Uniform large-area growth of nanotemplated high-quality monolayer MoS₂

Two-dimensional (2D) transition metal dichalcogenide (TMD) crystals have attracted extensive attention as next-generation semiconductor materials^1–4 due to a variety of applications in flexible and transparent electronics^5,6 and the potential for emergent phenomena due to the interplay of spin and valley degrees of freedom.^7–9 In particular, MoS₂ has emerged as one of the canonical members of this family with a variety of studies on its electrical, optoelectronic, and mechanical properties.^10–19 For example, bulk crystalline MoS₂ is a semiconductor with an indirect band gap of 1.2 eV,²⁰ while a single layer^1,21 exhibits a direct-gap of 1.8 eV and locking between the spin and valley degrees of freedom.^8,10,22–24 Recently, considerable work has been done to modify these properties via stacking and functionalization.^4,25–28 However, current approaches often require lithographic patterning to realize functional structures, inevitably leaving organic contaminants whose impact is exacerbated by the difficulty in cleaning these atomically thin materials. Encapsulation strategies, while effective for many applications,^13,29 do not allow for independent patterning of the constituent layers and thus limit the stacking of multiple functional layers with independent electrode geometries, channel widths, etc. The result is a clear need for high-quality, patterned 2D materials with pristine surfaces.

Here, we introduce a synthetic approach to templated MoS₂ growth with submicron features without the need for resists or other sources of contamination. Stencils made of SiN membranes³⁰ with feature sizes down to 100 nm are used as masks through which Mo films are deposited onto atomically stepped sapphire substrates.^31,32 These pre-patterned Mo films are subsequently sulfurized in a reducing atmosphere to grow MoS₂ films.^33–37 Combining the SiN stencil with the sulfurization growth technique allows centimeter-scale uniform growth and direct control of the number of layers in the resulting MoS₂ film which have an optical quality similar to that of exfoliated and suspended MoS₂^10,38 and the highest quality synthesized MoS₂ grown by other methodologies.^31,32 This growth technique can readily be expanded to a wide variety of other TMDs simply by changing the templated metallic film and has the potential to directly address the demand for patterned 2D materials with pristine surfaces suitable for subsequent modification via functionalization and mechanical stacking.

Over the past five years, many methodologies for MoS₂ growth have been developed.^{31,32,39–46} In particular, chemical vapor deposition (CVD) techniques whereby MoO₃ and S precursors are evaporated upstream of a heated growth substrate produce high-quality monolayer MoS₂ islands 10–100 μm in width that can form locally continuous films.^{31,32,42–44} Recently, this CVD technique has been augmented through the use of atomically stepped c-axis sapphire as a growth substrate,^31,32 promoting MoS₂ flake growth with a uniform lattice orientation and reducing the number of domains present in these coalesced crystalline films. An alternate route to MoS₂ growth, and the method employed here, is through the sulfurization of pre-deposited Mo films.^33,34 This technique produces uniform large-area MoS₂ films whose thickness is tuned by controlling the thickness of the starting Mo film.^35–37 While this sulfurization method is both facile and inexpensive, in the past, it has typically yielded polycrystalline and multilayer films.^33–37 

In several cases, both the CVD and sulfurization growth methods have been augmented to nucleate MoS₂ crystallization in predetermined locations by seeding⁴⁷ or patterning^48,49 the initial substrate and by templating the pre-deposited Mo film prior to sulfurization.^36,50 However, these techniques either rely on potentially contaminating lithographic processes or produce an erratically shaped and positioned material. The growth technique adopted here builds on these existing approaches through the use of SiN stencils to produce resist free, templated Mo films that are subsequently sulfurized, producing high-quality MoS₂ with large-area uniformity, sub-micron feature sizes, and precisely controlled shape and location.

This growth procedure is illustrated schematically in Fig. 1(a). The stencil is fabricated by focused ion beam (FIB) milling to pattern a SiN membrane, and sapphire substrates are pre-annealed for 1 h at 1000 °C in air. This pre-treatment creates atomically smooth steps⁵¹ (see supplementary material, Fig. S1) and facilitates the single-crystal domain growth of high-quality MoS₂.^31,43 Next, the SiN stencil is placed membrane-down on the sapphire substrate, and the Mo thin film is deposited through the stencil by electron beam evaporation. These Mo/sapphire samples are then loaded onto an alumina plate and placed in a custom two-zone furnace [Fig. 1(b)] for sulfurization. The furnace is designed such that the temperature of the Mo-templated sample and sulfur powder precursor can be individually controlled, allowing the sulfur temperature and flow during the sulfurization process to be systematically controlled, independent of the sample temperature.

