Mechanical Exfoliation Assisted by Molecular Tweezers for Production

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

pubs.acs.org/IECR

Mechanical Exfoliation Assisted by Molecular Tweezers for Production of Sulfur-Based Semiconducting Two-Dimensional Materials Chinedu Obiakara,† Chih-Kai Liao,† and Mahmoud A. Mahmoud*,†,‡,§ †

Chemical Engineering, Department of Biomedical Engineering, ‡Department of Chemistry, and §Department of Physics and Astronomy, The University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249, United States

Downloaded via BUFFALO STATE on July 30, 2019 at 23:52:28 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The liquid and mechanical exfoliation techniques, used to produce two-dimensional materials (2DM), are merged in a mechanical exfoliation assisted by molecular tweezers technique (MEAMT). Sulfur-based semiconducting materials, that is, WS2 or MoS2 microcrystals dispersed in a liquid, are functionalized with thiolated long chain polymers. The polymer chains bound to the surface of microcrystals are stretched by ultrasonication, which generates a strong mechanical force. The force of the polymer tweezers overcomes the van der Waals force that holds the sheets of the crystals. By changing the chain length of polymer tweezers, the dimensions and the yield percentage of the MoS2 and WS2 sheets produced by MEAMT are controlled. The prepared 2DM functionalized with polymer improve their applicability and stability. Upon comparison to the typical liquid exfoliation technique, the WS2 and MoS2 sheets produced by MEAMT resulted in a higher yield percentage and higher photoluminescence (PL) quantum efficiency. The higher PL efficiency is attributed to the increased surface homogeneity of the sheets and reduced defect density.

■

INTRODUCTION Bulk sulfur-based transition metal dichalcogenides, MS2 (M = W, Mo), are semiconductors.1 When MS2 are prepared in mono- or few-layers, exciting optical and optoelectrical properties are obtained.2,3 MS2 two-dimensional materials (2DM) have received significant interest due to their exciting electrical, mechanical, and optical properties.4−11 The 2DM have potential applications such as in electronics,7 photonics, transistors,9 light-emitting diodes, solar cells, touchscreens,12 energy storage devices,5,13 and catalysis.14,15 Unlike bulk MS2 semiconductors, excited 2DM MS2 exhibit a large exciton binding energy. This allows for the generation of charged trions in addition to the regular excitons.16−19 Two common strategies are used to prepare the 2DM. The first is down-top via chemical vapor deposition (CVD) for powder4,20 or metals-organic.21,22 CVD technique is extensively used to prepare 2DM of slightly good morphology and scalability.20 In the CVD technique, the 2DM are grown on a substrate by thermal vaporization of the metal precursor that reacts with the thermally evaporated chalcogen elements such as sulfur and selenium.4,20−22 However, it is a challenge to reduce the defects in the 2DM prepared by the CVD technique, and it is difficult to control their spatial homogeneity and stoichiometry. High-quality 2DM have been prepared by molecular beam epitaxy, but this method is not cost-effective.23 © XXXX American Chemical Society

The second strategy to prepare 2DM is the top-down by mechanical exfoliation technique5 (MET) and liquid exfoliation technique (LET).4,5,24−26 The MET involves peeling a thin layer off bulk crystals by using an adhesive. For example, 2DM can be cleaved from a crystal by using Scotch tape with the assistance of plastic tweezers.5 However, preparing 2DM on a large scale by MET is challenging. Alternatively, LET is based on the sonication of the bulk powder in different solvents.27−30 These 2DM prepared by LET are scalable, but their usability is limited due to their suspension in a solvent. Removal of the solvent and transfer to the surface of a substrate are useful for overcoming the usability limits of these 2DM if the sheets are assembled in separate layers. The functionalized WS2 and MoS2 2DM have been used in a variety of fields, in particular, for medical and biological applications such as photothermal and chemotherapy for cancer treatments.31 Such semiconducting 2DM were used as multifunctional theranostic agents for photoacoustic imaging.32 Organic compound functionalized 2DM have been researched extensively for their role in targeted drug delivery.32,33 Polymers were used to improve the mechanical properties of 2DM and enhance their applicability.34−36 Received: Revised: Accepted: Published: A

June 6, 2019 June 24, 2019 July 18, 2019 July 18, 2019 DOI: 10.1021/acs.iecr.9b03082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

min. The precipitate was dispersed in 150 mL of DI water. The absorption spectrum of the prepared sheets was measured using the StellarNet dual detector super range spectrometer, UVN-SR (UV−vis−NIR, CCD) plus NIR-512 (InGaAs PDA). The StellarNet spectrometer is sensitive in the range of 200− 1700 nm. JEOL 2010F was used to conduct the transmission electron microscopy (TEM) imaging. LabRAM HR Evolution confocal Raman microscopy spectrometer using a 100× objective and 532 nm excitation laser of power of 5 and 1 mW were used to conduct the Raman and PL measurements, respectively. The Raman and PL measurements were conducted using a one scan setting with 5 s acquisition time. The Raman and PL settings of low laser power, the short accumulation time, and a single scan measurement are to sustain the integrity of the sample during the measurements. Atomic force microscopy (AFM) Icon Bruker was used to image the topography of the WS2 sheets. Malvern Nano Zetasizer was used to measure the zeta potential.

It is aimed to produce sulfur-based semiconducting 2DM of different dimensions in a large scale by a mechanical exfoliation assisted molecular tweezers technique (MEAMT). This technique involves the sonication of WS2 and MoS2 microcrystals after being functionalized with thiolated polyethylene glycol (PEG) of different chain length in solvent. The thiol group of PEG is bound to the surface of the WS2 and MoS2 microcrystals where W or M central atoms are lacking sulfur atoms. Once functionalized with polymer, sonication of the microcrystals generates a strong mechanical force, a result from the vibration of the polymer chains. The strong mechanical force exfoliates 2DM sheets from the surface of the microcrystals. The covalent or the semicovalent bonds between PEG and the surface of the microcrystals is stronger than the van der Waals force is holding the WS2 or MoS2 layers together. PEG can bind to the sulfur atoms located on the surface of WS2 or MoS2 through a disulfide bond and covalent bond. Additionally, the thiol group of PEG can form a semicovalent bond weaker, in terms of bond energy, than the fully covalent bond but stronger than a dipole−dipole interaction with W or Mo atoms on the surface of the WS2 or MoS2 microcrystals. The dimensions and the yield percent of the resulting sheets are controlled by changing the chain length of PEG. Raman and photoluminescence (PL) measurements were conducted to examine the quality of the sulfurbased 2DM prepared by the traditional LET and MEAMT techniques. By comparing the Raman band corresponding to the longitudinal acoustic (LA) of sulfur-based 2DM prepared by LET and MEAMT, the sheets prepared by MEAMT exhibited lowered intensity. The lowered intensity of LA band indicates a reduction of defect density on the 2DM crystals. 2DM prepared by MEAMT produced sharper PL spectral peak due to a higher quality and reduced defect density. The PL results agreed well with the Raman measurements.

