[Paper Review] Nanowire Gold Chains: Formation Mechanisms and Conductance

Hannu Häkkinen, R. N. Barnett+2 more|arXiv (Cornell University)|Sep 11, 2000

Molecular Junctions and Nanostructures3 citations

TL;DR

This study uses ab initio density functional theory and conductance calculations to reveal that elongated gold nanowires transform from a double-strand to a bent-chain and finally to a linear, dimerized atomic chain under strain. The process features a stable ~1g₀ conductance plateau due to directional spd-hybridized bonding, explaining experimental observations of quantized conductance and structural evolution in gold nanowires during mechanical pulling.

ABSTRACT

Structural transformations, electronic spectra and ballistic transport in pulled gold nanowires are investigated with ab initio simulations, and correlated with recent measurements. Strain-induced yield of an initial double-strand wire results first in formation of a bent-chain which transforms upon further elongation to a linear atomic chain exhibiting dimerized atomic configurations. These structures are stabilized by directional local bonding with spd-hybridization. The conductance of the initial double-stranded contact is close to 2g_0 and it drops sharply to 1g_0 during the transformation to a single chain, exhibiting subsequently a 1g_0 plateau extending over an elongation well above typical Au-Au distances.

Motivation & Objective

• To elucidate the atomic-scale structural evolution and transformation mechanisms in elongated gold nanowires during mechanical pulling.
• To explain the experimentally observed conductance quantization and the sharp drop from 2g₀ to 1g₀ during the transition from double-strand to single-atom chain.
• To investigate the electronic structure and bonding characteristics responsible for stabilizing the linear atomic chain configurations.
• To clarify why such extended single-atom chains are stable in gold but not in simple metals like sodium.
• To distinguish the formation mechanism in controlled tip-pulling experiments from that in electron-bombarded nanobridge configurations with anomalously large interatomic distances.

Proposed method

• Large-scale ab initio density functional theory (DFT) simulations were performed to model structural and electronic properties of gold nanowires under strain.
• Conductance was calculated using the Landauer-Büttiker formalism, with transmission eigenchannels derived from non-equilibrium Green's function methods.
• The system was modeled as two 29-atom gold electrodes with (110) orientation, separated by a variable distance L, with atomic positions fully relaxed at each elongation step.
• Structural evolution was tracked by increasing L from 11.5 Å to 18.0 Å in 0.25–1.0 Å increments, with energy and conductance computed at each step.
• Orbital analysis and charge density mapping were used to characterize bonding, including hybridization of s, p, and d orbitals in the wire region.
• The simulations focused on the transition from double-strand (ds) to bent-chain (bc) to linear-chain (lc) configurations, with detailed analysis of dimerization patterns (inner vs. end-atom dimerization).

Experimental results

Research questions

• RQ1 What is the atomic-scale mechanism by which a double-strand gold nanowire transforms into a single-atom linear chain under mechanical strain?
• RQ2 Why does the conductance drop sharply from ~1.79g₀ to ~1g₀ during the transition from double-strand to bent-chain configuration?
• RQ3 What role do spd-hybridized orbitals play in stabilizing the linear atomic chain and enabling a long conductance plateau near 1g₀?
• RQ4 How do the electronic structure and bonding characteristics differ between the double-strand, bent-chain, and linear-chain configurations?
• RQ5 Why are stable, extended single-atom gold chains observed in tip-pulling experiments but not in electron-irradiated nanobridge configurations?

Key findings

• The double-strand to bent-chain transition is driven by strain accumulation and is accompanied by a sharp drop in conductance from 1.79g₀ to 1.68g₀ and a drop in pulling force.
• The bent-chain configuration is energetically favorable and transforms into a linear chain with dimerized atomic configurations, stabilizing the system under further elongation.
• The linear chain exhibits a long conductance plateau of ~1g₀ extending over 4–4.5 Å of elongation, consistent with experimental observations.
• The stability of the linear chain is due to directional bonding involving significant s, p, and d orbital hybridization, particularly sd-hybridization at the chain ends and σ-like bonding from s and dz² orbitals on inner atoms.
• Inner-dimerization (id) occurs first, followed by end-atom dimerization (ed), with the latter showing strong s and d-orbital contributions (s⁰.⁴⁸dₓ²⁻ʸ²⁰.⁴⁶) on end atoms and s⁰.⁸⁶p_z⁰.¹¹d_z²⁰.⁰³ on inner atoms.
• The formation of stable single-atom chains is specific to gold due to its unique spd-hybridization capabilities; such chains are not stable in simple metals like sodium, explaining the absence of similar behavior in other systems.