Adsorption on the carbon nanotubes

Adsorption on single walled carbon nanotubes (SWCNTs) is a subject of growing experimental and theoretical interest. The possible adsorbed patterns of atoms and molecules on the single-walled carbon nanotubes vary with the diameters and chirality of the tubes due to the confinement. The curvature of the carbon nanotube surface enlarges the distance of the adsorbate atoms and thus enhances the stability of high coverage structures of adsorbate. There exist two novel high-coverage stable structures of potassium adsorbed on SWCNTs, which are not stable on graphite. The electronic properties of SWCNTs can be modified by adsorbate atoms and metal-semiconductor and semiconductor-semi-conductor transitions can be achieved by the doping of alkali atoms.     

Optimization of the calculations of the electronic structure of carbon nanotubes

A method is proposed for calculating the electronic structure and physical properties (in particular, Young’s modulus) of nanotubes, including single-walled carbon nanotubes. This method explicitly accounts for the periodic boundary conditions for the geometric structure of nanotubes and makes it possible to decrease considerably (by a factor of 10–10³) the time needed to calculate the electronic structure with minimum error. In essence, the proposed method consists in changing the geometry of the structure by partitioning nanotubes into sectors with the introduction of the appropriate boundary conditions. As a result, it becomes possible to reduce substantially the size of the unit cell of the nanotube in two dimensions, so that the number of atoms in a new unit cell of the modified nanotube is smaller than the number of atoms in the initial unit cell by a factor equal to an integral number. A decrease in the unit cell size and the corresponding decrease in the number of atoms provide a means for drastically reducing the computational time, which, in turn, substantially decreases with an increase in the degree of partition, especially for nanotubes with large diameters. The results of the calculations performed for carbon and non-carbon (boron nitride) nanotubes demonstrate that the electronic structures, densities of states, and Young’s moduli determined within the proposed approach differ insignificantly from those obtained by conventional computational methods.     

氮掺杂手性碳纳米管的电子结构和输运特性的理论研究

运用第一性原理的密度泛函理论,结合非平衡格林函数,研究了氮原子取代掺杂手性单壁(6,3)碳纳米管的电子结构和输运特性.计算结果表明:不同构形和不同数目的氮原子取代掺杂对手性碳管的输运性质有很复杂的影响.研究发现,氮原子掺杂明显改变了碳管的电子结构,使金属型手性碳管的输运性能降低,电流-电压曲线呈非线性变化,而且输运性能随着杂质原子间间距的变化而发生显著改变.在一定条件下,金属型碳管向半导体型转变. 关键词： 手性单壁碳纳米管 氮掺杂 电子结构 输运性能     

Prismatic modifications of single-walled carbon nanotubes and their electronic properties: Regular adsorption of fluorine atoms on graphene surfaces of nanotubes

The regular adsorption of fluorine atoms on surfaces of single-walled carbon nanotubes along their axes can lead to a modification of cylindrical carbon cores of these single-walled carbon nanotubes to carbon cores that have a nearly prismatic shape (prismatic modification). In faces of these modified single-walled carbon nanotubes, there can arise quasi-one-dimensional isolated carbon conjugated subsystems (tracks) with different structures. It has been established that the main characteristics of the single-walled carbon nanotubes thus modified are rather close to the corresponding characteristics of the related isostructural polymer conjugated systems (such as cis-polyenes, polyphenylenes, poly(periacenes), or polyphenantrenes). Fragments of model nanotubes of the (n, n) and (n, 0) types that contain up to 360 carbon atoms and their derivatives doped with fluorine atoms have been calculated using the semiempirical parametric method 3.     

Tamm states and quantum dots in carbon and heteroatomic nanotubes

The electronic structure of C-BN nanotubes is discussed in the π approximation. Two types of such structures with (n,0)-tubulet topology are investigated: 1) semiinfinite C-BN and C nanotubes and 2) C-BN nanotubes, consisting of two semiinfinite BN nanotubes coupled by a ring-shaped carbon fragment C_mn. It is shown that, in the first case, energy levels (Tamm levels) whose wave functions are localized on the terminal fragment can exist under certain conditions. In the second case, bound states localized on atoms of the carbon fragment exist. It is established that if a quite extended, cylindrical, carbon cluster is present at the end of a semiinfinite BN nanotube, then such a system can be viewed as a very simple model of a quantum dot. C-BN nanotubes where the carbon fragment couples two semiinfinite BN nanotubes can also be interpreted similarly. A simple analytic method is proposed for finding the Tamm energy levels in heteroatomic nanotubes. Fiz. Tverd. Tela (St. Petersburg) 41, 1515–1519 (August 1999)     

