Abrupt Thermal Shock of (NH4)2Mo3S13 Leads to Ultrafast Synthesis

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Functional Inorganic Materials and Devices

Abrupt Thermal Shock of (NH4)2Mo3S13 Leads to Ultrafast Synthesis of Porous Ensembles of MoS2 Nanocrystals for High Gain Photodetectors Saiful M. Islam, Vinod K. Sangwan, Yuan Li, Joohoon Kang, Xiaomi Zhang, Yihui He, Jing Zhao, Akshay A. Murthy, Shulan Ma, Vinayak P. Dravid, Mark C Hersam, and Mercouri G. Kanatzidis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12406 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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ACS Applied Materials & Interfaces

Abrupt Thermal Shock of (NH4)2Mo3S13 Leads to Ultrafast Synthesis of Porous Ensembles of MoS2 Nanocrystals for High Gain Photodetectors Saiful M. Islam,1,2 Vinod K. Sangwan3, Yuan Li,3 Joohoon Kang,3 Xiaomi Zhang,3 Yihui He,2 Jing Zhao2, Akshay Murthy,3 Shulan Ma2, Vinayak P. Dravid,3,4 Mark C. Hersam2,3,5,6 Mercouri G. Kanatzidis2,3* 1Deapartment

of Chemistry, Physics and Atmospheric Sciences, Jackson State University, Jackson, MS 39217, United States 2Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States 3Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States 4International Institute for Nanotechnology, Northwestern University, Evanston, Illinois 60208, United States 5Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, USA 6Applied Physics Graduate Program, Northwestern University, Evanston, Illinois 60208, USA

ABSTRACT: Ultrafast synthesis of high quality transition metal dichalcogenide (TMDQ) nanocrystals such as molybdenum disulfide (MoS2) is technologically relevant for large-scale production of electronic and optoelectronic devices. Here, we report a rapid solidstate synthesis route for MoS2 using the chemically homogeneous molecular precursor, (NH4)2Mo3S13·H2O, resulting in nanoparticles with estimated size down to 25 nm only in 10 sec at 1000 oC. Despite the extreme non-equilibrium conditions, the resulting porous MoS2 nanoparticles remain aggregated to preserve the form of the original rod shape bulk morphology of the molecular precursor. This ultrafast synthesis proceeds through the rapid decomposition of the precursor and rearrangement of Mo and S atoms coupled with simultaneous efficient release of massive gaseous species, to create nanoscale porosity in the resulting isomorphic pseudocrystals which are composed of the MoS2 nanoparticles. Despite the very rapid escape of massive amounts of NH3, H2O, H2S and S gases from the (NH4)2Mo3S13·H2O mm sized crystals, they retain their original shape as they convert to MoS2 rather than undergo explosive destruction from the rapid escape process of the gases. The 1 ACS Paragon Plus Environment

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obtained

pseudocrystals

are

made

of

aggregated

MoS2

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nanocrystals

exhibit

a

Brunauer−Emmett−Teller (BET) surface area of ~35 m2/g with an adsorption average pore width of ~160 Å. The nanoporous MoS2 crystals are solution-processable by dispersing in ethanol and water and can be cast into large-area uniform composite films. Photodetectors fabricated from these films show more than two orders of magnitude higher conductivity (~6.25 × 10-6 S/cm) and photoconductive gain (20 mA/W) than previous reports of MoS2 composite films. The optoelectronic properties of this nanoporous MoS2 imply that the shallow defects that originate from the ultrafast synthesis act as sensitizing centers that increase the photocurrent gain via two-level recombination kinetics.

Keywords: solid state synthesis, porous MoS2 nanocrystal, thin films, photodetector

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Transition metal dichalcogenides (TMDQs), especially from group IV-VII, crystallize in a layered structure that leads to strong out-of-plane anisotropy in their electrical, optical, mechanical, and thermal properties.1-5 In the class of TMDQs, MoS2 is among the most extensively studied because of its high charge carrier mobility, strong light-matter interactions, and high chemical stability. Monolayer MoS2 exhibits a thickness of ~6.5 Å and features strong covalent Mo-S bonding. Multilayer MoS2 consists of stacks of these monolayers, which are coupled by weak van der Waals interactions that allow the crystal to readily cleave along the basal planes. Because of its propensity to exfoliate into 2D sheets, MoS2 has been explored for a variety of applications including electronics,3, 6 energy storage,7 sensing,8 and catalysis.9,10 Specifically, due to its large optical absorption coefficient, high charge carrier mobility, and high photoresponsivity, MoS2 is of broad interest in high-performance photodetectors.11-14 Ultrathin MoS2 nanosheets have been prepared via a variety of bulk crystal synthesis methods such as hydrothermal synthesis, chemical vapor deposition, metal-organic chemical vapor deposition, physical vapor deposition, often coupled with micromechanical, chemical, and liquid phase exfoliation (LPE). 9, 15-27 Although micromechanical exfoliation can produce high quality 2D nanosheets, this approach is not suitable for large scale applications. Chemical exfoliation via lithiation produces large quantities of 2D nanosheets, but undesirable phase transformation from the semiconducting 2H phase to the metallic 1T phase occurs during this process.28 Alternatively, LPE of MoS2 yields large quantities without chemical phase transformation,29-33 although the exfoliation yields tend to be poor.34-35 Overall, current 3 ACS Paragon Plus Environment


