Optical properties and charge transfer effects in single-walled carbon nanotubes filled with functionalized adamantane molecules

Carbon 109 (2016) 87e97

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Optical properties and charge transfer effects in single-walled carbon nanotubes filled with functionalized adamantane molecules A.A. Tonkikh a, *, D.V. Rybkovskiy a, A.S. Orekhov b, c, A.I. Chernov d, e, A.A. Khomich d, C.P. Ewels f, E.I. Kauppinen g, S.B. Rochal a, A.L. Chuvilin h, i, E.D. Obraztsova d a

Faculty of Physics, Southern Federal University, 344090, Rostov-on-Don, Russia University of Eastern Finland, Department of Physics and Mathematics, Joensuu, 80101, Finland National Research Center «Kurchatov Institute», 123182, Moscow, Russia d A.M. Prokhorov General Physics Institute, RAS, 119991, Moscow, Russia e National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), 115409, Moscow, Russia f Institut des Mat eriaux Jean Rouxel (IMN), Universit e de Nantes, CNRS UMR 6502, FR-44322, Nantes, France g Department of Applied Physics, Aalto University, School of Science, P.O. Box 15100, FI-00076, Espoo, Finland h CIC nanoGUNE Consolider, Tolosa Hiribidea 76, 20018, Donostia-San Sebastian, Spain i IKERBASQUE Basque Foundation for Science, Maria Diaz de Haro 3, E-48013, Bilbao, Spain b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 May 2016 Received in revised form 20 July 2016 Accepted 26 July 2016 Available online 3 August 2016

The filling of single-walled carbon nanotube films with 1-adamantanemethanol and 1bromoadamantane molecules was carried out by a gas-phase procedure. Optical absorption and Raman spectroscopies revealed the effects of charge transfer for samples treated with 1bromoadamantane. Using density functional simulations we show that the charge transfer takes place when bromine is detached from the carbon cage and accepts electrons as free-standing atoms or as polybromide structures. Transmission electron microscope images confirm the presence of individual bromine atoms inside carbon nanotubes, treated with 1-bromoadamantane molecules. The observed charge transfer is, therefore, a marker of the dehalogenization reaction, a critical step in the synthesis process of narrow 1D sp3 nanostructures. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction The filling of single-walled carbon nanotubes (SWCNTs) with impurity atoms is a promising way to modify the properties of the SWCNTs and to create novel one-dimensional structures inside the nanotube channels. Doping of SWCNT films with inorganic materials can increase the film conductivity and change the optical absorption spectra [1e5]. At the same time, a large variety of 1Dnanostructures, which are unstable as freestanding systems, can be formed inside SWCNT channels [1,6e9]. Beside the use of inorganic materials as dopants, there has been a large effort to fill nanotubes with organic molecules with the aim to synthesize novel carbon-based 1Dstructures. This typically involves filling the nanotube channels with organic molecules and subsequent annealing, leading to the polymerization of the molecules inside

* Corresponding author. E-mail address: [email protected] (A.A. Tonkikh). http://dx.doi.org/10.1016/j.carbon.2016.07.053 0008-6223/© 2016 Elsevier Ltd. All rights reserved.

the CNTs. Small diameter inner tubes have been formed in this way from fullerene-filled CNTs [10,11]. A similar method is used to synthesize narrow graphene nanoribbons from sp2 bonded molecules [12e14]. Linear-chain polyyne molecules are used to form 1D carbyne chains [15,16]. Another intriguing task is the formation of narrow 1D sp3 bonded structures. Diamondoid molecules are promising building blocks for this kind of system, but there are relatively few studies of such structures to date. The only existing works consider carbon nanotube filling with adamantane [17,18] and diamantane dicarboxylic acid with a subsequent formation of 1D structures inside the CNTs [19]. The results indicate that the encapsulated diamondoid molecules are difficult to detect with spectroscopic techniques, used to study carbon nanotubes. A wide range of diamondoid-derived molecules exist, which differ by functional groups, and the correct choice of the particular functional group may simplify the molecule encapsulation and polymerization. An important factor in the polymerization procedure is a weak coupling between the functional groups and the carbon atoms, which results in simple precursor dissociation.

