8231
The initial lateral etching profile of the Si ( 111 ) substrates shows a 2 - fold rotational symmetry using 4 % ammonia solution , which provides guidance for initial scrolling of one - end - fixed nanoribbons to form helical structures .
 Chemical: Si; Chemical: ammonia
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: Si; Chemical: ammonia