Photoelectron emission microscopy:
Facts and fiction
Wolfgang Kuch, B3
Photoelectron emission microscopy (PEEM):
spectroscopic and microscopic information
“spectromicroscopy”
“microspectroscopy”
copyright W. Kuch 2007
why spectromicroscopic imaging of surfaces?
images look nicer than spectra, people get more easily impressed
study lateral interactions: growth, chemical reactions, magnetic
interactions
study properties of small individual objects
fast way to study thickness dependence in thin film systems
(wedges)
surfaces may exhibit laterally varying properties, spectra only
capture average
obtain real space information (complementary to diffraction and
scattering techniques)
copyright W. Kuch 2007
fiction:
“PEEM is useful only for very special cases”
facts:
• PEEM proves useful in different fields of physics, chemistry, material
science or life science
• several contrast mechanisms allow to address different questions
• systems investigated range from meteorite pieces to biologic tissues
copyright W. Kuch 2007
photoelectron emission microscopy (PEEM)
variant of electron microscopy
screen
lens
photons
electrons
specimen
first use:
E. Brüche, Z. Phys. 86 (1933) 448;
J. Pohl, Zeitschr. f. techn. Physik 12 (1934) 579
copyright W. Kuch 2007
cathode lenses for electron emission microscopy
electrostatic
tetrode
magnetic
triode
E. Bauer, Rep. Prog. Phys. 57 (1994) 895
copyright W. Kuch 2007
cathode lens for electron emission microscopy
electrostatic tetrode lens
ra
contrast aperture
real starting angle 0
virtual starting angle 0
sample
+
HV
Uex
–
'
0
eU ex
E0
virtual
sample
sample is part of optical system
accepted solid angle
1
E0
copyright W. Kuch 2007
averaged image intensity (arb. units)
photoelectron spectrum using a cathode lens
binding energy (eV)
80
60
40
20
0
300
10 ML Fe pattern on W(001)
h = 95 eV
250
200
U ex = 3.4 keV
2ra = 150 µm
150
Fe 3p W 4f
Fe 3d
100
0 :
25
50
0
0
18° 8°
20
40
60
kinetic energy (eV)
80
2°
0
eU ex
E0
accepted solid angle
1
E0
copyright W. Kuch 2007
fiction:
“In photoelectron emission microscopy, as we know from the name,
photoelectrons are used to image the sample”
facts:
• In PEEM, photoexcited electrons are used to image the sample:
– for photon energies close to the vacuum threshold, these are basically
photoelectrons (Hg lamp, laser)
– for higher photon energies, these are basically secondary electrons
(as long as no energy filtering is used)
copyright W. Kuch 2007
schematics of an electrostatic PEEM
CCD camera
fluorescent screen
channelplate
projection lenses
HV
+
–
photons
copyright W. Kuch 2007
cathode lens: influence of sample topography
S. A. Nepijko et al., Ann. Phys. 9 (2000) 441
copyright W. Kuch 2007
PEEM contrast: topographic
J. Stöhr and S. Anders, IBM J. Res. Develop. 44 (2000) 535
copyright W. Kuch 2007
fiction:
“very special samples are needed for PEEM”
facts:
• Samples have to be flat
– Rule of thumb: required flatness 1/10 of desired resolution
• Samples should not charge under illumination
• The information depth is determined by the escape depth of secondary
electrons: typical 1/e length: 2 nm (in metals)
copyright W. Kuch 2007
PEEM contrast: work function
work function contrast
from coarse-grained Au
Hg lamp (h = 4.9 eV)
H. Seiler, “Abbildung von Oberflächen”, Bibliographisches Institut, Mannheim (1968)
copyright W. Kuch 2007
PEEM contrast: work function
spatiotemporal pattern evolution in surface reactions
PEEM images of CO-Oxidation on patterned Pt, FOV: 400 m
J. Wolff et al., Science 294 (2001) 134
copyright W. Kuch 2007
PEEM contrast: work function
CO-oxidation reaction front on Pt(110), being dragged by a laser spot
that locally heats the sample. FOV: 1.5 1.1 mm2
J. Wolff et al., Science 294 (2001) 134
copyright W. Kuch 2007
PEEM contrast: work function
islands of pentacene molecules on Si, FOV: 65 m
F. Meyer zu Heringdorf et al., Nature 412 (2001) 517
copyright W. Kuch 2007
PEEM contrast: spectroscopic
x-ray absorption spectroscopy
LUMO
HOMO
h
absorption
XAS-spectrum
photon energy h
copyright W. Kuch 2007
PEEM contrast: spectroscopic
elemental
chemical
J. Stöhr and S. Anders, IBM J. Res. Develop. 44 (2000) 535
copyright W. Kuch 2007
PEEM contrast: spectroscopic
CK
chemical
(imaging and
analysis of
wear tracks
on a hard
disk)
FK
J. Stöhr and S. Anders, IBM J. Res. Develop. 44 (2000) 535
copyright W. Kuch 2007
PEEM contrast: spectroscopic
magnetic
copyright W. Kuch 2007
magnetic trilayers: layer-resolved images
0–20 ML FeMn
0–8 ML Co
FeMn thickness (ML)
14
Fe
12
10
8
6
4
2
0
3 4 5 6 7 8
Co thickness (ML)
6 ML FeNi
Cu(001)
h
[100]
20 µm
FeMn thickness (ML)
14
Co
12
10
8
6
4
2
0
W. Kuch et al., Nature Materials 5 (2006) 128
3 4 5 6 7 8
Co thickness (ML)
copyright W. Kuch 2007
fiction:
“Only very few groups have access to PEEM”
facts:
• Rich groups can just buy a PEEM
• Many synchrotrons (nearly all) offer PEEM user end stations
– Good groups can apply for beamtime at these instruments
copyright W. Kuch 2007
commercial PEEMs
Specs
(Tromp design,
magnetic lens,
sample on –HV)
Staib
(electrostatic, no
sample holder)
Omicron
Elmitec
(Bauer design, magnetic
lens, sample on –HV)
(Schönhense design,
electrostatic, sample on
ground)
copyright W. Kuch 2007
PEEMs at synchrotrons
place
type
photon spot size
Berlin (BESSY)
Elmitec PEEM
5 5 m2
Villigen (SLS @ PSI)
Elmitec LEEM/PEEM
30 100 m2
Trieste (Elettra)
Elmitec LEEM/PEEM
20 5 m2
Berkeley
custom-built
electrostatic PEEM
30 30 m2
PEEM end stations also exist at synchrotrons in Japan, Taiwan;
others are being set up at Diamond (UK), Soleil (France), Canada, ...
