Controlled Plasma Thinning of Bulk MoS2 Flakes for Photodetector Fabrication

Layer-by-layer thinning of two-dimensional materials

Etching technology - one of the representative modern semiconductor device makers - serves as a broad descriptor for the process of removing material from the surfaces of various materials, whether partially or entirely. Meanwhile, thinning technology represents a novel and highly specialized approach within the realm of etching technology. It indicates the importance of achieving an exceptionally sophisticated and precise removal of material, layer-by-layer, at the nanoscale. Notably, thinning technology has gained substantial momentum, particularly in top-down strategies aimed at pushing the frontiers of nano-worlds. This rapid development in thinning technology has generated substantial interest among researchers from diverse backgrounds, including those in the fields of chemistry, physics, and engineering. Precisely and expertly controlling the layer numbers of 2D materials through the thinning procedure has been considered as a crucial step. This is because the thinning processes lead to variations in the electrical and optical characteristics. In this comprehensive review, the strategies for top-down thinning of representative 2D materials (e.g., graphene, black phosphorus, MoS2, h-BN, WS2, MoSe2, and WSe2) based on conventional plasma-assisted thinning, integrated cyclic plasma-assisted thinning, laser-assisted thinning, metal-assisted splitting, and layer-resolved splitting are covered in detail, along with their mechanisms and benefits. Additionally, this review further explores the latest advancements in terms of the potential advantages of semiconductor devices achieved by top-down 2D material thinning procedures.

From Bulk Molybdenum Disulfide (MoS2) to Suspensions of Exfoliated MoS2 in an Aqueous Medium and Their Applications

Two-dimensional (2D) layered materials like graphene, transition-metal dichalcogenides (TMDs), boron nitrides, etc., exhibit unique and fascinating properties, such as high surface-to-volume ratio, inherent mechanical flexibility and robustness, tunable bandgap, and high carrier mobility, which makes them an apt candidate for flexible electronics with low consumption of power. Because of these properties, they are in tremendous demand for advancement in energy, environmental, and biomedical sectors developed through various technologies. The production and scalability of these materials must be sustainable and ecofriendly to utilize these unique properties in the real world. Here, in this current review, we review molybdenum disulfide (MoS2 nanosheets) in detail, focusing on exfoliated MoS2 in water and the applicability of aqueous MoS2 suspensions in various fields. The exfoliation of MoS2 results in the formation of single or few-layered MoS2. Therefore, this Review focuses on the few layers of exfoliated MoS2 that have the additional properties of 2D layered materials and higher excellent compatibility for integration than existing conventional Si tools. Hence, a few layers of exfoliated MoS2 are widely explored in biosensing, gas sensing, catalysis, photodetectors, energy storage devices, a light-emitting diode (LED), adsorption, etc. This review covers the numerous methodologies to exfoliate MoS2, focusing on the various published methodologies to obtain nanosheets of MoS2 from water solutions and their use.

MoS2 Field-Effect Transistor Performance Enhancement by Contact Doping and Defect Passivation via Fluorine Ions and Its Cyclic Field-Assisted Activation

MoS2-based field-effect transistors (FETs) and, in general, transition metal dichalcogenide channels are fundamentally limited by high contact resistance (RC) and intrinsic defects, which results in low drive current and lower carrier mobilities, respectively. This work addresses these issues using a technique based on CF4 plasma treatment in the contacts and further cyclic field-assisted drift and activation of the fluorine ions (F-), which get introduced into the contact region during the CF4 plasma treatment. The F- ions are activated using cyclic pulses applied across the source-drain (S/D) contacts, which leads to their migration to the contact edges via the channel. Further, using ab initio molecular dynamics and density functional theory simulations, these F- ions are found to bond at sulfur (S) vacancies, resulting in their passivation and n-type doping in the channel and near the S/D contacts. An increase in doping results in the narrowing of the Schottky barrier width and a reduction in RC by ∼90%. Additionally, the passivation of S vacancies in the channel enhances the mobility of the FET by ∼150%. The CF4 plasma treatment in contacts and further cyclic field-assisted activation of F- ions resulted in an ON-current (ION) improvement by ∼90% and ∼480% for exfoliated and CVD-grown MoS2, respectively. Moreover, this improvement in ION has been achieved without any deterioration in the ION/IOFF, which was found to be >7-8 orders.

