Applied Physics A

, 123:239 | Cite as

Raman spectroscopy study of the doping effect of the encapsulated terbium halogenides on single-walled carbon nanotubes

  • M. V. Kharlamova
  • C. Kramberger
  • A. Mittelberger
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Abstract

In the present work, the doping effect of terbium chloride, terbium bromide, and terbium iodide on single-walled carbon nanotubes (SWCNTs) was compared by Raman spectroscopy. A precise investigation of the doping-induced alterations of the Raman modes of the filled SWCNTs was conducted. The shifts of the components of the Raman modes and modification of their profiles allowed concluding that the inserted terbium halogenides have acceptor doping effect on the SWCNTs, and the doping efficiency increases in the line with terbium iodide, terbium bromide, and terbium chloride.

1 Introduction

The unique physical, chemical, and mechanical properties of single-walled carbon nanotubes (SWCNTs) have caused major scientific and technological interest, which was steadily growing over the last 20 years [1, 2]. Special attention was dedicated to the filling of the internal channels of SWCNTs, which affected the properties of both, the filling materials and nanotubes [2, 3, 4, 5]. Different substances were encapsulated into SWCNTs, such as metal halides [6, 7, 8, 9, 10] and chalcogenides [11, 12], pure metals [13, 14, 15, 16, 17], and molecules [18, 19, 20] using appropriate filling methods [2, 3, 4, 5].

Lanthanide (or rare-earth) halogenides are a group of chemical compounds possessing interesting magnetic and optical properties, whose crystallographic structure, phase transformations, and properties were deeply explored several decades ago [21, 22, 23, 24, 25]. The incorporation of lanthanide halides inside the one-dimensional (1D) nanotube channels opens a way to study their structure and properties in a confined geometry and understanding low dimensionally confined phenomena. In addition, being encapsulated inside the SWCNTs, these compounds can modify the chemical and physical properties of the host. It causes large application potential of such nanostructures.

In 1998, SWCNTs were first filled with ruthenium chloride [26]. Later on, other lanthanide halogenides MX3 (M = La, Nd, Sm, Eu, Gd, Tb, Tm, Er, Yb, X = Cl, I) were encapsulated into the SWCNT channels both experimentally [27, 28, 29, 30, 31] and theoretically [32, 33]. Most of these works were dedicated to the investigation of atomic structure of one-dimensional nanocrystals templated inside the SWCNT hollow cavities and their comparison with the corresponding bulk structures. However, the physical properties of filled nanotubes that are important for their applications were not properly addressed. Thus, to date, the assessing of application potential of these nanostructures in new device architectures is limited by the scarcity of experimental data on their modified physical properties.

In the present contribution, the doping effect of the encapsulated terbium chloride (TbCl3), terbium bromide (TbBr3), and terbium iodide (TbI3) on the host SWCNTs is studied by Raman spectroscopy. The filling of SWCNTs is confirmed by high-resolution and scanning transmission electron microscopy (HRTEM and STEM). The doping-induced alterations of the radial breathing mode (RBM) and G-mode in the Raman spectra of the filled SWCNTs are analyzed. It is found that the encapsulated terbium halogenides have an acceptor doping effect on the nanotubes; however, the doping level depends on the compound. The incorporated terbium iodide causes the smallest doping of SWCNTs, whereas terbium chloride results in the largest doping.

2 Experimental details

Pristine 1.4 nm-diameter SWCNTs were synthesized by the arc-discharge method using 0.8 cm-diameter graphite rods with Y/Ni catalyst at 73.3 kPa helium pressure and a current of 100–110 A. To remove the metallic catalyst, the nanotubes were purified by repeated oxygenation at 350–450 °С in air and rinsing with hydrochloric acid. The purified SWCNTs were annealed at 500 °С in dry air for 30 min, to open the ends of nanotubes. The obtained sample of SWCNTs (m = 0.025 g) was grinded with anhydrous TbCl3, TbBr3 or TbI3 (Aldrich, 99.999 wt%) in a molar ratio of 1:2 in a glove box, to prevent the hydration of the salt. The mixture was evacuated in a quartz ampoule to a pressure below 10−5 Torr for 2 h and sealed. The ampoule was heated at a rate of 1 °C/min to the temperature of 688 °C (TbCl3), 927 °C (TbBr3) or 1057 °C (TbI3). This temperature was kept for 10 h, after that the sample was slowly cooled down at a rate of 0.02–1 °C/min to facilitate better crystallization of the salt inside the SWCNT channels.