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solution based synthesis and exfoliation strategies for MoS2 fail to produce the combination of materials quality and scalability that are required for high-performance applications such as photodetectors.36-38 In an effort to overcome these issues, this study reports an ultrafast synthesis method for porous MoS2 nanoparticles using well-defined molecular precursors. Previously, we report that the thermal decomposition of (NH4)2Mo3S13·H2O results in isomorphic pseudocrystals of MoS2 aggregated nanoparticles.39 Here we studied the ultrafast limit of this molecular precursor decomposition and demonstrated the remarkable transformation at 1000 oC to MoS2 nanocrystals down to 25 nm in less than 10 sec. The results of this synthesis is crystalline MoS2 with a highly porous aggregate structure that efficiently exfoliates into 2D nanosheets via LPE, allowing subsequent assembly into uniform thin films. The resulting material possesses desirable optoelectronic characteristics with improved photodetector metrics such as dark conductivity (~6.25 × 10-6 S/cm), photoconductivity (~3 × 10-5 S/cm) and photoresponsivity (20 mA/W) which is two orders of magnitude higher that existing reports on MoS2 composite films. MoS2 nanocrystals were synthesized via solid-state synthesis by rapidly heating mm size crystals of the polysulfide precursor, (NH4)2Mo3S13·H2O, in an evacuated sealed fused silica tube (see details in Supporting Information, Figure S1). In order to understand the effect of temperature on the synthesis and crystallization of MoS2, the polysulfide precursor, (NH4)2Mo3S13·H2O was heated with a variable but short duration ranging from 11 to 15 min at 500 oC and from 5 to 60 sec at 1000 oC. With a loading of ~100 mg of (NH4)2Mo3S13·H2O crystals 4 ACS Paragon Plus Environment

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in a fused sealed silica tube (outer diameter, OD=10 mm), we observed that at 500oC the shortest possible conversion time for the mm scale crystals of (NH4)2Mo3S13·H2O to isomorphic pseudocrystal of MoS2 is 12 min; whereas, at 1000oC, complete conversion was achieved in 10 sec, which is the fastest MoS2 synthesis time reported to date. Synthesis at 500 oC for 11 min or 1000 oC for 5 sec resulted in incomplete conversion (NH4)2Mo3S13·H2O to MoS2 as shown in the PXRD in Figures 1A and 1B. According to PXRD, MoS2 was assigned as 2H phase which is the most thermodynamically stable phase. The 1T phase is thermodynamically unstable and relaxes to 2H over time even at temperatures below 100 oC.28, 40-41 We have synthesized the MoS2 at temperatures ≥ 500 oC. Under these conditions the formation of 1T phase is unlikely. However, the conversion of the 2H MoS2 through the 3R phase cannot be completely ruled out. The remarkable fact is that despite the very rapid escape of massive amounts of NH3, H2O, H2S and S gases from the (NH4)2Mo3S13·H2O (Equation 1)39 mm sized crystals, they retain their original shape as they convert to MoS2 and do not undergo explosive destruction from the rapid escape process of the gases. The resulting products were characterized by powder X-ray diffraction (PXRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) (Figure 1).

PXRD peak widths and intensities are comparable for all MoS2 samples synthesized over times ranging from 12 to 15 min at 500 oC (Figure 1A). The weak and broad reflections in the PXRD patterns illustrate nanosized nature of the structures that could be attributed to short coherence lengths, inhomogeneous strain, crystal lattice imperfections and stacking



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faults. Scherrer analysis of the peak broadening of the [110] reflections of the sample synthesized at 500 oC for 12 min yielded an estimated average domain size of 7 nm along the stacking direction. However, the MoS2 sample synthesized in 10 sec at 1000 oC (Figure 1B) exhibited domain sizes of ~ 4 nm for both [002] and [110] planes. As expected, the domain size increases with annealing time, leading to ~8 nm and ~9 nm for the [002] and [110] planes, respectively, following a 60 sec heat treatment (Figure 1B). -1

