High Refractive Index Polythiophene for 3-D High Refractive Index Polythiophene for 3-D Photonic Crystals with Complete Band GapsPhotonic 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 Collaborative Center in Polymer Photonics between AFRL Materials and Manufacturing Directorate Materials and Manufacturing Directorate

and The University of Akronand The University of Akron

Polymer Photonics WorkshopPolymer Photonics Workshop

Photonics, Photonic Crystal and Photonics, Photonic Crystal and Photonic Band GapPhotonic 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 PhotonicsApplications of Photonics

Fiber optics

Optical switches

Light emitting diodes

PhotovoltaicsOptical amplifiers

Applications of Photonic CrystalsApplications of Photonic Crystals

Waveguides

Thresholdless Lasers

Photonic Computers

Signal Filters

Loss-less Mirrors

Dimensionality of Photonic CrystalsDimensionality of Photonic Crystals

Periodic in one dimension

Periodic in two dimensions

Periodic in three dimensions

Joannopoulos, D. D. et al. Photonic Crystals, Princeton University, 1995.

Different colors represent different refractive indices.How does the degree of refractive index variation affect the property of a photonic crystal?

2

2

1

2

2

1

2

1

1

N

N

nnnn

R

R: peak reflectivity in the band gapN: multilayer number: wavelength in the center of

photonic band gap: bandwidth of band gap

ni, ti are refractive indices and thicknesses of corresponding layers.

Assuming n1 > n2 and n1t1 = n2t2 = /4:

21

211

/1

/1sin

4

nn

nn

n1/n2 (refractive index contrast) is important for both R and !

n1

n2

One-dimensional Photonic Band Gap-One-dimensional Photonic Band Gap-Layered Dielectric StructureLayered Dielectric Structure

Yeh, P. Optical Waves in Layered Media, John Wiley & Sons: New York, 1988.

3D Complete Photonic Band Gap3D 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 Threshold RI Contrasts for Complete Band Gaps in 3-D Photonic CrystalsGaps in 3-D Photonic Crystals

Diamond:1.87

Single Gyroid: 2.28

HCP: 3.10

Inversed Opal: 2.80

Inversed Square Spiral: 2.20

• 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 Indices of MaterialsRefractive Indices of Materials

Ge (633 nm) 5.5

Si (633 nm) 3.8

Air 1

Polysulfone (589 nm) 1.63

Polystyrene (589 nm) 1.59

Polypropylene (589 nm) 1.51

Refractive Index and Molecular StructureRefractive Index and Molecular Structure

n – Refractive IndexNA – Avogadro’s constantMw – Molar weight – Density – Molecular polarizability

w

A

MN

n

34

1

322

• 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: Conjugated Polymers: A Source of Achieving Higher RI ContrastA Source of Achieving Higher RI Contrast

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

polyacetylene(PA)

S

S

n

polythiophene(PT)

n

polyphenylenevinylene(PPV)

Predicted Refractive Indices of Predicted Refractive Indices of Conjugated PolymersConjugated Polymers

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!

Predicted Refractive Indices

Yang; C., Jenekhe, S. Chem. Mater. 1995, 7, 1276

Refractive Indices: Refractive Indices: Calculations versus ExperimentsCalculations versus Experiments

Polymer npred. nexp.

trans-PA 2.442.5 m 2.331

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 synthesized1. Yang; C., Jenekhe, S. Chem. Mater. 1995, 7, 12762. Burzynski, R.; Prasad, P. N.; Karasz, F. E. Polymer 1990, 31, 6273. Hamnett, A.; Hillman, A. R. J. Electrochem. Soc. 1988, 135, 25174. Yassae, A. et al. J. Appl. Phys. 1992, 72, 15

Why Electrochemical Synthesis?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 ParadoxPolythiophene Paradox

• Electro-polymerization must begin with the electro-oxidation 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 Lewis Acid-assisted Low-potential PolymerizationLow-potential Polymerization

Ct = 0.1 mole/L

The oxidation potential of thiophene was lowered to 1.3 V, degradation of polymer can be avoided!

BF3•Et2O

CH3CN

3 mole/L AlCl3/CH3CN

Borontrifluoridediethyl ether

Proton-free Low-potential Proton-free Low-potential Polymerization of ThiophenePolymerization 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.

N

2,6-di-tert-butylpyridine (DTBP)

Spectroscopic Characterization of Spectroscopic Characterization of Polythiophene FilmsPolythiophene Films

With DTBP

Without DTBP

Amount of saturated units was greatly reduced.

Red-shift of max indicates a more extended conjugated structure.

10 15 20 25 30 35

with DTBP without DTBP

2 (°)

Wide-angle X-ray Scattering of Wide-angle X-ray Scattering of Polythiophene FilmsPolythiophene Films

S

S

S

S

S

S

S

S

S

S

S

S

0.5 nm

0.35 nm

0.5 nm

0.35 nm

Packing was improved with introducing proton scavenger.

=1.512 g cm-3

=1.495 g cm-3

• Conductivity: up to 1300 S cm-1

– Comparable to regio-regular poly(3-alkyl-thiophenes)– 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%

Electric and Mechanical PropertiesElectric and Mechanical Properties

Refractive Index Dispersion of a Highly Refractive Index Dispersion of a Highly Conjugated Polythiophene FilmConjugated Polythiophene Film

Courtesy of AFRL Materials and Manufacturing Directorate

Threshold RI Contrasts for Complete Threshold RI Contrasts for Complete Band Gaps in 3-D Photonic CrystalsBand Gaps in 3-D Photonic Crystals

Diamond:1.87

Single Gyroid: 2.28

HCP: 3.10

Inversed Opal: (FCC) 2.80

Inversed Square Spiral: 2.20

Electrochemical Fabrication of a PT Electrochemical Fabrication of a PT Inversed Opal Photonic CrystalInversed Opal Photonic Crystal

Addition of monomer

Electro-synthesis of polythiophene

Removal of colloid spheres

FCC single crystal Partial fusion of colloids

Dedoping of polythiophene

n1 = 2.9 n2 = 1

FCC and HCPFCC and HCP

Volume fraction = 0.7405Coordination # = 12Sequence = ABCABC

Volume fraction = 0.7405Coordination # = 12Sequence = ABAB

FCC HCPG = 0.005kBT per particle

Bolhuis, P. B.; Frenkel, D.; Mau, S. and Huse, D. Nature 1997, 388, 235

FCC is more stable than HCP with a very small energy difference.

Colloid CrystallizationColloid Crystallization

50 m

Polystyrene colloid, d = 269 nm

FCC:refl. 640 nm

HCP:refl. 600 nm

HCP

FCC

Mechanical AnnealingMechanical Annealing

Colloid crystal

Piezoelectric element

Oscillator

Phase Flipping Phase Flipping with Mechanical Annealingwith Mechanical Annealing

HCP FCC conversion was achieved by mechanical annealing.

50 m50 m

Phase Structure of an Inversed Opal Phase Structure of an Inversed Opal Photonic CrystalPhotonic Crystal

SummarySummary

• 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.