1. The Structure Liquid Surfaces P. S. Pershan DEAS & Dept. Of Physics, Harvard Univ. Cambridge, MA 02421 DMR-0124936; NSF 03-03916; DE-FG02-88-ER45379 O. Shpyrko, A. Grigoriev, R. Streitel, P. Pershan, B. Ocko, and M. Deutsch,” Surface Freezing and Quasi-2D Phase Transitions in Binary Metal Liquids,” (March APS 2005) O. G. Shpyrko, A. Y. Grigoriev, R. Streitel, D. Pontoni, P. S. Pershan, M. Deutsch, B. M. Ocko, B. Lin, M Meron, T. Graber, J. Gebhardt, "Atomic-scale surface demixing in a eutectic liquid BiSn alloy.Phys. Rev.Lett. "Submitted (2005). Other Principal Collaborators: J. Als-Nielsen, E. DiMasi, E. Kawamoto, O. Gang, P. Huber, O. Magnussen, K. Penanen, M. Regan,,M.Schlossman, D.Schwartz, H. Tostmann,

2. Pershan: APS Colloq. Jun’05 Condensed Matter: 20th Century Solids: Bulk (3d) Structure  Band Gaps, exotic Fermi Surfaces, etc Surfaces (2d)  Localized Electron States, physisorption, metal/semiconductor interface (rectification), etc. Liquids:Absence of Structure  Less Phenomena Bulk (3d): Liquid Structure Factor Surfaces (2d): Surface tension, Langmuir monolayers, wetting. Ancient History of Liquid Surfaces: Pliny the Elder (~50 AD) & Ben Franklin Surfactants (oil) calm water surface waves Footprint of whale (biomaterial on surface of sea). http://web.mit.edu/1.63/www/ (Chiang C, Mei and T. R Akylas http://web.mit.edu/1.63/www/

3. Pershan: APS Colloq. Jun’05 Modern Era of Surface Science Ref: A. Zangwill, Physics at Surfaces (Cambridge University Press,1988) True emergence of solid state surface physics:  Electron Spectroscopy (Brundle, 1974) & Auger Spectroscopy (Harris, 1974) followed by STM, AFM, etc Synchrotron: SSRL(1973), NSLS (1984), APS (1998) X-rays & Surfaces (Solid): Reflectivity (Parratt ’ 54) Grazing Incidence Diffraction(GID) (Marra, Eisenberger, Cho ’ 79) New tool: probe buried interfaces and structure far below the surface (i.e. GaAs- Al interface) Liquid Surfaces: Reflectivity: Als-Nielsen and Pershan ‘ 82 (Liquid Crystal) & ’ 85 (Water): GID: Dutta ‘ 84 (Langmuir monolayer on water). THIS TALK: X-RAY AND LIQUID SURFACES

4. Pershan: APS Colloq. Jun’05 X-Rays and Solid Surfaces Truncation Rods from Crystal Surface Bragg Scattering From Crystal Fresnel Reflectivity from Flat Surface A. H. Compton and S. K. Allison ‘35: Surface Information: Intensity along truncation rods Extra Peaks due to Surface Phases (reconstruction)

5. Pershan: APS Colloq. Jun’05 Liquid vs Solid Surfaces Liquids: Bulk Liquid Diffuse Scattering 3d Crystal If Liquid Surface was FLAT Specular would be the Only Truncation Rod Surfaces ARE NOT FLAT! Liquid Surface Information: Surface Structure Factor  (Q z ) Extra peaks due to Langmuir monolayer or Surface Frozen phases.

6. Pershan: APS Colloq. Jun’05 Surface Roughness For a solid: Fourier Transform  Reflectivity  Structure Factor + Debye -Waller

7. Pershan: APS Colloq. Jun’05 Fluctuations of Surface of Bulk Liquid q max ~1/Atom Not  (Q xy )

8. Pershan: APS Colloq. Jun’05 Effect of Resolution Scan Detector  s Increasing Q z or Liquid: Large  i  ~Capillary Effects Dominate Solid: Effect of Resolution on R(Q z ) is Minor Liquid: Small  i  ~Nearly Solid Like Small angles liquids are like solids / large angles they are not!

9. Pershan: APS Colloq. Jun’05 Simulated Detector Scan The Liquid Surface Reflectometer HasyLab: Als-Nielsen, Christensen, Pershan, PRL (`82). NSLS: X22B, X19C APS: CHEMMATCARS, CMC,  CAT ESRF: ID15A (Alternate Design) H. Reichert ‘03

10. Pershan: APS Colloq. Jun’05 Data for Water with increasing  Q y (1/Å) 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

11. Pershan: APS Colloq. Jun’05 Surface Induced Layering:  (Q z ) Nematic Surface  Smectic-A Order z Surface Induced Smectic Isotropic/Nematic/Smectic-A

12. Pershan: APS Colloq. Jun’05 Nematic Phase: 1st Observed Surface Induced Layering First Data (Pershan, Als-Nielsen.PRL, ‘ 84) R F (Q z ) T-T NA 0.05 C 2.8 11.6 1/Width Surface vs. Bulk |  (Q z )| 2

13. Pershan: APS Colloq. Jun’05 Reflectivity & Surface Structure Factor (Layers) Structure Factor  (Q z )| 2 Thermal Factor R(Q z ) =R F (Q z ) Simple Surface Surface Structure Factor When do surface layers appear? Quantitative Measure of  (Q z )! Prediction: Constructive Interference Q z =(4  )sin  =(2  /D)

