基于碳纳米管分子封装的一维电子波函数扫描

利用第一性原理,设计并研究了一类基于单臂碳纳米管的分子封装的分子体系．计算表明,半环葫芦脲类化合物可有效封装碳纳米管,引入微弱的分子间相互作用,对碳纳米管的电子态能级结构分布 仅带来微弱影响．半环葫芦脲分子与碳纳米管在管径方向的一维电子态波函数充分耦合,进而有效改变了一些前沿分子轨道的波函数在管径两头的分布以及相应的电子布居浓度．基于电子输运的模拟,发现半环葫芦脲分子在碳纳米管一维方向滑动时的某个电压下的电导变化可准确反映电子态波函数在相应分子导电通道上的一维分布信息．     

Controllable growth and characterization of isolated single-walled carbon nanotubes catalyzed by Co particles

Discrete Co catalytic nanoparticles with small diameters are obtained by pulsed vacuum arc evaporation on Si/SiO₂ substrates, which are used for the growth of isolated single-walled carbon nanotubes (SWNTs) by an ethanol chemical vapor deposition approach (CVD). The distributions of catalytic nanoparticles change with the number of arc pulses, which allows control of the nanotubes formation. We find that an increase of ethanol pressure during CVD growth can change SWNTs from isolated ones into bundles. A new growth mechanism which combines a tip and base model for SWNT growth has been tentatively proposed. It is suggested that the small size catalytic particles prepared by pulsed arc evaporation have a potential advantage for small diameter SWNT growth. PACS 78.67.Ch; 78.67.Bf; 78.67.-n; 81.07.De; 61.46.-w     

Transport properties of poly(GACT)-poly(CTGA) deoxyribonucleic acid: A ladder model approach

In this paper, based on the tight-binding Hamiltonian model and within the framework of a generalized Green’s function technique, the electronic conduction through the poly(GACT)-poly(CTGA) DNA molecule in SWNT/DNA/SWNT structure has been numerically investigated. In a ladder model, we consider DNA as a planar molecule containing M cells and four further sites (two base pair sites and two backbone sites) in each cell, sandwiched between two semi-infinite single-walled carbon nanotubes (SWNT) as the electrodes. Having relied on Landauer formalism, we focussed on studying the current-voltage characteristics of DNA, the effect of the coupling strength of SWNT/DNA interface and the role of tube radius of nanotube contacts on the electronic transmission through the foregoing structure. Finally, a characteristic time was calculated for the electron transmission, which measures the delay caused by the tunnelling through the SWNT/DNA interface. The results clearly show that the calculated characteristic time and also the conductance of the system are sensitive to the coupling strength between DNA molecule and nanotube contacts.     

Demetalization of single-walled carbon nanotube thin films with microwave irradiation

The microwave irradiation effects on purified HiPCO and CoMoCat single-walled carbon nanotube (SWNT) thin films are investigated. The surface conductivities of the SWNT films are extracted from the measured THz transmission coefficients to provide a direct indication of the metallic content in the films. The observed drastic conductivity decrease indicates a significant metallic content reduction after the microwave irradiation. Two different laser excitations are applied for Raman spectroscopy to reveal the response of different nanotube species. The Raman spectra of both HiPCO and CoMoCat thin films confirm the decrease of metallic carbon nanotubes. The observed microwave-induced effects may potentially lead to a convenient scheme for demetalization of single-walled carbon nanotube mixtures.     

Biophysical and electrochemical properties of Self-assembled noncovalent SWNT/DNA hybrid and electroactive nanostructure

DNA self-assembled hybrid nanostructures are widely used in recent research in nanobiotechnology. Combination of DNA with carbon based nanoparticles such as single-walled carbon nanotube (SWNT), multi-walled carbon nanotube (MWNT) and carbon quantum dot were applied in important biological applications. Many examples of biosensors, nanowires and nanoelectronic devices, nanomachine and drug delivery systems are fabricated by these hybrid nanostructures. In this study, a new hybrid nanostructure has been fabricated by noncovalent interactions between single or double stranded DNA and SWNT nanoparticles and biophysical properties of these structures were studied comparatively. Biophysical properties of hybrid nanostructures studied by circular dichroism, UV–vis and fluorescence spectroscopy techniques. Also, electrochemical properties studied by cyclic voltammetry, linear sweep voltammetry, square wave voltammetry, choronoamperometry and impedance spectroscopy (EIS). Results revealed that the biophysical and electrochemical properties of SWNT/DNA hybrid nanostructures were different compare to ss-DNA, ds-DNA and SWNT singly. Circular dichroism results showed that ss-DNA wrapped around the nanotubes through π-π stacking interactions. The results indicated that after adding SWNT to ss-DNA and ds-DNA intensity of CD and UV–vis spectrum peaks were decreased. Electrochemical experiments indicated that the modification of single-walled carbon nanotubes by ss-DNA improves the electron transfer rate of hybrid nanostructures. It was demonstrated SWNT/DNA hybrid nanostructures should be a good electroactive nanostructure that can be used for electrochemical detection or sensing.     

