Surface Analysis with Many Positrons

Transcription

1 Surface Analysis with Many Positrons Alex Weiss Physics Department The University of Texas at Arlington TX 76019, USA

2 Acknowledgments: Current Students:: 1. Saurabh Mukherjee Ph.D. Program in Physics, 2. Karthik Shastry, M.S. Physics Undergraduate Students: Aale Naqvi Former Students: Ph.D.-1. M. Jibally,.2. D.Mehl, 3. K.H. Lee, 4. G. Yang, 5. H.Q. (Amy) Zhou, 6. E. Jung, 7. J.H. Kim 8. Wu-Chi Chen, 9.S. Xie 10. J. Zhu, 11. M. Nadesalingam, 12 R. Sundaramoorthy MS: 1. C. Lei 2. K.H. Lee 3. L.-W.Tyan 4. J.H. Kim 5. S. Wheeler 6. R. Venkataraman 7. A. Nangia 8. R. Nayak 9. N. Jiang 10. S. Starnes 11. J. Yan 12. S. Kim 13. R. Sundaramoorthy 14. M. Nadesalingam 15. S. Mukherjee Collaborators (UTA): A. R. Koymen,.J.L.Fry, N. Fazleev, W. Chen, M. Tao, K. Rajeshwar, C. Kim, (Japan): Hasegawa, Nagai, (Germany): G. Brauer, (BNL)S. Hulbert, R. Bartynski Former Post-Docs/Collaborators: J. Kaiser UTA, K.O. Jensen - U. of East Anglia, U.K., G. A. Mulhollan SLAC, Rulon Mayer, Arnum Schwab BNL, Anat Eshed - MIT Previous and Current Funding: NSF, DOE, Texas-ARP, DOE, The Welch Foundation

3 Lecture I: Positron Surface Spectroscopies Review of important positron surface interactions Overview of Positron Surface Spectroscopies Introduction to PAES spectroscopy Lecture II: Applications Examples of Applications of Positron Surface Spectroscopies Lecture III: High Flux e+ Beams: New Possibilities Surface island traps: possible means of increasing e+ densities New kinds of surface measurements

4 Why study surfaces with positrons?

5 Corrosion Electrical Contacts Adhesion Ultrathin Layers Corrosion Heterogeneous Catalysts Solid or Liquid Electrical Contacts >95% of all products synthesised by Catalysts ~20% of world economy depends on Catalysts Adhesion Ultra Thin layers Catalysis

6 Many Techniques to Probe Surfaces Technique Depth Probed in Atomic Layers Comments LEED 3-5 Need theory SIMS 1-5 Destructive of thin films. Hard to qualify XPS 3-10 e - Auger 3-10 STM ~1 Difficulty with element identification e + Auger ~1 Significant advantages over E-AES

7 Capable of detecting both structure and elemental composition Highly surface selective due to trapping in surface state Capable of probing active sites and chemical mechanisms Capable of probing internal surfaces of porous materials Low Background Very low damage low energy dose to surface

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9 e + beam Thermalized Energy Loss: -plasmons -electron-hole pairs -Phonons Ps e + Annihilation in the bulk Positron surface state Diffusion After: Schultz and Lynn, Rev. Mod. Phys. 60, 1988

10 Ps self-annihilation γ Surface State γ Thermally Desorbed Annihilation with Surface Electrons γ e + beam Diffusion γ

11 Recently Demonstrated Direct Surface Trapping Mechanism Positron may give up all or most of its kinetic energy to an electron at or near the Fermi level and directly drop into the surface state; Thus transferring the surface state binding energy to the electron in the process. The maximum kinetic energy for an electron emitted in this process is given by: E K max = Ep + E ss ϕ = E Φ + Metal Vacuum E= E E ss Surfaces potential Vacuum level Now, ϕ = 5eV, Ess = 3eV so for E K max > 0eV, EP > 2eV

12 ε e+ Positron emission (ε e+ ) ε Ps Positronium emission (ε Ps ) ε S Surface State Trapping (ε S ) J. Phys. C: Solid State Phys. 21 (1988) Branching ratios for electron-volt positrons at a Cu(ll0) surface J A Baker, M Touat and P G Coleman

13 LEPD, positron work function, positron reemission spectroscopy, positron tunneling spectroscopy positron backscattering positrons in positrons out positronium gamma rays out positronium emission spectroscopy Ps diffraction out positrons in or Ps in positrons in surface ACAR positrons in electrons out PAES, positron-induced secondary electrons

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15 Principal Advantages of PAES: 1. Higher surface selectivity than electron spectroscopies. 2. Very low background-elimination of primary beam induced secondary electron background. 3. Selective sensitivity to nano structures and impurities.

