Single-walled carbon nanotubes as atemplate for coronene stack formation

Alexander I. Chernov*,1,2, Pavel V. Fedotov1, Ilya V. Anoshkin3, Albert G. Nasibulin3,4, Esko I. Kauppinen3,Vladimir L. Kuznetsov5,6, and Elena D. Obraztsova1,2

1 Prokhorov General Physics Institute, RAS, 38 Vavilov str., Moscow 119991, Russia2 National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe hwy. 31, Moscow 115409, Russia3 Department of Applied Physics, Aalto University School of Science, P.O. Box 15100, FI-00076 Espoo, Finland4 Skolkovo Institute of Science and Technology, 100 Novaya st., Skolkovo, Odintsovsky district, Moscow Region 143025, Russia5 Boreskov Institute of Catalysis, SB RAS, Lavrentieva ave. 5, Novosibirsk 630090, Russia6 Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia

Received 30 April 2014, revised 15 August 2014, accepted 18 August 2014Published online 10 September 2014

Keywords columns, coronene stacks, nanoreactor, single-walled carbon nanotubes

* Corresponding author: e-mail al.chernov@nsc.gpi.ru, Phone: þ7 4995038206, Fax: þ7 4995038206

Single-walled carbon nanotubes can serve as a template forformation of various structures inside. Depending on conditions,the nanotube filling with coronenemolecules may lead to creationof coronene stacks, graphene nanoribbons, or inner tubes. In thiswork, the formation of coronene stacks inside single-walledcarbon nanotubes was studied using photoluminescence (PL)spectroscopy. We demonstrate the difference in the excitation PLmaps between the spectral features of coronene stacks inside thenanotubes and products formed outside the nanotube interior. Theoptical investigation was supported by calculations of adsorptionenergy of coronene molecules inside and outside nanotubes. Thetheoretical calculations predict the most energetically favorableconfigurations of coronene molecules with respect to the diameterof the outer nanotubes. Orientation of coronene stacks depends onthe nanotube diameter.

Models of ten coronene molecules placed inside (10,10) and(11,11) nanotubes (upper and lower image part, respectively).

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1 Introduction Single-walled carbon nanotubes(SWCNTs) have been demonstrated to serve as thenanoscale reactors [1]. The space inside SWCNTs providesgeometrical conditions for formation of one- [2] andzero-dimensional [3] structures. Molecules placed insideSWCNTs are well protected from the external environmentand can form inner structures, which are thermodynamicallyunstable in the absence of encapsulating template. Confine-ment determines the behavior of molecules encapsulated inSWCNTs. Therefore, the diameter of nanotube not onlydetermines whether the molecule can get inside, but alsoaffects the molecules motion and possibility to form newstructures. Various molecules can be inserted into

SWCNTs [4–6]. Increase of the system temperature triggersthe modifications of internal structures, turning the nano-tubes in the nanoreactors [7]. One of the first reactions ofsuch type was the transformation of the filled fullerenes intothe inner nanotubes [8, 9]. Recently, the polycyclic aromatichydrocarbon (PAH) molecules were used as precursors forformation of graphene nanoribbons [10] and their furthertransformation into new nanotubes inside the host tubes [11].Reaction of polymerization leads to modification of singlemolecules into a sole spiral nanoribbon. However, thediameter of the outer nanotube plays an important role in suchreactions and governs the shape of the interior. For example,a preferential configuration inside the small diameter

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nanotubes is an array of the planar coronene molecules(C24H12) oriented perpendicular to the nanotube axis, so-called“stacks” [12, 13], instead of the nanoribbons. In such stackscoronene molecules are supported by the outer nanotube andline up due to the p–p interaction between each other.

Photoluminescence (PL) spectroscopy is an efficienttechnique for identification of semiconducting SWCNTs [14,15], PAH molecules [16] and hydrogen-terminated nano-ribbons [17, 18]. It allows accessing the energy band gap ofthe material and therefore discriminating between the typesof structures presented in the sample. SWCNTs emit light inthe near IR range, while nanoribbons and PAH moleculesexhibit emission usually in a visible spectral range. Despitethe fact that the emission of the latter structures can be wellresolved, the only complicating fact, which may occur, isthe distinction of the spectral features assignment to thestructures formed inside or outside nanotubes. For example,during the thermal mediated filling of SWCNTs withcoronene molecules some of them can easily attach to thenanotube external surface or form molecular crystals [19].

