Complex sentence: Both are expected to have the characteristics of massless Dirac fermions that carry charge, but it this is only predicted for silicene and has not been observed, likely because it is expected to only occur with free-standing silicene which has not been synthesized.
Split into simpler sentences: Both are expected to have the characteristics of massless Dirac fermions that carry charge, but it is only predicted for silicene and has not been observed. This is because it is expected to only occur with free-standing silicene which has not yet been synthesized.