Positional and Rotational Molecular Degrees of Freedom in a Metal-Organic Surface Alloy: the Copper-Fullerene System and its Multiple Structural Phases

TL;DR

This study explores the complex structural phases of copper-fullerene surface alloys formed on Cu(111), revealing multiple coexisting phases with diverse molecular arrangements and degrees of freedom, modeled by a new energy framework.

Contribution

It introduces a comprehensive analysis of copper-fullerene surface alloys, highlighting the multiple structural phases and degrees of freedom, and develops a competing-interaction energy model based on ab initio calculations.

Findings

Ten distinct copper-fullerene surface alloys coexist.

Fullerenes exhibit in-plane and out-of-plane positional degrees of freedom.

A new energy model accurately describes observed phases.

Abstract

Mixing two chemical elements at the surface of a substrate is known to produce rich phase diagrams of surface alloys. Here, we extend the concept of surface alloying to the case where the two constituent elements are not both atoms, but rather one atom (copper) and one molecule (fullerene). When deposited at room temperature on a Cu(111) surface, fullerenes intermix with the metal substrate. Surprisingly, 10 distinct copper-fullerene surface alloys are found to coexist. The structure of these alloys, i.e. their composition and commensurability relationship with the substrate, is resolved using scanning tunneling microscopy and density functional theory calculations. This diversity in the alloying process is associated to the multiple possibilities a fullerene can bind to the Cu surface. The molecules are indeed found to have in-plane and out-of-plane positional degree of freedom: the molecular alloys have elastic in-plane properties and can buckle. In addition, the molecules can rotate on their binding sites, conferring extra degrees of freedom to the system. We introduce a competing-interaction energy model, parametrized against the results of the \textit{ab initio} calculations, that describes well all the phases we observe experimentally.