The HRTEM investigations were carried out on a JEOL 2100 microscope at 200 kV. The STEM studies were performed on Nion UltraSTEM 100 microscope at 60 kV. The samples for the measurements were prepared by dispersing the nanotubes in isopropanol. After that several drops of this dispersion were put onto carbon-coated copper grids. Two setups were combined for the acquisition of multifrequency Raman spectroscopy data. The first one was a Renishaw InVia Raman system with a fixed excitation wavelength of 785 nm (1.58 eV). The other system was a Horiba Jobin Yvon LabRAM HR800 spectrometer equipped with a HeNe laser operated at a wavelength of 633 nm (energy of 1.96 eV) and a tunable ArKr mixed gas laser (Coherent Innova 70c) operated at wavelengths of 458, 514, 531, 568, and 647 nm (energies of 2.71, 2.41, 2.34, 2.18, and 1.92 eV, respectively). For the measurements, the samples were dispersed in hexane and bath sonicated, to ensure the homogeneity of material. After that the SWCNT dispersion was dropped onto silicon wafers. A 600 mm−1 grating (normal resolution mode) was employed. A constant power of 0.5 mW was used. The RBM- and G-bands of Raman spectra were fitted with Voigtian and Fano peaks and the area intensities were calculated using PeakFit v4.12. The error in peak positions was ±2 cm−1.

3 Results and discussion

Figure 1 shows the TEM data of the terbium halogenide-filled SWCNTs. It proves the successful filling of nanotubes with the salts. The HRTEM micrograph of the TbCl3-filled SWCNTs (Fig. 1a) shows contrast elements (corresponding to individual atoms of the encapsulated salt) inside the inner channels of nanotubes. They are placed not periodically, which corresponds to the presence of amorphous salt inside SWCNTs. The STEM data provide further insight into the structure of the encapsulated terbium halogenides. The STEM micrograph of TbBr3-filled SWCNTs (Fig. 1b) confirms continuous filling of the SWCNT channels. The high-magnification micrographs of TbBr3@SWCNT in Fig. 1c and d prove the formation of well-ordered 1D crystals of the salt within the interior space of SWCNTs. The atoms of the encapsulated salt are placed periodically along the SWCNT axis, and two projections of the 1D crystal are seen in Fig. 1c, d. The STEM micrographs of TbI3-filled SWCNTs (Fig. 1e, f) also confirm the filling of SWCNTs with 1D crystals.

Fig. 1

HRTEM micrograph of SWCNTs filled with TbCl3 (a) and STEM micrographs of SWCNTs filled with TbBr3 (bd) and TbI3 (e, f)

The interactions in the filled one-dimensional nanostructures are accessible in resonant Raman spectroscopy via changes in optical transitions as well as phonon frequencies. A Raman spectrum of SWCNTs contains four main regions: the RBM-band at frequencies below 300 cm−1, which corresponds to symmetric radial vibrations of carbon atoms, the D-line between 1300 and 1400 cm−1, which is a symmetry forbidden zone boundary in-plane mode of the graphene sheets that is enabled by structural defects and disorder, the G-band at frequencies ranging from 1500 to 1700 cm−1, which is assigned to the longitudinal (LO) and transversal (TO) in-plane phonon, and the 2D-band between 2500 and 2800 cm−1, which is the symmetry allowed overtone of the D-line [34].

Figure 2 demonstrates the RBM-, D-, G-, and 2D-bands of Raman spectra of the pristine and TbCl3-filled SWCNTs acquired at laser wavelengths of 458, 514, 531, 568, 633, 647, and 785 nm. Different lasers excite electronic transitions in nanotubes with different diameters [35]. The RBM-bands of the spectra of the pristine SWCNTs demonstrate peaks at frequencies in the range from 149 to 182 cm−1 (Fig. 2a; Table 1). Taking into consideration the fact that the RBM frequency (wRBM) is inversely proportional to the tube diameter (dt) by the formula \({{\omega _{{\text{RBM}}}} = \frac{{227}}{{{d_t}}}\sqrt {1 + Cd_t^2} }\), where C = 0.05786 nm−2 [36], these peaks correspond to nanotubes with diameters ranging from 1.31 to 1.64 nm.

