NANO EXPRESS Open Access

Synergistic Exfoliation of MoS2 byUltrasound Sonication in a SupercriticalFluid Based Complex SolventXi Tan, Wenbin Kang, Jingfeng Liu and Chuhong Zhang*

Abstract

Molybdenum disulfide (MoS2) is an extremely intriguing low-D layered material due to its exotic electronic, optical,and mechanical properties, which could be well exploited for numerous applications to energy storage, sensing,and catalysis, etc., provided a sufficiently low number of layers is achieved. A facile exfoliation strategy that leadsto the production of few-layered MoS2 is proposed wherein the exfoliation efficacy could be synergistically boostedto > 90% by exploiting ultrasound sonication in supercritical CO2 in conjunction with N-methyl-2-pyrrolidone (NMP)as the intercalating solvent, which is superior to general practiced liquid exfoliation methods wherein only thesupernatant is collected to avoid the majority of unexfoliated sediments. The facile and fast exfoliation techniquesuggests an exciting and feasible solution for scalable production of few-layered MoS2 and establishes a platformthat contributes to fulfilling the full potential of this versatile two-dimensional material.

Keywords: Molybdenum disulfide (MoS2), Exfoliation, Supercritical CO2, Ultrasound sonication

IntroductionTwo-dimensional (2D) transition metal dichalcogenides(TMD) have attracted substantial attention due to theatomically thin layer as well as unique and versatile elec-tronic properties spanning across being semiconductingto superconducting depending on the particular compos-ition and structure [1–4]. As a quintessential member inthe TMD family, molybdenum disulfide (MoS2) consistsof hexagonally arranged Mo atoms sandwiched by S atomsin an alternatingly occurring manner. The layered materialpossesses strong covalent bonds in a plane while the layersout of the plane are held together by weak van der Waalsbond, which in principle makes possible the exfoliation ofsuch a material into individually separate thin layers [5]. Ithas been reported that new physiochemical propertiesarise accompanying the exfoliation of MoS2 into a few-lay-ered structure such as enhanced-specific surface area, in-direct to direct bandgap transition, and improved surfaceactivity [6, 7].Thus, the great advantages of MoS2 remain hitherto elu-

sive until it is thin enough to induce the aforementioned

properties that could make MoS2 very appealing for vari-ous applications such as energy storage, catalysis, opticaldevices, and sensor [7–11].However, a facile and feasible exfoliation technique that

renders scalable production of high-quality few-layeredMoS2 remains to be highly sought after in order to fully tapinto the huge potential of MoS2 not only for small scalelaboratory demonstration or miniature microelectronicapplications but also for large scale practical utilization interms of, say, energy storage applications [12, 13]. Thesestringent requirements thus rule out currently popularproduction methods like CVD growth which is time-con-suming and involves high temperature and large energy in-put [14], micromechanical cleavage which suffers extremelylow yield and reproducibility [15], ion intercalation methodthat requires strong reducing intercalants and strict inertreaction atmosphere [16], and hydrothermal reaction thatinduces defects [17]. This leaves a liquid-phase exfoliation,a compelling strategy that could potentially strike an excel-lent balance among ease of exfoliation, quality, and scalabil-ity. Notwithstanding, in traditional liquid-phase exfoliation,common issues such as the use of surfactants difficult to re-move in the post-treatment defiles the purity and the in-trinsic electronic property of the 2D material [18] and

© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link tothe Creative Commons license, and indicate if changes were made.

* Correspondence: chuhong.zhang@scu.edu.cnState Key Laboratory of Polymer Materials Engineering, Polymer ResearchInstitute of Sichuan University, Chengdu 610065, China

Tan et al. Nanoscale Research Letters (2019) 14:317 https://doi.org/10.1186/s11671-019-3126-4

prolonged sonication time in order to enhance layer separ-ation and yield inevitably increase the density of defectsunder strong cavitation [19].Herein, an improved liquid-phase exfoliation method

is proposed that exploits the unique physiochemicalproperties and synergistic function of supercritical CO2

and N-methyl-2-pyrrolidone (NMP), which enables facileintercalation and simultaneously penalty reduction ofsystem enthalpy increase from exfoliation. The noveltactic promotes an effective and rapid exfoliation ofMoS2 into a few-layered 2D structure with a high yield,which sets a highly rewarding demonstration and holdsgreat promise for facile and scalable production of notonly exfoliated MoS2 but also possibly a library of itstwo-dimensional analogs.

