Single-Wall Carbon Nanotube Growth from Graphite Layers-a Tight Binding Molecular Dynamics Simulation

The growth of single-wall carbon nanotube from graphite layers is studied by tight binding molecular dynamics simulation. Given temperature of 2500 K or 3500 K and an interval of 0.25 nm for the two layers of graphite, a single-wall carbon nanotube with a zigzag shell will be produced. On the other conditions the carbon nanotube cannot grow or grows with too many defects. All carbon nanotube ends have pentagons which play an important role during the tube ends closing.

Density Functional Theory and Tight Binding-Based Dynamical Studies of Carbon Metal Systems of Relevance to Carbon Nanotube Growth

Density functional theory(DFT)and tight binding(TB)models have been used to study systems containing single-walled carbon nanotubes(SWNTs)and metal clusters that are of relevance to SWNT growth and regrowth.In particular,TB-based Monte Carlo(TBMC)simulations at 1000 or 1500 K show that Ni atoms that are initially on the surface of the SWNT or that are clustered near the SWNT end diffuse to the nanotube end so that virtually none of the Ni atoms are located inside the nanotube.This occurs,in part,due to the lowering of the Ni atom energies when they retract from the SWNT to the interior of the cluster.Aggregation of the atoms at the SWNT end does not change the chirality within the simulation time,which supports the application of SWNT regrowth(seeded growth)as a potential route for chirality-controlled SWNT production.DFT-based geometry optimisation and direct dynamics at 2000 K show that Cr and Mo atoms in Cr5Co50 and Mo5Co50 clusters prefer to be distributed in the interior of the clusters.Extension of these calculations should deepen our understanding of the role of the various alloy components in SWNT growth.

The Elastic Continuum Limit of the Tight Binding Model

The authors consider the simplest quantum mechanics model of solids, the tight binding model, and prove that in the continuum limit, the energy of tight binding model converges to that of the continuum elasticity model obtained using Cauchy-Born rule. The technique in this paper is based mainly on spectral perturbation theory for large matrices.

Nonorthogonal Tight-binding Study of the Geometries and Electronic Properties of Ge_n(n=2-20)Clusters

The universal parameter nonorthogonal tight binding scheme proposed by Menon and Subbaswamy was used to optimize the geometrical structures, binding energies and electron affinities of small germanium clusters Ge n ( n =2—20). A complete agreement with available ab initio results from the lowest energy structures for Ge 2—Ge 6 was obtained and reasonable structures for these clusters were predicted and compared with those of corresponding silicon clusters in the range of n =7—20 . The averaged discrepancy with experiments in binding energies for n =2—7 is about 6% and the calculated electron affinities agree well with the measured values in the range of n =2—8 as well.

TIGHT BINDING MOLECULA RDNAMICS SIMULATION FORH C_(60)COLLISION:APOSSIBILITY OF H@ C60

Collision between C60 — and Hatom areinvestigatedbytight binding molecular dynamicssimu lation . When Hatom with kineticenergy 5eVhitthecenter of a six membered ring of C60 — , or with kineticenergy 6ev hitsthecenterofa five memberedring of C60 , H@ C60 iscreated. Ifthekineticenergyislower,the Hatom staysoutside, and C60 is deformed bytheshock.

TIGHT BINDING STUDIES ON THE STRUCTURAL PROPERTIES OF SILICON CLUSTERS

Tight binding molecular dynamicsand simulated annealingtechniquesareemployed tostudythestructuralpropertiesofsilicon clusterscontaining 2 14 atoms.Itisfoundthatour results for Si2 Si6 agree well with thoseobtained using abinitiotechniques. Further Si cluster re search which givethesignificantprediction hasbeen made.

Programmable repulsive potential for tight-binding from Chen-Möbius inversion theorem

An accurate total energy calculation is essential in materials computation.To date,many tight-binding(TB)approaches based on parameterized hopping can produce electronic structures comparable to those obtained using first-principles calculations.However,TB approaches still have limited applicability for determining material properties derived from the total energy.That is,the predictive power of the TB total energy is impaired by an inaccurate evaluation of the repulsive energy.The complexity associated with the parametrization of TB repulsive potentials is the weak link in this evaluation.In this study,we propose a new method for obtaining the pairwise TB repulsive potential for crystalline materials by employing the Chen-Möbius inversion theorem.We show that the TB-based phonon dispersions,calculated using the resulting repulsive potential,compare well with those obtained by first-principles calculations for various systems,including covalent and ionic bulk materials and twodimensional materials.The present approach only requires the first-principles total energy and TB electronic band energy as input and does not involve any parameters.This striking feature enables us to generate repulsive potentials programmatically.

Improved double-gate armchair silicene nanoribbon field-effect-transistor at large transport bandgap

The electrical characteristics of a double-gate armchair silicene nanoribbon field-effect-transistor （DG ASiNR FET） are thoroughly investigated by using a ballistic quantum transport model based on non-equilibrium Green＇s function （NEGF） approach self-consistently coupled with a three-dimensional （3D） Poisson equation. We evaluate the influence of variation in uniaxial tensile strain, ribbon temperature and oxide thickness on the on-off current ratio, subthreshold swing, transconductance and the delay time of a 12-nm-length ultranarrow ASiNR FET. A novel two-parameter strain mag- nitude and temperature-dependent model is presented for designing an optimized device possessing balanced amelioration of all the electrical parameters. We demonstrate that employing HfO2 as the gate insulator can be a favorable choice and simultaneous use of it with proper combination of temperature and strain magnitude can achieve better device performance. Furthermore, a general model power （GMP） is derived which explicitly provides the electron effective mass as a function of the bandgap of a hydrogen passivated ASiNR under strain.

