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Polymer Photonics Workshop
High Refractive Index Polythiophene for 3-D
Photonic Crystals with Complete Band Gaps
Shi Jin, Matt Graham, Frank W. Harris
and Stephen Z. D. Cheng
Maurice Morton Institute and Department of Polymer Science
The University of Akron
Timothy J. Bunning, Richard A. Vaia and Barry L. Farmer
AFRL Materials and Manufacturing Directorate
Collaborative Center in Polymer Photonics between AFRL
Materials and Manufacturing Directorate
and The University of Akron
Photonics, Photonic Crystal and
Photonic Band Gap
• Photonics: “The technology of generating and
harnessing light and other forms of radiant energy
whose quantum unit is the photon.”1
• Photonic Crystals: (photonic band gap materials), are
materials with periodic variation of refractive index. A
photonic crystal can control the flow of electromagnetic
waves, if its periodicity is comparable to their
wavelengths.
• Photonic band gap: a frequency band in which
electromagnetic waves are forbidden.
1. Photonic Dictionary at www.photonics.com
Applications of Photonics
Fiber optics
Light emitting diodes
Optical switches
Optical amplifiers
Photovoltaics
Applications of Photonic Crystals
Loss-less Mirrors
Waveguides
Photonic Computers
Signal Filters
Thresholdless Lasers
Dimensionality of Photonic Crystals
Periodic in
Periodic in
Periodic in
one dimension two dimensions three dimensions
Different colors represent different refractive indices.
How does the degree of refractive index variation
affect the property of a photonic crystal?
Joannopoulos, D. D. et al. Photonic Crystals, Princeton University, 1995.
One-dimensional Photonic Band GapLayered Dielectric Structure
Assuming n1 > n2 and
n1t1 = n2t2 = /4:
n1
n2


R:
N:
:
:
peak reflectivity in the band gap
multilayer number
wavelength in the center of
photonic band gap
bandwidth of band gap
ni, ti are refractive indices and
thicknesses of corresponding
layers.

4

sin
1
1  n1 / n2
1  n1 / n2
2N
 


n
2
 
1  

  n1  
R
2N 
  n2  
 
1  

  n1  
2
n1/n2 (refractive index
contrast) is important for
both R and !
Yeh, P. Optical Waves in Layered Media, John Wiley & Sons: New York, 1988.
3D Complete Photonic Band Gap
• Complete photonic band gap: a frequency
band in which electromagnetic waves
propagation is forbidden along all directions.
• Complete photonic band gaps can only be
opened up under favorable circumstances:
– Right structures
– Sufficient (threshold) refractive index
contrast
Yablonovitch, E. J. Phys.: Condens. Matter 1993, 5, 2443.
Threshold RI Contrasts for Complete Band
Gaps in 3-D Photonic Crystals
HCP: 3.10
Single Gyroid:
2.28
Inversed Opal:
2.80
Inversed Square
Spiral: 2.20
Diamond:
1.87
Refractive Indices of Materials
Ge (633 nm)
Si (633 nm)
Air
5.5
3.8
1
Polysulfone (589 nm)
Polystyrene (589 nm)
Polypropylene (589 nm)
1.63
1.59
1.51
• 3-D photonic crystals with complete band gaps were
fabricated using Ge, Si (inversed opal).
• These inorganic materials are brittle and difficult to
process.
• Polymers are desired for better physical properties.
• Inorganic nano-particles were incorporated to improve
refractive indices of polymers
• Can we have polymers with high refractive indices?
Refractive Index and Molecular Structure
3
n 2
4 NA
1 

