1. University of California, Berkeley March 9, 2007 Tony van Buuren Nanoscale Synthesis and Characterization Laboratory Lawrence Livermore National Laboratory and UC Merced School of Natural Sciences Use of NEXAFS in Materials Science

2. Outline Overview of the X-ray absorption process How do you measure NEXAFS or XANES –Element specific –Measures partial density of empty states –Sensitive to local bonding –Polarization depended Applications of NEXAFS: –Surface Chemistry Catalysts –Environmental Chemistry Oxy-state -> mobility –Material science experiments using NEXAFS Quantum dots Self assembled monolayer Chemical mapping (Imaging) Magnetic structures

3. Density of states from x-ray absorption unoccupied, CB occupied, VB variable h core level emission  XANES=NEXAFS XANES = x-ray absorption near-edge structure NEXAFS = near-edge x-ray absorption fine structure XAS = x-ray absorption EELS = electron energy loss spectroscopy, provides very similar information to XANES

4. XANES: Partial density of unoccupied states unoccupied, CB occupied, VB variable h core level  W i  f ~  f  T  i  2  (E f -E i -E) I l (E) ~  l-1 (E)M l-1 (E) 2 +  l+1 (E)M l+1 (E) 2 Dipole selection rules apply (l  1): s  p p  s and d d  p and f f  d and g Quadrupole transitions (l  2 or 0) are typically much (10 2 -10 3 times) weaker. Element-specific, angular-momentum resolved density of unoccupied states XANES edges: 1s – K edge 2s, 2p – L edges 3s, 3p, 3d – M edges 4s, 4p, 4d, 4f – N edges

5. Zinc K-edge X-ray Absorption Spectroscopy Spectrum -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) -0.6 -0.4 -0.2 0 0.2 0.4 0.6 9450965098501005010250 Photon Energy (eV) Signal (Arb. Units) Pre- Edge XANES EXAFS Pre-Edge Region Prior to adsorption of the atom of interest Composed of the absorption ‘tails’ of elements with lower binding energies X-ray Absorption Near Edge Structure (XANES) Absorption induces internal electronic transitions Oxidation state is obtained from the position of the ‘edge’ Edge features are characteristic of the local environment of the atom of interest Extended X-ray Absorption Fine Structure (EXAFS) Oscillations dependant upon type, position and number of neighbouring atoms A monatomic gas would not display the fine structure Extend for up to 1000eV beyond the edge Only elastically scattered electrons contribute to the EXAFS – local order Unlike the XANES, interpretation of the EXAFS by inspection is limited

6. Extended X-ray Absorption Fine Structure (EXAFS) Irradiate the sample with X-ray photons stepwise over a range encompassing the binding energy of a core electron Adjacent atoms backscatter the ejected photoelectrons which then interfere with outgoing wave Constructive interference near the nucleus promotes X-ray photon absorption, destructive interference reduces absorption XAS is: Element specific Non-intrusive Whilst probing: Short-range order

7. Structure Extraction From the EXAFS Quantitative analysis of the EXAFS was first realised by Sayers et al. working with polycrystalline and amorphous Ge samples – Fourier transformation of c(k) into real space – Peaks correspond to shells of atoms distributed around the central atom and comprise an entire radial structure function Data analysis Subtract pre- and post-edge backgrounds Create structural models and computer generate EXAFS and FTs from them to compare with the real data Least squares regression gives a fit-parameter, R Model parameters varied include: Position of backscatterers and their identity Co-ordination numbers Thermal disorder (Debye-Waller factors) Atomic potentials

8. NEXAFS spectra can be recorded in different ways. The most common methods are transmission and electron yield measurements. Note that the absorption coefficient µ is obtained either as the logarithm or the direct ratio of the detected intensities I t and I e and incident intensity I o www-ssrl.slac.stanford.edu/stohr/nexafs

9. NEXAFS measurements are element specific www-ssrl.slac.stanford.edu/dichroism/xas X-ray absorption spectra of a wedge sample, revealing the composition at various points along the wedge.