MethodsMaterialsThe molybdenum disulfide powders (MoS2, 99.5%) andN-methylpyrrolidone (NMP, 99.9%) were purchased fromAladdin Reagent (Shanghai) and used without furtherpurification. Absolute ethanol (99.5%) was purchased fromChengdu Kelong chemicals. Purified water was purchasedfrom Sichuan Uppulta-pure Technology. CO2 with 99.5%purity was purchased from Chengdu Qiyu Gas.

Exfoliation processThe exfoliation device mainly consists of a high-pressurechamber which can be pressured up to 20MPa and an

ultrasonic probe. All exfoliation experiments wereperformed in the stainless-steel reactor chamber with amaximum volume of 250 mL. In a typical experiment,MoS2 powder (100 mg) was added and dispersed in aspecified solvent (150 mL), then the device was heatedup to a preset temperature by an electric heating jacketbefore CO2 was subsequently pumped into the reactorup to 14MPa using a manual pump. After thetemperature and pressure reached the preset level, theultrasonic probe was started for 1 h under the power of600W. After exfoliation, the pressure was released andthe chamber was opened, and the obtained MoS2 nano-sheets were thereafter repeatedly washed and collectedvia filtration before drying.

CharacterizationThe crystal structure was examined by X-ray diffraction(XRD, Rigaku Co., Japan) analysis under CuKα radiationat 10–80°with a scan rate of 10°/min. Raman spectrawere recorded on a laser Raman spectrometer (ThermoFisher Co., America) with a He-Ne laser at 532 nm atroom temperature. The number of layers and topog-raphy of the exfoliated samples were probed by atomicforce microscopy (ANSYS, Co., America) in a tappingmode with the sample prepared from solution-casting ofMoS2 nanosheets dispersion onto mica. The Brunauer–Emmett–Teller (BET) surface areas were analyzed froma Tristar 3020 apparatus (Micromeritics Instrument Co.,America) over a P/P0 range automatically determined by

Fig. 1 A schematic showing the exfoliation procedure and the concerted intercalation of supercritical CO2 and NMP

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Quadrawin. Sample surface chemistry was investigatedusing X-ray photoelectron spectroscopy (XPS) withmonochromated Al Kα X-ray source (excitation energyof 1486.6 eV) on XPS ESCALAB 250Xi. High-resolutiontransmission electron microscopy (HRTEM, QuantaAmerica) was carried out to determine the surfacemorphology and thickness. The examined sample wasprepared by dropping diluted dispersion of exfoliatedMoS2 onto a holey carbon-covered copper grid.

Results and discussionA schematic showing the exfoliation procedure is pre-sented in Fig. 1, and the detailed description could befound in the experimental section. Briefly, bulk MoS2 issuspended in a complex solvent made up of supercriticalCO2 and NMP followed by ultrasound sonication to initi-ate exfoliation. The critical factor that determines effectiveexfoliation lies in the employment of a complex solventconstituted by supercritical CO2 and NMP. For one thing,once the supercritical state is reached, CO2 deliversunique properties that stride between gas and liquidwherein a low viscosity, zero surface tension, and high dif-fusivity resembling those of gas set in, and at the sametime, it bears a certain density and behaves as a liquidsolvent. This peculiar combination makes supercriticalCO2 a surprisingly outstanding intercalating molecule thatinserts between MoS2 layers to weaken the Van der Waalsinteraction between adjacent layers given its small mo-lecular size in conjunction with the unrestrained mobility.On the other hand, it is established by Coleman that inorder to facilitate liquid-phase exfoliation, a careful choiceof solvent with matching surface tension to the surfaceenergy of the layered material so as to compromise thegain in enthalpy of mixing during exfoliation is of para-mount significance [19, 20]. Besides, according to Hansensolubility parameter theory [21, 22], solvents that enablesuccessful exfoliation ought to contain dispersive, polar,and H-bonding components of the cohesive energy dens-ity within a certain reasonable range. The end resultpoints to NMP as a matching solvent that reduces barrierfor solvent intercalation and improves the dispersion ofMoS2 [23–25]. Considering NMP is miscible with super-critical CO2, the concerted function of the dual solventsystem not only thermodynamically reduces the exfoli-ation threshold but also weakens the interlayer force

