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Liquid Phase Exfoliation of MoS2 Nano-sheets and Observation of Resistive Switching Memory in MoS2 Nano-sheets-PVDF-HFP Composite Films
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ScienceDirect Materials Today: Proceedings 18 (2019) 5447–5453
www.materialstoday.com/proceedings
ICMPC-2019
Liquid Phase Exfoliation of MoS2 Nano-sheets and Observation of Resistive Switching Memory in MoS2 Nano-sheets-PVDF-HFP Composite Films Deepaka, Rajesh Deba, Manjula G. Naira, Sudipta Halderb, A. L. Sharmac and Saumya R. Mohapatraa* b
a Department of Physics, National Institute of Technology Silchar, Silchar – 788010, Assam Department of Mechanical Engineering, National Institute of Technology Silchar, Silchar – 788010, Assam c Centre for Physical Sciences, Central Univeristy of Punjab, Bhatinda - 151001
Abstract To realise the true and versatile application potentials of 2-D materials like MoS2, the preparation of mono-layer or few layer nano-sheets holds the key. In the present study, we report the successful exfoliation of bulk MoS2 into few layer nano-sheets by adopting a two-step process of grinding and then two hours of ultrasonication by using a probe sonicator. The yield of exfoliation was 32 mg/10 ml and the exfoliated MoS2 was very stable without any re-stacking for more than one month. Further, the exfoliated MoS2 nano-sheets are added to Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) to prepare the nanocomposite thin films. Two-terminal devices are prepared with ITO and aluminium as bottom and top electrodes, respectively on a plastic substrate. Its electrical properties are investigated to observe the electrical bistability. Nanocomposite based devices showed bipolar resistive switching memory. For composite film with 1wt.% of MoS2, resistance switching is observed with SET and RESET voltages at 2.74 V and -0.9 V respectively. Resistance ratio of the order of 104 is achieved between the ON and OFF states. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: MoS2 nano-sheets; Liquid-phase method; polymer composite; resistance switching memory.
* Corresponding author. Tel.: +91-3842-242914; Fax: +91-3842 22479. E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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1. Introduction Since the discovery of graphene in 2004 [1] and subsequent exploration of its alluring properties, a whole new class of materials known as 2-D layered materials started emerging. Though the layered materials like transition metal dichalcogenides (TMDCs) [2], clays [3] and carbonitrides [4] were quite known for long time, the 2-D material class represents their atomically thin mono-layers or few-layer nano-sheets/platelets. By delaminating these layered materials to mono-layer or nano-sheets can enormously increase the surface area and can confine the electrons in two-dimension. This can significantly affect the overall electronic and optical properties. From the large pool of TMDC materials, MoS2 is the most widely studied, and explored for lubrication, catalysis, energy storage, hydrogen storage and opto-electronics applications due to its thickness dependent bandgap, availability in abundance in nature [5-6]. Bandgap of MoS2 varies from indirect type with a value 1.29 eV to direct type of 1.8 eV in monolayer MoS2 [6]. Similarly in nano-sheets of MoS2, the catalytic activity of the MoS2 is mainly due to its exposed edges whereas its basal planes in general remain inactive [7]. Also, MoS2 in the form of either monolayer or nano-sheets with odd number of layers can show piezoelectric effect. This can have applications in the area or nano-generators and stretchable electronics [8]. Hence, to explore these unique properties of MoS2, preparation of monolayer or few-layer nano-sheets of MoS2 is very important. It is observed from literatures that there are many exfoliation methods viz. mechanical exfoliation, ion-intercalated exfoliation and liquid-phase exfoliation exist for preparing such 2-D materials [5, 9 and 10]. Among them, liquid-phase exfoliation method stands out for its simple processing steps and low cost advantage. However, with this exfoliation method the stability of the exfoliated phase always remains a concern. The restacking or aggregation of layered platelets into bulk form over a long period of time or after the removal of the solvent is seemingly a possibility. Hence, by making composites with polymers while the MoS2 is still in the exfoliated phase can offer stability to the exfoliated MoS2. Also such composites can offer new functionalities due to the polymer matrix. In fact, there are many reports of MoS2 based composites with polymers like PVA, PMMA, PEO, and PVDF showing improved thermal, mechanical and electrical properties [11-13]. Recently, MoS2 nanosheets based PVP composites also