13832
Almost near - unity absorption and near - zero reflectance result in this layered nanostructure , over a broad range of wavelengths and a wide range of angles of incidence , due to the low packing density of a - Si and the smooth transition of the refractive index from the air to the Si substrate across both the nanograss and NF layers .
 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