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Bridging the gap between experiments and theory
- studies of single crystal surfaces and interfaces -
Guido Grundmeier
O. Ozcan, B. Torun, C. Kunze
Institute for Polymer Materials and Processes
Technical and Macromolecular Chemistry
University Paderborn, Germany
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How to understand molecular polymer/oxide adhesion?
Bridging experiment and theory
Preparation of
model surfaces
(1) Single crystalline!
(2) Large atomically flat
plateaus!
(3) Stable in electrolytes!
Probing surfaces with
single molecules
(1) Desorption of
single molecules
by means of AFM!
(2) Does it work
on oxides?
Theoretical description
of the interfaces
Adsorption
of monomolecular
films
(1) Interface Microscopy/
Spectroscopy!
(2) Interfacial Binding?
(2) Orientation?
3 3
Bridging Experiment and Theory in Adhesion Science Model Substrates
Al2O3
ZnO
MgO
Surf
ace
oxi
de
s o
f co
mm
on
en
gin
ee
rin
g al
loys
Surfaces via DFT
Al2O3
ZnO
MgO
DFT Studies of Adsorption on Selected Surfaces
X / Ang.
Y /
Ang.
33.60
34.40
35.20
36.00
36.80
37.60
38.40
39.20
40.00
40.80
41.60
42.40
43.20
44.00
44.80
Adsorption of organosilanes on ZnO
Adsorption of organophosphonates on Al2O3
Stability
against water Binding Mechanisms
mono-dental
bi-dental
tri-dental
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Substrate Structure
• Ideal crystal
• 1 atom per unit cell
• Cutting along a plane does not change the positions
Ideal surface structure: „Substrate structure“
Bulk crystal structure + rel. orientation of the cutting plane
Miller notation:
• Intersections: cutting plane with crystal axes
in units of lattice constants
• take invers values
• multiply values with factor to achieve
smallest triple of integer values
• negative number:
• notation {hkl} specifies (hkl)-planes
n
Coordinates (3,1,2)
Inverse (1/3, 1, 1/2)
Miller indices (263)
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Preparation of ZnO(0001)-Zn surfaces
M. Valtiner, S. Borodin, G. Grundmeier,
PCCP Vol. 9, 2406-2412 (2007)
Zn
1/2 e-
At the surface truncation: 1/2 electron per Zn left!
E.g. an OH ad-group can accept these electrons
• Each OH can accept 1 electron to reach the full
valency 1 OH can accept one electron from
2 Zn-atoms.
• An ad-atom structure with 1/2-ML of OH can be
expected to be stable.
E.g. a 2x1 OH ad-group structure.
e.g. transfer to adlayer atoms
1/2 ML OH
H
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Preparation of ZnO(0001)-Zn surfaces
Preparation conditions:
(1) Alkaline etching
(2) Rinsing with water
(3) Dry in stream of nitrogen
(4) Thermal treatment in
humidified oxygen atmosphere
M. Valtiner, S. Borodin, G. Grundmeier,
PCCP Vol. 9, 2406-2412 (2007)
A: Dry O2/ 1223K
B: 4% H2O in O2/ 1223K
63eV
w3w3
B
A
M. Valtiner,M. Todorova, G. Grundmeier,
J. Neugebauer, PRL 2009
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Phase diagram of ZnO(0001) surface was calculated for different H and O chemical potentials and the structures favored at different pH values were determined.
M. Valtiner, M. Todorova, G. Grundmeier, J. Neugebauer, Physical Review Letters 103, 6, (2009) Diebold U., Koplitz L.V., Dulub O., App. Surf. Sci. 237 (2004)
UHV
Preparation
Preparation
under atmospherical
conditions
Stabilisation of ZnO (0001) surface
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Surface spectroscopy of ZnO(0001)-Zn (1/2 OH)
surface after heat treatment
M. Valtiner, S. Borodin, G. Grundmeier,
PCCP Vol. 9, 2406-2412 (2007)
AR-XPS
90°
10°
OH- OH-
Surface is obviously OH covered and thereby stabilized
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Characterization of the interface: PZC
ZnO(0001)-Zn-OH
Charged surface/ interface
At pH > 6.5
carboxylic acid
entirely de-protonated
Tip.
Au-S-(CH2)14-COOH (SAM)
Electrolyte.
1mM NaClO4
pH adjustment with 1mM NaOH/ HClO4 -
-> constant ionic strengh
PZC. pH 8.7±0.2
(O/OH coverage = √3√3
Structure)
Negatively charged above
PZC.
