Homogenous Dispersion of MoS2

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Homogenous dispersion of MoS2 nanosheets in polyindole matrix at air-water interface assisted by Langmuir technique. Richa Mishra, Narsingh R. Nirala, Rajiv Kumar Pandey, Ravi Prakash Ojha, and Rajiv Prakash Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03019 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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Homogenous dispersion of MoS2 nanosheets in polyindole matrix at air-water interface assisted by Langmuir technique Richa Mishra, Narsingh R. Nirala, Rajiv Kumar Pandey, Ravi Prakash Ojha, Rajiv Prakash* School of Materials Science and Technology, Indian Institute of Technology, Banaras Hindu University, Varanasi-221005, India *Email: [email protected]

Abstract: Two-dimensional (2D) inorganic layered materials when embedded in organic polymer matrix exhibit exotic properties that is grabbing contemporary attention for their various applications. Here, nanosheet morphology of molybdenum disufide (MoS2) synthesized via onepot facile hydrothermal reaction are exfoliated in benign aqueous medium in presence of indole to obtain stable dispersion. These exfoliated nanosheets then act as host to template the controlled polymerization of indole. The pre-assembled MoS2-polyindole (MoS2-PIn) nanostructures are re-organised at air-water interface using Langmuir method to facilitate maximum interfacial interaction between nanosheet and polymer. This report emphasizes large area, homogenous dispersion of uniform sized MoS2 nanosheets (40-60 nm diameter) in PIn matrix and formation of stable and uniform film via Langmuir-Schaefer (LS) method. These self-assembled, MoS2 decorated PIn LS films are characterized using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The fabricated LS films in sandwiched structure Al/MoS2-PIn/ITO as Schottky diode portrayed remarkable enhancement in charge transport properties. Our study illustrates the potential of MoS2-PIn LS film in electronic applications and opens new dimension for uniform dispersion of 2D materials in other polymers via Langmuir method for device fabrication and enhancement of electrical property. Keywords: Air-water interface; In-situ decoration; Langmuir-Schaefer film; MoS2 nanosheets; Polyindole; Self-assembly.

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Introduction: Advent of new era of materials science is manifested by unearthing of nanomaterials with applications in electronics, sensors, electrocatalysis, optoelectronics, bio related applications field, etc.1-4 These nano sized materials allow us to tailor their physical properties by varying experimental conditions, thus enabling their applicability in various fields. Further, it is conducive to employ rather green synthesis protocols for these nanostructures utilizing environmental friendly precursors and medium.5,6 Among the nanomaterials, 2D layered materials (graphene and its analogues, like transition metal dichalcogenides) have grabbed greater attention due to their distinctive material properties and their applicability in sensors, electronics,

catalysis,

energy-storage

and

optoelectronic

fields.7-13

Among

transition

dichalcogenides, molybdenum disulfide (MoS2) displays highest conductivity and has relieved much from the scalability and economic issues to a large extent. 8-11 Various reported synthetic procedures for MoS2 are micromechanical exfoliation, liquid phase exfoliation, physical vapor deposition, chemical solution process, chemical vapor deposition, etc. that require severe experimental conditions, but hydrothermal route (solution-based process) offers best control over MoS2 nanosheet morphology in mild condition with maximum exfoliation in liquid phase. 8,9,12-17 MoS2 in bulk state is a 3 atoms (S-Mo-S) layered structure stacked via van der Waals interactions, those need to be intervened in order to exploit its few layers intrinsic properties either by exfoliating chemically or physically.14,17 MoS2 nanosheets with minimal interference from the counter ions synthesized via hydrothermal route are generally few layered, that need to be exfoliated or dispersed in proper dispersion medium to avoid the risk of re-stacking.13,14 Coleman et al.14 has demonstrated exfoliation via sonication in non-volatile organic solvents (non-ecofriendly) to obtain single or a few layered nanosheets of MoS2, but the organic wrapping 2 ACS Paragon Plus Environment

