222
A. Kleibert
is a versatile technique with a variety of detection modes and is used in many research
fields ranging from magnetism, mineralogy, catalysis, organic matter, electrochemistry, to battery research and nanodevices [9, 10]. In the following sections we will
briefly introduce the instrumentation needed for XPEEM investigations of individual nanoparticles, the working principle of XPEEM for magnetism studies, and the
particular sample requirements for experiments on individual nanoparticles.
9.2.1 Instrumentation
XPEEM investigations on magnetic systems require highly intense, tunable and
monochromatic radiation with variable polarization. In the soft x-ray range one
probes most frequently the L edges of the 3d transition metals and the M edges
of the rare earth elements at dedicated beamlines of large scale synchrotron facilities. A recent list of beamlines equipped with XPEEM instruments can be found in
[33]. Access to XPEEM instruments is usually possible through user operation programs of the individual facilities. Figure 9.1a displays the schematics of an XPEEM
instrument suited for nanoparticle studies. The x-rays impinge at a grazing angle of
incidence with respect to the sample surface. For XPEEM imaging the sample is
placed in the x-ray beam at a typical working distance between 1 and 2 mm in front
of the objective lens. Usually, the sample is held at a negative high voltage in a range
from –10 to –20 kV in order to accelerate the photo-excited electrons to high energies
and into the electron optics. The latter consists of an energy analyzer and a number
of electromagnetic or electrostatic lenses used for fast switching between different
imaging/detection modes and field of views, which usually range from 1 to 100 µm.
The instrument illustrated in Fig. 9.1a is in addition equipped with an electron gun
used for spectroscopic LEEM (SPELEEM) applications. A more detailed description of the various possible instrument configurations and operation modes used in
low energy electron microscopy can be found in [9]. As detector, a microchannel
plate assembly combined with a phosphorescent screen and a charge-coupled device
(CCD) camera are mostly used. More recently, pixelated direct electron detectors
with enhanced quantum efficiency, virtually infinite dynamic range, and calibrated
flat field have been introduced [34]. Such detectors may in future offer advanced
detection schemes such as single electron time–of–flight detection with promising
potential for nanoparticle investigations.
9.2.2 XPEEM Fundamentals
Magnetic and chemical characterization in XPEEM is usually achieved by probing
the local x-ray absorption cross-section via spatially resolved detection of secondary
electrons. The latter originate from non-radiative intra-atomic relaxation processes
following the primary absorption of an x-ray photon [36]. The formation of secondary
A. Kleibert
is a versatile technique with a variety of detection modes and is used in many research
fields ranging from magnetism, mineralogy, catalysis, organic matter, electrochemistry, to battery research and nanodevices [9, 10]. In the following sections we will
briefly introduce the instrumentation needed for XPEEM investigations of individual nanoparticles, the working principle of XPEEM for magnetism studies, and the
particular sample requirements for experiments on individual nanoparticles.
9.2.1 Instrumentation
XPEEM investigations on magnetic systems require highly intense, tunable and
monochromatic radiation with variable polarization. In the soft x-ray range one
probes most frequently the L edges of the 3d transition metals and the M edges
of the rare earth elements at dedicated beamlines of large scale synchrotron facilities. A recent list of beamlines equipped with XPEEM instruments can be found in
[33]. Access to XPEEM instruments is usually possible through user operation programs of the individual facilities. Figure 9.1a displays the schematics of an XPEEM
instrument suited for nanoparticle studies. The x-rays impinge at a grazing angle of
incidence with respect to the sample surface. For XPEEM imaging the sample is
placed in the x-ray beam at a typical working distance between 1 and 2 mm in front
of the objective lens. Usually, the sample is held at a negative high voltage in a range
from –10 to –20 kV in order to accelerate the photo-excited electrons to high energies
and into the electron optics. The latter consists of an energy analyzer and a number
of electromagnetic or electrostatic lenses used for fast switching between different
imaging/detection modes and field of views, which usually range from 1 to 100 µm.
The instrument illustrated in Fig. 9.1a is in addition equipped with an electron gun
used for spectroscopic LEEM (SPELEEM) applications. A more detailed description of the various possible instrument configurations and operation modes used in
low energy electron microscopy can be found in [9]. As detector, a microchannel
plate assembly combined with a phosphorescent screen and a charge-coupled device
(CCD) camera are mostly used. More recently, pixelated direct electron detectors
with enhanced quantum efficiency, virtually infinite dynamic range, and calibrated
flat field have been introduced [34]. Such detectors may in future offer advanced
detection schemes such as single electron time–of–flight detection with promising
potential for nanoparticle investigations.
9.2.2 XPEEM Fundamentals
Magnetic and chemical characterization in XPEEM is usually achieved by probing
the local x-ray absorption cross-section via spatially resolved detection of secondary
electrons. The latter originate from non-radiative intra-atomic relaxation processes
following the primary absorption of an x-ray photon [36]. The formation of secondary