Visible and near-infrared excited-state dynamics of single-walled carbon nanotubes

Excited-state lifetimes of isolated single-walled semiconducting carbon nanotubes (SWNTs) have been measured for the first time; these excited states, observed over the 400- to 1800-nm spectral domain, possess lifetimes that range from several ps to more than 100 ps. Sub-ps to ps decay components are assigned to relaxation in SWNT bundles. Interrogation of the samples with different SWNT mean diameters further confirms the dependence of the excited-state lifetime on roll-up vector. The ratio of fast and slow decaying component contributions in the first van Hove band can be viewed as a measure of the bundle content. PACS 78.67.Ch; 78.47.+p; 61.46.+w; 73.22.-f     

Economic assessment of single-walled carbon nanotube processes

The carbon nanotube market is steadily growing and projected to reach $1.9 billion by 2010. This study examines the economics of manufacturing single-walled carbon nanotubes (SWNT) using process-based cost models developed for arc, CVD, and HiPco processes. Using assumed input parameters, manufacturing costs are calculated for 1 g SWNT for arc, CVD, and HiPco, totaling $1.9 billion by 2010. This study examines the economics of manufacturing single-walled carbon nanotubes (SWNT) using process-based cost models developed for arc, CVD, and HiPco processes. Using assumed input parameters, manufacturing costs are calculated for 1 g SWNT for arc, CVD, and HiPco, totaling 1,906, 1,706, and1,706, and 485, respectively. For each SWNT process, the synthesis and filtration steps showed the highest costs, with direct labor as a primary cost driver. Reductions in production costs are calculated for increased working hours per day and for increased synthesis reaction yield (SRY) in each process. The process-based cost models offer a means for exploring opportunities for cost reductions, and provide a structured system for comparisons among alternative SWNT manufacturing processes. Further, the models can be used to comprehensively evaluate additional scenarios on the economics of environmental, health, and safety best manufacturing practices.     

Determination of band gaps of self-assembled carbon nanotube films using Tauc/Davis–Mott model

The band gaps of self-assembled single-walled carbon nanotube (SWNT) films have been determined through curve fitting using the semi-empirical Tauc and Davis–Mott model, based on the measurement of optical absorption at the visible and near infrared range. This study provides a practicable option for the determination of band gaps for ultra-thin SWNT films or multi-walled carbon nanotube films whose vHs peaks cannot be well resolved in absorption spectra.     

Immobilization of redox mediators on functionalized carbon nanotube: A material for chemical sensor fabrication and amperometric determination of hydrogen peroxide

Chemical functionalization of single-walled carbon nanotubes with redox mediators, namely, toluidine blue and thionin have been carried out and the performance of graphite electrode modified with functionalized carbon nanotubes is described. Mechanical immobilization of functionalized single-walled nanotube (SWNT) on graphite electrode was achieved by gently rubbing the electrode surface on carbon nanotubes supported on a glass slide. The electrochemical behaviour of the modified electrodes was investigated by cyclic voltammetry. The SWNT-modified electrodes showed excellent electrocatalytic effect for the reduction of hydrogen peroxide. A decrease in overvoltage was observed as well as an enhanced peak current compared to a bare graphite electrode for the reduction of hydrogen peroxide. The catalytic current was found to be directly proportional to the amount of hydrogen peroxide taken.     

Optical transition energies for carbon nanotubes from resonant Raman spectroscopy: environment and temperature effects

This Letter reports the laser energy dependence of the Stokes and anti-Stokes Raman spectra of carbon nanotubes dispersed in aqueous solution and within solid bundles, in the energy range 1.52-2.71 eV. The electronic transition energies (E(ii)) and the radial breathing mode frequencies (omega(RBM)) are obtained for 46 different (18 metallic and 28 semiconducting) nanotubes, and the (n,m) assignment is discussed based on the observation of geometrical patterns for E(ii) versus omega(RBM) graphs. Only the low energy component of the E(M)(11) value is observed from each metallic nanotube. For a given nanotube, the resonant window is broadened and down-shifted for single wall carbon nanotube (SWNT) bundles compared to SWNTs in solution, while by increasing the temperature, the E(S)(22) energies are redshifted for S1 [(2n+m) mod 3=1] nanotubes and blueshifted for S2 [(2n+m) mod 3=2] nanotubes.     

