Local stiffness and work function variations of hexagonal boron nitride on Cu(111).

Abhishek Grewal,Matthias Münks,Markus Ternes,Yuqi Wang,Klaus Kern

doi:10.3762/bjnano.12.46

Abstract

Combined scanning tunnelling and atomic force microscopy using a qPlus sensor enables the measurement of electronic and mechanic properties of two-dimensional materials at the nanoscale. In this work, we study hexagonal boron nitride (h-BN), an atomically thin 2D layer, that is van der Waals-coupled to a Cu(111) surface. The system is of interest as a decoupling layer for functional 2D heterostructures due to the preservation of the h-BN bandgap and as a template for atomic and molecular adsorbates owing to its local electronic trapping potential due to the in-plane electric field. We obtain work function (Φ) variations on the h-BN/Cu(111) superstructure of the order of 100 meV using two independent methods, namely the shift of field emission resonances and the contact potential difference measured by Kelvin probe force microscopy. Using 3D force profiles of the same area we determine the relative stiffness of the Moiré region allowing us to analyse both electronic and mechanical properties of the 2D layer simultaneously. We obtain a sheet stiffness of 9.4 ± 0.9 N·m−1, which is one order of magnitude higher than the one obtained for h-BN/Rh(111). Using constant force maps we are able to derive height profiles of h-BN/Cu(111) showing that the system has a corrugation of 0.6 ± 0.2 Å, which helps to demystify the discussion around the flatness of the h-BN/Cu(111) substrate.

Highlights

Two-dimensional hexagonal boron nitride (h-BN) is among the list of materials that garnered tremendous interest following the exfoliation of mono- and few-layer thick graphene films [1,2].Unique properties, such as high thermal stability and conductivity, immense intra-sheet stiffness, and excellent dielectric properties, make h-BN interesting for technological applica-Beilstein J
We observe an upward shift of the surface state onset of Cu(111) from approx. −455 meV on the bare substrate to approx. −320 meV on h-BN/Cu(111) (Figure 1c) [32]
Probing the force perpendicular to the substrate, F⟂, at varying tip–sample separations z, the effective stiffness of a nanostructure can be evaluated by comparing the F⟂(z) behaviour at different areas of the Moiré superstructure [23]

Summary

Introduction

Two-dimensional hexagonal boron nitride (h-BN) is among the list of materials that garnered tremendous interest following the exfoliation of mono- and few-layer thick graphene films [1,2].Unique properties, such as high thermal stability and conductivity, immense intra-sheet stiffness, and excellent dielectric properties, make h-BN interesting for technological applica-Beilstein J. To shed more light on this controversy we use an alternative method to verify the mechanical properties of the monolayer by measuring the stiffness of the h-BN layer at different locations of the superstructure and comparing these results with concomitantly recorded local work function variations.

Results

Conclusion