Perylene tetracarboxylate surfactant assisted liquid phase exfoliationof graphite into graphene nanosheets with facile re-dispersibility inaqueous/organic polar solvents
Rekha Narayan, Joonwon Lim, Taewoo Jeon, Dong Jun Li, Sang Ouk Kim*
National Creative Research Initiative Center for Multi-Dimensional Directed Nanoscale Assembly, Department of Materials Science & Engineering, KAIST,Daejeon, 34141, Republic of Korea
a r t i c l e i n f o
Article history:Received 27 January 2017Received in revised form11 April 2017Accepted 29 April 2017Available online 2 May 2017
Liquid phase exfoliation (LPE) is a promising method for graphene production particularly in terms ofcost effectiveness and scale up. Nonetheless, it is still challenging to synchronize prime goals of highquality, good yield, large sheet size, stable long term storage and low cost eco-friendly processing. Wepresent a simple and inexpensive green route for large scale production of exfoliated graphene disper-sions exploiting the non-covalent surface chemistry between graphene and perylene tetracarboxylate(PTCA) aromatic semiconducting surfactant. Direct sonication of graphite flakes in aqueous PTCA solu-tions produced high yield of single and few-layer graphene sheets with minimal basal plane defects asrevealed by XPS, Raman and FTIR spectroscopy. Uniquely for LPE protocol, the lateral graphene flakedimensions extended upto 10e12 mm range. The exfoliated dispersions exhibited high colloidal stabilitywith shelf-life exceeding a year. Facile re-dispersibility of the dried graphene/PTCA powders wasobserved in water as well as many polar organic solvents. Significantly, pure aromatic semiconductingnature of surfactant without dielectric moiety ensures tight electrical contact among graphene sheets inthin films. The approach exploiting the simple molecular design of aromatic charged surfactants forgraphene exfoliation holds a great prospect for solution processed graphene based nanomaterials anddevices.
Discovery of graphene has ignited a sheer revolution across themultiple disciplines of science and technology [1,2]. After the initialgush of research activities demonstrating the astonishing facets ofthis wonder material, it is time to focus on the integration of gra-phene into real-life applications. Unfortunately, it is still a chal-lenging task to produce high quality graphene sheets in a costeffective scalable way. To date, many different bottom up and topdown approaches have been investigated for graphene production,including mechanical cleavage [1], epitaxial growth [3], chemicalvapor deposition (CVD) [4,5], graphene oxide reduction [6,7] andliquid-phase graphite exfoliation [8e15]. Among them, LPE ofpristine graphite has been considered a promising methods toobtain graphene in a large scale. Several research groups including
).
Coleman et al. have demonstrated the successful LPE of graphite inorganic solvents (N-methylpyrrolidone, ortho-dichloro benzeneetc.), whose Hansen solubility parameters match well with that ofgraphene [8,16]. In an attempt to use less toxic and inexpensivesolvent, aqueous exfoliation methods have been actively pursuedusing surfactants, ionic liquids etc. [16e18]. Noteworthy that thehigh quality of as-produced graphene flakes has been the primefocus among those previous works, with a relatively little attentionpaid to the long term colloidal stability, overall cost of materials andprocess and, more significantly, the presence of insulating disper-sants which is unfavorable for the final material properties aftersolution processing. Many of those works reported high qualitygraphene production but in sub-microgram quantities and thelateral flake size remained typically between 500 nm and 1 mm.
Non-covalent chemistry is attractive for the surfactant systemsof LPE owing to its simplicity and the preservation of graphene p-conjugated system intact. In this work, we demonstrate a tetra-anionic polycyclic aromatic semiconducting compound, Perylene-3,4,9,10-tetracarboxylate (PTCA) as a new exfoliant-cum-dispersant
for graphene in aqueous media. PTCA is a water soluble derivativeof the archetypal p-conjugated n-type organic semiconductormolecule PTCDA (perylene tetracarboxylic acid dianhydride) [19].Owing to its planar conjugated nanographene structure, PTCA cannoncovalently adhere to graphene surface via p-p stacking totrigger the exfoliation in water, whereas the tetra-anionic stateprovides the necessary inter-flake repulsive electrostatic forces forstable dispersions. For a truly scalable LPE process, the surfactant isdesired to be readily synthesized in a large quantity from inex-pensive raw materials. Presently, a few relevant reports are avail-able where the perylene core based dendritic macromolecules orpolymers are used for the effective exfoliation or functionalizationof carbon nanotubes and graphene, but all of them require multi-step organic synthesis and tedious purification procedures result-ing in low yields [13,20,21]. By contrast, synthesis of PTCA is verysimple and single step with a high yield. Moreover, intrinsic sem-iconducting character of this surfactant without electrically insu-lating moiety is highly beneficial for the electrical properties ofsolution processed final product, while ensuring good electricalcontacts among neighboring stacked graphene flakes.
2. Experimental section
2.1. Materials
The starting materials graphite flakes (þ100 mesh (�75% min)batch no. 52996AJ) and perylene-3,4,9,10-tetracarboxylic dianhy-dride (PTCDA) were obtained from Sigma Aldrich and used withoutfurther purification. KOH was purchased from Junsei. All solutionswere prepared using deionized water, which was also used forwashing and dialysis of samples.
2.2. Characterization
The morphology of graphene sheets were imaged using a field-emission SEM (Hitachi S-4800) and TEM (FEI Technai G2 F30). AFMimages were obtained with scanning probe microscope (Multi-Mode 8, Bruker). For photophysical studies, absorption spectrawere measured on UVevis spectrometer (UV-3600, Shimadzu).Steady state fluorescence emission and time resolved fluorescencelifetime measurements (TCSPC time-correlated single photoncounting) were performed by Edinburgh Instrument Model-FLS920. Raman spectroscopy was carried out with ARAMIS(Horiba Jobin Yvon, France) using a 514.5 nm laser. XPS measure-ments were carried out on VG ESCA2000 X-ray photoelectronspectrometer using a 400 mm MXR1 gun and analyzed withAvantage software. Cyclic voltammetry was conducted with elec-trochemical analysis instrument (VersaSTAT3 electrochemicalwork station, Princeton Applied Research Bio-Logic SAS SP-200model). X-ray diffraction measurements were obtained usingRigaku D/MAX-2500 (Cu Ka radiation, 40 kV, 300 mA). Sheetresistance was measured with CMT-SR1000 N 4-point probe sys-tem. The zeta potential was measured using a Photal (Japan) ModelELS-Z2 -Zeta potential analyzer instrument.
