Semiconductor nanoparticles.
http://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&sqi=2&ved=0CEMQFjAH&url=http%3A%2F%2Fykuo.ncue.edu.tw%2Freport%2F052-Yen%2520Sheng_Horng-Vegard%27s%2520law%2520deviation%2520in%2520band%2520gap.ppt&ei=stUdU-vjO6S1iQea4YGwBw&usg=AFQjCNHbwcPagW8uxgyTeangxFCheVlmjQ&sig2=OjGdXp03nFLeoDB-5SqnZw