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Synthesis and characterization of MoS2 nanosheets
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Nanotechnology Nanotechnology 27 (2016) 075604 (10pp)
doi:10.1088/0957-4484/27/7/075604
Synthesis and characterization of MoS2 nanosheets G Deokar1, D Vignaud2, R Arenal3,4, P Louette5 and J-F Colomer1 Research Group on Carbon Nanostructures (CARBONNAGe), University of Namur, 61 Rue de Bruxelles, 5000 Namur, Belgium 2 Institute of Electronics, Microelectronics and Nanotechnology (IEMN), University of Lille 1, Av. Poincaré CS 60069, 59652 Villeneuve D’Ascq, France 3 Laboratory of Advanced Microscopies (LMA), Institute of Nanoscience of Aragón (INA), Universidad de Zaragoza, c/Mariano Esquillor, 50018 Zaragoza, Spain 4 Fundacion ARAID, 50004 Zaragoza, Spain 5 Research Center in Physics of Matter and Radiation (PMR), University of Namur, 61 Rue de Bruxelles, 5000 Namur, Belgium 1
E-mail: [email protected] Received 13 October 2015, revised 30 November 2015 Accepted for publication 14 December 2015 Published 20 January 2016 Abstract
Here, we report on the synthesis of MoS2 nanosheets using a simple two-step additive-free growth technique. The as-synthesized nanosheets were characterized to determine their structure and composition, as well as their optical properties. The MoS2 nanosheets were analyzed by scanning electron microscopy, transmission electron microscopy (TEM), including highresolution scanning TEM imaging and energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy (XPS), Raman spectroscopy and photoluminescence (PL). The asproduced MoS2 nanosheets are vertically aligned with curved edges and are densely populated. The TEM measurements confirmed that the nanosheets have the 2H-MoS2 crystal structure in agreement with the Raman results. The XPS results revealed the presence of high purity MoS2. Moreover, a prominent PL similar to mechanically exfoliated few and mono-layer MoS2 was observed for the as-grown nanosheets. For the thin (50 nm) nanosheets, the PL feature was observed at the same energy as that for a direct band-gap monolayer MoS2 (1.83 eV). Thus, the as-produced high-quality, large-area, MoS2 nanosheets could be potentially useful for various optoelectronic and catalysis applications. S Online supplementary data available from stacks.iop.org/NANO/27/075604/mmedia Keywords: MoS2, nanosheets, vapour-solid reactions, TEM, photoluminescence (Some figures may appear in colour only in the online journal) 1. Introduction
forces. MoS2 is one of the most extensively studied of the various transition-metal dichalcogenides [2]. Bulk MoS2 has been used for several decades as a solid lubricant or an additive for lubricating oils and greases. As compared with bulk MoS2, nanoscale MoS2 has more favourable properties, including large specific surface areas, strong visible light absorbing ability and, extreme flexibility [2–5]. In nanometric size, it shows prominent photoluminescence with energies in the visible range (1.8–1.9 eV) [3, 6] which makes it a potential material for the fabrication of efficient optoelectronic devices [5]. MoS2 nanosheet-based semiconductor
In the last decade, there has been a huge interest in the twodimensional (2D) transition-metal dichalcogenides such as MX2 (M stands for Mo or W, Nb etc, and Ta, X=S, Se etc). Unlike the zero band-gap graphene, the 2D dichalcogenides are promising for the next generation of nanoelectronics devices beyond graphene limits [1, 2]. The layered structure is constructed by unit X-M-X atomic tri-layers in which the bonding between metal and chalcogens is covalent, and the tri-layers themselves are held together by weak van der Waals 0957-4484/16/075604+10$33.00
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devices, such as field-effect transistors [7] and digital circuits [8] have been successfully fabricated. Besides, large (148 meV) spin-splitting of the uppermost valence band at the K point for MoS2 is very much suitable for spintronics applications [9]. MoS2 has been obtained in various forms such as nanoflowers [10], nanotubes [11], nanowires [12], nanoplatelets [13], etc. These different morphologies are related to the anisotropic structure of MoS2, which is a consequence of its chemical bonding. These structures that predominantly expose the edges of the layers exhibit high surface energy. Such edge-terminated films are metastable structures of MoS2, lead to diverse catalytic applications [14] in hydrodesulfurization catalysis [15], hydrogen evolution reaction [16], lithium storage [17], lithium batteries [10] as well as for some biological applications [18]. Various methods have been developed so far to produce nanosize MoS2. The mechanical exfoliation is a good technique for the production of high quality mono- to few-layer MoS2 nanosheets [3]. But, it entails the lack of thickness control and it is rather unreliable for large-scale material production [3]. The MoS2 nanosheets produced via a facile low-temperature chemical or liquid exfoliation method require longer synthesis times (8–84 h) [19]. Again, uncertainty in control over the flake size and thickness uniformity limits their use in practical applications [19, 20]. Hence, a significant scope remains to develop a simple technique to fabricate MoS2 nanosheets of high quality and with a controlled layer number. Recently, the chemical vapour deposition (CVD) method has shown great promise for producing high quality one- to few-layer MoS2 [4, 21]. However,few reports on the growth of large area vertical MoS2 nanosheets are available using the CVD method [22–24]. Vertical MoS2 nanosheets with both the large specific surface area and sharp, active edges are strongly desirable due to their potential applications as a catalyst, sensors and field emitters. Therefore, a significant objective remains to develop a new and cost-effective CVD- based growth technique to synthesize vertical MoS2 nanosheets. Herein, we report a simple two-step CVD growth strategy of vertically aligned petal-like MoS2 nanosheets by rapid sulfurization. The characteristics of the produced samples were investigated by using well-established techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM, including high-resolution scanning TEM (HR-STEM) imaging and energy-dispersive x-ray spectroscopy (EDS)), x-ray photoelectron spectroscopy (XPS), Raman spectroscopy and photoluminescence (PL). Moreover, a possible growth mechanism of the MoS2 nanosheet is discussed on the basis of the experimental facts. Vertically aligned thick, thin MoS2 nanosheets with sharp edges showing PL were produced using a simple experimental setup without employing any additional chemicals. This work would be highly useful for the future development of the large-scale MoS2 nanosheets production for various applications such as catalyst, sensors, field emitters and optoelectronic devices.
