Introduction

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are a versatile family of nanomaterials with more than forty layered composites. They have the general formula MX2, where M is a transition metal of the groups 4–10 of the periodic table, and X is the respective chalcogen [1,2,3,4]. Molybdenum (IV) disulfide (MoS2) is one in this type of 2D layered materials that come up with a variety of promising applications in nanoelectronics, photonics, sensing, energy storage, and optoelectronics [3, 5,6,7,8]. For instance, MoS2 exhibits weak interlayer Van der Waals interactions that allow it to act as an efficient host for a variety of electron donating atoms, such as lithium or sodium, to produce nano-batteries [9]. In bulk, MoS2 exhibits a semiconductor behavior with an indirect bandgap of 1.29 eV, but it drastically differs when becomes a monolayer, having a transition to a direct bandgap of around 1.8 eV [8, 10]. The displayed properties are directly related to its crystalline structure and particular three-dimensional arrangement. One of the biggest challenges in the synthesis of MoS2 materials is advancing reliable methods to produce specific nanometric-size arrangements and fine tune their properties. Among the different methods employed to modify and control the structural properties of MoS2 are: mechanical synthesis [11], wet chemistry [12], exfoliation [13], and chemical vapor deposition (CVD) [14]. CVD is particularly useful for large-scale production of 2D MoS2, enabling direct growth of materials over-activated solid substrates using gas-phase precursors. An efficient control of the reaction conditions allows the production of uniform thin layers [15], or vertically aligned MoS2 films [16]. Moreover, this method enables the overall shape control of deposited layers, using elemental molybdenum and sulfur as precursors, to produce high-quality single-layer MoS2 thin flakes, with triangular and hexagonal shapes, extended on solid substrates [17, 18]. Shape and structure effect the properties and consequently the applications of multilayered materials; e.g., a preferred pyramidal arrangement of layers exposes a succession of edges over the pyramidal steps which have potential catalytic applications [19,20,21,22]. Hence, the detailed study of morphological and structural features of 2D materials becomes of paramount importance to understand their evolution and properties.

Previous reports described the growth of 2D MoS2 as a screw-dislocation-driven process [23,24,25,26,27,28,29], and the study of layered microstructures has been conducted using atomic force microscopy (AFM) and Raman spectroscopy. Typically, MoS2 exhibits a centrosymmetric 2H-hexagonal phase described by an ABAB stacking of S–Mo–S layers, with coplanar S–Mo–S atoms in the first layer opposite to the second layer. However, other non-centrosymmetric phases have been reported specially when dealing with few layered materials and screw-dislocation pyramidal morphologies [24, 25, 30, 31]. In this work, pyramidal microstructures made of MoS2 were produced in a CVD system. Their structural characteristics were investigated using spectroscopy and electron microscopy techniques, namely Raman, X-ray photoelectron spectroscopy (XPS), ultra-high-resolution scanning electron microscopy (UHR-SEM), focused ion beam (FIB), and transmission electron microscopy (TEM). Emphasis placed on the structural characteristics is aimed to expand the understanding on the growth of MoS2 microstructures.

Experimental section

Synthesis

MoS2 microstructures were synthesized via CVD, protocol adapted from Chen et al. [24]. The experiment setup is illustrated in Fig. 1. A 5 × 5 mm section of ultra-flat [100] SiO2 wafer was used as a substrate for the preferential growth of MoS2. The SiO2 piece was previously cleaned using an isopropyl alcohol/acetone (1:1 volume) solution. In a typical growth reaction, 0.3 g of molybdenum trioxide (MoO3) powder (99%, Sigma-Aldrich) was placed in an alumina boat located at the center of a quartz tube (2.5 cm in diameter and 60 cm long). The boat position was at the center of the heating zone of the furnace (Lindberg Blue STF55433). Next, 0.6 g of sulfur powder (99% Sigma-Aldrich) was placed in the inlet of the furnace 15 cm away from the center. During a typical procedure, the furnace temperature was set to 900 °C following a linear ramp of 10 °C min−1 under a 20 sccm flow of N2 gas. The system was maintained during 20 min at 900 °C, time in which the sulfur evaporated entirely and reached the center of the reactor to interact with the MoO3 [32, 33]. Finally, the system was cooled down to room temperature under N2 atmosphere.

