Reactive Vapor Deposition of Conjugated Polymer Films on Arbitrary Substrates

Podsumowanie

This paper presents a protocol for reactive vapor deposition of poly(3,4-ethylenedioxythiophene), poly(3,4-propylenedioxythiophene), and poly(thieno[3,2-b]thiophene) films on glass slides and rough substrates, such as textiles and paper.

Streszczenie

We demonstrate a method of conformally coating conjugated polymers on arbitrary substrates using a custom-designed, low-pressure reaction chamber. Conductive polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-propylenedioxythiophene) (PProDOT), and a semiconducting polymer, poly(thieno[3,2-b]thiophene) (PTT), were deposited on unconventional highly-disordered and textured substrates with high surface areas, such as paper, towels and fabrics. This reported deposition chamber is an improvement of previous vapor reactors because our system can accommodate both volatile and nonvolatile monomers, such as 3,4-propylenedioxythiophene and thieno[3,2-b]thiophene. Utilization of both solid and liquid oxidants are also demonstrated. One limitation of this method is that it lacks sophisticated in situ thickness monitors. Polymer coatings made by the commonly used solution-based coating methods, such as spin-coating and surface grafting, are often not uniform or susceptible to mechanical degradation. This reported vapor phase deposition method overcomes those drawbacks and is a strong alternative to common solution-based coating methods. Notably, polymer films coated by the reported method are uniform and conformal on rough surfaces, even at a micrometer scale. This feature allows for future application of vapor deposited polymers in electronics devices on flexible and highly textured substrates.

Wprowadzenie

Polymeric conducting and semiconducting materials have unique properties, such as flexibility¹, stretchability², transparency³, and low density,⁴ which provide extraordinary opportunities for creating next-generation electronic devices on nontraditional substrates. Currently, many researchers are endeavoring to take advantage of the unique properties of polymeric materials to create flexible and/or wearable electronics^5,6 and smart textiles⁷. However, the ability to conformally coat highly textured surfaces and non-robust substrates, such as paper, fabrics and threads/yarns, remains unmastered. Most commonly, polymers are synthesized and coated on surfaces using solution methods.^8,9,10,11,12 Although solution methods provide polymer coated fibers/textiles, the coatings thus obtained are often non-uniform and easily damaged by small physical stresses^13,14 . Solution methods are also not applicable to coating paper because of wetting problems.

Reactive vapor deposition can create conformal conjugated polymer films on a diverse range of substrates, irrespective of surface chemistry/composition, surface energy and surface roughness/topography¹⁵. In this approach, conjugated polymers are synthesized in the vapor phase by simultaneously delivering monomer and oxidant vapors to a surface. Polymerization and film formation occurs on the surface in a single, solvent-free step. This method is theoretically applicable to any conjugated polymer that can be synthesized by oxidative polymerization using solution methods. However, to date, protocols for depositing only a narrow set of conjugated polymer structures are known.¹⁵

Here, we demonstrate the deposition of conductive poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3,4-propylenedioxythiophene) (PProDOT), and semiconducting poly(thieno[3,2-b]thiophene) (PTT) films via reactive vapor deposition. Two kinds of oxidants, solid FeCl₃ and liquid Br₂, are used in the process. The corresponding polymers are named Cl-PProDOT, Cl-PTT, and Br-PEDOT. Both conventional substrates, glass slides, and unconventional textured substrates, such as paper, towels and fabrics, were coated with the polymer films.

This protocol describes the setup of the custom-built vapor deposition chamber and the details of the deposition process. It is intended to help new practitioners to build their deposition system and avoid common pitfalls associated with vapor-phase synthesis.

Protokół

Read MSDS for reagents and follow all chemical safety measures as required by your institution.

