E, k x,y, z, point group, spin E kin,, , h, polarization, spin Electron Spectrometer Synchrotron Radiation Mott Detector Spectroscopy of Nanostructures.

E , kx,y, z , point group, spin

Ekin , ,, h, polarization, spin

Electron Spectromete

r

Synchrotron Radiation

Mott Detector

Spectroscopy of

Nanostructures

Angle-resolved Photoemission (ARPES, UPS)

Measures all quantum numbers of an electron in a solid.

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

States within kBT of the Fermi level

EF determine transport,

superconductivity, magnetism,

electronic phase transitions.

-10

-8

-6

-4

-2

0

2

4

XK

Ni

0.7 0.9 1.1 k Å

E

(eV)

k

EF

E

(eV)

E,k multidetection: Energy bands on TV

s,p-band

3d-bands

Spin filter: Magnetic doping with Fe in permalloy (Ni0.9Fe0.1) shortens the lifetime of and thereby selects .

Im() = Energy Width = ħ / Lifetime

Spectrometer with E,kx -

Multidetection

50x50 = 2500 spectra in one scan !

Angle Resolved Mode

Transmission Mode

Lens focused to

Energy Filter

Angular Multidetection

0 kx

EF

TheoryAtom chains on a silicon surface E

Spin-split band is similar to that in photoemission

Losio et al., PRL 86, 4632 (2001)

Sanchez-Portal et al., PRL 93, 146803 (2004)

0 ExperimentE

Mugarza et al., PR B 67, 0814014 (2003)

From Reciprocal Space to Real Space

Angular Pattern 1D Quantum Well States in Photoemission on a Terrace

Phase from iterated Fourier transform cycle

(r)|(k)|2

Imaging Molecular Orbitals by

Photoemission

Puschnig et al., Sciencexpress, 10 Sep. (2009)

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.

(A) HOMO of sexiphenyl reconstructed from the 2D photoemission momentum map.

(B) HOMO of an isolated sexiphenyl molecule from density functional theory (DFT).

ky

kx

Core Level Photoemission (XPS)

Element selective

Intermediate oxidation states of Si at the Si/SiO2 interface (key to Si technology !) .

Synchrotron

radiation X-ray tube (Al K)

h = 1400eV

Varying the Probing Depth

(A =

0.1

nm

)

Not enough energy to excite plasmons (≈15eV)

Fast electrons get farther

Si

Ge

GaAs

Photon energy h related to:

1) Core level Element

2) Valence orbital Bonding

X-Ray Absorption Spectroscopy

(XAS, NEXAFS, XANES)

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)

Detection Modes: Electron and Fluorescence Yield

Empty states

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

900

200

900 200

C-H

C-C

Alkanethiol self-assembled mono-layer (SAM )

Chemistry of Bio-Interfaces

*

*

• 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)

Double-stranded DNA

Mean Free Path of Photons vs. Electrons in Water

Energy

C N O

Water

Window10 nm

102 nm

1 nm

0.1 nm

103 nm

104 nm

Water Window

10 eV 100 eV 1000 eV

Path

http://henke.lbl.gov/optical_constants/

Magnetis

m

Environme

nt

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

BioCatalysts

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.

Fe3+

Fe2+

Time-resolved X-Ray Absorption spectroscopy

X-ray absorption spectra of a solvated organic Fe complex for the low-spin ground state (blue) and an excited high-spin state

(red).

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.

Spatially Resolved X-Ray Absorption Spectroscopy

Want this chemically resolved Chemically resolved, but still insufficient spatial resolution

Fischer-Tropsch process for con-verting coal to liquid fuel.

De Smit et al., Nature 456, 222 (2008)

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.

Gilbert et al., JACS 130, 17519 (2008)

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.

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

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

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.

Magnetic Tunneling via Spin-Polarized Surface States

Wave function of

the spin-polarized

dz2 surface state on

Fe(100) and

Cr(100)

Caveat: Sample and Tip are Involved

Equally

An atom jumping onto the STM tip and back reverses the contrast between Cu and Mo.

Contrast reversal between Cu and Mo by changing the tunnel voltage.

Cu Mo