been tried for nonvolatile memory applications and showing great promise for next-generation electronic and optoelectronic devices [14]. In another report, Rehman et al. observed resistive switching memory in all-printed MoS2-PVA nanocomposite based devices. They demonstrated characteristic bistable, nonvolatile and rewritable resistive switching behavior at a low operating voltage [15]. In the present study, we report the successful exfoliation of MoS2 using DMF as a solvent. The exfoliated MoS2 was made composite with Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). The free-standing film of the composite was prepared to study structural and micro-structural properties. For electrical characterizations, two terminal devices were fabricated by depositing the composite solution on ITO coated PET substrate. The preliminary studies of current-voltage characteristics show stable bipolar resistive switching. 2. Experimental Methods The bulk MoS2 powders and dimethylformamide (DMF) were purchased from Alfa Aesar and Merck, respectively. DMF was chosen as the medium of exfoliation due to good miscibility and wettability of MoS2 in DMF [10]. PVDF-HFP (Mw ~400,000) was supplied by Alfa Aesar. Following the reports that mechanical grinding can improve the yield of exfoliation, initially the MoS2 powders were grinded for half an hour using a mortar pestle [16]. Small amount of DMF was added periodically during the grinding process. Then MoS2 and DMF paste are further diluted by adding 100 ml of DMF and then sonicated for 2 hours using a probe sonicator (model no. Q700, make QSonica). To separate the exfoliated MoS2 from those were not exfoliated; centrifugation at the rate of 2000 rpm for half an hour was carried out. Then the supernatant of the solutions were separated from those were precipitated at the bottom. These supernatants contain the exfoliated MoS2 nano-sheets which was stored for further characterization and preparation of composite with PVDF-HFP.
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Fig. 1 shows three step process of MoS2 exfoliation; (a) the grinding of MoS2 in DMF medium; (b) probe sonication of grinded MoS2 for 2 hours and (c) centrifugation at 2000 rpm for half an hour.
2.1 Preparation of MoS2 and PVDF-HFP nanocomposites To prepare the nanocomposite films with PVDF-HFP, 1 wt. % of the exfoliated MoS2 was added to the solution of PVDF-HFP and stirred for 12 hours to get a homogenous mixture. Then the composite film was prepared by solution cast method. For observing current-voltage characteristics in the nanocomposite films, two terminal sandwich structures were prepared on ITO coated PET substrate (Sigma-Aldrich).The solutions of PVDF-HFP and MoS2 nano-sheets in DMF are deposited by spin-coating at 2000 rpm on the ITO surface. Then these samples were dried in vacuum oven for 8 hours followed by deposition of top aluminum electrode by using thermal evaporation method. The diameter of the top aluminum electrode is about ~100 µm. 2.2 Characterization The absorption spectra of the solution containing exfoliated MoS2 nano-sheets and DMF was carried out using Agilent Technology UV-visible spectrometer having wavelength in the range of 200-800nm. For structural characterization, XRD pattern was acquired using Rigaku TT RAX III, Japan X-ray diffractometer with Cu Kα radiation (λ=1.5406 Å). Crystallite size of the material can be calculated using Scherrer equation. This equation is useful in calculating the number of MoS2 layers stacked along the C-axis in the MoS2 nano-sheets. The surface morphology of the composite films was studied using field emission scanning electron microscope (FESEM, make: Carl Zeiss, Germany). The impedance spectroscopy measurement was conducted in a frequency range of 100Hz to 1MHz using Hioki LCR Meter. The current-voltage characteristics of the prepared devices were studied using keithley 4200-scs semiconductor characterization system. 3. Results and Discussion 3.1 Calculation of yield of MoS2 exfoliation The exfoliated Mos2 exhibited good stability and no settlement or re-stacking of the nano-sheets of MoS2 was observed. Figure 2. (a) to (c) present the picture of exfoliated MoS2 nano-sheets in DMF stored in glass bottles showing no significant change in color for the storing time up to one month. To calculate the exfoliation yield of MoS2, a clean and dry glass petridish was taken and weighed initially. Then 10 ml of the supernatant (containing
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exfoliated MoS2 in DMF) collected after centrifugation was taken in that glass petridish. It was dried at 180 °C for 12 hrs to remove the DMF. Then the weight of petridish with dried MoS2 was measured again. Hence, the weight of the exfoliated MoS2 was calculated from the difference of the two weights. The detail calculation is given in the Table -1.
