Novel polygonized single-wall carbon nanotube bundles

We have synthesized novel crystalline ropes of "polygonized" single-wall carbon nanotubes (SWCNTs). The tubes exhibit rounded-hexagonal cross sections in contrast to the earlier observations of nearly circular tubes. To investigate the structural characteristics of the lattice of SWCNTs we have performed extensive molecular-dynamics simulations. We find several metastable structures of the lattice characterized by different tube cross sections, hexagonal, rounded-hexagonal, and circular, and increasing cell volume. The competition between different tube shapes as a function of tube diameter is analyzed and compared to experiments.     

Growth of patterned and aligned carbon nanotube bundles on micro-structured substrate

Microstructures are used as inducement for growth of patterned and aligned carbon nanotube (CNT) bundles by pyrolysis of iron phthalocyanine (FePc) under H₂/Ar. The flow of mixture gas can be influenced by geometry profile of microstructure, and the distribution density of catalyst will be different related to the different microstructure. Many types of substrates with different microstructures are used in this study, and several different profiles of CNT bundles are achieved under different process conditions, especially an apical dominance like plant growth is observed under specific H₂/Ar flow rate. Through using appropriate microstructures and controlling the flow rate, the density of CNT bundles can be adjusted, which is very important for weakening electric field shielding effect.     

Si表面间水平碳纳米管束的分子动力学模拟研究

本文采用分子动力学模拟方法研究了Si表面间单壁水平碳纳米管束SWCNT (10,10)的变形和摩擦特性.系统在弛豫平衡后,首先对碳纳米管束施加压力至碳纳米管或Si表面结构破坏.之后在无压力和高压力两种情况下使上表面沿水平方向做剪切运动以研究碳纳米管束的摩擦特性.结果表明,由于碳纳米管的柔韧性,碳纳米管束在加载过程中出现明显变形,但直至3.8 GPa高压下并无结构破坏.系统无压力时SWCNT (10,10)在原地轻微随机滚动,压力为3.8 GPa时,碳纳米管束出现了整体的轻微滑动,同时伴随无规律的轻微滚动, 关键词： 碳纳米管束 摩擦 分子动力学模拟     

4He adsorbed on the outer surface of carbon nanotube bundles

The results of diffusion Monte Carlo calculations on the behavior of 4He adsorbed on the external surface of a bundle of carbon nanotubes are presented. The corrugation effects are found to be very important, making the outside part of the bundles a quite inhomogeneous substrate. No stable solid helium monolayer at high density was found. Instead, helium atoms are promoted to a second quasi-one-dimensional phase on top of the liquid first layer. On increasing the helium intake, a two layer structure is formed in which the helium directly in contact with the carbon surface solidifies.     

Dynamics and damping of electromagnetic solitons in the carbon nanotube bundles

The possible existence of the electromagnetic solitons in carbon nanotubes is analyzed. Solitons appear as a result of a simultaneous change in the classical electron distribution function and the electric field produced by the nonequilibrium electrons in carbon nanotube. The effective equations are obtained for the dynamics of electromagnetic field with allowance for the interband transitions causing soliton damping. The numerical results are presented evidencing the existence of solitons in carbon nanotubes. The propagation dynamics is studied for the periodic electromagnetic waves in the bundles of carbon nanotubes. The shape of electromagnetic wave is found to change during its propagation.     

碳纳米管束中的正电子理论

采用中性原子叠加模型和有限差分方法(SNA-FD)计算了大范围内不同管径的单壁碳纳米管束中的正电子情况，发现对于单壁碳纳米管束，正电子的主要湮没区域，湮没对象和正电子寿命随碳纳米管管径的不同而发生规律性变化.计算得到管径范围在0.8—1.6nm的碳纳米管束的正电子寿命范围为332—470ps，与实验测得的394ps符合较好.     

