Kasper Nørgaard

Fabrication of 2D Gold Nanowires by Self-Assembly of Gold Nanoparticles on Water Surfaces in the Presence of Surfactants

Self-assembly of metallic nanostructures using 2D templates consisting of surfactant systems at the air/water interface is demonstrated (see Figure). The morphology of the resulting nanostructures can be partially controlled by adjusting parameters such as the molecular structure of the surfactant, the chemical properties of the assembly units, and the surface pressure.

Solution‐Processed Ultrathin Chemically Derived Graphene Films as Soft Top Contacts for Solid‐State Molecular Electronic Junctions

A novel method using solution-processed ultrathin chemically derived graphene films as soft top contacts for the non-destructive fabrication of molecular junctions is demonstrated. We believe this protocol will greatly enrich the solid-state test beds for molecular electronics due to its low-cost, easy-processing and flexible nature.

Adaptive chemistry of bifunctional gold nanoparticles at the air/water interface. A synchrotron X-ray study of giant amphiphiles

A series of ligand stabilized gold nanoparticles with diameters close to 3 nm were studied as Langmuir monolayers at the air/water interface by synchrotron X-ray diffraction and reflectivity. Alkylthiols with different length and/or terminal functional group (hydrophilic or hydrophobic) were introduced into the ligand shell by ligand place exchange reactions. Synchrotron grazing incidence X-ray diffraction (GIXD) and specular X-ray reflectivity reveal the well known hexagonally packed monolayers. In addition the mixed hydrophilic/hydrophobic ligand shell nanoparticles show a high degree of environmental responsiveness, as they adapt to an amphiphilic distribution of ligands around the gold core when spread at the water surface. Likewise nanoparticles of mixed long and short alkyl chains respond to lateral pressure by adopting a structure where the short alkyl chains determine the in-plane nearest neighbor distance while the long alkyl chains determine the film thickness. Based on X-ray reflectivity measurements, which quantitatively account for the electron density in the monolayers, combined with GIXD we calculate the average size and number of atoms of the individual gold particle cores, and estimate the number of passivating ligand on the particle surface. Typical values are dAU-core = 16 A, dnanoparticle = 26-37 A depending on the ligands, Mw = 30-40000 g mol(-1) number of ligands = 40-60. The thickness of the monolayers was determined by AFM after transfer of the monolayers to a solid support using the Langmuir Schaefer technique. The combination of the different techniques produce a very consistent picture of the structure and adaptive chemical nature of the nanoparticles studied, and reveal a surprisingly monodisperse particle distribution centered around 140 atoms in the gold core.

Ultrathin Reduced Graphene Oxide Films as Transparent Top‐Contacts for Light Switchable Solid‐State Molecular Junctions

A new type of solid-state molecular junction is introduced, which employs reduced graphene oxide as a transparent top contact that permits a self-assembled molecular monolayer to be photoswitched in situ, while simultaneously enabling charge-transport measurements across the molecules. The electrical switching behavior of a less-studied molecular switch, dihydroazulene/vinylheptafulvene, is described, which is used as a test case.

A clamp-like biohybrid catalyst for DNA oxidation

In processive catalysis, a catalyst binds to a substrate and remains bound as it performs several consecutive reactions, as exemplified by DNA polymerases. Processivity is essential in nature and is often mediated by a clamp-like structure that physically tethers the catalyst to its (polymeric) template. In the case of the bacteriophage T4 replisome, a dedicated clamp protein acts as a processivity mediator by encircling DNA and subsequently recruiting its polymerase. Here we use this DNA-binding protein to construct a biohybrid catalyst. Conjugation of the clamp protein to a chemical catalyst with sequence-specific oxidation behaviour formed a catalytic clamp that can be loaded onto a DNA plasmid. The catalytic activity of the biohybrid catalyst was visualized using a procedure based on an atomic force microscopy method that detects and spatially locates oxidized sites in DNA. Varying the experimental conditions enabled switching between processive and distributive catalysis and influencing the sliding direction of this rotaxane-like catalyst.

Supramolecular chemistry on water – towards self-assembling molecular electronic circuitry

This feature article describes the self-assembly of electroactive molecules at the air/water interface, emphasizing the structural and electronic characterizations of the resulting supramolecular architectures.

