D. W. Horsell

University of Exeter, Exeter, England, United Kingdom

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Publications (26)66.87 Total impact

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    ABSTRACT: Here we demonstrate the growth of transfer-free graphene on SiO2 insulator substrates from sputtered carbon and metal layers with rapid thermal processing in the same evacuation. It was found that graphene always grows atop the stack and in close contact with the Ni. Raman spectra typical of high quality exfoliated monolayer graphene were obtained for samples under optimised conditions with monolayer surface coverage of up to 40% and overall graphene surface coverage of over 90%. Transfer-free graphene is produced on SiO2 substrates with the removal of Ni in acid when Ni thickness is below 100 nm, which effectively eliminates the need to transfer graphene from metal to insulator substrates and paves the way to mass production of graphene directly on insulator substrates. The characteristics of Raman spectrum depend on the size of Ni grains, which in turn depend on the thickness of Ni, layer deposition sequence of the stack and RTP temperature. The mechanism of the transfer-free growth process was studied by AFM in combination with Raman. A model is proposed to depict the graphene growth process. Results also suggest a monolayer self-limiting growth for graphene on individual Ni grains.
    Carbon 08/2013; · 6.16 Impact Factor
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    ABSTRACT: Techniques for mass-production of large area graphene using an industrial scale thin film deposition tool could be the key to the practical realization of a wide range of technological applications of this material. Here, we demonstrate the growth of large area polycrystalline graphene from sputtered films (a carbon-containing layer and a metallic layer) using in-situ or ex-situ rapid thermal processing in the temperature range from 650 to 1000 oC. It was found that graphene always grows on the top surface of the stack, in close contact with the Ni or Ni-silicide. Raman spectra typical of high quality exfoliated monolayer graphene were obtained for samples under optimised conditions. A fast cooling rate was found to be essential to the formation of monolayer graphene. Samples with Ni atop SiC produced the best monolayer graphene spectra with ~40% surface area coverage, whereas samples with Ni below SiC produced poorer quality graphene but 99% coverage. The flexibility of the sputtering process allows further optimization of the growth, with possibility of transferring the graphene to any insulator substrate in vacuum. We present a potential route for the production of graphene-on-insulator wafers, which would facilitate easy integration of graphene into modern semiconductor device process flows.
    09/2012;
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    ABSTRACT: We have experimentally studied the nonlinear nature of electrical conduction in monolayer graphene devices on silica substrates. This nonlinearity manifests itself as a nonmonotonic dependence of the differential resistance on applied DC voltage bias across the sample. At temperatures below ~70K, the differential resistance exhibits a peak near zero bias that can be attributed to self-heating of the charge carriers. We show that the shape of this peak arises from a combination of different energy dissipation mechanisms of the carriers. The energy dissipation at higher carrier temperatures depends critically on the length of the sample. For samples longer than 10um the heat loss is shown to be determined by optical phonons at the silica-graphene interface.
    Physical review. B, Condensed matter 02/2012; 85(16). · 3.77 Impact Factor
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    ABSTRACT: Graphene shows great potential for future electronic devices due to its high carrier mobility and thermal conductivity [1, 2]. An important consideration for such devices is thermal cooling of charge carriers. The use of large current densities results in over heating of charge carriers, and this power must be dissipated to avoid thermal breakdown of the graphene sheet [3]. The main cooling mechanisms are (1) direct transfer of heat to the metallic contacts forming the source and drain of the device via diffusion of electrons [4], (2) transfer of heat to the graphene lattice via scattering of electrons by acoustic phonons of the graphene sheet [5], and (3) transfer of heat directly to the underlying substrate via scattering of electrons by surface mode phonons of the substrate [6].
    Nanotechnology (IEEE-NANO), 2012 12th IEEE Conference on; 01/2012
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    ABSTRACT: We measure the dependence of the conductivity of graphene as a function of magnetic field, temperature and carrier density and discover a saturation of the dephasing length at low temperatures that we ascribe to spin memory effects. Values of the spin coherence length up to eight microns are found to scale with the mean free path. We consider different origins of this effect and suggest that it is controlled by resonant states that act as magnetic-like defects. By varying the level of disorder, we demonstrate that the spin coherence length can be tuned over an order of magnitude.
