Photoemission (2)Surface Science
Iwao Matsuda
Synchrotron Radiation Laboratory,
the Institute for Solid State Physics,
the University of Tokyo, JAPANhttp://imatsuda.issp.u-tokyo.ac.jp/index.htm

Surface/Interface in scales
SpaceThe Earth/Vacuum interface

Climates
Sea/Air interface

Skins
Face/Air interface

1Surface/Interface in scales
Goods(solid/air interface)
Cooking(Oil/water interface)

Biomaterials surviving
in various environments

Molecules and atoms
in various conditions

Surface/Interface in scales
Things go on in a non-uniform system in any scale.
And there’re always interfaces (surfaces) that play their roles.
- Solid/Liquid- Solid/Gas- Liquid/Gas- Solid/Solid- Solid/Vacuum- ……..

2

My playground
Vacuum/Solid interface, solid surface

AtomQuantum film

Monolayer

Atomic wire
Nanowire

Quantum dot

Nanometer-scale and atomic-scale structures on a solid surface.
Chemistry・Catalysis reaction
・Ecology
・Solutions for energy-shortage problem

Physics
・Low-dimensional physics
・Quantum dynamics
・New physics

Applied Physics
・Bottom-up nanotechnology
・Atom technology
・New technological developments

Advantage of surface science:• Visualization of atomic configuration and electron density (LDOS) distribution in atomic scale• Direct determination of electronic structure (band, Fermi surface, etc…)

Surface Analyses
Varieties of Surface analysis techniques
Given examples…….

PhotonParticle

Nanoprobes
Scanning Tunneling Microscope (STM)
Atomic Force Microscope (AFM)

Nanoprobes
Particle
PhotonParticle-in / Particle-out
He scattering
Low-energy electron Diffraction (LEED)Transmission Electron Diffraction (TED)
Refection High-Energy Electron Diffraction (RHEED)
Refection High-Energy Positron Diffraction (RHEPD)
Photon-in / Photon-out
Grazing-angle incident X-ray Diffraction (GIXRD)

3

Surface Analyses
Photon-in / Particle-out
Photoemission Spectroscopy (PES), Ultraviolet Photoelectron Spectroscopy (UPS)Core-level Spectroscopy (CLS), X-ray Photoelectron Spectroscopy (XPS)
X-ray Absorption Spectroscopy (XAS)
Auger Electron Spectroscopy (AES)
Bands(unoccupied)ElectronPhoton
Bands(occupied)Core-levels

PES, UPS

CLS, XPSXAS

AESSpectroscopy with VUV~SXElectron mean free pathsurface-sensitive ~ bulk sensitive
Electron mean free path (nm)An atomic layer

Electron Energy, Kinetic Energy (eV)4Spectroscopy with VUV~SX

Absorption edge, resonance effectlanthanoid（M-edge）

transition metal（L-edge）

Spectroscopy with VUV~SXEnergy range to probe- atomic structure- electronic structure- spin structure● Diffraction● Absorption (EXAFS,NEXAFS,MCD)● Photoemission (ARPES, CLS, Spin-resolved PES, PED)
● X-ray emission○ surface sensitive ~ bulk sensitive○ specification of all elements○ structure determination with high accuracy○ spin magnetic moment, orbital magnetic moment○ direct determination of spin-resolved electronic structure5

Cutting a Si(111) crystal
Atomic structure

Cutting (expectation)
Silicon（Si)Diamond structure

Dangling-bonds

6

Scanning Probe Microscope
Scanning Tunneling Microscope (STM)

Rohrer and Binnig (1982)

STM

7

Scanning Tunneling Microscope

An experimental chamber- the Ultra High Vacuum condition- Isolation of vibration- Sample surface preparation

An STM image
Ideal surface

Real surface (STM image)G. Binnig, H. Rohrer et al., Phys. Rev. Lett. 50 (1983) 120.

The 7a1x1 (a1x1:3.8Å) periodicity:
The Si(111)7x7 surface8Scanning Tunneling Microscope
It’s just an image of atomic scale protrusions
measured through tunneling currents.
What are them?
Tunneling currents between unoccupied statesand occupied states near Fermi level (EF).

