Modelling the growth reaction pathways of zincone ALD/MLD hybrid thin films : a DFT study

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 3.0 https://creativecommons.org/licenses/by/3.0/ Modelling the growth reaction pathways of zincone ALD/MLD hybrid thin films : a DFT study © the Owner Societies 2024 Published version Mäkinen, Mario; Weckman, Timo; Laasonen, Kari Mäkinen, M., Weckman, T., & Laasonen, K. (2024). Modelling the growth reaction pathways of zincone ALD/MLD hybrid thin films : a DFT study. Physical Chemistry Chemical Physics, 26(24), 17334-17344. https://doi.org/10.1039/d4cp00249k 2024 17334 | Phys. Chem. Chem. Phys., 2024, 26, 17334–17344 This journal is © the Owner Societies 2024 Cite this: Phys. Chem. Chem. Phys., 2024, 26, 17334 Modelling the growth reaction pathways of zincone ALD/MLD hybrid thin films: a DFT study† Mario Ma ¨kinen, * a Timo Weckman b and Kari Laasonen a ALD/MLD hybrid thin films can be fabricated by combining atomic layer deposition (ALD) and molecular layer deposition (MLD). Even though this deposition method has been extensively used experimentally, the computational work required to acquire the reaction paths during the thin film deposition process is still in dire demand. We investigated hybrid thin films consisting of diethyl zinc and either 4-aminophenol or hydroquinone using both gas-phase and surface reactions to gain extensive knowledge of the complex phenomena occurring during the process of hybrid thin film deposition. We used density functional theory (DFT) to obtain the activation energies of these kinetic-dependent deposition processes. Different processes of ethyl ligand removal as ethane were discovered, and we found that the hydroxyl group of 4-aminophenol was more reactive than the amino group in the migration of hydrogen to an ethyl ligand within a complicated branching reaction chain. 1 Introduction Atomic layer deposition (ALD) is a method to fabricate thin films with thicknesses in the nanometer range. In addition to the exceptional thinness of the fabricated thin films, this method can also be used to fabricate homogeneous thin films on uneven surfaces. During ALD, gaseous precursors are released into the reactor individually and sequentially. Each precursor can only react with surface molecules, and, therefore, the precursor is attached to the surface as a layer with a theoretical thickness of one atom. Excess residue of this precursor is then removed from the reactor using some inert gas, usually nitrogen. 2 Based on ALD, another method, called molecular layer deposition (MLD), has also been developed, in which polymer chains are made from organic precursors. ALD has been developed since the 1970s and MLD has been developed since the 1990s. The atomic/molecular layer deposition (ALD/MLD) makes it possible to combine these methods and thus use both inorganic and organic layers of the atoms together as hybrid thin film materials. 3 Future applications of these hybrid thin films include electrode and coating materials for battery applications, solar cells, UV-active photoluminescence materials, flexible, lightweight magnets, catalytic applications, and protective coatings. 4 The fabrication process of hybrid thin films is still rather unknown from the perspective of chemical reaction mechanisms due to the very difficult experimental access to the properties of the transition state structure. The chemical reactions used to fabricate these hybrid materials are adsorption reactions, which can be studied using density functional theory (DFT) modelling. Thus, we can reveal the chemistry occurring in the reactor during the thin film fabrication process. This enables the fine-tuning of the deposition process and the feasibility comparison between precursors, which can enable the discovery of completely new precursors and deposition processes. Zinc is one of the most common elements in conventional ALD due to the high demand for ultrathin zinc oxide structures in microelectronic applications. 4,5 Similar to ALD, in the ALD/ MLD processes, diethyl zinc is the most common inorganic precursor, and it is often combined with an organic precursor hydroquinone. 