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We intentionally generated surface defects in WSe2 using a low energy argon (Ar+) ion-beam. We were unable to detect any changes in lattice structure through Raman spectroscopy as expected through simulation. Meanwhile, atomic force microscopy showed roughened surfaces with a high density of large protruding spots. Defect-activated Photoluminescence (PL) revealed a binding energy reduction of the W 4f core level indicating significant amounts of defect generation within the bandgap of WSe2 even at the lowest studied 300 eV ion-beam energy. The intensity ratio increase of direct PL peak demonstrated the decoupling of surface layers, which behave like consecutive defective monolayers. Electrical measurements after post-irradiation showed p-type ohmic contacts regardless of the ion-beam energy. The resulting ohmic contact contributed to an increased on/off current ratio, mobility enhancement of around 350 cm2V-1s-1 from a few cm2V-1s-1 in pristine devices and electron conduction suppression. Further increased ion-beam energy over 700 eV resulted in a high shift of threshold voltage and diminished subthreshold slope due to increased surface roughness and boosted interface scattering. The origin of the ohmic contact behavior in p-type WSe2 is expected to be from chalcogen vacancy defects of a certain size which pins the Fermi level near the valence band minimum. An optimized ion-beam irradiation process could provide solutions for fabricating ohmic contacts to transition metal dichalcogenides.

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Despite intensive studies on van der Waals heterostructures based on two-dimensional layered materials, isotype vdW heterojunctions, which consist of two different semiconductors with the same majority carrier, have received little attention. We demonstrate an n–n isotype field-effect heterojunction device composed of multilayer moly ditelluride (MoTe2) and tin disulfide (SnS2). The carrier transport flowing through the n-MoTe2/n-SnS2 heterojunction exhibits a clear rectifying behavior exceeding 103, even at a moderate source–drain voltage of 1 V in ambient environment. Owing to the large band offsets between the two materials, a potential barrier exceeding ~1 eV is formed, which is verified by comparing a numerical solution of Poisson’s equation and experimental data. In contrast to the conventional p–n heterostructure operating by diffusion of the minority carrier, we identify the carrier transport is governed by the majority carrier via the thermionic emission and tunneling-mediated process through the potential barrier. Furthermore, the gate voltage can completely turn off the device and even enhance the rectification. A ternary inverter based on the isotype MoTe2/SnS2 heterojunction and a SnS2 channel transistor is demonstrated for potential multivalued logic applications. Our results suggest that the isotype vdW heterojunction will become an able candidate for electronic or optoelectronic devices after suitable band engineering and design optimization.

Recently, attempts to overcome the physical limits of memory devices have led to the development of promising materials and architectures for nextgeneration memory technology. The selector device is one of the essential ingredients of high-density stacked memory systems. However, complicated constituent deposition conditions and thermal degradation are problematic, even with effective selector device materials. Herein, we demonstrate the highly stable and low-threshold voltages of vanadium pentoxide (V 2 O 5 ) nanosheets synthesized by facile chemical vapor deposition, which have not been previously reported on the threshold switching (TS) properties. The electrons occupying trap sites in poly-crystalline V 2 O 5 nanosheet contribute to the perfectly symmetric TS feature at the bias polarity and low-threshold voltages in V 2 O 5 , confirmed by high-resolution transmission electron microscopy measurements. Furthermore, we find an additional PdO interlayer in V 2 O 5 nanodevices connected with a Pd/Au electrode after thermal annealing treatment. The PdO interlayer decreases the threshold voltages, and the I on /I off ratio increases because of the increased trap density of V 2 O 5 . These studies provide insights into V 2 O 5 switching characteristics, which can support low power consumption in nonvolatile memory devices.