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Halogen-terminated precursor molecules, which dissociate at sufficiently low temperatures (~200
C) via the dehalogenization reaction, are frequently used for the formation of graphene nanoribbons [20,21]. In the present work we carry out the filling of different diameter SWCNT films with 1-adamantanemethanol and 1bromoadamantane molecules at the same conditions. We show that the 1-bromoadamantane molecules effectively dissociate inside the SWCNTs during the filling procedure and the features of this dissociation can be efficiently detected with use of common spectroscopic measurements as Raman and optical absorption. On this basis halogenated diamondoid molecules are promising candidates for the formation of 1D sp3 bonded carbon structures. 2. Experimental and theoretical methods Single walled carbon nanotubes (SWCNTs) synthesized by arc discharge and aerosol-chemical vapor deposition (CVD) were used for the diamondoid filling. Arc-SWCNTs were synthesized by “home-made“ growth [22] set-up with impurities and amorphous carbon subsequently removed by suspending 0,1e0,3% nanotube powder in 2 w/w% water solution of sodium cholate (SC) and centrifugation in a Beckman Coulter Ultra-Max-E. The diameter distribution of the arc-SWCNTs was 1.4 ± 35 nm. Thin films were made from purified arc-SWCNTs by liquid filtration using MCE (Millipore) filters with pores diameter of 100 nm and transferred to silicon and quartz substrates. The aerosol-SWCNTs were synthesized by floating catalyst chemical vapor deposition (FC-CVD) reactors from CO using ferrocene vapor based catalyst and collected on cellulose filters downstream the reactor [23]. These SWCNT films were transferred from the filters to the silicon and quartz substrates. The diameter distribution of aerosol-SWCNTs was d ¼ 1.9 ± 0.5 nm. After the transfer on the substrates the SWCNT films were annealed at 400
C at ambient pressure in air atmosphere during 15 min to open the nanotube ends. After that, the films were annealed for the second time at 200
C and vacuum about 103 mbar during a few hours to remove adsorbents. Based on the aerosol-SWCNTs a few special samples of free-standing SWCNT films were prepared to exclude the substrate effect during encapsulation and analysis by stretching them on a silicon frame to form membrane-like structure. Part of the spectroscopic measurements was carried out on such membrane-like filled SWCNT films. Molecules of 1-bromoadamantane (C10H15Br, Alfa Aesar, 99%) and 1-adamantanemethanol (C11H18O, Alfa Aesar, 98%) were used to fill the SWCNTs. The filling procedure was carried out in glass evacuated ampoules (vacuum about 103 mbar) for 13 h. The filling temperature was tested in the range of 100e200
C. Spectroscopic measurements revealed the lowest filling at 100
C, and high outerwall adsorption at 200
C. The final studies were performed at 160
C. After the filling all samples cleaned with CHCl3 to remove adsorbates. Raman spectroscopy of pristine SWCNT films and samples treated by diamondoids was carried out within spectral ranges of radial breathing modes (80-450 cm1) and G-modes (around 1591 cm1). Raman spectra were obtained on LabRAM HR Evolution equipped with HeNe laser at operating wavelength 632.8 nm. Additional Raman measurements at higher excitation energy (514 nm) were carried out with a JobinYvon S-3000 spectrometer and AreKr laser. The UVeviseNIR optical absorption spectra were recorded within spectral range 200e3000 nm with a spectrophotometer Lambda-950 (Perkin-Elmer). The FT-IR spectra were obtained within spectral range 400e3500 cm1 with a Perkin-Elmer Spectrum 100.

