Atomic Concentration Profiles Research Articles

Depth Analyses of Mg‐Acceptor and Si‐Donor Concentrations in GaN by Combining Stepwise Etching and Photoluminescence

The depth profiles of the Mg‐acceptor or Si‐donor concentration in GaN are visualized using stepwise etching and photoluminescence (PL) measurements. Low‐damage etching techniques are applied: multistep‐bias inductively coupled plasma reactive ion etching for p‐type GaN and electrochemical etching for n‐type GaN. To determine the concentration calibration curve, the PL intensity ratios of Mg‐neutral acceptor or Si‐neutral donor‐bound excitons to free excitons are plotted against Mg or Si atomic concentrations in etched GaN epitaxial layers having depth distributions of dopant concentrations, as estimated by secondary ion mass spectrometry. The quantitative accuracy of the calibration curve is ensured by the appropriate sample temperature and excitation density ranges for PL. The stepwise dry etching and the Mg acceptor quantification by PL using this calibration curve are applied to Mg‐ion‐implanted GaN after ultra‐high‐pressure annealing at 1300 °C for 1 min. The obtained depth profile of the Mg‐acceptor concentration is consistent with the Mg atomic concentration profile in the region deeper than the implantation peak, whereas the acceptor concentration in the shallow region is lower than the Mg atomic concentration. This analysis is useful for revealing the site substitution ratio of dopants in GaN materials partially involving inactive dopants.

High-temperature oxidation of acetylene by N2O at high Ar dilution conditions and in laminar premixed C2H2 + O2 + N2 flames

High-temperature oxidation of acetylene (C2H2) is studied behind reflected shock waves and in laminar flames. Atomic resonance absorption spectroscopy (ARAS) is employed to record oxygen atom concentration profiles for the mixture of 10 ppm C2H2 + 10 ppm N2O + argon and temperatures from 1688 K to 3179 K, extending the range of such data available from the literature. Laminar burning velocity of C2H2 in a diluted oxidizer with 11–13% O2 in the O2 + N2 mixture is measured using the heat flux method and compared to the literature data for the 13% O2 mixture. An updated detailed kinetic mechanism is presented to model and analyze the results, and the selection of rate constants in the C2H2 sub-mechanism, whose importance was identified by the sensitivity analysis, is discussed. The performance of the new model is compared against several reaction schemes available from the literature, and kinetic differences between them are outlined. The new shock-wave data helped to improve the performance of the present model compared to its previous version. For the laminar flames, a particular importance of reactions involving C2H3 is identified, however, the reasons for the observed differences in model predictions are to a large extent located outside the C2H2 sub-mechanism, which were also identified.

Maintaining atomically smooth GaAs surfaces after high-temperature processing for precise interdiffusion analysis and materials engineering

Arsenic's high vapor pressure leads to thermal instability during high-temperature processing (>370 °C) of GaAs, contributing to the performance degradation of subsequently fabricated devices. The resulting surface damage also obfuscates the exact quantitative characterization of the diffusion process, a critical step in device manufacturing. In this experiment, an encapsulant-and-sacrificial-layer procedure is employed to reduce arsenic sublimation and preserve a smooth surface. A capped GaAs/InGaAs/GaAs quantum well structure is subjected to rapid thermal annealing, and AFM, SEM, and energy-dispersive x-ray spectroscopy are used to compare the surface qualities of the postannealed encapsulated GaAs against the reference GaAs. For the encapsulated substrate, a smooth surface with an average root-mean-squared value of 6.5 Å is achieved after high-temperature processing. SIMS analysis is used to obtain the diffused indium atomic concentration profiles for a smooth and roughened GaAs surface and their corresponding diffusion parameters. The interdiffusion characterization process demonstrates in a GaAs/InGaAs system how precise diffusion parameter extraction requires preserving an atomically smooth surface when using surface-sensitive techniques.

