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The introduction of magnetism in two-dimensional (2D) materials represents an intense field of research nowadays and the quest to reach above-room-temperature ordering temperatures is still underway. Intrinsic ferromagnetism was discovered in 2017 in CrI$_{{3}}$ and Cr$_{{2}}$Ge$_{{2}}$Te$_{{6}}$ in the monolayer form with low Curie temperatures. An alternative method to introduce magnetism into conventional 2D materials is substitutional doping with magnetic impurities similarly to three-dimensional diluted magnetic semiconductors. The case of Mn-doped transition metal dichalcogenide (MoS$_{{2}}$, MoSe$_{{2}}$, WS$_{{2}}$, WSe$_{{2}}$) monolayers is very interesting because combining out-of-plane ferromagnetism and valley contrast leads to ferrovalley materials. In this work, we focus on the incorporation of Mn in MoSe$_{{2}}$ by molecular beam epitaxy on graphene which has been rarely addressed up to now. By using a multiscale characterization approach, we demonstrate that Mn atoms are incorporated into the MoSe$_{{2}}$ monolayer up to 5 atomic percent. However, when incorporated into the film, Mn atoms tend to diffuse to the grain edges forming undefined Mo$_{{{\it x}}}$Mn$_{{{\it y}}}$Se$_{{{\it z}}}$ phase at grain boundaries after completion of the MoSe$_{{2}}$ monolayer. This segregation leaves the crystalline and electronic structure of MoSe$_{{2}}$ unmodified. Above 5%, the saturation of Mn content in MoSe$_{{2}}$ leads to the formation of epitaxial MnSe clusters.

Developments in optoelectronics and spin-optronics based on transition metal dichalcogenide monolayers (MLs) need materials with efficient optical emission and well-defined transition energies. In as-exfoliated MoS2 MLs, the photoluminescence (PL) spectra even at low temperature consist typically of broad, overlapping contributions from neutral, charged excitons (trions) and localized states. Here, we show that in superacid treated MoS2 MLs, the PL intensity increases by up to 60 times at room temperature. The neutral and charged exciton transitions are spectrally well separated in PL and reflectivity at T = 4 K, with linewidth for the neutral exciton of 15 meV, but both transitions have similar intensities compared to the ones in as-exfoliated MLs at the same temperature. Time resolved experiments uncover picoseconds recombination dynamics analyzed separately for charged and neutral exciton emissions. Using the chiral interband selection rules, we demonstrate optically induced valley polarization for both complexes and valley coherence for only the neutral exciton. [ABSTRACT FROM AUTHOR]

Van der Waals heterojunctions composed of graphene and transition metal dichalcogenides have gain much attention because of the possibility to control and tailor band structure, promising applications in two-dimensional optoelectronics and electronics. In this report, we characterized the van der Waals heterojunction MoSe 2 /few-layer graphene with a high-quality interface using cutting-edge surface techniques scaling from atomic to microscopic range. These surface analyses gave us a complete picture of the atomic structure and electronic properties of the heterojunction. In particular, we found two important results: the commensurability between the MoSe 2 and few-layer graphene lattices and a band-gap opening in the few-layer graphene. The band gap is as large as 250 meV, and we ascribed it to an interface charge transfer that results in an electronic depletion in the few-layer graphene. This conclusion is well supported by electron spectroscopy data and density functional theory calculations. The commensurability between the MoSe 2 and graphene lattices as well as the band-gap opening clearly show that the interlayer interaction goes beyond the simple van der Waals interaction. Hence, stacking two-dimensional materials in van der Waals heterojunctions enables us to tailor the atomic and electronic properties of individual layers. It also permits the introduction of a band gap in few-layer graphene by interface charge transfer.