-1

Raman spectra of the MoS2 reveal bands centered at ~403 cm and ~378 cm , which represent the A1g and E12g vibrational modes of MoS2, respectively (Figures 1C and 1D).42,43 A broad feature of the Raman spectrum for the 1000 oC, 10 sec sample (Figure 1C) indicates the formation of poorly crystalline MoS2, while, a weak band centered at ~350 cm-1 hints at the presence of amorphous MoS3.44 This observation suggests that the transformation of (NH4)2Mo3S13·H2O to MoS2 proceeds through the formation of amorphous MoS3. XPS was used to track the formation of MoS2 and to determine the effect of different synthesis conditions including pristine (NH4)2Mo3S13·H2O and nanocrystalline MoS2 synthesized at 500 and 1000 oC (Figures 1E and 1F). The binding energies (BEs) observed from the (NH4)2Mo3S13·H2O crystals are 228.7 eV and 232.1 eV for Mo 3d states and a range of 164.4 - 161.9 eV for S 2p states. Such a broad range in the BEs for the S 2p states arises from the two different oxidation states of sulfur (S2- and S1-), variation of the local environments as well as the splitting of energy levels due to spin-orbit interactions.45 The BEs at 164.4 and 163.3 eV correspond to the S-1 2p3/2 and 2p1/2, respectively, while those centered at 162.0 and 163.0 eV,


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correspond to the S-2 2p3/2 and 2p1/2, respectively.46 As expected, the 11 min at 500 oC and 5 sec at 1000 oC samples show relatively intense S2- peaks (at ~162 eV for S 2p3/2), because of the formation of MoS2. However, we also see the presence of S-1 peaks at 164.4 and 163.3 eV, suggesting incomplete conversion. In contrast, the 12 min at 500 oC and 10 sec at 1000 oC samples show typical S2- peaks, albeit, a weak tail at 164.5 eV represents the presence of the disulfide fraction. The S-2 2p3/2 peak position shows a gradual blue shift in the interval of 12 to 15 min at 500 oC and in the interval of 10 to 60 sec at 1000 oC. This gradual change in BEs is indicative of the conversion of S22- residue of amorphous MoS3 to MoS2. However, the presence of residual MoS3 in the ultrafast synthesized final products cannot be ruled out. Thermogravimetric and in situ EXAFS studies show that the thermal decomposition of (NH4)2Mo3S13·H2O to MoS2 proceeds through the loss of volatile gaseous species (H2O, NH3, H2S and sulfur) with the subsequent rearrangements of the Mo and S atoms.47-48 Unlike slowly heating from room temperature,47-48 we heated the mm size rod shaped single crystals of (NH4)2Mo3S13·H2O in evacuated sealed fused silica tubes by placing them directly in a preheated furnace at 500 and 1000 oC (details in the SI). Surprisingly, despite such an abrupt thermal shock, the resulting MoS2 ensembles retain the original morphology of the rod-shaped crystals of (NH4)2Mo3S13·H2O (forming pseudo-crystals) instead of exploding by the ultrafast discharge of the numerous gaseous species from the polysulfide molecular precursor (Figures 2A-C). Optical AFM images of the pseudocrystals show rough surfaces and random porosities (Figures 2F and 2H). The presence of the pores in the pseudocrystals results from the loss of the gaseous species from the polysulfide precursor. AFM topography reveals the existence of 7 ACS Paragon Plus Environment


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dark and bright contrast regions that correspond to the pores and the MoS2 nanoparticles, respectively (Figures 2G and 2I). The AFM image of the 10 sec sample does not resolve the morphology of the nanoparticles, while the 60 sec sample shows the grain boundaries of the irregular shaped nanoparticles with diameters ranging from 20 to 40 nm. The existence of pores in the pseudocrystal of the nanoparticles can be further validated by surface area (Figure 2J) measurements. MoS2 synthesized at 1000oC exhibits a surface area in the range of 26 to ~35 m2/g. The ~35 m2/g sample exhibits an average pore width (4 V/Å by BET) of ~160 Å, and the pore volume is ~0.14 cm³/g. In our previous paper, we have shown the solid-state conversion of single crystal (NH4)2Mo3S13·H2O to isomorphic pseudocrystals of MoS2 nanoparticles as a function of time and temperature.39 The UV/vis transmission spectra of the exfoliated nanosheets from MoS2 synthesized at 1000oC exhibit two pairs of bands: the first pair appears at 675 and 625 nm corresponding to the A and B excitons, while the second intense broad pair centered at 443 and 413 nm corresponds to the C and D excitonic transitions.39 The porous crystalloids of MoS2 nanocrystals obtained at 1000 oC (treated as a single object) possess an electrical conductivity of σ ~7.4 × 10-4 S/cm (Figure 2K). This value is in agreement with that reported from bulk single crystals of MoS2 along the c-axis (σ ~ 4.6 × 104

S/cm).49 Our measured value is relatively high and similar to the out-of-plane electrical

conductivity of single-crystal MoS2.42 Therefore, despite the high density of pores, the spherical MoS2 nanoparticle aggregates are well connected electrically forming a conductive 3D network in the overall pseudocrystal. The ultrafast synthesis of MoS2 likely also gives rise


