Tracing the structural evolution of quasi-freestanding germanene on Ag(111).

In the last decade, research on 2D materials has expanded massively due to the popularity of graphene. Although the chemical engineering of two-dimensional elemental materials as well as heterostructures has been extensively pursued, the fundamental understanding of the synthesis of 2D materials is not yet complete. Structural parameters, such as buckling or the interface structure of a 2D material to the substrate directly affect its electronic characteristics. In order to proceed the understanding of the element-specific growth and the associated ability of tuning material properties of two-dimensional materials, we performed a study on the structural evolution of the promising 2D material germanene on Ag(111). This study provides a survey of germanium formations at different layer thicknesses right up to the arising of quasi-freestanding germanene. Using powerful surface analysis tools like low-energy electron diffraction, x-ray photoelectron spectroscopy, and x-ray photoelectron diffraction with synchrotron radiation, we will reveal the internal and interfacial structure of all discovered germanium phases. Moreover, we will present models of the atomic and chemical structure of a [Formula: see text] surface alloy and the quasi-freestanding germanene with special focus on the structural parameters and electronic interaction at the interface.

Surface and interface analysis of a low-dimensional Au-Si surface alloy on Au(110) by means of XPS and XPD.

The chemical and structural characteristics of a low-dimensional Au-Si surface alloy are presented in this work. Alloy formation was obtained by deposition of a sub-monolayer Si on Au(110). This preliminary phase to Si nano-ribbons is being investigated, as the transition from clean Au(110) to a silicon nano-ribbon coated surface is not yet understood. A multiple technique study has been carried out for detailed atomic structure determination and chemical investigation. Particular attention is paid to the clarification of the structural arrangement at the surface and at the interface. Using low-energy electron diffraction, the periodicity of the structure on long-range order could be examined. By means of high-precision photoemission measurements using synchrotron radiation, the electronic and atomic structure of the alloy can be presented. The investigation by photoelectron spectroscopy (XPS) using soft x-rays for a high surface sensitivity showed different chemical environments in the high-resolution spectra. The x-ray photoelectron diffraction (XPD) measurements, which are sensitive to the local atomic order, gave an approach to the structural configuration of the alloy. A new structural arrangement was found simulating both Au and Si XPD patterns. The results are compared to former proposed structure models. A deconvolution of the Si 2pXPD pattern revealed the origin of two chemically shifted XPS components.

A scanning reflection X-ray microscope for magnetic imaging in the EUV range.

The mechanical setup of a novel scanning reflection X-ray microscope is presented. It is based on zone plate optics optimized for reflection mode in the EUV spectral range. The microscope can operate at synchrotron radiation beamlines as well as at laboratory-based plasma light sources. In contrast to established X-ray transmission microscopes that use thin foil samples, the new microscope design presented here allows the investigation of any type of bulk materials. Importantly, this permits the investigation of magnetic materials by employing experimental techniques based on X-ray magnetic circular dichroism, X-ray linear magnetic dichroism or the transversal magneto-optical Kerr effect (T-MOKE). The reliable functionality of the new microscope design has been demonstrated by T-MOKE microscopy spectra of Fe/Cr-wedge/Fe trilayer samples. The spectra were recorded at various photon energies across the Fe 3p edge revealing the orientation of magnetic domains in the sample.

Fast and easy fabrication methodology of Fresnel zone plates for the extreme ultraviolet and soft x-ray regions.

Zone plate design and efficient methods for the fabrication of zone plates for extreme ultraviolet (EUV) and soft x-ray applications in a newly developed scanning reflection microscope are presented. Based on e-beam lithography, three types of transmission zone plates with focal lengths between 6 and 15 mm are reported: (i) phase-shifting zone plates made by 190 nm thick PMMA rings on Si3N4 membranes, (ii) absorbing zone plates made by 75 nm thick Au ring structures on Si3N4, and (iii) freestanding Au rings of 50 nm thickness and increased transmission in the EUV range. Experiments at the DELTA synchrotron facility reveal a minimum spot size and resulting spatial resolution of 9±3 µm, which is the theoretical limit resulting from the synchrotron beam parameters at 60 eV photon energy. Images of a Ti/Si chessboard test pattern are recorded exploiting the energy dependence of the element-specific reflectance.

Revealing the interfaces of the hybrid system MgO/Co/GaAs(0 0 1): a structural and chemical investigation with XPS and XPD.

Bcc metals and MgO are used in technological research for building magnetic tunnel junctions (MTJs), because they yield a high tunnel magnetoresistance. Thin insulating barriers are of great importance in realizing MTJs. Combined with electrons spin-injected into GaAs, tunneled electrons can be detected and manipulated. We report on a synchrotron radiation based x-ray photoelectron spectroscopy and x-ray photoelectron diffraction study on the system MgO/Co(bcc)/GaAs(0 0 1) for ultra-low Co and MgO coverages ([Formula: see text], [Formula: see text]). As a result, we obtain a Co3Ga alloy at the Co/GaAs interface in the rare D03 structure. This structure is only 6.07 Å thick, and serves as a template for the metastable Co(bcc) structure. Co(bcc) itself grows heavily distorted in the (0 0 1) direction for the first two unit cells, due to the D03 template. The MgO/Co interface reveals a weak bonding between MgO and Co(bcc) without Co oxidation, since no compound formation was observed. Additionally, MgO grows in an amorphous phase for a thickness of [Formula: see text]. At [Formula: see text], it crystallizes in a compressed unit cell where every second layer is shifted toward the (0 0 1) direction compared to the bulk halite structure.

Facing the interaction of absorbed silicon nano-ribbons on silver.

The atomic structure is one key property for any material. Despite great efforts during the last few years unveiling the internal structure of silicon nano-ribbons, analysis of the interfacial structure and bonding was neglected. We report on a comprehensive photoelectron spectroscopy and photoelectron diffraction study that reveals the weak interaction of silicon nano-ribbons with the underlying silver substrate identifying the specific locations of the individual silicon, as well as silver atoms. Furthermore, we provide unique experimental evidence that clarifies the origin of the two distinct chemically shifted components in the silicon photoelectron spectra.

Photoemission electron microscopy and scanning electron microscopy of Magnetospirillum magnetotacticum's magnetosome chains.

Magnetotactic bacteria are of great interdisciplinary interest, since a vast field of applications from magnetic recording media to medical nanorobots is conceivable. A key feature for a further understanding is the detailed knowledge about the magnetosome chain within the bacteria. We report on two preparation procedures suitable for UHV experiments in reflective geometry. Further, we present the results of scanning electron microscopy, as well as the first photoemission electron microscopy experiments, both accessing the magnetosomes within intact magnetotactic bacteria and compare these to scanning electron microscopy data from the literature. From the images, we can clearly identify individual magnetosomes within their chains.