Large-area high quality MoS₂ monolayers grown by sulfur vapor counter flow diffusion

Abstract

Large-area uniform monolayer MoS₂ films are prepared via an atmospheric pressure chemical vapor deposition method. Sulfur vapor counter flow diffusion lowers its concentration at the reaction zone and makes MoO₃ sulfurization slow and gentle, which is beneficial for monolayer MoS₂ growth. Suboxide MoO_3−x domains, co-existing MoS₂ mono- and multi-layers, monolayer MoS₂ films, triangular MoS₂ monolayer flakes are successively formed in the growth zone along the gas flow direction. Optical microscopy, atomic force microscopy, transmission electron microscopy, UV-vis, Raman and photoluminescence spectra demonstrate that the as-grown films are continuous monolayers over a large area, and are of high quality comparable to exfoliated monolayer MoS₂. This method can also be applied to synthesize MoS₂/MoO₂ microplates without the assistance of metal oxides.

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1. Introduction

Graphene has shown many exotic physical phenomena and remarkable properties,^1–7 but its zero band gap is not suitable for application in logic electronics.⁸ In this regard, transition-metal dichalcogenides (TMDs) have recently emerged as the most promising alternative to graphene. As a member of the TMD family, molybdenum disulfide (MoS₂) exhibits many excellent properties when thinned to the monolayer regime, such as intrinsic direct gap structure,^9,10 band structure tunability with strain,^11–14 valley-selective circular dichroism,^15–19 highly reactive edge sites for hydrogen evolution reaction (HER),^20–26 excellent on/off ratio of ∼10⁸ and high carrier mobility up to 200 cm² V⁻¹ s⁻¹ at room temperature,²⁷ integration versatility,^28,29 etc. Thus, there is a critical need to develop reliable and flexible synthesis methods for monolayer MoS₂ to utilize these fantastic properties.

Until now, two major strategies have been employed to obtain MoS₂ atomically thin layers. Initially the top-down methods relying on exfoliation of layered bulk crystals were widely adopted, including scotch tape assisted micromechanical exfoliation,³⁰ electrochemical Li-intercalation and exfoliation,³¹ liquid phase exfoliation by direct sonication in solvents,^32,33 and laser or plasma thinning.^34,35 These methods only allow us to produce nanometer- to micrometer-sized monolayers. Recently, several bottom-up approaches, such as thermolysis of ammonium thiomolybdate,^36,37 sulfurization of molybdenum^38–40 or molybdenum oxide film,⁴¹ physical vapor deposition,⁴² and atomic layer deposition,^43,44 have been exploited to synthesize MoS₂ on insulating substrates. However, none of the processes have the capability of large-area growth of uniform MoS₂ monolayers. Compared with the routes mentioned above, chemical vapor deposition (CVD) has been proven a more suitable technique for synthesizing MoS₂ film with control over layer number,^45–47 nucleation,^48–50 grain size,⁵¹ and lattice orientation^52–54 due to its broad tunability in growth parameters. Despite the above progresses, current growth of monolayer MoS₂ is still in a nascent stage.

Yu et al.⁴⁶ demonstrated that for an exclusive growth of MoS₂ monolayer in CVD process, partial pressure of gaseous MoS₂, P_Mo, should lie between the vapor pressures of MoS₂ monolayer (P^O_Mo,1) and bilayer (P^O_Mo,2) films as P^O_Mo,1 < P_Mo < P^O_Mo,2. So in previous works, low pressure is often employed in MoS₂ monolayer synthesis since P_Mo is proportional to the total pressure when precursor amount is constant and Mo precursor vapor is fully reacted. The synthesized film is polycrystalline with grain size ranges from tens of nanometers to micrometers.^46,51,52 While at atmospheric pressure, sulfur vapor concentration is so high that sulfurization reaction is violent and partial pressure of gaseous MoS₂ is too large for monolayer MoS₂ to grow (Fig. S1†). Therefore, moderating sulfurization reaction to lower the partial pressure of gaseous MoS₂ may be a feasible way to synthesize monolayer MoS₂ at atmospheric pressure.

