arXiv subjects

Conrad Escher

Publications and source records attributed to Conrad Escher.

At least 19 recordsLinked to original sources

arXiv2023

Metal adsorption and nucleation on free-standing graphene by low-energy electron point source microscopy (graphene intercalation)

The interaction of metals with carbon materials, specifically with graphene, is of importance for various technological applications. In particular, intercalation of alkali metals is believed to provide a means for tuning the electronic properties of graphene for device applications. While the macroscopic effects of such intercalation events can readily be studied, following the related processes at an atomic scale in detail and under well-defined experimental conditions constitutes a challenge. Here, we investigate in situ the adsorption and nucleation of the alkali metals K, Cs, and Li on free-standing graphene by means of low-energy electron point source microscopy. We find that alkali metals readily intercalate in between bilayer graphene. In fact, the equilibrium distribution of K and Cs favours a much higher particle density in between the bilayer than on the single layer graphene. We obtain a quantitative value for the difference of the free energy of binding between these two domains. Our study is completed with a control experiment introducing Pd as a representative of the non-alkali metals. Now, we observe cluster formation in equal measure on both, single and bilayer graphene, however no intercalation. Our investigations thus constitute the first in situ study of metal atom sorption of different specificity on free-standing graphene.

cond-mat.mes-hall↗

arXiv2019

Direct visualization of charge transport in suspended (or free-standing) DNA strands by low-energy electron microscopy

Low-energy electrons offer a unique possibility for long exposure imaging of individual biomolecules without significant radiation damage. In addition, low-energy electrons exhibit high sensitivity to local potentials and thus can be employed for imaging charges as small as a fraction of one elementary charge. The combination of these properties makes low-energy electrons an exciting tool for imaging charge transport in individual biomolecules. Here we demonstrate the imaging of individual deoxyribonucleic acid (DNA) molecules at the resolution of about 1 nm with simultaneous imaging of the charging of the DNA molecules that is of the order of less than one elementary charge per nanometer. The cross-correlation analysis performed on different sections of the DNA network reveals that the charge redistribution between the two regions is correlated. Thus, low-energy electron microscopy is capable to provide simultaneous imaging of macromolecular structure and its charge distribution which can be beneficial for imaging and constructing nano-bio-sensors.

cond-mat.mes-hall↗

arXiv2018

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.

cond-mat.mes-hall↗

arXiv2017

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.

cond-mat.mes-hall↗

arXiv2017

Imaging proteins at the truly single-molecule level

Imaging single proteins has been a long-standing ambition for advancing various fields in natural science, as for instance structural biology, biophysics and molecular nanotechnology. In particular, revealing the distinct conformations of an individual protein is of utmost importance. Here, we show the imaging of individual proteins and protein complexes by low-energy electron holography. Samples of individual proteins and protein complexes on ultraclean freestanding graphene were prepared by soft-landing electrospray ion beam deposition, which allows chemical- and conformational-specific selection and gentle deposition. Low-energy electrons do not induce radiation damage, which enables acquiring sub-nanometer resolution images of individual proteins (cytochrome C and bovine serum albumin) as well as of protein complexes (hemoglobin), which are not the result of an averaging process.

physics.bio-ph↗

arXiv2016

Direct observation of individual charges and their dynamics on graphene by low-energy electron holography

Visualizing individual charges confined to molecules and observing their dynamics with high spatial resolution is a challenge for advancing various fields in science, ranging from mesoscopic physics to electron transfer events in biological molecules. We show here, that the high sensitivity of low-energy electrons to local electric fields can be employed to directly visualize individual charged adsorbates and to study their behaviour in a quantitative way. This makes electron holography a unique probing tool for directly visualising charge distributions with a sensitivity of a fraction of an elementary charge. Moreover, spatial resolution in the nanometer range and fast data acquisition inherent to lens-less low-energy electron holography allows for direct visual inspection of charge transfer processes.

cond-mat.mes-hall↗

arXiv2016

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)].

cond-mat.mes-hall↗

arXiv2015

Low-energy electron holographic imaging of individual tobacco mosaic virions

Modern structural biology relies on NMR, X-ray crystallography and cryo-electron microscopy for gaining information on biomolecules at nanometer, sub-nanometer or atomic resolution. All these methods, however, require averaging over a vast ensemble of entities and hence knowledge on the conformational landscape of an individual particle is lost. Unfortunately, there are now strong indications that even X-ray free electron lasers will not be able to image individual molecules but will require nanocrystal samples. Here, we show that non-destructive structural biology of single particles has now become possible by means of low-energy electron holography. As an example, individual tobacco mosaic virions deposited on ultraclean freestanding graphene are imaged at one nanometer resolution revealing structural details arising from the helical arrangement of the outer protein shell of the virus. Since low-energy electron holography is a lens-less technique and since electrons with a deBroglie wavelength of approximately 1 Angstrom do not impose radiation damage to biomolecules, the method has the potential for Angstrom resolution imaging of single biomolecules.

physics.bio-ph↗

arXiv2015

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.

physics.ins-det↗

arXiv2015

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.

