Scalable Substitutional Re-Doping and its Impact on the Optical and Electronic Properties of Tungsten Diselenide.

Reliable, controlled doping of 2D transition metal dichalcogenides will enable the realization of next-generation electronic, logic-memory, and magnetic devices based on these materials. However, to date, accurate control over dopant concentration and scalability of the process remains a challenge. Here, a systematic study of scalable in situ doping of fully coalesced 2D WSe2 films with Re atoms via metal-organic chemical vapor deposition is reported. Dopant concentrations are uniformly distributed over the substrate surface, with precisely controlled concentrations down to <0.001% Re achieved by tuning the precursor partial pressure. Moreover, the impact of doping on morphological, chemical, optical, and electronic properties of WSe2 is elucidated with detailed experimental and theoretical examinations, confirming that the substitutional doping of Re at the W site leads to n-type behavior of WSe2 . Transport characteristics of fabricated back-gated field-effect-transistors are directly correlated to the dopant concentration, with degrading device performances for doping concentrations exceeding 1% of Re. The study demonstrates a viable approach to introducing true dopant-level impurities with high precision, which can be scaled up to batch production for applications beyond digital electronics.

Low-Power and Ultra-Thin MoS2 Photodetectors on Glass.

Integration of low-power consumer electronics on glass can revolutionize the automotive and transport sectors, packaging industry, smart building and interior design, healthcare, life science engineering, display technologies, and many other applications. However, direct growth of high-performance, scalable, and reliable electronic materials on glass is difficult owing to low thermal budget. Similarly, development of energy-efficient electronic and optoelectronic devices on glass requires manufacturing innovations. Here, we accomplish both by relatively low-temperature (<600 °C) metal-organic chemical vapor deposition growth of atomically thin MoS2 on multicomponent glass and fabrication of low-power phototransistors using atomic layer deposition (ALD)-grown, high-k, and ultra-thin (∼20 nm) Al2O3 as the top-gate dielectric, circumventing the challenges associated with the ALD nucleation of oxides on inert basal planes of van der Waals materials. The MoS2 photodetectors demonstrate the ability to detect low-intensity visible light at high speed and low energy expenditure of ∼100 pico Joules. Furthermore, low device-to-device performance variation across the entire 1 cm2 substrate and aggressive channel length scalability confirm the technology readiness level of ultra-thin MoS2 photodetectors on glass.

Study on the Growth Parameters and the Electrical and Optical Behaviors of 2D Tungsten Disulfide.

Transition-metal dichalcogenides (TMDCs) with atomic thickness are promising materials for next-generation electronic and optoelectronic devices. Herein, we report uniform growth of triangular-shaped (∼40 µm) monolayer WS2 using the atmospheric-pressure chemical vapor deposition (APCVD) technique in a hydrogen-free environment. We have studied the optical and electrical behaviors of as-grown WS2 samples. The absorption spectrum of monolayer WS2 shows two intense excitonic absorption peaks, namely, A (∼630 nm) and B (∼530 nm), due to the direct gap transitions at the K point. Photoluminescence (PL) and fluorescence studies reveal that under the exposure of green light, monolayer WS2 gives very strong red emission at ∼663 nm. This corresponds to the direct band gap and strong excitonic effect in monolayer WS2. Furthermore, the efficacy of the synthesized WS2 crystals for electronic devices is also checked by fabricating field-effect transistors (FETs). FET devices exhibit an electron mobility of µ ∼ 6 cm2 V-1 s-1, current ON/OFF ratio of ∼106, and subthreshold swing (SS) of ∼641 mV decade-1, which are comparable to those of the exfoliated monolayer WS2 FETs. These findings suggest that our APCVD-grown WS2 has the potential to be used for next-generation nanoelectronic and optoelectronic applications.

Mobility Deception in Nanoscale Transistors: An Untold Contact Story.

Mobility is a critical parameter that is routinely used for benchmarking the performance of field-effect transistors (FETs) based on novel nanomaterials. In fact, mobility values are often used to champion nanomaterials since high-performance devices necessitate high mobility values. The current belief is that the contacts can only limit the FET performance and hence the extracted mobility is an underestimation of the true channel mobility. However, here, such misconception is challenged through rigorous experimental effort, backed by numerical simulations, to demonstrate that overestimation of mobility occurs in commonly used geometries and in nanomaterials for which the contact interface, contact doping, and contact geometry play a pivotal role. In particular, dual-gated FETs based on multilayer MoS2 and WSe2 are used as case studies in order to elucidate and differentiate between intrinsic and extrinsic contact effects manifesting in the mobility extraction. The choice of 2D layered transition metal dichalcogenides (TMDCs) as the semiconducting channel is motivated by their potential to replace and/or coexist with Si-based aging FET technologies. However, the results are equally applicable to nanotube- and nanowire-based FETs, oxide semiconductors, and organic-material-based thin-film FETs.

Mimicking Neurotransmitter Release in Chemical Synapses via Hysteresis Engineering in MoS2 Transistors.

Neurotransmitter release in chemical synapses is fundamental to diverse brain functions such as motor action, learning, cognition, emotion, perception, and consciousness. Moreover, improper functioning or abnormal release of neurotransmitter is associated with numerous neurological disorders such as epilepsy, sclerosis, schizophrenia, Alzheimer's disease, and Parkinson's disease. We have utilized hysteresis engineering in a back-gated MoS2 field effect transistor (FET) in order to mimic such neurotransmitter release dynamics in chemical synapses. All three essential features, i.e., quantal, stochastic, and excitatory or inhibitory nature of neurotransmitter release, were accurately captured in our experimental demonstration. We also mimicked an important phenomenon called long-term potentiation (LTP), which forms the basis of human memory. Finally, we demonstrated how to engineer the LTP time by operating the MoS2 FET in different regimes. Our findings could provide a critical component toward the design of next-generation smart and intelligent human-like machines and human-machine interfaces.