Harvard University

Surface Structure of Liquid Metal and Liquid Metal Alloy Surfaces

Poster presentations
Publications

People working on the project:

Principal Non-Harvard Collaborators

 

The liquid metals project is carried out in collaboration with x-ray groups at the Physics Department of Brookhaven National Laboratory (BNL) , NY and Bar-Ilan University, Israel. Originally the experiments were performed at the beamlines X25and X22B of the National Synchrotron Light Source (NSLS); however, since Summer 2001 many experiments have been carried out at the Advanced Photon Source at Argonne National Laboratory (APS) sectors ID-15 ( ChemMat-CARS CAT) and ID-9 (CMC CAT).

This work is jointly supported at Harvard by the U.S. DOE Grant No. DE-FG02-88-ER45379. Measurements at the NSLS are supported by DOE grant DE-AC02-76CH00016 Investigators from other institutions that collaborate with us receive other support.

Outline of the project:

In the absence of a crystalline lattice, a liquid metal is considered to be a two-component fluid with the ions coupled to the nearly-free conduction electrons through Coulomb interactions. The ions can be treated classically as a positive background jellium, whereas the electron gas has to be described by quantum mechanics. In contrast to liquid metals, dielectric liquids can be well described as a one-component fluid consisting of hard spheres in the simplest cases.

The fundamental differences in the basic physics of liquid metals and liquid dielectric fluids are not manifested in the bulk structure of the two as revealed by x-ray or neutron diffraction. For example, the pair correlation function describing the structure of liquids in real space is almost identical for free-electron liquid metals such as Na and simple dielectric liquids such as liquid Ar. The structure of the surface on the other hand presents a relatively unique opportunity to study the difference between a liquid metal and a dielectric liquid because the interatomic interactions depend strongly on the changing density along the liquid-vapor interface only in the case of a liquid metal. In fact, the transition from the liquid to the vapor phase corresponds to a transition from a metallic state (nearly-free electrons) to a nonmetallic state (localized electrons) in the case of a liquid metal. By contrast, the type of interactions remains the same along the liquid-vapor transition in the case of a dielectric liquid (van der Waals interactions with electrons well localized at a single atom or molecule).

Apart from this more involved consideration, the macroscopically observed fact is that the surface tension of liquid metals is often at least one order of magnitude larger than the surface tension of dielectric liquids at comparable temperatures. Larger surface tension suppresses thermal fluctuations of the surface and allows measurement of the microscopic surface structure.

Both analytic jellium and pseudopotential calculations, as well as Monte Carlo computer simulations all lead to the prediction that the extremely flat surface created by the conduction electrons acts as a hard wall that suppresses positional fluctuations of the near surface ion cores. This should result in atomic layering at the surface of liquid metals, i.e. positional ordering of the ion cores normal to the surface. The electron density is therefore expected to display characteristical oscillations at the interface decaying to the bulk electron density after a few atomic distances. The existence of surface layering has been proven experimentally by our group for liquid mercury, gallium, tin, bismuth indium and potassium . In contrast to liquid metals, simple monotonic profiles without any oscillations have been simulated for dielectric liquids, i.e. the atoms or molecules show no order perpendicular to the liquid-vapor interface.
A cartoon depicting this fundamental difference and the density profiles normal to the surface for liquid metals and dielectric liquids can be seen here.

In addition to ordering normal to the surface (layering), the ions in both the Au-Si eutectic and the more complex Au-Cu-Si-Ag-Pd glass forming alloy. have been observed to be ordered within the surface layer (in-plane order). In principle, these two structural features of the liquid metal surface are accessible using two different x-ray scattering techniques: the technique of x-ray reflectivity for layering and the grazing incidence x-ray diffraction (GID) to obtain information about the in-plane order. At the moment there is no experimental evidence supporting in-plane order at the free surface of a pure liquid metal. Furthermore, the Au-Ge eutectic, which has nearly the same phase diagram as Au-Si does not have surface ordering.