Fig. 2 Picture of stored exfoliated MoS2; (a) Freshly prepared sample; (b) After two weeks and (c) After four weeks of exfoliation; (d) shows the UV-Visible absorption spectra of freshly prepared exfoliated MoS2.
Table -1: Yield of exfoliation of MoS2 nano-sheets Weight of petridish (a) 42.48g
Weight of petridish +dried MoS2 (b) 42.5195g
Weight of MoS2 (b-a) 0.032g
Yield of exfoliation 32mg/10 ml
The concentration of exfoliated MoS2 is 32mg/10 ml which is considered to be a good yield and comparable to the reported literatures [17, 18]. Figure 2. (d) shows the UV-VIS spectra of exfoliated MoS2. In the UV-VIS spectra two characteristic absorbance peaks were obtained at 619nm and 678nm for the exfoliated sample. These two peaks are related to the absorption due to direct transition at the K points of the Brillouin zone [2, 19]. They are usually known as B and A excitons respectively. Also, some broad absorption peaks known as C and D excitons are also observed in the exfoliated MoS2. 3.2 Structural characterization The structural characterization of the nano-composite film was carried out using XRD and shown in Fig. 3 (a). The presence of Bragg peaks for both MoS2 and PVDF-HFP confirms the composite formation. PVDF-HFP is a semi-crystalline polymer with its crystalline form exists in different phases known as α, β, γ and δ phases [20, 21]. The small peaks observed at 17.64° and 18.44° are due to the α-phase of PVDF-HFP. The most intense peak at 20.56° is due to the electroactive β and γ phases. So the PVDF-HFP in the composite exists with multiple phases among which β-phase is the most dominating. The crystallite size corresponding to the β-phase is found to be 3.1 nm as recorded in Table 2. Owing to our interest in exfoliated MoS2 nano-sheets, the peak (002) of MoS2 was separately investigated. The crystallite size corresponding to (002) peak represents the MoS2 size along the C-axis.
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Using Scherrer’s formula the crystallite size was calculated to be 5.8 nm. Taking the d- spacing as 5.96Å, roughly 10 number of MoS2 layers are expected to be stacked in the exfoliated nano-sheets. Fig. 3(c) shows the SEM micrographs of the nano-composite films. The presence of MoS2 on the surface shows the composite nature of the films. The lateral dimension of the exfoliated nano-sheets is found to be in the range 20-25 µm. The Fig. 3(d) shows the elemental composition of the composite films as calculated from the EDAX.
Fig. 3 (a) XRD pattern of MoS2 nano-sheets doped PVDF-HFP composite films ; (b) (002) peak of MoS2; (c) Surface morphology of the composite films as recorded using SEM ; (d) EDAX showing the elemental composition of the composite films. Table 2: Analysis of Bragg peaks for exfoliated MoS2 and β-phase of PVDF-HFP. Diffraction angle (2θ)/degree
d-spacing (Å)
Crystallite size (nm)
MoS2 (002) peak
14.84
5.96
5.8
β– phase peak (020) of PVDF-HFP
20.56
4.33
3.1
3.3. Electrical Characterization Electrical characterizations of the composite films were carried out using impedance spectroscopy. The Nyquist plot at the room temperature is shown in the Fig. 4(a). The straight spike observed parallel to the Zʺ-axis indicates the resistive nature of the composite films. The bulk resistance of the composite films was calculated from the high frequency end of the spectrum and found out to be 2.2 kΩ. This corresponds to the conductivity of the composite films is ~ 3.4 ×10-6 S cm-1.
