Science Interviews


Sun, 27th Jan 2008

The Blackest Black

Professor Pulickel Ajayan, Rensselaer Polytechnic

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One big thing thatís been in the news in the last weeks or so is a story about the blackest black thatís ever been seen.  Using carbon nanotubes Professor Pulickel Ajayan from Rensselaer Polytechnic has developed a material that he says absorbs over 99.9% of all the light that falls on it.  Thatís 30 times darker than our current benchmark, or what we call black. 

Chris - Hello Pulickel, thank you for joining us.

Pulickel - Thank you.

Chris - So why have you done this?  What is this stuff?

A side-view SEM image of the darkest material at a high magnification.Pulickel - Well, an ideal black material absorbs light at all wavelengths and all angles, basically.  There were some speculations published by Spanish and English scientists who said if you take these tiny carbon cylinders and organise them into an array you can get very high absorbancy and very low integral refraction.  Of course to get a very black substance you also need to minimise the reflectants on the surface and you can do that by creating a surface softness.

Chris - So how does your technique actually work?  If I was to zoom in with a very powerful microscope on the surface that youíve created that is very back, what would I see?

Pulickel - Right, you will see nano scale cylinders of these carbon materials.  A nanometre is about a billionth of a metre so you have several of these structures that are just randomly placed on the surface.  Thatís what actually decreases the surface reflectance.

Chris - So you have these tiny tubes.  I mean, looking at the scale of them that would make them 400 or 500 times thinner than a human hair wouldnít it?

Pulickel - It would be more than that, about 3 orders of magnitude smaller than..[a human hair]

Chris - So you have these peppered all over a surface?  How do you make them..

Pulickel - There is a periodicity of these structures in the bulk of the material.  In the surface you have a random array so it satisfies both criteria: that you have a very highly absorbent material and also it reflects very little on the surface.

Chris - So how does it actually work to mean that so little light comes back off?

Pulickel - Itís basically almost like the light enters this material and it gets trapped because of this periodicity.  Thatís why you have a very low index of refraction.

Chris - How would you see this being used?  Itís one thing to get into the Guinness Book of Records for the Worldís blackest substance but how is this actually useful?

Pulickel - It was done mostly from an academic curiosity but something that absorbs a lot of light would be useful as a solar collector.  Again, in order to make this into an appropriate device you would have to worry about some other losses from material as a result of light.  Certainly itíll be a very strong absorber.  It could also be useful in areas where you want a very black background so that you can have a high definition of the areas which are light and black.

 Chris - So you could use it to trap sunlight which would make solar cells more efficient.  What about other things?  Does it work outside the visible spectrum of light?  Of course, thereís much more to light than just what we can see: things like radio waves, microwaves and at the other end of the scale x-rays and gamma waves.

Pulickel - Absolutely, thatís our hope.  We have not done these things yet but we are in the process of doing the high end IR and other wavelengths.  Thatíll be much more interesting from various points of view.

Chris - Thank you very much, Pulickel.  Good to have you on the show.

New Standard of Black


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