Sequential Solvent Exchange Method for Controlled Exfoliation of

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A Sequential Solvent Exchange Method for Controlled Exfoliation of MoS2 suitable for Phototransistor Fabrication Foad Ghasemi, and Shams Mohajerzadeh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07211 • Publication Date (Web): 28 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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ACS Applied Materials & Interfaces

A Sequential Solvent Exchange Method for Controlled Exfoliation of MoS2 suitable for Phototransistor Fabrication Foad Ghasemi and Shams Mohajerzadeh* Nanoelectronic Lab, School of Electrical and Computer Eng, University of Tehran, Tehran, 14399-56191 Iran *Email: [email protected]

Abstract: In this study, flakes of Molybdenum disulfide (MoS2) with controlled size and thickness are prepared through sequential solvent exchange method by sonication in dimethylformamide (DMF) and N-Methyl-2-pyrrolidone (NMP) solvents. While NMP acts more effectively in reducing the thickness of flakes, DMF shows better potential in conserving the lateral size of nanosheets. The distribution of size and thickness of nanoflakes as a function of sonication time verifies that extended sonication results in dramatic drop of the dimension of the exfoliated flakes. This technique leads to the formation of few-layered MoS2 flakes without further drop of their lateral dimensions. It has been observed that by exposing the bulk MoS2 powders to oxygen plasma, the exfoliation process is accelerated without converting to 2H-MoS2 structures. Finally, a phototransistor has been fabricated based on few-layered MoS2 layers with a field effect mobility of ~2.1 cm2V-1s-1 showing a high response to laser excitation of 532 nm wavelength.

Keywords: Sonication assisted, exfoliation, MoS2, solvent, lateral size, phototransistor

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1. INTRODUCTION Over the past few years, layered materials have grasped significant interest due to their unique properties1. A variety of layered materials such as Graphene2-4, Hexagonal Boron Nitride5, Transition Metal Dichalcogenides (TMDs: MoS2, WS2, WSe2)6-7, Metal Oxides (MoO3)8-9, Metal Halides (MgBr2)10, etc., have been discovered in the last decade. Among these diverse families of layered materials, Transition Metal Dichalcogenides (TMDs) have gained more attention owing to their unique physical properties11. TMD crystals are composed of hexagonal layered metal atoms between two layers of chalcogen atoms coupling together via Van der Waals (VDW) interaction12. Molybdenum disulfide (MoS2) as a TMD material is one of the most promising 2D structure exhibiting outstanding semiconductor properties with great promise for electronic application7. In particular, mono and few layers of MoS2 possess distinctive properties in comparison with its bulk allotrope13. Typically, bulk MoS2 has an indirect band gap around 1.2 eV whereas its mono layers show a direct band gap with a higher value of 1.8 eV. This is a welcoming event which enables one to tailor the band structure of the material14-16. Emerging direct transition in few layered MoS2 offers broad optical and electrical applications such as FET-transistors 17-20, photo-transistors 21-22, Photodetectors 23-24, gas sensors25-26 and so on. To date, a variety of techniques have been introduced to synthesize few layered MoS2 structures, including mechanical exfoliation27-28, liquid phase exfoliation29-32, hydrothermal reaction33-35, and chemical vapor deposition36-39. Among these approaches, phase liquid exfoliation (ion intercalation and solvent based sonication) have been widely utilized because of its capability for low-cost mass production40-41. The ion intercalation approach uses tiny guest atoms (such as lithium) penetrating into the layers of MoS2, weakening the interlayer interaction and resulting in easy exfoliation of flakes in aqueous media42-43. However, sensitivity to ambient conditions such


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as humidity, temperature, structure converting (2H to 1T-MoS2) and decomposing into other materials as metal nanoparticles (Li2S for instance) are undesirable side-effects of such methods7, 44-45

. In a different approach, sonication-assisted exfoliations take advantage of organic solvents to

yield a high quality dispersion of few-layered MoS229, 41, 46. Based on the experimental studies, the surface energy of layered materials plays an essential role in the process of layered exfoliation. If a high concentration and well-exfoliated dispersion is desired, the surface energy of solvents should be close to the layered materials47. It has been reported that MoS2 is highly dispersed in solvents which have surface tension close to 40 mJ/m2

46-47

. Among various solvents, N-

Methylpyrrolidone (NMP) and N, N-dimethylformamide (DMF) with respective surface tensions of 40.79 and 37.10 mJ/m2, are considered to be suitable for MoS2 exfoliation48. Although these solvents provide high concentrate dispersion of nanosheets, prolonged bath or high-power point probe sonication is required for achieving mono and few-layered MoS2 flakes, which subsequently leads to small scale sheets along with low control on thickness and size of flakes. In general the attempts to achieve low thickness flakes are adversely associated with few-layered and small dimensions of flakes due to more sonication time and power. As a result, alternative approaches to obtain desired layered and dimensions are being highly investigated. In this paper, a method for controlled exfoliation of MoS2 layers is introduced based on the sonication assisted exfoliation of bulk powders in NMP and DMF solvents. Sonication is conducted under 400 watt power for different durations of 15, 30 and 60 mins and the size and thickness distributions of exfoliated flakes have been thoroughly investigated. Despite of achieving the lowest thickness for the sample prepared with 60-min sonication in NMP, the dimension of the flake is measured to be smallest. In contrast, sonication in DMF solvent yields in less decrease in the lateral size of nanosheets although the thickness of the MoS2 flakes is higher.