Figure 1
figure 1

The schematic shows the location of substrate and precursor materials in the CVD system used for the growth of MoS2 pyramids

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Characterization

Raman spectroscopy studies were performed with a LabRam HR Evolution from Horiba at ambient conditions with controlled light and a back-scattering configuration at a wavelength of 633 nm laser. For mapping, an acquisition time of 5 s and 10 accumulations were used for each point with a grid of 10 × 10, an objective lens of 100× Vis and grating of 500 nm. X-ray photoelectron spectroscopy (XPS) spectra were obtained in an ESCALAB 250Xi (Thermo Scientific) X-ray photoelectron spectroscope using monochromatic AlKα radiation (hν = 1486.7 eV). Images of the surface morphology of the MoS2 pyramids were acquired with an opto-digital microscope (Olympus DSX500), an ultra-high-resolution field-emission scanning electron microscope (UHR FE-SEM Hitachi 5500), and with a FIB-scanning electron microscope (FIB-SEM, ZEISS Crossbeam 340). A cross section of the pyramidal structure was obtained through FIB microsectioning using Ga ions at 30, 5, and 2 kV. The pyramid was previously coated with a layer of carbon and platinum to protect the sample from gallium ions. Plain-view imaging of the microstructure was performed with a high-resolution transmission electron microscope (HRTEM, JEOL 2010F). Experimental HRTEM images are compared with a simulated 2H.MoS2 structure generated by SimulaTEM software [34]. Atomic resolution imaging was carried out in a Cs-corrected transmission electron microscope (JEOL ARM-200F) operated in scanning mode (STEM) at 200 kV.

Results and discussion

The SiO2 substrate was explored through optical microscopy to determine the structures obtained through the CVD process (Fig. 2). It is possible to observe pyramidal microstructures obtained through the CVD process in the images of bright-field (2a–b) and dark-field (2c–d) mode, such structures are indicated with arrows.

Figure 2
figure 2

Bright-field (a, b) and selected area dark-field optical images (c, d) of the SiO2 piece after the CVD process

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The deposit of MoS2 on the SiO2 substrate as a result of the CVD process was observed with more detail through SEM imaging, Fig. 3. It´s noticed the formation of an irregular film (or “islands”) on the substrate, Fig. 3a. At the edge of this irregular film, it is possible to observe the abundant formation of triangular structures, with some degree of truncation or shape irregularity (Fig. 3b). Some of these microstructures present a perfect triangular shape, such as the one shown in Fig. 3c, with a size of 3.8 µm per side, where the stepped surface is visible.

Figure 3
figure 3

a SEM images of the “islands” and b and c pyramidal structures of MoS2 at different magnifications

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The Raman spectrum of the micropyramid are shown in Fig. 4, where the color of each spectrum colors corresponds to the color assigned to each circle shown in the mapped area. Mapping analysis revealed the appearance of characteristic MoS2 signals at 179 (A1g (M) − LA (M)), 230 (LA(M)), 383 (\( E_{2g}^{1} \)), 409 (A1g), 421 (\( B_{2g}^{2} + E_{1u}^{2} ) \), 454 (2LA), and 465 cm−1 (A2u) [35,36,37] corresponding to the spectra of the shown areas. However, there are some differences between the spectrum obtained from the central area of the micropyramid (red spectrum or labeled with the letter d) and the spectrum obtained on the edge labeled with the letter b, black circle. The spectrum obtained from the central area shows a value of the I2LA(M)/IA2u ratio of 0.98, while the I2LA(M)/IA2u ratio for the black spectrum was 1.05, which indicates that the 2LA(M) mode shows a high value of intensity at the edge of the micro pyramid. According to Chakraborty et al. [36], the relative intensity of the 2LA(M) mode respect to the A2u mode decreases as a function of the number of layers, correlating with our results since at the center of the micropyramid the number of layers is greater than in the edge. The relationship between the 2LA(M) and A2u modes for the spectra c and e indicates a 2H-type structure similar to the to the bulk material, while in the a spectrum the presence of MoS2 is null since the mapping was carried out outside the micropyramid. Another vibrational mode observed in the spectrum and taken at the edge of the micropyramid is located at 230 cm−1 (mode LA (M)) taken at the edge of the micropyramid. This mode is identified as first-order mode, and it is attributed to the dispersion of phonons due to defects induced in the structure and the presence of a few atomic layers. Finally, a slight signal was also observed around 300 cm−1, which may be due to the presence of molybdenum oxides remaining from the synthesis [38,39,40].