The optimized growth recipe, including the temperatures of the sulfur boat and substrate, is shown in Fig. 1(c). The chamber is initially purged for 10 min using 150  sccm of Ar. After purging, the gas flow is changed to 115  sccm of Ar and 5 sccm of H₂,^44,52 and the sample temperature is ramped to 650 °C at a rate of 20 °C/min for sulfurization. During this initial ramp, the sulfur zone is also ramped up to a pre-melting temperature of 110 °C. The sample is held at 650 °C for 30 min, while the sulfur zone temperature is raised to a temperature of 140 °C, beginning the sulfurization process. Subsequently, the sample temperature is increased to 950 °C at a rate of 10 °C/min for a second annealing step while maintaining the sulfur flux. This annealing step is found to increase the quality of the resulting MoS₂ films both in our own work and in the literature.^35,53 After it is complete, the sulfur zone temperature is reduced until the sulfur resolidifies (typically at a sulfur zone temperature of 50 °C) while maintaining the sample zone temperature at 950 °C to reduce the amount of free sulfur residue present on the substrate. Any residual sulfur is removed either through a post-growth anneal around 150 °C for 1–2 h or by soaking the sample in acetone for 2 min, rinsing in isopropanol, and finally drying with flowing nitrogen gas (see supplementary material, Fig. S2).

Characterization begins with the stencils, which are evaluated primarily through in situ scanning electron microscopy (SEM) and focused ion beam (FIB) imaging [Fig. 2(a)]. Pattern scaling is limited by the lateral dimensions of the SiN membrane (750 × 750 μm²), with feature sizes that range from 100 nm to tens of microns [Fig. 2(a)]. Molybdenum film thicknesses in the range of 1–10 nm (100–1000 s) are calibrated by atomic force microscopy (AFM), and this calibration is subsequently used to estimate the Mo thickness for shorter deposition times (50–100 s). Figure 2(b) shows the patterned film after a 50 s Mo deposition and subsequent sulfurization using the recipe shown in Fig. 1(c), demonstrating the fidelity of the templating procedure. Write fields spaced across the entire SiN membrane surface produce uniform MoS₂, regardless of the position on the 5 × 5 mm² sapphire substrates within the deposition region of the CVD system (roughly 3 × 8 cm²). In contrast to the bare Mo films, sulfurized films are visible with optical microscopy (see supplementary material, Fig. S3). When the deposited metal thickness is properly tuned, this templated sulfurization produces single-layer MoS₂ over the entire templated surface. For example, the AFM image in Fig. 2(c) shows a zoomed in image of the “E” feature of the “CEM” shown in the SEM micrograph in Fig. 2(b), with the line cut inset showing a uniform thickness of 0.7 nm, indicating a single atomic layer of MoS₂.

Figures 2(d)–2(g) show further AFM micrographs, demonstrating the utility of this growth technique in producing templated features with precise control of layer thickness. These micrographs of 2 μm diameter dots show the transition from single-layer to multi-layer MoS₂ films as the Mo metal deposition time is increased from 50 to 100 s, while all other growth parameters are held constant. These results are summarized in Fig. 2(h), which shows an AFM line cut for each dot, demonstrating the layer-by-layer thickness control down to a single monolayer of MoS₂ and an edge width of 20 ± 5 nm.

Further structural and electronic characterization of these thin films is performed using Raman and photoluminescence (PL) spectroscopies. Raman spectra are collected using a 514 nm (2.41 eV) laser for excitation, and a representative spectrum is shown in Fig. 3(a). We observe both the in-plane vibrational mode of the Mo and S atoms at 384.9 cm⁻¹ (E′) with a full width at half maximum (FWHM) of 3.5 cm⁻¹ and the out-of-plane vibration of S atoms at 405.1 cm⁻¹ (A′₁ peak) with a FWHM of 5.5 cm⁻¹. The E′ and A′₁ peak locations are characteristic of single-layer MoS₂ grown on sapphire,^31,54 and the sharpness of these peaks is an indicator of high structural order.⁵⁵ Additionally, the A′₁ and E′ peaks exhibit an intensity ratio close to one (1.02), indicating a slight n-type doping.⁵⁶ The small shoulder of the E′₁ peak at 381 cm⁻¹ and the extra peak at 416 cm⁻¹ are both attributed to the sapphire substrate (see supplementary material, Fig. S4).

