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It is well known that the laser-induced dynamics of a system is determined by three laser parameters, namely, wavelength, pulse width, and intensity. In this study, the laser polarization is presented as a new parameter that determines the dynamics of highly anisotropic materials. By performing first-principles electron-ion dynamics simulations assuming a laser wavelength of 800 nm, a full width at half-maximum of 10 fs, and a field intensity of 4 V/A, carbon nanotubes (CNTs) were found to be fragile or robust when the polarization was parallel or perpendicular, respectively, to the tube axis. This behavior was observed for both zigzag and armchair CNTs and is thus expected to be only weakly dependent on CNT chirality. The obtained results are anticipated to encourage experimental efforts to select CNTs of a particular orientation using pulsed lasers with controlled polarization.

Despite the thermal instability of carbon nanotubes (CNTs) with diameters of less than 1 nm, first-principles simulations indicate the possibility of selecting such narrow CNTs using laser pulses. The simulations suggested the possibility of selecting CNTs narrower than 1 nm under pulsed laser irradiation with a full width at half-maximum of 10 fs, a wavelength of 800 nm, and a maximum field intensity ranging from 4.5 to 5 V/Å when the polarization vector was set perpendicular to the CNT axis. This result was common to both zigzag and armchair CNTs, suggesting that the preferential survival of narrow CNTs is independent of the chirality. The mechanisms underlying the preferential survival of narrower CNTs are discussed from analogous simulations of graphene nanoribbons under various polarization directions of the laser field, and the possibility of selecting CNTs with subnanometer diameters is evaluated on the basis of the simulation results.

Laser ablation is often simulated by the two-temperature model in which electrons are assumed to be thermalized by laser irradiation, while an explicit representation of interaction between laser-field and electrons is challenging but beneficial as being free from any adjustable parameters. Here, an ab initio method based on the time-dependent density functional theory (TDDFT) in which electron-ion dynamics under a laser field are numerically simulated is examined as a tool for simulating femtosecond laser processing of metals. Laser-induced volume expansion in surface normal directions of Cu(111) and Ni(111) surfaces are simulated by using repeating slab models. The amount of simulated volume expansion is compared between Cu(111) and Ni(111) slabs for the same laser pulse conditions, and the Ni slab is found to expand more than the Cu slab despite the smaller thermal expansion coefficient of Ni compared with Cu. The analyzed electronic excitation and lattice motion were compared to those in the two-temperature model. The threshold fluence to release surface Cu atom deduced from current TDDFT approach is found to be comparable to those of Cu ablation reported experimentally.

Recently, all-inorganic perovskite quantum dots have become a hot research topic due to their unique optical response. We have studied barium titanate quantum dots and their hybrid structure consis...

Twin graphene is a new two-dimensional semiconducting carbon allotrope which is proposed recently. The structural, magnetic and electronic properties are investigated for 3d transition metal (TM) atom adsorbed twin graphene by means of GGA+U calculations. The results show most of single 3d transition metal atom except Zn can make twin graphene magnetization. The adsorption of single TM atom can also make the twin graphene systems turn to half metal (V adsorption), half-semiconductor (Fe adsorption) or metal (Sc, Cr, Mn, Co and Cu adsorption). The semiconducting nature still exists for Ti, Ni and Zn adsorption. All the 3d TM adatoms belong to n-type doping for transferring charge to the neighboring C atoms and have strong covalent bond with these C atoms. The influence of Hubbard U value on half-metallic V adsorbed system is also considered. As the U increases, the system can gradually transform from metal to half metal and metal. The effect of the coverage is investigated for two TM atoms (Sc-Fe) adsorption, too. We can know TM atoms adsorbed twin graphene have potentials to be spintronic device and nanomagnets from the results.

Optical properties of oxidized single-wall carbon nanotubes (SWCNTs) have attracted much attention because of the greater luminescence quantum yield than that of pristine CNTs, showing the possibility of optical applications such as near-infrared single-photon emitters and bioimaging. To advance the fundamental understanding of the optical properties, we theoretically investigated the energetics and the optical transitions for complex oxygen (O) adsorption structures on (6,5) CNTs, including adsorption of two O atoms. For CNTs adsorbed by isolated O atoms, four groups of optical transition levels were obtained below E 11 . We also found two additional groups of optical transition levels for adsorption structures of two O atoms, whose related adsorption structures are more stable than the isolated O-adsorption structures. These results explain the multiple emission peaks and their irradiation time dependence in the photoluminescence (PL) spectra for CNTs oxidized by ultraviolet ozone, providing a possible mechanism for significant brightening of the emission whose wavelength range is ideal for biological imaging. Our results are also consistent with the PL measurements of SWCNTs oxidized by O 2 molecules during single chirality separation processes. This comprehensive understanding is essential for further applications of oxidized SWCNTs.

