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Altering the Surface Chemistry of Substrates

Theory

When making thin films wetting of the substrate is vital for success. Insufficient wetting lead to solvent evaporation problems and non-uniform films. In the worst case scenario, the solution to be coated will fly off the surface of the substrate like the repelling of water droplets by the lotus leaf. Therefore it is desirable in certain circumstances to change the surface chemistry of the underlying substrate to achieve better wetting.

In general, surfaces exhibit either hydrophilic or hydrophobic behaviour. The methods outlined below aims to alter this preference of the substrate by chemical means. It is useful to note that generally metal oxides are hydrophilic and one can usually infer the behaviour of the substrate in the case of polymers by taking note of the solvent the substrate dissolves in a priori for example PDMS dissolves in heptane which is an organic solvent and non-polar therefore it is likely to be hydrophobic. A quick drop cast onto the substrate will test for hydrophilic and hydrophobic behaviour.

Practice

During the thin film making we primarily coated our first layers on silicon substrate because silicon has a high refractive index of 3.42 compared to reflectin's 1.59 allowing for easy visualisation of thin films.

Silicon

Silicon readily oxidises in air and is coated with a layer of silicon oxide which is a hydrophilic surface. It can be rendered readily hydrophobic by exposure to HMDS (hexamethyldisilazane) in a vacuum enclosure.

To Make Silicon Hydrophobic

1. Prepare vacuum chamber by sucking out all vapours for several minutes depending on size of enclosure
2. Place silicon wafer/s into enclosure along with HMDS either in its bottle with its lid off or in a beaker for 10 mins (Longer times may be required depending on size of wafer)
3. Remove silicon wafer and test hydrophobicity by drop casting water onto its surface. The water drop should bead up and be easily removed by blowing.

For the theory please refer to the section below on HMDS.

This method was initially tried because it was noted that dewetting seems to occur with the produced thin films which led to thoughts the protein may not like the hydrophilic layer.

PDMS

PDMS or Polydimethylsiloxane (CH3[Si(CH3)2O]nSi(CH3)3) belongs to the silicone family and is used in our studies as an intermediate layer for building bragg stacks for the following reasons:

1. Refractive index of 1.4, different to reflectin
2. Not re-dissolved by polar solvents such as HFIP allowing for coating of reflectin
3. Flexible substrate which can lead to further studies in flexible screens

It is a viscoelastic polymer which cross-links when heated. The degree of cross-linking is related to the heating time. This process is known as curing. After curing PDMS presents an external hydrophobic layer. HFIP is a polar solvent and Loligo Reflectin A1 which we are spin-coating is primarily hydrophilic. Therefore to achieve good wetting and solvent evaporation, it was found necessary to make the PDMS layer hydrophilic.

To Make PDMS temporarily Hydrophilic

1. Cure PDMS by heating for 10 mins at 100^o
2. Place in O² plasma for 2-10 secs. Longer times give better hydrophilic behaviour however one runs the risk of oxidising the PDMS layer leading to surface cracking and wrinkling.
3. Test for hydrophilic behaviour by drop casting
4. Depending on environmental conditions the PDMS will retain the hydrophilic behaviour for varying times.

The O² plasma adds Silanol (SiOH) groups to the surface which has hydrophilic tails thus imparting the hydrophilic behaviour.

General Notes on HMDS and Hydrophobicity

HMDS or Bis(trimethylsilyl)amine or hexamethyldisilazane, [(CH3)3Si]2NH is a common reagent used to make laboratory glassware hydrophobic. It achieves this by silylation, the process of adding silyl(SiH3) groups to a molecule or the addition of any similar radical in which one or more of the hydrogen atoms are replaced by organic groups.

Safety