2D-MoS2 goes 3D: transferring optoelectronic properties of 2D MoS2 to a large-area thin film

The ongoing miniaturization of electronic devices has boosted the development of new post-silicon two-dimensional (2D) semiconductors, such as transition metal dichalcogenides, one of the most prominent materials being molybdenum disulfide (MoS2). A major obstacle for the industrial production of MoS2-based devices lies in the growth techniques. These must ensure the reliable fabrication of MoS2 with tailored 2D properties to allow for the typical direct bandgap of 1.9 eV, while maintaining large-area growth and device compatibility. In this work, we used a versatile and industrially scalable MoS2 growth method based on ionized jet deposition and annealing at 250 °C, through which a 3D stable and scalable material exhibiting excellent electronic and optical properties of 2D MoS2 is synthesized. The thickness-related limit, i.e., the desired optical and electronic properties being limited to 2D single/few-layered MoS2, was overcome in the thin film through the formation of encapsulated highly crystalline 2D MoS2 nanosheets exhibiting a bandgap of 1.9 eV and sharp optical emission. The newly synthesized 2D-in-3D MoS2 structure will facilitate device compatibility of 2D materials and confer superior optoelectronic device function.


INTRODUCTION
Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as MoS 2 hold great potential for applications in nextgeneration optoelectronic devices, e.g., by overcoming the intrinsic limitation of graphene-related to its semi-metal character (i.e., zero bandgap) 1 . Unlike graphene, the bandgap of 2D TMDCs can be tuned from indirect to direct by varying the number of layers [2][3][4] . Moreover, the properties of MoS 2 such as chemical reactivity and electronic configuration make it a preferential candidate for a wide range of applications ranging from catalysis, such as water splitting and hydrogen generation for energy storage and harvesting [5][6][7] to biomedicine for cancer therapy and bone regeneration [8][9][10] . Hence, many synthesis routes have been recently pursued to grow large-area 2D MoS 2 . Top-down approaches such as exfoliation methods [11][12][13] , present severe limitations due to the small size of exfoliated layers and cumbersome large-area scalability, and due to the poor control of their morphological and structural properties. Bottom-up methods include chemical vapour deposition (CVD) [14][15][16] , thermal decomposition of Mo-based precursors 17,18 and sol−gel derived growth 19,20 , physical vapour deposition (PVD) 21 , in particular pulsed laser deposition (PLD) [22][23][24] and magnetron sputtering 25,26 . As a matter of fact, most of these bottom-up techniques still lack industrial scalability for simple and low-cost production of 2D MoS 2 with uniform and reproducible material properties (e.g., stoichiometry, high mobility). A major open issue lies in the nature of the substrates employed during the growth (e.g., pre-patterned, flexible, metallic, etc.), often limiting the processing temperature and the film size. Furthermore, the 2D material's fragility is challenging for the integration into the current device technology. For instance, if 2D materials are used as electrodes (usually in form of a single or few-layered flake), the devices can easily fail due to disruption of the layer integrity. These problems call for a synthesis method able to provide 2D materials possessing the desired properties, but with the strength of the bulk material.