First-principles simulations were used to investigate water (H2O) decomposition induced by a femtosecond laser with high flux ∼1 × 1020 photons/(sec cm2 ). One goal of our research is to find metamaterials that locally enhance the laser field to reduce the threshold laser intensity required to decompose H2O molecules. In this work, small-diameter (6.3 Å) single-walled carbon nanotubes were found to reduce the threshold power by 90% compared with the power required to decompose H2O in the gas phase. The present results suggest a strategy for the design of materials with high energy efficiency for H2O decomposition based on polarizability and morphology (curvature) to enhance the local field. We demonstrate that carbon nanotubes enhance the local field resulting in a power enhancement of approximately eight times., X.C. acknowledges financial support from the National Natural Science Foundation of China (Grant No. 11774248). A.R. acknowledges financial support from the JSPS Fellowship program, the European Research Council (Grant No. QSpec-NewMat ERC-2015-AdG-694097), and Grupos Consolidados (Grant No. IT578-13).

Recent advances in ultrashort laser pulse techniques have opened up a wide variety of applications in both fundamental physics and industrial fields. In this work, $ab\phantom{\rule{4pt}{0ex}}initio$ molecular dynamics simulations based on time-dependent density functional theory revealed a steady deceleration of lattice distortion propagation in an aluminum slab with increasing laser pulse intensity. Analyzing the interatomic force revealed a significant reduction in the harmonic terms and nonmonotonic growth of anharmonicity. This is characterized by spatially nonuniform force screening by plasmon under a nonperturbative excitation condition that is not captured in Born--Oppenheimer molecular dynamics, and is consistent with the current interpretation of laser-induced periodic structure patterning. This work highlights the significance of collective electronic excitations for modeling the structure formation in such a system.

Using time-dependent density function theory (TDDFT), we have carried out a systematic study of collective excitations of the AB-stacked and AA-stacked few-layer graphene nanostructures with rectangle geometries. We found little effect of stacking sequence on the excitation properties of few-layer graphene nanostructures when the polarized direction of impulse excitation is in the armchair-edge or in the zigzag-edge directions. However, the effect of the stacking sequence becomes significant when the polarized direction was perpendicular to the layers. The absorption strength in the bilayer/trilayer system is slightly more than twice/three times as strong as that in the monolayer system. With the increase of the number of the layers, in the lower-energy resonance zone, absorption spectra are blue-shifted. The increasing strength of the optical absorption has also been found when the interlayer distance is increased followed by red shift in the supreme absorption peak. We found high energy resonances of πplasmons at 4–8 eV are localized in the boundary region showing dipole-like character. The current findings may pave a way for the potential applications in UV regions if there are available nanomechanical devices to tune stacking and interlayer distance.

In this study, first-principles time-dependent density functional theory calculations were used to demonstrate the possibility to modulate the amplitude of the optical electric field (E-field) near a semiconducting graphene nanoribbon. A significant enhancement of the optical E-field was observed 3.34 Å above the graphene nanoribbon sheet, with an amplitude modulation of approximately 100 fs, which corresponds to a frequency of 10 THz. In general, a six-fold E-field enhancement could be obtained, which means that the power of the obtained THz is about 36 times that of incident UV light. We suggest the use of semiconducting graphene nanoribbons for converting visible and UV light into a THz signal., HZ and XLC acknowledge financial support from the National Natural Science Foundation of China (Grant No. 11474207 and Grant No. 11374217). YM acknowledges the fund from MEXT, Grant-in-Aid for Scientific Research on Innovative Areas “Science of Atomic Layers (SATL)”, Japan, and the support from Research organization of Information Science and Technology (RIST) at Tokyo. AR acknowledges financial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG-267374), Spanish Grant (FIS2013-46159-C3-1-P), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT578-13) and COST Actions CM1204 (XLIC) and MP1306 (EUSpec).

