A bimetallic CoCu alloy thin-film catalyst is developed that enables the growth of uniform, high-quality graphene at 750 °C in 3 min by chemical vapour deposition. The growth outcome is found to vary significantly as the Cu concentration is varied, with ∼1 at% Cu added to Co yielding complete coverage single-layer graphene growth for the conditions used. The suppression of multilayer formation is attributable to Cu decoration of high reactivity sites on the Co surface which otherwise serve as preferential nucleation sites for multilayer graphene. X-ray photoemission spectroscopy shows that Co and Cu form an alloy at high temperatures, which has a drastically lower carbon solubility, as determined by using the calculated Co-Cu-C ternary phase diagram. Raman spectroscopy confirms the high quality (ID/IG < 0.05) and spatial uniformity of the single-layer graphene. The rational design of a bimetallic catalyst highlights the potential of catalyst alloying for producing two-dimensional materials with tailored properties.

Transfer-induced contamination of graphene and the limited stability of adsorptive dopants are two of the main issues faced in the practical realization of graphene-based electronics. Herein, we assess the stability of HNO3, MoO3, and AuCl3 dopants upon transferred graphene with different extents of polymer contamination. Sheet resistivity measurements prove that polymer residues induce a significantly degenerative effect in terms of doping stability for HNO3 and MoO3 and a highly stabilizing effect for AuCl3. Further characterization by Raman spectroscopy and atomic force microscopy (AFM) provides insight into the stability mechanism. Together, these findings demonstrate the relevance of contamination in the field of adsorptive doping for the realization of graphene-based functional devices.

We investigate the interfacial chemistry between Fe catalyst foils and monolayer hexagonal boron nitride (h-BN) following chemical vapor deposition and during subsequent atmospheric exposure, using scanning electron microscopy, X-ray photoemission spectroscopy, and scanning photoelectron microscopy. We show that regions of the Fe surface covered by h-BN remain in a metallic state during exposure to moist air for ∼40 h at room temperature. This protection is attributed to the strong interfacial interaction between h-BN and Fe, which prevents the rapid intercalation of oxidizing species. Local Fe oxidation is observed on bare Fe regions and close to defects in the h-BN film (e.g., domain boundaries, wrinkles, and edges), which over the longer-term provide pathways for slow bulk oxidation of Fe. We further confirm that the interface between h-BN and metallic Fe can be recovered by vacuum annealing at ∼600 °C, although this is accompanied by the creation of defects within the h-BN film. We discuss the importance of these findings in the context of integrated manufacturing and transfer-free device integration of h-BN, particularly for technologically important applications where h-BN has potential as a tunnel barrier such as magnetic tunnel junctions.

We systematically dope monolayer graphene with different concentrations of nitric acid over a range of temperatures, and analyze the variation of sheet resistance under vacuum annealing up to 300 °C.

We demonstrate the growth of continuous monolayer graphene films with millimeter-sized domains on Cu foils under intrinsically safe, atmospheric pressure growth conditions, suitable for application in roll-to-roll reactors. Previous attempts to grow large domains in graphene have been limited to isolated graphene single crystals rather than as part of an industrially useable continuous film. With both appropriate pre-treatment of the Cu and optimization of the CH 4 supply, we show that it is possible to grow continuous films of monolayer graphene with millimeter scale domains within 80 min by chemical vapour deposition. The films are grown under industrially safe conditions, i.e., the flammable gases (H 2 and CH 4 ) are diluted to well below their lower explosive limit. The high quality, spatial uniformity, and low density of domain boundaries are demonstrated by charge carrier mobility measurements, scanning electron microscope, electron diffraction study, and Raman mapping. The hole mobility reaches as high as ~5,700 cm 2 V −1 s −1 in ambient conditions. The growth process of such high-quality graphene with a low H 2 concentration and short growth times widens the possibility of industrial mass production.

High sensitive sensors are one of the goals of the modern society due to an increasing necessity for real-time monitoring. To achieve this, new devices and materials are being developed nowadays. In particular, electroacoustic resonators have gained importance in the chemical and biological sensors field owing to their promising performances. Although important advances have been made in the electroacoustic field, additional work is still required to improve the quality factor of the acoustic resonators and their sensing layer. Carbon nanotubes (CNTs) are good candidate materials to act as sensing layers but they can also be used as electrodes. Here we use CNT forests as top electrodes for solidly mounted resonators (SMR) based in AlN thin films and we study their influence on the latters. We prove that, under specific conditions, SMRs do not degrade during the high-temperature CNTs growth process. Also, CNT forests have a weak influence on the SMRs acoustic characteristics. The main drawback regarding their use is a large parasitic resistance presented by the devices. However, this can be solved by depositing denser CNT forests or by growing them on metallic top electrodes layers. SMRs with CNT top electrode can be used as high sensitive and high selective gas sensors due to their high performance and increased functionalized area that CNTs offer.