(Adsorption of OH-)
Positively charged below
PZC.
(Adsorption of H+ onto
surface)
FD-approach curves
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XPS investigations on TiO2 powder samples
detector
In
monochromator analyzer
TiO2 particles
Indium foil
X-ray source
chemical composition of particles?
oxidation state of Ti?
immobilization of nanoparticles (d = 110 nm)
on an Indium foil
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XPS investigations on TiO2 powder samples
O K
LL
Ti
LM
M
Ti
LM
M1
Ti2
s O
1s
Ti2
p
C1
s
Ti3
s Ti3
p
O2
s
survey spectrum: XPS- and Auger-Peaks
12 12
C-C
C-O
x 102
2
4
6
8
10
12
CP
S
291 288 285 282Binding Energy (eV)
O
OH
x 102
20
30
40
50
60
70
80
CP
S
534 531 528 525Binding Energy (eV)
Ti2p3/2
Ti2p1/2
x 102
10
20
30
40
50
60
CP
S
468 464 460 456 452Binding Energy (eV)
Ti2p O1s
13.3 at% 37.6 at%
C1s
peak area of 1:2
is fixed by spin-
orbit coupling!
1.1 at%
30,1 at% 17.9 at%
XPS investigations on TiO2 powder samples quantification of sample composition via detail spectra
oxidation state of Ti is Ti4+ as followed by the Ti2p doublet
significant amount of carbon: residues of particle synthesis and/or contaminations
oxides and hydroxides, in total excess in oxygen compared to the 1:2 stochiometry of TiO2
no information about TiO2 modification (Anatase vs. Rutile vs. amorphous) in XPS
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Preparation of TiO2 single crystalline rutile surfaces
single crystalline surfaces are well suited for basic research
BUT: untreated TiO2 single crystal with poorly defined surface structure
Preparation:
etching for 10 to 20 min in 20 % HF
annealing for 24 to 36 h at 800 °C in air
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well defined surfaces for (110) and (100) orientation
step height of 0.3 nm corresponds to the rutile unit cell (c = 0.296 nm)
Preparation of TiO2 single crystalline rutile surfaces
TiO2 (100) unpolar TiO2 (110) polar
Requirement of
nanosized tip to
measure
adhesion forces
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LEED
P. Thissen, M. Valtiner, G. Grundmeier, Langmuir, 2010
AFM
Molecular Understanding of SAMs on Al2O3(0001)
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P. Thissen, M. Valtiner, G. Grundmeier, Langmuir, 2010
AFM
Molecular Understanding of SAMs on Al2O3(0001)
536 534 532 530 528
O2-OH
-
d)
c)
b)
a)
BE / eV
O2- OH-
17 17 P. Thissen, M. Valtiner, G. Grundmeier, Langmuir, 2010
CH3
(CH2)17
P
O O-
O-
Al
AlOOH
Adsorbate formation on Oxide Surfaces: ODPA on Al2O3
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Comparison of FTIR-data on different surfaces
Al/AlOOH
Al/Al2O3-OH (PVD)
Al2O3 (0001)
Al2O3 (1-102)
-2OORP = O
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In-situ AFM of Monolayer Desorption on Al2O3(0001)
P. Thissen, M. Valtiner,
G. Grundmeier, Langmuir, 2010
11 m
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Model of Interface Binding of ODPA on Al2O3(0001)
P. Thissen, M. Valtiner,
G. Grundmeier, Langmuir, 2010
Top and side view on the atomic
structure of the Al2O3(0001)
and Al2O3(1-102) surface.
Surface terminating Al-ions vary in the
distance to the next neighbor.
The stability of the resulting bonding is
based on three competing influences:
• interfacial bonding types,
• adsorption free energies in
competition with water and
• involved adsorption geometries.
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Nanoshaving: Principle • During conventional AFM imaging, applied forces on the
surface remains less than a Nanonewton (nN). Surface
remains unchanged.
• Nanoshaving: higher loads are applied in a controlled
manner in order to remove material.
Continuous PET Film
200 nm
Nanoshaved area.
• Thickness of the film
can be determined.
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Nanoshaving: Limitations
• “Scratching” a hard surface with a “soft” material is not
possible.
• The tip material (alternatively tip coating) has to be
selected carefully.
• Applied load has to be correctly adjusted.
Healthy tip. Worn Si tip used for
nanoshaving on Al2O3
Solution: Harder materials (e.g. Silicon-nitride) or anti-wear coatings (e.g. diamond like carbon.)