Recent progress in plasma modification of 2D metal chalcogenides for electronic devices and optoelectronic devices

Two-dimensional metal chalcogenides (2D MCs) present a great opportunity for overcoming the size limitation of traditional silicon-based complementary metal-oxide-semiconductor (CMOS) devices. Controllable modulation compatible with CMOS processes is essential for the improvement of performance and the large-scale applications of 2D MCs. In this review, we summarize the recent progress in plasma modification of 2D MCs, including substitutional doping, defect engineering, surface charge transfer, interlayer coupling modulation, thickness control, and nano-array pattern etching in the fields of electronic devices and optoelectronic devices. Finally, challenges and outlooks for plasma modulation of 2D MCs are presented to offer valuable references for future studies.

Paper-based broadband flexible photodetectors with van der Waals materials

Layered metal chalcogenide materials are exceptionally appealing in optoelectronic devices thanks to their extraordinary optical properties. Recently, their application as flexible and wearable photodetectors have received a lot of attention. Herein, broadband and high-performance paper-based PDs were established in a very facile and inexpensive method by rubbing molybdenum disulfide and titanium trisulfide crystals on papers. Transferred layers were characterized by SEM, EDX mapping, and Raman analyses, and their optoelectronic properties were evaluated in a wavelength range of 405–810 nm. Although the highest and lowest photoresponsivities were respectively measured for TiS₃ (1.50 mA/W) and MoS₂ (1.13 μA/W) PDs, the TiS₃–MoS₂ heterostructure not only had a significant photoresponsivity but also showed the highest on/off ratio (1.82) and fast response time (0.96 s) compared with two other PDs. This advantage is due to the band offset formation at the heterojunction, which efficiently separates the photogenerated electron–hole pairs within the heterostructure. Numerical simulation of the introduced PDs also confirmed the superiority of TiS₃–MoS₂ heterostructure over the other two PDs and exhibited a good agreement with the experimental results. Finally, MoS₂ PD demonstrated very high flexibility under applied strain, but TiS₃ based PDs suffered from its fragility and experience a remarkable drain current reduction at strain larger than ± 0.33%. However, at lower strains, all PDs displayed acceptable performances.

Plasma-assisted three-dimensional lightscribe graphene as high-performance supercapacitors

Graphene-based supercapacitors demonstrate extraordinary energy storage capacity due to their layered structure, large effective surface area, high electrical conductivity and acceptable chemical stability. Herein, reduced graphene oxide (rGO)-based supercapacitors were introduced in a simple, and fast method with considerable performance. For this purpose, graphene oxide (GO) was synthesized by the modified Hummers' method and then easily reduced to desired patterns of rGO using a commercial LightScribe DVD drive. In order to increase the effective surface area, as well as the electrical conductivity of rGO layers, oxygen/sulfur hexafluoride plasma was applied to the rGO followed by laser irradiation. By performing such sequential processes, an rGO-based supercapacitor was introduced with a capacitance of about 10.2 F/cm3, which had high stability for more than 1000 consecutive charge-discharge cycles. The fabrication steps of the electrodes were investigated by different analyses such as SEM, TEM, Raman, surface resistance, BET, and XPS measurements. Results showed that these rGO-based electrodes fabricated by sequential processes are very interesting for practical applications of energy storage.

Vertically aligned carbon nanotubes, MoS2-rGo based optoelectronic hybrids for NO2 gas sensing

A simple method is developed through drop-casting techniques to assemble a molybdenum disulfide (MoS2)-reduced graphene oxide (rGO) hybrid on vertically aligned carbon nanotubes (VACNTs) to perform as an optoelectronic device for nitrogen dioxide (NO2) gas sensing at room temperature. The VACNT not only forms an ohmic contact with the hybrid material, but also yields a weak charge impurity scattering in the rGo layers across the channel. These features dramatically affect the optical response of the device to the light through which improve the photoresponsivity by up to 236% and the response time by up to 40% compared to the Au contacted device. Next, the fabricated MoS2-rGo/VACNTs device is employed as a resistor gas sensor for NO2 under in situ exposure to the light at room temperature. Under laser illumination, the sensor demonstrates a high sensitivity of ~ 41% at an inlet NO2 concentration of 100 ppm with a complete recovery time of ~ 150 s which shows comparable improvements relative to the sensor performance in dark condition.