10. BN Thin Films: NEXAFS Determination of Bonding Cubic phase (sp3 bonded) Boron Nitride films and coatings are desirable for their hardness and electronic (wide band gap) properties (GM - G. L. Doll) Hexagonal (sp2, graphitic) BN is the energetically favorable phase Metastable growth conditions (magnetron sputtering, laser ablation) greatly affect the film’s bonding and morphology B 1s Photoabsorption, hBN and cBN vs BN film I. Jimenez, L. J. Terminello, et al. Appl. Phys. Lett. 68, 2816 (1996) sp2 sp3 p*

11. Polarization Dependent NEXAFS Synchrotron radiation sources: → high flux → polarized light BL8.2SSRL Bond/functional group orientation: NEXAFS resonance strength E.

12. Surface Chemistry: Catalysts SnO 2 aerogels are attractive gas sensor materials S. O. Kucheyev, PRB 72 (3): Art. No. 035404 (2005).

13. Surface Chemistry: Polymers This clearly illustrates the power of NEXAFS to distinguish chemical bonds and local bonding. In many ways it is superior to XPS, which doe s not provide local structural information. Often one can use a spectral "fingerprint" technique to identify the local bonding environment. Carbon K-edge NEXAFS spectra of different polymers, revealing the sensitivity to molecular functional groups. www-ssrl.slac.stanford.edu/stohr/nexafs

14. NEXAFS Imaging Chemical mapping of polymer blend C. Morin J. Electron Spectosc. 137-140 (2004) 785-794 XAS images at 283, 285.1, 288.4 and 290 eV, at the C 1s region of an annealed 28:72 (w/w) PS:PMMA blend thin film spun cast on native oxide Si (b) Spectra from the indicated spots. (c and d) Component maps of PS and PMMA derived by singular value decomposition of the C 1s image sequence. (e) Color coded composite map (red: PS; green: PMMA

15. Environmental Chemistry: Oxy state http://wwwssrl.slac.stanford.edu/research/highli ghts_archive/rocky_flats.pdf Comparison of plutonium LII XANES spectra for plutonium in oxidation states III, IV, V, and VI with RFETS soil and concrete samples

16. Self-Assembled Monolayers (SAMs): molecules which adsorb on a surface, spontaneously order via intramolecular forces. Most common type of SAM: alkanethiols on gold. Polarization depended NEXAFS to study self assembled monolayers extremely easy to make dip gold substrate in mM solution rinse in clean solvent relatively stable under ambient conditions ~hrs. Under N 2 > 1 year Mica or 5nm Ti on Si substrate Au(111) Sulfur bound to gold Van der Waals Interactions between chains cause alignment, ordering Headgroup of molecule changed for chemical functionality

17. “Hot” Applications 60nm switchable surfacesswitching interlocking molecules - molecular electronics trapping of proteins, viruses, etc. Barry Cheung et. Al., LLNL, to be published

18. C(1s) NEXAFS of Organothiol SAMs Collect and compare NEXAFS spectra at multiple angles of incidence Vary from grazing to normal angles of incidence between X-rays and sample Polarization dependent resonances denote well-defined orientation in the orbital of interest Peak direction in difference spectra provides a preliminary indication of functional group orientation C(1s) NEXAFS for MUA SAM on Au(111)

19. Obtaining Molecular Orientation from C(1s) NEXAFS Linear regression analysis provides a more quantitative measure of functional group orientation: → Peak fitting protocols resolve convoluted resonances and provide peak intensities → Linear regression analysis yields bond orientation to within ± 5° Orientation of MUA on Au(111)? Carboxyl Group: Carboxyl group tilted ~ 45° from the Au(111) surface normal Alkyl Chain: Hydrocarbon backbone tilted by ~ 42° T.M. Willey et. al., Langmuir, 2004, 20, 2746

20. NEXAFS Characterization of MBA SAMs on Au(111) 2-MBA 3-MBA 4-MBA

21. Quantum Confinement Effects in Semiconductor Nanocrystals (NCs) Photoluminescence/arb. units 3.02.0 Energy (eV) Absorbance/arb. units 37 Å 45 Å 60 Å 85 Å Semiconductor nanocrystals/‘Quantum dots’ → Unique, size-dependent, optical and electronic properties → Diverse range of potential technological applications Optoelectronic behavior explained in terms of quantum confinement effects: Particle in a Box

22. Silicon nanocrystals are prepared and deposited in situ out of the gas-phase. 19 Å 111 220 311 SAD BF AFM C. Bostedt, et al., J. Phys. Condens. Matter 15, 1017 (2003). TEM Crystalline particles with narrow size distribution.

