The influence of doping on the Raman intensity of the D band in single walled carbon nanotubes

The D band in the Raman spectra of single walled carbon nanotubes is considered as an indicator of defects in carbon nanotubes. However, its dependence on charge-transfer doping is generally ignored, despite the studied samples are often naturally doped. We studied the intensity of the D band, the ratio of the intensities of the D band and TG band (I_D/I_TG) and the ratio of the intensities of the D and G′ band (I_D/I_G′) in the Raman spectra of the single walled carbon nanotubes in dependence on a doping level. We tested two laser excitation energies viz 2.41 and 1.92 eV, which are in resonance with semiconducting and metallic tubes, respectively in our sample. It is shown that the D band intensity is significantly attenuated in doped carbon nanotubes sample for both semiconducting and metallic tubes. The I_D/I_TG ratio is weakly dependent on doping for semiconducting tubes but for metallic tubes the I_D/I_TG ratio exhibits strong dependence on doping. The I_D/I_G′ ratio is suggested for evaluation of the defects in carbon nanotubes samples since it is less sensitive to doping both for semiconducting and metallic tubes. Nevertheless, for highly doped samples even the I_D/I_G′ ratio exhibits significant dependence on doping level.     

Hydrothermally Grown ZnO Micro/Nanotube Arrays and Their Properties

We reported the optical and wettability properties of aligned zinc oxide micro/nanotube arrays, which were synthesized on zinc foil via a simple hydrothermal method. As-synthesized ZnO micro/nanotubes have uniform growth directions along the [0001] orientations with diameters in the range of 100–700 nm. These micro/nanotubes showed a strong emission peak at 387 nm and two weak emission peaks at 422 and 485 nm, respectively, and have the hydrophobic properties with a contact angle of 121°. Single ZnO micro/nanotube-based field-effect transistor was also fabricated, which shows typical n-type semiconducting behavior.     

Preparation of Aligned Ultra-long and Diameter-controlled Silicon Oxide Nanotubes by Plasma Enhanced Chemical Vapor Deposition Using Electrospun PVP Nanofiber Template

Well-aligned and suspended polyvinyl pyrrolidone (PVP) nanofibers with 8 mm in length were obtained by electrospinning. Using the aligned suspended PVP nanofibers array as template, aligned ultra-long silicon oxide (SiOx) nanotubes with very high aspect ratios have been prepared by plasma-enhanced chemical vapor deposition (PECVD) process. The inner diameter (20–200 nm) and wall thickness (12–90 nm) of tubes were controlled, respectively, by baking the electrospun nanofibers and by coating time without sacrificing the orientation degree and the length of arrays. The micro-PL spectrum of SiOx nanotubes shows a strong blue–green emission with a peak at about 514 nm accompanied by two shoulders around 415 and 624 nm. The blue–green emission is caused by the defects in the nanotubes.     

Formation of single-walled carbon nanotube thin films enriched with semiconducting nanotubes and their application in photoelectrochemical devices

Single-walled carbon nanotube (SWCNT) thin films, containing a high-density of semiconducting nanotubes, were obtained by a gel-centrifugation method. The agarose gel concentration and centrifugation force were optimized to achieve high semiconducting and metallic nanotube separation efficiency at 0.1 wt% agarose gel and 18,000g. The thickness of SWCNT films can be precisely controlled from 65 to 260 nm with adjustable transparency. These SWCNT films were applied in photoelectrochemical devices. Photocurrents generated by semiconducting SWCNT enriched films are 15-35% higher than those by unsorted SWCNT films. This is because of reducing exciton recombination channels as a result of the removal of metallic nanotubes. Thinner films generate higher photocurrents because charge carriers have less chances going in metallic nanotubes for recombination, before they can reach electrodes. Developing more scalable and selective methods for high purity semiconducting SWCNTs is important to further improve the photocurrent generation efficiency by using SWCNT-based photoelectrochemical devices.     

