Zweidimensionale Kohlenstoffkristalle : elektrischer Transport in Einzel- und Doppellagen-Graphen

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Berry Phase Transition in Twisted Bilayer Graphene

The electronic dispersion of a graphene bilayer is highly dependent on rotational mismatch between layers and can be further manipulated by electrical gating. This allows for an unprecedented control over electronic properties and opens up the possibility of flexible band structure engineering. Here we present novel magnetotransport data in a twisted bilayer, crossing the energetic border between decoupled monolayers and coupled bilayer. In addition a transition in Berry phase between pi and 2pi is observed at intermediate magnetic fields. Analysis of Fermi velocities and gate induced charge carrier densities suggests an important role of strong layer asymmetry for the observed phenomena.Comment: 20 pages main paper + 10 pages supporting informatio

Superlattice structures in twisted bilayers of folded graphene

The electronic properties of bilayer graphene strongly depend on relative orientation of the two atomic lattices. Whereas Bernal-stacked graphene is most commonly studied, a rotational mismatch between layers opens up a whole new field of rich physics, especially at small interlayer twist. Here we report on magnetotransport measurements on twisted graphene bilayers, prepared by folding of single layers. These reveal a strong dependence on the twist angle, which can be estimated by means of sample geometry. At small rotation, superlattices with a wavelength in the order of 10 nm arise and are observed by friction atomic force microscopy. Magnetotransport measurements in this small-angle regime show the formation of satellite Landau fans. These are attributed to additional Dirac singularities in the band structure and discussed with respect to the wide range of interlayer coupling models

Terahertz detectors based on graphene

In this study we present magnetotransport an magnetooptical data obtained in the magnetic field range 0T < B < 7T at detectors patterned in Corbino geometry on epitaxial graphene wafer using a Ge detector. We observed the cyclotron resonance of charge carriers in these wafers by measurement of the transmission of THz wafes through the unpatterned squares (about 4 × 4mm2) of the wafers as a function of the magnetic field B applied perpendicular to the wafer. Further, we performed measurements of the photocunductivity of graphene-based devices shaped in Corbino geometry, induced by terahertz (THz) radiation generated by a p-Ge laser (emitting in the energy range 7.5meV ≤ Eph ≤ 11meV). Our photoconductivity measurement imply that graphene devices are suitable for the detection of terahertz radiation

Large Thermoelectricity via Variable Range Hopping in Chemical Vapor Deposition Grown Single-Layer MoS₂

Ultrathin layers of semiconducting molybdenum disulfide (MoS₂) offer significant prospects in future electronic and optoelectronic applications. Although an increasing number of experiments bring light into the electronic transport properties of these crystals, their thermoelectric properties are much less known. In particular, thermoelectricity in chemical vapor deposition grown MoS₂, which is more practical for wafer-scale applications, still remains unexplored. Here, for the first time, we investigate these properties in grown single layer MoS₂. Microfabricated heaters and thermometers are used to measure both electrical conductivity and thermopower. Large values of up to ∼30 mV/K at room temperature are observed, which are much larger than those observed in other two-dimensional crystals and bulk MoS₂. The thermopower is strongly dependent on temperature and applied gate voltage with a large enhancement at the vicinity of the conduction band edge. We also show that the Seebeck coefficient follows <i>S</i> ∼ <i>T</i>^1/3, suggesting a two-dimensional variable range hopping mechanism in the system, which is consistent with electrical transport measurements. Our results help to understand the physics behind the electrical and thermal transports in MoS₂ and the high thermopower value is of interest to future thermoelectronic research and application

Transport Properties of Monolayer MoS₂ Grown by Chemical Vapor Deposition

Recent success in the growth of monolayer MoS₂ via chemical vapor deposition (CVD) has opened up prospects for the implementation of these materials into thin film electronic and optoelectronic devices. Here, we investigate the electronic transport properties of individual crystallites of high quality CVD-grown monolayer MoS₂. The devices show low temperature mobilities up to 500 cm² V^–1 s^–1 and a clear signature of metallic conduction at high doping densities. These characteristics are comparable to the electronic properties of the best mechanically exfoliated monolayers in literature, verifying the high electronic quality of the CVD-grown materials. We analyze the different scattering mechanisms and show that the short-range scattering plays a dominant role in the highly conducting regime at low temperatures. Additionally, the influence of optical phonons as a limiting factor is discussed