Applying surface-sensitive techniques to structural and chemical study of uncapped Sn-Sb-Te thin film. A density functional theory - based study

We present here a combined experimental and theoretical study of the structural and chemical properties of polycrystalline Sn-Sb-Te film with nominal composition SnSb2Te4 grown by pulsed laser deposition technique on mylar substrate. From the experimental side, surface-sensitive techniques as x-ray photoelectron spectroscopy (XPS), grazing incidence x-ray diffractometry (GIXRD) and 119mSn integral conversion electron Möβbauer spectroscopy (ICEMS) have been applied to the study of the film at room-temperature and under normal conditions of pressure. GIXRD showed that the Sn-Sb-Te film adopted a NaCl- type structure (Fm-3m), and in the detection limits, no other crystalline phase was revealed. ICEMS technique unambiguously indicated the coexistence of two different tin fractions: Sn(II), as expected for the SnSb2Te4 phase, and Sn(IV), suggesting oxidation of tin. Chemical in-depth profile obtained by means of XPS suggests the oxidation of all the constituent atoms at the topmost layers of the film and the progressive depletion of tin and antimony oxides going depth in the film. The in-depth atomic concentration profiles also reveals a notorious deficiency of Te in the sample. Theoretically, density functional theory-based calculations (assuming that the Sn-Sb-Te film adopts the Fm-3m structure) support the hypothesis that Te - vacancies sites are occupied by oxygen atoms during the natural oxidation process of Sn-Sb-Te film. Additionally, our calculations demonstrated that only the substitution of Te atoms by oxygen ones induces a semiconducting behavior of the otherwise metallic Sn-Sb-Te host.

Behavior of Na and RbF‐Treated CdS/Cu(In,Ga)Se2 Solar Cells with Stress Testing under Heat, Light, and Junction Bias

The effects of Na and RbF alkali treatment on the metastability behavior of CdS/Cu(In,Ga)Se2 solar cells are investigated with stress factors of heat, junction bias, and illumination. Four device types with and without Na or RbF treatments are subjected to heat‐ and light‐soaking under open‐ and short‐circuit (OC, SC) junction bias. Low‐Na devices show a higher bandgap due to increased minimum Ga content, higher recombination current, and lower open‐circuit voltage (VOC). Devices with RbF post‐deposition treatment (PDT) show an improvement in net doping density ≈1016 cm−3, VOC, and efficiency. Heat‐ and light‐soaking under OC junction bias provokes an increase in net carrier concentration and VOC irrespective of the alkali treatments. After SC stress, a decrease in VOC and net carrier concentration is observed, which can be stabilized by RbF‐PDT. An increase in Na and oxygen concentration in CIGS is observed for baseline and low‐Na devices, respectively, after OC stress. The oxygen concentration in CdS decreases after heat‐ and light‐soaking for devices without RbF‐PDT, whereas it remains unchanged for devices with RbF‐PDT. The atomic concentration profiles in CIGS significantly stabilize as a function of stress with the addition of RbF‐PDT.

The role of interface on the toughening and failure mechanisms of thermoplastic nanocomposites reinforced with nanofibrillated rubber.