In this work, we study the filling of SWCNTs withcoronene molecules, investigate the PL response of thestructures encapsulated in SWCNTs and differentiate itfrom the spectral features of side products. We present thetheoretical calculations of the adsorption energy of coronenemolecules placed inside and outside the nanotubes.

2 Experimental procedure SWCNTs with an aver-age diameter 1.3� 0.2 nm have been synthesized by anaerosol method [20] and tubes with an average diameter1.4� 0.15 nm have been formed by an electrical arc-discharge method using Ni/Y2O3 (1:1 ratio) catalyst in Aratmosphere. SWCNTs used in the experiments were either inthe form of bucky paper or in the form of film deposited onthe quartz substrate. The deposition was performed by asimple dry transfer technique described elsewhere [21]. Priorto the filling procedure, the nanotube caps were opened byheating in air for 25min at 450 8C. The filling was performedin the sealed glass tubes where nanotubes were spatiallyseparated from coronene powder (Angene). The treatmentwas performed in Ar atmosphere or in vacuum at 450 8C for13 h. After the synthesis, samples were washed in toluene inorder to remove the coronene and side products from thesurface of the nanotubes.

PL measurements have been performed on the solidsamples, without formation of nanotube suspensions. Spectrawere recorded with a Horiba Jobin-Yvon NanoLog-4 systemsupplied with the InGaAs CCD detector (850–1550 nm) andwith the photomultiplier (R928P) working in a spectralrange (180–850 nm). A PL excitation in the UV–Vis spectralrange was performed with a Xe lamp (250–900 nm). A PLexcitation mapping was performed with a 2 nm step inexcitation.

2.1 The calculation method details Molecularmechanics (MM) is widely used for the numerical studiesof adsorption energies of different molecules on graphite and

carbon nanotubes [22–26]. In the present paper, we use MMto characterize the adsorption of coronene moleculeson SWCNTs of various diameters. We used commonMMþ force field parameter set [27, 28] with a softwarepackage HyperChem 8.0 (Hypercube, Inc.) to get the mostfavorable geometry and to compare the energy of coronenemolecule adsorption on the external nanotube surface andinside nanotube.

Since the self-assembly of nanotube and coronenemolecules leads to non-chemical bonding between them, theoptimization of the adsorption geometry is only governed bythe non-bonding forces involving van der-Waals repulsiveforces at close range, Lennard-Jones long-range attractiveinteractions and electrostatic Coulomb forces. The adsorp-tion energy between the coronene molecules and thenanotubes were estimated using Eq. (1):

DEads ¼ ENTþcoronenes � ðENT þ EcoronenesÞ; ð1Þ

where ENTþcoronenes is the energy of the optimized structureof the nanotube and coronene molecules interaction, ENT isthe energy of the nanotube alone and Ecoronenes is the energyof isolated (gas phase) coronene molecules. Two typesof SWCNTs with diameters 1.36 nm (10,10) and 1.49 nm(11,11) were used.

For checking the applicability of MMþ calculations, wehave performed a comparative calculation of the adsorptionenergies of single coronene molecule positioned inside andon the external surface of a small fragment of (11,11) tubeusing the MM method and the semi-empirical quantum-chemical method CМ6 DH2 (Mopac2012) specially modifiedfor the calculations of van-der-Waals interactions andhydrogen bonding [29]. We have found that the data on theadsorption energy were in consistence with the data obtainedusing the semi-empirical quantum-chemical method CМ6 DH2(Mopac2012), i.e., DH2: �43.587 and �32.30 kcalmol�1,and MMþ: �43.84 and �32.39 kcalmol�1, respectively.