■

RESULTS AND DISCUSSION Effect of PEG Chain Length on Dimensions of Exfoliated 2D Sheets. A weak van der Waals force (vdWF) holds the S−W−S and S−Mo−S layers in the bulk WS2 and MoS2. This flexibility makes it easy to exfoliate single or multilayer sheets of WS2 and MoS2 from the surface of a large crystal by overcoming the vdWF holding the layers together, as in the case of LET.24 Sulfur-based 2DM can be electrochemically exfoliated using lithium ions. MoS2 and WS2 are usually present in a thermodynamically stable 2H crystal structure and are converted to the less stable 1T structure after doping with foreigner ions.4,37 The prepared sheets by electrochemical technique, doped with lithium ions, are likely 1T structure. Solvent mixtures such as NMP and water were used to improve the yield of the exfoliated sheets from the surface of the powder.25 The solvent molecules intercalated between the WS2 and MoS2 layers and enhanced their exfoliation.26,38,39 The yield and dimensions of the exfoliated sheets depend on the solubility and viscosity of the solvent used in the liquid exfoliation techniques.24 The interaction of the NMP−water mixture with the surface of MoS2 is high at low NMP/water ratio and decreases upon increasing the percent of water. The NMP−water heteroaggregates are formed between the MoS2 layers during the LET at low water percent, enhancing the exfoliation process. Functionalizing the surface of the sulfur-based semiconducting 2DM can enhance their applicability and increase their stability.40 Large-scale production of 2DM functionalized with polymer of different dimensions is useful for the production of optical and electrical devices and sensors.40 To produce WS2 and MoS2 2DM functionalized with PEG of various chain lengths, the bulk crystals dispersed in water−NMP 1:9 mixture were sonicated after functionalization with 1, 2, 5, 10, and 20k PEG. Figure 1A−F shows the TEM image of WS2 sheets obtained from the sonication of the large crystals before and after functionalization with thiolated PEG of chain length of 1, 2, 5, 10, and 20k, respectively. The surfaces of the WS2 sheets prepared in the presence of PEG are coated with polymer even after three times cleaning, precipitation by centrifugation, and redispersion in DI water. Figure S1 shows TEM image of MoS2 sheets prepared using PEG of different chain lengths. To investigate the effect of changing the chain length of polymer on the dimension of the prepared 2DM, the lengths and widths of over 100 isolated sheets were measured from 14 TEM

■

EXPERIMENTAL SECTION N-Methyl-2-pyrrolidone (NMP), MoS2, and WS2 powder of respective average diameters of 2 and 6 μm were purchased from Sigma-Aldrich, while mPEG-thiol of average molecular weights of 1, 2, 5, 10, and 20k g/mol was purchased from Laysan Bio, Inc. In a 200 mL beaker, 0.15 g of WS2 or MoS2 powder was added to 15 mL of aqueous solution of 50 μM PEG of 1, 2, 5, 10, or 20k. The resulting mixture was stirred by a magnet bar for 6 h at 700 rpm speed, and 135 mL of NMP was added. The resulting mixture was sonicated for 3 h, 5 s on and 5 s off, using Qsonica Q700 probe sonicator with 1/2 in. tip diameter. The solution was cooled in an ice bath during the sonication, with temperature kept bellow 15 °C. A similar exfoliation experiment was carried out in pure NMP. For this, 0.15 g of MoS2 powder was dispersed in 150 mL of solvent and sonicated. Furthermore, 0.15 g of MoS2 functionalized with 10k PEG and dispersed in 150 mL water was sonicated at the same setting. After the sonication, the resulting solution was diluted to 200 mL with deionized water (DI), and the large WS2 and MoS2 particles were removed by centrifugation in 50 mL tubes at a speed of 2000 rpm for 10 min in an Allegra 64R high-speed refrigerated centrifuge for 10 min. The high-quality sheets were decanted from the precipitated large particles, and the supernatant was centrifuged in 50 mL tubes at 9000 rpm for 20 min. The precipitated sheets were dispersed in 100 mL of DI water and sonicated for 10 min using ultrasonic cleaner bath, VWR Symphony. The aqueous solution of the MoS2 and WS2 was then centrifuged at 8000 rpm in 15 mL tubes for 10 B

DOI: 10.1021/acs.iecr.9b03082 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


width, and the surface area of the prepared WS2 and MoS2 sheets vary as the chain lengths of PEG was changed. Figure S4 shows the average diameters, widths, and surface area of WS2 and MoS2 prepared using PEG of various chain lengths. The values of the average diameters and widths of WS2 and MoS2 sheets are reported in Tables S1 and S2. The average length and width of WS2 sheets produced by the traditional LET using water−NMP 1:9 mixture are 322 ± 152 and 180 ± 97 nm, respectively. The average lengths of WS2 prepared in using 1, 2, and 5k PEG decreased to 274 ± 132, 193 ± 89, and 287 ± 131 nm, respectively. The average widths of WS2 prepared using 1, 2, and 5k PEG were also reduced to 130 ± 57, 134 ± 63, and 192 ± 85 nm, respectively. Conversely, using 10 and 20k PEG during the exfoliation increased the dimensions of the produced WS2 sheets. However, WS2 sheets of length of 367 ± 216 and 353 ± 151 nm and width of 198 ± 116 and 181 ± 91 nm are produced upon using 10 and 20k PEG, respectively. Thus, by comparing the surface area of the WS2 prepared in the absence and presence of PEG, the surface area of WS2 was decreased upon using 1, 2, and 5k PEG, while larger surface area sheets were obtained upon using 10 and 20k PEG. The smallest WS2 sheets were obtained when 2k PEG was used. The dimension of the MoS2 prepared without using PEG was 241 ± 102 nm length and 142 ± 64 nm width. The lengths of the prepared MoS2 reduced to 175 ± 82 and 202 ± 114 nm when 2 and 20k PEG were used, while the widths reduced to 125 ± 66 and 111 ± 68 nm, respectively. The opposite behavior trend was observed when 1, 5, and 10k PEG were used during the exfoliation of MoS2; the obtained sheets have ∼270 nm length larger than that produced in absence of PEG. Generally, PEG increased the surface area of the prepared MoS2 2DM except in the case of using 2 and 20k PEG, which is decreased, see Table S2. Optical Characterization of Sulfur-Based Semiconducting 2DM. The optical properties of semiconducting 2D materials depend on the dielectric function of the surrounding medium,9 the crystalline structures and crystal imperfections,41 applied strain on the sheets,42 chemical doping,41,43 the number of layers per sheet, and temperature.44 Comparing the optical properties of WS2 and MoS2 prepared by the traditional LET with those prepared in the presence of PEG

Figure 1. TEM image of WS2 prepared in water−NMP solvent mixture of 1:9 ratio by (A) exfoliation of pure WS2 crystals, and WS2 crystals functionalized with PEG of chain length of (B) 1k, (C) 2k, (D) 5k, (E) 10k, and (F) 20k.

images. Statistical analysis was carried out to determine the average lengths and widths of the prepared WS2 and MoS2 sheets as shown in Figures S2 and S3. The average length,

Figure 2. Absorption spectrum of (A) WS2 sheets and (B) MoS2 prepared without PEG and with PG of average molecular weight of 1, 2, 5, 10, and 20k. The absorption spectrum of WS2 and MoS2 prepared without PEG is less intense compared with those prepared using PEG. C


Article


yield is decreased from 0.055 mg/mL in case of pure NMP to 0.0126 mg/mL in case of using pure water as a solvent. Finally, 0.247 mg/mL was obtained when the 1 mg/mL MoS2 functionalized with 10k PEG was exfoliated while dispersed in water solvent. The slightly red shift of the main absorption peak of WS2 and MoS2 2DM prepared with PEG of various chain lengths suggested the following: (1) The thiol groups of PEG cause the n-doping of the WS2 and MoS2 sheets.4,37 (2) The thiol group of PEG binds to the defective spots on the surface of the WS2 and MoS2 replacing missing sulfur atoms,40 or bind with their sulfur atoms through the S−S bond.31 Raman spectra collected from the surface of MoS2 sheets prepared in the presence of PEG of different chain lengths showed Raman bands at 496 and 563 cm−1 assigned as S−S stretching and bending, respectively, see Figure S6B. These two Raman bands were not observed in the case of MoS2 sheets produced in the absence of PEG, which confirmed the formation of S−S bond between PEG and MoS2. (3) PEG did not induce any 2H and T1 phase transition of either WS2 or MoS2, as no blue shift in the absorption peak was observed, and (4) the large molecular weight of the polymer did not introduce any additional strain to the WS2 and MoS2 sheet while dispersed in water, due to the high solubility of the PEG in water, as no change in the optical signal was observed.49 Liquid Exfoliation Assisted Molecular Tweezers Technique. The dimensions and the percent yield of the obtained WS2 and MoS2 using PEG depend greatly on the average molecular weight of the polymer, Figure S5. It is useful to understand the role of PEG in the exfoliation of WS2 and MoS2 sheets. The solubility theory is commonly used to discuss the mechanism of liquid exfoliation.50 This theory is based on studying the thermodynamics involved in the solubility of nanomaterials in the solvent during the exfoliation. Generally, the solubility of two materials is dependent on the change in the entropy, ΔS, and the change in the enthalpy, ΔH. The exfoliation of the MS2 functionalized with PEG of different chain lengths can be described using the Gibbs free energy, ΔG (eq 1):