The interface plays a crucial role in the physical and functional properties of polymer nanocomposites, yet its effects have not been fully recognized in the setting of classical continuum-based modeling. In the present study, we investigate the roles of interface and interfiber interactions on the toughening effects of rubber nanofibers embodied in thermoplastic-based materials. Emphasis is placed on establishing comprehensive theoretical and atomistic descriptions of the nanocomposite systems subjected to pull-out and uniaxial extension in the longitudinal and transverse directions. Using the framework of molecular dynamics, the annealed melt-drawn nanofibers were spontaneously formed via the proposed four-step methodology. The generated nanofibers were then crosslinked using the proposed robust topology-matching algorithm, through which the chemical reactions arising in the crosslinking were closely assimilated. The interfiber interactions were also examined with respect to separation distances and nanofiber radius via a nanofiber-pair atomistic scheme, and the obtained results were subsequently incorporated into the pull-out and uniaxial test simulations. The results indicate that the compatibilizer grafting results in enhanced interfacial shear strength by introducing extra chemical interactions at the interface. In particular, it was found that the compatibilizer restricts the formation and coalescence of nanovoids, resulting in enhanced toughening effects. Together, we have shown that the presence of a small amount of well-dispersed rubber nanofibrillar network whose surfaces are grafted with maleic anhydride compatibilizer can dramatically increase the toughness and alter the failure mechanisms of the nanocomposites without any deterioration in the stiffness, which is also consistent with the recent experimental observations in our lab. The interfacial failure mechanism was also investigated by monitoring the changes in the atomic concentration profiles, mean square displacement and fractional free volume. The results obtained may serve as a promising alternative for the continuum-based modeling and analysis of interfaces.

Interaction of Te and Se interlayers with Ag or Au nanofilms in sandwich structures.

Noble metal nanolayers on flat substrates are often deposited with the use of semiconductor interlayers, which may strongly interact with the noble metal overlayer. We investigated the crystallinity, atomic concentration profile and optical parameters of ≈35 nm-thick silver and gold layers deposited on glass substrates with 2 nm-thick tellurium or selenium interlayers. Our study, based on X-ray reflectometry (XRR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and ellipsometric measurements, showed that using either of these interlayers introduces strain in nanocrystals of both plasmonic films. This, in turn, influences the migration of Se and Te into the metal layers. Selenium atoms migrate both in the silver and gold nanolayers, while tellurium atoms migrate only in silver. The Te concentration curve clearly suggests that this migration is an effect of the segregation of Te atoms in the silver structure. The differences in crystallinity, as well as the migration process, strongly influence the optical parameters of Ag and Au. In the permittivity of Ag deposited on either Te or Se, additional plasmonic bands originating from grain boundary segregation or diffusion occur, while for the Au layer, such resonances were not pronounced. In the permittivity of both materials, the intensity of the interband transition peaks is strongly altered, possibly due to the nano-alloy formation, but more likely due to the microstrain on metal grains.

X-ray photoelectron spectroscopy study of an exchange bias system on the basis of Co70Fe30/Mn83Ir17 interface

The consequences of 10 keV He+ ion bombardment on exchange biased Cu(50 nm)/Mn83Ir17(10 nm)/Co70Fe30(6.5 nm)/Ta(15 nm) thin film systems are studied by X-ray photoelectron spectroscopy. The modifications of the exchange bias field, the elemental depth concentration profiles, and the phase composition of the Mn83Ir17/Co70Fe30 interface zone have been investigated for selected doses. Measured atomic depth concentration profiles confirmed a broadening of the interface zone between ferromagnetic and antiferromagnetic layers upon bombardment; the width of the interface zone correlates with the relative change of the exchange bias field. The profiles of the Mn2p and Mn3s photoelectron peaks revealed the presence of a non-Mn83Ir17 phase containing quasiatomic manganese with mean local spin and magnetic moments greater than those in Mn within the Mn83Ir17 alloy in the interface zone. The magnitude of the exchange bias field correlates with the amount of manganese in this high-spin-Mn phase.

Updated SABER Night Atomic Oxygen and Implications for SABER Ozone and Atomic Hydrogen

AbstractUpdated night atomic oxygen concentration (O) profiles from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the National Aeronautics and Space Administration TIMED satellite are presented. These are derived from measurements of the OH(υ = 9 + 8) volume emission rates and photochemical balance relationships. The new night O concentrations are smaller than those originally derived in 2013 and yield physically realistic global annual mean energy budgets in the upper mesosphere and lower thermosphere. The update to the night O atom profiles is motivated by recent discovery and verification of large rates of collisional quenching of OH(υ) by atomic oxygen. The kinetic model relating the SABER‐observed OH emission rates to atomic oxygen is now consistent with these larger quenching rates and other literature values. The new, smaller SABER night O also confirms that SABER daytime ozone is too large. The new night O and OH(υ) model impacts the inference of day and night atomic hydrogen.

