Panoply of doping-induced reconstructions and electronic phases in Ni-doped 1T-MoS$_2$

Abstract

Monolayer MoS 2 has promising applications in catalysis and optoelectronics, which can be enhanced and tuned by transition-metal doping. The 1T phase is metallic and also has several known distorted structures with distinct electronic properties. In this work, we use density-functional theory to investigate the effect of Ni-doping in 1T-MoS 2 , considering adatom and substitutional sites, and find an array of distorted phases induced by Ni-doping, beyond the ones typically reported.Hollow is the most favorable adatom site, and doping in most cases is thermodynamically favored compared to undistorted 1T. Depending on concentration and site, Ni-doping induces reconstruc-, and 4 × 4. These doped phases become semiconducting in most cases, and a few are also magnetic. These phases are metastable after removal of the dopant, offering a potential route to experimental synthesis of pristine distorted phases. We find that the pristine phases have distinct metallic and semiconducting electronic structures, including band gaps away from the Fermi level. Our calculations show that Ni-doping of 1T offers a systematic route to different distorted phases of 1T-MoS 2 , both doped and pristine, with a variety of electronic properties. I. INTRODUCTIONThe crystal structure of MoS 2 has strong covalent bonds in-plane and weak van der Waals interactions out of plane, and offers different stackings and single-layer polymorphs.In bulk, MoS 2 can exist as 2H or 3R phase, which have similar energies and differ by stacking sequence and orientation of the 1H layers. In monolayer form, MoS 2 exists as 1H and 1T, which differ by coordination around Mo: 1H has trigonal prismatic coordination and 1T has octahedral bonding. 1T is higher in energy, and so most studies of MoS 2 have focused on the 2H or 1H phase, due to their greater stability. 2H-MoS 2 has already shown promising applications in lubrication, hydrodesulfurization, and optoelectronics [1-4]. Studies related to 2H and 1H MoS 2 also include tunability of band gap from direct to indirect [5, 6], good electron mobility [7], the possibility of defect engineering including creating quantum emitters [8], catalysis in solar hydrogen production [9], adsorption of CO a

Development and Application of a ReaxFF Reactive Force Field for Ni-Doped MoS2

“…We used 2 × 2, 3 × 3, and 4 × 4 in-plane supercells of the three-atom unit cell of 1H and 1T, where each supercell contained one Ni atom; these structures were then doubled to create orthogonal unit cells with two Ni atoms per cell. The different doping sites , were adatoms at the hollow position (3-fold hollow space between top S atoms), Mo atop (on top of Mo), or S atop (on top of S), Mo-substituted, or S-substituted. The pristine structures were also included for reference, which had been studied in the 2017 work .…”

“…Snapshots of representative structures (S atop) of different sizes (doping concentrations) are shown in Figure . The energy differences between 1H and 1T polytypes were compared between ReaxFF and DFT in Figure , using DFT results from Karkee et al and Karkee and Strubbe . Results show that the force field can capture the energies corresponding to Ni dopants and adatoms with a small underestimation, as in the pristine case …”

“…Other studies showed that Ni doping improves the catalytic performance of MoS 2 by decreasing the surface sulfur-metal bonding energy as well as weakening the S–H bond strength . Previous DFT-based studies have provided details about structures, bonding, thermodynamics, vibrational properties, elasticity, and interlayer binding in Ni-doped bulk 2H, bulk 3R, and monolayer 1H-MoS 2 in different phases. , DFT studies have also examined the energies and structural changes in frictional sliding of Ni-doped 2H and bilayer MoS 2 and the range of different reconstructed phases accessible by Ni doping of monolayer 1T-MoS 2 . However, such calculations are computationally demanding, limiting the time and size scales of model systems that can be studied.…”