1 1

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

2 2

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

4 4

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

6 6

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

9 9

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

10 10

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

27 27

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

29 29

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.

30 30

pH-dependant forces on the polar ZnO surface

Increasing double layer

repulsion

• Decreasing attractive

electrostatic attraction

• Coil less stretched

A B C D

31 31

PIA – SIS

Paderborn Integrated Analysis – System for Interfaces Science

32 32

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

33 33

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

34 34

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

35 35

UHV system setup

Analysis chamber

• XPS

• UPS

• SAM

• ISS

SPM chamber

AFM

STM

Preparation chamber

• LEED

• plasma-/sputter sources

Vacuum suitcase

36 36

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