Organic Molecules on Insulating Surfaces Organic Molecules on Insulating Surfaces Investigated by NC-AFMInvestigated by NC-AFM
February 26February 26thth, 200, 20044
MPI DresdenMPI Dresden, , GermanyGermany
Enrico GneccoEnrico Gnecco
Institute of Physics Institute of Physics University of Basel, SwitzerlandUniversity of Basel, Switzerland
Motivations
metallicsubstrate
electrodes
moleculeI
Insulating surfaces are potentially good candidates
Disadvantage:
• Spacers adaptation to the substrate changes in the electronic properties
The circuit architecture still remains a problem !
Advantage:
• Insulating spacers (porphyrins, landers)
Chemistry is important!
UHV atomic force microscope
• Surface preparation in vacuum
• Light-beam adjusted by motorized mirrors
L. Howald et al., APL 63 (1993) 117
Observing organic molecules with AFM:intrinsic problems
• The vertical resolution is ~ the same but...
• Long range contribution is detrimental for lateral resolution
• The tip sharpness is critical
NC-AFMSTM
• Cu-tetra porphyrin (Cu-TBPP) on Cu(100):
Observing organic molecules with AFM:intrinsic problems
different interaction potentials
different set points !
Despite the problems...
• Energetics of a single molecule can be studied:
• Comparing force-distance curves:
(i) on the molecule legs and
(ii) on the substrate:
Ch. Loppacher et al., Phys. Rev. Lett. 90, 066107 (2003)
Switching energy: W ~ 0.3 eV
Switching to insulators...
• “Atomic” resolution on KBr(100):
5 nm
a = 0.66 nmb = 0.47 nm
• Stable nanopatterns can be created:
E. Gnecco et al., Phys. Rev. Lett. 88, 215501 (2002)
50 nm
Trapping the molecules...
Step height: 0.35 nm
• How to reduce the mobility of the molecules?
• Heating at 380 °C Spiral pattern
K. Yamamoto et al., J. Cryst. Growth 94 (1989) 629
Cu-TBPP on KBr(100)
• The steps are decorated by “molecular wires”
The mobility of the molecules is still high
0 100 200 300
-2
-1
0
1
hei
gh
t (n
m)
distance (nm)
~ 1.5 nm
~ 3.3 nm
• Multi-layered structures
• No evidence of internal structures
• ½ ML on KBr(100) at room temperature:
L. Nony et al., Nanotechnology 15 (2004) 591
Lowering the mobility...
• KBr(100) irradiated with 1 kV e at 120 °C:
• Rectangular holes (~10 nm wide)
Holes as molecular traps?
R. Bennewitz et al., Surf. Sci. 474, L197 (2001)
• Mono-layer depth (0.33 nm)
“Legless” molecules in the holes
• The holes are empty or (partially) filled
• Perylene tetracarboxylic dianhydride (PTCDA):
topography
• No resolution of single molecules
damping
140 nm
4.55
4.6
4.65
4.7
4.75
4.8
4.85
4.9
4.95
0 20 40 60 80 100 120 140
Da
mp
ing
(eV
/cyc
le)
Cross section (nm)
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0 20 40 60 80 100 120 140
He
igh
t (
nm
)
Cross section (nm)
Towards polar molecules...
• Three fold symmetry
• Charge of the chlorine: 0.42 e
• Molecules with large dipole moment: Sub-phtalocyanine (SubPc)
S. Berner et al., Phys. Rev. B 68 (2003) 115410
d = 4.8 debye
SubPC molecules on e-irradiated KBr
• 1 ML on KBr(100) at 80 °C:
• Single molecules are resolved !
L. Nony, E. Gnecco, R. Bennewitz, A. Baratoff, and E. Meyer et al., in preparation
18 nm
Molecular confinement
• Height of the islands ~ 0.6 nm (+ hole depth = 1 nm)
• Some details:
• Along some edges the molecules are mismatched
• The molecules are aligned in rows oriented 45°
1.4 nm
Matching the substrate...
Potential arrangement of the molecules :
• Apparent size ~1 nm
• Alignment along the [110] axis
• Regular rows: 3b ~ 1.4 nm
• Distance between molecules in a row: 2b ~ 0.95 nm
Understanding the trapping mechanism
• Electrostatic field inside a hole:
• A dipole d ~ 1 debye can be trapped at the corner site!(U = d·E ~ 8 kBT)
Interpretation
• Both interactions are > kBT molecular confinement
• Expected arrangement of the molecules:
• Dipole-dipole interaction ~ Dipole-substrate interaction
• The sign of the corner site selects the growth direction
• Mismatch at edges due to 3-fold symmetry
Empty vs filled holes
• On larger scale...
• Only the holes < 15 nm in size are filled !
150 nm
Conclusions
• Holes created by e irradiation on KBr act as molecular traps
• Single organic molecules on insulators have been resolved by AFM
• The size of the holes is critical
Outlook
• Molecules with 4-fold symmetry
• How to contact electrodes?
• Theory of molecular confinement?
AcknowledgmentsUNI Basel
Ernst Meyer
Christoph Gerber
Laurent Nony
Alexis Baratoff
Roland Bennewitz (*)
Oliver Pfeiffer
Thomas Young
University of Tokyo
T. Eguchi
CNRS Toulouse
A. Gourdon
C. Joachim
This work was supported by
• The Swiss National Science Fundation
• The Swiss National Center of Competence in Research on Nanoscale Science
(*) Now at McGill University, Montreal