2. Materials and methods 2.1. Materials
All the chemical reagents were of analytical grade and used as received. A commercial MoO3 (purity 99.5%) and S (purity 99.5%) powders were purchased from Alfa Aesar. 2.2. Synthesis of MoS2 nanosheets
In order to be free from debris such as dust or grease on the substrate surface, samples were pre-cleaned with acetone and methanol. MO3 and S powders were used as source materials for MoS2 synthesis (figures 1(a) and (b)). These powders consist of particles of size varying over the range of a few micron upto a hundred microns. In a typical preparation, a mixture of 50 mg of MoO3 and 50 mg of S (Molar ratio of MoO3/S:1) powders was added in ethanol and then kept in an ultrasonicator for 10 min with pre-cleaned Si pieces immersed init. Thus, a MoO3 and S fine powder dispersion on the Si substrate with a few micron size particles aggregation and cracks was obtained at some places (figure 1(c)). It can be clearly noted that as compared to the as-received MoO3 and S powders in the sample with dispersion, the particle size is much smaller (0.1 to 10 μm). The as-treated sample was used for MoS2 nanosheets growth. Henceforth, it will be referred to as ‘Si sample with dispersion’. Then the two samples, one with dispersion and additional MoO3 powder (0 to 25 mg extra powder in addition to the already used MoO3 powder in the dispersion) and the other with S (50 to 150 mg) powder were introduced in a horizontal quartz reactor. Their positions in the quartz reactor were maintained so as to reach 850 °C and 400 °C respectively (figure 1(d)). It was purged with Ar (0.725 l/min) at atmospheric pressure for 60 min to remove residual oxygen. Meanwhile, the temperature of the ceramic tube furnace was raised to 850 °C at a rate of 6 °C/min. Then the quartz tube was inserted into a ceramic tube furnace and maintained conveniently at 850 °C for 30 min with a continuous Ar flow (0.725 l/min) at atmospheric pressure. Afterwards, the quartz reactor tube was moved out of the furnace and cooled down to room temperature naturally with continued Ar flow. The typical synthesis process is schematically illustrated in figure 1(d). After completion of the typical growth process, MoS2 deposited on the quartz tube was removed by annealing at 700 °C under O2 [33]. 2.3. Physical characterization techniques
The morphology of the samples was characterized by SEM (JEOL 7500F, operating voltage −3 kV). The TEM analysis was performed using a FEI Titan Low-Base microscope, working at 200 kV equipped with a Cs probe corrector, a HAADF (high angle annular dark field imaging) detector and EDS spectrometer. The samples for TEM were prepared using lacey-carbon Cu grid. Using a sharp knife some material from the surface of MoS2 sample was scratched over the laceycarbon Cu grid.
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Figure 1. SEM images of (a) MoO3 and (b) S powders as received, (c) MoO3 and S (1:1) solution in an ethanol dispersion on a Si substrate; inset: a high magnification image taken at the planar area marked by the red dotted circle in the panel (c). (d) Schematic of the synthesis process using a quartz tube furnace.
3. Results and discussion
The XPS analysis was performed on a Thermo Fisher ESCALAB 250 Xi instrument. This spectrometer uses a monochromatic Al Kα x-ray source (1486.6 eV) and a hemispherical deflector analyser working at constant pass energy. This mode allows a constant energy resolution on the whole spectrum. The intrinsic resolution of the spectrometer is 0.47 eV, measured on the Ag 3d5/2 line. The experiments were performed using a 250 μm diameter x-ray spot. The charge neutralization of the sample was achieved with a flood gun using low energy electrons and Ar+ ions. The base pressure in the analyser chamber was 2.10−8 Pascal, and during experiments an argon partial pressure of 3.10−5 Pascal was maintained for the flood gun operation. Survey spectra were recorded with a 150 eV pass energy, and this energy was decreased to 20 eV for high-resolution spectra. All core-level spectra were recorded with a take-off angle of 35° relative to the substrate excitation source. The chemical composition was obtained from the areas of the detected XPS peaks in the Mo 3d, S 2p, and O 1s regions, performing Shirley background subtraction and taking into account sensitivity factors for each element. The Raman and PL spectra were obtained by using a Horiba micro-Raman confocal microscopic system (LabRAM), at room temperature in an ambientair. A low laser power (1 mW) was used for spectra acquisition in order to avoid any oxidation of MoS2 nanosheets by heating effect [25]. The excitation laser line and spot size of the laser are 473 nm and 0.6 μm, respectively. The Raman emission was collected by a 100× (numerical aperture 0.9) objective and dispersed by a 1800 lines mm−1 grating while, for PL measurements, a 300 lines mm−1 grating was used. The typical integration times were 10 s for Raman and 20 s for PL.
