Axel Enders

Molecular adsorption on graphene.

Current studies addressing the engineering of charge carrier concentration and the electronic band gap in epitaxial graphene using molecular adsorbates are reviewed. The focus here is on interactions between the graphene surface and the adsorbed molecules, including small gas molecules (H(2)O, H(2), O(2), CO, NO(2), NO, and NH(3)), aromatic, and non-aromatic molecules (F4-TCNQ, PTCDA, TPA, Na-NH(2), An-CH(3), An-Br, Poly (ethylene imine) (PEI), and diazonium salts), and various biomolecules such as peptides, DNA fragments, and other derivatives. This is followed by a discussion on graphene-based gas sensor concepts. In reviewing the studies of the effects of molecular adsorption on graphene, it is evident that the strong manipulation of graphenes electronic structure, including p- and n-doping, is not only possible with molecular adsorbates, but that this approach appears to be superior compared to these exploiting edge effects, local defects, or strain. However, graphene-based gas sensors, albeit feasible because huge adsorbate-induced variations in the relative conductivity are possible, generally suffer from the lack of chemical selectivity.

Bottom-up solution synthesis of narrow nitrogen-doped graphene nanoribbons.

Large quantities of narrow graphene nanoribbons with edge-incorporated nitrogen atoms can be synthesized via Yamamoto coupling of molecular precursors containing nitrogen atoms followed by cyclodehydrogenation using Scholl reaction.

Surface state engineering of molecule-molecule interactions.

Engineering the electronic structure of organics through interface manipulation, particularly the interface dipole and the barriers to charge carrier injection, is of essential importance to improve organic devices. This requires the meticulous fabrication of desired organic structures by precisely controlling the interactions between molecules. The well-known principles of organic coordination chemistry cannot be applied without proper consideration of extra molecular hybridization, charge transfer and dipole formation at the interfaces. Here we identify the interplay between energy level alignment, charge transfer, surface dipole and charge pillow effect and show how these effects collectively determine the net force between adsorbed porphyrin 2H-TPP on Cu(111). We show that the forces between supported porphyrins can be altered by controlling the amount of charge transferred across the interface accurately through the relative alignment of molecular electronic levels with respect to the Shockley surface state of the metal substrate, and hence govern the self-assembly of the molecules.

A simple technique to measure stress in ultrathin films during growth

We demonstrate an easy implementation of the cantilever bending beam approach to measure stress during film growth in ultrahigh vacuum. Using a simple and compact optical deflection technique, film stress with sub‐monolayer sensitivity can be detected. A stress measurement during FeSi2 formation on Si(111) is presented.

Magnetic surface nanostructures

Recent trends in the emerging field of surface-supported magnetic nanostructures are reviewed. Current strategies for nanostructure synthesis are summarized, followed by a predominantly theoretical description of magnetic phenomena in surface magnetic structures and a review of experimental research in this field. Emphasis is on Fe- or Co-based nanostructures in various low-dimensional geometries, which are studied as model systems to explore the effects of dimensionality, atomic coordination, chemical bonds, alloying and, most importantly, interactions with the supporting substrate on the magnetism. This review also includes a discussion of closely related systems, such as 3d element impurities integrated into organic networks, surface-supported Fe-based molecular magnets, Kondo systems or 4d element nanostructures that exhibit emergent magnetism, thereby bridging the traditional areas of surface science, molecular physics and nanomagnetism.

The correlation between mechanical stress and magnetic properties of ultrathin films

The cantilever bending beam technique was applied to measure film stress, film magnetization and magneto-elastic coupling in nanometre Fe films grown epitaxially on W substrates. A simple optical deflection technique yielded sub-monolayer sensitivity for stress measurements and was used to determine magnetization and magnetostrictive properties of nanometre Fe films in situ. The combination of an electromagnet inside an ultra-high-vacuum chamber with a rotatable external magnet was employed to perform magneto-optical Kerr-effect measurements in the transversal, longitudinal and polar geometry in fields of up to 0.4 T. Examples for stress-driven structural changes in monolayer Fe films are discussed with respect to the unusual high coercivity found for sesquilayer Fe films and the re-orientation of the easy axis of magnetization in Stranski-Krastanov Fe films. The direct correlation between strain and magnetism was exploited to measure the magnetostrictive bending of the film-substrate composite. The magnitude and sign of the magneto-elastic coupling coefficient were found to depend on the film thickness, in contrast to the respective bulk values.

