2, Colorado School of Mines, Golden, Colorado, United States
Two-dimensional (2D) layered hexagonal boron nitride (hBN) is an insulator, isostructural to graphene, and is considered an ideal dielectric for a variety of 2D layer based nanoelectronic devices. In contrast to free-standing hBN layers, substrate-supported hBN can be metallic, semiconducting, or insulating, depending on the chemistry and electronic structure of the hBN/substrate interface. Control over these properties hinges on the ability to grow high-quality, wafer-scale hBN crystals. hBN layers are commonly grown on transition-metal substrates and the hBN layer structures, as well as their interactions with the transition-metals have been fairly well characterized. However, relatively few studies have investigated (i) the mechanisms underlying the growth of the hBN layers and (ii) influence of hBN domain orientation (with respect to the substrate) on its electronic properties.
Using in situ ultra-high vacuum scanning tunneling microscopy (UHV STM), we investigated the chemical vapor deposition (CVD) kinetics of hBN on Pd(111). STM images are acquired during CVD of borazine as a function of substrate temperature, borazine flux, and deposition time. We observe the nucleation and growth of chemisorbed borazine islands on the Pd surfaces. Furthermore, we investigated the surface structure of hBN domains on Pd(111) using STM, scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. STM images acquired from the hBN/Pd(111) sample reveal moiré patterns with at least six different periodicities (λ) corresponding to six rotational domains of hBN. From the STM images, we measured the surface corrugations in each of the moiré patterns and found that the corrugation amplitude △z depends on the tunneling bias and increases with increasing λ. We suggest that △z is a measure of hBN-Pd(111) interaction strength and attribute the higher corrugation amplitudes to stronger interactions between the hBN domains and the Pd(111). We expect that similar approach could be used to investigate growth kinetics and orientation-dependent interactions in other substrate-supported 2D layers as well as in 2D layered heterostructures.