Metal and Metal Halogenide-Filled Single-Walled Carbon Nanotubes: Kinetics, Electronic Properties, Engineering the Fermi Level

Here, we present a review of the major achievements in kinetics, electronic properties, and engineering in the Fermi level of single-walled carbon nanotubes (SWCNTs). Firstly, the kinetics of metal-filled SWCNTs were revealed with precision over several minutes. Secondly, the growth rates of nanotubes were calculated. Thirdly, the activation energies of nanotubes were measured. Fourthly, the methods of the quantitative analysis of the doping level were developed. Indeed, only qualitative analysis has been previously performed. The quantitative analysis allowed us to obtain quantitative data on charge transfer. Fifthly, the correlation between the physical properties, chemical properties, electronic properties of SWCNTs was elucidated.

Significant progress has been made in the understanding of kinetics and the electronic properties of filled SWCNTs and the precise control of the Fermi level, which put SWCNTs a step forward toward their use in applications. The kinetics and electronic properties were analyzed using Raman spectroscopy, near edge X-ray absorption fine structure spectroscopy (NEXAFS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and optical absorption spectroscopy (OAS) . It is high time to review tremendous progress that has been made over the last five years [10]. There are five major achievements: Firstly, the kinetics of metal-filled SWCNTs were revealed with the precision of several minutes. Secondly, the growth rates of nanotubes were calculated. Thirdly, the activation energies of nanotubes were measured. Fourthly, the methods of the quantitative analysis of the doping level were developed. Indeed, only qualitative analysis has previously been performed. The quantitative analysis allowed for the obtaining of quantitative data on charge transfer. Fifthly, the correlation between the physical properties, chemical properties, and electronic properties of SWCNTs were elucidated.
The goal of this review is to be the central reference for researchers aiming at a precise control of kinetics and electronic properties toward applications and to stimulate emerging applications utilizing the physical and chemical properties.
Kinetics and the electronic properties of filled SWCNTs and the engineering of their Fermi level are tailored for their use as applications. This review begins from the overview of metal and metal halogenide filled inside SWCNTs. Then, the kinetics of metal-filled

Filling of SWCNTs with Metals
The encapsulation of metals inside SWCNTs was performed in a saturated solution

Filling of SWCNTs with Metal Halogenides
The embedding of metal halogenides inside SWCNTs was performed using the melt method. Sealed quartz ampoule was heated above the melting temperature (Table 1), kept at this temperature for some time, and then cooled down. The filling of the compound inside SWCNTs occurred via capillary forces. This filling method allowed nanocomposites with a large filling ratio (up to 90%) of SWCNTs to be obtained. The control of the cooling procedure of the ampoule allowed for the crystallization of the salt inside SWCNTs and for one-dimensional nanocrystals to be obtained. The optical absorption spectroscopy (OAS) is an informative method used for the investigation of the electronic properties of filled SWCNTs. It provides data about charge transfer in filled SWCNTs. Many samples of filled SWCNTs can be investigated, and the method is simple and quick. A comparison of the spectra of pristine and filled SWCNTs reveals the modifications of the electronic structure of carbon nanotubes.
ing exposure time [129]. There are 11 , 22 , 33 , and 11 absorption bands. The 11 absorption band vanishes after 5 h of exposure to CuCl gas; moreover, with increasing exposure time, other absorption bands are also slightly reduced. This was attributed to canceling optical transitions [129].

Raman Spectroscopy
Raman spectroscopy is a very useful method of investigation for examining the electronic properties of filled SWCNTs. It is a simple, nondestructive, and informative technique used to investigate the vibronic properties of carbon nanotubes upon filling. The The suppression of the E S 11 absorption band was also revealed [123,124,129,[135][136][137][138]141,142,158]. Taking into account these data, the authors made a conclusion about doping. For metal chalcogenide-filled SWCNTs, there are no noticeable changes [176,179,[181][182][183][184][185][186][187][188].

