936 research outputs found
Effect of the cluster size in modeling the H_2 desorption and dissociative adsorption on Si(001)
Three different clusters, Si_9H_12, Si_15H_16, and Si_21H_20, are used in
density-functional theory calculations in conjunction with ab initio
pseudopotentials to study how the energetics of H_2 dissociativ e adsorption on
and associative desorption from Si(001) depends on the cluster size. The
results are compared to five-layer slab calculations using the same
pseudopotentials and high qu ality plane-wave basis set. Several
exchange-correlation functionals are employed. Our analysis suggests that the
smaller clusters generally overestimate the activation barriers and reaction
energy. The Si_21H_20 cluster, however, is found to predict reaction
energetics, with E_{a}^{des}=56 +- 3 kcal/mol (2.4 +- 0.1 eV), reasonably close
(though still different) to that obtained from the slab calculations.
Differences in the calculated activation energies are discussed in relation to
the efficiency of clusters to describe the properties of the clean Si(001)-2x1
surface.Comment: 10 pages, 6 figures, submitted to J. Chem. Phy
First-principles studies of kinetics in epitaxial growth of III-V semiconductors
We demonstrate how first-principles calculations using density-functional
theory (DFT) can be applied to gain insight into the molecular processes that
rule the physics of materials processing. Specifically, we study the molecular
beam epitaxy (MBE) of arsenic compound semiconductors. For homoepitaxy of GaAs
on GaAs(001), a growth model is presented that builds on results of DFT
calculations for molecular processes on the beta2-reconstructed GaAs(001)
surface, including adsorption, desorption, surface diffusion and nucleation.
Kinetic Monte Carlo simulations on the basis of the calculated energetics
enable us to model MBE growth of GaAs from beams of Ga and As_2 in atomistic
detail. The simulations show that island nucleation is controlled by the
reaction of As_2 molecules with Ga adatoms on the surface. The analysis reveals
that the scaling laws of standard nucleation theory for the island density as a
function of growth temperature are not applicable to GaAs epitaxy. We also
discuss heteroepitaxy of InAs on GaAs(001), and report first-principles DFT
calculations for In diffusion on the strained GaAs substrate. In particular we
address the effect of heteroepitaxial strain on the growth kinetics of
coherently strained InAs islands. The strain field around an island is found to
cause a slowing-down of material transport from the substrate towards the
island and thus helps to achieve more homogeneous island sizes.Comment: 12 pages, 7 figures, REVTeX, Final version to appear in Appl. Phys. A
(2002). Other related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
L-band ATS 5/Orion/S. S. Manhattan marine navigation and communication experiment Final report
L-band signals relayed by synchronous satellite for navigation and data communicatio
Thermal oxidative degradation reactions of perfluoroalklethers
The objective of this contract was to investigate the mechanisms operative in thermal and thermal oxidative degradation of Fomblin Z and hexafluoropropene oxide derived fluids and the effect of alloys and additives upon these processes. The nature of arrangements responsible for the inherent thermal oxidative instability of the Fomblin Z fluids has not been established. It was determined that this behavior was not associated with hydrogen end-groups or peroxy linkages. The degradation rate of these fluids at elevated temperatures in oxidizing atmospheres was found to be dependent on the surface/volume ratio. Once a limiting ratio was reached, a steady rate appeared to be attained. Based on elemental analysis and oxygen consumption data, -CF2OCF2CF2O-, not -CF2CF2O-, is one of the major arrangements present. The action of the M-50 and Ti(4 Al, 4 Mn) alloys was found to be much more drastic in the case of Fomblin Z fluids than that observed for the hexalfuoropropane oxide derived materials. The effectiveness of antioxidation/anticorrosion additives, P-3 and phospha-s-triazine, in the presence of metal alloys was very limited at 316 C; at 288 C the additives arrested almost completely the fluid degradation. The phospha-s-triazine appeared to be at least twice as effective as the P-3 compound; it also protected the coupon better. The Ti(4 Al, 4 Mn) alloy degraded the fluid mainly by chain scission processes; this took place to a much lesser degree with M-50
Determination of Traveltime in the Delaware River, Hancock, New York, to the Delaware Water Gap by Use of a Conservative Dye Tracer
Traveltime of a soluble substance was determined for a 120-mile reach of the Delaware River from the confluence of the East Branch Delaware River and the West Branch Delaware River at Hancock, N.Y. to the Delaware Water Gap. Dye studies were conducted at the 85-95 percent and the 25-30 percent flow durations. Discharges ranged from 500-1,740 cubic feet per second during the 85-95 percent flow duration and 3,070-7,500 cubic feet per second for the 25-30 percent flow duration. The data were used to develop a set of time-concentration curves that would enable estimation of the traveltime of a spill at any point in the river within the study reach for 10 flow durations. The leading edge of a contaminant spill at Buckingham Access would take about 70 hours to reach the Delaware Water Gap when flows are at the 30-percent flow duration. The trailing edge (location of the dye cloud when concentrations would decrease to 10 percent of the peak concentration) would take about 50 hours after the arrival of the leading edge
International Intercomparison of In Situ Chlorophyll-a Measurements for Data Quality Assurance of the Swedish Monitoring Program