Theoretical calculations of thermophysical properties of single-wall carbon nanotube bundles

Carbon nanotube bundles are promising thermal interfacial materials due to their excellent thermal and mechanical characteristics.In this study,the phonon dispersion relations and density of states of the single-wall carbon nanotube bundles are calculated by using the force constant model.The calculation results show that the inter-tube interaction leads to a significant frequency raise of the low frequency modes.To verify the applied calculation method,the specific heat of a single single-wall carbon nanotube is calculated first based on the obtained phonon dispersion relations and the results coincide well with the experimental data.Moreover,the specific heat of the bundles is calculated and exhibits a slight reduction at low temperatures in comparison with that of the single tube.The thermal conductivity of the bundles at low temperatures is calculated by using the ballistic transport model.The calculation results indicate that the inter-tube interaction,i.e.van der Waals interaction,hinders heat transfer and cannot be neglected at extremely low temperatures.For(5,5) bundles,the relative difference of the thermal conductivity caused by ignoring inter-tube effect reaches the maximum value of 26% around 17 K,which indicates the significant inter-tube interaction effect on the thermal conductivity at low temperatures.     

Zero-bias conductance in carbon nanotube quantum dots

We present numerical renormalization group calculations for the zero-bias conductance of quantum dots made from semiconducting carbon nanotubes. These explain and reproduce the thermal evolution of the conductance for different groups of orbitals, as the dot-lead tunnel coupling is varied and the system evolves from correlated Kondo behavior to more weakly correlated regimes. For integer fillings N=1, 2, 3 of an SU(4) model, we find universal scaling behavior of the conductance that is distinct from the standard SU(2) universal conductance, and concurs quantitatively with experiment. Our results also agree qualitatively with experimental differential conductance maps.     

碳纳米管量子点的电子输运

在紧束缚近似下,利用常量相互作用模型和Landauer-Bütticker公式,计算了扶手椅型和金属锯齿型碳纳米管量子点的电导。发现,根据碳纳米管量子点的长度的不同,扶手椅型碳纳米管量子点的电导可以具有两电子或四电子的壳层结构。而锯齿型碳纳米管量子点的电导却仅有四电子的壳层结构,与长度无关;这些理论结果与之前的实验结果符合的很好。     

Fabrication of carbon nanotube bundles and measurement of field electron emission properties

Carbon nanotube (CNT) bundles are synthesized on rough polycrystalline ceramic wafers by pyrolyzing ferrocene/melamine mixtures through a three-step process in a single stage furnace in an Ar atmosphere. The CNTs are multi-walled and have outer diameters from 10 to 90 nm and lengths from 20 to 100 microns. These CNTs display a bamboo-like structure with open graphite layers and defects at the outer surfaces. Field electron emission (FEE) measurements show that the turn-on electrical field is 2.9 V/m and the field enhancement factor is 2700. PACS 61.46.+w; 82.30.Lp; 79.70.+q     

Linear and nonlinear transport across carbon nanotube quantum dots

We present a low energy-theory for non-linear transport in finite-size interacting single-wall carbon nanotubes. It is based on a microscopic model for the interacting p_z electrons and successive bosonization. We consider weak coupling to the leads and derive equations of motion for the reduced density matrix. We focus on the case of large-diameter nanotubes where exchange effects can be neglected. In this situation the energy spectrum is highly degenerate. Due to the multiple degeneracy, diagonal as well as off-diagonal (coherences) elements of the density matrix contribute to the nonlinear transport. At low bias, a four-electron periodicity with a characteristic ratio between adjacent peaks is predicted. Our results are in quantitative agreement with recent experiments.     

Adsorption of argon on carbon nanotube bundles and its influence on the bundle lattice parameter

We report experimental studies of the adsorption characteristics and structure of both 36Ar and 40Ar on single-wall carbon nanotube bundles. The structural studies make use of the large difference in coherent neutron scattering cross section for the two Ar isotopes to explore the influence of the adsorbate on the nanotube lattice parameter. We observe no dilation of the nanotube lattice with 40Ar, and explain the apparent expansion of this lattice upon 36Ar adsorption by the location of the adsorbed Ar atoms on the outer bundle surface.     

