Raman Spectroscopy Study of the Doping Effect of the Encapsulated Iron , Cobalt , and Nickel Bromides on Single-Walled Carbon Nanotubes

In this contribution the modification of the electronic properties of single-walled carbon nanotubes (SWCNTs) filled with nickel bromide, cobalt bromide, and iron bromide was studied by Raman spectroscopy. The doping-induced alterations of the radial breathing mode (RBM) and G-mode in the Raman spectra of the filled SWCNTs were analyzed in detail. The observed shifts of the components of the Ramanmodes and changes of their profiles allowed concluding that the embedded compounds have an acceptor doping effect on the SWCNTs, and the doping level increases in the line with nickel bromide-cobalt bromide-iron bromide.

Raman spectroscopy is a very useful method for studying vibrational and electronic properties of carbon nanotubes [36,37].The investigation of the filled SWCNTs by this technique showed significant modifications of the Raman spectra of nanotubes upon their filling.The shifts of peaks of the radial breathing mode (RBM) and G-mode as well as changes in their profiles were assigned to the alteration of the electronic structure of SWCNTs as a result of doping and hybridization effects.For SWCNTs filled with electron acceptors (TmCl 3 [22], TbCl 3 , ZnCl 2 [21], NiCl 2 , NiBr 2 [23], CoBr 2 [24], CdCl 2 [20,21], CdBr 2 , CdI 2 [20], FeCl 2 , FeBr 2 , and FeI 2 [25]) changes in the relative intensities of the RBM peaks were observed, which were attributed to the doping-induced shift of the resonance excitation energy of nanotubes.Also, the significant upshifts of the G-band peaks were reported, which were accompanied by the change of the band profile to semiconducting type.For SWCNTs filled with electron donors (Ag [26][27][28], Cu [28]) the shifts of the RBM and G-band peaks were observed, and they were typically accompanied by the switching of the G-band shape to metallic type.
In most reports only a qualitative description of the Raman spectra of the filled SWCNTs was performed.However, a detailed investigation of the spectra with a quantitative analysis of the relative intensities of the individual peaks in the RBM and G-bands opens a way to studying precisely the correlation between the type of the encapsulated compound and its influence on the electronic properties of SWCNTs, as well as the dependence of the Raman modes on the doping level.
In the present work a detailed Raman spectroscopy study of the doping effect of encapsulated nickel bromide (NiBr 2 ) [23], cobalt bromide (CoBr 2 ) [24], and iron bromide (FeBr 2 ) [25] on SWCNTs was performed.The doping-induced alterations of the radial breathing mode (RBM) and G-mode in the Raman spectra of the filled SWCNTs were analyzed.The data showed that the incorporated metal bromides have an acceptor doping effect on the nanotubes; however, the doping level depends on the compound.The inserted nickel bromide causes the smallest doping of SWCNTs, whereas iron bromide causes the largest.

Materials and Methods
SWCNTs with a mean diameter of 1.4 nm synthesized by the catalytic arc-discharge method were used as a starting material.The synthesis was conducted using 0.8 cm-diameter graphite rods with Y/Ni catalyst at 73.3 kPa helium pressure and a current of 100-110 A [38].The synthesized SWCNTs were purified by repeated oxygenation at 350-450 ∘ b in air and rinsing with hydrochloric acid (HCl).The purified samples had a SWCNT content of 78 wt.% and a catalyst content of 0.12 wt.%.
Before the filling procedure, the SWCNTs were annealed at 500 ∘ b in dry air for 30 minutes, in order to open their ends.The obtained nanotube sample ( = 0.025 g) was grinded with anhydrous NiBr 2 , CoBr 2 , or FeBr 2 (Aldrich, 99.999 wt.%) in a molar ratio of 1 : 1 in a glove box, in order to prevent hydration of the salts.The mixture was evacuated in a quartz ampoule to a pressure below 10 −5 Torr for 2 hours and sealed.The ampoule was heated at a rate of 1 ∘ C/min to the temperature of 1063 ∘ C (for NiBr 2 ), 778 ∘ C (for CoBr 2 ), or 784 ∘ C (for FeBr 2 ).This temperature was kept for 10 hours, after that the samples were slowly cooled down at a rate of 0.02-1 ∘ C/min [23][24][25].The obtained samples are labeled NiBr 2 @SWCNT, CoBr 2 @SWCNT, and FeBr 2 @SWCNT.
The Raman spectra were acquired using a Renishaw InVia Raman microscope equipped with 17 mW 633 nm ( ex = 1.96 eV) HeNe and 300 mW 785 nm ( ex = 1.58 eV) nearinfrared diode lasers, variable power neutral density filters (power range 0.00005-100%), and near-excitation tunable filters.The samples were prepared by dispersing the filled nanotubes in hexane with subsequent dropping this dispersion onto silicon wafers.The Raman spectra were fitted by the Lorentzian, Voigt, and Fano peaks with the PeakFit v4.12 program.

