Near-surface Density Research Articles

Doping-induced Ti3+ state and oxygen vacancies in TiO2: A single-chip combinatorial investigation

TiO2 is widely recognized as a high performance photocatalyst for photocatalytic applications. Oxygen vacancies (VO) and titanium defects are known to make a significant contribution to the enhanced photocatalytic efficiency of TiO2, accordingly control of these two centers is pivotal to optimizing the surface photochemical activity. In this work, the formation of VO and Ti3+ ions induced by n-type doping of TiO2 with Nb is demonstrated using a combinatorial investigation approach. Chemical Beam Vapour Deposition (CBVD) is used to fabricate Nb concentration-graded (NbxTi1-x)O2 anatase films with 0.009 ≤ x ≤ 0.033 over a 5 mm length on a single chip. Cathodoluminescence (CL) microanalysis shows the luminescence intensity decreases with increasing Nb concentration; however, the presence of a self-trapped exciton (STE) emission, characteristic of anatase TiO2, indicates that the Nb incorporation does not affect the structural properties of TiO2. Notably, the introduction of Nb donors induces the formation of the signature luminescence bands of VO and Ti3+ at 2.05 and 2.60 eV at 80 K, respectively, and their intensities rise by up to 28% with increasing Nb content. The formation of VO and Ti3+ is attributed to the effect of excess electrons locally produced by Nb donor doping. The thermal activation energies for the STE and defect-related emissions are found to be identical at 32 ± 4 meV, suggesting that the recombination kinetics is mediated by Nb donors. Significantly, we demonstrate that the near-surface VO density can be further increased by two different post-growth treatment approaches: (i) remote hydrogen plasma and (ii) low-energy electron beam irradiation (LEEBI), without effecting the valence state of Ti. Our results open opportunities to optimize the photocatalytic properties of TiO2 by defect engineering.

Surface doping of rubrene single crystals by molecular electron donors and acceptors.

The surface molecular doping of organic semiconductors can play an important role in the development of organic electronic or optoelectronic devices. Single-crystal rubrene remains a leading molecular candidate for applications in electronics due to its high hole mobility. In parallel, intensive research into the fabrication of flexible organic electronics requires the careful design of functional interfaces to enable optimal device characteristics. To this end, the present work seeks to understand the effect of surface molecular doping on the electronic band structure of rubrene single crystals. Our angle-resolved photoemission measurements reveal that the Fermi level moves in the band gap of rubrene depending on the direction of surface electron-transfer reactions with the molecular dopants, yet the valence band dispersion remains essentially unperturbed. This indicates that surface electron-transfer doping of a molecular single crystal can effectively modify the near-surface charge density, while retaining good charge-carrier mobility.

Characterization of Interfacial Properties in Electrochemical Nitrate Reduction to Optimize Ammonia Production

Nitrate pollution of wastewater from agricultural runoff and industrial waste streams is common around the globe. Nitrate negatively impacts the environment through harmful algal blooms and can be dangerous for human consumption. Rather than treat nitrate as a waste, we electrochemically reduce nitrate (NO3RR) to ammonia (NO3 - + 9H+ + 8e- → NH3 + 3H2O), a commonly used fertilizer and clean fuel. Electrochemical treatment can enable on-site, modular treatment powered by renewable energy. As an inner-sphere reaction involving multiple hydrogenation and electron-transfer steps, the activity and selectivity of electrochemical NO3RR to NH3 are strongly influenced by properties at the electrode–electrolyte interface. When potential is applied, the electric double layer (EDL) forms that comprises ordered molecular layers extending up to 10 Å from the surface. This EDL is the primary environment where the heterogeneous NO3RR occurs and can impact the reaction in several ways, including blocking catalytic sites and stabilizing reactants and reaction intermediates. To understand the molecular mechanisms of these impacts, the EDL structure is studied with x-ray reflectivity (XRR), which provides atomic-level resolution of the near-surface electron density profile and reveals the number of ordered layers, the distance of each layer from the surface, the ion surface coverage, and the degree of hydration of ions in each layer. We complement these investigations with operando attenuated total reflectance–surface-enhanced infrared absorption spectroscopy (ATR–SEIRAS) under similar reaction conditions to investigate the adsorbed reactants, intermediates, and local pH. Furthermore, composition and structure of the EDL can change with varying bulk electrolyte compositions and mass transport regimes, which also provide levers to control reaction activity and selectivity. We use a continuum model (generalized modified Poisson–Nernst–Planck, GMPNP) to develop the spatial profiles of the electric field, reactant nitrate concentration, cation concentrations, and pH outside the EDL and how they change as functions of applied potential, bulk electrolyte composition, and flow rate. Taken together, our results relate 1) bulk electrolyte properties with interfacial EDL properties and 2) interfacial properties with reaction activity and selectivity, informing the choice of operating parameters such as electrolyte composition and flow rate to optimize ammonia production in electrochemical NO3RR.