Fig. 2

RBM-, D-, G-, and 2D-bands of Raman spectra of the pristine SWCNTs (a) and TbCl3@SWCNT sample (b) acquired at laser wavelengths of 458, 514, 531, 568, 633, 647, and 785 nm. The spectra were normalized to the area intensity of the G-band. The individual spectra are offset for clarity

Table 1

Positions of the components of the RBM-band (C1, C2, and C3) and G-band (GBWF, G and G+) of the Raman spectra of the pristine SWCNTs and TbCl3@SWCNT sample acquired at laser wavelengths of 458, 514, 531, 568, 633, 647, and 785 nm

λex (eV)

Sample

RBM-band (cm−1)

G-band (cm−1)

C1

C2

C3

GBWF

G

G+

458

SWCNT

162

177

 

1554

1570

1594

TbCl3@SWCNT

163 (+1)

179 (+2)

 

1562 (+8)

1581 (+11)

1602 (+8)

514

SWCNT

155

170

182

1554

1570

1593

TbCl3@SWCNT

158 (+3)

171 (+1)

180 (−2)

1560 (+6)

1577 (+7)

1598 (+5)

531

SWCNT

159

173

181

1556

1571

1593

TbCl3@SWCNT

161 (+2)

172 (−1)

182 (+1)

1560 (+4)

1578 (+7)

1600 (+7)

568

SWCNT

163

175

 

1556

1571

1592

TbCl3@SWCNT

160 (−3)

176 (+1)

 

1558 (+2)

1576 (+5)

1600 (+8)

633

SWCNT

150

166

 

1539

1569

1593

TbCl3@SWCNT

166 (+16)

176 (+10)

 

1561 (+22)

1577 (+8)

1603 (+10)

647

SWCNT

149

167

 

1538

1566

1588

TbCl3@SWCNT

167 (+18)

176 (+9)

 

1560 (+22)

1577 (+11)

1604 (+16)

785

SWCNT

159

171

 

1551

1569

1594

TbCl3@SWCNT

161 (+2)

173 (+2)

 

1560 (+9)

1578 (+9)

1605 (+11)

The shifts of the component positions in comparison to the ones of the pristine SWCNTs are given in the parentheses. The experimental error in the peak positions is ±2 cm−1

It should be noted that since all spectra were normalized to the area intensity of the G-band, the observed different intensities of the RBM-bands reflect the strong variations in the resonance conditions at the different laser wavelengths.

The G-band of Raman spectra acquired at 458, 514, 531, and 568 nm has a narrow Lorenzian shape of semiconducting SWCNTs (Fig. 2a) [34, 37, 38]. The G-band includes an intense G+ component located at 1592–1594 cm−1, which stems from longitudinal optical (LO) phonon, a less intense G component at 1570–1571 cm−1, which stems from transversal optical (TO) phonon, and a low intensity shoulder at 1554–1556 cm−1 due to the resonant E2g mode in semiconducting SWCNTs [38]. In contrast, the G-band of the Raman spectra acquired at 633, 647, and 785 nm shows a broad asymmetric shape due to the Breit–Wigner–Fano component (GBWF) of metallic SWCNTs [34, 37].

The Raman spectra of the TbCl3-filled SWCNTs show noticeable differences as compared to the spectra of the pristine nanotubes (Fig. 2b). The RBM-bands of the spectra acquired at laser wavelengths of 458, 514, 531, and 568 nm, where semiconducting nanotubes are detected, exhibit a slight modification of the profile and shifts of the peaks by 1–3 cm−1 (Table 1). This testifies that the resonance excitation conditions of the host nanotubes stay unchanged in the TbCl3-filled SWCNTs. The G-band of the spectra acquired at these laser wavelengths shows more sufficient changes. They include the shift of the peaks of the G-band by 2–11 cm−1 toward higher frequencies (Table 1). This points to the doping-induced alteration of the electronic properties of the filled SWCNTs.

