179 research outputs found
Nanoscale periodicity in stripe-forming systems at high temperature: Au/W(110)
We observe using low-energy electron microscopy the self-assembly of
monolayer-thick stripes of Au on W(110) near the transition temperature between
stripes and the non-patterned (homogeneous) phase. We demonstrate that the
amplitude of this Au stripe phase decreases with increasing temperature and
vanishes at the order-disorder transition (ODT). The wavelength varies much
more slowly with temperature and coverage than theories of stress-domain
patterns with sharp phase boundaries would predict, and maintains a finite
value of about 100 nm at the ODT. We argue that such nanometer-scale stripes
should often appear near the ODT.Comment: 5 page
Labyrinthine Island Growth during Pd/Ru(0001) Heteroepitaxy
Using low energy electron microscopy we observe that Pd deposited on Ru only
attaches to small sections of the atomic step edges surrounding Pd islands.
This causes a novel epitaxial growth mode in which islands advance in a
snakelike motion, giving rise to labyrinthine patterns. Based on density
functional theory together with scanning tunneling microscopy and low energy
electron microscopy we propose that this growth mode is caused by a surface
alloy forming around growing islands. This alloy gradually reduces step
attachment rates, resulting in an instability that favors adatom attachment at
fast advancing step sections
Electron reflectivity measurements of Ag adatom concentrations on W(110)
The density of two-dimensional Ag adatom gases on W(110) is determined by
monitoring local electron reflectivity using low energy electron microscopy
(LEEM). This method of adatom concentration measurement can detect changes in
adatom density at least as small as 10 ML for a m size region of
the surface. Using this technique at high temperatures, we measure the
sublimation rates of Ag adatoms on W(110). At lower temperatures, where Ag
adatoms condense into monolayer islands, we determine the temperature
dependence of the density of adatoms coexisting with this condensed phase and
compare it with previous estimates.Comment: Presented at the ECOSS 23 Conference (Berlin 2005
Kinetics and thermodynamics of carbon segregation and graphene growth on Ru(0001)
We measure the concentration of carbon adatoms on the Ru(0001) surface that
are in equilibrium with C atoms in the crystal's bulk by monitoring the
electron reflectivity of the surface while imaging. During cooling from high
temperature, C atoms segregate to the Ru surface, causing graphene islands to
nucleate. Using low-energy electron microscopy (LEEM), we measure the growth
rate of individual graphene islands and, simultaneously, the local
concentration of C adatoms on the surface. We find that graphene growth is fed
by the supersaturated, two-dimensional gas of C adatoms rather than by direct
exchange between the bulk C and the graphene. At long times, the rate at which
C diffuses from the bulk to the surface controls the graphene growth rate. The
competition among C in three states - dissolved in Ru, as an adatom, and in
graphene - is quantified and discussed. The adatom segregation enthalpy
determined by applying the simple Langmuir-McLean model to the
temperature-dependent equilibrium concentration seriously disagrees with the
value calculated from first-principles. This discrepancy suggests that the
assumption in the model of non-interacting C is not valid
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In-Situ STM Studies of Strain Stabilized Thin Film Dislocation Networks Under Applied Stress
How metal films de-wet substrates - identifying the kinetic pathways and energetic driving forces
We study how single-crystal chromium films of uniform thickness on W(110)
substrates are converted to arrays of three-dimensional (3D) Cr islands during
annealing. We use low-energy electron microscopy (LEEM) to directly observe a
kinetic pathway that produces trenches that expose the wetting layer. Adjacent
film steps move simultaneously uphill and downhill relative to the staircase of
atomic steps on the substrate. This step motion thickens the film regions where
steps advance. Where film steps retract, the film thins, eventually exposing
the stable wetting layer. Since our analysis shows that thick Cr films have a
lattice constant close to bulk Cr, we propose that surface and interface stress
provide a possible driving force for the observed morphological instability.
