments that followed caused the desorption of Se and Si, resulting in the etching of the Si atoms formerly part of the silicene layer, and the formation of bare ZrB 2 (0001) surface area. In addition, a ZrB 2 (0001)-(√7 × 3)R40.9° surface reconstruction was observed, attributed to a Se-termination of the surface of the transition metal diboride thin film.

Formation of “negative” silicon whiskers

An approach has been proposed for producing “negative” whiskers in Si wafers which is based on the thermal migration of silicon–metal (Si + M) melt droplets in the field of a transverse temperature gradient. We have obtained regular systems of negative whiskers in the form of through holes in Si {111} wafers. It has been shown that the thermal migration of Si + M melt droplets is driven by the difference between the chemical potentials of Si in the liquid and crystalline phases, which arises from the difference in curvature between the liquid/vapor and solid/liquid interfaces and leads to Si dissolution and desorption.

Microscopic Mechanisms of Initial Formation Process of Graphene on SiC(0001) Surfaces: Selective Si Desorption from Step Edges

Simulations within the density functional theory framework clarify the microscopic mechanism responsible for the initial stage of graphene formation on the SiC(0001) surface. Favorable reaction pathways for the desorption of either Si or C atoms from the stepped surface have been found by determining the desorption and the subsequent migration pathways, quantified in terms of the corresponding energy barriers for the first time. We find that the energy barrier for the desorption of an Si atom at the step edge and the subsequent migration toward stable terrace sites is lower than that of a C atom by 0.75 eV, indicative of the selective desorption of Si from the SiC surface. We also find that the subsequent Si desorption is an exothermic reaction. This exothermicity comes from the energy gain due to the bond formation of C atoms being left near the step edges. This is likely to be a seed of graphene flakes.

Stability and Reactivity of [11-20] Step in Initial Stage of Epitaxial Graphene Growth on SiC(0001)

The energetics for the Si desorption and the C adsorption at a [11-20] step on SiC(0001) surface are studied using the first-principles calculation. It is found that the [11-20] step is stable and nonreactive. The stability of the step is thought to govern the surface morphology during the graphene formation. It is shown that the Si pressure and the temperature are the control parameters for the surface morphology and the graphene quality.

Si-adatom kinetics in defect mediated growth of multilayer epitaxial graphene films on 6H-SiC

We present a quantitative study on the growth of multilayer epitaxial graphene (EG) by solid-state decomposition of SiC on polar (c-plane Si and C-face) and non-polar (a and m planes) 6H-SiC faces, with distinctly different defect profiles. The growth rates are slower than expected from a mechanism that involves Si loss from an open and free surface, and much faster than expected for the nucleation of a defect-free EG layer, implying that defects in the EG play a critical role in determining the growth kinetics. We show that a Deal-Grove growth model, which assumes vertical diffusion of Si through these defects as the limiting factor for EG growth, is unsuitable for describing multilayer growth. Instead, we introduce a lateral “adatom” diffusion mechanism for Si out-diffusion, based on a modified Burton, Cabrera, and Frank model. In this model, defects in epitaxial graphene serve as sinks for Si desorption loss, taking the place of reactive sites, such as step edges for nucleation and growth of crystals produced with external precursors. This analysis shows that the surface diffusion of Si atoms to the grain boundaries of EG limits the growth on c-plane C-face and non-polar faces, rather than the purely vertical diffusion of Si through the grain boundaries described in the Deal-Grove model. However, for Si-face c-plane growth, diffusion of Si to the defects, as well as desorption of Si at the grain boundaries are both relevant, leading to a different temperature trend compared with the other faces. This distinct qualitative difference is ascribed to point-defects in Si-face growth, as contrasted with line defects/grain boundaries on the other faces. The size of the EG grains correlates with the surface diffusion length extracted from this model. The longer a Si adatom diffuses, the higher the quality of the grown EG film, an insight that provides valuable information on Si adatom kinetics for optimizing EG growth. We discuss the applicability of this model to growth of multilayer EG in an argon ambient at atmospheric pressure.

Phosphorus Profile Control in Ge by Si Delta Layers

The impact of Si spike layer on phosphorus diffusion in germanium is investigated. Ge cap/in-situ P-doped Ge/Ge buffer is deposited and post-annealed using PRCVD tool. In the case of samples without Si spikes, P diffusion/segregation and desorption from the Ge surface happened during Ge cap layer deposition at 550C resulting in dopant dose reduction and profile broadening. By interposing P-doped Ge by Si spikes, P diffusion is suppressed. The diffused P is piled-up at the position of the Si spikes. The P diffusion suppression effect by the Si spike is observed after postannealing even at 650C. XRD-RSM shows that the Si spike is pseudomorphic to Ge lattice. After postannealing, Si diffusion is observed but no Ge crystal degradation was evident. Enhanced Si diffusion is observed by presence of P. Based on these results, the P diffusion suppression seems to be caused by the replacement of substitutional Si by diffused P.