Results and Discussion
Using lasers with two energies 1.58 eV and 1.96 eV allowed exciting electronic transitions between van Hove singularities (vHs) of metallic and semiconducting SWCNTs of different diameters.As follows from the Kataura plot, the laser with an energy of 1.58 eV initiates the electronic transitions between the first vHs of metallic SWCNTs (   11 ) with a diameter of 1.4-1.5 nm.The laser with an energy of 1.96 eV excites the electronic transitions between the first vHs of metallic nanotubes with a diameter of 1.4 nm and between the third vHs of semiconducting SWCNTs with a diameter of ∼1.6 nm (  33 ) [39]. Figure 1 demonstrates the Raman spectra of the pristine SWCNTs, NiBr 2 @SWCNT, CoBr 2 @SWCNT, and FeBr 2 @SWCNT samples acquired at laser energies of 1.58 eV and 1.96 eV.Here two main regions of the Raman spectra are shown: a radial breathing mode (RBM), which corresponds to synchronous radial vibrations of carbon atoms (A 1g symmetry), and a G-band, which belongs to C-C bond vibrations (A, E 1 , and E 2 symmetries [36]) [40].The RBM band of the Raman spectrum of the pristine SWCNTs acquired at 1.58 eV contains two peaks at 157 and 171 cm −1 (Figure 1(a)), which are assigned to 1.5-and 1.4-nm metallic nanotubes, accordingly [41].The G-band of the spectrum includes three peaks at 1556, 1571, and 1593 cm −1 .The component at 1556 cm −1 belongs to the G − -mode, which originates from the longitudinal optical (LO) phonon in metallic SWCNTs.Two components at higher frequencies correspond to the G + -mode of semiconducting SWCNTs.The first component at 1571 cm −1 (G + TO ) belongs to the transversal optical (TO) phonon, and the second component at 1593 cm −1 (G + LO ) belongs to the longitudinal optical phonon [42,43].The G-band has a broad asymmetric shape, which is typical of metallic SWCNTs [40,44].The RBM-band of the Raman spectrum of the pristine nanotubes acquired at 1.96 eV contains two peaks at 151 and 169 cm −1 (Figure 1(b)), which can be assigned to ∼1.5-nm semiconducting SWCNTs and 1.4-nm metallic nanotubes, respectively [41].The G-band of the spectrum includes three components at 1546, 1565, and 1591 cm −1 (G − , G + TO , and G + LO , resp.) and has a metallic profile.
The Raman spectra of the NiBr 2 -, CoBr 2 -, and FeBr 2filled nanotubes acquired at laser energies of 1.58 eV and 1.96 eV show significant differences in comparison with the corresponding spectra of the pristine SWCNTs.The peaks in the RBM-band of the filled SWCNTs are upshifted by up to 14 cm −1 , and the RBM-band also alters the profile (Figure 1).This is possibly connected with changes in the resonance excitation conditions of the SWCNTs upon their filling.The peaks in the G-band of the filled SWCNTs are significantly shifted towards higher frequencies by up to 15 cm −1 .Additionally, the G-band modifies the profile from a broad asymmetric Breit-Wigner-Fano shape of metallic SWCNTs to a narrow Lorentzian shape of semiconducting  nanotubes [40,44].These changes are probably caused by the modification of the electronic structure of the filled SWCNTs due to a charge transfer from the nanotube walls to the encapsulated salts, that is, acceptor doping of nanotubes [23][24][25].
The RBM and G-bands of the Raman spectra of the NiBr 2 -, CoBr 2 -, and FeBr 2 -filled nanotubes demonstrate similar trends of shifts of the peaks and changes in the band profiles.However, the shift values and degrees of the profile change depend on the metal bromide.These changes are also dependent on the laser energy, at which the Raman spectra were acquired.
The detailed analysis of the Raman spectra of the pristine and metal bromide-filled SWCNTs acquired at a laser energy of 1.58 eV is shown in Figure 2. The RBM-bands of the Raman spectra are fitted with two components (C 1 and C 2), corresponding to SWCNTs of different diameters.The G-bands are fitted with one component of metallic SWCNTs (G − ) and two components of semiconducting nanotubes (G + TO and G + LO ), which were discussed above.Table 1 summarizes the peak positions and calculated relative integral intensities of the individual components.The RBM-band of the Raman spectrum of the NiBr 2 -filled SWCNTs does not show shifts of the components as compared to the spectrum of the pristine nanotubes (Figure 2(a)).However, their relative intensities change.The relative intensity of the component C 2 increases from 0.41 to 0.75, and the peak of 1.4 nm-metallic SWCNTs has the maximal intensity (Figure 2(b)).This fact implies the change in the resonance excitation energy of 1.5-nm metallic SWCNTs.As a result, the laser with an energy of 1.58 eV excites smaller diameter nanotubes.The RBM-bands of the spectra of the CoBr 2 -and FeBr 2 -filled SWCNTs show a small upshift of the components by 1-3 cm −1 (Figures 2(c) and 2(d)).Also, there is a larger increase in the relative intensity of the component C 2 to 0.77 for CoBr 2 @SWCNT and to 0.87 for FeBr 2 @SWCNT (Table 1).