High-Temperature Rate Constants for the Reaction of Hydrogen Atoms with Tetramethoxysilane and Reactivity Analogies between Silanes and Oxygenated Hydrocarbons.

The shock-tube technique has been used to investigate the H-abstraction reaction H + Si(OCH3)4 → H2 + Si(OCH2)(OCH3)3 behind reflected shock waves. C2H5I was used as a thermal in situ source for H atoms. The experiments covered a temperature range of 1111-1238 K, and pressures of 1.3-1.4 bar. H atom concentrations were monitored with atomic resonance absorption spectrometry (ARAS). Fits to the temporal H atom concentration profiles based on a developed chemical kinetics reaction mechanism were used for determining bimolecular rate constants. Experimental total H-abstraction rate constants were well represented by the Arrhenius equation ktotal( T) = 10-9.16±0.24 exp(-25.5 ± 5.6 kJ mol-1/ RT) cm3 s-1. Transition state theory (TST) calculations based on the G4 level of theory show excellent agreement with experimentally obtained rate constants, i.e., the theory values of k( T) deviate by less than 25% from the experimental results. Regarding H abstractions, we have compared the reactivity of C-H bonds in Si(OCH3)4 with the reactivity of C-H bonds in dimethyl ether (CH3OCH3). Present experimental and theoretical results indicate that at high temperatures, i.e., T > 500 K, CH3OCH3 is a good reactivity analog to Si(OCH3)4, i.e., kH+Si(OCH3)4( T) ∼ 1.5 × kH+CH3OCH3( T). On the basis of these results, we discuss the possibility of drawing reactivity analogies between oxygenated silanes and oxygenated hydrocarbons.

The study of C2F4Br2 dissociation kinetics using methods of atomic and molecular resonance absorption spectroscopy behind shock waves

Measurements of atomic bromine and CF2 radical concentration profiles, formed at the dissociation of C2F4Br2 molecules behind shock waves, were carried out by means of atomic resonance absorption spectroscopy and molecular resonance absorption spectroscopy. The experiments were performed in mixtures with various concentrations of C2F4Br2 in Ar at the temperatures of 1000–3500 K and pressures of 2–17 bar. As a result the mechanism of C2F4Br2 pyrolysis was investigated and the C2F4 radical dissociation rate constant was determined: kdC2F4 = A exp(-E/RT), where A = 4.76 × 107 s−1 is constant, E = 96 kJmole−1 is the activation energy, R is the perfect gas constant and T is the temperature.

Impurity-free quantum well intermixing for large optical cavity high-power laser diode structures

We report on the correlation of atomic concentration profiles of diffusing species with the blueshift of the quantum well luminescence from both as-grown and impurity free quantum wells intermixed on actual large optical cavity high power laser diode structures. Because it is critical to suppress catastrophic optical mirror damage, sputtered SiO2 and thermally evaporated SrF2 were used both to enhance and suppress quantum well intermixing, respectively, in these (Al)GaAs large optical cavity structures. A luminescence blueshift of 55 nm (130 meV) was obtained for samples with 400 nm thick sputtered SiO2. These layers were used to generate point defects by annealing the samples at 950 °C for 3 min. The ensuing Ga diffusion observed as a shifting front towards the surface at the interface of the GaAs cap and AlGaAs cladding, as well as Al diffusion into the GaAs cap layer, correlates well with the observed luminescence blue shift, as determined by x-ray photoelectron spectroscopy. Although this technique is well-known, the correlation between the photoluminescence peak blue shift and diffusion of Ga and Al during impurity free quantum well intermixing on actual large optical cavity laser diode structures was demonstrated with both x ray photoelectron and photoluminescence spectroscopy, for the first time.