can provide useful information about the quality and the yield percent of the prepared 2DM. Figure 2A shows the absorption spectrum of WS2 prepared by traditional LET and that prepared by exfoliation in the presence of PEG of different chain lengths. The optical measurements were conducted for cleaned WS2 dispersed in 150 mL of water after two-times dilution. The three characteristic absorption peaks of WS2 were observed at 639, 529, and 466 nm, which is equivalent to 1.94, 2.34, and 2.66 eV in the case of all the WS2 samples prepared without and with PEG of different chain lengths. The peaks at 1.94 and 2.34 eV are excitonic absorption of the direct band gap and identified as A and B peak, respectively.44−46 The energy difference between A and B band is ∼0.4 eV for the WS2 prepared with and without PEG. The energy difference between A and B is usually fixed and independent of the number of WS2 layers in each sheet.44−46 The third peak centered at 2.66 eV is labeled as C.47,48 The extinction coefficient of WS2 at 639 nm peak is 27.56 mL mg−1 cm−1.24 Consequently, the average concentrations of WS2 prepared without and with PEG of chain lengths of 1, 2, 5, 10, and 20k are 0.021 ± 0.003, 0.064 ± 0.002, 0.051 ± 0.002, 0.073 ± 0.003, 0.079 ± 0.004, and 0.067 ± 0.002 mg/mL, respectively. This suggests that the yield of the WS2 increased 310, 240, 350, 380, and 320% upon using PEG of average molecular weights of 1, 2, 3, 5, 10, and 20k, respectively. Interestingly, the yield of WS2 produced when PEG was increased in the same order of increasing the surface area of the sheets, see Figure S5. The 10k PEG produced the largest surface area and highest yield WS2 sheets. Comparatively, the WS2 sheets prepared without PEG do not follow the direct correlation trend of surface area and percent yield. Instead, a lower yield percent was observed at this larger surface area. The exfoliation process was carried out in the presence of nonthiolated 8k PEG to investigate the role of the thiol group of PEG in the exfoliation process of WS2. This possibly confirms whether the improved exfoliation efficiency is due to the functionalization of PEG to the surface of WS2 crystal or due to the increase of the viscosity of the solvent by the free PEG.24 Figure S6A shows the absorption spectrum of cleaned WS2 dispersed in water obtained from the LET in the presence of nonthiolated PEG after two-times dilution. The yield was slightly increased to 0.041 mg/mL, which was less than the yield obtained for WS2 in case of any thiolated PEG. The absorption spectrum of MoS2 prepared without and with PEG of different chain lengths after two-times dilution is shown in Figure 2B. The A, B, and C bands of MoS2 prepared without and with PEG of different chain lengths are located at 676, 616, and 464 nm, which are equivalent to 1.83, 2.01, and 2.67 eV, respectively. The energy difference between the A and B peaks is 0.18 eV. The average concentrations of the prepared MoS2 sheets were 0.067 ± 0.002, 0.092 ± 0.003, 0.079 ± 0.002, 0.087 ± 0.003, 0.107 ± 0.004, and 0.090 ± 0.003 mg/ mL when using no polymer and PEG of chain lengths of 1, 2, 5, 10, 20k, respectively. The extinction coefficient of the 676 nm peak of MoS2 is 34 mL mg−1 cm−1.24 The yield of the MoS2 slightly increased by ∼37, 17, 29, 59, 34% when PEG of average weights of 1, 2, 5, 10, and 20k was used, respectively. A systematic correlation between the surface area of the prepared MoS2 sheets and the yield percentage is observed, see Figure S5. To investigate the role of thiolated PEG in the exfoliation of sulfur-based semiconducting 2DM, MoS2 was exfoliated in the absence of PEG in pure NMP and in pure water, and in water after functionalization with 10k PEG. Interestingly, the

ΔGmix = ΔHmix − T ΔSmix

(1)

where ΔHmix and ΔSmix represent the changes of the enthalpy and entropy of the MS2−PEG after exfoliation from the surface of the large crystal and dispersion in the NMP−water solvent at temperature T. The estimated chain lengths of the stretched thiolated PEG of average molecular weights of 1, 2, 5, 10, and 20k polymer, based on the calculations of the bond lengths, are ∼10, 20, 50, 100, 200 nm, respectively. The molecular weight of each repeated (−CH2−CH2−O−) unit is 44 g/mol, and the length of the repeating monomer is 0.44 nm. Because of the large size of the MS2 flakes functionalized with PEG of large molecular weights, the dominating factor for mixing of the solutes and the solvent will be the enthalpy.29 Consequently, the enthalpy of mixing per unit volume can be used to express the energy of exfoliation of MS2−PEG sheets from the surface of the large crystal. The Gibbs equation can be simplified and modified to include the surface energy of the MS2−PEG sheets as shown in eq 2: ΔHmix = ϕ1ϕ2Vmix[δ MS2 − PEG − δSolvent ]2

(2)

where Vmix is the total volume mixture (solvent and MS2−PEG sheets), free PEG, δMS2 − PEG and δSolvent are the Hildebrand D


Article

Industrial & Engineering Chemistry Research solubility parameters of MS2−PEG system and solvents mixture, respectively, and are expressed as the square root of the cohesive energy densities, and ϕ1 and ϕ2 are the volume fractions of the MS2−PEG and solvent, respectively. The introduction of surface energy to eq 2 makes it possible to describe the effective mixing of the MS2−PEG and NMP− water solvent. The closer the difference between δMS2 − PEG and δSolvent to zero, the more the enthalpy of mixing is minimized and the greater exfoliation is taking place. Because of the complicated interactions of the MS2−PEG flakes with the NMP−water solvent, the simple Hildebrand solubility parameters, introduced in eq 2, cannot address the exfoliation process accurately.24 For better accuracy, the Hanson solubility parameters are used to describe the dispersive force D, the polar interaction P, and hydrogen bonding interaction H between MS2−PEG and the solvent.23,45 Upon applying the Hanson solubility parameters to eq 2, the enthalpy of mixing can be expressed as shown in eq 3 where d is the dimensionality of the solute in terms of the volume fraction (d = 0 for a small molecule, d = 1 for a rod, and d = 2 for a platelet).51 For the MS2−PEG system, d is equal to 2 when considering 2D sheets:

To estimate the initial molar ratio between PEG and WS2 or MoS2, the weights of 367 × 198 × 0.7 nm and 270 × 131 × 0.7 nm, sheets obtained in the case of using 10k PEG during the exfoliation of WS2 or MoS2 are 1.2 × 108 and 1.8 × 108 g/mol, respectively. Complete exfoliation of 1 mg/mL WS2 and MoS2 microcrystal produces 2D sheets of estimated concentrations of 12.4 and 4.3 nM, respectively. The initial addition of PEG has a concentration of 5 μM, and the grafting density, or distance between each neighboring polymer chains, of PEG functionalized with gold nanoparticles is 0.1 chain/nm2 for both 2 and 6k PEG.56,57 The high concentration of PEG and the large surface area of the individual WS2 and MoS2 sheet, ∼145 340 and ∼72 580 nm2, respectively, are responsible of coating the surface of the exfoliated sheets with a large number of PEG chains, see TEM images in Figure 1. This makes the values of the HSP of MS2−PEG depend more greatly on PEG rather than MS2. However, Ra of pure WS2 and MoS2 in the solvent mixture is ∼5.6 MPa1/2 lower than that of WS2 or MoS2 coated with PEG, with a value of ∼7.5 MPa1/2. Furthermore, the value Ra for the exfoliation of pure MS2 in water and in pure NMP is 36.4 and 3.8 MPa1/2, respectively, while the value of Ra of the exfoliation of MS2−PEG system in water is 38.1 MPa1/2. The increase in Ra value implies that upon addition of the polymer to the exfoliation process, a decrease in the yield percent is predicted; however, the experiment resulted in a higher yield percentage. Moreover, an increase in Ra value is expected from increasing the chain of the polymer. Experimentally, an increase in the exfoliated yield was observed in MS2−PEG system as the chain length was increased. On the basis of these two observations, one may attribute the increase of the efficiency of the exfoliation process upon using PEG as not due to a change in solubility, and the role of PEG during the exfoliation process would require further investigation. Because of the increase of the value of Ra of MS2 functionalized with PEG, the dispersibility of pure MS2 is higher than the MS2−PEG, especially in case of 1k and 2k PEG. Although MS2−1k and 2k PEG samples are easily agglomerated, they showed an excellent dispersibility upon 30 s sonication by a weak ultrasonic cleaner bath. Interestingly, no agglomeration was observed in case of MS2−PEG prepared using 5, 10, and 20k PEG. The surface charge of MS2 after functionalization with PEG exhibited no significant change. However, zeta potential measurement showed that MoS2 has −14 mV, which decreased to be −9.5 mV after functionalization with 10 PEG. The driving forces for PEG bound to the surface of neighboring sheets to assemble are the van der Waals attractive forces between the PEG chains on the surface of different sheets and PEG chains on one sheet and the surface of the other sheet. The attractive forces induce the sheet to agglomerate. Additionally, steric repulsion forces are generated between the PEG chains, which confine their assembly and sheet agglomeration. The agglomeration of the MoS2 functionalized with 1 and 2k PEG is because the van der Waals attractive forces exceed the steric repulsion forces. When polymers are sonicated, a mechanical force is generated due to the stretching and elongation of the polymer chains.56,57 This generated force leads to various arrangements of the polymer chains such as dumbbell, coiled, folded, or kinked.58,59 The mechanical response of the polymer, as in the case of PEG, depends on the ultrasound intensity, solvent vapor pressure and viscosity, temperature, solution concentration, initial polymer molecular weight, and the chemical structure of the polymer.58 Ultralong chain polymer, such as

ΔHmix i dy = jjj1 − zzzϕ(1 − ϕ)(δD ,MS2 − PEG − δD ,Solvent)2 Vmix 3{ k + (δP ,MS2 − PEG − δP ,Solvent)2 + (δH ,MS2 − PEG − δH ,Solvent)2

(3)

Eq 3 addresses the relationship between the enthalpy of mixing and the Hansen solubility parameters (HSP) well. The derivations of eq 3 do not require the 2D sheets to be rigid, thus making the equation applicable to the MS2−polymer system.51 To minimize the energy required to induce the exfoliation and increase the dispersity of MS2−PEG, all three Hanson solubility parameters of the NMP−water solvent must match those of MS2−PEG. Comparing the exfoliation of MS2 functionalized with PEG of various chain lengths, the Hanson parameters of the solvents and MS2 are fixed. However, the values of δD, δP, and δH are 17.8, 12.7, and 10.7 MPa1/2 for the solvent mixture and 18, 8.5, and 7 MPa1/2 for MS2 sheets, respectively. The value of Hanson parameters of PEG is slightly increased by increasing the chain length of the polymer,52−54 but the average values of δD, δP, and δH of PEG are 20.3, 9.6, and 6.0 MPa1/2, respectively.55 According to the introduced values of HSP, when the surface of MS2 is functionalized with PEG, the polymer is expected to increase the value of δD of MS2, thus increasing the difference between δD, MS2 − PEG and δD, Solvent. Conversely, PEG decreases the value of δH of MS2, which thus increases the δH, MS2 − PEG and δH, Solvent difference. Additionally, PEG decreases the difference between δP, MS2 − PEG and δP, Solvent slightly due to increasing the value of δP of MS2. The efficiency of the dissolution process can be simply expressed by HSP interaction distance, “Ra”, see eq 4. The lower value of Ra indicates the higher the efficiency of the exfoliation process: (Ra)2 = [4(δD ,MS2 − PEG − δD ,Solvent)2 + (δP ,MS2 − PEG − δP ,Solvent)2 + (δH ,MS2 − PEG − δH ,Solvent)2 ]

(4) E


Article

Industrial & Engineering Chemistry Research 2000K, suffers from chain scission after sonication, while shorter chain polymers tend to assemble into large aggregates.60 As the polymer is stretched out by sonication, the ultralong chain polymers rupture and shorten the number of units along the backbone by breaking the covalent C−C bonds.60 On the other hand, stretching short chain PEG, as in the case of all the PEG used in this study, increases their hydrophobicity.60 Consequently, when the PEG functionalized with the large WS2 or MoS2 crystals are elongated due to sonication, it is expected to induce the exfoliation of the sheets due to the generation of strong mechanical force that overcome the van der Waals force holding the sheets. As stated in the Experimental Section, PEG was allowed to bind initially to the surface of WS2 and MoS2 microcrystals before the exfoliation process. Despite this, sonication is used to enhance the thiol functionalization to the surfaces. It is useful to address whether PEG binds once initially or binds continuously to the crystal after removing the first layer during exfoliation. This can be accomplished by comparing the number of exfoliated flakes produced from the initial conjugation of PEG to the surface of the crystal and the total yield percentage obtained after 3 h exfoliation. The average diameters of the bulk WS2 and MoS2 crystals are 2 and 6 μm, which have estimated surface areas of ∼12.5 and ∼113 μm2, respectively. The number of flakes produced from the exfoliation of the first layer from the surface of the bulk crystals of WS2 and MoS2 functionalized with 10K should be ∼173 and ∼2993 flakes, respectively. The estimated yield obtained from of the first exfoliated layer is approximated to being ∼1.2 × 10−5 and ∼2.3 × 10−7 mg/mL, respectively. Therefore, the initial layer of PEG only produces a small fraction of the overall yield. On the basis of the experimental results shown in Figure S5, there is a systematic relationship between the chain length of the polymer with the surface area and yield percentage of the flakes produced. Thiolated 10k PEG improved the yield of exfoliation of WS2 two-times compared with the nonthiolated 8k PEG. As shown by the data, there is a systematic relationship between the flake surface area and yield percentage and the chain length of the polymer used during the exfoliation. The exfoliation of MS2, functionalized with PEG, proceeds by mechanical exfoliation assisted by molecular tweezers, is a merge between LET and MET. The surface of WS2 or MoS2 crystalline powder was functionalized with PEG through the thiol group. Despite the hydrophobic surfaces of the WS2 and MoS2 crystals, the PEG functionalization is driven by the strong binding affinity of the thiol group to these surfaces. The thiol groups can bind to sites that are deficient in sulfur atoms and to the surface sulfur atoms through the S−S bond.40,61 The energy supplied to the solution from the ultrasonicator induced the vibration of the PEG chains bound to the surface of WS2 or MoS2 generating a mechanical force that assisted the exfoliation of the sheets. The PEG chains bound to the surface of the microcrystal act as molecular tweezers to separate the flakes with greater efficiency. The van der Waals force holding the WS2 or MoS2 layers is weaker than the covalent bonds between the thiolated polymer and the surface of the powder. Additionally, changing the chain length of PEG varies the strength of the mechanical force induced by sonication; thus, the surface area and yield of the flakes depend on the chain length of PEG. Figure 3 is schematic depiction summarizing the MEAMT technique used to prepare sulfur-based 2DM. Sonication of the solution enhances the rate of functionaliza-