We have systematically studied the macroscopic adhesive properties of vertically aligned nanotube arrays with various packing density and roughness. Using a tensile setup in shear and normal adhesion, we find that there exists a maximum packing density for nanotube arrays to have adhesive properties. Too highly packed tubes do not offer intertube space for tube bending and side-wall contact to surfaces, thus exhibiting no adhesive properties. Likewise, we also show that the surface roughness of the arrays strongly influences the adhesion properties and the reusability of the tubes. Increasing the surface roughness of the array strengthens the adhesion in the normal direction, but weakens it in the shear direction. Altogether, these results allow progress toward mimicking the gecko’s vertical mobility.

We dope nanotube forests using evaporated MoO3 and observe the forest resistivity to decrease by 2 orders of magnitude, reaching values as low as ∼5 × 10(-5) Ωcm, thus approaching that of copper. Using in situ photoemission spectroscopy, we determine the minimum necessary MoO3 thickness to dope a forest and study the underlying doping mechanism. Homogenous coating and tube compaction emerge as key factors for decreasing the forest resistivity. When all nanotubes are fully coated with MoO3 and packed, conduction channels are created both inside the nanotubes and on the outside oxide layer. This is supported by density functional theory calculations, which show a shift of the Fermi energy of the nanotubes and the conversion of the oxide into a layer of metallic character. MoO3 doping removes the need for chirality control during nanotube growth and represents a step forward toward the use of forests in next-generation electronics and in power cables or conductive polymers.

We grow dense carbon nanotube forests at 450 °C on Cu support using Co/Al/Mo multilayer catalyst. As a partial barrier layer for the diffusion of Co into Mo, we apply very thin Al layer with the nominal thickness of 0.50 nm between Co and Mo. This Al layer plays an important role in the growth of dense CNT forests, partially preventing the Co-Mo interaction. The forests have an average height of ∼300 nm and a mass density of 1.2 g cm(-3) with tubes exhibiting extremely narrow inner spacing. An ohmic behavior is confirmed between the forest and Cu support with the lowest resistance of ∼8 kΩ. The forest shows a high thermal effusivity of 1840 J s(-0.5) m(-2) K(-1), and a thermal conductivity of 4.0 J s(-1) m(-1) K(-1), suggesting that these forests are useful for heat dissipation devices.

We report the growth of carbon nanotubes on tetrahedral-amorphous carbon using a Fe–Cu catalyst system at temperatures <500°C. By X-ray photoemission spectroscopy, we show that Cu forms an alloy with Fe during the process of catalyst pretreatment. This not only dramatically enhances the catalytic activity of Fe towards the nucleation of nanotubes at low temperatures, but simultaneously reduces its propensity to diffuse into the bulk of carbon-based substrates, thus minimising support damage. Such results prove the feasibility of growing nanotubes directly on carbon fibres, highlighting their potential for use in composites and fuel cell electrodes.

We grow vertically aligned carbon nanotube forests on refractory conductive films of TiSiN and achieve area densities of (5.1 ± 0.1) × 1012 tubes cm−2 and mass densities of about 0.3 g cm−3. The TiSiN films act as diffusion barriers limiting catalyst diffusion into the bulk of the support, and their low surface energy favours catalyst de-wetting, inducing forests to grow by the root growth mechanism. The nanotube area density is maximised by an additional discontinuous AlOx layer, which inhibits catalyst nanoparticle sintering by lateral surface diffusion. The forests and the TiSiN support show ohmic conduction. These results suggest that TiSiN is the favoured substrate for nanotube forest growth on conductors and liable of finding real applications in microelectronics.

We report the growth of multi-walled carbon nanotube forests employing an active-active bimetallic Fe-Co catalyst. Using this catalyst system, we observe a synergistic effect by which—in comparison to pure Fe or Co—the height of the forests increases significantly. The homogeneity in the as-grown nanotubes is also improved. By both energy dispersive spectroscopy and in-situ x-ray photoelectron spectroscopy, we show that the catalyst particles consist of Fe and Co, and this dramatically increases the growth rate of the tubes. Bimetallic catalysts are thus potentially useful for synthesising nanotube forests more efficiently.

We report the fabrication and characterization of hybrids of vertically-aligned carbon nanotube forests and gold nanoparticles for improved manipulation of their plasmonic properties. Raman spectroscopy of nanotube forests performed at the separation area of nanotube-nanoparticles shows a scattering enhancement factor of the order of 1 × 106. The enhancement is related to the plasmonic coupling of the nanoparticles and is potentially applicable in high-resolution scanning near-field optical microscopy, plasmonics, and photovoltaics.