3.1. Morphology characterization
The morphology of MoS2 samples was identified by SEM imaging (figure 2). The Si sample with dispersion and additional 25 mg of MoO3 powder was sulfurized with 150 mg of S powder at 850 °C for 30 min. As shown in figure 2(a), the deposition is continuous on the entire Si surface (1 cm2). It also reveals a high material yield and a controllable nanosheet-like morphology of the synthesized material. From SEM images in figure 2(b) and in its insert, it can be observed that the synthesized material is composed of numerous well-distributed, densely grown, lamellar nanosheets with sharp edges. Higher magnification images as shown in figures 2(b) and (c) shows MoS2 nanosheets with thicknesses between 80 and 250 nm and of lateral sizes about 2–20 μm. From the cross-section image in figure 2(b) insert, we clearly see that the nanosheets grow vertically. However, a few nanosheets are folded at the edges and gives a petal-like shape. These petal-like nanosheets appear thicker and were not considered to estimate the nanosheet thickness. Further, these samples were characterized using Raman spectroscopy. A typical Raman spectrum is shown in figure 2(d), where two pronounced peaks at 382 and 405 cm−1 are observed. The presence of these characteristic peaks indicates the as-grown MoS2 is of 2H-MoS2 type [26], which is later confirmed by TEM measurements as well. The appearance of the 382 cm−1 peak corresponds to the E12g vibrational mode of first order Raman active centre (figure 1(d)) related to the in-plane MoS phonon mode [25, 27]. The Raman peak at 405 cm−1 (A1g mode) is associated to the out-of-plane MoS2 phonon mode. This indicates that the vibration along the vertical layer direction of the bond between Mo and S is significant, which 3
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Figure 2. MoS2 nanosheets grown on a Si sample with dispersion and extra 25 mg of MoO3 by sulfurization using 150 mg of S powder at 850 °C for 30 min. (a), (b) and (c) SEM images with different magnification; inset of (b): a cross-section SEM image. (d) A typical Raman spectrum.
is also coincident with the presence of a (002) plane in electron diffraction measurements discussed later (figure 6). It was observed that the difference (Δf) between the two prominent Raman modes (A1g and E12g ) is 23 cm−1 indicating the nanosheets consists of 5 or more MoS2 layers stacked together [11, 27, 28]. The A1g and E12g peaks with full widths at half maximum (FWHM) of 11 and 12 cm−1, respectively, demonstrate the presence of nano-crystalline MoS2 [29]. Similar broadening of the E12g and A1g peaks has been observed for 20 nm (5 nm) do not exhibit photoluminescence [19]. Surprisingly, from the SEM images of these films it is noticed that the as-grown nanosheets are of 50 nm in thickness and still exhibit PL signal. This is contrary to the previous results of no PL signal detected for nanosheets thicker than 5 nm [41]. One of the reasons for the observed prominent PL signal could be understood using TEM results presented later (figure 6). Combining Raman and PL mapping results (figure 5) imply that the MoS2 nanosheets exhibit a high crystallinity while it shows a less optical uniformity. The observed PL features are comparable with those reported in previous studies for one to few layer MoS2. Thick (80–250 nm) nanosheets have an indirect band-gap-like PL features while, for thin (50 nm) nanosheets the PL feature appears at the same energy position as that of a direct band gap monolayer MoS2. This shows that there is a strong thickness effect on the optical properties of as-grown nanosheets. Additionally, the MoS2 nanosheet edges might have a significant role in the observed PL signal. Recently, a strong PL enhancement at the WS2 edges has been shown [42]. It was attributed to the
exciton accumulation or localized excitons near lattice defects (different structure and chemical composition of the WS2 platelet edges). The observed PL signal for both thick and thin nanosheets also may be due to the structural discontinuity at the nanosheet edges, which induces variation of the electronic structure of MoS2 nanosheets. Another possibility could be that the PL signal might be due to the Mo-O bonds (or S vacancy) present at the MoS2 nanosheet edges occurring during the growth of MoS2 [31]. There is a possibility of MoO bonds (or S vacancy) at the edges for thick (100 nm) MoS2 nanosheets since a weak MoO3 presence at the MoS2 nanosheet surface was observed from the XPS results (figure 3). However, no MoO3 was detected for the thin (50 nm) MoS2 nanosheet sample using either XPS (the XPS result is not shown) or EDS measurements (figure 6). This excludes the probability that Mo-O (or S vacancy) responsible for the observed PL signal. Understanding the exact mechanism leading to a PL signal in the as-grown MoS2 nanosheets will require more detailed studies.
3.4. Structural characterization
SEM allows inspection of a material’s morphological features at a larger field of view (10–1000× magnification). On the other hand, TEM can be used to quantitatively determine the crystal structure accurately, the layer thickness, the stacking and the defect density if any, over a small sampling volume and for a limited field of view (figure 6) [43]. In a low magnification HAADF-STEM image (figure 6(a)), MoS2 deposit bundles can be seen. Furthermore, the high-resolution image of the area marked by the dotted red square shown in figure 6(a) confirms a layered structural feature of nanosheets. 7
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The parallel MoS2 layers (∼28 layers) can be clearly seen. More details of MoS2 nanosheet nanostructure are seen in figure 6(c). The nanosheet layers present a 0.62 nm interlayer separation, which corresponds to the (002) oriented crystal planes of the 2H-MoS2 structure. The fast Fourier transform (FFT) pattern (figure 6(d)) of nanosheets in figure 6(c) reveals the presence of highly crystalline structures. Consistent with previous, experimental and theoretical worksa discrete kink formation, as indicated by the yellow arrow in figure 6(b), is observed because of delamination of the van der Waals nanosheets [44, 45]. The bending angles at kinks are 85° to 149.8°. However, no breakage of nanosheets at kinks demonstrates their high flexibility. The delamination possibly occurs to adjust the strain in MoS2 layers at the kink [44]. As compared to the 149.8° kink angle, for 85° more delaminated layers (figure 6(b)) are seen indicating a relatively more strain induced for kink angles less than 90°. The delaminated MoS2 layers might act as a single layer and shows PL. Moreover, the EDS mapping analysis (figures 6(f) and (g)) clearly indicates that Mo and S are uniformly distributed. Over the mapped area, no O was detected.