At this point in time the reason why some alloys have in-plane surface order and some do not is not explained.b>

Experimental


A sketch of the experimental set-up with a few explanatory notes can be viewed here.
In order to investigate the surface of liquid metals, several experimental challenges have to be taken into consideration :

 

  • Structural features of the surface occur on the length scale of a few Å. This implies that the measurements have to be taken out to large values of the momentum transfer, qz, the difference between incoming and scattered wavevector. Since the reflectivity R falls of drastically with q z (for a flat surface with q z4), high flux synchrotron x-rays have to be used whenever high q z's (corresponding to small length scales) are probed.
  • At large q z's, the directional scattering from the surface can often be obscured by diffuse scattering from the underlying bulk phase and special background subtraction techniques have to be developed to separate the interesting surface signal from the bulk diffuse scattering.
  • Thermally excited capillary waves roughen the surface and thus obscure the microscopic structure at the surface. This effect is the more pronounced the higher the temperature and the lower the surface tension. We developed techniques to retrieve the intrinsic density profile (not broadened by surface waves) from the experimentally determined reflectivity for metals with a relatively low melting point such as Hg, Ga or In. For liquid metals at high temperatures and/or comparatively low surface tensions it is not clear if these same intrinsic features of the electron density profile will be observable. We are currently studying the free surface of alkali metals, which do have low surface tensions, in order to explore this question.
  • If very thin wetting layers of liquid metals could be formed, the capillary wave roughness would be suppressed and layering could even be studied at high temperatures. We have some ideas how this might be done and we hope to start such experiments in the near future.

 

 

Apart from those more principal considerations, the following points are of importance for our particular experiments on liquid metals and alloys:

  • the surface of the liquid metal must be atomically clean. In the case of liquid mercury, the purest metal available, to some degree oxide contamination can be prevented by purging a vacuum tight cell with reducing hydrogen gas; however, for our final experiments the liquid was drawn into a preevacuated UHV chamber from the interior of a bulk liquids., In and Bi were investigated under UHV conditions and oxide monolayers are effectively removed by sputtering with an ion gun.
  • mechanically induced vibrations have a strong influence on the qz range available for measurements and the use of an active vibration isolation table effectively suppresses any outside vibrations.
  • because of the high surface tension mentioned above, liquid metals do not wet the substrate and their surface is curved. In the case of small samples special techniques for taking and analyzing the data have been developed.

 

Liquid Metals

Apart from stimulating experimental challenges, the liquid metals project offers insight into different topics lying far beyond the scope of simple structure examination:

  • layering has been observed so far only for those few liquid metals for which the melting temperature is low enough the Debye-Waller factor due to thermal capillary waves does not render their observation impossible. The electron density profiles are considerably different for Ga, In and Hg. For example, differences in the surface structure of liquid In and liquid Ga can be explained by the fact that Ga displays directional bonding in the bulk liquid whereas In behaves more like a nearly-free electron metal in the melt. Also, the statistical properties of the thermally induced surface roughening can be explained with simple capillary wave behavior in the case of liquid Ga and In whereas liquid Hg shows a more complicated variation of the surface structure with temperature. Obviously, the investigation of other liquid metal surfaces is mandatory to obtain detailed insight into the physics of surface ordering.

 

  • the formation of oxide films can be monitored. In the case of Ga, the formation, growth, thickness, stiffness, homogeneity and temperature dependence of a thin oxide layer has been observed. This thin oxide film completely protects the underlying bulk liquid from further oxidation similar to the oxide passivation observed for solid Al exposed to air.

    Liquid In shows a completely different behavior. Here, macroscopic oxide islands form once the liquid is exposed to the same amount of oxygen that created a homogeneous oxide film on liquid Ga. These oxide islands do not wet the LM surface and therefore do not effect the x-ray reflectivity. Upon further exposure to oxygen, the islands eventually grow into the center of the sample and the reflectivity drastically decreases due to the large roughness of the oxidized surface at this point. This oxidation or corrosion pattern ressembles the way solid Fe oxidizes upon exposure to air.
    Generally, the reaction of liquid metals with oxygen (and other gases such as ammonia) is of interest for physics, chemistry and materials science and this topic will pursued in the future

 

  • the formation of Langmuir monolayers can be observed, namely thiols on mercury. So far, monolayers have been investigated either on dielectric liquids such as water or on crystalline metal surfaces such as Au(111). Liquid metals provide an intermediate subphase for monolayers. Interestingly, the chemisorption of a thiol monolayer on liquid Hg does not suppress the surface induced layering in a significant way even though the the Hg-S bond is strong enough to be expected to effect the surface structure.

