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Onion-Structured Spherical MoS Nanoparticles Induced by Laser Ablation in Water and Liquid Droplets’ Radial Solidification/Oriented Growth Mechanism Le Zhou, Hongwen Zhang, Haoming Bao, Guangqiang Liu, Yue Li, and Weiping Cai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07784 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

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The Journal of Physical Chemistry

Onion-Structured Spherical MoS2 Nanoparticles Induced by Laser Ablation in Water and Liquid Droplets’ Radial Solidification/Oriented Growth Mechanism

Le Zhoua,b, Hongwen Zhanga*, Haoming Baoa,b, Guangqiang Liua , Yue Lia and Weiping Caia,b*

a

Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and Nanotechnology, Institute

of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, P.R. China b

University of Science and Technology of China, Hefei 230026, P.R. China

Weiping Cai email: [email protected]

1 Environment ACS Paragon Plus


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ABSTRACT: Spherical MoS2 nanoparticles are fabricated by laser ablation of MoS2 target in water. The obtained nanoparticles are mostly nearly perfectly spherical in shape with smooth surface, and tens to hundreds of nanometers in diameters. Such spherical MoS2 nanoparticles are built by concentrically curved {002} planes and show onion-like structure. Further examination has revealed that there exist shrinkage cavities (or voids) in central part of the MoS2 nanoparticles or small pores dispersed in the particles and a few tadpole-like long-tailed nanoparticles in the products, indicating the marks of melting and molten liquid droplets’ solidification during laser ablation. A model is thus presented based on laser-induced MoS2 liquid droplets’ generation and inward {002}-oriented growth via radial solidification, which reveals the growth mechanism of the spherical MoS2 nanoparticles with onion-like structure. Interestingly, such onion-like structured spherical MoS2 nanoparticles have exhibited much higher surface enhanced Raman scattering (SERS) effect than the MoS2 nanoplates prepared by conventional methods. This work not only presents the route to the spherical MoS2 nanoparticles with onion-like structure but also reveals the formation process for the MoS2 nanoparticles in laser ablation in water.


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1. INTRODUCTION Molybdenum disulfide (MoS2) is an important semiconducting material. Because of its hexagonal crystal system and the laminated structure with only very weak van der Waals forces between the adjacent layers, we can easily produce the plate-like MoS2 which is often used as lubricating additive.1,2 With the research upsurge of two-dimensional (2D) materials in recent years, MoS2 as a member of them has attracted much attention due to their excellent physical and electronic properties3-5 and exhibited the potential applications in such as electrocatalysis,6 photocatalysis,6,7 biomedical applications,8,9 sensors,10,11 and even surface enhanced Raman scattering (SERS),12-15 etc. A variety of methods have been developed for preparation of 2D MoS2 including mechanical exfoliation,16-18 chemical vapor deposition (CVD),19,20 liquid-phase exfoliation methods21 and wet chemical synthesis methods,22,23 and so on. The mechanical exfoliation using scotch-tape is one of the techniques to fabricate MoS2 nanosheets with various thicknesses even down to monolayer.16,17 Chemical vapor deposition can be used to prepare MoS2 sheets with high quality, controllable planar size and thickness19,20 but this process is relatively harsh due to needing high temperature, high vacuum and specific substrates. Liquidphase exfoliation methods (including ultrasonic exfoliation) and wet chemical synthesis methods are two typical categories of solution-based techniques for preparation of MoS2 nanosheets.24 All these methods mentioned above almost induced MoS2 nanosheets. Reports on the spherical MoS2 nanoparticles were very limited probably due to the preferential and anisotropic growth induced by its laminated crystal structure, although Lou et al



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prepared nanosheets-built MoS2 hollow nanospheres by two step sequential hydrothermal method.25 Recently laser ablation in liquid, which can induce quick formation of nanoparticles,2629

has been used to fabricate spherical MoS2 nanoparticles.9,30,31 For instance, Wang et al9 and

