View
233
Download
0
Category
Preview:
Citation preview
copyright W. Kuch 2007
Photoelectron emission microscopy:
Facts and fiction
Wolfgang Kuch, B3
Photoelectron emission microscopy (PEEM):
spectroscopic and microscopic information
“spectromicroscopy” “microspectroscopy”
copyright W. Kuch 2007
images look nicer than spectra, people get more easily impressed
surfaces may exhibit laterally varying properties, spectra only
capture average
study lateral interactions: growth, chemical reactions, magnetic
interactions
study properties of small individual objects
obtain real space information (complementary to diffraction and
scattering techniques)
fast way to study thickness dependence in thin film systems
(wedges)
why spectromicroscopic imaging of surfaces?
copyright W. Kuch 2007
fiction:
facts:
“PEEM is useful only for very special cases”
• 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)
photonselectrons
lens
screen
specimen
first use:
E. Brüche, Z. Phys. 86 (1933) 448;
J. Pohl, Zeitschr. f. techn. Physik 12 (1934) 579
variant of electron microscopy
copyright W. Kuch 2007
E. Bauer, Rep. Prog. Phys. 57 (1994) 895
electrostatic
tetrode
magnetic
triode
cathode lenses for electron emission microscopy
copyright W. Kuch 2007
cathode lens for electron emission microscopy
electrostatic tetrode lens
sample is part of optical system
ra
real starting angle
virtual starting angle
0
'
HV0
contrast aperture
sample
+
–
virtualsample
Uex0
eUex
E0
1E0
accepted solid angle
copyright W. Kuch 2007
photoelectron spectrum using a cathode lens
0
eUex
E0
1
E0
accepted solid angle
300
250
200
150
100
50
0
av
era
ge
d i
ma
ge
in
ten
sit
y
(arb
. u
nit
s)
806040200
kinetic energy (eV)
80 60 40 20 0
binding energy (eV)
Fe 3p Fe 3dW 4f
25
10 ML Fe pattern on W(001)
h = 95 eV Uex = 3.4 keV
2ra = 150 µm
0 : 18° 8° 2°
copyright W. Kuch 2007
fiction:
facts:
“In photoelectron emission microscopy, as we know from the name,
photoelectrons are used to image the sample”
• 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
photonsHV +–
projection lenses
copyright W. Kuch 2007
S. A. Nepijko et al., Ann. Phys. 9 (2000) 441
cathode lens: influence of sample topography
copyright W. Kuch 2007
PEEM contrast: topographic
J. Stöhr and S. Anders, IBM J. Res. Develop. 44 (2000) 535
copyright W. Kuch 2007
fiction:
facts:
“very special samples are needed for PEEM”
• 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
H. Seiler, “Abbildung von Oberflächen”, Bibliographisches Institut, Mannheim (1968)
Hg lamp (h = 4.9 eV)
copyright W. Kuch 2007
PEEM images of CO-Oxidation on patterned Pt, FOV: 400 m
J. Wolff et al., Science 294 (2001) 134
spatiotemporal pattern evolution in surface reactions
PEEM contrast: work function
copyright W. Kuch 2007
J. Wolff et al., Science 294 (2001) 134
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
copyright W. Kuch 2007
islands of pentacene molecules on Si, FOV: 65 m
F. Meyer zu Heringdorf et al., Nature 412 (2001) 517
PEEM contrast: work function
copyright W. Kuch 2007
LUMOHOMO
PEEM contrast: spectroscopic
x-ray absorption spectroscopy
h
photon energy
absorption
XAS-spectrum
h
copyright W. Kuch 2007
PEEM contrast: spectroscopic
J. Stöhr and S. Anders, IBM J. Res. Develop. 44 (2000) 535
elemental
chemical
copyright W. Kuch 2007
PEEM contrast: spectroscopic
J. Stöhr and S. Anders, IBM J. Res. Develop. 44 (2000) 535
chemical
(imaging and
