Simone Pisana

Monitoring dopants by Raman scattering in an electrochemically top-gated graphene transistor

We demonstrate electrochemical top gating of graphene by using a solid polymer electrolyte. This allows to reach much higher electron and hole doping than standard back gating. In-situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, while the 2D peak shows a different response to holes and electrons. Its position increases for hole doping, while it softens for high electron doping. The variation of G peak position is a signature of the non-adiabatic Kohn anomaly at

Breakdown of the adiabatic Born-Oppenheimer approximation in graphene.

\Gamma

Raman fingerprint of charged impurities in graphene

. On the other hand, for visible excitation, the variation of the 2D peak position is ruled by charge transfer. The intensity ratio of G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor charges.The recent discovery of graphene has led to many advances in two-dimensional physics and devices. The graphene devices fabricated so far have relied on SiO(2) back gating. Electrochemical top gating is widely used for polymer transistors, and has also been successfully applied to carbon nanotubes. Here we demonstrate a top-gated graphene transistor that is able to reach doping levels of up to 5x1013 cm-2, which is much higher than those previously reported. Such high doping levels are possible because the nanometre-thick Debye layer in the solid polymer electrolyte gate provides a much higher gate capacitance than the commonly used SiO(2) back gate, which is usually about 300 nm thick. In situ Raman measurements monitor the doping. The G peak stiffens and sharpens for both electron and hole doping, but the 2D peak shows a different response to holes and electrons. The ratio of the intensities of the G and 2D peaks shows a strong dependence on doping, making it a sensitive parameter to monitor the doping.

Phonon renormalization in doped bilayer graphene

Engineering Department, Cambridge University, 9 JJ Thomson Avenue, Cambridge CB3 0FA,UK IMPMC, Universités Paris 6 et 7, CNRS, IPGP, 140 rue de Lourmel, 75015 Paris, France Department of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, UK The Born-Oppenheimer approximation (BO) [1] is the standard ansatz to describe the interaction between electrons and nuclei. BO assumes that the lighter electrons adjust adiabatically to the motion of the heavier nuclei, remaining at any time in their instantaneous ground-state. BO is well justified when the energy gap between ground and excited electronic states is larger than the energy scale of the nuclear motion. In metals, the gap is zero and phenomena beyond BO (such as phonon-mediated superconductivity or phonon-induced renormalization of the electronic properties) occur [2]. The use of BO to describe lattice motion in metals is, therefore, questionable [3, 4]. In spite of this, BO has proven effective for the accurate determination of chemical reactions [5], molecular dynamics [6, 7] and phonon frequencies [9, 8, 10] in a wide range of metallic systems. Graphene, recently discovered in the free state [11, 12], is a zero band-gap semiconductor [13], which becomes a metal if the Fermi energy is tuned applying a gate-voltage Vg [14, 12]. Graphene electrons near the Fermi energy have twodimensional massless dispersions, described by Dirac cones. Here

Ultra-high coercivity small-grain FePt media for thermally assisted recording (invited)

We report strong variations in the Raman spectra for different single-layer graphene samples obtained by micromechanical cleavage. This reveals the presence of excess charges, even in the absence of intentional doping. Doping concentrations up to ∼1013cm−2 are estimated from the G peak shift and width and the variation of both position and relative intensity of the second order 2D peak. Asymmetric G peaks indicate charge inhomogeneity on a scale of less than 1μm.

Thermal and chemical vapor deposition of Si nanowires: Shape control, dispersion, and electrical properties

We report phonon renormalization in bilayer graphene as a function of doping. The Raman G peak stiffens and sharpens for both electron and hole doping as a result of the nonadiabatic Kohn anomaly at the Gamma point. The bilayer has two conduction and valence subbands, with splitting dependent on the interlayer coupling. This gives a change of slope in the variation of G peak position with doping which allows a direct measurement of the interlayer coupling strength.

Graphene Magnetic Field Sensors

Thermally assisted magnetic recording (TAR), a promising approach to extend data storage densities beyond 1 terabit/in.2, requires high anisotropy granular magnetic media with small grains and a tight grain size distribution. We demonstrate sputtered chemically ordered granular L10 Fe45Pt45Ag10 media using carbon segregant on glass substrates. X-ray diffraction and transmission electron microscopy reveal high chemical ordering, an average grain size of 〈D〉 = 7.2 nm and a size distribution as low as σD/〈D〉=16%. Magnetic properties studied with a vibrating sample magnetometer show Hc = 4.8 T, Hk > 9 T, and Ku > 4.5 × 107 erg/cm3. Drag testing of this media shows recording areal densities of 620 Gb/in.2.

Ion Beam Doping of Silicon Nanowires

We investigate and compare complementary approaches to SiNW production in terms of yield, morphology control, and electrical properties. Vapor-phase techniques are considered, including chemical vapor deposition (with or without the assistance of a plasma) and thermal evaporation. We report Au-catalyzed nucleation of SiNWs at temperatures as low as 300°C using SiH4 as precursor. We get yields up to several milligrams by metal-free condensation of SiO powders. For all processes, we control the final nanostructure morphology. We then report concentrated and stable dispersions of SiNWs in solvents compatible with semiconducting organic polymers. Finally, we investigate the electrical response of intrinsic SiNWs grown by different methods. All our SiNWs exhibit p-type behavior and comparable performance, though in some cases ambipolar devices are observed. Thus, processing and morphology, rather than the growth technique, are key to achieve optimal samples for applications.

Tunable Nanoscale Graphene Magnetometers

Graphene extraordinary magnetoresistance (EMR) devices have been fabricated and characterized in varying magnetic fields at room temperature. The atomic thickness, high carrier mobility and high current carrying capabilities of graphene are ideally suited for the detection of nanoscale sized magnetic domains. The device sensitivity can reach 10 mV/Oe, larger than state of the art InAs 2DEG devices of comparable size and can be tuned by the electric field effect via a back gate or by imposing a biasing magnetic field.

Resolving the role of femtosecond heated electrons in ultrafast spin dynamics

We demonstrate n- and p-type field-effect transistors based on Si nanowires (SiNWs) implanted with P and B at fluences as high as 10(15) cm (-2). Contrary to what would happen in bulk Si for similar fluences, in SiNWs this only induces a limited amount of amorphization and structural disorder, as shown by electrical transport and Raman measurements. We demonstrate that a fully crystalline structure can be recovered by thermal annealing at 800 degrees C. For not-annealed, as-implanted NWs, we correlate the onset of amorphization with an increase of phonon confinement in the NW core. This is ion-dependent and detectable for P-implantation only. Hysteresis is observed following both P and B implantation.