Daniela Künzel

Experimental analysis of charge redistribution due to chemical bonding by high-resolution transmission electron microscopy

The electronic charge density distribution or the electrostatic atomic potential of a solid or molecule contains information not only on the atomic structure, but also on the electronic properties, such as the nature of the chemical bonds or the degree of ionization of atoms. However, the redistribution of charge due to chemical bonding is small compared with the total charge density, and therefore difficult to measure. Here, we demonstrate an experimental analysis of charge redistribution due to chemical bonding by means of high-resolution transmission electron microscopy (HRTEM). We analyse charge transfer on the single-atom level for nitrogen-substitution point defects in graphene, and confirm the ionicity of single-layer hexagonal boron nitride. Our combination of HRTEM experiments and first-principles electronic structure calculations opens a new way to investigate electronic configurations of point defects, other non-periodic arrangements or nanoscale objects that cannot be studied by an electron or X-ray diffraction analysis.

Hierarchically self-assembled host-guest network at the solid-liquid interface for single-molecule manipulation

The manufacture of functional molecular devices is one of the key research topics in nanotechnology. For applications in molecular storage and quantum computing, molecules must be arranged in a repetitive structure and also be addressable and manipulable in a controlled fashion. The self-assembly of molecular building blocks with hydrogen-bonding capabilities is a suitable method for generating highly ordered and porous two-dimensional (2D) hydrogen-bonded networks (HBNs). These porous 2D HBNs can be used to immobilize organic and inorganic guest molecules in a spatially wellordered arrangement, predetermined by the host network structure. The controlled manipulation of guest molecules was demonstrated for various functional guest molecules by means of scanning tunneling microscopy (STM) experiments but has been limited so far to the controlled desorption or the lateral manipulation of single molecules. In contrast to ultrahigh-vacuum (UHV) conditions where the reservoir of manipulable molecules is restricted to the number of adsorbed species, the supernatant liquid phase at the solid–liquid interface in principle offers an almost unlimited depot of molecules (“ink”) and is therefore the perfect experimental environment for tip-controlled adsorption of guest molecules into the HBN. The “ink” attribute of a supernatant solution is used in scanning-probe-based lithographic techniques such as replacement lithography and dip-pen lithography to tailor the chemical composition and structure of a surface on the 100 nm scale. So far, these lithographic techniques are limited to a resolution of about 15 nm. For the spatially controlled adsorption of guest molecules in an HBN, the host–guest system must fulfill the following requirements: 1) the host network must be inert towards the manipulation process; 2) the dynamics of the manipulated components must be slow enough in order to follow the result of the manipulation with STM; and 3) the occupation of the cavities with guest molecules should be low such that unoccupied host cavities are available. All of these requirements need well-balanced adsorbate–adsorbate and substrate–adsorbate interactions. Here we present a host–guest network that meets the demands for a spatially tip-controlled single-molecule manipulation. After describing the outstanding properties of our host–guest system, we demonstrate the spatially tip-controlled desorption of guest molecules from the cavities of the host network. Moreover, we show for the first time the tipcontrolled adsorption of single solvated guest molecules at the solid–liquid interface. The C2v-symmetric HBN building block 3,3’-BTP forms a polymorphic supramolecular HBN on highly ordered pyrolytic graphite (HOPG). The porous 2D network was used to generate a hierarchically self-assembled host–guest architecture with copper(II) phthalocyanine (CuPc) as guest molecule. The occupation of individual HBN cavities with CuPc can be altered with a voltage pulse applied to the tip (“erasing” and “writing”). As we recently

Adsorption of Supramolecular Building Blocks on Graphite: A Force Field and Density Functional Theory Study

Supramolecular building blocks: The adsorption of the oligopyridine isomers 2,4′-BTP and 3,3′-BTP on graphite (see picture) is studied with force field and dispersion-corrected density functional theory (DFT-D) methods. Whereas the used force fields yield different adsorption geometries and strongly varying adsorption energies, the adsorption energy obtained with DFT-D is in rather good agreement with experiment.

