24796
Images acquired over a range of positive and negative tip - sample bias voltages show a shift in contrast , with the acetylene linker appearing brighter than the ferrocenes at positive sample bias ( where unoccupied states primarily contribute ) and the reverse contrast at negative bias .
 acetylene, ferrocenes
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 acetylene, ferrocenes