The RBM-bands of the spectra of the TbCl3-filled SWCNTs acquired at laser wavelengths of 633, 647, and 785 nm, where metallic nanotubes are detected, show significant changes as compared to the pristine SWCNTs (Fig. 2b). The RBM-band of the filled SWCNTs demonstrates upshifts of the peaks by up to 18 cm−1 (Table 1) and changes in the profile, because of the altered relative intensities of the peak components. This is related to changes in the resonance excitation conditions of the host nanotubes. The G-band of the spectra of the filled SWCNTs acquired at these laser wavelengths demonstrates significant shifts of the peaks toward higher frequencies by up to 22 cm−1 (Table 1) and changes in the profile from a broad asymmetric Breit–Wigner–Fano shape of metallic nanotubes to a narrow Lorenzian shape of semiconducting SWCNTs.

The D- and 2D-bands of the Raman spectra of the filled SWCNTs (Fig. 2b) show the shift of the peak positions towards lower frequencies with increasing laser wavelengths, as in the case of the pristine SWCNTs (Fig. 2a). The intensity of the D- and 2D-bands is almost independent on the laser wavelength.

The whole data set of multifrequency Raman spectroscopy testifies to changes in the electronic structure of the filled SWCNTs as result of the charge transfer from the nanotubes to the encapsulated salt. The observed effects are in agreement with previously reported Raman spectra of nanotubes filled with electron acceptors [39, 40, 41, 42, 43].

Figure 3 compares the RBM- and G-bands of the Raman spectra of the pristine SWCNTs and terbium halogenide-filled SWCNTs acquired at a laser wavelength of 633 nm. 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 (GBWF) and two components of semiconducting nanotubes (G and G+). The RBM-band of the Raman spectrum of the pristine nanotubes contains two peaks at 150 and 166 cm−1, which can be assigned to ~1.5-nm semiconducting SWCNTs and 1.4-nm metallic nanotubes, respectively [36]. The G-band of the spectrum includes three components at 1539, 1569, and 1593 cm−1 (GBWF, G, and G+, respectively) and has a metallic profile.

Fig. 3

Fitting of the RBM- and G-bands of the Raman spectra of the pristine SWCNTs (a), TbCl3@SWCNT (b), TbBr3@SWCNT (c), and TbI3@SWCNT (d) samples acquired at a laser wavelength of 633 nm. The RBM-bands include two components, which belong to the nanotubes of different diameters. The G-bands include one component of metallic SWCNTs (GBWF at the lowest frequencies) and two components of semiconducting SWCNTs (G at lower and G+ at higher frequencies)

The Raman spectra of the TbCl3-, TbBr3-, and TbI3-filled SWCNTs show significant differences in comparison with the spectrum of the pristine SWCNTs. The RBM-bands of the Raman spectra of the TbCl3- and TbBr3-filled SWCNTs show a significant shift of the components by up to 16 cm−1 for C 1 and 9 cm−1 for C 2. 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 0.65 for SWCNTs to 0.45 for TbCl3@SWCNT and 0.47 for TbBr3@SWCNT (Table 2), which corresponds to a change in the resonance excitation conditions for the filled nanotubes. For TbI3@SWCNT, there are only insignificant shift of the RBM peak components by 1 cm−1 and no change in the RBM profile.

Table 2

Results of the fitting of the RBM- and G-bands of the Raman spectra of the pristine SWCNTs, TbCl3@SWCNT, TbBr3@SWCNT, and TbI3@SWCNT samples acquired at laser wavelengths of 633 nm. Given are the positions (RS) and relative intensities (I) of the components of the RBM-band (C1 and C2) and G-band (GBWF, G and G+)

Sample

RBM-band (cm−1)

G-band (cm−1)

C1

C2

GBWF

G

G+

RS

I

RS

I

RS

I

RS

I

RS

I

SWCNT

150

0.32

166

0.65

1539

0.70

1569

0.09

1593

0.21

TbCl3@SWCNT

166 (+16)

0.55

176 (+10)

0.45

1561 (+22)

0.01

1577 (+8)

0.20

1603 (+10)

0.79

TbBr3@SWCNT

164 (+14)

0.53

175 (+9)

0.47

1558 (+19)

0.11

1576 (+7)

0.12

1602 (+9)

0.77

TbI3@SWCNT

151 (+1)