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The current-voltage characteristic of the composite film is presented in Fig. 4(b). The voltage was initially swept from 0V to 3V. The current suddenly rises from nA range to 0.1 mA at 2.74V which makes the ON state. When swept from 0V to -2V, current rises very sharply up to 1.5 mA and then abruptly drops to nA range. This confirms the RESET process. This typical I-V characteristic suggests that the composite film shows bipolar resistive switching where the SET occurs in the positive polarity at 2.74V while RESET occurs in the negative polarity at 0.9V. The compliance current is set at 0.1mA. The OFF/ON resistance ratio is found to be of the order of 10 -10 .
Fig. 4 (a) Nyquist (Z՜ vs. Z՜՜) plot of MoS2 nano-sheets doped PVDF-HFP nano-composite films; (b) current-voltage characteristics of the composite films showing bipolar resistive switching. The inset shows the semi-log plot of the switching cycle.
The switching mechanism could be due to electron trapping and de-trapping on the MoS2 nano-sheets present in the composite films [15, 22, and 23]. However, a detailed study of the switching parameters and supporting experiments need to be carried out to ascertain the exact switching mechanism. 4. Conclusion Bulk MoS2 powders are successfully exfoliated into few-layer nano-sheets using a probe sonicator. DMF was chosen as the medium of exfoliation. A very good yield of 32mg per 10 ml is achieved by this liquid phase exfoliation method. Further, nano-composite film was prepared using PVDF-HFP as the polymer matrix. The XRD and SEM micrograph confirms the composite formation. The number of MoS2 layers in the nano-sheets are approximately found to be 10 as calculated from the (002) peak of the MoS2 using Scherrer’s formula. The room temperature conductivity of the composite film is calculated to be ~ 3.4 ×10-6 S cm-1from the Nyquist plot. CurrentVoltage (I-V) characteristic suggests electrical bistability and hence shows great promise in bipolar non-volatile memory applications. Acknowledgements Authors gratefully acknowledge the support received from Dr. A. K. Thakur, IIT Patna and Mr. Ujjal Das, NIT Silchar for carrying out the XRD measurements and I-V characteristic studies of the composite films respectively.
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References [1] A. K.Geim, K. S. Novoselov, Nature Materials 6 (2007)183-191. [2] A. R. Beal, J. C. Knights and W. Y. Liang, J. Phys. C: Solid State Phys. 5(1972) 3450-3551. [3] S. R. Mohapatra, A. K. Thakur and R. N. P. Choudhary, Journal of Power Sources 191 (2009) 601-613. [4] W. Lengauer, S. Binder, K. Aigner, P. Ettmayer, A. Guillou, J. Debuigne and G. Groboth, J Alloy Compd 217 (1995), 137-147. [5] S. Ravula, J. B. Essner and G.A. Baker, Chem. Nano. Mater. 1 (2015) 167 – 177. [6] X. Li and H. Zhu, Journal of Materiomics 1(2015) 33-44. [7] X. Huang, M. Leng, W. Xiao, M. Li, J. Ding, T. L. Tan, W. S. V. Lee and J. Xue, Adv. Funct. Mater. 