Figure 4
figure 4

Raman spectra of different points on the MoS2 pyramids grown by CVD (633 nm laser)

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The XPS analysis of the MoS2 pyramids is displayed in Fig. 5. In the high-resolution spectrum of the S2p region (Fig. 5a), the 2p1/2 and 2p3/2 signals of the MoS2 are observed at 163.1 and 161.8 eV. Also, two signals are observed at 162.4 and 161.3 eV, which can be assigned to non-stoichiometric MoxSy due to the presence of surface defects introduced during the CVD growth [41, 42].The S2s-Mo3d region is displayed in Fig. 5b. The S2s (226.50 eV) and Mo 3d (229.1 and 232.2 eV for 3d5/2 and 3d3/2, respectively) correspond to signals of MoS2 are observed [43,44,45]. Other signals located at 233.3 eV and 236 eV, are observed, assigned to the 3d5/2 and 3d3/2 of the Mo(VI) species [46,47,48]. These results show that the obtained microstructures are almost entirely composed of MoS2; however, some molybdenum oxide and non-stoichiometric MoxSy phases presented.

Figure 5
figure 5

High-resolution XPS spectrum of S2p a and S2s-Mo3d b regions of the MoS2 micropyramids

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A more detailed morphological analysis was carried out by SEM tilting the sample up to 54o, which allows the observation of a distinguishable stepped surface of the micropyramids. The pyramid has a triangular base consisting of several layers of MoS2. The pyramids correspond to a truncated trigonal morphology with a high density of surface steps, which can be correlated to the speed of growth in the direction of the [0001] axis. It slightly varies since each step has a different number of layers. The obtained pyramidal morphology is related to screw dislocations occurring during the growth of the nucleus along the vertical axis, and to the triangular-shaped layers in the lateral dimension that form the MoS2 pyramid. A spiral structure of MoS2 pyramids can be clearly seen, and the direction of growth that occurred in a clockwise direction is indicated with white arrows in Fig. 6a. A schematic of the MoS2 pyramid growth with a spiral structure is shown in Fig. 6b. The different steps that are formed during the growth of the pyramid are clearly observed in Fig. 6c. The image shows defects at the edges, which cause deviation from the perfect triangular shape. The combination of morphological features between hexagonal and triangular shapes in the microstructures is due to a complex growth that includes two dislocations that share a common core [29]. In some of the MoS2 pyramids, we observed the formation of small particles at the edges of the layers and some larger particles dispersed throughout the pyramid. These round particles corresponded to nucleation points for the growth of monolayers of MoS2 [14]. These images help to understand the growth mechanism of the triangular crystallites. The growth proceeded along emerging screw dislocation on the surface, as demonstrated by the Burton, Cabrera, and Frank theory (BCF) [49, 50]. This type of growth mechanism has been reported in other nanomaterials such as cobalt, nickel hydroxide, zinc hydroxy sulfate, and other pure metals [51,52,53,54]. The formation of a screw dislocation in MoS2 requires the generation of slipped planes (screw defects) in the first layers of the pyramid; otherwise, the growth will preferentially follow an in-plane growth mode, producing monolayers of MoS2 [25, 55]. Screw dislocations developed under low supersaturation conditions created staggered edges in the bottom layer that act as nucleation sites for the addition of the following precursor atoms, driving the growth of the second layer in the top of the bottom layer. This gradually leads to the continuous growth of spirals, resulting in a pyramidal growth [50]. These conditions of low-saturation conditions were reached in our experiment set up by the low sulfur concentration present at the center of the reactor and by controlling the low drag flow velocity. For most of the reported experiments for the growth of MoS2 using CVD the rate of the carrier gas is usually high, ranging from 100 to 500 sccm [14, 56, 57], in comparison with our process at only 20 sccm.