We evaluate carbon nanotube growth by employing AlSi, TiSiN and TiN as conductive catalyst supports. Using a wide range of chemical vapour deposition conditions, we find that only AlSi and TiSiN yield homogeneously‐sized nanoparticles, which are stable throughout both catalyst preparation and nanotube synthesis processes. This favours the growth of forests with area densities of the order of 1012 nanotubes cm−2. TiN, in contrast, yield lower density forests in a very narrow window process. The forests and the three screened catalyst supports show ohmic conductivity. TiSiN, however, is the only conductor that leads to the very robust growth process. This suggests TiSiN is useful for applications requiring forest growth on conductors and thus warrants further assessment for reducing nanotube diameter and improving area density of the forests.

We evaluate the growth kinetics and growth mechanism of ultrahigh mass density carbon nanotube forests. They are synthesized by chemical vapor deposition at 450 °C using a conductive Ti/Cu support and Co-Mo catalyst system. We find that Mo stabilizes Co particles preventing lift off during the initial growth stage, thus promoting the growth of ultrahigh mass density nanotube forests by the base growth mechanism. The morphology of the forest gradually changes with growth time, mostly because of a structural change of the catalyst particles. After 100 min growth, toward the bottom of the forest, the area density decreases from ∼ 3-6 × 10(11) cm(-2) to ∼ 5 × 10(10) cm(-2) and the mass density decreases from 1.6 to 0.38 g cm(-3). We also observe part of catalyst particles detached and embedded within nanotubes. The progressive detachment of catalyst particles results in the depletion of the catalyst metals on the substrate surfaces. This is one of the crucial reasons for growth termination and may apply to other catalyst systems where the same features are observed. Using the packed forest morphology, we demonstrate patterned forest growth with a pitch of ∼ 300 nm and a line width of ∼ 150 nm. This is one of the smallest patterning of the carbon nanotube forests to date.

We report the growth of vertically-aligned nanotube forests, of up to 0.2mm in height, on an 85:15 sp2:sp3 carbon support with Fe catalyst. This is achieved by purely-thermal chemical vapour deposition with the catalyst pretreated in inert environments. Pretreating the catalyst in a reducing atmosphere causes catalyst diffusion into the support and the growth of defective tubes. Other sp2:sp3 compositions, including graphite, tetrahedral amorphous carbon, and pure diamond, also lead to the growth of defective carbon morphologies. These results pave the way towards controlled growth of forests on carbon fibres. It could give rise to applications in enhanced fuel cell electrodes and better hierarchical carbon fibre-nanotube composites.

We have grown carbon nanotubes using Fe and Ni catalyst films deposited by atomic layer deposition. Both metals lead to catalytically active nanoparticles for growing vertically aligned nanotube forests or carbon fibres, depending on the growth conditions and whether the substrate is alumina or silica. The resulting nanotubes have narrow diameter and wall number distributions that are as narrow as those grown from sputtered catalysts. The state of the catalyst is studied by in-situ and ex-situ X-ray photoemission spectroscopy. We demonstrate multi-directional nanotube growth on a porous alumina foam coated with Fe prepared by atomic layer deposition. This deposition technique can be useful for nanotube applications in microelectronics, filter technology, and energy storage.

We use oxygen plasma to increase the height of forests from similar to 0.2 to >2 mm. The effectiveness of treating alumina by oxygen plasma, prior to iron nanoparticle formation, is studied using cycles of nanotube growth, nanotube burning, and regrowth. This demonstrates that plasma-treated alumina is more resistant to iron bulk diffusion than an untreated one. Secondary ion mass spectroscopy shows there is negligible iron diffusion into the bulk of treated alumina. Plasma treatment of catalyst supports is potentially useful for growth of ultra-high-density nanotube forests for applications such as interconnects in integrated circuits and heat sinks.

We investigate the formation and stability of Fe nanoparticles on TiN and poly-crystalline PtSi films, and their ability to grow carbon nanotubes forests. Using different-microstructure films, coated with or without their native oxides, we show that, upon purely-thermal catalyst pretreatment, PtSi favours the formation of homogenously sized nanoparticles and forest growth, partly due to its low surface energy. TiN, in contrast, leads to much less controllable processes and only when coated with its native oxide, or with thick catalyst films, yields large diameter nanotube forests. The microstructure of the material can dramatically limit catalyst diffusion into the bulk of the support during nanotube growth. These results allow us to establish the general behaviour expected for nanotube growth on any conductive materials.