First-principles Studies of the Electronic and Thermoelectric Properties of Misfit Layered Phases of Calcium Cobaltite

The electronic and thermoelectric properties of two phases of calcium cobaltite, a misfit layered compound, are investigated and compared using first-principles DFT calculations. The two phases considered here include the conventional bulk phase that consists of alternating layers of Ca₂CoO₃ and CoO₂, and a new phase that consists of alternating layers of CaCoO₂ and CoO₂, which was recently discovered in nanotubes. Electronic structure calculations reveal that both phases are ferrimagnetic materials with one important difference: the bulk phase is metallic, whereas the nanotubular phase is semiconducting. The metal-to-semiconductor transition that accompanies the Ca₂CoO₃ to CaCoO₂ structural transition is shown to arise from the depletion of free carriers from the donor Ca atoms. The implications of the difference in electronic structure for the thermoelectric performance of these two phases are further examined with Boltzmann transport calculations. Relative to the metallic phase, the semiconducting phase displays appreciably higher Seebeck coefficients at minimal doping levels; these increased Seebeck coefficients compensate for the reduced conductivity and result in large power factors. In conjunction with the fact that the semiconducting phase is peculiar to 1D nanotubes, it is expected that additional effects from quantum confinement could render these low-dimensional materials as promising thermoelectric materials.     

Internal charge transfer in metallicity sorted ferrocene filled carbon nanotube hybrids

Site selective high resolution photoemission and X-ray absorption have revealed the complex interplay between ferrocene and single-walled carbon nanotubes (SWCNTs) upon filling. The use of ultraclean and metallicity sorted nanotubes as starting material and a subsequent ferrocene filling, yields metallicity sorted hybrids, where the details of the internal charge transfer from Fe to the SWCNTs have been distinctly unravelled. An n-type doping of the SWCNTs has been identified, with the peculiarity that purely metallic tubes are prone up to 30% higher doping than their semiconducting counterparts. Concomitantly, the average Fe valency changes from 2 in ferrocene to 2.3 in the ferrocene/semiconducting SWCNT hybrids and to 2.4 in the ferrocene/metallic SWCNT hybrids. The origin of this extra charge transfer has been revealed to be ionic by resonant photoemission, which excludes a finite hybridization between the ferrocene filler and the SWCNTs. This does not only substantiate that the internal charge transfer determines the resulting electronic transport properties in the filled 1D carbon hybrids, but that the metallic or semiconducting character of the encapsulating tubes is crucial towards tailoring and regulating tunable 1D electronic transport properties in these materials that are highly desirable in nanoelectronics.     

Transfer-matrix simulations of electronic transport in single-wall and multi-wall carbon nanotubes

We present simulations of electronic transport in single-wall and multi-wall carbon nanotubes, which are placed between two metallic contacts. We consider situations where the electrons first encounter a singe-wall nanotube (corresponding to either the inner or the outer shell of the (10, 10)@(15, 15)@(20, 20) and (10, 10)@(20, 10)@(20, 20) nanotubes), before encountering the multi-wall structures. The role of this two-step procedure is to enforce the electrons to enter a single shell of the multi-wall nanotubes, and we study how from that point they get redistributed amongst the other tubes. Because of reflections at the metallic contacts, the conductance of finite armchair nanotubes is found to depend on the length of the tubes, with values that alternate between three separate functions. Regarding the transport in multi-wall nanotubes, it is found that the electrons keep essentially propagating in the shell in which they are initially injected, with transfers to the other tubes hardly exceeding one percent of the whole current. In the case where the three tubes are conducting, these transfers are already completed after four nanometers. The conductance and repartition of the current present then oscillations, which are traced to the band structure of the nanotube. The transfers between the shells and the amplitude of these oscillations are significantly reduced when the intermediate tube is semiconducting.     

Optical absorption matrix elements in single-wall carbon nanotubes

The optical absorption matrix element as a function of one-dimensional (1D) wave vector k, and subband index μ of a single wall carbon nanotube is given analytically for linearly polarized light with polarization parallel to the nanotube axis. For armchair nanotubes, it is found that the optical transitions for non-degenerate A symmetry bands are forbidden over the whole 1D k region and the transitions for all other bands are also forbidden at the k = 0 point. Near the Fermi level, the absorption for all metallic nanotubes is found to be approximately zero. For both metallic and semiconducting nanotubes, it is found that the absorption matrix element has a maximum absolute value at the van Hove singularity (vHS) k point around the Fermi energy for each band. The absorption dependence on diameter and chiral angle is also presented for semiconducting nanotubes. For light polarization perpendicular to the nanotube axis, on the other hand, the absorption for nanotubes is generally weak near a vHS.     