Liquid Metal Alloys

 

Our most recent experiments exploring how surface induced layering is affected by alloying two liquid metals. Typically, the surface layering is suppressed or destroyed when the two components differ considerably in size. In addition, new surface phases that depend on the type of bulk interactions between the two components can be studied. Examples are:

  • Weak interactions between the two components lead to ideal mixtures or alloys with a eutectic. We have studied the exemplary Ga-In system and found surface segregation of In, the lower surface tension component. In addition, we have proven experimentally that this surface segregation is confined to the very first atomic surface layer.


  • Repulsive interactions result in a miscibility gap. As an example, we studied Ga-Bi alloys for different temperatures. At temperatures below the monotectic temperature of 220 C , the Ga rich fluid coexists with solid Bi. We find the same monolayer segregation described above for the case of Ga-In.[2] Above this monotectic temperature,the Bi monolayer persists; however, a thick Bi rich film forms between the Bi monolayer and the Ga rich fluid. We believe this is the first direct measurment of the microscopic structure of a wetting layer for the surface of any binary LM. The Bi monolayer persists above the critical consolute point at 262 C where the subsurface fluid becomes homogeneous whereas the thick Bi rich film vanishes as expected.

    For more information about GaBi project, please refer to GaBi Poster (large JPEG file).

  • Attractive interactions between the two components lead to the formation of ordered intermetallic phases in the bulk solid phase. One objective is to find out if the pronounced order that prevails in the bulk solid but is not present in the bulk liquid is preserved at the liquid surface.

    In order to answer the latter question, we investigated several Bi-In alloys. Two of them corresponding to stoichiometric intermetallic phases in the bulk solid (InBi and In2Bi) and one with the concentration of the low melting eutectic. In addition to a layering peak found for these alloys, pronounced oscillations are present as well. The period of these oscillations (0.9â-1 corresponding to a structural feature of a length scale of 6Å is suggestive of the presence of Bi-In pairs at the surface of the liquid. Similar competition between surface layering and intermetallic phase formation at the surface has been observed in dilute Hg-Au alloys.

 

 

Partial List of the Earliest Poster Presentations:
More recent presentations are availble from the link in the side-bar to the home page

