harald etschmaier

University of Technology Graz - Institute of Solid State Physics

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Harald Etschmaier LB-films and SAMs in [email protected]

Langmuir-Blodgett films and chemically reactive Self Assembled Monolayers in

Organic Thin film Transistors

Harald Etschmaier

IF – Seminar, Graz, 9.1.2008


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Harald Etschmaier Seminar - Graz, 9.1.2008 LB-films and SAMs in [email protected]

Outline

Part I: Langmuir Blodgett films– The Method

– Characterization

– Modified OTFTs

Part II: Chemically Reactive SAMs– Theory

– Setup

– Measurements and results


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Motivation

LB-films form a very regular structure on the substrate and allow a easy control of the surface wettability.

We hope that these properties can yield a better grain formation when growing pentacene on top and hence result in a better mobility in the transistor.

Organic Electronics, Hagen Klauk, Wiley-VCH Verlag (2006)


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Idea of the LB-film

• Principle copied from nature

• Cell membrane is stabilized by surrounding water

www.molecularexpressions.com


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The technical approach

Binks, B.P. Adv. Colloid Interface Sci., 34 (1991) 343

• Apply non-soluble molecules on water surface

• transfer one by one monolayer to substrate by dipping and removing


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The Langmuir Blodgett Trough

• Force Sensor is measuring the surface pressure (=surface tension of pure water – surface tension of water with floating molecules)


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The moleculePentacosa-10,12-diynoic acid

so-called diacetylene

Polymerization upon exposure to UV stabilizes layer

Single Monolayer transistors have been build from these films

Monolayer Transistor Using a Highly Ordered Conjugated Polymer as the Channel, Scott et al., Nano letter Vol.6, 2006


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Layer formation

• Diacetylene is dissolved in chloroform and dropped on the surface

• The barrier compresses the molecules to a solid stateliquid

gaseous

solid

Multiple layers


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Layer transfer

Adam, G.; Läuger, P.; Stark, G.; Physikalische Chemie und Biophysik

• wettability of sample changes for each dipping

• in theory any number of monolayers possible


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Sample preparation

SiOx substrates with different pretreatments:

1. Oxygen plasma etched hydrophilic

2. HDMS layer hydrophobic

3. Perflourinated SAM super hydrophobic

Optional polymerization on water surface or directly on substrate

Cd-Ions in the head group of the diacetylene to stabilize the layer and improve XRR-contrast


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Recorded area transfer

outin

in

out

Decrease of surface area gives a good feedback for layer deposition


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XRR - Different layer thickness

0 3 6

3 LB layers 9 LB layers

Inte

nsity

[a.u

.]

2[deg.] 5.2 5.6


Inte

nsi

ty [a

.u.]

2[deg.]

1.6 2.0


Inte

nsi

ty [a

.u.]

2[deg.]

Some kind of wetting layer is always the same

The Bragg Peak develops with increasing number of LB layers

DI Heinz-Georg Flesch


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XRR – illuminated and not illuminated

0 1 2 3 4 5 6 7

3 LB layers 3 LB layers (no cross-linking) 9 LB layers

Inte

nsi

ty [a

.u.]

2[deg.]

-Smaller and broader Bragg Peak for not polymerized sample (worse multilayer stacking)

-Minimum shifts upon polymerization, but stays the same independent from number of LB layers



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XRR

D-spacing of layers approximately 4.8 nm (one repeated unit consists of two LB layers)


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AFM

Center of sample (LB-film)

Edge of sample (SIOx substrate)

AFM can proof that something was deposited



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AFM

0.0 2.0x10-7 4.0x10-7 6.0x10-7 8.0x10-7 1.0x10-61.0x10-9

1.5x10-9

2.0x10-9

2.5x10-9

3.0x10-9

3.5x10-9

4.0x10-9

he

igh

t

distance

cross section 1

~1,25nm

~2nm

AFM data gives no clear result for the layer thickness



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Pentacene DepositionNo LB-film 1 monolayer diacetylene

Dr. A

nja Haase

•Higher nucleation density, three dimensional grains

•Better homogeneity in depth and a lower density of defects

•Disordered solid made of dendritic grains

•High concentration of defects , mainly located at grain boundaries

Kalb et al. / Synthetic Metals 146 (2004) 279-282


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Light Microscope

Pentacene on LB-film

Pentacene on SiOx

SiOx

Pentacene on SiOx


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FETs

No improvement achieved so far – Further experiments in process!


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Part II

Time resolved measurements of chemically reactive SAMs


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Setup

Application of CSTS SAM on SiOx substrate

Spin coating of P3HT Evaporation of Au

contacts Electrical characterization

e- assisted Pt coating

Au contact

P3HT

CSTS

SiOX

Si

165nm SiOX

CSTS

P3HT

VS VD

VG

AuAu

S

S

R1

R1

P3HT

Pacher et al., submitted


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Doping and Dedoping mechanism

SiOx

SiO O

O

S

OH

OO

SiOx

SiO O

O

S

O-

OO

SS

R

R

n

SS

R

R

nH

SS

R

R

n

SiOx

SiO O

O

S

O-NH4+

OO

Doping Dedoping

NH3

SiOx

SiO O

O

S

Cl

OO

SiOx

SiO O

O

S

Cl

OO

SS

R

R

n

SS

R

R

n

SS

R

R

n

SiOx

SiO O

O

S

NH2

OO

Doping

2 NH3NH4

+Cl-

T-SA

T-SC

Pacher et al., submitted

Shifts the threshold voltage by up to 60V to negative values


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Creating the gas mixture

Argon

AmmoniaFlow meters

Valves

Measuring cell


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The measuring cell

Potassic-bromide windows

Three-way valves

Spring contacts

Source Drain Gate


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Results – threshold voltage


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Results – threshold voltage


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Results - mobility


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Conclusions from preliminary experiments

Ammonia increases hysteresis: Molecules diffuse into bulk and cause additional trap states

Ammonia decreases mobility: Additional scattering centres

The threshold voltage drops irreversibly upon exposure to Ammonia by. Quantity is dependent on time and concentration. further experiments


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Additional observations

After reaction with ammonia FETs are very sensitive to light: Turn-on-voltage shifts by up to 70V upon illumination.


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Additional observations

Measurements influence device characteristics


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Co-Workers

FETs– Peter Pacher, Andrej Golubkov, Egbert Zojer

SAMs– Alexandra Lex, Gregor Trimmel, Christian Slugovc

AFM, XRR– Heinz Georg Flesch, Roland Resel, Anja Haase

LB-films– Martin Weis, Julius Cirak

harald etschmaier

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