Scalable heterojunctions based on two-dimensional transitional metal dichalcogenides are of great importance for their applications in the next generation of electronic and optoelectronic devices. However, reliable techniques for the fabrication of such heterojunctions are still at its infancy. Here we demonstrate a simple technique for the scalable fabrication of lateral heterojunctions via selective chemical doping of TMD thin films. We demonstrate that the resistance of large area MoS 2 and MoSe 2 thin film, prepared via low pressure chalcogenation of molybdenum film, decreases by up to two orders of magnitude upon doping using benzyl viologen (BV) molecule. X-ray photoelectron spectroscopy (XPS) measurements confirms n-doping of the films by BV molecules. Since thin films of MoS 2 and MoSe 2 are typically more resistive than their exfoliated and co-evaporation based CVD counterparts, the decrease in resistance by BV doping represents a significant step in the utilization of these samples in electronic devices. Using selective BV doping, we simultaneously fabricated many lateral heterojunctions in 1 cm 2 MoS 2 and 1 cm 2 MoSe 2 films. The electrical transport measurements performed across the heterojunctions exhibit current rectification behavior due to a band offset created between the doped and undoped regions of the material. Almost 84% of the fabricated devices showed rectification behavior demonstrating the scalability of this technique.

Chemical vapor deposition (CVD) is a powerful method employed for high-quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD-based growth of monolayer molybdenum disulfide (MoS 2 ) by using thermally evaporated thin films of molybdenum trioxide (MoO 3 ) as the molybdenum (Mo) source for coevaporation. Uniform evaporation rate of MoO 3 thin films provides uniform Mo vapors which promote highly reproducible single-crystal growth of MoS 2 throughout the substrate. These high-quality crystals are as large as 95 μm and are characterized by scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy. The bottom-gated field-effect transistors fabricated using the as-grown single crystals show n-type transistor behavior with a good on/off ratio of 10 6 under ambient conditions. Our results presented here address the precursor vapor control during the CVD process and is a major step forward toward reproducible growth of MoS 2 for future semiconductor device applications.

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Palladium diselenide (PdSe2) is a novel member of the transition metal dichalcogenide (TMD) family with layer dependent bandgap in the infrared (IR) regime with potential applications in many electronic and optoelectronic devices. Low pressure CVD (LPCVD) could be an effective way to synthesize large area 2D PdSe2 materials at low growth temperatures creating new opportunities for the widescale applications of PdSe2. Here, we report LPCVD growth of PdSe2 for the first time at a growth temperature down to 250 oC, which is significantly lower than what was previously reported. The 2 nm Pd films became 8 nm PdSe2 after selenization in the temperature range of 250 - 375 oC and no thickness variation with growth temperature was observed in our AFM study. Raman study showed narrowing of PdSe2 related peaks with increasing growth temperature suggesting improved structural quality of the films. XPS study confirmed complete selenization of the thin films to the lowest growth temperature of 250 oC. Electrical transport properties study showed resistance of the devices decrease with increasing growth temperature possibly due to the improvement of crystallinity. We also found that the devices show p-type behavior with mobilities up to 1 cm2/Vs. The good electrical quality of the film was further confirmed by demonstrating its application in fabricating PdSe2/MoSe2 vertical heterojunction which showed rectification behavior with a rectification ratio of up to 232. Kelvin probe force microscopy confirmed that the rectification behavior was originated from the work function difference of 0.76 eV between MoSe2 and PdSe2.

Palladium diselenide (PdSe2) is an emerging 2D material with exotic optical and electrical properties and widely tunable layer dependent band gap in the infrared regime. The ability to further tune the electronic properties of PdSe2 via doping is of fundamental importance for a wide range of electronic and optoelectronic device applications. Here, surface charge transfer doping of chemical vapor deposition grown p‐type PdSe2 thin film using benzyl viologen (BV) molecules is reported. The electrical transport measurements of the PdSe2 device show an increase in resistance by ≈1700 percent from 2.1 MΩ for the pristine sample to 36.2 MΩ upon BV doping, revealing electrons are transferred from BV molecules to PdSe2 resulting in an n‐doping. Raman characterization shows a red‐shift and broadening of A3g characteristic peak for the doped sample, while X‐ray photoelectron spectroscopy shows a negative shift in Pd 3d and Se 3d binding energies confirming n‐doping by BV. Kelvin force probe microscopy measurements show a ≈0.3 eV decrease in work function for doped PdSe2, consistent with the n‐doping by BV molecules. A selective doping of PdSe2 channel is implemented for the fabrication of lateral heterojunction device which shows good current rectifying behavior with a rectification ratio of up to ≈55.