about the technique

The end station of the High Energy branch (HE) at APE beamline is devoted to the investigation of the magnetic and electronic properties of surfaces, interfaces and nanostructures via X-ray Photoemission Spectroscopy (XPS), X-ray Absoprtion Spectrscopy (XAS) and X-ray Magnetic Circular Dichroism (XMCD) analysis.

Photoelectron Spectroscopy at APE-HE can be exploited as a surface chemical analysis technique: the valence band DOS reveals orbital contributions through cross-section analysis (photon energy dependence) and core level photoemission is sensitive to the local environment of a given atomic specie in the sample or at the surface. The photon energy range available allows to excite relatively deep core levels (> 1000 eV) or to have relatively high kinetic energies for shallow core levels, probing variable depths of material.

X-ray Absoprtion Spectrscopy (XAS) at APE-HE can be exploited to detect surface or buried atomic species displaying characteristic absorption edges in the relevant energy range and to exploit the linear polarization of the beamline (horiziontal and vertical) as a "search light" and for X-ray Linear Magnetic Dichroism (XMLD) as well as the circular polarization for the XMCD spectroscopy and magnetometry (atom specific ferromagnetic hysteresis loops, sum rule analysis).

i
i

The APE-HE apparatus

The end station of the high energy branch (figure) is an UHV chamber equipped with a 4 axis manipulator and an hemispherical electron energy analyzer. It is built as a vibration-free sample chamber with mu-metal shielding for reducing the background H-field. It can be fitted with microfocussing zone-plate option (not in use as standard configuration). The sample manipulator allows X-Y scanning on the surface with 1 micrometer precision and is therefore suitable to perform experiment on microstructures (see figure). The sample environment allows for temperature control (He-flow cryostat and heating stages) with feedback, for H-field application (in the surface plane) and for E-field application between contacted surfaces and substrate. Detection of total electron yield is achieved by measuring the drain current with a high sensitivity electrometer and highly shielded connections, or with an in vacuum electron multiplier. Detection of total fluorescent yield is achieved by an in-vacuum photodiode. Continuous scanning of the monochromator for XAS/XMCD spectra is available for superior signal/noise ratio and speed. Step-by-step scanning is also available and used to perform XMCD while switching the magnetization of the sample at each point (see figure). Recent developments are focussing on the in-operando facility that allows to collect synchrotron radiation spectroscopy signals as a function of applied E and H fields.

The new set-up for performing XAS spectra while keeping the sample at ambient pressure is sketched in figure 3. The system is composed by a DN 100 CF flange on which are fixed 3 inox sticks that support the reactor and connect it rigidly on the flange. The movement of the system is then ensured by a XY table mounted on the rear flange of the APE-HE chamber on which the cell is attach. In this geometry the X ray beam arrive along the axis of the cell and the cell can be moved in the plane perpendicular to the beam propagation direction in order to center the Silicon nitride window which is of only 500X500 microns. The cell has two independent electrical contacts (fig 3) one on the sample and one on the membrane that allow to polarize the membrane at positive bias voltage and to record the total electron yield on the sample by measuring the drain current. In this way is possible to record XAS spectra of the sample while keeping the sample at ambient pressure and exposed to a desired gas (or a mixture of gases). The XAS spectroscopy provides a number of information on the electronic state of the material starting from the oxidation state of each element and going in greater details like the symmetry of the ligands. In figure 4 is depicted the  TEY spectrum of the Ce M4;5  absorption edge in Ceria/YSZ, using the fast data acquisition mode. As is clearly visible from fig 4 the fast acquisition mode the XAS spectra permits the recording of XAS spectra with a great statistics in a time scale between second and minutes. This will allow to follow the evolution of the reaction and not only the initial and final products of a reaction.

 

Fig 1: Schematic circuit of the XAS and XMCD experiment with applied electric field.

Fig 1: Schematic circuit of the XAS and XMCD experiment with applied electric field.

Fig 2: Picture of the different elements of the reactor cell.

Fig 2: Picture of the different elements of the reactor cell.

Fig 3: Schematic cross section of the reactor at the sample level. The sample is depicted in green while the orange part represents the SiN membrane.

Fig 3: Schematic cross section of the reactor at the sample level. The sample is depicted in green while the orange part represents the SiN membrane.

Fig 4:  XAS spectrum of CeO2 pure powder acquired at 1 Bar of pressure of pure Helium at room temperature. The total acquisition time of the spectrum is 60 seconds.

Fig 4: XAS spectrum of CeO2 pure powder acquired at 1 Bar of pressure of pure Helium at room temperature. The total acquisition time of the spectrum is 60 seconds.

Technical specifications

Source Apple II Undulator
Photon energy range 150-1600 eV
Polarization Variable (horizontal, vertical, circular ±)
Resolution (E/dE) up to 5000
Beam size on the sample (H x V, ) 500 x 200 microns
Entry slit reduction to 100x100 microns possible
Zone Plate optical condenser reduction to 1x3 microns possible
base pressure in end station <2x10E-10 mbar
manipulator degrees of freedom X,Y,Z + Polar and Azimuthal rotations
Sample position procision 1 micrometer
Exlectron Energy Analyzer Omicron EA 125 analyser
Sample drain current measurements Keithley 6514 picoammeter (for XAS/XMCD/XMLD)
Photon detectors 10x10 mm silicon photodiode
total ejected electron yield detector 20mm high gain channeltron
Sample temperature LT stage (while measuring) from 50 K to 300 K (He-flow cryostat)
Sample temperature HT stage (while measuring) from 300 K to 500 K
Magnetic field up to 1000 Oe in pulse mode up to 200 Oe in continuous mode
Accepted sample size 5 x 5 mm
------Ambient pressure XAS------ --------------------------
Photon energy range 150-1600 eV
Polarization Variable (horizontal, vertical, circular ±)
Resolution (E/dE) up to 5000
Silicon Nitride membrane size 500 x 500 microns
Silicon Nitride membrane thickness 100 nm
Pressure in the cell From 1x4E-10 mbar to 1 Bar
manipulator degrees of freedom X,Y
Sample position precision 1 micrometer
Sample drain current measurements Keithley 6514 picoammeter (for XAS/XMCD/XMLD)
Sample temperature (while measuring) from RT to 150 Celsius
Magnetic field up to 2000 Oe
Accepted sample size 10 x 10 mm
End station of the APE-HE line with PES analyzer, TEY multiplier and drain current measurements

End station of the APE-HE line with PES analyzer, TEY multiplier and drain current measurements

The map of techniques

at MM building at Q2 building at Elettra experimental hall at CNR-IOM cloud at Fermi-T-Rex laboratory Surface & Nano Science Lab, STM/STS PLD XPS & ambient pressure XAS ARPES & Spin ARPES MOKE & Masked deposition system XPS MBE Oxides SPRINT laboratory SEM XRD PVD data repository open data data analysis
add to wishlist

Scientists in charge