Photoelectron emission microscopy: Facts and fiction · Photoelectron emission microscopy: Facts...

copyright W. Kuch 2007

Photoelectron emission microscopy:

Facts and fiction

Wolfgang Kuch, B3

Photoelectron emission microscopy (PEEM):

spectroscopic and microscopic information

“spectromicroscopy” “microspectroscopy”


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?


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


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


E. Bauer, Rep. Prog. Phys. 57 (1994) 895

electrostatic

tetrode

magnetic

triode

cathode lenses for electron emission microscopy


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


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°


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)


schematics of an electrostatic PEEM

CCD camera

fluorescent screen

channelplate

photonsHV +–

projection lenses


S. A. Nepijko et al., Ann. Phys. 9 (2000) 441

cathode lens: influence of sample topography


PEEM contrast: topographic

J. Stöhr and S. Anders, IBM J. Res. Develop. 44 (2000) 535


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)


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)


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



J. Wolff et al., Science 294 (2001) 134


CO-oxidation reaction front on Pt(110), being dragged by a laser spot

that locally heats the sample. FOV: 1.5 1.1 mm2


islands of pentacene molecules on Si, FOV: 65 m

F. Meyer zu Heringdorf et al., Nature 412 (2001) 517



LUMOHOMO

PEEM contrast: spectroscopic

x-ray absorption spectroscopy

h

photon energy

absorption

XAS-spectrum

h




elemental

chemical




chemical

(imaging and

analysis of

wear tracks

on a hard

disk)

C K

F K



magnetic


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


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


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)


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, ...


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 ...”


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


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


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


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


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)


sample

objective lens

magnetic sector field

electron gun

illuminationcolumn

imaging

column

imaging unit

CCD camera

low energy electron microscopy (LEEM)


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)


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


ON

OFFON

OFF

focal plane

image plane

real space k space

imaging of the diffraction plane

sample


Fermi surface mapping by PEEM

M. Kotsugi et al., Rev. Sci. Instrum. 74 (2003) 2754

photon energy 95 eV


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...

Photoelectron emission microscopy: Facts and fiction · Photoelectron emission microscopy: Facts...

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