Electrochemical Fabrication of One

Transcript Electrochemical Fabrication of One

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 Optical switches Light emitting diodes Optical amplifiers Photovoltaics

Applications of Photonic Crystals

Loss-less Mirrors Photonic Computers Waveguides Signal Filters Thresholdless Lasers

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

n 2 n 1

:  :  

peak reflectivity in the band gap multilayer number wavelength in the center of photonic band gap bandwidth of band gap

n i

t i

are refractive indices and thicknesses of corresponding layers

Assuming

n 1 n 1 t 1

n 2 t 2

n 2

=  /4: and     4  sin  1 1 

1 1 

1 / /

        1 1          

n n

2 1     2

    2

       2

n 1 /n 2

(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) contrast refractive index 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

Single Gyroid: 2.28

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

2  2  1  4 3  3

N A M w



– Refractive Index

N A M w

 

– Avogadro’s constant – Molar weight – Density – Molecular polarizability

• • •

Higher



Higher

  

higher n 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)

polyphenylenevinylene (PPV)

S 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.

Predicted Refractive Indices of Conjugated Polymers

Predicted Refractive Indices

Polymer

n 700nm n 1064nm

trans-PA PPV PT

2.28

3.90

2.47

2.04

3.04

n 2500nm

2.44

1.95

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 trans-PA PPV

n pred.

2.44

2.5



m 2.28

700 nm

n exp.

2.33



1 2.09

633 nm 2 PT* 3.9

700 nm 1.4

633 nm 3

However, 6T shows n 633nm = 2.15

4 !

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 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

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 BF 3 •Et 2 O 3 mole/L AlCl 3 /CH 3 CN CH 3 CN C t = 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

With DTBP Without DTBP 0.8

0.6

0.4

0.2

3200 3000

Wavenumber (cm -1 )

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 0.5 nm 0.35 nm S S S S S S S S S S S S 0.5 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-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

3.2

3.0

2.8

1.0

0.8

0.6

0.4

0.2

0.0

2.6

2.4

2.2

2.0

1.8

1.6

1.4

1.2

n R n I n = n R -

n i Inversed opal 450 500 550 600 650 700 Wavelength (nm) 750 800 Courtesy of AFRL Materials and Manufacturing Directorate

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

HCP: 3.10

Inversed Opal: (FCC) 2.80

Single Gyroid: 2.28

Inversed Square Spiral: 2.20

Diamond: 1.87

Electrochemical Fabrication of a PT Inversed Opal Photonic Crystal

FCC single crystal n 1 = 2.9

n 2 = 1 Partial fusion of colloids Addition of monomer Dedoping of polythiophene Removal of colloid spheres Electro-synthesis of polythiophene

FCC

FCC and HCP



G = 0.005k

B T per particle

HCP

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

HCP



FCC 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

HCP  annealing.



m 50



FCC conversion was achieved by mechanical

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 .