钯金属吸附对半导体性碳纳米管电输运的影响

利用物理蒸发技术,在半导体性的碳纳米管上沉积钯金属,利用导电原子力显微镜检测钯吸附对碳纳米管电输运的影响.结果表明:沉积的钯在碳纳米管上形成纳米颗粒,随着钯颗粒密度的增加,半导体性碳纳米管逐渐向金属性转变.利用第一性原理计算了吸附有钯原子的半导体性单壁碳纳米管的能带结构.研究发现,钯的覆盖率越高,其禁带宽度越窄,直至为零,定性说明了实验结果的合理性. 关键词： 单壁碳纳米管 钯纳米颗粒 导电原子力显微镜 第一性原理计算     

Quantum-dot transport in carbon nanotubes

Transport measurements on a bundle of single-walled carbon nanotubes have been made below 4.2 K as a function of side gate and source–drain bias voltage. The transport of an individual nanotube is described by the Coulomb blockade effect. The zero-dimensional quantum states of the nanotube become clear for measurements of large bias voltage. In addition, we present preliminary results of microwave application to the SWNT dot, and the results can be qualitatively explained by classical coupling to the dot.     

Chemically induced conductance switching in carbon nanotube circuits

The chemical reactivity of carbon nanotubes in H2SO4 is investigated using individual, single-walled carbon nanotubes (SWNTs) incorporated into electronic devices. Exploiting the device conductance as a sensitive indicator of chemical reactions, discrete oxidation and reduction events can be clearly observed. During oxidation, a SWNT opens circuits to a nanometer-scale tunnel junction with residual conduction similar to Frenkel-Poole charge emission. When electrochemically reduced, a SWNT returns to its original conductance. This redox cycle can be repeated many times, suggesting a novel chemical method of reversibly switching SWNT conductivity.     

Photoluminescence from single-walled carbon nanotubes: a comparison between suspended and micelle-encapsulated nanotubes

Single-walled carbon nanotubes (SWNTs) are luminescent. Up to now, two preparation methods, both of which isolate individual SWNTs, have enabled the detection of nanotube bandgap photoluminescence (PL): encapsulation of individual SWNTs into surfactant micelles and direct growth of individual SWNTs suspended in air between pillars. This paper compares the PL obtained from suspended SWNTs to published PL data obtained from encapsulated SWNTs. We find that emission peaks are blueshifted by 28 meV on average for the suspended nanotubes as compared to the encapsulated nanotubes. Similarly, the resonant absorption peaks at the second set of van Hove singularities are blueshifted on average by 16 meV. Both shifts depend weakly on the particular chirality and diameter of the SWNT. PACS 78.67.Ch; 78.55.-m     

Molecular dynamics simulations of hydrogen adsorption/desorption by palladium decorated single-walled carbon nanotube bundle

Utilising molecular dynamics simulations, the hydrogen molecules adsorption isotherms of the (8,?0) palladium decorated single-walled carbon nanotube (SWNT) were obtained. The hydrogen adsorption was studied on the external, interstial and internal surfaces of the SWNT bundle at several temperatures ranging from 77 to 400?K. The results were compared with the bare single-walled carbon nanotube bundle under the same conditions. The decorated carbon nanotube bundle hydrogen adsorption was significantly higher than that of the bare one. The hydrogen desorption and readsorption were studied using temperature as the readsorption/desorption variable. The rate constants were calculated for the hydrogen desorption at different temperatures. The calculated decorated SWNT bundle hydrogen desorption activation energy was higher than that for the bare SWNT bundle. The calculated activation energies for the hydrogen desorption in both nanotube bundles specified the temperature dependency of hydrogen desorption.     

