Transcript Liquid Metal Surfaces P. S. Pershan SEAS & Dept of Physics

Liquid Metal Surfaces
P. S. Pershan
SEAS & Dept of Physics, Harvard Univ., Cambridge, MA,
USA
• Colleagues
Balagurusamy, V. S. K.
Berman, E.
Deutsch, M.
DiMasi, E.
Fukuto, M.
Gebhardt , J.
Gog, T.
Graber, T.
Grigoriev, A.
Huber, P.
Kawamoto, E. H.
Kuzmenko, I.
Lin, B. H.
Magnussen, O. M.
Mechler, S.
Meron, M.
Ocko, B. M.
Pontoni, D.
Harvard, Non-Harvard, Beam Line
Pershan/ESRF
Regan, M. J.
Sellner, S.
Shpyrko, O. G.
Steimer, C.
Stoltz, S.
Streitel, R.
Tostmann, H.
Yahel, E
1st Synchrotron Studies: Liquid Crystal Surfaces
Idea!
Als-Nielsen, Christensen, Pershan,(1982)
Tilt Monochromator to Steer beam downward by a
Horizontal liquid surface.
Liquid Crystal:
Isotropic/Nematic/Smectic-A
Surface Induced
Smectic
Reflectivity
Reflectivity
z
Pershan/ESRF
Normalized
Kinematics & Reflectivity: Flat Surface
Reflectivity:Flat Surface a   r
ki  2 xˆ cosa  zˆ sina
z
r
d Q
r
d Q
dQxy2
dQxy2
Qxy  0
   
x
Fresnel
Resolution:
QxySlit  k 2 sin  
Q
Electron Density
(Liq. Xtal)
Slit
xy
Reflectivity 
2  0
&
  
r
Q
2
r
 Qxy
2
F
r
r
Not
True
for
d
Q

(
Q
) Liquids
1

Qxy 0
Slit
AQxy
2
2
xy
xy
Reflectivity R Qz   RF Qz   Qz 
2
Fresnel
RF Qz   Qc 2Qz 
4
Structure Factor
 Qz 
   z   iQz z
1
dz
e


z
R Qz  RF Qz    Qz 
Temperature Dependence
of Liq. Xtal Surface.
Pershan/ESRF
2
No Layering for Water and Simple Liquids
Liquid Crystal
A. Braslau et al. PRL (1985).
l
u<l
u≥a
RF Qz  Layer
Surface
Roughness
Molecular Simulations
Chapela et al. (1977)
Pershan/ESRF
u
Simple Liquid
a
Surface Defines a Layer
Surface Does Not
Define a
Hard WallLayer
Free Surface ✕
Layers
Free Surface of Liquid Metal: Hard Wall
Metallic Liquids (D’Evelyn & Rice ‘83)
Liquid:
Positive Ions in
Sea of Negative
Fermi Liquid
Interface
Vapor: Neutral Atoms
Suppression of
Local Fluctuations
Local Hard Wall.

Layers
Hg
Ga
Hg. Magnussen et al. (1995).
Ga Regan et al.(1995).
Goal: Measure Electron/Atom Density Profile!
Pershan/ESRF
In
Capillary Waves & Thermal Roughness
Rough Phase Shift
r
r
 rxy  Qz u rxy 
r
d 2 Q
r
d Q
dQxy2
dQxy2
 ~  
Flat surface:
(Qz<<1)

2D Liquid Surface

r
r
r
iQz • rxy
2
2
d rxy e
~  Qxy
 
Sinha et al.’88
r
  
iQz u(rrxy )u(0)
r
i
Q
2
xy • rxy
d rxy e
e
~  2 
 Qxy 
r
r
r d Q
d Q
Qxy 0
2
AQxySlit d Qxy dQxy2 ~ dQxy2
2
Pershan/ESRF


r
Q
 
F
Signal
F
2
2
r
r
r
iQxy • rxy iQz u(rxy )u(0)
2
d rxy e
e
r
r
d Q
d Q
Qxy0 2 r
 AQxySlit d Qxy dQ2 ~ dQ2
xy
xy
2


r
Q

F
2
u rrxy  u 0 ~ kBT ln(rxyQxymax )
2

  CW , Q 
r
Q
AQxySlit  k 2 sin  
Slit
xy
k BT 2
Qz
2

~ Q
Slit
xy
max
xy
Q


2
Capillary Effects: H20 & Ga
Water (Schwartz ’90):
log  R Q  R Q   Log  exp Q  
z
F
2
 cap

2
z
z
k BT
2

Slit
Qz2 ln Qmax Qxy
2
cap

Slits
5.0 mm
2.0 mm
0.8 mm
(Qz) for Liquid Ga (Regan, ’96)
R Qz  RF Qz CW (,QxySlit )   Qz 
2
R Qz  RF Qz    Qz  CW ,Q
2
k BT 2

