Velimir Meded

Organometallic Benzene-Vanadium Wire: A One-Dimensional Half-Metallic Ferromagnet

Using density functional theory we perform theoretical investigations of the electronic properties of a freestanding one-dimensional organometallic vanadium-benzene wire. This system represents the limiting case of multidecker Vn(C6H6)(n+1) clusters which can be synthesized with established methods. We predict that the ground state of the wire is a 100% spin-polarized ferromagnet (half-metal). Its density of states is metallic at the Fermi energy for the minority electrons and shows a semiconductor gap for the majority electrons. We find that the half-metallic behavior is conserved up to 12% longitudinal elongation of the wire. Ab initio electron transport calculations reveal that finite size vanadium-benzene clusters coupled to ferromagnetic Ni or Co electrodes will work as nearly perfect spin filters.

Expanding the Coordination Cage: A Ruthenium(II)−Polypyridine Complex Exhibiting High Quantum Yields under Ambient Conditions

A mononuclear ruthenium(II) polypyridyl complex with an enlarged terpyridyl coordination cage was synthesized by the formal introduction of a carbon bridge between the coordinating pyridine rings. Structurally, the ruthenium(II) complex shows an almost perfect octahedral N6 coordination around the central Ru(II) metal ion. The investigation of the photophysical properties reveals a triplet metal-to-ligand charge transfer emission with an unprecedented quantum yield of 13% and a lifetime of 1.36 mus at room temperature and in the presence of air oxygen. An exceptional small energy gap between light absorption and light emission, or Stokes shift, was detected. Additionally, time-dependent density functional theory calculations were carried out in order to characterize the ground state and both the singlet and triplet excited states. The exceptional properties of the new compound open the perspective of exploiting terpyridyl-like ruthenium complexes in photochemical devices under ambient conditions.

[Au14(PPh3)8(NO3)4]: An Example of a New Class of Au(NO3)-Ligated Superatom Complexes†

Ultrasmall gold nanoparticles (usNPs) have been of considerable interest because of their unique properties, which arise from size quantization effects and thus make them attractive for applications in different areas of nanoscience and nanotechnology. 4] Among usNPs of defined composition, and especially for gold clusters, an outstanding so-called “magic” stability has been observed for certain compositions. This stability has been explained through their specific geometric or electronic structure. In the full-shell cluster model, an usNP is considered as a cut-out of the bulk fcc structure. 6] Accordingly, a central metal atom is surrounded by shells of closely packed metal atoms, each shell having (10n+2) atoms, n indicating the number of shells. Clusters possessing fully occupied shells consist of 13, 55, 147, ... atoms and are supposed to show high stability for geometric reasons, such as the well-studied Schmid cluster [Au55(PPh3)12Cl6]. [7] In another model, which goes back to the counting rules introduced by Mingos, metal clusters are considered as superatom complexes formulated as [LS·ANXM] z with electron-withdrawing ligands X or weak Lewis base ligands L attached to the core with metal atoms A and an overall core charge z. 10] Accordingly, for Au the 6s electrons are counted and corrected by the number of electrons that are located at electron-withdrawing ligands and corrected by the charge of the cluster. The remaining 6s electrons are placed within the binding delocalized orbitals of the cluster, which is analogous to valence electrons of the atomic theory. Exceptional stability is associated with a total number of: n* 1⁄4 2, 6, 12, 20, 30, . . . ð1Þ

Spin transition in arrays of gold nanoparticles and spin crossover molecules.

