Painting at the Atomic Level

  • Published
  • By Robert White
  • Air Force Office of Scientific Research
ARLINGTON, Va. --  We are all familiar with what is required to paint a room: paint rollers, brushes, drop cloths, etc. But have you given thought to what is required to paint at the atomic level--a layer of "paint" that is one atom thick? That is what Steven M. George, a professor of chemistry and chemical engineering at the University of Colorado, Boulder, does.

Funded by the Air Force Office of Scientific Research since 1999, Dr. George has been responsible for several unique breakthroughs in the field of Atomic Layer Deposition, or ALD. The ALD method allows one to control the physical and chemical properties of materials one atomic layer at a time, and depending on what materials you are depositing on a surface, you get different results--and some are incredibly unique.

ALD did not originate with Dr. George. This process was actually preceded by Atomic Layer Epitaxy (ALE) beginning in the 1970s with applications in use through the 1990s. But while ALE could do some things well, it had limitations. Initially, the only materials you could put down via the ALE process were inorganic materials, and most of those were compound materials--binary materials--things like Zinc Sulfide, which ruled out carbon, organic entities, and metals. The transition in the name from ALE to ALD occurred because many deposited materials had no epitaxial match to the underlying substrate or were amorphous. ALD can now deposit a wide range of materials from organic to inorganic to specific metals, or even a hybrid composition of organic with inorganic. With the ability to combine such things at the atomic level in sequential alternating layers that have never been in combination before, you may end up with a chemistry that yields unique new properties. This possibility was the driver behind AFOSR's support for Dr. George's program.

When Dr. George first described the ALD process to AFOSR program manager Dr. Michael Berman in 1999, Berman was struck by two things: "The exquisite control it provided for material growth on the atomic scale, and the generality of the method that would enable it to have a tremendous range of open-ended applications." This process seemed to offer a method that would give unprecedented and revolutionary control over the size, composition, and properties of nanoscale materials.

This control is achieved by the inherent simplicity of the ALD process due to its self-limiting characteristic. During the deposition process when gaseous reactants are exposed to a surface area, these precursors react with the surface and add to the surface by one atomic layer, then the reaction stops. Subsequently, a new gaseous reactant is introduced for the next layer of growth to continue. By breaking the chemistry down into this elegant two-step process that is self limiting, incredible control is gained over the growth process, and this method can be applied to a wide range of systems that made its application extremely general. The resultant ultra-thin coatings have a wide range of uses.

For example, the size of an orifice can be controlled just by growing a layer of material to shrink the gap one atomic layer at a time. Incredibly thin protective layers can be deposited on materials that benefit from ALD's ability to create conformal and pinhole free coatings. These coatings can be used to make extremely thin barriers to water vapor, ranging from the most prosaic, such as inexpensive, thin, transparent non-metallic high tech potato chip bags that ensure freshness, to the creation of new types of flexible waterproof solar cells.

There is one area where ALD can do something not otherwise possible--coatings for Micro-Electro-Mechanical Systems (MEMS), where there is literally no other technique that can coat small MEMS widgets--micron size or smaller--where they are not only exquisitely small, but you do not have line-of-sight to reliably reach and coat all surface areas. For MEMS coating requirements, the ALD application process is revolutionary.

Another revolutionary use: ALD coatings to protect the electrodes of lithium ion batteries. For example, you run your laptop and after a year your batteries have half the lifetime compared to when they were new. That is the result of the electrodes in the battery degrading by a number of different mechanisms, and one of the main culprits is the interaction between the electrode and the electrolyte in the battery which causes a slow loss of battery capacity. Dr. George's group found that protective ALD films, when deposited on the surface of the electrode, with a thickness of only four to five angstroms, can significantly help stabilize the battery's capacity to maintain its charge. After a couple hundred cycles the battery may degrade to fifty percent of capacity without the ALD coating; with the ALD coating, the battery degrades five to ten percent. This significantly enhances lithium ion battery performance--good news for warfighters weighed down by numerous battery packs and for future electric vehicles.

The ALD process can be applied to an extremely wide range of systems including coatings on aircraft canopies, protective coatings on particles and nano-particles, creating insulators in integrated circuits, and controlling catalysts and sensors. What remains to be seen are the even more unique coatings on an increasingly wider range of materials, hence enabling new potential applications.


ABOUT AFOSR:
The Air Force Office of Scientific Research (AFOSR), located in Arlington, Virginia, continues to expand the horizon of scientific knowledge through its leadership and management of the Air Force's basic research program. As a vital component of the Air Force Research Laboratory (AFRL), AFOSR's mission is to discover, shape, and champion basic science that profoundly impacts the future Air Force.