4–12 To prevent hydroquinone from reacting twice within the same precursor pulse, Sood et al. 13 suggested the use of 4-aminophenol, which replaces one of the two hydroxyl groups of hydroquinone with an amino group, to promote more sequential growth of the thin film due to the different reactivities of the reacting functional groups. The hybrid thin films under investigation consisted of diethyl zinc and an organic phenol, either 4-aminophenol or hydroquinone, depending on the thin film. These zincone thin films have been studied using both gas-phase models and a Department of Chemistry and Materials Science, School of Chemical Engineering, Aalto University, Kemistintie 1, 02150, Espoo, Finland. E-mail: [email protected] b Department of Chemistry, University of Jyva ¨skyla ¨, Survontie 9 B, 40500, Jyva ¨skyla ¨, Finland †Electronic supplementary information (ESI) available: The xyz coordinates of the reactions studied are included in the supplementary material. With these coordinates the systems can be visualized, and with an appropriate DFT code our results can be reproduced. This data, along with the relevant metadata and the results of the Bader charge analysis, are also presented in the Harvard Dataverse. 1 See DOI: https://doi.org/10.1039/d4cp00249k Received 18th January 2024, Accepted 23rd May 2024 DOI: 10.1039/d4cp00249k rsc.li/pccp PCCP PAPER Open Access Article. Published on 24 May 2024. Downloaded on 6/25/2024 5:53:23 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online View Journal | View Issue This journal is © the Owner Societies 2024 Phys. Chem. Chem. Phys., 2024, 26, 17334–17344 | 17335 significantly more computationally expensive surface models. The surface models consisted of two different ethyl-saturated zinc oxide surfaces and the adsorbing organic precursor of the first precursor pulse. The organic precursor is adsorbed onto the zinc oxide surface via a branching chain reaction pathway, leading to either a ligand exchange or a dissociation reaction. 4-Aminophenol reacts faster with its hydroxyl group than with its amino group. The key step in the total hybrid thin film growth reaction was discovered to be the removal of the ethyl group as ethane. Different ethyl removal processes were discovered and found to depend on the coverage of the ethyl ligands on the zinc oxide surface. 1.1 Experimental results by other groups Hybrid thin films made of diethyl zinc with either hydroquinone 6–12 or 4-aminophenol 13,14 have been extensively researched experimentally. Hydroquinone and diethyl zinc were used by Yoon et al. 8 to fabricate hybrid thin films. With a deposition temperature of 150 1C, uniform and electron-conducting thin films were fabricated. The growth rate was 1.6 Å per cycle, and the total number of cycles was 1000. Yoon et al. 9 also combined hydroquinone and diethyl zinc layers with layers made of water and diethyl zinc to fabricate conductive and transparent thin films. The electron conductivity of the thin films was controlled by varying their composition. Even though pure zincone thin films have much lower electron conductivity than zinc oxide thin films, the thin films containing both structures combined can have a higher conductivity than pure zinc oxide thin films. Liu et al. 10 discovered that thermal conductivity can also be varied by varying the content of water, diethyl zinc, and hydroquinone in these films. This effect of varying the thermal conductivity was also discovered by Tynell et al., 11 who discovered that the thermal conductivity of zinc oxide can be lowered by implementing one layer of hydroquinone every 50th or 100th deposition cycle. Karttunen et al. 12 deposited these hybrid thin films on cotton textile substrates. Sood et al. 13 investigated the use of heterobifunctional 4-aminophenol in zincone hybrid thin films. Two different functional groups and the stiff backbone of the aromatic ring should prevent double reactions of the precursor within the same precursor pulse. These double reactions are undesirable because they hinder the growth of the thin films, as the precursor cannot react further with the diethyl zinc of the next precursor pulse. Zincone hybrid thin films were deposited on top of 40 zinc oxide layers. The deposition temperature ranged from 140 to 330 1C. The growth of the thin film was linear, with an approximate growth rate of 1.1 Å. The hybrid films were stable in the ambient atmosphere, even at relatively high humidity levels, when capped with few zinc oxide layers. The films were reported to be amorphous, and the FTIR spectra revealed that both the hydroxyl and amino groups were bonded to zinc. Hybrid thin films fabricated with varying ratios of 4-aminophenol, diethyl zinc, and water were investigated by Sundberg et al. 14 The ratio between the components was found to alter the hardness, chemical stability, surface roughness, density, and crystallinity of thin films. 4-Aminophenol can be removed from the structure using acetone, but this can be prevented by a 10 nm-thick topmost layer of zinc oxide. 