27 (2017) 1604943(1-8). [8] W. Wu, L. Wang, Y. Li, F. Zhang, L. Lin, S. Niu, D. Chenet, X. Zhang, Y. Hao, T. F. Heinz, J. Hone and Z. L. Wang, Nature 2 (2014) 13792. [9] V. Nicolosi, M. Chhowalla, M. G. Kanatzidis, M. S. Strano, J. N. Coleman, Science 340 (2013) 1226419(1-18). [10] J. Shen, Y. He, J. Wu, C. Gao, K. Keyshar, X. Zhang, Y. Yang, M. Ye, R. Vajtai, J. Lou and P. M. Ajayan, Nano Lett. 15 (2015) 5449 – 5454. [11] K. Zhou, S. Jiang, C. Bao, L. Song, B. Wang, G. Tang, Y. Hu and Z. Gui, RSC Advances 2 (2012) 11695–11703. [12] K. Zhou, J. Liu, B. Wang, Q. Zhang, Y. Shi, S. Jiang, Y. Hu and Z. Gu, Materials Letters 126 (2014) 159–16. [13] Q. Jia, X. Huang, G. Wang, J. Diao, and P. Jiang, J. Phys. Chem. C 120 (2016) 10206−10214. [14] J. Liu, Z. Zeng, X. Cao, G. Lu, L. Wang, Q. Fan, W. Huang and H. Zhang, Small 8 (2012) 3517 – 3522. [15] M. M. Rehman, G. U. Siddiqui, J. Z. Gul, S.W. Kim, J. H. Lim and K. Choi, Scientific Reports 6 (2016) 36195. [16] E. P. Nguyen, B. J. Carey, T. Daeneke, J. Zhen Ou, K. Latham, S. Zhuiykov and K. Kalantar-zadeh, Chem. Mater. 27 (2015) 53−59. [17] Y. Yao, L. Tolentino, Z. Yang, X. Song, W. Zhang, Y. Chen, and C. P. Wong, Adv. Funct. Mater. 23 (2013) 3577-3583. [18] V. H. Pham, K. H. Kim, D. W. Jung, K. Singh, E. S. Oh and J. S. Chung, Journal of Power Sources 144 (2013) 280-286. [19] K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz Phys. Rev. Lett. 105 (2010) 136805. [20] D. Yuan, Z. Li, W. Thitsartarn, X. Fan, J. Sun, H. Li and C. He, J. Mater. Chem. C 3 (2015) 3708-3713. [21] D. Mondal, A. L. Gayen, B. K. Paul, P. Bandyopadhyay, D. Bera, D. S. Bhar, K. Das, P. Nandy, S. Das, Journal of Materials Science: Materials in Electronics, 29(2018) 14535–14545. [22] M. G. Nair, M. Malakar, S. R. Mohapatra and A. Chowdhury, AIP Conference Proceedings 1953 (2018) 030171. [23] S. M. Shinde, G. Kalita and M. Tanemura, Journal of Applied Physics 116 (2014) 214306.
ScienceDirect Materials Today: Proceedings 18 (2019) 5447–5453
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ICMPC-2019
Liquid Phase Exfoliation of MoS2 Nano-sheets and Observation of Resistive Switching Memory in MoS2 Nano-sheets-PVDF-HFP Composite Films Deepaka, Rajesh Deba, Manjula G. Naira, Sudipta Halderb, A. L. Sharmac and Saumya R. Mohapatraa* b
a Department of Physics, National Institute of Technology Silchar, Silchar – 788010, Assam Department of Mechanical Engineering, National Institute of Technology Silchar, Silchar – 788010, Assam c Centre for Physical Sciences, Central Univeristy of Punjab, Bhatinda - 151001
Abstract To realise the true and versatile application potentials of 2-D materials like MoS2, the preparation of mono-layer or few layer nano-sheets holds the key. In the present study, we report the successful exfoliation of bulk MoS2 into few layer nano-sheets by adopting a two-step process of grinding and then two hours of ultrasonication by using a probe sonicator. The yield of exfoliation was 32 mg/10 ml and the exfoliated MoS2 was very stable without any re-stacking for more than one month. Further, the exfoliated MoS2 nano-sheets are added to Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) to prepare the nanocomposite thin films. Two-terminal devices are prepared with ITO and aluminium as bottom and top electrodes, respectively on a plastic substrate. Its electrical properties are investigated to observe the electrical bistability. Nanocomposite based devices showed bipolar resistive switching memory. For composite film with 1wt.% of MoS2, resistance switching is observed with SET and RESET voltages at 2.74 V and -0.9 V respectively. Resistance ratio of the order of 104 is achieved between the ON and OFF states. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: MoS2 nano-sheets; Liquid-phase method; polymer composite; resistance switching memory.
