26801
We find that optical second - harmonic generation ( SHG ) in reflection from a chemical - vapor - deposition graphene monolayer transferred onto a SiO2 / Si ( 001 ) substrate is enhanced about 3 times by the flow of direct current electric current in graphene .
 Chemical: graphene; Chemical: SiO2; Chemical: Si; 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: SiO2; Chemical: Si; Chemical: graphene