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Using ab initio electronic structure calculations and grand canonical Monte Carlo simulations, we investigate the storage capacity of hydrogen molecules in a potential well created inside potassium-intercalated graphite oxide layers. We show that the binding energy of hydrogen located between layers of graphite oxide mainly originates from the dispersion interaction, and it is further increased slightly by induced dipole interactions. Its strength is fairly insensitive to the precise positions of the hydrogen molecules on the graphene plane, so the system may be described as a quasi-two-dimensional potential well. In this situation, the storage capacity is enhanced by the corresponding Boltzmann factor based on equilibrium thermodynamics. The trend of storage capacity with different geometries and chemical compositions of the scaffold materials is explained. With the present model, the density functional theory calculations and grand canonical Monte Carlo simulations predict a 2.5 wt% hydrogen storage capacity at room temperature at 10 MPa. For a model with increased potential depth, the storage capacity is predicted to increase up to 5.5 wt%.

The gas-phase reaction mechanism of magnesium amide (Mg(NH2)2) and lithium hydride (LiH) to produce H2 is theoretically investigated using ab initio electronic structure and total energy calculations based on the density functional theory. The reaction with the addition of KH which has been reported to lower the activation energy is also considered. We identify intermediate complexes and transition states in the reaction pathways. Detailed processes are analyzed to show how each reactant (Mg(NH2)2 or LiH) contributes a H atom to produce a H2 molecule and KH induces a modified reaction mechanism with a lower reaction energy.

Hydrogen is an excellent energy carrier free of carbon dioxide emission, but safe and efficient storage of hydrogen has been a bottleneck for the commercial use of hydrogen as a fuel. Here, we present a strategy based on simple thermodynamic principles that the density of a gas residing in a potential well increases exponentially relative to the ambient gas by the corresponding Boltzmann factor. This mechanism allows for enormously enhanced H2 storage in the form of delocalized gas permeating throughout the void space of a material, in contrast to conventional storage localized to specific adsorption sites. We create mesoporous graphene oxide that provides a two-dimensional potential well and efficient hydrogen diffusion pathways. The gravimetric storage density measured with quartz-crystal microbalance reaches 4.65 wt% reproducibly at a modest pressure of 40 atm at room temperature. Our work demonstrates the attainability of the long-standing goal of room-temperature hydrogen storage.

Metal-doped graphene produces magnetic moments that have potential application in spintronics. Here we use density function theory computational methods to show how the magnetic interaction between metal atoms doped in graphene can be controlled by the degree of flexure in a graphene membrane. Bending graphene by flexing causes the distance between two substitutional Fe atoms covalently bonded in graphene to gradually increase and these results in the magnetic moment disappearing at a critical strain value. At the critical strain, a carbon atom can enter between the two Fe atoms and blocks the interaction between relevant orbitals of Fe atoms to quench the magnetic moment. The control of interactions between doped atoms by exploiting the mechanical flexibility of graphene is a unique approach to manipulating the magnetic properties and opens up new opportunities for mechanical-magnetic 2D device systems.

Graphene nanoribbons (GNRs) have recently been shown by Cao, Zhao, and Louie [Cao, T.; Zhao, F.; Louie, S. G. Phys. Rev. Lett. 2017, 119, 076401] to possess distinct topological phases in general, characterized by a [Formula: see text]

Dirac line node (DLN) semimetals are a class of topological semimetals that feature band-crossing lines in momentum space. We study the type-I and type-II classification of DLN semimetals by developing a criterion that determines the type using band velocities. Using first-principles calculations, we also predict that Na3N under an epitaxial tensile strain realizes a type-II DLN semimetal with vanishing spin-orbit coupling (SOC), characterized by the Berry phase that is Z2-quantized in the presence of inversion and time-reversal symmetries. The surface energy spectrum is calculated to demonstrate the topological phase, and the type-II nature is demonstrated by calculating the band velocities. We also develop a tight-binding model and a low-energy effective Hamiltonian that describe the low-energy electronic structure of strained Na3N. The occurrence of a DLN in Na3N under strain is captured in the optical conductivity, which we propose as a means to experimentally confirm the type-II class of the DLN semimetal., 14 pages, 9 figure

