13831
A double - layer nanostructure comprising amorphous Si nanograss on top of Si nanofrustums ( NFs ) with a total height of 680 nm exhibits ultralow reflection .
 Chemical: Si; Chemical: Si
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: Si