Part II. X-ray Absorption Spectroscopy (XAS)

Transcription

1 Part II XAFS: Principles XANES/NEXAFS Applications 1 X-ray Absorption Spectroscopy (XAS) X-ray Absorption spectroscopy is often referred to as - NEXAFS for low Z elements (C, N, O, F, etc. K-edge, Si, P, S, L-edges) or - XAFS (XANES and EXAFS) for intermediate Z and high Z elements. 2

2 NEXAFS, XANES and EXAFS NEXAFS (Near Edge X-ray Absorption Fine Structures) describes the absorption features in the vicinity of an absorption edge up to ~ 50 ev above the edge (for low Z elements for historical reasons). It is exactly the same as XANES ( X-ray Absorption Near Edge Structures), which is often used together with EXAFS (Extended X-ray Absorption Fine Structures) to describe the modulation of the absorption coefficient of an element in a chemical environment from below the edge to ~ 50 ev above (XANES), then to as much as 1000 ev above the threshold (EXAFS) NEXAFS and XANES are often used interchangeably XAFS and XAS are also often used interchangeably 3 XAFS of free atom In rare gases, the pre-edge region exhibits a series of sharp peaks arising from bound to bound transitions (dipole: 1s to np etc.) called Rydberg transitions 4

3 XAFS of small molecules Small molecules exhibit transitions to LUMO, LUMO + and virtual orbital, MO in the continuum trapped by a potential barrier (centrifugal potential barrier set up by high angular momentum states and the presence of neighboring atoms), or sometimes known as multiple scattering states 5 XAFS: the physical process Dipole transition between quantum states core to bound states (Rydberg, MO below vacuum level, -ve energy, the excited electron remains in the vicinity of the atom)- long life time-sharp peaks core to quasi-bound state (+ve energy, virtual MO, multiple scattering states, shape resonance, etc.); these are the states trapped in a potential barrier, and the electron will eventually tunnel out of the barrier into the continuum-short lifetime, broad peaks core to continuum (electron with sufficient kinetic energy to escape into the continuum) - photoelectric effect. XPS &EXAFS 6

4 XAFS: the physical process cont Scattering of photoelectron by the molecular potential how the electron is scattered depends on its kinetic energy Low kinetic energy electrons - Multiple scattering (typically up to ~ 50 ev above the threshold, the region where bound to quasi-bound transitions take place); e is scattered primarily by valence and shallow inner shell electrons of the neighboring atoms - XANES region High kinetic energy electrons ( ev) are scattered primarily by the core electrons of the neighboring atoms, single scattering pathway dominates - EXAFS region 7 Electron scattering Free electron (plane wave) scattered by an atom (spherical potential) travels away as a spherical wave Electrons with kinetic energy > 0 in a molecular environment is scattered be the surrounding atoms Low KE e - is scattered by valence electrons, undergoes multiple scattering in a molecular environment High KE e - is scattered by core electrons, favors single scattering Multiple Scattering Single Backscattering 8

5 9 10

6 NEXAFS of free atom & unsaturated diatomic molecules States trapped in the potential barrier are virtual MO s or multiple scattering states (quasi bound states) Free atom diatomic with unsaturated bonding N 2, NO, CO etc. π* free e with KE >0 bound states Asymptotic wing of the coulomb potential supports the Rydberg states 11 NEXAFS of Free Atom: Ar L 3,2 -edge Relative Intensity p 3/2-4s 123meV Photon Energy 2p 1/2-4s 3d d 5d 6d Photon Energy (ev) Ar (gas) [Ne]3s 2 3p 6 2p 6 2p 5 4s 1 3d 4d Core hole 5d 6d hv 2p 1 4s 1 j = l ± s Hund s rule 2p 1/2 2p 3/2 (lower binding) 12

7 NEXAFS of atom and small molecules Kr M 5,4 (3d 5/2, 3/2 ) HBr M 5,4 (3d 5/2, 3/2 ) MO Rydberg transitions in HBr remain strong and a broad transition to molecular orbital emerges 13 NEXAFS of Nitrogen π* IP σ* Intensity (arb. units) N Photon Energy (ev) 1s - π* Vibronic structures in the 1 s to π* transition 14

