Tunneling electron induced rotation of a copper phthalocyanine molecule on Cu(111)

TL;DR

This paper models how tunneling electrons induce frustrated rotation in copper phthalocyanine molecules on Cu(111), explaining experimental observations and revealing lobe-specific excitation due to hybridization effects.

Contribution

It provides a theoretical framework combining scattering theory and electronic structure calculations to explain STM-induced molecular rotation and lobe selectivity.

Findings

Tunneling electrons cause frustrated rotation of CuPc molecules.

The excitation is dominated by two molecular lobes due to hybridization.

The model matches experimental telegraph noise observations.

Abstract

The rates of a hindered molecular rotation induced by tunneling electrons are evaluated using scattering theory within the sudden approximation. Our approach explains the excitation of copper phthalocyanine molecules (CuPc) on Cu(111) as revealed in a recent measurement of telegraph noise in a scanning tunneling microscopy (STM) experiment [Schaffert \textit{et al.}, Nat. Mat. {\bf 12}, 223 (2013)]. A complete explanation of the experimental data is performed by computing the geometry of the adsorbed system, its electronic structure and the energy transfer between tunneling electrons and the molecule's rotational degree of freedom. The results unambiguously show that tunneling electrons induce a frustrated rotation of the molecule. In addition, the theory determines the spatial distribution of the frustrated rotation excitation, confirming the striking dominance of two out of four molecular lobes in the observed excitation process. This lobe selectivity is attributed to the different hybridizations with the underlying substrate.