The Use of Synchrotron Radiation in Modern Research

Transcription

1 The Use of Synchrotron Radiation in Modern Research Physics Chemistry Structural Biology Materials Science Geochemical and Environmental Science Atoms, molecules, liquids, solids. Electronic and geometric structure, magnetism,... Atomic physics, solid state physics, surface physics,... Atoms, molecules, liquids, solids. Chemical reactions,... Catalysis, photo-chemistry,... Proteins, virus,... Structure determinations by X-ray diffraction, microscopy,... Cement, nano-structured materials,... Structure determinations by X-ray diffraction, EXAFS, microscopy,... Soil, chemical waste, old ships (WASA),... Elemental and chemical analysis, microscopy,... Medicine Imaging, radiation therapy,... +

2 Properties of Synchrotron Radiation Tunable Infra-red to hard X-rays Intense Many photons per energy interval and second. >10 15 ph/sec at 1% bandwidth Brilliant Small source and well collimated light (approaching lasers) Polarization Plane or circular polarization, controllable Time structure Bunches in storage rings, Free electron lasers reach down into the femto-second range

3 Energy Levels of Atoms and Solids All electron spectrocopy methods rely on the electronic structure of atoms, molecules, and solids Increasing binding energy of electrons Note: Energies not to scale

4 Methods XAS and EXAFS (X-ray Absorption Spectroscopy and Extended X-ray Absorption Fine Structure) Photoemission Auger spectroscopy X-ray emission spectroscopy X-ray diffraction

5 Mean Free Path (or Escape Depth) The intensity removed (-di) per length travelled (dx) Many methods rely on the short mean free path of low energy electrons in solids for achieving surface sensitivity. -di = σ N I dx (σ: cross section for inelastic processes) (N : Scattering centers per cm 3 ) I(x) = I 0 e -σn x = I 0 e -x/λ where λ= (σn ) -1 is the mean free path I(x) is the intensity of electrons that have not lost any energy after they have travelled the distance x in the solid. So, if you made all atoms in a solid emit electrons at a given energy of around say 70 ev and detected all electrons coming out of the sample with that energy, the majority of the electrons would come from the first few atomic layers.

6 Mean Free Path (or Escape Depth) II Probability of an electron travelling the distance d through a material without losing energy ( λ : mean free path ) P(d) = e -d/λ (remember λ=λ(e)) Detector d Emitting atom As you see virtually no electrons make it for more than 5λ without loosing energy. Actually most of the electrons which escape from a surface without loosing energy have originated from within 1-2 * λ below the surface. Remember the minimum λ is about 5 Å.

7 Photoemission Principle Schematic experiment From Energy Conservation (E sample is the total energy of the sample before and after the electron is emitted) IN hν (mono-energetic) e - OUT hν + E sample (before) = E sample (after) + E kin (e - ) i.e. a Binding Energy E B (or if you like, BE) can be defined E B = hν E kin (e - ) = E sample (after) - E sample (before) Sample To beam line For the outgoing electrons we measure the number of electrons versus their kinetic energy. In addition the direction of the electrons may be detected (and in some cases their spin). NOTE the direction of the Binding Energy (BE) scale

8 Photoemission, what s it used for? A) What elements are present in the surface region. Different elements have different binding energies of the inner (core) levels. B) Often, also the chemical state of the elements can be determined, eg. Al-metal can be distinguished from Al-oxide. The exact binding energy of a core level depends on the chemical state. Chemical shifts. C) The surface geometry can be determined. Using diffraction effects and/or the chemical shifts of the binding energies (and imagination) D) The band-structure of the solid can be measured. Measuring the emission from the valence band in an angle resolved manner E) Chemically sensitive microscopy is possible. (Note: Chemically, not just element specific) Combine the above with either focusing of the incoming photons or magnifying electron optics. Some 10 nanometers to micrometers are typical values for the spatial resolution. In other words, chemical composition, geometrical structure and electronic structure. Quite complete information!