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to a large density of defects that may further contribute charge carriers to the porous structure.50 To gain a deeper morphologial and structural insight, we used transmission electron microscopy (TEM) to examine both the 500 oC (12 min) and 1000 oC (10 and 30 sec) synthesized MoS2 (Figure 3). TEM provides further evidence of the formation of randomly oriented MoS2 with no obvious separation between the nanoflakes. The light contrast in varous areas of the TEM images highlights the ultrathin nature of the crystallites (Figures 3A, C and E) for both the 500 and 1000 oC samples. For the 500 oC samples, the flakes consist of 3 to 9 layers, while the 1000 oC at 10 and 30 sec growth samples consist of 3 to 12 layers and 4 to 15 layers, respectively (Figures 3B, D and E). The interlayer distance varies between 0.62 nm and 0.72 nm, which is typical for the MoS2. The length of the flakes ranges from 5 to 25 nm. The MoS2 nanoparticles appear to have random edge-on orientations as they aggregate with other nanoparticles (Figure 3) resulting in large surface curvature in the mesostructure of the nanocrystallites. These features ultimately result in high surface to volume ratios with exposed edge sites that are relevant for hydrogen evolution reaction (HER) and hydrodesulfurization (HDS) catalysis.17-28 Since one of the most promising applications of MoS2 is in high gain photodetectors, we demonstrate utility of the materials in the said devices. In an effort to isolate individual MoS2 nanocrystals and allow the formation of percolating thin films, LPE was performed in a surfactant-free co-solvent system of ethanol (EtOH) and water. This process of exfoliation not 9 ACS Paragon Plus Environment


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only minimizes processing residues but also allows controlled tuning of surface energy by varying the ratio of co-solvents to match with surface energy of 2D materials.51 Figure 4A shows stable dispersions of MoS2 obtained by low-power sonication followed by centrifugation up to 15k rpm (see Methods). MoS2 flakes were then assembled into ~200 nm thick films using vacuum filtration (see Methods). MoS2 photodetectors were subsequently fabricated via evaporation of Au electrodes through shadow mask (Figures 4B and C). MoS2 devices were annealed at 80 oC for 5 min to evaporate residual solvents. In comparison, MoS2 films from LPE of commercial crystals required thermal annealing at 200 oC for 30 min to achieve any measurable electrical conductivity. The conductivity of the resulting MoS2 film is ~6.25 × 10-6 S/cm, which is two orders of magnitude higher than previously reported MoS2 films deposited from commercial powders.34 The high conductivity from nanoporous MoS2 suggests that there may be a large density of defects and trap states in this material due to the ultra-fast growth process. It could also be due to the small size of the nanoparticles that are expected to yield a compact composite film with dense percolating conduction pathways.50 Raman spectroscopy of the thin film confirms that solution-processed MoS2 flakes remain intact (Figure 4D). The large optical absorption coefficient, high charge carrier mobility, and photoresponsive properties of MoS2 make it well-suited for high-performance photodetector applications.11-12 Photocurrent measurements for the as-prepared films were conducted in vacuum using a laser diode (ex = 515.6 nm) with variable power (see Methods). Currentvoltage (I-V) characteristics (Figure 5A) reveal that the current increases by up to an order of


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magnitude under laser illumination (Ilight) as compared to the dark current (Idark), suggesting high photoconductivity. Linear I-V characteristics rule out the formation of significant Schottky barriers with the Au contacts, and thus most of the photocurrent (Ipc = Ilight - Idark) is expected to originate from the MoS2 channel. Laser power (P) dependence of the photocurrent in Figure 5B follows a power law (Ipc ~P1.49). Such super-linear power dependence has been frequently observed in disordered semiconductors including solution-processed MoS2 thin films,34,

52

and the mechanism has been explained by a two-center Shockley-Read-Hall

photoconductivity model that predicts a power factor between 1 and 2.53 The photodetector gain is typically measured in terms of the responsivity, Rλ = Ipc/P, which increases with laser intensity (Figure 5C). The Rλ value of 20 mA/W at 0.75 W/cm2 for the present device surpasses other 2D composite photodetectors by at least two orders of magnitude.34, 54 Certainly, the value that we obtained is lower than that of a single MoS2 layer but is about two orders of magnitude more than the reported photoresponsivity of the solution processed MoS2 nanocrystalline film.13, 55 By controlling the annealing as a function of time and temperature, we previously showed that we can tune the size, shape, morphology, and even defects of the MoS2 nanocrystals.39 Thus, one can expect the tuning of the photodetector property of the MoS2 film can be accomplished by controlling the size, shape and defects of the crystallites. The synthesis of porous MoS2 is scalable, and thus commercially more viable than MoS2 mono layer. Timeresolved photoconductivity measurements also show bi-exponential rise and decay characteristics with large time constants of 1 to 8 sec (Figure 5D and Supporting Fig. S2).36-37 Large time constants are common in highly photoconductive media with a large density of



