AVS 70 Session TF-WeM: Vapor Synthesis of Hybrid, Organic, and Polymeric Materials (VSHOP II)

Porphyrins have been intensively investigated as catalysts and sensing materials. The properties of porphyrin-based catalysts and sensors can be tailored from the careful selection of the central metal ion chelated at the centre of the macrocycleand its peripheral substituents. Beyond, several studies have highlighted the cooperative effect promoted by conjugated covalent links between porphyrins on both their catalytic and sensing properties. However, in spite of the many synthetic approaches developed towards the synthesis of porphyrin-based conjugated assemblies, the engineering and integration of porphyrin-based conjugated assemblies has been limited by their weak solubility.

Oxidative chemical vapor deposition (oCVD) was recently demonstrated as a convenient method for the simultaneous synthesis and deposition of porphyrin-based conjugated polymers.^[1] Porphyrins possessing free meso-positions^[1-4] and/or free beta-positions^{[5] and} porphyrins bearing thienyl or aminophenyl substituents^[6] have both been successfully polymerised using oCVD to yield the formation of formation fused porphyrin tapes and bridged porphyrins covalent organic frameworks, respectively. Importantly in the perspective of practical application, including heterogeneous electrocatalysis and gas sensing, the porphyrin-based conjugated polymers are readily deposited on virtualy any substrate in the form of smooth and thickness-controlled thin films.

Up-to-date, porphyrin-based conjugated polymer thin films prepared by oCVD have been successfully investigated for the electrochemical hydrogen evolution reaction,^[2] nitrate reduction reaction,^[6] oxygen reduction reaction,^[6] oxygen evolution reactions.^[3] In addition, porphyrin-based conjugated polymer thin films prepared by oCVD have also been successfully investigated as chemiresistive sensors for ammonia detection.^[4] Experimental and theoretical data demonstrate the impact of both the central metal cations and substituents on the catalytic activitiesand sensing properties of the porphyrin conjugated polymer thin films.^[2-6] The approach reported in this work circumvents many limitations of solution-based approaches and pave the way to the facile engineering and integration of efficient electrocatalysts and selective sensing materials from porphyrins.

[1] Bengasi et al., Angew. Chem., Int. Ed. 2019. [2] Huerta-Flores et al., ACS Appl. Energy Mater. 2020. [3] Bansal et al., J. Mater. Chem. A 2023. [4] Bengasi et al., Adv. Elect. Mat. 2020. [5] Bansal et al., Chem. Eur. J. 2024. [6] Mohamed et al., Adv. Mater. 2024.

Initiated Chemical Vapor Deposition (iCVD) has been increasingly studied in the solvent-free synthesis and manufacturing of polymeric thin films. It eliminates bulk solvents, thus improving the environmental sustainability of polymer synthesis. Its scalability points to a new pathway for accelerating the development and environmentally friendly manufacturing of polymeric nanomaterials. While prior research has predominantly leveraged the all-dry and low-pressure reaction environment to achieve ideal gas behavior and thereby controlled synthesis conditions, recent advances in iCVD synthesis have unlocked an opportunity to depart from the ideal gas regime. In this talk, I will discuss our results on introducing non-covalent interactions to preorganize molecules and access new chemical synthesis pathways to broaden the palette of attainable polymer chemistry and morphologies. Leveraging hydrogen bonding, we demonstrated that complexing 4-vinylpyridine with hexafluoroisopropanol could increase the polymer molar mass by 700%, which in turn led to unprecedented material hardness and surface morphologies. Building from this demonstration, we tackled the challenge of undesirable chain transfer (e.g., to the imidazole group during the polymerization of 1-vinylimidazole), which has limited the synthesis of an important class of amine/imidazole-containing polymers using iCVD polymerization. Using 1-vinylimidazole as an example, we protonated the transfer-to location via a vapor complexation with acetic acid, rendering the chain transfer energetically unfavorable and enabling synthesis of poly(1-vinylimidazole) films with unprecedented purity, molar mass, and highly controllable polymerization kinetics. This new synthetic capability, in turn, led to the discovery of a novel bioactive material.

