Structural coloration of a zeolite monolayer

[Dyn Doeth]

TL;DR Summary

a monolayer of crystals on a glass plate seems to be coloured red at a specific angle under fluorescent light

I have a monolayer of zeolite crystals on glass with the thickness of the zeolites about 800nm.
When I look at the reflection of fluorescent light bulbs on this monolayer, the reflection is white except at specific angles.
At around 45 deg, the colour appears red/magenta and at a lower angle, this changes to green.
A double layer at 45 deg will also look green.

I have no idea what a good explanation of this phenomenon is. Thin-film interference would be a good first guess because the wavelength of the light matches the thickness of the crystals. However, thin-film interference would suggest that all colours should be reflected at some angle, I do not observe the other colours.

In summary, i am looking for a theory that can explain why a certain colour appears at a certain angle while there is no colour at other angles.

The zeolite powder is white and there is nothing in the zeolite that should have a colour.

 

[Lord Jestocost]

For constructive interference: 2t = (m+1/2) λ (t is the film thickness, m an integer and λ the wavelength of light)

"For white light, the above criteria yield constructive interference for some wavelengths, and destructive interference for others. Thus, the light reflected back from the film exhibits those colours for which the constructive interference occurs."
https://farside.ph.utexas.edu/teaching/302l/lectures/node152.html

 

[Baluncore]

Welcome to PF.

In summary, i am looking for a theory that can explain why a certain colour appears at a certain angle while there is no colour at other angles.

I think you should start by identifying the spectrum of the florescent light you are using. The only colours you see will be the colours of the phosphors used in the tube. It is not a continuous spectrum as there is no light between those peaks. Your eyes see the three or four colours of light as white.

Look at the florescent light reflected from a CD. It will show what colours are present. Are the colours still seen when the sample is illuminated with a filament lamp or sunlight ?

How thick is the glass plate ?

 

[Dyn Doeth]

For constructive interference: 2t = (m+1/2) λ (t is the film thickness, m an integer and λ the wavelength of light)

"For white light, the above criteria yield constructive interference for some wavelengths, and destructive interference for others. Thus, the light reflected back from the film exhibits those colours for which the constructive interference occurs."
https://farside.ph.utexas.edu/teaching/302l/lectures/node152.html

This explanation only explains the colour of a film when the light is perpendicular to the surface. When the angle of reflection changes, the path length through the film changes as well.
I did not observe any other colours except some red/magenta at ~45°.
If thin-film interference were to be true, then I should see every colour appearing as the angle of reflection changes, right?

Something like this.
But I only see two colours and white light in between.
I was thinking that a grating might be a better explanation, but the width of the crystals is at least 2-3 micron

 

[Dyn Doeth]

Welcome to PF.

I think you should start by identifying the spectrum of the florescent light you are using. The only colours you see will be the colours of the phosphors used in the tube. It is not a continuous spectrum as there is no light between those peaks. Your eyes see the three or four colours of light as white.

Look at the florescent light reflected from a CD. It will show what colours are present.Are the colours still seen when the sample is illuminated with a filament lamp or sunlight ?

How thick is the glass plate ?

I couldn't see the colours in sunlight. But maybe I just couldn't get the angle right because it was cloudy.

I will go back to see what happens on a CD. I thought all TL tubes used the same phosphor, so I thought that a common spectrum of a tube light might suffice.

the glass plate is 0.16-0.19mm thick

 

[Dyn Doeth]

I couldn't see the colours in sunlight. But maybe I just couldn't get the angle right because it was cloudy.

I will go back to see what happens on a CD. I thought all TL tubes used the same phosphor, so I thought that a common spectrum of a tube light might suffice.

the glass plate is 0.16-0.19mm thick

If the crystals filter the colours from the fluorescent tubes, (through thin film interference) then the colour should gradually fade right?
But what i observe is more like an on/off phenomenon.

If I change the angle of reflection it is white-red-white-green.
With no other colours in between.

 

[Lord Jestocost]

This explanation only explains the colour of a film when the light is perpendicular to the surface.

Sorry. Please, check https://www.tf.uni-kiel.de/matwis/amat/semitech_en/kap_3/basics/b3_1_1.html

 

[Andy Resnick]

Summary:: a monolayer of crystals on a glass plate seems to be coloured red at a specific angle under fluorescent light

I have a monolayer of zeolite crystals on glass with the thickness of the zeolites about 800nm.
When I look at the reflection of fluorescent light bulbs on this monolayer, the reflection is white except at specific angles.
At around 45 deg, the colour appears red/magenta and at a lower angle, this changes to green.
A double layer at 45 deg will also look green.

I have no idea what a good explanation of this phenomenon is. Thin-film interference would be a good first guess because the wavelength of the light matches the thickness of the crystals. However, thin-film interference would suggest that all colours should be reflected at some angle, I do not observe the other colours.

In summary, i am looking for a theory that can explain why a certain colour appears at a certain angle while there is no colour at other angles.

The zeolite powder is white and there is nothing in the zeolite that should have a colour.

This is an interesting phenomenon!

A couple of quick questions, tho:

1) I'm a little confused that you first said "monolayer of zeolite crystals [...] thickness of the zeolites about 800nm" and ended with: "The zeolite powder [...]". I'm not entire sure what configuration of zeolite crystals were used- for example, the first description seems to imply a solid polycrystalline layer. Can you better describe your sample geometry?

2) I'm not totally familiar with these compounds, but zeolites appear to be a class of materials with a characteristic cage-like crystal structure. I can't seem to find any relevant 'prior literature' concerning optical properties of zeolites (e.g absorption or reflection spectra in the visible waveband, birefringence properties, fluorescence properties, etc.). Can you provide a little more information about your compound?