  1. Lead-Free Solder Binary Alloys: X-Ray Studies of BiSn
    Oleg Shpyrko, Alexei Grigoriev, Diego Pontoni, Peter Pershan, Ben Ocko, Moshe Deutsch, Binhua Lin, Jeff Gebhardt, Timothy Graber, Mati Meron
    The 8th International Conference on Surface X-Ray and Neutron Scattering, Germany 2004;
    APS Users Meeting, ANL, 2004
  2. Surface Oxidation of Liquid Sn
    Alexei Grigoriev, Oleg Shpyrko, Christoph Steimer, Peter Pershan, Ben Ocko, Moshe Deutsch, Binhua Lin, Jeff Gebhardt, Timothy Graber, Mati Meron
    The 8th International Conference on Surface X-Ray and Neutron Scattering, Germany 2004;
    APS Users Meeting, ANL, 2004
  3. Surface Structure Study of Liquid Eutectic Alloys:AuSi and AuGe
    (2004 NSLS Users Meeting Poster Presentation Winner)
    Alexei Grigoriev, Oleg Shpyrko, Christoph Steimer, Peter Pershan, Ben Ocko, Moshe Deutsch, Binhua Lin, Jeff Gebhardt, Timothy Graber, Mati Meron
    NSLS Users Meeting, BNL, May 2004; APS Users Meeting, ANL, May 2004; The 8th International Conference on Surface X-Ray and Neutron Scattering, Germany 2004
  4. Atomic Layering Structure at the Surface of Liquid Sn
    Oleg Shpyrko, Alexey Grigoriev, Christoph Steimer, Peter Pershan, Ben Ocko, Moshe Deutsch, Binhua Lin, Mati Meron, Tim Graber, Jeff Gebhardt
    APS Users Meeting, ANL, May 2004; The 8th International Conference on Surface X-Ray and Neutron Scattering, Germany 2004
  5. Surface studies of water: Are all liquids intrinsically layered?
    Oleg Shpyrko, Masa Fukuto, Peter Pershan, Ben Ocko, Moshe Deutsch, Thomas Gog, Ivan Kuzmenko
    APS Users Meeting, ANL, May 2004; The 8th International Conference on Surface X-Ray and Neutron Scattering, Germany 2004
  6. Tetra Point Wetting at the Free Surface of a Binary Liquid Metal
    Patrick Huber, Oleg Shpyrko, Peter Pershan, Holger Tostmann, Elaine Dimasi, Ben Ocko, Moshe Deutsch
    International TRI/Princeton Workshop on Nanocapillarity, Princetion , NJ, June 2001
  7. Short-Range Wetting Probed at the Free Surface of a Liquid Gallium-Bismuth Alloy
    Patrick Huber, Oleg Shpyrko, Peter Pershan, Holger Tostmann, Elaine DiMasi, Ben Ocko, Moshe Deutsch
    Fifth Liquid Matter Conference, University of Konstanz, Germany, Sep 2002
  8. Breaking the Gibbs Adsorption Rule: Resonant X-ray Reflectivity from a Liquid Bismuth-Indium Alloy
    Elaine DiMasi, Ben Ocko, Holger Tostmann, Oleg Shpyrko, Patrick Huber, Peter Pershan, Moshe Deutsch
    NSLS Research Highlights, Brookhaven National Laboratory


Partial List of the Earliest Publications relevant to the project:
More recent publications are availble from the link in the side-bar to the home page

