Conrad Escher

Nondestructive imaging of individual biomolecules

Radiation damage is considered to be the major problem that still prevents imaging an individual biological molecule for structural analysis. So far, all known mapping techniques using sufficient short wavelength radiation, be it x rays or high energy electrons, circumvent this problem by averaging over many molecules. Averaging, however, leaves conformational details uncovered. Even the anticipated use of ultrashort but extremely bright x-ray bursts of a free electron laser shall afford averaging over 10{6} molecules to arrive at atomic resolution. Here, we present direct experimental evidence for nondestructive imaging of individual DNA molecules. In fact, we show that DNA withstands coherent low energy electron radiation with deBroglie wavelength in the Angstrom regime despite a vast dose of 10{8} electrons/nm{2} accumulated over more than one hour.

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 objects shapes and distribution. Furthermore, a Moire effect has been observed with free-standing graphene multi-layers.

Ultraclean 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. The authors discovered a method for preparing ultraclean 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 ultraclean freestanding graphene.

Highly collimated electron beams from double-gate field emitter arrays with large collimation gate apertures

Electron collimation in field emitter arrays with electron extraction gate and collimation gate electrodes is studied with the goal to develop a high-brightness high current cathode. Using metallic field emitter arrays prepared by the molding method, we fabricated a stacked double-gate device with the two gates differing in diameter by a process utilizing focused-ion beam milling. We measured the field-emission beam characteristics and demonstrated a reduction of the emission angle by a factor of 7.1±0.8 with minimal emission current decrease under collimating conditions, resulting in a current density increase by a factor of 13.9±1.0.Electron collimation in field emitter arrays with electron extraction gate and collimation gate electrodes is studied with the goal to develop a high-brightness high current cathode. Using metallic field emitter arrays prepared by the molding method, we fabricated a stacked double-gate device with the two gates differing in diameter by a process utilizing focused-ion beam milling. We measured the field-emission beam characteristics and demonstrated a reduction of the emission angle by a factor of 7.1±0.8 with minimal emission current decrease under collimating conditions, resulting in a current density increase by a factor of 13.9±1.0.

Imaging proteins at the single-molecule level

Significance We report a method to image and reveal structural details of proteins on a truly single-molecule level. Low-energy electron holography is used to image individual proteins electrospray deposited on freestanding graphene. In contrast to the current state of the art in structural biology, we do away with the need for averaging over many molecules. This is crucial because proteins are flexible objects that can assume distinct conformations often associated with different functions. Proteins are also the targets of almost all the currently known and available drugs. The design of new and more effective drugs relies on the knowledge of the targeted proteins structure in all its biologically significant conformations at the best possible resolution. 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 subnanometer resolution images of individual proteins (cytochrome C and BSA) as well as of protein complexes (hemoglobin), which are not the result of an averaging process.

Non-destructive imaging of an individual protein

Imaging a single biomolecule at atomic resolution without averaging over different conformations is the ultimate goal in structural biology. We report recordings of a protein at nanometer resolution obtained from one individual ferritin by means of low-energy electron holography. One single protein could be imaged for an extended period of time without any sign of radiation damage. Since the fragile protein shell encloses a robust iron cluster, the holographic reconstructions could also be cross-validated against transmission electron microscopy images of the very same molecule by imaging its iron core.

Field-Emission Characteristics of Molded Molybdenum Nanotip Arrays With Stacked Collimation Gate Electrodes

Double-gate field-emission characteristics of metallic field-emitter-array (FEA) cathodes fabricated by molding with stacked collimation gate electrodes with planar end plane are reported. The collimation of field-emission electron beam with minimal reduction of emission current is demonstrated when a negative bias is applied to the collimation gate, whereas when the two electrodes are at the same potential, the emission characteristic of the double-gate device is the same as that of the single-gate device that shows an emission current of ~1 mA from 40 × 40 tip arrays. Results indicate that the device structure of the fabricated double-gate FEAs is promising for high-brilliance cathode applications.

Vacuum ion emission from solid electrolytes: An alternative source for focused ion beams

A bright ion source based on the solid electrolyte (AgI)0.5(AgPO3)0.5 has been developed. The solid electrolyte source provides stable currents of Ag+ in the microampere regime that make it suitable for focused ion beam applications. Similar conditions are expected for different solid electrolyte materials and their corresponding ions. This opens a broad field of applications in structuring and modifying devices on a nanometer scale using focused ion beams.

Fabrication of metallic double-gate field emitter arrays and their electron beam collimation characteristics

The fabrication of double-gate metallic field emitter arrays with large collimation gate apertures and their field emission beam characteristics are reported. The device fabrication steps, including the molding technique for array fabrication, the electron extraction gate fabrication by the self-aligned resist etch-back method, and the fabrication of the collimation gate electrode using a focused ion beam assisted method are described in detail. The experimental results of 2 × 2 tip arrays with the proposed double-gate structure demonstrate an order of magnitude enhancement in beam brightness with minimal current loss. A similarly high beam brightness enhancement was achieved with a 20 × 20 tip array device, showing the scalability of the proposed structure. The observation of improved current-voltage characteristics with the 20 × 20 tip array is ascribed to the difference in gate aperture shape. The possibility of further improving the beam characteristics of double-gate field emitter arrays and the redu...

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. 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.Modern structural biology relies on Nuclear Magnetic Resonance (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 1 nm 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 deBrogl...