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Chapter 8 LEEM and SPLEEM 655
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657
9
Photoemission Electron
Microscopy (PEEM)
Jun Feng and Andreas Scholl
1 Introduction
1.1 A Brief History of Photoemission Electron Microscopy
Photoemission electron microscopy (PEEM) has proven to be a power-
ful tool in material science, surface physics and chemistry, thin fi lm
magnetism, polymer science, and biology. PEEM, also called photoelec-
tron microscopy (PEM), is the most widely used type of emission
microscopy and is closely related to the more recently developed low-
energy electron microscopy (LEEM) (Bauer, 1994). In 1905, Einstein
proposed that light quanta are quantized and explained the photoelec-
tric effect in terms of a photon’s energy to release an electron from a
surface (Einstein, 1905). A photoemission electron microscope forms an
image of a solid surface based on the spatial distribution of electrons
emitted by the absorption of photons with energy that exceeds the
sample work function. Historically, the invention of PEEM dates to the
early 1930s shortly after the introduction of electron lenses. The fi rst
working photoemission electron microscope was built by Brüche in
1932 using ultraviolet (UV) light to image photoelectrons emitted from
a metal (Brüche, 1932; 1933) (Knoll et al., 1932). The principal design of
his PEEM is still used. In Brüche’s PEEM, UV light from a mercury lamp
was focused by a quartz lens onto the sample cathode. The emitted
electrons were accelerated by a potential difference of 10–30 kV between
the cathode and the anode of the PEEM and focused onto a phosphor
screen. At about the same time, Knoll, Hontermans, and Schulzer con-
structed an emission microscope using two magnetic lenses (Langmuir,
1937). The fi rst calculation of the resolution in emission microscopy was
published by Langmuir in 1937, and subsequent treatments were pub-
lished by Reckangel (1941). PEEM techniques were refi ned and improved
by Engel (1966). Engel’s PEEM achieved an order-of-magnitude increase
in resolution over previous instruments and served as a prototype for
the KE-3 emission microscopes developed by Wegmann in the 1970s
(1972). At this time it was recognized that PEEM is a surface technique,
which requires proper attention to the condition of the sample surface
on an atomic scale (Engel, 1966; Griffi th and Rempfer, 1987). A detailed
658 J. Feng and A. Scholl
description of the early history of electron lenses and electron micros-
copy can be found in Griffi th and Engel (1991) and in Bauer (1994).
In the development of PEEM, the Oregon microscope project, lead by
Griffi th and Rempfer, played an important role. Griffi th and Rempfer
built an ultrahigh vacuum UV-PEEM with a spatial resolution of 10 nm
and produced a steady stream of beautiful micrographs of biological
samples (Griffi th and Rempfer, 1987; Rempfer and Griffi th, 1989).
Rempfer also investigated the spatial resolution of PEEM and the prop-
erties of electron lenses of different shape and using different operation
voltages, both in experiment and in theory (Rempfer, 1985; Rempfer
and Griffi th, 1989). Bauer and Telieps built an electron microscope with
both LEEM and PEEM modes (Telieps and Bauer, 1987). Instead of
photoelectrons, the LEEM employs electron diffraction of low-energy
electrons and excels at the investigation of crystalline surfaces, epitaxial
fi lms, and fi lm growth.
A relatively recent development is the use of X-rays instead of UV
radiation. X-ray photoemission electron microscopy (X-PEEM) was
demonstrated for the fi rst time in 1988 by Tonner and Harp (1988). In
1993 Stöhr et al. (1993) showed that X-PEEM can image magnetic
domains at high resolution. X-PEEM instrumentation has evolved
rapidly during the past decade, and almost every major synchrotron
facility is now home to a PEEM instrument (Anders et al., 1999;
Heyderman et al., 2003; Kuch et al., 2000; Schneider and Schönhense,
2002; Wei et al., 2003).
PEEM is limited in resolution by the chromatic and spherical aberra-
tions of the electron lenses. It has been shown that an aberration cor-
rector can improve the resolution down to 1 nm (Fink et al., 1997).
