Elemental Identification by Combining Atomic Force Microscopy and Kelvin Probe Force Microscopy

Abstract

There are currently no experimental techniques that combine atomic-resolution imaging with elemental sensitivity and chemical fingerprinting on single molecules. The advent of using molecular-modified tips in noncontact atomic force microscopy (nc-AFM) has made it possible to image (planar) molecules with atomic resolution. However, the mechanisms responsible for elemental contrast with passivated tips are not fully understood. Here, we investigate elemental contrast by carrying out both nc-AFM and Kelvin probe force microscopy (KPFM) experiments on epitaxial monolayer hexagonal boron nitride (hBN) on Ir(111). The hBN overlayer is inert, and the in-plane bonds connecting nearest-neighbor boron and nitrogen atoms possess strong covalent character and a bond length of only ∼1.45 Å. Nevertheless, constant-height maps of both the frequency shift Δf and the local contact potential difference exhibit striking sublattice asymmetry. We match the different atomic sites with the observed contrast by comparison with nc-AFM image simulations based on the density functional theory optimized hBN/Ir(111) geometry, which yields detailed information on the origin of the atomic-scale contrast.

Benchmarking van der Waals-treated DFT: The case of hexagonal boron nitride and graphene on Ir(111)

“…In the related system of gr/Ir(111), nc-AFM measurements showed that, as a result, even within a single island, the corrugation of individual moiré unit cells varies smoothly by approximately ±8%, with occasional unit cells yielding even up to 20% larger corrugation [39]. Overall, our nc-AFM results are in agreement with the value obtained by Farwick zum Hagen et al from XSW (1.55 Å) [43], as well as with recent DFT results [42,43,53] and thus confirm that h-BN/Ir(111) belongs to the class of large-corrugation moiré systems.…”

“…[47]. nc-AFM measurements were carried out in a Createc LT-STM/AFM equipped with a qPlus tuning fork sensor [50] housed within the same UHV system at a temperature of 5 K. The attractive short-range interactions between the probe tip and the h-BN surface were minimized by passivating the tip apex by a carbon monoxide molecule (CO) [51][52][53].…”

Atomically Resolved Chemical Reactivity of Small Fe Clusters

“…However, Refs. [21,48] report that the properties, i.e., the electric field of CO tips, strongly depend on the tip backgrounds behind the terminating CO molecule. Hence, we conclude that the bond between the CO molecule and the metal tip is different for the various CO tips, although all these metal tips were terminated with a single metal atom prior to CO functionalization, leading to quantitatively different F z;SR ðz; 0Þ curves (see Fig.…”

Quantifying the evolution of atomic interaction of a complex surface with a functionalized atomic force microscopy tip

“…A major breakthrough in nc-AFM was the ability to resolve the internal structure of simple organic molecules in real space 2 , which was enabled by functionalizing a metallic AFM tip apex with a carbon-monoxide (CO) molecule. Since then, CO-terminated tips (CO tips) have been widely applied in AFM experiments to study molecular adsorbates [3][4][5][6][7][8][9] and various types of surfaces and adsorbates with atomic resolution [10][11][12][13][14][15] . The interaction of a CO tip with a sample surface is composed of different physical mechanisms, including van der Waals attraction and Pauli repulsion, which can be described by a Lennard-Jones potential 12,16,17 , and electrostatic interaction between the complex electric field of the CO tip and the sample electron density 12,13,18 .…”