1. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 1 Surface Structure and Chemical Composition of Liquid Metal Alloys P. S. Pershan HSEAS & Dept. of Physics, Harvard Univ. I.Liquid Surfaces: Basic Ideas II.Experimental Methods for Studying Liquid Surfaces III.Liquid Metals III.Simple Surfaces: Ga, In, K, Hg(?) IV.Subtler Sufaces: Sn, Bi V.Alloys: Gibbs Adsorption, SnBi, AuSn VI.Au-Eutectics: Surface Crystals HSEAS

2. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 2 Colleagues V. Balagurusamy, R. Streitel, O. Shpyrko, P. S. Pershan, M. Deutsch, and B. Ocko, "Surface X- ray Scattering Studies of Liquid AuSn alloy ",Phys. Rev. B, (2006), to appear.. G. Shpyrko, R. Streitel, V. S. K. Balagurusamy, A. Y. Grigoriev, M. Deutsch, B. M. Ocko, M. Meron, B. H. Lin, and P. S. Pershan, "Surface crystallization in a liquid AuSi alloy",Science 313, 77 (2006). O. G. Shpyrko, A. Y. Grikgoriev, R. Streitel, D. Pontoni, P. S. Pershan, M. Deutsch, and B. M. Ocko, "Atomic-scale surface demixing in a eutectic liquid BiSn alloy."Phys. Rev. Lett. 95, 106103 (2005). A.Grigoriev, O. G. Shpyrko, C. Steimer, P. Pershan, B. Ocko, M. Deutsch, B. Lin, M. Meron, T. Graber and J. Gebhardt "Surface Oxidation of Liquid Sn", Surf. Sci. 575, 3, 223 (2005). B.G. Shpyrko, A. Grigoriev, C. Steimer, P. S. Pershan, B. Lin, M. Meron, T. Graber, J. Gerbhardt, B. M. Ocko, and M. Deutsch, "Anomalous layering at the liquid Sn surface",Phys. Rev. B 70, 224206 (2004 ). Stefan Sellner (New to Group)

3. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 3 Modern Era of Surface Science: Solid Surfaces Electron Spectroscopy (Brundle, 1974) & Auger Spectroscopy (Harris, 1974) followed by STM, AFM, etc Coincidentally: Synchrotron: SSRL(1973), NSLS (1984), APS (1998) Synchrotron Radiation Enabled First Atomic Scale Studies of Liquid Surfaces But these techniques can not be used on Liquids!

4. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 4 Solid vs Liquid Surfaces Non-metallic, Atomic, H 2 O, etc Free Surface: Defined by Only Gravity & Surface Tension Liquid Solid Surface: Defined by Hard Wall Liquid Surfaces: Most of What We Know Molecular Simulations Solid Surface: Defined by Rigid Lattice Extensive Studies: Reconstruction, etc Width of Interface   Surface Tension  Properties of Interfaces Hard Wall  Atomic Layering Long Wavelength Capillary Fluctuations (To be discussed later).

5. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 5 Free Surfaces: Induced Order: When? Properties? Liquid Crystals: Fluctuations <<Molecular Size Different Interactions Suppress Local Fluctuations Local Layering Vapor: Neutral Atoms Liquid: Positive Ions in Sea of Negative Fermi Liquid Metallic Liquids (D’Evelyn & Rice ‘83) Goal : Measure surface induced order!

6. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 6 Surface Tension vs Interfacial Structure Heuristic Discussion: Young, Poisson, etc. ~1800 Nearest Neighbor Attractive Interaction: -  Number of Neighbors for Bulk Atom:Z B Enthalpy per Bulk Atom;-  Z B Number of Neighbors for Surface Atom:Z S <Z B Enthalpy per Surface Atom;-  Z S Surface Enthalpy:  Enthalpy = -  Z S -(-  Z B ) =+  (Z B -Z S )>0 S` Fluctuations of Surface Atoms: Z S` ≠Z S Interfacial Structure  Total =  Enthalpy +  Entropy

7. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 7 Liquid Metal Alloys J. W. Gibbs ~1920 A/B Alloy If Surface Tension:  A >  B Surface Rich in “B”. Eutectic Alloys Immiscible Solid Repulsive Pair-wise Interactions Surface Layering, Adsorption & 2D Ordering! Approx. Theories of Surface: Guggenheim(1944), Defay-Prigogine (1950), Strohl-King(1989 ) Entropy of Mixing!

8. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 8 How Liquid Surfaces Have Been Studied! Surface Tension Ellipsometry: Drude (1889) For nearly 200 Years: Measured Integrated Properties of Interface More Recent: Non-Linear Optics (Sum/Difference Frequency) Local Property: Requires Non-Trivial Theory ~200 years with little progress in understanding. X-rays now yield atomic structure  Hope for Theory !

9. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 9 X-Ray Reflectivity: Basics Snell’s Law Critical Angle: Fresnel Reflectivity From A Structureless Flat Surface

10. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 10 Surface Structure Structure Factor Reflectivity Grazing Incidence Diffraction Layers

11. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 11 X-ray Scattering Experiments Specular Reflectivity GID

12. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 12 SurfaceRoughness Solids Solid 1 Fourier Transform  Effect of Roughness Debye-Waller

13. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 13 Liquid Roughness S. K. Sinha et al Phys. Rev. 38, 2297 (1988). q max ~1/Atom r >> 1/q max Q z <<1 or  <<1  Solid Like Otherwise,  ~1 Very Different

14. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 14 Solid Liquid: Diffuse Scattering vs Specular Reflection. Liquid No True Specular Reflection for Liquids:

15. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 15 Solid vs Liquid I (Reflectometer) Solid : Q max ~ 2 to 3 Å -1 E~10 keV   Als-Nielsen, ‘82 Solid Specular Reflectivty: Rotate Sample! Liquid : Liquid: Scan Incident Beam/Sample Height

16. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 16 Liquid Surface Reflectometer HASYLAB: BW1 NSLS: X22B, X19C APS: CHEMMATCARS, CMC,  CAT ESRF: ID10B & ID15A (Alternate Design) H. Reichert ‘03 HasyLab: Als-Nielsen, Christensen, Pershan, PRL (`82). L

17. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 17 Data for H 2 O Q y (1/Å)=(2π/  cos(  I )-cos(  s )] Q z (or  i  Increasing 0.3 Å -1 to 1 Å -1  Increasing 0.08 to ~ 1 Shpyrko, Fukuto, Pershan, Ocko, Gog, I. Kuzmenko, Deutsch,,Phys. Rev. B (2004). CMC CAT Peak vanishes for slight increase in Q z

18. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 18 Typical Liquid Metal Measurements Hg In Ga Effect of T (Liquid Ga) Structure FactorThermal Factor Observe Apparent Difference Magnussen, Ocko, Regan, Penanen, PershanM. Deutsch,PRL (1995). Regan, Kawamoto, Pershan, Maskil, Deutsch, Magnussen, Ocko, L. E. Berman, PRL (1995). Tostmann,DiMasi, Pershan, Ocko, Shpyrko, M. Deutsch, PRB (1999).

19. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 19 Removal of Thermal Factor Liquid Ga Electron Density Profile  (z)> Ga & In with T-effects removed  (z)> Indium T- effects Not Removed T-effects Removed

20. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 20 Metallic Layering Is not Due to High Surface Tension (  ) R/(R F x Thermal) for Ga, In and K   In(~550mN/m) Ga(~750mN/m) K(~100mN/m) H 2 O(73mN/m) H 2 O vs Liquid Metals H2OH2O K

21. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 21 Anomalous Layering of Liquid Sn R(Q z ) Bump Not seen in Ga,In Bump  Surface Density Is Higher Than Bulk! No Theoretical Explanation Why Sn Should be This Way ! 1st Layer is ~10 % Thinner

22. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 22 Anomalous Layering of Liquid Sn &Bi Bi: Equal Spacing Bi: ~8% Higher Density Model Properties: Number of Atoms 1st layer vs others  Z/Z Spacing of 1st layer vs others  d/d KGaInSnBiHg  Z/Z +8%  d/d -10% No Theory

23. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 23 Liquid Metal Alloys J. W. Gibbs ~1920 A/B Alloy Surface Adsorption If Surface Tension:  A >  B Surface is Rich in “B”. Approx. Theories of Surface: Guggenheim(1944), Defay-Prigogine (1950), Strohl-King(1989 ) There is No Serious Theory of Effects to be Described!

24. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 24 Gibbs Surface Adsorption(BiSn)  Bi =378,  Sn =560, Alloy: Bi and Sn  (Bi) ≈ 398  (Sn)≈567 dyne/cm Energy Dispersion: f(E) Adsorption Scat. Ampl.

25. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 25 Surface Freezing AuSiGe Eutectics Au 82 Si 18 1st Order Transition R/R F

26. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 26 Grazing Incidence Diffraction Electron Density High T Low T Au 82 Si 18 Continued Standard Low T High T Q z Dependence of Bragg Peak Proves 2D 2D Surface Crystals

27. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 27 Au 82 Si 18 Continued: 2D-Liquid Diffuse Scattering From H 2 O Diffuse Scattering From Au 82 Si 18

28. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 28 Why?? Au 82 Si 18 There is no theoretical explanation! Some Speculations! 2-X. Li et al "Gold as hydrogen. … bonding in disilicon gold clusters Si 2 Au n -(n=2,4), J.P.Chem A 109(‘05). 3- J. Weissmuller, "Reduced Short-Range Order in Amorphous-Si/Au-Alloys",J. Non-Cryst. Solids 142(‘92). Si 1-x Au x : Covalent  Metallic vs x. Possible Surface Network with Covalent Structure 1- Gibbs: Si should adsorb to surface.

29. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 29 Silicon vs Germanium

30. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 30 Reflectivity of the Au- eutectics Why are AuSi and AuGe eutectics different? Could it be that Si is more covalent?

31. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 31 Another Mystery Surface Tension and Order Croxton, Stat. Mec. of the Liq. Surf. (1980). C. J. Aidinis,..”.. liquid metal field ion emitter for the production of Si ions", Microelec. Eng. 73-74(‘04). 0.8%Ge: Nearly Same as 0% Ge- But Differences (next) Croxton’s Idea?

32. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 32 GID: Au 81.9 Si 17.3 Ge 0.8 0.8% Ge: GID Scans Fluctuate 0% Ge: GID Scans Reproduceable Average 0.8%Ge: Fluctuating Coarse Powder Partial Powder Average 0%Ge: Fine Powder Lattices are Identical

33. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 33 Temperature Range Au 81.9 Si 17.3 Ge 0.8 vs Au 82 Si 18 Au 82 Si 18 18°C Ge has a major effect! Why! No Explanation!

34. 9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06). 34 Summary I.Liquid vs Solid Surfaces  Capillary Roughness vs Rigid Lattice  Different Experimental Methods II.No True Reflectivity from Liquid Surfaces  Experiments on Water III.Liquid Metals  Simple (Ga, In, K, Hg)  Anomalous (Sn, Bi)  Gibbs Adsorption (SnBi)  Surface Freezing (AuSiGe Eutectics) IV.Need for Theory!