Figure 3. Exfoliation of sulfur-based microcrystals powders into 2DM by the mechanical exfoliation assisted molecular tweezers technique. This technique involves functionalization of the sulfur-based microcrystals with PEG, ultrasonication to generate a strong mechanical force from the vibrating polymer chains, and the induction of the exfoliation of sheets from the surface of the microcrystals.

tion of PEG with the surface of bulk crystal due to the following: (1) Sonication stretches the polymer and increases the exposure of the single thiol group of PEG, which can be hidden inside the long chain polymer, to defect sites. (2) Sonication improves the mobility of PEG chains, which increases the probability of collision with the surface of the microcrystal, thus increase the crystal-PEG binding efficiency. (3) Stretching the free PEG by sonication decreases the hydrophilicity of PEG,60 which increases the hydrophobic microcrystal polymer interactions. The yield and the dimensions of the MS2 (M = Mo or W) produced by MEAMT depend on the chain length of PEG. Generally, the mechanical force generated as a result of PEG chain vibration increased by increasing the chain length of PEG. However, the yield did not increase linearly by increasing the chain length of PEG. Instead, the minimum yield was obtained in the case of 2k PEG, while 10k PEG produced the highest yield. This suggests that other factors influence the exfoliation of MS2 such as polymer−polymer interactions resulting from van der Waals attraction and steric repulsion forces. Our recent study showed that PEG of different chain lengths functionalized to the surface of WS2 2DM is found to assemble into different structures depending on their chain lengths.62 However, the attraction and repulsion forces depend on the chain length of PEG. Thus, the polymer chain interactions play a role during the production of MS2 sheets by MEAMT. The decreasing of the yield of MoS2 and WS2 produced by 20k PEG compared with 10k PEG could be attributed to 20k PEG chain rupture.60 Effect of Chain Length of PEG on Optoelectrical Properties of Sulfur-Based 2D Materials. The electrical and photoelectrical properties of the 2DM depend greatly on the charge carriers generation, mobility, and interaction.63,64 Semiconducting 2DM exhibit strong light−exciton interactions.65,66 2DM have large surface area and a great possibility of having defects, which can trap the excitation energy and lower their photoelectrical efficiency.67 Raman spectroscopy is used to determine the number of layers, stacking, and defects of 2D materials such as WS2 and MoS2.68 The Raman spectrum of 2DM shows a characteristic peak that corresponds to longitudinal acoustic (LA) phonons at the edge of the Brillouin zone (M point).69 The LA peak is attributed to the presence of defects, and its intensity is proportional to the density of defects.70 This makes it possible to quantify the defects in the MS2 using Raman spectroscopy by simply dividing the intensity of the LA band by the intensity of the F


Article


Figure 4. Raman spectrum of (A) WS2 and (B) MoS2 prepared with LET and MEAMT using different chain lengths of PEG. The inset is the ratio between the value of the intensity of 2LA and E′ Raman bands for (A) WS2 and (B) MoS2 prepared by MEAMT. This value is directly related to changes of the chain length of PEG suggesting the change of the defect density. The minimum defect density was observed for 1k in case of WS2 and 10k in case of MoS2. The LA Raman band attributed to the defect density.

Figure 5. PL spectrum of: (A) WS2 and (B) MoS2 prepared with LET and MEAMT using different chain lengths of PEG. The PL efficiency of sheets prepared by MEAMT using 1, 2, and 10k PEG showed higher PL intensity compared with that prepared with LET. All the MoS2 sheets prepared with MEAMT showed better PL efficiency compared with that prepared by LET.

first-order Raman modes associated with in-plane vibrations. The unit cell in the single layer of such materials belongs to the D3h point group symmetry.69 The point group of the unit cell of the MS2 depends on the number of layers; however, point group is D3h for odd number of layers and D3d for even number of layers due to the existence of inversion symmetry i.69 Figure 4A shows the Raman spectrum of WS2 and WS2 functionalized with PEG of different chain lengths. The characteristic Raman bands of WS2 corresponding to E′ and A′1 were observed.70 A′1 appeared at 418 cm−1, while the E′ band is overlapped with the 2LA band. The broad band centered at 351 cm−1 was deconvoluted via Lorentzian curve fitting into two peaks at 348 and 353 cm−1 corresponding to 2LA and E′, respectively, see Figure S7. The resonance of 2LA and E′ bands is observed only in the WS2 monolayer, which suggests that the prepared WS2 sheets are composed of single layers.70,71 To examine the effect of PEG chain length on the quality of the prepared WS2 sheets, the ratio between the intensities of the 2LA and E′ Raman bands (I2LA/E′) was determined and reported in the inset of Figure 4A. The value of I2LA/E′ is ∼1.5 in case of WS2

prepared by the traditional LET, which dropped to 0.8 for WS2 prepared by MEAMT using 1k PEG. The highest value of I2LA/E′ was observed in case of WS2 prepared using 5k PEG. The 2k and 20k PEG showed I2LA/E′ values higher than that of WS2 prepared by LET technique. High-resolution TEM imaging of WS2 produced by 1k PEG showed no defects confirming the Raman results, see Figure S9. Figure 4B shows the Raman spectrum of MoS2 sheets prepared by LET and MEAMT at different chain lengths of PEG. The E′ and A′1 Raman bands were observed at ∼381 and ∼403 cm−1, respectively, while the 2LA band appeared at ∼451 cm−1. The I2LA/E′ ratio for MoS2 is shown in the inset of Figure 4B. Interestingly, the values of all the MoS2 samples prepared by MEAMT showed an I2LA/E′ value lower than that prepared by LET. The weakest I2LA/E′ value was observed for 10k PEG. The electronic transitions in the 2DM can be identified from their photoluminescence (PL) spectra.72,73 The emitting states can also be identified from the PL spectrum of the 2DM.3 The defect density on the MS2 sheets can be estimated from the PL measurements.72,73 G


Article


was used to examine the crystal structure and the defect density of the prepared 2DM. 2DM prepared by MEAMT resulted in lowered defect density. Crystal defects trap the excitation energy and lower the photoelectrical efficiency of the semiconducting 2D materials. PL measurements suggested that the WS2 and MoS2 prepared by MEAMT have better optoelectrical properties than by preparation by LET due to improved degree of crystallinity and surface homogeneity. The limitation of LET is that it produces 2D materials with high defect density. Several of these defect sites along the surface contain sulfur vacancies. Interestingly, the thiol groups of PEG used in MEAMT has good affinity to bind to Mo or W atoms in the sheet. The thiol groups could substitute the missing sulfur atoms in the defect sites, thus repairing the structure of the sheet and improving the optoelectrical properties.

