Rapid Growth of Crystalline Mn5O8 by Self-Limited Multilayer Deposition using Mn(EtCp)2 and O3

Role of Surface Chemistry in Pyrrole Autoxidation

Chemical compounds in liquid hydrocarbon fuels that contain five-membered pyrrole (Py) rings readily react with oxygen from air and polymerize through a process known as autoxidation. Autoxidation degrades the quality of fuel and leads to the formation of unwanted gum deposits in fuel storage vessels and engine components. Recent work has found that the rate of formation of these gum deposits is affected by material surfaces exposed to the fuel, but the origins of these effects are not yet understood. In this work, atomic layer deposition (ALD) is employed to grow aluminum oxide, zinc oxide, titanium dioxide, and manganese oxide films on silicon substrates to control material surface chemistry and study Py adsorption and gum nucleation on these surfaces. Quartz crystal microbalance (QCM) studies of gas-phase Py adsorption indicate 1.5-2.8 kcal/mol exergonic adsorption of Lewis basic Py onto Lewis acidic surface sites. More favorable Py adsorption onto Lewis acidic surfaces correlates with faster polypyrrole (PPy) film nucleation in vapor phase oxidative molecular deposition (oMLD) polymerization studies. Liquid-phase studies of Py autoxidation reveal primarily particulate formation, indicating a homogeneous PPy propagation step rather than a completely surface-based polymerization mechanism. The amount of PPy particulate formation is positively correlated with more acidic surfaces (lower pH-PZC values), indicating that the rate-limiting step for Py autoxidation involves Lewis acidic surface sites. These studies help to establish new mechanistic insights into the role of surface chemistry in the autoxidation of pyrrolic species. We apply this knowledge to demonstrate a polymer coating formed by vapor phase polymer deposition that slows autoxidation by 2 orders of magnitude.

Atomic layer deposition of thin-film sodium manganese oxide cathode materials for sodium ion batteries

To improve the performance of sodium ion batteries (NIBs), we need to better understand the materials chemistry occurring at the surface of NIB cathode materials. In this work, we aim to form thin films of sodium manganese oxide (NMO) cathode materials for NIBs using atomic layer deposition (ALD) with the vision to isolate and study these interfacial processes in the absence of bulk NMO. We combine established chemistries for ALD of manganese oxide (MnO_x) using Mn(thd)₃/O₃ and sodium hydroxide (NaOH) using NaO^tBu/H₂O and adjust the sequence and ratios of these two chemistries to form Na_xMn_yO alloy films. We identify that increasing the O₃ exposure during Mn(thd)₃/O₃ ALD beyond previously reported values increases the growth rate of MnO_x from 0.23 to 0.62 Å per cycle and provides improved uniformity, yielding predominantly Mn₅O₈. Furthermore, alloying Mn(thd)₃/O₃ with NaO^tBu/H₂O mutually enhances the growth rate of both ALD chemistries, yielding a growth rate of ∼9 Å per supercycle for a 1 : 1 cycle ratio. This enhancement in growth arises from sub-surface reactions, including the reaction of NaO^tBu to a depth of ∼1.3 nm into bulk MnO_x to form Na₂MnO_x. By tuning cycle ratios and growth conditions, we demonstrate control over the Na_xMn_yO composition and measure different electrochemical properties depending on the composition. The formation of NMO thin films with controlled thickness and composition established in this work provides a means to systematically study interfacial processes occurring in NMO cathode materials for NIBs.

Synthesis of Ilmenite-type ε-Mn2O3 and Its Properties

In contrast to the corundum-type A₂X₃ structure, which has only one crystallographic site available for trivalent cations (e.g., in hematite), the closely related ABX₃ ilmenite-type structure comprises two different octahedrally coordinated positions that are usually filled with differently charged ions (e.g., in Fe²⁺Ti⁴⁺O₃ ilmenite). Here, we report a synthesis of the first binary ilmenite-type compound fabricated from a simple transition-metal oxide (Mn₂O₃) at high-pressure high-temperature (HP-HT) conditions. We experimentally established that, at normal conditions, the ilmenite-type Mn²⁺Mn⁴⁺O₃ (ε-Mn₂O₃) is an n-type semiconductor with an indirect narrow band gap of E_g = 0.55 eV. Comparative investigations of the electronic properties of ε-Mn₂O₃ and previously discovered quadruple perovskite ζ-Mn₂O₃ phase were performed using X-ray absorption near edge spectroscopy. Magnetic susceptibility measurements reveal an antiferromagnetic ordering in ε-Mn₂O₃ below 210 K. The synthesis of ε-Mn₂O₃ indicates that HP-HT conditions can induce a charge disproportionation in simple transition-metal oxides A₂O₃, and potentially various mixed-valence polymorphs of these oxides, for example, with ilmenite-type, LiNbO₃-type, perovskite-type, and other structures, could be stabilized at HP-HT conditions.

