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 MoS2_2/MoSe2_2/MoS2_2 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$_2$ is completely recovered if the material is sandwiched in MoS$_2$/MoSe$_2$/MoS$_2$ 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$_2$ layers are donated to heal Se vacancy defects in the middle MoSe$_2$ layer. In addition, the trilayer structure exhibits a considerable charge-transfer mediated valley polarization of MoSe$_2$ 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 $\rm{VI_3}$, 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 $T_C~=~57~K$ (significantly larger than in the bulk; $T_C^{\rm bulk}~=~50~K$), 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 $T_C$. They show that below $T~\simeq~40$ 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 $\rm{VI_3}$ layers. Our results provide new information about magnetism in exfoliated $\rm{VI_3}$ 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