Low-energy electron transmission imaging of clusters on free-standing graphene by Jean-Nicolas Longchamp, Tatiana Latychevskaia, Conrad Escher, Hans-Werner Fink, read more

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 Moiré effect has been observed with free-standing graphene multi-layers.

Non-destructive imaging of an individual protein by J.-N. Longchamp, T. Latychevskaia, C. Escher, and H.-W. Fink, read more

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.

When holography meets coherent diffraction imaging. We discovered a mathematical relationship between a hologram and a diffraction pattern allowing us to merge the two prominent phase retrieval methods into one leading to fast and unambiguous object reconstructions while not requiring the oversampling condition anymore. read more

Coherent low-energy electron diffraction of individual nanometer sized objects. The first coherent diffraction pattern of an individual free-standing carbon nanotube using low-energy electrons has been obtained. read more

Fabrication and characterization of low aberration micrometre-sized electron lenses. We have developed methods for the routine fabrication of micron sized electron lenses and evaluated their performance in a dedicated UHV vacuum system. A detailed evaluation of the spherical aberrations shows that these lenses have a quality that can otherwise only be reached by elaborate magnetic objective lenses in modern electron microscopes.  read more

Pulsed electron holography has been invented to circumvent the disturbing effects of residual vibrations, drift and ac-magnetic fields limiting the coherence of the low energy electron wave. It has been possible to acquire a hologram of a single DNA within 10 micro-seconds. A set of several 100 such holograms of one and the same molecule is used to build up a record with a high signal to noise level. For this, the maximum of the cross correlation between subsequently acquired holograms is computed prior to shifting and superimposing the whole set of pulsed holograms.

Ref.: Master thesis of Matthias Germann

Quantitative Radiation Damage studies in Low Energy Electron Point Source Microscopy. While it seemed qualitatively apparent for quite some time that electron waves, associated with kinetic energies below 300eV, introduce little respectively non-detectable radiation damage on such fragile biological objects like an individual DNA molecule, a quantitative study of radiation damage has only recently been attempted.  

A set of holograms of a freshly prepared DNA sample has been acquired and the cross correlation between subsequent taken records been computed. Thereafter, the DNA molecule has been subject to continues radiation for one hour corresponding to a total effective dose of several orders of magnitude larger than what is considered tolerable in high energy TEM studies of biological samples.
Following this, another set of pulsed holograms has been taken. A cross correlation coefficient close to unity between the set of holograms taken at the beginning of the experiment and that following one hour of continues radiation, indicates that a free-standing DNA molecule has not been damaged in such a way that it causes structural changes on a scale associated with the present interference resolution of pulsed holography which is in the sub-nanometer regime.

It should be noted that similar experiments have been carried out at slightly higher energies, namely above 300 eV. Under these conditions, a single DNA decomposes within a few 10 seconds of observation time. Future experiments should reveal if there is a well defined threshold for damage or if there exist discrete windows between a few 10 and a few 100 eV.

Ref.: Master thesis of Matthias Germann Phys. Rev. Lett. 104, 095501 (2010)

Solution of the Twin Image Problem in Holography. The reconstruction of a hologram is the final step that leads to the three dimensional shape of the scattered object wave that eventually reveals the structural detail of the object. While the quality of the reconstructed image depends on the quality of detection of amplitude and phase of the object wave in the holographic record, one intrinsic problem remains. It is the so-called “twin image problem” recognized since the invention of holography by Dennis Gabor. It is a consequence of the foundation of coherent optics that the out of focus conjugated twin is entangled with the wanted image and obscures its recognition.

This long standing problem of coherent optics has recently been solved and provides object reconstructions no longer obscured by the twin image.

Ref.: Phys. Rev. Lett. 98, 233901 (2007)

A more popular written account of the above described work can be found here:

June 8, 2007: Molecular holograms are coming into focus: New Scientist comments on the twin image removal.

June 19, 2007: Getting rid of the twin image that plagues holography: PhysOrg comments on the twin image removal.

Energetics of a single DNA Molecule: Exploring the dynamics of individual DNA molecules has been subject of various theoretical studies and models over many decades, but only during the last 10 years the ability to actually carry out experiments on individual molecules has become routine. However, direct insight into the energetics on a single molecule level has not been obtained so far. We have carried out  temperature dependent experiments with individual lambda-DNA molecules and provide the first quantitative information about the internal energetics of a single molecule in solution. From the mean square displacement of the center of mass of a DNA random coil we derive the diffusion coefficient from Einstein equation and from its temperature dependence the activation energy for diffusion to be roughly 1/6 eV. Following these control experiment we determine the activation energy associated with the transition from the straightened to the lowest free energy random coil configuration. The statistics of a total of 600 measurements on individual lambda-DNA molecules at 12 different temperatures reveals an activation free energy for the transition between these two prominent configurations of 1/4 eV. This indicates that the DNA random coil equilibrium configuration is not entirely given by entropy in contrast to current models.