We use time-dependent density functional theory to study laser-pulse induced decomposition of H2O molecules above the two-dimensional (2D) materials graphene, hexagonal boron nitride, and graphitic carbon nitride. We examine femtosecond-laser pulses with a full width at half maximum of 10 or 20 fs for laser-field intensity and wavelengths of 800 or 400 nm by varying the intensity of the laser field from 5 to 9 V/Å, with the corresponding range of fluence per pulse up to 10.7J/cm2. For a H2O molecule above the graphitic sheets, the threshold for laser-field H2O decomposition is reduced by more than 20% compared with that of an isolated H2O molecule. We also show that hole doping enhances the water adsorption energy above graphene. The present results indicate that the graphitic materials should support laser-induced chemistry and that other 2D materials that can enhance laser-induced H2O decomposition should be investigated., Y.M. acknowledges funding from JSPS KAKENHI Grants No. JP16H00925, No. JP16H04103, and No. JP16K05049. Y.M. also acknowledges support from the Research Organization of Information Science and Technology (RIST), Tokyo. H.Z. and X.C. acknowledge financial support from the National Key R&D Program of China 2017YFA0303603 and the National Natural Science Foundation of China (Grants No. 11474207 and No. 11374217). A.R. acknowledges financial support from the JSPS Fellowship program and from the European Research Council (QSpecNewMat ERC-2015-AdG-694097) and Grupos Consolidados (IT578-13).

Recently, the two-dimensional (2D) semiconductors arsenene and antimonene, with band gaps larger than 2.0 eV, have attracted tremendous interest, especially for potential applications in optoelectronic devices with a photoresponse in the blue and UV range. Motivated by this exciting discovery, types of highly stable wide-band-gap 2D nitride semiconductors were theoretically designed. We propose single-layer $1T\ensuremath{-}X{\mathrm{N}}_{2}$ ($X=\mathrm{S}$, Se, and Te) via first-principles simulations. We compute $1T\ensuremath{-}X{\mathrm{N}}_{2}$ ($X=\mathrm{S}$, Se, and Te) with indirect band gaps of 2.825, 2.351, and 2.336 eV, respectively. By applying biaxial strain, they are able to induce the transition from a wide-band-gap semiconductor to a metal, and the range of absorption spectra of $1T\ensuremath{-}X{\mathrm{N}}_{2}$ ($X=\mathrm{S}$, Se, and Te) obviously extend from the ultraviolet region to the blue-purple light region. With an underlying graphene, we find that $1T\ensuremath{-}X{\mathrm{N}}_{2}$ can completely shield the light absorption of graphene in the range of 1--1.6 eV. Our research paves the way for optoelectronic devices working under blue or UV light, and mechanical sensors based on these 2D crystals.

Compared with conventional metal plasmonic materials, surface plasmons in graphene are advantageous in terms of higher confinement, relative low loss, flexible featuring, and good tunability. However, the working frequencies of the pristine graphene (undoped graphene) surface plasmons are located in the terahertz and infrared regions, which limit their applications. Here we show high-frequency plasmons in nitrogen (N)-doped graphene nanostructures investigated by time-dependent density functional theory. We found the optical absorption strength of systems containing two layers to be more than twofold stronger than that of systems with monolayers. The optical absorption strength increases as the interlayer distance increases, and the absorption spectra are red-shifted for impulse excitations polarized in the armchair edge direction $(x$ axis). For microstructures of more than two layers, the optical absorption strength increases as number of layers of the N-doped graphene nanostructures increases. In addition, when the number of layers becomes elevated at low-energy resonances, the absorption spectra are seen to blue-shift. The plasmon energy resonance points are located in the visible and ultraviolet regions. The N-doped graphene provides an effective strategy for nanoscale plasmon devices in the visible and ultraviolet regions, despite their weaker absorption intensities when compared with the pristine graphene.

Graphene zigzag edges are known to show the spin polarized ferromagnetic states, which are well described by the mean field treatment of Hubbard model. The parameter of onsite Coulomb interaction U is estimated to be comparable to the kinetic hopping parameter t so as to fit the electronic band structures obtained by the spin–polarized density functional theory (DFT). In this paper, we propose a simple way to transfer the electronic band structures obtained by DFT onto the mean-field Hubbard Hamiltonian by adopting site-dependent U parameter, which is taken as the decaying function from the edge. This approach is applicable to both anti-ferromagnetic and ferromagnetic states between two edges of graphene nanoribbons and will serve to perform the further large-scale simulation of electro-magnetic transport properties of graphene-based nanodevices.

