Liquid-assisted tip manipulation: fabrication of twisted bilayer graphene superlattices on HOPG

Abstract

We use the tip of a scanning tunneling microscope (STM) to manipulate single weakly bound nanometer-sized sheets on a highly oriented pyrolytic graphite (HOPG) surface through artificially increasing the tip and sample interaction by pretreatment of the surface using a liquid thiol molecule. By this means it is possible to tear apart a graphite sheet against a step and fold this part onto the HOPG surface and thus generate graphene superlattices with hexagonal symmetry. The tip and sample surface interactions, including the van der Waals force, electrostatic force and capillary attraction force originating from the Laplace pressure due to the formation of a highly curved fluid meniscus connecting the tip and sample, are discussed quantitatively to understand the formation mechanism of a graphene superlattice induced by the STM tip. The capillary force plays a key role in manipulating the graphite surface sheet under humid conditions. Our approach provides a simple and feasible route to prepare controllable superlattices and graphene nanoribbons and also to better understand the process of generation of a graphene superlattice on the surface of HOPG with the tip.

Capillary force-induced superlattice variation atop a nanometer-wide graphene flake and its moiré origin studied by STM

“…There have been reports both in support [21,25] as well as against [26–27] the simple moiré theory. Changes of the superlattice periodicity in space [25] and time [17] have been reported before. However, there has been no report on a real-time observation of a change in the periodicity with concurrent and direct measurement of the orientation of the top graphene layer before and after its rotation.…”

“…Roy et al demonstrated the STM manipulation of folding of graphene under UHV conditions [30], although tip-driven surface layer deformation may occur more easily in air than in vacuum [31]. Under humid or liquid conditions, capillary forces are present and might offer a substantial amount of force for rotation or translation of the top layer [17,31–33]. The presence of an organic solution, thus, may facilitate the rotation of the top-layer graphene yielding superlattices on graphite [7,17,29].…”

“…Under humid or liquid conditions, capillary forces are present and might offer a substantial amount of force for rotation or translation of the top layer [17,31–33]. The presence of an organic solution, thus, may facilitate the rotation of the top-layer graphene yielding superlattices on graphite [7,17,29]. Here, we report a rare occurrence of an isolated, nanometer-wide graphene flake and its live transformation from one superlattice to another.…”

“…Since an STM image is a map of the local electronic density of states (LDOS), such electronic modifications may be visualized in real space. When STM is operated at solid–liquid interfaces, the capillary force due to the meniscus formed between the tip and the surface could be utilized for mechanically manipulating graphene flakes on the surface [17]. According to calculations, the pressure within the meniscus could be large enough to cause small deformations in crystalline materials such as graphite [17–18].…”

“…When STM is operated at solid–liquid interfaces, the capillary force due to the meniscus formed between the tip and the surface could be utilized for mechanically manipulating graphene flakes on the surface [17]. According to calculations, the pressure within the meniscus could be large enough to cause small deformations in crystalline materials such as graphite [17–18]. Here we try to use the capillary force to displace a nanometer-wide graphene flake with the intent of altering the superlattice on the flake.…”