Here we report the CVD synthesis of large-area uniform monolayer MoS₂ film at atmospheric pressure by placing sulfur powder downstream relative to the gas flow direction instead of upstream as usually adopted in previous studies. The dependence of products on the distance away from MoO₃ precursor and the flow rate of carrier gas were investigated. The quality of the as-grown film was evaluated by Raman, photoluminescence, UV-vis and transmission electron microscope measurements. Furthermore, MoS₂/MoO₂ microplates can also be grown using this method without the need of adding metal oxides to assist the synthesis.

2. Experimental section

2.1 CVD growth of monolayer MoS₂ on sapphire

The experimental setup to synthesize monolayer MoS₂ on sapphire is illustrated in Fig. 1b. A tubular furnace equipped with a quartz tube (length 100 cm, inner diameter 3.2 cm) was employed to conduct the growth. The length of the heating zone of the furnace is 18 cm. 6 mg of MoO₃ powder was placed at the upstream entry of the furnace and 200 mg of sublimated sulfur was located downstream. Well cleaned sapphire (0001) substrates were put downstream in a distance of 6.5 cm away from the center of furnace (the distance between MoO₃ and sapphire is 17.5 cm). The CVD growth was performed at atmospheric pressure. After flushing the tube with Ar carrier gas for two times, the furnace was heated up to 820 °C at a rate of 20 °C min⁻¹ with 50 sccm Ar (sccm denotes standard cubic centimeter per minute), held at 820 °C for 20 min, and then naturally cooled down to room temperature.

Fig. 1 Synthesis and optical characterizations of monolayer MoS₂ on sapphire. (a) Schematic view of the surface growth. (b) Experimental setup of the CVD system. (c) Photograph of a bare sapphire substrate and one nearly full covered by MoS₂ film. (d and e) High and low magnification optical microscope images of monolayer MoS₂ film. (f) Optical microscope image of coalescence of MoS₂ domains, with lateral size of about 150 μm. (g) XPS scans for Mo and S binging energies of bulk and CVD MoS₂. (h and i) Typical Raman and normalized PL spectra comparisons between our synthesized MoS₂ and an exfoliated one. The PL intensity is normalized by Raman A_1g phonon peak at ∼525.4 nm (400.7 cm⁻¹ in Raman shift). Inset is the magnified spectra showing characteristic Raman peaks. (j) UV-vis absorbance spectrum of monolayer MoS₂ film.

2.2 Micromechanical exfoliation of monolayer MoS₂ on sapphire

To obtain monolayer MoS₂ from its bulk form, scotch tape assisted micromechanical exfoliation method was adopted. Firstly a MoS₂ flake was mechanically exfoliated from a bulk MoS₂ crystal using 3M scotch tape. Then the tape was folded, pressed gingerly and peeled slowly so that the MoS₂ flake cleaved smoothly into two, this step was repeated for about six to eight times. Next, the cleaved MoS₂ flake that remained stuck to the tape was laid onto a sapphire substrate and rubbed by using a plastic tweezer. After that, the tape was peeled off slowly with the plastic tweezer. Finally, the sapphire substrate was placed under an optical microscope to find the monolayer MoS₂ sample (Fig. S2†).

2.3 Characterization

Optical images were taken on an Olympus BX51M microscope. X-ray photoemission spectroscopy (XPS) measurements were performed on Thermo Scientific Escalab 250Xi system using an Al Kα X-ray source and energy calibrations were made against C 1s peak to eliminate the charging of sample during analysis. Raman and photoluminescence (PL) spectra were collected by JY Horiba HR800 micro-Raman spectrometer with an excitation wavelength of 514 nm. UV-vis absorbance spectrum was measured using Shimadzu SolidSpec-3700DUV spectrophotometer. Tapping-mode atomic force microscope (AFM) measurements were done on a Bruker Dimension Icon microscope. Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) pattern were captured using JEOL JEM-2010F operated at 200 kV.

3. Results and discussion

Fig. 1a schematically illustrates the MoS₂ growth process on sapphire (0001) substrate. In brief, upstream MoO₃ was partially reduced by sulfur vapor from downstream S source to form volatile suboxide MoO_3−x species. These suboxide compounds were conveyed by Ar carrier gas and reacted with sulfur to give rise to the formation of MoS₂ on sapphire. The stepwise reaction process of MoO₃ and S can be described by the following equations:⁵⁵

The CVD setup employed in this work is depicted in Fig. 1b. Macroscopic photograph in Fig. 1c clearly presents the color contrast between a bare substrate and an as-grown sample. Further optical microscope images (Fig. 1d and e) demonstrate that the as-prepared monolayer MoS₂ film is uniform and continuous over a large area, with the constituent domains of about 150 μm in edge length (Fig. 1f).