Raman Spectroscopy
Raman spectroscopy is a very useful method of investigation for examining the electronic properties of filled SWCNTs. It is a simple, nondestructive, and informative technique used to investigate the vibronic properties of carbon nanotubes upon filling. The nanotubes have a radial breathing mode (RBM) and D, G, 2D bands of Raman spectra, which are characteristic for a nanotube with a certain diameter, metallicity type, and chiral angle.
The Raman spectrum of pristine SWCNTs shows two dominant peaks in RBM (C1, and C2) [110]. They are located at 156 and 172 cm −1 (Figure 3a) and belong to maindiameter semiconducting and metallic SWCNTs, accordingly [189]. The G-line reveals three components at 1540, 1567, and 1591 cm −1 (Figure 3a). The positions of the individual peaks are very close to the predicted peak positions [190][191][192]. The G-band further backs up this interpretation, as this shape is reminiscent of metallic SWCNTs [190,193,194].
The Raman spectra of Ag-filled SWCNTs show differences ( Figure 3b). In the RBMband, there are just minuscule downshifts of the peaks by 3 cm −1 . In the G-band, there are shifts from 2 to 6 cm −1 . The relative strength of the metallic component increases from 0.70 to 0.79. There is the charge transfer between the SWCNTs and silver with a rigid band shift as the overall metallicity increases while the resonance conditions are preserved.
Besides silver [109,112,179], there are also similarities to copper [109,115]. This is in line with the n-doping of SWCNTs, which is expected from the lower work functions of metals as compared to nanotubes.
The Raman spectra of pristine SWCNTs, electron acceptor PbCl 2 -filled SWCNTs [145], and electron donor RbI-filled SWCNTs [168] are shown in Figure 4. In the RBM-band, there are shifts in the peaks and an alteration in their relative intensities. In the D and 2D-bands, there are shifts in the peaks and changes in their intensities. The G-band shows changes in the peak positions and modifications inthe peak profiles [145,168].

angle.
The Raman spectrum of pristine SWCNTs shows two dominant peaks in RBM (C1, and C2) [110]. They are located at 156 and 172 cm −1 (Figure 3a) and belong to main-diameter semiconducting and metallic SWCNTs, accordingly [189]. The G-line reveals three components at 1540, 1567, and 1591 cm −1 (Figure 3a). The positions of the individual peaks are very close to the predicted peak positions [190,191,192]. The G-band further backs up this interpretation, as this shape is reminiscent of metallic SWCNTs [190,193,194]. The Raman spectra of Ag-filled SWCNTs show differences ( Figure 3b). In the RBMband, there are just minuscule downshifts of the peaks by 3 cm −1 . In the G-band, there are shifts from 2 to 6 cm −1 . The relative strength of the metallic component increases from 0.70 to 0.79. There is the charge transfer between the SWCNTs and silver with a rigid band shift as the overall metallicity increases while the resonance conditions are preserved.
Besides silver [109,112,179], there are also similarities to copper [109,115]. This is in line with the n-doping of SWCNTs, which is expected from the lower work functions of metals as compared to nanotubes.
In the RBM-band of the RbI-filled SWCNTs (Figure 5c), peak positions are shifted by 5-10 cm −1 . However, the intensities are slightly altered due to resonance conditions. In the G-band, peaks are upshifted by 9, 5, and 1 cm −1 as compared to the pristine nanotubes. This was consistent with the n-doping of SWCNTs observed using the encapsulated RbI.
Modifications were also found to depend on the p-doping level of nanotubes with CuCl [129]. The RBM-peaks were upshifted at low doping levels, and they were completely suppressed at high doping levels. In the G-band, there is a gradual upshift when the doping level increases.
The Raman spectra of pristine SWCNTs, electron acceptor PbCl2-filled SWCNTs [145], and electron donor RbI-filled SWCNTs [168] are shown in Figure 4. In the RBM-band, there are shifts in the peaks and an alteration in their relative intensities. In the D and 2Dbands, there are shifts in the peaks and changes in their intensities. The G-band shows changes in the peak positions and modifications inthe peak profiles [145,168].  [168,190,193].
In the RBM-band of the PbCl2-filled SWCNTs (Figure 5b), there are peaks at 165 and 173 cm −1 , and relative intensities are changed from 1:1.83 to 1.63:1. In the G-band, there

