Inhibiting Metal Oxide Atomic Layer Deposition: Beyond Zinc Oxide

Atomic layer deposition (ALD) of several metal oxides is selectivity inhibited on alkanethiol self-assembled monolayers (SAMs) on Au, and the eventual nucleation mechanism is investigated. The inhibition ability of the SAM is significantly improved by the in situ H₂-plasma pretreatment of the Au substrate prior to the gas-phase deposition of a long-chain alkanethiol, 1-dodecanethiol (DDT). This more rigorous surface preparation inhibits even aggressive oxide ALD precursors, including trimethylaluminum and water, for at least 20 cycles. We study the effect that the ALD precursor purge times, growth temperature, alkanethiol chain length, alkanethiol deposition time, and plasma treatment time have on Al₂O₃ ALD inhibition. This is the first example of Al₂O₃ ALD inhibition from a vapor-deposited SAM. The inhibitions of Al₂O₃, ZnO, and MnO ALD processes are compared, revealing the versatility of this selective surface treatment. Atomic force microscopy and grazing-incidence X-ray fluorescence further reveal insight into the mechanism by which the well-defined surface chemistry of ALD may eventually be circumvented to allow metal oxide nucleation and growth on SAM-modified surfaces

Epitaxial Atomic Layer Deposition of Sn-Doped Indium Oxide

Coherently strained, epitaxial Sn-doped In₂O₃ (ITO) thin films were fabricated at temperatures as low as 250 °C using atomic layer deposition (ALD) on (001)-, (011)-, and (111)-oriented single-crystal Y-stabilized ZrO₂ (YSZ) substrates. Resultant films possess cube-on-cube epitaxial relationships with the underlying YSZ substrates and are smooth, highly conductive, and optically transparent. This epitaxial ALD approach is favorable compared to many conventional growth techniques as it is a large-scale synthesis method that does not necessitate the use of high temperatures or ultrahigh vacuum. These films may prove valuable as a conductive growth template in areas where high-quality crystalline thin film substrates are important, such as solar energy materials, light-emitting diodes, or wide bandgap semiconductors. Furthermore, we discuss the applicability of this ALD system as an excellent model system for the study of ALD surface chemistry, nucleation, and film growth

Low-Temperature Atomic Layer Deposition of CuSbS₂ for Thin-Film Photovoltaics

Copper antimony sulfide (CuSbS₂) has been gaining traction as an earth-abundant absorber for thin-film photovoltaics given its near ideal band gap for solar energy conversion (∼1.5 eV), large absorption coefficient (>10⁴ cm^–1), and elemental abundance. Through careful in situ analysis of the deposition conditions, a low-temperature route to CuSbS₂ thin films via atomic layer deposition has been developed. After a short (15 min) postprocess anneal at 225 °C, the ALD-grown CuSbS₂ films were crystalline with micron-sized grains, exhibited a band gap of 1.6 eV and an absorption coefficient >10⁴ cm^–1, as well as a hole concentration of 10¹⁵ cm^–3. Finally, the ALD-grown CuSbS₂ films were paired with ALD-grown TiO₂ to form a photovoltaic device. This photovoltaic device architecture represents one of a very limited number of Cd-free CuSbS₂ PV device stacks reported to date, and it is the first to demonstrate an open-circuit voltage on par with CuSbS₂/CdS heterojunction PV devices. While far from optimized, this work demonstrates the potential for ALD-grown CuSbS₂ thin films in environmentally benign photovoltaics

Porphyrins as Templates for Site-Selective Atomic Layer Deposition: Vapor Metalation and in Situ Monitoring of Island Growth

Examinations of enzymatic catalysts suggest one key to efficient catalytic activity is discrete size metallo clusters. Mimicking enzymatic cluster systems is synthetically challenging because conventional solution methods are prone to aggregation or require capping of the cluster, thereby limiting its catalytic activity. We introduce site-selective atomic layer deposition (ALD) on porphyrins as an alternative approach to grow isolated metal oxide islands that are spatially separated. Surface-bound tetra-acid free base porphyrins (H₂TCPP) may be metalated with Mn using conventional ALD precursor exposure to induce homogeneous hydroxide synthetic handles which acts as a nucleation point for subsequent ALD MnO island growth. Analytical fitting of in situ QCM mass uptake reveals island growth to be hemispherical with a convergence radius of 1.74 nm. This growth mode is confirmed with synchrotron grazing-incidence small-angle X-ray scattering (GISAXS) measurements. Finally, we extend this approach to other ALD chemistries to demonstrate the generality of this route to discrete metallo island materials

