about the technique

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

Photoelectron Spectroscopy (XPS) at APE-HE can be exploited as a surface chemical analysis technique: the valence band DOS reveals the 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 (up to 1600 eV) or to have relatively high kinetic energies for shallow core levels, probing variable depths of the material.

X-ray Absorption Spectroscopy (XAS) at APE-HE can be exploited to detect surface or buried atomic species displaying characteristic absorption edges in the relevant energy range (150-1600 eV) and to exploit the linear polarization of the beamline (horizontal 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 (element specific ferromagnetic hysteresis loops, sum rule analysis).

Three picoammeters are used during each XAS spectrum. One records the drain current at the entry of the end station (mesh) for intensity normalization, a second one measures the drain current of the sample, and a third one records the drain current of a reference sample, covering the whole range of energies, which is used as a reference of the photon energy.

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The APE-HE apparatus

APE-HE end station (figure) is a 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. The sample manipulator allows X-Y scanning on the surface with 1 micrometer precision and is therefore suitable to perform experiments 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 plan up to 0.1 T and for E-field application up to 500 V between contacted surfaces and substrate. The detection of total electron yield (TEY) is achieved by measuring the drain current with a high sensitivity electrometer and highly shielded connections, or with an in vacuum electron multiplier. The detection of total fluorescent yield (TFY) is achieved by an in-vacuum photodiode. Continuous scanning of the monochromator for XAS spectra is available for superior speed. Step-by-step scanning is also available and used to perform highly sensitive XMCD while switching the magnetization of the sample at each point (see figure).

Two independent electrical connections on the measuring position of the sample allow applying electric bias during or in-between the measurement. The possibility of combining electric and magnetic fields allows studying the chemical and magnetic behaviour of multiferroic materials and heterostructures.

Together with the UHV setup, APE-HE allows performing experiments at ambient pressure, as 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 manipulator mounted on the rear flange of the APE-HE chamber on which the cell is attach. In this geometry the X ray beam impinges normal to the cell, which can be moved in the plane perpendicular to the beam in order to center the 500x500µm Silicon nitride window. The cell has two independent electrical contacts (fig 3), one on the sample and one on the membrane; this allows polarizing the membrane at positive bias voltage and recording the total electron yield (TEY) on the sample by measuring its drain current. In this way it is possible to record XAS spectra of the sample while keeping the sample at ambient pressure and/or expose it 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 it is clearly visible from fig 4, the fast acquisition mode the XAS spectra allows the recording of XAS spectra with a great statistics in a time scale between seconds and few minutes. This will allow following 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
Base pressure in end station <2x10E-10 mbar
Manipulator degrees of freedom X,Y,Z + Polar and Azimuthal rotations
Sample position precision 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 0.1 T in pulse mode up to 200 mT 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 400 Celsius
Magnetic field up to 0.2 T
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
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Scientists in charge