A competitive approach for low-cost and industrially scalable growth of materials is the ionized jet deposition (IJD), which is a pulsed electron deposition method developed by Noivion S.r.l. (Italy) [27][28][29] and optimized for MoS 2 deposition in collaboration with the IMEM-CNR institute (Italy) 30 . It is based on an ionized gas jet, which acts as source and carrier gas of pulsed electrons and in a later step as carrier gas (plasma) for the material to be deposited. Briefly, after ultra-short electric discharges in the MW range are generated by a controlled and geometrically confined cascade ionization 31 , the dense and highly energetic pulsed electrons are directed towards a solid target constituted by the material-ofinterest, where they generate a superficial explosion with consequent emission (ablation) of the material that finally condenses on a substrate kept in front of the target. Using a single IJD source, uniform deposition of diameters up to 10 cm (e.g., 4-inch wafer) can be realized; by assembling several IJD sources that are mounted in-line, surfaces can be uniformly coated without any size limitations (more details about the IJD technique can be found in Supplementary Fig. 1). By controlling the deposition parameters 30 , it is possible to tune the stoichiometry of the deposited film and replicate one of the targets. In contrast to PLD, where a complex and expensive laser source and ultrahigh vacuum (UHV) conditions are needed [22][23][24] , the pulsed electron source of IJD operates in vacuum with 10 −7 mbar base pressure. These vacuum conditions make IJD a compact, costcompetitive, and industrially up-scalable technique, compatible with current micro-and nanofabrication processes (e.g., top-down lithography, wiring, bonding, and micro-/nanomanipulation), providing high deposition rates, low substrate heating, and a great tunability of the growth parameters. Due to material ablation instead of sputtering, almost any material can be deposited with very efficient material/target use. The IJD method has been successfully used to grow thin films from a wide range of complex materials on various substrates, such as hard metal carbides and nitrides on silicon, aluminium alloy and stainless steel substrates 32 , yttria-stabilized zirconia on SiO 2 , borosilicate, titanium and poly(ether ether ketone) 33 , tin sulfide layers on sodalime glass 34 , soft polymers on stainless steel and glass substrates 35 , and bone apatite-like films on silicon 36,37 . Hence, the IJD technique represents a versatile, very safe, and low-temperature process, which can be explored for TMDC growth, e.g., to obtain stoichiometric and crystalline 2H-MoS 2 without the use of hazardous gases such as hydrogen and sulfur, and carbon compounds, or other molecules (often used as catalysators in the synthesis).
In this contribution, we demonstrate for the first time the growth of large-area MoS 2 thin films employing IJD. The material is very attractive for technological and industrial-relevant scalable applications due to the relatively low temperature needed for the annealing process (250°C), which is compatible with some highperformance flexible polymers such as polyimide. The films with thicknesses of~200 nm remarkably display electronic and optical properties like encapsulated 2D MoS 2 (single or few layers). More specifically, they exhibit a bandgap of 1.86 eV and well-resolved photoluminescence spectra with an excitonic linewidth of less than 10 meV. These properties are given by the 3D structure obtained via the IJD thin film growth and subsequent annealing. The deposited material is initially composed of an amorphous film of Mo x S y nanoclusters, which rearrange after annealing into crystalline hexagonal and p-doped 2D MoS 2 nanosheets (1-4 layers) encapsulated by amorphous, stoichiometric MoS 2 . The high quality of the IJD-grown MoS 2 thin film for electronic applications is demonstrated on a memristive device.

Sample preparation
The MoS 2 thin films (with thicknesses of~200 nm) were grown on Pt and Au/Si 3 N 4 substrates via IJD (see Supplementary Fig. 1 for details on deposition and geometry) and subsequently annealed at low temperature. The formation of a stoichiometric crystalline MoS 2 structure is obtained at the annealing temperature of 250°C in combination with the optimization of the IJD parameters.

