Doping-controlled phase transitions in single-layer MoS2

The electronic properties of single-layer MoS2 make it an ideal two-dimensional (2D) material for application in electronic devices. Experiments show that MoS2 can undergo structural phase transitions. Applications of single-layer MoS2 will require firm laboratory control over the phase formation. Here we compare the stability and electronic structure of the three experimentally observed single-layer MoS2 phases, 2H,1T, and 1T′, and an in-plane metal/semiconductor heterostructure. We reveal by density-functional theory calculations that charge doping can induce the phase transition of single-layer MoS2 from the 2H to the 1T structure. Further, the 1T structure undergoes a second phase transition due to the occurrence of a charge-density wave (CDW). By comparing the energies of several possible resulting CDW structures, we find that the 1T′ orthorhombic structure is the most stable one, consistent with experimental observations and previous theoretical studies. We show that the underlying CDW transition mechanism is not due to Fermi surface nesting, but nonetheless, can be controlled by charge doping. In addition, the stability landscape is highly sensitive to charge doping, which can be used as a practical phase selector. We also provide a prescription for obtaining the 1T′ structure via growth or deposition of MoS2 on a Hf substrate, which transfers electrons uniformly and with minimal structural distortion. Finally, we show that lateral heterostructures formed by the 2H and 1T′ structures exhibit a low interfacial energy of 0.17 eV/Å, a small Schottky barrier of 0.3 eV for holes, and a large barrier of 1.6 eV for electrons.