Diffusion in the Layered Li-Ni-Mn-Co Oxide Based Core-Shell/Gradient Materials during Sintering Simulated with Pellets in Contact

Core-shell structured positive electrode materials based on layered Li-Ni-Mn-Co oxide could be the next generation of positive electrode materials for high energy density lithium-ion batteries. This is because a high energy core material with poor stability against the electrolyte can be protected by a thin layer of a stable shell material. In our previous report1,2, Li and Mn-rich materials were used as the protecting shell, and Ni-rich materials were used as the core. It was shown that the Mn-rich shell can effectively protect the Ni-rich core from reactions with the electrolyte while the Ni-rich core renders a high and stable average voltage.2However, it was also found that diffusion of the cations between the core and shell phases occurs during sintering. Figure 1 shows energy dispersive X-ray spectroscopy (EDS) results of the core-shell precursor and lithiated samples. It is demonstrated that a Mn-rich shell was still maintained whereas the Co, which was only in the shell in the precursor, was approximately homogeneous throughout the particles after sintering at 900oC compared to the prepared hydroxide precursors. This diffusion phenomena in core-shell/gradient materials has not been previously examined in detail, to our knowledge, despite the large amount of work on the nickel rich NMC (LiNixMnyCozO2) (without excess lithium) gradient cathode material, which has been published and cited.3–5In this work, a series of experiments to measure the interdiffusivity of the cations were designed. Pellets of core material and shell material were placed in contact and heated to induce cation diffusion over long, measurable distances. Based on the analyzed composition profiles, interdiffusion constants can be extracted. Li1.09(Ni0.8Mn0.2)0.81O2, Li1.0 5(Co0.9Mn0.1)0.8 5O2, LiNi0.8Co0.2O2 and LiCoO2 powders were produced at 900oC in oxygen for 10 h, while Li2MnO3 was made in air. Powder a was first compressed to 15 MPa for 5 minutes, then powder b was added to the mould on top of the pellet a. The powders were compressed again to 50 MPa for 10 mins to ensure a good contact at the interface. The pellets were heated to 900oC for 10 h and then cooled slowly. The heated pellets were purposely fractured in to smaller pieces, which were subsequently embedded into Crystal BondTM, and eventually polished to a mirror surface finish for scanning electron microscopy (SEM) and EDS experiments. Figure 2 shows the back scattered SEM image of the cross-section of a pellet that had LiCoO2 powder on the left side and LiNi0.8Co0.2O2 on the right side. Small pores are observed at the interface. Figure 3a shows the Ni and Mn atomic concentration profile against position for the Li1.09(Ni0.8Mn0.2)0.81O2/Li2MnO3 pellet measured using EDS line scans. The symbols show the original data points. The concentration of Ni decreased from 80% to about 0% in about 2 µm while the concentration of Mn increased from 20% to about 100% over the same distance. Figure 3b shows the Ni and Co atomic concentration profiles versus position for the LiCoO2/LiNi0.8Co0.2O2 pellet measured using EDS. Figure 3b shows that the atomic concentration of Co decreased from 100% to about 20% in about 20 µm, which is about 10 times larger than that for the Ni and Mn shown in Figure 3a, indicating the interdiffusion between Ni3+ and Co3+ is much larger than that between Ni3+ and Mn4+. This result agrees well with the trends observed in the core-shell samples shown in Figure 1. Interdiffusion constants of Ni, Co and Mn in various couples as a function of temperature will be extracted and will be reported in the presentation.

