Transcript Slide 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
1905
Einstein: Photoemission is a quantum effect
Emax = h - 
1965-1975
Photoemission with surface control
Probing depth of 0.5-5 nm, “nano” in one direction
1975-1985
E(k) angle-resolved, tunable synchrotron radiation
1985-1995
Empty states , inverse photoemission, pump-probe
1995-2005
Line shape with resolution < kBT
 Electron propagator, self-energy , lifetime
2005- …
Fourier transform from k-space to real space
E(k) from Angle-resolved Photoemission
E (eV)
E (eV)
4
Ni
2
EF
0
3d-bands
-2
0.7
0.9
1.1
k (Å-1)
E,k multidetection: Energy bands on TV
-4
-6
-8
-10

s,p-band
k
K
States within kBT of the Fermi level EF
X
determine transport, superconductivity,
magnetism, electronic phase transitions.
Im() = Energy Width  =
ħ / Lifetime
Spin filter: Magnetic doping with Fe
in permalloy (Ni0.9Fe0.1) shortens the
lifetime of and thereby selects .
Spectrometer with E,kx - Multidetection
50x50 = 2500 spectra in one scan !
Exit Slit
Entrance Slit
Lens
Lens
Sample
Angular Multidetection
Angle Resolved Mode
Lens focused to 
Energy
Filter
Atom chains on a silicon surface
Theory
E
Spin-split band is
similar to that
in photoemission
E0
Experiment
EF
0
Losio et al., PRL 86, 4632 (2001)
kx
Sanchez-Portal et al., PRL 93, 146803 (2004)
From Reciprocal Space to Real Space
Angular Pattern
in Photoemission
|(k)|2
1D Quantum Well States
on a Terrace
(r)
Phase from iterated
Fourier transform cycle
Mugarza et al., PR B 67, 0814014 (2003)
Imaging Molecular Orbitals by Photoemission
ky
kx
(A) HOMO of sexiphenyl reconstructed from
the 2D photoemission momentum map.
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.
(B) HOMO of an isolated sexiphenyl molecule
from density functional theory (DFT).
Puschnig et al., Sciencexpress, 10 Sep. (2009)
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 !) .
Varying the Probing Depth
(A = 0.1 nm)
Fast electrons
get farther
Not enough energy
to excite plasmons
(≈15eV)
Si
Ge
GaAs
X-Ray Absorption Spectroscopy
(XAS, NEXAFS, XANES)
Photon energy h related to:
1) Core level

Element
2) Valence orbital 
Bonding
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 (100-1000 nm)
• Total Electron Yield (TEY): Surface sensitive ( ≈ 5nm)
Information about Molecular Orientation
Dipole selection rules:
900
200
• l  l 1, here s  p

C-H
• Electric field vector E parallel to the
orientation of the molecular orbital
Alkanethiol selfassembled monolayer (SAM )
900
200
C-C
Chemistry of Bio-Interfaces
*
Double-stranded DNA
p*
p*
• The N 1s edge selects the
p*-orbitals of the base pairs
• All p* orbitals are parallel to
the axis of the double-helix
Crain et al., JAP 90, 3291 (2001)
Mean Free Path of Photons vs. Electrons in Water
Path
104
C N O
nm
103 nm
Water
Window
Wa
ter
102 nm
Wi
ndo
10 nm
w
1 nm
0.1 nm
10 eV
100 eV
1000 eV
Energy
http://henke.lbl.gov/optical_constants/
Chemical Information from X-Ray Absorption Spectroscopy
Core to Valence Transitions : 1s  2p (p*, *) , 2p  3d, …
Sharp levels (<1keV) for bond orbitals
Deep levels (>1keV) for dilute species
Magnetism
Environment
Catalysts
Bio
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+
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).
Spatially Resolved X-Ray Absorption Spectroscopy
Want this chemically resolved
Chemically resolved, but still
insufficient spatial resolution
Fischer-Tropsch
process for converting coal to
liquid fuel.
De Smit et al.,
Nature 456,
222 (2008)
PEEM and LEEM
Photoemission Electron Microscope:
Low Energy Electron Microscope:
Accelerate photoelectrons and run
them through the magnifying optics
of an electron microscope.
Use diffracted electrons instead.
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, 17519 (2008)
Scanning Tunneling Spectroscopy (STS)
Atomic resolution
Scanning Probe Microscopy, ed. by R.
Wiesendanger, Nanoscience and Technology,
Springer, Berlin 1998, ISBN 3-540-63815-6
Mapping the Density of States (DOS) by STS
The density of states is given the differential conductance dI/dV:
DOS  dI/dV
I/V
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.
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 orbital, producing a stable lone pair.
Magnetic Tunneling via Spin-Polarized Surface States
Wave function of
the spin-polarized
dz2 surface state on
Fe(100) and Cr(100)
Cu
Mo
Contrast reversal between Cu and
Mo by changing the tunnel voltage.
Caveat: Sample and Tip
are Involved Equally
An atom jumping onto the STM tip and back
reverses the contrast between Cu and Mo.