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Langmuir-Blodgett films and chemically reactive Self Assembled Monolayers in Organic Thin film Transistors

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Harald Etschmaier Langmuir-Blodgett films and chemically reactive Self Assembled Monolayers in Organic Thin film Transistors IF Seminar, Graz, 9.1.2008 – PowerPoint PPT presentation

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Title: Langmuir-Blodgett films and chemically reactive Self Assembled Monolayers in Organic Thin film Transistors


1
Langmuir-Blodgett films and chemically reactive
Self Assembled Monolayers in Organic Thin film
Transistors
  • Harald Etschmaier

IF Seminar, Graz, 9.1.2008
2
Outline
  • Part I Langmuir Blodgett films
  • The Method
  • Characterization
  • Modified OTFTs
  • Part II Chemically Reactive SAMs
  • Theory
  • Setup
  • Measurements and results

3
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.
  • Two-dimensional charge transport in
    self-organized, high-mobility conjugated
    polymers, Sirringhaus et al., Nature Vol 401,
    1999
  • Improved organic thin-film transistor
    performance using novel self-assembled
    monolayers, McDowell, Hill, APL 88, 2006

Organic Electronics, Hagen Klauk, Wiley-VCH
Verlag (2006)
4
Idea of the LB-film
  • Principle copied from nature
  • Cell membrane is stabilized by surrounding water

www.molecularexpressions.com
5
The technical approach
  • Apply non-soluble molecules on water surface
  • transfer one by one monolayer to substrate by
    dipping and removing

Binks, B.P. Adv. Colloid Interface Sci., 34
(1991) 343
6
The Langmuir Blodgett Trough
  • Force Sensor is measuring the surface pressure
    (surface tension of pure water surface tension
    of water with floating molecules)

7
The molecule
Pentacosa-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
8
Layer formation
  • Diacetylene is dissolved in chloroform and
    dropped on the surface
  • The barrier compresses the molecules to a solid
    state

Multiple layers
solid
liquid
gaseous
9
Layer transfer
  • wettability of sample changes for each dipping
  • in theory any number of monolayers possible

Adam, G. Läuger, P. Stark, G. Physikalische
Chemie und Biophysik
10
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

11
Recorded area transfer
in
out
in
out
Decrease of surface area gives a good feedback
for layer deposition
12
XRR - Different layer thickness
DI Heinz-Georg Flesch
Some kind of wetting layer is always the same The
Bragg Peak develops with increasing number of LB
layers
13
XRR illuminated and not illuminated
  • 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

DI Heinz-Georg Flesch
14
XRR
  • D-spacing of layers approximately 4.8 nm (one
    repeated unit consists of two LB layers)

15
AFM
Center of sample (LB-film)
DI Heinz-Georg Flesch
Edge of sample (SIOx substrate)
AFM can proof that something was deposited
16
AFM
2nm
1,25nm
DI Heinz-Georg Flesch
AFM data gives no clear result for the layer
thickness
17
Pentacene Deposition
No LB-film
1 monolayer diacetylene
Dr. Anja 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
18
Light Microscope
SiOx
Pentacene on SiOx
Pentacene on SiOx
Pentacene on LB-film
19
FETs
No improvement achieved so far Further
experiments in process!
20
Part II
  • Time resolved measurements of chemically reactive
    SAMs

21
Setup
P3HT
  • Application of CSTS SAM on SiOx substrate
  • Spin coating of P3HT
  • Evaporation of Au contacts
  • Electrical characterization

Pacher et al., submitted
22
Doping and Dedoping mechanism
Pacher et al., submitted
Shifts the threshold voltage by up to 60V to
negative values
23
Creating the gas mixture
Flow meters
Ammonia
Argon
Valves
Measuring cell
24
The measuring cell
Spring contacts Source Drain Gate
Three-way valves
Potassic-bromide windows
25
Results threshold voltage
26
Results threshold voltage
27
Results - mobility
28
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

29
Additional observations
After reaction with ammonia FETs are very
sensitive to light Turn-on-voltage shifts by up
to 70V upon illumination.
30
Additional observations
Measurements influence device characteristics
31
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
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