Mechanical Properties of Short Monatomic Gold Chain

Abstract

A number of experiments have been conducted over the past decade to study the mechanical properties of a single string of gold atoms suspended between two gold tips, using a scanning tunneling microscope (STM) or an atomic force microscope (AFM). We consider four, five, six, seven, eight and nine atom monatomic chains of gold stretched between two end atoms whose positions remain fixed while those of the rest of the atoms are relaxed. Though there is not any difference in the behavior of these chains but in order to avoid overlapping in graphs we have differentiated chains on the basis of even (4, 6, 8) atom and odd (5, 7, 9) atom chains. We use the Gupta Potential which is a many body Glue Potential describing atom- atom interactions. The distance between the end atoms is the total length of the chain and the total energy of the chain is the sum of interaction energies between all pairs of atoms. The intervening atoms are allowed to adjust their positions till minimum energy is reached. The results from this procedure show that the minimum energy configurations are zig-zag chains which straighten when stretched. The Young’s modulus is in the range of a Gigapascal. On further stretching, the chain breaks. The maximum force for these short chains to break is of the order of 1.2, 1.4, 1.3 nN for even (4, 6, 8) chains and 1.7, 1.5 and 0.99 nN for odd (5, 7, 9) chains respectively. The region of length beyond the point of inflexion represents plasticity, and therefore cannot extend to the large strains. Hence this must be unphysical and we take the maximum force to be the breaking force. These results compare well with existing literature qualitatively and quantitatively. However, for long chains, containing 15 or more atoms, the zigzag configuration is not favoured. Experimental realizations of such long monatomic chains are awaited but experimental results are available for stretching up to eight atom chain.