8 Molecular orbital illustration N 2 CO MO axis C 2 H 2 σ* π* π* C 1s C 1s 15 16

9 E = (E σ* -E o ) 1/r Why would this correlation exist? i) Multiple scattering theory ii) Simple picture- particle in a boxthe shorter the bond the farther the energy separation iii) Scattering amplitude Z 17 18

10 NEXAFS of molecules oriented on a surface (angular dependence of resonance intensity) When molecules adsorb on a surface, their molecular axis is defined by the axis of the substrate. Therefore, angle dependent experiments can be made by rotating the substrate with respect to the polarization of the photon beam. Using selection rules, the orientation of the molecule on a surface can be determined. H H C=C H H C-C 19 Carbon Allotropes Hexagonal Diamond (Lonsdaleite) Diamond Graphene Graphite Nanotube 20

11 XANES of Benzene LUMO + 1 LUMO core 21 XANES systematic π* 1 C-H* π* 2 Tom Regier unpublished Increasing no. of benzene rings 22

12 Chemical systematic C 60 CN T 23 24

13 d - electron in transition metals The dipole selection rule allows for the probing of the unoccupied densities of states (DOS) of d character from the 2p and 3p levels M 3,2 and L 3,2 -edge XANES analysis (relevance: catalysis) 25 Transition metal systematic Filling of the d band across the period 3d 4d 5d 26 Decreasing whiteline intensity across the period as the d band gets filled

14 L 3,2 /M 3,2 Whiteline and unoccupied densities of d states 3d metal: the L-edge WL for early 3d metals is most complex due to the proximity of the L 3,L 2 edges as well as crystal field effect; the 3d spin-orbit is negligible 4d metals: the L 3,L 2 edges are further apart, WL intensity is a good measure of 4d hole population 5 d metals: The L 3 and L 2 are well separated but the spin orbit splitting of the 5d orbital becomes important, j is a better quantum number than l, WL intensity is a good measure of d5/3 and d3/2 hole populations 27 3d metals Hsieh et al Phys. Rev. B 57, (1998) 28

15 d- band filling across 5d row EFermi Ta W Pt Au 5d metal d band Intensity (area under the curve) of the sharp peak at threshold (white line) probes the DOS 3p3/2-5d 2p3/2-5d T.K. Sham et al. J. Appl. Phys. 79, 7134(1996) 29 d - charge redistribution in 4d metals Ag Pd 4d charge transfer upon alloying I. Coulthard and T.K. Sham, Phys. Rev. Lett., 77, 4824(1996) 30

16 Analysis of d hole in Au and Pt metal In 5 d metals with a nearly filled/full d band such as Pt and Au, respectively, spin orbit coupling is large so d 5/2 and d 3/2 holes population is not the same E F Selection rule: dipole, Pt l = ± 1, j = ± 1, 0 L 3, 2p 3/2 5d 5/2.3/2 L 2, 2p 1/2 5d 3/2 5d 5/2 5d 3/2 DOS T.K. Sham et al. J. Appl. Phys. 79, 7134(1996) 31 Au L 3 2p 3/2-5d 5/2,3/2 Au L 2 2p 1/2-5d 3/2 M. Kuhn and T.K. Sham Phys. Rev. 49, 1647 (1994) 32

17 * * Phys. Rev. B22, 1663 (1980) 33 Probing depth profile with light: applications surface near surface Interface bulk-like X-ray probes: Morphology photon Light-matter Interaction: Absorption & Scattering Structure e Electronic properties ion soft x-ray (photon) nm nm-10 nm nm hard x-ray 34

18 Soft X-ray vs. hard X-ray Soft x-ray usually cannot penetrate the entire sample measurements cannot be made in the transmission mode. Yield spectroscopy is normally used! One absorption length (µt 1 = 1 or t 1 = 1/ µ) is a good measure of the penetration depth of the photon Example of one absorption lengths Element density(g/cm 3 ) hv(ev) mass abs (cm 2 /g) t 1 (µm) Si x x Graphite x Soft x-ray spectroscopy: unique features XAFS measurement in the soft x-ray (short absorption length) region is often made using yield spectroscopy Electron yield (total, partial, Auger) X-ray fluorescence yield (total, selected wavelength) Photoluminescence yield (visible, light emitting materials) XEOL (X-ray excited optical luminescence) and TRXEOL (Time-resolved X-ray Excited Optical Luminescence) Since soft X - ray associates with the excitation of shallow core levels, the near-edge region (XANES/ NEXAFS) is the focus of interest for low z elements and shallow cores 36

