It can't be graphene if it has to be reflective, graphene is transparent.
These probes are a neat idea, but for now staying home and building better telescopes can get us better information faster than travelling there. In the next hundred years we can build telescopes that will be able to resolve great detail on systems out to 300 light years or so. With them we can know what resources are available in those systems before we get there.
Do both. Telescope spacecraft like Hubble and here are 5 reasons why the coming James Webb Space Telescope is a big deal.
Nice video about 2:40 long.