The G-band of the Raman spectra of the metal bromidefilled SWCNTs demonstrates upshifts of the components that increase from NiBr 2 @SWCNT to CoBr 2 @SWCNT to FeBr 2 @SWCNT.The upshift of the G − -component increases from 1 to 2 to 4 cm −1 , the upshift of the G + TO -component increases from 5 to 6 to 7 cm −1 , and the upshift of the G + LO -component increases from 8 to 9 to 13 cm −1 in this line (Table 1).Moreover, the Raman spectra of the filled SWCNTs show the change in the G-band profile, which is a consequence of the change of the relative intensities of the individual components (Figure 2).In particular, there is a significant decrease in the relative intensity of the component of metallic SWCNTs (G − ).It drops from 0.44 for the pristine SWCNTs to 0.09 for NiBr 2 @SWCNT, 0.08 for CoBr 2 @SWCNT, and 0.05 for FeBr 2 @SWCNT (Table 1).
Figure 3 illustrates the changes observed in the RBMand G-bands of the Raman spectra of the metal bromidefilled SWCNTs acquired at a laser energy of 1.58 eV in comparison with the spectrum of the pristine nanotubes.The observed alteration of the relative intensities of the RBM components and their shifts are caused by changes in the resonance excitation conditions of SWCNTs upon their filling.The modifications of the RBM band become larger in line with NiBr 2 -CoBr 2 -FeBr 2 (Figure 3(a)).The G-band of the Raman spectrum of the iron bromide-filled SWCNTs also shows the largest shifts of the components and the most significant decrease in the relative intensity of the metallic component, whereas the spectrum of the nickel bromide-filled SWCNTs demonstrates the smallest changes (Figure 3(b)).This indicates the strongest acceptor doping of SWCNTs by the encapsulated FeBr 2 and the weakest doping of SWCNTs by the introduced NiBr 2 .Figure 4 demonstrates the fitting of the RBM-and Gbands of the Raman spectra of the pristine and metal bromide-filled SWCNTs acquired at a laser energy of 1.96 eV with individual components.The RBM-bands of the Raman spectra of the filled SWCNTs show a significant shift of the components by up to 14 cm −1 for C 1 and 8 cm −1 for C 2, which is much larger than in the spectra acquired at a laser energy of 1.58 eV, discussed above.There is also the alteration of the relative intensities of the RBM components.In all spectra the relative intensity of the component C 2 decreases as compared to the pristine nanotubes.The value changes from 0.66 for SWCNTs to 0.54 for NiBr 2 @SWCNT, 0.65 for CoBr 2 @SWCNT, and 0.60 for FeBr 2 @SWCNT (Table 1), which corresponds to a slight change in the resonance excitation conditions for the filled nanotubes.However, no dependence of the modifications of the RBM band on the compound is observed.
The G-band of the metal bromide-filled SWCNTs demonstrates upshifts of the components that are larger than those observed in the Raman spectra acquired at a laser energy of 1.58 eV.They increase from NiBr 2 @SWCNT to CoBr 2 @SWCNT to FeBr 2 @SWCNT (Figure 4).The upshift of the G − -component increases from 8 to 10 to 12 cm −1 , the upshift of the G + TO -component increases from 10 to 11 to 13 cm −1 , and the upshift of the G + LO -component increases from 11 to 13 to 15 cm −1 in this line (Table 1).As for the case of the 1.58 eV-energy laser, the Raman spectra of the filled SWCNTs show the alteration of the G-band profile due to the significant decrease of the relative intensity of the component of metallic SWCNTs (G − ).It decreases from 0.47 for TO and G + LO ), and the relative intensity of the G − -component (b) in the Raman spectra of the pristine SWCNTs, NiBr 2 @SWCNT, CoBr 2 @SWCNT, and FeBr 2 @SWCNT samples, acquired at a laser energy of 1.58 eV.
Table 1: The positions (RS) and relative intensities () of the components of the RBM-bands (C 1 and C 2) and G-bands (metallic G − and semiconducting G + TO and G + LO ) of the Raman spectra of the pristine SWCNTs, NiBr 2 @SWCNT, CoBr 2 @SWCNT, and FeBr 2 @SWCNT samples, acquired at laser energies of 1.58 eV and 1.96 eV.In the parentheses the shifts of the component positions in comparison to ones of the pristine SWCNTs are given.The experimental error in the component positions equals ±0.5 cm −1 .1). Figure 5 summarizes the modifications observed in the Raman spectra of the filled SWCNTs acquired at a laser energy of 1.96 eV.Although the changes in the RBM-bands do not show a clear dependence on the compound, the shifts and alterations of the intensities of the components of the G-bands increase in line with nickel bromide-cobalt bromide-iron bromide.This implies that the encapsulated FeBr 2 causes the largest acceptor doping level of SWCNTs, whereas NiBr 2 causes the smallest.This conclusion is in agreement with the results obtained at a laser energy of 1.58 eV.

