Sampsa K. Hämäläinen

Molecular Self-Assembly on Graphene on SiO2 and h-BN Substrates

One of the suggested ways of controlling the electronic properties of graphene is to establish a periodic potential modulation on it, which could be achieved by self-assembly of ordered molecular lattices. We have studied the self-assembly of cobalt phthalocyanines (CoPc) on chemical vapor deposition (CVD) grown graphene transferred onto silicon dioxide (SiO2) and hexagonal boron nitride (h-BN) substrates. Our scanning tunneling microscopy (STM) experiments show that, on both substrates, CoPc forms a square lattice. However, on SiO2, the domain size is limited by the corrugation of graphene, whereas on h-BN, single domain extends over entire terraces of the underlying h-BN. Additionally, scanning tunneling spectroscopy (STS) measurements suggest that CoPc molecules are doped by the substrate and that the level of doping varies from molecule to molecule. This variation is larger on graphene on SiO2 than on h-BN. These results suggest that graphene on h-BN is an ideal substrate for the study of molecular self-assembly toward controlling the electronic properties of graphene by engineered potential landscapes.

Intermolecular Contrast in Atomic Force Microscopy Images without Intermolecular Bonds

Intermolecular features in atomic force microscopy images of organic molecules have been ascribed to intermolecular bonds. A recent theoretical study [P. Hapala et al., Phys. Rev. B 90, 085421 (2014)] showed that these features can also be explained by the flexibility of molecule-terminated tips. We probe this effect by carrying out atomic force microscopy experiments on a model system that contains regions where intermolecular bonds should and should not exist between close-by molecules. Intermolecular features are observed in both regions, demonstrating that intermolecular contrast cannot be directly interpreted as intermolecular bonds.

Templated Self-Assembly and Local Doping of Molecules on Epitaxial Hexagonal Boron Nitride

Using low-temperature scanning tunneling microscopy, we show that monolayer hexagonal boron nitride (h-BN) on Ir(111) acts as ultrathin insulating layer for organic molecules, while simultaneously templating their self-assembly. Tunneling spectroscopy experiments on cobalt phthalocyanine (CoPC) reveal narrow molecular resonances and indicate that the charge state of CoPC is periodically modulated by the h-BN moiré superstructure. Molecules in the second layer show site-selective adsorption behavior, allowing the synthesis of molecular dimers that are spatially ordered and inaccessible by usual chemical means.

Electronic states at the graphene - hexagonal boron nitride zigzag interface

The electronic properties of graphene edges have been predicted to depend on their crystallographic orientation. The so-called zigzag (ZZ) edges haven been extensively explored theoretically and proposed for various electronic applications. However, their experimental study remains challenging due to the difficulty in realizing clean ZZ edges without disorder, reconstructions, or the presence of chemical functional groups. Here, we propose the ZZ-terminated, atomically sharp interfaces between graphene and hexagonal boron nitride (BN) as experimentally realizable, chemically stable model systems for graphene ZZ edges. Combining scanning tunneling microscopy and numerical methods, we explore the structure of graphene-BN interfaces and show them to host localized electronic states similar to those on the pristine graphene ZZ edge.

Topographic and electronic contrast of the graphene moire on Ir(111) probed by scanning tunneling microscopy and noncontact atomic force microscopy

Epitaxial graphene grown on transition-metal surfaces typically exhibits a moir´e pattern due to the lattice mismatch between graphene and the underlying metal surface. We use both scanning tunneling microscopy (STM) and atomic force microscopy (AFM) to probe the electronic and topographic contrast of the graphene moir´e on the Ir(111) surface. STM topography is influenced by the local density of states close to the Fermi energy and the local tunneling barrier height. Based on our AFM experiments, we observe a moir´e corrugation of 35 ± 10 pm, where the graphene-Ir(111) distance is the smallest in the areas where the graphene honeycomb is atop the underlying iridium atoms and larger on the fcc or hcp threefold hollow sites.

Sample Corrugation Affects the Apparent Bond Lengths in Atomic Force Microscopy

Frequency modulation atomic force microscopy (AFM) allows the chemical structure of planar molecules to be determined with atomic resolution. Typically, these measurements are carried out in constant-height mode using carbon monoxide (CO) terminated tips. Such tips exhibit considerable flexibility, i.e., the CO molecule can bend laterally due to the tip-sample interaction. Using epitaxial graphene as a model system, we demonstrate experimentally that the apparent atomic positions measured by AFM depend on the sample corrugation. Using molecular mechanics simulations, we explain these observations by the interplay of the CO bending and the nonlinear background signal arising from the neighboring atoms. These effects depend nontrivially on the tip-sample distance and limit the achievable accuracy on the bond length determination based on AFM experiments.

Structure and local variations of the graphene moiré on Ir(111)

We have studied the incommensurate moire structure of epitaxial graphene grown on iridium(111) by dynamic low-energy electron diffraction [LEED I(V)] and noncontact atomic force microscopy (AFM) with a CO-terminated tip. Our LEED I(V) results yield the average positions of all the atoms in the surface unit cell and are in qualitative agreement with the structure obtained from density functional theory. The AFM experiments reveal local variations of the moire structure: The corrugation varies smoothly over several moire unit cells between 42 and 56 pm. We attribute these variations to the varying registry between the moire symmetry sites and the underlying substrate. We also observe isolated outliers, where the moire top sites can be offset by an additional 10 pm. This study demonstrates that AFM imaging can be used to directly yield the local surface topography with pm accuracy even on incommensurate two-dimensional structures with varying chemical reactivity.

Structural manipulation of the graphene/metal-interface with Ar+ irradiation

Controlled defect creation is a prerequisite for the detailed study of disorder effects in materials. Here, we irradiate a graphene/Ir(111)-interface with low-energy Ar+ to study the induced structural changes. Combining computer simulations and scanning-probe microscopy, we show that the resulting disorder manifests mainly in the forms of intercalated metal adatoms and vacancy-type defects in graphene. One prominent feature at higher irradiation energies (from 1 keV up) is the formation of line-like depressions, which consist of sequential graphene defects created by the ion channeling within the interface -- much like a stone skipping on water. Lower energies result in simpler defects, down to 100 eV where more than one defect in every three is a graphene single vacancy.

Synthesis of Extended Atomically Perfect Zigzag Graphene - Boron Nitride Interfaces

The combination of several materials into heterostructures is a powerful method for controlling material properties. The integration of graphene (G) with hexagonal boron nitride (BN) in particular has been heralded as a way to engineer the graphene band structure and implement spin- and valleytronics in 2D materials. Despite recent efforts, fabrication methods for well-defined G-BN structures on a large scale are still lacking. We report on a new method for producing atomically well-defined G-BN structures on an unprecedented length scale by exploiting the interaction of G and BN edges with a Ni(111) surface as well as each other.

Atomic-Scale Contrast Formation in AFM Images on Molecular Systems

Imaging individual molecules with atomic resolution is now possible using non-contact atomic force microscopy (AFM). In all cases where atomic resolution imaging of molecules was demonstrated, chemically passivated tips were used. This chapter will discuss the factors influencing the atomic scale imaging of molecular systems. We will first discuss the effect of the tip passivation on the atomic scale contrast. Subsequently, we will consider the factors affecting the quantitative details of the apparent atomic positions (background from the neighbouring atoms, flexibility of the tip apex and non-planar samples). Finally, we will discuss how the tip flexibility affects the appearance of the inter- and intramolecular bonds imaged with AFM.