Near Edge X-ray Absorption Fine Structure Spectroscopy
Near edge X-ray absorption fine structure spectroscopy (NEXAFS) is a synchrotronbased technique for the investigation of the electronic properties of filled SWCNTs. It provides information about hybridization in filled SWCNTs and the formation of new chemical bonds between the introduced substances and SWCNTs. It also reveals modifications in the band structure of SWCNTs upon filling; moreover, it can differ between chemical bonds for different substances and elements. pared to the pristine nanotubes. The relative intensity of the metallic component decreases from 0.40 to 0.06 because of the transition into a semiconducting state [159]. This is similar for other metal halogenides [122][123][124]129,[135][136][137][138][140][141][142][143][144][149][150][151]158,161,165,179] and metal chalcogenides [176,180].
In the RBM-band of the RbI-filled SWCNTs (Figure 5c), peak positions are shifted by 5-10 cm −1 . However, the intensities are slightly altered due to resonance conditions. In the G-band, peaks are upshifted by 9, 5, and 1 cm −1 as compared to the pristine nanotubes. This was consistent with the n-doping of SWCNTs observed using the encapsulated RbI.  NEXAFS allows the formation of chemical bonds between nanotubes and encapsulated substances to be investigated. In the literature, the C 1s NEXAFS spectra of SWCNTs filled with iron halogenides [138], nickel halogenides [135], zinc halogenides [137], cadmium halogenides [141], silver halogenides [123], copper halogenides [124], ErCl 3 [121], and HgCl 2 [140] were reported. Figure 6 compares the C 1s NEXAFS spectra of SWCNTs and HgCl 2 -filled SWC-NTs [140]. There is also the π*-resonance at~285 eV and the π*-resonance at~292 eV. There is, however, an additional pre-edge peak in filled SWCNTs (see label A in Figure 6) before the π*-resonance at~284.0 eV. This is due to the hybridization of the π-orbitals of SWCNTs with embedded mercury chloride.
in the band structure of SWCNTs upon filling; moreover, it can differ between chemical bonds for different substances and elements.
NEXAFS allows the formation of chemical bonds between nanotubes and encapsulated substances to be investigated. In the literature, the C 1s NEXAFS spectra of SWCNTs filled with iron halogenides [138], nickel halogenides [135], zinc halogenides [137], cadmium halogenides [141], silver halogenides [123], copper halogenides [124], ErCl3 [121], and HgCl2 [140] were reported. Figure 6 compares the C 1s NEXAFS spectra of SWCNTs and HgCl2-filled SWCNTs [140]. There is also the *-resonance at ~285 eV and the *-resonance at ~292 eV. There is, however, an additional pre-edge peak in filled SWCNTs (see label A in Figure 6) before the *-resonance at ~284.0 eV. This is due to the hybridization of the -orbitals of SWCNTs with embedded mercury chloride. There is an emergence of similar additional peaks for other metal halogenides [121,123,124,135,137,138,141], because chemical bonds are formed between SWCNTs and embedded substances.

Photoemission Spectroscopy
The X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) allow the electronic properties of filled SWCNTs to be investigated. In XPS, the modifications in the spectra of regions are observed. They are characteristic for different fillers. This includes shifts of peaks and alterations of spectral shapes. In UPS, the changes in the valence band spectra are revealed. These changes highlight the modifications of the band structure.
Using photoemission spectroscopy (XPS and UPS) SWCNTs filled with Ag [109,110,179], Cu [109,115], and Eu [120] were investigated, and more reports are expected. There is an emergence of similar additional peaks for other metal halogenides [121,123,124,135,137,138,141], because chemical bonds are formed between SWCNTs and embedded substances.

