The potential of covalent organic frameworks (COFs) for realizing porous, crystalline networks with tailored combinations of functional building blocks has attracted considerable scientific interest ...in the fields of gas storage, photocatalysis, and optoelectronics. Porphyrins are widely studied in biology and chemistry and constitute promising building blocks in the field of electroactive materials, but they reveal challenges regarding crystalline packing when introduced into COF structures due to their nonplanar configuration and strong electrostatic interactions between the heterocyclic porphyrin centers. A series of porphyrin-containing imine-linked COFs with linear bridges derived from terephthalaldehyde, 2,5-dimethoxybenzene-1,4-dicarboxaldehyde, 4,4′-biphenyldicarboxaldehyde and thieno3,2-bthiophene-2,5-dicarboxaldehyde, were synthesized, and their structural and optical properties were examined. By combining X-ray diffraction analysis with density-functional theory (DFT) calculations on multiple length scales, we were able to elucidate the crystal structure of the newly synthesized porphyrin-based COF containing thieno3,2-bthiophene-2,5-dicarboxaldehyde as linear bridge. Upon COF crystallization, the porphyrin nodes lose their 4-fold rotational symmetry, leading to the formation of extended slipped J-aggregate stacks. Steady-state and time-resolved optical spectroscopy techniques confirm the realization of the first porphyrin J-aggregates on a > 50 nm length scale with strongly red-shifted Q-bands and increased absorption strength. Using the COF as a structural template, we were thus able to force the porphyrins into a covalently embedded J-aggregate arrangement. This approach could be transferred to other chromophores; hence, these COFs are promising model systems for applications in photocatalysis and solar light harvesting, as well as for potential applications in medicine and biology.
The verification of a successful covalent functionalization of graphene and related carbon allotropes can easily be carried out by Raman spectroscopy. Nevertheless, the unequivocal assignment and ...resolution of individual lattice modes associated with the covalent binding of addends was elusive up to now. Here we present an in situ Raman study of a controlled functionalization of potassium intercalated graphite, revealing several new bands appearing in the D-region of the spectrum. The evolution of these bands with increasing degree of functionalization from low to moderate levels provides a basis for the deconvolution of the different components towards quantifying the extent of functionalization. By complementary DFT calculations we were able to identify the vibrational changes in the close proximity of the addend bearing lattice carbon atoms and to assign them to specific Raman modes. The experimental in situ observation of the developing functionalization along with the reoxidation of the intercalated graphite represents an important step towards an improved understanding of the chemistry of graphene.
The search of ultra hard materials is inevitable in high pressure device applications. Nitrides of group-14 elements have been foreseen as potential candidates in replacing existing hard materials. ...In a recent experiment, pyrite structures of SiN2, GeN2 and SnN2 have been synthesized at high pressure and are shown to have high bulk modulus. Though their existence and bulk modulus are known, limited studies have been devoted to SiN2, GeN2 and SnN2. In the present work, we have performed the first principles calculations to investigate structural, electronic, mechanical and vibrational properties at ambient as well as high pressure condition. The physical properties of SnN2 and high pressure lattice dynamical properties of SiN2 and GeN2 are explored for the first time. SiN2 has higher bulk modulus among all MN2 (where M = Si, Sn and Ge), and increases further with increase in pressure. The increase in elastic moduli of MN2 have been related to the shortening of M-N bond length at high pressure. Electronic properties of these pyrites suggest that the bandgap increases with pressure. We further characterize these MN2 at high pressure using theoretically calculated Raman spectra. Frequency of N–N stretching Ag and Tg modes confirms the single bond character of nitrogen dimer in all studied MN2 compounds.
•Elastic moduli increases with pressure for MN2.•We predicted the electronic band gap of SiN2, GeN2 and SnN2 at high pressures.•The lattice dynamical stability of MN2 were confirmed by phonon dispersion calculations.•The high pressure structures of MN2 were further characterized using Raman spectra.
Graphyne-based two-dimensional (2D) carbon allotropes feature extraordinary physical properties; however, their synthesis as crystalline single-layered materials has remained challenging. We report ...on the fabrication of large-area organometallic Ag−bis-acetylide networks and their structural and electronic properties on Ag(111) using low-temperature scanning tunneling microscopy combined with density functional theory (DFT) calculations. The metalated graphyne-based networks are robust at room temperature and assembled in a bottom-up approach via surface-assisted dehalogenative homocoupling of terminal alkynyl bromides. Large-area networks of several hundred nanometers with topological defects at domain boundaries are obtained due to the Ag–acetylide bonds’ reversible nature. The thermodynamically controlled growth mechanism is explained through the direct observation of intermediates, which differ on Ag(111) and Au(111). Scanning tunneling spectroscopy resolved unoccupied states delocalized across the network. The energy of these states can be shifted locally by the attachment of a different number of Br atoms within the network. DFT revealed that free-standing metal−bis-acetylide networks are semimetals with a linear band dispersion around several high-symmetry points, which suggest the presence of Weyl points. These results demonstrate that the organometallic Ag−bis-acetylide networks feature the typical 2D material properties, which make them of great interest for fundamental studies and electronic materials in devices.
