Defect-Mediated Alloying of Monolayer Transition-Metal Dichalcogenides

Abstract

Alloying plays a central role in tailoring the material properties of 2D transition-metal dichalcogenides (TMDs). However, despite widespread reports, the details of the alloying mechanism in 2D TMDs have remained largely unknown and are yet to be further explored. Here, we combine a set of systematic experiments with ab initio density functional theory (DFT) calculations to unravel a defect-mediated mechanism for the alloying of monolayer TMD crystals. In our alloying approach, a monolayer MoSe 2 film serves as a host crystal in which exchanging selenium (Se) atoms with sulfur (S) atoms yields a MoS 2x Se 2(1−x) alloy. Our study reveals that the driving force required for the alloying of CVD-grown films with abundant vacancy-type defects is significantly lower than that required for the alloying of exfoliated films with fewer vacancies. Indeed, we show that preexisting Se vacancies in the host MoSe 2 lattice mediate the replacement of chalcogen atoms and facilitate the synthesis of MoS 2x Se 2(1−x) alloys. Our DFT calculations suggest that S atoms can bind to Se vacancies and then diffuse throughout the host MoSe 2 lattice via exchanging the position with Se vacancies, further supporting our proposed defect-mediated alloying mechanism. Beside native vacancy defects, we show that the existence of large-scale defects in CVD-grown MoSe 2 films causes the fracture of alloys under the alloying-induced strain, while no such effect is observed in exfoliated MoSe 2 films. Our study provides a deep insight into the details of the alloying mechanism and enables the synthesis of 2D alloys with tunable properties.

Synthesis of epitaxial monolayer Janus SPtSe

“…The internal electric field in Janus TMDCs may be exploited also in piezoelectric devices 6 and p-n junctions 7 . Anionic (chalcogen) substitution reactions have been used to tune the composition of TMDCs, with the main purpose of engineering the bandgap [8][9][10] . However, few reports addressed so far the selective substitution of the chalcogen atoms in a single layer (SL) to prepare ordered Janus compounds 11,12 .…”

Photocarrier‐Induced Active Control of Second‐Order Optical Nonlinearity in Monolayer MoS2

“…[38] To elucidate the impact of photocarriers on the SHG response of MoS 2 , we conduct a set of linear and second-order nonlinear time-resolved measurements in which, a broad probe beam or an 800 nm fundamental light (i.e., I ω ) interacts with a monolayer crystal, at various timeframes before and after the photoexcitation (characterization setup, Figure 1). [43][44][45][46] Although the modulated linear response of the MoS 2 crystal under both excitation wavelengths displays similar trends, the induced ΔOD signals at the excitonic energy levels (e.g., A exciton) reveal disparate correlations to the intensity of the two control signals (Figure 3d). Under the latter condition, the energy of control photons, E ctrl ≈ 2.05 eV, lays between the excitonic bandgap of MoS 2 , g exc E ≈ 1.85 eV, and the electronic bandgap of monolayers, g el E ≈ 2.3 eV.…”

Heterogeneities at multiple length scales in 2D layered materials: From localized defects and dopants to mesoscopic heterostructures

“…Introducing the third element usually causes lattice distortions and perturbations, impacting the crystal structure and electronic properties. For example, hightemperature sulfurization/selenization or deposition of transition metals are simple and rapid post-synthesis methods for doping chalcogens and metal atoms in TMDs [90]. However, they can also inadvertently create defects or distort the structure [90], thereby degrading the overall material quality or inducing a new structure to tailor the electronic and catalytic properties.…”

“…For example, hightemperature sulfurization/selenization or deposition of transition metals are simple and rapid post-synthesis methods for doping chalcogens and metal atoms in TMDs [90]. However, they can also inadvertently create defects or distort the structure [90], thereby degrading the overall material quality or inducing a new structure to tailor the electronic and catalytic properties. Therefore, fundamental questions for doping 2D materials are (1) which elements can be incorporated into the TMD lattice?…”

“…In addition to synthetic doping strategies [97], post-synthesis processing methods such as hightemperature sulfurization/selenization also enable random doping of chalcogens into 2D TMDs [98]. The vapor phase conversion usually requires a high temperature (500-1,000 °C) environment to accelerate the ion-exchange rate (i.e., Se-S or Te-S exchange) by annealing the 2D TMDs [90]. The key step for post-synthesis doping is to displace the constituents of 2D TMD first to generate vacancies as the host for incoming dopants.…”