**Preface to "Atomic Layer Deposition"**

Atomic layer deposition (ALD) is a process that is renowned for its ability to produce films with unrivaled thickness control, conformability to three-dimensional structures, control over composition, and versatility in the range of materials it can produce from quaternary compounds to elemental metals. It has expanded from a small-scale batch process to large scale production now also including continuous processing—spatial ALD. It has matured into an industrial technology essential for many areas of materials science and engineering from microelectronics to corrosion protection. Its attributes make it a key technology in studying new materials and structures over an enormous range of applications. This Special Issue contains six research articles and one review article that illustrate the breadth of these applications from energy storage in batteries or supercapacitors to catalysis via x-ray, UV, and visible optics. They deal with the details of the ALD processes that produce these various films, the properties of the devices, and simulations that illustrate how the ALD system configuration affects the deposition process. In their research article, de la Huerta et al. explored gas flow issues in atmospheric pressure spatial ALD by computational fluid dynamics. They demonstrate the influence of the size and uniformity of the spacing between the coating head and the substrate and how it affects the transition from ALD to CVD behavior. The importance of exhaust efficiency in removing the reaction byproducts in the transition from ALD to CVD has also been explored. They have shown how control of the substrate-head spacing could be used as a method to obtain selective area deposition. Lee and Chang used the inorganic precursor VCl4 to directly deposit crystalline VO2. The thermal stability of this inorganic precursor for V compared to the typically used metal-organic precursors allows a high enough deposition temperature to render the usual post-annealing process unnecessary. The films show a transition from monoclinic to rutile crystal structure between 30 ◦C and 90 ◦C corresponding to a semiconductor to metal phase transition. Muller et al. deposited Ru ¨ metal films for XUV and x-ray optical applications and compared the results with films produced by magnetron sputtering. The ALD films show lower stress but somewhat higher roughness. This, together with the existence of a thin oxide surface film, leads to lower reflectance in the wavelength range of interest. The process needs further development to improve the optical properties to fully exploit the advantages of ALD, such as 3D conformality, in short-wavelength optics. Krumpolec et al. investigated the deposition of the wide bandgap semiconductor γ-CuCl using pyridine hydrochloride and a Cu metal-organic compound as precursors. CuCl is of interest for UV optical applications because of its bandgap and high exciton binding energy. They have shown that films of CuCl could be deposited without any Cu2+ content and could be protected against hydrolysis by atmospheric moisture using an Al2O3 capping layer. Ivanova et al. investigated the use of silver oxide and silver-doped CeO2 ALD films for catalytic oxidation of diesel exhaust soot. In their work, a 1:10 composition of Ag in CeO2 deposited on stainless steel had the best performance, with complete combustion of the soot at 390 ◦C, lower than for pure Ag2O. The films showed consistent performance in repetitive tests whereas Ag2O showed fast deterioration. The activity was found to be caused by oxygen species bound to Ag+ sites. Fernandez-Men ´ endez et al. explored a new manufacturing ´ sequence for supercapacitors based on porous alumina with Al2O3:Zn conducting contact material and a dielectric layer consisting of either Al2O3 or a SiO2/TiO2/SiO2 triple layer. They show that the device containing the Al2O3 dielectric is better in terms of lower internal resistance, lower leakage current, and higher breakdown voltage. Nevertheless, the overall performance still needs improvement with lower resistance internal contact layers and better external contacts particularly required. The films showed characteristic free and bound excitonic emissions and structure in photo-luminescence and optical reflectance, respectively. The review by Mantym ¨ aki et al. covered ¨ the basics of Li-ion batteries and a discussion of metal fluorides as Li-ion battery materials, used as electrodes, electrode-electrolyte interphase layers, and solid electrolytes. The review demonstrates that metal fluorides have interesting properties that could provide high voltage and high capacity alternatives to oxide-based materials and that these are worthy of further research. They propose that the advantages of ALD processes, namely conformal coatings with precise thickness control for ultra-thin films could answer the demands of battery materials. They review the previous work on ALD of metal fluorides and suggest that these are ripe for future investigations. This paper will prove valuable to those investigating metal fluorides both for battery and other applications.

> **David C. Cameron** *Editor*

*Article*
