Variable-Temperature Scanning Tunneling Microscopy Studies of Growth and Characterization of Hexagonal Boron Nitride and Graphene Layers
Two-dimensional (2D) materials, such as graphene and hexagonal boron nitride (hBN) have attracted attention from both academia and industry owing to their unique atomically thin structure and associated properties. The goal of this dissertation is to provide fundamental insights of the growth kinetics, surface structure, and thermal stability of hBN and graphene layers grown via chemical vapor deposition (CVD) on metal substrates.
Scanning tunneling microscopy (STM) was used to investigate the surface structure of hexagonal boron nitride (hBN) domains on Pd(111) grown via dissociative chemisorption of borazine on Pd(111)/Al2O3(0001) thin films. STM images acquired from the hBN/Pd(111) sample reveal moir� patterns with different periodicities λ corresponding to rotational domains of hBN. Surface corrugations in each of the moir� patterns were measured from the STM images as a function of the STM tunneling parameters. The corrugation amplitude Δz was found to depend on the tunneling bias and increases with increasing λ. The observed tunneling-parameter dependence in Δz were attributed to the electronic structure of the hBN/Pd(111) system. The domains with the largest λ, exhibit a bifurcation behavior in which some domains are nearly flat, and others develop “blisters”, i.e., significant hills-and-valley geometric undulations. Hence, unlike any other monolayer hBN-on-metal system, hBN/Pd can have either mainly geometric or mainly electronic corrugation, depending on the domain orientation.
The growth kinetics of hBN monolayers grown via CVD using borazine (B3N3H6) on Pd(111) were investigated using in situ variable-temperature STM. STM images were acquired during CVD of borazine on Pd(111) as a function of the substrate temperature T, the borazine partial pressure P, and time t. The STM images reveal a T and P dependent change in the growth mode: at higher P and lower T, (lower P and higher T), hBN nucleation occurs primarily on Pd step edges (Pd terraces). Furthermore, the mechanisms promoting growth across the step edges (as a carpet across steps) are identified. These results provide new insights into the nucleation and growth of hBN monolayers and potentially other 2D materials and related heterostructures on metal substrates.
The thermal stability of hBN covered Pd(111) thin films was investigated using in situ variable-temperature STM. STM images were acquired from bare Pd(111) and hBN covered Pd(111) at temperatures between 600 K and 1000 K. It was found that the hBN overlayer enhances the stability of the underlying Pd surface by suppressing the surface mobility of adatoms. These results provide new, direct insights into the behavior of substrate supported 2D materials at elevated temperatures.
Finally, in situ VT-STM was used to investigate the growth of graphene using benzene (C6H6) on Pd(111). STM images were acquired during and after benzene deposition at temperatures between 300 K and 1100 K. Compared to growth of graphene on Pd(111) using other hydrocarbon precursors (e.g. ethylene), benzene enables the growth of graphene at lower T, and at significantly lower doses, likely due to an increased probability of forming C6 and/or C6Hx(x<6) clusters. These results provide new insights into the mechanisms leading to the growth of graphene using benzene on reactive metals with high carbon solubility.