Compagnini et al30 obtained so-called fullerene-like MoS2 nanoparticles by laser ablation of MoS2 target in water. The formation of fullerene-like MoS2 nanoparticles was attributed to the hitting by pulse laser-induced thermal shock waves, which leads to exfoliation of flakes and further diminution of particles, or to the oxidative environment induced by the plasma plume.30 Oztas et al31 also obtained fullerene-like MoS2 nanospheres by pulsed laser ablation of bulk MoS2 powders in methanol and suggested that the formation of fullerene-like structures was linked with laser-induced introduction of defects based on the deformation of planar structures. Totally, formation mechanism of such fullerene-structured spherical MoS2 is still unclear and to be addressed. In this work, we use laser ablation of MoS2 target in water to fabricate spherical MoS2 nanoparticles, which are of very smooth surface and tens to hundreds of nanometers in diameters. The spherical nanoparticles show onion-like structure and there exist internal shrinkage cavities. In addition, there also exist a few tadpole-like long-tailed nanoparticles in the products. These exhibit the obvious marks of melting and liquid droplets’ solidification. A model is thus presented, based on MoS2 liquid droplets’ radial solidification and inward {002}-oriented growth, to reveal the formation mechanism of the spherical MoS2 nanoparticles with onion-like structure. Interestingly, such onion-like structured spherical MoS2 nanoparticles show much


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higher SERS effect than MoS2 nanoplates. This study has revealed the formation mechanism of the onion-like structured spherical MoS2 nanoparticles and deepened understanding of the formation process for MoS2 nanoparticles in laser ablation in liquid. The details are reported in this article.

2. EXPERIMENTAL SECTION The laser ablation in water is schematically illustrated in Figure S1. Firstly, a molybdenum disulfide target (99.9% in purity, 3 cm× 3 cm× 0.3 cm in size) was purchased from Quanzhou Qi Jin New Material Co., LTD (In China), washed with deionized water by ultrasonic vibration and then dried out in air. The target was then put at the bottom of a beaker with 100 mL deionized water. The beaker was placed on a horizontal platform. The distance between the target in the water and optical lens was kept at 15 cm. Finally, vertical irradiation was carried out for 1 hour using a Nd: YAG laser with 1064 nm in wavelength, 5Hz in frequency, and 10 ns in pulse duration. The power of laser was around 50 mJ per pulse and the laser beam was focused on the target’s surface with about 2 mm in diameter. After the laser ablation, the colloidal solution was thus obtained. The final powder products were collected by centrifugation and ultrasonic cleaning for three times. For reference, the ultrasonic exfoliation was carried out, as previously reported.32 The platelike molybdenum disulfide target was put in a 250 mL beaker filled with water about 100 mL before the ultrasonic vibration with 150W. After ultrasonic vibration for 2 hours, the final



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products in the water were collected by the centrifugation and ultrasonic cleaning for three times before characterization. X-ray diffraction (XRD) patterns of the products were measured on an X’Pert Philips Diffractometer using Cu Kα radiation (0.15406 nm). The morphological observation was conducted on a field emission scanning electron microscope (FESEM, FEI Sirion 200) equipped with an Oxford IE250X-Max50 energy dispersion spectroscopy (EDS). The microstructural examination was performed on a transmission electron microscope (TEM, JEM-200CX). The optical absorbance spectrum was measured on a Shimadzu UV-2600 spectrometer by dispersion of the products in deionized water.

3. RESULTS AND DISCUSSION After laser ablation of the MoS2 target in the water for 1 hour, the water was changed from colorless to cinnamon in color. The solution shows obvious Tyndall effect, as illustrated in the inset of Figure 1(a), indicating the formation of colloidal particles in the water. Figure 1(a) gives the optical absorbance spectrum of the colloidal solution obtained by laser ablation without any treatment. The spectrum shows an absorption edge which increases with the reducing wavelength. Further analysis has revealed that the absorbance value in the edge region can be well described by the optical absorption edge expression of the semiconductors with indirect band gap:33

αhν = A(hν − E g )2


(1)