Fig. 2 a The XRD patterns of the exfoliated MoS2 from different co-solvents of NMP, ethanol, and water with supercritical CO2,respectively, as compared to the result from when supercritical CO2

is used as the only solvent and to that of the bulk sample. b TheXRD patterns of the exfoliated MoS2 under conditions of NMP andsupercritical CO2 used individually compared to used together toshow the synergistic effect. c Raman spectroscopy of the bulk andMoS2 exfoliated from the complex solvent of NMP andsupercritical CO2

Table 1 A summary of the exfoliation figure of merits (F.O.M)under different processing conditions

SC CO2 pressure NMP F.O.M

1 14 MPa 150ml 0.152

2 14 MPa None 0.676

3 0 MPa 150ml 0.778

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between MoS2 to accelerate exfoliation, which results infacile and rapid exfoliation as will be delineated below.To ascertain the critical role of NMP in promoting more

intense exfoliation and the involved fundamentals, a seriesof control experiments were conducted under the condi-tion of fixed sonication power, time, and the presence ofsupercritical CO2. Their corresponding XRD patternswere recorded as shown in Fig. 2a. The XRD peak inten-sity is here adopted as the major indicating parameter toreflect the exfoliation extent based on the knowledge thatwith the reduction in the number of layers of such 2D ma-terials, the loss in long-range order leads to weakened co-herent scattering which in turn results in the reduction inthe reflection intensity. It is found that when no co-solv-ent is used the exfoliation effect is weak, with the corre-sponding XRD peak intensity showing almost no changecompared to the bulk MoS2 sample, which suggests thedifficulty of supercritical CO2 alone to surmount the en-thalpy gain barrier resulting from exfoliation. Given thatwater poorly mix with supercritical CO2, the correspond-ing result suggests the phase separation between the twosolvents prevents any joint action on MoS2 and this barelyleads to any obvious exfoliation. The adoption of ethanol

and NMP with excellent miscibility with supercritical CO2

results in improved exfoliation. NMP shows the best ex-foliation efficacy reflected by the largely suppressed XRDpeak intensity. This leads to the conclusion that both anexcellent miscibility with supercritical CO2 and a match-ing surface tension to MoS2 that leads to reduced enthalpygain thus promoting facile exfoliation, need to be guaran-teed so as to achieve efficient exfoliation.A synergistic contribution from supercritical CO2 and

NMP to MoS2 exfoliation is discovered (Fig. 2b). Toquantitatively characterize the exfoliation efficiency fromeach exfoliation condition, a figure of merit (F.O.M) isdefined as the retention rate of the XRD peak intensityof plane (002) at 14.5° after exfoliation with respect tothat of the bulk sample, i.e., Iexfoliated/Ibulk (the lower thebetter exfoliation). It is particularly worth mentioningthat even the multiplied F.O.M value obtained from theexfoliation where NMP and supercritical CO2 wereemployed alone (0.526) is still much bigger than theF.O.M for when they were adopted simultaneously(0.152) (Table 1). This clearly verifies a strong synergisticeffect wherein the two miscible solvents are enhancingeach other in the exfoliation process with NMP lowering

Fig. 3 XPS survey spectra of a Mo 3d and b S 2p of the exfoliated MoS2 nanosheets

Fig. 4 a AFM topography of exfoliated MoS2 nanosheets and the cross-sectional height profiles obtained from line scanning in a (inset).b HRTEM images showing the exposed edge of an exfoliated nanosheet

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the exfoliation energy barrier while concurrently super-critical CO2 facilitates the subsequent intercalationbetween layers to initiate facile exfoliation.Raman spectroscopy was conducted on the bulk sample

as well as the exfoliated MoS2 from the complex solvent.The bulk sample exhibits typical E1