16 Time scales of physical processes underlying PAES Low energy (~10ev) positron in. Implantation, thermalization, diffusion, encountering the surface sec Positron trapped in surface state sec Annihilation of surface state positron with core electron sec Emission of Auger electron Higher energy ( ev) electron out.

17 Auger Electron Vacuum Level Valence Band φ E z E y γ E x γ E Auger = E x - E z - E y - φ

18 Elimination of Secondary Electron Background EAES 3 kev e - in PAES 30 ev e + in 30eV -1keV Auger e kev secondary e - 30eV -1keV Auger e ev secondary e -

19 Why is PAES more surface selective than other electron surface spectroscopies? metal vacuum Electrons: λ- escape depth; λ~ 4 10 Å Excited Atoms Incident electrons ~ kev 1000 Å EAES samples several layers below the surface ~50% of signal from 2 nd layer and below Positrons: λ ~ 1 Å Thermalized positron in surface state PAES probes only top layer ~95% of signal from top layer

20 Detailed Calculations: ~95% of positron in surface state on Cu annihilate with top atomic layer Construct positron potential: V+(r) = VH(r) + Vcorr(r). V ( image e 2 1 r )= - χ 4πε 4 0 Z (n_( r eff )) - Z 0 Solve for positron-surface-state wave function: 2 h 2m 2 ψ + i ( r ) + + [ ] Ei ψ + V ( r ) + V ( r )ψ ( r ) = ( r ) H Corr i i Calculate annihilation probability ( α positron-electron overlap) λ n, l = πr 2 o c d 3r ψ ( r) i ψ ( r) i n, l

21 PAES vs EAES ~1 ML Pd deposited on Cu PAES EAES Cu MVV Pd NVV Cu LVV Cu MVV Pd NVV Cu LVV Cu MVV Pd NVV Cu LVV

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23 UTA Time of Flight (TOF) PAES Spectrometer TOF method collects all energies simultaneously rather than sequentially stepping an energy window. 100 times the collection efficiency of previous PAES systems. Permits observation of Auger peaks from chemically important light elements (e.g. C, O, N).

24 1 m Sample Preparation Chamber Sample Position E X B Plates 22 Na Positron Source Retarding Tube MCP Accelerator Sample manipulator Ion Pump Ion Pump Positron Annihilation-Induced Auger Spectroscopy (PAES) System with Time-of-Flight (T-O-F) Energy Analyzer

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28 Schematic of T-O-F PAES system B = 40Gauss B = 100Gauss Permanent magnet γ Sample TOF retarding tube EXB Plates D e - MCP EXB Plates C Accelerator EXB Plates B Tungsten Barrier C EXB Plates A Tungsten Barrier B Moderator Tungsten Barrier A γ e + V bia s V rt d V EXB 2 V EXB 1 V m

29 Timing scheme of the Time-of-Flight (TOF) PAES spectrometer NaI Detector γ Sample TOF Retarding tube EXB Plates Micro-channel Plate (MCP) e - Fast pre-amplifier Magnet γ e + BaF 2 Detector stop start CFD Time to Amplitude converter (TAC) Counter Fast pre-amplifier CFD Time delay Multi-channel Analyzer (MCA) The energy distribution of outgoing electrons in the TOF technique can be calculated by the equation: E kinetic = 1 2 m v e 2 = 1 2 m e t stop L t start 2

30 TOF-PAES Flight Time Spectrum Count ts/sec Channel ns (#1118) Copper Sample 195.6ns (#1325) Flight time (ns) Clean Cu Surface Counts s/sec Channel Flight time (ns) 274.9ns (#1118) 132.3ns (#1483) 195.8ns (#1320) 115.1ns (#1527) 104.2ns (#1555) Cu Surface with adsorbates