Photoemission Spectroscopy
The X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) allow the electronic properties of filled SWCNTs to be investigated. In XPS, the modifications in the spectra of regions are observed. They are characteristic for different fillers. This includes shifts of peaks and alterations of spectral shapes. In UPS, the changes in the valence band spectra are revealed. These changes highlight the modifications of the band structure.
In Figure 7, in the C 1s XPS spectrum, the first component is unchanged and is assigned to carbon in the unfilled nanotubes, whereas the other two components belong to the silverfilled SWCNTs, whose chemical composition is confirmed by the Ag 3d XPS spectrum (see inset in Figure 7b) [110]. The second component appears upshifted by +0.33 eV due to the raised Fermi level. The third component is not interpreted [110]. The same effects are observed for copper-filled SWCNTs [109,115].
The C 1s peak of the Eu-filled SWCNTs was upshifted by 0.1 eV due to the incorporated europium walls as well as the further modification of the electronic structure [120]. The UPS data of the Eu-filled SWCNTs demonstrated a uniform upshift of peaks. These peaks are the consequence of the equal Fermi level upshift. These changes in the spectra are a clear signature of metallic atomic wires [120].
In Figure 7, in the C 1s XPS spectrum, the first component is unchanged and is assigned to carbon in the unfilled nanotubes, whereas the other two components belong to the silver-filled SWCNTs, whose chemical composition is confirmed by the Ag 3d XPS spectrum (see inset in Figure 7b) [110]. The second component appears upshifted by +0.33 eV due to the raised Fermi level. The third component is not interpreted [110]. The same effects are observed for copper-filled SWCNTs [109,115]. The C 1s peak of the Eu-filled SWCNTs was upshifted by 0.1 eV due to the incorporated europium walls as well as the further modification of the electronic structure [120]. The UPS data of the Eu-filled SWCNTs demonstrated a uniform upshift of peaks. These peaks are the consequence of the equal Fermi level upshift. These changes in the spectra are a clear signature of metallic atomic wires [120].
The authors of Refs. [123,[135][136][137][138][141][142][143]158] used the same interpretation of the components of the spectra as in the above-described case. They revealed the shift of the second component to lower binding energies, i.e., p-doping. The authors of Refs. [124,150] fitted the C 1s spectra with components of metallic and semiconducting SWCNTs. They showed larger p-doping for the metallic carbon nanotubes. The determination of the Fermi level shift was conducted using the secondary electrons' (SE) cutoff for the copper halogenide-filled SWCNTs [124]. This was reported to be −0.2, −0.6, and −0.65 eV for CuI, CuBr, and CuCl, accordingly. In [137], the SE cutoff spectra of SWCNTs and ZnBr2-filled SWCNTs allowed the Fermi level shift of −0.3 eV to be evaluated.
Additional valence band (VB) spectra measurements on copper halogenide- [124] and zinc bromide-filled [137] nanotubes were in line with the direct measurements of the work function in the SE cutoff spectra. Regarding the  -peaks in the VB spectra of SWCNTs, the copper halogenide-filled nanotubes originate from the photoemission from the -band of SWCNTs, and the -peaks originate from the photoemission from the band of SWCNTs [124]. The -peaks of filled SWCNTs are shifted to higher kinetic energies by 0.2-0.7 eV. This testifies to p-doping. The comparable effect was derived for ZnBr2 [137] and ErCl3 [121]. The authors of Refs. [123,[135][136][137][138][141][142][143]158] used the same interpretation of the components of the spectra as in the above-described case. They revealed the shift of the second component to lower binding energies, i.e., p-doping. The authors of Refs. [124,150] fitted the C 1s spectra with components of metallic and semiconducting SWCNTs. They showed larger p-doping for the metallic carbon nanotubes. The determination of the Fermi level shift was conducted using the secondary electrons' (SE) cutoff for the copper halogenide-filled SWCNTs [124]. This was reported to be −0.2, −0.6, and −0.65 eV for CuI, CuBr, and CuCl, accordingly. In [137], the SE cutoff spectra of SWCNTs and ZnBr 2 -filled SWCNTs allowed the Fermi level shift of −0.3 eV to be evaluated.
Additional valence band (VB) spectra measurements on copper halogenide- [124] and zinc bromide-filled [137] nanotubes were in line with the direct measurements of the work function in the SE cutoff spectra. Regarding the π -peaks in the VB spectra of SWCNTs, the copper halogenide-filled nanotubes originate from the photoemission from the π-band of SWCNTs, and the σ-peaks originate from the photoemission from the σ-band of SWCNTs [124]. The -peaks of filled SWCNTs are shifted to higher kinetic energies by 0.2-0.7 eV. This testifies to p-doping. The comparable effect was derived for ZnBr 2 [137] and ErCl 3 [121].