Inverse Stone-Thrower-Wales defect and transport properties of 9AGNR double-gate graphene nanoribbon FETs

Defect-based engineering of carbon nanostructures is becoming an important and powerful method to modify the electron transport properties in graphene nanoribbon FETs. In this paper, the impact of the position and symmetry of the ISTW defect on the performance of low dimensional 9AGNR double-gate graphene nanoribbon FET (DG-GNRFET) is investigated. Analyzing the transmission spectra, density of states and current-voltage characteristics shows that the defect effect on the electron transport is considerably varied depending on the positions and the orientations (the symmetric and asymmetric configuration) of the ISTW defect in the channel length. Based on the results, the asymmetric ISTW defect leads to a more controllability of the gate voltages over drain current, and drain current increases more than 5 times. The results have also con rmed the ISTW defect engineering potential on controlling the channel electrical current of DG-AGNR FET.

修正的非正交紧束缚总能模型在Si团簇中的应用

采用修正后的非正交紧束缚总能模型并结合遗传算法研究了半导体团簇Ｓｉ的基态能和基态构型，修正后的非正交紧束缚总能模型考虑了晶体场效应的影响，其结合能计算结果在实验结果的误差范围之内，通过它得出的基态构型与第一性原理的结果完全一致，这充分说明了修正后的非正交紧束缚总能模型的正确性。

Doping Effect of Poly(vinylidene fluoride)on Carbon Nanofibers Deduced by Thermoelectric Analysis of Their Melt Mixed Films

The effect of temperature on the electrical conductivity(σ)and Seebeck coefficient(S)of n-type vapor grown carbon nanofibers(CNFs)and poly(vinylidene fluoride)(PVDF)melt-mixed with 15 wt%of those CNFs is analyzed.At 40°C,the CNFs show stable n-type character(S=-4.8μV·K^(-1))with anσof ca.165 S·m^(-1),while the PVDF/CNF composite film shows anσof ca.9 S·m^(-1)and near-zero S(S=-0.5μV·K^(-1)).This experimental reduction in S is studied by the density functional tight binding(DFTB)method revealing a contact electron transfer from the CNFs to the PVDF in the interface.Moreover,in the temperature range from 40°C to 100°C,theσ(T)of the CNFs and PVDF/CNF film,successfully described by the 3D variable range hopping(VRH)model,is explained as consequence of a thermally activated backscattering mechanism.On the contrary,the S(T)from 40°C to 100°C of the PVDF/CNF film,which satisfactorily matches the model proposed for some multi-walled carbon nanotube(MWCNT)doped mats;however,it does not follow the increase in S(T)found for CNFs.All these findings are presented with the aim of discerning the role of these n-type vapor grown carbon nanofibers on theσand S of their melt-mixed polymer composites.

Band structure of monolayer of graphene, silicene and silicon-carbide including a lattice of empty or filled holes

We have developed a π-orbital tight-binding Hamiltonian model taking into account the nearest neighbors to study the effect of antidot lattices(two dimensional honeycomb lattice of atoms including holes) on the band structure of silicene and silicon carbide(SiC) sheets. We obtained that the band structure of the silicene antidot superlattice strongly depends on the size of embedded holes, and the band gap of the silicene antidot lattice increases by increasing of holes diameter. The band gap of SiC antidot lattice, except for the lattice of the small unit cell, is independent of the holes diameter and also depends on the distance between holes. We obtained that, the band gap of the SiC antidot lattice is the same as the band gap of the corresponding sheet without hole. Also, the electronic properties of the SiC antidot superlattice occupied either by carbon or by silicon atoms are investigated,numerically. Furthermore, we study the effect of occupation of graphene antidot by Si atoms and vice versa. Also,we have calculated the band structure of graphene and silicene antidot lattice filled by Si + C atoms. Finally, we compute the band structure of the SiC antidot lattice including the holes which are filled by C or by Si atoms.Really, in this paper we have generalized the method of paperabout graphene antidot with empty holes to the cases of filled holes by different atoms and also to the case of silicene and silicon carbide antidot lattices.

Surface brightens up Si quantum dots: direct bandgap-like size-tunable emission

Colloidal semiconductor quantum dots(QDs)constitute a perfect material for ink-jet printable large area displays,photovoltaics,light-emitting diode,bio-imaging luminescent markers and many other applications.For this purpose,efficient light emission/absorption and spectral tunability are necessary conditions.These are currently fulfilled by the direct bandgap materials.Si-QDs could offer the solution to major hurdles posed by these materials,namely,toxicity(e.g.,Cd-,Pb-or As-based QDs),scarcity(e.g.,QD with In,Se,Te)and/or instability.Here we show that by combining quantum confinement with dedicated surface engineering,the biggest drawback of Si—the indirect bandgap nature—can be overcome,and a‘direct bandgap’variety of Si-QDs is created.We demonstrate this transformation on chemically synthesized Si-QDs using state-of-the-art optical spectroscopy and theoretical modelling.The carbon surface termination gives rise to drastic modification in electron and hole wavefunctions and radiative transitions between the lowest excited states of electron and hole attain‘direct bandgap-like’(phonon-less)character.This results in efficient fast emission,tunable within the visible spectral range by QD size.These findings are fully justified within a tight-binding theoretical model.When the C surface termination is replaced by oxygen,the emission is converted into the well-known red luminescence,with microsecond decay and limited spectral tunability.In that way,the‘direct bandgap’Si-QDs convert into the‘traditional’indirect bandgap form,thoroughly investigated in the past.