Kagome Topology in Two-Dimensional Noble-Metal Monolayers

TL;DR

This study uses first-principles calculations to explore the stability and properties of kagome lattice monolayers of Cu, Ag, and Au, revealing strain and atomic size as key factors for their stability and potential applications.

Contribution

It provides the first comprehensive theoretical analysis of free-standing kagome monolayers of Cu, Ag, and Au, including stability conditions and mechanical properties.

Findings

Au monolayer has the highest in-plane Young's modulus.

Biaxial tensile strain stabilizes Ag and Au kagome monolayers.

Cu monolayer reconstructs to a trigonal lattice at room temperature.

Abstract

Two-dimensional (2D) metallic lattices with kagome topology provide a unique platform for exploring the interplay between geometric frustration, reduced coordination, and lattice stability in elemental systems. Motivated by the recent experimental realization of atomically thin gold layers and kagome goldene, we present a first-principles investigation of free-standing kagome monolayers of Cu, Ag, and Au. Using density functional theory combined with lattice dynamics and ab initio molecular dynamics, we systematically assess their structural, mechanical, dynamical, and thermal stability. All kagome monolayers satisfy the 2D Born criteria and exhibit relatively low in-plane stiffness compared to graphene and hexagonal goldene, reflecting the porous nature of the kagome lattice and its metallic bonding. Among the three systems, the Au-based lattice displays the highest in-plane Young's modulus. Phonon calculations reveal that the unstrained kagome phase is dynamically unstable for all metals. However, a moderate biaxial tensile strain of 5% stabilizes the Ag and Au monolayers, while Cu retains residual unstable modes. Finite-temperature simulations further show that Cu rapidly reconstructs toward a trigonal lattice, Ag remains metastable at low temperature but collapses at room temperature, and Au exhibits competing kagome and trigonal motifs at 300 K, indicating near-degeneracy between these phases. These results establish that strain engineering and atomic size are key determinants of the stability of metallic kagome monolayers and provide guidance for future substrate-supported realizations.