MoO3 (l) + 4S (g) 2MoS2 (s) + 3O2 (g)¼.@850 C
As the reaction proceeds, further adsorption of incoming species occurs at the already nucleated sites. This leads to the formation of MoS2 nanosheets on the substrate surface. A very fast reduction of MoO3 by S might have an important role in the formation of vertically aligned layered MoS2 nanosheets. The growth of nanosheets, occurs along three directions ([001], [010] and, [100]), with higher growth velocity along [001] leading to the formation of sheet-like structures. The vertical alignment of MoS2 nanosheets formation is possibly favoured by its anisotropic atomic bonding nature.
5. Conclusions We have demonstrated the synthesis of a large area (1 cm2), vertically aligned 2H-MoS2 nanosheets. This was achieved using a simple, two-step CVD technique without the use of any additional chemicals. By employing XPS, Raman spectroscopy, PL and STEM high-quality and high crystalline quality of the synthesized material was evidenced. The asgrown nanosheets showprominent PL. The thin MoS2 nanosheets (50 nm) showPL at 1.83 eV and thus exhibit an optical quality comparable to monolayer MoS2. The precursor’s quantity was found to be the key parameter to controlling the thickness and height of the MoS2 nanosheets. Based on the experimental results, a plausible growth mechanism of MoS2 nanosheets is proposed. This efficient and a simple synthetic route used for synthesizing a large area MoS2 nanosheets can be applied as a general method for the synthesis of other transition metal dichalcogenides. The asgrown MoS2 nanosheets could be a very interesting material for numerous nanotechnological applications in catalysis, optoelectronics, and nanoelectronics fields. The growth of MoS2 nanosheets arrays with uniform height will be probed in future studies.
4. MoS2 nanosheet growth mechanism Based on the above experimental results, we propose a twostep MoS2 nanosheet growth mechanism. In this work, MoO3 as a source material and S as reducing and sulfurization agent were used. In the first step, the dispersion of MoO3 and S (1:1 proportion solution in ethanol) spread on Si sample and in the second step, the growth of nanosheet by rapid sulfurization was achieved. As shown in figure 1(c), using dispersion of MoO3 and S, a fine powder distribution with a few particles was obtained. These particlesizes wererelatively much smaller than the as-received source material particles. We have seen that the sample with dispersion and without any extra MoO3 powder could produce thinner nanosheets (figure 5(d)) thanthe sample with dispersion and an extra 25 mg MoO3 powder added(figure 2). Thus, the nanosheet thickness was controlled simply by varying the source material quantity. During nanosheet growth, the temperature of the sample with dispersion and extra (0–25 mg) MoO3 powder addedwas maintained at a sufficiently higher temperature (850 °C) than the melting point (795 °C) of MoO3. The S powder (50–150 mg) was kept at a higher temperature (400 °C) than its boiling point (350 °C). So, the precursor was in liquid form while the reactant was in a vapour form in the reactor. The S vapour was transported by the carrier gas (Ar) towards deposition sample (sample with dispersion and extra (0–25 mg) MoO3 powder added) where MoO3 vapour reduction occurs by it (figure 1(d)). Then it might subsequently nucleate in the form of MoS2 nanoclusters onto the whole substrate surface. The reaction routes for the synthesis of MoS2 in our experiment could be expressed as follows (where, s=solid, l=liquid, and g=gas):
Acknowledgments This research work was financially supported by a grant of the University of Namur, Belgium. G Deokar is grateful to C Santos (University catholic of Louvain, 1, Place de Université, Louvain-la-Neuve, Belgium) for allowing initial Raman measurements of MoS2 samples. We are thankful to N Reckinger for careful proofreading of this manuscript. This research used resources of the Electron Microscopy Service and of the ELISE service at the University of Namur. These Services are members of ‘Plateformes Technologiques Morphologie – Imagerie and SIAM’, respectively. JFC is Research Associate of FRS-FNRS. The research leading to these results have received partial funding from the European Union Seventh Framework Program under the grant agreement n°604391 Graphene Flagship. D Vignaud acknowledges
S (s) S (g)¼ ¼@450 C MoO3 (s) MO3 (l)¼ ¼@850 C 8
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financial support from the Nord-Pas de Calais Regional Council and the Renatech network. The HR-STEM studies were conducted at the Laboratorio de Microscopias Avanzadas, Instituto de Nanociencia de Aragon, Universidad de Zaragoza, Spain. R Arenal gratefully acknowledges the support of ARAID foundation, of the INA—U. Zaragoza (Spain), from the Spanish Ministerio de Economia y Competitividad (FIS2013-46159-C3-3-P) and from the European Union Seventh Framework Program under Grant Agreement 312483 —ESTEEM2 (Integrated Infrastructure Initiative—I3) and from EU H2020 ETN project ‘Enabling Excellence’ Grant Agreement 642742.