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where α is the absorbance value, hν the energy of incident light, A is the constant and Eg corresponds to the edge position.33,34 The plot of (α hν)1/2 versus hν should be a straight line, which is in agreement with the results, as illustrated in Figure 1(b). The Eg value is thus be estimated, by fitting, to be about 3.38eV. This is significantly different from that of the colloidal solution prepared by ultrasonic exfoliation, which is about 2.30eV, as illustrated in Figure S2. 3.1. Morphology and Structure. After centrifugation and cleaning, the final powder products in milligram level were obtained. The corresponding morphological observations are given in Figure 2(a). The products consist mostly of spherical particles with a large size dispersivity from tens to hundreds of nanometers in diameter. All spherical nanoparticles have very smooth surface. Interestingly, there exist few particles in the products with tadpole-like or long tailed shape, as typically shown in the inset of Figure 2(a). In contrast, ultrasonic exfoliation only induces the plate-like products with several tens to hundred nanometers in thickness, about 500 nm in planar dimension and rough planar surface, as illustrated in Figure S3(a), which is in agreement with the previous reports.32 Here, it should be mentioned that the tadpole-like long tailed nanoparticles and some large particles could be separated from the as-prepared products by filtering the as-prepared colloidal solutions using disposable filter. Figure 2 (b) shows the typical results after filtering using 450 nm disposable filter and centrifugation. All tadpole-like nanoparticles and large particles in the



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original colloidal solution was removed and only spherical MoS2 nanoparticles with about 60 nm in mean size were left. The corresponding XRD measurements were conducted, as shown in Figure 3. Pattern for the products from ultrasonic exfoliation is well matched with the standard pattern of MoS2 (JCPDS No. 002-0132). For the products induced by laser ablation in water, most peaks match with those of MoS2, except the diffraction at 40.4o and 73.8o which are slightly smaller in the plane spcaings than those of (103) and (203) planes of MoS2, respectively, as more clearly shown in Figure 3 (b). No oxide was detected in the products. The crystal plane spacings were contracted by 1.3% for (103) and 0.5% for (203). The EDS measurements have revealed that the spherical nanoparticles are mainly composed of Mo and S, as typically illustrated in Figure S4. Further, TEM examination was carried out for the as-prepared products, as illustrated in Figure 4. The most particles are nearly perfectly spherical in shape (Figure 4a). The selected area electron diffraction (SAED) has confirmed that such spherical nanoparticles are MoS2 (see the inset of Figure 4a). The high resolution TEM (HRTEM) examination has revealed that the spherical MoS2 nanoparticles are of onion-structured or concentrically ringed lattice fringes with the spacing about 0.61 nm, which match well with the crystal plane spacing of {002} of MoS2, as typically illustrated in Figure 4(b). The corresponding Fourier transformation image shows a discontinuous ring, as shown in the inset of Figure 4(b). It means that such spherical MoS2 nanoparticles are actually polyhedral, and layer by layer and concentrically built by nearly spherical {002} planes, exhibiting the onion-structure. In addition, a close observation has found


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that for many MoS2 nanoparticles, there exist a hollow (or void) area in the central-part of the particles [as shown in Figure S5 or typically in Figure 4(c)], or the porosities dispersed in the particles (as typically illustrated in Fig.S6). The volume of such voids takes about 1.0% - 2.5% of the nanoparticle’s volume. Similarly, the tail part of a tadpole-like long-tailed nanoparticle is concentrically built by nearly cylindrical {002} crystal planes, and also a small hollow in the central-area of the tail part was observed, as typically shown in Figure 4(d). As for the MoS2 nanoplates prepared by the ultrasonic exfoliation, we can only observe the normal or flat lattice fringes of the planes (100), but not the curved or concentrically ringed ones, as illustrated in Figure S3(b). Finally, it should be mentioned that the yield of the MoS2 products was quite low (only few milligrams in each ablation experiment) in this work. However, the high productivities (in level of gram/hour) are achievable by this technique, as previously reported.35 3.2 Formation of the onion-structured MoS2 nanoparticles. As mentioned above, the value of the optical edge position Eg for the laser-induced spherical MoS2 colloidal solution is obviously different from that of the colloidal solution prepared by ultrasonic exfoliation. This could be attributed to their different structures and need special and intensive research. Here we are mainly focused on discussing the formation of the onionstructured spherical MoS2 nanoparticles during laser ablation in water. According to previous reports, 9, 30 laser ablation of MoS2 target in water could produce the spherical fullerene-like MoS2 nanoparticles. The formation of such MoS2 nanoparticles was attributed to pulse laser-induced thermal shock, which leads to the exfoliation of flakes and



