Gold-assisted mechanical exfoliation currently represents a promising method to separate ultralarge (centimeter scale) transition metal dichalcogenide (TMD) monolayers (1L) with excellent electronic and optical properties from the parent van der Waals (vdW) crystals. The strong interaction between Au and chalcogen atoms is key to achieving this nearly perfect 1L exfoliation yield. On the other hand, it may significantly affect the doping and strain of 1L TMDs in contact with Au. In this paper, we systematically investigated the morphology, strain, doping, and electrical properties of large area 1L MoS₂ exfoliated on ultraflat Au films (0.16–0.21 nm roughness) and finally transferred to an insulating Al₂O₃ substrate. Raman mapping and correlative analysis of the E′ and A₁′ peak positions revealed a moderate tensile strain (ε ≈ 0.2%) and p-type doping (n ≈ −0.25 × 10¹³ cm^–2) of 1L MoS₂ in contact with Au. Nanoscale resolution current mapping and current–voltage (I–V) measurements by conductive atomic force microscopy (C-AFM) showed direct tunneling across the 1L MoS₂ on Au, with a broad distribution of tunneling barrier values (Φ_B from 0.7 to 1.7 eV) consistent with p-type doping of MoS₂. After the final transfer of 1L MoS₂ on Al₂O₃/Si, the strain was converted to compressive strain (ε ≈ −0.25%). Furthermore, an n-type doping (n ≈ 0.5 × 10¹³ cm^–2) was deduced by Raman mapping and confirmed by electrical measurements of an Al₂O₃/Si back-gated 1L MoS₂ transistor. These results provide a deeper understanding of the Au-assisted exfoliation mechanism and can contribute to its widespread application for the realization of novel devices and artificial vdW heterostructures.

Semiconductor transition metal dichalcogenides (TMDs), such as MoS₂, are currently regarded as key‐enabling materials for sub‐1 nm channel transistors, beyond‐complementary metal–oxide–semiconductor electronic and optoelectronic devices and sensors. Owing to this wide application potential, several bottom‐up and top‐down synthesis approaches for these materials have been explored so far. Despite the huge progresses in scalable deposition methods (such as chemical vapor deposition, metal–organic chemical vapor deposition), exfoliated layers from bulk crystals still represent the benchmark for record electronic properties of TMDs. Among exfoliation approaches, metal‐assisted mechanical exfoliation emerges as the most effective method to separate large‐area (mm² to cm²) single‐crystalline monolayer membranes of TMDs (and many other 2D materials) from the parent bulk crystals. This paper reviews the state‐of‐the‐art in this field, from current understanding of MoS₂ exfoliation mechanisms on Au (considered as a model system), to the main device applications of as‐exfoliated and transferred large‐area MoS₂ membranes (including Au/MoS₂/Au memristors, MoS₂ photodetectors, and field‐effect transistors). Perspectives of this method in the realization of arrays of 2D heterojunction devices, including Moirè superlattice devices, and open challenges for its widespread application are finally discussed.

We present a complete study of the CVD synthesis of monolayer MoS₂ assisted by gold nanoparticles, resulting in (i) improved lateral growth, (ii) quenched exciton PL emission, and (iii) a positive threshold voltage in back-gated FETs.

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 E_2g–A_1g 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.

In this paper, we report a multiscale investigation of the compositional, morphological, structural, electrical, and optical emission properties of 2H-MoS2 obtained by sulfurization at 800 °C of very thin MoO3 films (with thickness ranging from ~2.8 nm to ~4.2 nm) on a SiO2/Si substrate. XPS analyses confirmed that the sulfurization was very effective in the reduction of the oxide to MoS2, with only a small percentage of residual MoO3 present in the final film. High-resolution TEM/STEM analyses revealed the formation of few (i.e., 2–3 layers) of MoS2 nearly aligned with the SiO2 surface in the case of the thinnest (~2.8 nm) MoO3 film, whereas multilayers of MoS2 partially standing up with respect to the substrate were observed for the ~4.2 nm one. Such different configurations indicate the prevalence of different mechanisms (i.e., vapour-solid surface reaction or S diffusion within the film) as a function of the thickness. The uniform thickness distribution of the few-layer and multilayer MoS2 was confirmed by Raman mapping. Furthermore, the correlative plot of the characteristic A1g-E2g Raman modes revealed a compressive strain (ε ≈ −0.78 ± 0.18%) and the coexistence of n- and p-type doped areas in the few-layer MoS2 on SiO2, where the p-type doping is probably due to the presence of residual MoO3. Nanoscale resolution current mapping by C-AFM showed local inhomogeneities in the conductivity of the few-layer MoS2, which are well correlated to the lateral changes in the strain detected by Raman. Finally, characteristic spectroscopic signatures of the defects/disorder in MoS2 films produced by sulfurization were identified by a comparative analysis of Raman and photoluminescence (PL) spectra with CVD grown MoS2 flakes.