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Synthesis and characterization of MoS2 nanosheets
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Nanotechnology Nanotechnology 27 (2016) 075604 (10pp)
doi:10.1088/0957-4484/27/7/075604
Synthesis and characterization of MoS2 nanosheets G Deokar1, D Vignaud2, R Arenal3,4, P Louette5 and J-F Colomer1 Research Group on Carbon Nanostructures (CARBONNAGe), University of Namur, 61 Rue de Bruxelles, 5000 Namur, Belgium 2 Institute of Electronics, Microelectronics and Nanotechnology (IEMN), University of Lille 1, Av. Poincaré CS 60069, 59652 Villeneuve D’Ascq, France 3 Laboratory of Advanced Microscopies (LMA), Institute of Nanoscience of Aragón (INA), Universidad de Zaragoza, c/Mariano Esquillor, 50018 Zaragoza, Spain 4 Fundacion ARAID, 50004 Zaragoza, Spain 5 Research Center in Physics of Matter and Radiation (PMR), University of Namur, 61 Rue de Bruxelles, 5000 Namur, Belgium 1
E-mail: [email protected] Received 13 October 2015, revised 30 November 2015 Accepted for publication 14 December 2015 Published 20 January 2016 Abstract
Here, we report on the synthesis of MoS2 nanosheets using a simple two-step additive-free growth technique. The as-synthesized nanosheets were characterized to determine their structure and composition, as well as their optical properties. The MoS2 nanosheets were analyzed by scanning electron microscopy, transmission electron microscopy (TEM), including highresolution scanning TEM imaging and energy-dispersive x-ray spectroscopy, x-ray photoelectron spectroscopy (XPS), Raman spectroscopy and photoluminescence (PL). The asproduced MoS2 nanosheets are vertically aligned with curved edges and are densely populated. The TEM measurements confirmed that the nanosheets have the 2H-MoS2 crystal structure in agreement with the Raman results. The XPS results revealed the presence of high purity MoS2. Moreover, a prominent PL similar to mechanically exfoliated few and mono-layer MoS2 was observed for the as-grown nanosheets. For the thin (50 nm) nanosheets, the PL feature was observed at the same energy as that for a direct band-gap monolayer MoS2 (1.83 eV). Thus, the as-produced high-quality, large-area, MoS2 nanosheets could be potentially useful for various optoelectronic and catalysis applications. S Online supplementary data available from stacks.iop.org/NANO/27/075604/mmedia Keywords: MoS2, nanosheets, vapour-solid reactions, TEM, photoluminescence (Some figures may appear in colour only in the online journal) 1. Introduction
forces. MoS2 is one of the most extensively studied of the various transition-metal dichalcogenides [2]. Bulk MoS2 has been used for several decades as a solid lubricant or an additive for lubricating oils and greases. As compared with bulk MoS2, nanoscale MoS2 has more favourable properties, including large specific surface areas, strong visible light absorbing ability and, extreme flexibility [2–5]. In nanometric size, it shows prominent photoluminescence with energies in the visible range (1.8–1.9 eV) [3, 6] which makes it a potential material for the fabrication of efficient optoelectronic devices [5]. MoS2 nanosheet-based semiconductor
In the last decade, there has been a huge interest in the twodimensional (2D) transition-metal dichalcogenides such as MX2 (M stands for Mo or W, Nb etc, and Ta, X=S, Se etc). Unlike the zero band-gap graphene, the 2D dichalcogenides are promising for the next generation of nanoelectronics devices beyond graphene limits [1, 2]. The layered structure is constructed by unit X-M-X atomic tri-layers in which the bonding between metal and chalcogens is covalent, and the tri-layers themselves are held together by weak van der Waals 0957-4484/16/075604+10$33.00
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devices, such as field-effect transistors [7] and digital circuits [8] have been successfully fabricated. Besides, large (148 meV) spin-splitting of the uppermost valence band at the K point for MoS2 is very much suitable for spintronics applications [9]. MoS2 has been obtained in various forms such as nanoflowers [10], nanotubes [11], nanowires [12], nanoplatelets [13], etc. These different morphologies are related to the anisotropic structure of MoS2, which is a consequence of its chemical bonding. These structures that predominantly expose the edges of the layers exhibit high surface energy. Such edge-terminated films are metastable structures of MoS2, lead to diverse catalytic applications [14] in hydrodesulfurization catalysis [15], hydrogen evolution reaction [16], lithium storage [17], lithium batteries [10] as well as for some biological applications [18]. Various methods have been developed so far to produce nanosize MoS2. The mechanical exfoliation is a good technique for the production of high quality mono- to few-layer MoS2 nanosheets [3]. But, it entails the lack of thickness control and it is rather unreliable for large-scale material production [3]. The MoS2 nanosheets produced via a facile low-temperature chemical or liquid exfoliation method require longer synthesis times (8–84 h) [19]. Again, uncertainty in control over the flake size and thickness uniformity limits their use in practical applications [19, 20]. Hence, a significant scope remains to develop a simple technique to fabricate MoS2 nanosheets of high quality and with a controlled layer number. Recently, the chemical vapour deposition (CVD) method has shown great promise for producing high quality one- to few-layer MoS2 [4, 21]. However,few reports on the growth of large area vertical MoS2 nanosheets are available using the CVD method [22–24]. Vertical MoS2 nanosheets with both the large specific surface area and sharp, active edges are strongly desirable due to their potential applications as a catalyst, sensors and field emitters. Therefore, a significant objective remains to develop a new and cost-effective CVD- based growth technique to synthesize vertical MoS2 nanosheets. Herein, we report a simple two-step CVD growth strategy of vertically aligned petal-like MoS2 nanosheets by rapid sulfurization. The characteristics of the produced samples were investigated by using well-established techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM, including high-resolution scanning TEM (HR-STEM) imaging and energy-dispersive x-ray spectroscopy (EDS)), x-ray photoelectron spectroscopy (XPS), Raman spectroscopy and photoluminescence (PL). Moreover, a possible growth mechanism of the MoS2 nanosheet is discussed on the basis of the experimental facts. Vertically aligned thick, thin MoS2 nanosheets with sharp edges showing PL were produced using a simple experimental setup without employing any additional chemicals. This work would be highly useful for the future development of the large-scale MoS2 nanosheets production for various applications such as catalyst, sensors, field emitters and optoelectronic devices.
