Soft X-Ray Spectroscopy with synchrotron radiation: A powerful tool for Materials Research

Transcription

1 Soft X-Ray Spectroscopy with synchrotron radiation: A powerful tool for Materials Research Alexander Moewes moewes@usask.ca University of Saskatchewan Department of Physics and Engineering Physics Engineering CLS Day, September 16 th, 2005

2 Outline of General part! (1) How do I obtain funding?! (2) How to choose a CLS beamline and how to know its status.! (3) How to collaborate?! (4) A few specific questions from you.

3 (2) How to choose a beamline and how to know its status Click on beamline

4 (3) How to collaborate 1. Identify the scientific question that you are trying to solve. 2. Does this require spatial, structural, chemical, or physical information? 3. Can you identify the photon energy range you need? Other questions: how much sample, energy scale of process, how deep do you want to probe, is sample vacuum compatible? 4. Discuss this with as many people with synchrotron background as possible to refine #2 and #3. 5. Ultimately one field will appear favorable to start with: Spectroscopy or Imaging or Diffraction (or a combination). Most importantly, you need to take into account: 5. You want to expand in some territory that is new for you. There will be a learning curve and this will take some time. But this is fun! 6. The sooner you start the better (and the less time you loose)!

5 (3) How to collaborate 1. Think science first (funding last it takes time but will come with good scientific questions). 2. Look for collaborator but do not expect an immediate answer to your question. Most likely your original question will lead to more questions before answering it (doesn t it always?) 3. Over the past 5 years, we developed > 21 new collaborations: Canada: U of S: Chang, Hirose, Kraatz, Lee, Tse, Xiao, Yang. UBC: Sawatzky, Tiedje. UWO: Sham. Europe: Riedel/Germany, Hague/France, Pivin/France, Kochur/Russia, Japan: Endo, Sekine, Taniguchi. US: Ching/Kansas City, Chang/MIT, Rehr/Washington, van Buuren/Livermoore.

6 Invitation to collaborate! First measurements can take place now! (October and December) to try a start.! Nothing is a better start than to start looking at some data.! By the time more beamlines are up and running, you will be an experienced user.! Invitation to collaborate: A large team is needed, join in if you are interested in soft x-ray spectroscopy moewes@usask.ca

7 A few specific questions from you (1) Microscopy for geological applications? For soft x-rays SM beamline (Adam Hitchcock / McMaster). For hard x-rays Microanalysis (DeTong Jiang / Guelph). (2) (a) Capabilities of different beamlines and (b) their capabilities with respect to various elements? for (a) see CLS website: for (b) see table with binding energies of elements. (3) Understanding molecular changes in agri-food and biological materials (when subjected to microwave and infra-red energy)? for hard x-ray region DeTong Jiang / Guelph for soft X-ray region Alex Moewes / U of S or Jeff Cutler / CLS.

8 Outline of the scientific part! I. Where do we carry out our research?! II. How does it work (in principle)?! III. Why synchrotron radiation?! IV. What do absorption and emission spectroscopy probe?! V. What are our general questions?! VI. Research examples from our (many) research interests: 1. Plasma deposited diamond films 2. Modifying polymers by ion bombardment 3. Conductivity of DNA 4. Ultra-hard materials: γ-si 3 N 4

9 What is materials science?! Materials science: synthesis, characterization and modeling of (new) materials.! Ultimate goal: tailor advanced materials with specific electrical, optical, magnetic, chemical, catalytic, thermal and other properties.! In order to tailor materials, we need to understand the structure of the materials.! Structure determines their properties and functionality such as: how molecules within living cells interact (such as proteins), Mechanical, electronic, optical, chemical, and other properties, band gap, conductivity, magnetic properties of materials.

10 I. Where do we carry out our research?

11 At the Advanced Light Source at Berkeley

12 and recently at CLS! August 28,30, 31, 2005: Our first run at CLS: XAS at SGM. Normalized sample current [arb. units] C 60 : C 1s XAS in TEY CLS SGM Beamline ALS Beamline Excitation Energy [ev] normalized sample current [arbitrary units] h-bn: N 1s XAS CLS SGM beamline TEY ALS Bl TEY Excitation energy [ev] Normalized sample current [arb. units] LiFePO 4 : Fe L 2,3 XAS CLS SGM ALS BL Excitation Energy [ev] We will have our REIXS beamline (Phase II), which is currently under construction (operational 2007).

13 II. How does it work (in principle)?

14 Soft X-rays let you see like light! Interaction of radiation with matter is the fundamental process allowing us to study materials. Light interacting with a sample does one of the following. It is absorbed, reflected, scattered, refracted, diffracted. Absorption of light leads to either (discrete) excitation of sample or to emission of the following products: electrons, photons, other fragments like ions and desorbed atoms or molecules.

