187 research outputs found
Nano scale phase separation in Au-Ge system on ultra clean Si(100) surfaces
We report on the formation of lobe-lobe (bi-lobed) Au-Ge nanostructures under
ultra high vacuum (UHV) conditions (\approx 3\times 10^{-10} mbar) on clean
Si(100) surfaces. For this study, \approx 2.0 nm thick Au samples were grown on
the substrate surface by molecular beam epitaxy (MBE). Thermal annealing was
carried out inside the UHV chamber at temperature \apprx 500{\deg}C and
following this, nearly square shaped Au_{x}Si_{1-x} nano structures of average
length \approx 48 nm were formed. A \approx 2 nm Ge film was further deposited
on the above surface while the substrate was kept at a temperature of \approx
500{\deg}C. Well ordered Au-Ge nanostructures where Au and Ge residing side by
side (lobe-lobe structures) were formed. In our systematic studies, we show
that, gold-silicide nanoalloy formation at the substrate (Si) surface is
necessary for forming phase separated Au-Ge bilobed nanostructures. Electron
microscopy (TEM, STEM-EDS, SEM) studies were carried out to determine the
structure of Au - Ge nano systems. Rutherford backscattering Spectrometry
measurements show gold inter-diffusion into substrate while it is absent for
Ge.Comment: 23 pages, 6 Figures, 1 Tabl
Increase of the mean inner Coulomb potential in Au clusters induced by surface tension and its implication for electron scattering
Electron holography in a transmission electron microscope was applied to
measure the phase shift induced by Au clusters as a function of the cluster
size. Large phase shifts Df observed for small Au clusters cannot be described
by the well-known equation Df=C_E V_0 t (C_E: interaction constant, V_0: mean
inner Coulomb potential (MIP) of bulk gold, t: cluster thickness). The rapid
increase of the Au MIP with decreasing cluster size derived from Df, can be
explained by the compressive strain of surface atoms in the cluster
Measurement of local crystal lattice strain variations in dealloyed nanoporous gold
Reversible macroscopic length changes in nanoporous structures can be achieved by applying electric potentials or by exposing them to different gases or liquids. Thus, these materials are interesting candidates for applications as sensors or actuators. Macroscopic length changes originate from microscopic changes of crystal lattice parameters. In this report, we show spatially resolved measurements of crystal lattice strain in dealloyed nanoporous gold. The results confirm theory by indicating a compression of the lattice along the axis of cylindrically shaped ligaments and an expansion in radial direction. Furthermore, we show that curved npAu surfaces show inward relaxation of the surface layer. (Figure presented) IMPACT STATEMENT We show spatially resolved measurements of strain in nanoporous gold confirming theory: Crystal lattice is compressed along the axis of cylindrical ligaments and expanded in radial direction, surfaces relax inward
Influence of plasmon excitations on atomic‑resolution quantitative 4D scanning transmission electron microscopy
Scanning transmission electron microscopy (STEM) allows to gain quantitative information on the
atomic‑scale structure and composition of materials, satisfying one of todays major needs in the
development of novel nanoscale devices. The aim of this study is to quantify the impact of inelastic,
i.e. plasmon excitations (PE), on the angular dependence of STEM intensities and answer the
question whether these excitations are responsible for a drastic mismatch between experiments
and contemporary image simulations observed at scattering angles below∼40 mrad. For the two
materials silicon and platinum, the angular dependencies of elastic and inelastic scattering are
investigated. We utilize energy filtering in two complementary microscopes, which are representative
for the systems used for quantitative STEM, to form position‑averaged diffraction patterns as well
as atomically resolved 4D STEM data sets for different energy ranges. The resulting five‑dimensional
data are used to elucidate the distinct features in real and momentum space for different energy
losses. We find different angular distributions for the elastic and inelastic scattering, resulting in an
increased low‑angle intensity (∼10–40 mrad). The ratio of inelastic/elastic scattering increases with
rising sample thickness, while the general shape of the angular dependency is maintained. Moreover,
the ratio increases with the distance to an atomic column in the low‑angle regime. Since PE are usually
neglected in image simulations, consequently the experimental intensity is underestimated at these
angles, which especially affects bright field or low‑angle annular dark field imaging. The high‑angle
regime, however, is unaffected. In addition, we find negligible impact of inelastic scattering on first‑
moment imaging in momentum‑resolved STEM, which is important for STEM techniques to measure
internal electric fields in functional nanostructures. To resolve the discrepancies between experiment
and simulation, we present an adopted simulation scheme including PE. This study highlights
the necessity to take into account PE to achieve quantitative agreement between simulation and
experiment. Besides solving the fundamental question of missing physics in established simulations,
this finally allows for the quantitative evaluation of low‑angle scattering, which contains valuable
information about the material investigated
Strontium doping in mullite-type bismuth aluminate: A vacancy investigation using neutrons, photons and electrons
We report on strontium doped dibismuth-nonaoxoaluminate(III) produced at 1023 K. Partial substitution of bismuth by strontium in the structure yields oxygen vacancies for charge balance. Introducing oxygen vacancies rearranged the associated Al2O7 double-tetrahedra forming “Al3O10” tri-clusters which were identified by multi-quantum 27Al MAS NMR. Both STEM-EDX and XPS showed homogeneous distribution of strontium in the bulk and on the surface, respectively. Moreover, XPS confirms the valence state of bismuth after doping. The orientations of bismuth 6s2 lone electron pairs were calculated using DFT methods. The amount of strontium in the crystal structure was further confirmed from the decomposition product SrAl12O19 formed during the temperature-dependent X-ray powder diffraction. The structural proof was carried out by refining the structure of (Bi0.94Sr0.06)2Al4O8.94 from powder neutron and X-ray diffraction data. Rietveld refinements clearly showed the under occupation of one oxygen site and the shift of two aluminum atoms from the double-tetrahedra to two tri-cluster sites
Entropy-limited topological protection of skyrmions
Magnetic skyrmions are topologically protected whirls that decay through singular magnetic configurations known as Bloch points. We used Lorentz transmission electron microscopy to infer the energetics associated with the topological decay of magnetic skyrmions far from equilibrium in the chiral magnet Fe1-xCoxSi. We observed that the lifetime tau of the skyrmions depends exponentially on temperature, tau similar to tau(0) exp(Delta E/k(B)T). The prefactor tau(0) of this Arrhenius law changes by more than 30 orders of magnitude for small changes of the magnetic field, reflecting a substantial reduction of the lifetime of skyrmions by entropic effects and, thus, an extreme case of enthalpy-entropy compensation. Such compensation effects, being well known across many different scientific disciplines, affect topological transitions and, thus, topological protection on an unprecedented level
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