Atomistic simulations and analytic elastic models show that surface and
interface stress can cause a dependence of film energy on thickness that leads
to an instability to simultaneous thinning and thickening. We observe that
de-wetting is also initiated at bunches of substrate steps in two other
systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are
converted into patterns of unidirectional stripes as the trenches that expose
the wetting layer lengthen along the W[001] direction. Finally, we observe how
3D Cr islands form directly during film growth at elevated temperature. The Cr
mesas (wedges) form as Cr film steps advance down the staircase of substrate
steps, another example of the critical role that substrate steps play in 3D
island formation
Cryptosporidium Priming Is More Effective than Vaccine for Protection against Cryptosporidiosis in a Murine Protein Malnutrition Model
Cryptosporidium is a major cause of severe diarrhea, especially in malnourished children. Using a murine model of C. parvum oocyst challenge that recapitulates clinical features of severe cryptosporidiosis during malnutrition, we interrogated the effect of protein malnutrition (PM) on primary and secondary responses to C. parvum challenge, and tested the differential ability of mucosal priming strategies to overcome the PM-induced susceptibility. We determined that while PM fundamentally alters systemic and mucosal primary immune responses to Cryptosporidium, priming with C. parvum (106 oocysts) provides robust protective immunity against re-challenge despite ongoing PM. C. parvum priming restores mucosal Th1-type effectors (CD3+CD8+CD103+ T-cells) and cytokines (IFNγ, and IL12p40) that otherwise decrease with ongoing PM. Vaccination strategies with Cryptosporidium antigens expressed in the S. Typhi vector 908htr, however, do not enhance Th1-type responses to C. parvum challenge during PM, even though vaccination strongly boosts immunity in challenged fully nourished hosts. Remote non-specific exposures to the attenuated S. Typhi vector alone or the TLR9 agonist CpG ODN-1668 can partially attenuate C. parvum severity during PM, but neither as effectively as viable C. parvum priming. We conclude that although PM interferes with basal and vaccine-boosted immune responses to C. parvum, sustained reductions in disease severity are possible through mucosal activators of host defenses, and specifically C. parvum priming can elicit impressively robust Th1-type protective immunity despite ongoing protein malnutrition. These findings add insight into potential correlates of Cryptosporidium immunity and future vaccine strategies in malnourished children
BCAA catabolism in brown fat controls energy homeostasis through SLC25A44.
Branched-chain amino acid (BCAA; valine, leucine and isoleucine) supplementation is often beneficial to energy expenditure; however, increased circulating levels of BCAA are linked to obesity and diabetes. The mechanisms of this paradox remain unclear. Here we report that, on cold exposure, brown adipose tissue (BAT) actively utilizes BCAA in the mitochondria for thermogenesis and promotes systemic BCAA clearance in mice and humans. In turn, a BAT-specific defect in BCAA catabolism attenuates systemic BCAA clearance, BAT fuel oxidation and thermogenesis, leading to diet-induced obesity and glucose intolerance. Mechanistically, active BCAA catabolism in BAT is mediated by SLC25A44, which transports BCAAs into mitochondria. Our results suggest that BAT serves as a key metabolic filter that controls BCAA clearance via SLC25A44, thereby contributing to the improvement of metabolic health
Field-effect transistors made of graphene grown on recycled copper foils
In this paper, recycled low purity copper foils (98%) are tested after a multiple-use process for obtaining singlelayer graphene (SLG). The graphene transfer technique based on water electrolysis (bubbling) is used for the preservation of the Cu foils in multiple graphene deposition cycles. Preliminary cleaning by O2 plasma remove carbon residues from the copper surface. The Cu foils are then cleaned with hydrochloric (HCl) or acetic acid
(CH3COOH) solutions, used as chemical baths, and the results compared. Atomic force microscopy used to check the Cu foil roughness, a critical parameter for the growth of SLG, shows root-mean-square roughness of 4.61, 28.00, 3.50 nm, for new Cu foil, after acetic acid, and after hydrochloric acid treatments, respectively. Full
coverage of SLG was obtained only during the third usage of Cu foils, i.e., after two recycling cycles. Carrier
mobility measured on graphene field-effect transistors fabricated after each recycling cycle, show values of 814
(1728) and 1847 cm2/V (1147 cm2/V), for electrons (holes) after one and two recycling cycles, respectively, thus demonstrating the improvement of the quality of the graphene with the number of Cu recycling cycles. Devices fabricated with graphene grown on the initial, low-purity, Cu foil did not show transistor behavior.FCT in the framework of the Strategic Funding UIDB/04650/2020, POCI-01-0145-FEDER-028114 (GRAPHSENS). NPq – Brazil. Work supported by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Funding UIDB/04650/2020 and by the project POCI-01-0145-FEDER-028114 (GRAPHSENS
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