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Britnell L.,University of Manchester | Gorbachev R.V.,University of Manchester | Jalil R.,University of Manchester | Belle B.D.,University of Manchester | And 12 more authors.
Nano Letters | Year: 2012

We investigate the electronic properties of ultrathin hexagonal boron nitride (h-BN) crystalline layers with different conducting materials (graphite, graphene, and gold) on either side of the barrier layer. The tunnel current depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field. It offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel. © 2012 American Chemical Society.


Wang L.,Wuhan University | Lin B.,Wuhan University | Zhou L.,Wuhan University | Shang Y.X.,Wuhan University | And 3 more authors.
Materials Letters | Year: 2012

ZnO is a wide band-gap material with excellent optical properties for optoelectronics applications. However, device fabrication has been hampered by difficulties in p-type doping. Nitrogen-doped ZnO nanorods were synthesized through thermal diffusion of nitrogen in an aqueous solution at 90 °C. Low-temperature photoluminescence measured at 10 K showed two peaks located at 3.353 and 3.242 eV, which were assigned to the acceptor-bound excitons and donor-acceptor pairs, respectively. The conductance of the nitrogen-doped ZnO nanorods increased 1.5 times compared with Al-doped samples and 5.8 times compared with undoped ZnO nanorods. The results show hydrothermal process to be an attractive technique for preparation of p-type nitrogen-doped ZnO nanorods. © 2012 Elsevier B.V. All rights reserved.


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.


Moldt T.,Free University of Berlin | Eckmann A.,University of Manchester | Klar P.,Free University of Berlin | Morozov S.V.,Institute for Microelectronics Technology | And 4 more authors.
ACS Nano | Year: 2011

We report large-yield production of graphene flakes on glass by anodic bonding. Under optimum conditions, we counted several tens of flakes with lateral size around 20-30 μm and a few tens of flakes with larger size. About 60-70% of the flakes have a negligible D peak. We show that it is possible to easily transfer the flakes by the wedging technique. The transfer on silicon does not damage graphene and lowers the doping. The charge mobility of the transferred flakes on silicon is on the order of 6000 cm2/V s (at a carrier concentration of 1012 cm-2), which is typical for devices prepared on this substrate with exfoliated graphene. © 2011 American Chemical Society.


Gorbachev R.V.,University of Manchester | Geim A.K.,University of Manchester | Katsnelson M.I.,Radboud University Nijmegen | Novoselov K.S.,University of Manchester | And 8 more authors.
Nature Physics | Year: 2012

Coulomb drag is a frictional coupling between electric currents flowing in spatially separated conducting layers. It is caused by interlayer electron-electron interactions. Previously, only the regime of weak (d ≫ l) to intermediate (d∼l) coupling could be studied experimentally, where d is the interlayer separation and l is the characteristic distance between charge carriers. Here we use graphene-boron-nitride heterostructures with d down to 1nm to probe Coulomb drag in the limit d ≪ l such that the two Dirac liquids effectively nest within the same plane, but can still be tuned and measured independently. The strongly interacting regime reveals many unexpected features. In particular, although drag vanishes because of electron-hole symmetry when either layer is neutral, we often find drag strongest when both layers are neutral. Under this circumstance, drag is positive in zero magnetic field but changes its sign and rapidly grows in strength with field. The drag remains strong at room temperature. The broken electron-hole symmetry is attributed to mutual polarization of closely spaced interacting layers. © 2012 Macmillan Publishers Limited. All rights reserved.


Nair R.R.,University of Manchester | Blake P.,University of Manchester | Blake J.R.,University of Manchester | Zan R.,University of Manchester | And 7 more authors.
Applied Physics Letters | Year: 2010

We demonstrate the application of graphene as a support for imaging individual biological molecules in transmission electron microscope (TEM). A simple procedure to produce free-standing graphene membranes has been designed. Such membranes are extremely robust and can support practically any submicrometer object. Tobacco mosaic virus has been deposited on graphene samples and observed in a TEM. High contrast has been achieved even though no staining has been applied. © 2010 American Institute of Physics.


Mayorov A.S.,University of Manchester | Gorbachev R.V.,University of Manchester | Morozov S.V.,University of Manchester | Morozov S.V.,Institute for Microelectronics Technology | And 8 more authors.
Nano Letters | Year: 2011

Devices made from graphene encapsulated in hexagonal boron-nitride exhibit pronounced negative bend resistance and an anomalous Hall effect, which are a direct consequence of room-temperature ballistic transport at a micrometer scale for a wide range of carrier concentrations. The encapsulation makes graphene practically insusceptible to the ambient atmosphere and, simultaneously, allows the use of boron nitride as an ultrathin top gate dielectric. © 2011 American Chemical Society.


Neubeck S.,University of Manchester | Ponomarenko L.A.,University of Manchester | Freitag F.,University of Manchester | Giesbers A.J.M.,Radboud University Nijmegen | And 5 more authors.
Small | Year: 2010

Quantum dots of around 20 nm in size are fabricated using local anodic oxidation. The behavior of the smallest dots in a magnetic field (see image) allows the identification of the charge-neutrality point and distinguishing of the states with one electron, no charge, and one hole left inside the quantum dot.


Mayorov A.S.,University of Manchester | Elias D.C.,University of Manchester | Mukhin I.S.,University of Manchester | Morozov S.V.,Institute for Microelectronics Technology | And 4 more authors.
Nano Letters | Year: 2012

The above question is frequently asked by theorists who are interested in graphene as a model system, especially in context of relativistic quantum physics. We offer an experimental answer by describing electron transport in suspended devices with carrier mobilities of several 10 6 cm 2 V -1 s -1 and with the onset of Landau quantization occurring in fields below 5 mT. The observed charge inhomogeneity is as low as ≈10 8 cm -2, allowing a neutral state with a few charge carriers per entire micrometer-scale device. Above liquid helium temperatures, the electronic properties of such devices are intrinsic, being governed by thermal excitations only. This yields that the Dirac point can be approached within 1 meV, a limit currently set by the remaining charge inhomogeneity. No sign of an insulating state is observed down to 1 K, which establishes the upper limit on a possible bandgap. © 2012 American Chemical Society.


Elias D.C.,University of Manchester | Gorbachev R.V.,University of Manchester | Mayorov A.S.,University of Manchester | Morozov S.V.,Institute for Microelectronics Technology | And 7 more authors.
Nature Physics | Year: 2011

In graphene, electron-electron interactions are expected to play a significant role, as the screening length diverges at the charge neutrality point and the conventional Landau theory that enables us to map a strongly interacting electronic liquid into a gas of non-interacting fermions is no longer applicable. This should result in considerable changes in graphene's linear spectrum, and even more dramatic scenarios, including the opening of an energy gap, have also been proposed. Experimental evidence for such spectral changes is scarce, such that the strongest is probably a 20% difference between the Fermi velocities v F found in graphene and carbon nanotubes. Here we report measurements of the cyclotron mass in suspended graphene for carrier concentrations n varying over three orders of magnitude. In contrast to the single-particle picture, the real spectrum of graphene is profoundly nonlinear near the neutrality point, and v F describing its slope increases by a factor of more than two and can reach ĝ‰̂3-106 ms -1 at n<1010 cm-2. No gap is found at energies even as close to the Dirac point as 0.1meV. The observed spectral changes are well described by the renormalization group approach, which yields corrections logarithmic in n. © 2011 Macmillan Publishers Limited. All rights reserved.

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