Spectroscopy of Nanostructures. Angle-resolved Photoemission (ARPES, UPS)

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1 Spectroscopy of Nanostructures Angle-resolved Photoemission (ARPES, UPS) Measures all quantum numbers of an electron in a solid. E, k x,y, z, point group, spin E kin, ϑ,ϕ, hν, polarization, spin Electron Spectrometer Synchrotron Radiation Mott Detector

2 1905 Einstein: Photoemission is a quantum effect E max = hν - Φ 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 < k B T Electron propagator, self-energy Σ, lifetime Fourier transform from k-space to real space

3 E(k) from Angle-resolved Photoemission E(eV) 4 Ni E(eV) 2 E F 0 3d-bands k (Å 1 ) -4 E,k multidetection: Energy bands on TV -6-8 s,p-band -10 Γ k K X States within k B T of the Fermi level E F determine transport, superconductivity, magnetism, electronic phase transitions.

4 Im(Σ) = Energy Width Γ = ħ / Lifetime Spin filter: Magnetic doping with Fe in permalloy (Ni 0.9 Fe 0.1 ) shortens the lifetime of and thereby selects.

5 Spectrometer with E,k x - Multidetection 50x50 = 2500 spectra in one scan!

6 Entrance Slit Angular Multidetection Exit Slit Sample Lens Lens Angle Resolved Mode Lens focused to Energy Filter

7 Atom chains on a silicon surface E Theory Spin-split band is similar to that in photoemission E 0 Experiment E F 0 k x Losio et al., PRL 86, 4632 (2001) Sanchez-Portal et al., PRL 93, (2004)

8 From Reciprocal Space to Real Space Angular Pattern in Photoemission 1D Quantum Well States on a Terrace ψ(k) 2 ψ(r) Phase from iterated Fourier transform cycle Mugarza et al., PR B 67, (2003)

9 Imaging Molecular Orbitals by Photoemission k y k x 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). Puschnig et al., Sciencexpress, 10 Sep. (2009)

10 Core Level Photoemission (XPS) Element selective Synchrotron radiation X-ray tube (Al K α ) hν = 1400eV Intermediate oxidation states of Si at the Si/SiO 2 interface (key to Si technology!).

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

12 X-Ray Absorption Spectroscopy (XAS, NEXAFS, XANES) Photon energy hν related to: 1) Core level Element 2) Valence orbital Bonding

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)

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 selfassembled monolayer (SAM )

15 Chemistry of Bio-Interfaces Double-stranded DNA σ* π* π The N1s edge selects the π*-orbitals of the base pairs All π* orbitals are parallel to the axis of the double-helix Crain et al., JAP 90, 3291 (2001)

16 Mean Free Path of Photons vs. Electrons in Water Path 10 4 nm C N O 10 3 nm Wa ter 10 2 nm Wi ndo 10 nm w Water Window 1 nm 0.1 nm 10 ev 100 ev 1000 ev Energy

17 Chemical Information from X-Ray Absorption Spectroscopy Core to Valence Transitions : 1s 2p (π*, σ*), 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. Fe 2+ Fe 3+

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 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)

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 CaCO 3 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 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 D tip is swept past the sample DOS D spl. Such a sweep corresponds to a convolution (represented by a star) : I(E) D tip * D spl = D tip (E-E ) D spl (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 δ-function. Convolution of a function with the δ-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.

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 orbital, producing a stable lone pair.

26 Magnetic Tunneling via Spin-Polarized Surface States Wave function of the spin-polarized d z 2 surface state on Fe(100) and Cr(100)

27 Cu Mo Caveat: Sample and Tip are Involved Equally 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.