Many factors contribute to determining the final state of a dip coated thin film. By controlling the functionalisation of the initial substrate surface, submersion time, withdrawal speed, number of dipping cycles, solution composition, concentration and temperature, number of solutions in each dipping sequence and environment humidity, a large variety of repeatable dip coated film structures and thicknesses can be fabricated. The technique of dip coating can give uniform, high quality films even on bulky, complex shapes.
The dip coating technique is used for the fabrication of thin films by self-assembly and the sol-gel technique. Self-assembly can give film thicknesses of exactly one monolayer. The sol-gel technique creates films of increased, precisely controlled thickness, determined mainly by the deposition speed and solution viscosity.
Self assembly is the process where components spontaneously organize or assemble into more complex objects, typically by bouncing around in a solution or gas phase until a stable structure of minimum energy is reached.
Self-assembly is crucial to the assembly of biomolecular nanotechnology, and is thus a promising method for assembling atomically precise devices. Components in self-assembled structures find their appropriate location based solely on their structural properties (or chemical properties in the case of atomic or molecular self-assembly). Self-assembly is by no means limited to molecules or the nanoscale and can be carried out on just about any scale, making it a powerful bottom-up assembly method.
Surfactant molecules can assemble into larger aggregates in solutions varying from round balls to circular rods and lamellar structures, whereas if a solid substrate is immersed in a liquid containing (functionalized) surfactant molecules a monolayer of these components can spontaneously form on the solid substrate either by physisorption, covalent binding or electrostatic interactions. A self-assembled monolayer (SAM) is a two-dimensional film, one molecule thick, covalently organized or assembled at an interface. The classical example of a SAM is the reaction of alkanethiols with a gold surface. Another example is the reaction of silanes with glass, quartz or SiO2 surfaces.
The sol-gel technique is deposition method widely used in material science to create protective coatings, optical coatings or ceramics to name a few. This technique starts with the hydrolysis of a liquid precursor (sol) which undergoes polycondensation to gradually obtain a gel. This gel is a bi-phasic system containing both a liquid phase (solvent) and a solid phase (integrated network, typically polymer network). Step by step, the proportion of liquid is reduced. The rest of liquid can be removed by drying and can be coupled with a thermal treatment to tailor the material properties of the solid.
The layer-by-layer assembly (LBL) is a simple and relatively cheap method to deposit alternate layers of materials. Thin films are created by depositing alternatively layers of opposite charges, providing a high degree of control of the film thickness: negatively and positively charged layers are deposited successively until the desired thickness is reached.