Adsorptive graphene doping: Effect of a polymer contaminant

© 2017 Author(s). 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

Stability of graphene doping with MoO<inf>3</inf>and I<inf>2</inf>

We dope graphene by evaporation of MoO_3 or by solution-deposition of I_2 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 MoO_3-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 MoO_3 for full graphene doping.This is the author's accepted manuscript. Copyright (2014) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Applied Physics Letters (volume 105) and may be found at http://scitation.aip.org/content/aip/journal/apl/105/10/10.1063/1.489502

Low temperature growth of fully covered single-layer graphene using a CoCu catalyst.

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

Low-Temperature Growth of Carbon Nanotube Forests Consisting of Tubes with Narrow Inner Spacing Using Co/Al/Mo Catalyst on Conductive Supports.

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.This work has been funded by the European projects Technotubes and Grafol. H.S. acknowledges a research fellowship from the Japanese Society for the Promotion of Science (JSPS).This is the accepted manuscript. The final version is available at http://pubs.acs.org/doi/abs/10.1021/acsami.5b04846

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.This work was partially supported by the European Commission through the project GRAFOL and the COST action IC1208 and by the Ministerio de Economía y Competitividad del Gobierno de España through project MAT2013-45957.This is the accepted manuscript. The final version is available at http://scitation.aip.org/content/aip/journal/apl/107/13/10.1063/1.4932197

Adsorptive graphene doping: Effect of a polymer contaminant

© 2017 Author(s). 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

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 ~5,7002 m(2) 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

Long-Term Passivation of Strongly Interacting Metals with Single-Layer Graphene

The long-term (>18 months) protection of Ni surfaces against oxidation under atmospheric conditions is demonstrated by coverage with single-layer graphene, formed by chemical vapor deposition. In situ, depth-resolved X-ray photoelectron spectroscopy of various graphene-coated transition metals reveals that a strong graphene-metal interaction is of key importance in achieving this long-term protection. This strong interaction prevents the rapid intercalation of oxidizing species at the graphene-metal interface and thus suppresses oxidation of the substrate surface. Furthermore, the ability of the substrate to locally form a passivating oxide close to defects or damaged regions in the graphene overlayer is critical in plugging these defects and preventing oxidation from proceeding through the bulk of the substrate. We thus provide a clear rationale for understanding the extent to which two-dimensional materials can protect different substrates and highlight the key implications for applications of these materials as barrier layers to prevent oxidation