We performed ab initio calculations combined with the nudged elastic band method to study the diffusion of oxygen atoms on graphene. The most stable adsorption configuration is epoxy structure, and the diffusion barrier height for single epoxy oxygen is 0.9 eV. The calculated diffusion barrier of the second epoxy oxygen atom shows that the epoxy oxygen atoms tend to align along the armchair direction on a flat graphene surface. To estimate the curvature effect of a graphene sheet, we have compared above results with the diffusion of an oxygen atom on carbon nanotubes (CNTs). The curvature effect induces the stretching of the C–C bond beneath epoxy oxygen and reduces the internal strain due to the local curvature, which significantly lowers the total energy of the system. For the armchair CNT, the oxygen atoms would align along the tube axis (zigzag direction). These facts suggest that the favored direction of oxygen alignment can be changed by the curvature of graphene.

Laser-patterning techniques are typically based on local-heating phenomena, whereas recently developed short-pulse lasers operating on the order of femtoseconds may also induce non-thermal processes in materials. As it is difficult to study non-thermal processes experimentally, ab initio simulations are crucial for understanding laser-patterning phenomena under these conditions. In this study, real-time time-dependent density functional simulations were performed to examine the kinetic energies of the atoms in a slab of α-quartz under a laser fluence per pulse of 10 J/cm2, a wavelength of 800 nm, and full width at half-maximum (FWHM) values ranging from 10 fs to 100 fs. The simulation results suggested the occurrence of thermal/non-thermal crossover at an FWHM value of approximately 50 fs. Smaller FWHM values resulted in non-thermal lattice dynamics. Even under a lower laser fluence per pulse, a shorter pulse initiated non-thermal dynamics with a duration of several tens of femtoseconds.

Non-equilibrium dynamics induced by the femtosecond laser shot has attracted high attentions in condensed matter community. Interaction between a strong field of light and materials cannot be treated by conventional perturbation theory that assumes both electron wave functions and ionic positions as insensitive to the field. Direct approach for computing real-time propagations of electron wave packet and ionic positions under presence of strong optical field is one of promising solutions. In this review, I present theoretical approaches based on the time-dependent density functional theory and show some examples of ultra-fast dynamics in materials induced by the femtosecond laser shots: exfoliation of graphene layer from graphite and photochemical reaction of molecules encapsulated inside carbon nanotubes.

Ab-initio simulations to explore photo-induced dynamics in nanotubes are presented. By using the time-dependent density functional theory, simulations of electron-ion dynamics in nanotubes were performed with aid of supercomputers. From the simulation results, efficient methods of detection of structural defects and elimination of oxygen impurities from nanotubes are proposed. Furthermore, hot-carrier decay process and modulation of optical field in nanotubes are analyzed. In this review, the computational scheme and the simulation results are introduced.

We address the mechanism responsible for the chirality-selective destruction of carbon nanotubes upon oxidation. The reaction barriers for breaking the C–C bonds of carbon nanotubes are obtained based on density functional calculations. The barrier height is associated with the local-curvature radii along with the breaking C–C bonds rather than the diameters of nanotubes. Furthermore, hole doping can alter the order of the reaction barrier heights. These findings can be utilized for the chirality selection of nanotubes using oxidative purification.

The chirality-dependent combustion of single-walled carbon nanotubes (SWCNTs) during oxidation in both air and hydrogen peroxide was investigated using photoluminescence and Raman spectroscopy. Under air oxidation conditions, SWCNTs with a higher chiral angle and smaller diameter were observed to decompose more rapidly. The decomposition rate of each chiral index was determined from the reaction rate analysis, and it was found that the reaction is governed by a "local curvature radius" along the C-C bond. The reaction barriers for breaking the C-C bonds after cycloaddition with oxygen molecules were obtained for 10 types of SWCNTs using first-principles calculations. The barrier heights were found to depend on the local curvature radius, which showed good agreement with the experimental results. On the other hand, for the oxidation reaction in hydrogen peroxide, oxygen radicals decomposed the smaller-radius SWCNTs more rapidly without any chirality selection. As a result, two different chirality distributions for SWCNTs with similar diameters were obtained by these oxidation processes.

In this article, I describe a method to perform molecular dynamics simulation under electronic excitation within a framework of the time-dependent density functional theory and the Ehrenfest approximation. I present computational procedure and its application to carrier dynamics in carbon nanotubes. The difficulty in dealing with non-adiabatic transition within a scheme of the density functional theory is also discussed.

