**3.4. Depositions techniques**

Two main deposition techniques have been widely used to prepare MTTFs, namely, spincoating or dip-coating.

Spin-coating is a simple process for a rapid deposition of thin films onto flat substrates. The substrate to be covered is held by some rotatable fixture (often using vacuum to clamp the substrate into place), and the coating solution is spread onto the surface; the action of spinning causes the liquid to spin radially outward by the centrifugal force until the thin film is formed. The initial volume of the fluid distributed onto the rotating substrate and the delivery rate have both a minor effect onto the final film thickness. On the other hand, the resulting viscosity of the fluid to be deposited and the chosen ultimate rotation velocity are both parameters that mainly control the resulting thickness of the film. High angular speed produces thinner film. At a constant speed, the film thickness initially rapidly decreases, but this decrease slows at longer times. Typical coating thickness values are often below 1 μ when spin-coating is used.

Dip-coating is a technique based on the deposition of a wet liquid film by withdrawal of a substrate from a liquid coating medium. This process is very simple, flexible, and economically advantageous. Nevertheless, it is necessary to make provisions for cleanliness in order to obtain a high-quality deposition. Dip-coating is one of the few techniques that allow a simultaneous double-sided coating which may be regarded as an advantage especially in production of optical filters. Typically, film thickness obtained with dip-coating ranges from a few nanometers to 200 nm for oxide coatings. The thickness of the liquid film depends mainly on two factors: (i) the viscosity of the solution and (ii) the speed rate used during the substrate withdrawn from the solution.

In dip-coating, mesoscale ordering is achieved almost instantly after the covered substrate is withdrawn from the solution. Even if the self-assembled structure may form by spincoating, the degree of ordering cannot be as high as in the case of dip-coating. This is probably due to the faster solvent evaporation occurring during spin-coating leading to a more viscous film deposit, which, in turn, makes the rearrangement of the titania and surfactant species in solution that is too sluggish to achieve a high degree of ordering in a short period of time [26].