1.2 Computational work by other groups Computational work related to the zincone hybrid thin films under study is relatively scarce. Zincone thin films were studied by Karttunen et al., 15 who studied superlattice structures, where a small layer of hydroquinone was surrounded by a ZnO crystal structure. They discovered that a chemical bond between every zinc atom of the zinc oxide surface and hydroquinone would lead to noticeable repulsive interactions between the adsorbed hydroquinone molecules, which can be lowered by halving the amount of hydroquinone on the surface. They also discovered that the band structure of the superlattice could be tailored by varying the organic content of the thin film. In addition, Choudbury et al. 16 studied zincone hybrid thin films using diethyl zinc (in the form of hydroxylated monoethyl zinc) and hydroquinone as precursors in a gas-phase model. They found that layers of zinc oxide stabilized the structure of the zincone hybrid thin film via a more exothermic reaction between the bonded monoethyl zinc and water than bonded monoethyl zinc and hydroquinone. They also discovered the hydroxylated monoethyl zinc molecule to have a reaction energy of 0.9 eV at room temperature when bonding with hydroquinone. Quite recently, computational research studies have been conducted on titanicone 17 and alucone 18,19 hybrid thin films. Tanskanen et al. 17 studied the thermodynamics of the reactions of both hydroquinone and 4-aminophenol with TiCl 4 , which were adsorbed onto a TiO 2 surface. They found that both the hydroxyl and amino groups of 4-aminophenol can bind to Ti 4+ - ions and that the hydroxyl group binds more strongly than the amino group. Muriqi et al. 18 studied the thermodynamics of reactions between the methyl-terminated Al 2 O 3 surface and both hydroquinone and 4-aminophenol. Both amino and hydroxyl groups bind favorably with either Al–O or Al–N bonds and eliminate CH 4 from the surface. The hydroxyl group was found to be more reactive than the amino group. Yang et al. 19 studied the gas-phase reaction between trimethyl aluminum and both 4-aminophenol and hydroquinone. They discovered that aluminum binds tightly to the hydroxyl group while eliminating H and CH 3 . This reaction is irreversible until all methyl groups are consumed. The reaction between the amino group and trimethyl aluminum is reversible. The reaction sequence was found to be considerably slower for the amino group than for the hydroxyl group. 1.3 Growth reactions of the ALD/MLD hybrid thin films The precursors responsible for the growth of the hybrid thin films investigated in this study are presented in Fig. 1. The surface reactions studied in this work are both the ligandexchange reaction presented in Fig. 2 and the dissociation reaction presented in Fig. 3. Later, the hydrogen donated in the dissociation reaction can migrate to an ethyl ligand, thus cleaving ethane from the surface. The migration reaction is presented in Fig. 4. Paper PCCP Open Access Article. Published on 24 May 2024. Downloaded on 6/25/2024 5:53:23 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 17336 | Phys. Chem. Chem. Phys., 2024, 26, 17334–17344 This journal is © the Owner Societies 2024 The macrostructure of the thin film can be either a more traditional hybrid thin film, where a single layer of organic molecules and a metal atom alternate, or a more unconventional superlattice structure, where layers of single organic molecules are surrounded by relatively thick zinc oxide layers. The differences between the hybrid thin film and superlattice structures are presented in Fig. 5. The reactions between the precursors presented in Fig. 1 were studied using DFT calculations. The energy DEof these reactions can be calculated by subtracting the energy of the initial state E init from the energy of the reacted state E fin , which corresponds to the final state of the reaction. The activation energy E act for the reaction between the precursors can be calculated using the energy of the highest energy state, i.e. the transition state E ts along the minimum energy path from the initial state to the final state of the reaction, and subtracting E init from E ts . One can also calculate the energy of the molecular adsorption DE m.ads . of the precursor molecule on the ethyl-saturated zinc oxide surface using eqn (1), DE m.ads. =E init E meox E adsorbate (1) where E init is the total energy of the molecular adsorption system, which is also the initial state of the chemisorption reaction. E meox is the energy of the ethyl-saturated zinc oxide slab, and E adsorbate is the total energy of an independent molecule 4-aminophenol. 2 Computational methods Modelling of the reaction pathways was conducted using GPAW 20 with an atomic simulation environment (ASE). 21 Visual molecular dynamics (VMD) 22 program was used in the visualization of the results. Density functional theory (DFT) was used with the Perdew–Burke–Ernzerhof (PBE) exchange and correlation functional. 