Effect of hydrogenation on the spectra of electronic and vibrational transitions in single-walled carbon nanotubes

Single-walled carbon nanotubes containing 5.4 wt% H are prepared under a hydrogen pressure of 50 kbar at the temperature T = 500°C. Analysis of the optical transmission spectra has revealed that the hydrogenation of single-walled carbon nanotubes brings about suppression of high-frequency conduction provided by free charge carriers in the nanotubes, the disappearance of interband electronic transitions, and the appearance of an absorption line at 2845 cm^?1 corresponding to stretching vibrations of the C-H bonds. The removal of hydrogen from hydrogenated single-walled carbon nanotubes owing to vacuum annealing at a temperature of 500°C is accompanied by a linear decrease in the intensity of this line as the hydrogen content in the system decreases. This phenomenon indicates that the greater part of the hydrogen atoms in single-walled carbon nanotubes are covalently bonded to the carbon atoms.     

Molecular dynamics study of the destruction of single-walled carbon nanotubes under tension

The deformation and destruction of single-walled open carbon nanotubes are investigated in the framework of the molecular dynamics model of charges localized at bonds, which takes into account the atomic (ionic) and electronic degrees of freedom. This approach allows one to study excited electronic states induced by electronic transitions and to investigate both the ionic and electronic subsystems simultaneously. The structural transformations of nanotubes with (10, 2) chirality under tension are investigated, and the tensile stress-strain diagrams of these nanotubes in the temperature range 100–1000 K are calculated. It is established that, at low and moderate temperatures, the destruction of the nanotubes has a similar nature: the interatomic covalent bonds are broken in the normal cross section of the nanotube. At high temperatures, the nanotube undergoes a scission along the stretching axis.     

Electronic structures and magnetic properties of rare-earth-atom-doped BNNTs

Stable geometries, electronic structures, and magnetic properties of (8,0) and (4,4) single-walled BN nanotubes (BNNTs) doped with rare-earth (RE) atoms are investigated using the first-principles pseudopotential plane wave method with density functional theory (DFT). The results show that these RE atoms can be effectively doped in BNNTs with favorable energies. Because of the curvature effect, the values of binding energy for RE-atom–doped (4,4) BNNTs are larger than those of the same atoms on (8,0) BNNTs. Electron transfer between RE-5d, 6s, and B-2p, N-2p orbitals was also observed. Furthermore, electronic structures and magnetic properties of BNNTs can be modified by such doping. The results show that the adsorption of Ce, Pm, Sm, and Eu atoms can induce magnetization, while no magnetism is observed when BNNTs are doped with La. These results are useful for spintronics applications and for developing magnetic nanostructures.     

扶手椅型碳化硅纳米管电子结构和光学性质的研究

本文基于密度泛函理论研究了扶手椅型碳化硅纳米管(SiCNT)的电子结构、成键机制以及其光学性质。研究结果表明:当碳和硅的原子比为1∶1时,SiCNT的结构最为稳定,并且表现出诸多的优良性质。通过分析计算结果我们发现,SiCNT是间接带隙材料,并且管子的带隙随着直径的增加而增加。从SiCNT的轨道图谱中我们看到碳和硅原子之间属于sp2杂化,同时硅原子周围的电子密度明显要低于碳原子周围的电子密度。对能态密度的计算我们得知碳原子和硅原子分别主导价带和导带。与其它纳米管(BN)有所不同,SiCNT的光学性质更接近于各向同性材料。     

Anti-prismatic modifications of single-walled carbon nanotubes and their electronic properties: Regular adsorption of fluorine atoms on graphene surfaces of nanotubes

The regular adsorption of fluorine atoms on the surfaces of single-walled carbon nanotubes along their cylindrical axes leads to a modification of cylindrical carbon skeletons of these single-walled carbon nanotubes into carbon skeletons that have a nearly “anti-prismatic” shape (anti-prismatic modifications). In the faces of these modified single-walled carbon nanotubes, there can arise quasi-one-dimensional isolated carbon conjugated subsystems (tracks) with different structures. Model fragments of nanotubes of the (n, 0) type that contain up to 360 carbon atoms and their derivatives with regularly adsorbed fluorine atoms on the graphene surface have been calculated using the semiempirical PM3 method. It has been found that the main properties of the single-walled carbon nanotubes modified by the above method are determined by the character of the conjugation of the electrons in isolated carbon tracks, which is close to the character of the conjugation of the electrons in the initial single-walled carbon nanotubes.     

