Effect of metal doping on structural characteristics of amorphous carbon system: A first-principles study

• The bond characteristics as 3d electrons changed from bonding, nonbonding to antibonding.
• The TM–C bond was a mixture of covalent and ionic characters.
• Reduced strength and directionality of TM–C bond led to small distortion energy change.
• The weak TM–C bond accounted for the reduced compressive stress caused by TM.

First-principles calculation was performed to investigate the effect of metal doping on the structural characteristics of amorphous carbon system, and the 3d transition metals (TM) were particularly selected as representative case. Results showed that the total energy in TM–C systems caused by distorting the bond angles was reduced distinctly for comparison with that in C–C system. Further electronic structure revealed that as the 3d electrons of doped TM increased, the bond characteristic of highest occupied molecular orbital changed from bonding (Sc, Ti) to nonbonding (V, Cr, Mn, Fe) and finally to antibonding (Co, Ni, Cu) between the TM and C atoms. Meanwhile, the TM–C bond presented a mixture of the covalent and ionic characters. The decrease of strength and directionality of TM–C bonds resulted in the total energy change upon bond angle distortion, which demonstrated that the bond characteristics played an important role in reducing residual stress of TM-doped amorphous carbon systems.