Atomic oxygen retrievals in the MLT region from SCIAMACHY nightglow limb measurements

Abstract. Vertical distributions of atomic oxygen concentration ([O]) in the mesosphere and lower thermosphere (MLT) region were retrieved from sun-synchronous SCIAMACHY/Envisat (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY on board the Environmental Satellite) limb measurements of the oxygen 557.7 nm green line emission in the terrestrial nightglow. A band pass filter was applied to eliminate contributions from other emissions, the impact of measurement noise and auroral activity. Vertical volume emission rate profiles were retrieved from integrated limb-emission rate profiles under the assumption that each atmospheric layer is horizontally homogeneous and absorption and scattering can be neglected. The radiative transfer problem was solved using regularized total least squares minimization in the inversion procedure. Atomic oxygen concentration profiles were retrieved from data collected for altitudes in the range 85–105 km with approximately 4 km vertical resolution during the time period from August 2002 to April 2012 at approximately 22:00 local time. The retrieval of [O] profiles was based on the generally accepted two-step Barth transfer scheme including consideration of quenching processes and the use of different available sources of temperature and atmospheric density profiles. A sensitivity analysis was performed for the retrieved [O] profiles to estimate maximum uncertainties assuming independent contributions of uncertainty components. Errors in photochemical model parameters depending on temperature uncertainties and random errors of model parameters contribute less than 50% to the overall [O] retrieval error. The retrieved [O] profiles were compared with reference [O] profiles provided by SABER/TIMED (Sounding of the Atmosphere using Broadband Emission Radiometry instrument on board the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics satellite) or by the NRLMSISE-00 (Naval Research Laboratory Mass Spectrometer and Incoherent Scatter radar Extended model, year: 2000) and SD-WACCM4 (Whole Atmosphere Community Climate Model with Specified Dynamics, version 4). A comparison of the retrieved [O] profiles with the reference [O] profiles led to the conclusion that the photochemical model taking into account quenching of O(1S) by O2, O(3P), and N2 and the SABER/TIMED model as a source of temperature and density profiles are the most appropriate choices for our case. The retrieved [O] profile time series exhibits characteristic seasonal variations in agreement with satellite observations based on analysis of OH Meinel band emissions and atmospheric models. A pronounced 11-year solar cycle variation can also be identified in the retrieved atomic oxygen concentration time series.

Heavy ion elastic recoil detection analysis of AlxOy/Pt/AlxOy multilayer selective solar absorber

An AlxOy/Pt/AlxOy multilayer solar absorber for use in solar-thermal applications has been deposited onto copper substrate by electron beam (e-beam) vacuum evaporation at room temperature. Different samples were annealed at different temperatures in air and characterized by spectrophotometry, emissometry, heavy ion elastic recoil detection analysis (HI-ERDA), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDS). The AlxOy/Pt/AlxOy multilayer solar absorbers heated up to 500°C were found to exhibit good spectral selectivity (α/ɛ) of 0.951/0.08. However, beyond 500°C the spectral selectivity decreased to 0.846/0.11, possibly due to thermally activated atomic interdiffusion profiles. HI-ERDA has been used to study depth-dependent atomic concentration profiles. These measurements revealed outward diffusion of the copper substrate towards the surface and therefore, the decrease in the constituents of the coating. The decrease in the intensity of Pt grains and formation of CuO and Cu2O phases at 700°C were confirmed by XRD and EDS.

Molecular Mechanism of Stabilization of Thin Films for Improved Water Evaporation Protection

All-atom molecular dynamics simulations and experimental characterization have been used to examine the structure and dynamics of novel evaporation-suppressing films where the addition of a water-soluble polymer to an ethylene glycol monooctadecyl ether monolayer leads to improved water evaporation resistance. Simulations and Langmuir trough experiments demonstrate the surface activity of poly(vinyl pyrrolidone) (PVP). Subsequent MD simulations performed on the thin films supported by the PVP sublayer show that, at low surface pressures, the polymer tends to concentrate at the film/water interface. The simulated atomic concentration profiles, hydrogen bonding patterns, and mobility analyses of the water-polymer-monolayer interfaces reveal that the presence of PVP increases the atomic density near the monolayer film, improves the film stability, and reduces the mobility of interfacial waters. These observations explain the molecular basis of the improved efficacy of these monolayer/polymer systems for evaporation protection of water and can be used to guide future development of organic thin films for other applications.