We investigate if the functionality of spin crossover molecules is preserved when they are assembled into an interfacial device structure. Specifically, we prepare and investigate gold nanoparticle arrays, into which room-temperature spin crossover molecules are introduced, more precisely, [Fe(AcS-BPP)2](ClO4)2, where AcS-BPP = (S)-(4-{[2,6-(dipyrazol-1-yl)pyrid-4-yl]ethynyl}phenyl)ethanethioate (in short, Fe(S-BPP)2). We combine three complementary experiments to characterize the molecule-nanoparticle structure in detail. Temperature-dependent Raman measurements provide direct evidence for a (partial) spin transition in the Fe(S-BPP)2-based arrays. This transition is qualitatively confirmed by magnetization measurements. Finally, charge transport measurements on the Fe(S-BPP)2-gold nanoparticle devices reveal a minimum in device resistance versus temperature, R(T), curves around 260-290 K. This is in contrast to similar networks containing passive molecules only that show monotonically decreasing R(T) characteristics. Backed by density functional theory calculations on single molecular conductance values for both spin states, we propose to relate the resistance minimum in R(T) to a spin transition under the hypothesis that (1) the molecular resistance of the high spin state is larger than that of the low spin state and (2) transport in the array is governed by a percolation model.

Catalytic subsurface etching of nanoscale channels in graphite

Catalytic hydrogenation of graphite has recently attracted renewed attention as a route for nanopatterning of graphene and to produce graphene nanoribbons. These reports show that metallic nanoparticles etch the surface layers of graphite or graphene anisotropically along the crystallographic zig-zag ‹11-20› or armchair ‹10-10› directions. The etching direction can be influenced by external magnetic fields or the supporting substrate. Here we report the subsurface etching of highly oriented pyrolytic graphite by Ni nanoparticles, to form a network of tunnels, as seen by scanning electron microscopy and scanning tunnelling microscopy. In this new nanoporous form of graphite, the top layers bend inward on top of the tunnels, whereas their local density of states remains fundamentally unchanged. Engineered nanoporous tunnel networks in graphite allow for further chemical modification and may find applications in various fields and in fundamental science research.

Selective Coordination Bonding in Metallo‐Supramolecular Systems on Surfaces

The confinement of molecular species in nanoscale environments strongly modifies the interaction pathways compared to homogenous, three-dimensional (bulk) conditions. A new field of chemistry featuring weak interactions, coordination bonding, and covalent chemistry at solid surfaces has recently emerged. In particular, the combination of surface-confined chemistry and scanning probe techniques with subnanometer resolution allows immediate insights into molecular self-organization processes on the nanometer level. Extended monolayers of open, two-dimensional (2D) coordination networks with high organizational periodicity, controlled symmetries, and modular dimensionality have been achieved by using designed, self-instructedmolecular building blocks. The deposition of mixtures of precursor molecules has led to more sophisticated architectures, mainly built on weak intermolecular interactions or weak interactions in combination with coordination bonding, that is, hierarchical motifs. The cooperative assembly of instructed mixtures of molecular bricks enables a high degree of structural control and functionality, for example, the stability and ordering of primary structures can be increased, or the dimensionality and geometry of supramolecular structures can be steered. Observations of molecular-level self-recognition and error correction have demonstrated collective dynamics in surface-confined supramolecular systems. A grand challenge in materials chemistry is the capability to design adaptive materials, that is, to develop systemic methods for tailored structure and function. To exploit the opportunities of systemic chemistry, a detailed understanding of the selectivity in the interaction mechanisms of molecular mixtures, if possible by direct studies at the single-molecule level, is of pivotal interest. Herein, we report on the observation of supramolecular selectivity in the simultaneous coordinative interaction of two different molecular ligands, aromatic bipyrimidines and dicarboxylic acids, with Cu and Fe atoms resulting in a selfsegregation into two distinct, surface-confined coordination network domains. The random mixture of ligands and metals separates into subdomains of pure bipyrimidine–Cu and carboxylate–Fe networks, while heteroleptic ligand combinations, though feasible, are not observed. Each 2D coordination network exhibits a tetragonal geometry with metal atom coordination nodes, but expresses unique molecular composition and spatial organization. The molecular components PBP (5,5’-bis(4-pyridyl)(2,2’bipyrimidine)) and BDA (1,4’-biphenyl-dicarboxylic acid, see Scheme 1) are co-evaporated in a 1:1 number ratio onto a Cu(100) substrate at room temperature under ultra-high vacuum (UHV) conditions. At this temperature, a diffusing copper adatom gas is present at the Cu(100) surface, which has been shown to be available for the formation of extended