  1. Surface Layering of Liquids: The Role of Surface Tension
    O. G. Shpyrko, M. Fukuto, P.S. Pershan, B.M. Ocko, M. Deutsch and I. Kuzmenko , to appear, Phys. Rev. B 69 2054XX, (2004), [arXiv.org/cond-mat/0406579] (Abstract) (PDF) (PS)
  2. Surface Oxidation of Liquid Tin
    A. Grigoriev, O. G. Shpyrko, C. Steimer, P.S. Pershan, B.M. Ocko, M. Deutsch, B. Lin, J. Gebhardt, M. Meron, T. Graber, in preparation for Surf. Sci. (2004) (PDF)
  3. Surface structure of Bismuth-Tin Binary Alloy
    O. G. Shpyrko, A. Grigoriev, R. Streitel, D. Pontoni, C. Steimer, P.S. Pershan, B.M. Ocko, M. Deutsch, B. Lin, M. Meron, in preparation (2004) (Poster)(Proposal)
  4. Anomalous Layering at the Liquid Sn Surface
    O. G. Shpyrko, A. Grigoriev, C. Steimer, P.S. Pershan, B.M. Ocko, M. Deutsch, B. Lin, J. Gebhardt, M. Meron, T. Graber, to be submitted in Phys. Rev. B (2004) [arXiv.org/cond-mat/0406583] (Abstract)(PDF)(PS)
  5. X-ray Resonant Studies of Gold-Germanium Binary Alloy
    O. G. Shpyrko, A. Grigoriev, C. Steimer, P.S. Pershan, B.M. Ocko, M. Deutsch, B. Lin, M. Meron, in preparation (2004) (HTML) (HTML) (Poster)
  6. X-ray Studies of Au-Si binary alloy
    A. Grigoriev, O. G. Shpyrko, C. Steimer, P.S. Pershan, B.M. Ocko, M. Deutsch, in preparation (2004) (HTML) (Poster)
  7. Experimental X-ray Studies of Liquid Surfaces
    O. G. Shpyrko, Ph.D. Thesis, Department of Physics, Harvard University (2003) (Abstract) (Abstract) (PDF) (PS) (HTML)
  8. X-ray Study of the Liquid Potassium Surface: Structure and Capillary Wave Excitations
    O. G. Shpyrko, P. Huber, P.S. Pershan, B.M. Ocko, H. Tostmann, A. Grigoriev and M. Deutsch, Phys. Rev. B 67, 115405 (2003), [arXiv.org/cond-mat/0406585] (Abstract) (PDF) (PS)
  9. Short-Range Wetting at Liquid Gallium-Bismuth Alloy Surfaces: X-ray Reflectivity Measurements and Square Gradient Theory P. Huber, O. G. Shpyrko, P.S. Pershan, B.M. Ocko, E. DiMasi, M. Deutsch, Phys. Rev. B 68, 085409 (2003) [arXiv.org/cond-mat/0406661] (Abstract) (PDF) (PS)
  10. Wetting at the Free Surface of a Liquid Gallium-Bismuth Alloy: An X-ray Reflectivity Study Close to the Bulk Monotectic Point P. Huber, O. Shpyrko, P.S. Pershan, E. DiMasi, B.M. Ocko, H. Tostmann and M. Deutsch, Colloids & Surfaces A. 206, 515 (2002). [arXiv.org/cond-mat/0406659](Abstract) (PDF) (PS)
  11. Tetra Point Wetting at the Free Surface of Liquid Ga-Bi P. Huber, O. G. Shpyrko, P.S. Pershan, B.M. Ocko, E. DiMasi, and M. Deutsch, Phys. Rev. Lett. 89, 035502 (2002). (Abstract) (PDF) (PS)
  12. Pairing interactions and Gibbs adsorption at the liquid Bi-In surface: a resonant x-ray reflectivity study
    DiMasi, E., Tostmann, H., Shpyrko, O. G., Huber, P., Ocko, B. M., Pershan, P. S., Deutsch, M. and Berman, L. E.
    Phys. Rev. Lett. 86, 1583 (2001) (Abstract) (PDF) (PS)
  13. Microscopic Structure of the Wetting Film at the Surface of Liquid Ga-Bi Alloys
    H. Tostmann, E. DiMasi, O.G. Shpyrko, P.S. Pershan, B.M. Ocko and M.Deutsch, Phys. Rev. Lett. 84 (2000) 4385. (Abstract) (PDF) (PS)
  14. Tostmann, H., DiMasi, E., Pershan, P. S., Ocko, B. M., Shpyrko, O. G. and Deutsch, M. (2000) Microscopic surface structure of liquid alkali metals. Phys. Rev. B 61.pdf
  15. Pershan, P. S. (2000) Effects of Thermal Roughness on X-ray Studies of Liquid Surfaces. Colloids & Surfaces A 171, 149-157.pdf
  16. DiMasi, E., Tostmann, H., Shpyrko, O. G., Deutsch, M., Pershan, P. S. and Ocko, B. M. (2000) Surface induced order in liquid metals and binary alloys. Journal of Physics: Condensed Matter 12, 209-214.pdf
