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Single - layer graphene has been shown to have intriguing prospects as a plasmonic material , as modes having plasmon wavelengths ~ 20 times smaller than free space ( lambda p ~ lambda 0 / 20 ) have been observed in the 2 - 6 THz range , and active graphene plasmonic devices operating in that regime have been explored .
 graphene, graphene
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 graphene, graphene