Alisson R. Cadore

Asymmetric effect of oxygen adsorption on electron and hole mobilities in bilayer graphene: long- and short-range scattering mechanisms.

We probe electron and hole mobilities in bilayer graphene under exposure to molecular oxygen. We find that the adsorbed oxygen reduces electron mobilities and increases hole mobilities in a reversible and activated process. Our experimental results indicate that hole mobilities increase due to the screening of long-range scatterers by oxygen molecules trapped between the graphene and the substrate. First principle calculations show that oxygen molecules induce resonant states close to the charge neutrality point. Electron coupling with such resonant states reduces the electron mobilities, causing a strong asymmetry between electron and hole transport. Our work demonstrates the importance of short-range scattering due to adsorbed species in the electronic transport in bilayer graphene on SiO2 substrates.

Thermally activated hysteresis in high quality graphene/h-BN devices

We report on gate hysteresis of resistance in high quality graphene/hexagonal boron nitride (h-BN) devices. We observe a thermally activated hysteretic behavior in resistance as a function of the applied gate voltage at temperatures above 375 K. In order to investigate the origin of the hysteretic phenomenon, we compare graphene/h-BN heterostructure devices with SiO2/Si back gate electrodes to devices with graphite back gate electrodes. The gate hysteretic behavior of the resistance is present only in devices with an h-BN/SiO2 interface and is dependent on the orientation of the applied gate electric field and sweep rate. We describe a phenomenological model which captures all of our findings based on charges trapped at the h-BN/SiO2 interface. Such hysteretic behavior in graphene resistance must be considered in high temperature applications for graphene devices and may open new routes for applications in digital electronics and memory devices.

Metal-graphene heterojunction modulation via H2 interaction

Combining experiment and theory, we investigate how a naturally created heterojunction (pn junction) at a graphene and metallic contact interface is modulated via interaction with molecular hydrogen (H2). Due to an electrostatic interaction, metallic electrodes induce pn junctions in graphene, leading to an asymmetrical resistance in electronic transport for electrons and holes. We report that the asymmetry in the resistance can be tuned in a reversible manner by exposing graphene devices to H2. The interaction between the H2 and graphene occurs solely at the graphene-contact pn junction and induces a modification on the electrostatic interaction between graphene and metallic contacts. We explain the experimental data with theory providing information concerning the length of the heterojunction and how it changes as a function of H2 adsorption. Our results are valuable for understanding the nature of the metal-graphene interfaces and have potential application for selective sensors of molecular hydrogen.Combining experiment and theory, we investigate how the naturally created heterojunction at a graphene and metallic contact is modulated via interaction with molecular hydrogen (H2). Due to electrostatic interaction, a Cr/Au electrode induces a pn junction in graphene, leading to an asymmetrical resistance between the charge carriers (electron and hole). This asymmetry is well modeled by considering the preferential charge scattering at the pn junction, and we show that it can be modulated in a reversible, selective and asymmetrical manner by exposing H2 to the metal-graphene interface. Our results are valuable for understanding the nature of the metal-graphene interfaces and demonstrate a novel route towards hydrogen sensor application. KEYWORDS: graphene, contact resistance,

Metal-doped carbon nanotubes interacting with vitamin C

In this paper, the structural, electronic and magnetic properties of carbon nanotubes doped with Al, Fe, Mn and Ti atoms interacting with vitamin C molecules are studied through first principles simulations based on the density functional theory. The charge transfers are obtained from the vitamins into the tubes for adsorption and substitutional doping cases. The highest binding energies of vitamin C molecules are calculated for the Al substitutional and Ti adsorbed cases, with values of about 1.20 and 3.26 eV, respectively. The results demonstrated that, depending on doping, the spin polarizations and the conductance characters of the systems can change, which could be relevant to improve the molecule adsorption on carbon nanostructures.