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.

Ab initio molecular orbital theory was used to examine the physisorption of a hydrogen molecule on planar and curved graphene clusters. The repulsive nature of a H2−H2 interaction and the small dependence of physisorption energies on physisorption sites favor a close-packed structure for molecular hydrogen physisorption on a planar graphene. It was also found that the physisorption energies are significantly increased on curved graphenes.

Chiral conductivity in nanotubes has recently been predicted theoretically. The realization and application of chiral conducting nanotubes can be of great interest from both fundamental and technological viewpoints. These chiral currents, if they are realized, can be detected by measuring the self-inductance. We have treated Maxwell's equations for chiral conducting nanotubes (nanocoils) and find that the self-inductance and the resistivity of nanocoils should depend on the frequency of the alternating current even when the capacitance of the nanocoils is not taken into account. This is in contrast to elementary treatment of ordinary coils. This fact is useful to distinguish nanocoils by electrical measurements.

The first GaS nanosheet-based photodetectors are demonstrated on both mechanically rigid and flexible substrates. Highly crystalline, exfoliated GaS nanosheets are promising for optoelectronics due to strong absorption in the UV-visible wavelength region. Photocurrent measurements of GaS nanosheet photodetectors made on SiO2/Si substrates and flexible polyethylene terephthalate (PET) substrates exhibit a photoresponsivity at 254 nm up to 4.2 AW(-1) and 19.2 AW(-1), respectively, which exceeds that of graphene, MoS2, or other 2D material-based devices. Additionally, the linear dynamic range of the devices on SiO2/Si and PET substrates are 97.7 dB and 78.73 dB, respectively. Both surpass that of currently exploited InGaAs photodetectors (66 dB). Theoretical modeling of the electronic structures indicates that the reduction of the effective mass at the valence band maximum (VBM) with decreasing sheet thickness enhances the carrier mobility of the GaS nanosheets, contributing to the high photocurrents. Double-peak VBMs are theoretically predicted for ultrathin GaS nanosheets (thickness less than five monolayers), which is found to promote photon absorption. These theoretical and experimental results show that GaS nanosheets are promising materials for high-performance photodetectors on both conventional silicon and flexible substrates.

In this report, numerical simulation on non-thermal dynamics in condensed matters conducted by femtosecond laser shot is presented. Electron–ion dynamics was treated within the framework of the time-dependent density functional theory for electrons coupled with classical molecular dynamics for ions. The formalisms and application of this simulation to photo-exfoliation of graphene from graphite surface and photo-disintegration of molecules inside a carbon nanotube are presented. Heavy tasks for memory access in this simulation scheme will also be mentioned.

By performing the first-principles simulation of electron-ion dynamics based on the time-dependent density-functional theory, we propose a way to produce graphene from graphene oxides by means of the laser-induced reduction without using chemical species. Epoxy and hydroxyl groups on graphene sheets can be completely removed upon irradiation with femtosecond laser without damaging the graphene sheet. By comparing the simulated results with different pulse shapes and intensities, optimum conditions of the femtosecond laser for reduction of graphene oxide were determined. The current works will be useful for further experimental researches.

Nanoscale molecular confinement substantially modifies the functionality and electronic properties of encapsulated molecules. Many works have approached this problem from the perspective of quantifying ground-state molecular changes, but little is known about the nonequilibrium dynamics of encapsulated molecular system. In this letter, we report an analysis of the nonequilibrium dynamics of acetylene (C2H2) inside a semiconducting carbon nanotube (CNT). An ultrashort high-intense laser pulse (2 fs width and 1015 W/cm2 intensity) brings the systems out of equilibrium. This process is modeled by comprehensive first-principles time-dependent density-functional simulations. When encapsulated, acetylene dimer, unlike a single acetylene molecule, exhibits correlated vibrational dynamics (C–C bond rotation and H–C–C bending) that is markedly different from the dynamics observed in the gas phase. This result highlights the role of CNT in modulating the optical electric field within the tube. At longer simulation timescales (> 20 fs) in the largest-diameter tube studied here [CNT(14,0)], we observe synchronized rotation about the C–C axes in the dimer and ultimately ejection of one of the four hydrogen atoms. Our results illustrate the richness of photochemical phenomena in confined geometries., Y.M. acknowledges the support of the Research Organization of Information Science and Technology at the Tokyo Office, and support by the Strategic Programs for Innovative Research, Ministry of Education, Cultural, Sports, Science, and Technology (Japan) and the Computational Materials Science Initiative (Japan). H.Z. is supported by the National Natural Science Foundation of China (under NSFC Grant 11074176) and the support from Research Fund for the Doctoral Program of Higher Education of China (Grant 20100181110080). A.R. acknowledges financial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG-Proposal 267374), the Spanish (FIS2011-65702-C02-01 and PIB2010US-00652 ), ACI Promociona (ACI2009-1036), Grupos Consolidados UPV/EHU del Gobierno Vasco (IT-319-07), and Ikerbasque foundation.

