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Compositional tunability, an indispensable parameter to modify materials' properties, can open up new applications for the class of van der Waals (vdW) layered materials such as transition-metal dichalcogenides (TMDCs). To-date, multi-element alloy TMDC layers are obtained via exfoliation from bulk polycrystalline powders. Here, we demonstrate direct deposition of high-entropy alloy disulfide, (VNbMoTaW)S2, layers with controllable thicknesses on free-standing graphene membranes and on bare and hBN-covered Al2O3(0001) substrates via ultra-high vacuum reactive dc magnetron sputtering of VNbMoTaW target in Kr and H2S gas mixtures. Using a combination of density functional theory calculations, Raman spectroscopy, X-ray diffraction, scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, we determine that the as-deposited layers are single-phase, 2H-structured, and 0001-oriented (V0.10Nb0.16Mo0.19Ta0.28W0.27)S2.44. Our synthesis route is general and applicable for heteroepitaxial growth of a wide variety of TMDC alloys and potentially other multielement alloy vdW compounds with the desired compositions. 

The observation of characteristic A1g and E2g1 peaks, at around 408 and 382 cm−1, respectively, in Raman spectroscopy is considered the evidence of 2H-structured MoS2, probably the most extensively studied transition-metal dichalcogenide. Here, using a combination of x-ray diffraction, x-ray photoelectron spectroscopy, and resonant Raman spectroscopy, we show that the detection of A1g and E2g1 modes in Raman spectra alone may not necessarily imply the presence of MoS2. A series of Mo–S films, ≈ 20-nm-thick, are grown on single-crystalline Al2O3(0001) substrates at 1073 K as a function of H2S partial pressure, pH2S (= 0, 0.01%, 0.1%, and 1% of total pressure) via ultra-high vacuum dc magnetron sputtering of a Mo target in 20 m Torr (2.67 Pa) Ar/H2S gas mixtures. In pure Ar discharges and with pH2S up to 0.1%, i.e., pH2S ≤ 2.67 × 10−3 Pa, we obtain body centered cubic (bcc), 110-textured films with lattice parameter a increasing from 0.3148 nm (in pure Ar) to 0.3151 nm (at pH2S = 2.67 × 10−4 Pa), and 0.3170 nm (at pH2S = 2.67 × 10−3 Pa), which we attribute to increased incorporation of S in the Mo lattice. With 1% H2S, i.e., pH2S = 2.67 × 10−2 Pa, we obtain 000l oriented 2H-structured MoS2.0±0.1 layers. Raman spectra of the thin films grown using 0.1% (and 1%) H2S show peaks at around 412 (408) and 380 cm−1 (382 cm−1), which could be interpreted as A1g and E2g1 Raman modes for 2H-MoS2. By comparing the Raman spectra of MoS2.0±0.1 and Mo:S thin films, we identify differences in A1g and E2g1 peak positions and intensities of defect-sensitive peaks relative to the A1g peaks that can help distinguish pure MoS2 from non-stoichiometric MoS2−x and multiphase Mo:S materials.