Despite being widely investigated for their memristive behavior, ferroelectrics are barely studied as channel materials in field-effect transistor (FET) configurations.

A predictive approach to grain size control from 10 nm to 100 μm is demonstrated in chemical vapor deposited MoS 2 monolayers. Such control is critical to enabling consistent 2D electronics. Physico-chemical modeling involving adsorption− diffusion−growth−desorption equilibrium has been used to correlate this variation to the change in supersaturation and kinetics on the growth surface. The intentional addition of reaction products to the source chemistry shows that nucleation density (and hence final grain size) is very sensitive to supersaturation in the very initial stage of growth. The steady-state nucleation and edge growth rates are diffusion-controlled by a ∼1 eV barrier. The different dependencies of the nucleation rate and edge growth rate on surface kinetics and supersaturation have been exploited to reduce nucleation density from 10 7 to 10 3 cm −2 while simultaneously increasing edge growth rates to as large as 3.3 μm/s. Rapid coverage, <1 min, over large areas by monolayers with 100 μm grain sizes is hence obtained. The microstructural improvement is shown to help increase field-effect electronic mobility from 0.1 to 17 cm 2 /V s.

A 2D/0D heteronanostructure (HNS) employing WSe2 as 2D nanosheets and Fe3O4 as 0D nanoparticles has been facilely synthesized at room temperature using a simple wet chemical route. The process involved liquid-phase exfoliation of WSe2 nanosheets, followed by a coprecipitation method for the subsequent nucleation of nanoparticles on the former. The hence-formed hybrid along with its pristine counterparts has been investigated for ammonia-sensing properties. Herein, WSe2 behaves as a p-type semiconductor and Fe3O4 as an n-type semiconductor as per the trends observed in the modulation of electrical conductivity in the presence of ammonia. As expected, the HNS demonstrated ultrasensitive (R % = 510% to 3 ppm) and selective response toward ammonia at room temperature when compared to WSe2 (53.2% to 3 ppm) and Fe3O4 (128% to 3 ppm) alone. The 10-fold increase in sensitivity for ammonia sensing achieved by fabricating a heterostructure enabled the detection down to 50 ppb with a response magnitude of 2.4%. Moreover, our sensor exhibits an ultrafast recovery of 13 s toward 50 ppb NH3 at room temperature without any external stimulus. Importantly, the repeatability and long-term stability over a period of few months seem to be promising. Therefore, the sensor can reliably be deployed in a real environment for practical gas-sensing applications. The exemplary gas-sensing performance achieved here can be ascribed to the enlarged specific surface area (219 m2/g) and the electronic effect of type II p–n heterostructures. This work can pave the way for the utilization of HNS of other 2D/0D materials for the ultrasensitive and selective gas-sensing applications.

Ion-sensitive field-effect transistors (ISFETs) have gained a lot of attention in recent times as compact, low-cost biosensors with fast response time and label-free detection. Dual gate ISFETs have been shown to enhance detection sensitivity beyond the Nernst limit of 59 mV pH−1 when the back gate dielectric is much thicker than the top dielectric. However, the thicker back-dielectric limits its application for ultrascaled point-of-care devices. In this work, we introduce and demonstrate a pH sensor, with WSe2(top)/MoS2(bottom) heterostructure based double gated ISFET. The proposed device is capable of surpassing the Nernst detection limit and uses thin high-k hafnium oxide as the gate oxide. The 2D atomic layered structure, combined with nanometer-thick top and bottom oxides, offers excellent scalability and linear response with a maximum sensitivity of 362 mV pH−1. We have also used technology computer-aided (TCAD) simulations to elucidate the underlying physics, namely back gate electric field screening through channel and interface charges due to the heterointerface. The proposed mechanism is independent of the dielectric thickness that makes miniaturization of these devices easier. We also demonstrate super-Nernstian behavior with the flipped MoS2(top)/WSe2(bottom) heterostructure ISFET. The results open up a new pathway of 2D heterostructure engineering as an excellent option for enhancing ISFET sensitivity beyond the Nernst limit, for the next-generation of label-free biosensors for single-molecular detection and point-of-care diagnostics.

NiFe layered double hydroxide (NiFe LDH) grown in the presence of MoS2 (rich in 1T phase) shows exceptional performance metrics for alkaline oxygen evolution reaction (OER) in this class of composites. The as-prepared NiFe LDH/MoS2 composite (abbreviated as MNF) exhibits a low overpotential (η10) of 190 mV; a low Tafel slope of 31 mV dec–1; and more importantly, a high stability in its performance manifested by the delivery of current output for 45 h. It is important to note that this could be achieved with an exceedingly low loading of 0.14 mg cm–2. The mass activity of this composite (97 A g–1) is about 14 times greater than that of the conventional RuO2 (7 A g–1) at η = 200 mV. When normalized with respect to the total metal content, a mass activity of 1000 A g–1 (η = 300 mV) was achieved. Impedance analysis further reveals that the significant reduction in charge-transfer resistance and hence high current density (5 times greater as compared to NiFe LDH at η = 300 mV) observed for MNF is associated with interfacial adsorption kinetics of intermediates (R 1). Significant enhancement in the intrinsic activity of MNF over LDH has been observed through normalization of current with the electrochemically active surface area. Computational studies suggest that the Ni centers in the composite act as the active sites for OER, which is well-corroborated with the observed postreaction appearance of Ni3+ species.

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