Structural Stability and Properties of Marokite-Type γ-Mn3O4

We synthesized single crystals of marokite (CaMn₂O₄)-type orthorhombic manganese (II,III) oxide, γ-Mn₃O₄, in a multianvil apparatus at pressures of 10-24 GPa. The magnetic, electronic, and optical properties of the crystals were investigated at ambient pressure. It was found that γ-Mn₃O₄ is a semiconductor with an indirect band gap E_g of 0.96 eV and two antiferromagnetic transitions (T_N) at ∼200 and ∼55 K. The phase stability of the γ-Mn₃O₄ crystals was examined in the pressure range of 0-60 GPa using single-crystal X-ray diffraction and Raman spectroscopy. A bulk modulus of γ-Mn₃O₄ was determined to be B₀ = 235.3(2) GPa with B' = 2.6(6). The γ-Mn₃O₄ phase persisted over the whole pressure range studied and did not transform or decompose upon laser heating of the sample to ∼3500 K at 60 GPa. This result seems surprising, given the high-pressure structural diversity of iron oxides with similar stoichiometries. With an increase in pressure, the degree of distortion of MnO₆ polyhedra decreased. Furthermore, there are signs indicating a limited charge transfer between the Mn³⁺ ions in the octahedra and the Mn²⁺ ions in the trigonal prisms. Our results demonstrate that the high-pressure behavior of the structural, electronic, and chemical properties of manganese oxides strongly differs from that of iron oxides with similar stoichiometries.

Inherently Area-Selective Atomic Layer Deposition of Manganese Oxide through Electronegativity-Induced Adsorption

Manganese oxide (MnO_x) shows great potential in the areas of nano-electronics, magnetic devices and so on. Since the characteristics of precise thickness control at the atomic level and self-align lateral patterning, area-selective deposition (ASD) of the MnO_x films can be used in some key steps of nanomanufacturing. In this work, MnO_x films are deposited on Pt, Cu and SiO₂ substrates using Mn(EtCp)₂ and H₂O over a temperature range of 80–215 °C. Inherently area-selective atomic layer deposition (ALD) of MnO_x is successfully achieved on metal/SiO₂ patterns. The selectivity improves with increasing deposition temperature within the ALD window. Moreover, it is demonstrated that with the decrease of electronegativity differences between M (M = Si, Cu and Pt) and O, the chemisorption energy barrier decreases, which affects the initial nucleation rate. The inherent ASD aroused by the electronegativity differences shows a possible method for further development and prediction of ASD processes.

Probing the Atomic-Scale Structure of Amorphous Aluminum Oxide Grown by Atomic Layer Deposition

Atomic layer deposition (ALD) is a well-established technique for depositing nanoscale coatings with pristine control of film thickness and composition. The trimethylaluminum (TMA) and water (H₂O) ALD chemistry is inarguably the most widely used and yet to date, we have little information about the atomic-scale structure of the amorphous aluminum oxide (AlO_x) formed by this chemistry. This lack of understanding hinders our ability to establish process-structure-property relationships and ultimately limits technological advancements employing AlO_x made via ALD. In this work, we employ synchrotron high-energy X-ray diffraction (HE-XRD) coupled with pair distribution function (PDF) analysis to characterize the atomic structure of amorphous AlO_x ALD coatings. We combine ex situ and in operando HE-XRD measurements on ALD AlO_x and fit these experimental data using stochastic structural modeling to reveal variations in the Al-O bond length, Al and O coordination environment, and extent of Al vacancies as a function of growth conditions. In particular, the local atomic structure of ALD AlO_x is found to change with the substrate and number of ALD cycles. The observed trends are consistent with the formation of bulk Al₂O₃ surrounded by an O-rich surface layer. We deconvolute these data to reveal atomic-scale structural information for both the bulk and surface phases. Overall, this work demonstrates the usefulness of HE-XRD and PDF analysis in improving our understanding of the structure of amorphous ALD thin films and provides a pathway to evaluate how process changes impact the structure and properties of ALD films.

Quantitative Analysis of the Incorporation Behaviors of Sr and Ti Atoms During the Atomic Layer Deposition of SrTiO3 Thin Films

The atomic layer deposition (ALD) of multication oxide films is complicated because the deposition behaviors of the component oxides are not independent of one another. In this study, the Ti and Sr atom incorporation behaviors during the ALD of SrTiO₃ films were quantitatively examined via the carefully designed ALD process sequences. H₂O and O₃ were adopted as the oxygen sources of the SrO subcycles, whereas only O₃ was used for the TiO₂ ALD subcycles. Apart from the general conjecture on the roles of the different types of oxygen sources, the oxygen source that was adopted for the subcycles of the other component oxide had almost complete control of the metal atom incorporation behaviors. This means that the first half-cycle of ALD played a dominant role in determining the metal incorporation rate, which revealed the critical role of the steric hindrance effect during the metal precursor injection for the ALD rate. O₃ had almost doubled its reactivity toward the Ti and Sr precursors compared with H₂O. Although these are the expected results from the common knowledge on ALD, the quantitative analysis of the incorporation behaviors of each metal atom provided insightful viewpoints for the ALD process of this technically important oxide material. Furthermore, the SrTiO₃ films with a bulk dielectric constant as high as 236 were obtained by the Ru-SrTiO₃-RuO₂ capacitor structure.

An aminopyridinato Mn(ii) compound as a novel CVD precursor for manganese-containing films

An aminopyridinato Mn(ii) compound as a novel CVD precursor for manganese-containing films.