Chemical and morphological properties
The X-ray photoemission spectroscopy (XPS) chemical analysis of the MoS 2 thin films is reported in Fig. 1a-c, where representative spectra (survey, Mo 3d + S 2s and S 2p core levels, respectively) are shown. The wide energy range spectrum is dominated by Mo and S species. Small contributions arising from carbon and oxygen are mainly due to residual gases in the IJD chamber (pressure during deposition: 10 −3 −10 −5 mbar) conditions. It should be noted that high vacuum conditions are compatible with scalable applications, differently from other PVD techniques working in UHV. The spectral features of the Mo 3d peak in Fig. 1b show that Mo has a 4+ oxidation state, indicating successful synthesis of MoS 2 . In particular, the Mo 3d 5/2 and 3d 3/2 peaks are located at binding energies (BEs) of 228.9 and 232.2 eV, respectively, and the S 2s peak is observed at 226.3 eV. The peaks at BE of 161.8 and 162.9 eV correspond to the S 2p 3/2 and 2p 1/2 of MoS 2 , respectively (see Fig. 1c). From quantitative analysis, the atomic ratio S/Mo is 2.11 ± 0.10, in good agreement with the nominal expected one. It is noteworthy that the Mo 3d and S 2p core levels of the annealed MoS 2 thin film are shifted by 0.6 eV to lower BEs with respect to the as-deposited case ( Supplementary  Fig. 1), indicating a change in the chemical configuration and electronic structure of the MoS 2 thin film after annealing. Overall, the spectra exhibit a narrow Mo 3d and S 2p line shape (as evaluated via full width at half maximum, FWHM = 1.0 and 0.9 eV, respectively), suggesting that the optimized parameters for IJD and annealing enable the formation of chemically pure MoS 2 . Figure 1d shows an X-ray diffraction (XRD) scan to investigate the crystalline structure of the MoS 2 films. The peaks are assigned to the MoS 2 crystal planes according to ref. 38 . The interlayer distance obtained via the Bragg equation results is 6.19 Å, in excellent agreement with the standard hexagonal 2H-MoS 2 crystal (6.14 Å) 39 . The surface morphology of the deposited films obtained by atomic force microscopy (AFM) is shown in Fig. 1e. The representative topography image (1 µm × 1 µm) shows that the MoS 2 thin film exhibits a continuous surface with nm-sized protrusions and pores. Large area AFM and scanning electron microscopy (SEM) images of the MoS 2 surface are displayed in Supplementary Fig. 2, evidencing the formation of large-area homogeneous thin films.
Optical properties Photoluminescence (PL) measurements (excitation wavelength of λ = 532 nm) were performed to investigate the optical properties of the IJD-grown MoS 2 thin films. PL allows to distinguish bulk (3D) from 2D MoS 2 , thanks to the indirect-to-direct bandgap semiconductor transition that MoS 2 undergoes when confined in the mono-and few-layer regime. Room temperature PL spectra of 2D MoS 2 are typically observed as a broad peak centred at~650 nm (1.9 eV), commonly ascribed to A exciton complexes, i.e., a convolution of neutral (X 0 ) and red-shifted charged excitons (trion, T) with linewidths around 50−100 meV 2,40-43 , whereas the X 0 and T excitons are resolved in low-temperature PL spectra of encapsulated 2D-MoS 2 reaching linewidths around 10 meV [44][45][46] . Fig. 2a reports representative PL spectra collected on a deposited MoS 2 thin film before and after annealing. Whereas before annealing no PL signal is detected (black curve in Fig. 2a), after annealing the MoS 2 remarkably displays a PL emission spectrum consisting of a minor X 0 peak centred at 1902 meV (652 nm) and a sharp peak stemming from charged excitons (labelled as T in Fig. 2a) located 47 meV below X 0 with a linewidth of less than 10 meV. In addition, a third peak (X L ) located 101 meV below X 0 is observed, which has been recently ascribed to atomic defects and vacancies in the MoS 2 structure 47 .
Additional PL spectra acquired in different sample positions, as well as with another excitation wavelength (λ = 632 nm) (Supplementary Fig. 3a, b), confirm the well-resolved PL emission despite the thickness of the samples.
Non-resonant Raman spectra (λ = 532 nm) were collected on the MoS 2 thin film before and after annealing (Fig. 2b). The spectra reveal two typical Raman active modes of hexagonal MoS 2 , namely in-plane vibrations (E 1 2g ) at 383.0 and 384.5 cm −1 before and after annealing, respectively, and out-of-plane (A 1g ) vibrations at 408.0 and 403.2 cm −1 , respectively. Similar to the PL emission, the wavenumbers difference between the two Raman modes is also a useful parameter to indicate confinement of MoS 2 from 3D to 2D as the difference decreases during this process 48,49 . Before annealing, the difference between the E 1 2g and A 1g modes is equal to 25.0 cm −1 , typical bulk MoS 2 , whereas for the annealed MoS 2 thin film, the measured difference decreases and, although different values have been measured on the annealed samples (see Supplementary Fig. 3d), they are systematically smaller than 25.0 cm −1 with a minimum measured difference of 19.3 cm −1 . The Raman measurements confirm the PL data, indicating the remarkable capability of the MoS 2 thin films to display properties typical for 2D MoS 2 .