Discrete Element Modeling of Aeolian-like Morphologies on Comet 67P/Churyumov-Gerasimenko

Context. Even after the Rosetta mission, some of the mechanical parameters of comet 67P/Churyumov-Gerasimenko’s surface material are still not well constrained. They are needed to improve our understanding of cometary activity or for planning sample return procedures. Aims. We discuss the physical process dominating the formation of aeolian-like surface features in the form of moats and wind taillike bedforms around obstacles and investigate the mechanical and geometrical parameters involved. Methods. By applying the discrete element method (DEM) in a low-gravity environment, we numerically simulated the dynamics of the surface layer particles and the particle stream involved in the formation of aeolian-like morphological features. The material is composed of polydisperse spherical particles that consist of a mixture of dust and water ice, with interparticle forces given by the Hertz contact model, cohesion, friction, and rolling friction. We determined a working set of parameters that enables simulations to be reasonably realistic and investigated morphological changes when modifying these parameters. Results. The aeolian-like surface features are reasonably well reproduced using model materials with a tensile strength on the order of 0.1–1 Pa. Stronger materials and obstacles with round shapes impede the formation of a moat and a wind tail. The integrated dust flux required for the formation of moats and wind tails is on the order of 100 kg m−2, which, based on the timescale of morphological changes inferred from Rosetta images, translates to a near-surface particle density on the order of 10−6–10−4 kgm−3. Conclusions. DEM modeling of the aeolian-like surface features reveals complex formation mechanisms that involve both deposition of ejected material and surface erosion. More numerical work and additional in situ measurements or sample return missions are needed to better investigate mechanical parameters of cometary surface material and to understand the mechanics of cometary activity.

Constraining the chemical depth profile of a manganese-rich surface layer in Gale crater, Mars

Geochemical analyses by X-ray spectrometry and laser-induced breakdown spectroscopy (LIBS) instruments on the surface of Mars enable detailed studies of surface materials. The two techniques are utilized in concert by rovers to glean information in a complementary fashion. However, fundamental differences in how these analytical techniques function can produce perceived discrepancies in results, such as those resulting from variation in sampling volume. Here we utilize data acquired by the APXS (X-ray spectrometer) and ChemCam (LIBS) instruments on the Curiosity rover to investigate a manganese-rich surface layer, and, in the process, provide an improved chemical depth profile. We also demonstrate a method whereby current and future spacecraft capable of utilizing both techniques can potentially improve estimates of martian material near-surface density.

Non-linear radar response to the radial structure of Europa plume fallout deposits

Observations of water vapor plumes emanating from Europa's surface provide direct evidence of on-going activity within the ice shell that may have important implications for the potential habitability of Europa's putative global ocean. However, the scarcity of repeated observations suggest that Europa plumes are sporadic and may prove elusive when attempting to sample them directly with the instruments on-board either NASA's Europa Clipper or ESA's JUpiter ICy moons Explorer (JUICE) spacecraft. An alternative approach is then to search for and characterize the remnant plume fallout deposits preserved on Europa's surface. To this end, in this study we investigate the radar response of surficial plume fallout deposits when illuminated by both the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument on-board Europa Clipper as well as the Radar for Icy Moons Exploration (RIME) instrument on-board JUICE. Our results reveal distinct patterns in radial coherent surface reflectivity associated with different families of plume heights. Uniform fallout depositional rates surrounding low (≤25 km) altitude plumes result in a radially-constant coherent surface reflectivity that oscillates as a function of plume duration. In contrast, fallout deposits from high (≥50 km) altitude plumes exhibit radially variable, ripple-like patterns in coherent surface reflectivity as the velocity at which a particle is ejected from the plume is size dependent. The amplitude of the reflectivity oscillations as a function of fallout deposit thickness is directly dependent on porosity, with greater oscillation amplitudes being associated with older fallout deposits that have undergone some degree of densification (as opposed to the young, porous deposits of very recently active plumes). Finally, the results highlight that in the presence of near-surface structure, there is no direct proportionality between observed radar surface reflectivity and near-surface bulk density.