The Raman spectra for samples of varying thicknesses [the same samples as those shown in Figs. 2(d) and 2(e)] are measured in order to verify the growth thickness. A summary of these data is shown in Fig. 3(b). We see the characteristic E′ peak shift downward by 1.2 cm⁻¹ and the A′₁ peak shift upward by 3.7 cm⁻¹ as the number of MoS₂ layers is increased from a single-layer sheet to a bulk film (>10 layers), consistent with the literature (see supplementary material, Fig. S5).⁵⁴ 

A typical photoluminescence (PL) spectrum is shown in Fig. 3(c) and is measured using a 633 nm (1.96 eV) laser for excitation. We observe an intense A excitonic peak at 1.86 eV with a FWHM of 56 meV (20 nm), which is comparable to that of suspended exfoliated MoS₂ flakes and the highest quality CVD grown MoS₂, is indicative of highly crystalline single-layer MoS₂ (for analysis details, see supplementary material, Fig. S6).^10,31,32 In addition to individual PL and Raman spectra, we have performed optical mapping of the “CEM” portion of the pattern using half micron steps with a spot size of 0.4–0.6 μm [Fig. 3(d)]. These maps demonstrate that the single-layer films are both optically and structurally uniform. The structural and electronic quality of the single-layer MoS₂ is further confirmed through the fabrication of ion-gel gated field-effect transistor (FET) devices⁵⁷ and shows an on/off ratio of 10⁶, comparable to other ion-gel gated measurements in the literature (see supplementary material, Fig. S7).⁵⁸ 

Finally, we verify the surface cleanliness and structural quality of our material by both scanning-tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS) (Fig. 4). For STM measurements, the MoS₂ was grown directly on graphene/SiO₂ substrates (purchased from Graphene Supermarket) using the same process described for the sapphire growths. Here, the single sheet of graphene replaces the sapphire as a template for the MoS₂ lattice and serves as a charge sink to drain the STM tunneling current, preventing charging and enabling for atomic resolution. Figure 4(a) shows a representative scan, where the hexagonal MoS₂ lattice is clearly visible with a lattice constant of 3.1 Å.^59,60 XPS scans [Fig. 4(b)] of the 3 nm thick MoS₂ indicate a ratio of sulfur to molybdenum of 2.04, and the high-definition scan of the Mo 3d doublet shows only peaks at 231.4 and 228.3 eV, typical of Mo-S bonding (225.5 eV is a S 2s peak). We do not observe a peak at 235.4 eV, which is typically seen for Mo-O bonds. These scans indicate that the molybdenum and sulfur have fully reacted to form MoS₂, with no oxidized Mo remaining.^59,61 Neither of these measurements show any sign of surface contamination beyond the trace atmospheric contaminants expected for any surface exposed to ambient conditions.

We have developed a high-quality MoS₂ growth technique wherein Mo is deposited onto atomically stepped sapphire substrates through a SiN stencil with feature sizes down to 100 nm and subsequently sulfurized at high temperature. These films have a quality comparable to the best MoS₂ prepared by other methodologies, and the thickness of the resulting MoS₂ patterns can be tuned layer-by-layer by controlling the initial Mo deposition. Raman and PL spectra are found to be of similar high quality to that of suspended exfoliated MoS₂ and high-quality MoS₂ grown by CVD methods,^10,31,32 and STM and XPS studies verify the structural quality and cleanliness of the as-grown surface. This stenciled sulfurization methodology presents a facile and inexpensive route to pre-patterned MoS₂ monolayer films that are completely free of the organic residues generated by lithographic patterning techniques. We believe that this approach should be extendable to other metallization techniques where comparable control over deposition thickness can be achieved (i.e., sputtering and thermal evaporation). The combination of high material quality and pristine surfaces demonstrates the potential of this growth technique for use in surface functionalization studies and the assembly of van der Waals heterostructures that comprise some of the most active frontiers in 2D materials.^4,25,27,28

SiN masks are etched from amorphous, silicon-rich, transmission electron microscopy membranes purchased from Ted Pella, Inc. Prior to FIB writing, masks are coated with a 50 nm thick Ag film to reduce charging effects and to dissipate heat during the FIB writing process. FIB writing is performed using an FEI Helios Nanolab 600 dual beam focused ion beam and a scanning electron microscope at an acceleration voltage of 30 kV with current ranging from 28 to 2800 pA depending on the feature size.