Structural and electronic properties
To further prove how the annealed MoS 2 thin film displays typical 2D features, despite being thicker than 2D, a combination of transmission electron microscopy (TEM) and scanning tunnelling microscopy (STM) was employed to explore the structural and morphological properties. The bright-field TEM image in  In Fig. 3b, a typical STM image of the MoS 2 thin film is depicted. A riffled structure corresponding to protruding nanosheets can be seen, which becomes more obvious in the 3D height profile in Fig. 3c. FFT analysis of an atomic-resolution STM image (see Supplementary Fig. 4) gives a surface periodicity of 0.28 nm, in good agreement with the sulfur-to-sulfur distance between individual MoS 2 layers. Figure 3d shows the local density of states (DOS) obtained by scanning tunnelling spectroscopy (STS) taken on the surface shown in Fig. 3c. The DOS is obtained by averaging over 30 different curves and then following the procedure described in ref. 51 to remove the exponential dependence on the tunnelling transmission probability of the electrons. This procedure avoids problems of obtaining a wide dynamic range in the tunnelling current which in general is a limiting factor in determining bandgap on semiconducting surfaces 51,52 . Thus, the obtained bandgap is (2.05 ± 0.20) eV, which lies within the range of previously reported STS bandgaps of 2D MoS 2 53-55 . The Fermi Level position lies in the lower half of the bandgap indicating a ptype character of the material.
The surface electronic properties of the MoS 2 thin film on Pt were further investigated by electron energy loss spectroscopy (EELS) and ultraviolet photoemission spectroscopy (UPS), see Fig. 4. The optical bandgap E g of the MoS 2 thin film was measured by EELS (see red curve Fig. 4a). The corresponding EELS spectrum of the thin film before annealing (i.e., amorphous as-deposited sample) can be found in Supplementary Fig. 5a, however, it cannot be considered as a reference for the electronic properties of stoichiometric and crystalline MoS 2 (neither 3D nor 2D). Thus, we compare the annealed sample with a 3D MoS 2 single crystal serving as a crystalline bulk reference (see blue curve in Fig. 4a). Furthermore, it should be noted that the exciton BE for MoS 2 grown on metal substrates is typically less than 0.1 eV 56 , therefore, optical and electronic bandgaps nearly coincide for the present MoS 2 thin film deposited on Pt. For the reference sample, E g is found to be 1.30 eV, in good agreement with previously reported EELS of bulk MoS 2 57 , whereas for the annealed MoS 2 thin film E g is found to be 1.86 eV. This value is comparable to the one measured for 2D MoS 2 displaying a direct bandgap 58 . Hence, the results obtained from STS and EELS confirm the Raman and PL findings of the 2D character of the annealed thin films despite its thickness of~200 nm, and support the presence of few-layered 2D MoS 2 nanosheets (as observed by TEM).  The work function (Φ) obtained by the secondary electron cut off (SECO) shown in Fig. 4b, is Φ = 4.95 eV for the MoS 2 thin film and of Φ = 4.33 eV for the reference MoS 2 crystal, suggesting a more p-type nature 59 of the thin film compared to the reference. This is also demonstrated by the valence band maximum (VBM) observed at about 0.6 eV, hence locating the Fermi level E F (BE = 0 eV) in the lower half of the bandgap, as also detected by the STS measurements (Fig. 3d), which is consistent with p-type behaviour 59 . The VB line shape is comparable to the one reported for MoS 2 single crystal 60 and other synthesized MoS 2 20,25,61 , and displays two-component centred at 1.7 and 3.5 eV, resulting from Mo 4d−S 3p hybridized orbitals 62,63 . A third minor component centred at 2.6 eV (green peak in Fig. 4c) is related to the presence of surface dangling bonds [64][65][66] . Figure 4d displays the resulting energy band diagram of the MoS 2 thin film as derived by EELS and UPS measurements.