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traps.36 Overall, the high conductivity and responsivity likely suggests that this ultra-fast growth process induces a large density of defects and trap states in this material that act as dopants and sensitizers. In conclusion, ultrafast synthesis of MoS2 nanocrystals has been accomplished from a chemically homogeneous molecular precursor at temperatures of 1000 oC in times as short as 10 sec. This direct synthetic pathway involves the abrupt thermal shock of (NH4)2Mo3S13·H2O single crystals which convert to 2D MoS2 nanocrystals that aggregate in the same original morphology of the host crystal, despite the extreme far from equilibrium synthesis conditions and the rapid escape of massive amount of gases. The resulting porous pseudocrystals show electrical conductivity that is comparable to that of out of plane MoS2 single crystals. Furthermore, these MoS2 nanocrystals can be exfoliated in solution, allowing the formation of percolating thin films with desirable optoelectronic properties such as high photoresponsivity which can be further improved by reducing the flake to flake resistance. This work establishes an ultrafast synthetic pathway that results in nanostructured MoS2 with potential utility in a range of high-performance applications. NOTE: Authors declare no competing financial interest. ASSOCIATED CONTENT *Supporting Information contains synthesis detailed, various characterization techniques. AUTHOR INFORMATION Corresponding Author 12 ACS Paragon Plus Environment

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*E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the ﬁnal version of the manuscript. ACKNOWLEDGEMENTS M.S.I., M.G.K., and V.D.P. thank the National Science Foundation (NSF) Materials Research Science and Engineering Center (NSF DMR-1720139). SEM, EDS, TEM, Raman, and XPS analyses were performed at the EPIC facility of the NUANCE Center at Northwestern University, which is partially supported by the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205), the Materials Research Science and Engineering Center (NSF DMR-1720139), the State of Illinois, and Northwestern University. V.K.S., J.K., and M.C.H. acknowledge the NSF 2-DARE program (NSF EFRI-1433510) for device fabrication and testing, and the NSF Division of Materials Research (NSF DMR-1505849) for solution processing.



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Figure 1: Broad features of the X-ray powder diffraction of MoS2 nanoparticles synthesized at 500 oC in 11 to 15 min (A) and at 1000 oC in 5 to 60 sec (B) reveal short range ordering of the atoms in the MoS2 nanocrystals. Raman spectra of the MoS2 nano particles synthesized at 500 (C) and 1000 oC (D) exhibit characteristic bands for MoS2. X-ray photoelectron spectroscopy tracks the synthetic route for the MoS2 nanoparticles at 500 (E) and 1000oC (F) from the (NH4)2Mo3S13·H2O molecular precursor.


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Figure 2: Image showing the bulk morphology of the rod-shaped pristine (NH4)2Mo3S13·H2O molecular precursor (A); SEM images of the MoS2 nanoparticles show the inherited morphology of the precursors (B and C). Elemental mapping of the MoS2 crystals shows homogeneous distribution of S and M atoms (D and E). Optical images obtained by AFM show the inherited morphology of MoS2 synthesized at 1000 oC in 10 and 60 sec (F and H). The inset for F and H show the magnified surface structure of the porous aggregated nanocrystallites. AFM topography shows random distribution of the porosity and nanoparticles for the 10 and 60 sec samples (G and I). BET-derived surface area of MoS2 is ~35 m2/g (J), which provides evidence of the porous aggregated nanocrystallites. (K) shows dark



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electrical conductivity, σ ~7.4 × 10-4 S/cm of the porous pseudosingle crystal of agglomerated nanocrystal comparable to that of single crystalline MoS2.

Figure 3: Transmission electron microscopy of the MoS2 synthesized at 500 oC (A and B). C D and E - F images obtained for the samples synthesized at 1000 oC in 10 and 30 sec, respectively. The 12 min (A) and 10 second (B) conversion to MoS2 show randomly oriented 16 ACS Paragon Plus Environment

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aggregated nanocrystals with no visual separation between the nanocrystallites. The 30 sec conversion in E tracks the pathways for the separation of flakes with increasing annealing duration. Figures B, D, F represent the high-resolution images of the MoS2 with “edge-on” random orientations with other aggregated nanoparticles.

Figure 4. (A) Photographs of liquid phase exfoliated MoS2 in a co-solvent system of ethanol and water. Vials show the as-prepared solution and after centrifugation at 5k and 15k rpm. (B) Optical micrograph of large-area MoS2 composite thin film photodetectors. (C) Optical 17 ACS Paragon Plus Environment


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micrograph of MoS2 film photodetectors with channel length and width of 50 μm and 2 mm, respectively. (D) Raman spectrum of a solution-processed MoS2 thin film, showing a gap between E12g and A1g peaks of ~24 cm-1.

Figure 5. (A) Current-voltage characteristics of a MoS2 thin-film photodetector in dark (Idark) and under illumination (Ilight) by a laser with wavelength = 515.6 nm. Photocurrent Ipc = Ilight – Idark. (B) Ipc versus laser power density (P) fitted with a power law Ipc ~ P1.49. (C) Responsivity (Rλ) versus P extracted from the data in (B). (D) Time response of the MoS2 photodetector for different operating biases showing bi-exponential rise and decay characteristics with time constants of ~1 and ~7 sec (see Supporting Figure S2 for details).


