Ultrafast and Low Temperature Synthesis of Highly Crystalline and Patternable Few-Layers Tungsten Diselenide by Laser Irradiation Assisted Selenization Process

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 <i>via</i> 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 cm² 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

Hyperacmosins H-J, three new polycyclic polyprenylated acylphloroglucinol derivatives from <i>Hypericum acmosepalum</i>

Three new polycyclic polyprenylated acylphloroglucinol derivatives, hyperacmosins H-J (1–3), with four known compounds (4–7), were isolated from the air-dried aerial parts of Hypericum acmosepalum. Especially, compounds 1 and 2 were identified as methylated polycyclic polyprenylated acylphloroglucinol derivatives (mPPAPs). Their structures were established by NMR, HRESIMS and experimental electronic circular dichroism (ECD) spectra. The hepatoprotective activity of seven compounds were evaluated. Compounds 1 and 5 exhibited hepatoprotective activity against paracetamol-induced HepG2 cell damage. </p

Direct Synthesis of Graphene with Tunable Work Function on Insulators via In Situ Boron Doping by Nickel-Assisted Growth

Work function engineering, a precise tuning of the work function, is essential to achieve devices with the best performance. In this study, we demonstrate a simple technique to deposit graphene on insulators with in situ controlled boron doping to tune the work function. At a temperature higher than 1000 °C, the boron atoms substitute carbon sites in the graphene lattice with neighboring carbon atoms, leading to the graphene with a p-type doping behavior. Interestingly, the involvement of boron vapor into the system can effectively accelerate the reaction between nickel vapor and methane, achieving a fast graphene deposition. The changes in surface potential and work function at different doping levels were verified by Kelvin probe force microscopy, for which the work function at different doping levels was shifted between 20 and 180 meV. Finally, the transport mechanism followed by the Mott variable-range hopping model was found due to the strong disorder nature of the system with localized charge-carrier states

Transfer-Free Growth of Atomically Thin Transition Metal Disulfides Using a Solution Precursor by a Laser Irradiation Process and Their Application in Low-Power Photodetectors

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

Wafer-Scale Growth of WSe₂ Monolayers Toward Phase-Engineered Hybrid WO_<i>x</i>/WSe₂ Films with Sub-ppb NO_<i>x</i> Gas Sensing by a Low-Temperature Plasma-Assisted Selenization Process

An inductively coupled plasma (ICP) process was used to synthesize transition metal dichalcogenides (TMDs) through a plasma-assisted selenization process of metal oxide (MO_<i>x</i>) 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. WSe₂ 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 WSe₂ on polyimide (30 × 40 cm²) 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 H₂/N₂ ratios, hybrid WO_<i>x</i>/WSe₂ films can be formed at the formation temperature of 250 °C confirmed by TEM and XPS. Remarkably, hybrid films composed of partially reduced WO_<i>x</i> and small domains of WSe₂ 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 <i>S</i>/<i>N</i> > 10 for the potential development of a low-cost plastic/wearable sensor with high sensitivity