2g and A1g bands with

their respective full width at half maximum (FWHM) of4.37 and 5.62 cm−1 (Fig. 2c). The reduced peak intensity of

the exfoliated sample along with the enlarged FWHM to13.44 and 13.56 cm−1 for E1

2g and A1g peaks due to phonon

nanoconfinement by facet boundaries [26, 27] indicatesthe decrease in the number of layers of MoS2 which tallieswith the results from XRD analysis.XPS analysis has been conducted to study the chem-

ical state of the exfoliated MoS2 sheets. High-resolutionXPS spectra for de-convoluted Mo (3d) and S (2p)peaks have been shown in Fig. 3a and b. The peak posi-tions at 229.2 eV and 232.3 eV refer to Mo 3d5/2 andMo 3d3/2, respectively, confirming the Mo4+ state [28, 29].Meanwhile, the doublet peaks for S 2p3/2 and S 2p1/2 at161.0 eV and 163.2 eV, respectively, confirm the sulfideS2− state [29, 30].Atomic force microscopy (AFM) analysis was conducted

in tapping mode on exfoliated MoS2 nanosheets solution-casted on mica substrate to identify their topography andlayer thickness. It is observed that the obtained MoS2nanosheets were exfoliated into sizes ranging from 100 to450 nm (Fig. 4a). The exfoliation end result could be prop-erly adjusted by tuning sonication power and time to avoidstrong cavitation and in-plane cracking of MoS2 sheetswhile increasing the chamber pressure to induce strongerintercalation of supercritical CO2 and weakening of inter-layer van der Waals force. Therefore, the maximum

Fig. 5 BET analysis on MoS2 exfoliated from various solvents

Fig. 6 Digital images of the MoS2 exfoliated a from the complex solvent (NMP and supercritical CO2) and b from supercritical CO2 alone, wherethe obtained MoS2 are re-dispersed in NMP for observation; and c, d their respective dispersing status after settling for 5 h

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dimension could be possibly enhanced to micrometerrange. Line scannings for the cross-sectional height profileon exfoliated MoS2 nanosheets reveal different layer thick-nesses from ~ 3 to ~ 9 nm as shown in Fig. 4a inset, whichindicates the number of layers distributed from 5 to 15considering the thickness of a single layer MoS2 being0.61 nm [31]. The number of layer distribution plot for ex-foliated MoS2 is shown in Additional file 1: Figure S1 withthe majority number sitting between 12 and 20 layers. Be-sides, HRTEM was employed to directly probe into thelayer thickness and number of layers by checking the lat-tice fringes on exposed nanosheet edges. The number oflayers of 18–19 is identified which corresponds to a thick-ness of ~ 11 nm (Fig. 4b).To estimate the average number of layers, Brunauer–

Emmett–Teller (BET) tests were conducted on the driedsample collected from each exfoliation condition. It has tobe highlighted that neither centrifugation nor decantingthe upper clear supernatant was employed to collect theexfoliated sample, but rather, the whole entity of productfrom the exfoliation chamber was taken for the test. Thisresults in a remarkably high product percentage yield thateasily surpasses 90% with the minor loss resulting fromsample washing and collection. As such, the herein pro-posed exfoliation technique represents a truly viable ap-proach for scalable exfoliation. This is in steep contrast togenerally practiced liquid exfoliation method wherein onlythe supernatant is garnered to avoid the majority of unex-foliated sediments, which inevitably brings about a lowyield [24, 32]. Efficiency-wise, the exfoliated product fromthe complex solvents delivers the highest specific surfacearea among all processing conditions with 36.86m2/g,which is congruent with previous discussions (Fig. 5). Thiscorresponds to an average exfoliated number of layers of17 by taking account of the theoretical specific surfacearea of single layer MoS2 of 636 m2/g [33]. Consideringthe large overall quantities of MoS2 exfoliated, it is soundto deem this approach highly efficient.When the exfoliated powders are re-dispersed in fresh

NMP, a stable dispersion without sedimentation in 5 h is ob-served (Fig. 6a, c). This implies the existence of stable finecolloidal particles, whereas when the re-dispersed MoS2 ofthe same concentration was prepared in NMP from thesample exfoliated in supercritical CO2 alone, a conspicuousamount of settled particles could be identified after 5 hsettlement (Fig. 6b, d). Furthermore, due to the synergisticexfoliation effect that intensively prompts the exfoliation,the whole process is completed rapidly in 1 h which is sub-stantially faster than some reported intercalation-basedexfoliation process that could even last up to 48 h [34].