23 The DFT description is known to provide a drastic underestimation of the band gap of the ZnO bulk band structure. 24 The effect of this underestimation on the reaction energies was tested using the Hubbard+U-correction 25 with parameters from Calzolari et al. 24 The effect on the reaction energies was found to be minute, and therefore, this correction was not used in the calculations. However, van der Waals correction TS09, by Tkatchenko and Scheffler, 26 was utilized to account for the weak interactions between a large aromatic Fig. 1 Precursors used to build ALD/MLD hybrid thin films investigated in this study: diethyl zinc (left) reacted either with hydroquinone (middle) or 4-aminophenol (right). Fig. 2 Ligand-exchange reaction investigated in this study, which grows ALD/MLD hybrid thin films: diethyl zinc reacted previously with the surface to create monoethyl zinc, which now reacts with 4-aminophenol resulting in the removal of gaseous ethane. Fig. 3 Dissociation reaction investigated in this study, which grows ALD/ MLD hybrid thin films: 4-aminophenol donates hydrogen to the surface structure, thus adsorbing to the surface without reacting with monoethyl zinc. Fig. 4 Migration reaction investigated in this study, which grows ALD/ MLD hybrid thin films: after dissociation reaction presented in Fig. 3, the hydrogen originally from 4-aminophenol can migrate to an ethyl ligand and cleave it as ethane from the surface. Fig. 5 The final composition of the film can be a hybrid thin film, where metal atoms and organic molecules alternate (presented on the left), or a superlattice structure, where organic molecule layers are surrounded by relatively thick zinc oxide layers (presented on the right). PCCP Paper Open Access Article. Published on 24 May 2024. Downloaded on 6/25/2024 5:53:23 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © the Owner Societies 2024 Phys. Chem. Chem. Phys., 2024, 26, 17334–17344 | 17337 precursor and ethyl ligands, which can have a noticeable contribution to the reaction energy of the system. The realspace grid spacing was 0.2 Å, and the sampling of the k-space was conducted with a 2 21 Monkhorst–Pack grid. The criterion for convergence was a force smaller than 0.05 eV Å 1 on all individual atoms, which offers a good balance between accuracy and computational efficiency. All of the reaction barriers were calculated using the nudged elastic band method with a climbing image, 27 utilizing a chain of 10 images of the system and a FIRE 28 optimizer. Bader analysis 29 was used to determine the absence of abnormal changes in the oxidation states of atoms during the reactions. These results are included in the Harvard Dataverse. 1 Due to the large number of rotational degrees of freedom of the ethyl ligands present in the studied ethyl-saturated zinc oxide surfaces, 3 ps molecular dynamics (MD) simulations with 1 fs time steps were used to effectively probe the potential energy surface. Then, geometry optimization was used for the generated structures between 500 fs intervals to find a minimum for the given surface structure. A small polarized double-z basis set was used for the dynamic simulations. The temperature was kept constant at 450 K using a Berendsen thermostat to prevent unwanted decomposition, which could be caused by a temperature that is too high. Additionally, relaxed surface scans were employed to determine an energetically favorable structure for the physisorption of 4-aminophenol onto the zinc oxide surface. The reaction coordinate was chosen to be the distance between the nitrogen or oxygen atom of the functional group of 4-aminophenol and the chosen zinc atom in the surface structure. Then, using constrained optimization, the surface was probed along the reaction coordinate. The use of DFT, especially when not paired with a relatively computationally expensive hybrid functional, can lead to an underestimation of activation energies of chemical reactions. 30 This study focuses on the relative comparison of the transition state energies and the detection of reaction paths with relatively high activation energies. Even if the real reaction barriers are slightly higher than our results, this would not change the conclusions of our work. Despite these limitations in the calculations of absolute energies, DFT is recognized as the most viable and, thus, widely used approach to gather information about the chemical reaction rates for complicated surface reactions. 31–33 DFT can also be utilized in the ab initio molecular dynamics, 34 which can aid in the research of dynamical systems, but this approach is useful only if reaction rates are fast, which is not the case for the chemical reactions investigated in this study. 