Currently, two aberration-corrected X-PEEMs are under construction,
the “SMART” PEEM at BESSY II (Hartel et al., 2002) and the PEEM-3 at
the Advanced Light Source (Wan et al., 2004). In Section 2 we will
discuss the basic layout of an X-PEEM experiment. We will also describe
the image generation due to X-ray absorption and discuss contrast
mechanisms with the aid of some examples. Section 3 will address the
electron optics of uncorrected PEEM, and Section 4 will extend the dis-
cussion to aberration correction. Section 5 will deal with a very impor-
tant application of X-PEEM: imaging of magnetic domains. The last
section will introduce time-resolved X-PEEM, sometimes called TR-
PEEM, which is a very recent development. TR-PEEM is used to image
magnetic processes on the nanoscale with picosecond time resolution.
2 X-Ray PEEM
2.1 X-Ray Sources
Electron storage rings, also called synchrotrons, are today’s premier
sources of polarized, intense X-rays. Synchrotron radiation is generated
when relativistic electrons are defl ected, for example, by a dipole
magnet. The radiation, which is polarized, spans a wide range of wave-
lengths from micrometers (infrared) to Ångstroms (hard X-rays), and
energies from millielectronvolts to kiloelectronvolts (Attwood, 2000).
Chapter 9 Photoemission Electron Microscopy (PEEM) 659
Bending magnets or so-called insertion devices are used as X-ray
sources. Radiation from a dipole or bending magnet is linearly polar-
ized in the storage ring plane. A few degrees above and below, the
radiation is right and left elliptically polarized with a high degree of
circular polarization of typically 80%. In a bending magnet beamline,
the polarization is chosen by a movable aperture, which masks out a
vertical slice of the beam. Insertion devices, in particular undulators
or elliptically polarizing undulators (EPUs), provide X-rays of much
higher intensity. Undulators are multipole magnet structures that
repeatedly defl ect the electron beam, generating X-rays at each bend
that are coherently superimposed. The magnetic structure in an EPU
can be adjusted to generate radiation of variable polarization: left and
right circularly polarized, linearly polarized in and perpendicular to
the ring plane, and often also linearly polarized at intermediate angles.
The X-rays from the storage ring are brought through an evacuated
beamline to the experiment (Figure 9–1). A grating monochromator in
the beamline generates a monochromatic beam with an energy resolu-
tion of typically several 100 meV. X-ray photoemission electron micro-
scopes are mostly used in an energy range between about 100 and
2000 eV, called the soft X-ray regime, which spans the absorption edges
of many important elements, in particular of the transition metal fer-
romagnets Fe, Co, and Ni. The X-rays are focused onto the sample
using X-ray mirrors to match the X-ray spot size to the fi eld of view of
the microscope, usually a few 10s or 100s of micrometers. The electron
optics of the X-PEEM experiment image the photoelectron yield of the
sample with magnifi cation onto a detector, usually a phosphor and a
charge-coupled device (CCD) camera. The CCD digitizes the signal,
which can later be processed by a PC.
2.2 The X-Ray Absorption and Photoemission Process
When X-rays are absorbed by matter, electrons are excited from core
levels into unoccupied states, leaving empty core states (Figure 9–2).
Secondary electrons are excited by the decay of the core hole. Auger
processes and inelastic electron scattering create a cascade of low-
energy electrons, of which some penetrate the sample surface, escape
Figure 9–1. Layout of X-ray source, beamline, and PEEM of the Advanced
Light Source PEEM-2 microscope.
660 J. Feng and A. Scholl
into vacuum, and are collected by the PEEM optics. A wide spectrum
of electrons is emitted, with energies between the energy of the illu-
mination and the work function of the sample. This wide electron dis-
tribution is the principal source of image aberration in the microscope
since electron lenses are chromatic. In the case of X-ray illumination
the distribution of the low-energy secondary electrons can be approxi-
mated by n
e
∼ E/(E + W)
4
, where W is the work function of the material
and E is kinetic energy (Tonner and Dunham, 1994). In Figure 9–3 the
electron distributions after X-ray and UV illumination are compared.