Density functional modeling of carbon nanotubes, filled with 1bromoadamantane, 1-adamantanemethanol molecules and polybromide structures was performed with the AIMPRO code [24,25], using Gaussian basis sets. The Perdew-Burke-Ernzerhof functional [26] is used for exchange and correlation, the wavefunctions are expanded using independent s-, p- and d- orbitals and the action of core electrons is modeled with the Hartwigsen-Goedecker-Hutter pseudopotentials [27]. The self-consistent field (SCF) cycle is calculated with a 12 k-point grid along the nanotube axis and a Fermi smearing with an effective temperature of 0.02 eV. After the SCF procedure the projected densities of states (PDOS) are calculated with a grid of 241 k-points. The structure and composition of aerosol-SWCNTs filled by 1adamantanemethanol and 1-bromoadamantane molecules from gas-phase was characterized by High-Resolution Transmission Electron Microscopy (HRTEM), Scanning Transmission Electron Microscopy (STEM) and X-ray Energy Dispersion Spectroscopy (EDS). The measurements were carried out on a Titan 60e300 TEM/ STEM electron microscope (FEI, Netherlands) equipped with x-FEG electron source, monochromator and Cs spherical aberration corrector, and operating at accelerating voltage of 80 kV. Characterization and interpretation of the experimental images were performed by comparison with simulated HRTEM images. 3. Results and discussion The samples of aerosol-SWCNTs and arc-SWCNTs transferred on silicon and quartz substrates were investigated by Raman spectroscopy after treating by 1-bromoadamantane and 1adamantanemethanol. Fig. 1 (a) shows the normalized Raman spectra of aerosol grown SWCNTs before (black line) and after (purple line) 1-adamantanemethanol encapsulation (1adamantanemethanol@aerosol-SWCNTs). To minimize the contribution of outer-wall adsorption for 1adamantanemethanol@aerosol-SWCNTs the nanotube films were cleaned with chloroform (CHCl3). The corresponding Raman spectrum is presented in Fig. 1 (a) by a navy line. Raman spectra of pristine aerosol-SWCNTs at the excitation wavelength 632.8 nm have clear features of semiconducting SWCNTs. The G-mode region has the most intensive band at 1591 cm1 (Gþ) and weak bands around 1570 cm1 (G-) which have been assigned to LO and TO phonons of semiconducting SWCNTs, respectively (LOS and TOS [28]). A simultaneous observation of multiple TOS bands is interpreted as a resonance with the excitation wavelength of the semiconducting SWCNTs with different geometries. Indeed, aerosol-SWCNTs in the low frequency region have a rich phonon spectrum. Up to 10 radial breathing modes are resolved at the operating wavelength 632.8 nm. The RBM region consists of two most intensive modes at 123 and 140 cm1, which can be assigned to SWCNTs with diameters of about 2 and 1.9 nm. Additionally, several weak RBMs were observed with frequencies up to 259 cm1 (a diameter of about 0.9 nm). Treatment with 1-adamantanemethanol leads to significant changes of aerosol-SWCNT spectra. In the G-mode region the intensity of LOS and TOS for all modes decreases due to the change of the resonance conditions because of heat treating, outer-wall adsorption and encapsulation of 1-adamantanemethanolin SWCNTs. The most interesting effect occurs in the RBM region. Beside the drop of RBM intensity a right-shift of RBM frequencies was detected. The right-shift was preserved after cleaning in CHCl3 solution. In view of gaseous treatment by 1-adamantanemethanol such a shift could be explained by either encapsulation inside the SWCNTs or outer-wall adsorption. RBM shifts have been discussed in different works devoted to filling of SWCNTs [29,30]. In these investigations the direction of the RBM shift depended on different

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factors such as geometry of the SWCNTs and nature of the filling material. Ultimately, the RBM shifts are a common consequence of the filling procedure. In our case, the values of RBM right-shift largely depend on SWCNT diameter. The magnitude of the rightshift decreases with increasing RBM frequency. The two most intense modes at 123 and 140 cm1 were shifted by 8 cm1 after filling. On the other hand, the RBM with the highest frequency 259 cm1 was shifted by only 1 cm1. Thus the highest shift of RBM was observed for SWCNTs with larger diameters. We believe this is related to the encapsulation of 1-adamantanemethanol molecules inside SWCNTs. As it was shown recently, SWCNTs with diameters less than 1 nm show only limited, or non-existent filling [31]. Since the RBM frequencies of small-diameter tubes are restored after cleaning with CHCl3, we conclude that their small shifts of 1 cm1 are due to outer-wall adsorption.

