Two-dimensional plumbum-doped tin diselenide monolayer transistor with high on/off ratio.

Doping can effectively regulate the electrical and optical properties of two-dimensional semiconductors. Here, we present high-quality Pb-doped SnSe2 monolayer exfoliated using a micromechanical cleavage method. X-ray photoelectron spectroscopy measurement demonstrates that Pb content of the doped sample is ∼3.6% and doping induces the downward shift of the Fermi level with respect to the pure SnSe2. Transmission electron microscopy characterization exhibits that Pb0.036Sn0.964Se2 nanosheets have a high-quality hexagonal symmetry structure and Pb element is uniformly distributed in the nanosheets. The current of the SnSe2 field effect transistors (FETs) was found to be very difficult to turn off due to the high electron density. The FETs based on the Pb0.036Sn0.964Se2 monolayer show n-type behavior with a high on/off ratio of 106 which is higher than any values of SnSe2 FETs reported at the moment. The estimated carrier concentration of Pb0.036Sn0.964Se2 is approximately six times lower than that of SnSe2. The results suggest that the method of reducing carrier concentration by doping to achieve high on/off ratio is effective, and Pb-doped SnSe2 monolayer has significant potential in future nanoelectronic and optoelectronic applications.

Thickness-Tunable Synthesis of Ultrathin Type-II Dirac Semimetal PtTe2 Single Crystals and Their Thickness-Dependent Electronic Properties.

The recent discovery of topological semimetals has stimulated extensive research interest due to their unique electronic properties and novel transport properties related to a chiral anomaly. However, the studies to date are largely limited to bulk crystals and exfoliated flakes. Here, we report the controllable synthesis of ultrathin two-dimensional (2D) platinum telluride (PtTe2) nanosheets with tunable thickness and investigate the thickness-dependent electronic properties. We show that PtTe2 nanosheets can be readily grown, using a chemical vapor deposition approach, with a hexagonal or triangular geometry and a lateral dimension of up to 80 µm, and the thickness of the nanosheets can be systematically tailored from over 20 to 1.8 nm by reducing the growth temperature or increasing the flow rate of the carrier gas. X-ray-diffraction, transmission-electron microscopy, and electron-diffraction studies confirm that the resulting 2D nanosheets are high-quality single crystals. Raman spectroscopic studies show characteristics Eg and A1g vibration modes at ∼109 and ∼155 cm-1, with a systematic red shift with increasing nanosheet thickness. Electrical transport studies show the 2D PtTe2 nanosheets display an excellent conductivity up to 2.5 × 106 S m-1 and show strong thickness-tunable electrical properties, with both the conductivity and its temperature dependence varying considerably with the thickness. Moreover, 2D PtTe2 nanosheets show an extraordinary breakdown current density up to 5.7 × 107 A/cm2, the highest breakdown current density achieved in 2D metallic transition-metal dichalcogenides to date.