Transport of bismuth atoms during liquid phase epitaxial growth of InSbBi and GaSbBi

We have used numerical methods to investigate the concentration profile of bismuth atoms during liquid phase epitaxial growth of InSbBi and GaSbBi. The one-dimensional diffusive model has been used to study the transport of bismuth atoms at equally spaced layers in the proximity of the epitaxial interface growing under normal conditions of liquid phase epitaxy. The parameters such as growth temperature, cooling rate and melt supercooling have been varied to find out the optimum conditions of growth. The model has also been used to estimate the thickness of the grown layers as a function of growth time.

Deposition of 236U from atmospheric nuclear testing in Washington state (USA) and the Pechora region (Russian Arctic)

Stratospheric fallout-derived 236U has been detected by sector field ICPMS at two field locations for which our laboratory possessed available archived samples: A) four soil cores from Washington state (northwestern USA) and B) sediment cores from three small lakes in the Pechora region (Russian Arctic). Four Washington state soil cores exhibit 236U inventories of 8.1 ± 1.3, 11.1 ± 0.9, 18 ± 2, and 30.2 ± 3.9 Tatoms/m2; the respective 239Pu contents are 52.9 ± 3.5, 67 ± 3, 71 ± 2, and 151 ± 2 Tatoms/m2. A 236U/239Pu atom ratio of 0.19 ± 0.04 (1 SD) has been determined from the Washington state soil cores. The three Pechora region lake cores each exhibit coincident maxima in their 236U and 239Pu atom concentration profiles. The 236U/238U atom ratios are controlled by two independent factors; 236U is from fallout deposition and 238U concentrations are a property of the geochemical distribution of naturally occurring U. A 236U/238U atom ratio as high as 8.9 × 10−6 has been observed for acid-leached soils containing Pu solely derived from bomb-test fallout. Accordingly, a non-zero 236U background from stratospheric fallout must be recognized and taken into account when detectable 236U is used to infer specific local or regional influences of reactor-irradiated U.

Modeling of gradient index solar selective surfaces for solar thermal applications

Reflectance of selective surfaces is numerically simulated from a continuous atomic concentration profile and then compared with experimental data, from a solar selective surface for solar thermal applications. The propagation of electromagnetic radiation is solved by using finite differences equation. Several theoretical models for optical properties of composite materials are compared and a proper mixing procedure is achieved. Results from Bruggeman effective medium theory are verified by comparison with experimental data. These data was obtained from Ni electrochemically impregnated into nanoporous alumina anodized onto an aluminum substrate. Concentration profile was measured using X-ray Photoelectron Spectroscopy. The procedure allows deducing measured reflectance from a very detailed model of the surface in depth, which uses three materials at the same time. This model could be used as a layout for a general three constituents mix for improving properties of solar thermal surfaces.

Atomic intermixing in short-period InAs/GaSb superlattices

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and laser-assisted local-electrode atom-probe (LEAP) tomography are utilizing for characterizing short-period InAs/GaSb superlattices with an emphasis on obtaining the atomic concentration profiles with sub-nm resolution. HAADF-STEM permits direct visualization and counting of atomic columns in individual sub-layers. The spatial resolution of HAADF-STEM is sufficient to resolve the anion-cation dumbbells and, on this basis, to follow the atomic distributions across a superlattice. Both methods confirm that InAs-on-GaSb interfaces are wider than GaSb-on-InAs interfaces. The interfacial widths deduced from LEAP tomographic measurements are slightly larger than those extracted from HAADF-STEM micrographs, with the maximum total width not exceeding 4.5 monolayers. LEAP tomographic analysis shows the presence of about 7 at. % of Sb atoms in the middle of the InAs sub-layers, as a result of As/Sb substitutions during growth.