  17. DiMasi, E., Tostmann, H., Ocko, B. M., Huber, P., Shpyrko, O. G., Pershan, P. S., Deutsch, M. and Berman, L. E. (2000) Resonant X-Ray Scattering From The Liquid Hg-Au Surface. In: Materials Research Society, Symposium AA: Applications of Synchrotron Radiation Techniques to Materials Science V Vol. 590. Eds. Mini, Perry and Stock. Materials Research Society.pdf
  18. Tostmann, H., DiMasi, E., Pershan, P. S., Ocko, B. M., Shpyrko, O. G. and Deutsch, M. (1999) Surface Structure of Liquid Metals and the Effect of Capillary Waves: X-Ray Studies on Liquid Indium. Phys. Rev. B 59, 783.pdf
  19. Tostmann, H., DiMasi, E., Ocko, B. M., Deutsch, M. and Pershan, P. S. (1999) X-ray Studies of Liquid Metal Surfaces. J. Non-Cryst. Solids 250-252, 182-190.
  20. Pershan, P. S. (1999) X-ray Scattering From Liquid Surfaces: Effects of Thermal Capillary Waves. Synchrotron Radiation News 12, 10.
  21. DiMasi, E., Tostmann, H., Ocko, B. M., Pershan, P. S. and Deutsch, M. (1999) Competition between surface layering and surface phase formation in dilute liquid HgAu alloys. J. Phys. Chem. B 103, 9952-9959. pdf
  22. Tostmann, H., DiMasi, E., Pershan, P. S., Ocko, B. M., Shpyrko, O. G. and Deutsch, M. (1998) Surface Phases in Binary Liquid Metal Alloys: An X-ray Study. Berichte der Bunsen-Gesellschaft 102, 1136-1141.
  23. DiMasi, E., Tostmann, H., Ocko, B. M., Pershan, P. S. and Deutsch, M. (1998) X-ray reflectivity study of temperature dependent surface layering in liquid Hg. Phys. Rev. B 58, 13419. pdf
  24. Regan, M. J., Tostmann, H. C., Pershan, P. S., Magnussen, O. M., DiMasi, E., Ocko, B. M. and Deutsch, M. (1997) Oxidation of Liquid Gallium Surfaces: X-ray Reflectivity Study. Phys. Rev. B 55, 10786-10790. pdf
  25. Regan, M. J., Pershan, P. S., Magnussen, O. M., Ocko, B. M., Deutsch, M. and Berman, L. E. (1997) X-ray Reflectivity Studies of Liquid Metal and Alloy Surfaces. Phys. Rev. B 55, 15874. pdf
  26. Deutsch, M., Magnussen, O. M., Ocko, B. M., Regan, M. J. and Pershan, P. S. (1997) The structure of alkanethiol films on liquid mercury: An x-ray study. In: Thin Films:Self Assembled Monolayers of Thiols. Ed. A. Ulman. Academic Press.
  27. Regan, M. J., Pershan, P. S., Magnussen, O. M., Ocko, B. M., Deutsch, M. and Berman, L. E. (1996) Capillary wave roughening of surface induced layering in liquid gallium. Phys. Rev. B. 54, 9730. pdf
  28. Regan, M. J., Magnussen, O. M., Kawamoto, E. H., Pershan, P. S., Ocko, B. M., Maskil, N., Deutsch, M., Lee, S., Penanen, K. and Berman, L. E. (1996) X-ray studies of atomic layering at liquid metal surfaces. J. Non-Cryst. Solids 207, 762-766.
  29. Magnussen, O. M., Regan, M. J., Kawamoto, E. H., Ocko, B. M., Pershan, P. S., Maskil, N., Deutsch, M., Lee, S., Penanen, K. and Berman, L. E. (1996) X-ray Reflectivity Studies of the Surface Structure of Liquid Metals, Proceedings of the Fourth International Conference on X-ray and Neutron Scattering. Physica B 221, 257-260.
  30. Magnussen, O. M., Ocko, B. M., Deutsch, M., Regan, M. J., Pershan, P. S., Abernathy, D., Grübel, G. and Legrand, J. F. (1996) Organic layers on liquid metals: An X-ray reflectivity study of thiols on mercury. Nature 384, 250-252.
  31. Regan, M. J., Kawamoto, E. H., Lee, S., Pershan, P. S., Maskil, N., Deutsch, M., Magnussen, O. M., Ocko, B. M. and Berman, L. E. (1995) Surface layering in liquid gallium: x-ray reflectivity study. Phys. Rev. Lett. 75, 2498. pdf
  32. Magnussen, O. M., Ocko, B. M., Regan, M. J., Penanen, K., Pershan, P. S. and Deutsch, M. (1995) X-ray reflectivity measurements of surface layering in liquid mercury. Phys. Rev. Lett. 74, 4444. pdf
  33. Kawamoto, E. H., Lee, S., Pershan, P. S., Deutsch, M., Maskil, N. and Ocko, B. M. (1993) X-ray scattering study of the surface of liquid gallium. Phys. Rev. B. 47, 6847.


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