In situ high temperature scanning tunneling microscopy of Ni3Al(111) oxidation

We use a high temperature STM with a novel sample holder and environmental cell design that will allow to perform in situ experiments at elevated pressures. The system is connected to an UHV chamber for surface science sample analysis (Low energy Electron Diffraction - LEED, Auger Electron Spectroscopy - AES and surface preparation tools). Figure 1 displays the photograph of the STM system and the attached UHV chamber.

Fig. 1: STM chamber (center - with glowing filament for sample heating in the in situ STM) and the attached UHV system for LEED AES and surface preparation (right).

Imaging catalytic processes on surfaces

Making use the capabilities of the above UHV-system, we could monitor the initial steps of the oxidation of the Ni3Al(111) alloy surface at 725-750 K which result in the formation of a one layer thick Al2O3 surface oxide. The properties of this layer and the material transport required for the buildup of the surface oxide layer were studied in detail and Fig. 2 shows the inside cover picture of PCCP of our recently published results [1].

Fig. 2. Inside cover of our recent publication using in situ Scanning Tunneling Microscopy (STM)[1] . The left sequence shows snapshots of a movie of the Ni3Al(111) surface during formation of the single layer thick surface oxide. The sequence on the right shows another movie with the metal transport required for the formation of the oxide layer. The sketch displays where metal atoms are removed and added in the surface oxide layer.

CVD growth of graphene on Cu

We use a home built quartz tube reactor to grow mono- and few-layer g on Cu. Figure 3 shows the reactor setup and the reactor tube with loaded Cu foils on which graphene (g) was grown. Applying thermodynamic considerations enabled us to setup a kinetic model that predicts the growth velocity of single hexagonally shaped graphene islands. Figure 4 displays the image of a CVD grown graphene island on Cu after producing the optical contrast to distinguish between the g covered (orange) and uncovered Cu surface (red).

Fig. 3: Upper image - Reactor setup for the graphene growth by Chemical vapor deposition (CVD). Lower image - Quartz glass tube with loaded Cu foil samples on which g was grown by CVD.

Fig. 4: Optical image of a CVD grown graphene island with a diameter of about 200 µm. Mild oxidation in air produces the contrast required for the resolution of graphene covered Cu areas: while the g covered Cu foil appears orange as it is protected from oxidation by the covering g film, the uncovered Cu is oxidized to red Cu2O.

Moiré patterns of g supported on transition metal (111) surfaces

Supported g on hexagonally ordered transition metal surfaces are of interest because its structural properties. Due to the lattice mismatch of the g layer and the transition metal support, beating frequencies between both lattices appear that lead to the formation of so-called moiré patterns. We provided an analysis capable of predicting possible commensurate cells of moirés on such systems and related it to the properties of diffraction data. Figure 5 shows a the case of two rotated g layers on Cu(111).

Fig. 5: LEED pattern of two slightly rotated graphene layers on top of Cu(111) (top graphs: ~ 0.8° rotated. lower graph: ~4.2° rotated). The light blue square in each commensurability plot (right) indicates the possible commensurate cells of the moiré. The properties of the moiré pattern may be derived at even higher precision (see [2]).

Cu faceting under supported g and its effect on freestanding CVD g

We apply for beamtime at the synchrotron light facitlity to perform Low Energy Electron Microscopy (LEEM) and PhotoElectron Emission Microscopy (PEEM) experiments. Figure 6 shows the setup of the Spectroscopic PhotoElectron Emission Microscope (SPELEEM) apparatus. A summary of the capabilities of such an instrument can be found here (https://roempp.thieme.de/roempp4.0/do/data/RD-19-06558).

Fig. 6: Sketch of the lens and aperture system in the SPELEEM apparatus at the nanospectroscopy beamline of Elettra.

Fig. 7: SEM image of a CVD grown monolayer g film on a Cu foil. Local etching of the Cu foil produces a local hole underneath the g and turns the layer into free standing graphene (top). Contrast enhancement shows the morphology of the free standing g (bottom).

Acquiring so called local area low energy electron diffraction data enables us to assign the striped appearance of the Cu foil underneath CVD grown grapheme to staircase morphology of alternating crystal surfaces (a sequence of (104) and (100) could be assigned qualitatively) and it was verified that the grown g layer follows this morphology in a one-to-one fashion [3]. Local etching of the Cu underneath the grown g layer turns the supported into freestanding graphene. Figure 7 shows a Scanning Electron Microscopy (SEM) of the graphene layer with an etched hole in the Cu foil underneath the free standing graphene film. Contrast enhancement of the SEM image proves that the morphology of the former Cu foil on which the g layer was CVD grown is still imprinted in the graphene film, even in its freestanding state.

Photoelectron spectroscopy through electron transparent membranes

Sealing a compartment with a leak tight and ultra thin g based membrane allows a new type of so called near ambient pressure XPS: The photoelectrons released from a vacuum incompatible specimen can penetrate the sealing membrane and reach the electron analyzer environment. Figure 8 shows a SEM image of a g membrane on which Au was evaporated on its backside. The Au 4f photoelectron spectrum acquired with local XPS in the ESCAMICROSCOPE at Elettra evidences that the Au 4f electron emission can pass the graphene membrane and reach the analyzer opposite to the Au film [4].

Fig. 8: left - SEM image of free standing grapehene membranes on a Cu foil. middle - Sketch of Au deposition on the membrane backside and geometry of the illumination and photoelectron emission in the XPS experiment. right - Au 4f photoelectron peak acquired with the electron analyzer facing the front side of the g membranes.

Laboratory based XPS

Our lab based X-ray Photoelectrons Spectroscopy (XPS) system (Leybold LH 10 analyzer) uses a transfer system that allows to insert arbitrary vacuum compatible specimen. The broad spot of a non-monochromatized X-ray tube disperses the ionizing (Mg K or Al K) radiation on a wide area so that low or non conducting samples like technical catalysts and clusters may be investigated. Another advantage of the system is the well characterized transmission function of the analyzer which allows for quantitative XPS analysis. Figure 9 displays the measurement and the preparation chamber of the system, where the sample can be heated and exposed to gases. This is shown in the right upper image of Figure 9. The image below displays a sample holder which can host powder samples in 5 pockets. The system provides XPS, AES and Ion Scattering Spectroscopy (ISS) surface characterization techniques. In several collaborations with partners of the CRC and other groups at TUM, we use the capabilities of the system.

Fig. 9: Photo of the XPS/AES/ISS system (left panel), the sample heating in the preparation chamber (right upper panel) and a sample holder that provides 5 pockets where powder samples may be inserted (right lower panel). Also other samples like foils, thin films or single crystals may be mounted on slightly different sample holders (not shown)

References:

1. Ma X, Günther S. Imaging the confined surface oxidation of Ni3Al(111) by in situ high temperature scanning tunneling microscopy. Physical Chemistry Chemical Physics. 2018;20(34):21844-55.

2. Patrick Z, Xinzhou M, Sebastian G. Indexing moiré patterns of metal-supported graphene and related systems: strategies and pitfalls. New Journal of Physics. 2017;19(1):013015.

3. Kraus J, Boecklein S, Reichelt R, Guenther S, Santos B, Mentes TO, Locatelli A. Towards the perfect graphene membrane? - Improvement and limits during formation of high quality graphene grown on Cu-foils. Carbon. 2013;64:377-90.

4. Kolmakov A, Gregoratti L, Kiskinova M, Günther S. Recent Approaches for Bridging the Pressure Gap in Photoelectron Microspectroscopy. Topics in Catalysis. 2016;59(5):448-68.