In this work, the first-principles computational scheme of electron-ion dynamics based on the time-dependent density functional theory is presented as a tool to study dynamical phenomena induced by light. Two applications of computations for photo-induced phenomena are shown. The one is structural change induced by intense and short laser shot with a purpose to simulate experiments using the femtosecond laser. The other is photo-excitation and subsequent carrier splitting into electrons and holes, which is a key process needed in photovoltaic materials.

We investigated the reactivity of ultra thin carbon nanotubes (UTCNTs) with diameter nm , which were synthesized in experiment. Tight-binding molecular dynamics (TBMD) simulations showed that isolated UTCNTs were thermally stable up to about 3000 K . By performing first principles calculations and TBMD simulations, we found that all pairs of UTCNTs coalesced into a single tube, without necessitating the presence of atomic defects.

By employing density-functional theory calculations we predict graphitic and tubular forms of silicon carbide (SiC) and proposes their synthesis. The present calculations suggest that the metastable SiC tubes can be synthesized by using a technique of extreme hole injection [J. M. Schon, Ch. Kloc, and B. Batlogg, Nature (London) 406, 702 (2000); 408, 549 (2000)]. This method is in contrast to the idea to synthesize new-tubular forms with a usage of a carbon nanotube as nucleation seed. Furthermore, the strain energies of SiC nanotubes are lower than those of the carbon nanotubes. The calculated band gaps of the investigated SiC tubes are either direct or indirect depending on the helicities. We expect that the SiC tubes with direct band gaps can be applied as nanoscale optoelectronics devices with strong oscillator strengths.

4 páginas, 5 figuras.-- PACS numbers: 82.37.Vb, 81.05.U-, 81.07.De, 82.50.-m, We show by first-principles simulations that ultrafast laser pulses induce different chemical reactions in a molecule trapped inside a nanotube. A strong laser pulse polarized perpendicular to the tube axis induces a giant bond stretch of an encapsulated HCl molecule in semiconducting carbon nanotube or in a BN nanotube. Depending on the initial orientation of the HCl molecule, the subsequent laser-induced dynamics is different: either complete disintegration or rebonding of the HCl molecule. Radial motion of the nanotube is always observed and a vacancy appears on the tube wall when the HCl is perpendicular to the tube axis. Those results are important to analyze confined nanochemistry and to manipulate molecules and nanostructures encapsulated in organic and inorganic nanotubes., Y.M. is supported by the Next Generation Supercomputing project conducted by MEXT Japan. H. Z. acknowledges financial support from the National Natural Science Foundation of China (NSFC, Grants No. 11074176 and No. 10676025), the NSAF (Grant No. 10976019), and the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20100181110080). A. R. acknowledges financial support from Spanish MEC (FIS2007-65702-C02-01), ACIPromociona (ACI2009-1036), ‘‘Grupos Consolidados UPV/EHU del Gobierno Vasco’’ (IT-319-07), the European Union through the FP7 e-I3 ETSF (Contract No. 211956), and THEMA (Contract No. 228539) projects.

We propose to use ultrashort laser pulses to detach intact graphene monolayers from a graphite surface, one at a time. As suggested by a combination of real-time ab initio time-dependent density functional calculations for electrons with molecular dynamics simulations for ions, this athermal exfoliation process follows exposure to femtosecond laser pulses with a wavelength of 800 nm and the full width at half maximum (FWHM) of 45 fs. Shorter pulses (FWHM=10 fs) with the same wavelength and intensity speed up the exfoliation and cause transient contraction in subsurface layers. Photoexfoliation should be capable of producing intact graphene monolayers free of contaminants and defects at a high rate.