physics.atm-clus↗

arXiv2014

Interfacing Ultraclean Graphene with Solid-State Devices

Interfacing graphene with solid-state devices and maintaining it free of contamination is a crucial step towards a functioning device, be it a semiconductor structure or any other device for technological applications. We take advantage of the catalytic properties of platinum metals to completely remove the polymer capping after the transfer of macroscopic graphene sheets to a solid-state device. For that purpose a platinum metal coated structure is brought in close proximity with the polymer capping. Subsequent annealing in air at a temperature between 175 and 350°C actuates a complete catalytic removal of the polymer. Finally, the platinum metal catalyst is removed revealing ultra clean graphene interfaced with an arbitrary device. Experiments to interface macroscopic graphene layers with oxidized silicon wafers demonstrate the general applicability of this approach. In repeating the procedure, multi-layer graphene sheets can also be produced. Direct evidence for the latter is provided by optical images of three overlapping graphene sheets. The exceeding level of cleanliness of the graphene is examined on the nanometer scale by means of low-energy electron transmission microscopy.

cond-mat.mes-hall↗

arXiv2014

Low-energy electron holographic imaging of gold nanorods supported by ultraclean graphene

An ideal support for electron microscopy shall be as thin as possible and interact as little as possible with the primary electrons. Since graphene is atomically thin and made up of carbon atoms arranged in a honeycomb lattice, the potential to use graphene as substrate in electron microscopy is enormous. Until now graphene has hardly ever been used for this purpose because the cleanliness of freestanding graphene before or after deposition of the objects of interest was insufficient. We demonstrate here by means of low-energy electron holographic imaging that freestanding graphene prepared with the Platinum-metal catalysis method remains ultraclean even after re-exposure to ambient conditions and deposition of gold-nanorods from the liquid phase. In the holographic reconstruction of the gold particles the organic shell surrounding the objects is imaged while it is not detectable in SEM images of the very same sample, demonstrating the tremendous potential of low-energy electron holography for the imaging of graphene-supported single biomolecules.

cond-mat.mtrl-sci↗

arXiv2013

On artefact-free reconstruction of low-energy (30-250 eV) electron holograms

Low-energy electrons (30-250 eV) have been successfully employed for imaging individual biomolecules. The most simple and elegant design of a low-energy electron microscope for imaging biomolecules is a lensless setup that operates in the holographic mode. In this work we address the problem associated with the reconstruction from the recorded holograms. We discuss the twin image problem intrinsic to inline holography and the problem of the so-called biprism-like effect specific to low-energy electrons. We demonstrate how the presence of the biprism-like effect can be efficiently identified and circumvented. The presented sideband filtering reconstruction method eliminates the twin image and allows for reconstruction despite the biprism-like effect, which we demonstrate on both, simulated and experimental examples.

physics.ins-det↗

arXiv2013

Pulsed Electron Holography

A technique of pulsed low-energy electron holography is introduced that allows for recording highly resolved holograms within reduced exposure times. Therefore, stacks of holograms are accumulated in a pulsed mode with individual acquisition times as short as 50 μs. Subsequently, these holograms are aligned and finally superimposed. The resulting holographic record reveals previously latent high-order interference fringes and thereby pushing interference resolution into the sub-nanometer regime. In view of the non-damaging character of low-energy electrons, the method is of particular interest for structural analysis of fragile biomolecules.

physics.ins-det↗

arXiv2013

Coherent diffraction and holographic imaging of individual biomolecules using low-energy electrons

Modern microscopy techniques are aimed at imaging an individual molecule at atomic resolution. Here we show that low-energy electrons with kinetic energies of 50-250 eV offer a possibility of overcome the problem of radiation damage, and obtaining images of individual biomolecules. Two experimental schemes for obtaining images of individual molecules, holography and coherent diffraction imaging, are discussed and compared. Images of individual molecules obtained by both techniques, using low-energy electrons, are shown.

physics.bio-ph↗

arXiv2013

Graphene Unit Cell Imaging by Holographic Coherent Diffraction

We have imaged a freestanding graphene sheet of 210 nm in diameter with 2 Angstrom resolution by combining coherent diffraction and holography with low-energy electrons. The entire sheet is reconstructed from just a single diffraction pattern displaying the arrangement of 660.000 individual graphene unit cells at once. Given the fact that electrons with kinetic energies of the order of 100 eV do not damage biological molecules, it will now be a matter to develop methods for depositing individual proteins onto such graphene sheets.

cond-mat.mtrl-sci↗

arXiv2012

Ultra-Clean Freestanding Graphene by Platinum-Metal Catalysis

While freestanding clean graphene is essential for various applications, existing technologies for removing the polymer layer after transfer of graphene to the desired substrate still leave significant contaminations behind. We discovered a method for preparing ultra-clean freestanding graphene utilizing the catalytic properties of platinum metals. Complete catalytic removal of polymer residues requires annealing in air at a temperature between 175 and 350°C. Low-energy electron holography investigations prove that this method results in ultra-clean freestanding graphene.

cond-mat.mtrl-sci↗

arXiv2012

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.

cond-mat.mes-hall↗