77 research outputs found
Holography and Coherent Diffraction with Low-Energy Electrons: A Route towards Structural Biology at the Single Molecule Level
The current state of the art in structural biology is led by NMR, X-ray
crystallography and TEM investigations. These powerful tools however all rely
on averaging over a large ensemble of molecules. Here, we present an
alternative concept aiming at structural analysis at the single molecule level.
We show that by combining electron holography and coherent diffraction imaging
estimations concerning the phase of the scattered wave become needless as the
phase information is extracted from the data directly and unambiguously.
Performed with low-energy electrons the resolution of this lens-less microscope
is just limited by the De Broglie wavelength of the electron wave and the
numerical aperture, given by detector geometry. In imaging freestanding
graphene, a resolution of 2 Angstrom has been achieved revealing the 660.000
unit cells of the graphene sheet from one data set at once. Applied to
individual biomolecules the method allows for non-destructive imaging and
imports the potential to distinguish between different conformations of
proteins with atomic resolution.Comment: 17 pages, 10 figures; Ultramicroscopy 201
Low-energy electron transmission imaging of clusters on free-standing graphene
We investigated the utility of free-standing graphene as a transparent sample
carrier for imaging nanometer-sized objects by means of low-energy electron
holography. The sample preparation for obtaining contamination-free graphene as
well as the experimental setup and findings are discussed. For incoming
electrons with 66 eV kinetic energy graphene exhibits 27% opacity per layer.
Hence, electron holograms of nanometer-sized objects adsorbed on free-standing
graphene can be recorded and numerically reconstructed to reveal the object's
shapes and distribution. Furthermore, a Moire effect has been observed with
free-standing graphene multi-layers
Mapping Unoccupied Electronic States of Freestanding Graphene by Angle-Resolved Low-Energy Electron Transmission
We report angle-resolved electron transmission measurements through
freestanding graphene sheets in the energy range of 18 to 30 eV above the Fermi
level. The measurements are carried out in a low-energy electron point source
microscope, which allows simultaneously probing the transmission for a large
angular range. The characteristics of low-energy electron transmission through
graphene depend on its electronic structure above the vacuum level. The
experimental technique described here allows mapping the unoccupied band
structure of freestanding two-dimensional materials as a function of energy and
probing angle, respectively in-plane momentum. Our experimental findings are
consistent with theoretical predictions of a resonance in the band structure of
graphene above the vacuum level [V. U. Nazarov, E. E. Krasovskii, and V. M.
Silkin, Physical Review B 87, 041405 (2013)]
Moire structures in twisted bilayer graphene studied by transmission electron microscopy
We investigate imaging of moire structures in free-standing twisted bilayer
graphene (TBG) carried out by transmission electron microscopy (TEM) in
diffraction and in-line Gabor holography modes. Electron diffraction patterns
of TBG acquired at typical TEM electron energies of 80 - 300 keV exhibit the
diffraction peaks caused by diffraction on individual layers. However,
diffraction peaks at the scattering angles related to the periodicity of the
moire structure have not been observed in such diffraction patterns. We show
that diffraction on moire structure can create intense diffraction peaks if the
energy of the probing electrons is very low, in the range of a few tens of eV.
Experimental diffraction patterns of TBG acquired with low-energy electrons of
236 eV exhibiting peaks attributed to the moire structure periodicity are
shown. In holography mode, the intensity of the wave transmitted through the
sample and measured in the far-field can be enhanced or decreased depending on
the atomic arrangement, as for example AA or AB stacking. Thus, a decrease of
intensity in the far-field must not necessarily be associated with some
absorption inside the sample but can simply be a result of a particular atomic
arrangement. We believe that our findings can be important for exploiting
graphene as a support in electron imaging
Imaging the potential distribution of individual charged impurities on graphene by low-energy electron holography
While imaging individual atoms can routinely be achieved in high resolution
transmission electron microscopy, visualizing the potential distribution of
individually charged adsorbates leading to a phase shift of the probing
electron wave is still a challenging task. Low-energy electrons (30 - 250 eV)
are sensitive to localized potential gradients. We employed low-energy electron
holography to acquire in-line holograms of individual charged impurities on
free-standing graphene. By applying an iterative phase retrieval reconstruction
routine we recover the potential distribution of the localized charged
impurities present on free-standing graphene
Design and Implementation of a Micron-Sized Electron Column Fabricated by Focused Ion Beam Milling
We have designed, fabricated and tested a micron-sized electron column with
an overall length of about 700 microns comprising two electron lenses; a
micro-lens with a minimal bore of 1 micron followed by a second lens with a
bore of up to 50 microns in diameter to shape a coherent low-energy electron
wave front. The design criteria follow the notion of scaling down source size,
lens-dimensions and kinetic electron energy for minimizing spherical
aberrations to ensure a parallel coherent electron wave front. All lens
apertures have been milled employing a focused ion beam and could thus be
precisely aligned within a tolerance of about 300 nm from the optical axis.
Experimentally, the final column shapes a quasi-planar wave front with a
minimal full divergence angle of 4 mrad and electron energies as low as 100 eV
Uptake and release kinetics of 22 polar organic chemicals in the Chemcatcher passive sampler
The Chemcatcher passive sampler, which uses Empore™ disks as sampling phase, is frequently used to monitor polar organic chemicals in river water and effluents. Uptake kinetics need to be quantified to calculate time-weighted average concentrations from Chemcatcher field deployments. Information on release kinetics is needed if performance reference compounds (PRCs) are used to quantify the influence of environmental conditions on the uptake. In a series of uptake and elimination experiments, we used Empore™ SDB disks (poly(styrenedivinylbenzene) copolymer modified with sulfonic acid groups) as a sampling phase and 22 compounds with a logK ow (octanol-water partitioning coefficient) range from −2.6 to 3.8. Uptake experiments were conducted in river water or tap water and lasted up to 25days. Only 1 of 22 compounds (sulfamethoxazole) approached equilibrium in the uptake trials. Other compounds showed continuing non-linear uptake, even after 25days. All compounds could be released from SDB disks, and desorption was proportionally higher in disks loaded for shorter periods. Desorption showed two-phase characteristics, and desorption was proportionally higher for passively sorbed compounds compared to actively loaded compounds (active loading was performed by pulling spiked river water over SDB disks using vacuum). We hypothesise that the two-phase kinetics and better retention of actively loaded compounds—and compounds loaded for a longer period—may be caused by slow diffusion of chemicals within the polymer. As sorption and desorption did not show isotropic kinetics, it is not possible to develop robust PRCs for adsorbent material like SDB disk
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