Many different methods are used to fabricate different nanomaterials, i.e., materials with a thickness between 1 and 100 nm. Because nanomaterials are a diverse range of materials, there are specific methods that are used to create these materials. However, all methods fall into one of two classes- bottom-up methods, where the material is built atom by atom, and top-down methods, where a smaller material is created from a larger bulk material. In this article, we look at both bottom-up and top-down nanofabrication methods.
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Bottom-Up Methods
Chemical Vapour Deposition
Chemical vapour deposition (CVD) is a technique that is widely known because of its use in creating single-layer graphene. However, it is a technique that can be used to create a wide variety of materials and coatings.
CVD is a generic name for a wide range of techniques that use the same basic principles, but the specific deposition mechanism differs. Overall, CVD is used under a vacuum with volatile precursor atoms, which are then vaporized in a reaction chamber (often using high temperatures). The process then deposits these gaseous atoms onto a substrate (often termed a wafer), and upon contact with the substrate, the atoms decompose and/or react to create a thin material on the surface of the substrate.
Physical Vapour Deposition
Physical vapour deposition (PVD) is the term used for another collective of methods that deposit atoms to create a thin material. PVD is also performed under a vacuum environment and first vaporizes a solid material, which then travels as a gas through a reaction chamber, before condensing onto a substrate.
There are many different PVD methods, with the most common being evaporation and sputtering methods, and all variations use the above fundamental steps. Where each PVD method differs, is in the methods used to generate the vaporized material and the methods used to deposit the material onto the substrate. PVD methods are used to create a wide variety of coatings and materials that are used in electronics, energy storage, and optical devices.
Nucleation-Growth
Nucleation-growth is a type of self-assembly process that nucleates and aggregates particles together to create a much larger material. It is a process often seen in nature to grow crystals, and the same principles can be applied in creating synthetic crystals. Aside from crystals, particles can be aggregated together to create larger systems using thermodynamic principles, especially those that involve entropy, phase changes, and phase transitions. However, unlike the other bottom-up methods, it is traditionally a random process, unless the growth can be controlled using additional means.
Top-Down Methods
Etching
There are various types of etching in the world, but reactive-ion etching (RIE) and its high aspect ratio variant, deep reactive-ion etching (DRIE), are the etching methods widely used in nanofabrication approaches.
RIE is a method that utilizes both chemical and physical etching methods to remove material so that a larger material can be made smaller and become nanoscale size. RIE is also a powerful tool for patterning the surface of a material by selectively removing areas.
RIE is performed by pumping chemically active plasma that contains a mixture of negatively and positively charged ions which then react and etch the surface of the material. Areas which do not wish to be etched (such as in patterning approaches) are covered with a mask so that they are protected from the plasma.
Lithography
Lithography is a method similar to etching, where a beam (of some description, which varies between the different types) is directed at a material to remove portions of it. Just like etching, lithography can also be used to pattern a material, where a mask is also employed to protect areas that are not wanting to be removed. The direct methods and mechanisms vary from type to type, but the most common lithography methods include photolithography (also known as optical lithography), electron-beam lithography (EBL), nanoimprint lithography (NIL), multiphoton lithography and scanning probe lithography (SPL).
Exfoliation
Exfoliation is one of the most common ways of creating large volumes of monolayer and multi-layered materials from larger bulk materials. The most common being in the commercial production of graphene (multi-layer, graphene nanoplatelets, and graphene flakes) from bulk graphite material. Exfoliation is the process of cleaving and removing the layers of bulk materials into fewer-layered materials by breaking the intermolecular forces that exist between these layers.
However, it can’t be completely controlled to an exact number of layers so variations in the number of layers between different samples can occur. Exfoliation methods also come in many forms, including chemical, solar, thermal, ultrasonic and mechanical exfoliation, which all use different means (as described in the name) to breakdown intermolecular bonds and cleave the layers of bulk material into smaller nanomaterials.
Sources:
MIT News: http://news.mit.edu/2015/explained-chemical-vapor-deposition-0619
AZoM: https://www.azom.com/article.aspx?ArticleID=1552
University of Oslo: http://www.uio.no/studier/emner/matnat/kjemi/KJM5100/h06/undervisningsmateriale/09KJM5100_2006_Chemical%20vapour%20deposition_d.pdf
Sigma Aldrich: https://www.sigmaaldrich.com/materials-science/material-science-products.html?TablePage=108832720
Missouri University of Science and Technology: http://web.mst.edu/~billf/nuc_growth.pdf
Melbourne Centre for Nanofabrication: http://nanomelbourne.com/services/etching/
London Centre for Nanotechnology: https://www.london-nano.com/research-and-facilities/themes/techniques/nanolithography
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