3 Results and discussion3.1 Photoluminescence of coronene stacks We

investigate the spectral properties of coronene stacks insideSWCNTs by PL excitation mapping. A high-resolutiontransmission electron microscope (HRTEM) image ofstudied material is presented in Fig. 1. Stacking is observedas a common type of encapsulation. Both of the nanotubetreatment procedures in Ar atmosphere and in high vacuumfor SWCNTs with chosen diameter distribution lead to thesame PL properties of materials. In Fig. 2, we show a typicalPL map of the sample prepared by annealing treatment at450 8C in Ar atmosphere followed by washing with toluene.The PL spectrum of coronene stacks inside SWCNTs waspreviously reported by Okazaki et al. [12]. However, somedoubts have been expressed [30], stating that the reportedspectral features might correspond to the side productsattached to the outer surface of the nanotubes instead of theinner coronene stacks. Nevertheless, both of the works wereconfirmed by the TEM images of the inner coronene stacks.

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In the paper of Botka et al. [30] authors demonstrateformation of coronene stacks inside nanotubes at varioustemperature regimes. Anoshkin et al. [13] demonstrated theformation of coronene stacks with different angles to thenanotube axis depending on the diameter of the host tubes.

In previous works the PL emission spectra of coroneneand carbon nanotube materials have been reported only at afew excitation wavelengths. Figure 2 represents the PLexcitation map of the annealed sample of SWCNTs filledwith coronene molecules. All of the spectral features arelocated within 475–650 nm emission wavelength range anddemonstrate the dependence on the excitation wavelength.In order to avoid complications due to the suspendingprocedures of carbon nanotubes, which might cause the non-desirable washing out of the studied objects, we haveperformed all the measurements on the solid samples washedwith toluene. We have revealed the difference in the PLspectra of the samples before and after washing with toluene(Fig. 3). Despite the fact that it is hard to completely excludethe influence from the side products, we can follow thedecrease of the emission intensity of the spectral componentsand dominantly of those, located below 480 nm. After thewashing procedure, the spectra demonstrate in average

three main components located at 498, 537, and 578 nm.These three components, which we refer to the contributionfrom the structures inside the nanotubes, at some momentafter the treatment start, become unaffected by the toluene.One of the possible reasons for such behavior is the carbonnanotube protection of the encapsulated structures. Anadditional heat treatment of the washed samples, such as ahigh temperature annealing or a laser beam irradiation, leadsto the formation of inner tubes, transforming the SWCNTswith encapsulated coronene stacks into the double-walledcarbon nanotubes (DWCNTs). Transformation of thecoronene-filled SWCNTs was previously reported [30, 31].However, prior to the DWCNT, the peapod-like structuresstart to form [13]. The origin of such reorganization andtheoretical investigation will be presented further in the text.Here, we demonstrate that the slight change in the diameterdistribution of host SWCNTs leads to the change of thecoronene stacks spectral features (Fig. 4a and b). We detectthe intensity redistribution and the emission peak positionschanges. Going into more details of the measured PLspectra, three components behave differently depending onthe excitation wavelength, type of the host nanotubediameter and slight changes of the synthesis parameters.The component at 498 nm emission wavelengths is notchanging its peak position, although the intensity can besignificantly decreased. The minimal emission intensity wasdetected for the 410 nm excitation wavelength and 20 8Cincreased treatment temperature (Fig. 4d). A relativeintensity of the peak in comparison with the intensity oftwo others inversely depends on the treatment temperature.The spectral component positioned at 537 nm only slightlychanges its position when the synthesis temperature ischanged and it is demonstrated to be dependent on theexcitation wavelength. The longest-wavelength spectralcomponent of the triplet strongly depends on the

Figure 1 The HRTEM image of coronene stacks inside SWCNTsprepared by treatment in vacuum. The arrow points to the areawhere the stacks are located. The sample was analyzed at anacceleration voltage of 80 kV.

Figure 2 The PLE contour plot of the coronene stacks insideSWCNTs measured on the quartz substrate. The average diameterof SWCNTs is 1.4 nm.

Figure 3 The photoluminescence emission spectra of the coronenestacks inside SWCNTs before and after the toluene treatmentmeasured at a 410 nm excitation wavelength. The average diameterof SWCNTs is 1.4 nm.