Device characterization
To evaluate the electrical performance of the MoS 2 thin film and to prove the versatile applicability of the herein reported IJD deposition technique for electronic components, we fabricated MoS 2 -based resistive random memory (ReRAM) devices, which are typically defined by an abrupt change in the material's resistivity after applying external voltages. Cylindrical Au electrodes (400 µm in diameter) were deposited on a Si 3 N 4 -on-quartz substrate integrating contact lines for external connection to perform the electrical characterization, as displayed in Fig. 5a-c. The optical micrograph and SEM image in Fig. 5a, b,

DISCUSSION
The MoS 2 thin films presented here exhibit optical and electronic properties (i.e., bandgap E g = 1.86 eV) similar to 2D MoS 2 , even though they are few hundreds of nm thick, large-area and continuous MoS 2 films. The findings can be ascribed to the fact that the large-area thin films are composed of randomly oriented luminescent and p-doped 2D MoS 2 nanosheets embedded in amorphous MoS 2 . The simple model shown in Fig. 6 accounts for the MoS 2 nanosheet formation. Upon IJD, molecular-sized species with undefined stoichiometry Mo x S y ablate from the MoS 2 target (see IJD details in Supplementary Fig. 1) and condense on the substrate forming an amorphous film of S-enriched Mo x S y nanoclusters (see Fig. 6a) over the entire deposited area. The typical atomic arrangement of as-deposited Mo x S y nanoclusters is depicted in the enlarged view in Fig. 6a. Mo and S atoms initially form various chemical species (e.g., Mo−S−S−Mo, Mo−Mo−S, Mo−S−S) with a high number of defects, dangling bonds, and different chemical environments of the Mo atoms, as evidenced by detailed XPS core-level analysis of the as-deposited thin film (see Supplementary Fig. 1). Residual gas atoms such as C, N, and O, as schematically represented by the red atoms in Fig. 6a, are present in trace amounts during deposition due to the relatively low base vacuum of the IJD apparatus (10 −7 mbar). The postdeposition annealing at 250°C favours the rearrangement of Mo x S y species into layered crystalline 2D MoS 2 nanosheets within the nanoclusters composing the thin film (Fig. 6b), as evidenced by TEM and STM analysis (see Fig. 3). Due to the nature of the growth process, we expect that the 2D MoS 2 nanosheets exhibit atomic defects such as Mo and S vacancies (represented by redcircled grey atoms in Fig. 6b) and that C, N, and O residual gas atoms are incorporated into the MoS 2 crystal structure (represented by red atoms in Fig. 6b). Since XPS analysis shows no extra peak in the Mo 3d and S 2p core levels (Fig. 1b, c) resulting in an overall atomic ratio S/Mo of~2, we assume that also the amorphous encapsulation matrix (light green in Fig. 6b) exhibits a final stoichiometric composition of MoS 2 . Even though the film is composed of two MoS 2 phases, namely crystalline nanosheets and amorphous matrix, the materials' electronic and optical properties are dominated by the 2D MoS 2 nanosheets (see Figs. 1, 2, and 4). The remaining amorphous phase does not contribute to the observed properties, but only serves as encapsulation/passivation material.
