The rich physical phenomena and properties associated with two-dimensional (2D) graphene layers have led to a surge in research efforts focused on synthesis, characterization, and, even large-scale production of a variety of 2D crystals. In the newly emergent class of 2D layered materials (and devices), some of the highly exciting developments are a result of leveraging the properties of individual layers via stacking of dissimilar 2D materials held together by van der Waals (vdW) forces. Such vdW-layers exhibit new properties (e.g., valleytronics) and are ideal for ultrathin flexible devices.

Graphene is semi-metallic, while its insulating 2D layered analog is hexagonal boron nitride (hBN). Graphene and hBN are isostructural and nearly lattice matched. Composite, lateral, and vertical heterostructures of graphene and hBN layers are expected to exhibit new physical phenomena and properties: tunable bandgap, magnetism, unique electronic and thermal transport properties, robust half-metallic behavior without applied electric fields, and more. Access to such new properties and hence novel applications, hinge on the ability to assemble the 2D materials in desired architecture. This requires fundamental understanding of the mechanisms governing the formation of epitaxial stacks of desired stacking sequences and layer thicknesses, atomically-abrupt graphene-hBN interfaces and domain widths within a single atomic layer, and desired composition within a 2D sheet. As a first step toward this goal, we investigated the chemical vapor deposition (CVD) kinetics of monolayer graphene and hBN on Pd(111) using ultra-high vacuum (UHV) variable-temperature scanning tunneling microscopy (VT-STM).

In this talk, we present in situ VT-STM observations of graphene growth using ethylene and hBN growth using borazine on Pd(111) as a function of substrate temperature, borazine flux, and deposition time. We observe the formation of monolayer graphene islands bounded by Pd surface steps. Using scanning tunneling spectroscopy (STS) measurements along with density functional theory (DFT) calculations, we show that graphene/Pd(111) exhibits a bandgap due to the strong interaction between graphene and the Pd substrate. During the CVD of borazine, we observe the nucleation and growth of chemisorbed borazine islands on Pd(111). Using STS, we determined the effect of hBN domain orientation with respect to Pd(111) on its bandgap. From the STM experiments, we identified the growth parameters critical to the formation of single-domain hBN layers on Pd(111). Our studies, which provide new insights into the growth of graphene and hBN monolayers, are useful in developing recipes for the growth of heterostructures of desired architecture.