2.3. Preparation of PTCA surfactant
Typically 1 g of red PTCDA powder was suspended in 100 mLethanol to which around 150 mL of 0.5 M aqueous KOH solutionwas added. The mixture was refluxed for 6hrs and cooled to roomtemperature. Excess ethanol is added to complete the precipitationof solid yellow PTCA potassium salt powder. The product wasfurther purified by recrystallization from ethanol water mixture.
2.4. Preparation of liquid phase exfoliated (LPE) graphene
In a typical preparation, solutions of PTCA surfactant was pre-pared in deionized water and stirred vigorously for completedissolution to get a greenish yellow fluorescent solution. Graphiteflakes were added to the aqueous PTCA solution so as to make afinal graphite concentration of 5 mg/mL and the mixture was stir-red overnight under ambient conditions. The resultant PTCAfunctionalized graphite flakes were then subjected to low powerwater bath ultrasonication (JAC Ultrasonic 2010P e 300 W) upto12 h under ambient conditions to get stable greenish black dis-persions. The water in sonication bath was regularly changed tomaintain bath temperatures below 30 �C. Details given in Sup-porting information.
3. Results and discussion
3.1. Graphite exfoliation with PTCA based non-covalent chemistry
We synthesized the PTCA dispersant starting from the dianhy-dride perylene precursor molecule PTCDA by alkaline hydrolysisusing KOH as described in Fig. 1a. The water insoluble bright redPTCDA powder is converted into yellow potassium salt of PTCAwitha high aqueous solubility, as characterized by strong greenish yel-low fluorescence (Fig. S2 ESI). Detailed preparation procedure(Scheme S1) and the conversion studies confirmed by FTIR exper-iments are discussed in the supporting information (Fig. S1).
Fig. 1b illustrates the PTCA assisted straightforward exfoliationof pristine graphite in water. For a highly stable and solution pro-cessable graphene dispersion with good yield, the following pro-cedure was successful. Briefly, graphite flakes (>100 mm lateraldimension) are suspended in aqueous PTCA solutions by magneticstirring overnight followed by low power bath sonication. Themassive un-exfoliated graphite sediments are separated by allow-ing the dispersions to stand overnight. Subsequently, the washedand centrifuged supernatant greyish black solutions with PTCAmolecules adsorbed on graphene are stored. (Details given inexperimental section ESI).
Notably, the first step of mixing graphite and PTCA is very crucialfor the efficient exfoliation. As shown in Fig. 1b, before the soni-cation step, highly planar aromatic PTCA molecules are allowed tonon-covalently attach at the surface of graphite via p-p stackinginteractions. This attachment eventually helps peeling of graphenesheets from graphite chunks during ultrasound agitations. Inaddition, PTCA being a strong electron-acceptor molecule, charge-transfer interaction with graphene would also favor the exfolia-tion process [22]. Upon suceesive sonication, continuous andvigorous vibrational motions of the individual graphene layersfacilitate more PTCA molecules penetrate through the interlayeropenings. As a consequence, the graphene-graphene interactionget weakened leading to the cleavage of exfoliated sheets.
In order to determine the optimum surfactant concentration forthe best exfoliation yield, we varied the PTCA concentration from0.2 to 1.0 mg/mL for fixed initial graphite feed concentration of5 mg/mL and sonication time of 3 h. The exfoliated graphene dis-persions were characterized by UVeVis spectroscopy. Fig. S3ashows the absorbance at 660 nm per unit length as a function ofPTCA concentration. The UV absorption from graphene is domi-nated by p-p* transition at lmax <300 nm. Usually the absorbance>500 nm is flat, but the optical density being concentrationdependent, the higher wavelength region (>500 nm) is often usedto correlate with dispersed graphene concentration following Beer-Lambert law [23]. The 660 nm wavelength is devoid of any ab-sorption from PTCA molecule, so the absorbance A660 is directlyproportional to the dispersed graphene concentration. It was found
Fig. 1. (a) Synthesis of PTCA surfactant from PTCDA precursor. (b) Schematic process for PTCA assisted liquid phase exfoliation of graphite. Graphite flakes are first non-covalentlyfunctionalized with PTCA in water via p-p stacking interactions. Subsequent low power sonication releases exfoliated graphene sheets stabilized by electrostatic repulsion given byadsorbed PTCA molecules. (A colour version of this figure can be viewed online.)
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that along with the PTCA concentration the A660 increased initially,reached an optimumvalue for 0.6 mg/mL PTCA, but decreased withhigher surfactant concentrations. The corresponding dispersedgraphene had the concentration range of 180e200 mg/mL. Note-worthy that, at a sufficiently high concentration of PTCA, theinitially adsorbed tetra-anionic molecules on graphite will producestrong Coloumbic repulsions that prevent more surfactants to comein close proximity to graphite chunks covered with PTCA. This self-limiting process will also reduce the amount of surfactant mole-cules penetrating into the interlayers and thus decrease the yield ofexfoliation. This behavior of the PTCA stabilized graphene disper-sions are in good agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory suggested by Coleman and others toexplain mechanism stabilizing graphene colloids by ionic surfac-tants [24]. According to the theory, the potential energy betweentwo approaching surface charged graphene sheets is composed ofdouble layer electrostatic repulsive part and the attractive van derWaals interactions as denoted by the equation below:
V ¼ 4Aε0εrkz2e�kD � Apr2C
.2D4
Here z is the surface zeta potential, k�1 ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiεrε0kT
�2e2n0
qis the
Debye screening length, n0 is the ionic surfactant concentration, Ais the sheet area, r is the number of atoms per unit area of sheets,AH ¼ p2Cr2 represents the Hamaker constant and D is the sheetseparation. As the concentration of ionic surfactant increases, thepotential barrier decreases, but the van der Waals forces increasesdestabilizing the charged graphene dispersions. As an example ofthis trend we found that upon increasing the PTCA concentrationup to 2.5 mg/mL the exfoliated graphene dispersions exhibitedstrong destabilization, as shown in Fig. S3b. Additionally, the
interaction of PTCA molecules with graphene scaffolds can bequalitatively traced from the signature perylene absorption bands.PTCA being a typical fluorophore, has characteristic absorptionbands in the UVevisible region corresponding to the S0-S1 elec-tronic transition from the perylene core at 465, 436, 413 nm arisingfrom the well-resolved vibronic structure from 0-0, 0e1 and 0e2transitions, respectively [25]. Upon the increase of PTCA concen-tration, these absorption peaks in 400e500 nm range got broad-ened and red shifted (Fig. S3ced) along with intensity reversal ofthe A0�0 and A0�1. Meanwhile, the exfoliated graphene absorbanceband around 268 nm increased with increasing PTCA concentrationtill 0.6 mg/mL and decreased beyond for higher PTCA amounts. At alower surfactant concentration of 0.2 mg/mL, the A0�0 to A0�1 in-tensity pattern is same as PTCA with a little peak broadening,indicating the surfactant molecules adsorb onto graphene surfacesprincipally as monomeric species. But at a higher PTCA concen-tration of 1.0 mg/mL all the perylene absorption peaks got mergedwith significant red shift indicating the aggregation of surfactantmolecules on graphene surfaces. Based on DLVO theory, it isconcluded that too much high concentration of ionic surfactantscan negatively influence the LPE of graphene.