15 The principle of photon-in photon-out experiments Sample Monochromator Spectrometer Obtain information on electronic structure and chemical bonding of materials Intensity [Counts] Emission Energy [ev]

16 III. Why synchrotron radiation?

17 Why synchrotron radiation? Our CL S beamline 109 Sun X-ray tube 1013 Light Bulb Intense or better Brilliant (small source small area with small BW) Tunable Polarized Time structure, coherent, collimated.

18 Soft X-ray region Visible range eV Soft X-rays Tunability is the key feature of SR!

19 Accessible binding energies in soft x-ray regime: 70 to 1200 ev C, N, O Si Definition: Binding energy is the energy required to remove an electron from the atom. Tunability is the key feature of SR! Binding energies are characteristic for each element! Spectra involving these energies are element specific.

20 IV. What do emission and absorption spectroscopy probe?

21 Excitation Absorption (XAS) What do XAS and XES probe? Band structure of γ-si 3 N 4 Relaxation Emission (XES) Conduction Band hν in Binding Energy [ev] Valence Band hν out XAS probes unoccupied pdos (CB) Si 2p at 99.2 ev XES probes occupied pdos (VB) Note: The selection rules for XAS and XES ( l = ± 1) only allow for transitions Si 3s / 3d 2p (or Carbon 2p 1s)

22 What are the questions we are asking in my group?! What happens when metal ions are implemented in a host matrix? Ion bombarded polymers DNA! Can we excite different sites selectively in order to learn more about complex systems? γ-si 3 N 4! In general: Can we link spectroscopic (structural) information to functionality (or even diseases)?

23 1. Plasma deposited diamond films

24 Counts 5000 XES (and XAS) are characteristic for each site Si L 2,3 XES: VB (3d3s) 2p C Kα emission : VB (2p) 1s Si SiC SiO 2 Si compounds porous Si K 8 Si 46 Counts C Kα XES CN x a-c HOPG Carbon compounds Counts diamond paste Diamond films Si nanowire Diamond γ-si 3 N 4 C 3 N Emission Energy [ev] Emission Energy [ev] Emission Energy [ev] 1. XES (& XAS) spectra are specific to the chemical environment. 2. XES (& XAS) spectra are element specific (and site specific). 3. Plasma produced Diamond films turned out to be diamond (commercial diamond paste did not).

25 2. Ion irradiated polymers

26 Ion irradiated polymers Polymer C 6 H 5 Si(OC 2 H 5 ) 3 Phenyltriethoxysilene or PTES Si L 2,3 emission : 3s3d (Valence Band) 2p Intensity [arb. units] Si L 2,3 XES 5x10 14 Au + cm -2 PTES unirrad. Result: PTES converts under ion irradiation to Si:O:C (ceramic!). Application: ceramic has different mechanical, electrical and thermodynamic properties than PTES: Scratch resistant, Electrical conductivity, Thermal conductivity. Intensity [arb. units] 2.5x10 15 Au + cm x10 15 Au + cm -2 PTES SiC SiO 2 c-si Emission Energy [ev] A new method to modify polymers! Kurmaev, Moewes et al., Phys. Rev. B 60, (1999)

27 3. Electronic properties of double strand DNA

28 Why is DNA interesting? Can easily synthesize a variety of structures Nanowires Self-assembling properties! High molecular recognition possibilities biological sensors. Great potential applications for DNA in Nanoelectronics but is DNA a good conductor?

29 The electronic structure of DNA (Deoxyribonucleic Acid) Nucleic acids are polymers of nucleotides. DNA bases (A, G, T or C): A nucleotide consists of -a nitrogenous base, - a pentose sugar and -a phosphate group. Sugar phosphate forms backbone. form complementary DNA base pairs : A T by 2 hydrogen bonds and G C by 3 hydrogen bonds.

30 The Controversy in the Literature: B-DNA Published results indicate totally different electrical properties: 4 references found insulating behavior E. Braun et al, Nature 391, 775, (1998). P.J. de Pablo et al, Phys. Rev. Lett. 85, 4992 (2000). 4 references found semi-conducting behavior D. Porath et al, Nature 403, 635, (2000). Results are difficult to compare because of differences in setup. 1 reference found superconductive behavior A. Kasumov et al. Science 291, 280 (2001). 1 reference found highly conductive behavior H.-W. Fink et al., Nature 398, 407 (1999). Our approach: Macroscopic instead of microscopic. Spectroscopic instead of I-V curves.

