STM and STS
at Solid-Liquid Interfaces
Jürgen P. Rabe
Department of Physics
Humboldt University Berlin
Outline
• Molecular Self-Assembly at Solid-Liquid Interfaces
• STM/STS at Solid-Liquid Interfaces
• Bilayers of Electron Donors and Acceptors
• Charge-Transfer Complexes as Nanoscopic FET-Gates
Molecular Self-Assembly
at Inert Solid-Liquid Interfaces
z
E
¾ Minimize enthalpy (H)
=> Maximum adsorbate density at interface
¾ Maximize entropy (S):
=> Adsorption of large and rigid molecules from solution
(since less translational and conformational entropy is lost)
ª Minimizing free energy (G): 2D cystals of large and rigid molecules
J.P.R, S. Buchholz, Science 253 (1991) 424; Phys. Rev. Lett. 66 (1991) 2096
P. Samori, N. Severin, K. Müllen, J.P.R. Adv. Mater. 12 (2000) 579
STM & STS
at Solid-Liquid Interfaces
feedback
control
piezo tube scanner
solution
tip
adsorbate
substrate
Solution:
Solvent:
Substrate:
Probe:
Molecular Imaging:
Almost saturated
1,2,4 trichlorobenzene (TCB)
Highly Oriented Pyrolytic Graphite
Pt/Ir (80%, 20%) etched tips
It= 0.01 - 1 nA
Ut= 0.1 - 1.2 V
J.P.R, S. Buchholz, Science 253 (1991) 424; Phys. Rev. Lett. 66 (1991) 2096
2D Single Crystal of a Rigid Rod Molecule
on the Basal Plane of Graphite (HOPG)
Unit cell: a=(1.83±0.11) nm
b=(3.42±0.12) nm
a=108 °
P. Samorì, V. Francke, K. Müllen, J.P.R., Chem. Eur. J. 5 (1999) 2312
Polycrystallinity
STM image of PPE trimer
P. Samorí, V. Francke, V. Enkelmann, K. Müllen, J.P.R., Chem. Mater. 15 (2003) 1032
Ostwald Ripening
P. Samorí, V. Francke, V. Enkelmann, K. Müllen, J.P.R., Chem. Mater. 15 (2003) 1032
Alkylated Nanographenes
C 12H25
(C12 -chain)
C 12H25
H 25 C12
H 25C12
C 12H25
H 25C 12
C 12H25
C 12H25
H25 C12
H25C12
C 12H25
H 25C 12
C 12H25
C12 H 25
C 12H25
H25C 12
C 12H25
H 25C 12
H 25C 12
C 12H25
C 12H25
~7.3eV
1
2
2.68eV
2.68 eV
IE~6.9eV
I~9.6eV
I(Graphite)~4.7eV
[
2.34eV
2.34 eV
2.03eV
2.03 eV
„C78“ on HOPG
Long Molecular Axis
Graphite-Axes
(zig-zag)
2nm
STM at constant height
Model for Orientation
S. Ito, P.T. Herwig, T. Böhme, J.P.R., W. Rettig, K. Müllen, J. Am. Chem. Soc. 122 (2000) 7698
I-V Characteristics
Different scans of one junction
Different junctions
I/nA 0.8
I/nA 0.8
0.4
0.4
U/V
0.0
-1.5
0.75
0.0
1.5
-0.75
-1.5
U/V
1.5
0.75
-0.75
-0.4
-0.4
Different tip-sample distances
Normalized tip-sample distances
I/nA 1.2
I/nA 0.8
0.6
0.8
0.4
0.4
0.2
U/V
0.0
-1.5
-0.75
0.75
U/V
0.0
1.5
-1.5
-0.75
-0.2
-0.4
0.75
1.5
Asymmetric I-V Characteristics
Through Single Symmetric Molecules
vacuum
Assumptions
• Resonantly enhanced tunneling
• Fermi-level non-symmetric
in HOMO-LUMO-gap
• Molecules closer to substrate
e-
Result
Enhanced tunneling due to
HOMO of HBC
at negative sample bias
e-
tip
HOPG
positive sample
bias
HOPG
tip
negative sample
bias
I-V Characteristics of
PAHs with Increasing Size
a.u.
2,75
Tunnelcurrent-Ratio at -1.4V:
2,50
2,25
2,00
1,75
1 : 2 : 3 : 5
1,50
1,25
1,00
Each Potential Shift ~0.15V
l
0,75
0,50
0,25
0,00
-0,25
-0,50
-0,75
-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
Substrate-Potential [V]
T. Böhme, C.D. Simpson, K. Müllen, J.P.R., Chem. Eur. J. (in press)
Frontier orbitals of a free C42 (PM3)
T. Schmitz-Hübsch et al. / Surface Science 445 (2000) 358-367
First HBC Adlayer
P. Samorí, N. Severin, C.D. Simpson, K. Müllen, J.P.R., J. Am. Chem. Soc. 124 (2002) 9454
Identical to UHV data (T. Schmitz-Hübsch et al., Surface Science 445 (2000) 358)
First and Second HBC Adlayer
First layer strongly perturbed
Second layer weakly perturbed
C42
3 nm
2.7·107 - 1.7·109 Ω
Tunneling junction impedance
1.1·109 - 8.4·109 Ω
- Perturbation decreases with the distance from the substrate
- Additional effect on electronic decoupling due to change of packing symmetry
P. Samorí, N. Severin, C.D. Simpson, K. Müllen, J.P.R., J. Am. Chem. Soc. 124 (2002) 9454
Donor-Stacks
STM:
Dimers
HBC
I/nA
0.5
U/V
0.0
-1.5
-1.0
-0.5
-0.5
0.5
1.0
1.5
STS:
Inequivalent dimers
-1.0
-1.5
-2.0
F. Jäckel, M.D. Watson, K. Müllen, J.P.R., Phys. Rev. B 73 (2006) 045423
P. Samorí, N. Severin, C.D. Simpson, K. Müllen, J.P.R., J. Am. Chem. Soc. 124 (2002) 9454
Model for HBC Bilayer
B
A
A
• HBCs in each row
electronically equivalent
• HBCs in each dimer
electronically not
equivalent
• off-sets 0.38/0.62 nm
F. Jäckel, M.D. Watson, K. Müllen, J.P.R.,
Phys. Rev. B 73 (2006) 045423
Aviram and Ratner’s 1974-Proposal
for a Molecular Rectifier
Donor-Bridge-Acceptor
no bias
+
+
electrode
electrode
-
electrode
electrode
electrode
-
forward bias
electrode
vacuum
reverse bias
A. Aviram, M.A. Ratner, Chem. Phys. Lett. 29, 1974, 277
Stacks of Donors and Acceptors?