A compressed stiff film on a soft substrate may exhibit wrinkles and, under increased compressive strain, post-buckling instabilities as well. We numerically analyze wrinkling behaviors of graphene attached on a polydimethylsiloxane (PDMS) substrate under lateral compression. The finite element method is used to simulate the equilibrium shape of the wrinkles as a function of compressive strain. Two-dimensional stretching and bending properties of graphene are obtained by density functional theory analysis, which are then converted to equivalent elastic properties of a continuum film with finite effective thickness. The PDMS is described using an Ogden or a neo-Hookean material model. Wrinkles first appear at extremely small strain. As the lateral compression increases, due to the nonlinear elasticity of the PDMS, sequential period-doubling bifurcations of the wrinkle mode are activated until the bifurcation stops and the film folds. We show that the bifurcations are consequences of a delicate balance between the deformations of the film and the substrate to minimize the total energy.

Because topological surface states of a single-crystal topological insulator can exist on all surfaces with different crystal orientations enclosing the crystal, mutual interactions among those states contiguous to each other through edges can lead to unique phenomena inconceivable in normal insulators. Here we show, based on a first-principles approach, that the difference in the work function between adjacent surfaces with different crystal-face orientations generates a built-in electric field around facet edges of a prototypical topological insulator such as Bi2Se3. Owing to the topological magnetoelectric coupling for a given broken time-reversal symmetry in the crystal, the electric field, in turn, forces effective magnetic dipoles to accumulate along the edges, realizing the facet-edge magnetic ordering. We demonstrate that the predicted magnetic ordering is in fact a manifestation of the axion electrodynamics in real solids.

Transition-metal (TM) atoms are known to form TM-H2 complexes, which are collectively called Kubas dihydrogen complexes. The TM-H2 complexes are formed through the hybridization of the TM d orbitals with the H2 σ and σ* orbitals. The adsorption energy of H2 molecules in the TM-H2 complexes is usually within the range of energy required for reversible H2 storage at room temperature and ambient pressure (−0.4 ~ −0.2 eV/H2). Thus, TM-H2 complexes have been investigated as potential Kubas-type hydrogen-storage materials. Recently, TM-decorated nanomaterials have attracted much attention because of their promising high capacity and reversibility as Kubas-type hydrogen-storage materials. The hydrogen storage capacity of TM-decorated nanomaterials is expected to be as large as ~9 wt%, which is suitable for certain vehicular applications. However, in the TM-decorated nanostructures, the TM atoms prefer to form clusters because of the large cohesive energy (approximately 4 eV), which leads to a significant reduction in the hydrogen-storage capacity. On the other hand, Ca atoms can form complexes with H2 molecules via Kubas-like interactions. Ca atoms attached to nanomaterials have been reported to be able to adsorb as many H2 molecules as TM atoms. Ca atoms tend to cluster less because of the small cohesive energy of bulk Ca (1.83 eV), which is much smaller than those of bulk TMs. These observations suggest thatKubas interactions can occur in d orbital-free elements, thereby making Ca a more suitable element for attracting H2 in hydrogen-storage materials. Recently, Kubas-type TM-based, hydrogen- stor ge materials were experimentally synthesized, and the Kubas-type interactions were measured to be stronger than the van der Waals interactions. In this review, the recent progress of Kubas-type hydrogen- storage materials will be discussed from both theoretical and experimental viewpoints.

Grain boundaries (GBs) in graphene can migrate when irradiated by electron beams from a transmission electron microscope (TEM). Here, we present an ab initio study on the atomic scale-mechanism for motion of GB with misorientation angle of ∼30° in graphene. From total energy calculations and energy barrier calculations, we find that a Stone–Wales (SW)-type transformation can occur more easily near GBs than in pristine graphene due to a reduced energy barrier of 7.23 eV; thus, this transformation is responsible for the motion of GBs. More interestingly, we find that a mismatch in the crystalline orientation at GBs can drive the evaporation of a carbon dimer by greatly reducing the corresponding overall energy barrier to 11.38 eV. After evaporation of the carbon dimer, the GBs can be stabilized through a series of SW-type transformations that result in GB motion. The GB motion induced by evaporation of the dimer is in excellent agreement with recent TEM experiments. Our findings elucidate the mechanism for the dynamics of GBs during TEM experiments and enhance the controllability of GBs in graphene.

