Surface Structure and Chemical Composition of Liquid Metal

Surface Structure and Chemical Composition of Liquid Metal Alloys P. S. Pershan HSEAS & Dept. of Physics, Harvard Univ. I. II. Liquid Surfaces: Basic Ideas 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, Sn. Bi, Au. Sn VI. Au-Eutectics: Surface Crystals 9 th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul. ’ 06). 1

Colleagues V. Balagurusamy, R. Streitel, O. Shpyrko, P. S. Pershan, M. Deutsch, and B. Ocko, "Surface Xray Scattering Studies of Liquid Au. Sn 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 Au. Si 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 Bi. Sn 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) 9 th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul. ’ 06). 2

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

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

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

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

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 Entropy of Mixing! Surface Layering, Adsorption & 2 D Ordering! Approx. Theories of Surface: Guggenheim(1944), Defay-Prigogine (1950), Strohl-King(1989) 9 th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul. ’ 06). 7

How Liquid Surfaces Have Been Studied! For nearly 200 Years: Measured Integrated Properties of Interface • Surface Tension • Ellipsometry: Drude (1889) 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 th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul. ’ 06). 8

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

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

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

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

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

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

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

Liquid Surface Reflectometer Hasy. Lab: Als-Nielsen, Christensen, Pershan, PRL (`82). L HASYLAB: BW 1 NSLS: X 22 B, X 19 C APS: CHEMMATCARS, CMC, CAT ESRF: ID 10 B & ID 15 A (Alternate Design) H. Reichert ‘ 03 9 th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul. ’ 06). 16

Data for H 2 O CMC CAT Shpyrko, Fukuto, Pershan, Ocko, Gog, I. Kuzmenko, Deutsch, , Phys. Rev. B (2004). Peak vanishes for slight increase in Qz Qz (or i Increasing 0. 3 Å-1 to 1 Å-1 Increasing 0. 08 to ~ 1 Qy(1/Å)=(2π/ cos( I)-cos( s)] 9 th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul. ’ 06). 17

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

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

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

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

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

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

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

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

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

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

Why? ? Au 82 Si 18 • There is no theoretical explanation! • Some Speculations! 1 - Gibbs: Si should adsorb to surface. 2 -X. Li et al "Gold as hydrogen. … bonding in disilicon gold clusters Si 2 Aun -(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. Aux: Covalent Metallic vs x. Possible Surface • Network with Covalent Structure 9 th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul. ’ 06). 28

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

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

Another Mystery Surface Tension and Order C. J. Aidinis, . . ”. . liquid metal field ion emitter for the production of Si ions", Microelec. Eng. 73 -74(‘ 04). Croxton, Stat. Mec. of the Liq. Surf. (1980). Croxton’s Idea? 0. 8%Ge: Nearly Same as 0% Ge- But Differences (next) 9 th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul. ’ 06). 31

GID: Au 81. 9 Si 17. 3 Ge 0. 8% Ge: GID Scans Fluctuate Average 0% Ge: GID Scans Reproduceable 0. 8%Ge: Fluctuating Coarse Powder Partial Powder Average 0%Ge: Fine Powder Lattices are Identical 9 th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul. ’ 06). 32

Au 82 Si 18 Temperature Range Au 81. 9 Si 17. 3 Ge 0. 8 vs Au 82 Si 18 Ge has a major effect! Why! No Explanation! 9 th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul. ’ 06). 18°C 33

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