We predict a new class of 2-D crystalline "bulk" magnets-the graphene nanohole (GNH) superlattices with each GNH acting like a "super" magnetic atom, using first principles calculations. We show that such superlattices can exhibit long-range magnetic order above room temperature, with a collective magnetic behavior governed by inter-NH spin spin interactions in additional to intra-NH spin ordering. Furthermore, magnetic semiconductors can be made by doping magnetic NHs into semiconducting NH superlattices. The possibility of engineering magnetic GNHs for storage media and spintronics applications is discussed. KEYWORDSGraphene, magnetism, spin, magnetic semiconductor, superlattice Nanostructured magnetic materials have a wide range of applications, such as being used for storage media. Magnetism is commonly associated with elements containing localized d or f electrons, i.e., the itinerant ferromagnetism [1,2]. In contrast, the elements containing diffuse sp electrons are intrinsically nonmagnetic, but magnetism can be induced in sp-element materials extrinsically by defects and impurities. There have been continuing efforts in searching for new magnetic nanomaterials, and much recent interest has been devoted to magnetism of carbon-based [3 7] , especially graphene-based structures [8 18] such as graphene nanoribbons [8,9,11,18] and nanoflakes [16,17]. Graphene nanoribbons [8,9,11,18] and nanoflakes [16,17] with zigzag edges have been shown to exhibit magnetism, originated from the localized edge states that give rise to a high density of states at the Fermi level rendering a spin-polarization instability [1]. However, these structures exhibit only local magnetization, lacking the long-range magnetic order with a well-defined transition temperature. Also, it is practically diffi cult to control the magnetic properties of individual nanoribbons and nanofl akes and integrate them into devices.Here we predict a new class of graphene-based magnetic nanostructures, the superlattices of graphene nanoholes (GNHs), which exhibit long-range magnetic order and collective "bulk" magnetism. This allows us to go beyond the current scope limited to the spins within a single nanoribbon or nanoflake. In fact, the superlattices consisting of a periodic array of NH spins form a unique family of magnetic 2-D crystals with the NH acting like a "super" magnetic atom. Their collective magnetic behavior depends on not only the local intra-NH spin property but also the long-range inter-NH spin spin interactions. The type of magnetic order, i.e., ferromagnetic (FM) vs antiferromagnetic (AF), can be controlled by using different NH shapes and superlattice symmetries, and the ordering Nano Research 57 Nano Res (2008) 1: 56 62 temperature can be tuned by NH size and density well above room temperature. It is also possible to combine magnetic NHs with nonmagnetic semiconducting NHs to form magnetic semiconductors. Our fi ndings represent a unique organic material exhibiting collective "bulk" magnetism, with significant implications in study...

Based on the underlying graphene lattice symmetry and an itinerant magnetism model on a bipartite lattice, we propose a unifi ed geometric rule for designing graphene-based magnetic nanostructures: spins are parallel (ferromagnetic (FM)) on all zigzag edges which are at angles of 0° and 120° to each other, and antiparallel (antiferromagnetic (AF)) at angles of 60° and 180°. The rule is found to be consistent with all the systems that have been studied so far. Applying the rule, we predict several novel graphene-based magnetic nanostructures: 0-D FM nanodots with the highest possible magnetic moments, 1-D FM nanoribbons, and 2-D magnetic superlattices.

We present first principles molecular dynamics simulations of stretched siloxane oligomers in an environment representative of that present in single molecule atomic force microscopy experiments. We determine that the solvent used (hexamethyldisiloxane) does not influence the stretching of the siloxane in the high force regime or the rupture process, but trace amounts of water can induce rupture before the maximum siloxane extension has been attained. This would result in a significantly lower rupture force. The simulations show that the rupture of a covalent bond through a reaction with a molecule from the environment, which would not normally occur between the species when the polymer is not stressed, is possible, opening a route to mechanically induced chemical reactions. The attack of the normally hydrophobic siloxane by water when it is stretched has wider implications for the material failure under tensile stress, where trace amounts of water could induce tearing of the material.