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Next we examine 1-bromoadamantane treated SWCNTs (Fig. 1 (b)). It was expected that the Raman spectra of SWCNTs, treated by 1-bromoadamantane at the same conditions as by 1adamantanemethanol, should be similar. Beside an intensity decrease for all SWCNT modes and analogous RBM shifts to those found for 1-adamantanemethanol, an additional right-shift of about 3 cm1 for LOS was observed for 1-bromoadamantane treated SWCNTs. Usually a LO shift is an indication of charge transfer between the nanotube and filling material [32]. The charge transfer leads to a Fermi level shift inside the valence or conduction band (depending on the nature of dopant) and to renormalization of the phonon energy by the electron-phonon interaction resulting in a shift of the Raman G-mode frequency [33]. Charge transfer could arise from filling of SWCNT channels by strong acceptors (or donors) [1,6] including halogens such as bromine or iodine [33,34].

Fig. 1. Normalized Raman spectra of aerosol synthesized SWCNTs, Left - RBM spectral region. Right - G mode region: (a) pristine aerosol-SWCNTs (black line); SWCNTs treated by 1adamantanemethanol (purple line); 1-adamantanemethanol-treated SWCNTs cleaned with CHCl3 (navy line); (b) pristine aerosol-SWCNTs (black line); SWCNTs treated by 1bromoadamantane (blue line); 1-bromoadamantane -treated SWCNTs cleaned with CHCl3 (green line). Excitation wavelength 633 nm. LOs, TOsephonons of semiconducting SWCNTs. (A colour version of this figure can be viewed online.)

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In our case the bromine atoms, injected inside the SWCNT as part of 1-bromoadamantane molecules, take the electrons from SWCNTs such that bromine plays the role of the acceptor. As was the case for 1-adamantanemethanol, cleaning by CHCl3 did not lead to restoration of modes for 1-bromoadamantane treated SWCNTs. To compare the encapsulation effect on different SWCNTs the arc-SWCNTs transferred on quartz substrates were investigated at the same excitation wavelength of 632.8 nm (Fig. 2 (a), (b)). Fig. 2 (a) is a Raman spectra of arc discharge grown SWCNTs before (black line) and after (purple line) 1-adamantanemethanol encapsulation (Further - 1-adamantanemethanol@arc-SWCNTs). Raman spectra of pristine arc-SWCNTs at excitation wavelength 632.8 nm are substantially different from spectra of pristine aerosol-SWCNTs. The G-mode region exhibits a set of equal-intensity bands within a wide range. Such a G-mode profile is associated with responses from semiconducting and metallic SWCNTs at the excitation wavelength of 632.8 nm. In this case the G-mode region has a few bands around 1591 cm1 (Gþ), which could be assigned to TO phonons and LO phonons of semiconducting SWCNTs (TOs and LOs)