3.2. Graphene/PTCA interaction
Once the optimum surfactant concentration was determined,we attempted to maximize the final dispersed graphene concen-tration principally by increasing the sonication time. Fig. 2a pre-sents the graphite flakes (5 mg/mL) non-covalently functionalizedwith optimized PTCA (0.6 mg/mL) in 400 mL water before exfoli-ation. Fig. 2b shows the corresponding exfoliated PTCA/graphenedispersion after 12hrs of sonication and purification. The optimized
Fig. 2. Digital photographs corresponding to aqueous solutions of (a) PTCA/graphite and (b) PTCA/graphene before and after exfoliation, respectively. (c) Tyndall effect demon-strated by the exfoliated graphene/PTCA dispersion and (d)e(e) its steady state absorption and emission spectra respectively. (A colour version of this figure can be viewed online.)
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exfoliation protocol could be readily scaled up to even larger vol-umes of 1e2 L scales (Fig. S4 ESI). The resulting dark black graphenedispersions had concentration in the range of 0.5e0.8 mg/mL fromdifferent batches, which is significantly higher thanmany other LPEmethods [16]. Noteworthy that the as-produced graphene disper-sions were highly stable inwater with shelf-life exceeding one year.Tyndall effect of the dispersion (Fig. 2c) verifies its true colloidalnature and high stability. Long term storage of our exfoliated gra-phene dispersions showed only less than 0.5% sedimentationwhichcan be attributed to the strong repulsive forces from the cloud ofnegatively charged carboxylate head groups of PTCA adsorbed ongraphene sheets.
Steady state absorption spectrum of the aqueous graphene/PTCA dispersions is compared with that of pure PTCA in Fig. 2d. Forthe graphene/PTCA, the absorption spectra extended upto NIR re-gion with the typical graphene absorption peak observed at~268 nm, arising from the p-plasmon resonance common forextended sp2 conjugated carbon sheets [26]. Concurrently, theextremely faint broad absorption bands in 450e600 nm region arealso seen, originating from the adsorbed PTCA molecules. Thisconfirms the non-covalent functionalization of graphenewith PTCAmolecules and indicates that very small amounts of PTCA are suf-ficient for providing the high colloidal stability. Here it is note-worthy that the dispersions are stored after thorough purificationsto remove excess unbound surfactants such that the absorptionbands purely originate from the molecules directly adhered tographene sheets. Compared to the pure PTCA spectrum with the
absorption maxima at 465 nm and resolved vibronic transitions,the spectrum for PTCA absorbed on graphene surface is signifi-cantly broadened and red shifted (visible in the enlarged spectragiven in inset of Fig. 2d. This is substantial evidence supporting thestrong electronic communication between PTCA and graphene inthe non-covalent hybrid structures [27].
Excited state photoluminescence experiments provided moreinsights into the nature of graphene/PTCA electronic interaction.Fig. 2e shows the steady state fluorescence spectra of pure PTCAand graphene/PTCA upon excitation at 430 nm. For the fluores-cence measurements, particular care was taken to keep the totalPTCA concentration similar in the graphene/PTCA and pure PTCAsolutions to get meaningful information. Firstly the UVevis spectrafrom graphene/PTCA was measured to determine the optical den-sity corresponding to PTCA absorbance peaks (400e550 nm range).Keeping this optical density value as reference, we performedquantitative dilutions on pure PTCA solutions so as to obtain thePTCA optical density matching with that found in graphene/PTCA.The resultant optimized solutions were measured for fluorescence.Pure PTCA revealed strong fluorescence with intensity maxima at511 nm (S1/S0 transition) along with broad low intensity peaks at550 nm and 590 nm. By contrast, the strong fluorescence emissionfrom PTCA was almost completely quenched in the presence ofgraphene. Inset given in Fig. 2e reveals the extremely weak emis-sion observed from graphene/PTCA. Digital photographs of the purePTCA and graphene/PTCA solutions illuminated under ambient andUV (365 nm) light verifies the fluorescence quenching by graphene
Table 1Summary of TCSPC results.
Sample FL lifetime (ns) Relativeamplitude %
c2
t1 t2 a1 a2
PTCA 4.78 e 100 e 1.082Graphene/PTCA 3.18 5.80 20.46 79.54 1.235
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(Fig. 3a).The fluorescence quenching can originate from the energy or/
and electron transfer process. Unfortunately, due to the absence ofany significant spectral overlap between PTCA emission and gra-phene absorption, it is difficult to ascertain the occurrence of en-ergy transfer process (FRET- Forster resonance energy transfer)[28]. Thus, we attribute the enhanced emission quenching to pro-ceed principally via photoinduced charge transfer route.We furthercarried out fluorescence decay lifetime measurements using time-correlated single photon counting (TCSPC) spectroscopy to getmore information on the quenching mechanism. Fig. 3b comparesthe TCSPC decay profiles for pure PTCA and graphene/PTCA,collected at 511 nm (PTCA emission) by excitation from 430 nm.Their lifetime values along with the relative distributions aresummarized in Table 1.
The decay profile of pure PTCAwas found to follow exclusively amonoexponential fit, with lifetime of 4.78 ns. By contrast, fluores-cence decay in graphene/PTCA could be best fitted bi-exponentiallyto one short and one long-lived component with the lifetimes of3.18 and 5.8 ns, respectively. The short lived component andsimultaneous evolution of a relatively long-lived transient speciesindicates the rapid deactivation of the perylene singlet excited stateto occur through the formation of charge separated states. Based onthese observations we propose the non-radiative quenching pro-cess of our graphene/PTCA to follow photoinduced Dexter typeelectron transfer or exchange from graphene to electron acceptingPTCAmolecules (Fig. 3c). The HOMO (�5.6 eV) and LUMO (�3.4 eV)levels of PTCA determined from cyclic voltammetry (Fig. S5) usingthe Bredas calculations are in a good agreement with the possibleelectron transfer from graphene Fermi level (�4.5 eV) to the semi-
Fig. 3. (a) Direct visualization of PTCA fluorescence quenching in graphene/PTCA dispersPlausible mechanism of electronic interaction between graphene and PTCA in the non-cova
filled HOMO of PTCA. Similar fluorescence bleaching associatedwith the electron transfer phenomena has been observed in singlewalled carbon nanotubes (SWNT)/perylene and electron-acceptingphthalocyanine/graphene systems, where SWNT/grapheneexhibited electron donating character [29,30]. The Dexterquenching mechanism proceeds through a fast transfer of excitedstate electron from the fluorophore to quencher to form themetastable charge separated state followed by charge recombina-tion [31].