Protrusions in STM could be surface atoms

Cutting a Si(111) crystal
Electronic structure

9Electronic states of Si(111)7x7
Bonding states
adatomBulk Conduction Band

restatom

DAS model

Dangling-bond states

Bulk Valence Band

Adatom# of dangling bondsin the 7x7 unit cell

Cornerhole

Restatomadatom: １２
restatom: ９
corner-hole: 1

Probing electronic statesphotoelectric effect
Dr. Albert Einstein

バンド分散（E,k//）10

Probing electronic statesPhotoelectron spectroscopy

Bands(unoccupied)ElectronPhoton
Bands(occupied)Core-levels

PES, UPS

CLS, XPS

What can be probed by photoemission
Vacuum level

Fermi level

Band Dispersions of- partially occupied states (metal)- fully occupied states (semicpnductor)

Energy shift of core-levels- Different elements- Different chemical sites (environments)

11Band mappingAngle-resolved photoemission spectroscopy(ARPES)

Band dispersion（E,k//）

ARPES measurement
Si(111)7x7
Conventional measurements
Energy spectra at various angles
along symmetric crystal axis

M

[101]

K

1x1 SBZ
P. Martensson et al.,Phys. Rev. B 36, 5974 (1987).

12

Dangling bonds
DAS model

Rest atom state(Fully occupied)

AdatomRestatom

Adatom state(Partially occupied)- Charge transfer- Metallic surface
R. Losio et al.,Phys. Rev. B 61, 10845 (2000).

Fermi surface mapping

Adatom stateSi(111)7x7

Photoemission intensity mapping
Intensity of a certain energy taken at various emission angles

R. Losio et al.,Phys. Rev. B 61, 10845 (2000).

13Core-levels
Surface components
C1: Atom binding to the adatom
C2: Rest atom
C3: Adatom
C4: Dimer atom
C5: Surface impurity atom
DAS model

AdatomRestatomR. I. G. Uhrberg et al.,Phys. Rev. B 58, R1730 (1998).

Scanning Tunneling Spectroscopy
Tunneling Spectroscopy at different sire

Differential Current Imaging Tunneling Spectroscopy

Adatom state @ - 0.35 V

Si(111)7x7

Rest atom state @ - 0.8 V

UPSIPS
Back bond state @ - 1.7 V
R. J. Hamers et al.,Phys. Rev. Lett. 56, 1972 (1986).

14

Cutting a Si(111) crystal
Adsorption

Variations of ordered surface phasesSi(111)7x7+ 1 ML Ag+ 0.4 ML Au

Si(111)5x2-Au

Si(111)√3x√3-Ag

Numerous numbers of ordered phases- Adsorbates (atom, molecule)- Coverage- Co-deposition- Substrate- Deposition temperature- Deposition rate
> 300 reported- Deposition steps
for Si substrate- ….

S. Hasegawa et al.,Prog. Surf. Sci. 60, 89 (1999).

15

Si(111)√3x√3-Ag

Atomic Structure and STM image

Si(111)√3x√3-Ag

Atomic Structure and CLS spectra

S1S2R.I.G Uhrberg et al., Phys. Rev. B 65, 081305(R) (2002).

16

Si(111)√3x√3-Ag
STM simulation

SiAgAgSi
Tunneling current distribution

Structure model

STM protrusions do not match the atom positions.
S. Watanabe et al., Phys. Rev. B 44, 8330 (1991).

Si(111)√3x√3-Ag
Electronic StructureBinding Energy (eV)Fermi level（EF）

Fermi surfacek// (Å-1)

17

Scanning Tunneling Spectroscopy
STM

G. Nicolay et al., Phys. Rev. B 62 1631 (2000).

Scanning Tunneling Spectroscopy (STS), dI/dV

X

HV-Amplifier

Y

HV-Amplifier

XY-RampX

Y
HV-Amplifier

Display

Signal (dI/dV)

dI/dVX

dIdVY

dI/dV
1.0
Sample Voltage (V)Z-ControlV’(I’)

I’U’

Reference

B

Z

Feedback
Electronics

Lock-in
Amplifier

C

X,Y,Z
Piezo
Tube
tip
sampleR. M. Feenstra et al.,Phys. Rev. B 66, 165204 (2002)

A

Bias Modulation: V’ = V +  sint
Current Modulation: I(V +  sint) = I(V) + dI  sintdV
Detection by phase matching～dIdV

Function Generator∝ LDOS (Local Density Of States)

C. Bai, Scanning Tunneling Microscopy and its Application(Springer,1992)

Si(111)√3x√3-Ag

Electron Standing wave
at monatomic step

I. Matsuda et al.,Phys. Rev. Lett. 93, 236801 (2004).

18

SummarySurface Science is everywhere. Global to Atomic scale.
For solid/vacuum interface (solid surface), we have advantages of• Visualization of atomic configuration and electron density (LDOS) distribution in atomic scale• Direct determination of electronic structure (band, Fermi surface, spin, etc…)

A combinations of nanoprobe techniques, electron diffraction, andspectroscopy with synchrotron radiationProper and detailed understandings of surface phenomenon in atomic scale19