High TMR in MXene-Based Mn2CF2/Ti2CO2/Mn2CF2 Magnetic Tunneling Junction.

We suggest an MXene-based magnetic tunnel junction (MTJ) design. The device characteristics of the MTJ were investigated by nonequilibrium Green's function formalism within the density functional theory. Inspired by the first synthesized magnetic MAX crystal of Mn2GaC, its two-dimensional (2D) counterpart of the half-metallic Mn2CF2 MXene layer was selected as the magnetic electrode. The tunneling barrier was chosen as Ti2CO2 MXene, which is one of the most studied MXenes in experimental and theoretical works. It is beneficial that both the electrodes and the tunneling barrier are 2D materials from the same material family and have similar structures. The common device problem of lattice mismatch does not occur in our MTJ design because the lattice parameters are compatible. In addition, the band gap of Ti2CO2 tunneling barrier is almost the same as the half-metallic gap of Mn2CF2 electrodes. Both the barrier and the electrodes have a common C layer that contributes the most to the transmission. Our MTJ design consists of structurally and electronically well-matched components. We find that the tunneling magnetoresistance ratio has a peak value of ≈106 and stays higher than ≈103 under the bias voltages up to 1 V. Since the applied bias voltages are within the energy gap of the tunneling barrier, the half-metallic character of the conduction is preserved up to 1 V. The tunneling-based transmission was observed in all of the three devices with varying tunneling barrier widths, and the current decreases with increasing width. The MXene-based MTJ has promising device characteristics.

Doped Sc2C(OH)2 MXene: new type s-pd band inversion topological insulator.

The electronic structures of Si and Ge substitutionally doped Sc2C(OH)2 MXene monolayers are investigated in density functional theory. The doped systems exhibit band inversion, and are found to be topological invariants in Z 2 theory. The inclusion of spin orbit coupling results in band gap openings. Our results point out that the Si and Ge doped Sc2C(OH)2 MXene monolayers are topological insulators. The band inversion is observed to have a new mechanism that involves s and pd states.

Structure-Photoproperties Relationship Investigation of the Singlet Oxygen Formation in Porphyrin-Fullerene Dyads.

A systematic structure-photoproperties relationship study of the interactions of porphyrin-fullerene dyads with molecular oxygen was conducted on a set of three porphyrin-fullerene dyads, as this approach of related applications - oxygen sensitivity and photo-induced singlet oxygen generation - of such dyads remained to be endeavoured. To promote energy transfers between the porphyrin and fullerene units and limit undesired charge separation, a particular attention was devoted to the choice of the solvents for the photoproperties determination. Toluene, in which in addition the compounds investigated are not aggregated, was selected accordingly. The molecular orbital levels and energy gaps of the dyads were determined by electrochemistry and theoretical calculations. Their ground state absorption, steady-state fluorescence-based oxygen sensitivity and photo-induced singlet oxygen generation were determined. The dyads were designed to benefit from a facilitated synthetic porphyrin-fullerene coupling thanks to an easy access to formyl-functionalized porphyrins. The effect of two structural parameters was investigated: the presence of electron-donating hexyloxy chains at the para position of the meso-phenyl, and the presence of a phenylacetylene spacer. This latest factor appeared to have the most predominant effect on all these properties.

Modulation of the electronic and spectroscopic properties of Zn(II) phthalocyanines by their substitution pattern.

Four isomerically pure octasubstituted zinc phthalocyanines with variations in the attachment atom and positions of the substituents were selected for a systematic investigation of the effect of the substitution pattern on their electronic and spectroscopic properties. Effects which were investigated are the position, the electron donating and withdrawing properties, and the donating force of the substituent. The results are discussed and interpreted based on theoretical and experimental determination of the orbital levels. This work allows us to highlight which substitution patterns are the most suitable considering different common applications of phthalocyanines.

Synthesis, spectroscopy, and characterization of some bis-nicotinamide metal(II) dihalide complexes.

We report synthesis of six new bis-nicotinamide metal(II) dihalide complexes [M(nia)(2)Cl(2); M = Mn, Co; nia:nicotinamide, M(nia)(2)Br(2); M = Mn, Hg; M(nia)(2)I(2); M = Cd, Cu], and their characterization by combining infrared spectroscopy with density functional theory (DFT) calculations. Infrared spectra indicate that ring-nitrogen is the active donor cite, and the atomic structure of the complexes is determined to be polymeric octahedral or distorted polymeric octahedral. Spin polarized electronic ground state is obtained for Mn, Co, and Cu halide complexes. The colors of the complexes also support the conclusion of octahedral coordination around the metal atoms, in agreement with DFT results.