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layers blocked its scope for usage in further applications. Later, Zhang and coworkers8, reported the exfoliation of MoS2 in an ethanol-water mixture that could be removed easily later. Usually, solvent assisted exfoliation weakens the interlayer interaction and enables the intercalation of small ions or molecules within the layers that can be removed later. Thus, dispersion medium and matrix play a significant role in controlling the extent of exfoliation for appropriate application.8 There have been many attempts to blend various polymers or form composite with other materials inorder to obtain desired properties.18-23 Similarily, MoS2 (possesing large surface area, high electrical conductivity, and heterogeneous electron transfer properties) when reinforced with various polymers have significantly enhanced composite’s thermal, mechanical and electrical properties synergistically.8,24-26 But the high cost, low productivity and poor fabricating techniques have limited their merits.8,14 Recently, Tang et al.26 have prepared polypyrrole-MoS2 nanocomposite via controlled polymerization of the monomer adsorbed on MoS2 sheets (exfoliated via

lithium ion). Here, we have adopted hydrothermal route directly yielding

nanosheet morphology, that are later exfoliated in ethanol-water mixture in the presence of monomer (indole) discarding the usage of other intercalating agent. The reason for interest in polyindole (PIn) among the most studied conducting polymers, is their excellent thermal stability, high redox activity, fast electrochromic response and better environmental stability with their prominence in sensors, electrocatalysis, electrochromic devices, energy storage, biosensor domains till date.29-36 Besides all these, poor electrical conductivity of PIn relative to Sulfur containing polymers with alkyl chain like Poly (3-alkylthiophene) (P3-AT), diminishes PIn family significance in organic electronics.36-40 In the past few decades, remarkable enhancement in redox and electrical properties of PIn family has been dealt via composites formation with 3 ACS Paragon Plus Environment

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CNT, graphene,etc. various conducting materials.29 Nevertheless, high cost, poor synthetic yield and failure in large area film formation of these materials reduce their industrial implication. These above issues still demand a simple, environmently benign and good synthetic yield protocols for PIn composite formation to enhance their film stability and conductivity. The major hurdle in their usage in electronic devices is their lack in formation of thin flexible films due to their poor solution processability in pure form (no surfactant). Solution processability issue has been already addressed in our previous report that clearly explains the formation of thin film of PIn via Langmuir-Blodgett (LB) method.35 Though the report states the stable film formation of PIn at air-water interface, their uniformity for multiple layers deposition onto substrates can be improved by varying the substrate nature and lifting style employing Langmuir-Schaefer (LS) method. Some previous reports have also preferred LS method for non-amphiphilic films like polymers as it minimizes the disruptive force disturbing the uniformity of rigid monolayer during vertical upliftment.27,28 Herein, we have employed LS method for film fabrication and maximum interfacial interaction between MoS2 and the dispersion matrix (PIn) at molecular level. Previously reported results promoted the addition of surfactants to MoS2 nanosheets to prevent their sinking in the trough and to polymers to aid their processability and orientation alongwith compromise with their intrinsic electrical properties.27,28,35,41,42 This issue has been addressed here properly via composite and film formation in their pure form without compromising with their intrinsic properties. Non-covalent interfacial interactions between MoS2 nanosheets and polymer matrix have been optimised via Langmuir in order to attain the desirable properties of the nanocomposite formed. 14,27,28 Herein we have demonstrated the insitu oxidation polymerization of indole monomer in presence of MoS2 nanosheets (synthesized by green strategy) in aqueous medium. Indole gets adsorbed 4 ACS Paragon Plus Environment

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on MoS2 nanosheets assisting its exfoliation and these nanosheets in turn act as template for insitu oxidative polymerization of indole. This work highlights the importance of Langmuir technique in attaining maximum interfacial interaction between MoS2 nanosheets and PIn at airwater interface. To the best of our knowledge, there is no report available till date on indole assisted exfoliation of MoS2 nanosheets and their homogenous dispersion via Langmuir method. Our work emphasizes on the simultaneous exploitation of excellent intrinsic properties of both MoS2 and PIn together via nanocomposite formation to attain desirable large area, thin film via LS method. We believe that this approach describing Langmuir assisted dispersion of 2D materials into polymer matrices (less soluble and lower conductivity) may content the desire to obtain their large-area uniform, compact and stable films for electronic device applications. Experimental Section Materials: Indole