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References 1. Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L. J.; Loh, K. P.; Zhang, H., The Chemistry of TwoDimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5 (4), 263-275. 2. Wilson, J. A.; Disalvo, F. J.; Mahajan, S., Charge-Density Waves and Superlattices in Metallic Layered Transition-Metal Dichalcogenides. Adv. Phys. 1975, 24 (2), 117-201. 3. Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S., Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7 (11), 699-712. 4. Yoffe, A. D., Low-Dimensional Systems - Quantum-Size Effects And Electronic-Properties Of Semiconductor Microcrystallites (Zero-Dimensional Systems) and Some Quasi-2-Dimensional Systems. Adv. Phys. 1993, 42 (2), 173-266. 5. Gandi, A. N.; Schwingenschlogl, U., Thermal Conductivity of Bulk And Monolayer MoS2. Epl. 2016, 113 (3). 6. Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A., Single-layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6 (3), 147-50. 7. Zhang, G.; Liu, H. J.; Qu, J. H.; Li, J. H., Two-Dimensional Layered MoS2: Rational Design, Properties and Electrochemical Applications. Ener. Envron. Sci. 2016, 9 (4), 1190-1209. 8. Late, D. J.; Huang, Y. K.; Liu, B.; Acharya, J.; Shirodkar, S. N.; Luo, J. J.; Yan, A. M.; Charles, D.; Waghmare, U. V.; Dravid, V. P.; Rao, C. N. R., Sensing Behavior of Atomically Thin-Layered MoS2 Transistors. ASC Nano 2013, 7 (6), 4879-4891. 9. Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F., Engineering the Surface Structure of MoS2 to Preferentially Expose Active Edge Sites For Electrocatalysis. Nat. Mater. 2012, 11 (11), 963-969. 10. Norskov, J. K.; Clausen, B. S.; Topsoe, H., Understanding the Trends in the Hydrodesulfurization Activity of the Transitin Metal Sulfides. Cat. Letters 1992, 13 (1-2), 1-8. 11. Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C., Emerging Device Applications For Semiconducting Two-Dimensional Transition Metal Dichalcogenides. ACS Nano 2014, 8 (2), 1102-1120. 12. Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A., Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497–501. 13. Xie, Y.; Zhang, B.; Wang, S.; Wang, D.; Wang, A.; Wang, Z.; Yu, H.; Zhang, H.; Chen, Y.; Zhao, M.; Huang, B.; Mei, L.; Wang, J., Ultrabroadband MoS2 Photodetector with Spectral Response from 445 to 2717 nm. Adv. Mater. 2017, 29 (17) 1605972. 14. Wang, S.; Yu, H.; Zhang, H.; Wang, A.; Zhao, M.; Chen, Y.; Mei, L.; Wang, J., Broadband Few-Layer MoS2 Saturable Absorbers. Adv Mater 2014, 26 (21), 3538-44. 15. Ding, Q.; Song, B.; Xu, P.; Jin, S., Efficient Electrocatalytic and Photoelectrochemical Hydrogen Generation Using MoS2 and Related Compounds. Chem. 2016, 1 (5), 699-726. 16. Samad, L.; Bladow, S. M.; Ding, Q.; Zhuo, J. Q.; Jacobberger, R. M.; Arnold, M. S.; Jin, S., LayerControlled Chemical Vapor Deposition Growth of MoS2 Vertical Heterostructures via van der Waals Epitaxy. Acs Nano 2016, 10 (7), 7039-7046. 17. Benck, J. D.; Hellstern, T. R.; Kibsgaard, J.; Chakthranont, P.; Jaramillo, T. F., Catalyzing the Hydrogen Evolution Reaction (HER) with Molybdenum Sulfide Nanomaterials. Acs Catal 2014, 4 (11), 3957-3971. 18. Voiry, D.; Salehi, M.; Silva, R.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M., Conducting MoS2 Nanosheets As Catalysts For Hydrogen Evolution Reaction. Nano Lett. 2013, 13 (12), 6222-7.