2. Materials and methods 2.1. Materials
All the chemical reagents were of analytical grade and used as received. A commercial MoO3 (purity 99.5%) and S (purity 99.5%) powders were purchased from Alfa Aesar. 2.2. Synthesis of MoS2 nanosheets
In order to be free from debris such as dust or grease on the substrate surface, samples were pre-cleaned with acetone and methanol. MO3 and S powders were used as source materials for MoS2 synthesis (figures 1(a) and (b)). These powders consist of particles of size varying over the range of a few micron upto a hundred microns. In a typical preparation, a mixture of 50 mg of MoO3 and 50 mg of S (Molar ratio of MoO3/S:1) powders was added in ethanol and then kept in an ultrasonicator for 10 min with pre-cleaned Si pieces immersed init. Thus, a MoO3 and S fine powder dispersion on the Si substrate with a few micron size particles aggregation and cracks was obtained at some places (figure 1(c)). It can be clearly noted that as compared to the as-received MoO3 and S powders in the sample with dispersion, the particle size is much smaller (0.1 to 10 μm). The as-treated sample was used for MoS2 nanosheets growth. Henceforth, it will be referred to as ‘Si sample with dispersion’. Then the two samples, one with dispersion and additional MoO3 powder (0 to 25 mg extra powder in addition to the already used MoO3 powder in the dispersion) and the other with S (50 to 150 mg) powder were introduced in a horizontal quartz reactor. Their positions in the quartz reactor were maintained so as to reach 850 °C and 400 °C respectively (figure 1(d)). It was purged with Ar (0.725 l/min) at atmospheric pressure for 60 min to remove residual oxygen. Meanwhile, the temperature of the ceramic tube furnace was raised to 850 °C at a rate of 6 °C/min. Then the quartz tube was inserted into a ceramic tube furnace and maintained conveniently at 850 °C for 30 min with a continuous Ar flow (0.725 l/min) at atmospheric pressure. Afterwards, the quartz reactor tube was moved out of the furnace and cooled down to room temperature naturally with continued Ar flow. The typical synthesis process is schematically illustrated in figure 1(d). After completion of the typical growth process, MoS2 deposited on the quartz tube was removed by annealing at 700 °C under O2 [33]. 2.3. Physical characterization techniques
The morphology of the samples was characterized by SEM (JEOL 7500F, operating voltage −3 kV). The TEM analysis was performed using a FEI Titan Low-Base microscope, working at 200 kV equipped with a Cs probe corrector, a HAADF (high angle annular dark field imaging) detector and EDS spectrometer. The samples for TEM were prepared using lacey-carbon Cu grid. Using a sharp knife some material from the surface of MoS2 sample was scratched over the laceycarbon Cu grid.
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Figure 1. SEM images of (a) MoO3 and (b) S powders as received, (c) MoO3 and S (1:1) solution in an ethanol dispersion on a Si substrate; inset: a high magnification image taken at the planar area marked by the red dotted circle in the panel (c). (d) Schematic of the synthesis process using a quartz tube furnace.
3. Results and discussion
The XPS analysis was performed on a Thermo Fisher ESCALAB 250 Xi instrument. This spectrometer uses a monochromatic Al Kα x-ray source (1486.6 eV) and a hemispherical deflector analyser working at constant pass energy. This mode allows a constant energy resolution on the whole spectrum. The intrinsic resolution of the spectrometer is 0.47 eV, measured on the Ag 3d5/2 line. The experiments were performed using a 250 μm diameter x-ray spot. The charge neutralization of the sample was achieved with a flood gun using low energy electrons and Ar+ ions. The base pressure in the analyser chamber was 2.10−8 Pascal, and during experiments an argon partial pressure of 3.10−5 Pascal was maintained for the flood gun operation. Survey spectra were recorded with a 150 eV pass energy, and this energy was decreased to 20 eV for high-resolution spectra. All core-level spectra were recorded with a take-off angle of 35° relative to the substrate excitation source. The chemical composition was obtained from the areas of the detected XPS peaks in the Mo 3d, S 2p, and O 1s regions, performing Shirley background subtraction and taking into account sensitivity factors for each element. The Raman and PL spectra were obtained by using a Horiba micro-Raman confocal microscopic system (LabRAM), at room temperature in an ambientair. A low laser power (1 mW) was used for spectra acquisition in order to avoid any oxidation of MoS2 nanosheets by heating effect [25]. The excitation laser line and spot size of the laser are 473 nm and 0.6 μm, respectively. The Raman emission was collected by a 100× (numerical aperture 0.9) objective and dispersed by a 1800 lines mm−1 grating while, for PL measurements, a 300 lines mm−1 grating was used. The typical integration times were 10 s for Raman and 20 s for PL.
