Electric field quenching of carbon nanotube photoluminescence

The effect of external electric fields on the photoluminescence intensity of single-walled carbon nanotubes was investigated for individual nanotubes and bulk samples in polymeric films. Fields of up to 10(7) V/m caused dramatic, reversible decreases in emission intensity. Quenching efficiency varied as the cosine of the angle between the field and nanotube axis and decreased with increasing optical band gap. Photoluminescence intensity was found to follow a reciprocal hyperbolic cosine dependence on electric field.     

Charge transfer induced polarity switching in carbon nanotube transistors

We probed the charge transfer interaction between the amine-containing molecules hydrazine, polyaniline, and aminobutyl phosphonic acid and carbon nanotube field effect transistors (CNTFETs). We successfully converted p-type CNTFETs to n-type and drastically improved the device performance in both the ON- and OFF-transistor states, utilizing hydrazine as dopant. We effectively switched the transistor polarity between p- and n- type by accessing different oxidation states of polyaniline. We also demonstrated the flexibility of modulating the threshold voltage (Vth) of a CNTFET by engineering various charge-accepting and -donating groups in the same molecule.     

Spraying carbon nanotube dispersions to prepare superhydrophobic films

Carbon nanotubes (CNTs) are a promising material for superhydrophobic coating. In the present study, we prepared superhydrophobic CNT films by spraying CNT dispersions, and discussed the factors that control superhydrophobicity of the CNT films. Three types of dispersions (i.e., CNTs/ethanol, CNTs functionalized with dodecyl groups/ethanol, and CNTs and trimethylsiloxysilicate (TMSS)/ethanol) were prepared as spraying solutions. As increasing the amount of spray-coated CNTs, hydrophobicity of the resulting films became higher, and eventually superhydrophobicity was observed. The spray-coated CNT films had hierarchical roughness, which is preferable morphology to show superhydrophobicity. The coexistence of TMSS in the film lowered the amount of CNTs required to show superhydrophobicity because TMSS acted like glue that prevented CNTs from aggregating on substrates. Interaction force of CNTs also affected the wettability of the CNT films. The π–π interaction of CNTs resulted in the aggregation of them on substrates, which lowered the wettability of the films. From this viewpoint, functionalization of CNTs with dodecyl groups was effective to form superhydrophobic films because the dodecyl groups on CNTs screened the π–π interaction of CNTs.     

Tritrichomonas foetus adhere to superhydrophilic vertically aligned multi-walled carbon nanotube surface

For the first time, we show that Tritrichomonas foetus can adhere on superhydrophilic vertically aligned carbon nanotubes (VACNT) films. Scanning electron microscopy shows an unusual adhesion with a higher membrane filopodium projection in all directions, directly attached to superhydrophilic VACNT tips.     

Magnetically induced field effect in carbon nanotube devices

Three-terminal devices with conduction channels formed by quasi-metallic carbon nanotubes (CNTs) are shown to operate as nanotube-based field-effect transistors under strong magnetic fields. The off-state conductance of the devices varies exponentially with the magnetic flux intensity. We extract the quasi-metallic CNT chirality as well as the characteristics of the Schottky barriers formed at the metal-nanotube contacts from the temperature-dependent magnetoconductance measurements.     

A vacuum sensor using field emitters made by multiwalled carbon nanotube yarns

A newly developed vacuum sensor using carbon nanotube (CNT) field-emission has been designed. The fabricated device is an ionization gauge with a silk-like CNT yarn cathode, and the vacuum is indicated by the ratio of the ion current to the electron-emission current. The metrological characteristics of the sensor were studied in a dynamic vacuum system. It showed good linearity ranged from 10⁻⁴ to 10⁻¹ Pa. Taking advantage of the field-emission cathode, the power consumption is only about 5.5 mW. Moreover, comparing it to the conventional thermionic cathode, the CNT yarn cathode is more miniature and a cold cathode with no obvious thermal outgassing effect. Due to these features, the sensor described here could have potential applications in measuring vacuum inside sealed and miniaturized devices.     

Voltage induced anomalous conductance switching behaviour in nematic liquid crystal-multiwalled carbon nanotube composite

Investigations on the voltage induced conductance switching behavior in the nematic liquid crystal-multiwalled carbon nanotube composite have performed. We have found that, the system formed electrical conduction pathways with increasing voltages. Anomalous conductance switching took place at a certain threshold voltage. Percolation threshold voltage depends on the frequencies of the measuring electric field. We have made an analogy between concentration dependent percolation and voltage induced percolation. A two dimensional (2D) renormalization model calculation reveals that if the CNTs form a 2D network then percolation threshold concentration (p(c)) should be 0.768. After careful analysis we have found that voltage analogy of percolation threshold concentration in our system is also very close to the above value. The connectivity exponent (beta) for 2D site percolation agrees well with our proposed voltage induced percolation connectivity exponent. Which gives a glimpse that voltage induced percolating network may forms a two dimensional network in this particular system.     

