Semiconductor−metal contacts as one major challenge have severely hindered the further progress of two-dimensional (2D) electronics.Here, we present a simple and effective strategy to improve the contacts and electrical performances by fabricating van der Waals (vdW) heterostructures with 2D semiconductor MoS 2 and type-II Dirac semimetal PtTe 2 . The semiconductor MoS 2 and Dirac semimetal PtTe 2 nanoflakes are synthesized through CVD routes separately, followed by systematic material characterizations to confirm their structures. Furthermore, we constructed MoS 2 / PtTe 2 vdW heterostructures via a transfer technology with as-grown MoS 2 and PtTe 2 nanoflakes. The field-effect transistor based on MoS 2 /PtTe 2 heterostructures shows ohmic contact and improved electrical performances, such as two-terminal carrier mobility (∼38.2 cm 2 •V −1 •s −1 ) and ON/OFF ratio (∼10 4 ). We ascribe the improvement of contact and electrical performances to the utilization of ultrahigh-conductive layered PtTe 2 as an interlayer. The theoretical calculations demonstrate that the vdW contact can eliminate the Fermi level pinning effect; meanwhile, the ultrastrong covalent-like interlayer coupling guarantees the high-efficiency carrier injection across PtTe 2 and MoS 2 . The concept that synergizes 2D semiconductors as the channel and Dirac semimetal PtTe 2 as an interlayer will offer a promising approach toward the design of high-performance 2D electronics.
Combining the intrinsic superiorities of two-dimensional materials and the emerging demand for neuromorphic computing, two-dimensional memristors have achieved huge advances in materials exploration and synaptic functionality emulation. However, their neuromorphic applications are still in the early stage since digital memristive behaviors for most of them are inconsistent with gradual biological synaptic plasticity. Here, we developed a simple approach to realize analog and thermal tunable memristive behaviors by introducing sulfur vacancies in CVD-grown In 2 Se 3 nanoflakes through secondary sulfurization treatment. The density functional theory and ab initio molecular dynamics simulations confirm that sulfurized and defective In 2 Se 3 can remain thermal stability at elevated temperatures up to 550 K. Therefore, we systematically investigated the temperature-dependent analog and tunable memristive behaviors and realized linear weight update with ultrawide dynamic range in sulfurized In 2 Se 3 at high temperatures. The developed memristive device successfully emulates bio-realistic synaptic functionalities including transformation from short-to long-term plasticity, paired-pulse facilitation, posttetanic potentiation, spike-amplitude-dependent plasticity, spike-rate-dependent plasticity, and spike-time-dependent plasticity effects. It is unveiled that the formation energy of sulfur vacancy is greatly smaller than that of selenium, while their roughly same migration barriers can be modulated by electric field and temperature. Therefore, we put forward that the applied electric field can mediate vacancy migration in the sulfurized In 2 Se 3 to gradually regulate the conductance, thereby realizing the emulation of synaptic plasticity. This work provides a promising approach to designing bio-plausible memristive devices for robust neuromorphic applications at high temperatures.
2D MoS2 films represent a promising direction for electronic and photonic devices, benefiting from their intrinsic semiconducting and ultrathin body. However, grain boundaries (GBs), as a common type of structural defect, are inevitable and significantly impair electrical performance and stability. Understanding the underlying forming mechanisms and influences of GBs is key to scalable MoS2 films with high electrical performance. Here, a phenomenon regarding the formation of twisted GBs and the exotic physical properties near the GB region are reported. A set of microscopies and spectroscopies complemented with theoretical calculations consistently show that the mechanical and electrostatic properties near the GB are distinct from the interior ones. The underlying mechanism is proposed to be that the edge stitching behavior of MoS2 single crystals with a misorientation angle leads to structural corrugation due to lattice deformation, which is responsible for the observed exotic properties. The proposed mechanism is further corroborated by the theoretical calculations of band structure as well as surface potentials for normal and twisted MoS2 monolayers. These results uncover an interesting interplay between the GB style and the physical properties and open new avenues for exploring applications in semiconducting MoS2.
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