1. Deposition of Cl-PProDOT and Cl-PTT

1. Build the structure of the custom-built tubular vapor deposition chamber as shown in Figure 1.
   1. Make a 1/4-in. (outer diameter, O.D.) fused quartz side inlet to a 2-in. (O.D.) fused quartz tube. Make a cold trap with a custom-built U-shape 1-in. stainless-steel tube and a Dewar flask.
   2. Connect the quartz tube with a vacuum gauge and cold trap using stainless-steel KF connectors and quick-connect couplings. Place the monomer in a quartz ampule and connect the ampule to the tubular chamber via 1/4-in. quick-connect couplings and a needle valve. Place the oxidant in a crucible in the chamber.
   3. Use separate heating tapes as heating sources for the oxidant, substrates and the monomer. Add a gas inlet at the right end of the chamber to introduce additional noble gases to control the process pressure if necessary.
2. Deposition of Cl-PProDOT
   1. Add 50 mg of 3,4-propylenedioxythiophene (ProDOT) in the monomer ampoule and connect it to the tubular chamber. Keep the needle valve open.
   2. Put substrates (glass slide, fabrics, paper, etc.) in the chamber. The size of the substrates is 1.3 cm x 2.5 cm.
   3. Add 50 mg of FeCl₃ in a 5-mL crucible and place it in the chamber.
      NOTE: The relative positions of the monomer inlet, substrates and the crucible are shown in Figure 1. The distance between the monomer inlet and the crucible is 13 cm.
   4. Turn on the pump. Close the valve on the right end of the chamber slowly. After the chamber pressure is below 525 mTorr (70 Pa), add liquid nitrogen in the cold trap.
   5. Wrap the three heating zones with heating tape and connect the heating tape to the temperature controllers.
   6. When the pressure decreases to the processing pressure (52.5 mTorr, 7 Pa), close the needle valve of the monomer container.
   7. Start heating the oxidant, the substrates and the monomer at 170 °C, 80 °C, and 80 °C, respectively. After ~10 min, FeCl₃ is vaporized and the red FeCl₃ solid is formed in the cool region.
   8. Open the needle valve of the monomer container.
      NOTE: Blue-colored thin films will be formed in the substrate region. Typical growth rates are ~10 nm/min. Ensure that the FeCl₃ vapor is formed in the chamber before opening the needle valve of the monomer container. Otherwise, the monomer will react with FeCl₃ solid in the crucible and form a polymer layer which prevents the further vaporization of the oxidant.
   9. Close the needle valve of the monomer container when the desired thickness is achieved. Turn off all heating tape and cool the system to room temperature.
   10. Open the gas inlet valve and turn off the pump.
   11. Take the samples out of the chamber. Carefully immerse the samples in methanol for 30 min to remove the residual oxidant and monomer.
       NOTE: Rinsing time should increase as film thickness increases. 30-min rinsing is typical for films thinner than 100 nm on glass slides. Films thicker than 500 nm might delaminate from the substrate when rinsing.
   12. Carefully blow dry the samples with nitrogen gas.
3. Deposition of Cl-PTT
   1. Add 50 mg of thieno[3,2-b]thiophene (TT) in the monomer ampoule and connect it to the tubular chamber. Keep the needle valve open.
   2. Repeat steps 1.2.2. to 1.2.12.

2. Deposition of Br-PEDOT

1. Deposition Chamber Setup
   1. Add an additional 1/4-in. side inlet for oxidants to the quartz tube and make it 8 in. apart from the monomer inlet. Place the liquid oxidant in a quartz ampule and connect the ampule to the tubular chamber the same way as the monomer (Figure 2).
2. Deposition of Br-PEDOT
   1. Add 2 mL of 3,4-ethylenedioxythiophene (EDOT) in the monomer ampule and connect the ampule to the tubular chamber. Keep the needle valve open.
   2. Place substrates (glass slide, fabrics, paper, etc.) in the tubular chamber near the monomer vapor inlet. The size of the substrate is 1.3 cm x 2.5 cm.
   3. In a fume hood, add 2 mL of Br₂ in the oxidant ampule, connect the ampule to the needle valve and keep the needle valve closed. Connect the needle valve to the quartz tube.
      Caution: Br₂ is a hazardous material. Use caution when handling.
   4. Turn on the pump. Close the valve on the right end of the chamber slowly. After the chamber pressure is below 525 mTorr (70 Pa), add liquid nitrogen in the cold trap.
   5. Wrap the monomer region with heating tape and connect it with a temperature controller. Maintain the substrate and oxidant region at room temperature.
   6. When the pressure decreases to the processing pressure of 52.5 mTorr (7 Pa), open the needle valve of the oxidant.
      NOTE: The reaction is very fast. Blue PEDOT films will form close to the monomer inlet because Br₂ is very volatile.
   7. Close the needle valves of both the monomer and the oxidant when desired thickness is achieved.
   8. Turn off the heating tape and cool the system to room temperature.
   9. Open the gas inlet valve and turn off the pump. Take the samples out of the chamber.
      NOTE: Rinsing is not needed for Br₂-doped polymers.

Wyniki

The thickness of Cl-PProDOT films formed on 1.3 cm x 2.5 cm glass slides placed at discrete lateral positions along the central tube were measured by a profilometer (Figure 3). Conductivities were calculated from resistivity measurements using a home-built four-point probe test station. The measured conductivity of a 100-nm thick Cl-PProDOT film on glass slides is 106 S/cm, which is sufficient to qualify this film as a potential electrode material.

Dyskusje

The mechanism of the reaction is oxidative polymerization. Polymer coating methods using the same mechanism include electropolymerization¹⁷ and vapor phase polymerization¹⁸. Electropolymerization requires a conductive substrate, lacks the advantage of uniform and conformal coating, and is an environmentally-unfriendly solution-based method¹⁹. The existing vapor phase polymerization method is similar to the method reported here but can only polymerize...

Materiały

Name	Company	Catalog Number	Comments
3,4-Ethylenedioxythiophene, 97%	Sigma Aldrich	483028
3,4-Propylenedioxythiophene, 97%	Sigma Aldrich	660485
Thieno[3,2-b]thiophene, 95%	Sigma Aldrich	702668
FeCl3, 97%	Sigma Aldrich	157740
Br2	Sigma Aldrich	207888