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The patterning toolbox FIB-o-mat: Exploiting the full potential of focused helium ions for nanofabrication
Focused beams of helium ions are a powerful tool for high-fidelity machining with spatial precision below 5 nm. Achieving such a high patterning precision over large areas and for different materials in a reproducible manner, however, is not trivial. Here, we introduce the Python toolbox FIB-o-mat for automated pattern creation and optimization, providing full flexibility to accomplish demanding patterning tasks. FIB-o-mat offers high-level pattern creation, enabling high-fidelity large-area patterning and systematic variations in geometry and raster settings. It also offers low-level beam path creation, providing full control over the beam movement and including sophisticated optimization tools. Three applications showcasing the potential of He ion beam nanofabrication for two-dimensional material systems and devices using FIB-o-mat are presented.
The interaction of chondroitin sulfate with a lipid monolayer observed by using nonlinear vibrational spectroscopy
The first vibrational sum-frequency generation (VSFG) spectra of chondroitin sulfate (CS) interacting with dipalmitoyl phosphatidylcholine (DPPC) at air–liquid interface are reported here, collected at a laser repetition rate of 100 kHz. By studying the VSFG spectra in the regions of 1050–1450 cm−1, 2750–3180 cm−1, and 3200–3825 cm−1, it was concluded that in the presence of Ca2+ ions, the head groups together with the head-group-bound water molecules in the DPPC monolayer are strongly influenced by the interaction with CS, while the organization of the phospholipid tails remains mostly unchanged. The interactions were observed at a CS concentration below 200 nM, which exemplifies the potential of VSFG in studying biomolecular interactions at low physiological concentrations. The VSFG spectra recorded in the O–H stretching region at chiral polarization combination imply that CS molecules are organized into ordered macromolecular superstructures with a chiral secondary structure.
Infrared and NMR Spectroscopic Fingerprints of the Asymmetric H7 + O3 Complex in Solution
Infrared (IR) absorption in the 1000-3700 cm-1 range and 1 H NMR spectroscopy reveal the existence of an asymmetric protonated water trimer, H7 + O3, in acetonitrile. The core H7 + O3 motif persists in larger protonated water clusters in acetonitrile up to at least 8 water molecules. Quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations reveal irreversible proton transport promoted by propagating the asymmetric H7 + O3 structure in solution. The QM/MM calculations allow for the successful simulation of the measured IR absorption spectra of H7 + O3 in the OH stretch region, which reaffirms the assignment of the H7 + O3 spectra to a hybrid-complex structure: a protonated water dimer strongly hydrogen-bonded to a third water molecule with the proton exchanging between the two possible shared-proton Zundel-like centers. The H7 + O3 structure lends itself to promoting irreversible proton transport in presence of even one additional water molecule. We demonstrate how continuously evolving H7 + O3 structures may support proton transport within larger water solvates.
Two-color two-dimensional terahertz spectroscopy: A new approach for exploring even-order nonlinearities in the nonperturbative regime
Nonlinear two-dimensional terahertz (2D-THz) spectroscopy at frequencies of the emitted THz signal different from the driving frequencies allows for exploring the regime of (off-)resonant even-order nonlinearities in condensed matter. To demonstrate the potential of this method, we study two phenomena in the nonlinear THz response of bulk GaAs: (i) The nonlinear THz response to a pair of femtosecond near-infrared pulses unravels novel fourth- and sixth-order contributions involving interband shift currents, Raman-like excitations of transverse-optical phonon and intervalence-band coherences. (ii) Transient interband tunneling of electrons driven by ultrashort mid-infrared pulses can be effectively controlled by a low-frequency THz field with amplitudes below 50 kV/cm. The THz field controls the electron–hole separation modifying decoherence and the irreversibility of carrier generation.
The 1-Megapixel pnCCD detector for the Small Quantum Systems Instrument at the European XFEL: system and operation aspects
The X-ray free-electron lasers that became available during the last decade, like the European XFEL (EuXFEL), place high demands on their instrumentation. Especially at low photon energies below 1 keV, detectors with high sensitivity, and consequently low noise and high quantum efficiency, are required to enable facility users to fully exploit the scientific potential of the photon source. A 1-Megapixel pnCCD detector with a 1024 × 1024 pixel format has been installed and commissioned for imaging applications at the Nano-Sized Quantum System (NQS) station of the Small Quantum System (SQS) instrument at EuXFEL. The instrument is currently operating in the energy range between 0.5 and 3 keV and the NQS station is designed for investigations of the interaction of intense FEL pulses with clusters, nano-particles and small bio-molecules, by combining photo-ion and photo-electron spectroscopy with coherent diffraction imaging techniques. The core of the imaging detector is a pn-type charge coupled device (pnCCD) with a pixel pitch of 75 µm × 75 µm. Depending on the experimental scenario, the pnCCD enables imaging of single photons thanks to its very low electronic noise of 3 e− and high quantum efficiency. Here an overview on the EuXFEL pnCCD detector and the results from the commissioning and first user operation at the SQS experiment in June 2019 are presented. The detailed descriptions of the detector design and capabilities, its implementation at EuXFEL both mechanically and from the controls side as well as important data correction steps aim to provide useful background for users planning and analyzing experiments at EuXFEL and may serve as a benchmark for comparing and planning future endstations at other FELs.