Note that for X-PEEM in contrast to UV-PEEM a much wider range of
energies contribute to the image. Therefore X-PEEM has signifi cantly
E
E
F
E
E
F
hν
E
E
F
E
E
F
hν
Figure 9–2. Excitation of
a core electron into an
empty valence state after
X-ray absorption (left)
and Auger decay of the
core hole (right). A
cascade of low-energy
electrons is created, of
which some escape into
a vacuum and are
detected.
Figure 9–3. Energy distribution of the emitted secondary electrons excited by
X-rays (solid line) and UV light (dashed line).
Chapter 9 Photoemission Electron Microscopy (PEEM) 661
lower resolution than UV-PEEM, if no aberration corrector is used. The
effective mean free path λ
el
of the low-energy secondary electrons sets
a limit to the probing depth of the technique. In metals a λ
el
of 1.5–2 nm
is typical (Nakajima et al., 1999). In polymers the effective mean free
path is at least twice as large (Ohara et al., 1999; Zharnikov et al., 2002).
Image contrast can still be obtained through cover layers with a thick-
ness of up to about three times the effective mean free path.
The total electron emission of the sample is proportional to the X-ray
absorption coeffi cient averaged over the probed thickness. PEEM
images the lateral electron emission and thereby maps the local X-ray
absorption of the sample as a function of X-ray energy. Local X-ray
absorption spectra are obtained by monitoring the local image inten-
sity while scanning the X-ray energy. Therefore, X-PEEM is called a
spectromicroscopy technique. PEEM is also a full-fi eld imaging tech-
nique, and spectra from many image points are taken in parallel.
2.3 Image Contrast in X-PEEM
Contrast in X-PEEM arises from the element-, chemistry-, and
magnetism-specifi c absorption of different materials. Magnetic con-
trast will be discussed in Section 5. Two other contrast mechanisms,
work function contrast and topographic contrast, appear using both
X-ray and UV radiation. Work function contrast is particularly strong
if a UV source is used with energy between the work function of two
materials that are present at the sample surface. Only the area with low
work function emits electrons and appears bright. Topographic con-
trast arises from rough or structured surfaces. Topographic features
shape the electrical potential close to the sample and lead to electron
defl ection, electron focusing, and defocusing. Usually, experimenters
seek to minimize topographic contrast by preparing smooth samples,
e.g., single crystals, polished surfaces, or fi lms grown on fl at substrates,
because sample roughness can lower the image resolution.
2.3.1 Elemental Contrast
Elemental contrast is a consequence of the large enhancement (by
factors of 10 and more) of the X-ray absorption right above core-level
absorption edges, compared to the absorption below the edge. Reso-
nant absorption into valence states can further increase the absorption
at particular photon energies. By tuning the photon energy to a specifi c
absorption edge, areas that predominantly contain that particular
element light up in the X-PEEM image. K edges of light elements from
C to Si, L edges of transition metals from Ti to Zn and of main group
elements from Ca to Br, and M edges of rare-earth metals from La to
Yb can be studied with soft X-rays. The technologically most important
magnetic elements, the transition metals Cr, Mn, Fe, Co, and Ni and
the rare-earth metals Sm, Eu, Gd, and Tb, have absorption edges
between 570 and 1300 eV, at which magnetism can be measured easily
and quantitatively, as will be discussed later.
The ultimate frontier of PEEM is the imaging and spectroscopy of
nanoscale objects. Although current instruments do not yet possess the
ability to spatially resolve a nanometer object, the high sensitivity of
PEEM allows us to detect the signal from a single nanoobject, if it is
662 J. Feng and A. Scholl
suffi ciently separated in space from neighboring objects. As an example
for the sensitivity of X-PEEM and for elemental contrast imaging,
measurements of 10-nm γ-F
2
O
3
clusters are shown in Figure 9–4
(Rockenberger et al., 2002). Clusters that were brought onto HF-etched
Si wafers from solution were studied. Images of these clusters were
obtained by subtracting a background image acquired before the Fe L
edge from an image acquired on the Fe L
3
peak (at 710 eV). This often-
used difference method augments the signal originating from a par-
ticular element. Individual clusters are visible as light dots. Local
spectra acquired from a single cluster (I) and the clean substrate (II)
show a clear Fe signal from a single cluster but only background from
the surrounding area.