Selective Dispersion of Large‐Diameter Semiconducting Single‐Walled Carbon Nanotubes with Pyridine‐Containing Copolymers

The purity of single-walled carbon nanotubes (SWNTs) is a key parameter for their integration in electronic, optoelectronic and photonic devices. Samples of pristine SWNTs are inhomogeneous in terms of electric behavior and diameter and contain a variety of amorphous carbon and catalyst residues. To obtain high performance devices, purification of SWNTs is required. Conjugated polymers have emerged as efficient solubilizing and sorting agents for small diameter SWNTs (HiPco tubes, 0.7 nm1.1 nm are lacking. Several pyridine-containing copolymers were synthesized for this purpose and showed efficient and selective extraction of semiconducting large diameter SWNTs (PLV tubes, Ø>1.1 nm). High concentration and high purity suspensions are obtained without the use of ultracentrifugation, which gives an up-scaling potential of the method. The emission wavelength is in near infrared region around 1550 nm and fits with broadly used telecommunication wavelength window. The processes taking place at the interface were simulated by a newly designed hybrid coarse-grain model combining density functional theory and geometrical calculation to yield insights into the wrapping processes with an unprecedented level of details for such large diameter SWNTs.

Spin-Crossover and Massive Anisotropy Switching of 5d Transition Metal Atoms on Graphene Nanoflakes

In spin crossover phenomena, the magnetic moment of a molecule is switched by external means. Here we theoretically predict that several 5d-transition metals (TMs) adsorbed on finite graphene flakes undergo a spin crossover, resulting from multiple adsorption minima, that are absent in the zero-dimensional limit of benzene and the two-dimensional limit of graphene. The different spin states are stable at finite temperature and can be reversibly switched with an electric field. The system undergoes a change in magnetic anisotropy upon spin crossover, which facilitates read-out of the spin state. The TM-decorated nanoflakes thus act as fully controlled single-ion magnetic switches.

QM/QM Approach to Model Energy Disorder in Amorphous Organic Semiconductors

It is an outstanding challenge to model the electronic properties of organic amorphous materials utilized in organic electronics. Computation of the charge carrier mobility is a challenging problem as it requires integration of morphological and electronic degrees of freedom in a coherent methodology and depends strongly on the distribution of polaron energies in the system. Here we represent a QM/QM model to compute the polaron energies combining density functional methods for molecules in the vicinity of the polaron with computationally efficient density functional based tight binding methods in the rest of the environment. For seven widely used amorphous organic semiconductor materials, we show that the calculations are accelerated up to 1 order of magnitude without any loss in accuracy. Considering that the quantum chemical step is the efficiency bottleneck of a workflow to model the carrier mobility, these results are an important step toward accurate and efficient disordered organic semiconductors simulations, a prerequisite for accelerated materials screening and consequent component optimization in the organic electronics industry.

Superexchange Charge Transport in Loaded Metal Organic Frameworks

In the past, nanoporous metal-organic frameworks (MOFs) have been mostly studied for their huge potential with regard to gas storage and separation. More recently, the discovery that the electrical conductivity of a widely studied, highly insulating MOF, HKUST-1, improves dramatically when loaded with guest molecules has triggered a huge interest in the charge carrier transport properties of MOFs. The observed high conductivity, however, is difficult to reconcile with conventional transport mechanisms: neither simple hopping nor band transport models are consistent with the available experimental data. Here, we combine theoretical results and new experimental data to demonstrate that the observed conductivity can be explained by an extended hopping transport model including virtual hops through localized MOF states or molecular superexchange. Predictions of this model agree well with precise conductivity measurements, where experimental artifacts and the influence of defects are largely avoided by using well-defined samples and the Hg-drop junction approach.