Structure of endohedral complexes of carbon nanotubes encapsulated with lithium and sodium

The article provides the results of ab initio calculations employing density functional theory of carbon nanotubes that contain clusters of lithium and sodium atoms. Stable positions of interstitial atoms, the electron density distribution in the system and the density of electronic states are determined. It is shown that the amount of charge transferred from the interstitial atoms in a cluster significantly differs from the corresponding value for a single atom. It is established that the density of electronic states of the system at low concentrations of atoms of the introduced element is determined by the electronic structure of a hollow nanotube, and as the concentration of interstitial atoms increases, this quantity becomes virtually independent on the type of alkali metal (lithium or sodium) and the initial type of the nanotube conductivity.     

Far-infrared to visible optical conductivity of single-wall carbon nanotubes

The reflectance of unoriented single-wall carbon nanotube films has been measured over a wide wavelength range (far-IR–UV). The results are consistent with the film being a mixture of conducting (armchair), small bandgap (n≡m, mod 3) and semiconducting nanotubes. The optical conductivity shows peaks corresponding to transitions between density-of-states peaks of these tubes, at energy locations consistent with 1.4 nm diameter tubes. In addition optical absorption spectroscopy of aligned single-wall carbon nanotubes shows that the optical transitions are well-aligned along the tube axis. This behavior is consistent with polarized resonant Raman and electronic structure calculations.     

单向压力对碳纳米管(6, 6)晶体电子结构的影响

通过第一性原理计算研究了垂直于碳纳米管轴向的单向压力对碳纳米管(6,6)晶体电子结构特性的影响.计算研究发现：由碳纳米管(6,6)组成的四方结构晶体(t相)具有金属特性,电子可以沿碳纳米管管壁运动;在单向压力作用下,t相发生结构相变形成非成键相,随着压力的进一步增大,碳纳米管间产生键合,形成了成键相;单向压力对碳纳米管(6,6)晶体的能带结构影响主要表现在π能带和π^*能带,伴随着单向压力的增加,碳纳米管晶体的电学性质经历从金属到半导体再到活泼金属的转变;非成键相的电子被局域在碳纳米管附近使晶体具有半导体特性,而成键相的电子不仅可以沿着碳纳米管管壁运动,还可以在碳纳米管之间(即成键方向)运动,从而使成健相晶体具有活泼的金属特性. 关键词： 碳纳米管晶体 第一性原理计算 金属—半导体转变     

Carbon systems of polymerized nanotubes: Crystal and electronic structures

Molecular dynamic calculations are carried out for the (P, T) phase diagram of a covalent compound of cross-linked carbon single-wall nanotubes (SWNT) and for the structures and electronic spectra of the novel crystals of polymerized carbon nanotubes. It is shown that the transformation of covalently bonded nanotubes in a close-packed conducting structure cardinally modifies their electronic properties. The P-SWNT crystal becomes semiconducting and, upon complete transformation of sp ²-hybridized carbon atoms into sp ³-hybridized ones, it becomes an insulator.     

Two-qubit cells made of boron nitride nanotubes for a quantum computer

The electronic structure of boron nitride nanotubes (8, 0) with intercalated alkali metal atoms and alkaline-earth metal ions is studied. It is shown by calculation that the spin density is localized on individual atoms or ions. The antiferromagnetic state of a linear chain of atoms and ions turns out to be energetically more favorable. Exchange interaction between spins is fairly weak. Such systems are suggested to be used as two-qubit cells for a quantum computer.     