Piperidine induced polarity conversion in single-walled carbon nanotube field effect transistors

Piperidine is found to be an efficient electron doping agent that converts as-prepared p-type single-walled carbon nanotube (SWCNT) field effect transistors (FETs) into n-type SWCNT-FETs. Electron transfer from the amine group in piperidine to the SWCNTs is suggested to be the origin of the p- to n-type conversion. The effect of electron doping is further supported by the Raman tangential G(+) and G(-)-peak downshift up to 3 cm(-1) without the peak broadening. No detectable change in the Raman D-peak suggests non-covalent attachment of piperidine to the SWCNTs. A low temperature (110?°C) Si(3)N(4) passivation layer is used to maintain the long term air stability of the converted n-type devices. A complementary SWCNT inverter is demonstrated through integrating the n- and p-type SWCNT-FETs.     

Observation of breathing-like modes in an individual multiwalled carbon nanotube

We study collective vibrational breathing modes in the Raman spectrum of a multiwalled carbon nanotube. In correlation with high-resolution transmission electron microscopy, we find that these modes have energies differing by more than 23% from the radial breathing modes of the corresponding single-walled nanotubes. This shift in energy is explained with intershell interactions using a model of coupled harmonic oscillators. The strength of this interaction is related to the coupling strength expected for few-layer graphene.     

电场诱导多壁碳纳米管有序排列对多壁碳纳米管/环氧树脂复合材料性能的影响

采用交流（AC）电场诱导法制备了多壁碳纳米管（MWCNTs）均匀分散且定向有序排列的MWCNTs/环氧树脂复合材料。采用SEM、偏振拉曼光谱等研究了电场强度、MWCNTs含量、加电时间及温度（黏度）等因素对MWCNTs定向排列的影响,讨论了MWCNTs有序排列对MWCNTs/环氧树脂复合材料电学和力学性能的影响。结果表明：MWCNTs沿电场方向有序排列;MWCNTs/环氧树脂复合材料施加AC电场后的拉曼强度明显高于未施加电场的情况;当MWCNTs含量从0wt%增加到0.025wt%时,MWCNTs/环氧树脂复合材料导电率从2.3×10^-12 S/cm增加到1.3×10^-8 S/cm,增加了约4个数量级;MWCNTs含量为2.5wt%时,MWCNTs/环氧树脂复合材料拉伸强度提高了26.3%。     

多壁碳纳米管超疏水涂层的制备和性能

以硬脂酸钠(SST)和多壁碳纳米管(MWCNT)为原料,采用溶液混合法制备了碳纳米管超疏水涂层。首先将硬脂酸钠与碳纳米管制备成分散溶液,然后过滤。硬脂酸钠可以和乙酸反应生成硬脂酸(STA),使得碳纳米管的表面附着上硬脂酸这种疏水物质。通过扫描电镜(SEM)观察发现薄膜表面有类似于荷叶结构的微小凸起,通过接触角测试仪测得薄膜的接触角为162°,表现出良好的超疏水效果。这种制备超疏水材料的方法简单易行,可重复性好,在需要大面积使用超疏水涂层的领域具有广阔的应用前景。     

Increasing mouse embryonic fibroblast cells adhesion on superhydrophilic vertically aligned carbon nanotube films

We have analyzed the adhesion of mouse embryonic fibroblasts (MEFs) genetically modified by green fluorescence protein (GFP) gene cultured on vertically-aligned carbon nanotubes (VACNTs) after 6 days. The VACNTs films grown on Ti were obtained by microwave plasma chemical vapor deposition process using Fe catalyst and submitted to an oxygen plasma treatment, for 2 min, at 400 V and 80 mTorr, to convert them to superhydrophilic. Cellular adhesion and morphology were analyzed by scanning electron, fluorescence microscopy, and thermodynamics analysis. Characterizations of superhydrophilic VACNTs films were evaluated by contact angle and X-Ray Photoelectron Spectroscopy. Differences of crowd adhered cells, as well as their spreading on superhydrophilic VACNTs scaffolds, were evaluated using focal adhesion analysis. This study was the first to demonstrate, in real time, that the wettability of VACNTs scaffolds might have enhanced and differential adherence patterns to the MEF-GFP on VACNTs substrates.     

Electrical properties of multiwalled carbon nanotube reinforced fused silica composites

Multiwalled carbon nanotube (MWCNT)-fused silica composite powders were synthesized by solgel method and dense bulk composites were successfully fabricated via hot-pressing. This composite was characterized by XRD, HRTEM, and FESEM. MWCNTs in the hot-pressed composites are in their integrity observed by HRTEM. The electrical properties of MWCNT-fused silica composites were measured and analyzed. The electrical resistivity was found to decrease with the increase in the amount of the MWCNT loading in the composite. When the volume percentage of the MWCNTs increased to 5 vol%, the electrical resistivity of the composite is 24.99 omega cm, which is a decrease of twelve orders of value over that of pure fused silica matrix. The electrical resistivity further decreases to 1.742 omega. cm as the concentration of the MWCNTs increased to 10 vol%. The dielectric properties of the composites were also measured at the frequency ranging from 12.4 to 17.8 GHz (Ku band) at room temperature. The experimental results reveal that the dielectric properties are extremely sensitive to the volume percentage of the MWCNTs, and the permittivities, especially the imaginary permittivities, increase dramatically with the increase in the concentration of the MWCNTs. The improvement of dielectric properties in high frequency region mainly originates from the greatly increasing electrical properties of the composite.     