Stratigraphic changes of Ge/Si, REE+Y and silicon isotopes as insights into the deposition of a Mesoarchaean banded iron formation

In order to determine the origin of silicon (Si) in banded iron formation (BIF), we have undertaken a multi-tracer study combining REE+Y data, Ge/Si ratios and Si isotopes (δ30Si) on stratigraphically resolved layers from a ∼2.95Ga BIF from the Pongola Supergroup, South Africa. Si in both Si-rich and Fe-rich layers has a common origin, represented by a seawater reservoir strongly influenced by continent-derived freshwaters (∼10%) and very limited (<0.1%) high-T hydrothermal fluids as indicated by Eu anomalies and Y/Ho ratios. The coevolution of δ30Si signatures of Si- and Fe-rich layers of the BIF coupled with similar Eu and Y anomalies in both types of layers is in accordance with a common silica precipitation promoted by Si adsorption onto Fe-oxyhydroxides from Archaean seawater. An increase in δ30Si values from −2.27‰ to −0.53‰ stratigraphically upwards in the BIF is inferred to be the result of two successive isotopic fractionation processes during (1) silicon adsorption onto the Fe-oxyhydroxide precursor and (2) silica precipitation at the sediment–water interface from pore fluid triggered by the local silica saturation consecutive to an early diagenetic Si desorption from the precursor Fe-oxyhydroxide. The first fractionation process depleted the parental water in 28Si while the second released 30Si back into the parental water, resulting in an increase of the δ30Si value of the parental water reservoir over time.

Surface complexation modeling of groundwater arsenic mobility: Results of a forced gradient experiment in a Red River flood plain aquifer, Vietnam

Three surface complexation models (SCMs) developed for, respectively, ferrihydrite, goethite and sorption data for a Pleistocene oxidized aquifer sediment from Bangladesh were used to explore the effect of multicomponent adsorption processes on As mobility in a reduced Holocene floodplain aquifer along the Red River, Vietnam. The SCMs for ferrihydrite and goethite yielded very different results. The ferrihydrite SCM favors As(III) over As(V) and has carbonate and silica species as the main competitors for surface sites. In contrast, the goethite SCM has a greater affinity for As(V) over As(III) while PO43− and Fe(II) form the predominant surface species. The SCM for Pleistocene aquifer sediment resembles most the goethite SCM but shows more Si sorption. Compiled As(III) adsorption data for Holocene sediment was also well described by the SCM determined for Pleistocene aquifer sediment, suggesting a comparable As(III) affinity of Holocene and Pleistocene aquifer sediments. A forced gradient field experiment was conducted in a bank aquifer adjacent to a tributary channel to the Red River, and the passage in the aquifer of mixed groundwater containing up to 74% channel water was observed. The concentrations of As (<0.013μM) and major ions in the channel water are low compared to those in the pristine groundwater in the adjacent bank aquifer, which had an As concentration of ∼3μM. Calculations for conservative mixing of channel and groundwater could explain the observed variation in concentration for most elements. However, the mixed waters did contain an excess of As(III), PO43− and Si which is attributed to desorption from the aquifer sediment. The three SCMs were tested on their ability to model the desorption of As(III), PO43− and Si. Qualitatively, the ferrihydrite SCM correctly predicts desorption for As(III) but for Si and PO43− it predicts an increased adsorption instead of desorption. The goethite SCM correctly predicts desorption of both As(III) and PO43− but failed in the prediction of Si desorption. These results indicate that the prediction of As mobility, by using SCMs for synthetic Fe-oxides, will be strongly dependent on the model chosen. The SCM based on the Pleistocene aquifer sediment predicts the desorption of As(III), PO43− and Si quite superiorly, as compared to the SCMs for ferrihydrite and goethite, even though Si desorption is still somewhat under-predicted. The observation that a SCM calibrated on a different sediment can predict our field results so well suggests that sediment based SCMs may be a feasible way to model multi-component adsorption in aquifers.

In situ high-temperature scanning tunneling microscopy study of bilayer graphene growth on 6H-SiC(0001)

Using in situ high-temperature (1395K), ultra-high vacuum, scanning tunneling microscopy (STM), we investigated the growth of bilayer graphene on 6H-SiC(0001). From the STM images, we measured areal coverages of SiC and graphene as a function of annealing time and found that graphene grows at the expense of SiC. Graphene domains were observed to grow, at comparable rates, at (I) graphene-free SiC step edges, (II) graphene–SiC interfaces, and (III) the existing graphene domain edges. Based upon our results, we suggest that the rate-limiting step controlling bilayer graphene growth is the desorption of Si from the substrate.