Simulated scanning-tunneling-microscopy (STM) images of stable and metastable adsorption sites for the O atom on the Si(100) surface have been obtained by first-principles electronic-structure calculations within the local-density approximation. In the simulated STM images of both sites, the positions of O atoms are not seen directly, while the neighboring Si atoms are found to become bright spots. As for the metastable site, the bright spot originating from the surface Si atom moves when the polarity of the tip's voltage is reversed. This phenomenon is due to the electrostatic field of negatively charged O atoms

Using a transmission electron microscope and a nanomanipulator, we explored the early head-to-head coalescence of two capped carbon nanotubes (CNTs) under induction of electric current. We measured detaching forces for coalesced CNTs, showing discrete identifiable values attributable to van der Waals interaction, single $s{p}^{2}$-like bonds, and double $s{p}^{3}$-like bonds by comparing them with forces obtained using molecular dynamics simulations. Our results underscore the feasibility of atomically controlled junctions of CNTs tuned by the amount of the electrical current.

A density functional technique for calculating interactions between highly charged ions and solids is presented, and the application of this technique to the simulation of an ${\mathrm{Ar}}^{7+}$ ion passing through a graphene sheet is demonstrated. The simulation is initiated with electronic states obtained by superimposing the individual electronic states of an ion and a solid. When the ion starts to move, the real-time propagation of all electron wave functions is treated. Also presented is a technique for gradually accelerating an ion up to a targeted velocity in order to minimize electronic excitations in the ion even without interaction with a solid surface. These techniques are suitable for analyzing bombardment of solids by highly charged, high-speed ions.

By combining ab initio time-dependent density functional calculations for electrons with molecular dynamics simulations for ions in real time, we investigate the microscopic mechanism of collisions between energetic protons and graphitic carbon nanostructures. We identify not only the amount of energy lost by the projectile, but also the electronic and ionic degrees of freedom of the target that accommodate this energy as a function of the impact parameter and projectile energy. Our results establish validity limits for the Born-Oppenheimer approximation and the threshold energy for defect formation in carbon nanostructures.

Combining time-dependent density functional calculations for electrons with molecular dynamics simulations for ions, we investigate the dynamics of excited carriers in a (3,3) carbon nanotube at different temperatures. Following an hν=6.8 eV photoexcitation, the carrier decay is initially dominated by efficient coupling to electronic degrees of freedom. At room temperature, the excitation gap is reduced to nearly half its initial value after ∼230 fs, where coupling to ionic motion starts dominating the decay. We show that the onset point and damping rate in the phonon regime change with initial ion velocities, a manifestation of temperature-dependent coupling between electronic and ionic degrees of freedom., A. R. was supported by the EC projects NANOQUANTA (No. NMP4-CT-2004-500198), SANES (No. NMP4-CT-2006-017310), Spanish MCyT, and the Humboldt Foundation. D. T. was supported by NSF NIRT Grants No. DMR-0103587, No. ECS-0506309, NSF NSEC Grant No. EEC-425826, and the Humboldt Foundation.

Graphite is the most stable form of carbon under room temperature and atmospheric pressure, and consists of two-dimensional honeycomb lattices with intralayer $s{p}^{2}$ bonding and rather weak van der Waals like interlayer interaction. When we supply gaseous small carbonic molecules such as methane to a patch of graphene, the patch will grow into graphite. Now, let us imagine a slightly different situation. Is a layered structure of graphite always formed, when we supply not methane molecules but another graphene patch? The answer from our computer simulations is ``No.'' Some graphene patches collide in parallel, but others at right angles, which result in a formation of junction structures (graphitic Y junctions). These junction structures are different from those of common sense for graphitic materials. Performing density functional calculations, we found that the reaction barrier height required for the formation of graphitic Y junctions are almost zero, and the binding energies per bond for each structure are $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Furthermore, tight-binding molecular dynamics simulations showed high thermal stability and high formation probabilities for these junction structures. As applications of graphitic Y junctions, we will present two interesting structures, where we focus on the magnetic properties of junction structures and nanotube T-junction structures which are different from conventional models. We expect that graphitic Y junctions might be hidden in graphitic soot and not characterized yet in experiment. 3D architectures constructed from those unit structures are expected to have various applications with lightweight, ferromagnet, high molecular storage and high thermal conductor.