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A novel three-step technique based on NMP, DMF and IPA solutions is introduced for controlled exfoliation of MoS2 with small thickness and large dimensions, leading to a considerable progress towards two-dimensional nanosheet preparation. In addition, a pre-treatment step on bulk MoS2 powders with O2 plasma remarkably enhances the exfoliation step. Recently, A Jawaid and his coworkers have demonstrated that dissolving O2 gas in NMP during the sonication process yields in more efficient exfoliation of bulk MoS2 crystal confirming the critical impact of O2 atoms on the sonication process 49. Here, our pre-treatment step combined with the solvent exchange has led to flakes with low thicknesses and large dimensions. This three-step technique allows one to prepare large scale MoS2 layers, suitable for the fabrication of electrical and optoelectronic devices. In this line of work, we have realized a MoS2-based phototransistor which shows a modest field effect mobility of ~2.1 cm2/Vs with photoresponsivity of 5 mA/W indicating a considerable performance compared to other MoS2 based phototransistors50-52 (see Table 1).

2. EXPERIMENTAL SECTION Materials: MoS2 powders (with flake sizes between 3 and 40 µm, purity of 99%) and NMP were purchased from Sigma Aldrich. DMF and IPA solvents were supplied from Merck Company and no special purification was performed. The SEM image of bulk MoS2 powders is given in Figure S2 in the supporting information. Preparation of MoS2 nanosheets: Based on the phase liquid exfoliation, 1 g bulk MoS2 powders were added to 15 ml NMP and DMF, separately. The obtained solutions were placed in an icewater bath followed by sonication using point probe at 400W (7 s “on” and 3 s “off”) for different durations (overall 15, 30 and 60 mins in “on” state). Afterwards, the prepared dispersions were


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centrifuged for 60 mins at a speed of 4000 rpm to eliminate or reduce the un-exfoliated flakes and the top supernatants were collected for the following analyses (Scheme 1). Characterizations: The AFM images were measured in a non-contact mode with NSG10 tip (~240 KHZ resonance frequency) by an NT-MDT system. To be able to take AFM images, the prepared MoS2 suspensions were drop-casted on a bare and clean Si wafer. To achieve uniform deposition of nano-sheets, samples were left for 24 hours at room temperature after drop-casting and subsequently were placed into a vacuum oven for another 24 hours at 70 oC in order to evaporate the residual solvents. Transmission electron microscopy (TEM) images of nanosheets were acquired by a Philips, CM 30 electron microscope operating at 150 kV. The analysis was conducted by dropping a few droplets of MoS2 dispersions on carbon grids. Scanning electron microscopy (SEM) analysis was carried out using FE-SEM, Hitachi, S-4160 operating at 30 kV. The dynamic light scattering (DLS) analysis was performed by Malvern instrument, USA by diluting obtained suspensions. Raman measurement was carried out by Senterra unit Germany with an excitation wavelength of 785 nm. In this case, dispersions were dropped onto 300 nm SiO2/Si substrate. PL and UV-Visible spectra were obtained by Cary Eclipse SpectrofluorimeterVarian and UV/Vis PG Instrument Ltd. For the latter experiments, dispersions were diluted 10 times in order to turn into transparent solutions. X-ray Photoelectron Spectroscopy (XPS) analysis has been carried out with Bestec instrument (X-ray source: Al Kα, hʋ=1486.6 eV, pressure > 10-7 Pa) and X-ray diffraction (XRD) data were collected by X’Pert PRO MPD instrument (Panalytical, Netherlands). The electrical characterization of the fabricated phototransistor was performed with IV Parameter Analyzer (Keithly K361, USA) at room temperature in an air ambient.



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Scheme 1. Schematic illustration of the preparation process of MoS2 nanosheets. 3. RESULTS AND DISCUSSION 3.1.

Effect of sonication time on thickness and size of flakes

The dependence of thickness and dimension of nanosheets on sonication times were investigated using solvent exfoliation method. In this method, bulk MoS2 powders were dispersed in DMF and NMP solvents (see experimental section). For each solvent, three sonication times were considered. Samples D15, D30 and D60 refer to 15, 30 and 60-min duration of sonication in DMF whereas samples with 15, 30 and 60-min sonication in NMP are called N15, N30 and N60, respectively. To characterize the thickness and size of exfoliated flakes, AFM and DLS measurements were conducted. The lateral size of nanosheets has been estimated based on the statistical data from DLS measurements by applying a correction introduced by Lotya et al.46, 53. Figure (1.a) shows the average thickness of exfoliated sheets for D15, D30, D60, N15, N30 and N60 samples using AFM analysis. The approximate values of 22, 18, 15, 18, 14 and 11 nm are measured as the extracted thicknesses for D15, D30, D60, N15, N30 and N60 samples, respectively. It is generally observed that increasing the sonication time (from 15 to 60 mins)