Ambient high-frequency seismic surface waves in the firn column of central west Antarctica

AbstractFirn is the pervasive surface material across Antarctica, and its structures reflect its formation and history in response to environmental perturbations. In addition to the role of firn in thermally isolating underlying glacial ice, it defines near-surface elastic and density structure and strongly influences high-frequency (> 5 Hz) seismic phenomena observed near the surface. We investigate high-frequency seismic data collected with an array of seismographs deployed on the West Antarctic Ice Sheet (WAIS) near WAIS Divide camp in January 2019. Cross-correlations of anthropogenic noise originating from the approximately 5 km-distant camp were constructed using a 1 km-diameter circular array of 22 seismographs. We distinguish three Rayleigh (elastic surface) wave modes at frequencies up to 50 Hz that exhibit systematic spatially varying particle motion characteristics. The horizontal-to-vertical ratio for the second mode shows a spatial pattern of peak frequencies that matches particle motion transitions for both the fundamental and second Rayleigh modes. This pattern is further evident in the appearance of narrow band spectral peaks. We find that shallow lateral structural variations are consistent with these observations, and model spectral peaks as Rayleigh wave amplifications within similarly scaled shallow basin-like structures delineated by the strong velocity and density gradients typical of Antarctic firn.

Near subsurface density reconstruction by full waveform inversion in the frequency domain

The work proposed is part of a global project dealing with the characterization of heterogeneous media by both electromagnetic and mechanical full waveform inversions. Indeed Full Waveform Inversion of seismic reflection or Ground Penetrating Radar data is an efficient approach to reconstruct subsurface physical parameters with high resolution. This paper focuses on the mechanical part, and more specifically on quantitative imaging of nearsurface density. Processing field data is challenging because the nature of the source and the sensors used impact the signal-to-noise ratio as well as the frequency range appearing in the recorded data. From then it becomes interesting to process the data in the frequency domain and work on a few representative frequencies of the recorded temporal signal. In this article, field data are simulated by noisy synthetic data. Different frequency strategies are used and their results are compared with each other. The inverse problem consists in assessing the density in the probed medium from the data on the displacement field measured at the detectors. Such a problem is known to be nonlinear and ill-posed. It is solved iteratively by a regularized Gauss-Newton algorithm, which relies on the Fréchet derivatives obtained through the generalized reciprocity principle equivalent to the well-known adjoint method. The numerical results show an optimal strategy, for which the convergence rate and the computation time are reasonable, the spatial resolution is improved and the density is well reconstructed.

Investigation of the near-surface matter density radial distribution in the skin explosion of cylindrical conductors

Investigations of the near-surface plasma formation process during skin explosion of cylindrical duralumin and copper conductors in rapidly increasing magnetic fields with their induction up to 500 T were carried out. The formation of plasma on the conductor surface was recorded by its glow in the visible range using a four-frame optical camera with an exposure time of each frame of 3 ns. The internal structure of the surface plasma, the assessment of the density of matter in it and its radial distribution were investigated using radiography pictures obtained by X-ray transmission with hv > 0.8 keV, which is formed at the “hot point” of the X-pinch. The dependences of the load substance density on its radius were determined and constructed from the obtained X-ray diffraction patterns at different points in time from the beginning of the current. So at 216 ns at a radius of 1.8 mm of a duralumin conductor with an initial radius of 1.485 mm, the density of the substance is estimated to be 0.0068 g/cm3.