Page 21 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19. Li, Y. P.; Yu, Y. F.; Huang, Y. F.; Nielsen, R. A.; Goddard, W. A.; Li, Y.; Cao, L. Y., Engineering the Composition and Crystallinity of Molybdenum Sulfide for High-Performance Electrocatalytic Hydrogen Evolution. ACS Cat. 2015, 5 (1), 448-455. 20. Li, G.; Zhang, D.; Qiao, Q.; Yu, Y.; Peterson, D.; Zafar, A.; Kumar, R.; Curtarolo, S.; Hunte, F.; Shannon, S.; Zhu, Y.; Yang, W.; Cao, L., All The Catalytic Active Sites of MoS2 for Hydrogen Evolution. J. Am. Chem. Soc. 2016, 138 (51), 16632-16638. 21. Benson, J.; Li, M. X.; Wang, S. B.; Wang, P.; Papakonstantinou, P., Electrocatalytic Hydrogen Evolution Reaction on Edges of a Few Layer Molybdenum Disulfide Nanodots. ACS Appl. Mater. Interfaces 2015, 7 (25), 14113-14122. 22. Ye, G.; Gong, Y.; Lin, J.; Li, B.; He, Y.; Pantelides, S. T.; Zhou, W.; Vajtai, R.; Ajayan, P. M., Defects Engineered Monolayer MoS2 for Improved Hydrogen Evolution Reaction. Nano Lett. 2016, 16 (2), 1097103. 23. Yan, Y.; Xia, B.; Ge, X.; Liu, Z.; Wang, J. Y.; Wang, X., Ultrathin MoS2 Nanoplates With Rich Active Sites As Highly Efficient Catalyst For Hydrogen Evolution. ACS Appl. Mater. Interfaces 2013, 5 (24), 12794-8. 24. Chang, Y. H.; Lin, C. T.; Chen, T. Y.; Hsu, C. L.; Lee, Y. H.; Zhang, W. J.; Wei, K. H.; Li, L. J., Highly Efficient Electrocatalytic Hydrogen Production by MoSx Grown on Graphene-Protected 3D Ni Foams. Adv. Mater. 2013, 25 (5), 756-760. 25. Yin, Y.; Han, J. C.; Zhang, Y. M.; Zhang, X. H.; Xu, P.; Yuan, Q.; Samad, L.; Wang, X. J.; Wang, Y.; Zhang, Z. H.; Zhang, P.; Cao, X. Z.; Song, B.; Jin, S., Contributions of Phase, Sulfur Vacancies, and Edges to the Hydrogen Evolution Reaction Catalytic Activity of Porous Molybdenum Disulfide Nanosheets. J. Am. Chem. Soc. 2016, 138 (25), 7965-7972. 26. Choi, Y. H.; Cho, J. S.; Lunsford, A. M.; Al-Hashimi, M.; Fang, L.; Banerjee, S., Mapping The Electrocatalytic Activity of MoS2 Across Its Amorphous To Crystalline Transition. J. Mater. Chem. A 2017, 5 (10), 5129-5141. 27. Kaindl, R.; Bayer, B. C.; Resel, R.; Müller, T.; Skakalova, V.; Habler, G.; Abart, R.; Cherevan, A. S.; Eder, D.; Blatter, M.; Fischer, G.; Meyer, J. C.; Polyushkin, D. K.; Waldhauser, W., Growth, Structure And Stability Of Sputter-Deposited MoS2 Thin Films. Beilstein J. Nanotechnol. 2017, 8, 1115–1126. 28. Heising, J.; Kanatzidis, M. G., Structure of Restacked MoS2 and WS2 Elucidated by Electron Crystallography. J. Am. Chem. Soc. 1999, 121 (4), 638-643. 29. Kang, J.; Sangwan, V. K.; Wood, J. D.; Hersam, M. C., Solution-Based Processing of Monodisperse Two-Dimensional Nanomaterials. Acc. Chem. Res. 2017, 50 (4), 943-951. 30. Kang, J.; Sangwan, V. K.; Wood, J. D.; Liu, X.; Balla, I.; Lam, D.; Hersam, M. C., Layer-by-Layer Sorting of Rhenium Disulfide via High-Density Isopycnic Density Gradient Ultracentrifugation. Nano Lett. 2016, 16 (11), 7216-7223. 31. Kang, J.; Seo, J. W. T.; Alducin, D.; Ponce, A.; Yacaman, M. J.; Hersam, M. C., Thickness Sorting of Two-Dimensional Transition Metal Dichalcogenides via Copolymer-Assisted Density Gradient Ultracentrifugation. Nat. Commun. 2014, 5, 5478. 32. Kang, J.; Wood, J. D.; Wells, S. A.; Lee, J.-H.; Liu, X.; Chen, K.-S.; Hersam, M. C., Solvent Exfoliation of Electronic-Grade, Two-Dimensional Black Phosphorus. ACS Nano 2015, 9 (4), 3596-3604. 33. Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N., Liquid Exfoliation of Layered Materials. Science 2013, 340 (6139), 1420. 34. Cunningham, G.; Khan, U.; Backes, C.; Hanlon, D.; McCloskey, D.; Donegan, J. F.; Coleman, J. N., Photoconductivity Of Solution-Processed MoS2 Films. J Mater Chem C 2013, 1 (41), 6899-6904. 35. Eda, G.; Yamaguchi, H.; Voiry, D.; Fujita, T.; Chen, M.; Chhowalla, M., Photoluminescence from chemically exfoliated MoS2. Nano Lett. 2011, 11 (12), 5111-5116.



