Introducing Semiconducting-to-Metallic Transitions into Wafer-Scale 2D PdSe₂ Layers by Low-Temperature Anion Exchange and Thickness Modulation

Two-dimensional (2D) palladium diselenide (PdSe₂) layers are projected to exhibit a number of intriguing electrical properties such as semiconducting-to-metallic transitions. Precisely modulating their morphology and chemistry is essential for realizing such opportunities, which is particularly demanded on a large dimension under flexible processing conditions toward broadening their practical device applicability. Herein, we explore a wafer-scale growth of 2D PdSe₂ layers and introduce semiconducting-to-metallic transitions into them at as low as 330 °C, a temperature compatible with a range of polymeric substrates as well as the back-end-of-line (BEOL) processes. Two independent physical and chemical approaches of thickness modulation and anion exchange are demonstrated to induce the low-temperature-driven electrical transitions. Wafer-scale 2D PdSe₂ layers grown from a scalable selenization of thin (∼2 nm) Pd exhibit p-type semiconducting characteristics, which completely vanish with increasing thickness. Furthermore, a postgrowth reaction involving an exchange of selenium (Se)-to-tellurium (Te) ions chemically introduces the semiconducting-to-metallic transitions through the conversion of PdSe₂-to-palladium ditelluride (PdTe₂). A significant reduction of the bandgap energy from 0.7 to 0 V is observed to be associated with the transitions, while the converted 2D layers remain to be highly metallic irrespective of thickness variations. These controlled transition characteristics are employed to fabricate "all-2D" flexible devices employing semiconducting 2D layer channels and metallic 2D layer electrodes on a wafer-scale.