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results in decreasing of flakes’ thickness. The lowest thickness (11nm) corresponds to 60-min exfoliation in NMP solvent. Basically, the difference in the surface tensions of NMP and DMF can be responsible for this result. If the surface tension of solvent is close to surface energy of MoS2, high concentration dispersion of well-exfoliated nanosheets can be achieved. Apart from the thickness of sheet, the average size of nanoflakes is collected in part (b) of Figure 1. The average dimension of nanoflakes in the D15, D30, D60, N15, N30, and N60 samples are measured to read 335, 300, 279, 319, 217 and 121 nm, respectively. Based on these findings, one can observe that the lateral dimension of nanosheets drops by the sonication time for both solvents. This effect is more evident in NMP solution than in DMF one. Therefore, MoS2 exfoliation in NMP results in smaller as well as thinner flakes whereas DMF solvent results in larger yet thicker flakes. Although none of these solvents can be successful in achieving both thin and large flakes, a combinational sequence of their application could be a suitable method to improve the size distribution of MoS2 nano-sheets.

Figure 1. (a) The average thickness of exfoliated sheets for different durations of sonication. (b) The average size of corresponding nano-flakes. Although both solvents depict a drop in size with sonication time, DMF solution seems to be less dependent on the sonication as opposed to NMP.



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The statistical data, extracted from AFM measurements, allows one to analyze the thickness and size distribution of obtained nanosheets. The histograms of the thickness and lateral size of D15, D30, D60, N15, N30 and N60 samples are illustrated in Figure 2. While each part in this figure represents the thickness distribution of the samples for a given sonication time, the inset in each part corresponds to the size distribution for the same samples. The height of the boxes in the histograms show the distributions (counts) of each size whereas the horizontal values are the average sizes of the flakes. We have also fitted a curve on the histogram plots to add to the clarity of the data. Based on these findings, it is observed that 15-min sonication results in a wide range distribution of thickness in both solvents (N15 and D15). Moreover, a comparison between N15 and D15 samples delivers that few-layered MoS2 are achieved mostly in N15 whereas D15 cannot result in a considerable exfoliation of flakes to realize thinner structures. By increasing the sonication time to 30 and 60 min, both NMP and DMF tend to make narrower thickness distributions. It is observed for N60 sample that around 33% of flakes are thinner than 5nm which is considered as “few-layered” stacks while more than 50% of them are between 5 and 10 nm thick. For D60 sample, this statistic is slightly degraded and only 25% are below 5nm. N60 and D60 samples have potential to produce few layered MoS2 dispersions as their stable aqueous solution contain 1/3 and 1/4 few layered nanosheets (with thickness less than 5 nm).


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Figure 2. The thickness distributions of prepared suspensions for different sonication times of: (a) 15 mins in NMP. (b) 15 mins in DMF. (c) 30 mins in NMP. (d) 30 mins in DMF. (e) 60 mins in NMP, and (f) 60 mins in DMF. Inset in each figure shows the corresponding size distribution.



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According to DLS measurements, D15 sample covers the range of 300-400 nm with median size of 335 nm while N15 consists mostly of flakes in the range of 200 to 500 nm with average dimension of 320 nm and wider distribution (insets in “a” and “b”). Furthermore, for N60 and D60 samples, these values are 80-200 nm and 220-340 nm, respectively. The median sizes of nanoflakes drop to 121 and 279 nm, correspondingly as seen from the insets in parts “e” and “f”. By increasing the sonication time, steep reduction in the lateral size of sheets for NMP samples is observed while flakes in DMF do not follow this sharp drop. At first glance, it may be deduced that re-aggregation of the exfoliated sheets in DMF causes to make flakes appear larger compared to NMP but the microscopy observations (TEM) do not support any re-stacking of flakes. Figure 3 provides the TEM images of prepared nanosheets for different sonication times and solvents and the insets represent the selected area diffraction patterns of such nanoflakes. Based on these images, layered structures of MoS2 are clearly observed and electron diffraction pattern confirms the crystalline nature of MoS2 layers.


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Figure 3. TEM images of nanoflakes in different suspensions for various sonication times of a) 30 mins in DMF. b) 60 mins in NMP c) 15 mins in DMF. d) 15 mins in NMP. e) 30 mins in DMF. f) 30 mins in NMP. g) 60 mins in DMF, and h) 60 mins in NMP. The inset shows the diffraction pattern of nanoflake. (Scale bar 100 nm) As seen, the lateral size of nanosheets are decreased as sonication time increases and the nanoflakes in NMP demonstrate a lower thickness compared to DMF prepared samples. Although DLS and AFM are used to arrive at a quantitative estimation of the size and thickness of flakes, electron microscopy has also been employed to corroborate the results. Usually, the image contrast in TEM microscope is related to the thickness of layered structures and by inspecting the layer edges one can obtain an estimation about layer numbers29, 41, 46. Sample D15 and N15 show large size of flakes but their thicknesses are higher while flakes in N60 and D60 suspensions possess lower



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thicknesses as well as smaller dimensional size. From thickness and size histogram of nanosheets in both solvents, it can be concluded that NMP accelerates thickness and lateral size reductions of flakes while DMF reduces thickness gradually without considerable reduction of flakes’ dimensions. Being inspired with the results in this part, we introduce a solvent exchange method to improve the lateral dimension of the flakes while their thickness is at the lowest values.

3.2.