Electrical transport properties of small diameter single-walled carbon nanotubes aligned on ST-cut quartz substrates

A method is introduced to isolate and measure the electrical transport properties of individual single-walled carbon nanotubes (SWNTs) aligned on an ST-cut quartz, from room temperature down to 2 K. The diameter and chirality of the measured SWNTs are accurately defined from Raman spectroscopy and atomic force microscopy (AFM). A significant up-shift in the G-band of the resonance Raman spectra of the SWNTs is observed, which increases with increasing SWNTs diameter, and indicates a strong interaction with the quartz substrate. A semiconducting SWNT, with diameter 0.84 nm, shows Tomonaga-Luttinger liquid and Coulomb blockade behaviors at low temperatures. Another semiconducting SWNT, with a thinner diameter of 0.68 nm, exhibits a transition from the semiconducting state to an insulating state at low temperatures. These results elucidate some of the electrical properties of SWNTs in this unique configuration and help pave the way towards prospective device applications.     

Production of predominantly semiconducting double-walled carbon nanotubes

The effects of growth conditions, such as methane flow rates and type of substrate on the distribution, structure and properties of nanotubes were examined. A scanning electron microscope equipped with a Raman spectrometer enabled us to obtain critical information about the structure and electrical properties of the nanotubes simultaneously, and it was shown that these were highly dependent on the methane flow rate. At a methane flow rate of 600 cc/min, we primarily obtained double-walled carbon nanotubes having predominantly semiconducting properties. At a higher methane flow rate (700 cc/min), a mixture of single-walled and double-walled carbon nanotubes was created, most of which were semiconducting. At low methane flow rates (300 and 500 cc/min), metallic multi-walled carbon nanotubes were predominated. Carbon nanotubes grown on a quartz substrate were between 4–10 μm in length, whereas those grown on silicon were longer (∼15–20 μm). The primary growth mechanism observed was base growth, although some cap growth did occur. Based on the results of this study, it is now possible to carefully control the synthesis conditions to produce carbon nanotubes that possess specific electrical properties that suit the desired application.     

Synthesis of vertically aligned carbon nanotubes on metal deposited quartz plates

Without plasma aid, we have successfully synthesized vertically aligned carbon nanotubes (CNTs) on iron-, cobalt- or nickel-deposited quartz plates by chemical vapor deposition with ethylenediamine as a precursor. The amine serves as both etching reagent for the formation of metal nanoparticles and carbon source for the growth of aligned carbon nanotubes. The carbon nanotubes were vertically aligned in high density on a large area of the plain silica substrates. The density and diameter of CNTs is determined by the thickness of the deposited metal film and the length of the tubes can be controlled by varying the reaction time. High-resolution transmission electron microscopy analysis reveals that the synthesized CNTs are multiwalled with a bamboo-like structure. Energy dispersive X-ray spectra demonstrate that the CNTs are formed as tip growths. Raman spectrum provides definite evidence that the prepared CNTs are multiwalled graphitic structure.     

Torsional buckling of double-walled carbon nanotubes

The mechanical instability of doubled-walled carbon nanotubes subject to torsion motion is investigated through molecular dynamics. A newly revealed buckling mode with one or three thin, local rims on the outer tube was discovered while the inner tube shows a helically aligned buckling mode in three dimensions. The distinct buckling modes of the two tubes imply the inapplicability of continuum mechanics modeling in which it is postulated that the buckling modes of the constituent tubes have the same shape. In view of this problem, a new concept of the equivalent thickness of double-walled carbon nanotubes is introduced, which enables the Kromm shell model to be applied to the derivation of the torsional buckling angle without the restraint of the two tubes having identical shapes.     

Selective laser treatment and laser patterning of metallic and semiconducting nanotubes in single walled carbon nanotube films

Single wall carbon nanotubes (SWCNT) synthesized using mass production methods such as pulsed arc deposition consist of a mixture of metallic and semiconducting nanotubes. In this work, we report on an approach for the selective removal of either metallic or semiconducting SWCNT by a heat-treatment process with cw-lasers and pulsed lasers with specific wavelengths. The results show that using ultraviolet–visible radiation (with wavelengths between 473 nm and 632 nm) it is possible to remove predominantly metallic nanotubes. In contrast, near infrared lasers with 785 nm and 1064 nm wavelengths can be used to remove predominantly the semiconducting nanotubes. Finally, the fabrication of SWCNT films with an anisotropic distribution of metallic and semiconducting nanotubes is demonstrated using a direct laser interference pattering method.     

Preparation of Aligned Ultra-long and Diameter-controlled Silicon Oxide Nanotubes by Plasma Enhanced Chemical Vapor Deposition Using Electrospun PVP Nanofiber Template

Well-aligned and suspended polyvinyl pyrrolidone (PVP) nanofibers with 8 mm in length were obtained by electrospinning. Using the aligned suspended PVP nanofibers array as template, aligned ultra-long silicon oxide (SiOx) nanotubes with very high aspect ratios have been prepared by plasma-enhanced chemical vapor deposition (PECVD) process. The inner diameter (20-200 nm) and wall thickness (12-90 nm) of tubes were controlled, respectively, by baking the electrospun nanofibers and by coating time without sacrificing the orientation degree and the length of arrays. The micro-PL spectrum of SiOx nanotubes shows a strong blue-green emission with a peak at about 514 nm accompanied by two shoulders around 415 and 624 nm. The blue-green emission is caused by the defects in the nanotubes.     

