Six independent papers reporting epitaxial formation of silicene in 2012
Since this is a Nature News article, usually it is a way of promoting a scientific article in one of the current Nature publications - that's why it can get away with stating "Six independent reports" without six citations attached.  In this case, it is citing this 2015 article: 


  Tao, L. et al. Nature Nanotechnol.
  http://dx.doi.org/10.1038/NNANO.2014.325 (2015)


There are many citations in that article from the 2010-2013 time period reporting silicene sheets and nanoribbons.  Keep in mind, the Fleurence et al. paper you mentioned might not be one of the six mentioned, because it uses a diboride substrate instead of silver.