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further diminution of particles,9 or to the oxidative environment created by the plasma plume.30 Obviously, these mechanisms can not explain the results in this study, since neither the laserinduced exfoliation nor the plasma plume-induced oxidative environment led to the formation of the concentrically spherical {002} planes-built MoS2 nanospheres shown in Figure 4 (a, b, c), and production of voids or porosities within the MoS2 nanoparticles [as shown in Fig.4(c), Figs.S5 and S6]. Also, it could not produce the tadpole-like long tailed nanoparticles (see the inset of Figure 2a). As mentioned above, the MoS2 nanoparticles obtained in this study are mostly of nearly perfectly spherical shape with smooth surface, and there also exist tadpole-like long tailed nanoparticles in the products and the voids or porosities within the MoS2 nanoparticles. All these indicate the marks of melting and liquid droplet’s solidification during the laser ablation, since the spherical and tadpole-like particles could originate from the solidification of the molten liquid droplets, and voids or porosities come from solidification-induced volume contraction. Based on these experimental evidences, here, we put forward to a model, or the laser-induced generation of MoS2 liquid droplets and inward {002}-oriented growth by radial solidification of the liquid droplets, to reveal the formation process of the spherical MoS2 nanoparticles with onion-like structure, as schematically illustrated in Figure 5. 3.2.1 Laser-Induced Generation of MoS2 Liquid Droplets and Surface Solidification. In our experimental conditions, when one pulse laser beam was shot on the MoS2 target in the water, the temperature on the irradiated area of the target surface would instantly increase and


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reach above the melting point of MoS2 due to absorption of the laser energy. Local melting on the target surface thus happened. Such locally molten MoS2 would be ejected into the water as liquid droplets in the next laser shot, as shown in Figure 5 (a). The ejected molten MoS2 liquid droplets would quench in the water but most of them could become spherical in water before solidification due to their surface tension. When the surface temperature of the droplets decreased below the melting point of MoS2, MoS2 nuclei would thus be formed randomly in such surface supercooling layer [see Figure 5 (b)]. The formed random-oriented MoS2 nuclei should preferentially grow along their {002} due to the laminated crystal structure. Obviously, only some nuclei, with {002} planes parallel to surface tangent planes of the spherical droplet, could effectively be growing along {002} plane within the surface super-cooling layer till contact with the neighboring growing nuclei or the surface of the droplet. As for the other-oriented nuclei within the supercooling layer, they could not grow up, as shown in Figure 5(c). With the completion of MoS2 nuclei growth within the supercooling layer, the surface solidified layer of the droplet was formed, as illustrated in Figure 5 (d). 3.2.2. Radial Solidification and Inward {002}-Oriented Growth. After surface solidification of the droplet, radial solidification would start due to the unceasing radial heatradiation from the droplet and induce the inward {002}-oriented growth. The thickness of the solidified layer was ever increasing with the inward growth and the molten MoS2 was constantly consumed, which induced shrinkages or volume contraction in the central part of the droplet due to the density difference between solid and liquid phase of MoS2, as shown in Figure 5 (e).



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Finally, the onion-structured spherical MoS2 nanoparticles, built of concentrically spherical {002} planes, were formed with a void in the central part or the dispersed pores depending on the local cooling condition in the water, as illustrated in Figure 5 (f) or Figure 4(c) and Figs.S5, 6. It means that the solidification contraction for MoS2 is about 1% ~2.5%, which is close to that of steel. 3.2.3. Formation of Tadpole-Like Long-tailed Nanoparticles. As for the formation of the tadpole-like nanoparticles, it could be attributed to the quick solidification of the molten tadpolelike liquid droplets, which were ejected from the target surface into the water, before the formation of spherical shape due to the surface tension. Similarly, due to the tadpole-like shape, during surface solidification, the MoS2 nuclei with {002} planes parallel to the surface tangent plane of the tadpole-like liquid droplet could grow along {002} plane till contact with the neighbouring growing nuclei, and the solidified layer, built with the cylindrically (or conically) curved {002} planes, was formed in the surface layer of the tadpole-like droplet. With inward radial solidification, the solidified layer increased in thickness, and finally the tadpole-like solid nanoparticle, built of concentrically and cylindrically curved {002} crystal planes, was formed with a small void in the central part, as typically shown in Figure 4 (d). 3.2.4 Size Dispersivity of MoS2 Nanoparticles. Based on the above formation process, the MoS2 nanoparticles originate from the radial solidification of the molten liquid droplets induced by laser shot. Such droplets, which were ejected by laser shot from the target surface into the water, should be of a large variation in size due to the randomicity in their generation, leading to


