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Phosphorus (P) has been predicted to possess many two-dimensional (2D) allotropes, which have attracted intensive research attention due to their excellent properties and application promises. While only 2D black P films have been successfully obtained by the exfoliation method, fabrication of other phosphorus structures by epitaxial growth remains challenging. Recently, the metal-phosphorus network (MPhoN), a superstructure consisted of P and metal atoms has been realized, which offers a wider platform to study P and related compounds. In this paper, we report the observation of platinum-phosphorus networks obtained by P adsorption on Pt(111). Our findings not only enrich the diverse structures of the MPhoN family but also provide insights into the formation mechanism of epitaxial P and its interaction with the noble metal substrate.

Many transition-metal dichalcogenides, such as MoSe2, are direct-gap semiconductors at monolayer thickness, which hold potentials in nano-electronics, optoelectronics, and some new concept spin- and valley-electronic applications. For device application, however, controllable doping of the materials is essential. Here we report hole doping of epitaxial MoSe2 by nitrogen (N) plasma treatment with the aim of understanding the defect structure and its electronic characteristics. Examinations by annular dark field scanning transmission electron microscopy clearly reveal substitutional doping of N by replacing Se atoms in MoSe2 monolayer upon N-plasma treatment, though creation of Se vacancies are also possible. Interestingly, we note an unexpectedly high concentration of 'dual defects', where both Se atoms in the top and bottom Se layers of MoSe2 at the same lattice site are substituted by N and/or become vacant, suggesting a catalytic effect of defect formation. X-ray photoelectron spectroscopy and electron energy loss spectroscopy confirm the presence of N–Mo bonds. Photoemission spectroscopy reveals an impurity band as well as the Fermi level shift, confirming the p-type doping effect in MoSe2 monolayer by N-plasma treatment. Consistent with the PES results, scanning tunneling spectroscopy measurement also reveal defect states peaked at 0.6–0.7 eV above the valance band maximum. The effectiveness of N-doping is discussed.

Interests in two-dimensional transition-metal dichalcogenides have prompted some recent efforts to grow ultrathin layers of these materials epitaxially using molecular-beam epitaxy. However, growths of monolayer and bilayer WSe2, an important member of the transition-metal dichalcogenides family, by the molecular-beam epitaxy method remain uncharted probably because of the difficulty in generating tungsten fluxes from the elemental source. In this work, we present a scanning tunneling microscopy and spectroscopy study of molecular-beam epitaxy-grown WSe2 monolayer and bilayer, showing atomically flat epifilm with no domain boundary defect. This contrasts epitaxial MoSe2 films grown by the same method, where a dense network of the domain boudaries defects is present. The scanning tunneling spectroscopy measurements of monolayer and bilayer WSe2 domains of the same sample reveal not only the bandgap narrowing upon increasing the film thickness from monolayer to bilayer, but also a band-bending effect across the boundary between monolayer and bilayer domains. This band-bending appears to be dictated by the edge states at steps of the bilayer islands. Finally, comparison is made between the scanning tunneling spectroscopy-measured electronic bandgaps with the exciton emission energies measured by photoluminescence, and the exciton binding energies in monolayer and bilayer WSe2/MoSe2 are thus estimated.