Air Stable p‑Doping of WSe₂ by Covalent Functionalization

Covalent functionalization of transition metal dichalcogenides (TMDCs) is investigated for air-stable chemical doping. Specifically, p-doping of WSe₂ <i>via</i> NO_<i>x</i> chemisorption at 150 °C is explored, with the hole concentration tuned by reaction time. Synchrotron based soft X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) depict the formation of various WSe_{2–<i>x</i>–<i>y</i>}O_<i>x</i>N_<i>y</i> species both on the surface and interface between layers upon chemisorption reaction. <i>Ab initio</i> simulations corroborate our spectroscopy results in identifying the energetically favorable complexes, and predicting WSe₂:NO at the Se vacancy sites as the predominant dopant species. A maximum hole concentration of ∼10¹⁹ cm^–3 is obtained from XPS and electrical measurements, which is found to be independent of WSe₂ thickness. This degenerate doping level facilitates 5 orders of magnitude reduction in contact resistance between Pd, a common p-type contact metal, and WSe₂. More generally, the work presents a platform for manipulating the electrical properties and band structure of TMDCs using covalent functionalization.