* Corresponding author. Tel.: +91-3842-242914; Fax: +91-3842 22479. E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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1. Introduction Since the discovery of graphene in 2004 [1] and subsequent exploration of its alluring properties, a whole new class of materials known as 2-D layered materials started emerging. Though the layered materials like transition metal dichalcogenides (TMDCs) [2], clays [3] and carbonitrides [4] were quite known for long time, the 2-D material class represents their atomically thin mono-layers or few-layer nano-sheets/platelets. By delaminating these layered materials to mono-layer or nano-sheets can enormously increase the surface area and can confine the electrons in two-dimension. This can significantly affect the overall electronic and optical properties. From the large pool of TMDC materials, MoS2 is the most widely studied, and explored for lubrication, catalysis, energy storage, hydrogen storage and opto-electronics applications due to its thickness dependent bandgap, availability in abundance in nature [5-6]. Bandgap of MoS2 varies from indirect type with a value 1.29 eV to direct type of 1.8 eV in monolayer MoS2 [6]. Similarly in nano-sheets of MoS2, the catalytic activity of the MoS2 is mainly due to its exposed edges whereas its basal planes in general remain inactive [7]. Also, MoS2 in the form of either monolayer or nano-sheets with odd number of layers can show piezoelectric effect. This can have applications in the area or nano-generators and stretchable electronics [8]. Hence, to explore these unique properties of MoS2, preparation of monolayer or few-layer nano-sheets of MoS2 is very important. It is observed from literatures that there are many exfoliation methods viz. mechanical exfoliation, ion-intercalated exfoliation and liquid-phase exfoliation exist for preparing such 2-D materials [5, 9 and 10]. Among them, liquid-phase exfoliation method stands out for its simple processing steps and low cost advantage. However, with this exfoliation method the stability of the exfoliated phase always remains a concern. The restacking or aggregation of layered platelets into bulk form over a long period of time or after the removal of the solvent is seemingly a possibility. Hence, by making composites with polymers while the MoS2 is still in the exfoliated phase can offer stability to the exfoliated MoS2. Also such composites can offer new functionalities due to the polymer matrix. In fact, there are many reports of MoS2 based composites with polymers like PVA, PMMA, PEO, and PVDF showing improved thermal, mechanical and electrical properties [11-13]. Recently, MoS2 nanosheets based PVP composites also been tried for nonvolatile memory applications and showing great promise for next-generation electronic and optoelectronic devices [14]. In another report, Rehman et al. observed resistive switching memory in all-printed MoS2-PVA nanocomposite based devices. They demonstrated characteristic bistable, nonvolatile and rewritable resistive switching behavior at a low operating voltage [15]. In the present study, we report the successful exfoliation of MoS2 using DMF as a solvent. The exfoliated MoS2 was made composite with Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). The free-standing film of the composite was prepared to study structural and micro-structural properties. For electrical characterizations, two terminal devices were fabricated by depositing the composite solution on ITO coated PET substrate. The preliminary studies of current-voltage characteristics show stable bipolar resistive switching. 2. Experimental Methods The bulk MoS2 powders and dimethylformamide (DMF) were purchased from Alfa Aesar and Merck, respectively. DMF was chosen as the medium of exfoliation due to good miscibility and wettability of MoS2 in DMF [10]. PVDF-HFP (Mw ~400,000) was supplied by Alfa Aesar. Following the reports that mechanical grinding can improve the yield of exfoliation, initially the MoS2 powders were grinded for half an hour using a mortar pestle [16]. Small amount of DMF was added periodically during the grinding process. Then MoS2 and DMF paste are further diluted by adding 100 ml of DMF and then sonicated for 2 hours using a probe sonicator (model no. Q700, make QSonica). To separate the exfoliated MoS2 from those were not exfoliated; centrifugation at the rate of 2000 rpm for half an hour was carried out. Then the supernatant of the solutions were separated from those were precipitated at the bottom. These supernatants contain the exfoliated MoS2 nano-sheets which was stored for further characterization and preparation of composite with PVDF-HFP.
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Fig. 1 shows three step process of MoS2 exfoliation; (a) the grinding of MoS2 in DMF medium; (b) probe sonication of grinded MoS2 for 2 hours and (c) centrifugation at 2000 rpm for half an hour.