monomer,

ammonium

persulfate

((NH4)2S2O8,

APS),

sodium

molybdate

(Na2MoO4.2H2O), L-Cysteine (HO2CCH(NH2)CH2SH) were purchased from Sigma Aldrich, India. Milli-Q (resistivity = 18.0 Ω cm−1 and pH 7.0) water has been used in all experiments. Synthesis of MoS2 nanosheets: MoS2 nanosheets are synthesized using facile, single step hydrothermal method.43 Sodium molybdate (Na2MoO4.2H2O) and L-cysteine (HO2CCH(NH2)CH2SH) (1:2 w/w) are dissolved in 25 ml of Milli-Q water in separate beakers and kept on constant stirring for 15 minutes. Then the two solutions are mixed slowly with continuous stirring maintaining temperature at ~40°C and pH at 3.0 with concentrated HCl. This solution is then transferred into stainless steel lined Teflon autoclave (100 ml capacity) maintained at ~200°C for 42 hours. After this, the content is left to cool down naturally, finally we obtain dark brown precipitate (nanosheets) settled at the bottom. 5 ACS Paragon Plus Environment

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These nanosheets are washed repeatedly via centrifugation and re-dispersion in water to obtain final product. Synthesis of MoS2-PIn nanocomposite: For the in-situ oxidative chemical polymerization, 0.05 M indole monomer was dissolved in minimal volume of EtOH and added to pre ice-cooled (~5ºC), nitrogen purged 0.1N HCl in a stoppered conical flask that already contained MoS2 nanosheets (0.4 mg/mL). Then this system was subjected to ultrasonication for 30 minutes followed by dropwise addition of 0.1 M APS solution (in 5mL distilled water) with continuous stirring. This setup was kept on stirring over night (24 h) in ice-bath (~5ºC) for complete polymerization. The brown precipitate so obtained was allowed to settle down before it was filtered and washed with water and ethanol several times. The final product was dried under vacuum and yield was calculated as 74%. The same procedure was followed for synthesis of pure PIn to further compare the characterizations with MoS2-PIn composite. Assembly of PIn and MoS2-PIn at air-water interface: PIn and MoS2-PIn were assembled at air-water interface by preparing its solution (dispersion) with a ratio of 1:10 DMSO/chloroform via co-solvents maintaining the final concentration of 1.0 mg/mL. Langmuir trough and all the substrates for film deposition were cleaned well prior to film study and the trough was filled with Milli-Q water (resistivity = 18.0 Ω cm−1 and pH 7.0, at 25 ◦C). The dispersion (400 µL) was spread onto water subphase in LB trough (model Apex, LB2007DC) with rate of 50 µL/min with hamilton syringe keeping barriers far apart. After complete spreading, the system is left undisturbed for 15 min to allow the material self-assemble itself via non-covalent interactions. Then, the barriers were compressed at 10 mm/min and the


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surface pressure vs area per molecule (π-A) was monitored by a sensor (wilhemy plate). The substrates were stamped over the surface in horizontal position and lifted slowly at an angle of 45° with the surface. This process is repeated as per number of layers required for various characterizations. These deposited films are dried in glove box (nitrogen atmosphere) at 40°C for 2 hours. Device fabrication: PIn and MoS2-PIn LS films were deposited at 30 and 40 mN/m film respectively as optimum surface pressure (SP) observed in π-A isotherm. For the charge transport study across the film, 5 layers LS films of PIn and MoS2-PIn were developed in sandwiched structures Al/PIn/ITO and Al/MoS2-PIn/ITO. Circular shadow masking of Al dots (electrode) of diameter 1 mm and thickness of 60 nm monitored via digital thickness monitor (DTM) was achieved on polymer and nanocomposite LS films via thermal deposition technique (Hind High Vac, India, model: 12A4D) maintaining pressure at 3x10-6 mbar. Characterization techniques: MoS2 nanosheets were synthesized in hydrothermal reactor (Hydroion Scientific Instrument Co. Ltd.). TEM micrographs of drop casted were investigated on FEI, TECHNAI G2 20 TWIN (Czech Republic) TEM instrument at accelerating voltage of 200 kV. Image acquirement and processing were performed using the Gatan Digital Micrograph software. XRD pattern was obtained on Miniflex 600 diffractometer, CuKα (Kα =1.54056 Å) radiation at scan rate of 3°/min. UV-visible (UV-Vis) spectrum was investigated by UV-Vis spectrophotometer (UV2600, Shimadzu) and vibrational spectrum using a Thermo 5700 FT-IR spectrometer, Germany model. Thermal stability was monitored in inert atmosphere at 10°C min-1 heating rate on Mettler Toledo (TGA STARE System, Switzerland). Cyclic voltammograms were recorded with 7 ACS Paragon Plus Environment

