Have you read some fundamental literature?

Wikipedia, for example can be enlightening:

https://en.wikipedia.org/wiki/Absorption_(electromagnetic_radiation)

   The graphene is a two dimensional semimetal that some of its electronic properties depend strongly of the substrate where it is put. It is known that on a periodic heterostructure it has an indirect gap and therefore the absorption of photons is always related with the exchange of phonons. This explains how the infrared absorption can transform in heat.

   I hope that this can helps.

http://garciadeabajos-group.icfo.es/publications/pdfs/paper245.pdf