Phonon softening of graphene under tensile strain is investigated based on ab initio density functional theory calculations. We demonstrate that, over a wide range of tensile strain directions, the strain-induced enhancement of phonon softening can give rise to phonon instabilities resulting in a mechanical failure of graphene. It is shown that there are two types of instabilities associated with phonons near K and Γ points, respectively, which induce symmetry-breaking structural distortions, and both of them lead to mechanical failure prior to the elastic failure commonly expected when the structural symmetry is retained.

A crack and its propagation is a challenging multiscale materials phenomenon of broad interest, from nanoscience to exogeology. Particularly in fracture mechanics, periodicities are of high scientific interest. However, a full understanding of this phenomenon across various physical scales is lacking. Here, we demonstrate periodic interlayer-mediated thin film crack propagation and discuss the governing conditions resulting in their periodicity as being universal. We show strong confinement of thin film cracks and arbitrary steering of their propagation by inserting a predefined thin interlayer, composed of either a polymer, metal, or even atomically thin graphene, between the substrate and the brittle thin film. The thin interlayer-mediated controllability arises from local modification of the effective mechanical properties of the crack medium. Numerical calculations incorporating basic fracture mechanics principles well model our experimental results. We believe that previous studies of periodic cracks in SiN films, self-de-bonding sol-gel films, and even drying colloidal films, along with this study, share the same physical origins but with differing physical boundary conditions. This finding provides a simple analogy for various periodic crack systems that exist in nature, not only for thin film cracks but also for cracks ranging in scale.

The Rashba effect is spin degeneracy lift originated from spin-orbit coupling under inversion symmetry breaking and has been intensively studied for spintronics applications. However, easily implementable methods and corresponding materials for directional controls of Rashba splitting are still lacking. Here, we propose organic-inorganic hybrid metal halide perovskites as 3D Rashba systems driven by bulk ferroelectricity. In these materials, it is shown that the helical direction of the angular momentum texture in the Rashba band can be controlled by external electric fields via ferroelectric switching. Our tight-binding analysis and first-principles calculations indicate that S = 1/2 and J = 1/2 Rashba bands directly coupled to ferroelectric polarization emerge at the valence and conduction band edges, respectively. The coexistence of two contrasting Rashba bands having different compositions of the spin and orbital angular momentum is a distinctive feature of these materials. With recent experimental evidence for the ferroelectric response, the halide perovskites will be, to our knowledge, the first practical realization of the ferroelectric-coupled Rashba effect, suggesting novel applications to spintronic devices.

We present a minimal but crucial microscopic theory for epitaxial graphene and graphene nanoribbons on the 4 H -SiC(0001) surface – prototypical materials to explore physical properties of graphene in a large scale. Coarse-grained model Hamiltonians are constructed based on the atomic and electronic structures of the systems from first-principles calculations. From the theory, we unambiguously uncover origins of several intriguing experimental observations such as broken-symmetry states around the Dirac points and new energy bands arising throughout the Brillouin zone, thereby establishing the role of substrates in modifying electronic properties of graphene. We also predict that armchair graphene nanoribbons on the surface have a single energy gap of 0.2 eV when their widths are over 15 nm, in sharp contrast to their usual family behavior.

Pore structure of carbide-derived carbon (CDC) was tunable by chlorination of Ti(C 1− x A x ) solid–solution carbides (A = O or N). High-energy ball milling method was used to synthesize various nanocrystalline Ti(C 1− x A x ) phases. We were able to obtain specific dimension of pore volumes in the range of micropore ( 0.7 O 0.3 ) CDCs were found 1.55 cm 3 /g, 1.72 cm 3 /g and 3100 m 2 /g, respectively. In contrast, Ti(C 0.5 N 0.5 ) CDCs showed enhancement of mesopore formation with 3.34 cm 3 /g, 3.45 cm 3 /g and 522 m 2 /g for mesopore volume density, total pore volume density and SSA, respectively.