Chlorophyll-a is an essential climate variable. Chlorophyll-a in situ measurements are usually used for the validation of satellite images. Previous intercomparisons have shown that there can be substantial differences between in situ laboratories. In order to shed light on these differences, we arranged international chlorophyll-a intercomparisons with eight participating laboratories during 1–2 July 2021. We performed two dedicated transects through Bråviken bay (NW Baltic proper) and sampled four stations in each transect along a chlorophyll-a gradient. We took three surface replicates per laboratory and per station, i.e., 24 samples per laboratory. The samples were filtered through Whatman GF/F filters, and filters were frozen in liquid nitrogen and distributed in dry ice to all laboratories together with chlorophyll-a standards. The results between labs compared quite well. The mean normalized bias (MNB) of the standard measurements ranged between −23% and +19% for all laboratories and −7% to +19% for the Baltic Sea laboratories compared to high-performance liquid chromatography. The MNB of the two Bråviken transects ranged between −23 and +17% for all laboratories (compared to the median of all spectrophotometric and fluorometric measurements) and between −2 and +17% for the Baltic Sea laboratories. On average, the chlorophyll-a concentrations measured by the fluorometric method were about 13% higher than those measured by spectrophotometry, and fluorometry samples tended to have more scatter. The largest uncertainties seem to be caused by variable storage and extraction methods and are not fully captured in this intercomparison. This is demonstrated by analyzing historical comparisons revealing very large uncertainties (root mean square difference (RMSD) up to 109% and bias up to 68%), possibly due to too low filtration volumes and due to different extraction and storage methods. Our recommendation is to flash-freeze samples in liquid nitrogen and store them at −80°C. After storage, they should be extracted and measured at room temperature within 6–24 h. Our results also indicate that ethanol is much more efficient in extracting Chl-a than acetone. Last but not least, we would like to point out that the uncertainties in measuring chlorophyll-a by satellite are now within the range of in situ data, as shown here by comparing the in situ results from this study with published remote sensing results from the literature.publishedVersio
Interaction between pentacene molecules and monolayer transition metal dichalcogenides
Using first-principles calculations based on density-functional theory, we
investigated the adsorption of pentacene molecules on monolayer two-dimensional
transition metal dichalcogenides (TMD). We considered the four most popular
TMDs, namely, MoS, MoSe, WS and WSe, and we examined the
structural and electronic properties of pentacene/TMD systems. We discuss how
monolayer pentacene interacts with the TMDs, and how this interaction affects
the charge transfer and work function of the heterostructure. We also analyse
the type of band alignment formed in the heterostructure and how it is affected
by molecule-molecule and molecule-substrate interactions. Such analysis is
valuable since pentacene/TMD heterostructures are considered to be promising
for application in flexible, thin and lightweight photovoltaics and
photodetectors.Comment: 13 pages, 4 figure
Anisotropic ferromagnetism in carbon doped zinc oxide from first-principles studies
A density functional theory study of substitutional carbon impurities in ZnO
has been performed, using both the generalized gradient approximation (GGA) and
a hybrid functional (HSE06) as exchange-correlation functional. It is found
that the non-spinpolarized C impurity is under almost all
conditions thermodynamically more stable than the C impurity which
has a magnetic moment of , with the exception of very O-poor
and C-rich conditions. This explains the experimental difficulties in sample
preparation in order to realize -ferromagnetism in C-doped ZnO. From GGA
calculations with large 96-atom supercells, we conclude that two
C-C impurities in ZnO interact ferromagnetically, but
the interaction is found to be short-ranged and anisotropic, much stronger
within the hexagonal -plane of wurtzite ZnO than along the c-axis. This
layered ferromagnetism is attributed to the anisotropy of the dispersion of
carbon impurity bands near the Fermi level for C impurities in
ZnO. From the calculated results, we derive that a C
concentration between 2% and 6% should be optimal to achieve
-ferromagnetism in C-doped ZnO.Comment: 9 pages, 7 figure
The effects of metals and inhibitors on thermal oxidative degradation reactions of unbranched perfluoroalkylethers
Thermal oxidative degradation studies were performed on unbranched perfluoroalkylethers at 288 C in oxygen. Metals and alloys studied included Ti, Al, and Ti (4 Al, 4 Mn). The mechanism of degradation was by chain scission. Ti and Al promoted less degradation than Ti (4 Al, 4 Mn). The two inhibitors investigated (a perfluorophenyl phosphine and a phosphatriazine) reduced degradation rates by several orders of magnitude. Both inhibitors were effective for the same duration (75 to 100 hours). The phosphatriazine appeared to provide more surface protection
Selection of highly specific and sensitive mRNA biomarkers for the identification of blood
In the present work, we have evaluated eight reportedly blood-specific mRNA markers (HBB, HBA, ALAS2, CD3G, ANK1, PBGD, SPTB, AQP9) in an attempt to determine the most suitable ones for use in forensic applications based on their sensitivities, specificities and performance with casework samples. While varying levels of expression were observed, all markers were relatively sensitive requiring as little as 1 ng of RNA input into the reverse transcription (RT) reaction. In singleplex reactions, seven of the eight analyzed blood markers (all except AQP9) demonstrated a high degree of specificity for blood. In multiplex reactions, non-reproducible cross-reactivity was observed for several of the mRNA markers, which was reduced and, in most cases, eliminated when less input total RNA was used. Additionally, some cross-reactivity was observed with tissue and animal samples. Despite differences in the observed sensitivity and specificity of the blood markers examined in this study, a number of the candidates appear to be suitable for inclusion in appropriately validated multiplex mRNA-based body fluid identification systems
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