A PL spectral peak appears at a higher energy for MS2 sheets; however, defected MS2 sheets have an additional PL peak at a lower energy, which is attributed to the excitons binding with defects.70,74,75 This makes it possible to characterize the quality of the MS2 samples prepared by MEAMT. Figure 5A shows the PL spectrum of WS2 prepared by LET and MEAMT at different chain lengths of PEG after 532 nm laser excitation. A sharp and intense PL spectrum was observed for WS2 prepared with 1 and 10k PEG. Conversely, WS2 prepared with 5k showed the weakest and the broadest PL spectrum, in addition to the PL peak corresponding to the defect-trapped excitons observed at 1.9 eV. Interestingly, the intensity of the PL of WS2 prepared by LET and MEAMT at different PEG chain length followed the same trend as the I2LA/E′ of Raman bands, see the inset of Figure 4A. The PL spectra of MoS2 prepared with by LET and MEAMT are shown in Figure 4B. The 10k PEG showed the sharpest PL peak, which suggested a decrease of defects in the sheets. The 20k PEG produced MoS2 sheets with a strong PL, but a secondary PL peak was observed at 1.75 eV corresponding to the defect-trapped excitons. The weakest PL spectrum was observed for MoS2 sheets prepared by LET. Also, 1k, 5k, and 2k PEG produced MoS2 sheets with low PL spectrum. The PL results accorded well with the Raman measurements in the inset of Figure 4B. The Raman and PL measurements suggest that WS2 sheets prepared by MEAMT using 1k and 10k have higher quality than that prepared with traditional LET, while the higher quality MoS2 sheets were obtained with 10k and 20k PEG in case of using the MEAMT. The PL spectrum of the MS2 prepared by MEAMT is red-shifted compared to the pure MS2, which is attributed to the electron injection by thiol group, n-doping. This confirms the functionalization of PEG to the surface of MS2 prepared by MEAMT through the thiol group. The optical and Raman measurements suggested that the prepared WS2 and MoS2 by MEAMT are single layers. To confirm the monolayer structure of the MS2 prepared by MEAMT, AFM imaging was carried out for WS2−1k PEG, see Figure S8. The average thickness of the WS2 sheets functionalized with 1k PEG is 0.9 ± 0.3 nm, which correspond to a single layer WS2. The surface roughness of the sheets is attributed to the PEG chains assembly.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b03082. TEM images; statistical analysis; values of average diameters, widths, and percent yield; average surface area and yield percent; absorption spectrum; Raman spectra; AFM spectrum; HR-TEM image (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mahmoud A. Mahmoud: 0000-0002-1986-1797 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the UTSA startup funding. This contribution was identified by Dr. Harish Kumar, MIT as the Best Presentation in the 2018 ACS Fall National Meeting in Boston.

■

■

CONCLUSIONS Mechanical exfoliation assisted by molecular tweezers was used to produce sulfur-based semiconducting 2D materials functionalized with PEG. Polymer chains bound via the thiol group to the surface of MoS2 and WS2 microcrystals vibrate when exposed to ultrasonication and act as molecular tweezers to exfoliate sheets from the surface of the crystals. The strength of the mechanical force generated by the polymer bound to the surface of the crystals depends on the chain length of the polymer. Consequently, the dimensions of the exfoliated sheets and the yield percentage varied upon changing the chain length of PEG. The Hansen solubility parameters that are usually used to describe the liquid exfoliation process were not valid to describe the MEAMT technique, which confirmed the mechanical effect of polymer in the exfoliation process. The advantages of the MEAMT are as follows: (1) This technique produces 2D materials functionalized with polymer. The polymer improves the mechanical properties of the sheets and facilitate their assembly can be controlled. (2) The 2D materials produced by MEAMT have high yield percentage and tunable dimensions. The Raman spectroscopy technique

REFERENCES

(1) Mukherjee, S.; Maiti, R.; Katiyar, A. K.; Das, S.; Ray, S. K. Novel Colloidal MoS2 Quantum Dot Heterojunctions on Silicon Platforms for Multifunctional Optoelectronic Devices. Sci. Rep. 2016, 6, 29016− 2926. (2) Zeng, H.; Dai, J.; Yao, W.; Xiao, D.; Cui, X. Valley Polarization in MoS2Monolayers by Optical Pumping. Nat. Nanotechnol. 2012, 7, 490−493. (3) Mak, K. F.; He, K.; Shan, J.; Heinz, T. F. Control of Valley Polarization in Monolayer MoS2 by Optical Helicity. Nat. Nanotechnol. 2012, 7, 494−498. (4) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (5) Li, H.; Wu, J.; Yin, Z.; Zhang, H. Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS2 and WSe2 Nanosheets. Acc. Chem. Res. 2014, 47, 1067−1075. (6) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666−669. (7) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699− 712. H


Article


by Water-Assisted Cosolvent Liquid-Phase Exfoliation. Chem. Mater. 2016, 28, 7586−7593. (26) Varrla, E.; Backes, C.; Paton, K. R.; Harvey, A.; Gholamvand, Z.; McCauley, J.; Coleman, J. N. Large-Scale Production of SizeControlled MoS2 Nanosheets by Shear Exfoliation. Chem. Mater. 2015, 27, 1129−1139. (27) Altavilla, C.; Sarno, M.; Ciambelli, P. A Novel Wet Chemistry Approach for the Synthesis of Hybrid 2D Free-Floating Single or Multilayer Nanosheets of MS2@oleylamine (M=Mo, W). Chem. Mater. 2011, 23, 3879−3885. (28) Smith, R. J.; King, P. J.; Lotya, M.; Wirtz, C.; Khan, U.; De, S.; O’Neill, A.; Duesberg, G. S.; Grunlan, J. C.; Moriarty, G.; Chen, J.; Wang, J.; Minett, A. I.; Nicolosi, V.; Coleman, J. N. Large-Scale Exfoliation of Inorganic Layered Compounds in Aqueous Surfactant Solutions. Adv. Mater. 2011, 23, 3944−3948. (29) Coleman, J. N. Liquid-Phase Exfoliation of Nanotubes and Graphene. Adv. Funct. Mater. 2009, 19, 3680−3695. (30) Cunningham, G.; Lotya, M.; Cucinotta, C. S.; Sanvito, S.; Bergin, S. D.; Menzel, R.; Shaffer, M. S. P.; Coleman, J. N. Solvent Exfoliation of Transition Metal Dichalcogenides: Dispersibility of Exfoliated Nanosheets Varies Only Weakly between Compounds. ACS Nano 2012, 6, 3468−3480. (31) Chen, X.; McDonald, A. R. Functionalization of TwoDimensional Transition-Metal Dichalcogenides. Adv. Mater. 2016, 28, 5738−5746. (32) Liu, T.; Wang, C.; Gu, X.; Gong, H.; Cheng, L.; Shi, X.; Feng, L.; Sun, B.; Liu, Z. Drug Delivery with PEGylated MoS2 Nano-sheets for Combined Photothermal and Chemotherapy of Cancer. Adv. Mater. 2014, 26, 3433−3440. (33) Karunakaran, S.; Pandit, S.; Basu, B.; De, M. Simultaneous Exfoliation and Functionalization of 2H-MoS2 by Thiolated Surfactants: Applications in Enhanced Antibacterial Activity. J. Am. Chem. Soc. 2018, 140, 12634−12644. (34) Nguyen, H. K.; Fujinami, S.; Nakajima, K. Elastic Modulus of Ultrathin Polymer Films Characterized by Atomic Force Microscopy The Role of Probe Radius. Polymer 2016, 87, 114−122. (35) Cheng, X.; Putz, K. W.; Wood, C. D.; Brinson, L. C. Characterization of Local Elastic Modulus in Confined Polymer Films via AFM Indentation. Macromol. Rapid Commun. 2015, 36, 391−397. (36) Xia, W.; Hsu, D. D.; Keten, S. Molecular Weight Effects on the Glass Transition and Confinement Behavior of Polymer Thin Films. Macromol. Rapid Commun. 2015, 36, 1422−1427. (37) Py, M. A.; Haering, R. R. Structural Destabilization Induced by Lithium Intercalation in MoS2 and Related Compounds. Can. J. Phys. 1983, 61, 76−84. (38) Jawaid, A.; Nepal, D.; Park, K.; Jespersen, M.; Qualley, A.; Mirau, P.; Drummy, L. F.; Vaia, R. A. Mechanism for Liquid Phase Exfoliation of MoS2. Chem. Mater. 2016, 28, 337−348. (39) Carey, B. J.; Daeneke, T.; Nguyen, E. P.; Wang, Y.; Zhen Ou, J.; Zhuiykov, S.; Kalantar-zadeh, K. Two Solvent Grinding Sonication Method for the Synthesis of Two-Dimensional Tungsten Disulphide Flakes. Chem. Commun. 2015, 51, 3770−3773. (40) Chou, S. S.; De, M.; Kim, J.; Byun, S.; Dykstra, C.; Yu, J.; Huang, J.; Dravid, V. P. Ligand Conjugation of Chemically Exfoliated MoS2. J. Am. Chem. Soc. 2013, 135, 4584−4587. (41) Lin, Y.-C.; Li, S.; Komsa, H.-P.; Chang, L.-J.; Krasheninnikov, A. V.; Eda, G.; Suenaga, K. Revealing the Atomic Defects of WS2 Governing Its Distinct Optical Emissions. Adv. Funct. Mater. 2018, 28, 1704210−1704217. (42) Castellanos-Gomez, A.; Roldán, R.; Cappelluti, E.; Buscema, M.; Guinea, F.; van der Zant, H. S. J.; Steele, G. A. Local Strain Engineering in Atomically Thin MoS2. Nano Lett. 2013, 13, 5361− 5366. (43) Sim, D. M.; Kim, M.; Yim, S.; Choi, M.-J.; Choi, J.; Yoo, S.; Jung, Y. S. Controlled Doping of Vacancy-Containing Few-Layer MoS2 via Highly Stable Thiol-Based Molecular Chemisorption. ACS Nano 2015, 9, 12115−12123. (44) Zhu, B.; Chen, X.; Cui, X. Exciton Binding Energy of Monolayer WS2. Sci. Rep. 2015, 5, 9218−9222.