9 Chemical shifts and why they exist The core level binding energies are found to depend on the chemical state of the atom under investigation. Example : Ti metal versus Ti oxide A simple (and not entirely sufficient) explanation : WHY? Strange as the core levels do NOT take part in the bonding

10 Irrespective of why, chemical shifts exist. Can we make use of them? Example 1 : Distinguishing Si and Si oxide Chemical shifts Example 2 : Distinguishing the surface atoms on a Rh(111) surface Photon Energy SiO 2 Si

11 Using the substrate core level shifts: CO/Rh(111) Theory results Total energies: almost degenerate for CO in top and 3fold sites (which they should be!!) Rh 3d shifts: Clean: 500 mev CO ind. (top): CO ind. (3-fold): 220 mev +450 mev (no buckling) +240 mev (+0.2Å buckling) Conclusion CO in on-top sites on a buckled surface

12 Using the adsorbate levels as fingerprints CO in various overlayers on Rh(111) For adsorbed CO the C1s binding energy provides a good fingerprint of the adsorption site. Nearest neighbors. Ex. CO on Rh(111), pure CO, and coadsorbed with O and K Large shifts even when ground-state total energies are almost degenerate General rule: The C 1s binding energy for CO decreases as the coordination to the substrate increases

13 Stepped surfaces: Rh(553), seeing the step atoms and following what happens when oxygen is adsorbed

14 Chemical shifts, examples Chemical reactions can be followed Dissociation of CO on Mo(110) At low temperature CO adsorbs on MO(110) as an intact molecule, at higher temperatures it dissociates General rule: CO Atomic C C1s around ev C1s below ~283 ev

15 Time resolved core level photoemission If we can measure spectra fast enough, we can follow which species are present on the surface during a chemical reaction. Example, reduction of oxygen overlayers on Rh(111) by CO. This is how CO is catalytically converted into CO 2 in a car exhaust cleaning catalyst. 2x1 Chemisorbed O 9x9 Oxide

16 Spectromicroscopy = Spectral and spatial information Spectral information from small areas of the sample and Image contrast obtained from different spectral features Chemical Microscopy Two ways to obtain both spectra and images: Sequential or scanning Microspectroscopy Parallel or imaging Spectromicroscopy analyser with large acceptance position sensitive detector (PSD) sample focused beam scanning sample imaging lens broad beam Typical resolutions: 1-1/10 μm nm

17 Data acquisition using focusing optics Spectra : As without microscope, but the spectrum is only from a small part of the sample Images : Pick one binding energy, scan the sample and record the intensity at that binding energy Image acquisition Spectrum acquisition photoemission intensity photoemission intensity as a function of kinetic energy fix ed electro n energy scanning the electron energy x- and y-positions sample at a fixed position scanning sample in x- and y-direction

18 Photoelectron spectromicroscopy Temperature induced void growth in SiO 2 overlayers on Si(100) 99 ev binding energy, Si 105 ev binding energy, SiO 2 Annealing temperature 1100 C Voids in the oxide layer grow with annealing time All voids are circular and of approximately the same size Yellow indicates SiO 2 rich areas, dark areas show Si from the substrate

19 The VUV Scanning Photoelectron Microscope (SPEM) at MAX-lab 100mm hemispheric al analyzer From undulator

20 Photoemission Electron Microscope (PEEM) Using an electron microscope lens system we can magnify the lateral distribution of electrons emitted by the sample. Objective lens Intermediate lens Projective lens Multichannel plate YAG-screen Sample on manipulator Sample at -5 to -30 kv CCD camera Field aperture Stigmator, deflector Intermediate image plane Image plane Contrast aperture Photons Synchrotron radiation (XPEEM) UV lamp Mercury lamp, etc

21 PEEM pictures of pentacene on Si Contrast either by Work Function or DOS Pentacene: 5 Benzene Rings Contrast Changes During Deposition FoV = 65µm 1 Layer 2 Layers 3 Layers

22 What did we do A) The many uses of SR + SR properties B) Methods XAS and EXAFS (X-ray Absorption Spectroscopy and Extended X-ray Absorption Fine Structure) Photoemission Auger spectroscopy X-ray emission spectroscopy X-ray diffraction Principle Mean free path Chemical shifts Examples Microscopy using photoemission