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Siloxanes are versatile elastomers with an exceptional chemical and physical stability that allows them to be used as adhesives, coatings, and sealants [1] in applications ranging from biomedical to aerospace. Although these materials are exceptionally strong, they are limited by the ease of propagation of cracks through the elastomer when subjected to tensile stress. The current method of improving the material strength is to add silica filler particles, [2,3] which hinder tearing in the bulk elastomer. However, the chemical mechanism that facilitates crack propagation and the way in which the filler particles hinder it have not been defined at the molecular level. Understanding these processes entails a full description of the electronic structure of a system during the process of bond rupture and the subsequent reactions between ruptured fragments to correctly determine the underlying chemistry.Information on the response of individual chemical bonds subjected to a tensile load is accessible via single-molecule atomic force microscopy (AFM) experiments, [4][5][6] which can be interpreted with theoretical studies. [7][8][9][10][11][12][13][14][15][16][17][18][19] In the experiments, a single polymer is stretched between a substrate and an AFM tip until one of the backbone bonds ruptures. Factors that can affect the magnitude of the measured rupture force, such as the polymer length and pulling velocity, [15,17] the solvent, [16,18] the presence of knots in the polymer chain, [11,12] and the pulling of a molecule from a substrate, [13,14,19] have been examined by using Car-Parrinello molecular dynamics (CPMD) simulations. These CPMD studies provide valuable insights into the characteristics of bond rupture within single molecules, because a full electronic structure calculation is performed on the fly for a molecular dynamics trajectory, which allows a complete description of the stretching of an oligomer in the highforce regime.Of interest in understanding stress-induced material failure in the bulk elastomer is what subsequently happens to the rupture products-in particular, whether chemical reactions between the rupture products lead to permanent weakening of the material or not. Building on the results of our previous studies of the rupture of isolated siloxane oligomers, here we investigate what happens on the molecular scale when neighboring oligomers are ruptured simultaneously.CPMD simulations have previously been used to examine how rupture occurs at a knot in polyethylene chains when stretched, and how the rupture products can further react with neighboring chains to cause a tear.[20] The CÀC bonds of the backbone ruptured via a radical mechanism and then reacted with neighboring alkane chains to induce further bond rupture. Also, a disproportionation involving hydrogen transfer was observed. In our previous CPMD study of the rupture of isolated polydimethylsiloxane (PDMS) oligomers, we determined that as the siloxanes are stretched, the backbone SiÀO bonds become increasingly polarized, until rupture ...

The dissymmetrical naphthalene-bridged complexes [Cp′Fe(μ-C 10 H 8 )FeCp*] (3; Cp* = η 5 -C 5 Me 5 , Cp′ = η 5 -C 5 H 2 -1,2,4-tBu 3 ) and [Cp′Fe(μ-C 10 H 8 )RuCp*] (4) were synthesized via a one-pot procedure from FeCl 2 (thf) 1.5 , Cp′K, KC 10 H 8 , and [Cp*FeCl(tmeda)] (tmeda = N,N,N′,N′tetramethylethylenediamine) or [Cp*RuCl] 4 , respectively. The symmetrically substituted iron ruthenium complex [Cp*Fe(μ-C 10 H 8 )RuCp*] (5) bearing two Cp* ligands was prepared as a reference compound. Compounds 3−5 are diamagnetic and display similar molecular structures, where the metal atoms are coordinated to opposite sides of the bridging naphthalene molecule. Cyclic voltammetry and UV/vis spectroelectrochemistry studies revealed that neutral 3−5 can be oxidized to monocations 3 + −5 + and dications 3 2+ −5 2+ . The chemical oxidation of 3 and 4 with [Cp 2 Fe]PF 6 afforded the paramagnetic hexafluorophosphate salts [Cp′Fe(μ-C 10 H 8 )FeCp*]PF 6 ([3]PF 6 ) and [Cp′Fe(μ-C 10 H 8 )RuCp*]PF 6 ([4]PF 6 ), which were characterized by various spectroscopic techniques, including EPR and 57 Fe Mossbauer spectroscopy. The molecular structure of [4]PF 6 was determined by X-ray crystallography. DFT calculations support the structural and spectroscopic data and determine the compositions of frontier molecular orbitals in the investigated complexes. The effects of substituting Cp* with Cp′ and Fe with Ru on the electronic structures and the structural and spectroscopic properties are analyzed.

The synthesis and characterization of a shape-persistent triphenylene-butadiynylene macrocycle formed by intermolecular Glaser-coupling of two ''half-rings'' and also by intramolecular coupling of the appropriate open dimer, respectively, are described in detail. The investigation of the photophysics has revealed that-compared to its open dimer-the macrocycle is more conjugated in the ground state and less so in the excited state, a result of the diacetylene bending in the macrocycle due to its constrained topology. The macrocycle is decorated with flexible side groups that support its adsorption on highly oriented pyrolytic graphite (HOPG) where a concentration-dependence of the 2D-structure is observed by means of scanning tunnelling microscopy (STM). The flexible side groups also guarantee a high compound solubility even in nonpolar solvents (cyclohexane). However, solvophobic interactions lead to the formation of a tube-like superstructure, as revealed by dynamic light scattering, X-ray scattering and atomic force microscopy.