2.1 Preparation of MoS2 and PVDF-HFP nanocomposites To prepare the nanocomposite films with PVDF-HFP, 1 wt. % of the exfoliated MoS2 was added to the solution of PVDF-HFP and stirred for 12 hours to get a homogenous mixture. Then the composite film was prepared by solution cast method. For observing current-voltage characteristics in the nanocomposite films, two terminal sandwich structures were prepared on ITO coated PET substrate (Sigma-Aldrich).The solutions of PVDF-HFP and MoS2 nano-sheets in DMF are deposited by spin-coating at 2000 rpm on the ITO surface. Then these samples were dried in vacuum oven for 8 hours followed by deposition of top aluminum electrode by using thermal evaporation method. The diameter of the top aluminum electrode is about ~100 µm. 2.2 Characterization The absorption spectra of the solution containing exfoliated MoS2 nano-sheets and DMF was carried out using Agilent Technology UV-visible spectrometer having wavelength in the range of 200-800nm. For structural characterization, XRD pattern was acquired using Rigaku TT RAX III, Japan X-ray diffractometer with Cu Kα radiation (λ=1.5406 Å). Crystallite size of the material can be calculated using Scherrer equation. This equation is useful in calculating the number of MoS2 layers stacked along the C-axis in the MoS2 nano-sheets. The surface morphology of the composite films was studied using field emission scanning electron microscope (FESEM, make: Carl Zeiss, Germany). The impedance spectroscopy measurement was conducted in a frequency range of 100Hz to 1MHz using Hioki LCR Meter. The current-voltage characteristics of the prepared devices were studied using keithley 4200-scs semiconductor characterization system. 3. Results and Discussion 3.1 Calculation of yield of MoS2 exfoliation The exfoliated Mos2 exhibited good stability and no settlement or re-stacking of the nano-sheets of MoS2 was observed. Figure 2. (a) to (c) present the picture of exfoliated MoS2 nano-sheets in DMF stored in glass bottles showing no significant change in color for the storing time up to one month. To calculate the exfoliation yield of MoS2, a clean and dry glass petridish was taken and weighed initially. Then 10 ml of the supernatant (containing
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exfoliated MoS2 in DMF) collected after centrifugation was taken in that glass petridish. It was dried at 180 °C for 12 hrs to remove the DMF. Then the weight of petridish with dried MoS2 was measured again. Hence, the weight of the exfoliated MoS2 was calculated from the difference of the two weights. The detail calculation is given in the Table -1.
Fig. 2 Picture of stored exfoliated MoS2; (a) Freshly prepared sample; (b) After two weeks and (c) After four weeks of exfoliation; (d) shows the UV-Visible absorption spectra of freshly prepared exfoliated MoS2.
Table -1: Yield of exfoliation of MoS2 nano-sheets Weight of petridish (a) 42.48g
Weight of petridish +dried MoS2 (b) 42.5195g
Weight of MoS2 (b-a) 0.032g
Yield of exfoliation 32mg/10 ml
The concentration of exfoliated MoS2 is 32mg/10 ml which is considered to be a good yield and comparable to the reported literatures [17, 18]. Figure 2. (d) shows the UV-VIS spectra of exfoliated MoS2. In the UV-VIS spectra two characteristic absorbance peaks were obtained at 619nm and 678nm for the exfoliated sample. These two peaks are related to the absorption due to direct transition at the K points of the Brillouin zone [2, 19]. They are usually known as B and A excitons respectively. Also, some broad absorption peaks known as C and D excitons are also observed in the exfoliated MoS2. 3.2 Structural characterization The structural characterization of the nano-composite film was carried out using XRD and shown in Fig. 3 (a). The presence of Bragg peaks for both MoS2 and PVDF-HFP confirms the composite formation. PVDF-HFP is a semi-crystalline polymer with its crystalline form exists in different phases known as α, β, γ and δ phases [20, 21]. The small peaks observed at 17.64° and 18.44° are due to the α-phase of PVDF-HFP. The most intense peak at 20.56° is due to the electroactive β and γ phases. So the PVDF-HFP in the composite exists with multiple phases among which β-phase is the most dominating. The crystallite size corresponding to the β-phase is found to be 3.1 nm as recorded in Table 2. Owing to our interest in exfoliated MoS2 nano-sheets, the peak (002) of MoS2 was separately investigated. The crystallite size corresponding to (002) peak represents the MoS2 size along the C-axis.