Anomalous Nonlinear Optical Response of Graphene Near Phonon Resonances

In this work we probe the third-order nonlinear optical property of graphene and hexagonal boron nitride and their heterostructure by the use of coherent anti-Stokes Raman spectroscopy. When the energy difference of the two input fields matches the phonon energy, the anti-Stokes emission intensity is enhanced in h-BN, as usually expected, while for graphene an anomalous decrease is observed. This behavior can be understood in terms of a coupling between the electronic continuum and a discrete phonon state. We have also measured a graphene/h-BN heterostructure and demonstrate that the anomalous effect in graphene dominates the heterostructure nonlinear optical response.

Infrared Fingerprints of Natural 2D Talc and Plasmon–Phonon Coupling in Graphene–Talc Heterostructures

Two-dimensional (2D) materials occupy noteworthy place in nanophotonics providing for subwavelength light confinement and optical phenomena dissimilar to those of their bulk counterparts. In the mid-infrared, graphene-based heterostructures and van der Waals crystals of hexagonal boron nitride (hBN) overwhelmingly concentrate the attention by exhibiting real-space nano-optics from plasmons, phonon-polaritons and hybrid plasmon phonon-polaritons quasiparticles. Here we present the mid-infrared nanophotonics of talc, a natural atomically flat layered material, and graphene-talc (G-talc) heterostructures using broadband synchrotron infrared nano-spectroscopy. We achieve wavelength tuning of the talc resonances, assigned to in- and out-of-plane vibrations by changing the thickness of the crystals, which serves as its infrared fingerprints. Moreover, we encounter coupling of the graphene plasmons polaritons with surface optical phonons of talc. As in the case of the G-hBN heterostructures, this coupling configures hybrid surface plasmon phonon-polariton modes causing 30 % increase in intensity for the out-of-plane mode, blue-shift for the in-plane mode and we have succeeded in altering the amplitude of such hybridization by varying the gate voltage. Therefore, our results promote talc and G-talc heterostructures as appealing materials for nanophotonics, like hBN and G-hBN, with potential applications for controllably manipulating infrared electromagnetic radiation at the subdiffraction scale.

Infrared nano-imaging and -spectroscopy of N graphene layers on SiO2 and hexagonal boron nitride substrates.

The optical response of exfoliated graphene on different surfaces (silicon dioxide (SiO2) and hexagonal boron nitride (hBN)) is investigated via scattering-type scanning near-field optical microscopy (s-SNOM) using broadband infrared synchrotron radiation. Basically, we use a commercial s-SNOM microscope integrated into the infrared synchrotron-based beamline to investigate with nanoscale resolution the optical response of different graphene layers on SiO2 or hBN substrates. Comparing atomic force microscopic topography and broadband mid-infrared images (lateral resolution of 30 nm), we confirm that optical response of both systems depends on the specific interactions between graphene and substrate as well as on the number of graphene layers. This dependence is explained by particular interactions of graphene and SiO2, wherein graphene plasmons couple to surface phonon-polaritons of SiO2. In the case of graphene and hBN, we observe coupling of the graphene plasmon to the hyperbolic phonon-polaritons of hBN.

Tailoring resistive switching properties of TiO2 with controlled incorporation of oxide nanoparticles

Reversible resistance states were extensively observed in thin film systems, and their physical properties were in most cases determined by the electric behavior of the dielectric layer placed between contacts. Here we include SnO2 nanoparticles on TiO2 dielectric films, inducing modifications of the resistive switching behavior. We show that the choice of oxide nanoparticles with dielectric constant smaller than the dielectric constant of the main oxide film guides conductive channels, increasing the extension of the Fowler–Nordheim (tunneling) conduction regime during their electroforming as the density of nanoparticles rises. It is found that the SnO2 nanoparticles show reduced impact on the resistive switching response of devices produced following this methodology. The formation of Ti4O7 conductive channels is discussed based on electric measurements as well as on scanning probe and electron microscopy techniques.