Effect of solvent exchange:

Based on the obtained results for exfoliation of MoS2 in DMF and NMP, it can be speculated that combination of two-step sonication with NMP and DMF solvents might end up with an efficient exfoliation of flakes in both size and thickness. For this reason, two dispersions were introduced by three-step preparation process (Scheme 2). Firstly, N15 and D15 suspensions were prepared separately as outlined before. Then, procured suspensions were diluted by a factor of 10 with isopropyl alcohol (IPA) and centrifuged for 30 min at 1000 rpm. Subsequently, the top supernatants were collected, followed by placing in a vacuum oven at 70 oC for 24 hours to evaporate their solvents. The treated dried sheets were named D15I and N15I in which index “I” refers to IPA in these samples. IPA, a low boiling point alcohol (82.6 oC) with surface tension of 23 mJ/m2, is known as a poor solvent for exfoliation of MoS2 which has been utilized in this study to remove thicker exfoliated flakes. In other words, IPA allows thicker nanoflakes to be considerably filtered in the rather low speed of centrifugations and its low surface tension improves sedimentation of thick sheets. The median thickness of nanoflakes for N15 and D15 were recorded as 18 and 22 nm with around 37% and 48% flake thicknesses more than 20 nm, respectively, showing a high abundance of thick flakes. After adding IPA to N15 and D15 and centrifuging, the average thicknesses were decreased to 12 and 10 nm, correspondingly.


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Scheme 2. Schematic illustration of the preparation N15D15 and D15N15 suspensions.

The thickness of nanosheets reads around 22%, 51% and 84% less than 5, 10, and 20 nm for N15I containing around 16% sheets with thicknesses more than 20 nm. For D15I suspension, flake’s thicknesses less than 5, 10, and 20 nm were counted to be around 38%, 56%, and 89% and only 11% are larger than 20 nm, indicating a more efficient thickness reduction compared to N15I. Moreover, to investigate the effect of centrifugation on pure N15 and D15 compared to N15I and



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D15I, the prepared suspensions (N15 and D15) were centrifuged at 1000 rpm for 30 mins which resulted in no particular reduction of thickness highlighting the substantial effect of IPA on the decreasing process. At the final step in this solvent exchange method, 10 ml NMP and DMF were added to D15I and N15I treated sheets and they were further sonicated for 15 mins at 400 W (7 s “on”, 3 s “off”). The procured suspensions were called D15N15 and N15D15, where first indexes assigned to first step sonication in DMF and NMP followed by diluting in IPA while second indexes specified the last step of sonication in NMP and DMF, preceding by evaporating IPA solvent in these samples. Thickness and size distribution of nanosheets are illustrated in Figure 4. Both samples have the same thickness probabilities which are comparable with D30 and N30 (or even D60 and N60), since they considerably have lower thickness even with less sonication time. For D15N15, flake’s thicknesses were measured to be around 77%, 89%, and 96 % less than 5, 10, and 20 nm with average thickness and size of 4.7 and 215 nm as seen in part “a” of this figure. On the other side, N15D15 contains 83%, 95%, and 98% sheet thickness below the 5, 10, and 20 nm while median thickness and size were 3.6 and 306 nm, respectively (part “b”). Compared to the results presented in previous figures, almost all the flakes are below 20 nm.


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Figure 4. Thickness distribution of a) D15N15 and b) N15D15 suspensions. Inset in each figure corresponds to the size distribution histograms. Although the overall sonication time of these samples is 30 mins (15 +15), it represents remarkable results in both size and thickness compared to D30 and N30 samples. In summary, N15D15 shows 20% reduction in nanosheet thickness compared to D30 without undergoing a considerable change in the lateral size of nanoflakes. Furthermore, around 34% reduction is observed in the thickness of flakes for D15N15 without any drop in lateral size of nanoflakes. These results reveal that the three-step technique substantially improves the exfoliation of MoS2 powders with a significant reduction in the thickness of flakes without adversely affecting the lateral size (dimension) of the flakes. A comparison of thickness and size of flakes are represented in Table S1 (in the supplementary document) for all prepared samples. As it can be seen, extreme reductions occur in thickness for N15D15 and D15N15 while no significant change is observed for their dimensions. Table S1 indicates that the introduced three-step technique (last two rows) shows the desirable nanoflakes with largest lateral size and fewest thickness in comparison to one step sonication just in one solvent.



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For further characterization of produced nanosheets, photoluminescence (PL) measurement was carried out. Figure 5 provides the PL spectra of N15D15 suspension at five various excitation wavelengths of 300, 350, 400, 450, and 500 nm. Based on these measurements, two characteristic peaks at ~625 nm and ~680 nm are observed for 500 nm excitation wavelength related to the Kpoint transition in the Brillioun zone of monolayers of MoS2 flakes which is an indication of the direct bandgap of the obtained flakes13, 54. In addition, for higher excitation energies (300-450 nm), broad and low intensity peaks appear similar to those reported for MoS2 quantum dots or small sized sheets55. We believe that the presence of some small nanoflakes of MoS2 in the solution is responsible for these broad peaks. Furthermore, the polydispersity of these few nanoflakes results in the blue-shifting of the PL spectra from the K point. As excited wavelength is increased, a redshift is observed in the emission peaks associated with quantum confinement effects56-57. Hence, low energy photons just excite large flakes while higher energy photons can also excite smaller flakes resulting in the red-shift of emission peaks.


