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

Fabian Schulz,Adam S Foster,Juha Ritala,Ondrej Krejčí,Peter Liljeroth,Ari Paavo Seitsonen

doi:10.1021/acsnano.7b08997

Fabian Schulz, Adam S Foster + Show 4 more

Open Access

https://doi.org/10.1021/acsnano.7b08997

Copy DOI

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.

Highlights

Atomic-resolution microscopies are key enabling techniques in modern materials research
When operated in the frequency modulation mode,[15] ncAFM measures atomic-scale forces between the tip on an oscillating cantilever and the sample surface through changes of the resonance frequency (Δf) of the cantilever. noncontact atomic force microscopy (nc-AFM) can yield atomic resolution, which has been amply demonstrated on elemental semiconductors as well as on heteroatomic surfaces of compound semiconductors and polar insulators such as alkali halides and oxides.[1,3,6,16−18] On such heteroatomic surfaces, typically only one type of atom is imaged with a given tip, because the polar nature of the compounds results in a strong variation in the short-range forces above the negatively
Atomic-resolution nc-AFM studies can be extended to molecular systems through chemical passivation of the tip apex, e.g., by controlled pick-up of a single carbon monoxide (CO) molecule.[13,14,28−36] With these tips, it is possible to enter a regime where the tip−sample interaction is dominated by the Pauli repulsion between the last atom of the tip and the sample atom directly under it.[28,37−39] In addition to molecules, this technique has been used to measure atomic positions and surface corrugations of two-dimensional materials.[32,40−44]

Summary

Introduction

Atomic-resolution microscopies are key enabling techniques in modern materials research. Nc-AFM image simulations based on the density functional theory (DFT)-optimized hBN/Ir(111) geometry allow us to match the two distinct atomic sites with the boron and the nitrogen sublattices. In Figure 2k, we plot the difference (black) between the two inequivalent top sites (green, red) of the hBN lattice and the corresponding force difference (magenta) recovered from Δf via the Sader−Jarvis method.[61] Taking the adjacent-averaged force data, the difference between the boron and nitrogen sublattice at typical imaging distances is no more than 10 pN.

Results

Conclusion