Mechanistic Insights into Oxidative Molecular Layer Deposition of Conjugated Polymers

Abstract

Oxidative molecular layer deposition (oMLD) promises to enable molecular-level control of polymer structure through monomer-by-monomer growth via sequential, self-limiting, gas-phase surface reactions of monomer(s) and oxidant(s). However, only a few oMLD growth chemistries have been demonstrated to date, and limited mechanistic understanding is impairing progress in this field. Here, we examine oMLD growth using 3,4-ethylenedioxythiophene (EDOT), pyrrole (Py), p-phenylenediamine (PDA), thiophene (Thi), and furan (Fu) monomers. We establish key insights into the surface reaction mechanisms underlying oMLD growth. We specifically identify the importance of a two-electron chemical oxidant with sufficient oxidation strength to oxidize both a surface and a gas-phase monomer to enable oMLD growth. The mechanistic insights we report enable rational molecular assembly of copolymer structures to improve electrochemical capacity. This work is foundational to unlock molecular-level control of redox-active polymer structure and will enable the study of previously intractable questions regarding the molecular origins of polymer properties, allowing us to control and optimize polymer properties for energy storage, water desalination, and sensors.

Effects of film thickness on electrochemical properties of nanoscale polyethylenedioxythiophene (PEDOT) thin films grown by oxidative molecular layer deposition (oMLD)

“…In prior work, we reported saturating dose conditions for sequential pulses of EDOT and MoCl 5 at 150 °C using in situ quartz crystal microbalance (QCM) measurements. 22 These saturating conditions consisted of an MoCl 5 dose time of 100 s, an EDOT dose time of 20 s, and 100 s purge times after each precursor dose. Here, we employ these growth conditions to deposit PEDOT thin films on Si and pyrolytic graphite sheet (PGS) samples using between 20 and 175 oMLD cycles.…”

“…We note that the oMLD PEDOT films contain residual MoCl x , oxidant and are in a fully oxidized state as-formed. 22,28 These fully oxidized PEDOT films exhibit minimal optical absorption at wavelengths <900 nm, [4][5][6][7]43,44 allowing for facile SE thickness fitting using a simple Cauchy model for thicknesses from 0-60 nm. This SE fitting yielded a refractive index of 1.61 at a wavelength of 589 nm, consistent with a value of n = 1.6 reported previously.…”

“…We employed a hot-walled viscous flow reactor 70 with precursor configurations for oMLD deposition described previously. 21,22 The reactor was held at 150 °C using PID temperature control and ∼0.85 torr of inert argon carrier gas baseline pressure (UHP, 99.999% Ar) is maintained using a mass flow controllers. EDOT (A) and MoCl 5 (B) were each held at 100 °C in stainless steel flow-over bubblers.…”

“…The benefits of nanoscale film thicknesses to enhance charge capacity have been shown, for example, in prior work on nanoscale PANI, which measured specific charge capacities of >130 mA h g −1 at thicknesses of <30 nm, 20 and in prior work on nanoscale PPY, which exhibited capacities of >250 mA h g −1 for <30 nm thicknesses. 21,22 However, conventional techniques for PEDOT synthesis such as (a) homogeneous oxidative chemical polymerization using a chemical oxidant such as iron(III) chloride (FeCl 3 ), 2 or (b) electrochemical oxidation 4 cannot easily deliver uniform nanoscale films of <100 nm.…”

Ultraviolet–Visible Absorption Spectroscopy of MoCl5, MoOCl4, and MoO2Cl2 Vapors

“…There is limited evidence for this in the literature. Work by Wyatt and co-workers 47 on the surface oxidative polymerization of 3,4-ethylenedioxythiophene shows that MoCl 5 exposure times on the order of minutes are required to saturate a surface containing adsorbed monomer at 150 °C. The desorption of weakly bound MoCl 5 from the same surface is shown to take even longer, which mirrors our experience and may be due to dimer formation in the condensed state.…”