Quantification of Charge Transfer in SWCNTs Filled with Inorganic Compounds
For the applications of filled nanotubes, one should quantify the charge transfer. In [132], the calculation was performed using the photoemission data. Alkali metals are very reactive, and the charge transfer is assumed to be practically one electron per K + ion; thus, Fermi level shifts can be linked to charge transfer densities [195]. Using the calibration plot (Figure 8), the number of transferred electrons for AgCl-filled SWCNTs was calculated to be 0.0024 e − per carbon. The charge transfer density along the SWCNT amounted to 0.0406 e − /Å [132].
The crucial properties of fillers that influence the doping type and doping level of SWCNTs are the work function of inorganic compounds and elementary substances. There are three doping effect-defining parameters: metallicity type, diameter of SWCNTs, and filling ratio. The introduction of metals leads to a high-yield filling of the nanotubes. It was shown that metals result in n-doping, and the Fermi level shift amounted to~+0.3 eV.

Quantification of Charge Transfer in SWCNTs Filled with Inorganic Compounds
For the applications of filled nanotubes, one should quantify the charge transfer. In [132], the calculation was performed using the photoemission data. Alkali metals are very reactive, and the charge transfer is assumed to be practically one electron per K + ion; thus, Fermi level shifts can be linked to charge transfer densities [195]. Using the calibration plot (Figure 8), the number of transferred electrons for AgCl-filled SWCNTs was calculated to be 0.0024 eper carbon. The charge transfer density along the SWCNT amounted to 0.0406 e − /Å [132]. Figure 8. The calibration plot for potassium. The data are from Ref. [195] are plotted.
The crucial properties of fillers that influence the doping type and doping level of SWCNTs are the work function of inorganic compounds and elementary substances. There are three doping effect-defining parameters: metallicity type, diameter of SWCNTs, and filling ratio. The introduction of metals leads to a high-yield filling of the nanotubes. It was shown that metals result in n-doping, and the Fermi level shift amounted to ~+0.3 eV.

Conclusions
This review has demonstrated how filling leads to a precise control over their electronic properties and how it has widened the scope of possible applications, raising great interest in science and technology as a result. The ability to tailor the electronic properties according to the specific requirements of individual applications renders these fascinating materials viable in the advancement of cutting-edge and fundamental applied research.
Transparency, conductivity, and mechanical robustness are key challenges in the application of nanoelectronics. While the diameter is only 1 to 2 nm, the contact length and channel length can be reduced further. Filled SWCNTs can have better contacts, requiring Figure 8. The calibration plot for potassium. The data are from Ref. [195] are plotted.

Conclusions
This review has demonstrated how filling leads to a precise control over their electronic properties and how it has widened the scope of possible applications, raising great interest in science and technology as a result. The ability to tailor the electronic properties according to the specific requirements of individual applications renders these fascinating materials viable in the advancement of cutting-edge and fundamental applied research.
Transparency, conductivity, and mechanical robustness are key challenges in the application of nanoelectronics. While the diameter is only 1 to 2 nm, the contact length and channel length can be reduced further. Filled SWCNTs can have better contacts, requiring shorter contact lengths and changes in mechanical stiffness. Thermoelectric applications based on filled SWCNTs have to aim at a higher conversion efficiency and a decreased thermal conductivity. Electrochemical energy storage in a filled SWCNT facilitates effective charge transfer throughout the bulk of composite material. In applications in catalysis, it is desirable to improve the lifetime of the catalytic particles. It is crucial to maximize the filling ratio and purity of the filled SWCNTs. For gas sensing, one aims to achieve sensitivity and selectivity. It is also required to determine the appropriate filling for spintronic applications. For magnetic recording, the filling might offer a way to shift the paramagnetic limit. Bioimaging applications of filled SWCNT will greatly benefit from higher spatial resolution as well as from the imaging of deeper layers of tissue. Reduced cytotoxicity and better biodegradability constitute the desired improved biocompability of filled SWCNTs. The targeting systems have to be specifically developed for the different use cases. The therapeutic options may be further expanded in combined therapies, where two or more therapeutics are co-delivered on a single platform. Filled SWCNTs are also a potential electrode material for solar cells.