Temperature effects on resistance of aligned multiwalled carbon nanotube films

Electrical transport in vertically aligned films of multiwalled carbon nanotubes has been investigated in the -150 degrees C to 300 degrees C temperature range (all the tests were conducted in air at atmospheric pressure). In all the cases, the nanotube film exhibited a semi-conducting behavior, with the film resistance decreasing with increasing temperature. Removal of amorphous carbon contamination (via plasma etching) significantly improved the nanotube film's sensitivity to temperature changes (particularly in the 20 degrees C to 200 degrees C temperature range). All the of films tested in this study showed a consistent, repeatable behavior that was independent of the nanotube film length. The temperature sensitivity of the nanotube films was also found to be independent of the heating/cooling rates and without hysteresis. Because of the excellent repeatability and stability of the results, it is conceived that miniaturized temperature sensors could be designed using such aligned multiwalled nanotube films.     

Synthesis and characterization of multiwalled carbon nanotube/FeCo nanocomposites

Multiwalled carbon nanotube/FeCo nanocomposites were produced by Catalytic Chemical Vapour Deposition using highly porous FeCo-SiO2 aerogels with different loadings and dimensions of FeCo nanoparticles as catalysts. Multiwalled carbon nanotubes with average number of walls depending on the size of the catalyst nanoparticles were obtained. Inside the nanotubes spherical or elliptical FeCo nanoparticles are retained, and the magnetic properties of the resulting nanocomposites were characterized in detail.     

Highly oriented carbon nanotube papers made of aligned carbon nanotubes

Paper-like carbon nanotube (CNT) materials have many important applications such as in catalysts, in filtration, actuators, capacitor or battery electrodes, and so on. Up to now, the most popular way of preparing buckypapers has involved the procedures of dispersion and filtration of a suspension of CNTs. In this work, we present a simple and effective macroscopic manipulation of aligned CNT arrays called 'domino pushing' in the preparation of the aligned thick buckypapers with large areas. This simple method can efficiently ensure that most of the CNTs are well aligned tightly in the buckypaper. The initial measurements indicate that these buckypapers have better performance on thermal and electrical conductance. These buckypapers with controllable structure also have many potential applications, including supercapacitor electrodes.     

Electric field induced switching of the fluorescence of single semiconductor quantum rods

The exceptional fluorescence properties of single CdSe quantum rods (QRs) arising from internal and external electric fields are studied. Reversible external field induced switching of the emission in single QRs is reported for the first time. This effect was correlated with local field induced emission intensity reduction and newly observed darkening mechanism. Bimodal spectral jumps under a zero field were also observed and assigned to charged exciton emission, a phenomenon that was likewise directly controlled through an external field. These phenomena point to the use of single QRs as spectrally tunable charge sensitive fluorophores with polarized emission in fluorescence tagging and optical switching applications.     

超疏水叠杯状碳纳米管薄膜的制备(英文)

采用化学气相沉积法制备了高纯度的叠杯状碳纳米管薄膜。生长15 min后,薄膜厚度可达～300μm。热重分析表明薄膜的纯度高达99.9%。透射电镜表征表明碳纳米管中石墨烯片层与轴向存在一定偏角呈叠杯状排列,其直径为80 nm～230 nm。通过对催化剂与叠杯状碳纳米管的结构分析,提出其根部生长机制。该叠杯状碳纳米管薄膜易于转移且具有良好的柔性。疏水性测试表明其具有超疏水表面,接触角约为155°,因而有望作为防水和自清洁保护层。     

Evidence of sequential lift in growth of aligned multiwalled carbon nanotube multilayers

We synthesized aligned multiwalled carbon nanotube multilayers by aerosol-assisted catalytic chemical vapor deposition through sequential injections of aerosols containing both carbon and catalyst precursors. Each sequence was traced by a specific duration or precursor mixture, with the carbon source being possibly enriched in (13)C isotope labels. We discovered that any sequence involved the growth of a new layer on the substrate surface, under any pre-existing one by lifting it up, giving definitive evidence of a base-growth mechanism.     

Dispersion control and characterization in multiwalled carbon nanotube and phenylethynyl-terminated imide composites

The inherent multifunctional properties of carbon nanotubes provide an opportunity to create novel composites, but their dispersion into a polymer matrix is challenging due to nanotube dimensions, interparticle forces, and poor interaction with the polymer. In this study, we used melt mixing to disperse multiwalled carbon nanotubes (MWNTs) in a polyimide resin under various process conditions to understand the efficacy of the process and the energy required to achieve dispersion and distribution. Through controlled variation of process conditions, we achieved various degrees of nanotube dispersion and distribution. The different dispersion and distribution states were observed by microscopy and correlated with the magnitude of the changes seen in the glass transition temperature and viscosity when compared to the neat resin. The results of these studies will be used to assess the compatibility of nanocomposite resins with composite fabrication methods and predict appropriate processing conditions for producing multiscale composites.