The unique characteristics of the laser, such as ultra-high-peak power density and short pulse duration, far exceed conventional heating methods, such as electric or gas heating, thus making it possible to exfoliate monolayers of two-dimensional materials. The van der Waals interaction between the layers of TMDC materials can be broken by the laser, which enables the efficient and controllable preparation of TMDCs with a specific number of layers in a relatively short time. In addition, this technique has now been used to exfoliate a variety of 2D TMDC materials such as MoS₂, MoSe₂, and WS₂ under different conditions (Schuffenhauer et al., 2005; Hu et al., 2006; An et al., 2018; Gao et al., 2019; Zhai et al., 2021; Zuo et al., 2021; Kimiagar and Abrinaei, 2023).

TMDCs are layered materials that form an MX₂ crystal structure composed of strongly covalently bonded X-M-X sandwiches, and the X-M-X layers are held together by weak van der Waals forces (An et al., 2018). Zhai et al. (2021) created a method for exfoliating 2H-phase MoS₂ (2H-MoS₂), that is, both efficient and free from contaminants, and also the exfoliation process can be directly observed by optical microscopy (Figure 1A). Compared to other exfoliation techniques, this method is significantly faster and easier. The thickness of the irradiated region underwent a reduction approximately from 100 nm to 5 nm, and the area of the peeled region is also much larger than the diameter of the laser spot used. They believed that the light illumination and water medium are essential for the laser exfoliation of 2D TMDCs. When 2H-MoS₂ layers are exposed to laser irradiation, the rapid vaporization of water molecules between the layers results in the exfoliation of MoS₂ sheets, and the exfoliation process can be controlled by applying a bias voltage. The related research results indicated that water molecules could be incorporated into the interlayer spaces of 2H-MoS₂ sheets (Levita and Righi, 2017; Ma et al., 2018). The phase transition may accompany the exfoliation process. Gao et al. (2019) achieved a one-step exfoliating bulk 2H-MoS₂ into 2H and 1T MoS₂ nanosheets using pulsed laser irradiation. Figure 1B shows a conceptual model of this experimental mechanism, suggesting that the 2H-phase MoS₂ can be reversed into 1T MoS₂ with the assistance of the chemically doped Fe³⁺ ions. In addition, the capability of the protic solvent also provides the proton to the reaction environment and plays a crucial role in triggering the phase transition.

Laser thinning techniques have provided a direct and site-specific method for removing layers and obtaining “on-demand” 2D TMDCs (Castellanos Gomez et al., 2012; Lu et al., 2014; Sunamura et al., 2016; Kim et al., 2017; Park et al., 2017; Gong et al., 2018; Nagareddy et al., 2018; Rho et al., 2019; Tran-Khac et al., 2019; Wang et al., 2020; Kang et al., 2021). Unlike the traditional gas or plasma etching thinning method, laser thinning shows high accuracy on thinned layers, flexible programmable patterning mode, and high thinning efficiency. Nagareddy et al. (2018) demonstrated an efficient laser thinning method, which was constructed by the Raman spectroscopy and coupled with an atomic force microscope. The experimental setup was placed in an airtight environment with strictly controlled humidity. Using ultra-low laser power, the thickness of MoTe₂ film can be controlled layer by layer from multilayer to monolayer, as shown in Figure 1C. In addition, the thickness reduction shows the linear correlation between the thickness reduction and the number of scans. The laser thinning process is achieved through the sublimation of the top layer, mainly caused by the conversion of light absorbed by the material into heat energy (Lu et al., 2014). The heat generated by the conversion of absorbed laser into energy is difficult to dissipate through the substrate due to the poor coupling between TMDC thin layers that are mediated by van der Waals forces (Su et al., 2021). Thus, until the laser power is increased, the bottom layer remains in close contact with the SiO₂/Si substrate, acting as a heat sink to prevent removal.

