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In the past decade, atomic layer deposition (ALD) has become an important thin film deposition technique for applications in nanoelectronics, catalysis, and other areas due to its high conformality on 3-D nanostructured substrates and control of the film thickness at the atomic level. The current applications of ALD primarily involve binary metal oxides, but for new applications there is increasing interest in more complex materials such as doped, ternary, and quaternary materials. This article reviews how these multicomponent materials can be synthesized by ALD, gives an overview of the materials that have been reported in the literature to date, and discusses important challenges. The most commonly employed approach to synthesize these materials is to combine binary ALD cycles in a supercycle, which provides the ability to control the composition of the material by choosing the cycle ratio. Discussion will focus on four main topics: (i) the characteristics, benefits, and drawbacks of the approaches that currently exist for the synthesis of multicomponent materials, with special attention to the supercycle approach; (ii) the trends in precursor choice, process conditions, and characterization methods, as well as underlying motivations for these design decisions; (iii) the distribution of atoms in the deposited material and the formation of specific (crystalline) phases, which is shown to be dependent on the ALD cycle sequence, deposition temperature, and post-deposition anneal conditions; and (iv) the nucleation effects that occur when switching from one binary ALD process to another, with different explanations provided for why the growth characteristics often deviate from what is expected. This paper provides insight into how the deposition conditions (cycle sequence, temperature, etc.) affect the properties of the resultant thin films, which can serve as a guideline for designing new ALD processes. Furthermore, with an extensive discussion on the nucleation effects taking place during the growth of ternary materials, we hope to contribute to a better understanding of the underlying mechanisms of the ALD growth of multicomponent materials.
Electrochemical processes for ammonia synthesis could potentially replace the high temperature and pressure conditions of the Haber-Bosch process, with voltage offering a pathway to distributed fertilizer production that leverages the rapidly decreasing cost of renewable electricity. However, nitrogen is an unreactive molecule and the hydrogen evolution reaction presents a major selectivity challenge. An electrode of electrodeposited lithium in tetrahydrofuran solvent overcomes both problems by providing a surface that easily reacts with nitrogen and by limiting the access of protons with a nonaqueous electrolyte. Under these conditions, we measure relatively high faradaic efficiencies (ca. 10 %) and rates (0.1 mA cm À 2 ) toward NH 3 . We observe the development of a solid electrolyte interface layer as well as the accumulation of lithium and lithium-containing species. Detailed DFT studies suggest lithium nitride and hydride to be catalytically active phases given their thermodynamic and kinetic stability relative to metallic lithium under reaction conditions and the fast diffusion of nitrogen in lithium.[a] Dr.
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