Stability concerns have hindered the practical use of perovskite solar cell (PSC) devices. One significant factor contributing to this issue is the inherent acidity of the commonly used hole transport layer, poly(3,4-ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS), potentially jeopardizing the stability of PSC devices. To address this challenge, this study explores employing oxidative chemical vapor deposition (oCVD) with antimony pentachloride (SbCl5) as a liquid oxidant for fabricating PEDOT-Cl thin film. This technique is utilized to create stable and ultrathin PEDOT films with high conformity, presenting a promising alternative as a hole transport layer in PSCs. The resulting oCVD-grown PEDOT-Cl thin films, showcase exceptional optoelectronic properties, precise nanostructure control, stability, and integration capabilities. These attributes establish them as a robust and effective choice to be used as the hole transport layer in PSC device. Integration of oCVD PEDOT-Cl thin films as the hole transport layer in PSCs yields a remarkable power conversion efficiency (PCE) of 20.74%, surpassing the 16.53% PCE achieved by spin-coated PEDOT:PSS thin films treated with dimethyl sulfoxide (DMSO) as a polar solvent. Additionally, PSCs incorporating oCVD PEDOT-Cl thin films exhibit a noteworthy 2.5× improvement in stability compared to their PEDOT:PSS-DMSO counterparts.

Keywords: Oxidative Chemical Vapor Deposition, PEDOT, SbCl₅ Oxidant, Perovskite Solar Cells

The implementation of extreme ultraviolet (EUV) lithography in semiconductor manufacturing promises to extend Moore’s Law by enabling the patterning of sub-20 nm feature sizes. However, further device scaling is dependent on the implementation of EUV-tailored photoresists that meet requirements in sensitivity, resolution, and line edge roughness. With feature sizes approaching the nanometer scale, stochastic variation in the photoresist molecular structure also affects pattern quality, resulting in the need for new resist chemistries with uniform chemical distribution.

Metal-organic thin films deposited via hybrid molecular layer deposition (MLD) are a promising class of materials to address the challenges of designing new EUV-compatible resist chemistries, by incorporating EUV-absorbing metal centers into the polymer network while exhibiting Å-level thickness control and atomic-scale homogeneity. In this work, we investigated a series of hybrid MLD-derived aluminum alkoxide (alucone) resists, deposited via trimethylaluminum (TMA) and a series of alcohol counter-reactants (glycerol, ethylene glycol, and sequential dosing of methanol/ethylene glycol). This process yielded thin films that are chemically akin, as demonstrated by x-ray photoelectron spectroscopy (XPS), but exhibit notable differences in networking/crosslinking density. The resist performance was evaluated via electron beam lithography, a well-established proxy for EUV. The alucone resists exhibited negative-tone patterning, attributed to the loss of organic ligands and alumina formation resulting from electron-induced reactions. Decreasing the crosslinking density enhanced the resist contrast, thus improving the resolution. On the other hand, increasing the crosslinking density resulted in drastically reduced developer solubility. The variation observed between these hybrid materials underscores the importance of structure-property relationships for the rational design of metal-organic EUV resists.

Poly-2,5-dimercapto-1,3,4-thiadiazole (pDMCT) is a redox-active polymer consisting of heterocyclic monomer units connected by disulfide bonds. pDMCT has been used as a battery material, biocide, and corrosion inhibitor. Upon electrochemical reduction, the disulfide bonds in pDMCT break to form thiolate anions that readily coordinate with cations, providing lithium ion conductivity for battery applications and allowing for capture of heavy metal cations. In this work, our goal is to study the vapor-phase formation of thin films of pDMCT using oxidative molecular layer deposition (oMLD) for use as a protective coating in solid state lithium-ion batteries. We employ alternating vapor exposures of the 2,5-dimercapto-1,3,4-thiadiazole (DMCT) monomer and a molybdenum pentachloride (MoCl₅) chemical oxidant to perform oMLD growth in a custom viscous-flow reactor at 150 °C and ~1 Torr. We employ in situ quartz crystal microbalance (QCM) studies during growth to understand the film growth chemistry, as well as ex situ spectroscopic and electrochemical characterization to confirm formation of pDMCT. QCM studies and ex situ thickness measurements indicate controlled linear growth. We examine the effect of precursor dose times on polymer chain length, and the effect of polymer chain length and film thickness on electrochemical properties. The controlled growth of ultrathin films of pDMCT shows promise for the application of this chemistry as a protective coating for lithium-ion battery applications and for passive uptake of heavy metal ions. More broadly, these studies establish that oMLD can be used to create polymers connected by disulfide linkages, opening a new class of polymers accessible by oMLD synthesis.