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Figure 5. PL spectra of the prepared N15D15 dispersion for various excitation wavelengths of 300, 350, 400, and 450 nm. Inset shows the PL spectrum of N15D15 for excitation wavelength of 500 nm. “A” and “B” in this figure correspond to 625 and 680 nm due to direct bandgap transitions. To better observe the effectiveness of the three-step solvent exchange method, TEM apparatus has been employed. The TEM images of N15D15 and D15N15 flakes have been depicted in Figure 6, corroborating the results of DLS and AFM measurements. The N15D15 sample indicates larger size of flakes (Figure 6.b) while D15N15 contains smaller size sheets with a thicker thickness (Figure 6.a). Flakes with sizes around 300-400 nm have been obtained.



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Figure 6. TEM images of nanoflakes in a) D15N15 and b) N15D15 suspensions. (The scale bar is 60 nm for both images) To further clarify the importance of N15D15 and D15N15 samples, we have also prepared N15N15 and D15D15 samples (Figure S3). The obtained results show that N15N15 contains smaller thicknesses compared to N30, N60, D15N15, and even N15D15. But, the lateral size of these samples is not desirable. For D15D15 sample the lateral size is large but the flakes are thick and inappropriate. Hence, N15N15 leads to small thicknesses but inappropriate lateral dimensions while D15D15 contributes to flakes with bigger sizes which are unfavorably thick. Furthermore, to investigate the effect of various sonication times in first and third steps of this technique, we have employed two other dispersions: N10D20 and D20N10 to demonstrate how sonication times can affect the final size and thickness of flakes (Figure S4). Based on this study, N10D20 sample contains large and thick flakes while thin and small flakes are associated with D10N20 sample. We believe that the near optimum procedure would be 15 min sonication in NMP followed by another 15 min sonication in DMF to arrive at mono and few-layered structures with suitable lateral sizes.

3.3.

Effect of oxygen and hydrogen bombarded MoS2 on exfoliation process


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The three-step exfoliation process is a powerful technique to arrive at few-layered nano-sheets of MoS2 with relatively large sizes. To further improve this process, we have introduced a presonication H2 and O2 plasma treatment. In this pre-sonication step, bulk powders of MoS2 were exposed to H2 and O2 plasma separately under 200 sccm flow of corresponding gas for 15 min. This plasma treatment has been conducted with a power of 200 W and the samples are named as H-200 and O-200, respectively. In the next step, 1 g of bombarded powders was dispersed in 15 ml DMF followed by probe-sonicating at 400 W (7 s “on”, 3 s “off”) for 15 mins. The resulted suspensions were then centrifuged at 3000 rpm for 60 mins followed by collecting the top supernatants for the next analyses. In order to investigate the effect of plasma on the exfoliation of bulk powders, UV-Visible analysis was carried out for pristine, O-200, and H-200 MoS2 samples. Figure 7 represents the A and B characteristics peaks of exfoliated nanosheets referring to the direct transition of excitons corresponding to the presence of few-layered flakes in the suspensions29. According to the spectrum, it is deduced that O2-MoS2 offers a more desirable result for exfoliation of bulk powders while H2-MoS2 maintains the same result as pristine MoS2 dispersion (Figure 7.a).



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Figure 7. (a) UV-Visible spectra of pure MoS2 dispersion and pre-treated MoS2 exposing to the 200 Watt power, 200 sccm flow and 15 mins duration of O2 and H2 plasma. (b) The spectra of pretreated MoS2 exposing to the different O2 power plasma. To further study the effectiveness of O2 plasma on MoS2 nano-sheets, various samples with different plasma powers including O-50, O-100, O-150, and O-200 were prepared where indexes refer to plasma power. Other preparation parameters are kept the same. As shown in Figure 7.b, by enhancing the power of O2 plasma, more efficient exfoliation of nanoflakes are resulted confirming the promising impact of O2 plasma on final exfoliation. These results are in general agreement with the data presented in A. Jawaid’s report which shows how O2 gas can lead to the efficient exfoliation of MoS249. In order to investigate the effect of O2 plasma on the structure of MoS2, X-Ray diffraction (XRD) analysis was conducted. XRD patterns of pristine and O-200 are shown in Figure 8. The characteristic peaks clearly confirm the 2H-MoS2 structure with d-spacing 6.17Å of (002) diffraction peak58. Compared with the pristine pattern, O2-MoS2 possesses the same peaks with lower intensity indicating some degree of exfoliation by O2 plasma while no specific peaks were identified for MoO2 or MoO3 structures. Moreover, a shift in the peaks towards lower angles might be assigned to the presence of smaller sheets which in the case of (002) direction, the


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d-spacing shifts to 6.18 Å showing a slight expansion of interlayer distance59. Therefore, lowintensity and shifted peaks are believed to result from slight exfoliation of sheets for O2-MoS2 in comparison with the pristine MoS258-60.

