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.     

Controlled synthesis of a large fraction of metallic single-walled carbon nanotube and semiconducting carbon nanowire networks

Controlled synthesis of both single-walled carbon nanotube and carbon nanowire networks using the same CVD reactor and Fe/Al(2)O(3) catalyst by slightly altering the hydrogenation and temperature conditions is demonstrated. Structural, bonding and electrical characterization using SEM, TEM, Raman spectroscopy, and temperature-dependent resistivity measurements suggest that the nanotubes are of a high quality and a large fraction (well above the common 33% and possibly up to 75%) of them are metallic. On the other hand, the carbon nanowires are amorphous and semiconducting and feature a controlled sp(2)/sp(3) ratio. The growth mechanism which is based on the catalyst nanoisland analysis by AFM and takes into account the hydrogenation and temperature control effects explains the observed switch-over of the nanostructure growth modes. These results are important to achieve the ultimate control of chirality, structure, and conductivity of one-dimensional all-carbon networks.     

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.     

Synthesis of Y-junction single-wall carbon nanotubes

Y-junction single-wall carbon nanotubes (SWNTs) are synthesized using controlled catalysts by chemical vapor deposition. Mo-doped Fe nanoparticles supported by aluminum oxide particles are used as catalysts for growing Y-junction single-wall carbon nanotubes. Distribution of Mo-doped Fe particles plays an important role in Y-junction formation. Transmission electron microscopy confirmed the formation of single-walled structures of Y-junctions with diameters of 2-5 nm. Radial breathing mode peaks in Raman spectra show that our sample has both metallic and semiconducting nanotubes, indicating the possible formation of Y-branching with different electrical properties. The different electrical properties of branch and stem can be utilized in nanoscale three terminal electronic devices. The growth mechanism of Y-junction SWNTs is proposed.     

Growth of carbon nanotubes by open-air laser-induced chemical vapor deposition

Carbon nanotubes have remarkable mechanical, electronic and electrochemical properties, but the full potential for application will be realized only if the growth of high quantity and quality carbon nanotubes can be optimized and well controlled. In this study, carbon nanotubes have been successfully grown on fused quartz rods by a novel open-air laser-induced chemical vapor deposition (LCVD) technique with gold palladium nanoparticles as catalyst material. In this LCVD technique, a curtain of inert nitrogen gas was used to shield the deposition zone from the surrounding environment and allows the growth of the nanotubes to occur under open-air conditions. A 35-W continuous CO₂ laser was used as a heat source to induce a local temperature rise on the substrate surface covered with metal nanoparticles, subsequently resulting in deposition of multi-wall carbon nanotubes. The carbon nanotubes deposited in this study are derived from a precursor mixture that consists of propane and hydrogen, and are in tangled form with different diameters (10-250 nm) and structures. Raman spectroscopy, transmission and scanning electron microscopy are used to investigate the microstructure and composition of the carbon 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.     

Beryllium doping graphene,graphene-nanoribbons,C60-fullerene,and carbon nanotubes

Beryllium substitutional doping within graphene, graphene nanoribbons, and carbon nanotubes are investigated using first-principles density functional theory calculations. Nanoribbons with armchair and zigzag edges, semiconducting (10,0) and metallic (6,6) carbon nanotubes, and C₆₀ fullerene structures are analyzed. Binding energy, doping energy, band structure, electronic density of states (DOS), and magnetic ordering are calculated. Our results demonstrate that conversely to perfect graphene, Be-doped graphene reveals a semiconducting behavior with an indirect band gap of 0.298 eV. Formation energy analysis reveals that Be into graphene and ribbons is more energetically favorable, but the energies involved are larger than those obtained for B- and N-doped nanocarbons. For nanoribbons, two different ways of incorporating the Be atom are explored (dopant placed in the center or edge), demonstrating that armchair nanoribbons preserve the semiconducting behavior with a reduced band-gap whereas that zigzag nanoribbons exhibit a half-metallic behavior with magnetic order along the edges. Results on Be-doping zigzag (10,0) semiconducting and armchair (6,6) metallic nanotubes and C₆₀ fullerene reveal the appearance of additional electronic states around the Fermi level. We envisage that the present investigation could motivate the realization of future experiments to introduce Be into sp² graphite-like lattice using high temperature chemical vapor deposition method.     