Electronic structure and magnetism of Fe-doped SiC nanotubes

The electronic structure and magnetic properties of Fe-doped SiC nanotubes are investigated by using the first-principles method based on density functional theory(DFT) in the local spin density approximation(LSDA).The calculation results indicate that the SiC nanotube of Fe substitution for C exhibits antiferromagnetism while ferromagnetism features prominently when Fe substitutes Si.This is a kind of half-metal magnetic material.The formation energy calculation results show that the formation energy of ferromagnetic structure is 3.2 eV lower than that of antiferromagnetic structure.Fe atoms are more likely to replace Si atoms.Spin-orbit coupling induces electron spin polarization in the ground state.Also,the doping Fe atoms make relaxation towards the outside of the tube to some extent and larger geometric distortion occurs when Fe substitutes C,but the whole geometric structure of SiC nanotubes is not damaged due to the doping.It is revealed in the calculation of energy band structure and density of states that more dispersed distribution of energy levels is produced near the Fermi level.For Fe substitution for Si,obviously there are spin-split and intense p-d hybrid effects by Si 3p electron spins and Fe 3d electron spins localized at the exchanging interactions between magnetic transitional metal(TM) impurities.Spin electronic density results indicate that system magnetic moments are mainly generated by the unpaired 3d electrons of Fe atoms.All these results show that the transition metal doping SiC nanotube could be a potential route to fabricating the promising magnetic materials.     

Electron-energy characteristics of double-walled carbon nanotubes doped with alkali metal atoms

A computational scheme based on density functional theory adapted for systems with translational symmetry is applied to calculating the electronic structure and energy spectrum of double-walled carbon nanotubes doped with alkali metal atoms (Li, Na, and K). The specific features of the electron-energy characteristics, including the potential curves for the interaction of nanotubes with “guest” atoms moving in the radial direction or along the circumference of the tube, are established. It is shown that doped double-walled nanotubes have an increased electron emissivity.     

Effects of Cs treatment on field emission properties of capped carbon nanotubes

Within the methodology [M. Khazaei, A.A. Farajian, Y. Kawazoe, Phys. Rev. Lett. 95 (2005) 177602] based on first-principles electronic structure calculations, the effects of Cs treatment on current emissions and emission patterns of capped carbon nanotubes are considered at low deposition densities when the nanotubes are under an electric field 0.2 V/Å. The results show that the current emission from the cap with one adsorbed Cs is 3.4 times larger than the cap without any Cs. It is 9.6 times larger in the cap with two adsorbed Cs atoms. After Cs deposition the emission patterns become asymmetric (current emission from the carbon atoms located at the topmost pentagon ring close to Cs atoms is larger than the other atomic sites). There are very few localized states on Cs atoms. Hence, although the tunneling probability of electron emission from Cs atoms is significant, there is no current from Cs atoms. In addition, the effect of Cs on work function reduction of nanotubes is explained in terms of Cs deposition densities and the surface dipole moments.     

Density and thermodynamics of hydrogen adsorbed inside narrow carbon nanotubes

A model is proposed for calculating the thermodynamic functions and the equilibrium density of a one-dimensional chain of molecules (atoms) adsorbed inside a narrow nanotube. The model considers both the interaction between introduced atoms (molecules) and their interaction with the nanotube walls. The quantum-mechanical effects resulting in discrete energy levels of a particle and in its smeared position between neighbors are taken into account. In calculating the free energy at a nonzero temperature, the phonon contribution and the particle transitions to excited levels are considered. The model is applied to calculate the thermodynamic parameters of adsorbed hydrogen molecules inside extremely narrow single-wall carbon nanotubes of the (3,3) and (6,0) type. It is shown that external pressure gives rise to a sequence of first-order phase transitions, which change the density of adsorbed hydrogen molecules.     

Interactions of Fe atom with single wall armchair SiC nanotubes: an ab initio study

A systematic study of Fe atom encapsulation and adsorption in armchair SiC nanotubes (SiCNT) with diameters in the range of 5.313 to 10.582 Å has been performed using hybrid density functional theory and a finite cluster approximation. A detailed comparison of the binding energies, equilibrium positions, Mulliken charges, and spin magnetic moments of Fe atoms has been performed for three types of nanotubes. The electronic states, HOMO–LUMO gaps, and changes in gaps with respect to the bare nanotube gaps have been investigated as well. Our results show that the properties of SiCNT can be modified by Fe atom encapsulation and adsorption. Binding energies of the encapsulated and adsorbed systems indicate that these structures are stable and show site dependence. For both cases a significant band gap decrease is observed for type 1 nanotubes enabling band gap tailoring. This decrease is not observed for the other two types with a larger diameter. All structures are found to have magnetic ground states with high magnetic moments indicating the possibility of them being used in spintronics applications.