Two-dimensional hexagonal composite materials (BN) n (C 2 ) m (n, m = 1, 2, ...), which all are isoelectronic with graphene and hexagonal boron nitride (h-BN), have been studied by density functional theory (DFT) with a focus on the relative energies of different material isomers and their band gaps. The well-established chemical concepts of conjugation and aromaticity were exploited to deduce a rationale for identifying the thermodynamically most stable isomer of the specific composites studied. We find that (BN) n (C 2 ) m materials will not adopt structures in which the B, C, and N atoms are finely dispersed in the 2D sheet. Instead, the C atoms and CÀC bonds, which provide for improved conjugation when compared to BÀN bonds, gather and form all-carbon hexagons and paths; that is, the (BN) n (C 2 ) m materials prefer nanostructured distributions. Importantly, there are several isomers of similarly low relative energy for each (BN) n (C 2 ) m composite type, but the band gaps for these nearly isoenergetic isomers differ by up to 1.0 eV. This feature in the band gap variation of the most stable few isomers is found for each of the four composites studied and at two different DFT levels. Consequently, the formation of a distinct (BN) n (C 2 ) m material isomer with a precise (small) band gap will likely be nontrivial. Therefore, one likely has to invoke nonstandard preparation techniques that exploit nanopatterned h-BN or graphene with voids that can be filled with the complementary all-carbon or boron nitride segments.

We have performed density functional calculations as well as employed a tight-binding theory, to study the effect of passivation of zigzag graphene nanoribbons (ZGNR) by Hydrogen. We show that each edge C atom bonded with 2 H atoms open up a gap and destroys magnetism for small widths of the nanoribbon. However, a re-entrant magnetism accompanied by a metallic electronic structure is observed from 8 rows and thicker nanoribbons. The electronic structure and magnetic state are quite complex for this type of termination, with sp 3 bonded edge atoms being non-magnetic, whereas the nearest neighboring atoms are metallic and magnetic. We have also evaluated the phase stability of several thicknesses of ZGNR, and demonstrate that sp 3 bonded edge atoms, with 2 H atoms at the edge, should be stable at temperatures and pressures which are reachable in a laboratory environment.

One of the primary objectives in molecular nanospintronics is to manipulate the spin states of organic molecules with a d-electron center, by suitable external means. In this Letter, we demonstrate by first principles density functional calculations, as well as second order perturbation theory, that a strain induced change of the spin state, from S=1→S=2, takes place for an iron porphyrin (FeP) molecule deposited at a divacancy site in a graphene lattice. The process is reversible in the sense that the application of tensile or compressive strains in the graphene lattice can stabilize FeP in different spin states, each with a unique saturation moment and easy axis orientation. The effect is brought about by a change in Fe-N bond length in FeP, which influences the molecular level diagram as well as the interaction between the C atoms of the graphene layer and the molecular orbitals of FeP.

One of the key factors behind the rapid evolution of molecular spintronics is the efficient realization of spin manipulation of organic molecules with a magnetic center. The spin state of such molecules may depend crucially on the interaction with the substrate on which they are adsorbed. In this paper we demonstrate, using ab initio density functional calculations, that the stabilization of a high spin state of an iron porphyrin (FeP) molecule can be achieved via chemisorption on magnetic substrates of different species and orientations, viz., Co(001), Ni(001), Ni(110), and Ni(111). The signature of chemisorption of FeP on magnetic substrates is evident from broad features in N K x-ray absorption (XA) and Fe L 2,3 x-ray magnetic circular dichroism (XMCD) measurements. Our theoretical calculations show that the strong covalent interaction with the substrate increases Fe-N bond lengths in FeP and hence a switching to a high spin state (S = 2) from an intermediate spin state (S = 1) is achieved. Due to chemisorption, ferromagnetic exchange interaction is established through a direct exchange between Fe and substrate magnetic atoms as well as through an indirect exchange via the N atoms in FeP. The mechanism of exchange interaction is further analyzed by considering structural models constructed from ab initio calculations. Also, it is found that the exchange interaction between Fe in FeP and a Ni substrate is almost 4 times smaller than with a Co substrate. Finally, we illustrate the possibility of detecting a change in the molecular spin state by XMCD, Raman spectroscopy, and spin-polarized scanning tunneling microscopy.