    Physical review. B, Condensed matter 08/2011; · 3.77 Impact Factor
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    Phys. Rev. B. 07/2011; 85(7):075435.
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    ABSTRACT: The electrical properties of graphene are known to be modified by chemical species that interact with it. We investigate the effect of doping of graphene-based devices by toluene (C6H5CH3). We show that this effect has a complicated character. Toluene is seen to act as a donor, transferring electrons to the graphene. However, the degree of doping is seen to depend on the magnitude and polarity of an electric field applied between the graphene and a nearby electrode. This can be understood in terms of an electrochemical reaction mediated by the graphene crystal.
    Carbon. 01/2011; 49(12):3829-3834.
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    ABSTRACT: Near infrared pump-probe spectroscopy has been used to measure the ultrafast dynamics of photoexcited charge carriers in monolayer and multilayer graphene. We observe two decay processes occurring on 100 fs and 2 ps timescales. The first is attributed to the rapid electron-phonon thermalisation in the system. The second timescale is found to be due to the slow decay of hot phonons. Using a simple theoretical model we calculate the hot phonon decay rate and show that it is significantly faster in monolayer flakes than in multilayer ones. In contrast to recent claims, we show that this enhanced decay rate is not due to the coupling to substrate phonons, since we have also seen the same effect in suspended flakes. Possible intrinsic decay mechanisms that could cause such an effect are discussed.
    Physical review. B, Condensed matter 12/2010; 83(12). · 3.77 Impact Factor
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    ABSTRACT: We study fluctuations of the conductance of micron-sized graphene devices as a function of the Fermi energy and magnetic field. The fluctuations are studied in combination with analysis of weak localization which is determined by the same scattering mechanisms. It is shown that the variance of conductance fluctuations depends not only on inelastic scattering that controls dephasing but also on elastic scattering. In particular, contrary to its effect on weak localization, strong intervalley scattering suppresses conductance fluctuations in graphene. The correlation energy, however, is independent of the details of elastic scattering and can be used to determine the electron temperature of graphene structures.
    Solid State Communications 03/2009; · 1.53 Impact Factor
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    ABSTRACT: We propose a method of measuring the electron temperature T_{e} in mesoscopic conductors and demonstrate experimentally its applicability to micron-size graphene devices in the linear-response regime (T_{e} approximately T, the bath temperature). The method can be especially useful in case of overheating, T_{e}>T. It is based on analysis of the correlation function of mesoscopic conductance fluctuations. Although the fluctuation amplitude strongly depends on the details of electron scattering in graphene, we show that T_{e} extracted from the correlation function is insensitive to these details.
    Physical Review Letters 02/2009; 102(6):066801. · 7.73 Impact Factor
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    ABSTRACT: We have fabricated graphene devices with a top gate separated from the graphene layer by an air gap-a design which does not decrease the mobility of charge carriers under the gate. This gate is used to realize p-n-p structures where the conducting properties of chiral carriers are studied. The band profile of the structures is calculated taking into account the specifics of the graphene density of states and is used to find the resistance of the p-n junctions expected for chiral carriers. We show that ballistic p-n junctions have larger resistance than diffusive ones. This is caused by suppressed transmission of chiral carriers at angles away from the normal to the junction.
    Nano Letters 08/2008; 8(7):1995-9. · 13.03 Impact Factor
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    ABSTRACT: We show that the manifestation of quantum interference in graphene is very different from that in conventional two-dimensional systems. Because of the chiral nature of charge carriers, it is not only sensitive to inelastic, phase-breaking scattering, but also to a number of elastic scattering processes. We study weak localization in different samples and at different carrier densities, including the Dirac region, and find the characteristic rates that determine it. We show how the shape and quality of graphene flakes affect the values of the elastic and inelastic rates and discuss their physical origin.
    Physical Review Letters 03/2008; 100(5):056802. · 7.73 Impact Factor
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    ABSTRACT: We demonstrate quantitative experimental evidence for a weak localization correction to the conductivity in monolayer and bilayer graphene systems. We show how inter- and intra-valley elastic scattering control the correction in small magnetic fields in a way which is unique to graphene. A clear difference in the forms of the correction is observed in the two systems, which shows the importance of the interplay between the elastic scattering mechanisms and how they can be distinguished. Our observation of the correction at zero-net carrier concentration in both systems is clear evidence of the inhomogeneity engendered into the graphene layers by disorder.
    Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 02/2008; 366(1863):245-50. · 2.89 Impact Factor
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    ABSTRACT: We experimentally demonstrate that weak localisation (WL) exists in graphene fabricated by mechanical exfoliation at different carrier densities. We show that it is controlled not only by inelastic dephasing mechanisms, but also by elastic intra- and inter-valley scattering. By reducing the width of the graphene sample, inter-valley scattering is found to be limited by the sample boundaries. Intravalley scattering, which suppresses WL in one valley, is found to be much faster than inter-valley scattering. In the Dirac point, where there is a zero-net carrier density, we observe a distinct change in both elastic and inelastic scattering.
    Physica E Low-dimensional Systems and Nanostructures 01/2008; · 1.86 Impact Factor
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    ABSTRACT: We report the first experimental study of the quantum interference correction to the conductivity of bilayer graphene. Low-field, positive magnetoconductivity due to the weak localisation effect is investigated at different carrier densities, including those around the electroneutrality region. Unlike conventional 2D systems, weak localisation in bilayer graphene is affected by elastic scattering processes such as intervalley scattering. Analysis of the dephasing determined from the magnetoconductivity is complemented by a study of the field- and density-dependent fluctuations of the conductance. Good agreement in the value of the coherence length is found between these two studies.
    09/2007;
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    ABSTRACT: We show that the manifestation of quantum interference in graphene is very different from that in conventional two-dimensional systems. Due to the chiral nature of charge carriers, it is sensitive not only to inelastic, phase-breaking scattering, but also to a number of elastic scattering processes. We study weak localization in different samples and at different carrier densities, including the Dirac region, and find the characteristic rates that determine it. We show how the shape and quality of graphene flakes affect the values of the elastic and inelastic rates and discuss their physical origin.
    08/2007;
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    ABSTRACT: We have performed the first experimental investigation of quantum interference corrections to the conductivity of a bilayer graphene structure. A negative magnetoresistance--a signature of weak localization--is observed at different carrier densities, including the electroneutrality region. It is very different, however, from the weak localization in conventional two-dimensional systems. We show that it is controlled not only by the dephasing time, but also by different elastic processes that break the effective time-reversal symmetry and provide intervalley scattering.
    Physical Review Letters 05/2007; 98(17):176805. · 7.73 Impact Factor
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    ABSTRACT: We study fluctuations in the conductance of a narrow GaAs/AlGaAs quantum wire. In strong magnetic fields we show that these fluctuations originate from a complex mixture of transport processes. Here back-scattering of electrons is determined by the position in energy of the bulk Landau level. We experimentally resolve a diffusion-tunnelling transition as the Fermi level is moved between integer filling factors.
    AIP Conference Proceedings 04/2007; 893:741.
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    ABSTRACT: We have performed the first experimental investigation of quantum interference corrections to the conductivity of a bilayer graphene structure. A negative magnetoresistance - a signature of weak localisation - is observed at different carrier densities, including the electro-neutrality region. It is very different, however, from the weak localisation in conventional two-dimensional systems. We show that it is controlled not only by the dephasing time, but also by different elastic processes that break the effective time-reversal symmetry and provide invervalley scattering. Comment: 4 pages, 4 figures (to be published in PRL)
    Physica E Low-dimensional Systems and Nanostructures 01/2007; · 1.86 Impact Factor
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    ABSTRACT: We investigate experimentally the conductance fluctuations in a high-mobility quantum wire. These fluctuations are found to differ from the universal-type expected for a diffusive system. With applied magnetic field the amplitude and quasi-period of the fluctuations are both enhanced at the field at which maximum boundary-roughness scattering occurs. We explain our results in terms of ballistic trajectories, prevalent in the centre of the wire, being deflected by the field into the impurities in the boundaries which scatter them diffusively.
    Physica E Low-dimensional Systems and Nanostructures 04/2006; 34:580-583. · 1.52 Impact Factor