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Autolab (PG STAT 302, The Netherlands) with NOVA software. Glassy carbon electrode (GCE) as working and platinum (Pt) as counter electrodes were polished well with alumina powder followed by ultrasonication in acetone and distilled water prior to experiment. Langmuir trough of working area 650 mm x 200 mm (Model No. LB-2007DC; Apex) fitted with double barrier was employed to determine optimum surface pressure for film deposition. The LB system with WinLB software provides expansion/compression resolution of 0.005 mm and surface pressure resolution of ±0.02 mN/m. The films were lifted manually in horizontal fashion representing as LS technique. The surface structure and phase contrast micrograph of LS films were studied on atomic force microscope (AFM) (model: NT-MDT, Russia). Monolayer LS film thicknesses on SiO2 substrate were checked via AFM tip. Current-voltage (J-V) characteristics of LS films in sandwiched structure of Al/PIn/ITO and Al/PIn/MoS2/ITO were studied on Keithley (model 2612A). Results and discussion Characterization of MoS2 nanosheets: As observed from TEM micrograph (Fig.1a-d), synthesized MoS2 reveals nanosheet like structure.8,14,17 Selected area electron diffraction (SAED) pattern (inset of Fig.1d) displays ringlike mode confirming their polycrystalline nature. STEM-HAADF image, elemental mapping image (Fig.S1) and elemental analysis (Fig.1e) confirm the presence of Mo and S together as MoS2. X-ray powder diffraction (XRD) pattern (Fig.1f) of the bulk MoS2 shows diffraction peaks at 2θ = 14, 33, 44 and 59° that can be unambigously assigned to (002), (100), (103) and (110) planes of hexagonal phase of MoS2 (JCPDS No. 37-1492) of MoS2.13,14,24,26,43,44 HRTEM measurement was also done to get insight of the crystal structure of MoS2 that clearly shows stacked layers. The interlayer distance (marked in yellow bars) was measured using Gatan 8 ACS Paragon Plus Environment

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software and found to be around 0.63 nm from an intensity profile graph.13 This d-spacing corresponds to (002) plane which is in good agreement with the present XRD results (Fig.1f) obtained for MoS2 and previous reports.44,45 On the basis of obtained results, we proposed a plausible mechanism (supporting information,S1) that explains the significant catalytic role of H+ ions in formation of MoS2 nanosheets. Further, low pH condition also negates the formation of Mo oxides thus favoring the formation of MoS2. 43,44 Interaction studies of MoS2-PIn nanocomposite: Interactions between the nanosheet and PIn is investigated via UV-Vis and FTIR spectroscopy. UV-Vis spectra (Fig.2) of precursors in pristine form (PIn and MoS2 nanosheets) and their MoS2-PIn composite was recorded to investigate the spectral changes after the composite formation. MoS2 nanosheets exhibited characteristic absorptions at 390, 445, 608 and 660 nm (Fig.2 a.(i) red star marked) which are absent in composite signifying the interaction of nanosheets with PIn.14,24 Fig.2a. (ii) showed characteristic absorptions of PIn at 253, 306, 347 (ππ*), 398 nm (n- π*) and polaronic transition (450-550 nm) consistent with reported literatures.29-33 These absorptions were consistent with the composite (Fig.2a (iii)) justifying the polymerization of indole along with 20 nm shift in the first peak (characteristic of indole; blue star marked). This red shift could be attributed to the increased conjugation length and controlled polymerization of indole on MoS2 nanosheets.30,33 FTIR spectroscopy aided in investigation of the non-covalent interaction between the PIn and MoS2 nanosheets in their composite. For pure MoS2 nanosheets (Fig. 2b (i)), peak at 470 cm-1 was assigned to Mo-S stretching vibration and the other two peaks at 1633 and 3434 cm-1 correspond to O-H stretching vibrations due to intercalated water molecules.24,46,47 Characteristic N-H stretching vibration at 3400 cm-1 for PIn and 3403 cm-1 for MoS2-PIn (Fig.2b (i) and (ii)) 9 ACS Paragon Plus Environment