Note that the XPS core levels of the annealed MoS 2 thin film are shifted to lower BEs (by 0.6 eV) with respect to the as-deposited sample ( Supplementary Fig. 1b, c). Such BE shifts can be related to two different processes; first, the formation of MoS 2 nanostructures was previously found to result in shifts to lower BEs with respect to the bulk values 70 , which is in agreement with the present MoS 2 nanosheet formation. Second, the annealing leads to a strong change of the electronic properties of the MoS 2 thin film. The VBM at 0.6 eV below E F (Fig. 4c), the increased work function (4.95 eV), and the bandgap of 1.86 eV indicate that the crystalline MoS 2 nanosheets exhibit p-type behaviour 71 (see the position of E F in Fig. 4d). It has been reported that encapsulation of 2D MoS 2 can effectively lower the position of E F 45 , which is in agreement with our finding of p-type doping of encapsulated 2D MoS 2 nanosheets. Moreover, unsaturated sulfur vacancies effectively induce p-type behaviour when they are not backfilled, e.g., when the vacancy-rich MoS 2 surface is kept under UHV conditions 66 . Since encapsulation prevents the reaction with ambient gases [44][45][46] , we assume that the p-type doping of encapsulated 2D MoS 2 nanosheets is mainly related to unsaturated vacancies.
A second mechanism that can induce p-type doping might be related to residual gas atoms that either react with some of the vacancies leading to the incorporation of substitutional/interstitial C, N, and O atoms into the MoS 2 lattice 59,72,73 (even though undetectable via XPS) or induce charge transfer 74,75 due to adsorption at the MoS 2 nanosheet surface.
The most striking feature of the crystalline 2D MoS 2 nanosheets is the observed bandgap of 1.86 eV (Figs. 3d and 4a) comparable to the one found for monolayer MoS 2 , and a well-resolved PL signal (Fig. 2) with the most prominent peak stemming from charged excitons (T). The high intensity of the T peak with respect to the neutral X 0 is in good agreement with our finding of p-type behaviour (high-density charged carriers) 44 . Up to date, such narrow PL emissions were only measured at low temperatures (4-10 K), for encapsulated monolayer MoS 2 [44][45][46][47] . In the present study, the detection of very narrow optical emission signals at room temperature might be related to the fact that the p-doped 2D MoS 2 nanosheets are highly crystalline but with local atomic defects (e.g., Mo and S vacancies), which act as a source of high-density charged carriers/photon emitters encapsulated in amorphous MoS 2 . On one hand, the amorphous MoS 2 encapsulation electrically shields the individual nanosheets from each other and prevents interaction with the substrate (i.e., no extrinsic doping or charge transfer from the substrate), therefore reducing electric field fluctuations 44 and consequently suppressing related inhomogeneous contributions to the exciton linewidth. On the other hand, as the crystalline 2D MoS 2 nanosheets maintain embedded during their growth, they are protected against physisorption of impurities from the environment 45,46 , allowing to keep the locally disordered structure of the nanosheets (e.g., Mo and S vacancies) that is likely responsible for the sharp PL emission (T and X L peaks in Fig. 2a).
It should be noted that the relatively low annealing temperature of 250°C was chosen to grow thin films compatible with polymer-based electronics and to avoid complete crystallization of the material, as the presence of the amorphous MoS 2 matrix is expected to stabilize and passivate the luminescent few-layered 2D-MoS 2 nanosheets. Different to single-phase materials such as exfoliated and CVD-grown flakes, the IJD-derived composite material (i.e., the 2D-MoS 2 nanosheets encapsulated in amorphous MoS 2 ) is expected to have stable optical and electronic properties over time, hence ensuring the quality of the thin films even after months.
The realized ReRAM device with an abrupt ON/OFF ratio of more than two orders of magnitude finally proves the excellent qualities of the MoS 2 nanosheet-containing thin film as a semiconductor material for electronic applications. It is important to note that the MoS 2 thin film works as a whole and not via single 2D-MoS 2 nanosheets. In other words, the dimensions of the metal contacts for device fabrication are large enough to ensure that the electrical behaviour is averaged over an area larger than a single MoS 2 nanosheet, proving that the amorphous MoS 2 encapsulation is conductive enough for device operation. The device was stable over dozens of cycles and reproducible in terms of retention time over a few days, highlighting the possibility to perform multicycle writing and reset operations. The growth of a stable 3D bulk material with 2D MoS 2 properties makes the IJD technique an excellent candidate for practical use in optoelectronic devices.
