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Liu Z.J.,University of Manchester | Zhong X.,University of Manchester | Liu H.,University of Manchester | Tsai I.-L.,Manchester Center for Mesoscience and Nanotechnology | And 2 more authors.
Electrochimica Acta | Year: 2015

The formation of anodic oxide films on the Ti6Al4V alloy in the NaTESi electrolyte has been studied in the present paper. An anodic film with a shallow pore-like texture was formed after anodizing to 10 V. Porous anodic films with increased porosity were generated after anodizing from 20 to 40 V, and the pores were developed mainly within the α phase. Significant amounts of sodium species were incorporated in the films, and the amount increased with increasing anodizing voltage. The current efficiency for the anodic film growth increased from 10 to 30 V, but decreased from 30 to 40 V due to the occurrence of more oxygen evolution. The film thicknesses determined by RBS were 15 nm, 65 nm, 115 nm and 250 nm at 10, 20, 30 and 40 V respectively. The film thickness generated at 10 V showed good agreement with the thickness of 11 nm revealed by transmission electron microscopy. The Raman spectra indicated that the degree of crystallinity of the anodic film increased at higher voltages. The dielectric permittivity of the film was estimated as ∼118 according to the results from transmission electron microscopy and electrochemical impedance spectroscopy. Single-lap shear bonding tests were employed to compare the strength of adhesively joined titanium alloy anodized to different voltages. The results revealed appreciable increase in bond strength with increasing anodic film thickness. © 2015 Elsevier Ltd. All rights reserved. Source

Lehtinen O.,University of Ulm | Tsai I.-L.,Manchester Center for Mesoscience and Nanotechnology | Jalil R.,Manchester Center for Mesoscience and Nanotechnology | Nair R.R.,Manchester Center for Mesoscience and Nanotechnology | And 3 more authors.
Nanoscale | Year: 2014

Irradiation with high-energy ions has been widely suggested as a tool to engineer properties of graphene. Experiments show that it indeed has a strong effect on graphene's transport, magnetic and mechanical characteristics. However, to use ion irradiation as an engineering tool requires understanding of the type and detailed characteristics of the produced defects which is still lacking, as the use of high-resolution transmission microscopy (HRTEM) - the only technique allowing direct imaging of atomic-scale defects - often modifies or even creates defects during imaging, thus making it impossible to determine the intrinsic atomic structure. Here we show that encapsulating the studied graphene sample between two other (protective) graphene sheets allows non-invasive HRTEM imaging and reliable identification of atomic-scale defects. Using this simple technique, we demonstrate that proton irradiation of graphene produces reconstructed monovacancies, which explains the profound effect that such defects have on graphene's magnetic and transport properties. This finding resolves the existing uncertainty with regard to the effect of ion irradiation on the electronic structure of graphene. © The Royal Society of Chemistry 2014. Source

Nair R.R.,Manchester Center for Mesoscience and Nanotechnology | Tsai I.-L.,Manchester Center for Mesoscience and Nanotechnology | Sepioni M.,Manchester Center for Mesoscience and Nanotechnology | Lehtinen O.,University of Helsinki | And 7 more authors.
Nature Communications | Year: 2013

Control of magnetism by applied voltage is desirable for spintronics applications. Finding a suitable material remains an elusive goal, with only a few candidates found so far. Graphene is one of them and attracts interest because of its weak spin-orbit interaction, the ability to control electronic properties by the electric field effect and the possibility to introduce paramagnetic centres such as vacancies and adatoms. Here we show that the magnetism of adatoms in graphene is itinerant and can be controlled by doping, so that magnetic moments are switched on and off. The much-discussed vacancy magnetism is found to have a dual origin, with two approximately equal contributions; one from itinerant magnetism and the other from dangling bonds. Our work suggests that graphene's spin transport can be controlled by the field effect, similar to its electronic and optical properties, and that spin diffusion can be significantly enhanced above a certain carrier density. © 2013 Macmillan Publishers Limited. All rights reserved. Source

Nair R.R.,Manchester Center for Mesoscience and Nanotechnology | Sepioni M.,Manchester Center for Mesoscience and Nanotechnology | Tsai I.-L.,Manchester Center for Mesoscience and Nanotechnology | Lehtinen O.,University of Helsinki | And 6 more authors.
Nature Physics | Year: 2012

The possibility to induce a magnetic response in graphene by the introduction of defects has been generating much interest, as this would expand the already impressive list of its special properties and allow novel devices where charge and spin manipulation could be combined. So far there have been many theoretical studies (for reviews, see refs 1-3) predicting that point defects in graphene should carry magnetic moments μ ∼ μ B and these can in principle couple (anti)ferromagnetically 1-12. However, experimental evidence for such magnetism remains both scarce and controversial 13-16. Here we show that point defects in graphene - (1) fluorine adatoms in concentrations x gradually increasing to stoichiometric fluorographene CF x=1.0 (ref. 17) and (2) irradiation defects (vacancies) - carry magnetic moments with spin 1/2. Both types of defect lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene's low-temperature magnetic properties, despite the fact that the maximum response we could achieve was limited to one moment per approximately 1,000 carbon atoms. This limitation is explained by clustering of adatoms and, for the case of vacancies, by the loss of graphene's structural stability. Our work clarifies the controversial issue of graphene's magnetism and sets limits for other graphitic compounds. © 2012 Macmillan Publishers Limited. All rights reserved. Source

Ponomarenko L.A.,University of Manchester | Geim A.K.,University of Manchester | Geim A.K.,Manchester Center for Mesoscience and Nanotechnology | Zhukov A.A.,Manchester Center for Mesoscience and Nanotechnology | And 11 more authors.
Nature Physics | Year: 2011

Disordered conductors with resistivity above the resistance quantum h/e 2 should exhibit an insulating behaviour at low temperatures, a universal phenomenon known as a strong (Anderson) localization. Observed in a multitude of materials, including damaged graphene and its disordered chemical derivatives, Anderson localization has not been seen in generic graphene, despite its resistivity near the neutrality point reaching ‰h/e 2 per carrier type. It has remained a puzzle why graphene is such an exception. Here we report a strong localization and the corresponding metal-insulator transition in ultra-high-quality graphene. The transition is controlled externally, by changing the carrier density in another graphene layer placed at a distance of several nm and decoupled electrically. The entire behaviour is explained by electron-hole puddles that disallow localization in standard devices but can be screened out in double-layer graphene. The localization that occurs with decreasing rather than increasing disorder is a unique occurrence, and the reported double-layer heterostructures presents a new experimental system that invites further studies. © 2011 Macmillan Publishers Limited. All rights reserved. Source

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