The molybdenum metal is deposited via electron beam evaporation from Mo pellets purchased from Kurt J. Lesker (99.95% purity) at a deposition rate of 0.1 Å/s as measured by using a quartz crystal monitor. Epi-ready, c-axis sapphire substrates are 0.432 mm thick, single-side-polished HEMCOR wafers purchased from Alfa Aesar (#45019). Substrates are pre-annealed before Mo deposition for 1 h at 1000 °C in air.

Sulfurization is performed in a 2 in. diameter quartz tube in a modified Carbolite tube furnace, using Eurotherm temperature controllers and n-type thermocouples for substrate temperature control. The additional sulfur zone is defined by a Cu sleeve wrapped in a heating cable placed on a section of the quartz tube extending beyond the main furnace and with temperature measured by using a k-type thermocouple. Sulfur powder is purchased from Sigma-Aldrich at a purity of 99.98% (#414980–250G). Typically 50 mg of S is used during a single growth. Sulfur powder and Mo coated sapphire substrates are both loaded on alumina sample holders. The tube pressure is maintained at a pressure of 3–5 Torr during all growth steps.

Physical characterization was performed via SEM and FIB using a FEI Helios Nanolab 600 dual beam focused ion beam and a scanning electron microscope at a voltage of 10 kV and a current of 21 pA. AFM measurements were performed using a Bruker AXS dimension icon atomic/magnetic force microscope and Bruker TESPA-V2 Si probes in the tapping mode.

MoS₂ samples were characterized optically by Raman scattering and PL. Both were performed on a Renishaw InVia Raman spectrometer equipped with a CCD detector with the sample at room temperature and in ambient atmosphere. Raman spectra were collected using a 514 nm laser for excitation, while a 633 nm laser was used for PL excitation.

XPS was performed by using a Physical Electronics, Inc. (PHI) Versaprobe System using Al kα x-rays with a spot size of 100 μm and a power of 25 W under ultrahigh vacuum (UHV) conditions. Photoelectrons were collected through a series of energy analyzer adjustment lenses, and kinetic energy was measured using a semispherical analyzer. The survey was performed with a pass energy of 117.4 eV and a 1 eV step size, and the S 2p and Mo 3d regions were collected with a pass energy of 23.5 eV and a 0.05 eV step size.

Scanning Tunneling Microscopy (STM) images were acquired in a Createc LT-STM in UHV (10⁻¹¹ mbar) at 5 K. The MoS₂/graphene/SiO₂ sample was baked at a temperature of 120 °C for 30 min in UHV to remove surface adsorbates. The graphene/SiO₂ substrates were purchased from Graphene Supermarket. The MoS₂ was grown on these purchased substrates using the same growth techniques as those described for sapphire substrates. STM Images were processed using WsXM.⁶² 

Single-layer MoS₂ FET devices were fabricated using standard electron-beam lithographic techniques utilizing a FEI Helios Nanolab 600 dual beam focused ion beam and a scanning electron microscope. Contacts were metalized through the thermal deposition of 70 nm of Au. After wire-bonding to the sample, the solid polymer electrolyte, polyethylene oxide (PEO) and LiClO₄ were dissolved in anhydrous acetonitrile and drop-cast onto the device in a glovebox for use as a top-gate. Transport measurements were carried out at room temperature.

See supplementary material for the AFM of annealed sapphire substrates, AFM showing MoS₂ growths before and after the sulfur cleaning process, optical microscopy of templated MoS₂ growth, Raman spectra for different thicknesses of MoS₂, peak fitting for Raman and PL spectra, and electronic transport data.

This research was primarily supported by funding from the Center for Emergent Materials: an NSF MRSEC, Award No. DMR-1420451. Raman spectroscopy was supported by NSF Grant No. 0639163. Technical support was provided by the NanoSystems Laboratory.