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Using Scherrer’s formula the crystallite size was calculated to be 5.8 nm. Taking the d- spacing as 5.96Å, roughly 10 number of MoS2 layers are expected to be stacked in the exfoliated nano-sheets. Fig. 3(c) shows the SEM micrographs of the nano-composite films. The presence of MoS2 on the surface shows the composite nature of the films. The lateral dimension of the exfoliated nano-sheets is found to be in the range 20-25 µm. The Fig. 3(d) shows the elemental composition of the composite films as calculated from the EDAX.
Fig. 3 (a) XRD pattern of MoS2 nano-sheets doped PVDF-HFP composite films ; (b) (002) peak of MoS2; (c) Surface morphology of the composite films as recorded using SEM ; (d) EDAX showing the elemental composition of the composite films. Table 2: Analysis of Bragg peaks for exfoliated MoS2 and β-phase of PVDF-HFP. Diffraction angle (2θ)/degree
d-spacing (Å)
Crystallite size (nm)
MoS2 (002) peak
14.84
5.96
5.8
β– phase peak (020) of PVDF-HFP
20.56
4.33
3.1
3.3. Electrical Characterization Electrical characterizations of the composite films were carried out using impedance spectroscopy. The Nyquist plot at the room temperature is shown in the Fig. 4(a). The straight spike observed parallel to the Zʺ-axis indicates the resistive nature of the composite films. The bulk resistance of the composite films was calculated from the high frequency end of the spectrum and found out to be 2.2 kΩ. This corresponds to the conductivity of the composite films is ~ 3.4 ×10-6 S cm-1.
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The current-voltage characteristic of the composite film is presented in Fig. 4(b). The voltage was initially swept from 0V to 3V. The current suddenly rises from nA range to 0.1 mA at 2.74V which makes the ON state. When swept from 0V to -2V, current rises very sharply up to 1.5 mA and then abruptly drops to nA range. This confirms the RESET process. This typical I-V characteristic suggests that the composite film shows bipolar resistive switching where the SET occurs in the positive polarity at 2.74V while RESET occurs in the negative polarity at 0.9V. The compliance current is set at 0.1mA. The OFF/ON resistance ratio is found to be of the order of 10 -10 .
Fig. 4 (a) Nyquist (Z՜ vs. Z՜՜) plot of MoS2 nano-sheets doped PVDF-HFP nano-composite films; (b) current-voltage characteristics of the composite films showing bipolar resistive switching. The inset shows the semi-log plot of the switching cycle.
The switching mechanism could be due to electron trapping and de-trapping on the MoS2 nano-sheets present in the composite films [15, 22, and 23]. However, a detailed study of the switching parameters and supporting experiments need to be carried out to ascertain the exact switching mechanism. 4. Conclusion Bulk MoS2 powders are successfully exfoliated into few-layer nano-sheets using a probe sonicator. DMF was chosen as the medium of exfoliation. A very good yield of 32mg per 10 ml is achieved by this liquid phase exfoliation method. Further, nano-composite film was prepared using PVDF-HFP as the polymer matrix. The XRD and SEM micrograph confirms the composite formation. The number of MoS2 layers in the nano-sheets are approximately found to be 10 as calculated from the (002) peak of the MoS2 using Scherrer’s formula. The room temperature conductivity of the composite film is calculated to be ~ 3.4 ×10-6 S cm-1from the Nyquist plot. CurrentVoltage (I-V) characteristic suggests electrical bistability and hence shows great promise in bipolar non-volatile memory applications. Acknowledgements Authors gratefully acknowledge the support received from Dr. A. K. Thakur, IIT Patna and Mr. Ujjal Das, NIT Silchar for carrying out the XRD measurements and I-V characteristic studies of the composite films respectively.
Deepak et al./ Materials Today: Proceedings 18 (2019) 5447–5453
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