Self-Assembled Monolayers - Deposition in supercritical CO2

Self-Assembled Monolayers
Deposition in Supercritical CO2
In labs, SAM layers are generally deposited in liquid solvents, as this method does not require specific equipments. But liquid processing faces strong limitations due to the poor diffusivity of this medium:
  • Long process time (several hours) leading to uncontrolled polymerization of the molecules
  • No deposition inside complex structures (microchannels)
Lifetime of the bath, solvent consumption, rinsing and drying are other issues. 
Vapor phase deposition process is totally on the op posite, with a dry process and a high diffusivity under vacuum. But as "big mole cules" with reduced vapor tension can be easily dissolved in solvents, it is much more difficult, and even impossible, to put them in vapor phase. Thus, vapor phase deposition has also strong limitations:
  • Use of low vapor tension molecules only 
  • Nebulizers and feed line cleaning issues
  • Memory effect
As a synthesis between solvent and vacuum, supercritical CO2 has properties generally considered as antagonistic : density close to solvents, zero surface tension, low viscosity and high diffusivity.
SC CO2 process combines advantages of the 2 other methods:
  • Fast process (15 - 30 min)
  • Wide range of molecules
  • Good diffusion through porous materials or microchannels
  • Dry process compatible with fragile structures
Compared density, viscosity and diffusivity of gases, liquids and SC Fluids