Bottom-up

Manufacturing - Bottom-up - About the sector Text

Bottom-up processes

In bottom-up processes, the material is synthesised from atoms or molecules through a physical or chemical reaction, e.g. through gas and vapour deposition, liquid phase methods and self-assembly, each of which is considered below with an explanation of the process and examples of the types of nanomaterials produced.

Gas and vapour phase

There are several methods of producing nanomaterials by deposition from a gas or vapour:

  • Physical Vapour Deposition (PVD) is the physical depositing of material from a vapour onto a surface. The starting material can be in solid or liquid form and is vaporised, possible at high-temperature in a vacuum, and then condensed onto a surface. Nanomaterials produced include coatings and films.
  • Chemical Vapour Deposition (CVD) is a method by which a material is deposited from a gas or vapour (gaseous state) onto a substrate via a chemical reaction. The process is temperature activated – at high temperature (for Thermal CVD) or a lower temperature (compared to Thermal CVD) if a plasma (an ionized gas) is used (Plasma-Enhanced CVD, PECVD). Metal oxides are routinely produced by the CVD method (also known as Metal-Oxide CVD or MOCVD). Atomic layer deposition is a form of CVD which results in the deposition of single layers of atoms which can be built up sequentially into a film or coating. Nanomaterials produced include coatings, films, nanotubes, graphene, dendrimers, fullerenes, wires and rods.
  • Plasma-arcing uses a plasma (ionised gas) produced with a high current of electricity and contained within a vacuum chamber. The nanomaterial is deposited either as free particles (e.g. carbon nanotubes) or as a surface coating. In effect, the process is consuming one material and depositing it either as the same or a modified material in a different structural form. Nanomaterials produced include particles, coatings and nanotubes.
  • Molecular Beam Epitaxy (MBE) is an evaporation method that uses beams of molecules focussed onto a surface to grow a structure such as a crystalline film. By minimising contamination, it is possible to produce a very pure layer of a uniform structure or high-purity and regular crystals. Nanomaterials produced include particles, coatings and films.
  • Spraying is a method in which a gas or liquid in aerosol form (or a mixture of both) is decomposed by putting it into a high-energy flame (plasma or laser-generated). The constituents decompose and form (typically particles) by nucleation and growth. Spraying can be used to dry a material and/or deposit it as a coating onto a surface which may be heated. Examples of this method include thermal spraying, plasma spraying and flame spraying. Nanomaterials produced include particles and coatings.

Liquid phase

  • In the sol-gel method, a colloid (a solid in suspension) is first created. Hydrolysis, condensation and polymerisation then take place, followed by agglomeration of the substance into a thickened gel. Sol-gel materials (e.g. silica gels) can also form precursors for other nanomaterials. Fibres can be produced from the colloid and powders from the gel. The gel can also be dried and sintered (heated) to form ceramics. Nanomaterials produced include gel, particles and fibres.
  • Solution phase synthesis and molecular seeding can be used to grow materials out of solution by arrested precipitation. Nanomaterials produced include quantum dots and dendrimers.
  • Electro-spinning involves taking a drop of dilute polymer solution and charging is with a high voltage so that a cone of material forms and a fibre (or bundles of fibres) spins out of the end of the cone in the form of a fine jet. Nanomaterials produced include fibres.

Self-assembly

Chemical and biological self-assembly encourage materials to form due to selective bonding of molecules to other molecules (e.g. on surfaces), preferential self-ordering and self-docking, and bio-molecular recognition and attachment. In effect, the materials build themselves because of attractive forces due to their (largely chemical or biological) characteristics. Nanomaterials produced include dendrimers, biomaterials such as cells, lipids, peptides and micelles.