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the large size dispersivity for the MoS2 nanoparticle products, as shown in Figure 2(a). Obviously, it is difficult to directly produce monodisperse MoS2 nanoparticles using laser ablation in water. However, we could obtain near monodisperse nanoparticles by filtering the asprepared colloidal solutions using disposable filters and centrifugation, as typically shown in Figure 2(b). , 3.2.5 Influence of Laser Power. According to the forementioned formation mechanism, the laser power should be large enough. Otherwise, if the laser power is relatively low, the laser shot is not enough to induce the local melting on the target surface and generate the molten MoS2 liquid droplets. The spherical MoS2 nanoparticles would not be obtained. For confirmation, further experiment was conducted by laser ablation with much smaller power (25 mJ per pulse). Figure 6 shows the morphology of the products, which mainly consist of the plate-like objects. Such plate-like objects are attributed to the pulse laser-induced local thermal shock on the target, which would lead to exfoliation and formation of the plate-like products. If the laser power was too low, no product was produced. 3.2.6 Chemical Stability during Laser Ablation. Finally, although the reactive materials tend to be oxidized when their nanoparticles are produced by laser ablation in water, as previously reported,36 no oxide was detected in our experiment, as shown in Figure 3. The reasons could be as follows: (i) MoS2 target instead of Mo target was employed. The MoS2 is relatively stable compared with Mo; (ii) Although MoS2 could be starting slow oxidation in atmosphere at 315oC or higher, the laser ablation was performed in water; (iii) The MoS2



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nanospheres were formed due to the quick solidification of the molten MoS2 liquid droplets in water. MoS2 is more stable in water than in air. 3.3. A New Performance: Onion Structure-Enhanced SERS Effect. As mentioned in the section Introduction, the nanostructured MoS2 materials possess many special properties and hence extensive potential applications. The onion structured spherical MoS2 nanoparticles presented in this study have also exhibited some unique performances. Typically, such onion structured spherical MoS2 nanoparticles show much higher SERS effect than those of the MoS2 nanoplates induced by the conventional methods (such as ultrasonic exfoliation). Briefly, MoS2 nanoparticles-built films were prepared by alternatively dropping the MoS2 colloidal solutions on silicon wafers and drying, as described in details in Supporting Information. The film thickness was about tens of microns. The Rhodamine 6G (R6G) was chosen as the probe molecules. Then a drop of R6G solution (10-4 M) about 5 µL was dropped onto the MoS2 films or the blank silicon wafer before dying and Raman spectral measurements (the details are seen in the Supporting Information). Figure 7 shows the Raman spectra of R6G molecules on the different substrates. The Raman signals were observed for the R6G molecules on the MoS2 nanoplates-built film and obviously higher than those of R6G on the blank silicon wafer [see curves (II) and (III)], showing Raman enhancement effect. Further, for the R6G molecules on the spherical MoS2 nanoparticles-built film, the Raman signals are much stronger, and more than 7 times as high as those on the MoS2 nanoplates, exhibiting significantly enhanced SERS effect.


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There have been some reports on the SERS effect to R6G molecules for the MoS2 nanoplates.12-15 Since MoS2 possesses the semiconducting electronic properties and the chemical bonds of polar covalent bond (Mo-S), its enhancement mechanism includes charge transfer and dipole–dipole coupling, which induce magnification of the polarizability of the adsorbed molecules and hence enhancement of the Raman signal.12 Furthermore, the Fermi level of MoS2 (4.6 eV) locates between LUMO (3.28 eV) and HOMO (5.35 eV) energy level of R6G molecules,15 which promotes the photoinduced charge transfer between the R6G and MoS2 substrate, as previously reported.37. In our case, the Raman signals for R6G molecules on the spherical MoS2 nanoparticles-built film is much stronger compared with those on the MoS2 nanoplates-built film. This could mainly be attributed to the special structure of the spherical MoS2 nanoparticles. Such special structure could enhance the photoinduced charge transfer between R6G and MoS2. In addition, the spherical nanoparticles-built film has the higher exposed surface area and hence could adsorb more R6G molecules than the nanoplates-built film in which the nanoplates are easy to be overlapped and lead to the reduction of exposed surface area. However, further study is needed to reveal such onion structure -enhanced SERS effect.