3.3. Morphology of exfoliated graphene
We characterized the morphology, layer number and lateraldimensions of the exfoliated graphene sheets by scanning electronmicroscopy (SEM), atomic force microscopy (AFM) and trans-mission electron microscopy (TEM). Fig. 4bee shows the SEM im-ages of our exfoliated product in comparison to the startinggraphite flakes (Fig. 4a).
The samples for SEM imaging were prepared by spin coatingexfoliated aqueous graphene suspensions onto Si/SiO2 substrates.
ion. (b) Fluorescence decay life-time measurements from TCSPC measurements. (c)lent structure. (A colour version of this figure can be viewed online.)
Fig. 4. SEM images corresponding to the staring (a) graphite flakes and (bee) exfoliated graphene sheets.
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Notably, compared to the pristine graphite flakes, we observedreduction in the lateral size of the graphene sheets, which is un-avoidable in sonication assisted exfoliation process. Analysis ofmultiple SEM images indicated that our exfoliated samplescomprised of a large number of flakes varying widely in terms oflayer thickness and lateral dimensions. Majority of the sheetsappeared few-layer with lateral size of the order of hundreds ofnanometers. Very interestingly, there were many extremely thinand large graphene sheets (>2 mm) with well-defined edges thatappeared semi-transparent under SEM electron beam (Fig. 4cee).The lateral width of such sheets extended from 2 to 12 mm and thesmaller sheets beneath the larger sheets were clearly visible pre-dicting the possibility of graphene monolayers. The surfaces ofthese sheets appeared smooth.
More SEM images are provided in the supporting info Fig. S6. Tothe best of our knowledge, such large lateral dimensions of exfo-liated graphene sheets are rare thus far among previously reportedsolvent or surfactant assisted LPE procedures [16]. However theformation of many large thinner graphene sheets along withmixture of thicker very small size graphene particles is still a majorlimitation of our procedure. Such broader size distribution lowersthe overall monolayer percentage and can also affect many appli-cations that demand controlled nanosheet sizes. In fact to addressthis challenge, we attempted to concentrate the proportion of thelarger graphene sheets by liquid cascade centrifugation methoddemonstrated by Coleman et al. to enrich the thinner layers [32].But unfortunately in our experiments, despite the iterative centri-fugation cascades with stepwise increasing centrifugation rates anddiscarding the denser residues, we found the endmixture to consistof reduced number of larger graphene sheets along with thesmaller particles still present. We logically assume, due to thematching densities of the very small thicker particles with that ofthe larger yet thinner graphene sheets renders the separationdifficult.
A more precise picture of the exfoliated state of dispersed gra-phene was obtained from TEM imaging, for which few microliters
of highly diluted solutions was pipetted onto TEM grids. Consistentwith the SEM images, TEM analysis revealed the presence of manymono-, bi- to few layer sheets (<10 layers). Fig. 5a shows therepresentative low resolution TEM image of highly transparent,large size (>5 mm) monolayer graphene sheets with foldings andpartially overlapping each other. Unlike the SEM images, the sur-faces of graphene sheets under TEM showed the presence of someparticles (inset Fig. 5a) which is believed to be the surfactant ag-gregations remaining after solvent evaporation. Interestingly, thesesheets exhibited significant folding and curling that substantiatestheir ultrathin 2D nature. Counting the number of lines at thegraphene sheet edges is a common method to determine the layerthickness [8,23,33]. High resolution TEM (HR-TEM) image in Fig. 5bcorresponding to the yellow square region from Fig. 5a, clearlyrevealed the pristine graphitic lattices with edge count to one,confirming monolayer graphene sheets.
Additional evidence for monolayer thickness is provided by theSAED (selected area electron diffraction) pattern displayed in theinset of Fig. 5b. The diffraction pattern depicted very sharp six-foldsymmetry, typical of hexagonal crystalline lattice of graphenemonolayer [34,35]. From the electron diffraction analysis, in addi-tion to typical six fold symmetry pattern, based on computationalstudies some literature reports predict the inner hexagonal spots{1110 planes} be more intense relative to the outer spots {2110planes} for single layer graphene, meanwhile high intensity {2110}spots relative to {1110} spots are correlated with bi- and/or multi-layer graphene sheets [8]. At the same time many other relevantliterature studies demonstrated the relative intensity of the innerand outer hexagons equivalent in single layer graphene and forbilayer graphene the intensity of outer hexagon are shown twice tothose of inner spots [36e39]. For our graphene sheets shown inFig. 5a the relative intensity of the two set of inner and outer spotsare equivalent, proving them monolayers in agreement with theaforementioned references in addition to the proof provided by theedge counting. Conversely since the intensity of the inner spots arenot lower compared to the outer, the possibility of bi/few layer can
Fig. 5. (a & c) Selected TEM images of single and few layer graphene sheets respectively, drop cast from the PTCA stabilized dispersions (Inset in (a) shows the zoomed region withsurfactant aggregation). (b & d) HRTEM images corresponding to the yellow square marked area in (a) & (b) with inset showing the respective SAED patterns. (A colour version ofthis figure can be viewed online.)
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be excluded and the graphene sheets shown in Fig. 5a can beaffirmed as overlapped single layers. Fig. 5c and d presents the lowand high resolution TEM images of few layer graphene sheets ob-tained from the exfoliated dispersion. The SAED pattern corre-sponding to the few-layer had doubled hexagonal reflexes whichare suggestive of disoriented stacking graphene layers or loss of ABBernal stacking of graphite [40]. This occurs presumably due to therestacking of the separate exfoliated graphene layers during drying.Many sheets were found folded and irregularly stacked; additionalTEM images are provided in the supporting info (Fig. S7).