Nitrogen-Doping Induced Self-Assembly of Graphene Nanoribbon-Based Two-Dimensional and Three-Dimensional Metamaterials

Narrow graphene nanoribbons (GNRs) constructed by atomically precise bottom-up synthesis from molecular precursors have attracted significant interest as promising materials for nanoelectronics. But there has been little awareness of the potential of GNRs to serve as nanoscale building blocks of novel materials. Here we show that the substitutional doping with nitrogen atoms can trigger the hierarchical self-assembly of GNRs into ordered metamaterials. We use GNRs doped with eight N atoms per unit cell and their undoped analogues, synthesized using both surface-assisted and solution approaches, to study this self-assembly on a support and in an unrestricted three-dimensional (3D) solution environment. On a surface, N-doping mediates the formation of hydrogen-bonded GNR sheets. In solution, sheets of side-by-side coordinated GNRs can in turn assemble via van der Waals and π-stacking interactions into 3D stacks, a process that ultimately produces macroscopic crystalline structures. The optoelectronic properties of these semiconducting GNR crystals are determined entirely by those of the individual nanoscale constituents, which are tunable by varying their width, edge orientation, termination, and so forth. The atomically precise bottom-up synthesis of bulk quantities of basic nanoribbon units and their subsequent self-assembly into crystalline structures suggests that the rapidly developing toolset of organic and polymer chemistry can be harnessed to realize families of novel carbon-based materials with engineered properties.

Stress, strain and magnetostriction in epitaxial films

The application of the cantilever bending technique to stress measurements at surfaces and in epitaxial films is elucidated. The role of elastic anisotropy in quantitative cantilever curvature analysis is discussed. The stress in Co monolayers is measured during epitaxial growth on Cu(001). The Co-induced stress is found to oscillate with a period of one atomic layer. Simultaneous stress and medium-energy electron diffraction identify maximum stress for filled Co layers. Strain relaxation in Co islands leads to the reduced stress contribution of 2.9 GPa in the partially filled top layer as compared to 3.4 GPa for the filled layers. The cantilever technique is also applied to measure magnetoelastic properties of nanometre thin epitaxial films. Our measurements reveal that the magnetoelastic-coupling coefficients in epitaxial Fe, Co and Ni films differ from the respective bulk values. It is proposed that the epitaxial misfit strain is of key importance for this peculiar magnetostrictive behaviour of ultrathin films.

Structure and morphology of Ni monolayers on W(110)

Scanning tunneling microscopy (STM ) is employed to investigate the structural and morphological properties of Ni monolayers (ML) deposited on W(110) at room temperature. We observe almost perfect layer-by-layer growth for the first three atomic layers. At a coverage of about 6 ML, a transition to three-dimensional growth is found. The analysis of STM images in the submonolayer regime indicates that the coalescence of the islands triggers the formation of a 8◊1 coincidence structure. In contrast to the preferential orientation expected from the twofold symmetric substrate, the islands in the first atomic layer have an irregular shape. Elongated Ni islands are observed in the second layer. The anisotropic island shape is ascribed to the anisotropic diVusion due to anisotropic strain in the underlying first layer. The edges of the Ni islands are oriented along the closed packed 110 directions of the growing Ni(111) film for Ni coverages above 5 ML.

Strain dependence of the magnetic properties of nm Fe films on W(100)

The thickness dependence of the magneto-elastic coupling B1, the intrinsic film stress, and the magnetic in-plane anisotropy K4 of Fe films on W(100) are measured with an in situ combination of a highly sensitive optical deflection technique with magneto-optical Kerr-effect measurements. We find that both B1 and K4 depend strongly on the Fe film thickness. The thickness dependence of B1 can be described by considering a second order magneto-elastic coupling constant D=1 GJ/m3 as a strain dependent correction of B1. We tentatively ascribe the deviation of K4 from its bulk value to the tetragonal lattice distortion caused by an effective tensile in-plane strain of 5.3% in the pseudomorphic region and of 0.2% in thicker films.The thickness dependence of the magneto-elastic coupling B1, the intrinsic film stress, and the magnetic in-plane anisotropy K4 of Fe films on W(100) are measured with an in situ combination of a highly sensitive optical deflection technique with magneto-optical Kerr-effect measurements. We find that both B1 and K4 depend strongly on the Fe film thickness. The thickness dependence of B1 can be described by considering a second order magneto-elastic coupling constant D=1 GJ/m3 as a strain dependent correction of B1. We tentatively ascribe the deviation of K4 from its bulk value to the tetragonal lattice distortion caused by an effective tensile in-plane strain of 5.3% in the pseudomorphic region and of 0.2% in thicker films.