Feb 12, 2016
Atomic layer deposition – tooling up nanotechnology step by step
Tools are useful when they meet all the demands of a particular objective, and invaluable when they continue to meet requirements that are ever evolving over time. Atomic layer deposition (ALD) was already a useful tool for thin films in the 1980s, although commercial use was then limited to electroluminescent displays. By the 1990s use of ALD was making inroads in the microelectronics industry, but it wasn’t until the end of that decade that the great match between the fabrication requirements in nanotechnology and the precision and control the technique can achieve became apparent. Since then, more than 1200 research papers, 80 reviews and two books have been published on ALD in nanotechnology. Over the past year, a Nanotechnology focus collection has been highlighting the latest research using the technique to fabricate devices for energy, environment and sustainability, where ALD is still crucial to developments at the frontiers of nanotechnology.
A kind of variation on chemical vapour deposition, ALD involves exposing the surface to the atomic elements of the chemicals to be deposited in separate stages, and then clearing the excess between each stage. In a paper published in the late 1990s exploring how ALD might be useful in nanofabrication, Mikko Ritala and Markku Leskelä list the merits this tweak introduces – “accurate and simple film thickness control, sharp interfaces, uniformity over large areas, excellent conformality, good reproducibility, multilayer processing capability, and high film qualities at relatively low temperatures.” These attributes have proved particularly valuable for attempts to improve the efficiency and scalability of alternative energy technologies.
Coating electrolytes on 3D structures
Many of the processes key to energy harvesting and storage devices are improved by the increased surface area that 3D structures bring. In lithium ion batteries (LIBs), a higher surface area means higher energy and power densities, as well as space to accommodate the addition and removal of lithium ions, which can introduce mechanical strain during use. LIB researchers are increasingly looking towards solid-state as opposed to liquid electrolytes to avoid risks of leakage and corrosion. As a result, sophisticated coating techniques to add electrolyte and counter-electrode layers to complex 3D nanostructures are in high demand. ALD not only provides pinhole-free coatings of solid-state electrolytes on 3D electrode structures, but it also offers the ability to tailor the coating thickness, which can significantly improve performance. As Tsun-Kong Sham and Xueliang Sun and colleagues at the University of Western Ontario point out in their report, “It is expected that the lithium phosphate thin films prepared by ALD can find potential applications as solid-state electrolytes for 3D all-solid-state micro-LIBs, which, as an emerging area, deserves more extensive investigation in the coming future.” Sun and colleague Jian Liu at the University also summarize key points in the design of electrodes and electrode/electrolyte interfaces in a separate paper reviewing ALD in LIB research .
Supercapacitors are a particularly attractive energy resource in nanoelectronics, where space is at a premium. As with LIB research, the advantages of 3D structures with solid-state electrolytes have been noted. In the focus collection, Giuseppe Fiorentino, Frans D Tichelaar and their colleagues at Delft University of Technology report on supercapacitors comprising carbon nanotube bundles as high-aspect-ratio electrodes that can improve the capacitance by a factor of five. The electrode is coated with aluminium oxide as the electrolyte, followed by titanium nitride as the counterelectrode, using ALD in work that not only demonstrates “the first known example of large-scale manufacturable nanostructured capacitors”, but also provides useful insights for coating such high-aspect ratio nanostructures.
Electrochemical cell structures can also take on elaborate hierarchical forms as demonstrated by Xudong Wang and colleagues at the University of Wisconsin-Madison and the Forest Products Laboratory in the US. They combine ALD and cellulose nanofibres to produce extensively branched structures for photoelectrochemical water splitting. “Such a 3D TiO2 fibre-nanorod heterostructure offers a new route for a cellulose-based nanomanufacturing technique, which can be used for large-area, low-cost and green fabrication of nanomaterials as well as their utilizations for efficient solar energy harvesting and conversion,” say the researchers.
With respect to solar energy harvesting, ALD is already a well entrenched technique in the field, and has been used to coat the inverse opals and other 3D structures often employed to reduce the distances charge carriers must travel to reach electrodes. Alfred Iing Yoong Tok and colleagues at Nanyang Technological University in Singapore and the University of New South Wales in Australia review the application of ALD in solar-power technologies, focusing on its use for surface passivation, surface sensitization and band-structure engineering.
The potential for lateral confinement
As well as providing an excellent tool for coatings, it has long been recognized that ALD offers great potential for controlled lateral confinement. This aspect is the subject of detailed scrutiny in the catalysis work of Marcel A Verheijen at Philips Innovation Services and Eindhoven University of Technology in the Netherlands. Coating thickness is still key for catalysis performance, as demonstrated by Ai-Dong Li and colleagues at Nanjing University in China. However, as the work by Verheijen and his colleagues in the group of Wilhelmus Kessels at the university identifies, ALD can also help with size matters. In a study of four ALD processes for the preparation of nanoparticle catalysts made from platinum and palladium, they identify the potential for size control, as well its dependence on process conditions.
It is perhaps the ability to accommodate innovation that makes ALD so invaluable. The use of plasmonic metamaterial absorbers has only really gained notice in the past few years, and ALD is already proving invaluable for work exploring the essential mechanisms in these systems too. Reporting in this collection, Xin Chen and colleagues at the Shanghai Institute for Technical Physics exploit ALD control over dielectric layers in order to distinguish between different plasmon modes. “We demonstrated inversed plasmonic metamaterial absorber architectures with a tunable ALD spacer layer, and thus identified the contributions of the gap plasmon and the interference-enhanced local surface plasmon resonance to the superior absorption in a step-by-step manner,” they conclude.
It is typical of science to break down problems into manageable pieces that can be tackled step by step. By breaking down deposition to an atomic level with step-by-step self-limiting stages, ALD mirrors this approach and in so doing seems to provide a multipurpose tool for a diverse range of applications, as the Nanotechnology focus collection highlights.
About the author
Anna Demming is editor of nanotechweb.org