扶手椅型单壁碳纳米管生长机理的理论研究

用Gaussian03程序中的AM1方法对扶手椅型单壁碳纳米管的生长机理进行了研究. 结果表明, 若碳纳米管生长的碳源是C2自由基, 则有一条反应途径可能是：C2自由基首先与碳纳米管的开口端形成一个中间体, 然后该中间体经过一个过渡状态, 形成产物；从(3, 3), (4, 4), (5, 5)到(6, 6), 其生长反应的活化能逐渐降低. 同时 研究发现, 活化能的高低与碳纳米管共轭程度的大小有关, 碳纳米管的共轭程度越大, 活化能越低；在靠近新形成的六元环的两侧, 碳纳米管可能优先继续生长.     

Tunable rectification and negative differential resistance induced by asymmetric doping in phosphorene nanoribbon

By using first-principles calculations based on density functional theory and non-equilibrium Green's function, we present the electronic transport properties of two kinds of devices based on armchair phosphorene nanoribbons, namely, A device, and B device. In A device, the phosphorus atoms in the center of armchair phosphorene nanoribbon have been replaced by impurity atoms of the S and Si, whereas in the B device, the impurity atoms are at the edge of ribbon. The results show that the current–voltage characteristics for both devices have striking nonlinear features and the rectifying behaviors strongly depend on the positions of impurity atoms. The highest rectification ratio is obtained about 125992 at 0.8 V bias for B device. Moreover, only for A device, robust negative differential resistance is observed with a high peak–valley ratio 27500 in the bias range $[?0.2,?0.6]\text{ V}$. The mechanism of the rectification behavior is analyzed in terms of the evolution of energy levels of the related electrodes and transmission spectra as well as the projected self-consistent Hamiltonian eigenvalues with the applied bias voltage. The results indicate that the asymmetric doping of the impurity atoms can lead to a robust rectification which can be utilized to design phosphorene-base rectifier with good performance.     

Theoretical Studies on the Growth Mechanism of Armchair Single-walled Carbon Nanotube

The growth mechanism of armchair single-walled carbon nanotube was studied theoretically by AM1 method as implemented in Gaussian03 program. The following results were obtained. (1) Let C₂ radicals be the carbon source for the growth of the carbon nanotube, then the most likely growth mechanism would be as follows. An intermediate is formed firstly by the direct addition of C₂ radical to the open end of the carbon nanotube without an energy barrier, then via a transition state the reaction produces the product, i.e., C₂ becomes the component of the hexagon of the nanotube. (2) From (3,3) to (6,6), the activation energy decreases (from 66.8 to 46.1 kJ·mol^?1), whereas the conjugation of the nanotube increases. (3) The distribution of the frontier molecular orbitals indicates that the two edges of the newly formed hexagon maybe grow easily.     

First-principles study of the electronic structure and transport properties of armchair graphene nanoribbons with adsorbed super-halogen LiF2 and super-alkali Li3 clusters

The electronic structure and quantum transport properties of pristine armchair graphene nanoribbons (AGNRs) and AGNRs adsorbing super-halogen LiF₂ and super-alkaline Li₃ clusters (Li₃/AGNRs/LiF₂) were investigated using density functional theory and non-equilibrium Green's function calculations. It was found that LiF₂ and Li₃ clusters are stably adsorbed on the AGNRs, and the adsorption of Li₃ and LiF₂ endows AGNRs with the characteristics of n-type and p-type semiconductors, respectively. The Li₃/AGNRs/LiF₂ structure reduces the band gap and the turn-on voltage, and improves the transmission coefficient of the ANGRs device. This structure also exhibit the rectification characteristics of a pn junction with the forward bias current greater than the reverse bias current. This shows that adsorption of super-alkali and super-halogen clusters in different regions of AGNRs is a feasible approach for obtaining AGNRs with pn junction characteristics.     

Effect of triangular vacancy defect on thermal conductivity and thermal rectification in graphene nanoribbons

We investigate the thermal transport properties of armchair graphene nanoribbons (AGNRs) possessing various sizes of triangular vacancy defect within a temperature range of 200–600 K by using classical molecular dynamics simulation. The results show that the thermal conductivities of the graphene nanoribbons decrease with increasing sizes of triangular vacancy defects in both directions across the whole temperature range tested, and the presence of the defect can decrease the thermal conductivity by more than 40% as the number of removed cluster atoms is increased to 25 (1.56% for vacancy concentration) owing to the effect of phonon–defect scattering. In the meantime, we find the thermal conductivity of defective graphene nanoribbons is insensitive to the temperature change at higher vacancy concentrations. Furthermore, the dependence of temperatures and various sizes of triangular vacancy defect for the thermal rectification ration are also detected. This work implies a possible route to achieve thermal rectifier for 2D materials by defect engineering.     

Thermal properties of triangle nitrogen-doped graphene nanoribbons

We investigate the thermal properties of triangle nitrogen-doped graphene nanoribbons (TNGNs) with different nitrogen-doped concentrations (0.11% to 2.31%) at different temperatures ($200\text{K}～600\text{K}$) using non-equilibrium molecular dynamics. The results show that the nitrogen atoms doped at the edge of the defect can increase the thermal conductivity of graphene nanoribbons, but with the increase of the nitrogen-doped concentrations from 0.11% to 2.31%, the thermal conductivity decreases sharply. In addition, nitrogen atoms reduces the sensitivity of the thermal conductivity to temperature. Besides, the thermal rectification is found, and it increases with the raise of nitrogen-doped concentration. Finally, in order to verify the correctness of the thermal rectification, we calculate the phonon power spectra of TNGNs with nitrogen-doped concentrations of 0.11% and 2.31% at 300 K. These research has important reference value for the control of heat in microelectronic devices.     

Insulator-semimetallic transition in quasi-1D charged impurity-infected armchair boron-nitride nanoribbons

We address control of electronic phase transition in charged impurity-infected armchair-edged boron-nitride nanoribbons (ABNNRs) with the local variation of Fermi energy. In particular, the density of states of disordered ribbons produces the main features in the context of pretty simple tight-binding model and Green's functions approach. To this end, the Born approximation has been implemented to find the effect of π-band electron-impurity interactions. A modulation of the π-band depending on the impurity concentrations and scattering potentials leads to the phase transition from insulator to semimetallic. We present here a detailed physical meaning of this transition by studying the treatment of massive Dirac fermions. From our findings, it is found that the ribbon width plays a crucial role in determining the electronic phase of disordered ABNNRs. The obtained results in controllable gap engineering are useful for future experiments. Also, the observations in this study have also fueled interest in the electronic properties of other 2D materials.     

Differential conductance of armchair single-wall carbon nanotubes due to presence of electron–phonon interaction

We have theoretically investigated the first correction to conductance of armchair single wall carbon nanotubes (SWCNTs) with finite length, embedded between two electrodes, due to the presence of electron–transversal phonon interaction. The perturbative scheme has been used with finite length real space nearest neighbors tight binding method. Both radial breathing and tangential modes are investigated separately. It is found that not only the conductance correction crucially depends on source-drain voltage but also it strongly depends on the length and diameter of SWCNT. So, this work opens up opportunities to control the electrical conductance of SWCNT and increases yield of micro or nanodevices based on carbon nanotube.