14. Pershan: APS Colloq. Jun’05 Density Profile vs Depth Solid-liquid interface Hard wall Molecular Simulations G. A. Chapela, G. Saville, S. M. Thompson, and J. S. Rowlinson, "Computer simulation of a gas-liquid interace",J. C. S. Faraday Trans II 73, 1133 (1977). Lennard-Jones (12,6) molecules Accepted Lore: Density Profile at Free Surface is Monotonic Liquid Crystals are Different: Why? What else is different? Liquid vapor interface

15. Pershan: APS Colloq. Jun’05 Simple Thoughts on Surface Layering Order Parameter  (r): Electron, Mass, or Particle Density Bulk (3d)Correlation Function: Characteristic Wavevector: Q 0 & Correlation Length:  Bulk Susceptibility: Z surf (Q 0  ) Surface Induced Layering: Surface Field: h(z=0) Translation Energy vs Entropy

16. Pershan: APS Colloq. Jun’05 In Plane Surface Order Langmuir Monolayer on H 2 O or Hg 2D Liquid 2D Crystal Surface Freezing 2D Surface Crystal SubSurface 3D Liquid Long Chain Alkanes Metallic Alloys Solid/Liquid Interface Commensurate/Incommensurate 2d Layering

17. Pershan: APS Colloq. Jun’05 Digest of Liquid Surface Order LayeringIn-Plane Surface Order Nematic/Isotropic Liquid XtalYesNo MicroemulsionsYesNo Long Chain Alkanes, Alcohols, etc NoYes H2OH2ONo Elemental Liquid MetalsYesNo Alloys of Liquid MetalsYes Langmuir MonolayersNoYes Experiments: Simulations: Atom & Small-Non-Metallic MoleculesNo

18. Pershan: APS Colloq. Jun’05 Why are Liquid Metals Different? Simuation (Lennard-Jones Liquids) D'Evelyn &. Rice, J. Chem. Phys., 1983. For Metals Particle-Particle Interactions Change Across The Surface Interactions are Same in Vapor and Liquid Dielectric Liquids Vapor: Neutral Atoms Liquid: Positive Ions in Sea of Negative Fermi Liquid Different Interactions Metallic Liquids This influences the structure of the surface! Goal: Measure Intrinsic Surface Structure Factor  (Q z )

19. Pershan: APS Colloq. Jun’05 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).

20. Pershan: APS Colloq. Jun’05 Removal of Thermal Factor Liquid Ga Electron Density Profile Indium T- effects removed & not removed Ga & In with T-effects removed

21. Pershan: APS Colloq. Jun’05 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 K H 2 O vs Liquid Metals

22. Pershan: APS Colloq. Jun’05 Gibbs Absorption: GaBi Alloy  (Bi)= 398 mN/m  (Ga)=750 mN/m Monolayer of Bi Coats Liquid Surface Thick Wetting Layer of Bi-Rich Liquid vs Temperature

23. Pershan: APS Colloq. Jun’05 Liquid Metals of Electronic Interest (I): BiSn O. G. Shpyrko, A. Y. Grigoriev, R. Streitel, D. Pontoni, P. S. Pershan, M. Deutsch, B. M. Ocko, Meron, B. Lin.Phys. Rev.Lett. "Submitted (2005). Energy Dispersive Reflectivity 142 °C T m =138°C  (Bi) ≈ 398  (Sn)≈567 dyne/cm

24. Pershan: APS Colloq. Jun’05 Liquid Metals of Electronic Interest (II) Au 80.5 Si 19.5 eutectic alloy Detector (  S )-Scan Alloy is Liquid  =780 dynes/cm

25. Pershan: APS Colloq. Jun’05 Surface Phase Transition vs T Reflectivity/(R F )  2 Phases Simple Layering Model (i.e. Ga or In)  1200 dyne/cm  718 dyne/cm Not Divided by Thermal

26. Pershan: APS Colloq. Jun’05 Model Density Profiles AuSi(Preliminary) High T Phase Low T Phase Typical Profiles Ga & In

27. Pershan: APS Colloq. Jun’05 2D Order of Surface Phases (GID) Truncation Rod  Monolayer

28. Pershan: APS Colloq. Jun’05 In-plane structure model: AuSi 2 Low Temperature Phase 7.4 C 9.4 C 12 atoms: 4 Au, 8 Si

29. Pershan: APS Colloq. Jun’05 AuSi, AuGe vs Elements AuSi ~ 10 x AuGe & elements

30. Pershan: APS Colloq. Jun’05 The Future The Buried Liquid/Solid Interface Effects: Layering induced by hard wall! Surface induced in-plane order! Problems with conventional approach: Absorption in Liquid Bulk Diffuse Scattering Path > 20 mm Ga Abs. Length 0.05mm(10 KeV) 0.13mm( 30KeV ) Si Abs. Length~17 mm(70 Kev) H. Reichert, et a;/ Physica B-Condensed Matter (03). van der Veen and Reichert, MRS Bulletin (04). Beam Height~10  m Path~ 5 mm

31. Pershan: APS Colloq. Jun’05 Summary  Solid vs Liquid Surfaces  Reviewed X-ray Methods of Surfaces Special Requirements of Liquids CARS,  -CAT, CMC  Surface Roughness: Capillary Waves  Examples of Liquid Surface Order  Liquid Metals vs Non-Metals  Alloys: AuSi >10 x Others: Surface Freezing  Future: Buried Interfaces (Not DONE @APS)