We study the effects of charge self-consistency within the combination of density functional theory (DFT; WIEN2K) with dynamical mean field theory (DMFT; W2DYNAMICS) in a basis of maximally localized Wannier orbitals. Using the example of two cuprates, we demonstrate that even if there is only a single Wannier orbital with fixed filling, a noteworthy charge redistribution can occur. This effect stems from a reoccupation of the Wannier orbital in k-space when going from the single, metallic DFT band to the split, insulating Hubbard bands of DMFT. We analyze another charge self-consistency effect beyond moving charge from one site to another: the correlation-enhanced orbital polarization in a freestanding layer of SrVO 3 .

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Graphene is a two-dimensional material with a capability of gas sensing, which is here shown to be drastically improved by inducing gentle disorder in the lattice. We report that by using a focused ion beam technique, controlled disorder can be introduced into the graphene structure through Ga(+) ion irradiation. This disorder leads to an increase in the electrical response of graphene to NO(2) gas molecules by a factor of three in an ambient environment (air). Ab initio density functional calculations indicate that NO(2) molecules bind strongly to Stone-Wales defects, where they modify electronic states close to the Fermi level, which in turn influence the transport properties. The demonstrated gas sensor, utilizing structurally defected graphene, shows faster response, higher conductivity changes and thus higher sensitivity to NO(2) as compared to pristine graphene.

We predict a transition to metallicity when a sufficient amount of disorder is induced in graphene. Calculations were performed by means of a first principles stochastic quench method. The resulting amorphous graphene can be seen as nanopatches of graphene that are connected by a network of disordered small and large carbon rings. The buckling is minimal and we believe that it is a result of averaging of counteracting random in-plane stress forces. The linear response conductance is obtained by a model theory as function of lattice distortions. Such metallic behaviour is a much desired property for functionalisation of graphene to realize a transparent conductor, e.g. suitable for touch-screen devices.Since the discovery of graphene, and all its unique physical properties [1][2][3][4][5], attention is now turned to functionalization of this material to suit specific applications. One example is the suggestion of graphane, where H atoms are adsorbed on graphene. Graphane, which is an insulator, was predicted from first principles theory [6], and was subsequently realized experimentally [7]. Further theoretical works [8,9] addressed e.g. the value of the band gap, which was found to be of order 5.7 eV. In a similar fashion, fluorine can also be found to adsorb, and in theoretical works a band gap of 7.4 eV is found [10,11], which is larger than the measured value of 3.4 eV [12]. The reason that a large band gap opens up when hydrogen or fluorine is adsorbed on graphene is that sp 2 bonded C atoms become sp 3 bonded. Hence it seems possible to decrease the conductivity by chemical functionalization, and turn the semi-metallic graphene to a semi-conductor or even an insulator. Smaller values of the gap can be reached by a functionalization by organic molecules [13].The ways to increase the conductivity by chemical means has also been discussed, although here significantly less success can be identified. Adsorption by single impurities [14] have been explored, as well as the replacement of C atoms for B or N atoms [15], or even the creation of structural defects in the C matrix [16][17][18]. The electronic structure of non-crystalline graphene, e.g. around grain boundaries have also been under consideration, both from tight-binding analysis [19] and within the framework of first principles theory [20] in combination with calculations of transport properties. In the works mentioned above, the conducting properties are still those of a doped semiconductor, with relatively few charge carriers. A fully metallic behavior was however suggested in an amorphous structure of graphene [21], where Stone-Wales defects were introduced into graphene and geometry optimization was done according to a Keating-like potential. After geometry optimization, the electronic structure was calculated from a tight-binding Hamiltonian, and a non-zero value of the density of states (DOS) at the Fermi level (E F ) was obtained, suggesting that metallic conductivity is possible. The possibility of a transparent, mono-atomic thin material, has a g...