Si diffusion path for pit-free graphene growth on SiC(0001)

Density functional theory calculations reveal that the interfacial 6$\sqrt{3}$ \ifmmode\times\else\texttimes\fi{} 6$\sqrt{3}$ structure [a warped graphene layer with periodic inclusions of pentagon-hexagon-heptagon (H${}_{5,6,7}$) defects] facilitates a Si diffusion path vertically through the interface layer during epitaxial growth of graphene on SiC(0001). The calculated diffusion barrier is 4.7 eV, competitive with Si interstitial diffusion of \ensuremath{\sim}3.5 eV in SiC [M. Bockstedte et al., Phys. Rev. B 68, 205201 (2003)]. Scanning tunneling microscopy study shows that, for growth in an Ar background, where Si desorption is suppressed and all diffusion channels contribute, graphene films with reduced pit density can be grown on nominally flat SiC substrates. On the other hand, for Si diffusion-limited growth in ultrahigh vacuum, the Si interstitial diffusion is the energetically favorable path where the step edges serve as the necessary outlet toward Si desorption. The much higher density of step edges on vicinal substrates also facilitates the growth of pit-free graphene.

Epitaxial Growth of Graphene on SiC Surfaces

Graphene is epitaxially grown on vicinal SiC(0001) surfaces via high temperature annealing in vacuum. To obtain higher quality graphene films and to investigate growth mechanisms it is beneficial to use well-defined SiC surface structures, which can be achieved by high temperature hydrogen etching before graphitization. The SiC surfaces with the periodic nanofacet structure resulted from the etching are thermally decomposed and graphitized to form epitaxial graphene layers. Temperature and time dependences of graphene growth are mainly investigated by means of low energy electron microscopy (LEEM). The graphene growth is accomplished by the layer-by-layer mode with strong anisotropy in the direction parallel to the steps and is limited by Si desorption from the surface/interface, where the 0th layer (buffer layer) is nucleated.

Molecular dynamics simulations of large fluorine cluster impact on silicon with supersonic velocity

The etching process by very large reactive gas cluster impact was investigated by molecular dynamics (MD) simulations. Fluorine-molecule clusters with the size up to 100,000atoms (50,000 F2 molecules) were irradiated on silicon (100) targets at supersonic velocity regime (0.1–1eV/atom, 1.0–3.2km/s). The MD simulations revealed that the existence of threshold energy-per-atom around 0.3eV/atom (1.75km/s) to cause surface deformation and enhancement of Si desorption. When the incident energy-per-atom is less than the threshold, the incident cluster breaks up itself on the target without surface deformation. The fluorine molecules in the cluster spread in the lateral direction along the target surface, and some part of them decompose and adsorbs on the target to form silicon fluoride composites. On the other hand, the clusters penetrate the surface of silicon target when the energy-per-atom is larger than 0.3eV/atom. In these collisional processes, the target surface is deformed to create shallow crater shape. The incident fluorine molecules are preferentially concentrated at the bottom of the crater, which resulted in high desorption yield of silicon as in the form of SiF2, SiF3 and SiF4.

Anisotropic layer-by-layer growth of graphene on vicinal SiC(0001) surfaces

Epitaxial graphene is formed on vicinal SiC(0001) surfaces via high temperature annealing in vacuum. Steps act as a significant ``kicker'' of graphene nucleation to feed C atoms. At elevated temperatures, graphene growth is controlled by the decomposition of Si-C bonds at step edges, Si desorption, and C diffusion on the surface. The limited Si desorption is due to the dependence of the growth rate on the thickness of graphene layers. The fabricated graphene layer(s) acts as a Si-diffusion barrier, which in turn induces local thermal equilibrium between the graphene layer and the SiC surface. C atoms preferentially diffuse along the steps, resulting in anisotropic layer-by-layer growth, which is characteristic in this system.

Radical Production Processes on Heated Metal Catalyst Surfaces

Free radical species, such as H, O, OH, CH3, and SiH3, can be ejected efficiently to the gas phase by catalytic decomposition of material gases on heated metal surfaces. The detailed reaction mechanisms for the production of these radicals are still hard to be specified, but the mechanisms can be presumed by measuring the radical densities in the gas phase under various conditions. In this review, the results on the detection of these radical species are summarized, and the possible mechanisms taking place on the catalyst surfaces are discussed. The experimental procedures to identify the radical species are also reviewed shortly. In the production of Si atoms from SiH4, the rate determining step changes from Si desorption to SiH4 adsorption with the increase in the catalyst temperature. In the decomposition of H2O, the OH desorption energy depends on the surface coverage. In O2 systems, decomposition takes place not only on the metal surfaces, but also on the second adsorbed layers.

