9
$\begingroup$

Graphene is always in the news now a days and its key features are that it is; very strong, conductive and transparent. It is so transparent that each layer of graphene will only absorb 2% of Light passing through it.

But what is it about the structure of Graphene which makes it (almost) transparent?

$\endgroup$
  • 11
    $\begingroup$ I'm not sure... but a mono-atomic layer of anything is never going to be very opaque, is it? $\endgroup$ – John Dvorak Dec 6 '14 at 17:30
  • $\begingroup$ it's related to graphene's band structure, which defines what's the required photon wavelength (energy). but I don't really understand what's the meaning of having zero bandgap for high values of k (momentum). $\endgroup$ – Sparkler Dec 6 '14 at 18:50
22
$\begingroup$

Graphene is only transparent because it is very thin (one atom thick). If it absorbs 2% per layer then just a few hundred layers would absorb almost all light and that would still be a very thin sheet of graphite.

The question should be why does graphene absorb so much light compared to diamond which really is transparent? A simplified answer is that graphene is a very good conductor because it has only three covalent bonds per atom compared to the full four in diamond. This makes it possible for electrons to move freely over a sheet of graphene to conduct electricity. Like metals this means it will absorb or reflect light because the free electrons can absorb the small amount of energy in the photon. In diamond the photons would need to have enough energy to release an electron from the covalent bonds. For visible light this is not possible so the photons pass right through the diamond and are only stopped or deflected by impurities.

$\endgroup$
  • $\begingroup$ Thanks for the very interesting and enlightening answer. $\endgroup$ – zordman Dec 17 '14 at 17:00
4
$\begingroup$

I assume the biggest factor is the thickness. Graphene is a layer of carbon one atom thick. Light is absorbed/reflected by the top layers of a material and if you make any material into a layer one atom thick you'll find it increases transparency a lot.

The thing that is special about graphene is that it forms bonds in a 2D layer where most materials don't.

$\endgroup$
  • $\begingroup$ Holes don't really affect anything as they are much smaller than visible wavelengths. A Faraday cage also has lots of holes, but still it isn't transparent for radio waves. $\endgroup$ – Ruslan Dec 6 '14 at 19:14
  • $\begingroup$ @Ruslan - Good point, I've removed reference to this as it's confusing. Philip's subsequent answer is better anyway :) $\endgroup$ – Quantumplate Dec 6 '14 at 22:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.