(8) Lee, K.; Kim, H.-Y.; Lotya, M.; Coleman, J. N.; Kim, G.-T.; Duesberg, G. S. Electrical Characteristics of Molybdenum Disulfide Flakes Produced by Liquid Exfoliation. Adv. Mater. 2011, 23, 4178− 4182. (9) Wang, H.; Yu, L.; Lee, Y.-H.; Shi, Y.; Hsu, A.; Chin, M. L.; Li, L.J.; Dubey, M.; Kong, J.; Palacios, T. Integrated Circuits Based on Bilayer MoS2 Transistors. Nano Lett. 2012, 12, 4674−4680. (10) Castellanos-Gomez, A. Why All The Fuss About 2D Semiconductors? Nat. Photonics 2016, 10, 202−204. (11) Mahler, B.; Hoepfner, V.; Liao, K.; Ozin, G. A. Colloidal Synthesis of 1T-WS2 and 2H-WS2 Nanosheets Applications for Photocatalytic Hydrogen Evolution. J. Am. Chem. Soc. 2014, 136, 14121−14127. (12) Khan, U.; Kim, T.-H.; Ryu, H.; Seung, W.; Kim, S.-W. Graphene Tribotronics for Electronic Skin and Touch Screen Applications. Adv. Mater. 2017, 29, 1603544. (13) Shehzad, K.; Xu, Y.; Gao, C.; Duan, X. Three-Dimensional Macro-structures of Two-Dimensional Nanomaterials. Chem. Soc. Rev. 2016, 45, 5541−5588. (14) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. Biomimetic Hydrogen Evolution MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308−5309. (15) Parzinger, E.; Miller, B.; Blaschke, B.; Garrido, J. A.; Ager, J. W.; Holleitner, A.; Wurstbauer, U. Photocatalytic Stability of Single- and Few-Layer MoS2. ACS Nano 2015, 9, 11302−11309. (16) Mak, K. F.; He, K.; Lee, C.; Lee, G. H.; Hone, J.; Heinz, T. F.; Shan, J. Tightly Bound Trions in Monolayer MoS2. Nat. Mater. 2013, 12, 207−211. (17) Neumann, A.; Lindlau, J.; Colombier, L.; Nutz, M.; Najmaei, S.; Lou, J.; Mohite, A. D.; Yamaguchi, H.; Högele, A. Opto-valleytronic Imaging of Atomically Thin Semiconductors. Nat. Nanotechnol. 2017, 12, 329−334. (18) Srivastava, A.; Sidler, M.; Allain, A. V.; Lembke, D. S.; Kis, A.; Imamoglu, A. Optically Active Quantum Dots in Monolayer WSe2. Nat. Nanotechnol. 2015, 10, 491−496. (19) He, Y.-M.; Clark, G.; Schaibley, J. R.; He, Y.; Chen, M.-C.; Wei, Y.-J.; Ding, X.; Zhang, Q.; Yao, W.; Xu, X.; Lu, C.-Y.; Pan, J.-W. Single Quantum Emitters in Monolayer Semiconductors. Nat. Nanotechnol. 2015, 10, 497−502. (20) Amani, M.; Burke, R. A.; Ji, X.; Zhao, P.; Lien, D.-H.; Taheri, P.; Ahn, G. H.; Kirya, D.; Ager, J. W.; Yablonovitch, E.; Kong, J.; Dubey, M.; Javey, A. High Luminescence Efficiency in MoS2 Grown by Chemical Vapor Deposition. ACS Nano 2016, 10, 6535−6541. (21) Kang, K.; Xie, S.; Huang, L.; Han, Y.; Huang, P. Y.; Mak, K. F.; Kim, C.-J.; Muller, D.; Park, J. High-mobility Three-Atom-Thick Semiconducting Films with Wafer-Scale Homogeneity. Nature 2015, 520, 656−660. (22) Eichfeld, S. M.; Hossain, L.; Lin, Y.-C.; Piasecki, A. F.; Kupp, B.; Birdwell, A. G.; Burke, R. A.; Lu, N.; Peng, X.; Li, J.; Azcatl, A.; McDonnell, S.; Wallace, R. M.; Kim, M. J.; Mayer, T. S.; Redwing, J. M.; Robinson, J. A. Highly Scalable, Atomically Thin WSe2 Grown via Metal−Organic Chemical Vapor Deposition. ACS Nano 2015, 9, 2080−2087. (23) Yue, R.; Barton, A. T.; Zhu, H.; Azcatl, A.; Pena, L. F.; Wang, J.; Peng, X.; Lu, N.; Cheng, L.; Addou, R.; McDonnell, S.; Colombo, L.; Hsu, J. W. P.; Kim, J.; Kim, M. J.; Wallace, R. M.; Hinkle, C. L. HfSe2 Thin Films 2D Transition Metal Dichalcogenides Grown by Molecular Beam Epitaxy. ACS Nano 2015, 9, 474−480. (24) Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; Shvets, I. V.; Arora, S. K.; Stanton, G.; Kim, H.-Y.; Lee, K.; Kim, G. T.; Duesberg, G. S.; Hallam, T.; Boland, J. J.; Wang, J. J.; Donegan, J. F.; Grunlan, J. C.; Moriarty, G.; Shmeliov, A.; Nicholls, R. J.; Perkins, J. M.; Grieveson, E. M.; Theuwissen, K.; McComb, D. W.; Nellist, P. D.; Nicolosi, V. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568−571. (25) Manna, K.; Huang, H.-N.; Li, W.-T.; Ho, Y.-H.; Chiang, W.-H. Toward Understanding the Efficient Exfoliation of Layered Materials I


