Group publications

7. Persistent hysteresis in graphene-mica van der Waals heterostructures

J. Mohrmann, K. Watanabe, T. Taniguchi and R. Danneau.

Nanotechnology 26, 015202 (2015).

We report the study of electronic transport in graphene-mica van der Waals heterostructures. We have designed various graphene field-effect devices in which mica is utilized as a substrate and/or gate dielectric. When mica is used as a gate dielectric we observe a very strong positive gate voltage hysteresis of the resistance, which persists in samples that were prepared in a controlled atmosphere down to even millikelvin temperatures. In a double-gated mica-graphene-hBN van der Waals heterostructure, we found that while a strong hysteresis occurred when mica was used as a substrate/gate dielectric, the same graphene sheet on mica substrate no longer showed hysteresis when the charge carrier density was tuned through a second gate with the hBN dielectric. While this hysteretic behavior could be useful for memory devices, our findings confirm that the environment during sample preparation has to be controlled strictly.



6. Charge distribution in metallic single walled carbon nanotube-graphene junctions

P.T. Robert and R. Danneau.

New J. Phys. 16, 013019 (2014).

We report numeric and analytic calculations of the electrostatic properties for armchair carbon nanotube–graphene junctions. Using a semi-empirical method we first demonstrate that the equilibrium distance between a carbon nanotube and a graphene sheet varies with respect to the diameter of the carbon nanotube. We find significantly reduced values compared to AB-stacked graphene sheets in graphite, while even smaller value is found for a fullerene C60 implying a dimensionality dependence of the equilibrium distance between graphene and the other sp2 carbon allotropes. Then, we use conformal mapping and a charge–dipole model to study the charge distribution of the carbon nanotube–graphene junctions in various configurations. We observe that the charges are accumulated/depleted at and near the vicinity of the junctions and that capped carbon nanotubes induce a significantly smaller charge concentration at their ends than the open-end nanotubes. We demonstrate that the carbon nanotube influence on the graphene sheet is limited to only few atomic rows. Such an influence strongly depends on the distance between carbon nanotube and the graphene sheet and scales with the carbon nanotube radius, while the potential difference does not modify the length over which the charge concentration is disturbed by the presence of the tube. By studying the potential landscape of carbon nanotube–graphene junctions, our work could be used as a starting point to model the charge carrier injection in these unconventional systems.





5. High-quality Si3N4 circuits as a platform for graphene-based nanophotonic devices

N. Gruhler, C. Benz, H. Jang, J.-H. Ahn, R. Danneau and W.H.P. Pernice.

Opt. Express 21, 31678 (2013).

Hybrid circuits combining traditional nanophotonic components with carbon-based materials are emerging as a promising platform for optoelectronic devices. We demonstrate such circuits by integrating single layer graphene films with silicon nitride waveguides as a new architecture for broadband optical operation. Using high-quality microring resonators and Mach-Zehnder interferometers with extinction ratios beyond 40 dB we realize flexible circuits for phase-sensitive detection on chip. Hybrid graphene-photonic devices are fabricated via mechanical transfer and lithographic structuring, allowing for prolonged light-matter interactions. Our approach holds promise for studying optical processes in low dimensional physical systems and for realizing electrically tunable photonic circuits.






4. Graphene on boron nitride microwave transistors driven by graphene nanoribbon back-gates

C. Benz, M. Thürmer, F. Wu, Z. Ben Aziza, J. Mohrmann, H.v. Löhneysen, K. Watanabe, T. Taniguchi and R. Danneau.

Appl. Phys. Lett. 102, 033505 (2013).

We have designed ultra-thin graphene microwave transistors by using pre-patterned metal or graphene nanoribbon back-gates and hexagonal boron nitride as a dielectric substrate. Despite the inhomogeneities induced by the graphene transfer process, we show that it is possible to operate these types of devices across a broad range of microwave frequencies. For the graphene nanoribbon gates, we observe a deviation of the current gain from the usual 1/f trend that can be attributed to the large gate resistance of these systems as we demonstrate with our small-signal model. The scattering parameter analysis shows a very limited back-action from the channel onto the graphene nanoribbon gates. Our work thus proves that graphene microwave transistors could be driven by graphene nanoribbon gates.



3. Graphene microwave transistors on sapphire substrates

E. Pallecchi, C. Benz, A.C. Betz, H.v. Löhneysen, B. Plaçais and R. Danneau.

Appl. Phys. Lett. 99, 113502 (2011).

We have developed metal-oxide graphene field-effect transistors (MOGFETs) on sapphire substrates working at microwave frequencies. For monolayers, we obtain a transit frequency up to ~80 GHz for a gate length of 200 nm and a maximum oscillation frequency of about 3 GHz for this specific sample. Given the strongly reduced charge noise for nanostructures on sapphire, the high stability and high performance of this material at low temperature, our MOGFETs on sapphire are well suited for a cryogenic broadband low-noise amplifier.






2. Shot noise and conductivity at high bias in bilayer graphene: Signatures of electron-optical phonon coupling

A. Fay, R. Danneau, J.K. Viljas, F. Wu, M.Y. Tomi, J. Wengler, M. Wiesner and P.J. Hakonen.

Phys. Rev. B 84, 245427 (2011).

We have studied electronic conductivity and shot noise of bilayer graphene (BLG) sheets at high bias voltages and low bath temperature T0 = 4.2 K. As a function of bias, we find initially an increase of the differential conductivity, which we attribute to self-heating. At higher bias, the conductivity saturates and even decreases due to backscattering from optical phonons. The electron-phonon interactions are also responsible for the decay of the Fano factor at bias voltages V > 0.1 V. The high bias electronic temperature has been calculated from shot-noise measurements, and it goes up to ∼1200 K at V = 0.75 V. Using the theoretical temperature dependence of BLG conductivity, we extract an effective electron-optical phonon scattering time τe-op. In a 230-nm-long BLG sample of mobility μ = 3600 cm2 V−1 s−1, we find that τe-op decreases with increasing voltage and is close to the charged impurity scattering time τimp = 60 fs at V = 0.6 V.





1. Shot noise suppression and hopping conduction in graphene nanoribbons

R. Danneau, F. Wu, M.Y. Tomi, J.B. Oostinga, A.F. Morpurgo, and P.J. Hakonen.

Phys. Rev. B 82, 161405(R) (2010).

We have investigated shot noise and conduction of graphene field-effect nanoribbon devices at low temperature.
By analyzing the exponential I-V characteristics of our devices in the transport gap region, we found out that transport follows variable range hopping laws at intermediate bias voltages 1 < Vbias < 12 mV. In parallel, we observe a strong shot noise suppression leading to very low Fano factors. The strong suppression of shot noise is consistent with inelastic hopping, in crossover from one- to two-dimensional regime, indicating
that the localization length lloc<W in our nanoribbons.