12001
Since nitrogen derived defects are mostly buried below the surface , they are not accessible for a chemical interaction with metal nanoparticles ; however , the amorphization induced by the ion beam in the outermost layers of the substrate beneficially affects the metal morphology , limiting the size of the nanoparticles and improving their thermal stability .
 Chemical: nitrogen
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: nitrogen