We propose to use photodesorption as a noninvasive tool to clean carbon nanotubes from oxygen, offering a clear advantage over thermal and chemical treatments. The detachment of chemisorbed oxygen from atomic vacancies is triggered by a resonant Auger process, initiated by an O1s→O2p transition, which leaves two holes in the O2s level. In the electronically excited state, oxygen desorbs spontaneously, with no damage to the carbon network or the cylindrical nanotube shape. Subsequent reaction of oxygen atoms with H2 molecules is shown to prevent reoxidation of the nanotube., A.R. acknowledges support from the European Community 6th framework Network of Excellence NANOQUANTA (NMP4-CT-2004-500198) Spanish MCyT and the University of the Basque Country. D.T. was partly supported by the NSF-NIRT Grant No. DMR-0103587.

We combined resonant photoabsorption and vibration spectroscopy with scanning tunneling microscopy (STM) to unambiguously identify the presence of Stone-Wales (SW) defects in carbon and boron nitride nanotubes. Based on extensive time-dependent ab initio density functional calculations, we propose to resonantly photoexcite SW defects in the infrared and ultraviolet regime as a means of their identification. Onset of nonradiative decay to a local defect vibration with a frequency of 1962 cm-1 serves as a fingerprint of such defects in carbon nanotubes. The bias dependence of the STM images shows distinct features associated with the presence of SW defects., Y.M. was supported by NAREGI Nanoscience Project, Ministry of Education, Culture, Sports, Science and Technology, Japan. A.R. acknowledges support from the EC grants (HPRN-CT-2000-00128 and HPRN-CT-2000-00167) and Spain MCyT. M.Y. and D.T. acknowledge partial support by NSF-NIRT grant DMR- 0103587.

We extend the time-dependent density functional formalism to study the microscopic response of defective nanotubes to electronic excitations. We find the lifetime of electronic excitations in these nanostructures to be several orders of magnitude longer than in solids, necessitating the use of excited-state molecular dynamics to correctly describe the atomic motion. We find that electronically excited nanotubes with monatomic vacancies show an unexpected self-healing ability, which is intimately linked to their nanometer size. © 2004 Elsevier B.V. All rights reserved., This work was performed under the management of the Frontier Carbon Technology supported by NEDO. D.T. and M.Y. acknowledge partial support by NSF-NIRT grant DMR-0103587. A.R. was supported by the European Community research and training network COMELCAN (HPRN-CT-2000- 00128), MCyT of Spain Grant MAT2001-09, and Basque Country University.

One of challenging application of carbon nanotubes is nano-scaled electronic device, in which precise control of defects and carriers is required in analogy of silicon-based technology. In this work, we show that optical excitations can be promising tools to analyze and control defects in nanotubes being alternative to conventional heattreatments. We performed ab initio calculations, which solve the time-dependent Schrodinger equations for electrons on real-time axis as well as classical Newton’s equations of motions for ions. This method is also useful to investigate carrier dynamics under finite temperatures. These works were done under collaboration with Prof. Angel Rubio, Prof. David Tomanek, Dr. Savas Berber, and Miss Mina Yoon.

Presentado al "Tsukuba Symposium on Carbon Nanotube" in commemoration of the 10th Anniversary of its discovery., Stability test of nanotubes with presence of single vacancies has been performed by means of tight-binding molecular dynamics and electron–ion dynamics within the framework of the density functional theory. A4Â diameter nanotube having a single vacancy with three dangling bonds has been found to retain its cylindrical shape under high temperature around 4000K, despite its large internal strain energy. Meanwhile, an electronic excitation of vacancy-related state has shown considerable atomic displacement, which may cause extraordinary large lattice vibration or nanotube decay. Furthermore, the single vacancy can stabilize itself by making carbon dimer to remain with only one dangling bond. Narrower nanotubes tend to prefer this self-stabilization and thus could be tolerant to the presence of defects

Using density functional theory, we calculate the charge redistribution incurred upon forming multiwall carbon nanotubes, or by sandwiching initially isolated single-wall nanotubes between graphene layers. In these systems, we observe a significant charge transfer between the \ensuremath{\pi} electron system of the tube walls and a newly formed interlayer state. We discuss the direction of charge flow in terms of the interlayer hybridization and work function differences in the composite systems.

We investigated the coalescence of ultrathin carbon nanotubes (UTCNTs) using the tight-binding molecular dynamics simulation technique. We have found that two UTCNTs having the same or different chirality can coalesce without initially introducing atomic defects to enhance the reaction. The chiral index of the coalesced nanotube was found to be given as a vector sum of the indices of the original nanotubes, which can be explained geometrically in terms of the developments of the nanotubes. The results clearly show that the chirality can be changed through chemical reaction, which might suggest a possibility of chirality control.