Qz
2
 ~ (0.5to0.9)
Pershan/ESRF
Slit
xy

 Eff z,T  / 
 z / 
Diffuse Scattering  Surface Tension()
k BT 2

Qz
2
r
d Q
2
r
d Q
 ~  
2
xy
dQ
2
xy
r
Q

dQ
F
2
1
Qxy
2
Qxy  Qxy
Compare Ga/In
Hg
Ga
In
Solid Line
No Adjustable
Parameters
Pershan/ESRF
Diffuse Scattering for In Compare (z)
a
In Ga
Simplest Surface Structure Model
DCM (Magnussen ’95)

 z 
d
2

exp   z  nd  / 2 n2 
  n0  n 2
 n2   02  n 2

  z 
1
 Qz  
dz
exp iQz z 

   
z

 Qz d  exp iQz dj  exp   n2Qz2 / 2 
n0
exp   02Qz2 / 2 
 Qz d
1  exp iQz d  exp   2Qz2 
Pershan/ESRF
Elemental Liquid Metals Studied
K
Ga
In
Sn
Bi
Hg
DCM
DCM
DCM
+1
+1
?


☐

☐
R Qz 

RF Qz CW , QxySlit
Measureable
Difference in 1st
Layer
Pershan/ESRF

r
  Q

2
•Why are 1st Layers
for Bi and Sn
different from K, Ga
and In?
•Why is Hg different
from all others?
Eutectic Alloys
J. W. Gibbs ~1920
Surface Adsorption: A/B Alloy
If Surface Tension: A > B Surface is Rich in “B”.
AxB1-x
A)/B)
H*
(mixing)
Concentration of Surface Layers
1st
2nd
GaxBi1-x
718/378=1.90
+4
Liquid-Liquid Phase Sep.
Ga83.5In16.5
718/556=1.29
+5
97%In
In78Bi22
556/378=1.47
-1
35%Bi
Sn57Bi43
560/378=1.48
+1
96%Bi
25%Bi
53%Bi
Au71Sn29
1100/560=1.96
-10
96%Sn
<1%Sn
24%Sn
Au72Ge28
1100/621=1.77
-21
No Gibbs Absorption
Au82Si18
1100/865=1.27
-30
4-layers, 2DXtal (AuSi2)
Pd81Ge19
1500/621=2.4
-44
~40 Å wetting layer (No Measureable Gibbs
Absorption)
*(kJ/mol)Takeuchi and Inoue, Mater. Trans. 46 (2005)
Pershan/ESRF
34d
Gibbs Surface Adsorption(BiSn)
Alloy: Bi
and Sn
Bi=378, Sn=560,
Energy Dispersion: f(E)
Adsorption
Scat. Ampl.
9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06).
(Bi)≈ 398
(Sn)≈567 dyne/cm
12
Surface Freezing Au82Si18Eutectic
R/RF × 20 DCM
R/RF
DCM
1st Order
Transition
2D Surface
Crystals:
Grazing Incidence
Diffraction
There is no
theoretical
explanation!
9th Int. Conf on Surf. X-ray and Neutron Scan (Taiwan, Jul.’06).
13
AuGe Eutectic(Should be Similar to Au-Si)
Au-Si
Au-Si
Pershan/ESRF
×0.8
2
11.915 kev
11.05 kev
f`(E) @AuL3-Edge
Au-Ge
(Au)/Si or Ge)
H
Au72Ge28
1100/621=1.77
-21
Au82Si18
1100/865=1.27
-30
1. Bumphigher
density in 1st layer.
2. No Energy effect
 Ge in 1st layer
≤40atm%.
•Small Gibbs (Different
from Au-Sn, etc)!
•No Enhanced Layering or
2D order
(Different from Au-Si)!
AuSiGe-Ternary Eutectic
Ge
Time average
0.8atm%Ge
Eutectic
Line
Au
Si
Surface Frozen Ge≤6.5 atm%
18atm%Si
0% Si
What is the physics of the cross over from Si type to
Ge type surface between 2.5 atm% and 6.5 atm%?
Pershan/ESRF
Pd81Ge19(Dec.’08)
Au82Si18
Pd81Ge19
Glass
former
yes
better
H
-30
-44
Expected same 2D surface order for Pd81Ge19 as Au82Si18!
Not found; however, something new! Metallic Clusters (Giant Unit Cells)
Small angle oscillations!
~4%
Mg32(Al,Zn)49
14nm
/
Preliminary fit.
Ref: Urban &Feuerbacher,
J.Non-Crys.Sol.(04)
Quenched Icosahedral Clusters
Others: NaCd2
YbCu4.5
Al3Mg2
Pershan/ESRF
30Å
44-49Å
28Å
Summary
•
Metal/Vapor InterfaceAtomic Layering:
•
Surface Structure Factor - (Qz): Measurement affected by thermal
roughness. Requires knowledge of surface tension.
•
Surface tension: measured with diffuse scattering:
•
Surface tension effect demonstrated for Ga/In
•
Subtle differences in elemental surfaces (Ga, In, K vs. Sn, Bi vs Hg)
•
Alloys: Surface tension vs. Enthalpy of Mixing
Gibbs absorption is not simple. No reliable theory.
•
Au82Si18 anomalously strong layering and 2D order.
Why are Au82Si18, Au72Ge28 and Pd81Ge19 all different?
•
Need for THEORY!
•
New Result (Preliminary):
Surfaces & Icosahedral Metallic Clusters
Pershan/ESRF