In the present study, we showed that IJD is able to efficiently grow MoS 2 thin films over inches-wide areas required for industrial up-scalable deposition. Crystalline 2D MoS 2 nanosheets encapsulated in an amorphous MoS 2 shell compose the film after a post-annealing step at low temperatures (i.e., 250°C). This makes the IJD technique compatible with all substrates and/or supports typically used for (opto-)electronic applications, including the family of flexible polymeric materials used for displays, wearable electronics, etc.
The as-deposited Mo x S y nanoclusters rearrange after annealing and form highly crystalline 2H-MoS 2 nanosheets encapsulated in amorphous MoS 2 . The material exhibits a bandgap of 1.86 eV and excellent photoluminescence properties, comparable to those reported for encapsulated single-layer MoS 2 . The MoS 2 thin films were shown to be applicable in ReRAM devices with low erasing and writing voltage thresholds.
In contrast to MoS 2 grown via other techniques, where an increase in thickness of more than a few layers unavoidably leads to the loss of the desired optical and electronic properties, the MoS 2 thin films grown by IJD containing few-layered crystalline 2D-MoS 2 nanosheets show properties comparable to few-layer 2H-MoS 2 even over a large area and independent on the film thickness. This is due to the encapsulation of the crystalline MoS 2 nanosheets (responsible for the 2D properties) in an amorphous MoS 2 matrix, forming a mechanically stable two-phase thin film (bulk). The 2D-embedded-in-bulk composition of the thin film ensures easier handling and makes the IJD technique preferential for mass production of electronic devices, where the fabrication process must be simplified as much as possible to guarantee low costs, but at the same time keep high-quality material and production standards.
In addition, the IJD technique enables the possibility to work with selenide-based TMDCs, such as WSe 2 , NbSe 2 , etc., ensuring the right composition of the thin films without the need of highly toxic selenium in the gas phase. Different from other deposition techniques, where multi-step processes are required to fabricate TMDC heterostructures, the IJD technique paves the way to onesolution in-situ multi-target deposition for TMDC heterostructures with desired electronic, optical, and magnetic properties.

Ionized jet deposition of MoS 2 thin films
Unless otherwise stated, the present MoS 2 thin films were grown on Pt (50 nm)-on-Si substrates (see ref. 76 for details of substrate preparation) to avoid self-oxidation and sulfur interaction with the substrate. The IJD growth was carried out at room temperature in a lab-grade IJD system through the ablation of a cylindrical MoS 2 target (Testbourne Ltd., Ø = 5 cm, thickness = 0.5 cm) with a target-to-substrate distance of 150 mm, a base pressure of p~10 −7 mbar, and using Ar as working gas (during deposition: p = 10 −3 −10 −5 mbar). The IJD discharge parameters were set at f = 150 Hz and V = 15 kV, with a total deposition time of 1 h. The asdeposited thin films were subjected to HV (p = 1 × 10 −6 mbar) postdeposition annealing at T = 250°C for 3 h. Electrical characterization was carried out on electronic devices that were fabricated by deposition of annealed MoS 2 thin films on Si 3 N 4 -on-quartz substrates. The substrates were pre-patterned with Au pads (Ø = 500 µm) serving as bottom-contacts. Before MoS 2 deposition, the pre-patterned Si 3 N 4 substrates were masked with polymethyl methacrylate (PMMA), leaving the Au bottom-contacts uncovered for subsequent MoS 2 and Au topcontact deposition. In a final step, the PMMA was peeled off.