RBM-band, cm
The doping of SWCNTs by the introduced metal bromides is a result of a large difference in the work functions of the compounds and nanotubes.It leads to the charge transfer from the SWCNTs to the inserted salts, accompanied by equalizing their Fermi levels.The observed differences in the doping levels for the NiBr 2 , CoBr 2 , and FeBr 2 -filled nanotubes are probably caused by their different chemical properties.These compounds differ only by the metal cation and, therefore, the metal type influences the doping effect on nanotubes.Indeed, the chemical properties (e.g., atomic and cation radii, electron affinity) of transition metals evenly change from nickel to cobalt to iron, and this fact may explain the observed tendency in the increase of the acceptor doping effect of SWCNTs from the incorporated NiBr 2 to CoBr 2 to FeBr 2 .

Conclusions
In conclusion, in this contribution the doping effect of the encapsulated nickel bromide, cobalt bromide, and iron bromide on the SWCNTs was compared on the basis of the Raman spectroscopy data.The conducted detailed analysis of the Raman spectra of the pristine and filled SWCNTs allowed precise investigation of the doping-induced modifications of the Raman modes.The observed shifts of the components of the RBM and G-bands of the Raman spectra and alteration of the band profiles allowed concluding that the incorporated metal bromides have acceptor doping effect on the SWCNTs, and the doping efficiency increases in the line with nickel bromide-cobalt bromide-iron bromide.TO , and G + LO ) and the relative intensity of the G − -component (b) in the Raman spectra of the pristine SWCNTs, NiBr 2 @SWCNT, CoBr 2 @SWCNT, and FeBr 2 @SWCNT samples, acquired at a laser energy of 1.96 eV.

Figure 2 :
Figure 2: The fitting of the RBM-and G-bands of the Raman spectra of the pristine SWCNTs (a), NiBr 2 @SWCNT (b), CoBr 2 @SWCNT (c), and FeBr 2 @SWCNT (d) samples acquired at a laser energy of 1.58 eV.The RBM-bands include two components, which belong to the nanotubes of different diameters.The G-bands include one component of metallic SWCNTs (G − at the lowest frequencies) and two components of semiconducting SWCNTs (G +TO at lower and G + LO at higher frequencies).

Figure 3 :
Figure 3: The relative intensity of the RBM components (C 1 and C 2) (a), the shift of the G-band components (G − , G +TO and G + LO ), and the relative intensity of the G − -component (b) in the Raman spectra of the pristine SWCNTs, NiBr 2 @SWCNT, CoBr 2 @SWCNT, and FeBr 2 @SWCNT samples, acquired at a laser energy of 1.58 eV.

Figure 4 :
Figure 4: The fitting of the RBM-and G-bands of the Raman spectra of the pristine SWCNTs (a), NiBr 2 @SWCNT (b), CoBr 2 @SWCNT (c), and FeBr 2 @SWCNT (d) samples acquired at a laser energy of 1.96 eV.The RBM-bands include two components, which belong to the nanotubes of different diameters.The G-bands include one component of metallic SWCNTs (G − at the lowest frequencies) and two components of semiconducting SWCNTs (G +TO at lower and G + LO at higher frequencies).

Figure 5 :
Figure 5: The relative intensity of the RBM components (C 1 and C 2) (a), the shift of the G-band components (G − , G +TO , and G + LO ) and the relative intensity of the G − -component (b) in the Raman spectra of the pristine SWCNTs, NiBr 2 @SWCNT, CoBr 2 @SWCNT, and FeBr 2 @SWCNT samples, acquired at a laser energy of 1.96 eV.