Figure 8. XRD spectra of pristine and O2 pre–treated MoS2. The effect of oxygen plasma seems to be more effective on the surface of the flakes and is not fully observed in XRD analysis. To further minimize the effect of bulk MoS2 flakes, we have dispersed the obtained powders in NMP solvent and sonicated for 15 mins following by drop-casting on bare silicon wafers. Despite a reduction in the intensity of the peaks, the XRD analysis does not seem to be able to observe any minute formation of MoO3 constituents (please observe Figure S5 in the supporting information file). To elaborate on this important effect, X-ray photoelectron spectroscopy (XPS) analysis was utilized on pristine and O2 treated bulk MoS2 powders. Pristine



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and O2 treated samples were prepared through dispersing of 1 g of pristine and 200-O2-MoS2 powders in the NMP solvents following by sonication and drop casting on clean bare silicon wafers. Figure 9 provides the results of XPS analysis on the pristine and O2-treated MoS2 samples. The characteristic peaks of Mo4+ 3d3/2, Mo4+ 3d5/2 and S 2s at ~229 eV, ~232 eV and ~225.4 eV correspond to Mo-S bonds in the MoS2 crystal, respectively (parts (a) and (b) in Figure 9)61-62. An additional peak at ~226 eV is also related to polysulfide states63. For the O2-treated sample, two extra peaks at ~232.7 eV and ~235 eV originate from Mo6+ 3d5/2 and Mo6+ 3d3/2 states, respectively. As reported in the literature, these peaks attribute to the formation of Mo-O bonds suggesting some degree of MoO3-like structures in the O2-treated sample43, 63-65. According to the atomic percentage, Mo and S atoms in the pristine MoS2 were calculated to be ~90 % and ~10 % with Mo/S ratio of 8.9 while ~73 % and ~12 % were measured for Mo and S atoms in the O2treated sample with Mo/S ratio of 6.1. Furthermore, after plasma treatment, ~15 % of MoS2 has been changed to MoO3 structure based on the normalized intensity of fitted Mo6+, Mo4+ and S 2s peaks. The envelope curves have been included in the Supporting information (Figure S6).


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Figure 9. XPS spectra of (a) pristine and (b) O2-treated MoS2 samples. To complete this investigation and to examine the effect of O2 plasma on exfoliation of MoS2 powders, 200 O2-N15D15 and 200 O2-D15N15 suspensions were prepared based on the procedure outlined in the previous experimental section. DLS and AFM measurements were carried out to characterize size and thickness of nanoflakes. As presented in Figure 10, the thickness distribution of obtained dispersions dramatically decline to lower thicknesses mostly below 5 nm while flake’s dimensions appear to be larger. This point is an important observation where lower thickness flakes are achieved without reduction in their lateral size compared to the previous sections. The 200 ON15D15 suspension contains thinnest and largest flakes, compared to the previous section. For 200 O-D15N15, 73% and 97% of flakes have thickness below 5 and 10 nm while 200 O-N15D15 have %97 flakes below 5 nm. Moreover, the average sizes of sheets are 310 and 344 nm for 200 O-D15N15 and 200 O-N15D15 dispersions, respectively.



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Figure 10. Thickness distributions of (a) 200 O2- D15N15 and (b) 200 O2-N15D15 obtained nanoflakes. Inset in each part depicts the size distribution. Referring to the data obtained in this part, one can deduce that pre-sonication oxygen plasma treatment is an effective method to arrive at efficient exfoliation (Figure S7). We speculate that O2 plasma can improve the exfoliation process of bulk powders via increasing electrostatic charging of layers, creating Coulombic repulsion between layers and subsequent weakening the Van der Waals (VDW) interaction, resulting in an easy exfoliation of flakes considerably with larger lateral size49. As seen in Figure 11, the TEM images of processed nanosheets demonstrate transparent layers in which sample 200 O-N15D15 contains fewest layers with larger surface area (Figure 11.b). Moreover, no trace of re-aggregation or porous structure is observed in the TEM images of nanosheets showing no special deformation of layers due to exposing to O2 plasma.


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Figure 11. TEM images of prepared MoS2 nanosheets based on the a) 200 O2-D15N15. b) 200 O2-N15D15 approaches. (The scale bar 60 nm in both images) To measure the concentration of all procured dispersions, 1 g MoS2 powders were added to 15 ml NMP and sonicated for 15 mins followed by centrifugation at 3000 rpm for 60 mins and finally its sediments were collected. By weighing residual sediments, final concentration of N15 dispersion was calculated (~18 mg/ml). For other suspensions, the Lambert-Beer law was utilized using N15 concentration as a reference. The UV-Visible absorbance spectra of all other samples have been obtained and presented in the supporting information document (Figure S8, Table S2). Equation S-1 in the supporting information presents a formula to estimate the concentration of the procured dispersions through UV-Visible spectra at particular absorbance wavelength. Although the threestep technique leads to a decrease of final concentration, the results are comparable with other reports (Table S3)29-30, 32, 41, 46-47, 66-68. One should bear in mind that the final concentration of O2treated suspensions are higher than D15N15 and N15D15 samples owning to partially oxidize of bulk MoS2 powders49. Figure 12 depicts the AFM image of obtained nanoflakes by 200 O-N15D15 approach. The height profile of image shows a thickness of 2-3 nm particularly referring to few layers MoS2.