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validate 1,3 linkage of monomer unit during polymerization.30,33 Both Mo-S vibration peak and other characteristic vibration peaks for PIn are consistent in the MoS2-PIn composite (Table.1) along with slight shifts reflecting successfully polymerization of indole onto MoS2 nanosheets. These red shifts (3-15 cm-1) towards higher wavenumber can be attributed to interaction between the host MoS2 nanosheet and guest PIn.14,30,35 XRD pattern of pure PIn and MoS2-PIn nanocomposite powders are shown in Fig.3a. Diffraction of pure PIn (Fig.3a (ii)) displayed characteristic weak reflections in the range of 15-25°.30,33 The MoS2-PIn composite unambiguously showed diffraction peaks corresponding to PIn along with some extra peaks suggesting successful polymerization growth on MoS2 nanosheets with controlled kinetics. While MoS2 did not give its signature in composite diffraction pattern. There are two reasons for this: (1) PIn matrix prevented the re-stacking of as-prepared MoS2 nanosheets (also evident from TEM image; Fig.4) and (2) MoS2 is present in very low wt% composition in composite viz. beyond the detection limit of our instrument.26 This clearly indicates that MoS2 nanosheets are few atomic layers in the composite. This MoS2 guided polymerization of indole in a controlled fashion yielded smaller polymeric fragments as compared to the chemically synthesized pristine PIn. This statement is in support with the FAB mass result that states 5-6 monomer units linked together. This result also confirms that MoS2 did not affect the PIn diffraction peak positions reflecting that it was not incorporated into the polymer structure, instead it formed non-covalent linkage during polymerization. Thermal analysis shown in Fig.3b clearly distinguishes the degradation temperatures for PIn and MoS2-PIn composite revealing the improved stability of the nanocomposite synthesized. At marked temperatures (a) 190°C, the weight loss difference was ~4% but at higher temperature (b) 600°C the difference raised to 12% indicating enhancement in thermal stability of the 10 ACS Paragon Plus Environment

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nanocomposite. All the above results validate the existence of non-covalent interaction between the polymerized indole and MoS2 nanosheet with synergistic enhancement in conjugation length, polymer crystallinity and thermal stability.26,29-33 TEM micrographs (Fig. 4) of the in-situ polymerized PIn on MoS2 as template shows dark black patches representing MoS2 nanosheets that are embedded onto the lighter backgrounds representing PIn. Fig. 4c (higher resolution) clearly reveals the upper few layers of MoS2 over the base layer of PIn. STEM image and elemental mapping for the same as shown in Fig.S1 supports the plausible mechanism (Scheme. 1) of homogenous growth of PIn over nanosheets.26 The image clearly corresponds to differents elements such as Mo, S, C and N uniformly distributed over the patched area. MoS2 concentrations in increasing order namely MoS2-PIn-1, MoS2-PIn-2 and MoS2-PIn-3 were also employed for nanocomposite formation and optimized using CV and TEM measurements. Cyclic voltammograms for pure PIn, MoS2 and various MoS2-PIn-1,2,3 combinations were obtained in a 5 mM [Fe(CN)6]3−/4−mixture (1:1); 0.1 M KCl (see Fig.S4). This clearly illustrates the enhancement in both anodic and cathodic peak current upto highest composition MoS2/PIn-3 and decrease in potential separation (peak to peak) as compared to pure PIn and MoS2. This can be attributed to the synergistic effect between the components of nanocomposite leading to increment in conductivity.8,25 TEM images of the above two extreme compositions (1 and 3) reveal very low dispersion of nanosheets in PIn matrix (see Fig.S5 a), whereas aggregation of MoS2 (see Fig.S5 b) in the highest composition MoS2/PIn-3 (dark patches marked with arrow represent MoS2). Hence, MoS2-PIn-2 composition representing well grown PIn over MoS2 nanosheets (Fig.4) consented by TEM and CV studies together, is the optimum one to carry on other studies. 11 ACS Paragon Plus Environment