Topology and layer thickness of the exfoliated graphene sheetswere examined by AFM measurements. AFM sample preparationwas same as that adopted for SEM analysis on Si/SiO2 substrates,but high dilutions were preferred in-order to prevent the extensivesheet agglomerations upon solvent evaporation. Fig. 6a presentsthe typical tappingmode AFM image of graphene sheets containingmono- and few-layers, in agreement with the SEM and TEM ana-lyses. From their corresponding height profiles shown in Fig. 6b, theaverage thickness of sheets was in the range 1.5e3 nm. Theoreti-cally, single layer graphene sheet is ca. 0.34 nm thick, however thisideal case is never observed as the apparent AFM thickness includesthe chemical and van der Waals contrast [1,41]. In reality, aninstrumental offset of ca. 0.5 nm (which itself is greater than0.34 nm) always exists and this leads to overestimation of actual
graphene layer number. Additionally, the height of graphene sheetsvia AFM measurement is substrate dependent; monolayer sampleis commonly appeared to have a height of ~1 nm on SiO2 and~0.4 nm on mica [42], meanwhile for functional molecule coveredsingle graphene sheets thickness of 1e2 nm is typical [43].Considering these facts, we count a 5 nm thick sheet to consist offive single graphene layers.
Fig. 6c shows the statistical thickness analysis from AFM stepheight measurements analyzed for about 82 randomly selectedgraphene sheets. About 30% of the graphene sheets were less than2 nm thick and the yield of monolayers is approximated to 8%. Wealso note that the presence of 5 nm thick range few-layers in nearly9% yield. Additional representative AFM images and their corre-sponding height profiles are given in supporting information(Fig. S8).
3.4. High quality of exfoliated graphene
In addition to themorphology analysis, it is equally significant tocharacterize the quality of exfoliated graphene. We assessed thegraphene sheets using Raman spectroscopy, which is a versatiletool that can provide critical information in terms of defect level,layer number as well as any dopant effect. In our exfoliationstrategy the PTCA molecules attached at graphene sheets provides
Fig. 6. (a) AFM image of exfoliated graphene sheets spin coated on Si/SiO2 substrate and (b) the corresponding height profiles taken along the broken lines. (c) Statistical thicknessanalysis of the graphene sheets from AFM measurements of randomly selected nearly 82 flakes. (A colour version of this figure can be viewed online.)
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the necessary colloidal stability in solution. But for experimentssuch as Raman spectra, the attached PTCA molecules can interferewith the true analysis of the graphene sheets. The PTCAmolecule isstrongly fluorescent in the region from 450 to 620 nm, which re-sults in extremely weak Raman spectral response dominated byfluorescing features (Fig. S9a). Meanwhile, we found that theRaman spectrum of exfoliated graphene/PTCA sheets were pre-dominantly superimposed by strong PTCA signals as depicted(Fig. S9b). The visibility of PTCA Raman peaks with significantreduction in the emission background is possibly due to its fluo-rescence quenching by graphene and this provides indirect evi-dence for the electronic communication between the two. Here wenote the recent applications of graphene in fluorescence quenchingmicroscopy [44]. However, for the sake of performing an unam-biguous Raman analysis of graphene sheets, we subjected thesample coated Si/SiO2 (300 nm) substrates to washing withdeionized water. Fig. 7a shows the resultant Raman spectra ofrepresentative various layers of exfoliated graphene sheets ac-quired with a 514 nm excitation laser and compared with startinggraphite flakes. The results are averaged from multiple arbitrarilyselected different spots. The Raman spectra of all graphene sheetsrevealed three major peaks: D-band around 1358-1360 cm�1, G-band around 1583-1587 cm�1 and 2D-band around 2691-2717 cm�1. In comparison, the starting graphite material showedonly G-band (1578 cm-1) and 2D-band (2730 cm�1). Fairly notice-able changes were observed in the 2D peak pattern and positionamong different graphene layers as visible from Fig. 7a(i-iii). The2D-band which is the overtone of D-peak does not require anydefects for its activation but is very sensitive to the number ofgraphene layers. The spectral shape fitting of 2D peak to singleLorentzian is identification for monolayer graphene, whereas abilayer requires four Lorentzians [45,46]. As the layer number
increases, the spectral patterns are known to resemble pristinegraphite where the 2D peak splits only to two [47]. Fig. 7b showsfitted 2D bands by Lorentzian functions corresponding to thespectra given in Fig. 7a. A sharp symmetrical and single Lorentzialfit generated in Fig. 7b(i) identifies a monolayer graphene sheetcompared to two component fitting obtained for bulk graphite(Fig. 7b(iv)).
Another key feature of the 2D peak from single layer pure gra-phene is narrow FWHM (full width half maximum) in the range~25e39 cm�1 [48]. But the FWHM of the 2D peak in Fig. 7b(i) iscalculated to ~58 cm�1, which is very broad compared to thosereported for single layer graphene sheets with no functionaliza-tions. Turbostratic graphite (lacking AB Bernal stacking) are knownto exhibit such broader 2D peaks along with an upshift ~20 cm�1 inthe peak position [45]. But in our sample such upshift is notobserved. Further for surfactant exfoliated graphene samples, sin-gle layers with broad as well as narrow 2D peak FWHM are re-ported [13,49]. However broader 2D peaks appear more feasible forsurfactant attached graphene sheets, especially if electrondonating/withdrawing functionalities are present on the surfactant[22,50]. Hence we assume the broader 2D peak FWHM of our singlelayer graphene is highly likely due to doping effects from surfaceadsorbed PTCA molecules that are present even after washing.
The four components splitting of the 2D spectrum in Fig. 6b(ii) istypical of bilayers [47]. Notably, the spectral pattern in Fig. 7b(iii) issimilar to graphite, but could be fitted to three components sug-gesting few-layer graphene (�5 layers) [41,45]. Thus, Ramanspectral analysis of the 2D-band ascertains that our method hassuccessfully exfoliated bulk graphite into mono- and few-layergraphenes.
One principal feature of graphitic Raman spectra is the G-bandand its frequency shift is frequently used to monitor the doping
Fig. 7. (a) Raman spectra of exfoliated graphene sheets with different layer numbers compared to starting graphite. (b) The corresponding 2D peak fittings by Lorentzian functions.(A colour version of this figure can be viewed online.)
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nature. Rao et al. have shown that the G-band gets softened (shift tolower frequencies) by electron donor molecules and stiffened(shifted to higher frequencies) by electron withdrawing molecules[12]. By comparing the G-bands of our monolayer graphene ob-tained from PTCA assisted LPE and starting graphite (Fig. S9d givenin ESI), it is revealed that the G-band is stiffened by 8 cm�1 con-firming the p-type doping induced by PTCA molecules. Moreover,the doping effects are also known to result in G-band broadening,which was observed as well [50]. Here the charge transfer inter-action between PTCA and graphene is well corroborated by theRaman analysis.
