Electric field-induced self-assembly
Z. Suo
Division of Engineering and Applied Science
Harvard University
Work with
W. Hong, Harvard University
Y.F. Gao, Brown University
G. Scoles, Princeton University
Forces that move adsorbates
V
++++++++
+++++
−−−−−−−−
−−−−−
Conducting substrate
•Entropy
•Inter-adsorbate attraction (phase coarsening)
•Dipole-dipole repulsion (phase refining)
•Dipole-electrode interaction (guiding force)
Surface potential
φ
+
0 -
+ +
+q
-
- q
-
_ _ _ _ _ _−_σ_ _ _ _ _ _ _ _
φβ
a
substrate
a << h
h
ε0
E=
φα − φβ
h
A
φα
++++++++++++++
+σ
A molecular capacitor:
φ≈
qa
ε
Kelvin method
σ = ε0
φα − φβ
h
1⎞
⎛
∆σ = ε 0 (φα − φβ )∆⎜ ⎟
⎝h⎠
Adsorbates carry electric dipoles
tail group
X
alkyl chain
H
C
H
H
C
H
H
C
H
S
thiol group
CnH2n+1SH
H
X
X
X
X
Evans, Ulman, Chem. Phys. Lett. 170, 462 (1990)
S
S
S
S
gold substrate
Cesium on GaAs
Whitman et al.
Science 251, 1206 (1991)
350 nm
3V for 0.1s
Equation of motion
φ=ζC
- - + + + + - - - -σ
---+++
conductor
Regular solution
∂C M 2 ⎛ ∂g
⎞
2
− 2h∇ C − ζσ ⎟
= 2∇ ⎜
∂t Λ
⎠
⎝ ∂C
g(C) = ΛkT[ClnC+(1−C) ln(1−C) +ΩC(1−C)]
∇2Ψ = 0
Electrostatic B.V.P.
Charge at the surface
Ψ ( x1 , x2 ,0 ) = ϕ ( x1 , x2 ) = ζC ( x1 , x2 )
Ψ is prescribed at electrodes
∂Ψ
σ ( x1 , x2 ) = −ε
, x3 = 0
∂x3
Suo, Gao, Scoles, JAM, in press
mask:
a template
Mobile
molecular
dipoles
-----
Gao & Suo, JAP, 93, 4276-4282 (2003).
-----
+
+ ++ +
+ + ++ + +
-
-
-
-- -
-
--
Pattern on the mask
--
substrate
C0 = 0.4
t = 1τ
t = 10τ
t = 100τ
t = 1000τ
t = 1e4τ
t = 1e5τ
Write one book, print many copies
Field-Directed Assembly
t=0
t = 100
t = 500
t = 1000
t = 5000
t =10000
Re-Configurable Assembly (RCA)
W. Hong
The molecular car
electrodes
Dielectric
Turning
Splitting (or merging)
v = 0.02
v = 0.05
v = 0.1
Speeding
Modular architecture
passenger
receptor
dipole
binder
+
-
pavement
on-chip
infrastructure
Add a dipole to a molecule
p = 0.9 ×10 −29 Cm
H S
H O
S
(CH 2 )n O
S
H
(CH 2 )m H
Evans, Urankar, Ulman, Ferris, J. Am. Chem. Soc., 113 4121 (1991)
Highway-on-a-wall
•Gravity is negligible
•Real estate is expensive
Adsorption and Mobility
Eb ~ 1eV
Em ~ 0.1eV
kT = 0.025eV
Strong adsorption: deep well,
High mobility:
E
b
>>
kT
⎛ Em ⎞
2
E
small m, D ~ ν a exp ⎜ − kT ⎟
⎠
⎝
Barth, Surface Science Report 40, 75 (2000).
The Authority of Electrode
V
U = − pE
R
x
U trap ~ 0.1eV
kT = 0.025eV
( )
p ~ ea ~ (10 −19 C )(10 −10 m ) = 10 −29 Cm
E ~ V / d ~ (1V )/ 10 −9 m = 109 V/m
More numbers
Em ⎞
⎛ Em ⎞ ( 13 )( −10 )2
⎛
2
D ~ νa exp⎜ −
⎟ ~ 10 / s 10 m exp⎜ −
⎟
⎝ kT ⎠
⎝ 0.025eV ⎠
= (10 −7 m 2 /s )exp(− 40 Em )
(
)(
)(
)
f = ∇(p ⋅ E ) ~ p(V / d ) / R ~ 10 −29 Cm 109 V/m / 10 −8 m = 1pN
Em
0.1 eV
D
X = 2 Dt
u=
D
f
kT
0.5 eV
1.0 eV
10 −9 m 2 /s
10 −16 m 2 /s
10 − 25 m 2 /s
10 −5 m
10 −8 m
10 −13 m
10 −1 m/s
10 −8 m/s
10 −17 m/s
Molecular boat?
air
Lipids on air/water interface
Lee, Klingler, McConnell.
Science 263, 655 (1994)
water
Why the molecular car now?
• Scanning probes (imaging, electrode). Tools to
search for the engines, wheels, pavements.
• Nanofabrication (~100 nm in fabs, ~10 nm in labs).
Tools to make on-chip infrastructure.
• Molecular synthesis. Tools to make the car.
• Computation. Tools to design the car and its onchip infrastructure.
Suo, Chen, Maynard, Saif, Sehitoglu
In search of engines, wheels, and pavements
car molecule
pavement molecules
substrate
Scanning probe: an imaging tool and a loading tool
X = 2 Dt
1 2
pE
U = − − αE
2
U ⎞
⎛
Prob ∝ exp⎜ − ⎟
⎝ kT ⎠
What is the molecular car good for?
• Microfluidics, nanofluidics, molecular cars
(ultimate frontier of matter-transport-on-a-chip).
• Drug discovery (combinatorial chemistry).
• Cancer detection (medical diagnostics).
• Proteomics (identity and function).
Summary
• Adsorbates carry electric dipoles.
• Adsorbates move.
• Electric field directs their motion.
• Self-assembly.
• Guided assembly.
• Molecular car.
www.deas.harvard.edu/suo
Publications 130, 140, 150