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Nanografting: Principle • If nanoshaving is performed in presence of a precursor in
liquid medium, adsorption takes place on the available
sites. Hence, nanografting takes place.
• Al2O3 (0001)-single crystal surface is covered with a shorter molecule
(Hexyl-phosphonic acid, HPA, 6 carbon atoms in the chain).
• Precursor solution contains longer molecules. (Octadecyl phosphonic
acid with 17-Carbon atoms.)
• Positive height difference is observed. No termination difference,
therefore no friction contrast.
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Molecular de-adhesion on terraces and step edges
30nm
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Desorbing Single Macromolecules - Theory
H E Gaub et al, J. Phys.: Condens. Matter 16 (2004) S2369–S2382
Rupturing of one specific bond
coupled to a polymer-spacer.
Rupturing of numerous bonds
in series.
Rupturing of numerous bonds
in series in an equilibrium process. Height of desorption plateau reflects the equilibrium desorption force required to peel the polymer off the surface segment by segment.
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Single Molecule Desorption Spectroscopy on ZnO(0001)
M. Valtiner, G. Grundmeier, Langmuir, 2010
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Single Molecule Desorption Spectroscopy
At edges:
-COO- coordinatively binds to Zn surface atoms
M. Valtiner, G. Grundmeier, Langmuir, 2010
28 28
Influence of electrostatics and polymer conformation
pH = 10 pH = 9 pH = 8
pH = 6 pH = 5 pH = 4
PZC
pKa
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Influence of macromolecular conformation
- - - - - - - - - - - - - - - - neg.
- -
- -
- -
-
-
-
+++++++++++++++++ pos.
- -
- -
- -
+++++++++++++++++ pos.
Above PZC
Below PZC
Below pKa
Polymer is negatively charged and due to repulsive intra-chain forces
tertiary structure reflects a „wormlike“ structure.
Surface is negatively charged, therefore REPULSION and consequently very
short desorption plateaus.
Polymer is still negatively charged an tertiary structure reflects a „wormlike“
structure.
Surface is positively charged, therefore attraction and consequently
desorption plateaus.
Polymer is less and less charged due to attractive intra-chain forces (H-
bonding etc.) the tertiary structure reflects a connected internal structure.
Therefore, no adsoprtion on to the substrate in form of long chains. Just
„bulky“ polymer bunch adsorbs.
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pH-dependant forces on the polar ZnO surface
Increasing double layer
repulsion
• Decreasing attractive
electrostatic attraction
• Coil less stretched
A B C D
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PIA – SIS
Paderborn Integrated Analysis – System for Interfaces Science
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Preparation Chamber
techniques for sample preparation
– load lock
• inward transfer of samples
• transfer to preparation chamber or UHV suitcase
– plasma and sputter source for sample preparation
• Ar, He, O2, N2, H2O
– sample heating up to 1100 K for high-temperature preparation
analytical technique: – low energy electron diffraction (LEED)
• Surface crystallinity and adsorbate structures
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Analysis Chamber
Analytical techniques:
– X-Ray Photoelectron Spectroscopy: XPS
• chemical surface composition
• oxidation state
• type of chemical bonds
– Scanning Auger Microscopy: SAM
• high resolution surface composition (150 nm resolution)
– Scanning Electron Microscopy: SEM
• sample imaging (100 nm resolution)
– UV Photoelectron Spectroscopy: UPS
• valence band structure of semiconductors
5-50 nm
Continuum X-rays
X-ray
Fluorescence
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SPM subsystem
Scanning Probe Microscopy: SPM
– decoupled from preparation/analysis system due to noise and vibration reduction
– Atomic Force Microscopy: AFM
• surface imaging up to atomic resolution
• force interaction and adhesion
– Scanning Tunneling Microscopy: STM
• surface imaging up to atomic resolution on
conducting samples
– variable sample temperature
• 120 to 500 K
– gas dosing system
• e.g. in-situ water adsorption experiments
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UHV system setup
Analysis chamber
• XPS
• UPS
• SAM
• ISS
SPM chamber
AFM
STM
Preparation chamber
• LEED
• plasma-/sputter sources
Vacuum suitcase
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Conclusions, Recommendations
The use of single crystalline surfaces enables to measure forces on
atomically defined surface structures
The synthesis of single crystals which are stable under environmental
conditions and their analysis is complex and time consuming
At least two single crystalline surface orientation have to be investigated
to predict the behaviour of amorphous surfaces
Surfaces of single crystals can be calculated concerning their structure and
surface energy
Single crystals thus serve as basis for the combination of theoretical and
experimental approaches