A chlorine-assisted method for synthesizing porous boron nitride (BN) is developed. Large scale fabrication is possible by this method under moderate conditions. B and N sources, TiB2 and TiN, respectively, are reacted under a chlorine atmosphere to yield a porous turbostratic BN that displayed a higher surface area (960 m2 g−1) and a larger micropore volume (0.38 cm3 g−1) than have been reported by others. The products exhibit a high hydrogen uptake, as high as 1.01 wt% at 77 K and 1 bar. These results also indicate that the porous BN is a good candidate for testing as a catalyst support and gas sorption material.

Securing a semiconducting bandgap is essential for applying graphene layers in switching devices. Theoretical studies have suggested a created bulk bandgap in a graphene layer by introducing an asymmetry between the A and B sub-lattice sites. A recent transport measurement demonstrated the presence of a bandgap in a graphene layer where the asymmetry was introduced by placing a graphene layer on a hexagonal boron nitride (h-BN) substrate. Similar bandgap has been observed in graphene layers on metal substrates by local probe measurements; however, this phenomenon has not been observed in graphene layers on a near-insulating substrate. Here, we present bulk bandgap-like features in a graphene layer epitaxially grown on an h-BN substrate using scanning tunneling spectroscopy. We observed edge states at zigzag edges, edge resonances at armchair edges, and bandgap-like features in the bulk.

Here we show the formation of the complex between a DNA aptamer and a single-walled carbon nanotube (SWNT) and its reaction with its target protein. The aptamer, which is specifically bound with thrombin, the target protein in this study, easily wraps and disperses the SWNT by noncovalent π-π stacking.

In the two-dimensional electron gas (2DEG) emerging at the transition metal oxide surface and interface, it has been pointed out that the Rashba spin-orbit interaction, the momentum-dependent spin splitting due to broken inversion symmetry and atomic spin-orbit coupling, can have profound effects on electronic ordering in the spin, orbit, and charge channels, and may help give rise to exotic phenomena such as ferromagnetism-superconductivity coexistence and topological superconductivity. Although a large Rashba splitting is expected to improve experimental accessibility of such phenomena, it has not been understood how we can maximally enhance this splitting. Here, we present a promising route to realize significant Rashba-type band splitting using a thin film heterostructure. Based on first-principles methods and analytic model analyses, a tantalate monolayer on BaHfO$_3$ is shown to host two-dimensional bands originating from Ta $t_{2g}$ states with strong Rashba spin splittings - up to nearly 10% of the bandwidth - at both the band minima and saddle points due to the maximal breaking of the inversion symmetry. Such 2DEG band structure makes this oxide heterostructure a promising platform for realizing both a topological superconductor which hosts Majorana fermions and the electron correlation physics with strong spin-orbit coupling., Comment: 11 pages, 7 figures

This article briefly summarizes the research activities in the field of hydrogen storage in sorbent materials and reports our recent works and future directions for the design of such materials. Distinct features of sorption-based hydrogen storage methods are described compared with metal hydrides and complex chemical hydrides. We classify the studies of hydrogen sorbent materials in terms of two key technical issues: ( i ) constructing stable framework structures with high porosity, and ( ii ) increasing the binding affinity of hydrogen molecules to surfaces beyond the usual van der Waals interaction. The recent development of reticular chemistry is summarized as a means for addressing the first issue. Theoretical studies focus mainly on the second issue and can be grouped into three classes according to the underlying interaction mechanism: electrostatic interactions based on alkaline cations, Kubas interactions with open transition metals, and orbital interactions involving Ca and other nontransitional metals. Hierarchical computational methods to enable the theoretical predictions are explained, from ab initio studies to molecular dynamics simulations using force field parameters. We also discuss the actual delivery amount of stored hydrogen, which depends on the charging and discharging conditions. The usefulness and practical significance of the hydrogen spillover mechanism in increasing the storage capacity are presented as well.

We report first-principles calculations of atomic and electronic structures of epitaxial single-layer graphene on Si-terminated 4H-SiC(0 0 0 1) surface under homogeneous transverse electric fields. We find that atomic positions are insensitive to applied electric fields, but the electronic band structures of the graphene layer are shifted in energy, depending strongly on the applied electric fields, while those of the buffer layer are almost unchanged. This effect finally results in field-induced closing of the energy gap at the Dirac energy point and recovery of the conic feature of the low-energy band structures of free-standing graphene, which are verified and analyzed further with a tight-binding model consisting of the single-layer and the buffer-layer graphene only. The recovery of conical dispersion of the single-layer graphene and ambipolar field-effect behavior, despite the band-gap closure under electric field, makes epitaxial single-layer graphene one of the promising alternatives to current state-of-the-art transistors for radiofrequency applications.

