Günther Kassier

Visualization of Multimerization and Self-Assembly of DNA-Functionalized Gold Nanoparticles Using In-Liquid Transmission Electron Microscopy

Base-pairing stability in DNA-gold nanoparticle (DNA-AuNP) multimers along with their dynamics under different electron beam intensities was investigated with in-liquid transmission electron microscopy (in-liquid TEM). Multimer formation was triggered by hybridization of DNA oligonucleotides to another DNA strand (Hyb-DNA) related to the concept of DNA origami. We analyzed the degree of multimer formation for a number of samples and a series of control samples to determine the specificity of the multimerization during the TEM imaging. DNA-AuNPs with Hyb-DNA showed an interactive motion and assembly into 1D structures once the electron beam intensity exceeds a threshold value. This behavior was in contrast with control studies with noncomplementary DNA linkers where statistically significantly reduced multimerization was observed and for suspensions of citrate-stabilized AuNPs without DNA, where we did not observe any significant motion or aggregation. These findings indicate that DNA base-pairing interactions are the driving force for multimerization and suggest a high stability of the DNA base pairing even under electron exposure.

Low-dose cryo electron ptychography via non-convex Bayesian optimization

Electron ptychography has seen a recent surge of interest for phase sensitive imaging at atomic or near-atomic resolution. However, applications are so far mainly limited to radiation-hard samples, because the required doses are too high for imaging biological samples at high resolution. We propose the use of non-convex Bayesian optimization to overcome this problem, and show via numerical simulations that the dose required for successful reconstruction can be reduced by two orders of magnitude compared to previous experiments. As an important application we suggest to use this method for imaging single biological macromolecules at cryogenic temperatures and demonstrate 2D single-particle reconstructions from simulated data with a resolution up to 5.4 Å at a dose of 20e−/Å2. When averaging over only 30 low-dose datasets, a 2D resolution around 3.5 Å is possible for macromolecular complexes even below 100 kDa. With its independence from the microscope transfer function, direct recovery of phase contrast, and better scaling of signal-to-noise ratio, low-dose cryo electron ptychography may become a promising alternative to Zernike phase-contrast microscopy.

Split ring resonator based THz-driven electron streak camera featuring femtosecond resolution

Through combined three-dimensional electromagnetic and particle tracking simulations we demonstrate a THz driven electron streak camera featuring a temporal resolution down to a femtosecond. The ultrafast streaking field is generated in a resonant THz sub-wavelength antenna, which is illuminated by an intense single-cycle THz pulse. Since electron bunches and THz pulses are generated by the same laser system, synchronization between the two is inherently guaranteed.

Visualization of Cellular Components in a Mammalian Cell with Liquid-Cell Transmission Electron Microscopy

We present liquid-cell transmission electron microscopy (liquid-cell TEM) imaging of fixed and non-fixed prostate cancer cells (PC3 and LNCaP) with high resolution in a custom developed silicon nitride liquid cell. Fixed PC3 cells were imaged for 90-120 min without any discernable damage. High contrast on the cellular structures was obtained even at low electron doses (~2.5 e-/nm2 per image). The images show distinct structures of cell compartments (nuclei and nucleoli) and cell boundaries without any further sample embedding, dehydration, or staining. Furthermore, we observed dynamics of vesicles trafficking from the cell membrane in consecutive still frames in a non-fixed cell. Our findings show that liquid-cell TEM, operated at low electron dose, is an excellent tool to investigate dynamic events in non-fixed cells with enough spatial resolution (few nm) and natural amplitude contrast to follow key intracellular processes.

Optical fiber-based photocathode

We present the design of a back-illuminated photocathode for electron diffraction experiments based on an optical fiber, and experimental characterization of emitted electron bunches. Excitation light is guided through the fiber into the experimental vacuum chamber, eliminating typical alignment difficulties between the emitter metal and the optical trigger and position instabilities, as well as providing reliable control of the laser spot size and profile. The in-vacuum fiber end is polished and coated with a 30 nm gold (Au) layer on top of 3 nm of chromium (Cr), which emits electrons by means of single-photon photoemission when femtosecond pulses in the near ultraviolet (257 nm) are fed into the fiber on the air side. The emission area can be adjusted to any value between a few nanometers (using tapered fibers) and the size of a multi-mode fiber core (100 μm or larger). In this proof-of-principle experiment, two different types of fibers were tested, with emission spot diameters of 50 μm and 100 μm, res...