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.     

Simultaneous synthesis of vertically aligned carbon nanotubes and amorphous carbon thin films on stainless steel

Carbonaceous nanostructures such as carbon nanotubes, graphene and transparent carbon-based thin films are envisioned to be part of the next generation of electronic devices, mechanical structures, and energy-storage systems. To synthesize these nanostructures on a large scale by chemical vapor deposition, large-area, flexible substrates are needed. Here, we studied the role of a metallic foil, stainless steel, as a self-catalytic substrate for carbon nanostructure synthesis. As a result, vertically aligned carbon nanotubes and amorphous carbon thin films were simultaneously obtained. We further showed that the evolution of the stainless steel foil during the different steps of the process played a critical role in carbon nanotubes and carbon thin film growth. A better understanding of how the growth of these carbon nanostructures is affected by stainless steel evolution under chemical vapor deposition conditions will enable the synthesis of hybrid carbon nanotubes/amorphous carbon nanostructures and pave the way to scale-up of their low-cost production.     

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.     

Water‐Processable Multiwalled Nanotube: Phenol‐Induced Reversible Oxidation Process and Ambipolar Charge Transport Property

Due to the fascinating electronic, thermal, and mechanical properties of single‐walled carbon nanotubes (SWCNTs), extensive efforts have been devoted to the development of SWCNT‐based materials. These materials' semiconducting properties and related applications, such as field‐effect transistors (FETs), have been investigated by researchers for many years. However, despite the significant progress achieved, it remains challenging to separate semiconducting and metallic nanotubes from the mixtures of as‐grown SWCNTs. In a few studies, composites of water‐processable phenol formaldehyde resin/multiwalled carbon nanotubes (MWCNTs) have been found to exhibit a quasireversible oxidation process and to behave as semiconductors or field‐effect transistors. This finding has rarely been reported for MWCNTs, and it differs greatly from findings regarding intrinsic semiconductive SWCNTs. Significantly, field‐effect transistors fabricated with MWCNT composites as their semiconductor active layers have shown ambipolar charge transport characteristics. The results provide a high value‐added application pathway for the application of polymer/MWCNTs as the FET materials for electronic devices that offer higher performance at a lower cost.     

煤基碳纳米管制备技术和生长机理研究进展

我国煤炭资源丰富,但现阶段以燃烧、热解和气化等为主的传统利用方式存在资源浪费、环境污染和经济效益低等问题,且我国以煤炭资源为主题的能源结构在短期内不会发生改变,因此,清洁、高效利用是新时期煤炭资源的立足点和首要任务。另一方面,碳纳米管因其独特的一维结构在力学、电学和热学等方面具有优异的特性,使其在复合材料、电子材料和能源材料等领域具有广泛的应用,但是碳纳米管制备成本偏高的问题较为突出,严重限制了其大规模的应用,现阶段急需开发新型、环境友好的碳纳米管制备技术。宏量、低成本的煤基碳纳米管制备技术可以同时较好地解决上述2个问题。笔者基于文献重点分析了反应原料、放电气氛和催化剂等因素对电弧放电法和等离子体射流法2种煤基直接制备碳纳米管技术的影响,讨论了原料种类、催化剂、反应温度、升温速率和反应气氛等因素对化学气相沉积法-煤基间接碳纳米管制备技术的影响过程。分析发现,在电弧放电法和等离子体射流法中,原料种类对碳纳米管产物的产量具有重要作用,放电气氛对碳纳米管产物的类型具有重要影响,催化剂对碳纳米管产物产量和类型均具有重要影响;在化学气相沉积法中,原料种类对碳纳米管产物形貌、长径比和有序度等性质具有重要影响,催化剂对碳纳米管产物的生长过程具有重要影响,反应温度和升温速率对碳纳米管产物的管径变化和类型具有重要影响,反应气氛可改变催化剂的催化效果。此外,总结了煤基碳纳米管直接和间接制备技术中碳纳米管的生长机理的类型及特点:其中,直接制备技术中碳纳米管生长过程符合碎片式生长机理,而间接制备技术中碳纳米管生长过程可分为气-液-固(VLS)、气-固-固(VSS)、气相成核(VPN)和阶梯式等类型。分析认为应当深入开展以下工作:探究煤、煤热解气和商业煤气等廉价原料制备碳纳米管的过程,进而建立和完善原料与碳纳米管产物之间的关系体系;开发新型、高效的煤基碳纳米管催化剂制备技术;建立新的碳纳米管生长模型,进一步丰富和完善碳纳米管生长模型体系。     

