138 research outputs found
Electron scattering in atomic force microscopy experiments
It has been shown that electron transitions, as measured in a scanning
tunnelling microscope (STM), are related to chemical interactions in a
tunnelling barrier. Here, we show that the shape and apparent height of
subatomic features in an atomic force microscopy (AFM) experiment on Si(111)
depend directly on the available electron states of the silicon surface and the
silicon AFM tip. Simulations and experiments confirm that forces and currents
show similar subatomic variations for tip-sample distances approaching the bulk
bonding length.Comment: 5 pages and 4 figure
Focussing quantum states
Does the size of atoms present a lower limit to the size of electronic
structures that can be fabricated in solids? This limit can be overcome by
using devices that exploit quantum mechanical scattering of electron waves at
atoms arranged in focussing geometries on selected surfaces. Calculations
reveal that features smaller than a hydrogen atom can be obtained. These
structures are potentially useful for device applications and offer a route to
the fabrication of ultrafine and well defined tips for scanning tunneling
microscopy.Comment: 4 pages, 4 figure
Local spectroscopy and atomic imaging of tunneling current, forces and dissipation on graphite
Theory predicts that the currents in scanning tunneling microscopy (STM) and
the attractive forces measured in atomic force microscopy (AFM) are directly
related. Atomic images obtained in an attractive AFM mode should therefore be
redundant because they should be \emph{similar} to STM. Here, we show that
while the distance dependence of current and force is similar for graphite,
constant-height AFM- and STM images differ substantially depending on distance
and bias voltage. We perform spectroscopy of the tunneling current, the
frequency shift and the damping signal at high-symmetry lattice sites of the
graphite (0001) surface. The dissipation signal is about twice as sensitive to
distance as the frequency shift, explained by the Prandtl-Tomlinson model of
atomic friction.Comment: 4 pages, 4 figures, accepted at Physical Review Letter
Distance dependence of the phase signal in eddy current microscopy
Atomic force microscopy using a magnetic tip is a promising tool for
investigating conductivity on the nano-scale. By the oscillating magnetic tip
eddy currents are induced in the conducting parts of the sample which can be
detected in the phase signal of the cantilever. However, the origin of the
phase signal is still controversial because theoretical calculations using a
monopole appoximation for taking the electromagnetic forces acting on the tip
into account yield an effect which is too small by more than two orders of
magnitude. In order to determine the origin of the signal we used especially
prepared gold nano patterns embedded in a non-conducting polycarbonate matrix
and measured the distance dependence of the phase signal. Our data clearly
shows that the interacting forces are long ranged and therefore, are likely due
to the electromagnetic interaction between the magnetic tip and the conducting
parts of the surface. Due to the long range character of the interaction a
change in conductivity of m can be
detected far away from the surface without any interference from the
topography
Moir\'e patterns on STM images of graphite from surface and subsurface rotated layer
We have observed with STM moir\'e patterns corresponding to the rotation of
one graphene layer on HOPG surface. The moir\'e patterns were characterized by
rotation angle and extension in the plane. Additionally, by identifying border
domains and defects we can discriminate between moir\'e patterns due to
rotation on the surface or subsurface layer. For a better understanding of
moir\'e patterns formation we have studied by first principles an array of
three graphene layers where the top or the middle layer appears rotated around
the stacking axis. We compare the experimental and theoretical results and we
show the strong influence of rotations both in surface and subsurface layers
for moir\'e patterns formation in corresponding STM images.Comment: 5 pages, 6 figure
Spin S=1 chain model for BaMoP2O8
The reported study was funded by RFBR according to the research project № 18-32-00018
Substrate-induced band gap opening in epitaxial graphene
Graphene has shown great application potentials as the host material for next
generation electronic devices. However, despite its intriguing properties, one
of the biggest hurdles for graphene to be useful as an electronic material is
its lacking of an energy gap in the electronic spectra. This, for example,
prevents the use of graphene in making transistors. Although several proposals
have been made to open a gap in graphene's electronic spectra, they all require
complex engineering of the graphene layer. Here we show that when graphene is
epitaxially grown on the SiC substrate, a gap of ~ 0.26 is produced. This gap
decreases as the sample thickness increases and eventually approaches zero when
the number of layers exceeds four. We propose that the origin of this gap is
the breaking of sublattice symmetry owing to the graphene-substrate
interaction. We believe our results highlight a promising direction for band
gap engineering of graphene.Comment: 10 pages, 4 figures; updated reference
Molecular beam growth of graphene nanocrystals on dielectric substrates
We demonstrate the growth of graphene nanocrystals by molecular beam methods
that employ a solid carbon source, and that can be used on a diverse class of
large area dielectric substrates. Characterization by Raman and Near Edge X-ray
Absorption Fine Structure spectroscopies reveal a sp2 hybridized hexagonal
carbon lattice in the nanocrystals. Lower growth rates favor the formation of
higher quality, larger size multi-layer graphene crystallites on all
investigated substrates. The surface morphology is determined by the roughness
of the underlying substrate and graphitic monolayer steps are observed by
ambient scanning tunneling microscopy.Comment: Accepted in Carbon; Discussion section added; 20 pages, 6 figures (1
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