Doping of two-dimensional materials provides them tunable physical properties and widens their applications. Here, we demonstrate the postgrowth doping strategy in monolayer and bilayer tungsten disulfide (WS2) crystals, which utilizes a metal exchange mechanism, whereby Sn atoms become substitutional dopants in the W sites by energetically favorable replacement. We achieve this using chemical vapor deposition techniques, where high-quality grown WS2 single crystals are first grown and then subsequently reacted with a SnS precursor. Thermal control of the exchange doping mechanism is revealed, indicating that a sufficiently high enough temperature is required to create the S vacancies that are the initial binding sites for the SnS precursor and metal exchange occurrence. This results in a better control of dopant distribution compared to the tradition all-in-one approach, where dopants are added during the growth phase. The Sn dopants exhibit an n-type doping behavior in the WS2 layers based on the decreased threshold voltage obtained from transistor device measurements. Annular dark-field scanning transmission electron microscopy shows that in bilayer WS2 the Sn doping occurs only in the top layer, creating vertical heterostructures with atomic layer doping precision. This postgrowth modification opens up ways to selectively dope one layer at a time and construct mixed stoichiometry vertical heterojunctions in bilayer crystals.
Keywords: 2D materials; Chemical vapor deposition; alloys; doping; transition metal dichalcogenides; tungsten disulfide.