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The gold-assisted exfoliation is a very effective method to produce large-area (cm2-scale) membranes of molybdenum disulfide (MoS2) for electronics. However, the strong MoS2/Au interaction, beneficial for the exfoliation process, has a strong impact on the vibrational and light emission properties of MoS2. Here, we report an atomic force microscopy, micro-Raman, and micro-photoluminescence (μ-PL) investigation of 2H-MoS2 with variable thickness exfoliated on Au and subsequently transferred on an Al2O3/Si substrate. The E2g–A1g vibrational mode separation Δω (typically used to estimate MoS2 thickness) exhibits an anomalous large value (Δω ≈ 21.2 cm−1) for monolayer (1L) MoS2 on Au as compared to the typical one (Δω ≈ 18.5 cm−1) measured on 1L MoS2 on Al2O3. Such substrate-related differences, explained in terms of tensile strain and p-type doping arising from the MoS2/Au interaction, were found to gradually decrease while increasing the number of MoS2 layers. Furthermore, μ-PL spectra for 1L MoS2 on Au exhibit a strong quenching and an overall redshift of the main emission peak at 1.79 eV, compared to the 1.84 eV peak for 1L MoS2 on Al2O3. After PL spectra deconvolution, such redshift was explained in terms of a higher trion/exciton intensity ratio, probably due to the higher polarizability of the metal substrate, as well as to the smaller equilibrium distance at the MoS2/Au interface. [ABSTRACT FROM AUTHOR]

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al0.5Ga0.5N/sapphire templates by propane (C3H8) chemical vapor deposition at a temperature of 1350 °C. After optimization of the C3H8 flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a ∼2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy combined with electron energy loss spectroscopy, which also showed the presence of a bilayer of Gr with partial sp2/sp3 hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size ∼7 nm) and compressively strained. A Gr sheet resistance of ∼15.8 kΩ sq−1 was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles. [ABSTRACT FROM AUTHOR]

The use of metal nanoparticles is an established paradigm for the synthesis of semiconducting one-dimensional nanostructures. In this work we study their effect on the synthesis of two-dimensional semiconducting materials, by using gold nanoparticles for chemical vapor deposition growth of two-dimensional molybdenum disulfide (MoS 2 ). In comparison with the standard method, the employment of gold nanoparticles allows us to obtain large monolayer MoS 2 flakes, up to 20 μm in lateral size, even if they are affected by the localized overgrowth of MoS 2 bilayer and trilayer islands. Important modifications of the optical and electronic properties of MoS 2 triangular domains are reported, where the photoluminescence intensity of the A exciton is strongly quenched and a shift to a positive threshold voltage in back-gated field effect transistors is observed. These results indicate that the use of gold nanoparticles influences the flake growth and properties, indicating a method for possible localized synthesis of two-dimensional materials, improving the lateral size of monolayers and modifying their properties., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)