Background-oriented schlieren measurement of near-surface density field in surface dielectric-barrier-discharge

Surface dielectric-barrier-discharge (DBD) is widely utilized for flow control actuators called DBD plasma actuator (DBDPA). With the aim of ensuring an accurate background-oriented schlieren (BOS) measurement of the near-surface density field in surface DBD, we investigate the effects of the depth of field (DoF), the wall surface and the background image deformation on measurement results. Experiments using a glass plate as the measurement target reveal that there is no appreciable effect of whether the DoF includes the measurement target. Additionally, the DoF should be shallow from the viewpoint of the error introduced by the wall surface. Moreover, it is found that the error introduced by the wall surface and the dot deformation can be characterized by specific dimensionless parameters. Finally, we conduct the BOS measurement of the DBDPA. We confirm that the density field is qualitatively valid from a physical respect, and we present the density field while discussing specific measurement errors.

Electron distribution and electrical resistance in nanostructured mixed oxides CeO2-In2O3

The electronic structure of nanoparticles in binary mixtures of oxides are considered by the example of CeO2-In2O3. The effect of catalytically active CeO2 nanoclusters on charge distribution in nanoparticles with a high concentration of conduction electrons (In2O3) is studied. The spillover of oxygen atoms from CeO2 nanoclusters to In2O3 nanoparticles, which enhances the inhomogeneous distribution of conduction electrons in the In2O3 nanoparticle, is taken into account, and it is shown that exactly spillover leads to depletion of the near-surface layer. The film resistance rises with an increase in the CeO2 concentration, due to a decrease in the near-surface electron density in In2O3 nanoparticles and the overlap of the current flow paths by CeO2 nanoclusters. An expression for the temperature dependence of a nanostructured film resistance is found, taking into account the inhomogeneous distribution of electrons in nanoparticles. The temperature dependence of the CeO2-In2O3 film was measured and the results were compared with theory. A good agreement between theory and experiment was obtained.

Electric Resistance of Binary Oxides CeO2–In2O3 Structured at the Nanolevel

Binary mixtures of oxides structured at the nanoscale are considered using the example of CeO2–In2O3. The role of catalytically active CeO2 nanoclusters is clarified during charge distribution in nanoparticles (In2O3) with a high concentration of electrons. The flow of oxygen atoms from the CeO2 nanoclusters on nanoparticles In2O3, which enhances the inhomogeneous distribution of the conduction electrons in the In2O3 nanoparticle and depletes the near-surface layer, is taken into account. It is shown that since the conductivity of the system is determined by the near-surface electron density, the film resistance increases with an increase in the concentration of the CeO2.

Modeling, analysis, and code/data validation of DIII-D tokamak divertor experiments on ELM and non-ELM plasma tungsten sputtering erosion

We analyzed recent DIII-D tokamak tungsten divertor probe experiments using advanced, coupled, sputter erosion/redeposition, plasma, and surface response code packages. Modeling is done for ELMing H-mode, and L-mode plasmas, impinging on various size tungsten deposits on Divertor Material Evaluation System (DiMES) carbon probes. The simulations compute 3D, full kinetic, sub-gyromotion, impurity sputtering and transport, including changes in tungsten surface composition and response due to mixed deuterium and carbon ions irradiation. Per our analysis, ELM (edge localized mode) plasma sputtering in DIII-D mostly involves free-streaming high energy (∼500–1000 eV) D+ and C+6 ions, with high near-surface plasma density. L-Mode sputtering is due to impurity sputtering (C, W) only, with lower density. All cases show complete redeposition of tungsten on the divertor, with significant redeposition on the tungsten spots themselves, and low self-sputtering. Comparison of ELM plasma gross tungsten erosion simulation results with in-situ spectroscopic data is good, as are code/data comparisons of net erosion using post-exposure Rutherford backscattering (RBS) data for the L-mode probes. The analysis, extrapolated to a full tungsten divertor, implies low net erosion and negligible plasma contamination from sputtering. These results support the use of high-Z plasma facing surfaces in ITER and beyond.

