It is because of the thickness of the film. Heat treated coating is different in thickness compared with as coated coating. The interference between incident and reflected light is one factor to consider.

I feel, it is due to irregular thickness distribution. Not only in the sol gel method, in physical methods also , however I will come back with suitable example

If the coating does not absorb light the colour is due to interference fenomena in the film, that are controlled by the film thickness and refraction index, your hints about trasmitted and reflected radiation are basically correct. The variation of the colour after heat treatmentare due to variation in the thickness/refraction index

It is because of the thickness of the film. Heat treated coating is different in thickness compared with as coated coating. The interference between incident and reflected light is one factor to consider.

Hello everyone,

I am having the same problem also with FTO slides. Sometimes, the reflection color is observed on the as-coated film or after the heat treatment . I will appreciate more discussion regarding this matter.

Regards

Alexis

Color of sol-gel films on silica glass or soda-lime glass depend on both their film thicknesses and refractive indexes. The thermal treatment may change both of them. In addition, if you do not use filters during the sol-gel film coating, precipitated materials enhance the film inhomogeneity. In this case, I recommend you to use a syringe with an appropriate filter during the coating to improve its film quality.

I agree with all these people above.. The appearance of colours (like rainbow) in the deposited film indicates that the film thickness is not uniform.. The deposited film with uniform thickness will not display rainbow colours.. And this can also happen with any other experimental technique..

There are numerous factors that can affect changes in coloration. Please have alook at my published presentation on this subject, maybe it will be of interest to others as well.

Best regards, Michael

I fully agree with E.Irene comment that in transparent thin films the change in colour is due to change in film thickness, I also observed the same in case of silicon dioxide thin films.

Interference occurs between the light reflected from top surface of the film and the light reflected from the interface between substrate and film. Constructive or destructive interference of this two reflected light depends on the on the path deference i.e. film thickness and refractive index of the film. During high temperature heat treatment almost all organic and some inorganic material get burnt out. So this leads to change in film thickness (path length) and refractive index of film. This is reason why that particular wavelength of light under constructive interference (visible to eye) and other destructive (invisible).

The same thing can be correlated to the concept of anti reflective coating but in opposite manner

Absolutely agree with Pallapothu. Just to mention that the method of thickness determination through visual inspection of color for thermally grown SiO2 was widely spread in electronics. The color charts for SiO2 on Si substrate can be easily found.

By a simple research on Google ("googling") you can find these two references that explain all that you need to know about your main question (changes in the colour of your films by seen the reflecting light that illuminates them, as a function of their thickness):

http://en.wikipedia.org/wiki/Interference_(wave_propagation)

http://en.wikipedia.org/wiki/Thin-film_interference

There are even some excellent images and photos that illustrates this phenomena.

And the color chart for SiO2 films deposited onto Si substrate can be found here:

http://www.htelabs.com/appnotes/sio2_color_chart_thermal_silicon_dioxide.htm

All you need to do in order to get the approximative thickness of YOUR film, is to apply a simple proportion (how many times the refractive index of your film is higher than those of the SiO2).

You're served!

the change in color of a thin transparent film after heating is due to optical interference through the film. The interference is a function of the optical path which is equal to the product of the real part of the refractive index and the film thickness. Both of these properties can change with heating. Change can be followed by using ellipsometry which has the ability to separate the index and thickness for suitable films on a highly reflecting surfaces.

All above answers are right. But, based on my own experience, color change after annealing can be simply the oxidation of the film.