ConclusionsA modified liquid phase exfoliation approach benefitingfrom the synergistic effect from supercritical CO2 and

NMP for facile MoS2 exfoliation into a few-layeredstructure is realized. The concerted function of the com-plex solvent system reduces the exfoliation energy bar-rier while simultaneously promotes easy insertion ofsupercritical CO2 into MoS2 interlayers to initiate facileexfoliation. This technique is not only highly efficientbut also permits scalable production of few-layeredMoS2 with a high yield (> 90%), and thus, it creates aprospectively valuable opportunity to promote the versa-tile applications of MoS2.

Additional file

Additional file 1: Figure S1. The distribution plot for the number oflayers of exfoliated MoS2 nanosheets. (DOCX 37 kb)

AbbreviationsAFM: Atomic force microscopy; BET: Brunauer–Emmett–Teller; F.O.M: Figureof merit; FWHM: Full width at half maximum; HRTEM: High-resolutiontransmission electron microscopy; MoS2: Molybdenum disulfide; NMP: N-Methyl-2-pyrrolidone; TMD: Transition metal dichalcogenides; XPS: X-rayphotoelectron spectroscopy; XRD: X-ray diffraction

AcknowledgementsNot applicable

Authors’ contributionsCZ conceived the idea and designed the experiments. XT and JL performedthe experiments and collected data. WK and XT analyzed the results. Allauthors read and approved the final manuscript.

FundingNational Key R&D Program of China (no. 2017YFE0111500), the NationalNatural Science Foundation of China (no. 51673123 and 51222305), andSichuan Science and Technology Project (no. 2016JQ0049).

Availability of data and materialsThe datasets used for analysis can be provided on a suitable request, by thecorresponding author.

Competing interestsThe authors declare that they have no competing interests.

Received: 8 April 2019 Accepted: 19 August 2019

References1. Mak KF, Lee C, Hone J et al (2010) Atomically Thin MoS2 : A New Direct-

Gap Semiconductor. Phys Rev Lett 105(13):136805.2. Lu JM, Zheliuk O, Leermakers I et al (2015) Evidence for two-dimensional

Ising superconductivity in gated MoS2. Sci 350(6266):1353–13573. Wang H, Yu L, Lee YH et al (2012) Integrated circuits based on bilayer MoS2

transistors. Nano Lett 12(9):4674–46804. Saito Y, Nakamura Y, Bahramy MS et al (2016) Superconductivity protected

by spin–valley locking in ion-gated MoS2. Nat Phys 12(2):144–1495. Eda G, Yamaguchi H, Voiry D et al (2011) Photoluminescence from

chemically exfoliated MoS 2. Nano Lett 11(12):5111–51166. Roxlo CB, Chianelli RR, Deckman HW et al (1987) Bulk and surface optical

absorption in molybdenum disulfide. J Vac Sci Technol A 5(4):555–5577. Kam KK, Parkinson BA (1982) Detailed photocurrent spectroscopy of the

semiconducting group VIB transition metal dichalcogenides. J Phys Chem86(4):463–467

8. Li T, Galli G (2007) Electronic properties of MoS2 nanoparticles. J Phys ChemC 111(44):16192–16196

9. David L, Bhandavat R, Singh G (2014) MoS2/graphene composite paper forsodium-ion battery electrodes. ACS Nano 8(2):1759–1770

Tan et al. Nanoscale Research Letters (2019) 14:317 Page 6 of 7

10. Li Y, Wang H, Xie L et al (2011) MoS2 nanoparticles grown on graphene: anadvanced catalyst for the hydrogen evolution reaction. J Am Chem Soc133(19):7296–7299

11. Wang N, Wei F, Qi Y et al (2014) Synthesis of strongly fluorescentmolybdenum disulfide nanosheets for cell-targeted labeling. ACS ApplMater Interfaces 6(22):19888–19894

12. Gan X, Zhao H, Quan X (2017) Two-dimensional MoS2: a promising buildingblock for biosensors. Biosens Bioelectron 89:56–71