3 Mw
2
•
•
•
n – Refractive Index
NA – Avogadro’s constant
Mw – Molar weight
 – Density
 – Molecular polarizability
Higher   higher n
Higher   higher n
What kinds of polymers are expected to show high
 values?
Yang; C., Jenekhe, S. Chem. Mater. 1995, 7, 1276
Conjugated Polymers:
A Source of Achieving Higher RI Contrast
S
n
polyacetylene
(PA)
n
polyphenylenevinylene
(PPV)
S
polythiophene
(PT)
Conjugated polymers possess higher polarizability than
classical polymers, thus higher refractive indices are
expected.
• They are often referred to as conducting polymers.
• Most of them are semiconductors in pristine state.
• They become conducting upon doping (partial
oxidation/reduction).
• Higher conductivity  better conjugation  higher RI
• Unsubstituted conjugated polymers are preferred over
their functionalized analogues.
n
Predicted Refractive Indices of
Conjugated Polymers
Predicted Refractive Indices
Polymer n700nm n1064nm
n2500nm
trans-PA
2.47
2.44
PPV
2.28
2.04
1.95
PT
3.90
3.04
2.77
According to calculation, polythiophene has
the refractive index comparable to inorganic
materials!
Yang; C., Jenekhe, S. Chem. Mater. 1995, 7, 1276
Refractive Indices:
Calculations versus Experiments
Polymer
npred.
nexp.
trans-PA
2.442.5 m
2.331
PPV
2.28700 nm
2.09633 nm2
PT*
3.9700 nm
1.4633 nm3
However, 6T shows n633nm = 2.154!
What are the problems with electrochemically
synthesized polythiophene films?
*Electrochemically synthesized
1. Yang; C., Jenekhe, S. Chem. Mater. 1995, 7, 1276
2. Burzynski, R.; Prasad, P. N.; Karasz, F. E. Polymer 1990, 31, 627
3. Hamnett, A.; Hillman, A. R. J. Electrochem. Soc. 1988, 135, 2517
4. Yassae, A. et al. J. Appl. Phys. 1992, 72, 15
Why Electrochemical Synthesis?
• Unsubstituted polythiophene is preferred for
maximizing refractive index.
• Most of other methods only can produce
polythiophene powders.
• Advantages of electrochemical synthesis:
• Direct grafting of the doped conducting
polymer films onto the electrode surface
• Easy control of the film thickness by the
deposition charge
Polythiophene Paradox
• Electro-polymerization must begin with the electrooxidation of thiophene monomers;
• The electro-oxidation of thiophene occurs at
potentials higher than 1.6 V vs. SCE in conventional
solvents;
• Over-oxidation of formed polythiophene occurs at
potentials above 1.4 V vs. SCE;
• Polythiophene degrades at potentials that are
required to synthesize it, a paradox.
• Conjugation is rather limited in polythiophene films
electro-synthesized in conventional solvents.
Refractive indices are thus low.
Roncali, J. Chem. Rev. 1992, 92, 711
Lewis Acid-assisted
Low-potential Polymerization
Borontrifluoride
diethyl ether
BF3•Et2O
3 mole/L
AlCl3/CH3CN
CH3CN
Ct = 0.1 mole/L
The oxidation potential of thiophene was lowered to
 1.3 V, degradation of polymer can be avoided!
Proton-free Low-potential
Polymerization of Thiophene
• Elimination of protons
– Protons have a negative impact to the structural
integrity.
– Lewis acid is needed to avoid degradation of formed
polymers.
– A proton scavenger that exclusively reacts with
protons could solve the problem.
2,6-di-tert-butylpyridine (DTBP)
N
Spectroscopic Characterization of
Polythiophene Films
1.0
Without DTBP
0.8
490--501---
With DTBP
0.6
0.4
0.2
3200
3000
-1
Wavenumber (cm )
Amount of saturated units
was greatly reduced.
2800
400 450 500 550 600
Wavelength (nm)
Red-shift of max indicates a
more extended conjugated
structure.
Wide-angle X-ray Scattering of
Polythiophene Films
with DTBP
without DTBP
S
S
S
S
S
S
S
S
0.5 nm
S
0.5 nm
S
S
S
0.35 nm
0.35 nm
=1.512 g cm-3
=1.495 g cm-3
10
15
20
25
2  (°)
30
35
Packing was improved with introducing proton scavenger.
Electric and Mechanical Properties
• Conductivity: up to 1300 S cm-1
– Comparable to regio-regular poly(3-alkylthiophenes)
– Compare with ~100 S cm-1 without DTBP
– High refractive indices are expected.
• Mechanical properties
– Tensile strength: ~135 MPa
– Tensile modulus: 4 GPa
– Elongation at break: 4%
Refractive Index Dispersion of a Highly
Conjugated Polythiophene Film
nR
nI
n = nR-ini
Inversed opal
3.2
3.0
2.8
2.6
Refractive Index
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
450
500
550
600
650
700
750
800
Wavelength (nm)
Courtesy of AFRL Materials and Manufacturing Directorate
Threshold RI Contrasts for Complete
Band Gaps in 3-D Photonic Crystals
HCP: 3.10
Single Gyroid:
2.28
Inversed Opal:
(FCC)
2.80
Inversed Square
Spiral: 2.20
Diamond:
1.87
Electrochemical Fabrication of a PT
Inversed Opal Photonic Crystal
FCC single crystal
n1 = 2.9
Partial fusion of
colloids
Addition of
monomer
Removal of
colloid spheres
Electro-synthesis of
polythiophene
n2 = 1
Dedoping of
polythiophene
FCC and HCP
FCC
Volume fraction = 0.7405
Coordination # = 12
Sequence = ABCABC
G = 0.005kBT
per particle
HCP
Volume fraction = 0.7405
Coordination # = 12
Sequence = ABAB
FCC is more stable than HCP with a very small energy difference.
Bolhuis, P. B.; Frenkel, D.; Mau, S. and Huse, D. Nature 1997, 388, 235
Colloid Crystallization
HCP
FCC
50 m
Polystyrene colloid, d = 269 nm
refl.
FCC:
 640 nm
refl.
HCP:
 600 nm
Mechanical Annealing
Colloid crystal
Piezoelectric element
Oscillator
Phase Flipping
with Mechanical Annealing
50 m
50 m
HCP  FCC conversion was achieved by mechanical
annealing.
Phase Structure of an Inversed Opal
Photonic Crystal
Summary
• Oxidation potential of thiophene monomer was
lowered by a Lewis acid system so that
degradation of the polymer is avoided.
• Acid-initiated addition polymerization was
suppressed by introducing a proton trap.
• Highly conjugated polythiophene films were
obtained with the refractive index comparable to
dielectric inorganics.
• HCP FCC conversion was successfully
carried out via mechanical annealing.