analysis of
wear tracks
on a hard
disk)
C K
F K
copyright W. Kuch 2007
PEEM contrast: spectroscopic
magnetic
copyright W. Kuch 2007
14
12
10
8
6
4
2
0
FeM
n t
hic
kn
ess
(ML
)
Co thickness (ML)
83 4 5 6 7[100]
20 µmh
14
12
10
8
6
4
2
0
FeM
n t
hic
kn
ess
(ML
)
Co thickness (ML)
83 4 5 6 7
Fe
Co
Cu(001)
6 ML FeNi
0–8 ML Co
0–20 ML FeMn
magnetic trilayers: layer-resolved images
W. Kuch et al., Nature Materials 5 (2006) 128
copyright W. Kuch 2007
fiction:
facts:
“Only very few groups have access to PEEM”
• 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
Staib
(electrostatic, no
sample holder)
Elmitec
(Bauer design, magnetic
lens, sample on –HV)
Omicron
(Schönhense design,
electrostatic, sample on
ground)
Specs
(Tromp design,
magnetic lens,
sample on –HV)
copyright W. Kuch 2007
PEEMs at synchrotrons
30 30 m2custom-built
electrostatic PEEM
Berkeley
20 5 m2Elmitec LEEM/PEEMTrieste (Elettra)
30 100 m2Elmitec LEEM/PEEMVilligen (SLS @ PSI)
5 5 m2Elmitec PEEMBerlin (BESSY)
photon spot sizetypeplace
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:
facts:
“The resolution of PEEM is ...”
or: “our PEEM has a resolution of ...”
• 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
Typical “routine” values
(point resolution, flat samples)
20–50 nmLEEM
50–150 nmPEEM, magnetic lens, with synchrotron
radiation
100–300 nmelectrostatic PEEM with synchrotron
radiation
50–150 nmelectrostatic PEEM with Hg lamp
PEEM spatial resolution
copyright W. Kuch 2007
chromatic aberration
spherical aberration
diffraction error
ds
dc
dD
Cs Cc
magnetic
electrostatic 10f
f
4f
f
ds =1
2Cs
3
dc = CcE
E
dD1
2
aberrations in optical imaging
copyright W. Kuch 2007
theoretical resolution
(magnetic triode, 25 kV/3 mm, E =
2.5 eV, E = 0.25 eV)
E. Bauer, Surf. Rev. Lett. 5 (1998) 1275
d = ds2
+ dc2
+ dD2rA
chromatic aberration
spherical aberration
diffraction error
ds =1
2Cs
3
dc = CcE
E
dD1
2
resolution limit
d/nm
copyright W. Kuch 2007
D. Preikszas and H. Rose, J. Electr. Micr. 1 (1997) 1
Th. Schmidt et al., Surf. Rev. Lett 9 (2002) 223
improved resolution by aberration correction
“SMART” target parameters:
E 2
+ E2
E + …Chromatic aberr.
1/1/Diffraction
53 + …Spherical aberr.
with
correction
without
correction
Resolution limit
copyright W. Kuch 2007
fiction:
facts:
“PEEM can do everything”
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
sample
objective lens
magnetic sector field
electron gun
illuminationcolumn
imaging
column
imaging unit
CCD camera
low energy electron microscopy (LEEM)
copyright W. Kuch 2007
20
18
16
14
12
10
86
42
0
20181614121086420
W. Kuch, FUB, K. Fukumoto, J. Wang, MPI-MSP,
C. Quitmann, F. Nolting, T. Ramsvik, PSI-SLS, unpublished.
topographic LEEM contrast
atomic steps at the surface of Cu(001)
copyright W. Kuch 2007
optical imaging: ideal lens
lens
focal plane:
beams starting under
identical angles meet
image plane:
beams starting at same
position meet
F F
p q
f
f
1
p
1
q
1= +
P
Q
Q
P= q
p
copyright W. Kuch 2007
ON
OFFON
OFF
focal plane
image plane
real space k space
imaging of the diffraction plane
sample
copyright W. Kuch 2007
Fermi surface mapping by PEEM
M. Kotsugi et al., Rev. Sci. Instrum. 74 (2003) 2754
photon energy 95 eV
copyright W. Kuch 2007
LEEM image of atomic terraces on Si(100), FOV: 4 m
G. L. Kellogg, Sandia Natl. Lab., Albuquerque
...it’s time for a (coffee) break!
when samples start looking at you...
Recommended