Chemical state and composition of the products were analyzed through XPS. As seen from Fig. 1g, there are two prominent groups of peaks. One is the peaks associated with Mo⁴⁺, peaking at 230.2 eV (3d_5/2) and 233.3 eV (3d_3/2), and the other can be assigned to S²⁻, peaking at 163.1 eV (2p_3/2) and 164.3 eV (2p_1/2). These peaks closely match with those of the bulk MoS₂ (Mo 3d_5/2 and 3d_3/2 states at binding energies of 230.1 and 233.2 eV and 2p_3/2 and 2p_1/2 states of S at binding energies of 162.9 and 164.1 eV, respectively).

Raman and PL measurements were performed to check the quality and layer number of the as-grown MoS₂ film. As a comparison, exfoliated monolayer MoS₂ on sapphire was also included as a control sample. Fig. 1h shows the acquired Raman spectra, from which two characteristic Raman modes can be found. The A_1g mode correlates with the out-of-plane vibration of S atoms and the E¹_2g mode relates with the in-plane vibration of Mo and S atoms. For the synthesized MoS₂ film, the full width at half maximum (FWHM) of A_1g and E¹_2g are 6.4 and 2.4 cm⁻¹, respectively. Spacing between the two modes (Δk) is ∼17.7 cm⁻¹. All of them are close to the exfoliated monolayer MoS₂ (FWHM values of A_1g and E¹_2g are 4.6 and 1.7 cm⁻¹, respectively, with Δk ∼ 17.7 cm⁻¹). Meanwhile, similar PL peaks can be found from both synthesized and exfoliated MoS₂ monolayers, as shown in Fig. 1i, where the PL spectra were normalized by Raman A_1g peak intensity to rule out the external effects.¹⁰ Two peaks located at ∼627 and ∼678 nm are originated from direct excitonic transitions between minimum of conduction band and maxima of split valence bands (B and A excitons) in MoS₂.^9,10 Note that the PL peak width of our CVD monolayer MoS₂ (128 meV) is a little narrower than that of exfoliated MoS₂ (133 meV). These results confirm the monolayer nature and high quality of our MoS₂ film as compared with the exfoliated one.

Taking advantage of the optical transparency of sapphire substrate, we further conducted UV-vis absorption characterization of the as-deposited monolayer MoS₂ sample (Fig. 1j). The resulting spectrum exhibited A and B excitonic absorption bands at 664 nm (1.871 eV) and 616 nm (2.017 eV), with energy difference of about 0.146 eV, which is in good agreement with the theoretical value of 0.148 eV calculated for monolayer MoS₂.⁵⁶ This demonstrates not only the fine electronic structure but also the high optical quality and uniformity of our obtained monolayer MoS₂ film.

Although A and B direct excitonic transitions can be readily identified from the UV-vis absorbance spectrum in Fig. 1j, PL spectra in Fig. 1i show obvious the A exciton peak, while the peak relates to B exciton is rather limited. However, in ref. 10, B exciton peak can be clearly observed in exfoliated MoS₂ monolayer, bilayer, and hexalayer samples on SiO₂/Si substrate. We attribute this phenomenon to different substrate-induced electron doping effects. Mak et al.⁵⁷ showed that the A exciton peak intensity is highly dependent on doping, the higher the electron doping density, the lower the A exciton peak intensity; while the B exciton peak intensity has little dependence on doping. For freestanding MoS₂ monolayers without electron doping from the substrate, the A exciton peak intensity is so high that the B exciton peak can not be observed.⁹ Recent studies found that SiO₂ substrate introduces more charge doping than Gel-film, few-layer graphene, hexagonal boron nitride, Au, mica, LaAlO₃, and SrTiO₃ substrates, which causes the low intensity of A exciton peak and both A and B exciton peaks are prominent.^58,59 In our case, we think that the sapphire substrate induces much less electron transfer than the SiO₂ substrate, which results in strong A exciton peak and limited B exciton peak in the PL spectra.

