20334
Our friction measurements revealed that graphene , once exfoliated from the bulk crystal , tends to maintain its corrugation level even after it is folded onto an atomically flat substrate and that ultra - flatness in both graphene and the substrate is required to achieve the intimate contact necessary for strong adhesion .
 Chemical: graphene; Chemical: graphene
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: graphene; Chemical: graphene