For all exfoliated graphene sheets we observed the presence ofdefect activated D-band (1358 cm�1 region) and D'-band(1620 cm�1 region). It is well-known that the D-band arising fromthe breathing modes of six-atom sp2 rings and D'-band fromintravalley double resonance are easily activated by the presence ofeven edge defects [45,51]. D-bands are often very weak or often notseen in the pristine graphite flakes due to the absence of any sig-nificant structural defects or edges that are hundreds of micro-meters long. Eckmann et al. have shown that the experimentalintensity ratio I(D)/I(D0) can be used to probe defect nature ofgraphene; I(D)/I(D0) z 13 indicated sp3 defects, decreases to I(D)/
R. Narayan et al. / Carbon 119 (2017) 555e568564
I(D0) z 7 for basal plane vacancy defects and minimizes to I(D)/I(D0) z 3.5 for edge (or boundary) defects [52]. Considering thepractical error range, Paton et al. revised this ratio for edge defectsto 3 � [I(D)/I(D0)]edge defects � 4.5 [49]. Following this method, wetraced the I(D)/I(D0) ratio in our graphene samples through Lor-entzial fittings as shown in Fig. S9e. The calculated values werearound 3.77. This is a strong indication that our exfoliation strategyhas createdminimal basal plane defects in agreement with the TEMobservation. However, note that the FWHM of the D-band is~53 cm�1, but we believe the broadening is due the presence ofremnant PTCA molecules. Additionally, the intensity of D-bandrelative to G-band in our graphene sheets were quite different fromthose shown for reduced graphene oxide samples and the ID/IGratio were typically around 0.38e0.45 similar to many high qualityun-oxidized graphene sheets reported from LPE methods[23,33,53]. For typical chemically or thermally reduced grapheneoxide, the ID/IG ratio generally approximates to 1.2e1.5, which isconsiderably higher than the ratios obtained with our graphene[54]. At the same time, exceptional reports are also present whichdemonstrated high quality raman spectra for reduced grapheneoxide from wet chemical synthesis [55,56].
In an attempt to elucidate the chemical structure and measurethe oxidative defects we carried out XPS analysis of the exfoliatedgraphene samples. The high resolution C1s core level XPS spectra of
Fig. 8. (a, b) High resolution C1s XPS spectra of graphite and exfoliated graphene peak fittinlayer numbers compared to starting graphite. (d) XRD profiles of graphite, PTCA and graph
pristine graphite as a reference and graphene/PTCA sample afterwashing are depicted in Fig. 8a and (b), respectively. Not less thanthree types of carbon species were detected by the fitted plots ofstarting graphite itself. The low binding energy of 284.6eV peakcorresponding to the graphitic C]C/CeC domainwas obviously themost dominant peak for both graphite and graphene. As typicallyobserved for carbon materials with high sp2 content satellite peakstowards higher binding energy were observed at 286.1, 287.0,289.5 eV in graphite spectrum, which can be assigned to CeO, C]Oand p-p*, respectively [57]. These peaks could be originated fromthe physisorbed oxygen or CO2 molecules on bulk graphite [58].
Meanwhile, the washed graphene samples exhibited a verysimilar graphite-like C1s spectrum, but the satellite peak positionsrevealed a CeO peak at 286.04eV, whose relative contribution waslower than that in parent graphite, emergence of a new OeC]Opeak at 288.16 (absent in graphite) and broad p-p* transition peakat 291.1eV. These comparative analysis strongly suggests the lowoxidation levels of our exfoliated graphene in comparison to typicalgraphene oxide which generates a huge CeO peak at 286.2eVusually higher intense than C]C (284.5 eV) peak [6,59]. Here thelowcontent of oxygen functionalities present in our graphene couldbe originated from the residual PTCAmolecules adsorbed via strongp-p interactions. This was in fact revealed from complete XPSsurvey conducted on exfoliated graphene/PTCA samples before and
gs by Lorentzian functions. (c) FTIR spectra of exfoliated graphene sheets with differentene. (A colour version of this figure can be viewed online.)
R. Narayan et al. / Carbon 119 (2017) 555e568 565
after washing in comparison with the precursor graphite and PTCA(Fig. S10a).
As tabulated in Fig. S10b, the atomic C/O ratio in the exfoliatedgraphene/PTCA sample before washing was ~2.24 which is veryclose to that of pure PTCA. By contrast, we found that after washing,the C/O ratio of the graphene/PTCA showed a shoot up to ~9.2,which clearly substantiates that the PTCA molecules principallycontribute to the oxygen content. Based on this if we assume theoxygen content in pure PTCA as 100% coverage, the surfacecoverage of PTCA over graphene can be approximated to be nearly82% before wasing and about 27% after washing, estimated fromtheir respective oxygen contents. This is again evident if weconsider the appearance and intensity reduction of the OeC]Opeak (~288 eV) in the C1s spectra of graphene/PTCA samples before(Fig. S10c) and after washing, which was absent in graphite butpresent in PTCA (Fig. S10d). The O1s spectra of the graphene/PTCAsamples before and after washing showed contributions from COO-(~530 eV) and CeO (~531eV) as depicted in Figs. S10e and frespectively which substantiate the oxygen content to mainlyarise from PTCA molecules adsorbed on graphene surface.
We clarified the issue of oxide defects in our samples based onFTIR spectroscopic investigations that can confirm the exact pres-ence of specific functional groups. As nicely evidenced in Fig. 8c, theFTIR analysis supports the existence of residual PTCA moleculesadsorbed at the exfoliated graphene sheets even after washing.Parent graphite exhibited almost featureless FTIR spectrum, exceptsome broad reflections around 1000 cm�1 region typical of CeOstretching vibrations. In the IR spectra of pure PTCA the majorbroad signal around 3301 cm�1 and the narrow bands at 1558 and1414 cm�1 originates from the tetra-carboxylate ions. The sharp1596 cm�1 peak of PTCA is due to aromatic perylene ring (C]C)stretching vibrations [60]. Discernibly, the graphene/PTCA samplesalso contained only those characteristic vibrations that are presentin pure PTCA and starting graphite. In addition, the IR spectrum ofwashed graphene/PTCA samples showed extremely weaker signalsfor the carboxylate functional groups (3300 cm�1 region) comparedto the unwashed sample spectrum. This functional group intensityreduction upon washing clearly stipulates all the functional groupspresent on the exfoliated graphene samples to originate from theadsorbed PTCA molecules. Here it also needs to be recognized thattypical graphene oxide FTIR spectra shows characteristic peaks at1720 and 1200 cm�1 regions due to COOH, C]O, CeO or CeOeCgroups at relatively high intensities [61]. In our case, no peak isobserved in the region 1600e1800, which is a strong support infavor of unoxidized graphene sheets.