19 E.G.: the Interplay of TEY and FLY hv TEY FLY Diamond-like Graphite-like Amorphous c-bn h-bn a-bn Si(100) 80 nm Cubic BN film grown on Si(100) wafer? Preferred orientation of the h-bn film? 37 Boron K-edge normal hv TEY FLY c-bn h-bn a-bn Si(100) Inthesity (a. u.) Intensity (a. u.) Photon Energy (ev) 2 3 TEY FLY 4 4 a b π* ε I (1s - π*) minimum glancing π* ε I (1s - π*) maximum X.T. Zhou et.al. JMR 2, 147 (2006) Photon Energy (ev) 38

20 XAFS and XEOL (Optical XAFS) XAFS E CB PLY Mono XEOL Edge hv ex Abs. VB E F hv op Wavelength (nm) 39 XEOL - Energy Domain X-ray photons in, optical photons out Illustrations BN nanowires Si nanowires ZnS nanoribbons 40 40

21 Boron nitride nanotube (BNNT) B-O bond Oxygen BNNT W.-Q. Han et al. Nanoletter. 8, (2008) Liu et. al. (UWO) unpublished h-bn 41 Si K-edge XEOL of silicon nanowires Intensity (arb. units) S i S i O 2 h v e x ( e V ) S i K - e d g e P h o t o n E n e r g y n m n m n m Intensity ( a ) 2 ( b ) TEY W a v e l e n g t h ( n m ) e V e V W a v e l e n g t h ( n m ) d i f f e r e n c e c u r v e T.K. Sham et al. Phys. Rev. B 70, (2004) 42

22 Photoluminescence: Si K-edge Si nanowire TEY 40 FLY Intensity (arb. units) PLY zero order 460 nm 530 nm nm 0 Si SiO Photon Energy (ev) 43 ZnS hetero-crystalline nano-ribbon X.-T. Zhou et al., J. Appl. Phys. 98, (2005) 520 nm 520 nm 280 K 10 K Total 332 nm ZnS OLnw (zinc ZB blend) XAS Wavelength (nm) Energy (ev) nm Total 0-14 ns ZnS nw (wurtzite) OL W XAS R.A. Rosenberg et al., Appl. Phys. Lett. 87, (2005) Wavelength (nm) Energy (ev)

23 XEOL - Time Domain Timing - resolved XEOL (TRXEOL) Illustrations ZnO: Nanodeedle vs Nanowire CdSe-Si: Hetero nanostructures TRXEOL at APS T.K. Sham & R.A. Rosenberg, ChemPhysChem 8, (2007) 46 46

24 Time-gated spectroscopy Select time window(s) Obtain spectra /yields of photons arriving within that window ns 100 Time (ns) 150 Fast time window Slow time window CdSe -Si hetero nanoribbons hv = 1100 ev CdSe 0-20 ns Si X.H. Sun et al. J. Phys. Chem., C, 111, 8475(2007) total optical yield ns R.A. Rosenberg et. al. Appl. Phys. Lett., 89, (2006) 48 48

25 CdSe-Si Heterostructure: Se L 3,2 - edge 0.2 Se L 3,2 -edge TEY Intensity (arb. units) 0.1 Un-gated 0-20 ns ns Photon Energy (ev) 49 STXM: Scanning Transmission X-ray Microscopy the SM Courtesy of A.P. Hitchcock, McMaster Fresnel Zone Plates 30nm J.G. Zhou, J. Wang, H.T. Fang, C.X. Wu, J.N. Cutler, T.K. Sham, Chem. Comm. 46, 2778 (2010) 50 50

26 Mico/nano spectroscopy of N-CNT TEM STXM S7 S1 S2 S3 S8 S9 500 nm (S2) J. Zhou, J. Wang et al. J. Phys. Chem. Lett. (2010) DOI: /jz100376v 51