We present a theoretical model capable of identifying multiple adsorption sites in a gas storage material with different binding energies and different amounts of adsorbed molecules. By applying this model to experimental data on the hydrogen storage in MOFs, we extract hydrogen adsorption properties of MOFs with coexisting strong and weak adsorption sites.

We perform ab initio density functional theory calculations to investigate the hydrogen storage capacity in the Fe-decorated, OH-functionalized isoreticular metal organic framework 16. The hydroxyl...

We developed a two-step doping strategy for chemical modification of covalent organic frameworks (COFs) as hydrogen storage materials. The first step was the boron (B) doping of organic building units, suppressing the clustering of subsequent metal dopants on frameworks. The second step was the doping of metal atoms, forming trapping centers for H2 molecules on B-doped COFs. Using ab initio calculations, we explored the doping processes and studied the dependence of the structural stability and electronic properties of doped building units on the B concentration. In organic building units of COF-1, two-B para substitution was energetically more favorable than other B substitutions. The clustering of Sc, Ti, and Ca was suppressed on such B-doped COF-1. Sc and Ti preferred the double-sided adsorption, while Ca only favored the single-sided adsorption, due to their different interactions with the substrates. Each Sc and Ti atom bound four H2 molecules through the Kubas interaction, while each Ca atom could a...

The feasibility to store hydrogen in calcium-decorated metal organic frameworks (MOFs) is explored by using first-principles electronic structure calculations. We show that substitution of boron atoms into the benzene ring of the MOF linker substantially enhances the Ca binding energy to the linker as well as the H 2 binding energy to Ca. The Kubas interaction between H 2 molecules and Ca added in the MOF gives rise to a large number of bound H 2 's (8H 2 's per linker) with the binding energy of 20 kJ/mol, which makes the system suitable for reversible hydrogen storage under ambient conditions.

A field effect transistor (FET) measurement of a SWNT shows a transition from a metallic one to a p-type semiconductor after helical wrapping of DNA. Water is found to be critical to activate this metal-semiconductor transition in the SWNT-ssDNA hybrid. Raman spectroscopy confirms the same change in electrical behavior. According to our ab initio calculations, a band gap can open up in a metallic SWNT with wrapped ssDNA in the presence of water molecules due to charge transfer., Comment: 13 pages, 6 figures

We investigate the electronic structures and electron emission properties of alkali-doped boron-nitride nanotubes (BNNTs) using density-functional theory calculations. We find that the nearly free-electron (NFE) state of the BNNT couples with the alkali atom states, giving rise to metallic states near the Fermi level. Unlike the cases of potassium-doped carbon nanotubes, not only the s but the d orbital state substantially takes part in the hybridization, and the resulting metallic states preserve the free-electron-like energy dispersion. Through first-principles electron dynamic simulations under applied fields, it is shown that the alkali-doped BNNT can generate an emission current 2 orders of magnitude larger than the carbon nanotube. The nodeless wave function at the Fermi level, together with the lowered work function, constitutes the major advantage of the alkali-doped BNNT in electron emission. We propose that the alkali-doped BNNT should be an excellent electron emitter in terms of the large emission current as well as its chemical and mechanical stability.

Saturated hydrocarbons, though relatively inert in nature, may become more reactive by functionalization. Using first-principles computational methods, we perform a systematic search for functional groups to be attached to saturated hydrocarbons for high-capacity storage of molecular hydrogen. When a saturated hydrocarbon such as propane is functionalized with OH, NH 2 , or SH, a transition metal atom may be strongly bound to each functional group. We find that hydrogen molecules are adsorbed on transition metal atoms with the binding energy suitable for reversible processes (adsorption and desorption) under practical hydrogen storage conditions. We also show that the functional groups may help reduce the tendency for transition metal clustering. The maximum gravimetric storage densities range from 8.7 wt% to 12.6 wt% depending on different materials. We explain the mechanism of multi-dihydrogen adsorption on a transition metal atom by generalizing the Kubas model.

