12 research outputs found
Zweidimensionale Kohlenstoffkristalle : elektrischer Transport in Einzel- und Doppellagen-Graphen
[no abstract
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 MoS<sub>2</sub>
Ultrathin layers of semiconducting
molybdenum disulfide (MoS<sub>2</sub>) 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 MoS<sub>2</sub>, which is more
practical for wafer-scale applications, still remains unexplored.
Here, for the first time, we investigate these properties in grown
single layer MoS<sub>2</sub>. 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 MoS<sub>2</sub>. 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><sup>1/3</sup>, 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 MoS<sub>2</sub> and the high
thermopower value is of interest to future thermoelectronic research
and application
Transport Properties of Monolayer MoS<sub>2</sub> Grown by Chemical Vapor Deposition
Recent
success in the growth of monolayer MoS<sub>2</sub> 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 MoS<sub>2</sub>.
The devices show low temperature mobilities up to 500 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> 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