We traced the XRD profiles of the exfoliated graphene sheets incomparison to starting materials in Fig. 8d. Pristine graphite hadfour well defined peaks at 26.4� (d ¼ 0.33 nm), 42.4�, 44.5� and54.6� 2q positions corresponding to the (002), (100), (101) and(004) graphitic planes, respectively [62]. It is well iterated that thehigh intensity 2q peak at ~26� (d z 0.34 nm) corresponds to theinterlayer p-p stacking distance between graphene sheets. ThePTCA, itself being a nanographene molecule, also contains anintense peak at ~25.9� originating from the aromatic perylene unitsstacking. However, the exfoliated graphene by contrast exhibitedonly a broad and relatively low intensity peak centered around23.6� (d ¼ 0.37 nm). This shift in the (002) reflection compared toparent graphite should originate from increase in interlayer dis-tances and can be correlated to high degree of graphite exfoliation.The peak broadening indicates the mixture of misaligned graphenesheets of different size and thickness. Significantly, the grapheneXRD spotted the absence of any (004) trace which is the signaturemultilayer stacking reflection in graphite, proves the loss of anylong range order upon exfoliation [63]. Noteworthy that we couldnot observe any PTCA peaks in the XRD pattern of exfoliated
graphene sheets before or after washing procedures. This indicatesdisordered arrangements of PTCA in graphene plane.
3.5. Exfoliation and colloidal stabilization mechanism
For the detailed analysis of graphite LPE mechanism with PTCA,a control experiment was conducted following the same exfoliationprocedure in water, but without PTCA. After 12 h of bath sonicationin pure water, the resultant dispersion exhibited a strong grey colorreminiscent of graphite in comparison to the black graphene/PTCAdispersions. Upon SEM observation, it was found that sonication inwater alone has caused only chopping down of the graphite flakesto nanoemicrometer sized chunks, but no sheet exfoliation(Figs. S11a and b). Not surprisingly, these graphene/water disper-sions sedimented almost completely overnight due to the absenceof any stabilizing effect (Fig. S11c). This control experiment provesthat graphene sheets are peeled off from aqueous graphite/PTCAsolutions only due to the molecular adsorption of PTCA on graphitewhich helps weaken the van der Waals forces during sonication asillustrated in Fig. 1b. Furthermore, on basis of the surfactant design,the high colloidal stability of our exfoliated graphene/PTCA dis-persions can be attributed to the presence of surface anioniccharges. In order to experimentally verify this dispersion stabilitymechanism the zeta potential (z) of the dispersions were measuredas a function of pH, which was adjusted by the addition of 1 M HClor NaOH. The zeta potential represents the magnitude of electro-static interactions and therefore can be directly correlated to thedispersion stability. Fig. 9a shows the pH dependent variation of thezeta potential and absorbance (A660nm) of the graphene/PTCA dis-persions in water. The as-prepared dispersion had a pH value closeto 4 and exhibited the highest zeta potential value of ~ �40 mV.
Increasing or decreasing the pH (1e14) retained the overalldispersion zeta potentials within negative scale, indicating thegraphene sheets dispersed in water to be negatively charged at anypH. The colloid's negative charge obviously originates from theadsorbed PTCA molecules, which have ionized carboxylic groups insolution. The PTCA surfactant can be considered as a nano-graphene-oxide molecule due to its amphiphilic structurewithhydrophobic basal plane and hydrophiliccarboxylate group edges.
Hence the adsorption of PTCA renders the local graphene sur-face negatively charged and screens its direct interaction withwater minimizing the interfacial energy. Overall, the exfoliatedgraphene sheets are conferred with long term colloidal stability inwater owing to electrostatic repulsions endowed by adsorbed PTCAmolecules.
The zeta potential of the graphene/PTCA dispersions were foundto decrease dramatically to the range of -(13e15) mV for pH valuesless than 4 and greater than 12. Under strongly acidic (pH ¼ 1) orbasic (pH ¼ 14) conditions, in consistent with the zeta potentialvalues the dispersions become unstable as evident from the visualcomparison given in Fig. 9b. Meanwhile, in the pH range of 4e10,the dispersions had zeta potentials between �40 and �31 mV,which is more negative than�30mVmagnitude generally reportedfor highly stable colloidal dispersions [64]. In this pH range thedispersions were highly homogeneous with no flocculates visible(see inset Fig. 9a). Photographs corresponding to the dispersionsover the broad pH range are provided in supporting information(Fig. S12). In addition to the direct visualization, the absorbancemeasurements matched well with this trend. We explain this trendbased on classical DLVO theory that assumes the colloidal behaviorof charged particles to be governed by the competition betweenattractive van der Waals and repulsive electrostatic interactions[65]. Accordingly, the zeta potential fall off observed in the high pHregion (>12) could be ascribed to the increasing ionic strength thatincreases the electrostatic repulsive forces beyond threshold limits
Fig. 9. (a) Zeta potential and absorbance of exfoliated graphene/PTCA dispersions as a function of pH value. (Measurement error: zeta potential ± 1 mV, absorbance ± 0.02) (b)Photographs showing the effect of pH on the stability of exfoliated graphene/PTCA dispersions. (c) Visual comparison of the lyophilized graphene/PTCA powder with that of startinggraphite flakes and demonstration of Tyndall effect from the instantly re-dispersed powder in water. (d) Photographs corresponding to the lyophilized graphene/PTCA powders re-dispersed in polar organic solvents, but insoluble in non-polar oils. (e) Photograph showing the use of graphene thin films by vacuum filtration in electrical conductivity test. (f)Schematic illustrating the gluing of graphene sheets by PTCA molecules at interflake junctions. (A colour version of this figure can be viewed online.)
R. Narayan et al. / Carbon 119 (2017) 555e568566
that themselves starts destabilizing the system. By contrast, atlower pH values, protonation of the PTCA carboxylic groups reducethe surface charge repulsion and the attractive vanderWaals/hy-drophobic interactions dominate. This substantiates the role ofelectrostatic repulsive forces from the ionized PTCA carboxylicgroups in the complete dissolution of exfoliated graphene sheets inwater. We also note that pH dependent colloidal behavior exhibitedby our LPE dispersion is similar to aqueous solutions of chemicallyconverted graphene oxide [44,66,67].