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experimental parameters and the excitation wavelength. Themaximal shift of the peak position is 7 nm. In ourexperiment, the PL emission spectra of the identicallyprepared samples of the filled coronene stacks inside carbonnanotubes with an average diameter 1.3� 0.2 nm (Fig. 4a)and an average diameter 1.4� 0.15 nm (Fig. 4b) demonstratethe different spectral positions of two components, while theone at 498 nm stays unchanged. The peak positions shiftsmay reach up to 8 nm. However, the overall type of thespectral band is similar for both diameter distributions ofnanotubes. The change of intensity and positions of thespectral components can be explained by the fact thatcoronene molecules inside the nanotubes have differentorganization. Recently, Anoshkin et al. [13] demonstratedthe dependence between the stack axis angle and the hostSWCNT diameter. The nanotube diameter value determinesthe configuration of encapsulated molecules. Important tomention, that in our study three peak type of the emissionspectrum is typical only for the coronene–nanotubes systemsformed in the narrow diameter SWCNTs, which are close tothe filling limit with coronene molecules [12, 17]. In case oflarger diameter nanotubes, the formation of coronene stacksbecomes not favorable and gives a way to the formation ofother forms of encapsulated structures [18, 11, 13], whichexhibit completely different PL emission [18].

3.2 Molecular models The models of tubes with teninserted coronene molecules are presented in Fig. 5. It wasfound that the adsorption of coronene molecules inside tubesis more energetically favorable than the adsorption ofmolecules on the external surface of tubes (for the (10,10)

tube: 47.35–32.24 kcal mol�1; for the (11,11) tube: 44.86–33.01 kcal mol�1, respectively; data were normalized perone coronene molecule). A significant deformation of thetube with a smaller diameter after the coronene adsorptionwas observed. The angle between the nanotube axis and theplane of the coronene molecules depends on the nanotubediameter.

We have also analyzed a possibility of the formation ofDWCNT using a thermal decomposition of pre-adsorbedcoronene molecules inside the SWCNT. Thus, we comparethe volume of forming internal nanotube with the volume ofpre-adsorbed coronenemolecules. The thermal decompositionof coronene molecules even in the case when they completelyoccupy the internal channel of the nanotubes cannot providethe formation of prolonged ideal double-walled nanotubebecause of the lack of carbon atom required per the unit lengthof internal nanotube. One also must remember that thehydrogen atoms filled a significant part of the nanotubeinternal space after adsorption of coronene molecules. We canpropose the formation of defective prolonged internalnanotube or the formation of short closed nanotubes separatedfrom each other (see Fig. 6). The formation of peapod-typestructures was recently reported [13].

3.3 A comparison between stacks and coronenewires As it was previously mentioned, the synthesistechnique used for formation of coronene stacks insideSWCNTs may also lead to formation of side products.The influence of the treatment temperature on the types ofthe grown structures has been discussed in detail in the paperof Talyzin et al. [32]. It is very important to clearlydistinguish the spectroscopic response from the inner

Figure 4 The photoluminescence emission spectra of coronenestacks inside SWCNTs obtained with different experimentalparameters. (a) SWCNTs with an average diameter 1.3 nm,360 nm excitation wavelength. (b–e) SWCNTs with an averagediameter 1.4 nm and (c) 410 nm excitation wavelength; (b) 470 8Ctreatment temperature, 360 nm excitation wavelength; (d) 470 8Ctreatment temperature, 410 nm excitation wavelength; (e) 440 8Ctreatment temperature, 360 nm excitation wavelength.

Figure 5 Models of ten coronene molecules placed inside (10,10)and (11,11) nanotubes (upper and lower part, respectively; in themiddle – the projections along the nanotube axis are presented).For clarity, the coronene molecules are presented using spheres ofVDW radius, while the carbon nanotubes are in a stick modelrepresentation. The view along tube axis demonstrates thesignificant deformation of (10,10) SWCNT. The angle betweenthe nanotube axis and the coronene molecule plane depends onthe nanotube diameter.