In addition to optical characterizations, electronic performance is another important parameter for evaluating the quality of CVD synthesized MoS₂. Early reports of the mobility of CVD synthesized monolayer MoS₂ film is about 0.003–0.03 cm² V⁻¹ s⁻¹,^46,48 which is much lower than that of exfoliated monolayer MoS₂ (0.1–10 cm² V⁻¹ s⁻¹).²⁷ The reason for this difference is not clear at present, but grain boundaries⁵¹ and charged interfacial states due to the dielectrics in contact⁶⁰ can be responsible for the degradation in mobility. Recent researches suggest that the high density of localized states prohibits access to the band transport regime,⁶¹ and interfacial contamination on the growth substrate as well as residual tensile strain resulting from the high temperature growth process limit device performance of the CVD synthesized MoS₂ monolayers.⁶² Besides, the silicon dioxide layer of the commonly used SiO₂/Si substrate may become defective and lose its function as an insulating layer for backgating after the high temperature growth process.⁶³ Continual engineering efforts on improving sample quality and reducing grain boundary density, as well as developing an appropriate transfer method are needed for potential applications of CVD synthesized MoS₂.

To verify that the luminescence property of our MoS₂ film is due to excitonic transitions, PL measurement at different excitation laser power is conducted, which can be used to characterize excitonic, donor–acceptor pair, and free-to-bound like transitions in semiconductors. Fig. 2 displays the dependence of PL spectra on excitation laser power at room temperature. The higher the excitation laser power is, the stronger the PL intensity and narrower the peak width will be. The relation between PL intensity (I_PL) and excitation laser power (I_EXC) can be expressed as I_PL = nIα_EXC, where n is the emission efficiency and exponent α represents the radiative recombination mechanism.^64,65 Specifically, α ranges generally from 1 to 2 for free- and bound-exciton recombination and is less than 1 for impurity-related recombination processes. The log–log plot of PL intensity as a function of excitation laser power is shown in inset of Fig. 2, and the slope found by linear fit is 1.0324, which is close to the result of 1.2133 in ref. 66. This confirms that the origin of MoS₂ luminescence property is excitonic transitions rather than an impurity related process.

Fig. 2 Laser power dependent PL spectra of MoS₂ film, inset is the curve of PL intensity versus laser power plotted in log–log form.

To quantitatively assess the uniformity of MoS₂ film over a large area, Raman spectra were measured every 1 mm along the line across the substrate (Fig. 3a). We find that peak positions of the A_1g and E¹_2g modes almost remain identical, and peaks profile shows negligible difference. These confirm the homogeneity of MoS₂ film over the substrate. We also performed mapping of the Raman frequency difference Δk and normalized PL intensity within a 15 × 15 μm² area (Fig. 3b and c). The Δk range of 17.9–19.1 cm⁻¹, combined with the homogeneous normalized PL intensity, indicate the film a uniform monolayer. Furthermore, film thickness of ∼0.7 nm obtained from the section-view analysis across film edge (Fig. 3d) and a continuous and smooth surface (roughness ∼ 0.2 nm) over an area of 5 × 5 μm² (Fig. 3e) measured by AFM directly demonstrate the monolayer uniformity of our prepared MoS₂ film.

Fig. 3 Large-area uniformity of the synthesized monolayer MoS₂. (a) Raman spectra of MoS₂ film collected every 1 mm along the line as illustrated in the inset. (b and c) Mapping of the Raman frequency difference Δk and normalized PL intensity within a 15 × 15 μm² area from the monolayer MoS₂ film, respectively. (d) AFM image and corresponding section view along the white dash line. (e) Perspective view of a typical AFM image of the as-prepared MoS₂ film.

TEM was also applied to investigate the structure of as-made MoS₂ film. Fig. 4a is a typical low magnification TEM image of the monolayer MoS₂ film, where the folds, cracks, and holes were created during the transfer process. SAED pattern in Fig. 4b shows hexagonal symmetry of the MoS₂ structure. Edge folding is a common phenomenon in two-dimensional materials that can be effectively utilized to determine the number of layers.⁶⁷ We occasionally found folded edges on TEM grid and observed a single dark line in the prepared MoS₂ sample (Fig. 4c), which could be correlated to a monolayer. FFT filtered high resolution TEM image in Fig. 4d shows a honeycomb arrangement of atoms with lattice spacing of 2.7 and 1.6 Å, assigning to the (100) and (110) planes, respectively.