In the frame work of density functional theoretical calculations, the electronic and lattice dynamical properties of graphene (multilayers and supercell) have been systematically investigated and ...analyzed using the plane wave pseudopotentials within the generalized gradient approximation and local density approximation functional. We have also studied the functionalization of graphene by adsorption and absorption of transition metals like Al and Ag. We find that the electronic properties exhibit large sensitivity to the number of layers and doping. The Al and Ag doped graphene exhibits peak at Fermi level in the density of states arising from the flat bands near Fermi level. The bonding of metal atoms and graphene leads to a charge transfer between them and consequently shift Fermi level with respect to the conical point at K-point. The adsorption of Ag/Al atoms suggests an effective interaction between the adatoms and graphene layers without disturbing the original graphene structure of lower graphene layers. Compared to single layer graphene, the optical phonon E2g mode and out of plane ZA mode at Γ-point splits in the bi-, tri- and four- layer graphene. We observe a shift for highest optical branch at Dirac K- point. We find that the different derivatives of graphene have different phonon dispersion relations. We demonstrate that there is removal of degeneracy of ZO/ZA modes at K- point with transition metal doping. The highest optical phonon branch becomes flat at Dirac point with doping of transition metals. Our study points that the substituted graphene sheets can have potential applications in ordered-disordered separated quantum films with two to four layers of atoms and new nano devices using graphene.
We have studied the strain induced modifications in the structural, electronic, and vibrational properties of beryllium chalcogenide (groups II–VI) BeX (where X = S, Se, and Te) compounds by ...employing first-principles calculation within the framework of density-functional theory based on the plane-wave pseudopotential method with implementation of the local-density approximation functional. We have gradually applied biaxial strain on BeX compounds and observed changes in the structure of these compounds. The calculated structural properties such as lattice parameter, bulk modulus, and bandgap are in good agreement with the previously reported theoretical and experimental values. It is found that the calculated bandgaps are indirect from the Γ → X direction and vary with the applied strain. It is also confirmed by the density of states calculations. To study the dynamical stability of BeX compounds, we have calculated the vibrational properties. The calculated bandgap and phonon frequencies vary with the applied strain. For a higher value of the applied strain, we found imaginary phonon frequencies for all three compounds. The bandgap and phonon frequencies decrease as we move from a lower to a higher atomic number (i.e., from S to Te) for BeX compounds.
The interaction of single‐layer hexagonal boron nitride (h‐BN) on Ni(111) with molecular oxygen from a supersonic molecular beam led to a covalently bonded molecular oxygen species, which was ...identified as being between a superoxide and a peroxide. This is a rare example of an activated adsorption process leading to a molecular adsorbate. The amount of oxygen functionalization depended on the kinetic energy of the molecular beam. For a kinetic energy of 0.7 eV, an oxygen coverage of 0.4 ML was found. Near‐edge X‐ray adsorption fine structure (NEXAFS) spectroscopy revealed a stronger bond of h‐BN to the Ni(111) substrate in the presence of the covalently bound oxygen species. Oxygen adsorption also led to a shift of the valence bands to lower binding energies. Subsequent temperature‐programmed X‐ray photoelectron spectroscopy revealed that the oxygen boron bonds are stable up to approximately 580 K, when desorption, and simultaneously, etching of h‐BN set in. The experimental results were substantiated by density functional theory calculations, which provided insight to the adsorption geometry, the adsorption energy and the reaction pathway.
Activated adsorption: The interaction of single‐layer hexagonal boron nitride (h‐BN) on Ni(111) with molecular oxygen from a supersonic molecular beam led to a covalently bonded molecular oxygen species, which was identified as being between a superoxide and a peroxide species. This is a rare example of an activated adsorption process leading to a molecular adsorbate.
In this work, we present biaxial strain-induced modification in the structural and electronic properties of a MoS2 hybrid structure made of a metallic (1T) ribbon embedded in the semiconducting (2H) ...phase. The results are based on density-functional theory. Biaxial strain is gradually applied on the hybrid structure, and the structural modifications are monitored. The MoS2 hybrid material was found to be stable up to 6% (extension) and −4% (compression) strain. The onset of bending and breaking of the 2D material was identified and correlated to its electronic behavior. The alteration of the density of states with biaxial strain was also investigated and revealed the enhancement of either the metallic or the semiconducting character of the hybrid depending on the amount and direction of strain. There is also a clear mapping of the structural asymmetry of the interfaces in the material to the anisotropy in its electronic features. This anisotropy becomes more pronounced as the strain on the material increases. Our results shed light on the relevance of the morphology and electronic properties and allow us to tailor these properties through straining. In the end we discuss the relevance of this material in realizing novel nanoelectronic devices with tunable properties related to sensing, nanopore materials for sequencing, etc.
The present paper reports the calculated vibrational and elastic properties of some two dimensional carbon allotropes such as graphene, α-, β- and γ-graphynes using first principles density ...functional theory. The phonon modes of graphynes show quite distinct behavior than graphene and have real frequency throughout the Brillouin zone thus indicating dynamically stable structures. The out of plane, ZA mode is more dispersive in the case of graphynes. We have discussed the implications of phonon modes to the thermal conductance in graphynes and graphene. We have also calculated the elastic constants for graphene and graphynes. Calculated elastic constants of graphynes show more anisotropic conformer nature than graphene.
•Lattice dynamical and elastic properties of graphene and graphynes have been studied respectively.•The PDCs for α, β and γ-graphyne show dynamical stability.•The out of plane, ZA mode is more dispersive in the case of graphynes.•Elastic constants of graphynes show more anisotropic conformer nature than graphene.
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