Unless your institution's style manual for an MS thesis requires that you include the name(s) of the author(s) of every cited reference -- and I can't believe that there's any institution that stupid -- I'd eliminate all names unless they're needed in a specific discussion (sometimes they are when you're discussing the work of many authors who need to be differentiated). Even the absurd APA Style Manual doesn't demand that.
Even the phrase "has been demonstrated" can be eliminated simply because you have a reference citation, # [8], to support the claim about negative conductivity in graphene. So you can say it less verbosely, probably as a parenthetical remark in another sentence, or just as a simple declarative with the reference citation in square brackets, as in your example sentence:
"Graphene is negatively conductive [8]."
or
"The dynamic negative ac conductivity of graphene [8] {means / causes / implies} that..."
or
"The absolute negative conductivity of graphene [8] {means / causes / implies} that..."
I make this kind of edit all the time in biomedical papers where nobody cares who said what, unless it was Crick & Watson (The Double Helix). Authors include that information because they don't have to think so much about how to craft their sentences if they begin them with, e.g., Jones et al. [1] {demonstrated / reported / said / observed} that... or As shown in Figure 1,....
Cut the unnecessary words. Use author names only when necessary.