31 Electron Energy Spectrum Intensity (count/sec) Cu M 2,3 VV (59.3 ev) Experimental data 5 point smoothing (b) Intensity (count/sec) Cu M 2,3 VV (58.3 ev) Experimental data 5 point smoothing Cu M 1 VV (108.7 ev) Energy (ev) Cu M 1 VV C KLL (108.0 ev) (268.0 ev) N KLL O KLL Energy (ev) Clean Cu Surface Cu with adsorbates: C (C KLL), N (KLL) and O (KLL)

32 Energy spectrum of graphite surface C KLL (258.4 ev) Counts/sec O KLL (497.5 ev) Energy (ev) Auger spectrum of graphite initial surface

33 Core level annihilation probability Table 2: adsorbate Copper Surface Core level annihilation probability (%) Cu 3p Cu 3s C 1s N 1s O 1s Cu (polycrystalline) Adsorbate surface Cu (110) * with C Adsorbate surface Cu (110) * with N Adsorbate surface Cu (110) * with O Adsorbate surface *is the theoretical value from reference (Jensen and Weiss), all the adsorbate coverage is 0.5 relative to the substrate atomic density for theoretical results.

34 Core level annihilation probability Theory Experimental p s Comparison of Calculated core-annihilation probabilities (Jensen and Weiss) and experimental results of TOF-PAES as functions of the binding energy of the core levels

35 Positron removed from surface state by thermal desorption of Ps

36 Thermaldesorption of Ps Rate of Ps desorption: φē b Function of z/γ (desorption rate/ss annihilation rate) A.P. Mills, Sol. St. Comm., 31 (1979) S. Chu, A.P. Mills, C.A. Murray, PRB, 23 ( 1981)

37 In the positron in a potential well picture: φ - E b Where E b is the binding energy of a positron in the surface state In the Ps in a potential well picture: E a is the binding energy of Ps in the well Energy available when e+ from outside sticks to surface E b =E a -φ - + 1/2Ry

38 PAES f Ps Ps self-annihilation No core e- f ss = 1-f Ps (neglecting free e+ channel) 1-f Ps Surface state annihilation ~1-10% surface atom core e- followed by Auger emission

39 Repulsive + van der Waals Where α(ω) is Ps polarizability and ε(ω) is the bulk dielectric function. Repulsive term approximation: v R (z) = V 0 e (z zo)/l

40 Positrons can pair up with electrons as Ps at a surface of quartz and Ps can stick to the surface. Saniz, B. Barbiellini, P. M. Platzman, and A. J. Freeman, PRL 99, , (2007); PRL 100, , (2008). Michael Schirber, Phys Rev. Focus 20, story 7

41 Ground state : ev ~1/z 3 Excited state: ev

42 Theory used to calculate PAES annihilation probabilities: Positron in potential well at the surface Nieminen and Puska, PRL 50 (1983) Construct positron potential: V + (r) = V H (r) + V corr (r). Jensen and Weiss, PRB 41 (1990) V ( image e 2 1 r )= - χ 4πε 0 4 Z (n_( r )) - Z eff 0 ~1/z Solve for positron-surface-state wave function: 2 h 2m 2 ψ + i ( r ) + + [ ] Ei ψ + V ( r ) + V ( r )ψ ( r ) = ( r ) H Corr i i Calculate annihilation probability ( α positron-electron overlap) λ = πr 2 o c n l d 3r, ψ ( r) i ψ ( r) i n, l

43 Core level annihilation probability Theory Experimental p s Comparison of Calculated core-annihilation probabilities (Jensen and Weiss) and experimental results of TOF-PAES as functions of the binding energy of the core levels

44 Properties of positron interactions with surfaces give positrons significant advantages as probes of surfaces and surface processes. PAES allows measurements of the elemental content of the top most atomic layer. Temperature dependence of PAES intensities consistent with surface state origin of PAES signal. Theory: Positron in a single particle potential convenient and accurate calculations of PAES intensities.