Please cite this article as: Süle, P., Szendrő, M., Hwang, C., Tapasztó, L., Rotation misorientated graphene moiré superlattices on Cu(111): Classical molecular dynamics simulations and scanning tunneling microscopy studies, Carbon (2014), doi: http://dx. AbstractGraphene on copper is a system of high technological relevance, as Cu is one of the most widely used substrates for the CVD growth of graphene. However, very little is known about the details of their interaction. One approach to gain such information is studying the superlattices emerging due to the mismatch of the two crystal lattices. However, graphene on copper is a low-corrugated system making both their experimental and theoretical study highly challenging. Here, we report the observation of a new rotational moiré superlattice of CVD graphene on Cu(111), characterized by a periodicity of (1.5 ± 0.05) nm and corrugation of (0.15 ± 0.05)Å , as measured by Scanning Tunneling Microscopy (STM). To understand the observed superlattice we have developed a newly parameterized Abell-Tersoff potential for the graphene/Cu(111) interface fitted to nonlocal van der Waals density functional theory (DFT) calculations. The interfacial force field with time-lapsed classical molecular dynamics (CMD) provides superlattices in good quantitative agreement with the experimental results, for a misorientation angle of (10.4 ± 0.5 • ), without any further parameter adjustment. Furthermore, the CMD simulations predict the existence of two nonequivalent high-symmetry directions of the moiré pattern that could also be identified in the experimental STM images.

The accurate molecular dynamics simulation of weakly bound adhesive complexes, such as supported graphene, is challenging due to the lack of an adequate interface potential. Instead of the widely used Lennard-Jones potential for weak and long-range interactions we use a newly parameterized Tersoff-potential for graphene/Ru(0001) system. The new interfacial force field provides adequate moiré superstructures in accordance with scanning tunnelling microscopy images and with DFT results. In particular, the corrugation of ξ ≈ 1.0 ± 0.2Å is found which is somewhat smaller than found by DFT approaches (ξ ≈ 1.2Å) and is close to STM measurements (ξ ≈ 0.8 ± 0.3Å). The new potential could open the way towards large scale simulations of supported graphene with adequate moiré supercells in many fields of graphene research. Moreover, the new interface potential might provide a new strategy in general for getting accurate interaction potentials for weakly bound adhesion in large scale systems in which atomic dynamics is inaccessible yet by accurate DFT calculations. keywords: atomistic and nanoscale simulations, molecular dynamics simulations, corrugation of graphene, moire superstructures INTRODUCTIONSince the reliable ab initio density functional theory (DFT) methods with various van der Waals (VDW) functionals can be used only up to ∼ 1000 atoms [1] the development of an adequate classical interfacial force-field for supported graphene (gr) is crucial. Although, classical molecular dynamics (CMD) simulations can not be used for the study of electronic structure, however, many important properties of graphene, such as surface topography, moire supercells and interfacial binding characteristics, adhesion as well as defects and many other features can in principle be studied by CMD simulations [2, 3] without the explicit consideration of the electron density or orbitals.The size-limit of DFT is especially serious if accurate geometry optimization or molecular dynamics simulations should be carried out. Moreover, various VDW DFT functionals provide diverging results [1, 4]. In particular, accurate DFT potential energy curves for the interface of gr-substrate systems has been obtained only by the extremely expensive random phase approximation (RPA) for small modell systems which fail, however, to include the full moire-superstructure [4][5][6]. Using CMD simulations no size-limit problem occurs. An important limitation here, however, is the limited availability of accurate interface potential for weak interactions. The widely accepted choice is the simple pairwise Lennard-Jones (LJ) potential. In the present paper we point out that this potential is inadequate, however, for gr/Ru(0001) due to the improper prediction of the binding sites (registry). Therefore, the development of reliable interatomic potentials for gr-support systems is inevitable.First principles calculations (such as density functional theory, DFT) have widely been used in the last few years to understand corrugation of nanoscale gr sheets on various substra...

The behavior of single layer van der Waals (vdW) materials is profoundly influenced by the immediate atomic environment at their surface, a prime example being the myriad of emergent properties in artificial heterostructures. Equally significant are adsorbates deposited onto their surface from ambient. While vdW interfaces are well understood, our knowledge regarding atmospheric contamination is severely limited. Here we show that the common ambient contamination on the surface of: graphene, graphite, hBN and MoS2 is composed of a self-organized molecular layer, which forms during a few days of ambient exposure. Using low-temperature STM measurements we image the atomic structure of this adlayer and in combination with infrared spectroscopy identify the contaminant molecules as normal alkanes with lengths of 20-26 carbon atoms. Through its ability to self-organize, the alkane layer displaces the manifold other airborne contaminant species, capping the surface of vdW materials and possibly dominating their interaction with the environment.

Despite their application potential, the structure of CVD grown MoSe2 single layers remained relatively unexplored. Here we report the rotationally aligned CVD growth of MoSe2 single layers on graphite. Such MoSe2 layers are characterized by grain boundary structures with significantly reduced disorder and display a much lower grain boundary density compared to samples grown by MBE. We show that the grain boundaries of CVD MoSe2 on graphite are predominantly mirror twin boundaries (MTBs) and distinguish two classes of such MTBs based on their orientation relative to the zigzag directions of the MoSe2 lattice. By combining atomic resolution scanning tunneling microscopy (STM) and spectroscopy measurements with DFT calculations, we demonstrate that the predominantly present MTBs running along zigzag directions only slightly perturb the electronic structure of the MoSe2 sheet. This enables the CVD growth of large-area MoSe2 layers with high structural and electronic quality. Atomic resolution STM investigations also revealed a high density (1012 cm–2) of native point defects identified as Mo vacancies. Our DFT calculations predict that Mo vacancies in MoSe2 are magnetic, and their magnetic moment can be efficiently controlled electrically by tuning the Fermi level position.