Robert Drost

Ultra-narrow metallic armchair graphene nanoribbons

Graphene nanoribbons (GNRs)—narrow stripes of graphene—have emerged as promising building blocks for nanoelectronic devices. Recent advances in bottom-up synthesis have allowed production of atomically well-defined armchair GNRs with different widths and doping. While all experimentally studied GNRs have exhibited wide bandgaps, theory predicts that every third armchair GNR (widths of N=3m+2, where m is an integer) should be nearly metallic with a very small bandgap. Here, we synthesize the narrowest possible GNR belonging to this family (five carbon atoms wide, N=5). We study the evolution of the electronic bandgap and orbital structure of GNR segments as a function of their length using low-temperature scanning tunnelling microscopy and density-functional theory calculations. Already GNRs with lengths of 5 nm reach almost metallic behaviour with ∼100 meV bandgap. Finally, we show that defects (kinks) in the GNRs do not strongly modify their electronic structure.

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.

Site-Dependent Coordination Bonding in Self-Assembled Metal-Organic Networks.

The combination of organic linkers with metal atoms on top of inorganic substrates offers promising perspectives for functional electronic and magnetic nanoscale devices. Typically, coordination bonds between electron-rich end groups and transition-metal atoms lead to the self-assembly of metal-organic nanostructures, whose shape and electronic and magnetic properties crucially depend on the type of ligand. Here, we report on the site-selective bonding properties of Co atoms to the dichotomic dicyanoazobenzene molecule with its carbonitrile and diazo N-based moieties as possible ligands. Using low-temperature scanning tunneling microscopy (STM) and spectroscopy measurements, we resolve the formation of self-assembled metal-organic motifs. Cobalt atoms exhibit a clear spectroscopic fingerprint dependent on the different coordination site, which is further used to map their position, otherwise not clearly visible in the topographic STM images. Density functional theory corroborates the observed bonding patterns and evidences their coordinative nature.

Topological states in engineered atomic lattices

Individual vacancies in a chlorine monolayer on copper can be manipulated with scanning tunnelling microscopy to engineer artificial lattices that have topologically nontrivial electronic states.

Enhanced flux pinning in Bi-2212 single crystals by planar defects introduced via Ti-substitution

Abstract Ti-doped Bi-2212 single crystals were grown by the travelling solvent floating zone technique. Concentrations of about 1 at% and 2 at% (with respect to Cu) were shown to be incorporated in the structure which leads to lowering of T c to about 73 K from 85 K. High resolution microscopy revealed high densities of planar defects parallel to the a , c -planes. The flux pinning properties at elevated temperatures are significantly improved with respect to undoped single crystals.

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.

Superconductivity in the BaPb1−xBix/2Sbx/2O3 system

Abstract The system BaPb 1− x Bi x /2 Sb x /2 O 3 has been synthesized and investigated. The parent compound, BaBi 0.5 Sb 0.5 O 3 , is a rhombohedral perovskite, having an ordered arrangement of Bi 3+ and Sb 5+ ions. Upon Pb-doping the ordered arrangement vanishes at x ≈0.60, but the system remains insulating. Superconductivity occurs in the composition range of about 0.2≤ x T c (magnetically determined) ranging from 2 K to 6 K. The most important feature of this system is that the Bi exists only as Bi 3+ . Therefore, the occurrence of superconductivity may shed the light on whether the pairing mechanism in the bismuthate superconductors can be described in terms of local pairs.

Microstructural changes of LFZ Bi-2212 thin rods due to Ti addition

The effects of 2 at.% Ti addition on the microstructure of laser floating zone textured Bi-2212 thin rods have been analyzed. It has been found that Ti induces a great number of nucleation centers in the molten zone and, in consequence, it reduces the grain size one order of magnitude. In addition, using the same growth conditions, the texture of the sample is strongly degraded and the superconducting properties at 77 K deteriorate.

Magnetic relaxation in the “Bragg-glass” phase in BSCCO

Magnetic relaxation in the Bragg-glass phase of overdoped Bi 2 Sr 2 CaCu 2 O 8 crystals was investigated using time-resolved magneto-optical visualisation of the flux distribution. This has permitted us to extract the current-voltage characteristic, which can be well described by a power-law, although fits to a stretched exponential E ∼ exp(-j c /j) μ with 0.3 < p < 0.8 are possible at long times in excess of 100 s.