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Figure 12. AFM image of obtained MoS2 nanoflakes for 200 O2-N15D15 suspension. (Scale bar 300 nm)

Raman spectroscopy is another powerful analysis used to confirm the proper exfoliation of bulk powders to arrive at few-layered structures. Raman spectrum of MoS2 possesses two characteristic peaks of E12g (in-plane phonon oscillation) and A1g (out-plane phonon oscillation) which in the case of few-layered MoS2, a shift occurs in the position of these peaks41, 69. A comparison between the Raman spectra of obtained nanoflakes (200:O-N15D15) and pristine bulk MoS2 clearly shows a remarkable shift in the peaks proving well-exfoliation of bulk powders to few layered flakes (Figure 13).


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Figure 13. Raman spectra of few layered (200 O2-N15D15) and pristine MoS2.

4. Optical response of MoS2 layers To finally investigate the electrical and optical response of MoS2 dispersion, we have used 200:ON15D15 MoS2 suspension for the formation of phototransistors and studied its optoelectronic performance under laser illumination. The fabrication of these devices is initiated by deposition of 5 nm Ti and 50 nm gold on 280 nm thick SiO2/ p-type Si substrates by means of a radio frequency (13.56 MHz) sputtering unit. A standard photolithography technique was employed to realize an interdigital pattern with 4μm separation. The oxide layer acts as a back gate oxide for this structure while the Au/Ti interdigital features act as source and drain electrodes. To make contact to the silicon back gate, the SiO2 layer has been etched away in a buffered HF solution in desired regions. A droplet of 200:O-N15D15 MoS2 suspension was drop-casted on the prepared electrodes



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following by drying at 70 0C. This step is repeated for three times to ensure the complete coverage of MoS2 flakes over Au/Ti electrode spacing. Parts (a) and (b) of Figure 14 demonstrate the SEM images of the fabricated transistor based on few-layer MoS2 sheets. It is worth mentioning that to take SEM images, the fabricated device has been coated with 10 nm gold layer. This overcoating could end up with a film which is thicker than its real value. The AFM profile and Raman spectrum of layers in the channel are given at Figure S9 where the thickness is as expected. As shown in part (b), the MoS2 nanoflakes completely cover the whole 4µm spacing between electrodes. A schematic representation of MoS2 phototransistor is also illustrated in Figure 14 (c). In this configuration, the source electrode is connected to ground.


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Figure 14. (a), (b) SEM images of the fabricated phototransistor. (c) A schematic illustrator of fabricated phototransistor based on the 200:O-N15D15 MoS2 suspension. P-type silicon acts as a back gate to control the drain-source current. The current-voltage characteristic of the device at different gate voltages is shown in Figure 15. Schottky barriers in metal-MoS2 contacts are assumed to be responsible for the nonlinear and rectifying behavior of Ids-Vds curves. By applying a positive voltage on the gate, the Schottky barriers are reduced and subsequently more drain current passes through the device. In contrast, applying a negative gate voltage raises Schottky barriers and leads to a smaller current to travel through the MoS2 layer. Moreover, the fabricated MoS2 transistor shows an n-type behavior with a field effect mobility (µFE) of 2.1 cm2/Vs and an on/off current ratio (Ion/Ioff) of more than 103 while the threshold voltage (Vth) is measured to be around 5 V, as seen in the inset of Figure 15.



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Furthermore, pristine MoS2 transistor exhibits a field effect mobility of 3.19 cm2/Vs better than O2-treated MoS2. The lower field-effect mobility of oxygen-treated MoS2 layers could be due to the formation of surface damages as a result of oxygen bombardment64. Although, O2-treated transistor demonstrates slightly lower mobility, it contains larger nanoflakes which can show great potential in the fabricating of electronic devices. We believe that a low temperature annealing (200-500 0C) can minimize surface damages to recover field-effect mobility. Further investigation on the plasma treatment on the electrical characteristics of these phototransistors is in underway. The field effect mobility was extracted based on the well-known following equation: µ=(L/WCoxVds)/(dIds/dVg) Eqn. (1) where Cox is the gate oxide capacitance (ε0εr/d), L and W are the channel length (4 µm) and width (3 µm), Vds is the drain-source voltage and Vg is the gate voltage. Here the thickness of oxide (d) is 285nm, ε0 the vacuum permittivity and εr is the relative permittivity of SiO2.


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Figure 15. Drain- source current and voltage characteristic of MoS2 transistor versus different applied gate voltages. Inset shows the Ids-Vg characteristic of fabricated phototransistor. The photoresponse characteristic of the device is shown in Figure 16 under excitation laser wavelength of 532 nm (1 mW) and dark state. The laser spot is close to 500 µm and covers the whole interdigital area of the device. As can be seen, generation of electron-hole pairs associate with a significant enhancement of the current in the illuminated device compared to the dark current (Figure 16.a). Moreover, Figure 16.b demonstrates the photocurrent response of the device to 10 seconds illumination of laser excitation with various drain and gate voltages. Accordingly, the photo-current increases to approximately 2.5 µA as drain voltage bias increases from 4 to 8 V. In addition, applying a higher gate voltage results in further extraction of photocurrent in this device due to lowering Schottky barrier height in the contacts. It is observed that the maximum photoresponse occurs when both drain bias and gate voltages have positive values.