0.27

166

0.73

1557 (+16)

0.11

1576 (+7)

0.12

1601 (+8)

0.77

The shifts of the component positions in comparison to the ones of the pristine SWCNTs are given in the parentheses. The experimental error in the peak positions is ±2 cm−1

The G-bands of the Raman spectra of terbium halogenide-filled SWCNTs show significant upshifts of the peaks by up to 22 cm−1. They decrease from TbCl3@SWCNT to TbBr3@SWCNT to TbI3@SWCNT. The upshift of the GBWF-component decreases from 22 to 19 to 16 cm−1, the upshift of the G-component decreases from 8 to 7 to 7 cm−1, and the upshift of the G+-component decreases from 10 to 9 to 8 cm−1 in this line (Table 2). In addition, the G-band modifies the profile from a broad asymmetric shape of metallic SWCNTs to a narrow shape of semiconducting nanotubes [34, 37]. This is due to the significant decrease of the relative intensity of the component of metallic SWCNTs (GBWF). It decreases from 0.70 for the SWCNTs to 0.01 for the TbCl3@SWCNT, 0.11 for the TbBr3@SWCNTs, and 0.11 for the TbI3@SWCNTs (Table 2). These changes are caused by modifications of the electronic structure of the filled SWCNTs due to a charge transfer from the nanotube walls to the inserted salts, that is, acceptor doping of nanotubes [39, 40, 41, 42, 43].

Figure 4 summarizes the changes observed in the RBM and G-bands of the Raman spectra of the terbium halogenide-filled SWCNTs 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 the line with TbI3–TbBr3–TbCl3 (Fig. 4a). The G-band of the Raman spectrum of the terbium chloride-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 terbium iodide-filled SWCNTs demonstrates the smallest changes (Fig. 4b). This indicates the strongest acceptor doping of SWCNTs by the encapsulated TbCl3 and the weakest doping of SWCNTs by the introduced TbI3.

Fig. 4

Relative intensity of the RBM components (C 1 and C 2) (a), the shift of the G-band components (GBWF, G and G+), and the relative intensity of the GBWF-component (b) in the Raman spectra of the pristine SWCNTs, TbCl3@SWCNT, TbBr3@SWCNT, and TbI3@SWCNT samples, acquired at a laser wavelength of 633 nm

The doping of SWCNTs by the introduced terbium halogenides is a result of the large difference in the work functions of the compounds and nanotubes. It leads to the charge transfer from the SWCNTs to the inserted salts, by aligning their Fermi levels. The observed differences in the doping levels for the TbCl3, TbBr3, and TbI3-filled nanotubes are due to their different chemical composition. The compounds differ only by the halogen anion and, therefore, the halogen type influences the doping effect on nanotubes. Indeed, the chemical properties (e.g., anion radii and electron affinity) of halogen evenly change from chlorine to bromine to iodine, and this falls in the line with the observed trend in the increase of the acceptor doping effect of SWCNTs from the incorporated TbI3 to TbBr3 to TbCl3.

4 Conclusions

To summarize, in this work, terbium chloride, terbium bromide, and terbium iodide-filled SWCNTs were synthesized. The successful filling was directly confirmed by the HRTEM and STEM data, revealing a distinctly one-dimensional morphology of terbium halogenides inside SWCNTs. The doping effect of encapsulated salts on SWCNTs was characterized by Raman spectroscopy. The shifts of the RBM and G-band components as well as the modification of their profiles in the Raman spectra demonstrate that terbium halogenide fillings cause acceptor doping of SWCNTs. The doping level of SWCNTs increases in the line from terbium iodide to terbium bromide to terbium chloride. These findings identify terbium halogenides as a class of filler materials to realize targeted stable doping of SWCNTs.

Notes

Acknowledgements

SWCNTs were synthesized by Dr. A.V. Krestinin (Institute of Problems of Chemical Physics RAS, Chernogolovka). Authors thank Dr. A.V. Egorov (Lomonosov Moscow State University) for the HRTEM measurements.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2017

Authors and Affiliations

  • M. V. Kharlamova
    • 1
  • C. Kramberger
    • 1
  • A. Mittelberger
    • 1
  1. 1.Faculty of PhysicsUniversity of ViennaViennaAustria

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