3 Results 3.1 Gas-phase reactions Gas-phase reactions were used to model the ligand elimination reactions that occur on the surface during the deposition of organic precursors. The studied gas-phase reactions consisted of a 4-aminophenol or hydroquinone precursor that reacted with a diethyl zinc molecule. 4-Aminophenol can either react with an amino or a hydroxide group, and therefore, reactions with both functional groups were investigated. Due to the similarities between the reactions of the hydroxide group in both 4-aminophenol and hydroquinone molecules, the surface reactions of both hydroquinone and 4-aminophenol were studied using only a 4-aminophenol molecule. Along with the data regarding hydroquinone, the activation energies of both reactions containing 4-aminophenol (presented in Fig. 6 and 7) are listed in Table 1. 3.2 Surface reactions Two ethyl-ligand-saturated zinc oxide surface slabs were used in this study. These surfaces model the experimental system presented in the experimental work by Sood et al., 13 where the base layer of hybrid thin films consists of 40 ALD cycles of alternating diethyl zinc and water pulses, thus forming zinc oxide. The final diethyl zinc pulse leaves the surface saturated with monoethyl zinc, and thus the first organic precursor pulse of the ALD/MLD-thin film reacts with monoethyl zinc. As the forming thin film of an ALD process is dependent on numerous variables such as temperature, the actual structure of the model benefits when it is benchmarked against Fig. 6 Initial state (left), transition state (middle), and final state (right) of the reaction between the hydroxyl group of 4-aminophenol and diethyl zinc. This reaction is very similar to that between hydroquinone and diethyl zinc. The colors of the atoms in the figure: oxygen in red, nitrogen in blue, hydrogen in white, zinc in light grey, and carbon in black. Fig. 7 Initial state (left), transition state (middle), and final state (right) of the reaction between the amino group of 4-aminophenol and diethyl zinc molecule. The atom colors are the same as those in Fig. 6. Table 1 Reaction parameters of the studied gas-phase reactions: diethyl zinc reacts with either 4-aminophenol or hydroquinone Precursor reacting with diethyl zinc E a (eV) DE(eV) Hydroquinone 0.9 0.7 4-Aminophenol (hydroxyl group) 0.8 0.7 4-Aminophenol (amino group) 1.5 0.1 Paper PCCP Open Access Article. Published on 24 May 2024. Downloaded on 6/25/2024 5:53:23 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 17338 | Phys. Chem. Chem. Phys., 2024, 26, 17334–17344 This journal is © the Owner Societies 2024 experimental thin films. Hence, ethyl-saturated zinc oxide surface slabs were adopted from the work of Weckman and Laasonen, 35,36 and are presented in Fig. 8. These zinc oxide structures are rigorously validated estimates of thin film structures fabricated at lower (approximately 100–140 1C) and higher (approximately 140–180 1C) deposition temperatures. The structure deposited at lower temperatures, denoted as case 1, occurs when the hydroxyl groups of the hydroxylated (100) zinc oxide surface react with five diethyl zinc molecules in a ligand-exchange reaction, as shown in eqn (2). Hydroxylation was conducted using four dissociated water molecules on three layers of (ZnO) 8 . Five ligand-exchange reactions eliminated a total of five hydrogen atoms from the dissociated water from the surface, making the chemical formula of case 1 surface (ZnO) 24 (ZnC 2 H 5 ) 5 O 4 H 3 . Five monoethyl zincs were chosen, as higher concentrations made the structure unstable, thus making the final coverage of monoethyl zinc 7.1 nm 2 . 35 ||Zn OH + Zn(C 2 H 5 ) 2 -||Zn OZn(C 2 H 5 )+C 2 H 6 (2) In the higher deposition temperature structure, denoted case 2, the structure is similar to case 1, as three layers of (ZnO) 8 were initially hydroxylated with four dissociating water molecules. However, at higher deposition temperatures, monoethyl zinc can also react further, thus creating bare zinc atoms on the surface, as illustrated in eqn (3). For the case 2 structure, it was assumed that all hydrogens on the surface were consumed in the ligand-exchange reactions, as presented in eqn (2) and (3). The most stable structure included four monoethyl zinc groups and two zinc atoms; thus, the chemical formula of the surface in case 2 was (ZnO) 24 (ZnC 2 H 5 ) 4 Zn 2 O 4 . The final coverage of monoethyl zinc was 5.7 nm 2 . 