Using first-principles density-functional electronic structure calculations, we propose functionalized organic molecules decorated with titanium atoms as high-capacity hydrogen storage materials. We study six kinds of functional groups which form complexes with Ti atoms and find that each complex is capable of binding up to six H2 molecules. Among such complexes, Ti-decorated ethane-1,2-diol can store H2’s with the maximum gravimetric density of 13 wt% and, under ambient conditions, a practically usable capacity of 5.5 wt%. We also present various forms of storage materials which are obtained by modifying well-known nanomaterials using Ti-functional group complexes.

We report from ab initio calculations that the band-gap sensitive side-wall functionalization of a carbon nanotube is feasible with the fluorine molecule (F2), which can provide a route to the extraction of semiconducting nanotubes by etching away metallic ones. In the small diameter cases like (11, 0) and (12, 0), the nanotubes are easily functionalized with F2 regardless of their electronic properties. As the diameter becomes larger, however, the fluorination is favoured on metallic CNTs with smaller activation barriers than those of semiconducting ones. Our results suggest that low-temperature exposure to F2 molecules in the gas phase can make a dominant portion of fluorinated metallic nanotubes and unfluorinated semiconducting ones. This is consistent with recent experimental reports.

Understanding of the electronic structure and the electrical transport properties on the nanoscale becomes increasingly important for the development of the next-generation nanodevices. We have developed a first-principles pseudopotential method to calculate the quantum conductance as well as the self-consistent charge distributions of nanostructures and studied the electronic structure and quantum conductance of carbon nanotubes with impurities or defects. Even if the carbon nanotube is metallic instead of semiconducting, boron and nitrogen atoms create acceptor-like and donor-like states which act as scattering centers for conducting electrons. Various defect geometries such as Stone–Wales defects are considered which give rise to interesting localized states and the corresponding conductance characteristics. These localized states are in resonance with the extended states of the metallic nanotube and form quasi-bound states with broadened energy levels leading to novel conductance behaviors. For semiconducting carbon nanotubes, it is shown that various defects located at the junction of two different tubes can produce both shallow and deep defect levels. Theoretical predictions are closely compared with recent scanning tunneling microscopy and scanning tunneling spectroscopy data.

We report results of our first-principles investigation on the energetics and electronic structures of bromine-adsorbed carbon nanotubes. While the bromine molecule binds preferentially to the outer wall of metallic nanotubes, the binding energy of adsorbed atomic bromines are found to depend on the radius as well as the energy gap. A recent experiment on the nanotube separation using bromines is discussed based on our computational data. The formation of strong C–Br chemical bonds at the zigzag edge of graphite demonstrates a close relationship between the density of states at the Fermi level and the binding strength. 2005 Elsevier B.V. All rights reserved.

Using a first-principles method, we study field emission properties of double-wall carbon nanotubes with various end geometries and combinations of inner and outer walls. The open double-wall nanotubes exhibit substantial contribution to the emission current from the extended π states. These states are insensitive to the detailed tip structure or the presence of foreign adsorbed molecules and the resulting emission current is expected to be stable under moderate vacuum environment. The mechanical stiffness also increases significantly compared to the single-wall nanotube, ensuring prolonged device lifetime under high-field operating conditions of the field emission display.

International audience; The results of the some practical applications of InAlN layers in a device hetrostructures grown by MOVPE is presented. It is shown that use of InAlN allows not only create high-quality DBRs and HEMT structures, but also effectively modify properties of InAlN/GaN/InGaN light-emitting devices.

A microscopic model for the resistance drift in the phase-change memory is proposed based on the first-principles results on the compressed amorphous Ge2Sb2Te5. First, it is shown that the residual pressure in the phase-change memory cell can be significant due to the density change accompanying the phase transformation. Our previous first-principles calculations showed that the energy gap is reduced and the density of localized in-gap states increases as the cell is pressurized. This indicates that the compressed amorphous Ge2Sb2Te5 is more conducting than those made under stress-free conditions. In addition, the crystallization dynamics was also accelerated under compressive stress. Based on these theoretical results, we propose a mechanism for the resistance drift in which the relaxation process in the amorphous Ge2Sb2Te5 corresponds to the growth of the crystalline nuclei inside the amorphous matrix, thereby lowering the internal stress. Our model can consistently explain several experimental observations such as the dependence of the drift exponent on the amorphous size.

