Vapour-phase self-assembly of organic monolayers

Background

This vacuum-based monolayer formation uses commercial equipment and replaces the existing solution-based self-assembly of alkyl phosphonic acids. The monolayer growth is self-limiting and ideally suited for large-area, roll-to-roll fabrication. Initially intended for use as ultra-thin gate dielectric in thin-film transistors operated below 2 volts, the monolayer would also find use in bio-electronics and modification of surfaces to lower their surface energy.

Alkyl phosphonic acids are linear organic molecules with strong chemical anisotropy. One end of the molecule readily binds to oxides, e.g. aluminium oxide, while the aliphatic tail orients away from the binding surface, changing the hydrophilic surface of oxides to hydrophobic one.

The formed organic monolayer is a very poor conductor of electricity, making it a perfect ultra-thin dielectric with electric breakdown field surpassing that of common inorganic dielectrics. Depending on the length of the aliphatic tail of the chosen molecule, the typical monolayer thickness is 1 to 2 nm. The outer surface of the monolayer is hydrophobic with low surface energy and natural ability to repel water.

The applications range from gate dielectrics in thin-film transistors (organic, metal oxides) to bio-electronics and smart packaging.

Technology

We have developed a ‘dry’ method for depositing monolayers of alkyl phosphonic acids. This new method replaces existing solution-based self-assembly. Unlike the solution assembly, the Strathclyde monolayer formation can be accomplished in minutes rather than hours and is ideally suited for large-area, roll-to-roll fabrication.

The technology employs standard vacuum thermal evaporation. Initially, the thickness corresponding to several monolayers is deposited to assure complete surface coverage. Molecules in the first monolayer chemically bond to the surface of underlying oxide. Molecules above them are not chemically bonded and are subsequently removed. Among all phosphonic acids, the process of monolayer formation was thoroughly optimized for n-octylphosphonic acid.

The Strathclyde monolayers have been successfully inserted into thin-film transistors that pose the highest-known demand on the structure of the monolayer. The monolayers achieve the performance levels of their solution-assembled counterparts.

Key benefits

  • uses vacuum thermal evaporation and commercially-available equipment
  • applicable to large-area, roll-to-roll fabrication
  • high-throughput, low-temperature process compatible with plastics
  • assures complete monolayer formation
  • good step coverage achieved in transistors
  • method applicable to other linear molecules with chemical anisotropy, eg carboxylic acids on oxides, alkane thiols on gold, etc.

Markets & applications

  • flexible displays, e-books
  • disposable RFID tags
  • smart packaging

Licensing & development

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