Fig. 4 TEM characterizations of monolayer MoS₂ film. (a) Low magnification TEM image of MoS₂ film. (b) SAED pattern acquired from the area marked with a white dashed circle in (a). (c) High resolution TEM image of the square area in (a). (d) FFT filtered high resolution TEM image of the marked area in (c).

As mentioned above, the reaction process of MoO₃ with S involves the stepwise reduction and sulfurization, that is, MoO₃ is first partially reduced by vaporized S to form volatile suboxide MoO_3−x species, which are further sulfurized to give rise to the formation of MoS₂. Sulfur serves as the reductant and sulfurization agent at the same time.⁵⁵ And it is expected that, in the growth zone along the gas flow direction, MoO₃ vapor concentration decreases while S vapor concentration increases. Therefore more distance away from MoO₃ precursor, more amount of MoO₃ should be reduced and the reduction will be more complete. Meanwhile, MoS₂ vapor concentration is anticipated to decrease in the gas flow direction from the region where MoO₃ vapor is fully reacted, which would lead to different morphologies of MoS₂ on the substrates. To confirm these hypotheses, a series of sapphire substrates were placed successively in the growth zone, as depicted in Fig. 5a. Fig. 5b shows optical microscope images of the products formed at different distance between MoO₃ precursor and sapphire substrate (D_dist): when D_dist is less than 10.0 cm, the supply of sulfur is low but sufficient MoO₃ is evaporated, little MoO₃ is initially reduced and small black particles are nucleated, which are proven to be Mo₄O₁₁ by Raman spectrum (Fig. S3†). With the increase of D_dist, rhomboidal shaped MoO₂ domains are formed at D_dist = 11.0 cm and become bigger and denser because of the raised sulfur vapor concentration as well as the enhanced reduction. For D_dist = 15.0 cm, parts of the vaporized MoO₃ are fully reacted, resulting in the formation of monolayer MoS₂ around the MoO₂ domains (see Fig. S4† for the larger version of the image and corresponding Raman measurements). When D_dist reaches 16 cm, sulfur is plentiful enough for the full reaction of MoO₃ vapor, giving rise to high levels of MoS₂ vapor concentration, which in turn causes the simultaneous growth of single- and multi-layered MoS₂. As MoS₂ vapor concentration reduces along the gas flow direction, multilayer MoS₂ domains gets smaller and fewer, and finally disappears at D_dist = 17.5 cm, leaving only the complete monolayer MoS₂ film. With the further increase of D_dist, monolayer MoS₂ film becomes discontinuous and discrete triangular MoS₂ flakes evolve, whose edges are sulfur terminated zigzag ones identified empirically.⁶⁸ These isolated triangles shrink in size and number, and finally vanish with the cut off of MoS₂ supply when D_dist ≥ 19.8 cm. All the results are consistent with the aforementioned hypothesizes and suggest that sulfur vapor flows to growth zone is retarded by Ar gas flow, which moderates MoO₃ sulfurization and benefits monolayer MoS₂ growth.

Fig. 5 Distance-dependent of products in the CVD growth. (a) Schematic illustration of the successively placed sapphire substrates in the growth zone. (b) Optical microscope images of the products with different MoO₃ precursor and sapphire substrate distance (D_dist), as shown on the upper right corner of each image. Scale bars: 10 μm.

It is worth noting that the Ar flow rate would affect readily the sulfur vapor concentration and in turn the resultant MoS₂ monolayers. As shown in Fig. 6a–c, when the Ar flow rate is 5 sccm, sulfur vapor concentration is so high that MoO₃ vapor is fully reacted before it flows into the growth zone, no products are formed on the sapphire substrates placed in the growth zone. With the increase of Ar flow rate to 30 sccm, sulfur vapor concentration decreases but is still high, irregularly shaped MoS₂ monolayers interspersed with bulk MoS₂ particles or multilayer patches are formed on the sapphire substrates located at the center of furnace (D_dist = 11.0 cm). Further increase of the Ar flow rate to 300 sccm leads to very low sulfur vapor concentration, which in turn causes the incomplete reaction of MoO₃ vapor and the formation of Mo₄O₁₁ particles on the sapphire substrates in the growth zone (Fig. S5†). So a proper Ar flow rate is needed for the synthesis of high quality MoS₂ monolayers. In our experiment, 50 sccm Ar is appropriate to grow large-area high quality monolayer MoS₂ film.