23. First, the size-dependent properties are investigated on sub-monolayer depositions of nanocrystals. Dilute systems need element specific measurements Film-morphology: Individual nanocrystals Substrate: Surface-passivated germanium AFM:

24. X-ray absorption and Emission measurements show shift in valence and conduction band of isolated Si clusters Valence Band Soft x-ray emission of Si nanoparticles Conduction Band Si 2p absorption from nanoparticles

25. Quantum Confinement in Nanoparticles Measured and Compared to Theory T. van Buuren, L. Dinh, L. L. Chase, L. J. Terminello, Phys. Rev. Lett. 80, 3803 (1998) CB and VB band edge shift as a function of particle size. Band gap as a function of particle size Ratio of CB to VB shift is 1:2

26. Germanium exhibits much stronger confinement effects than silicon. C. Bostedt, T. van Buuren, APL (2004) T. van Buuren, Phys. Rev. Lett. 80, 3803 (1998). The band-gap of the Ge becomes larger than Si at particles sizes below 2.0 nm

27. CdSe NCs: An Model System of Technological Importance → Readily synthesized with narrow size distributions → Exhibit size-dependent photoluminescence → Extensively studied Archetypal nanocrystalline binary semiconductor for technological applications Model system for the study of quantum confinement Murray, et. al., J. Am. Chem. Soc., 115, 8706 (1993) 5nm TEM Image of CdSe- TOPO Synthesis of CdSe NCs But… Theories on electronic structure conflict with one another and experimental results

28. Cd L 3 -edge XAS: We can probe the bottom of the CB (vacant) DOS directly using this technique Cd L 3 -edge XAS L 3 -edge formally 2p → s, d empty states Bottom of CB comprised of Cd 5s states Hybridized pd states located ~ 4-5 eV above CB minimum We find that only the ‘s’-states move as a function of particle size J. Lee R. Meulenberg PRL 2007

29. Magnetic properties of materials can be studied by X-Ray Magnetic Circular Dichroism (XMCD) spectroscopy Electronic transitions in conventional L-edge x-ray absorption (a), and x- ray magnetic circular x-ray dichroism (b,c), illustrated in a one-electron model. The transitions occur from the spin- orbit split 2p core shell to empty conduction band states. In conventional x-ray absorption the total transition intensity of the two peaks is proportional to the number of d holes. By use of circularly polarized x-rays the spin moment (b) and orbital moment (c) can be determined from linear combinations of the dichroic difference intensities A and B, according to other sum rules. www-ssrl.slac.stanford.edu/dichroism/xas

30. Circular dichroism at the Iron L-edge www-ssrl.slac.stanford.edu/dichroism/xas If the photoelectron originates from the p3/2 level (L3 edge), the angular momentum of the photon can be transferred in part to the spin through the spin-orbit coupling. Right circular photons (RCP) transfer the opposite momentum to the electron as left circular photons (LCP) photons, and hence photoelectrons with opposite spins are created in the two cases. Since the p3/2 (L3) and p1/2 (L2) levels have opposite spin-orbit coupling, the spin polarization will be opposite at the two edges. In the absorption process, "spin-up" and "spin-down" are defined relative to the photon helicity or photon spin.

31. Backup slides

32. NEXAFS Quantitative Orientation - Vector Define polarization: Intensity is electric field and TDM dot product squared Due to 3-fold or higher substrate symmetry, Squaring the dot product and averaging over azimuthal angle, For raw intensities, use the ratio method by fitting to experimental data. (From J. Stohr et. al., Phys. Rev. B, 1987, 36, 7891) Through this method we can determine quantitatively how molecules in ultrathin organic layers are oriented on surfaces.

33. NEXAFS Quantitative Orientation - Plane Define polarization: Intensity: square of projection of E onto plane or sin(ε) Due to 3-fold or higher substrate symmetry, Squaring the dot product and averaging over azimuthal angle, For raw intensities, use the ratio method by fitting to experimental data. (From J. Stohr et. al., Phys. Rev. B, 1987, 36, 7891) Through this method we can determine quantitatively how molecules in ultrathin organic layers are oriented on surfaces.

34. NEXAFS Quantitative Orientation - Difference Use vector or plane intensity: Take difference spectra between two incident angles Through this method we can determine quantitatively how molecules in ultrathin organic layers are oriented on surfaces. vector: plane: Determine parameter SP from a reference sample with known tilt Solve for α or γ as all parameters are now known. In either case, Run linear regressions of D vs.with multiple spectra