Carbon nanotubes produced by fluidized bed catalytic CVD: first approach of the process

Multi-walled carbon nanotubes have been produced with high yield on an iron supported catalyst by catalytic chemical vapor deposition in a fluidized bed reactor. The choice of such a technique allows to reach high selectivity towards the desired material. A remarkable feature of this process is the huge bed expansion observed during the nanotubes growth that affects the fluidization regime due to the evolution of the apparent density of the composite powder. The catalytic powder, the composite material and the purified nanotubes have been analyzed by SEM, TEM and BET nitrogen adsorption.     

Controlling the diameter of aligned single-walled carbon nanotubes on quartz via catalyst reduction time

One of the main objectives of chemical vapor deposition (CVD) growth of single-walled carbon nanotubes (SWCNT) is control over their diameter and type (metallic or semiconducting). Here, we investigate the evolution of iron catalyst particles on quartz substrates depending on the duration of the reduction step with hydrogen and its effect on the growth of horizontally aligned SWCNT. We find a strong dependence of catalyst particle size and size distribution on the initial iron film thickness and the reduction time at 630 °C. Initial decrease of the particle size is followed by an unexpected increase. Statistical analysis of the Raman radial breathing modes of the SWCNT over large areas gave reliable and reproducible diameter distributions that correlated directly with the catalyst particle size distributions. By changing the reduction time it was possible to reproducibly shift the average SWCNT diameter from 1.5 (±0.3) nm to 1.2 (±0.2) nm while maintaining a nanotube density of 5–6 SWCNT/μm. In order to describe the evolution of the particle size during the reduction process at this particular temperature, we propose a model that includes the diffusion of iron into the quartz and its re-emergence.     

Carbon nanotubes filled with metallic nanowires

A facile method is proposed to use LaNi₂ hydrogen storage alloy as a catalyst precursor to produce metallic nickel filled carbon nanotubes. Multi-walled carbon nanotubes filled with long continuous nickel nanowire with several microns in length are synthesized through chemical vapor deposition at low temperature (550 °C). It is more efficient to fill Ni nanowires into nanotubes after the oxidation treatment of LaNi₂ alloy at low temperatures, while the oxidation treatment at high temperature results in the forming of herringbone carbon nanofibers with tips of Ni nanoparticles. The metallic Ni nanowires inside the cores of carbon nanotubes could not be eliminated during the purification process in concentrated hydrochloric acid. The analysis of transmission electron microscopy (TEM), selected area electron diffraction (SAED) and X-ray diffraction (XRD) reveals that the metallic nickel nanowires filled inside carbon nanotubes exist as a single crystalline with fcc structure.     

