Commun. Comput. Phys. doi: 10.4208/cicp.270110.290610a Vol. 9, No. 4, pp. 843-858 April 2011 Computational Study of Interstitial Hydrogen Atoms in Nano-Diamond Grains Embedded in an Amorphous Carbon Shell Amihai Silverman 1 , Alon Hoffman 2 and Joan Adler 3, * 1 Taub Computer Center, Technion-IIT, Haifa 32000, Israel. 2 Schulich Faculty of Chemistry, Technion-IIT, Haifa 32000, Israel. 3 Department of Physics, Technion-IIT, Haifa 32000, Israel. Received 27 January 2010; Accepted (in revised version) 29 June 2010 Available online 13 October 2010 Abstract. The properties of hydrogen atoms in a nano-diamond grain surrounded by an amorphous carbon shell are studied with Tight Binding computer simulations. Our samples model nano-diamond grains, of a few nanometers in size, that nucleate within an amorphous carbon matrix, as observed in deposition from a hydrocarbon rich plasma. The calculations show that the average hydrogen interstitial formation energy in the amorphous region is lower than in the nano-diamond core, therefore hydrogen interstitial sites in the in the amorphous region are more stable than in the nano-diamond core. This formation energy difference is the driving force for the dif- fusion of hydrogen atoms from nano-diamond grains into amorphous carbon regions. An energy well was observed on the amorphous side of the nano-diamond amorphous carbon interface: hydrogen atoms are expected to be trapped here. This scenario agrees with experimental results which show that hydrogen retention of diamond films in- creases with decreasing grain size, and suggest that hydrogen is bonded and trapped in nano-diamond grain boundaries and on internal grain surfaces. PACS: 68.55.Ln, 71.15.Pd, 79.20.Uv, 81.05.Uw, 81.15.Gh Key words: Nano-diamond, amorphous-carbon, DOS, NEXAFS, molecular-dynamics, tight- binding. 1 Introduction Hydrogen is an essential component of the gas mixture used for micro and nano crys- talline diamond film nucleation and growth by Chemical Vapor Deposition (CVD) meth- ods [1–3]. Extensive research has been carried out to determine the role of hydrogen in * Corresponding author. Email address: phr76ja@technion.ac.il (J. Adler) http://www.global-sci.com/ 843 c 2011 Global-Science Press