35 ||Zn OZn(C 2 H 5 )+||Zn OH -(||Zn O) 2 Zn + C 2 H 6 (3) 3.2.1 Physisorption reactions on both surfaces. Due to the large number of degrees of freedom in the adsorption system, finding an energetically favorable physisorption site is challenging. A combination of molecular dynamics, relaxed surface scans, and constrained geometry optimization was used to identify an exothermic physisorption site for 4-aminophenol. The final configuration for physisorption was always acquired without any constraints, apart from the constraint of holding the lowest layer of atoms in place. For the lower deposition temperature system (denoted case 1), the physisorption energy was 0.2 eV for physisorption with both functional groups. For the higher deposition temperature system (denoted case 2), the physisorption energy was 0.3 eV for both functional groups. 3.2.2 Chemisorption reactions on the case 1 surface. The direct and indirect elimination of the ethyl ligand from the studied surface structures is caused by the donation of a hydrogen atom originally located at either the amino or hydroxyl group of 4-aminophenol. In the direct ligand elimination reaction, 4-aminophenol donates its hydrogen to an ethyl ligand while adsorbing to the surface by utilizing the ligandexchange reaction, thus removing gaseous ethane from the surface. Indirect ligand elimination begins with the dissociative adsorption of 4-aminophenol, followed by the migration of hydrogen from the hydroxyl group of the surface to the ethyl ligand. This hydroxyl group initially receives its hydrogen atom from the dissociatively adsorbed 4-aminophenol molecule. For indirect ligand elimination in the case 1 structure, there is only one feasible ligand for the hydrogen to migrate from its adsorbing site (i.e. the hydroxyl group of the surface), which is the same ligand as in the direct elimination reaction. The reactions and reaction parameters for the direct and indirect elimination reaction pathways are illustrated in Fig. 9 and 10. 3.2.3 Chemisorption reactions on the case 2 surface. Both the direct and indirect elimination of the ethyl ligand are also available on the case 2 surface. After dissociative adsorption of the indirect ligand elimination reaction path, there are three available ligands for hydrogen migration, from which one is the same ligand as in the direct elimination reaction. The other two are neighboring ligands with a feasible pathway for the adsorbed hydrogen to migrate to. All studied reaction pathways for the elimination reaction through hydrogen donation are presented in Fig. 11, where the functional group of 4-aminophenol that donates a hydrogen atom is the hydroxyl group, and in Fig. 12, where the amino group of 4-aminophenol donates a hydrogen atom. 4 Discussion 4.1 Gas-phase model The gas-phase reactions between diethyl zinc and either hydroquinone or 4-aminophenol proceeded as expected. Either amino or hydroxyl groups donated hydrogen to eliminate ethane from diethyl zinc. Simultaneously, oxygen or nitrogen formed a bond with the zinc of diethyl zinc. All of the studied gas-phase reactions were exothermic. The hydroxyl groups of 4aminophenol and hydroquinone have similar activation and Fig. 8 Structures of ethyl-saturated zinc oxide surface slabs used in this study: case 1 (left) is an estimate of the structure at lower deposition temperatures, whereas case 2 (right) is an estimate of the structure at higher deposition temperatures. The atom colors are the same as those in Fig. 6. PCCP Paper Open Access Article. Published on 24 May 2024. Downloaded on 6/25/2024 5:53:23 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © the Owner Societies 2024 Phys. Chem. Chem. Phys., 2024, 26, 17334–17344 | 17339 reaction energies. Therefore, the surface model was investigated using only 4-aminophenol as the organic precursor. The amino group of 4-aminophenol reacts more slowly than the hydroxide group, both kinetically and thermodynamically. Fig. 9 Reaction pathways for the direct and indirect ligand elimination reactions on the case 1 surface, when the hydroxide group of the 4-aminophenol acts as the hydrogen-donating group. This hydrogen atom can be donated either directly from the physisorbed 4-aminophenol, or it can be initially chemisorbed to an oxygen atom in the surface structure, from where it will migrate to the ethyl ligand. The atom colors are the same as those in Fig. 6. The most feasible reaction pathway is shown in bold. Migrating hydrogen atoms are marked with an asterisk. Fig. 10 Reaction pathways for the direct and indirect ligand elimination reactions on the case 1 surface, when the amino group of the 4-aminophenol acts as the hydrogen-donating group. This hydrogen atom can be donated either directly from the physisorbed 4-aminophenol, or it can be initially chemisorbed to an oxygen atom in the surface structure, from where it will