Lorenzo D'Arsié

Low temperature growth of ultra-high mass density carbon nanotube forests on conductive supports

We grow ultra-high mass density carbon nanotube forests at 450 °C on Ti-coated Cu supports using Co-Mo co-catalyst. X-ray photoelectron spectroscopy shows Mo strongly interacts with Ti and Co, suppressing both aggregation and lifting off of Co particles and, thus, promoting the root growth mechanism. The forests average a height of 0.38 μm and a mass density of 1.6 g cm−3. This mass density is the highest reported so far, even at higher temperatures or on insulators. The forests and Cu supports show ohmic conductivity (lowest resistance ∼22 kΩ), suggesting Co-Mo is useful for applications requiring forest growth on conductors.

Stability of graphene doping with MoO3 and I2

We dope graphene by evaporation of MoO3 or by solution-deposition of I2 and assess the doping stability for its use as transparent electrodes. Electrical measurements show that both dopants increase the graphene sheet conductivity and find that MoO3-doped graphene is significantly more stable during thermal cycling. Raman spectroscopy finds that neither dopant creates defects in the graphene lattice. In-situ photoemission determines the minimum necessary thickness of MoO3 for full graphene doping.

Growth of Continuous Monolayer Graphene with Millimeter-sized Domains Using Industrially Safe Conditions

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 CH4 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 (H2 and CH4) 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 cm2 V−1 s−1 in ambient conditions. The growth process of such high-quality graphene with a low H2 concentration and short growth times widens the possibility of industrial mass production.

Stable, efficient p-type doping of graphene by nitric acid

We systematically dope monolayer graphene with different concentrations of nitric acid over a range of temperatures, and analyze the variation of sheet resistance after vacuum annealing up to 300 °C. The optimized HNO3 doping conditions yield sheet resistances as low as 180 Ω sq.−1, which is significantly more stable under vacuum annealing than previously reported values. Raman and photoemission spectroscopy suggest that this stable graphene doping occurs by a bi-modal mechanism. Under mild conditions the dopants are weakly bonded to graphene, but at high acid temperatures and concentrations, the doping is higher and more stable upon post-doping annealing, without causing significant lattice damage. This work shows that large, stable hole concentrations can be induced by transfer doping in graphene.

Growth of continuous graphene by open roll-to-roll chemical vapor deposition

We demonstrate the growth of high-quality, continuous monolayer graphene on Cu foils using an open roll-to-roll (R2R) chemical vapor deposition (CVD) reactor with both static and moving foil growth conditions. N2 instead of Ar was used as carrier gas to reduce process cost, and the concentrations of H2 and CH4 reactants were kept below the lower explosive limit to ensure process safety for reactor ends open to ambient. The carrier mobility of graphene deposited at a Cu foil winding speed of 5 mm/min was 5270–6040 cm2 V−1 s−1 at room temperature (on 50 μm × 50 μm Hall devices). These results will enable the inline integration of graphene CVD for industrial R2R production.

Growth kinetics and growth mechanism of ultrahigh mass density carbon nanotube forests on conductive Ti/Cu supports.

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.

The synergistic effect in the Fe-Co bimetallic catalyst system for the growth of carbon nanotube forests

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.

Carbon nanotube forests as top electrode in electroacoustic resonators

We grow carbon nanotube forests on piezoelectric AlN films and fabricate and characterize nanotube-based solidly mounted bulk acoustic wave resonators employing the forests as the top electrode material. The devices show values for quality factor at anti-resonance of ∼430, and at resonance of ∼100. The effective coupling coefficient is of ∼6%, and the resonant frequencies are up to ∼800 MHz above those observed with metallic top electrodes. AlN promotes a strong catalyst-support interaction, which reduces Fe catalyst mobility, and thus enforces the growth of forests by the base growth mechanism.

Metal-Catalyst-Free Growth of Graphene on Insulating Substrates by Ammonia-assisted Microwave Plasma-Enhanced Chemical Vapor Deposition

We study the metal-catalyst-free growth of uniform and continuous graphene films on different insulating substrates by microwave plasma-enhanced chemical vapor deposition (PECVD) with a gas mixture of C2H2, NH3, and H2 at a relatively low temperature of 700–750 °C. Compared to growth using only C2H2 and H2, the use of NH3 during synthesis is found to be beneficial, in transforming vertical graphene nano-walls into a layer-by-layer film, reducing the density of defects, and improving the graphene quality. The effect of different insulating substrates (including Al2O3 and SiO2) on the growth of graphene was studied under different growth temperatures, implying that the growth-temperature window and catalytic effect vary among insulators. The low activation energy barrier of Al2O3 proves it to be a more suitable substrate for the metal-catalyst-free growth of graphene at low temperature. These directly grown graphene films on insulators can be conveniently integrated into various electronic and optoelectronic applications avoiding any post-growth transfer processes.

Adsorptive graphene doping: Effect of a polymer contaminant

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.