and at 1564 and 1544 cm1 (G) which correspond to LO phonons of metal SWCNTs. The low frequency region has several RBMs around 170 cm1 (diameters of about 1.45 nm) with the two most intense bands at 164 and 180 cm1. Filling of arc-SWCNTs was accompanied by a change of Raman spectra similar to the case of aerosol-SWCNT. After the 1-adamantanemethanol filling the Raman spectra in the G-region exhibit minor redistribution and shifts of the LO and TO bands due to the annealing process (Fig. 2(a)). The RBMs of arc-SWCNTs were shifted under 1adamantanemethanol filling as well as for aerosol-SWCNTs. The shifts of RBMs around 170 cm1 for both aerosol-SWCNTs and arcSWCNTs did not exceed 8 cm1. The G-mode region for 1bromoadamantane filled exhibited a right-shift of LOm bands which is most sensitive to electron-phonon interaction. Thus, the right-shift of LO bands, associated with charge transfer, was observed for both semiconducting and metallic SWCNTs synthesized by different methods (arc discharge and aerosol-CVD methods) which were filled by 1-bromoadamantane at the same conditions. The shift at same manner of RBMs observed in all cases

Fig. 2. Normalized Raman spectra of arc-synthesized SWCNTs, Left - RBM spectral region. Right - G mode region: (a) pristine arc-SWCNTs (black line); SWCNTs treated by 1adamantanemethanol (purple line); (b) pristine arc-SWCNTs (black line); SWCNTs treated by 1-bromoadamantane (blue line). Excitation wavelength 633 nm. LOm, TOm e phonons for metal SWCNTs. (A colour version of this figure can be viewed online.)

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Fig. 3. Normalized Raman spectra of aerosol synthesized SWCNTs, Left - RBM spectral region. Right - G mode region: Pristine aerosol-SWCNTs (black line); SWCNTs treated by 1adamantanemethanol (blue line); SWCNTs treated by 1-bromoadamantane (green line). Excitation wavelength 514 nm. LOs, TOs e phonons for semiconducting SWCNTs. (A colour version of this figure can be viewed online.)

of filling was associated with encapsulation of 1bromoadamantane and 1-adamantanemethanol in SWCNTs or their bundles. We performed additional Raman measurements of our samples with higher excitation energy (wavelength of 514 nm). Beside the shifts of the RBM and G-modes we observed an additional peak in the low-frequency region at 204 cm1 for the aerosol-SWCNTs, treated with 1-bromoadamantane (Fig. 3). The position of this new peak is close to the one found in nanotube samples filled with bromine and may correspond to polybromide structures, formed inside the SWCNTs [35,36]. The appearance of this peak is associated with the electronic resonance of polybromide at excitation wavelength of 514 nm. On this basis we assume that bromine dissociates from the carbon cages and acts as an electron acceptor within the nanotubes.

Due to its sensitivity to charge transfer in SWCNTs, UVevis-IR spectroscopy was applied. Fig. 4 (a) and (b) show normalized UVevis-IR spectra of pristine, 1-adamantanemethanol filled and 1bromoadamantane filled SWCNTs synthesized by aerosol-CVD (Fig. 4(a)) and arc discharge (Fig. 4 (b)) methods. UVevis-IR spectra of pristine aerosol- and arc-SWCNTs consist of a set of absorption bands, associated with optical transitions between symmetric van Hove singularities of the SWCNTs. The first (E11s) and second (E22s) electron transitions for semiconducting SWCNTs and first (E11m) electron transition can be resolved for both aerosol-CVD and arc-CVD. The E11s and E22s bands associated with electron transitions are partially suppressed for 1bromoadamantane@aerosol-SWCNTs (Fig. 4 (a)- red line). The E11s for 1-bromoadamantane@arc-SWCNTs is also partially suppressed (Fig. 4 (b)- red line).

Fig. 4. UVeviseNIR optical absorption spectra for (a) aerosol synthesized SWCNTs (black line), 1-adamantanemethanol treated aerosol-SWCNTs (blue line) and 1bromoadamantane treated aerosol-SWCNTs (red line); (b) arc-synthesized SWCNTs (black line), 1-adamantanemethanol treated arc-SWCNTs (blue line) and 1bromoadamantane treated arc-SWCNTs (red line); (c) DOS sketch of the nanotube with a shifted Fermi level (EF) and allowed and forbidden optical transitions (E11s, E22s,E11m). (A colour version of this figure can be viewed online.)

