High Refractive Index Polythiophene for 3-D Photonic Crystals with Complete Band Gaps

1. 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

2. 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

3. Optical switches Applications of Photonics Fiber optics Light emitting diodes Photovoltaics Optical amplifiers

4. Waveguides Thresholdless Lasers Photonic Computers Signal Filters Loss-less Mirrors Applications of Photonic Crystals

5. Dimensionality of Photonic Crystals Periodic in one dimension Periodic in two dimensions Periodic in 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.

6. One-dimensional Photonic Band Gap-Layered Dielectric Structure Assuming n1> n2and n1t1= n2t2= /4: n1 n2 R: peak reflectivity in the band gap N: multilayer number : wavelength in the center of photonic band gap : bandwidth of band gap n1/n2 (refractive index contrast)is important for bothR and ! ni, ti are refractive indices and thicknesses of corresponding layers. Yeh, P. Optical Waves in Layered Media, John Wiley & Sons: New York, 1988.

7. 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.

8. HCP: 3.10 Inversed Opal: 2.80 Threshold RI Contrasts for Complete Band Gaps in 3-D Photonic Crystals Inversed Square Spiral: 2.20 Single Gyroid: 2.28 Diamond: 1.87

9. Polysulfone (589 nm) 1.63 Polystyrene (589 nm) 1.59 Polypropylene (589 nm) 1.51 Ge (633 nm) 5.5 Si (633 nm) 3.8 Air 1 Refractive Indices of Materials • 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?

10. Refractive Index and Molecular Structure 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

11. polyacetylene (PA) polythiophene (PT) polyphenylenevinylene (PPV) Conjugated Polymers: A 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.

12. Predicted Refractive Indices of Conjugated Polymers Predicted Refractive Indices According to calculation, polythiophene has the refractive index comparable to inorganic materials! Yang; C., Jenekhe, S. Chem. Mater. 1995, 7, 1276

13. Refractive Indices: Calculations versus Experiments However, 6T shows n633nm = 2.154! What are the problems with electrochemically synthesized polythiophene films? *Electrochemically synthesized • Yang; C., Jenekhe, S. Chem. Mater. 1995, 7, 1276 • Burzynski, R.; Prasad, P. N.; Karasz, F. E. Polymer 1990, 31, 627 • Hamnett, A.; Hillman, A. R. J. Electrochem. Soc. 1988, 135, 2517 • Yassae, A. et al. J. Appl. Phys. 1992, 72, 15

14. 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

15. Polythiophene 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

16. 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!

17. 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)

18. With DTBP Without DTBP Spectroscopic Characterization of Polythiophene Films Amount of saturated units was greatly reduced. Red-shift of max indicates a more extended conjugated structure.

19. 0.5 nm 0.35 nm S S S S S S S S S S S S Wide-angle X-ray Scattering of Polythiophene Films 0.5 nm 0.35 nm =1.512 g cm-3 =1.495 g cm-3 Packing was improved with introducing proton scavenger.

20. Electric and Mechanical Properties • 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%

21. Refractive Index Dispersion of a Highly Conjugated Polythiophene Film Courtesy of AFRL Materials and Manufacturing Directorate

22. HCP: 3.10 Inversed Opal: (FCC) 2.80 Threshold RI Contrasts for Complete Band Gaps in 3-D Photonic Crystals Diamond: 1.87 Inversed Square Spiral: 2.20 Single Gyroid: 2.28

23. Electrochemical Fabrication of a PT Inversed Opal Photonic Crystal FCC single crystal Partial fusion of colloids Addition of monomer n1 = 2.9 n2 = 1 Dedoping of polythiophene Removal of colloid spheres Electro-synthesis of polythiophene

24. FCC and HCP FCC HCP G = 0.005kBT per particle Volume fraction = 0.7405 Coordination # = 12 Sequence = ABCABC 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

25. Colloid Crystallization FCC: refl.  640 nm HCP: refl.  600 nm HCP 50 m FCC Polystyrene colloid, d = 269 nm

26. Mechanical Annealing Colloid crystal Piezoelectric element Oscillator

27. Phase Flipping with Mechanical Annealing 50 m 50 m HCP  FCC conversion was achieved by mechanical annealing.

28. Phase Structure of an Inversed Opal Photonic Crystal

29. 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.