Growth dynamics and kinetics of monolayer and multilayer graphene on a 6H-SiC(0001) substrate

Using Scanning Tunnelling Microscopy (STM), the transformation from the commonly known carbon-rich (6√3×6√3)R30° reconstructed surface to graphene on the 6H-SiC(0001) substrate is systematically investigated with the aid of adsorbing cobalt (Co) which acts as a tracer to map the evolution of these surfaces. The formation of graphene is observed to begin from the step-edges as Si desorption occurs and the growth process continues akin to that of a step flow growth mode. Analysis of the surface step-height evolution at various stages of graphitization shows that as the initial (6√3×6√3)R30° surface converts to form graphene, three Si-C bilayers beneath collapse to regenerate a C-rich structure which also has a (6√3×6√3)R30° periodicity at the interface between graphene and the SiC bulk. Based on these observations, a structural mechanism for the growth of mono- and multilayer graphene is proposed. In addition, we also examine the rate at which the initial (6√3×6√3)R30° surface coverts to graphene as a function of time and temperature. Kinetic analysis of the growth process reveals that the transformation occurs with an activation energy of 3.0 ± 0.4 eV, a value close to the breaking of a Si-C bond.

Impact of soil weathering degree on silicon isotopic fractionation during adsorption onto iron oxides in basaltic ash soils, Cameroon

The sequestration of silicon in soil clay-sized iron oxides may affect the terrestrial cycle of Si. Iron oxides indeed specifically adsorb aqueous monosilicic acid (H 4SiO 4 0), thereby influencing Si concentration in soil solution. Here we study the impact of H 4SiO 4 0 adsorption on the fractionation of Si isotopes in basaltic ash soils differing in weathering degree (from two weathering sequences, Cameroon), hence in clay and Fe-oxide contents, and evaluate the potential isotopic impact on dissolved Si in surrounding Cameroon rivers. Adsorption was measured in batch experiment series designed as function of time (0–72 h) and initial concentration (ic) of Si in solution (0.61–1.18 mM) at 20 °C, constant pH (5.5) and ionic strength (1 mM). After various soil-solution contact times, the δ 30Si vs. NBS28 compositions were determined in selected solutions by MC-ICP-MS (Nu Plasma) in medium resolution, operating in dry plasma with Mg doping with an average precision of ±0.15‰ (±2σ SEM). The quantitative adsorption of H 4SiO 4 0 by soil Fe-oxides left a solution depleted in light Si isotopes, which confirms previous study on synthetic Fe-oxides. Measured against its initial composition (δ 30Si = +0.02 ± 0.07‰ (±2σ SD)), the solutions were systematically enriched in 30Si reaching maximum δ 30Si values ranging between +0.16‰ and +0.95‰ after 72 h contact time. The enrichment of the solution in heavy isotopes increased with increasing values of three parameters: soil weathering degree, iron oxide content, and proportion of short-range ordered Fe-oxide. The Si-isotopic signature of the solution was partly influenced by Si release, possibly through mineral dissolution and Si desorption from oxide surfaces, depending on soil type, highlighting the complex pattern of natural soils. Surrounding Cameroon rivers displayed a mean Si-isotopic signature of +1.19‰. Our data imply that in natural environments, H 4SiO 4 0 adsorption by soil clay-sized Fe-oxides at least partly impacts the Si-isotopic signature of the soil solution exported to water streams.

The effect of oxygen during irradiation of silicon with low energy Cs+ ions

The effect of oxygen flooding during ultralow energy SIMS depth profiling of silicon with Cs+ primary ions is presented. New experimental data show the effective sputtering yield of silicon in the presence of oxygen, as well as the energy distribution of the secondary Si− ions. It is found that the component sputtering yield of Si is very sensitive to minute amounts of oxygen in the proximity of the sputtered surface. At these very low flooding pressures (in the 10−9–10−8mbar range), one cannot account for a full oxidation in the time frame of a typical measurement; it is concluded that in this particular case, which is technologically very relevant, small traces of oxygen will change the desorption characteristics of cesium as well as silicon. It shows that oxygen that is adsorbed to the silicon surface provides an attachment site for impinging cesium ions and thereby increases the retention capacity of cesium tremendously. Also, oxygen changes the secondary ion yield and can even promote the desorption of Si from the analysis surface.