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Fig. 5. Normalized FTIR spectra of (a) aerosol synthesized SWCNTs (black line), 1-adamantanemethanol@ SWCNTs (blue line) and 1-adamantanemethanol (purple line); (b) aerosol synthesized SWCNTs (black line), 1-bromoadamantane@ SWCNTs (navy line) and 1-bromoadamantane (green line). (A colour version of this figure can be viewed online.)

We attribute this change in the optical absorption to the Burstein-Moss effect [37,38], which is observed in degenerate semiconductors with high degree of doping due to the depletion of the valence band maximum (or filling of the conduction band minimum) as shown in Fig. 4 (c). In the present case, this effect may be caused by electron transfer from the SWCNT to bromine atoms or molecules, which act as electron acceptors [39,40]. In the case of 1-adamantanemethanol @ SWCNTs the suppression of electron transitions and thus charge transfer does not occur. However, there is a red-shift of the absorption bands for 1-adamantanemethanol filled both aerosol- and arc-SWCNTs. This shift has been associated with dielectric screening phenomena, usually observed in SWCNTs under the change of dielectric background of the surrounding media [41,42]. Moreover, dielectric-screening was observed for SWCNTs filled with water molecules [43].

FT-IR spectroscopy was applied to detect molecules of 1bromoadamantane and 1-adamantanemethanol within SWCNT samples after the filling procedure. Fig. 5 shows normalized FT-IR spectra of pristine arc- and aerosol- SWCNTs, molecules of 1bromoadamantane and 1-adamantanemethanol and treated SWCNT samples. The rise of CeH vibrations was observed under treating procedure for all cases. The CeH vibrations were most intensive bands which were attributed to the presence of diamondoid molecules in the treated samples. Summarizing the spectroscopic measurements, the most significant changes in the optical spectra are observed in 1bromoadamantane filled samples and originate from charge transfer effects. As it was previously noted, halogens act as acceptors when put inside carbon nanotubes, however, it remains unclear if the bromine atoms receive electrons from the nanotube as a

Fig. 6. The unit cells of (a,b) 1-Adamantanemethanol and (c,d) 1-Bromoadamantane encapsulated inside a (20,0) SWCNT. The molecule is oriented (a,c) parallel and (b,d) perpendicular to the nanotube axis. Coloring within the molecule indicates carbon (gray), hydrogen (white), oxygen (red) and bromine (orange) atoms. (A colour version of this figure can be viewed online.)

Fig. 7. The projected densities of states, calculated for the 1-Adamantanemethanol ((a), (b), (e), (f)) and 1-Bromoadamantane ((c), (d), (g), (h)) molecules, encapsulated in the (20,0) and (14,0) SWCNT's. Figures (a), (c), (e), (g) correspond to the molecule orientation parallel to the tube axis, and figures (b), (d), (f) and (h) correspond to the molecule orientation perpendicular to the tube axis. The origin of the energy is taken to be equal to the center of the SWCNT band gap. The black and red lines depict the DOS of the nanotube and the molecules, respectively. The calculated Fermi energy, marked as a grey dashed line is located in the center of the gap, indicating the absence of charge transfer between the molecules and the nanotube. (A colour version of this figure can be viewed online.)

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Fig. 8. Left panel: The unit cells of a bromine atom (a), Br3 molecule (b) and polybromide chain (c), encapsulated within the (20,0) carbon nanotube, used to model the Br@SWCNT structures. Right panel: the corresponding projected densities of states. The Fermi energy, depicted as a gray dashed line, is shifted inside the nanotube valence band. (A colour version of this figure can be viewed online.)

