67 research outputs found
Electrical Transport Properties of Single-Layer WS2
We report on the fabrication of field-effect transistors based on single and
bilayers of the semiconductor WS2 and the investigation of their electronic
transport properties. We find that the doping level strongly depends on the
device environment and that long in-situ annealing drastically improves the
contact transparency allowing four-terminal measurements to be performed and
the pristine properties of the material to be recovered. Our devices show
n-type behavior with high room-temperature on/off current ratio of ~106. They
show clear metallic behavior at high charge carrier densities and mobilities as
high as ~140 cm2/Vs at low temperatures (above 300 cm2/Vs in the case of
bi-layers). In the insulating regime, the devices exhibit variable-range
hopping, with a localization length of about 2 nm that starts to increase as
the Fermi level enters the conduction band. The promising electronic properties
of WS2, comparable to those of single-layer MoS2 and WSe2, together with its
strong spin-orbit coupling, make it interesting for future applications in
electronic, optical and valleytronic devices
Identification of single nucleotides in MoS2 nanopores
Ultrathin membranes have drawn much attention due to their unprecedented
spatial resolution for DNA nanopore sequencing. However, the high translocation
velocity (3000-50000 nt/ms) of DNA molecules moving across such membranes
limits their usability. To this end, we have introduced a viscosity gradient
system based on room-temperature ionic liquids (RTILs) to control the dynamics
of DNA translocation through a nanometer-size pore fabricated in an atomically
thin MoS2 membrane. This allows us for the first time to statistically identify
all four types of nucleotides with solid state nanopores. Nucleotides are
identified according to the current signatures recorded during their transient
residence in the narrow orifice of the atomically thin MoS2 nanopore. In this
novel architecture that exploits high viscosity of RTIL, we demonstrate
single-nucleotide translocation velocity that is an optimal speed (1-50 nt/ms)
for DNA sequencing, while keeping the signal to noise ratio (SNR) higher than
10. Our findings pave the way for future low-cost and rapid DNA sequencing
using solid-state nanopores.Comment: Manuscript 24 pages, 4 Figures Supporting Information 24 pages, 12
Figures, 2 Tables Manuscript in review Nature Nanotechnology since May 27th
201
Disorder engineering and conductivity dome in ReS2 with electrolyte gating
Atomically thin rhenium disulphide (ReS2) is a member of the transition metal
dichalcogenide (TMDC) family of materials characterized by weak interlayer
coupling and a distorted 1T structure. Here, we report on the electrical
transport study of mono- and multilayer ReS2 with polymer electrolyte gating.
We find that the conductivity of monolayer ReS2 is completely suppressed at
high carrier densities, an unusual feature unique to monolayers, making ReS2
the first example of such a material. While thicker flakes of ReS2 also exhibit
a conductivity dome and an insulator-metal-insulator sequence, they do not show
a complete conductivity suppression at high doping densities. Using dual-gated
devices, we can distinguish the gate-induced doping from the electrostatic
disorder induced by the polymer electrolyte itself. Theoretical calculations
and a transport model indicate that the observed conductivity suppression can
be explained by a combination of a narrow conduction band and Anderson
localization due to electrolyte-induced disorder.Comment: Submitted versio
Defect healing and charge transfer mediated valley polarization in MoS/MoSe/MoS trilayer van der Waals heterostructures
Monolayer transition metal dichalcogenides (TMDC) grown by chemical vapor
deposition (CVD) are plagued by a significantly lower optical quality compared
to exfoliated TMDC. In this work we show that the optical quality of CVD-grown
MoSe is completely recovered if the material is sandwiched in
MoS/MoSe/MoS trilayer van der Waals heterostructures. We show by
means of density-functional theory that this remarkable and unexpected result
is due to defect healing: S atoms of the more reactive MoS layers are
donated to heal Se vacancy defects in the middle MoSe layer. In addition,
the trilayer structure exhibits a considerable charge-transfer mediated valley
polarization of MoSe without the need for resonant excitation. Our
fabrication approach, relying solely on simple flake transfer technique, paves
the way for the scalable production of large-area TMDC materials with excellent
optical quality.Comment: Just accepted for publication in Nano Letters
(http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b00904
Probing magnetism in exfoliated VI3 layers with magnetotransport
We perform magnetotransport experiments on exfoliated multilayers of
, a 2D magnetic semiconductor reported to exhibit Ising
ferromagnetism both in bulk and 2D form. Measurements of the magnetoconductance
performed on field-effect transistors and tunnel barriers consistently show
that the Curie temperature is (significantly larger than in the
bulk; ), irrespective of thickness (between 7 and
20~nm), in agreement with recent magneto-optical experiments. Measurements on
tunnel barriers allow magnetism to be probed down to temperatures much lower
than . They show that below K the magnetoconductance in a
perpendicular magnetic field becomes negative, and that a pronounced hysteresis
appears when the field is applied parallel to the planes. These phenomena,
which have no observable counterpart in magneto-optical measurements, indicate
an evolution of the magnetic state that deviates from that expected from a
simple Ising ferromagnet. We attribute the effect to the large in-plane
component of the magnetization, which we suggest to point in different
directions in different layers. Our results provide new information
about magnetism in exfoliated crystals and illustrate the
complementarity of magnetotransport and magneto-optical measurements to probe
the state of atomically thin 2D magnets.Comment: 10 pages, 5 figure
Observation of ionic Coulomb blockade in nanopores
Emergent behaviour from electron-transport properties is routinely observed in systems with dimensions approaching the nanoscale(1). However, analogous mesoscopic behaviour resulting from ionic transport has so far not been observed, most probably because of bottlenecks in the controlled fabrication of subnanometre nanopores for use in nanofluidics. Here, we report measurements of ionic transport through a single subnanometre pore junction, and the observation of ionic Coulomb blockade: the ionic counterpart of the electronic Coulomb blockade observed for quantum dots. Our findings demonstrate that nanoscopic, atomically thin pores allow for the exploration of phenomena in ionic transport, and suggest that nanopores may also further our understanding of transport through biological ion channels
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