Sandwich growth of carbon nanotubes

Direct formation of structures that comprise freestanding CNTs connected to two surfaces was, thus far, not possible. In this article we report a novel approach to grow structured, highly oriented carbon nanotubes that are vertically aligned between a substrate and a massive cover. Growth is feasible at pre-determined, e.g., lithographically defined sites on metallic, semiconducting, or glass substrates. A novel, sandwiched catalyst structure and microwave plasma chemical vapor deposition (CVD) led to the formation of freestanding, small diameter carbon nanotubes. Our new technology offers a simple and scalable pathway to create 3D structured nanotube-based two-terminal electronic devices, device arrays, sensors and corresponding electronic circuits.     

Growth of dense single-walled carbon nanotubes in nano-sized silicon dioxide holes for future microelectronics

Dense and aligned single-walled carbon nanotubes (SWCNTs) were synthesised in nano-sized silicon dioxide holes patterned using electron beam lithography for microelectronics applications. Carbon nanotubes are new materials with potential uses for interconnects and field effect transistors (FETs) of LSI. As single-walled carbon nanotubes have lower resistance than multi-walled carbon nanotubes in close-packed arrangements and show both metallic and semiconducting behaviour, there is a great deal of interest in using dense SWCNTs for low resistive interconnects and high current transistors. Here, we report not only a method for fabrication of SWCNTs in nano-sized holes, but also differences in growth rate and Raman spectroscopy of CNTs in holes of various sizes. The growth rate of CNTs in the holes decreased as the hole size was reduced, due to the amount of carbon radicals diffusing to the catalyst particles at the bottom of the holes.     

Nucleation and aligned growth of multi-wall carbon nanotube films during thermal CVD

The development during early growth of a multi-wall carbon nanotube film by thermal CVD with acetylene (C₂H₂) and hydrogen at 750 °C has been characterized in detail by cross-section transmission electron microscopy. The studies provide information on the nanotube growth mechanisms and the complex catalyst transformations that are essential for the onset of different growth stages. An initial random growth catalysed by supported particles is followed by aerosol growth of aligned tubes. This results in a two-layered film structure, where a film of aligned nanotubes is lifting up an initially formed nanotube network from the substrate.     

Carbon nanotubes from organometallic precursors

Multiwalled as well as single-walled carbon nanotubes are conveniently prepared by the pyrolysis of organometallic precursors such as metallocenes and phthalocyanines in a reducing atmosphere. More importantly, pyrolysis of organometallics alone or in mixture with hydrocarbons yields aligned nanotube bundles with useful field emission and hydrogen storage properties. By pyrolysis of organometallics in the presence of thiophene, Y-junction nanotubes are obtained in large quantities. The Y-junction tubes have a good potential in nanoelectronics. Carbon nanotubes prepared from organometallics are useful to prepare nanowires and nanotubes of other materials such as BN, GaN, SiC, and Si(3)N(4).     

氢气在大量高纯度定向碳纳米管束中的吸附

Multi-walled carbon nanotubes with uniform diameters and automatically assembled into densely aligned bundles prepared by a bulky yield and high purity were applied as hydrogen absorbant at room temperature under moderate pressure to absorb H_2. Both the surface of nanotubes and the interstitial space among tubes are possible sites for hydrogen molecules to adhere. The hydrogen uptake capacity is 0.82% (mass)and the interstitial position is considered as a promising site for hydrogen absorption.     

Vertically aligned carbon nanotube field-effect transistors

Vertically aligned carbon nanotube field-effect transistors (CNTFETs) have been developed using pure semiconducting carbon nanotubes. The source and drain were vertically stacked, separated by a dielectric, and the carbon nanotubes were placed on the sidewall of the stack to bridge the source and drain. Both the effective gate dielectric and gate electrode were normal to the substrate surface. The channel length is determined by the dielectric thickness between source and drain electrodes, making it easier to fabricate sub-micrometer transistors without using time-consuming electron beam lithography. The transistor area is much smaller than the planar CNTFET due to the vertical arrangement of source and drain and the reduced channel area.