We show that van der Waals (vdW)-bonded hexagonal boron nitride (hBN) promotes heteroepitaxial growth of semiconducting MoS x (0001) (x = 2.0 ± 0.1) thin films on Al 2 O 3 (0001) substrates. hBN layers are grown on Al 2 O 3 (0001) via pyrolytic cracking of borazine (∼6 × 10 4 L) at 1373 K and the MoS x layers are deposited in an ultrahigh-vacuum system via reactive directcurrent magnetron sputtering of Mo in an Ar/H 2 S gas mixture at 1073 K on bare and hBN-covered Al 2 O 3 (0001). Using in situ low-energy electron diffraction and Auger electron spectroscopy along with ex situ X-ray diffraction, X-ray photoelectron and Raman spectroscopies, and transmission electron microscopy, we determine the as-deposited MoS x layer composition and crystallinity. We obtain highly 0001-oriented, ∼20-nm-thick, 2H-structured MoS x multilayers with better crystalline quality on hBN/Al 2 O 3 (0001) than on Al 2 O 3 (0001). We suggest that hBN buffer layer enhances surface diffusion of depositing species, compared to bare Al 2 O 3 (0001), leading to an observed improvement in the crystallinity of MoS x layers. We expect that our results are likely to have broad implications in nanoelectronic device fabrication.

As high-entropy alloying provides an increasingly important avenue for widening the set of functional materials for a variety of applications, it is useful to uncover synthesis routes that do not rely on large temperatures for achieving entropic stabilization. Focusing on transition-metal dichalcogenides, we present direct computational evidence from density functional theory calculations that high-entropy disulfide (HES) alloys with five cations from groups 4–6 are thermodynamically stable at temperatures routinely achievable in conventional deposition systems. While all 126 sulfide combinations with five group 4–6 transition metals are thermodynamically favorable at low (<800 K) or medium (<1200 K) temperatures, we show that electronegativities, valence electron concentrations, and atomic radii of cations can help predict whether an HES alloy is stable in the 1-H or the 1-T structure. Furthermore, replacing one of the five cations with another, from outside groups 4–6, can still yield HES alloys with nearly planar layer morphologies and stabilization temperatures below 1200 K, albeit with some localized defects. These results demonstrate that a wide range of stable HES alloys can be synthesized experimentally as 2D layers, thereby providing facile ways for expanding the materials’ space with potential applications in electrochemical devices, catalysis, energy storage, or sensing.

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.