Resolving the Chemically Discrete Structure of Synthetic Borophene Polymorphs

Atomically thin two-dimensional (2D) materials exhibit superlative properties dictated by their intralayer atomic structure, which is typically derived from a limited number of thermodynamically stable bulk layered crystals (e.g., graphene from graphite). The growth of entirely synthetic 2D crystals, those with no corresponding bulk allotrope, would circumvent this dependence upon bulk thermodynamics and substantially expand the phase space available for structure–property engineering of 2D materials. However, it remains unclear if synthetic 2D materials can exist as structurally and chemically distinct layers anchored by van der Waals (vdW) forces, as opposed to strongly bound adlayers. Here, we show that atomically thin sheets of boron (i.e., borophene) grown on the Ag(111) surface exhibit a vdW-like structure without a corresponding bulk allotrope. Using X-ray standing wave-excited X-ray photoelectron spectroscopy, the positions of boron in multiple chemical states are resolved with sub-angström spatial resolution, revealing that the borophene forms a single planar layer that is 2.4 Å above the unreconstructed Ag surface. Moreover, our results reveal that multiple borophene phases exhibit these characteristics, denoting a unique form of polymorphism consistent with recent predictions. This observation of synthetic borophene as chemically discrete from the growth substrate suggests that it is possible to engineer a much wider variety of 2D materials than those accessible through bulk layered crystal structures

Real-Time Observation of Atomic Layer Deposition Inhibition: Metal Oxide Growth on Self-Assembled Alkanethiols

Through in situ quartz crystal microbalance (QCM) monitoring, we resolve the growth of a self-assembled monolayer (SAM) and subsequent metal oxide deposition with high resolution. We introduce the fitting of mass deposited during each atomic layer deposition (ALD) cycle to an analytical island-growth model that enables quantification of growth inhibition, nucleation density, and the uninhibited ALD growth rate. A long-chain alkanethiol was self-assembled as a monolayer on gold-coated quartz crystals in order to investigate its effectiveness as a barrier to ALD. Compared to solution-loading, vapor-loading is observed to produce a SAM with equal or greater inhibition ability in minutes vs days. The metal oxide growth temperature and the choice of precursor also significantly affect the nucleation density, which ranges from 0.001 to 1 sites/nm². Finally, we observe a minimum 100 cycle inhibition of an oxide ALD process, ZnO, under moderately optimized conditions

Liquid Water- and Heat-Resistant Hybrid Perovskite Photovoltaics via an Inverted ALD Oxide Electron Extraction Layer Design

Despite rapid advances in conversion efficiency (>22%), the environmental stability of perovskite solar cells remains a substantial barrier to commercialization. Here, we show a significant improvement in the stability of inverted perovskite solar cells against liquid water and high operating temperature (100 °C) by integrating an ultrathin amorphous oxide electron extraction layer via atomic layer deposition (ALD). These unencapsulated inverted devices exhibit a stable operation over at least 10 h when subjected to high thermal stress (100 °C) in ambient environments, as well as upon direct contact with a droplet of water without further encapsulation

Liquid Water- and Heat-Resistant Hybrid Perovskite Photovoltaics via an Inverted ALD Oxide Electron Extraction Layer Design

Despite rapid advances in conversion efficiency (>22%), the environmental stability of perovskite solar cells remains a substantial barrier to commercialization. Here, we show a significant improvement in the stability of inverted perovskite solar cells against liquid water and high operating temperature (100 °C) by integrating an ultrathin amorphous oxide electron extraction layer via atomic layer deposition (ALD). These unencapsulated inverted devices exhibit a stable operation over at least 10 h when subjected to high thermal stress (100 °C) in ambient environments, as well as upon direct contact with a droplet of water without further encapsulation

In Situ X‑ray Study of the Solid Electrolyte Interphase (SEI) Formation on Graphene as a Model Li-ion Battery Anode

The solid electrolyte interphase (SEI) plays a critical role in the performance and safety of Li-ion batteries, but the crystal structure of the materials formed have not been previously studied. We employ the model system of epitaxial graphene on SiC to provide a well-defined graphitic surface to study the crystallinity and texture formation in the SEI. We observe, via in situ synchrotron X-ray scattering, the formation and growth of LiF crystallites at the graphene surface, which increase in size with lithiation dose and are textured such that the LiF (002) planes are approximately parallel to the graphene sheets. Furthermore, X-ray photoelectron spectroscopy (XPS) reveals the composition of the SEI formed in this system to consist of LiF and organic compounds similar to those found previously on graphite. SEI components, other than LiF, do not produce X-ray diffraction peaks and are categorized as amorphous. From high-resolution transmission electron microscopy, the LiF crystallites are seen in near proximity to the graphene surface along with additional apparently amorphous material, which is likely to be other SEI components detected by XPS and/or misoriented LiF. This new understanding that LiF crystallites grow on the graphene surface with strong texturing will assist future efforts to model and engineer the SEI formed on graphitic materials

Water Oxidation Catalysis via Size-Selected Iridium Clusters

The detailed mechanism and efficacy of four-electron electrochemical water oxidation depend critically upon the detailed atomic structure of each catalytic site, which are numerous and diverse in most metal oxides anodes. In order to limit the diversity of sites, arrays of discrete iridium clusters with identical metal atom number (Ir₂, Ir₄, or Ir₈) were deposited in submonolayer coverage on conductive oxide supports, and the electrochemical properties and activity of each was evaluated. Exceptional electroactivity for the oxygen evolving reaction (OER) was observed for all cluster samples in acidic electrolyte. Reproducible cluster-size-dependent trends in redox behavior were also resolved. First-principles computational models of the individual discrete-size clusters allow correlation of catalytic-site structure and multiplicity with redox behavior