Research on stacked 2D materials has tremendously grown in the last decade. This can be attributed to a combination of their low mutual van der Waals hybridization and the translational as well as rotational degrees of freedom, which allow effective control over their stacking geometries and entailed functionalities. The growth of graphene on hexagonal boron nitride (h-BN) is particularly charming. Although SiO2 is the most common substrate for fabricating graphene devices, the application of h-BN as a support substrate has several advantages over SiO2. The absence of dangling bonds coupled with the inert nature of h-BN plays a crucial role in efficiently decoupling graphene from the environment and, thus, retaining its original properties. Indeed, based on previous studies, graphene-h-BN stacking has been shown to enhance ballistic charge transport, fractional quantum Hall effects and electron mobility.
Graphene stacking on h-BN is a major experimental challenge. In most cases, the fabrication of the heterostructure involves transferring exfoliated or chemically grown graphene onto h-BN. This method is associated with increased graphene rippling as well as trapped contaminants at the interface of graphene and h-BN. Alternative methods for stacking graphene onto h-BN like chemical vapor deposition (CVD) have been reported. Typically, CVD entails the thermal decomposition of molecular precursors facilitated by the catalytic activity of a metal surface. To preserve the quasi-free state and properties of the resulting 2D materials, it is recommendable to couple the 2D layers with a favorable metal. However, some metals such as nickel hinder retaining such properties due to considerable hybridization. Even though plasma-enhanced CVD improved the accessibility of support substrates for stacking, there is a need for metal substrates with low coupling to the 2D materials and efficient catalytic performance for stacking graphene onto h-BN.
On this account, Dr. Alexander Mehler, Dr. Nicolas Néel and Professor Jörg Kröger from Technische Universität Ilmenau in collaboration with Professor Elena Voloshina and Professor Yuriy Dedkov from Shanghai University studied the epitaxial growth of graphene atop h-BN on a (111)-oriented platinum surface via thermal decomposition of molecular precursors and catalytic assistance of platinum. The temporary deposition of platinum films served as a catalyst for the efficient fabrication of the graphene sheet. “One first has to cover h-BN with a thick film of platinum, then you grow graphene on top. At increased temperature, platinum seeps through the h-BN mesh and lets graphene softly land on top of it”, says Alex Mehler, first author of the studies. A combination of scanning tunneling microscopy (STM), density functional calculations and inelastic electron tunneling spectroscopy (IETS) were used to characterize the resulting graphene-h-BN stacking. Their research work has been published in Small.
Besides the controlled growth of large patches of the heterostacking, the authors’ findings revealed that the moiré pattern visible in STM images is not due to the twisted 2D sheets; unexpectedly, it rather results from the interface between h-BN and the Pt(111) surface. Moreover, STM images unveiled that the graphene honeycomb cells arrange themselves in a honeycomb superstructure, challenging the accompanying simulations. Elena Voloshina who performed the calculations mentions “that the extremely large supercells used for the simulations contain hundreds of atoms and pushed the computations close to their limits.” Beyond the interesting structural properties of the stacking, the occurrence of only weak signals in IETS of vibrational quanta is at odds with the generally assumed weak mutual interaction between the 2D materials.
In summary, the subsequent growth of epitaxial graphene on h-BN with the catalyst-assisted thermal decomposition of molecular precursors was successfully achieved on Pt(111) and a temporary intermediate platinum film. The novel approach enables the growth of stackings with minimized defects and the control over the number of layers. In a statement to Advances in Engineering, Professor Jörg Kröger noted that the obtained insights of the studies will help fabricate stackings of other 2D materials.
Mehler, A., Néel, N., Voloshina, E., Dedkov, Y., & Kröger, J. (2021). Second Floor of Flatland: Epitaxial Growth of Graphene on Hexagonal Boron Nitride. Small, 17(36), 2102747.