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On the basis of above results, we proposed a schematic (Scheme.1) describing MoS2 nanosheet synthesis via facile hydrothermal method, addition of indole to the MoS2 nanosheets followed by ultrasonication. Herein, ultrasonication in an ethanol/aqueous mixture in presence of indole facilitated adsorption of indole unit onto nanosheets coordinating to the metal chalcogenide center via nitrogen. This aqueous dispersion when subjected to APS (oxidant), assisted in controlled and assembled synthesis of PIn particles on MoS2 nanosheets serving as template. Once the polymer was formed, it directed the orientation of nanosheets. Monomers adsorbed on template, prior to polymerization results in controlled and ordered polymer formation playing significant role in solution processability.35,48,49 Floating film isotherm studies: Molecular interaction at air-water interface was investigated via pressure-area (π-A) isotherm study (Fig.5) of PIn and MoS2-PIn composite in LB trough. Various parameters that determine the film properties were optimised according to our previous reports.35,36 As per our previous report on Langmuir-Blodgett (LB) film of PIn, it was observed that vertical lifting of the film resulted in decrement of transfer ratio for multilayer.27,28,35 This was probably due to reverting back of the film into subphase during vertical upliftment. So, horizontal or LS method of film lifting especially for non-amphiphilic molecules is highly preferrable to obtain good quality films.27,28,35 Chemically synthesized PIn and MoS2-PIn were taken in very dilute concentration (~1.0 mg/mL) for spreading over langmuir trough. Scheme 2. very well depicts the process of self-assembly of MoS2-PIn initially and after barrier compression at the rate of 10 mm/min. On compression, the floating film islands of composite start arranging themselves to form continous film. Pressure-area (π-A) isotherm of both PIn and MoS2-PIn in Fig.5(a) and (b) illustrates the effect of increasing surface pressure on mean monomeric area of the sample. Both isotherms 12 ACS Paragon Plus Environment

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have been sub-divided into four distinct regions as per our earlier reports marked as region I for gaseous phase (molecules far apart) reflecting weak intermolecular interactions, II for liquid condensed phase, III for solid condensed and IV for solid collapse phase (monolayers start collapsing).35,36 As evident from Fig.5, mean monomeric area for (b) MoS2-PIn is larger than that for (a) PIn that can be attributed to the MoS2 nanosheets forming strong secondary interactions with PIn occupying larger surface coverage alongwith reduction in film rigidity. Chain extension of PIn anchored on MoS2 nanosheet occurs widely over the subphase giving rise to monolayer and uniform film. Being non-amphiphilic, PIn alone forms pseudomonolayer (thicker layer) which is evident from AFM results discussed later.8,14,35 Moreover, higher collapse point for the MoS2-PIn isotherm also demonstrates relative better monolayer stability at air-water interface than PIn. Blodgett style (vertical deposition) film lifting leads to aggregate and discontinous film formation especially in the case of multilayer deposition.27,48 This was consistent with our observations of diminishing transfer ratio from 0.95 to 0.45 for multilayer deposition as observed in Table.S1. Moreover, there is risk of monolayer to revert back into the subphase during vertical lifting. Therefore, for non-amphiphilic molecules like PIn, LS technique is more preferable. The substrate is stamped horizontally and lifted very slowly (manually) making an angle of 45° with the subphase. This is referred as semi-dry process as the substrate lifting doesnot directly disrupt the langmuir monolayer unlike vertical uplifting. Topographical and structural analysis of LS films: TEM and AFM studies were performed to investigate the uniform dispersion of MoS2 nanosheets in PIn matrix. TEM micrographs (Fig.6a and b) of PIn LS film shows its expanded sheet like structure wrinkled at places probably due to lifting flaws on carbon grid. TEM image (Fig.6c and d) of MoS2-PIn LS films clearly exhibit MoS2 nanosheets (dark patches) decorated 13 ACS Paragon Plus Environment

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uniformly in PIn matrix. These nanosheets are uniform in size ranging from 40-60 nm diameter decorated uniformly. Percentage of surface area coverage of MoS2 nanosheets on PIn LS film as calculated via ImageJ from Fig.6(d) was found to be 20±3 %. Moreover, STEM image and elemental mapping of the MoS2-PIn LS films (see Fig.S4) also validated the dark patches as MoS2 nanosheets on PIn matrix. Surface topography images of PIn and MoS2-PIn LS films were investigated along with Phase contrast images simultaneously as shown in Fig.7(a-d). AFM images of PIn LS films (Fig.7a) reveal the uniformity and compactness with average height profile of ~ 4.5 nm similar to our previous report.35 Average height profile of nanocomposite comes to be ~ 3.5 nm which suggest the formation of well dispersed three or fewer layers of MoS2 on PIn matrix. This finding also complements the larger mean monomeric area observed for composite (Fig.5b). Moreover, nanocomposite forms more uniform and smooth film as compared to pure polymer that is clearly evident from rms roughness values of 0.895 nm in comparison to 0.200 nm obtained for PIn. The difference in topography from previously reported work on PIn can be attributed to different lifting method of langmuir films and different synthetic route for polymerization of indole.35 Phase contrast image (Fig.7b) of PIn LS film doesnot reveal much information except the same composition throughout. AFM image of MoS2-PIn nanocomposite LS films (Fig.7c) shows white patches of uniform size homogenously distributed over the matrix. The same could be observed in its phase contrast image (Fig. 7d). Phase contrast images sensitive to compositional difference, very well depict the presence of two phases in nanocomposite. This observance strongly supports the TEM result (Fig.6c and d) signifying homogenous distribution of uniform sized MoS2 nanosheet on PIn matrix facilitated by Langmuir method self-assembly. Electrical Characterization: 14 ACS Paragon Plus Environment

