An inductively coupled plasma (ICP) process was used to synthesize transition metal dichalcogenides (TMDs) through a plasma-assisted selenization process of metal oxide (MO x ) at a temperature as low as 250 °C. In comparison with other CVD processes, the use of ICP facilitates the decomposition of the precursors at low temperatures. Therefore, the temperature required for the formation of TMDs can be drastically reduced. WSe 2 was chosen as a model material system due to its technological importance as a p-type inorganic semiconductor with an excellent hole mobility. Large-area synthesis of WSe 2 on polyimide (30 × 40 cm 2 ) flexible substrates and 8 in. silicon wafers with good uniformity was demonstrated at the formation temperature of 250 °C confirmed by Raman and X-ray photoelectron (XPS) spectroscopy. Furthermore, by controlling different H 2 /N 2 ratios, hybrid WO x /WSe 2 films can be formed at the formation temperature of 250 °C confirmed by TEM and XPS. Remarkably, hybrid films composed of partially reduced WO x and small domains of WSe 2 with a thickness of ∼5 nm show a sensitivity of 20% at 25 ppb at room temperature, and an estimated detection limit of 0.3 ppb with a S/N > 10 for the potential development of a low-cost plastic/wearable sensor with high sensitivity.

Recently, a few attempts to synthesize monolayers of transition metal dichalcogenides (TMDs) using the chemical vapor deposition (CVD) process had been demonstrated. However, the development of alternative processes to synthesize TMDs is an important step because of the time-consuming, required transfer and low thermal efficiency of the CVD process. Here, we demonstrate a method to achieve few-layers WSe2 on an insulator via laser irradiation assisted selenization (LIAS) process directly, for which the amorphous WO3 film undergoes a reduction process in the presence of selenium gaseous vapors to form WSe2, utilizing laser annealing as a heating source. Detailed growth parameters such as laser power and laser irradiation time were investigated. In addition, microstructures, optical and electrical properties were investigated. Furthermore, a patternable WSe2 concept was demonstrated by patterning the WO3 film followed by the laser irradiation. By combining the patternable process, the transfer-free WSe2 back gate field effect transistor (FET) devices are realized on 300 nm-thick SiO2/P(+)Si substrate with extracted field effect mobility of ∼0.2 cm(2) V(-1) s(-1). Similarly, the reduction process by the laser irradiation can be also applied for the synthesis of other TMDs such as MoSe2 from other metal oxides such as MO3 film, suggesting that the process can be further extended to other TMDs. The method ensures one-step process to fabricate patternable TMDs, highlighting the uniqueness of the laser irradiation for the synthesis of different TMDs.

To date, the chemical vapor deposition (CVD) process is the most popular approach because of its high yield and quality. Nevertheless, the need for a high temperature and the relatively long process time within each cycle hinder the commercial development in terms of production cost. In this work, we demonstrate a fast (<3 min) and feasible approach to synthesizing a few-layers of WSe 2 and MoSe 2 on arbitrary substrates by a microwave-assisted selenization process. The transition metal dichalcogenides (TMDs) can be patterned by standard photolithography. Furthermore, controllable layered growth from horizontal to vertical alignment can be achieved, leading to an enhanced chemical catalytic activity caused by large edge sites (exposed areas). As a proof, a highly sensitive NO gas sensor based on vertical WSe 2 was fabricated. Moreover, our microwaveassisted selenization process can be further extended to achieve other two-dimensional TMD materials because of the simplicity of the process.

Although chemical vapor deposition is the most common method to synthesize transition metal dichalcogenides (TMDs), several obstacles, such as the high annealing temperature restricting the substrates used in the process and the required transfer causing the formation of wrinkles and defects, must be resolved. Here, we present a novel method to grow patternable two-dimensional (2D) transition metal disulfides (MS2) directly underneath a protective coating layer by spin-coating a liquid chalcogen precursor onto the transition metal oxide layer, followed by a laser irradiation annealing process. Two metal sulfides, molybdenum disulfide (MoS2) and tungsten disulfide (WS2), are investigated in this work. Material characterization reveals the diffusion of sulfur into the oxide layer prior to the formation of the MS2. By controlling the sulfur diffusion, we are able to synthesize continuous MS2 layers beneath the top oxide layer, creating a protective coating layer for the newly formed TMD. Air-stable and low-power photosensing devices fabricated on the synthesized 2D WS2 without the need for a further transfer process demonstrate the potential applicability of TMDs generated via a laser irradiation process.