The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy

TLDR

Surface microscopy has long aimed to resolve individual atoms, yet scanning tunneling microscopy struggles to image single atoms within adsorbed molecules due to its sensitivity to local electron density near the Fermi level. The study demonstrates that noncontact atomic force microscopy can image molecules with unprecedented atomic resolution by probing short‑range chemical forces. The technique uses noncontact atomic force microscopy with a tip functionalized by atomically defined terminations such as CO to probe short‑range chemical forces. Experiments, supported by ab initio density functional theory, reveal that Pauli repulsion provides the atomic resolution while van der Waals and electrostatic forces contribute only a diffuse attractive background.

Abstract

Resolving individual atoms has always been the ultimate goal of surface microscopy. The scanning tunneling microscope images atomic-scale features on surfaces, but resolving single atoms within an adsorbed molecule remains a great challenge because the tunneling current is primarily sensitive to the local electron density of states close to the Fermi level. We demonstrate imaging of molecules with unprecedented atomic resolution by probing the short-range chemical forces with use of noncontact atomic force microscopy. The key step is functionalizing the microscope's tip apex with suitable, atomically well-defined terminations, such as CO molecules. Our experimental findings are corroborated by ab initio density functional theory calculations. Comparison with theory shows that Pauli repulsion is the source of the atomic resolution, whereas van der Waals and electrostatic forces only add a diffuse attractive background.