Fiber tip-based electron source

The authors report on the first experimental characterization of a fiber tip-based electron source, where electron emission can be triggered by both electric field and optical excitation. Our approach consists of coating the open aperture of a commercial 100 nm apex size near-field scanning optical microscopy fiber tip with a 10 nm thick tungsten (W) layer, which is back-illuminated by a 405 nm continuous-wave laser beam in the presence of an extraction electric field. Despite the very low optical transmission of the fiber due to the subwavelength aperture size, measurements show a clearly enhanced emission when photoexciting the W layer with respect to pure field emission. The emission response time is slower than the optical trigger time, suggesting that thermal effects are predominant in the studied regime. To back up this hypothesis, the authors fabricated a nanometric thermocouple probe based on a Pt/Au junction and measured the temporal response of the tip temperature. The measured switch-on time fo...

Photochromism of Amphiphilic Dithienylethenes as Langmuir–Schaefer Films

Surface pressure-area isotherms were recorded under different irradiation conditions for single-component Langmuir films of three photochromic amphiphilic dithienylethenes. Nonirradiated films of these photochromic amphiphiles were mechanically stable. In addition, a shift of the isotherms to larger mean molecular areas was observed for films prepared from UV-light-irradiated dithienylethenes. Unexpectedly, a significant expansion was observed for a film prepared from visible-light-irradiated dithienylethene incorporating large branched alkyl chains. Upon further study, atomic force microscopy and transmission electron microscopy images of Langmuir-Schaefer films revealed that this pronged dialkyl derivative undergoes a photoinduced change in morphology, as circular aggregates coalesce into larger continuous aggregated structures. Nevertheless, its photoisomerization was completely reversible as single-component multilayer thin films upon direct UV or visible light irradiation.

Fabrication and characterization of a focused ion beam milled lanthanum hexaboride based cold field electron emitter source

We report on a method of fabricating lanthanum hexaboride (LaB6) cold field emission tips with sub-100-nm apices by using a combination of electrochemical etching and focused ion beam milling. The primary advantage of combining the two methods is rapid fabrication while maintaining reproducibility. The LaB6 tips have low work functions and high mechanical stabilities and are chemically inert to residual gases. Field emission characterization was performed on three tips, with apex sizes of 15, 85, and 80 nm yielding 10 nA cold field emission currents at 0.76, 3.9, and 3.6 kV extraction potentials, respectively. All three tips showed excellent emission current stability for periods exceeding 30 min in a 5 × 10−9 mbar vacuum. We report on a method of fabricating lanthanum hexaboride (LaB6) cold field emission tips with sub-100-nm apices by using a combination of electrochemical etching and focused ion beam milling. The primary advantage of combining the two methods is rapid fabrication while maintaining reproducibility. The LaB6 tips have low work functions and high mechanical stabilities and are chemically inert to residual gases. Field emission characterization was performed on three tips, with apex sizes of 15, 85, and 80 nm yielding 10 nA cold field emission currents at 0.76, 3.9, and 3.6 kV extraction potentials, respectively. All three tips showed excellent emission current stability for periods exceeding 30 min in a 5 × 10−9 mbar vacuum.

Optical fiber-driven low energy electron gun for ultrafast streak diffraction

Here, we present an optical fiber-based electron gun designed for the ultrafast streaking of low-energy electron bunches. The temporal profile of the few tens of the picosecond long electron bunch composed of 200 electrons is well characterized using a customized streak camera. Detailed analysis reveals that the stretched optical trigger pulse owing to the dispersion effects inside the waveguide dominantly determines the temporal length of the low density electron bunch. This result illustrates the capability to control the observable time-window in the streak diffraction experiment by tailoring geometrical parameters of the fiber source and its coupling condition. With the electrostatic Einzel lens system integrated on the fiber-based cathode, we also demonstrate spatial focusing of the electron beam with the RMS spot size of 98 μm and imaging of the static low-energy electron diffraction pattern of monolayer graphene in the electron kinetic energy range of 1.0–2.0 keV.Here, we present an optical fiber-based electron gun designed for the ultrafast streaking of low-energy electron bunches. The temporal profile of the few tens of the picosecond long electron bunch composed of 200 electrons is well characterized using a customized streak camera. Detailed analysis reveals that the stretched optical trigger pulse owing to the dispersion effects inside the waveguide dominantly determines the temporal length of the low density electron bunch. This result illustrates the capability to control the observable time-window in the streak diffraction experiment by tailoring geometrical parameters of the fiber source and its coupling condition. With the electrostatic Einzel lens system integrated on the fiber-based cathode, we also demonstrate spatial focusing of the electron beam with th...

Development of a LaB 6 based ultra-bright cold field emitter electron source

We present our work on the development and characterization of a practical and robust LaB6 based cold field emitter electron source. To this end we prepared a Tantalum mounted single crystal LaB6 rod that was chemically etched, yielding an apex size of 2μm. The obtained tip is loaded into a ultra high vacuum chamber for pulsed emission in the μs regime, using extraction voltage pulsing. Further processing using focused ion beam (FIB) milling, and conditioning using a build-up technique will be used to reduce the apex size to the nm range.