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Figure 16. The optical response of fabricated phototransistor to 532 nm laser excitation (a) IdsVds results under illumination and at dark states. (b) Photocurrent of device as a function of various drain bias and gate voltages. To explain the operation mechanism of these phototransistors, the energy band diagram of structure is depicted in Figure 17. Under an equilibrium state (Vds = 0, Vg = 0, no illumination), contacts are dominated by Schottky barriers as Fermi energy of metals are different from MoS2 flakes (Figure 17.a). During illumination (Vds > 0, Vg = 0), electron-hole pairs are excited via absorbing incident photons and enhance the photocurrent as applied drain bias is increased (Figure 17.b). Applying positive gate voltage (Vds > 0, Vg > 0) results in lowering Schottky barrier height and allows more electron-hole pairs to contribute to photocurrent (Figure 17.c). In addition, thermionic and tunneling phenomena participate in the overall current of device especially in the latter condition (Vds > 0, Vg > 0).


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Figure 17. A schematic representation of Fermi energy diagram of Metal and MoS2 structure under (a) Equilibrium state. (b) Drain voltage biasing with illumination, and (c) drain voltage biasing with applied gate voltage and illumination. To further investigate the opto-electrical behavior of the device, we have plotted the rise and fall of the current upon consecutive exposure to laser illumination (Figure 18). Through this experiment, the rise and fall times of phototransistor are measured to be close to 0.5 and 0.7 s, respectively, under a drain bias voltage of 8 V (Figure 18.a). Fifteen cycles of photo response as a function of illumination time are plotted in part (b). Based on these examinations, this device exhibits a stable performance under consecutive switching with no particular hysteresis. Further elaboration of the electrical and opto-electrical behavior of these devices is underway.



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Figure 18. Photo switching performance of device to 532 nm laser illumination (a) Rise and fall times of response. (b) Fifteen cycles of photocurrent response with consequence switching modes. At last, Table 1 provides some MoS2 optoelectronics devices (transistors) reported in the literatures including their field effect mobility, On/Off ratio and their photo-responsivity. It should be born in mind that the overall performance of devices depends on the experimental conditions such as laser excitation power and laser spot size during photo-sensing. The photo-responsivity of phototransistors to various laser excitations (514, 532, 561, and 670 nm) ranges from 0.42 mAW1

– 2200 AW-1 values. Our fabricated phototransistor consists of mono and few layers MoS2 which

exhibits a considerable performance comparable with other reports.


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Table 1. Comparison of some fabricated MoS2 Opto-Electronics devices in literature. MoS2 Device

Field Effect Mobility

On/Off

Excitation Laser

PhotoResponsivity

Response Time

Ref

PhotoTransistor

2.1 cm2v-1s-1

>103

532 nm

5 mAW-1

Rise=0.5s, Fall=0.7s

here

PhotoDetector

4 cm2v-1s-1

-

561 nm

880 AW-1

Rise=4s,Fall=9s

23

Rise=50ms, Fall=50ms

21

2 -1 -1

PhotoTransistor

0.11 cm v s

~10

Optical Switch

-

PhotoTransistor

0.23 cm2v-1s-1

3

-1

670 nm

0.42 mAW

-

532 nm

0.57 AW-1

Rise=70µs, Fall=110µs

70

105

532 nm

2200 AW-1

Fall=3s

71

5. CONCLUSION We have investigated a sonication-assisted exfoliation of bulk MoS2 powders to arrive at nanosheets. The impact of sonication time and power of different solvents (NMP and DMF) on both size and thickness of the resulted flakes have been examined. While NMP solvent acts more superior over DMF in achieving thinner sheets, it leads to an undesirable reduction in the lateral size of these sheets. In contrast DMF solvent has shown a better potential in obtaining larger flakes with increased thickness. By combining two solvents in a three-step approach, we have introduced a solvent exchange method to achieve larger flakes with smallest thicknesses. Besides, we have demonstrated that by exposing the initial bulk powders to oxygen plasma, one can accelerate the exfoliation process of flakes via Coloumbic repulsion between layers. Utilizing O2 treated MoS2 powder along with this three-step technique, we have successfully achieved large flakes with small thickness. We believe the sequential solvent exchange method can also be employed for exfoliation of other 2-D materials. Phototransistor devices have been realized using these layered structures and the electrical and optical behavior of such devices have been examined. The fabricated transistors demonstrate suitable field-effect mobility with high photocurrent and photoresponsivity.



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6. ACKNOLEDGEMENTS This research was partially supported by the Research Council at the University of Tehran. Authors want to thank Mr. Farzad Ghasemi for drawing the schematics and we are also immensely grateful to Mrs. Fatemeh Salehi, Mr. Ali Akhavan and Mr. Siyamak Sohrabi for their supports and technical assistants.

7. SUPPORTING INFORMATION DISCRIPTION SEM image of bulk MoS2 powders, Concentration calculations, UV-Visible spectra of dispersions, AFM image and Raman spectrum of transistor channel, as well as AFM images and photos of procured suspensions are mentioned in supporting information. 8. AUTHOR INFORMATION Corresponding Author [email protected]

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Sequential Solvent Exchange Method for Controlled Exfoliation of

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