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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

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high refractive index polythiophene for 3 d photonic crystals with complete band gaps

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, 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

Optical switches

Applications of Photonics

Fiber optics

Light emitting diodes

Photovoltaics

Optical amplifiers

applications of photonic crystals

Waveguides

Thresholdless Lasers

Photonic Computers

Signal Filters

Loss-less Mirrors

Applications of Photonic Crystals
dimensionality of photonic crystals
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.

one dimensional photonic band gap layered dielectric structure
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.

3d complete photonic band gap
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

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

refractive indices of materials

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?
refractive index and molecular structure
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

conjugated polymers a source of achieving higher ri contrast

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.
predicted refractive indices of conjugated polymers
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

refractive indices calculations versus experiments
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
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 paradox
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

lewis acid assisted low potential polymerization
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
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)

spectroscopic characterization of polythiophene films

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.

wide angle x ray scattering of polythiophene films

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.

electric and mechanical properties
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%
refractive index dispersion of a highly conjugated polythiophene film
Refractive Index Dispersion of a Highly Conjugated Polythiophene Film

Courtesy of AFRL Materials and Manufacturing Directorate

threshold ri contrasts for complete band gaps in 3 d photonic crystals22

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

slide23

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

slide24

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

colloid crystallization
Colloid Crystallization

FCC:

refl.  640 nm

HCP:

refl.  600 nm

HCP

50 m

FCC

Polystyrene colloid, d = 269 nm

mechanical annealing
Mechanical Annealing

Colloid crystal

Piezoelectric element

Oscillator

phase flipping with mechanical annealing
Phase Flipping with Mechanical Annealing

50 m

50 m

HCP  FCC conversion was achieved by mechanical annealing.

slide29

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.