Revealing subsurface vibrational modes by atom-resolved damping force spectroscopy.

We propose to use the damping signal of an oscillating cantilever in dynamic atomic force microscopy as a noninvasive tool to study the vibrational structure of the substrate. We present atomically resolved maps of damping in carbon nanotube peapods, capable of identifying the location and packing of enclosed Dy@C_{82} molecules as well as local excitations of vibrational modes inside nanotubes of different diameter. We elucidate the physical origin of damping in a microscopic model and provide quantitative interpretation of the observations by calculating the vibrational spectrum and damping of Dy@C_{82} inside nanotubes with different diameters using ab initio total energy and molecular dynamics calculations.

Molecular self-assembly of functionalized fullerenes on a metal surface.

We present a combined experimental and theoretical study of the self-assembly of C60 molecules functionalized with long alkane chains on the (111) surface of silver. We find that the conformation of the functionalized C60 changes upon adsorption on Ag(111) and that the unit cell size in the self-assembled monolayer is determined by the interactions between the functional groups. We show that C60 molecules can be assembled in ordered 2D arrays with intermolecular distances much larger than those in compact C60 layers, and propose a novel way to control the surface pattern by appropriate chemical functionalization.

Direct observation of optically induced transient structures in graphite using ultrafast electron crystallography.

We use ultrafast electron crystallography to study structural changes induced in graphite by a femtosecond laser pulse. At moderate fluences of < or =21 mJ/cm2, lattice vibrations are observed to thermalize on a time scale of approximately 8 ps. At higher fluences approaching the damage threshold, lattice vibration amplitudes saturate. Following a marked initial contraction, graphite is driven nonthermally into a transient state with sp3-like character, forming interlayer bonds. Using ab initio density functional calculations, we trace the governing mechanism back to electronic structure changes following the photoexcitation.

Transforming carbon nanotubes by silylation: an ab initio study.

We use ab initio density functional calculations to study the chemical functionalization of single-wall carbon nanotubes and graphene monolayers by silyl (SiH(3)) radicals and hydrogen. We find that silyl radicals form strong covalent bonds with graphene and nanotube walls, causing local structural relaxations that enhance the s p(3) character of these graphitic nanostructures. Silylation transforms all carbon nanotubes into semiconductors, independent of their chirality. Calculated vibrational spectra suggest that specific frequency shifts can be used as a signature of successful silylation.

Spin currents in rough graphene nanoribbons: universal fluctuations and spin injection.

We investigate spin conductance in zigzag graphene nanoribbons and propose a spin injection mechanism based only on graphitic nanostructures. We find that nanoribbons with atomically straight, symmetric edges show zero spin conductance but nonzero spin Hall conductance. Only nanoribbons with asymmetrically shaped edges give rise to a finite spin conductance and can be used for spin injection into graphene. Furthermore, nanoribbons with rough edges exhibit mesoscopic spin conductance fluctuations with a universal value of rmsG_{s} approximately 0.4e/4pi.

Self-assembly of long chain alkanes and their derivatives on graphite.

We combine scanning tunneling microscopy (STM) measurements with ab initio calculations to study the self-assembly of long chain alkanes and related alcohol and carboxylic acid molecules on graphite. For each system, we identify the optimum adsorption geometry and explain the energetic origin of the domain formation observed in the STM images. Our results for the hierarchy of adsorbate-adsorbate and adsorbate-substrate interactions provide a quantitative basis to understand the ordering of long chain alkanes in self-assembled monolayers and ways to modify it using alcohol and acid functional groups.

Hydrogenation of single-wall carbon nanotubes using polyamine reagents: combined experimental and theoretical study.

We combine experimental observations with ab initio calculations to study the reversible hydrogenation of single-wall carbon nanotubes using high boiling polyamines as hydrogenation reagents. Our calculations characterize the nature of the adsorption bond and identify preferential adsorption geometries at different coverages. We find the barrier for sigmatropic rearrangement of chemisorbed hydrogen atoms to be approximately 1 eV, thus facilitating surface diffusion and formation of energetically favored, axially aligned adsorbate chains. Chemisorbed hydrogen modifies the structure and stability of nanotubes significantly and increases the inter-tube distance, thus explaining the improved dispersability in solvents like methanol, ethanol, chloroform, and benzene.

Unique structural and transport properties of molybdenum chalcohalide nanowires.

