E.g. crystalline YIG films appear yellow, whereas amorphous YIG is black.
This is explained by the concept of "color centers" in Solid State Physics which has some complex tensor-based math to quantitatively explain something which is a lot easier to understand in words ...
A perfect crystal (like a high-grade diamond or quartz) is optically transparent, light moves through the crystal without any particular absorption of any specific frequency. But of course, that would make for a boring world, so Mother Nature gives us a gift when she makes crystals, in the form of defects. Luckily everything in the world isn't perfect, and as those crystals grow, the randomness which is a part of the quantum world begins to take its effect, which creates point defects in the crystal lattice. One defect leads to another, and these defects begin to show a pattern all on their own.
As the light passes through this otherwise perfect volume of the crystal and then meets these defects, light responds to the defect in different ways. If the defect is significant enough and shaped in the right way, all of the visible light is trapped in it, and that part of the crystal, (or even the entire crystal) appears opaque. But when the defect is slight, it will absorb certain light frequencies, but not others. This produces the effect of color, because the whitish light that goes in may appear reddish or bluish or greenish (etc.) coming out, due to the fact that some of the light energy that went in was captured, and the light that came out was missing some bands.
To make things even more interesting, these defects in the crystal can be enhanced or cleared with more radiation (usually high energy radiation). So a clear crystal left in sunlight may absorb enough UV energy over the years to become slightly clouded, or even a bit yellowish. On the other hand, a colored crystal left in the sunlight with enough UV energy may gradually lose its vibrancy. Very high energy radiation can make this happen much faster, for instance inferior diamonds can be irradiated with high energy to make them appear clear, or else clear crystals can be placed in a high energy neutron flux or x-ray beam to give them a beautiful color.
To take this even further (and this is the really mind-blowing part) some seemingly unchanging materials can and will change dramatically for very (very) brief periods of time when exposed to light. Take a pencil out of a dark drawer, and hold it in the sunlight ... for a few femtoseconds, some of the graphite in that pencil actually converted to diamond due to the light absorption. Similarly, that expensive diamond when taken out of a drawer and exposed to light actually changed to nearly worthless graphite for a few femtoseconds. This doesn't last though (and the pencil graphite and diamond change back because these states are metastable, and they prefer to fall back to their more stable energy configuration.
http://physics.aps.org/story/v22/st5
Just off the top of my head, grain boundaries as well as their character (ie: high-angle grain boundaries versus low-angle grain boundaries as well as grain boundary impurity phases) can affect light scattering off of thin films. However, I am not aware of a qualitative relationship between the degree of crystallinity and the color of thin films; it probably depends on the materials as well as a variety of other factors.
amorphous material contains the hydroxide contain, for crystalline films the hydroxide phase is removed so that may be one reason for getting different color. Same behavior u will observe for CdO in hydrous form it is white with crystalline nature it change color to yellow brownish.
This is explained by the concept of "color centers" in Solid State Physics which has some complex tensor-based math to quantitatively explain something which is a lot easier to understand in words ...
A perfect crystal (like a high-grade diamond or quartz) is optically transparent, light moves through the crystal without any particular absorption of any specific frequency. But of course, that would make for a boring world, so Mother Nature gives us a gift when she makes crystals, in the form of defects. Luckily everything in the world isn't perfect, and as those crystals grow, the randomness which is a part of the quantum world begins to take its effect, which creates point defects in the crystal lattice. One defect leads to another, and these defects begin to show a pattern all on their own.
As the light passes through this otherwise perfect volume of the crystal and then meets these defects, light responds to the defect in different ways. If the defect is significant enough and shaped in the right way, all of the visible light is trapped in it, and that part of the crystal, (or even the entire crystal) appears opaque. But when the defect is slight, it will absorb certain light frequencies, but not others. This produces the effect of color, because the whitish light that goes in may appear reddish or bluish or greenish (etc.) coming out, due to the fact that some of the light energy that went in was captured, and the light that came out was missing some bands.
To make things even more interesting, these defects in the crystal can be enhanced or cleared with more radiation (usually high energy radiation). So a clear crystal left in sunlight may absorb enough UV energy over the years to become slightly clouded, or even a bit yellowish. On the other hand, a colored crystal left in the sunlight with enough UV energy may gradually lose its vibrancy. Very high energy radiation can make this happen much faster, for instance inferior diamonds can be irradiated with high energy to make them appear clear, or else clear crystals can be placed in a high energy neutron flux or x-ray beam to give them a beautiful color.
To take this even further (and this is the really mind-blowing part) some seemingly unchanging materials can and will change dramatically for very (very) brief periods of time when exposed to light. Take a pencil out of a dark drawer, and hold it in the sunlight ... for a few femtoseconds, some of the graphite in that pencil actually converted to diamond due to the light absorption. Similarly, that expensive diamond when taken out of a drawer and exposed to light actually changed to nearly worthless graphite for a few femtoseconds. This doesn't last though (and the pencil graphite and diamond change back because these states are metastable, and they prefer to fall back to their more stable energy configuration.
http://physics.aps.org/story/v22/st5
In addition to the color center in crystals , there is the phenomenon of light intereference which can cause the birth of beatiful colors. More generally, amorphos and crystalline films are different materials, and can have different colors and reflectivity, as happens in films used to make wriable CDs.
Very true Giuseppe! Amazing how Nature works to give us all this beautiful color eh?
Just re-stating some points from Michael, Guiseppe, and Claire
Amorphous Vs Crystalline:
- Different materials with different optical properties
- Addition of grain boundaries / defects
- Changes in film thickness from on state to another
- Changes at the substrate-film interface
These are just some starting points. As Claire mentioned, "it probably depends on the materials as well as a variety of other factors."
i think the participation of Ms Michael wofsey is enplane this question
If I may ask a question. In what mode are you talking about. Is your observation based on transmitted light/reflected light?
Amorphicity increases bandgap due to band edge tailing and also the Burstein-Moss effect leads to widening.
As you talked about reflected light - this might be the reason - increased amorphicity!
Reflection from different plane of lattice gives the interference pattern is the cause of different color.
The high school discussion of colours of thin films. Interference condition. Satisfied? What is the wavelength? That colour appears! Sunandana
Remember that thin film colour is not only related to cristallinity, but it is influenced by the stoichiometry of the film. For TiN film- under or over stoichiometric films are brown and stoichiometric film is yelow. Of couse you have to evaluate the influence of the stoichiometry not only on cristallinity but on the growth morphology of the film.
Useful to remember this. Visible wavelengths are from 200- 600 nm while lattice parameters are ~10 nm at the most.
So crystallinity is perceived more readily by X-rays rather than by visible light
Of couse the use of XRD is very important to verify crystallinity and must be done.
Color appearence of material is related to the refectance (wavelength) so i thinkk in case of amorphous no light will reflect back so appering black....and in case of crystalline light is reflecting back so looking yellow...........
The problem of passage and retrieval of a beam of light through a single/multilayer film usually deposited on a substrate is a hopelessly nonlinear problem. Why? Because it involves several stratified media each with its own index function. That too changing abruptly at the interfaces of the various strata. Of course mode;s are available that involve simple approximations involving thickness of film and refractive indices besides of course the wavelength. There are simple experimental signatures of crystallinity and amorphicity of thin films. A good crystalline film exhibits a well resolved interference pattern containing many extrema. While an amorphous film gives only a few such extrema-say 2 or 3.
It would be interesting to follow an amorphous thin film through optical transmission as it crystallizes upon programmed annealing. Finally a paper by Swanpoel may be downloaded for analysis of thin film transmission spectra
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