Charge-carrier dynamics in single-wall carbon nanotube bundles: a time-domain study

We present a real-time investigation of ultra-fast carrier dynamics in single-wall carbon nanotube bundles using femtosecond time-resolved photoelectron spectroscopy. The experiments allow us to study the processes governing the sub-picosecond and the picosecond dynamics of non-equilibrium charge carriers. On the sub-picosecond time scale the dynamics are dominated by ultra-fast electron–electron scattering processes, which lead to internal thermalization of the laser-excited electron gas. We find that quasiparticle lifetimes decrease strongly as a function of their energy up to 2.38 eV above the Fermi level – the highest energy studied experimentally. The subsequent cooling of the laser-heated electron gas to the lattice temperature by electron–phonon interaction occurs on the picosecond time scale and allows us to determine the electron–phonon mass-enhancement parameter λ. The latter is found to be over an order of magnitude smaller if compared, for example, with that of a good conductor such as copper. Received: 4 March 2002 / Accepted: 7 March 2002 / Published online: 3 June 2002     

Isotope engineering of carbon nanotube systems

The synthesis of a unique isotope engineered system, double-wall carbon nanotubes with natural carbon outer and highly 13C enriched inner walls, is reported from isotope enriched fullerenes encapsulated in single-wall carbon nanotubes (SWCNTs). The material allows the observation of the D line of the highly defect-free inner tubes that can be related to a curvature induced enhancement of the electron-phonon coupling. Ab initio calculations explain the inhomogeneous broadening of inner tube Raman modes due to the distribution of different isotopes. Nuclear magnetic resonance shows a significant contrast of the isotope enriched inner SWCNTs compared to other carbon phases and provides a macroscopic measure of the inner tube mass content. The high curvature of the small diameter inner tubes manifests in an increased distribution of the chemical shift tensor components.     

Shot-noise detection in a carbon nanotube quantum dot

An on-chip detection scheme for high frequency signals is used to detect noise generated by a quantum dot formed in a single wall carbon nanotube. The noise detection is based on photon assisted tunneling in a superconductor-insulator-superconductor junction. Measurements of shot noise over a full Coulomb diamond are reported with excited states and inelastic cotunneling clearly resolved. Super-Poissonian noise is detected in the case of inelastic cotunneling.     

Adiabatic charge pumping in carbon nanotube quantum dots

We investigate charge pumping in carbon nanotube quantum dots driven by the electric field of a surface acoustic wave. We find that, at small driving amplitudes, the pumped current reverses polarity as the conductance is tuned through a Coulomb blockade peak using a gate electrode. We study the behavior as a function of wave amplitude, frequency, and direction and develop a model in which our results can be understood as resulting from adiabatic charge redistribution between the leads and quantum dots on the nanotube.     

Size effect of quantum conductance in single-walled carbon nanotube quantum dots

The quantum conductance of two kinds of carbon nanotube quantum dots (CNQD) composed of (5,5) and (10,0) tubes, namely (10,0)/(5,5)/(10,0) and (5,5)/(10,0)/(5,5) with different quantum sizes, are calculated. It is shown that for (10,0)/(5,5)/(10,0) CNQD, one on-resonant peak at the Fermi energy exists only for special QD sizes, and the width of the conductance gap increases from 1.0 eV to 3.2 eV with the increase of size. The positions of peaks around the Fermi energy are obtained by the electronic structure of individual finite (5,5) tubes. We also find that the (5,5)/(10,0)/(5,5) CNQDs behave as a quantum dot, and its localized QD states are different from that of the former CNQD because of the existence of the interface states between (5,5)/(10,0) junctions. For (5,5)/(10,0)/(5,5) CNQD, there is no conductance gap with QDs size smaller than 7 layers, and the conductance peak around the interface quasilocalized state -0.26 eV disappears with QD sizes larger than 23 layers. In addition, for the (5,5)/(10,0)/(5,5) CNQD, the connection method can change the degree of electronic localization of intermediate (10,0) tube.Received: 8 August 2003, Published online: 23 December 2003PACS: 61.48. + c Fullerenes and fullerene-related materials - 71.20.Tx Fullerenes and related materials; intercalation compounds - 72.80.Rj Fullerenes and related materials - 68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.     

Correlated tunneling in intramolecular carbon nanotube quantum dots

We investigate correlated electronic transport in single-walled carbon nanotubes with two intramolecular tunneling barriers. We suggest that below a characteristic temperature the long-range nature of the Coulomb interaction becomes crucial to determine the temperature dependence of the maximum G(max) of the conductance peak. Correlated sequential tunneling dominates transport yielding the power law G(max) proportional, variant T(alpha(end-end)-1), typical for tunneling between the ends of two Luttinger liquids. Our predictions are in agreement with recent measurements.