C60 on SiC Nanomesh

Multiscale modeling of structure formation of C60 on insulating CaF2 substrates

Morphologies of adsorbed molecular films are of interest in a wide range of applications. To study the epitaxial growth of these systems in computer simulations requires access to long time and length scales, and one typically resorts to kinetic Monte Carlo (KMC) simulations. However, KMC simulations require as input transition rates and their dependence on external parameters (such as temperature). Experimental data allow only limited and indirect access to these rates, and models are often oversimplified. Here, we follow a bottom-up approach and aim at systematically constructing all relevant rates for an example system that has shown interesting properties in experiments, buckminsterfullerene on a calcium fluoride substrate. We develop classical force fields (both atomistic and coarse-grained) and perform molecular dynamics simulations of the elementary transitions in order to derive explicit expressions for the transition rates with a minimal number of free parameters.

Imaging prototypical aromatic molecules on insulating surfaces: a review

Insulating substrates allow for in-plane contacted molecular electronics devices where the molecule is in contact with the insulator. For the development of such devices it is important to understand the interaction of molecules with insulating surfaces. As substrates, ionic crystals such as KBr, KCl, NaCl and CaF₂ are discussed. The surface energies of these substrates are small and as a consequence intrinsic properties of the molecules, such as molecule-molecule interaction, become more important relative to interactions with the substrates. As prototypical molecules, three variants of graphene-related molecules are used, pentacene, [Formula: see text] and PTCDA. Pentacene is a good candidate for molecular electronics applications due to its high charge carrier mobility. It shows mainly an upright standing growth mode and the morphology of the islands is strongly influenced by dewetting. A new second flat-lying phase of the molecule has been observed. Studying the local work function using the Kelvin method reveals details such as line defects in the center of islands. The local work function differences between the upright-standing and flat-lying phase can only be explained by charge transfer that is unusual on ionic crystalline surfaces. [Formula: see text] nucleation and growth is explained by loosely bound molecules at kink sites as nucleation sites. The stability of [Formula: see text] islands as a function of magic numbers is investigated. Peculiar island shapes are obtained from unusual dewetting processes already at work during growth, where molecules 'climb' to the second molecular layer. PTCDA is a prototypical semiconducting molecule with strong quadrupole moment. It grows in the form of elongated islands where the top and the facets can be molecularly resolved. In this way the precise molecular arrangement in the islands is revealed.

Self-templating 2D supramolecular networks: a new avenue to reach control over a bilayer formation

One of the greatest challenges in 2D self-assembly at interfaces is the ability to grow spatially controlled supramolecular motifs in the third dimension, exploiting the surface as a template. In this manuscript a concentration-dependent study by scanning tunneling microscopy at the solid-liquid interface, corroborated by Molecular Dynamics (MD) simulations, reveals the controlled generation of mono- or bilayer self-assembled Kagomé networks based on a fully planar tetracarboxylic acid derivative. By programming the backbone of the molecular building blocks, we present a strategy to gain spatial control over the adlayer structure by conferring self-templating capacity to the 2D self-assembled network.

Molecular-scale investigation of C60/p-sexiphenyl organic heterojunction interface

In situ low-temperature scanning tunneling microscopy (LT-STM) and ultraviolet photoelectron spectroscopy (UPS) experiments have been carried out to investigate the interface properties at the C(60)∕p-sexiphenyl (6P) organic-organic heterojunction interface, including the interfacial energy level alignment and the supramolecular packing structures. As revealed by UPS measurements, the vacuum level is almost aligned at the C(60)∕6P interface, suggesting that the interface is dominated by weak intermolecular interactions, such as van der Waals and π-π interactions. In situ LT-STM experiments also indicate the formation of a molecularly sharp C(60)∕6P interface with hexagonally-close-packed C(60) layers nucleated atop 6P layer on graphite.

Self-assembly of enantiopure domains: the case of indigo on Cu(111)

The adsorption of indigo molecules on Cu(111) was investigated by low temperature (5 K) scanning tunneling microscopy from the isolated single molecule regime to one monolayer. Structural optimization and image calculations demonstrate that the molecules are in a physisorbed state. Because of the reduced symmetry at the surface, single molecules acquire a chiral character upon adsorption leading to a two-dimensional (2D) chirality. They adopt two adsorption configurations, related by a mirror symmetry of the substrate, each with a distinct molecular orientation. Consequently, the 2D chirality is expressed by the orientation of the molecule. For higher coverage, molecules self-assemble by hydrogen bonding in nearly homochiral molecular chains, whose orientation is determined by the orientation taken by the isolated molecules. When the coverage approaches one monolayer, these chains pack into domains. Finally, the completion of the monolayer induces the expulsion of the molecules of the wrong chirality that are still in these domains, leading to perfect resolution in enantiopure domains.

Modern Nanotemplates Based on Graphene and Single Layer h-BN

This mini-review focuses on the recently discovered nanomeshes and nanotemplates made from carbon, boron nitride and their mixtures with a thickness of just a single atomic layer. Typically they exist on some transition metal or semiconductor substrate, the interaction with it playing a crucial role in nanopattern formation. We review systems such as graphene/SiC(0001), graphene/Ru(0001), h-BN/Ru(0001), and h-BN/Rh(111), their atomistic models, synthesis routes, as well as possible applications as templates for nanoperiodic arrays of clusters and molecules. Scanning tunneling microscopy (STM), a technique with ultimate resolution in real space, is stressed as an indispensable tool for a comprehensive characterization of the given systems.

Origin of anomalous electronic structures of epitaxial graphene on silicon carbide

On the basis of first-principles calculations, we report that a novel interfacial atomic structure occurs between graphene and the surface of silicon carbide, destroying the Dirac point of graphene and opening a substantial energy gap there. In the calculated atomic structures, a quasiperiodic 6x6 domain pattern emerges out of a larger commensurate 6 sqrt [3] x 6 sqrt [3]R30 degrees periodic interfacial reconstruction, resolving a long standing experimental controversy on the periodicity of the interfacial superstructures. Our theoretical energy spectrum shows a gap and midgap states at the Dirac point of graphene, which are in excellent agreement with the recently observed anomalous angle-resolved photoemission spectra. Beyond solving unexplained issues in epitaxial graphene, our atomistic study may provide a way to engineer the energy gaps of graphene on substrates.

Orientationally ordered C60 on p-sexiphenyl nanostripes on Ag111

Long range orientational ordering within C(60) monolayers is observed on p-sexithiophene (6P) monolayer nanostripes on Ag(111) at 77 K. Low-temperature scanning tunneling microscopy studies reveal that the C(60)-6P intermolecular interaction constrains all C(60) molecules to adsorb on their hexagons atop 6P molecules. The orientation-dependent bond-to-bond Coulomb interaction between charge deficient single bonds and double bonds with excess charge in neighboring C(60) molecules results in the in-plane orientational ordering and contributes to the lowering of the total energy of the orientationally ordered C(60) islands.

Surface transfer p-type doping of epitaxial graphene

Epitaxial graphene thermally grown on 6H-SiC(0001) can be p-type doped via a novel surface transfer doping scheme by modifying the surface with the electron acceptor, tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Synchrotron-based high-resolution photoemission spectroscopy reveals that electron transfer from graphene to adsorbed F4-TCNQ is responsible for the p-type doping of graphene. This novel surface transfer doping scheme by surface modification with appropriate molecular acceptors represents a simple and effective method to nondestructively dope epitaxial graphene for future nanoelectronics applications.