part of the 1-bromoadamantane molecules or are dissociated from the carbon cage and act as individual atoms or polybromide structures. The latter case would be a sign of 1-bromoadamantane molecule dissociation, being an important step towards polymerization of diamondoid molecules. For a deeper understanding of charge transfer on a microscopic level, we perform density functional modeling of the electronic structure for nanotubes encapsulating 1-adamantanemethanol, 1-bromoadamantane and polybromide structures. We use a semiconducting zig-zag (20,0) nanotube with ~1.5 nm diameter to model the molecule@ SWCNT system. The tube diameter corresponds to the mean diameter of the experimental arc-SWCNT samples. The 1-adamantanemethanol and 1-bromoadamantane molecules are placed in the center of the nanotube unit cell which is made long enough to avoid interaction between molecules in neighboring periodic images. For each of the hybrid structures we consider two different molecule orientations, with the functional group aligned either parallel or perpendicular to the tube axis. The resulting unit cells are shown in Fig. 6. To model the situation when the molecules are closely packed inside a SWCNT and interact more strongly with the nanotube walls, we also consider a (14,0) SWCNT with a diameter of ~1.1 nm. For the (20,0) nanotube (Fig. 7 (a)-(d)), due to its large diameter there is almost no difference between the peak positions corresponding to the molecules oriented parallel (Fig. 7 (a) and (c)) or perpendicular (Fig. 7 (b) and (d)) to the SWCNT axis. The highest occupied molecular orbital levels of the 1-adamantanemethanol and 1-bromoadamantane molecules lie about 2 eV and 2.2 eV below the Fermi energy respectively, and the corresponding lowest unoccupied molecular orbital energies lie about 3.5 eV and 2.8 eV above the Fermi level of the hybrid system. In the case of the (14,0) nanotube (Fig. 7 (e)-(h)) the peaks from molecules oriented perpendicular to the tube axis are smeared (Fig. 7 (f) and (h)). This is due to some hybridization between the molecule and SWCNT electronic states. At the same time, in both cases, the calculated Fermi energy lies in the center of the nanotube band gap and there is no charge transfer between the molecules and SWCNTs.

Alternatively, the charge transfer may originate from bromine atoms dissociated from the 1-bromoadamantane molecules. Previous theoretical investigations revealed that bromine can form polybromide anions inside SWCNTs [44]. We calculate the PDOS and Fermi energies for a single bromine atom, a Br3 molecule and a periodic bromine chain, encapsulated in a (20,0) nanotube. Following the result obtained in Refs. [44], we use BreBr bond lengths for the Br3 molecule of 2.53 Å and a distance between the Br atoms and the SWCNT wall of 3.4 Å (the Br chain is thus located not on the tube axis, but is placed closer to the wall). For the periodic polybromide chain, we use a 2.55 Å bond length and translate the nanotube unit cell 6 times to achieve commensurability between the lattice vectors. The k-point grids have been scaled for these three systems to ensure a similar k-point density in all cases. The

Fig. 9. EDS spectra form the bunch of 1-Bromoadamantane@SWCNT from the framed area in the STEM image. (A colour version of this figure can be viewed online.)

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unit cell geometries, used in our calculations, together with the resulting PDOS plots and calculated Fermi levels are shown in Fig. 8. The Fermi energies lie 0.10, 0.07 and 0.11 eV below the top of the nanotube valence band for the Br@(20,0), Br3@(20,0) and Brchain@(20,0), respectively. The corresponding mean electron transfer is 0.51, 0.25 and 0.15 electrons per bromine atom. The Fermi energy drop results in a shift of the Raman G-band and empties the topmost van Hove singularity of the nanotube. This is the cause of the Burstein-Moss effect observed in the optical absorption spectra. Thus these results support the hypothesis that the bromine atoms are dissociated from the 1-Bromoadamantane molecules inside the carbon nanotubes, resulting in charge transfer with the tube, and the remaining C10H15 fragments may form polymerized adamantane structures. To further investigate the results of the filling of SWCNTs with 1bromoadamantane we performed a TEM study. Unlike bromine, the diamondoid molecules inside the nanotubes cannot be recognize by chemical composition control in TEM measurements. This leads to the difficulty of the control of filling results if the filling material does not contain heavy elements. The typical EDS spectrum is shown in Fig. 9. STEM image of the nanotubes bundle and selected area (white box), used for EDS, are present on the insert of Fig. 9. Bromine Kand L-lines were detected for 1bromoadamantane@SWCNT. Low signal from Fe, which can be attributed to the presence of catalytic particles in the samples, was also observed. The samples treated with 1-adamantanemethanol exhibit similar features excluding the bromine lines.

