van der Waals (vdW) heterostructures composed of multiple vertical stacks of two-dimensional materials exhibit unique optoelectronic properties compared with their single constituent counterparts. The interlayer coupling between adjacent layers directly affects the transfer of excitons and charges, thereby governing the device performance. Herein, we report that the interlayer energy transfer occurring in a transition-metal dichalcogenide/graphene vdW heterostructure strongly depends on the interlayer distance and modulates photocurrent generation. MoSe 2 /graphene and MoSe 2 / hexagonal boron nitride (h-BN)/graphene heterostructures comprising chemical-vapor-deposition-grown layers show different degrees of photoluminescence (PL) quenching of MoSe 2 with respect to the number of layers and the types of adjacent layers. Comparisons of the Raman and PL spectra revealed that the h-BN interlayer can modulate the long-range exciton energy transfer from MoSe 2 to graphene, as corroborated by the photocurrent measurements from the photoconductor devices. These results underscore the effect of modulating the interlayer coupling in vdW heterostructures on the fabrication and control of optoelectronic devices.
High-precision artificial synaptic devices compatible with existing CMOS technology are essential for realizing robust neuromorphic hardware systems with reliable parallel analogue computation beyond the von Neumann serial digital computing architecture. However, critical issues related to reliability and variability, such as nonlinearity and asymmetric weight updates, have been great challenges in the implementation of artificial synaptic devices in practical neuromorphic hardware systems. Herein, a robust three-terminal two-dimensional (2D) MoS 2 artificial synaptic device combined with a lithium silicate (LSO) solid-state electrolyte thin film is proposed. The rationally designed synaptic device exhibits excellent linearity and symmetry upon electrical potentiation and depression, benefiting from the reversible intercalation of Li ions into the MoS 2 channel. In particular, extremely low cycle-to-cycle variations (3.01%) during long-term potentiation and depression processes over 500 pulses are achieved, causing statistical analogue discrete states. Thus, a high classification accuracy of 96.77% (close to the software baseline of 98%) is demonstrated in the Modified National Institute of Standards and Technology (MNIST) simulations. These results provide a future perspective for robust synaptic device architecture of lithium solid-state electrolytes stacked with 2D van der Waals layered channels for high-precision analogue neuromorphic computing systems.
Recent
progress in the chemical vapor deposition technique toward growing
large-area and single-crystalline two-dimensional (2D) transition
metal dichalcogenides (TMDs) has resulted in an electronic/optoelectronic
device performance that rivals that of their top-down counterparts,
despite the extensive use of hydrogen, a common reducing agent that
readily generates defects in TMDs. Herein, we report that 2D MoSe2 domains containing oxide seeds are resistant to hydrogen-induced
defect generation. Specifically, we observed that the etching of the
edges of seed-containing MoSe2 was significantly less than
that of pristine MoSe2, without apparent seed particles,
under the same H2 annealing conditions. Our systematic
approach for controlling the H2 exposure time indicates
that the oxidation of Mo and the edge roughening of seedless MoSe2 coincidentally increase after H2 exposure owing
to the formation of Se vacancy followed by Mo oxidation, which is
not the case with seed-containing MoSe2. An ab
initio calculation indicates that hydrogen preferentially
adsorbs more onto O bonded to Mo than onto Se, providing further evidence
of the resistance of seeded MoSe2 to hydrogen etching.
This finding provides an insight into controlling defect formation
in 2D TMDs by employing sacrificial adsorption sites for reactive
species (i.e., hydrogen).
Wearable electronic devices with next‐generation biocompatible, mechanical, ultraflexible, and portable sensors are a fast‐growing technology. Hardware systems enabling artificial neural networks while consuming low power and processing massive in situ personal data are essential for adaptive wearable neuromorphic edging computing. Herein, the development of an ultraflexible artificial‐synaptic array device with concrete‐mechanical cyclic endurance consisting of a novel heterostructure with an all‐solid‐state 2D MoS2 channel and LiSiOx (lithium silicate) is demonstrated. Enabled by the sequential fabrication process of all layers, by excluding the transfer process, artificial van der Waals devices combined with the 2D‐MoS2 channel and LiSiOx solid electrolyte exhibit excellent neuromorphic synaptic characteristics with a nonlinearity of 0.55 and asymmetry ratio of 0.22. Based on the excellent flexibility of colorless polyimide substrates and thin‐layered structures, the fabricated flexible neuromorphic synaptic devices exhibit superior long‐term potentiation and long‐term depression cyclic endurance performance, even when bent over 700 times or on curved surfaces with a diameter of 10 mm. Thus, a high classification accuracy of 95% is achieved without any noticeable performance degradation in the Modified National Institute of Standards and Technology. These results are promising for the development of personalized wearable artificial neural systems in the future.
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