migrate to the ethyl ligand. The atom colors are the same as those in Fig. 6. The most feasible reaction pathway is shown in bold. Migrating hydrogen atoms are marked with an asterisk. Paper PCCP Open Access Article. Published on 24 May 2024. Downloaded on 6/25/2024 5:53:23 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online 17340 | Phys. Chem. Chem. Phys., 2024, 26, 17334–17344 This journal is © the Owner Societies 2024 This result suggests that 4-aminophenol first reacts with its hydroxyl group. According to our surface model, additional surface reaction pathways are available at the interface between zinc oxide and the organic layer of these films, which are not taken into account in the gas-phase model; however, these do not change this result. The relatively high activation energies, especially for the reactions of the amino group, are in agreement with the slow growth of the zincone thin film presented by Sood et al. 13 Interestingly, the tendency for 4-aminophenol to react with the hydroxyl rather than the amino group first has been discovered computationally also for titanicone 17 and alucone 18,19 hybrid thin films. Due to the amorphous structure 13 of these films, the growth should be more polymer-like in later precursor pulses, which in practice corresponds to more degrees of freedom for the reactants. This further indicates that gas-phase reactions are beneficial in the discovery of the growth reactions in later pulses, as they offer a hard limit on the effect of degrees of freedom for the system, as an amorphous or crystal structure is always more rigid than a gas-phase structure. 4.2 Surface model At the higher ethyl-ligand concentration surface (case 1), the most feasible reaction path for 4-aminophenol adsorption Fig. 11 Reaction pathways for the direct and indirect ligand elimination reactions on the case 2 surface, initiated by the donation of a hydrogen atom from the 4-aminophenol’s hydroxide group. The ligand-exchange reaction is hindered by dissociative adsorption, which will lead to a reverse reaction back to the initial state or to a very slow hydrogen migration to the ethyl ligand. The atom colors are the same as those in Fig. 6. The most feasible reaction pathway is shown in bold. Migrating hydrogen atoms are marked with an asterisk. PCCP Paper Open Access Article. Published on 24 May 2024. Downloaded on 6/25/2024 5:53:23 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online This journal is © the Owner Societies 2024 Phys. Chem. Chem. Phys., 2024, 26, 17334–17344 | 17341 utilizes an exothermic ligand-exchange reaction to react with the hydroxyl group first, with an activation energy of 0.9 eV. For reactions with the hydroxyl group, this reaction path is more feasible than the indirect reaction pathway, which starts with the dissociation reaction, due to the lower activation energy of the reaction. For the amino group on the case 1 surface, the high 1.4 eV activation energy of the direct ligand-exchange reaction causes the dissociation reaction to be the most viable reaction path. However, the viability of this reaction path to eliminate ethane from the surface is hindered by the fact that the desorption of molecular 4-aminophenol (with an activation energy of 0.3 eV) is approximately equal in reaction rate to the migration of hydrogen (with an activation energy of 0.4 eV), which eliminates ethane from the surface. This causes a dynamic reaction, where the reaction alternates between the initial physisorbed state and the dissociatively chemisorbed state, and approximately half of the dissociatively adsorbed 4-aminophenol proceeds to eliminate the ethyl ligand as ethane. To summarize, on the case 1 surface, the main growth reaction of the hybrid thin film is the ligand-exchange reaction, where 4-aminophenol first reacts with its hydroxyl group. The ALD/MLD process is mainly driven by reaction kinetics due to the relatively short pulsing times, and the 0.9 eV barrier of the direct ligand elimination is the lowest on the case 1 surface. Fig. 12 Reaction pathways for the direct and indirect ligand elimination reactions on the case 2 surface, initiated by donation of a hydrogen atom from the amino group of 4-aminophenol. The activation energies of these reactions are relatively high, and therefore, the reactions of the hydroxyl group presented in Fig. 11 are significantly more feasible on the case 2 surface. The atom colors are the same as in Fig. 6. The most feasible reaction pathway is in bold. The migrating hydrogen atom is marked with an asterisk. Paper PCCP Open Access Article. Published on 24 May 2024. Downloaded on 6/25/2024 5:53:23 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. View Article Online