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A series of HRTEM images with 0.4 s exposure were recorded (Fig. 10 (a)e(e)). Carbon structures and single Br atoms were observed in nanotubes filled with 1-bromoadamantane during TEM studies. Bromine atoms, which are detected as bright spots (marked by arrows), migrate inside the tube during recording. Some of them occur as free-standing atoms or are embedded in the carbon structures. For proper contrast interpretation of the of HRTEM images, a SWCNT with an inner carbon tube and a free bromine atom were simulated taking into account the TEM optical parameters (Fig. 10(g)). The relative intensity profiles through the marked line (red line) from simulated and experimental images are shown in Fig. 10 (h)e(k). Free bromine atom inside SWCNT can be easily identified by high contrast difference due to atomic number. Good agreement is observed for experimental and calculated profiles. 4. Conclusion In summary, we carried out a filling procedure of SWCNT films with mean tube diameters of 1.5 nm and 1.9e2.0 nm with 1adamamntanemethanol and 1-bromoadamantane molecules. We investigated the obtained samples with optical absorption and Raman spectroscopy. For both molecules, filling of the nanotubes with diameters above 1 nm was confirmed by a shift of the RBM modes. Additionally, 1-bromoadamantane filling led to a shift of the Raman G-mode, associated with charge transfer from the nanotubes to bromine atoms or molecules. This charge transfer was also observed in the optical absorption spectra as a suppression of the

Fig. 10. Series of HRTEM images of 1-Bromoadamantane@SWCNT for visualization of free bromines atoms and carbon structures moving inside the carbon nanotube (aee). Experimental (the framed area in (b)), simulated HRTEM images and atomic model of SWCNT with Br atom (f), (g), (l). The relative intensity profiles through the marked line for experimental and simulated images (h) and (k). (A colour version of this figure can be viewed online.)

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electronic transitions due to the Fermi energy shift and the depletion of the uppermost valence band. On the basis of DFT simulations we have shown that such charge transfer takes place when bromine atoms are detached from the molecules and act as electron acceptors, either as individual atoms or polybromide structures. Moreover, isolated bromine atoms inside the nanotubes, filled with 1-bromoadamantane were observed during the TEM study. This shows that 1-bromoadamantane molecules effectively dissociate during the SWCNT filling procedure at relatively low temperatures (~160
C) and that bromine functionalized adamantanes may be considered as perspective building blocks for 1D sp3 bonded systems. Moreover, the charge transfer, observed in Raman and optical absorption may be used as a marker of bromine dissociation inside the carbon nanotubes. Acknowledgments A. A. Tonkikh, D. V. Rybkovskiy and S. B. Rochal carried out the nanotube filling experiments, optical experiments and computational modeling. A. S. Orekhov and A.L. Chuvilin performed HRTEM measurements. A. A. Khomich performed FT-IR measurements. A. A. Tonkikh, D. V. Rybkovskiy, A. S. Orekhov, A. I. Chernov, A. A. Khomich, C. P. Ewels, E. I. Kauppinen, S. B. Rochal, A.L. Chuvilin and E. D. Obraztsova actively participated in the data analysis, discussions and wrote the paper. A. A. Tonkikh, D. V. Rybkovskiy and S. B. Rochal acknowledge financial support of Russian Science Foundation (grant number 1512-10004). C. Ewels acknowledges the European Commission under FP7 Project IRSES “NanoCF” Grant number 612577. E. Kauppinen acknowledges the Aalto University AEF program project MOPPI.

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Appendix A. Supplementary data [22]

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.carbon.2016.07.053. [23]

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