copyright W. Kuch 2007
fiction:
“The resolution of PEEM is ...”
or: “our PEEM has a resolution of ...”
facts:
• The resolution depends on:
– aberrations (spherical, chromatic, diffraction)
– noise (electrical, magnetic, mechanical)
– sample
– the way it is measured
• One has to distinguish “best” and “routine” values, the latter are rarely
published
• “The resolution of the images presented here, determined as ..., is ...”
copyright W. Kuch 2007
copyright W. Kuch 2007
PEEM spatial resolution
Typical “routine” values
(point resolution, flat samples)
electrostatic PEEM with Hg lamp
50–150 nm
electrostatic PEEM with synchrotron
radiation
100–300 nm
PEEM, magnetic lens, with synchrotron
radiation
50–150 nm
LEEM
20–50 nm
copyright W. Kuch 2007
aberrations in optical imaging
spherical aberration
ds =
ds
1
C s 3
2
chromatic aberration
dc = Cc
dc
E
E
diffraction error
1
dD 2
magnetic
electrostatic
Cs
Cc
f
f
10f
4f
dD
copyright W. Kuch 2007
resolution limit
spherical aberration
theoretical resolution
1
d s = C s 3
2
(magnetic triode, 25 kV/3 mm, E =
2.5 eV, E = 0.25 eV)
chromatic aberration
E
E
diffraction error
dD d/nm
dc = Cc
1
2
E. Bauer, Surf. Rev. Lett. 5 (1998) 1275
rA
2
d = d 2s + dc2 + dD
copyright W. Kuch 2007
improved resolution by aberration correction
“SMART” target parameters:
Resolution limit
without
correction
with
correction
Spherical aberr.
3 + …
5
Chromatic aberr.
E + …
E 2
+ E2 Diffraction
1/
1/
D. Preikszas and H. Rose, J. Electr. Micr. 1 (1997) 1
Th. Schmidt et al., Surf. Rev. Lett 9 (2002) 223
copyright W. Kuch 2007
fiction:
“PEEM can do everything”
facts:
You can do a lot more than just take images
• combine with low-energy electron microscopy (LEEM)
• image diffraction plane
• use electron energy filtering
• do full-image microspectroscopy (limit spectromicroscopy =
microspectroscopy)
copyright W. Kuch 2007
low energy electron microscopy (LEEM)
sample
illumination
column
objective lens
magnetic sector field
imaging
column
CCD camera
electron gun
imaging unit
copyright W. Kuch 2007
topographic LEEM contrast
atomic steps at the surface of Cu(001)
2
4
6
8
10
12
14
16
18
20
20
18
16
14
12
10
8
6
4
2
0
0
W. Kuch, FUB, K. Fukumoto, J. Wang, MPI-MSP,
C. Quitmann, F. Nolting, T. Ramsvik, PSI-SLS, unpublished.
copyright W. Kuch 2007
lens
optical imaging: ideal lens
p
P
p
=
q
Q
q
f
1
f
P
F
=
1
p
+
1
q
F
Q
focal plane:
image plane:
beams starting under beams starting at same
identical angles meet position meet
copyright W. Kuch 2007
imaging of the diffraction plane
real space
k space
ON
OFF
OFF
ON
image plane
focal plane
sample
copyright W. Kuch 2007
Fermi surface mapping by PEEM
photon energy 95 eV
M. Kotsugi et al., Rev. Sci. Instrum. 74 (2003) 2754
copyright W. Kuch 2007
when samples start looking at you...
...it’s time for a (coffee) break!
LEEM image of atomic terraces on Si(100), FOV: 4 m
G. L. Kellogg, Sandia Natl. Lab., Albuquerque
copyright W. Kuch 2007