Spectroscopy of Nanostructures Angleresolved Photoemission ARPES UPS Measures

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 < k. BT Electron propagator, self-energy 2005 - … Fourier transform from k-space to real space , lifetime

E(k) from Angle-resolved Photoemission E (e. V) Ni EF 3 d-bands 0. 7 0. 9 1. 1 k (Å-1) E, k multidetection: Energy bands on TV s, p-band States within k. BT of the Fermi level EF k determine transport, superconductivity, magnetism, electronic phase transitions.

Im( ) = Energy Width = ħ / Lifetime Spin filter: Magnetic doping with Fe in permalloy (Ni 0. 9 Fe 0. 1) shortens the lifetime of and thereby selects.

Spectrometer with E, kx -Multidetection 50 x 50 = 2500 spectra in one scan !

Angular Multidetection Lens focused to Energy Filter

Atom chains on a silicon surface Theory E Spin-split band is similar to that in photoemission E 0 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 1 D 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 2 D photoemission momentum map. Photoemission momentum map (square root of the intensity) for sexiphenyl on Cu (110) at a binding energy of 1. 9 e. V 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 = 1400 e. V Intermediate oxidation states of Si at the Si/Si. O 2 interface (key to Si technology !).

Varying the Probing Depth (A = 0. 1 nm) Fast electrons get farther Not enough energy to excite plasmons (≈15 e. V) Si Ge Ga. As

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 ( ≈ 5 nm)

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 * * • The N 1 s 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)

Mean Free Path of Photons vs. Electrons in Water Path C N O 104 nm 103 nm Water Window Wa ter 102 nm Wi ndo 10 nm w 1 nm 0. 1 nm 10 e. V 1000 e. V Energy http: //henke. lbl. gov/optical_constants/

Chemical Information from X-Ray Absorption Spectroscopy Core to Valence Transitions : 1 s 2 p ( *, *) , 2 p 3 d, … Sharp levels (<1 ke. V) for bondorbitals Deep levels (>1 ke. V) for dilute species Magnetism Environment Catalysts Bio

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

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 Ca. CO 3 act like bricks connected by a protein glue. Hard, but flexible to prevent cracking. May 17 -18, 2000 Your Name 22 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 d. I/d. V: DOS d. I/d. V 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’) d. E’ 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 d. I/d. V 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)7 x 7 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 dz 2 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.