4. CONCLUSION In summary, we have fabricated the spherical MoS2 nanoparticles with smooth surface via laser ablation of MoS2 target in water. Such spherical nanoparticles are tens to hundreds of nanometers in diameters and concentrically curved {002} planes-built, showing onion-like structure. Further examination has revealed the marks of melting and liquid droplet’s



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solidification during laser ablation, such as shrinkage cavities or dispersed porosities within the MoS2 nanoparticles, few tadpole-like long-tailed nanoparticles, and nearly perfectly spherical shape. On such bases, we have put forward to a model or laser-induced generation of molten MoS2 liquid droplets and inward {002}-oriented growth by radial solidification of the liquid droplets, which has clarified the formation process of the spherical MoS2 nanoparticles with onion-like structure. Interestingly, such onion-like structured spherical MoS2 nanoparticles show much stronger Raman signal to R6G molecules than the conventionally-prepared MoS2 nanoplates, exhibiting significantly onion structure-enhanced SERS effect. This study has not only presented the way to fabricate the spherical MoS2 nanoparticles with onion-like structure but also revealed the formation mechanism of the MoS2 nanoparticles during laser ablation in water. Supplementary Information Available: The details of preparation of SERS substrates and Raman spectral measurements, schematic illustration of the laser ablation, the optical absorbance spectrum of the colloidal solution, EDS results, FESEM and (HR)TEM images of the MoS2 nanosheets and nanospheres. ACKNOWLEDGMENTS This work is financially supported by the National Key Research and Development Program of China (Grant No 2017YFA0207101), Natural Science Foundation of China (Grant No. 51531006, 11574313, 51771182 and 51571188) and the CAS/SAF International Partner-ship Program for Creative Research Teams.


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Figure 1 Le Zhou, et al

Figure 1 (a): Optical absorbance spectrum of the colloidal solution obtained by laser ablation without any treatment. The inset is the photo of the colloidal solution in a bottle (left); the right bottle shows Tyndall effect when a laser beam illuminates. (b): The corresponding plot of (αhυ)1/2 versus hυ in the edge region.


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Figure 2 Morphology of the products. (a): FESEM image of the as-prepared products. The inset: the magnified image of a tadpole-like long tailed particle in the products. (b): FESEM image of the products after filtering using 450 nm disposable filter. The up-left inset: a local magnification; the down-right inset: size distribution of spherical particles.



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Figure 3 XRD patterns of the products prepared by ultrasonic exfoliation (curve I) and laser ablation (curve II). The line spectrum corresponds to the standard pattern of MoS2 powders (JCPDS No. 002-0132). (b): The enlarged curves of (a).


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Figure 4 Microstructural examination of the as-prepared MoS2 nanoparticles. (a): TEM image of the spherical MoS2 nanoparticles. The inset is the SAED pattern and shows MoS2 phase. (b): and (c): HRTEM images of spherical MoS2 nanoparticles. The inset in (b): the corresponding Fourier transformation pattern. (d): HRTEM image of the area marked in the inset. The inset: TEM image of a single tadpole-like nanoparticle.



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Figure 5 Schematic illustration for the formation of onion-structured spherical MoS2 nanoparticles. (a): Laser-induced target surface local melting and ejection of liquid droplets; (b): Random formation of MoS2 nuclei in the liquid droplet’s surface supercooling layer; (c): The {002} preferential growth of the random-oriented nuclei, but only some nuclei, with {002} plane parallel to surface tangent planes of the droplet, effectively grow within the layer; (d): The formation of surface solidified layer; (e): Inward {002}-oriented growth via radial solidification, with formation of shrinkages in the central liquid; (f): Completion of solidification, and formation of onion-structured spherical MoS2 nanoparticles with a void in the central part.


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Figure 6 The FESEM image of the products via laser ablation of MoS2 target in water with 25 mJ/pulse in power.



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Figure 7 The Raman spectra of R6G molecules on the different substrates. Curve (I): on the onion structured spherical MoS2 nanoparticles-built film; Curve (II): on the MoS2 nanoplatesbuilt film; Curve (III): on the blank silicon wafer. (Excited at 633 nm).


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TOC Graphic .


Onion-Structured Spherical MoS2 Nanoparticles ... - ACS Publications

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