Figure 9–4. High-resolu-
tion micrograph of 10-nm
iron oxide clusters (γ-Fe
2
O
3
)
imaged by X-PEEM at the
Fe edge. Below: local
absorption spectra
acquired at a cluster posi-
tion (I) and on the substrate
(II). [Reprinted with per-
mission from Rocken-
berger et al. (2002).
Copyright 2002 by the
American Institute of
Physics.]
Chapter 9 Photoemission Electron Microscopy (PEEM) 663
2.3.2 Chemical Contrast
The fi ne structure, which is superimposed onto the step-like increase
in the absorption at photon energies close to the absorption edge, is
called near edge X-ray absorption fi ne structure, or NEXAFS. NEXAFS
is selective regarding the electronic structure and chemical environ-
ment of the atom that absorbed the X-ray photon (Stöhr, 1992). This
chemical and electronic sensitivity is due to the fact that close to the
absorption edge photoexcited core electrons are excited into and there-
fore probe valence states. The density of these states is determined by
the electronic band structure and therefore the electronic, chemical,
and magnetic properties of the material. In hydrocarbons spectral fea-
tures can often be associated with particular bonds and molecular
orbitals, allowing a fi ngerprint analysis of spectra (Stöhr, 1992;
Urquhart et al., 1999). In inorganic compounds, the NEXAFS spectrum
is often used to determine oxidization states. Figure 9–5 shows Fe and
Mn images and spectra of a manganese nodule found on the fl oor of
the deep ocean. Regions that are rich in a material containing Mn and
Fe ions appear brighter, when imaged at energies that are specifi c for
the oxidization states of ions. Local spectra acquired in selected regions
distinguish the materials that dominate in the core and rim region of
the studied growth band, providing information about the genesis of
the material (Smith et al., 2003).
0.00
0.05
0.10
0.15
0.20
0.25
0.30
700 705 710 715
Rim interior
Core region
Fe L
3
Photon Energy (eV)
Intensity (arb.units)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
635 640 645 650 655
Rim exterior
Rim interior
Core region
Intensity (arb.units)
Photon Energy (eV)
Mn L
2,3
Mn
2+
– 638.8 eV Mn
3+
– 642.2 eV
Fe
3+
– 707.5 eV
10µm
1
2
3
Figure 9–5. Chemical imaging of a manganese nodule from the ocean fl oor. X-PEEM micrographs
and spectra show areas with different Fe and Mn oxidization states. [Reprinted with permission from
Smith et al. (2003).
35
Copyright 2003 by EDP Sciences.]
664 J. Feng and A. Scholl
3 Uncorrected PEEM Microscopes
Most photoemission electron microscopes have some common char-
acteristics. Modern microscopes use at least three lenses—an objec-
tive, an intermediate, and a projective lens—to achieve optimum
resolution and magnifi cation. The outline of the electron optics is
sketched in Figure 9–6. Some microscopes use magnetic or compound
magnetic–electrostatic lenses, but here a microscope based on simple
electrostatic lenses will be described. The sample, which is illumi-
nated by UV or X-ray radiation, is located in front of an immersion
objective lens. In contrast to light microscopes, the sample is an inte-
gral part of the fi rst electrostatic lens of the imaging system. The
design of the sample stage in a PEEM is critical. Any vibration or drift
is magnifi ed by the optics and reduces the microscope resolution.
Signifi cant misalignment of the sample causes coma and also a loss
of resolution. The sample–objective lens optics consist of two parts:
an acceleration fi eld and a unipotential lens. A strong acceleration
fi eld of about 20 kV over 2 mm accelerates the electrons between the
sample and the fi rst electrode of the objective lens. The strong accel-
eration limits the relative energy spread and the angular spread of
the electrons that are transported into the microscope. The objective
lens together with a contrast aperture in the back focal plane deter-
mines the resolution and transmission of the system. By choosing
different aperture sizes, resolution can be traded for microscope
Figure 9–6. Schematic electron optical layout of a photoemission electron
microscope.