Phase transitions in transition metal dichalcogenides (TMDCs) have garnered significant attention due to their potential in advanced electronic and optoelectronic applications. Among these transitions, the structural transformation from the semiconducting 2H phase to the metallic 1T phase is particularly notable. This transition can be induced by various external stimuli, including lithium intercalation, strain engineering, and laser irradiation. These methods enable precise control over the phase transition, allowing for the creation of metastable phases with unique properties. The ability to manipulate these phases opens new avenues for applications such as resistive switching devices, where controlled phase transitions can modulate electrical resistance in a non-volatile manner.

Lithium intercalation is one of the most widely studied methods for inducing phase transitions in TMDCs. When lithium ions are inserted between the layers of a TMDC, such as MoS2 or WS2, the electron transfer from lithium to the host material destabilizes the 2H phase, leading to a transition to the 1T phase. This process is often reversible, with the 1T phase persisting as a metastable state even after lithium removal. The degree of intercalation and the resulting phase transition can be finely tuned by controlling the concentration of lithium and the intercalation time. Studies have shown that a lithium concentration of approximately 1.0 to 1.5 atoms per formula unit is sufficient to induce a complete 2H-to-1T transition in MoS2. The metastable 1T phase exhibits distinct electronic properties, including higher conductivity and altered optical responses, making it suitable for applications requiring tunable electronic states.

Strain engineering offers another pathway to induce phase transitions in TMDCs. Applying mechanical strain to TMDC monolayers or few-layer systems can modify the atomic arrangement, leading to phase transformations. For instance, uniaxial or biaxial strain can reduce the energy barrier between the 2H and 1T phases, facilitating the transition. Strain-induced phase transitions are highly sensitive to the magnitude and direction of the applied strain, with critical strain thresholds typically ranging from 5% to 10% depending on the specific TMDC material. The resulting metastable phases can exhibit unique mechanical and electronic properties, which are exploitable in flexible electronics and strain sensors. However, the stability of these phases under ambient conditions remains a challenge, as strain relaxation can revert the material to its original phase.

Laser irradiation provides a non-contact and localized method for inducing phase transitions in TMDCs. Pulsed laser beams can deliver precise energy doses to the material, triggering the 2H-to-1T transition without the need for chemical intercalants or mechanical deformation. The mechanism involves localized heating and electron excitation, which disrupt the atomic bonding and favor the formation of the 1T phase. The spatial resolution of laser-induced phase transitions can reach sub-micrometer scales, enabling the patterning of phase domains for device applications. The metastable 1T phase created by laser irradiation often exhibits enhanced stability compared to chemically induced phases, though the exact stability depends on the laser parameters and material thickness.

Characterization of these phase transitions relies on advanced techniques such as transmission electron microscopy (TEM) and X-ray diffraction (XRD). TEM provides atomic-scale resolution, allowing direct visualization of the lattice rearrangement during the transition. High-resolution TEM can distinguish between the 2H and 1T phases based on their distinct atomic configurations, with the 1T phase exhibiting octahedral coordination compared to the trigonal prismatic coordination of the 2H phase. Electron diffraction patterns further confirm the phase transition by revealing changes in the crystal symmetry. XRD complements TEM by providing bulk-sensitive information about the phase composition and crystallographic orientation. Shifts in diffraction peaks and the appearance of new reflections are indicative of the phase transition, while quantitative analysis can determine the relative fractions of the 2H and 1T phases.

The metastable phases created through these methods have significant implications for resistive switching devices. Resistive random-access memory (RRAM) relies on reversible resistance changes between high-resistance and low-resistance states, which can be achieved through controlled phase transitions in TMDCs. The 2H-to-1T transition, for example, can serve as the switching mechanism, with the semiconducting 2H phase representing the high-resistance state and the metallic 1T phase representing the low-resistance state. The transition can be triggered electrically, thermally, or optically, offering versatility in device operation. The non-volatile nature of the metastable 1T phase ensures data retention, while the fast switching kinetics enable high-speed operation. Device performance metrics such as endurance, switching ratio, and retention time are influenced by the stability and reversibility of the phase transition, making material optimization critical.

In summary, phase transitions in TMDCs induced by lithium intercalation, strain, or laser irradiation provide a powerful means to engineer metastable phases with tailored properties. These transitions are characterized by advanced techniques such as TEM and XRD, which reveal the structural and electronic changes at atomic and macroscopic scales. The ability to control these transitions has profound implications for resistive switching applications, where the reversible modulation of resistance enables next-generation memory and logic devices. Future research will likely focus on improving the stability of metastable phases and refining the methods for inducing transitions at scale, paving the way for practical implementations in advanced electronics.