The exploration and manipulation of a novel phase of matter is a primary pursuit for materials research. The emergence of atomically 2D TMDCs has enabled the examination of diffusive, displacive, and quantum phase transitions. Cho et al. (2015) demonstrated a novel method of localized polymorph engineering and realized the laser-induced phase transition of MoTe₂. The multilayer 2H-MoTe₂ flake with about 30 layers was obtained by mechanical exfoliation. The schematic diagram of the phase transition mechanism is illustrated in Figure 1D. Under laser irradiation, the thickness of the irradiated region in 2H-MoTe₂ decreased by a few layers and transformed to 1T′ phase. During the irradiation process, the 1T′ MoTe₂ layer remained due to the heat sink effect of the SiO₂ substrate. It should be noticed that the phase transition is irreversible from 1T′ to 2H phase even under the higher energy or intensity of the laser. The driving force for the one-way phase transition is caused by the irreversible Te vacancy created under laser irradiation. Compared with conventional methods such as heat treatment, strain engineering, charge transfer, and plasma irradiation, laser-induced phase transition engineering allows for specific phase transition sites and patterned processing without impurities during the phase transition (Chen Z. R. et al., 2019).

Direct laser writing technology can directly pattern 2D TMDC materials without mask and lithography, reducing the process flow of device fabrication and the risk of chemical contamination. Generally, the fiber laser (1.06 μm), carbon dioxide laser (10.6 μm), and femtosecond laser (780 nm) have been adopted for the synthesis of 2D TMDCs (Hu et al., 2018). Xu et al. (2021a) reported a method for the efficient synthesis of wafer-scale MoS₂ using a direct laser writing technique, and the workflow is shown in Figure 2A. With heating by a 1.06 μm commercial fiber laser, the MoS₂ can be programmable with text, lines, patterns, and matrices in a few minutes. The laser interacts with the precursor, and the reaction temperature facilitates the thermal decomposition of (NH₄)₂MoS₄ into MoS₂. Apart from the thermal decomposition, the photochemical reaction induced by two-photon absorption can also be adopted for synthesizing 2D TMDCs. As shown in Figure 2B, the femtosecond direct laser writing was carried out to synthesize MoS₂ under an objective lens. Molybdenum acetylacetonate and carbon disulfide were adopted as the molybdenum and sulfur sources. The two-photon absorption occurs in the focal point, synthesizing MoS₂ from the molybdenum metal complex via a photochemical reaction. With the assistance of a femtosecond laser, a minimal line width of approximately 780 nm can be achieved.

The direct laser writing technique allows the fabrication of single 2D TMDCs and the direct preparation of 2D TMDC heterostructures. Compared with the transfer method, the laser heterostructure process can be programmable patterned, avoiding material damage and pollution (Li et al., 2020). In addition, the synthesis efficiency of heterostructures is greatly improved compared with CVD. Park et al. (2020a) successfully prepared WS₂–MoS₂ heterostructures vertically by two-step laser scribing, as shown in Figure 2C. The MoS₂ layer was first synthesized by the thermal decomposition of (NH₄)₂MoS₄ when the temperature increased over 700°C under laser irradiation. While for the heterostructures, due to the different optical absorption coefficients, the MoS₂ layer shows a minor temperature increase, and only the (NH₄)₂WS₄ layer absorbed the laser energy and selectively decomposed into WS₂. Thus, the selective growth process by laser effectively produces layer-by-layer 2D TMDC heterostructures in a programmable pattern.

The impurity atom can modulate the physical and chemical properties of semiconductors such as electrical and optical. A small amount of elemental doping could affect the energy bands enhancing semiconductor conductivity. However, doping elements in 2D TMDCs are relatively difficult and generate more defects in the 2D TMDCs. In addition, the weak interaction will result in unstable interfaces due to the interaction between doped elements and 2D TMDCs (Mak et al., 2010). Laser provides a simple way for 2D TMDC doping, which is promising for future electronic applications (Chen J. et al., 2019). One way to achieve laser doping in 2D TMDCs is by dissolving the doping substance in the precursor, which results in doping during formation (Hu et al., 2020). In addition, doping of elements in preprepared 2D TMDCs can also be accomplished by introducing gas molecules or solids containing the desired doping atoms (Afaneh et al., 2018; Rho et al., 2019; Huo et al., 2021). For example, Huo et al. (2021) reported an Au-doped MoS₂ process by doping a solid source, as shown in Figure 2D. MoS₂ was transferred to the Au electrode, and with femtosecond laser irradiation, the Au atom can be intercalated and diffused into MoS₂. The interface diffusion and chemical bonding of Au reduced the Schottky barrier of the metal–semiconductor interfaces, which could enhance the performance of the devices.