We present an atomic resolution structural study of covalently bonded dopant pairs in the lattice of monolayer graphene. Two iron (Fe) metal atoms that are covalently bonded within the graphene lattice are observed and their interaction with each other is investigated. The two metal atom dopants can form small paired clusters of varied geometry within graphene vacancy defects. The two Fe atoms are created within a 10 nm diameter predefined location in graphene by manipulating a focused electron beam (80 kV) on the surface of graphene containing an intentionally deposited Fe precursor reservoir. Aberration-corrected transmission electron microscopy at 80 kV has been used to investigate the atomic structure and real time dynamics of Fe dimers embedded in graphene vacancies. Four different stable structures have been observed; two variants of an Fe dimer in a graphene trivacancy, an Fe dimer embedded in two adjacent monovacancies and an Fe dimer trapped by a quadvacancy. According to spin-sensitive DFT calculations, these dimer structures all possess magnetic moments of either 2.00 or 4.00 μB. The dimer structures were found to evolve from an initial single Fe atom dopant trapped in a graphene vacancy.

Silicene is a two-dimensional honeycomb lattice made of silicon atoms, which is considered to be a new Dirac fermion system. Based on first-principles calculations, we examine the possibility of the formation of solitonlike topological domain walls (DWs) in silicene. We show that the DWs between regions of distinct ground states of the buckled geometry should bind electrons when a uniform electric field is applied in the perpendicular direction to the sheet. The topological origin of the electron confinement is demonstrated based on numerical calculations of the valley-specific Hall conductivities, and possible experimental signatures of the quantum valley Hall effects are discussed using simulated scanning tunneling microscopy images. Our results strongly suggest that silicene could be an ideal host for the quantum valley Hall effect, thus providing a pathway to the valleytronics in silicon-based technology.

The effects of vacancies on mechanical properties of the transition metal carbides and nitrides are studied using the ab initio pseudopotential approach. Calculated shear elastic stiffness and electronic structures show that the vacancy produces entirely different effects on the mechanical strength of groups IVb nitrides and Vb carbides. It is found that the occupation of shear-unstable metallic dd bonding states changes essentially in an opposite way for the carbides and nitrides in the presence of vacancies, resulting in different responses to shear stress. Our study provides an atomistic understanding of the anomaly in hardness for these substoichiometric materials.

A low-temperature growth of carbon nanotubes (CNTs) on a sodalime glass substrate at 550°C was achieved by a thermal chemical vapor deposition (CVD) of acetylene gas. Acetylene gas was heated in the first zone of the reactor at 850°C and then brought into the second zone maintained at 550°C for the nanotube growth on the glass substrate. The sodalime glass did not exhibit thermal deformation or material degradation throughout the process. Electron emission sufficient for flat panel displays was observed with the practical range of the applied field, i.e., 1 mA/cm 2 at 5 V/μm. This development is an important progress in the integrated process for the mass production of glass-sealed CNT-based field emission displays (FEDs).

It has been known that single-strand DNA wraps around a single-walled carbon nanotube (SWNT) by pi-stacking. In this paper it is demonstrated that such DNA is dissociated from the SWNT by Watson-Crick base-pairing with a complementary sequence. Measurement of field effect transistor characteristics indicates a shift of the electrical properties as a result of this "unwrapping" event. We further confirm the suggested process through Raman spectroscopy and gel electrophoresis. Experimental results are verified in view of atomistic mechanisms with molecular dynamics simulations and binding energy analyses.

A family of crystal structures of carbon composed of alternating ${\mathrm{sp}}^{2}$ and ${\mathrm{sp}}^{3}$ bonds is investigated. Graphitic strips are connected by ${\mathrm{sp}}^{3}$ bonds to form an array of hexagonal pillars exhibiting a honeycomb lattice in the perpendicular plane. The electronic structure and elastic properties of this family of structures are calculated using an ab initio pseudopotential as well as the environment-dependent tight-binding method. Their electronic structure has a similar size dependence to zigzag nanotubes; they are metallic if twice the strip width is a multiple of three hexagonal units, and otherwise semiconducting with a wider range of the band gap than for carbon nanotubes. The structural stability is studied and compared with other carbon structures.