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The charge transport properties and electronic parameters of layer by layer deposited PIn LS film was investigated from sandwiched structure of Al/PIn/ITO and Al/PIn-MoS2/ITO. 38,48-51 Fig.8a shows the J-V characteristics of PIn LS film and PIn-MoS2 LS film respectively. However, Fig.8b demonstrate the semi-log plot of Fig.8a under dark condition at room temperature. The fabricated sandwiched structure fabricated device ITO/Semiconductor/Al Schottky junction is shown in the schematic diagram (inset of Fig.8a). The rectifying behavior of the sandwiched structure devices were evaluated by assuming the standard thermionic emission theory which can be expressed as38,49 =

=

− 1 (1)

∆ (2) ∆

Where J0 denotes the saturation current density in absence of external bias, η is the ideality factor of device, q is electronic charge, and V is the applied voltage, T is temperature in Kelvin and k is the Boltzmann constant. J0 is associated with the Schottky barrier height, ɸB as

= ∗ exp Or

&' =

! $ (3) "# *

ln ,∗+# - $ (4) "#

Where A* is the effective Richardson constant (value=120 A cm−2 K−2). Based on the above equations, we calculated the saturation current density J0, barrier height φB and ideality factor η of devices Al/PIn/ITO and Al/PIn-MoS2/ITO given in Table 2. The ideality factor of Al/PIn/ITO device demonstrates deviation from ideal diode behaviour and much higher value in comparison to ideal diode ideality factor (=1). However, the ideality factor 15 ACS Paragon Plus Environment

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of both devices viz Al/PIn/ITO and Al/PIn-MoS2 is larger than one. It is important to note that the ideality factor between 1.0 and 2.0 arises due to carrier drift diffusion process and the Sah– Noyce–Shockley generation recombination process. There are several phenomena that can attribute to the exceeding value of ideality factor such as trap-assisted tunneling, carrier leakage, and barrier inhomogeneity, etc. Therefore, ideality factor more than 2 in both case LS films is due to layer by layer deposition (upto five layers) causing increase in trap-assisted tunneling, carrier leakage, and barrier inhomogeneity. Multilayer deposited PIn LS films devices with the deviation from ideal behavior is consistent with our previous assumptions.48,49 However, addition of MoS2 nanosheets in polymer film matrix induces betterment in the ideality factor from 4.55 to 3.54. The current obtained for Al/MoS2-PIn/ITO devices demonstrate significantly ~9 fold enhancement in forward current as compared to pure Al/PIn/ITO. This enhancement in forward current can be explained by using Mott’s law of variable-range hopping for variety of disordered materials. It is noteworthy, that three type of conduction pathways are possible in the MoS2-PIn materials. First one, along the conjugation length of polymer chains; second one, hopping between the polymer chains and third one, conduction via MoS2 conducting filler. This conducting filler plays a vital role in this remarkable enhancement in conductivity because fillers provide an easy path for charge conduction. It means nanofiller MoS2 contributes remarkably towards the significant enhancement in current. Further, addition of MoS2 nanofillers cause improvement in saturation current and successive reduction in barrier height from 0.683 to 0.676 as shown in Table 2. Finally, it is noteworthy to mention that the MoS2-PIn LS film demonstrated 10 fold enhancement in rectification ratio as compared to pure PIn LS film. The remarkable increment in


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rectification ratio can be attributed to the formation of smooth surface, reduction in barrier height and enhancement in charge injection efficiency.