Figure 6
figure 6

a SEM image of MoS2 pyramid showing the spiral growth mechanism. b Schematic model of the MoS2 pyramid. c SEM image of a MoS2 pyramid is showing the multiple layered steps

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Dichalcogenide MoS2 present typically a 2H-hexagonal phase with an ABAB stacking. This stacking order can vary depending on the morphology, thickness, growth mechanism, etc. [24, 25, 31, 58]. To confirm the structural arrangement of the produced MoS2 pyramids, HRTEM and atomic-resolution STEM imaging were performed. First, complete structures were recovered by scraping the surface of the SiO2 substrate with a thin blade, then placed into an ethanol solution and resuspended with an ultrasonic bath for 20 min. Afterward, 7µL of the ethanol solution was placed onto a carbon-coated TEM grid, and the solvent was allowed to evaporate at room temperature. A second sample was prepared by FIB cross-sectioning, attaching the fabricated lamella on a copper lift-out grid. Figure 7a (scraped sample) shows an HR-TEM micrograph of a scraped MoS2 pyramid, confirming the crystalline arrangement achieved during the CVD production process. No perceivable defects were found in the analyzed areas. The three interplanar distances were 0.27 nm, with 60° between them. These distances matched with the \( \left( {0\bar{1}10} \right) \), \( \left( {1\bar{1}00} \right) \), and \( \left( {10\bar{1}0} \right) \) planes of the 2H-MoS2 structure, viewed along the \( \left[ {0001} \right] \) direction. The corresponding electron diffraction pattern is shown in Fig. 7b. The pattern shows sharp reflections characteristic of 2H-MoS2. Satellite reflections following the hexagonal symmetry of the pattern are also observed, indicating a rotation of ~ 2.6° of some layers around the [0001] axis due to the spiral growth. Micrographs of the pyramid cross section, Fig. 7c, exhibit an area with a periodicity of layers or steps forming the pyramidal arrangement. The overall imaged area was approximately 1 µm2, and it is composed of about 68 layers. The interlayer distance is 0.6 nm, corresponding to {0001} planes. The electron diffraction pattern of the MoS2 (Fig. 7c, up inset) confirmed the [0001] growth direction. A magnified area of the image indicated with the blue square in Fig. 7a is presented in the left panel in Fig. 7e highlighting the lattice pattern. An HRTEM image of a MoS2 flake with 2H-ABAB structure viewed from the [0001] axis zone was simulated, right panel in Fig. 6e, taking into consideration the experimental acquisition conditions. By tilting the sample around the \( \left[ {10\bar{1}0} \right] \) axis, a second HRTEM image was taken along the \( \left[ {03\bar{3}1} \right] \) axis zone, and its corresponding simulated image is also shown, right panel in Fig. 7f. As noticed, the simulated images are in good agreement with the observed experimental lattice patterns along two different axis zones.

Figure 7
figure 7

a TEM image of MoS2 pyramid viewed from different zone axis b electron diffraction pattern of MoS2 pyramid viewed from [0001] direction. c Cross section area and d electron diffraction pattern of the pyramid and composed of 68 layers, their evenly space of 0.65 nm which correspond to the (0002) planes of MoS2. e, f Experimental (left) and simulated (right) HRTEM images of a small area of the single pyramid. The lattice pattern is in agreement with a 2H-ABAB layer stacking