Characterization
The thickness of the annealed MoS 2 thin films was determined via crosssectional SEM images (e.g., see inlet in Supplementary Fig. 2d). Photoluminescence (PL) and non-resonant Raman spectra of as-deposited and annealed MoS 2 thin films were acquired using a LabRAM Aramis (Horiba Jobin-Yvon) instrument, which is equipped with an optical microscope and a 100× objective. A diode-pumped solid-state laser source of 532 and 632 nm was used for the excitation of the PL and Raman signals. The luminescence and Raman signals were monitored with an air-cooled charge-coupled device. The slit width of the spectrometer was typically set at 100 µm. A diffraction grating with 1800 lines mm −1 was used for the collection of all Raman spectra with an overall spectral resolution of~2 cm −1 . PL and Raman spectra were collected with an overall acquisition time of 10 s by setting the laser power to 200 µW.
Ultraviolet photoelectron spectroscopy (UPS) was carried out using the He I photon with an energy at~21.2 eV, and X-ray photoelectron spectroscopy (XPS) was performed with a non-monochromatized Mg Kα source (emission line at 1253.6 eV). The photoelectron signals were detected with a VSW HSA100 hemispherical analyzer equipped with PSP electronic power supply and control. The total energy resolution is about 0.1 eV for UPS and 0.8 eV for XPS. The binding energy (BE) scale of UPS spectra was referred to the Fermi level of the same Au clean substrate, whereas XPS BEs were calibrated using the Au 4f peak at 84.0 eV as reference. The SECO spectra were measured with a sample bias of −7.0 V.
EELS was performed with a Leybold electron gun (EQ22/35) operated at 1000 eV of primary beam energy and a CLAM2 (VG Microtech) electron analyzer, with a resolution of 0.1 eV. The electron beam current on the sample was 2 nA.
XRD spectra were detected with a Rigaku D/max III diffractometer in glancing incidence configuration, using Cu Kα radiation and a graphite monochromator in the diffracted beam. An asymmetric scan geometry was adopted to enhance the signal stemming from the MoS 2 thin film. Typical measurements were performed in the (2θ) range of 10−70°with an incidence angle of 1°, sampling interval of 0.05°, and counting time of 8 s.
For TEM analysis, the annealed MoS 2 thin film was transferred from the substrate by gently rubbing on standard carbon-coated copper grids. Samples were imaged with a field-emission JEM-2200FS JEOL TEM equipped with an in-column Omega filter operated at 200 kV.
The STM images were acquired in an UHV (p < 10 −10 mbar) chamber in constant current mode at room temperature, using an Omicron multiscan system. The Pt/Ir tip was prepared by electrochemical etching in saturated CaCl 2 aqueous solution and subsequently cleaned in UHV by electron bombardment. Before the measurements, all samples were degassed at approximately 120°C at a pressure below 5 × 10 −10 mbar for a minimum of 3 h up to 10 h and subsequently moved to the scanner without breaking the vacuum. The STM images were recorded in constant current mode with the tip to sample bias ranging from −0.7 to −2 V and tunnelling current ranging from 0.5 to 2 nA. STS data were taken by the sample-and-hold technique at room temperature. For each point, the I/V and the dI/dV curves were collected. The dI/dV curve was obtained by applying to the bias potential, a modulating voltage of 20 mV, and collecting the derivative with a lock-in amplifier. The stabilizing conditions for the different points are V bias in the −1.6−2.0 V range, I in the −0.5−2.0 nA range. Each curve was obtained by averaging over more than 20 individual spectra to improve the signal-tonoise ratio. The DOS was calculated following eq. 3 of ref. 51 , e.g., from the (dI/dV)/(I/V), that is independent of the tip to sample distance.
Electrical characterization (current−voltage (I−V) measurements) of annealed MoS 2 thin films deposited on Au-covered Si 3 N 4 -on-quartz substrates were acquired at room temperature and in the air using an Imina miBots probe station with Tungsten tips connected to a Keithley 4200 A parameter analyzer.