31 The effect of the buffer materials for DNA normalized current [arb. units] Counts 1.5 TRIS 1.0 Phosphate 0.5 Boric Acid Salt Excitation Energy [ev] 5000 hν exc = ev TRIS Phosphate Boric Acid 0 Salt Emission Energy [ev] A. Moewes, J. MacNaughton et al, J. Electr. Spectr. 137, 817 (2004). Choice of buffer material strongly affects the electronic structure of DNA!

32 C 1s absorption of Nucleobases J. MacNaughton, A. Moewes, E.Z. Kurmaev, J. Phys. Chem. B 109, 7749 (2005).

33 N 1s XAS N and O absorption O 1s XAS

34 Bases versus DNA normalized current [arb. units] N 1s absorption Guanine Adenine Thymine Cytosine BDNA Spectra cannot be assembled from spectra of its components because stacking interaction, additional sugar and added phosphate affect electronic structure of B-DNA Excitation Energy [ev]

35 DNA is extremely large complex biomolecule. Each exp. microscopic setup is difficult and varies. Our approach 1. Systematic approach: understand building blocks and synthesis. 2. Macroscopic study (instead of microscopic set-up). 3. Spectroscopic study (instead of electric I-V curves). Our results 1. Nucleobases are understood. 2. Many control parameters to control: segment length, sequence, environment, contacts, substrate type, sample synthesis (buffer). 3. Each parameter matters and affects electronic structure of DNA. 4. Study in liquid form necessary (very dilute!).

36 4. Ultra-hard new Si phase: γ-si3n4

37 Why spinel Silicon nitride γ-si 3 N 4? Newest synthesized phase: at high T (1800 K) and p (13 GPa) [Zerr et al, Nature 400, 340 (1999)]. Considered 3 rd hardest material (next to diamond and c-bn). Hardness, thermal stability, resistance to oxidation are interesting for applications (LED & other semiconductor devices). Theory: large gap semiconductor: 3.45 ev (direct) UV [Mo et al, PRL 83, 5046 (1999)]. Theory: tunable gap when doped with Al, O [Oba et al, APL 78, 1577 (2001)]. Our Questions: What is the band-gap? How different are tetrahedrally and octahedrally coordinated Si? Can we spectroscopically distinguish the non-equivalent Si sites?

38 The three phases of Si 3 N 4 and Si 2 N 2 O: Original Spinel (mineral): MgAl 2 O 4 Four forms of Si 3 N 4 observed: α-si 3 N 4 β-si 3 N 4 γ-si 3 N 4 Amorphous Si 3 N 4 Si 2 N 2 O Cubic spinel structure γ-si 3 N 4 Three possible locations of Si atom 1 Tetrahedral atom 2 Octahedral atoms

39 Problem: No single crystal available, band gap measurements are difficult. Solution: Measure occupied (XES) and unoccupied (XAS) density of states: Emission The band gap of γ-si 3 N 4 Absorption Intensity (arb. units) Si SiO eV 8.45eV S. Leitch, A. Moewes et al, J. Phys. Cond. Matt. 16, 6469 (2004). γ-si 3 N eV Intesity (arb. units) Si 1.14eV Energy (ev) γ-si 3 N eV Energy (ev) Our experimental determination of band gap for γ-si 3 N 4 : 4.3 ± 0.25 ev Theory: 3.45 ev [Mo PRL 83, 5046 (1999), own calc. (Ching)]. Other Exp. value (UV abs.) : 3.3 ev [Zerr et al, Act. Crystallogr. 58, C47 (2002)]. Without accounting for exciton, the band gap value would be 3.42 ev.

40 Summary In general:! Synchrotron radiation is used for its tunability and brightness.! All relevant information on CLS can be found easily on the CLS website.! Collaborations can start today! Absorption (XAS) and emission spectroscopy (XES):! Absorption and emission are element specific and site specific.! XAS and XES allow to tune photon energy to probe directly the chemical environment of a specific element and even particular site.! XES and XAS measure directly local partial density of occupied and unoccupied states. DNA:! The electronic structure of the nucleobases is understood.! B-DNA bandgap changes with buffer material, which is likely reason for confusion regarding conductivity of DNA.

41 Acknowledgements Synthesis of B-DNA by J.S. Lee / U of S, Dept. of Biochemistry and H.-B. Kraatz / U of S, Dept. of Chemistry Funding by NSERC CFI Canada Research Chair Program U of S & Dept. of Physics (scholarships)

42 The group Janay MacNaughton Ph.D. student Electr. Struct. of DNA Regan Wilks Ph.D. student Self-assembl.Proteins Tor Pedersen M.Sc. student Organic magnetic devices David Muir M.Sc. student Spectrometer Adrian Hunt M.Sc. student Half-metallic systems Mark Boots USRA student Efficiency of diffraction gratings Dr. M. Yablonskikh Postdoctoral fellow Heusler alloys/spintronics Spectrometer optics