Tropical Atlantic Mixed Layer Buoyancy Seasonality: Atmospheric and Oceanic Physical Processes Contributions

This study investigates the physical processes controlling the mixed layer buoyancy using a regional configuration of an ocean general circulation model. Processes are quantified by using a linearized equation of state, a mixed-layer heat, and a salt budget. Model results correctly reproduce the observed seasonal near-surface density tendencies. The results indicate that the heat flux is located poleward of 10° of latitude, which is at least three times greater than the freshwater flux that mainly controls mixed layer buoyancy. During boreal spring-summer of each hemisphere, the freshwater flux partly compensates the heat flux in terms of buoyancy loss while, during the fall-winter, they act together. Under the seasonal march of the Inter-tropical Convergence Zone and in coastal areas affected by the river, the contribution of ocean processes on the upper density becomes important. Along the north Brazilian coast and the Gulf of Guinea, horizontal and vertical processes involving salinity are the main contributors to an upper water change with a contribution of at least twice as much the temperature. At the equator and along the Senegal-Mauritanian coast, vertical processes are the major oceanic contributors. This is mainly due to the vertical gradient of temperature at the mixed layer base in the equator while the salinity one dominates along the Senegal-Mauritania coast.

Evaluation of Northern Hemisphere surface wind speed and wind power density in multiple reanalysis datasets

Reanalysis products have become more and more popular for wind energy scientific community to analyze the wind speed variability and get long-term wind power estimations. The present study evaluates the biases of near-surface wind speed and wind power density in four of the most reputable reanalysis datasets, which include ERA-Interim, JRA-55, CFS and MERRA-2. The results indicate that the abilities of reanalysis products to reproduce the variabilities of wind speeds are different in different regions. JRA-55 and CFS offer the best estimates of annual and seasonal variabilities of surface wind speeds over the Northern Hemisphere. In detail, JRA-55 is the best to reproduce surface wind speeds in Asia, CFS has the best performance in Europe, and MERRA-2 just can reproduce the central part of North America. All the four datasets show decreasing tendencies in surface winds over the Northern Hemisphere during 1980–2016, although the trends are largely diverse among them. The most significant disagreements of wind speed trends are encountered between JRA-55 and MERRA-2, which are likely related to the different methodologies from the lowest model level that reanalyses use. The main drivers of wind speed trends are the changes of large-scale circulation, urbanization, and aerosol emissions.

Plasma expansion towards an electrically insulated surface

The problem of plasma expansion into a vacuum is revisited with the addition of a finite boundary condition; an electrically insulated surface. As plasma expands towards a charge-accumulating surface, the leading electron cloud charges the surface negatively, which in turn repels electrons and attracts ions. This plasma–surface interaction is shown to result in a feedback process which accelerates the plasma expansion. In addition, we examine the decrease in (negative) surface potential and associated near-surface electron density. To investigate this plasma coupling with an electrically floating surface, we develop an analytic model including four neighbouring plasma regions: (i) undisturbed plasma, (ii) quasi-neutral self-similar expansion, (iii) ion front boundary layer and (iv) electron cloud. A key innovation in our approach is a self-contained analytic approximation of the ion front boundary layer, providing a spatially continuous electric field model for the early phase of bounded plasma expansion.

Meltwater percolation, impermeable layer formation and runoff buffering on Devon Ice Cap, Canada

AbstractThe retention of meltwater in the accumulation area of the Greenland ice sheet and other Arctic ice masses buffers their contribution to sea level change. However, sustained warming also results in impermeable ice layers or ‘ice slabs’ that seal the underlying pore space. Here, we use a 1-D, physically based, high-resolution model to simulate the surface mass balance (SMB), percolation, refreezing, ice layer formation and runoff from across the high-elevation area of Devon Ice Cap, Canada, from 2001 to 2016. We vary the thickness of the ‘impermeable’ ice layer at which underlying firn becomes inaccessible to meltwater. Thick near-surface ice layers are established by an initial deep percolation, the formation of decimetre ice layers and the infilling of interleaving pore space. The cumulative SMB increases by 48% by varying impermeable layer thickness between 0.01 and 5 m. Within this range we identify narrower range (0.25–1 m) that can simulate both the temporal variability in SMB and the observed near-surface density structure. Across this range, cumulative SMB variation is limited to 6% and 45–49% of mass retention takes place within the annually replenished snowpack. Our results indicate cooler summers after intense mid-2000s warming have led to a partial replenishment of pore space.

