Bifurcation and orientation-dependence of corrugation of 2D hexagonal boron nitride on palladium

Abstract

Using ultra-high vacuum scanning tunneling microscopy (STM) and density functional theory (DFT), we investigated the surface structure of 2D hexagonal boron nitride (hBN) domains on Pd(1 1 1). STM images of polydomain hBN monolayers, grown via dissociative chemisorption of borazine on Pd(1 1 1)/Al2O3(0 0 0 1) thin films, are acquired as a function of tunneling current and bias. The images reveal moiré patterns with four periodicities λ = 0.6 ± 0.05 nm, 1.8 ± 0.14 nm, 2.7 ± 0.20 nm, and 2.8 ± 0.14 nm, corresponding to different orientations on Pd(1 1 1). We find that the apparent surface corrugation Δz in STM changes little with tunneling current, exhibits an oscillatory dependence on the bias voltage, and increases from Δz ~ 14 pm for domains with λ = 0.6 nm to Δz ~ 200 pm for λ = 2.8 nm. We attribute the observed tunneling-parameter dependence in Δz to the electronic structure of the hBN/Pd(1 1 1) system. Unlike any other monolayer hBN-on-metal system, we suggest that hBN/Pd can have either mainly geometric or mainly electronic corrugation, depending on the domain orientation. Furthermore, for the largest periodicities, we observe a bifurcation behavior in which some domains are nearly flat, and others develop significant hill-and-valley geometric corrugations. We expect a similar behavior for other substrates for which the interaction energy with hBN is intermediate, i.e. neither mostly chemical nor van der Waals binding: for these substrates, a similar approach can help identify interlayer interactions and electronic structure modifications.