13. Zheng J, Zhang H, Dong S et al (2014) High yield exfoliation of two-dimensional chalcogenides using sodium naphthalenide. Nat Commun2014 5:2995

14. Lopez-Sanchez O, Lembke D, Kayci M et al (2013) Ultrasensitivephotodetectors based on monolayer MoS2. Nat Nanotechnol 8(7):497–501

15. van der Zande AM, Huang PY, Chenet DA et al (2013) Grains and grainboundaries in highly crystalline monolayer molybdenum disulphide. NatMater 12(6):554–561

16. Ambrosi A, Sofer Z, Pumera M (2015) Lithium Intercalation CompoundDramatically Influences the Electrochemical Properties of Exfoliated MoS2.Small 11(5):605–612.

17. Chang K, Chen W (2011) L-cysteine-assisted synthesis of layered MoS2/graphene composites with excellent electrochemical performances forlithium ion batteries. ACS Nano 5(6):4720–4728

18. Smith RJ, King PJ, Lotya M et al (2011) Large-scale exfoliation of inorganiclayered compounds in aqueous surfactant solutions. Adv Mater 23(34):3944–3948

19. Coleman JN, Lotya M, O’Neill A et al (2011) Two-dimensional nanosheetsproduced by liquid exfoliation of layered materials. Sci 331(6017):568–571

20. Zhou KG, Mao NN, Wang HX et al (2011) A mixed-solvent strategy forefficient exfoliation of inorganic graphene analogues. Angew Chem Int Ed50(46):10839–10842

21. Hansen CM (2004) 50 years with solubility parameters—past and future.Prog Org Coat 51(1):77–84

22. Hansen CM (1969) The universality of the solubility parameter. Ind EngChem Prod Res Dev 8(1):2–11

23. Xu S, Li D, Wu P (2015) One-pot, facile, and versatile synthesis of monolayerMoS2/WS2 quantum dots as bioimaging probes and efficientelectrocatalysts for hydrogen evolution reaction. Adv Funct Mater 25(7):1127–1136

24. Bang GS, Nam KW, Kim JY et al (2014) Effective liquid-phase exfoliation andsodium ion battery application of MoS2 nanosheets. ACS Appl MaterInterfaces 6(10):7084–7089

25. Backes C, Berner NC, Chen X et al (2015) Functionalization of liquid-exfoliated two-dimensional 2H-MoS2. Angew Chem Int Ed 54(9):2638–2642

26. Ramakrishna Matte HSS, Gomathi A, Manna AK et al (2010) MoS2 and WS2analogues of graphene. Angew Chem Int Ed 49(24):4059–4062

27. Thripuranthaka M, Kashid RV, Sekhar Rout C et al (2014) Temperaturedependent Raman spectroscopy of chemically derived few layer MoS2 andWS2 nanosheets. Appl Phys Lett 104(8):081911.

28. Eda G, Yamaguchi H, Voiry D et al (2011) Photoluminescence fromchemically exfoliated MoS2. Nano letters, 11:5111-5116.

29. Tao P, He J, Shen T et al (2019) Nitrogen-doped MoS2 foam for fast sodiumion storage. In: Advanced materials interfaces, p 1900460

30. Gu W, Yan Y, Zhang C et al (2016) One-step synthesis of water-soluble MoS2quantum dots via a hydrothermal method as a fluorescent probe forhyaluronidase detection. ACS Appl Mater Interfaces 8:11272–11279

31. Wakabayashi N, Smith HG, Nicklow RM (1975) Lattice dynamics ofhexagonal Mo S 2 studied by neutron scattering. Phys Rev B 12(2):659–663.

32. O’Neill A, Khan U, Coleman J N (2012) Preparation of High ConcentrationDispersions of Exfoliated MoS2 with Increased Flake Size. Chem Mater24(12):2414–2421.

33. Zhang Y, Xu L, Walker WR et al (2017) Langmuir films and uniform, largearea, transparent coatings of chemically exfoliated MoS2 single layers. JMater Chem C 5(43): 11275–11287.

34. Jawaid A, Nepal D, Park K et al (2016) Mechanism for Liquid PhaseExfoliation of MoS2. Chem Mater 28(1):337–348.

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