Transient dielectric functions of Ge, Si, and InP from femtosecond pump-probe ellipsometry

Transient dielectric functions with a 120 fs time resolution of Ge, Si, and InP were acquired from 1.7 to 3.5 eV with a femtosecond pump-probe rotating-compensator ellipsometer. The intensity of the pump laser (with 1.55, 3.10, or 4.65 eV photon energy) was adjusted to create an initial near-surface carrier density of 1020 cm−3. In Ge, there is a significant (∼15%) decrease in the E1 and E1 + Δ1 critical point absorption and a Kramers–Kronig consistent change in the refractive index because photoexcited electrons at L block these transitions and reduce their amplitudes. Only a small redshift of the E1 critical point is observed, which we attribute to lattice heating and exchange-correlation effects. Minimal changes were found for Si and InP, where electrons near Δ and Γ do not participate in interband transitions between 1.7 and 3.5 eV.

Response of the Salinity-Stratified Bay of Bengal to Cyclone Phailin

AbstractCyclone Phailin, which developed over the Bay of Bengal in October 2013, was one of the strongest tropical cyclones to make landfall in India. We study the response of the salinity-stratified north Bay of Bengal to Cyclone Phailin with the help of hourly observations from three open-ocean moorings 200 km from the cyclone track, a mooring close to the cyclone track, daily sea surface salinity (SSS) fromAquarius, and a one-dimensional model. Before the arrival of Phailin, moored observations showed a shallow layer of low-salinity water lying above a deep, warm “barrier” layer. As the winds strengthened, upper-ocean mixing due to enhanced vertical shear of storm-generated currents led to a rapid increase of near-surface salinity. Sea surface temperature (SST) cooled very little, however, because the prestorm subsurface ocean was warm.AquariusSSS increased by 1.5–3 psu over an area of nearly one million square kilometers in the north Bay of Bengal. A one-dimensional model, with initial conditions and surface forcing based on moored observations, shows that cyclone winds rapidly eroded the shallow, salinity-dominated density stratification and mixed the upper ocean to 40–50-m depth, consistent with observations. Model sensitivity experiments indicate that changes in ocean mixed layer temperature in response to Cyclone Phailin are small. A nearly isothermal, salinity-stratified barrier layer in the prestorm upper ocean has two effects. First, near-surface density stratification reduces the depth of vertical mixing. Second, mixing is confined to the nearly isothermal layer, resulting in little or no SST cooling.

Marked decrease in the near-surface snow density retrieved by AMSR-E satellite at Dome C, Antarctica, between 2002 and 2011

Abstract. Surface snow density is an important variable for the surface mass balance and energy budget. It evolves according to meteorological conditions, in particular, snowfall, wind, and temperature, but the physical processes governing atmospheric influence on snow are not fully understood. A reason is that no systematic observation is available on a continental scale. Here, we use the passive microwave observations from AMSR-E satellite to retrieve the surface snow density at Dome C on the East Antarctic Plateau. The retrieval method is based on the difference of surface reflections between horizontally and vertically polarized brightness temperatures at 37 GHz, highlighted by the computation of the polarization ratio, which is related to surface snow density. The relationship has been obtained with a microwave emission radiative transfer model (DMRT-ML). The retrieved density, approximately representative of the topmost 3 cm of the snowpack, compares well with in situ measurements. The difference between mean in situ measurements and mean retrieved density is 26.2 kg m−3, which is within typical in situ measurement uncertainties. We apply the retrieval method to derive the time series over the period 2002–2011. The results show a marked and persistent pluri-annual decrease of about 10 kg m−3 yr−1, in addition to atmosphere-related seasonal, weekly, and daily density variations. This trend is confirmed by independent active microwave observations from the ENVISAT and QuikSCAT satellites, though the link to the density is more difficult to establish. However, no related pluri-annual change in meteorological conditions has been found to explain such a trend in snow density. Further work will concern the extension of the method to the continental scale.