Graphene Oxide: A One- versus Two-Component Material

The structure of graphene oxide (GO) is a matter of discussion. While established GO models are based on functional groups attached to the carbon framework, another frequently used model claims that GO consists of two components, a slightly oxidized graphene core and highly oxidized molecular species, oxidative debris (OD), adsorbed on it. Those adsorbents are claimed to be the origin for optical properties of GO. Here, we examine this model by preparing GO with a low degree of functionalization, combining it with OD and studying the optical properties of both components and their combination in an artificial two-component system. The analyses of absorption and emission spectra as well as lifetime measurements reveal that properties of the combined system are distinctly different from those of GO. That confirms structural models of GO as a separate oxygenated hexagonal carbon framework with optical properties governed by its internal structure rather than the presence of OD. Understanding the structure of GO allows further reliable interpretation of its optical and electronic properties and enables controlled processing of GO.

Molecular Junctions Based on SAMs of Cruciform Oligo(phenylene ethynylene)s

Cruciform oligo(phenylene ethynylene)s (OPEs) with an extended tetrathiafulvalene (TTF) donor moiety (OPE5-TTF and OPE3-TTF) and their simple analogues (OPE5-S and OPE3) without conjugated substituents were used to form high-quality self-assembled monolayers (SAMs) on ultraflat gold substrates. Molecular junctions based on these SAMs were investigated using conducting-probe atomic force microscopy (CP-AFM). The TTF substituent changes the molecular orbital energy levels and decreases the HOMO-LUMO energy gap, resulting in a 9-fold increase in conductance for both TTF cruciform OPEs compared to the unsubstituted analogues. The difference in electrical transport properties of the SAMs was reproduced by the theoretical transport calculations for the single molecules.

A Comprehensive Study of Extended Tetrathiafulvalene Cruciform Molecules for Molecular Electronics: Synthesis and Electrical Transport Measurements

Cruciform-like molecules with two orthogonally placed π-conjugated systems have in recent years attracted significant interest for their potential use as molecular wires in molecular electronics. Here we present synthetic protocols for a large selection of cruciform molecules based on oligo(phenyleneethynylene) (OPE) and tetrathiafulvalene (TTF) scaffolds, end-capped with acetyl-protected thiolates as electrode anchoring groups. The molecules were subjected to a comprehensive study of their conducting properties as well as their photophysical and electrochemical properties in solution. The complex nature of the molecules and their possible binding in different configurations in junctions called for different techniques of conductance measurements: (1) conducting-probe atomic force microscopy (CP-AFM) measurements on self-assembled monolayers (SAMs), (2) mechanically controlled break-junction (MCBJ) measurements, and (3) scanning tunneling microscopy break-junction (STM-BJ) measurements. The CP-AFM measurements showed structure-property relationships from SAMs of series of OPE3 and OPE5 cruciform molecules; the conductance of the SAM increased with the number of dithiafulvene (DTF) units (0, 1, 2) along the wire, and it increased when substituting two arylethynyl end groups of the OPE3 backbone with two DTF units. The MCBJ and STM-BJ studies on single molecules both showed that DTFs decreased the junction formation probability, but, in contrast, no significant influence on the single-molecule conductance was observed. We suggest that the origins of the difference between SAM and single-molecule measurements lie in the nature of the molecule-electrode interface as well as in effects arising from molecular packing in the SAMs. This comprehensive study shows that for complex molecules care should be taken when directly comparing single-molecule measurements and measurements of SAMs and solid-state devices thereof.

Anisotropic growth of gold nanoparticles using cationic gemini surfactants: effects of structure variations in head and tail groups

A library of gemini surfactants is employed to study surfactant directed anisotropic growth of gold nanoparticles. The surfactants are modified with respect to the length and type of the tails, as well as of the spacer group. By analyzing the structure of the anisotropic nanoparticles, it is possible to extract information on how the structure of the surfactants influences the anisotropic gold nanocrystal growth. We find that the tail length of the surfactants has a greater influence on the resulting nanoparticle aspect ratio compared to the chemical nature of the spacer group. While clear trends between the aspect ratio and the tail as well as spacer length remain elusive, we observe that surfactants with a critical micelle concentration of ∼1 mM produce particles with the highest aspect ratio. A crystallographic analysis of nanorods obtained using gemini surfactants reveals that they grow along 〈100〉 and are bound by {310} facets. This observation, which is specific for gemini surfactants, is explained by taking into account the preferential alignment of gemini surfactants with surface steps as suggested by electronic structure calculations.