Ferrocene is a stable molecule with well understood redox activity, and has been utilized extensively in areas such as bio-sensing, organometallic chemistry, and materials science.^1–3 This work leverages the utility of ferrocene precursors for solution and vapour phase depositions that prepare ferrocene-based thin films. There was an affinity for the deposition to occur selectively on metallic substrates, and therefore the area-selective deposition of ferrocene-containing precursors was demonstrated, which has potential applications for onwards application in nanoscale device fabrication.⁴ Preparation of the small molecule precursors will be discussed as well as the analysis of the fabricated thin film/monolayers via X-ray photoelectron spectroscopy (XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), Atomic Force Microscopy (AFM), surface Raman spectroscopy, and X-ray Reflectivity (XRR)

(1) Ferrocene—Beauty and Function. Organometallics 2013, 32 (20), 5623–5625. https://doi.org/10.1021/om400962w.

(2) van Staveren, D. R.; Metzler-Nolte, N. Bioorganometallic Chemistry of Ferrocene. Chem. Rev. 2004, 104 (12), 5931–5986. https://doi.org/10.1021/cr0101510.

(3) Hauquier, F.; Ghilane, J.; Fabre, B.; Hapiot, P. Conducting Ferrocene Monolayers on Nonconducting Surfaces. J. Am. Chem. Soc. 2008, 130 (9), 2748–2749. https://doi.org/10.1021/ja711147f.

(4) Parsons, G. N.; Clark, R. D. Area-Selective Deposition: Fundamentals, Applications, and Future Outlook. Chem. Mater. 2020. https://doi.org/10.1021/acs.chemmater.0c00722.

Thin films of Ni₂(BDC)₂DABCO, where BDC is benzendicarboxylic acid and DABCO is 1,4-diazabicyclo[2.2.2]octane, were made by a hot vapor-assisted conversion technique. The thin films were characterized by x-ray diffraction, vibrational spectroscopy and scanning electron microscopy. The polycrystalline films were determined to be phase pure by powder x-ray diffraction. Infrared and Raman spectroscopy reveals the carboxylic acid ligands were converted into carboxylates. The films were activated by heating under high vacuum and adsorption/desorption isotherms were measured for several volatile organic compounds, including alcohols, nitrobenzene, toluene, and methyl iodide vapors at room temperature. The isotherms revealed a Type I behavior for ethanol, isopropanol, toluene nitrobenzene, and methyl iodide vapors. In contrast, the isotherm for methanol has a characteristic S-shape, which is characteristic of a flexible lattice. The adsorption isotherm for methanol has distinct steps, which is attributed to lattice expansion. Upon removal of methanol from the thin film, the lattice relaxes back to the original structure. The results are compared to other pillared microporous coordination compounds.

Molecular nanolayers (MNLs) at inorganic thin film interfaces are known to improve chemical stability, and stimulate completely unexpected mechanical responses and electrical and thermal transport behaviors^[1]. Stacking inorganic/organic thin film interfaces with nanoscale proximity, e.g., in high-interface-fraction multilayers, offers promise to access emergent properties via superposition of nanomolecular effects. Synthesizing such structures requires low-temperature depositions of high-quality ultrathin inorganic nanolayers to preserve the integrity of the MNLs. Here, we will demonstrate the synthesis of titania/organo-diphosphonate^[2] and AlO_xN_y/hydroquinone multilayers with sharp interfaces by sequential atomic- and molecular-layer deposition (ALD/MLD) cycles. Our results from electron microscopy, X-ray diffraction, and ion beam and photoelectron spectroscopy measurements show that MNLs can significantly alter the inorganic nanolayer growth rate, composition, surface roughness, and phase stability. Examples include up to ~50% decrease in titania growth rates by diphosphonate MNLs, and near complete aluminum nitride to oxide conversion by hydroquinone MNLs. Atomistic mechanisms underpinning these changes will be discussed in terms of the impact of the MNL backbone structure and terminal chemistry on the surface reaction pathways. Acoustic pump-probe spectroscopy measurements on our hybrid multilayers reveal unusual acoustic damping responses which will be described in terms of the MNL interface chemistry and nanolayer periodicity. Further understanding and harnessing such MNL-induced effects and their correlations with emergent properties is crucial for designing high-interface-fraction hybrid nanolaminates for applications.

[1] Engineering inorganic interfaces using molecular nanolayers, G. Ramanath, C. Rowe, G. Sharma, V. Venkataramani, J. G. Alauzun, R. Sundararaman, P. Keblinski, D. G. Sangiovanni, P. Eklund, H. Pedersen, Appl. Phys. Lett. 122, 260502 (2023).

[2] Nanomolecularly-induced effects at titania/organo-diphosphonate interfaces for stable hybrid multilayers with emergent properties, C. Rowe, A. Kashyap, G. Sharma, N. Goyal, J. G. Alauzun, S. T. Barry, N. Ravishankar, A. Soni, P. Eklund, H. Pedersen, G. Ramanath, ACS Appl. Nano Mater. (2024).