Inspired by the good exfoliating and stabilizing ability of PTCAfor graphene, we exploited the same strategy for other 2Dmaterialslike molybdenum disulfide (MoS2). Unfortunately, the results werenot so encouraging, which we assume to be due to lack of anystrong non-covalent interactions such as p-p stacking. By contrast,we found that PTCA could very efficiently exfoliate multiwall car-bon nanotubes (MWCNT) in a good yield and long shelf-life (>12months) without any aggregation (Fig. S13a). Unbundled CNTswere clearly revealed from SEM (Fig. S13b) and TEM images(Figs. S13ced), confirming that PTCA is an excellent exfoliant cumdispersant for 1D carbon nanomaterials as well.
3.6. Re-dispersion of exfoliated graphene
Another interesting aspect of our exfoliation strategy is the easy
storage of the dispersed graphene sheets in the form of powder andre-dispersion ability in water and many organic solvents. Asevident from Fig. 9c, the freeze dried graphene powder exhibitdistinctly different dark black appearance compared to startinggraphite, which has a greyishmetallic lustre. The SEM images of thefreeze dried graphene/PTCA powder given in Fig. S14 shows thatthe graphene sheets arrangewith minimal restacking. But the mainhighlight of these powders is their instantaneous dissolution inwater without any sonication step. A direct visualization of thisbehavior is provided in Supplementary video 1. We note thatrepeated washing-centrifugation steps has removed the excess/loosely bound PTCA molecules from the graphene/PTCA and theblack residues obtained after centrifugation were easily re-dispersed with high colloidal stability and freeze dried. Theremarkable re-dispersability of the resultant graphene/PTCA pow-der is clearly attributed to the high binding strength of PTCA mol-ecules that remain strongly adhered to the graphene surface evenafter freeze-drying. For many important applications like opto-electronics it is highly desirable that the graphene dispersions to beprocessable in volatile non-aqueous solvents. We examined the re-dispersibility of graphene powder in common organic solvents. Thegraphene powders got easily dispersed in polar organic solventslike ethanol (EtOH), isopropyl alcohol (IPA), N-methyl pyrrolidone(NMP), N, N-dimethylformamide (DMF) upon sonicating for less
R. Narayan et al. / Carbon 119 (2017) 555e568 567
than 5 min (Fig. 9d). By contrast, the graphene powders showed nosign of dispersibility in strongly hydrophobic solvents like tolueneeven after ultrasonicating for 30 min. These observations deducethat amphiphilic structure of PTCA is a necessary and sufficientcondition that provides good compatibility for the graphene sheetstowards aqueous and polar organic solvents simultaneously.
Supplementary video related to this article can be found athttp://dx.doi.org/10.1016/j.carbon.2017.04.071.
3.7. Graphene film electrical properties
Finally, we investigated the electrical performance of the filmsassembled from exfoliated graphene sheets. Thin graphene films(~250 nm thickness) were prepared by simple vacuum filtrationthrough flexible cellulose acetate membranes. We demonstrate theas-prepared graphene films could act as conductive wires to lightup LED lamp (3 V), completing the electrical circuit as shown inFig. 9e. The sheet resistance of graphene film was ~660 U/,, asmeasured by four-probe instrument and the thickness could bevaried by adjusting the volume of filtered dispersion. The corre-sponding bulk electrical conductivity is 6000 Sm-1. The surfacemorphology and cross-sectional imaging of these thin films usingSEM showed that the graphene sheets are aligned parallel to eachother, but not in AB stack fashion (Fig. S15). This random stackingwould favor the exfoliated thin layers to retain the electronicproperties distinguishable from graphite. Noteworthy that theelectrical performances of our graphene films are better thanchemically oxidized graphene based films [68] and comparable tothe films prepared from NMP-based LPE methods (6500 Sm-1) [8].In graphene flake stacked films, the electrical transport is domi-nated by the inter-flake junction resistance and the number of suchcontacts in the percolation pathway. Normally, the presence ofelectrically insulating surfactants at the interface of two graphenesheets tends to increase the contact resistance. However, in com-parison to the typical dielectric surfactants having insullatingcomponents of long flexible chains tailored to impart solubility inwater or organic media, the PTCA is a purely aromatic wide bandgap organic semiconductor (Eg ¼ 2.19 eV). In particular, the per-ylene cores are known to readily self-organize into p-stacks withadjacent planar molecules that support favorable charge transportpathways, such that electrical conductivity upto 3.0 Sm-1 has beenreported in single crystalline PTCDA nanowires [69]. Consideringthe semiconducting nature and large aromatic p-structure of PTCA,it is reasonably surmised that continuity of the conductive pathwayin our graphene films should be smoothened by the surfactants atthe junctions as schematically depicted in Fig. 9f.
4. Conclusion
Noncovalent chemistry of graphene with amphiphilic tetra-anionic polycyclic aromatic hydrocarbon e PTCA (perylene tetra-carboxylate) is introduced for the effective large scale LPE of gra-phene. Inexpensive raw material and straightforward single stepsynthesis render PTCA distinctly attractive for large scale LPE.Excellent exfoliation efficiency is demonstrated for the productionof litres of high quality graphene dispersions at a concentration upto 0.8 mg/mL. However broad distribution in terms of layer thick-ness and lateral dimensions is a major limitation of the strategy,which restricts the overall monolayer graphene yield to ~8%. Theseparation of very small, but thicker graphene chunks from thethinner large sheets still remain a challenge. Nevertheless, themechanism of PTCA induced electrostatic stabilization of graphenecould be verified by pH dependent zeta potential measurements.Unique polyaromatic amphiphilic structure of this surfactant offerssolvent dispensability readily switchable to polar organic media as
well as low boiling solvents such as ethanol, IPA. Noticeably, purelysemiconducting nature of PTCA without dielectric moiety ensurestight electrical contact among neighboring graphene flakes aftersolution processing. Our approach, devoid of any harsh reactionconditions or cumbersome synthetic steps should be an importantstep towards reliable large scale LPE of graphene, which is desiredfor many different applications requiring scalable solutionprocessability.
Acknowledgements
This work was financially supported by the Multi DimentionalDirected Nanoscale Assembly Research Center (Creative ResearchInitiative 2015R1A3A2033061) and the Nano$Material TechnologyDevelopment Program through the National Research Foundationof Korea (NRF) funded by the Ministry of Science, ICT and FuturePlanning (2016M3A7B4905613).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.carbon.2017.04.071.
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