Page 22 of 24

36. Buscema, M.; Island, J. O.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A., Photocurrent Generation With Two-Dimensional Van Der Waals Semiconductors. Chem. Soc. Rev. 2015, 44 (11), 3691-3718. 37. Koppens, F. H. L.; Mueller, T.; Avouris, P.; Ferrari, A. C.; Vitiello, M. S.; Polini, M., Photodetectors Based On Graphene, Other Two-Dimensional Materials and Hybrid Systems. Nat. Nano 2014, 9 (10), 780-793. 38. Lee, H. S.; Min, S. W.; Chang, Y. G.; Park, M. K.; Nam, T.; Kim, H.; Kim, J. H.; Ryu, S.; Im, S., MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy Gap. Nano Lett. 2012, 12, 36953700. 39. Islam, S. M.; Cain, J. D.; Shi, F.; He, Y.; Peng, L.; Banerjee, A.; Subrahmanyam, K. S.; Li, Y.; Ma, S.; Dravid, V. P.; Grayson, M.; Kanatzidis, M. G., Conversion of Single Crystal (NH4)2Mo3S13·H2O to Isomorphic Pseudocrystals of MoS2 Nanoparticles. Chem. Mater. 2018, 30, 3847–3853. 40. Chou, S. S.; Huang, Y. K.; Kim, J.; Kaehr, B.; Foley, B. M.; Lu, P.; Dykstra, C.; Hopkins, P. E.; Brinker, C. J.; Huang, J.; Dravid, V. P., Controlling the metal to semiconductor transition of MoS2 and WS2 in solution. J Am Chem Soc 2015, 137 (5), 1742-5. 41. Heising, J.; Kanatzidis, M. G., Exfoliated and Restacked MoS2 and WS2: Ionic or Neutral Species? Encapsulation and Ordering of Hard Electropositive Cations. J. Am. Chem. Soc. 1999, 121, 11720-11732. 42. Li, H.; Zhang, Q.; Yap, C. C. R.; Tay, B. K.; Edwin, T. H. T.; Olivier, A.; Baillargeat, D., From Bulk to Monolayer MoS2: Evolution of Raman Scattering. Adv. Funct. Mater. 2012, 22 (7), 1385-1390. 43. Bertrand, P. A., Surface-phonon Dispersion of MoS2. Phys. Rev. B Condensed Matter. 1991, 44 (11), 5745-5749. 44. Chang, C. H.; Chan, S. S., Infrared and Raman Studies Of Amorphous Mos3 And Poorly Crystalline MoS2. J. Catalysis 1981, 72, 139-148. 45. Moulder, J. F.; Stickle, W. F.; Sohol, P. E.; Bomben, K. D., Handbook of X-ray photoelectron spectroscopy. Physical Electronics, Inc.: Eden Prairie, MN, 1995. 46. Kibsgaard, J.; Jaramillo, T. F., Molybdenum Phosphosulfide: An Active, Acid-Stable, EarthAbundant Catalyst for the Hydrogen Evolution Reaction. Angew. Chem. Int. Edit. 2014, 53 (52), 1443314437. 47. Diemann, E.; Mueller, A., The thermal decomposition of (NH4)2Mo3S13.H2O . nH2O. Z. Anorg. Allg. Chem. 1981, 459, 191-198 48. Hibble, S. J.; Feaviour, M. R., An in situ structural study of the thermal decomposition reactions of the ammonium thiomolybdates, (NH4)2Mo2S12.2H2O and (NH4)2Mo3S13.2H2O. J. Mater. Chem. 2001, 11, 2607–2614. 49. Evans, B. L.; Young, P. A., Optical Absorption and Dispersion Inmolybdenum Disulphide. P oc. R. Soc. Lon. Ser-A 1965, 284 (1398), 402-&. 50. Kim, I. S.; Sangwan, V. K.; Jariwala, D.; Wood, J. D.; Park, S.; Chen, K.-S.; Shi, F.; Ruiz-Zepeda, F.; Ponce, A.; Jose-Yacaman, M.; Dravid, V. P.; Marks, T. J.; Hersam, M. C.; Lauhon, L. J., Influence of Stoichiometry on the Optical and Electrical Properties of Chemical Vapor Deposition Derived MoS2. ACS Nano 2014, 8 (10), 10551-10558. 51. Zhou, K. G.; Mao, N. N.; Wang, H. X.; Peng, Y.; Zhang, H. L., A mixed-solvent strategy for efficient exfoliation of inorganic graphene analogues. Angew. Chem. Int. Edit. 2011, 50 (46), 10839-10842. 52. Bube, R. H., Infrared Quenching and a Unified Description of Photoconductivity Phenomena in Cadmium Sulfide and Selenide. Phys. Rev. 1955, 99 (4), 1105-1116. 53. Bube, R. H., Photoelectronic Properties of Semiconductors. Cambridge University Press: Vambridge, 1992. 54. Li, J.; Naiini, M. M.; Vaziri, S.; Lemme, M. C.; Östling, M., Inkjet Printing of MoS2. Adv. Funct. Mater. 2014, 24 (41), 6524-6531. 22 ACS Paragon Plus Environment

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55. Mak, K. F.; Shan, J., Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides. Nat. Photonics 2016, 10, 216–226.



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Abrupt Thermal Shock of (NH4)2Mo3S13 Leads to Ultrafast Synthesis

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