Mass production of high-quality multi-walled carbon nanotube bundles on a Ni/Mo/MgO catalyst

A new catalyst (Ni/Mo/MgO) is reported, with which one can synthesize multi-walled carbon nanotube (MWNT) bundles with a yield of more than 45 times the amount of the pristine catalyst, using a methane-hydrogen mixture as precursor. Powder X-ray diffraction, Raman spectroscopy and thermal gravimetric analysis show that the purity of the as-prepared MWNTs is over 97%. The diameter of the carbon nanotubes is 9-20 nm, measured by high-resolution electron microscopy on 421 individual MWNTs. The high purity of the as-prepared MWNTs allows us to omit the usual complex purification process required for carbon nanotubes synthesized by chemical vapor deposition. Because of its durable high activity, the Ni/Mo/MgO catalyst in its pristine state is ideal for mass production of high-quality MWNTs. The synergism of nickel and molybdenum is considered the main reason for the high yield of carbon nanotubes.     

The synthesis and characterization of carbon nanotubes grown by chemical vapor deposition using a stainless steel catalyst

Multi wall carbon nanotubes (MWCNTs) were grown on a stainless steel (SS) sheet by chemical vapor deposition without the addition of external metal catalyst. We found that the key for highly efficient growth includes the nanoscale roughness of the SS surface, as shown by scanning tunneling microscopy, that acts as catalyst/template in the nanotube formation. Raman spectroscopy and electron microscopy were used to check the nature and quality of the synthesized nanotubes. We conclude that stainless steel favors a base-growth mechanism. Transmission electron energy loss spectroscopy performed on single metallic particles found inside the nanotubes clarified the atomic nature of the catalytic particles supplied by the steel. Only unoxidized iron was found and no traces of nickel and chromium were detected. In addition, the SS substrate has been used for a second growth process after carefully removing the synthesized CNTs, proving that a continuous production of CNTs from the same substrate is achievable.     

Semiconducting properties of cup-stacked carbon nanotubes

The current-voltage characteristics of individual cup-stacked carbon nanotubes (CSCNTs) were investigated in situ inside the transmission electron microscope. Different from other quasi-1D carbon structures such as multi-walled carbon nanotubes, carbon nanofibers or graphitic fibers that normally behave as a metallic conductor of electrons, individual CSCNTs were found to exhibit unexpectedly semiconducting behaviors due to the special stacking microstructure of graphene layers. The band gap of the CSCNTs was obtained with the value of about 0.44 eV, in contrast to the zero-gap semiconducting quasi-2D graphene. These findings provide new information about the effect of the stacking graphene layers on their electronic properties, and will widen the usefulness of such stacking structure for the application in nanoelectronics.     

Two-step pyrolysis process to synthesize highly dispersed Pt-Ru/carbon nanotube catalysts for methanol electrooxidation

Highly dispersed Pt-Ru particles with different atomic ratios supported on carbon nanotubes were synthesized using an easy two-step synthesis method including adsorption and pyrolysis. In this method, the functionalized carbon nanotubes act as adsorption sites for metallic ions and subsequently act as nucleation center for catalyst deposition in the pyrolysis process. The deposited Pt-Ru nanoparticles disperse on the carbon nanotubes surface uniformly, and the bulk composition of the Pt-Ru particles can be adjusted simply by changing atomic ratios of the metallic solution for adsorption. Finally, the electrocatalytic activity of the as-prepared catalysts supported on carbon nanotubes toward oxidation of methanol was studied. Results showed that their electrocatalytic activity, having long-term stability, strongly depends on the atomic ratio of Pt to Ru. The higher the concentration of Pt in the binary system is, the greater the electrocatalytic activity will be.     

Effects of catalyst pre-treatment on the growth of single-walled carbon nanotubes by microwave CVD

Layers of carbon nanotubes were deposited by microwave CVD on oxidized silicon substrates coated with Al-Fe-Mo catalyst films. To achieve a tube growth at about 973 K, the ion bombardment of the catalyst surface has to be avoided. The appropriate pre-treatment of the substrates is essential for the deposition of single-walled carbon nanotubes. Annealing in air is preferable to the frequently used reducing pre-treatment prior to the deposition as a higher area density of the tubes and a better reproducibility of deposition can be obtained. To figure out this finding, selected samples were investigated by analytical transmission electron microscopy and Raman spectroscopy. It is shown that the pre-treatment has a strong effect on the size and distribution of the catalyst particles.