![]() ![]() ![]() From a theoretical point of view, the accurate band edge positions should be the quasiparticle energy, which can be obtained from first-principles many-electron Green function approach within the GW approximation, where electron-electron correlation effects are treated properly 13, 31, 32, 33, 34, 35, 36, 37, 38. It is well known that the common LDA and GGA fail to do so. Because the SBH at the metal-semiconductor interfaces depends on the difference between the Fermi level ( E f) of the metal and the band edge positions of the semiconductor, the band edge positions of the semiconductor must be accurately determined 29, 30. There are two open issues concerning this validity of the DFT energy band approach to treat the SBH of a transistor. Although there are several energy band calculations based on single particle density functional theory (DFT) to examine ML MoS 2-metal interfaces 22, 23, 25, 27, 28, a comprehensive energy band calculation for BL MoS 2-metal interfaces is still lacking at present. For example, there is a significant SBH between Ti and ML MoS 2 24, 25 by contrast, Ti forms an Ohmic contact with BL MoS 2 at room temperature and a Schottky contact with a small SBH of ~0.065 eV at a low temperature 11, 26. Lower work function metal and more MoS 2 layer number favor a smaller SBH. The SBH of a 2D MoS 2-metal contact depends on the work function of metal and the layer number of MoS 2. The formation of low-resistance metal contacts is the biggest challenge that masks the intrinsic exceptional electronic properties of 2D MoS 2, and many efforts have been made to study 2D MoS 2-metal contact so as to reduce the Schottky barrier height (SBH) 21, 22, 23. In a real device, semiconducting 2D MoS 2 needs a contact with metal electrodes, and a Schottky barrier is often formed in semiconductor-metal interface, which impedes the carrier transport. (3) Zeeman-like spin splitting is nearly intact by a vertical electric field in ML MoS 2 but it becomes tunable in BL MoS 2 because top and bottom MoS 2 feel different electric potentials 16. ![]() By contrast, inversion symmetric BL MoS 2 is not a VHI, but it can be transformed into a VHI with a tunable valley magnetic moment by a vertical electric field, which destroys the inversion symmetry 5. (2) ML MoS 2 is inversion asymmetric and serves as an ideal valley Hall insulator (VHI) 1. ![]() Correspondingly, photoluminescence is dramatically enhanced in ML MoS 2 6, 20. They show quite interesting differences and make up a pair of complementary materials: (1) ML MoS 2 has a larger direct band gap, while BL MoS 2 possesses a smaller indirect band gap due to the strong interlayer coupling. Very recently, wafer-scale high performance 2D MoS 2 FETs have been fabricated in batch mode, paving the way towards atomically thin integrated circuitry 19. A variety of prototype devices based on 2D MoS 2 have been fabricated, such as field-effect transistors (FETs) 7, 8, 9, inverters 10, fully integrated circuits 11, sensors 12, photoelectronic devices 13, phototransistors 14, 15, spintronic devices 16, and valleytronic devices 17, 18. Owing to their excellent properties, two-dimensional (2D) molybdenum disulfide MoS 2 has attracted much recent attention 1, 2, 3, 4, 5, 6. BL MoS 2-metal contacts generally have a reduced SBH than ML MoS 2-metal contacts due to the interlayer coupling and thus have a higher electron injection efficiency. By contrast, an ab initio quantum transport device simulation better reproduces the observed SBH in 2D MoS 2-Sc interface and highlights the importance of a higher level theoretical approach beyond the energy band calculation in the interface study. The extensively adopted energy band calculation scheme fails to reproduce the observed SBHs in 2D MoS 2-Sc interface. A comparison between the calculated and observed Schottky barrier heights (SBHs) suggests that many-electron effects are strongly suppressed in channel 2D MoS 2 due to a charge transfer. We provide a comprehensive ab initio study of the interfacial properties of a series of monolayer (ML) and bilayer (BL) MoS 2-metal contacts (metal = Sc, Ti, Ag, Pt, Ni, and Au). Although many prototype devices based on two-dimensional (2D) MoS 2 have been fabricated and wafer scale growth of 2D MoS 2 has been realized, the fundamental nature of 2D MoS 2-metal contacts has not been well understood yet. ![]()
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