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Aberration-corrected HAADF-STEM imaging was employed to get a deeper insight into the pyramid´s. Figure 8a shows a MoS2 pyramid coated with a layer of carbon and platinum to protect it before sectioning. The prepared pyramid´s cross section is observed in Fig. 8b. This process enabled to determine that the sample height was of approximately 125 nm close to its center with ~ 200 monolayers of MoS2, offering a considerable amount of chemically active reactive sites for efficient catalytic applications or to act as support for other functional nanostructured materials. The cross section view revealed four steps clearly defined, the height of each step depending on the number of layers of MoS2 conforming them (Fig. 8c). In the analyzed area, each step was formed by 3 to 7 layers, which corresponded to a size of 1.8 and 4.2 nm, respectively (Fig. 8d). A schematic of the pyramid showing the different number of layers in each step is presented in Fig. 8e. The cross-sectional atomic-resolution HAADF-STEM image in Fig. 8f enables direct visualization of the stacking order of the pyramid layers. The brightest atomic columns correspond to projected Mo atomic columns, while the dim ones are exclusively S columns. An atomistic model is inserted in the image for comparison. This arrangement corresponds to an ABAB stacking of MoS2 layers, characteristic of the 2H phase, viewed along the \( \left[ {1\bar{1}00} \right] \) direction. An atomistic model is inserted in the image for comparison.

Figure 8
figure 8

a TEM image of the MoS2 pyramid with a white rectangle where the cross section cut was made. b SEM image of cross section obtained using the FIB-SEM. c, d Aberration-corrected STEM imaging showing the position and height of the atomic steps and separation between the MoS2 layers e atomic model of the MoS2 structure showing the steps. f Atomic resolution imaging of the lattice of a MoS2 with the superimposed model of the structure. g Atomic resolution imaging of the layer stacking of MoS2. h Deconvolution of a line profile spectrum of MoS2 (black line) with peak overlaps of Mo (Blue line), and S (purple line) elements, taken from figure (f)

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An intercalated stacking of MoS2 originated by the screw dislocation closer to the center of the pyramid is observed in Fig. 8g. It is noted that the ABAB stacking order is retained with very small deviations due to layers deformation or rotation, which can cause the satellite reflections observed in the diffraction pattern in Fig. 6b. The nucleation at the initial stage generates a dislocation center with high spiral activity, which allows maintaining the screw dislocation in the MoS2 layers. A deconvoluted intensity profile of the selected area shown in Fig. 8f highlights the location of heavy atomic columns (Mo) and light atomic columns (S) throughout the line profile (Fig. 8h). Interplanar distances of 0.6 nm and 0.15 nm were measured, corresponding to interlayer Mo-Mo columns and in-plane Mo-Mo columns, respectively. The difference in the intensities of the S columns in the profile is due to a slight deviation of the sample from the exact axis zone. These cross-sectional images present a direct observation of the MoS2 layer intercalation produced by the screw-dislocation-driven growth and confirm the 2H-ABAB stacking order of the produced pyramids in agreement with spectroscopy results [29].

Conclusions

MoS2 micropyramids were produced by using a CVD growth method using at high temperatures and low gas rates. Raman spectroscopy demonstrated the characteristics signals of MoS2 on the SiO2 substrate and highlighted the differences between the vibrational modes at the center and in the edge of pyramids. Signals at 230 and 454 cm−1 attributed to few layers MoS2 were located at the edge of the pyramids suggesting a one-step formation, while at the center, the signal corresponds to the bulk material. XPS confirmed the presence of the MoS2 phase in its majority, however, the presence of the Mo (VI) signals are also observed attributed to the presence of an oxide phase. UHR-SEM imaging revealed the preferential formation of pyramidal structures ranging in size between 1 and 4 µm per side. In combination with FIB-SEM, it was possible to determine that the growth mechanism of these materials is dominated by a spiral growth at screw dislocations, particularly due to the low concentration of the sulfur used during the optimized process. Cross sections of MoS2 pyramids obtained by FIB allowed to reveal the internal arrangement of the materials. In combination with atomic resolution, S/TEM imaging allowed the direct determination of the 2H-ABAB structural stacking along the [0001] direction of the produced pyramids, with horizontal growth along the \( \left[ {10\bar{1}0} \right] \) and \( \left[ {0\bar{1}10} \right] \) directions. The high contrast and resolution of the HAADF signal undoubtedly showed the precise organization of MoS2 into defined atomic steps of S-Mo-S, and the intercalation of layers one by one originated by the screw-dislocation-driven growth.