Fig. 6 Influence of Ar flow rate on the resultant MoS₂ monolayers. (a–c) Optical microscope images of the products synthesized with different Ar flow rate of 5, 30, and 300 sccm, respectively. Scale bars: 10 μm.

In ref. 45, metal oxide powder (MnO₂, V₆O₁₃, or Co₃O₄, etc.) was added to react with extra sulfur at the initial stage of MoO₃ reduction to allow MoO₂ grow up to microplates, which were used as templates to synthesize MoS₂ flakes. Too much sulfur would result in MoS₂ particles because the growth of MoO₂ was terminated by the complete coverage of MoS₂ due to high sulfur vapor concentration. Inspired by the results in Fig. 5 and the fact that sulfur vapor concentration is relatively low in the growth zone for our experiment, we wonder if it is possible to grow MoS₂/MoO₂ plates on SiO₂/Si substrates in configuration Fig. 1b without the need of metal oxides. It is found that like the growth procedure for monolayer MoS₂ film, rhomboidal shaped microplates were formed, as seen from Fig. 7a, when SiO₂/Si substrates were located at the center of furnace and MoO₃ powder was placed 1 cm closer to the center of furnace (the distance between MoO₃ powder and SiO₂/Si substrates is 10.0 cm) as well as Ar gas flow rate was 20 sccm. Raman spectrum of the as-made microplates in Fig. 7b shows eleven peaks in the range of 150–800 cm⁻¹, which can be assigned to MoS₂ (E¹_2g mode ∼ 382 cm⁻¹ and A_1g mode ∼ 406 cm⁻¹), MoO₂ (∼204, 230, 346, 364, 460, 497, 570, and 743 cm⁻¹),⁶⁹ and Si (∼520 cm⁻¹). This confirms the rhomboidal shaped microplates to be MoS₂/MoO₂. Metal oxide powder is not needed to assist the synthesis in our experiment (the Ar flow rate also has an influence on the resultant MoS₂/MoO₂ microplates, see Fig. S6 in the ESI†).

Fig. 7 Growth of the MoS₂/MoO₂ microplates. (a) Optical microscope image and (b) Raman spectrum of the as-synthesized MoS₂/MoO₂ plates. Peaks marked with ∗ and # correspond to MoS₂ and MoO₂, respectively.

4. Conclusion

In summary, we present a facile method to grow large-area uniform monolayer MoS₂ films via a new CVD configuration. MoO₃ is first partially reduced by vaporized S to form volatile suboxide MoO_3−x species, which are further sulfurized into MoS₂ on the substrate. Mo₄O₁₁ particles, MoO₂ domains, MoS₂ multilayers, monolayer MoS₂ film, triangular MoS₂ monolayer flakes are successively formed in the growth zone along the gas flow direction. Optical microscope, AFM, Raman and PL mapping show that the as-grown MoS₂ films are uniform monolayers over a large area. Meanwhile, FWHM values of Raman and PL peaks of our MoS₂ film are close to the exfoliated monolayer sample, indicating the as-grown films are of high quality comparable to the exfoliated MoS₂. Rhomboidal shaped MoS₂/MoO₂ microplates can also be grown using our method without the need of adding metal oxides to assist the synthesis. Moderating MoO₃ sulfurization offers an effective way to grow monolayer MoS₂ films.

Acknowledgements

This work was financially supported by National Basic Research Program of China (Grant No. 2011CB921400 and 2013CB921800), the Natural Science Foundation of China (Grant No. 11374280 and 50772110), and the Natural Science Foundation of Anhui province (Grant No. 1408085ME91).

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Footnote

† Electronic supplementary information (ESI) available: Optical microscope images of MoS₂ synthesized in configuration Fig. S1a, optical microscope image and atomic force microscope image of the exfoliated monolayer MoS₂ sample, Raman measurements of the intermediate oxides, and optical microscope images as well as Raman measurements of the products synthesized with different Ar flow rate. See DOI: 10.1039/c6ra03641d

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