Conclusions MoS2 nanostructures successfully synthesized via hydrothermal reaction that yielded its nanosheet morphology, guided the controlled growth of PIn. The non-covalent interaction between MoS2 nanosheet and nitrogen atom of indole unit was the driving force of exfoliation in aqueous medium and assembly at air-water interface. These pre-assembled structures were tailored using Langmuir technique for large area uniform dispersion of MoS2 nanosheets on PIn matrix. This hypothesis was validated by TEM and AFM results. LS technique offered remarkable opportunity to nanocomposite to self-assemble and disperse themselves at the airwater interface. MoS2 nanosheets formed larger interface with higher surface to mass ratio on PIn matrix in langmuir trough leading to stable and uniform film. Furthermore, this simple approach for homogenous distribution of uniform sized MoS2 nanosheets in PIn matrix exhibited significant enhancement in electrical properties of the polymer. We hope that this work will encourage the usage of Langmuir technique for obtaining homogenously dispersed 2D materials in other polymer matrices too. Acknowledgements Richa Mishra is thankful to DST-INSPIRE, India for providing her fellowship and the authors also acknowledge Central Instrument Facility (CIF), IIT (BHU) for providing TEM and AFM facilities. We are also thankful to Dr. Chandan Upadhyay for providing us Langmuir facilities and fruitful discussions.


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Supporting Information Supporting information includes HAADF-STEM and elemental mapping image of MoS2, drop casted and LS film of MoS2-PIn nanocomposite, CV curves of various combinations of MoS2PIn (1,2,3) nanocomposite in [Fe(CN)6]3−/4− and TEM micrographs of LS films of (a) MoS2-PIn1 and (b) MoS2-PIn-3 combination. It also includes table summarising transfer ratio of different number of layers (I, III, V) deposited by LB and LS method.

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Legends to Figures: Figure.1 Characterizations of MoS2 nanosheets synthesized via hydrothermal process (a-d) TEM micrographs at different scales, inset of (d) showing SAED pattern, (e) EDS elemental analysis, (f) XRD pattern. Figure.2 (a) UV-vis and (b) vibrational spectroscopy of (i) MoS2 nanosheets, (ii) PIn and (iii) MoS2-PIn composite. Figure.3 (a) XRD pattern and (b) TGA of (i) PIn and (ii) MoS2-PIn nanocomposite (bulk). Figure.4 TEM micrographs of drop casted MoS2-PIn nanocomposite at different scale bars (a) 500 nm (b) 200 nm (c) 50 nm. Figure.5 Pressure vs. Area (π-A) isotherm of (a) PIn and (b) MoS2-PIn nanocomposite. Figure.6 TEM micrographs of LS films of (a,b) PIn and (c,d) MoS2-PIn. Figure.7 Tapping mode AFM and phase contrast images of LS films of (a,b) PIn and (c,d) MoS2-PIn. Figure.8 (a) Current density-voltage (J-V) measurement (b) Semi-log plot for 5 layered LS film fabricated Al/PIn/ITO (black curve) and Al/ MoS2-PIn/ITO (red curve) device structure. Table 1. Vibrational spectroscopy band assignment of MoS2, PIn and MoS2-PIn. Table 2. Electronic parameters of device. Scheme 1. Schematic presentation of synthesis of MoS2 nanosheets, their exfoliation and MoS2PIn nanocomposite formation. Scheme 2. Schematic representation of assembly of MoS2-PIn on water subphase initially, after barrier compression and LS film fabrication. 22 ACS Paragon Plus Environment

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Figure.1


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Figure.2


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Figure.3


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Figure.4


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Figure.5


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Figure.6


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Figure.7


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Figure.8


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Band assignment (in cm-1) Mo-S str O-H str N-H str N-H bend C=C str (aromatic) C-N str C-H bend Table.1

MoS2 470 1633, 3434 -

PIn 3400 1616 1600-1400 1454 741

MoS2-PIn 470 3403 1621 1615-1415 1461 747

Parameter→ Devices ↓

η

J0 (A/cm2)

ΦB (eV)

Rectification ratio

Al/PIn/ITO

4.55

1.15x10-6

0.684

22

Al/MoS2-PIn/ITO

3.54

1.45x10-6

0.676

240

Table.2


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Scheme.1


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Scheme.2


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Homogenous dispersion of MoS2 nanosheets in polyindole matrix at air-water interface assisted by Langmuir technique Richa Mishra, Narsingh R. Nirala, Rajiv K. Pandey, Ravi Prakash Ojha, Rajiv Prakash* School of Materials Science and Technology, Indian Institute of Technology, Banaras Hindu University, Varanasi-221005, India *Email: [email protected] Table of content


Homogenous Dispersion of MoS2

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