We combine ab initio density functional and quantum transport calculations based on the nonequilibrium Green's function formalism to compare structural, electronic, and transport properties of Mo6S6-xIx nanowires with carbon nanotubes. We find systems with x=2 to be particularly stable and rigid, with their electronic structure and conductance close to that of metallic (13,13) single-wall carbon nanotubes. Mo6S6-xIx nanowires are conductive irrespective of their structure, more easily separable than carbon nanotubes, and capable of forming ideal contacts to Au leads through thio groups.

Atomic and electronic structures of carbon nanotubes on Si(001) stepped surfaces.

We report first-principles total-energy calculations that provide energetics and electronic structures of adsorbed carbon nanotubes (CNTs) on stepped Si(001) surfaces. We find that adsorption energies strongly depend on the directions of CNTs, and that there are several metastable adsorption sites both on terraces and near step edges. We also find that the electronic structure of adsorbed metallic CNTs becomes semiconducting or remains metallic, depending on the adsorption site. Charge redistribution upon adsorption is prominent mainly at the CNT-surface interface.

Interplay between structure and magnetism in Mo12S9I9 nanowires.

We investigate the equilibrium geometry and electronic structure of Mo12S9I9 nanowires using ab initio density functional calculations. The skeleton of these unusually stable nanowires consists of rigid, functionalized Mo octahedra, connected by flexible, bistable sulfur bridges. This structural flexibility translates into a capability to stretch up to approximately 20% at almost no energy cost. The nanowires change from conductors to narrow-gap magnetic semiconductors in one of their structural isomers.

Effect of SOCl2 treatment on electrical and mechanical properties of single-wall carbon nanotube networks.

Chemical modification by SOCl2 of an entangled network of purified single-wall carbon nanotubes, also known as 'bucky paper', is reported to profoundly change the electrical and mechanical properties of this system. Four-probe measurements indicate a conductivity increase by up to a factor of 5 at room temperature and an even more pronounced increase at lower temperatures. This chemical modification also improves the mechanical properties of SWNT networks. Whereas the pristine sample shows an overall semiconducting character, the modified material behaves as a metal. The effect of SOCl2 is studied in terms of chemical doping of the nanotube network. We identified the microscopic origin of these changes using SEM, XPS, NEXAFS, EDX, and Raman spectroscopy measurements and ab initio calculations. We interpret the SOCl2-induced conductivity increase by p-type doping of the pristine material. This conclusion is reached by electronic structure calculations, which indicate a Fermi level shift into the valence band, and is consistent with the temperature dependence of the thermopower.

Zipper mechanism of nanotube fusion: theory and experiment.

We propose a new microscopic mechanism to explain the unusually fast fusion process of carbon nanotubes. We identify the detailed pathway for two adjacent (5,5) nanotubes to gradually merge into a (10,10) tube, and characterize the transition states. The propagation of the fused region is energetically favorable and proceeds in a morphology reminiscent of a Y junction via a zipper mechanism, involving only Stone-Wales bond rearrangements with low activation barriers. The zipper mechanism of fusion is supported by a time series of high-resolution transmission electron microscopy observations.

Thermal contraction of carbon fullerenes and nanotubes.

We perform molecular dynamics simulations to study shape changes of carbon fullerenes and nanotubes with increasing temperature. At moderate temperatures, these systems gain structural and vibrational entropy by exploring the configurational space at little energy cost. We find that the soft phonon modes, which couple most strongly to the shape, maintain the surface area of these hollow nanostructures. In nanotubes, the gain in entropy translates into a longitudinal contraction, which reaches a maximum at T approximately 800 K. Only at much higher temperatures do the anharmonicities in the vibration modes cause an overall expansion.

Magnetism in all-carbon nanostructures with negative Gaussian curvature.

We apply the ab initio spin density functional theory to study magnetism in all-carbon nanostructures. We find that particular systems, which are related to schwarzite and contain no undercoordinated carbon atoms, carry a net magnetic moment in the ground state. We postulate that, in this and other nonalternant aromatic systems with negative Gaussian curvature, unpaired spins can be introduced by sterically protected carbon radicals.

Bonding and energy dissipation in a nanohook assembly.

Combining total energy and molecular dynamics calculations, we explore the suitability of nanotube-based hooks for bonding. Our results indicate that a large force of 3.0 nN is required to disengage two hooks, which are formed by the insertion of pentagon-heptagon pairs in a (7,0) carbon nanotube. Nanohooks based on various nanotubes are resilient and keep their structural integrity during the opening process. Arrays of hooks, which are permanently anchored in solid surfaces, are a nanoscale counterpart of velcro fasteners, forming tough bonds with a capability of self-repair.