Computational analysis of effect of modification on the interfacial characteristics of a carbon nanotube-polyethylene composite system

In this study, the non-covalent association of single-walled nanotube (SWNT) with polyethylene (PE) molecule and the influence of sidewall modification on the interfacial bonding between the SWNTs and polymer were investigated using molecular mechanics (MM) and molecular dynamics (MD) simulations. The model of interaction between the initially separated PE and SWNT fragments, which can be either wrapping or filling, was computed. The possible extension of polymers wrapping or filling SWNTs can be used to structurally bridge the SWNTs and polymers to significantly improve the load transfer between them when SWNTs are used to produce nanocomposites. The interfacial bonding characteristics between the single-walled nanotubes, on which -COOH, -CONH₂, -C₆H₁₁, or -C₆H₅ groups have been chemically attached, and the polymer matrix were also investigated by performing pullout simulations. The results show that appropriate functionalization of nanotubes at low densities of functionalized carbon atoms drastically increase their interfacial bonding and shear stress between the nanotubes and the polymer matrix, where chemisorption with -C₆H₅ groups to as little as 5.0% of the nanotube carbon atoms increases the shear stress by about 1700%. Furthermore, this suggests the possibility to use functionalized nanotubes to effectively reinforce other kinds of polymer-based materials as well.     

Surfactant-nanotube interactions in water and nanotube separation by diameter: atomistic simulations

A non-destructive sorting method to separate single-walled carbon nanotubes (SWNTs) by diameter was recently proposed. By this method, SWNTs are suspended in water by surfactant encapsulation and the separation is carried out by ultracentrifugation in a density gradient. SWNTs of different diameters are distributed according to their densities along the centrifuge tube. A mixture of two anionic surfactants, namely sodium dodecylsulfate (SDS) and sodium cholate (SC), presented the best performance in discriminating nanotubes by diameter. Unexpectedly, small diameter nanotubes are found at the low density part of the centrifuge tube. We present molecular dynamics studies of the water-surfactant-SWNT system to investigate the role of surfactants in the sorting process. We found that surfactants can actually be attracted towards the interior of the nanotube cage, depending on the relationship between the surfactant radius of gyration and the nanotube diameter. The dynamics at room temperature showed that, as the amphiphile moves to the hollow cage, water molecules are dragged together, thereby promoting the nanotube filling. The resulting densities of filled SWNT are in agreement with measured densities.     

Giant thermopower effects from molecular physisorption on carbon nanotubes

Results are presented of in situ studies of the thermoelectric power and four-probe resistance of single-walled carbon nanotube films during the adsorption of cyclic hydrocarbons C(6)H(2n) (n=3-6). The size of the change in these transport parameters is found to be related to the pi electron population of the molecule, suggesting the coupling between these pi electrons and those in the nanotube wall may be responsible for the observed effects. A transport model for the SWNT film behavior is presented, incorporating the effects of a new scattering channel associated with the adsorbed molecules.     

碳纳米管掺杂对聚合物聚(2-甲氧基-5-辛氧基)对苯乙炔-PbSe量子点复合材料性能的影响

基于碳纳米管的良好导电性、激子传输性能和量子点聚合物复合材料高的光电转换性能, 采用原位缩合法制备了聚合物聚(2-甲氧基-5-辛氧基)-对苯乙炔(MOPPV)功能化碳纳米管(SWNT)-PbSe量子点复合材料, 通过对复合材料的X射线衍射、透射电子显微镜和紫外可见吸收光谱研究, 发现MOPPV, SWNT与PbSe量子点可以有效地复合, 且SWNT与MOPPV形成网状结构; PbSe量子点尺寸为5.75 nm, 其可均匀地分散在MOPPV-SWNT基体中形成包覆或镶嵌结构, 并发生了光诱导电荷转移.通过对复合材料的光电性能研究发现, 当MOPPV, SWNT, PbSe三者的质量比为1: 0.3 : 1 时其光电性能最好, 开路电压为0.556 V, 短路电流为2.133 mA, 填充因子为34.48%, 转换效率为0.452%, 与聚合物MOPPV-PbSe量子点复合材料材料相比, 光电性能提高了2–3倍. 关键词： 量子点 碳纳米管 复合材料 转换效率     

Intrinsic electrical properties of individual single-walled carbon nanotubes with small band gaps

Individual single-walled carbon nanotubes (SWNT) exhibiting small band gaps on the order of 10 meV are observed for the first time in electron transport measurements. Transport through the valence or conduction band of a small-gap semiconducting SWNT (SGS-SWNT) can be tuned by a nearby gate voltage. Intrinsic electrical properties of the Ohmically contacted SGS-SWNT are elucidated. An SGS-SWNT exhibits metal- or semiconductorlike characteristics depending on the Fermi level position in the band structure.