1. Spectroscopy of Nanostructures
Angle-resolved Photoemission (ARPES, UPS)
Measures all quantum numbers of an electron in a solid.
E , kx,y, z , point group, spin
Ekin , ,, h, polarization, spin
Electron Spectrometer
Synchrotron Radiation
Mott Detector

2. 1905 Einstein: Photoemission is a quantum effect Emax = h - 
Photoemission with surface control Probing depth of nm, “nano” in one direction
E(k) angle-resolved, tunable synchrotron radiation
Empty states , inverse photoemission, pump-probe
Line shape with resolution < kBT
 Electron propagator, self-energy , lifetime
2005- … Fourier transform from k-space to real space

3. E(k) from Angle-resolved Photoemission
E (eV)
E (eV) Ni EF
3d-bands
k (Å-1)
E,k multidetection: Energy bands on TV
s,p-band
States within kBT of the Fermi level EF determine transport, superconductivity, magnetism, electronic phase transitions. k

4. Im() = Energy Width  = ħ / Lifetime
Spin filter: Magnetic doping with Fe in permalloy (Ni0.9Fe0.1) shortens the lifetime of and thereby selects .

5. Spectrometer with E,kx - Multidetection
50x50 = spectra in one scan !

6. Angular Multidetection
Lens focused to 
Energy Filter
Angular Multidetection

7. Atom chains on a silicon surfacekx EF
Theory E
Spin-split band is similar to that in photoemission
Experiment E
Losio et al., PRL 86, 4632 (2001)
Sanchez-Portal et al., PRL 93, (2004)

8. Phase from iterated Fourier transform cycle
From Reciprocal Space to Real Space
Angular Pattern D Quantum Well States
in Photoemission on a Terrace
|(k)|2
(r)
Phase from iterated Fourier transform cycle
Mugarza et al., PR B 67, (2003)

9. Imaging Molecular Orbitals by Photoemissionky kx
(A) HOMO of sexiphenyl reconstructed from the 2D photoemission momentum map.
(B) HOMO of an isolated sexiphenyl molecule from density functional theory (DFT).
Photoemission momentum map (square root of the intensity) for sexiphenyl on Cu (110) at a binding energy of 1.9 eV which corresponds to the HOMO.
Puschnig et al., Sciencexpress, 10 Sep. (2009)

10. Core Level Photoemission (XPS)
Element selective
Synchrotron radiation
X-ray tube (Al K)
h = 1400eV
Intermediate oxidation states of Si at the Si/SiO2 interface (key to Si technology !) .

11. Varying the Probing Depth
(A = 0.1 nm)
Not enough energy to excite plasmons (≈15eV)
Fast electrons get farther Si Ge
GaAs

12. X-Ray Absorption Spectroscopy
(XAS, NEXAFS, XANES)
Photon energy h related to:
Core level  Element
Valence orbital  Bonding 12

13. Detection Modes: Electron and Fluorescence Yield
Empty states
Detect the absorption of photons indirectly by looking at the decay products:
Fluorescence Yield (FY): Bulk sensitive ( nm)
Total Electron Yield (TEY): Surface sensitive ( ≈ 5nm) 13

14. Information about Molecular Orientation
Dipole selection rules:
l  l 1, here s  p
Electric field vector E parallel to the
orientation of the molecular orbital 
C-H
C-C
Alkanethiol self-assembled mono-layer (SAM )
900
200 14

15. Chemistry of Bio-Interfaces* *
Double-stranded DNA p*
The N 1s edge selects the
*-orbitals of the base pairs
All * orbitals are parallel to
the axis of the double-helix
Crain et al., JAP 90, 3291 (2001)

16. Water Window Mean Free Path of Photons vs. Electrons in Water Energy
C N O
104 nm
103 nm
Water Window
Water
Window
102 nm
10 nm
1 nm
0.1 nm
10 eV 100 eV eV
Energy

17. Chemical Information from X-Ray Absorption Spectroscopy
Core to Valence Transitions : 1s  2p (*, *) , 2p  3d, …
Sharp levels (<1keV) for bond orbitals
Deep levels (>1keV) for dilute species
Magnetism
Catalysts
Bio
Environment

18. Transition Metals: 2p  3d Absorption Edge
Can detect the oxidation state, spin state, and the electric field of the ligands for one Fe atom inside a complex molecule.
Fe2+
Fe3+

19. Time-resolved X-Ray Absorption spectroscopy
These measurements provide information about spin excitations with about 100 picoseconds (ps) time resolution. To see atomic vibrations one would need <100 femtoseconds (fs) time resolution, to follow electrons in real time about 1 fs . The velocity of electrons in a metal is about 1 nm/fs at the Fermi level.
X-ray absorption spectra of a solvated organic Fe complex for the low-spin ground state (blue) and an excited high-spin state (red).

20. Spatially Resolved X-Ray Absorption Spectroscopy
Chemically resolved, but still insufficient spatial resolution
Want this chemically resolved
Fischer-Tropsch process for con-verting coal to liquid fuel.
De Smit et al., Nature 456, 222 (2008)

21. PEEM and LEEM Photoemission Electron Microscope:
Accelerate photoelectrons and run them through the magnifying optics of an electron microscope.
Low Energy Electron Microscope:
Use diffracted electrons instead.

22. Orientation of Nacre Platelets from PEEM with Polarized Light
Oriented single crystals of CaCO3 act like bricks connected by a protein glue.
Hard, but flexible to prevent cracking.
Gilbert et al., JACS 130, (2008)

23. Scanning Tunneling Spectroscopy (STS)
Atomic resolution
Scanning Probe Microscopy, ed. by R. Wiesendanger, Nanoscience and Technology, Springer, Berlin 1998, ISBN

24. Mapping the Density of States (DOS) by STS
The density of states is given the differential conductance dI/dV:
DOS 
Explanation: For small bias voltages («1 V) and for a metallic tip the density of occupied tip states can be approximated by a step function. In an I(V) scan this tip DOS Dtip is swept past the sample DOS Dspl Such a sweep corresponds to a convolution (represented by a star) :
I(E)  Dtip * Dspl =  Dtip(E-E’) · Dspl(E’) dE’ E = e·V
The derivative of a convolution is the convolution with the derivative, and the derivative of the step function is the -function. Convolution of a function with the -function replicates this function. The result is that dI/dV is proportional to the sample DOS.
The derivative is obtained by modulating the sample voltage sinusoidally and picking up the oscillating component of the current with a lock-in amplifier. Electronic noise at all other frequencies is filtered out.
dI/dV
I/V

25. Density of States from STS, Photoemission and Inverse Photoemission
The Si(111)7x7 surface (the most stable surface of silicon) has two types of broken (“dangling”) bonds:
Adatoms trade 3 broken bonds for one (yellow).
Rest atoms are part of the original truncated silicon surface (red).
An electron is transferred from adatoms to a rest atom and thereby completely fills its broken bond or-bital, producing a stable lone pair.

26. Magnetic Tunneling via Spin-Polarized Surface States
Wave function of the spin-polarized dz2 surface state on Fe(100) and Cr(100)

27. Caveat: Sample and Tip are Involved EquallyCu Mo
Contrast reversal between Cu and Mo by changing the tunnel voltage.
An atom jumping onto the STM tip and back reverses the contrast between Cu and Mo.