C10H21
O
O
C8H17
HBC
N
N C8H17
O
O
C10H21
Coronene-di-imide (CDI)
zigzag-HBC
K. Müllen et al., MPI-P Mainz
Mixed Donor-Acceptor Layer
a α
b
A
A
Ut=1450 mV
It=140 pA
a=(1.85± 0.15) nm ; b=(1.80± 0.15) nm ; α=(86±5)°
P. Samorí, N. Severin, C.D. Simpson, K. Müllen, J.P.R.,
J. Am. Chem. Soc. 124 9454 (2002)
F. Jäckel
I-Vs through Mixed
Donor-Acceptor Layer
I/nA
0.8
0.4
0.0
-1.5
-1.0
0.5
1.0 U/V 1.5
-0.5
-0.4
Donor
on Donor
-0.8
-1.2
F. Jäckel
-1.6
Acceptor
on Donor
Reverse Orientation of Molecular Diodes?
=> I-Vs through Donors and Acceptors
vacuum
Assumptions
• Resonantly enhanced tunneling
• Fermi-level non-symmetric
in HOMO-LUMO-gap
• Molecules closer to substrate
Result
Enhanced tunneling due to
HOMO of Donor and LUMO of
Acceptor at opposite bias
e-
e-
Donor
Acceptor
tip
HOPG
positive sample
bias
HOPG
tip
negative sample
bias
HBC-Anthraquinone6
R
R
0.8
R = C 12H 25
I/nA
R
R
O
R = (CH 2) 11
R
R
0.4
O
O
a)
0.0
1.0
-1.0
O
U/V
HBCs
alkyl chains
-0.4
AQs
tunneling current in nA
-0.8
2,1
1,4
HBC
0,7
AQ
0,0
alkyl chain
-0,7
-1,5 -1,0 -0,5 0,0
0,5 1,0 1,5
sample bias in V
F. Jäckel, Z. Wang, M.D. Watson, K. Müllen, J.P.R., Chem. Phys. Lett. 387 (2004) 372
Donors and Acceptors
vacuum
0.35
0.97
2.28
4.7
6.15
6.55
8.47
HOPG
DMA
HBC
AQ
Dimethoxyanthracene (DMA) as donor and AQ as acceptor:
known to form CT complexes in the solid state
(E. Ostuni et al., Liquid Crystals 26, 1999, 541)
STM of HBC-Anthraquinone6 + DMA
no DMA
co-adsorbed
5 nm
Us = +1.4 V
It = 300 pA
15 nm
Us = -1.4 V
It = 108 pA
5 nm
Us = -1.2 V
It = 270 pA
Charge Transfer Complexes
F. Jäckel, M.D. Watson, K. Müllen, J.P.R., Phys. Rev. Lett. 92 (2004) 188303
STS of HBC-Anthraquinone6 + DMA
I/nA 0.8
DMA co-adsorbed
DMA not co-adsorbed
0.4
0.0
5 nm
1.0
-1.0
OFF
ON
-0.4
...
AQ-DMA-Complex
HBC
U/V
...
F. Jäckel, M.D. Watson, K. Müllen, J.P.R., Phys. Rev. Lett. 92 (2004) 188303
I-V-Master-Curve
I/nA
0.8
with DMA, normalized
without DMA
0.6
0.4
0.2
0.0
1.0 U/V
-1.0
-0.2
Shift by 120 mV and normalization
F. Jäckel, M.D. Watson, K. Müllen, J.P.R., Phys. Rev. Lett. 92 (2004) 188303
Interface-Dipole-Model
vacuum
-
+
no interface dipole
LUMO
with interface dipole
EF
HOMO
interface normal
Helmholtz equation:
∆φ =
N µe
ε 0ε r
calculations: µ ~ 3 Debye
=> ∆φ ~ 120 meV
Prototypical Single-Molecule Chem-FET
with a Nanometer-Sized Gate
SOURCE
U
DMA
GATE
AQ
CT complex
I
HBC
DRAIN
F. Jäckel, M.D. Watson, K. Müllen, J.P.R., Phys. Rev. Lett. 92 (2004) 188303
Why STM & STS at Solid-Liquid Interfaces?
• General processing scheme for large molecules
• Clean & self-repairing due to strong interfacial forces
• High similarity to UHV results (if available)
• Ambient conditions attractive for applications