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structures and from the side products, which may be formedoutside nanotubes. A PL study of the materials synthesizedat different temperatures shows that PL mapping may be aconvenient tool to detect the change of the spectral featurescorresponding to various materials. We have performed thesame synthesis procedure as in the case of coronene stacksinside SWCNTs, but without the nanotubes. As the result,when no nanotubes are presented during the synthesis, wehave obtained the yellow color coronene narrow wiresaround 1 cm long. The PL excitation map of such wires ispresented in Fig. 7. Two main differences can be easilynoticed when we compare the PL map of coronene wires andcoronene stacks inside SWCNTs (Fig. 2). For the coronenewires, a larger number of spectral features are presented andthe excitation range is much broader (up to 575 nm). Despitethe fact that three spectral components, which we previouslydiscussed for encapsulated molecules, are presented in themap, their spectral positions are shifted. The peakspositioned at 537 and 578 nm in Fig. 2 are shifted to 529and 568 nm, respectively. The best similarities in the peakpositions are found for coronene stacks formed inside thenanotubes with an average diameter 1.3� 0.2 nm (Fig. 4a),

with the main difference in a smaller range of excitationwavelengths. In the solid coronene, the molecules arepositioned in the adjacent stacks and are nearly perpendicu-lar to each other. The plane of coronene molecules is inclinedat approximately 458 to the crystal axis [19]. Such crystalstructure gives the origin for a larger number of emissionpeaks in comparison to the PL spectra of coronene stacksinside SWCNTs.

The increase of the treatment temperature in theexperiment leads to the formation of additional sideproducts. For example, when the temperature is raised upto 530 8C without the presence of nanotubes, we obtain thered color material. Previously, such material was determinedas a dicoronylene crystal [32, 33]. In contrast to thepreviously reported PL emission spectra of dicoronylene atroom temperature [16], for the red color material, obtained inour process in Ar atmosphere, we get only one broad peakpositioned around 661 nm (Fig. 8). According to the PLexcitation mapping, the spectral features are not resonant andmolecules emit light while excited in a wide spectral region.The shape and position of the PL emission peak of the red-colored material look the most similar to the quaterry-lene [16], although the peak position is 10 nm shifted. InFig. 3e, we show the PL response of the encapsulatedSWCNTs. Such spectral line shape was reported to be asignature of dicoronylene [16, 31]. In contrast to theaforementioned works, we demonstrate, that changing thediameter of the host nanotubes and treatment temperaturemodifies the intensity distribution between the triplet peaksand drastically changes the spectral line shape, together withthe spectral peak positions. In Fig. 8, we demonstrate thedifference in PL emission spectra between the molecularcoronene crystal, red color dicoronylene crystal andcoronene stacks inside SWCNTs.

Figure 6 Size comparison of a closed nanotube model containing240 carbon atoms (black) and a set of closely packed ten coronenemolecules (red) also containing 240 carbon atoms inside the (11,11)nanotube. One can see that the closed nanotube has a smallermolecular volume than a set of ten coronene molecules.

Figure 7 The PLE contour plot of the molecular coronene crystalformed without the presence of nanotubes.

Figure 8 The photoluminescence emission spectra of the solidcoronene wires (black), solid dicoronylene (blue) and coronenestacks inside SWCNTs (red color dashed line) measured at a410 nm excitation wavelength.

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4 Conclusions In this contribution, we show theimportance of the PL excitation mapping in a wide spectralrange in order to clearly detect the difference between thespectral features of the coronene stacks inside the nanotubesand products, which can be formed outside the nanotubeinterior. The coronene and dicoronylene crystals formedoutside the nanotubes and coronene stacks inside SWCNTspossess their own intrinsic resonant and non-resonantspectral features. We demonstrate the PL response of thecoronene stacks inside different diameter SWCNTs formedat various treatment temperatures. Theoretically estimatedadsorption energies of coronene molecules inside andoutside the nanotubes predict the most favorable nanotubeparameters for the formation of coronene stacks. It wasfound that adsorption of coronene molecules inside the tubesis more energetically favorable than the adsorption of themolecules on the tube external surface. Orientation ofcoronene stacks depends on the nanotube diameter. Anextensive interest in the area of utilization of carbonnanotubes as the reactors for transformation of filled PAHmolecules requires an efficient tool for the control ofsynthesized materials. We demonstrate that PL spectroscopycan serve as such tool and provide the basis for further studyof the peculiar properties of novel materials.

Acknowledgements The work was supported by RFBRGrants 13-02-01354, 14-02-31829, 14-32-50113, 14-02-00777,RAS research projects, Ministry of Education and Science ofRussian Federation Grants No. 14.513.12.003 and SP-7362.2013.3.

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