3.1. Morphology characterization
The morphology of MoS2 samples was identified by SEM imaging (figure 2). The Si sample with dispersion and additional 25 mg of MoO3 powder was sulfurized with 150 mg of S powder at 850 °C for 30 min. As shown in figure 2(a), the deposition is continuous on the entire Si surface (1 cm2). It also reveals a high material yield and a controllable nanosheet-like morphology of the synthesized material. From SEM images in figure 2(b) and in its insert, it can be observed that the synthesized material is composed of numerous well-distributed, densely grown, lamellar nanosheets with sharp edges. Higher magnification images as shown in figures 2(b) and (c) shows MoS2 nanosheets with thicknesses between 80 and 250 nm and of lateral sizes about 2–20 μm. From the cross-section image in figure 2(b) insert, we clearly see that the nanosheets grow vertically. However, a few nanosheets are folded at the edges and gives a petal-like shape. These petal-like nanosheets appear thicker and were not considered to estimate the nanosheet thickness. Further, these samples were characterized using Raman spectroscopy. A typical Raman spectrum is shown in figure 2(d), where two pronounced peaks at 382 and 405 cm−1 are observed. The presence of these characteristic peaks indicates the as-grown MoS2 is of 2H-MoS2 type [26], which is later confirmed by TEM measurements as well. The appearance of the 382 cm−1 peak corresponds to the E12g vibrational mode of first order Raman active centre (figure 1(d)) related to the in-plane MoS phonon mode [25, 27]. The Raman peak at 405 cm−1 (A1g mode) is associated to the out-of-plane MoS2 phonon mode. This indicates that the vibration along the vertical layer direction of the bond between Mo and S is significant, which 3
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Figure 2. MoS2 nanosheets grown on a Si sample with dispersion and extra 25 mg of MoO3 by sulfurization using 150 mg of S powder at 850 °C for 30 min. (a), (b) and (c) SEM images with different magnification; inset of (b): a cross-section SEM image. (d) A typical Raman spectrum.
is also coincident with the presence of a (002) plane in electron diffraction measurements discussed later (figure 6). It was observed that the difference (Δf) between the two prominent Raman modes (A1g and E12g ) is 23 cm−1 indicating the nanosheets consists of 5 or more MoS2 layers stacked together [11, 27, 28]. The A1g and E12g peaks with full widths at half maximum (FWHM) of 11 and 12 cm−1, respectively, demonstrate the presence of nano-crystalline MoS2 [29]. Similar broadening of the E12g and A1g peaks has been observed for 20 nm (5 nm) do not exhibit photoluminescence [19]. Surprisingly, from the SEM images of these films it is noticed that the as-grown nanosheets are of 50 nm in thickness and still exhibit PL signal. This is contrary to the previous results of no PL signal detected for nanosheets thicker than 5 nm [41]. One of the reasons for the observed prominent PL signal could be understood using TEM results presented later (figure 6). Combining Raman and PL mapping results (figure 5) imply that the MoS2 nanosheets exhibit a high crystallinity while it shows a less optical uniformity. The observed PL features are comparable with those reported in previous studies for one to few layer MoS2. Thick (80–250 nm) nanosheets have an indirect band-gap-like PL features while, for thin (50 nm) nanosheets the PL feature appears at the same energy position as that of a direct band gap monolayer MoS2. This shows that there is a strong thickness effect on the optical properties of as-grown nanosheets. Additionally, the MoS2 nanosheet edges might have a significant role in the observed PL signal. Recently, a strong PL enhancement at the WS2 edges has been shown [42]. It was attributed to the
exciton accumulation or localized excitons near lattice defects (different structure and chemical composition of the WS2 platelet edges). The observed PL signal for both thick and thin nanosheets also may be due to the structural discontinuity at the nanosheet edges, which induces variation of the electronic structure of MoS2 nanosheets. Another possibility could be that the PL signal might be due to the Mo-O bonds (or S vacancy) present at the MoS2 nanosheet edges occurring during the growth of MoS2 [31]. There is a possibility of MoO bonds (or S vacancy) at the edges for thick (100 nm) MoS2 nanosheets since a weak MoO3 presence at the MoS2 nanosheet surface was observed from the XPS results (figure 3). However, no MoO3 was detected for the thin (50 nm) MoS2 nanosheet sample using either XPS (the XPS result is not shown) or EDS measurements (figure 6). This excludes the probability that Mo-O (or S vacancy) responsible for the observed PL signal. Understanding the exact mechanism leading to a PL signal in the as-grown MoS2 nanosheets will require more detailed studies.
3.4. Structural characterization
SEM allows inspection of a material’s morphological features at a larger field of view (10–1000× magnification). On the other hand, TEM can be used to quantitatively determine the crystal structure accurately, the layer thickness, the stacking and the defect density if any, over a small sampling volume and for a limited field of view (figure 6) [43]. In a low magnification HAADF-STEM image (figure 6(a)), MoS2 deposit bundles can be seen. Furthermore, the high-resolution image of the area marked by the dotted red square shown in figure 6(a) confirms a layered structural feature of nanosheets. 7
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The parallel MoS2 layers (∼28 layers) can be clearly seen. More details of MoS2 nanosheet nanostructure are seen in figure 6(c). The nanosheet layers present a 0.62 nm interlayer separation, which corresponds to the (002) oriented crystal planes of the 2H-MoS2 structure. The fast Fourier transform (FFT) pattern (figure 6(d)) of nanosheets in figure 6(c) reveals the presence of highly crystalline structures. Consistent with previous, experimental and theoretical worksa discrete kink formation, as indicated by the yellow arrow in figure 6(b), is observed because of delamination of the van der Waals nanosheets [44, 45]. The bending angles at kinks are 85° to 149.8°. However, no breakage of nanosheets at kinks demonstrates their high flexibility. The delamination possibly occurs to adjust the strain in MoS2 layers at the kink [44]. As compared to the 149.8° kink angle, for 85° more delaminated layers (figure 6(b)) are seen indicating a relatively more strain induced for kink angles less than 90°. The delaminated MoS2 layers might act as a single layer and shows PL. Moreover, the EDS mapping analysis (figures 6(f) and (g)) clearly indicates that Mo and S are uniformly distributed. Over the mapped area, no O was detected.