Article


Advances in Two-Dimensional Materials beyond Graphene. ACS Nano 2015, 9, 11509−11539. (64) Dickinson, W. W.; Kumar, H. V.; Adamson, D. H.; Schniepp, H. C. High-Throughput Optical Thickness and Size Characterization of 2D Materials. Nanoscale 2018, 10, 14441−14447. (65) Ye, Z.; Cao, T.; O'Brien, K.; Zhu, H.; Yin, X.; Wang, Y.; Louie, S. G.; Zhang, X. Probing Excitonic Dark States in Single-Layer Tungsten Disulphide. Nature 2014, 513, 214−218. (66) Ugeda, M. M.; Bradley, A. J.; Shi, S.-F.; da Jornada, F. H.; Zhang, Y.; Qiu, D. Y.; Ruan, W.; Mo, S.-K.; Hussain, Z.; Shen, Z.-X.; Wang, F.; Louie, S. G.; Crommie, M. F. Giant Bandgap Renormalization and Excitonic Effects in a Monolayer Transition Metal Dichalcogenide Semiconductor. Nat. Mater. 2014, 13, 1091− 1095. (67) Amani, M.; Lien, D.-H.; Kiriya, D.; Xiao, J.; Azcatl, A.; Noh, J.; Madhvapathy, S. R.; Addou, R.; KC, S.; Dubey, M.; Cho, K.; Wallace, R. M.; Lee, S.-C.; He, J.-H.; Ager, J. W.; Zhang, X.; Yablonovitch, E.; Javey, A. Near-unity Photoluminescence Quantum Yield in MoS2. Science 2015, 350, 1065−1068. (68) Terrones, H.; Corro, E. D.; Feng, S.; Poumirol, J. M.; Rhodes, D.; Smirnov, D.; Pradhan, N. R.; Lin, Z.; Nguyen, M. A. T.; Elías, A. L.; Mallouk, T. E.; Balicas, L.; Pimenta, M. A.; Terrones, M. New First Order Raman-active Modes in Few Layered Transition Metal Dichalcogenides. Sci. Rep. 2015, 4, 4215−4223. (69) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2 A New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805−136808. (70) McCreary, A.; Berkdemir, A.; Wang, J.; Nguyen, M. A.; Elías, A. L.; Perea-López, N.; Fujisawa, K.; Kabius, B.; Carozo, V.; Cullen, D. A.; Mallouk, T. E.; Zhu, J.; Terrones, M. Distinct Photoluminescence and Raman Spectroscopy Signatures for Identifying Highly Crystalline WS2 Monolayers Produced by Different Growth Methods. J. Mater. Res. 2016, 31, 931−944. (71) Azizi, A.; Zou, X.; Ercuis, P.; Zhang, Z.; Laura Elias, A.; PereaLópez, N.; Terrones, M.; Yakobson, B. I.; Alem, N. Atomic-scale Observation of Grains and Grain Boundaries in Monolayers of WS2. Microsc. Microanal. 2014, 20, 1084−1085. (72) Kheng, K.; Cox, R. T.; d’ Aubigné, M. Y.; Bassani, F.; Saminadayar, K.; Tatarenko, S. Observation of Negatively Charged Excitons X− in Semiconductor Quantum Wells. Phys. Rev. Lett. 1993, 71, 1752−1755. (73) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147− 150. (74) Chow, P. K.; Jacobs-Gedrim, R. B.; Gao, J.; Lu, T.-M.; Yu, B.; Terrones, H.; Koratkar, N. Defect-Induced Photoluminescence in Monolayer Semiconducting Transition Metal Dichalcogenides. ACS Nano 2015, 9, 1520−1527. (75) Mignuzzi, S.; Pollard, A. J.; Bonini, N.; Brennan, B.; Gilmore, I. S.; Pimenta, M. A.; Richards, D.; Roy, D. Effect of Disorder on Raman Scattering of Single-Layer MoS2. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 195411−195417.

(45) Zhao, W.; Ghorannevis, Z.; Chu, L.; Toh, M.; Kloc, C.; Tan, P.H.; Eda, G. Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2. ACS Nano 2013, 7, 791−797. (46) Ross, J. S.; Wu, S.; Yu, H.; Ghimire, N. J.; Jones, A. M.; Aivazian, G.; Yan, J.; Mandrus, D. G.; Xiao, D.; Yao, W.; Xu, X. Electrical Control of Neutral and Charged Excitons in A Monolayer Semiconductor. Nat. Commun. 2013, 4, 1474−1479. (47) Zhang, C.; Wang, H.; Chan, W.; Manolatou, C.; Rana, F. Absorption of Light by Excitons and Trions in Monolayers of Metal Dichalcogenide MoS2 Experiments and Theory. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 205436−205446. (48) Zhu, B.; Zeng, H.; Dai, J.; Gong, Z.; Cui, X. Anomalously Robust Valley Polarization and Valley Coherence in Bilayer WS2. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 11606−11611. (49) Conley, H. J.; Wang, B.; Ziegler, J. I.; Haglund, R. F.; Pantelides, S. T.; Bolotin, K. I. Bandgap Engineering of Strained Monolayer and Bilayer MoS2. Nano Lett. 2013, 13, 3626−3630. (50) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’Ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.; Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. High-yield Production of Graphene by Liquid-Phase Exfoliation of Graphite. Nat. Nanotechnol. 2008, 3, 563−568. (51) Hughes, J. M.; Aherne, D.; Coleman, J. N. Generalizing Solubility Parameter Theory to Apply to One- and Two-Dimensional Solutes and to Incorporate Dipolar Interactions. J. Appl. Polym. Sci. 2013, 127, 4483−4491. (52) Adamska, K.; Voelkel, A. Hansen Solubility Parameters for Polyethylene Glycols by Inverse Gas Chromatography. J. Chromatogr. A 2006, 1132, 260−267. (53) Kawakami, M.; Egashira, M.; Kagawa, S. Measurements of the Interactions between Polyethylene Glycol and Organic Compounds by Gas Chromatographic Technique. Bull. Chem. Soc. Jpn. 1976, 49, 3449−3453. (54) Ö zdemir, C.; Güner, A. Solubility Profiles of Poly(ethylene glycol)/solvent systems, I: Qualitative Comparison of Solubility Parameter Approaches. Eur. Polym. J. 2007, 43, 3068−3093. (55) Adamska, K.; Voelkel, A.; Berlińska, A. The Solubility Parameter for Biomedical PolymersApplication of Inverse Gas Chromatography. J. Pharm. Biomed. Anal. 2016, 127, 202−206. (56) Mahmoud, M. A. Dynamic Template for Assembling Nanoparticles into Highly Ordered Two-Dimensional Arrays of Different Structures. J. Phys. Chem. C 2015, 119, 305−314. (57) Emilsson, G.; Schoch, R. L.; Feuz, L.; Höök, F.; Lim, R. Y. H.; Dahlin, A. B. Strongly Stretched Protein Resistant Poly(ethylene glycol) Brushes Prepared by Grafting-To. ACS Appl. Mater. Interfaces 2015, 7, 7505−7515. (58) Caruso, M. M.; Davis, D. A.; Shen, Q.; Odom, S. A.; Sottos, N. R.; White, S. R.; Moore, J. S. Mechanically-Induced Chemical Changes in Polymeric Materials. Chem. Rev. 2009, 109, 5755−5798. (59) Perkins, T. T.; Smith, D. E.; Chu, S. Single Polymer Dynamics in an Elongational Flow. Science 1997, 276, 2016−2021. (60) Duval, M.; Gross, E. Degradation of Poly(ethylene oxide) in Aqueous Solutions by Ultrasonic Waves. Macromolecules 2013, 46, 4972−4977. (61) Tuxen, A.; Kibsgaard, J.; Gøbel, H.; Lægsgaard, E.; Topsøe, H.; Lauritsen, J. V.; Besenbacher, F. Size Threshold in the Dibenzothiophene Adsorption on MoS2 Nanoclusters. ACS Nano 2010, 4, 4677− 4682. (62) Liao, C.-K.; Phan, J.; Herrera, M.; Mahmoud, M. A. Modifying the Band Gap of Semiconducting Two-Dimensional Materials by Polymer Assembly into Different Structures. Langmuir 2019, 35, 4956−4965. (63) Bhimanapati, G. R.; Lin, Z.; Meunier, V.; Jung, Y.; Cha, J.; Das, S.; Xiao, D.; Son, Y.; Strano, M. S.; Cooper, V. R.; Liang, L.; Louie, S. G.; Ringe, E.; Zhou, W.; Kim, S. S.; Naik, R. R.; Sumpter, B. G.; Terrones, H.; Xia, F.; Wang, Y.; Zhu, J.; Akinwande, D.; Alem, N.; Schuller, J. A.; Schaak, R. E.; Terrones, M.; Robinson, J. A. Recent J


Mechanical Exfoliation Assisted by Molecular Tweezers for Production

Recommend Documents