Real-time numerical system convertor via two-dimensional WS2-based memristive device.

The intriguing properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs) enable the exploration of new electronic device architectures, particularly the emerging memristive devices for in-memory computing applications. Implementation of arithmetic logic operations taking advantage of the non-linear characteristics of memristor can significantly improve the energy efficiency and simplify the complexity of peripheral circuits. Herein, we demonstrate an arithmetic logic unit function using a lateral volatile memristor based on layered 2D tungsten disulfide (WS2) materials and some combinational logic circuits. Removable oxygen ions were introduced into WS2 materials through oxygen plasma treatment process. The resistive switching of the memristive device caused by the thermophoresis-assisted oxygen ions migration has also been revealed. Based on the characteristics of excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), and spike rate dependent plasticity (SRDP), a real-time numerical system convertor was successfully accomplished, which is a significant computing function of arithmetic logic unit. This work paves a new way for developing 2D memristive devices for future arithmetic logic applications.

Defect-suppressed submillimeter-scale WS2 single crystals with high photoluminescence quantum yields by alternate-growth-etching CVD.

Defects, such as uncontrollable vacancies, will intensively degrade the material properties and device performance of CVD-grown transition metal dichalcogenides (TMDs). Although vacancies can be repaired by some post-processing measures, these treatments are usually time-consuming, complicated and may introduce uncontrollable chemical contaminants into TMDs. How to efficiently suppress the uncontrollable defects during CVD growth and acquire intrinsic high-quality CVD-grown TMDs without any after-treatment remains a critical challenge, and has not yet been well resolved. Here, an alternate-growth-etching (AGE) CVD method was demonstrated to fabricate defect-suppressed submillimeter-scale monolayer WS2 single crystals. Compared with normal CVD, the grain size of the as-grown WS2 can be enlarged by 4-5 times (∼520 µm) and the growth rate of ∼14.4 µm min-1 is also at a high level compared to reported results. Moreover, AGE-CVD can efficiently suppress atomic vacancies in WS2. In every growth-etching cycle, the etching of WS2 occurs preferentially at the defective sites, which will be healed at the following growth stage. As a result, WS2 monolayers obtained by AGE-CVD possess higher crystal quality, carrier mobility (8.3 cm2 V-1 s-1) and PL quantum yield (QY, 52.6%) than those by normal CVD. In particular, such a PL QY is the highest value ever reported for in situ CVD-grown TMDs without any after-treatment, and is even comparable to the values of mechanically exfoliated samples. This AGE-CVD method is also appropriate for the synthesis of other high-quality TMD single crystals on a large-scale.

Ultrafast growth of submillimeter-scale single-crystal MoSe2 by pre-alloying CVD.

The synthesis of large-scale monolayer single-crystal MX2 (M = Mo, W; X = S, Se), a typical transition metal dichalcogenide (TMD), is the premise for their future applications. Compared with insulating substrates such as SiO2 and sapphire, Au is more favourable for the fast growth of TMDs by chemical vapor deposition (CVD). Recently, large-scale single-crystal WX2 was successfully grown and transferred on Au. In sharp contrast, the growth and transfer for monolayer MoX2 is still very challenging, because Au has a higher solubility of Mo and stronger interaction with MoX2 than WX2. Compared with the most studied MoS2, MoSe2 is superior in many aspects because of the narrower band gap and tunable excitonic charging effects. However, the synthesis of large-scale single-crystal MoSe2 on Au has not been reported so far. Here, a pre-alloying CVD method was developed to solve the problems for the growth and non-destructive transfer of MoX2. It has realized the ultrafast growth (30 s) of submillimeter-scale (560 µm) single-crystal MoSe2 for the first time. As-grown samples are strictly monolayers with good optical and electrical properties, which can be easily transferred without sacrificing Au foils by the electrochemical bubbling method. It was found that pre-alloying not only passivates the energetically active sites on Au but also weakens the interaction between Au and MoSe2, which is responsible for the ultrafast growth and easy transfer of MoSe2. This method is also universal for the fast growth and non-destructive transfer of other 2D TMDs.