MoO3 (l) + 4S (g) 2MoS2 (s) + 3O2 (g)¼.@850 C
As the reaction proceeds, further adsorption of incoming species occurs at the already nucleated sites. This leads to the formation of MoS2 nanosheets on the substrate surface. A very fast reduction of MoO3 by S might have an important role in the formation of vertically aligned layered MoS2 nanosheets. The growth of nanosheets, occurs along three directions ([001], [010] and, [100]), with higher growth velocity along [001] leading to the formation of sheet-like structures. The vertical alignment of MoS2 nanosheets formation is possibly favoured by its anisotropic atomic bonding nature.
5. Conclusions We have demonstrated the synthesis of a large area (1 cm2), vertically aligned 2H-MoS2 nanosheets. This was achieved using a simple, two-step CVD technique without the use of any additional chemicals. By employing XPS, Raman spectroscopy, PL and STEM high-quality and high crystalline quality of the synthesized material was evidenced. The asgrown nanosheets showprominent PL. The thin MoS2 nanosheets (50 nm) showPL at 1.83 eV and thus exhibit an optical quality comparable to monolayer MoS2. The precursor’s quantity was found to be the key parameter to controlling the thickness and height of the MoS2 nanosheets. Based on the experimental results, a plausible growth mechanism of MoS2 nanosheets is proposed. This efficient and a simple synthetic route used for synthesizing a large area MoS2 nanosheets can be applied as a general method for the synthesis of other transition metal dichalcogenides. The asgrown MoS2 nanosheets could be a very interesting material for numerous nanotechnological applications in catalysis, optoelectronics, and nanoelectronics fields. The growth of MoS2 nanosheets arrays with uniform height will be probed in future studies.
4. MoS2 nanosheet growth mechanism Based on the above experimental results, we propose a twostep MoS2 nanosheet growth mechanism. In this work, MoO3 as a source material and S as reducing and sulfurization agent were used. In the first step, the dispersion of MoO3 and S (1:1 proportion solution in ethanol) spread on Si sample and in the second step, the growth of nanosheet by rapid sulfurization was achieved. As shown in figure 1(c), using dispersion of MoO3 and S, a fine powder distribution with a few particles was obtained. These particlesizes wererelatively much smaller than the as-received source material particles. We have seen that the sample with dispersion and without any extra MoO3 powder could produce thinner nanosheets (figure 5(d)) thanthe sample with dispersion and an extra 25 mg MoO3 powder added(figure 2). Thus, the nanosheet thickness was controlled simply by varying the source material quantity. During nanosheet growth, the temperature of the sample with dispersion and extra (0–25 mg) MoO3 powder addedwas maintained at a sufficiently higher temperature (850 °C) than the melting point (795 °C) of MoO3. The S powder (50–150 mg) was kept at a higher temperature (400 °C) than its boiling point (350 °C). So, the precursor was in liquid form while the reactant was in a vapour form in the reactor. The S vapour was transported by the carrier gas (Ar) towards deposition sample (sample with dispersion and extra (0–25 mg) MoO3 powder added) where MoO3 vapour reduction occurs by it (figure 1(d)). Then it might subsequently nucleate in the form of MoS2 nanoclusters onto the whole substrate surface. The reaction routes for the synthesis of MoS2 in our experiment could be expressed as follows (where, s=solid, l=liquid, and g=gas):
Acknowledgments This research work was financially supported by a grant of the University of Namur, Belgium. G Deokar is grateful to C Santos (University catholic of Louvain, 1, Place de Université, Louvain-la-Neuve, Belgium) for allowing initial Raman measurements of MoS2 samples. We are thankful to N Reckinger for careful proofreading of this manuscript. This research used resources of the Electron Microscopy Service and of the ELISE service at the University of Namur. These Services are members of ‘Plateformes Technologiques Morphologie – Imagerie and SIAM’, respectively. JFC is Research Associate of FRS-FNRS. The research leading to these results have received partial funding from the European Union Seventh Framework Program under the grant agreement n°604391 Graphene Flagship. D Vignaud acknowledges
S (s) S (g)¼ ¼@450 C MoO3 (s) MO3 (l)¼ ¼@850 C 8
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financial support from the Nord-Pas de Calais Regional Council and the Renatech network. The HR-STEM studies were conducted at the Laboratorio de Microscopias Avanzadas, Instituto de Nanociencia de Aragon, Universidad de Zaragoza, Spain. R Arenal gratefully acknowledges the support of ARAID foundation, of the INA—U. Zaragoza (Spain), from the Spanish Ministerio de Economia y Competitividad (FIS2013-46159-C3-3-P) and from the European Union Seventh Framework Program under Grant Agreement 312483 —ESTEEM2 (Integrated Infrastructure Initiative—I3) and from EU H2020 ETN project ‘Enabling Excellence’ Grant Agreement 642742.
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