Quantum Size Effect and Electronic Stability of Freestanding Metal Atom Wires

Abstract

Fabrication of freestanding or supported metal atom wires may offer unprecedented opportunities for investigating exotic behaviors of one-dimensional systems, including the possible existence of non-Fermi liquids. Many recent efforts have been devoted to the formation of different kinds of metal atom wires in freestanding forms by novel techniques like mechanical break junction or deposited on substrates via self-assembly, focusing on their mechanical, chemical and electronic properties. Various atom wires with different lengths can be obtained during fabricating processes. Their size distributions have been extensively analyzed, which exhibit diverse features.Although several factors such as strain and substrate effects have been employed to interpret these phenomena, the stability of atom wire itself is largely ignored. Using density functional theory calculations, we present a thorough study on freestanding metal atom wires, including s, sd and sp electron prototypes, to examine the size effect in their stabilities. We find that the total energy of all systems oscillates within wire length, which clearly indicates the existence of some preferred lengths. Increasing the length of atom wires, s electron system shows even-odd oscillation following a a/x + b/x 2 trend in the stability, due to both electrons pairing up and one-dimensional quantum confinement. Meanwhile, sd electron systems show a similar oscillation within wire length although s-d hybridization is presented. In sp electron systems, some oscillations beyond the even-odd one are exhibited due to unpaired p orbitals resulting in some nontrival filling rule. Our findings clearly demonstrate that electronic contribution is quite critical to the stability of freestanding atom wires and is also expected to dominate even when atom wires are deposited on substrates or under strain. This study sheds light on the formation of metal atom wires and helps understanding relevant phenomena.