Influence of the solvent on the stability of bis(terpyridine) structures on graphite

Summary The effect of solvation on the adsorption of organic molecules on graphite at room temperature has been addressed with force-field molecular dynamics simulations. As a model system, the solvation of a bis(terpyridine) isomer in water and 1,2,4-trichlorobenzene was studied with an explicit solvation model. The inclusion of solvation has a noticeable effect on adsorption energies. Although the results of the various considered force fields differ quite significantly, they all agree that the adsorption of BTP from the TCB solvent is almost thermoneutral. The substrate simply acts as a template to allow a planar arrangement of the network, which is stabilized by the intermolecular interaction. Using an atomic thermodynamics approach, the order of the stability of various network structures as a function of the chemical potential is derived yielding a sequence in agreement with the experiment.

Simulation of bonding effects in HRTEM images of light element materials.

Summary The accuracy of multislice high-resolution transmission electron microscopy (HRTEM) simulation can be improved by calculating the scattering potential using density functional theory (DFT) [1–2]. This approach accounts for the fact that electrons in the specimen are redistributed according to their local chemical environment. This influences the scattering process and alters the absolute and relative contrast in the final image. For light element materials with well defined geometry, such as graphene and hexagonal boron nitride monolayers, the DFT based simulation scheme turned out to be necessary to prevent misinterpretation of weak signals, such as the identification of nitrogen substitutions in a graphene network. Furthermore, this implies that the HRTEM image does not only contain structural information (atom positions and atomic numbers). Instead, information on the electron charge distribution can be gained in addition. In order to produce meaningful results, the new input parameters need to be chosen carefully. Here we present details of the simulation process and discuss the influence of the main parameters on the final result. Furthermore we apply the simulation scheme to three model systems: A single atom boron and a single atom oxygen substitution in graphene and an oxygen adatom on graphene.

Bonding Effects in Nitrogen Doped Graphene and Hexagonal Boron Nitride

We report on the effects of chemical bonding on high-resolution transmission electron microscopy images of covalently bonded light elements (B, C, N). We follow the procedure of Deng and Marks [1] (with some modifications) to calculate the accurate electrostatic potential of the bonded configuration using the WIEN2k DFT code [2]. The conventional TEM simulation, based on the independent atom model (IAM), is shown for comparison. We have studied single-layer hexagonal boron nitride and nitrogen-doped graphene membranes by calculation and experiment. Nitrogen doped graphene membranes are prepared by CVD methods [3] with the addition of ammonia into the reaction [4]. TEM imaging is carried out using an image-side Cs-corrected Titan 80-300, operated at 80kV. The spherical aberration is 20 m and a defocus of f1 = −90 A (Scherzer defocus) and f2 = −180 A is used.Image calculations for nitrogen doped graphene are based on a DFT relaxed atomic configuration. This configuration was obtained with the VASP DFT code [5], and agrees with previous works [6, 7] in that the bond length changes after replacing C by N are less than 2pm. According to conventional (IAM) TEM simulation, a nitrogen substitution in graphene would be invisible (a contrast of less than 0.1%) for all these relaxed configurations. Image simulations based on the electrostatic potentials from the DFT calculation, however, predict a detectable signal. Our charged defect produces predominantly low -er spatial frequency information in the projected potentials. This is mostly cut off by the CTF in the Scherzer-focus image of the Cs-corrected microscope (f1 , top row in Fig. 1), but visible at larger defo -cus. A further analysis of the projected potentials and electron distribution shows that the chemical shift here is not localized on the nitrogen substitution, but is actually spread onto the neighbouring car -bons. We note that a physical phase plate (not present in the current experiment) would be highly useful to study this non-local bonding effect to include all frequencies in one image. The third row in Fig. 1 shows an example experimental image from nitrogen doped single layer graphene membranes recorded at f2 (a drift-compensated average of 34 individual exposures). Two defects in excellent agreement with expectations (a dark contrast of ca. 0.5%) are visible in this image. In in-focus (f1) images of the same noise level of the same defects, the substitutions could not be detected above the noise (in agreement with the calculation). We also briefly consider the IAM and DFT calculation results for single-layer hexagonal boron nitride (h BN). Fig. 2 shows the projected electrostatic potentials for IAM and DFT, withdifferent resolutionlimiting apertures. The IAM simulation predicts a small but significant contrast difference for the two