Tuning the Electronic Properties of Single-Walled Carbon Nanotubes by Filling with Electron Donor and Acceptor Compounds

: The endohedral chemical functionalization of single-walled carbon nanotubes (SWCNTs) allows for tuning their electronic properties toward applications. It was demonstrated that SWCNTs can be filled with elementary substances, chemical compounds and molecules. In this work, we performed the filling of SWCNTs with metal halogenide (cobalt iodide, CoI 2 ) and metal carbide (nickel carbide, Ni 3 C). The filling of SWCNTs with CoI 2 was conducted by the melt method. The filling of SWCNTs with Ni 3 C was performed by the thermal treatment of nickelocene-filled nanotubes. The filled SWCNTs were investigated by the high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The HRTEM data prove the encapsulation of compounds inside the SWCNTs. By combining the Raman spectroscopy and XPS data, it was shown that the encapsulated CoI 2 causes p -doping of nanotubes accompanied by the downshift of the Fermi level of nanotubes. The embedded Ni 3 C leads to n -doping of SWCNTs with upshifting of the Fermi level of nanotubes. The obtained results allow for applying filled SWCNTs in arange of fields such as nanoelectronics, energy storage, sensors, catalysis and biomedicine.


Introduction
Filled single-walled carbon nanotubes (SWCNTs) are attracting ever-increasing attention of the research community due to their extraordinary physical and chemical properties. Tuning the electronic properties of SWCNTs by filling with electron donor and acceptor compounds opens the way for applications of filled nanotubes in various fields, such as nanoelectronics, energy storage, sensors and nanomedicine [1][2][3]. The filled SWCNTs have homogenous properties, including a defined metallic or semiconducting conductivity type and electronic properties [4]. It was demonstrated that the SWCNTs can be filled with elementary substances, chemical compounds and molecules [4].
In our work, we chose an electron acceptor (cobalt iodide (CoI2)) and electron donor (nickel carbide (Ni3C)) for encapsulation inside SWCNTs and investigated the modification of the electronic properties of SWCNTs. The properties of filled SWCNTs were analyzed by high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS).

Materials and Methods
The filling of SWCNTs with CoI2 was performed by the melt method. The 1.4 nmdiameter mixed metallicity SWCNTs and CoI2 were sealed inside a quartz ampoule under high vacuum, and it was heated in a tube furnace up to atemperature that exceeded the melting point of salt by 100 °C (615 °C) and kept at this temperature for 10 h. Then, the ampoule was slowly cooled down to room temperature to obtain homogeneous crystallization of the salt. The filling of SWCNTs with Ni3C was performed by the two-step method. The 1.7 nm diameter mixed metallicity SWCNTs and nickelocene powder were sealed inside a quartz ampoule under high vacuum, and it was heated up to 50 °C and kept at this temperature for 5 days. Then, the ampoule was heated up to 250 °C and kept at this temperature for 2 h to convert nickelocene to nickel carbide.

Results
The HRTEM data prove the filling of internal channels of SWCNTs with both compounds. In the case of CoI2, one-dimensional nanocrystals of the salt were formed. For nickel carbide, the formation of ~1-2 nm nanoclusters was observed. The Raman spectroscopy data of the CoI2-filled SWCNTs show the modifications of the radial breathing mode (RBM) and G-bands of SWCNTs. The RBM and G-bands of Raman spectra of the pristine and filled SWCNTs were fitted with individual components. The data show the shifts and changes in peak intensities in the RBM-band of the filled SWCNTs as compared to the pristine SWCNTs. This demonstrates changes in the electronic properties of SWCNTs due to doping accompanied by the charge transfer between the nanotubes and salt [5]. The Raman spectroscopy data of nickel carbide-filled SWCNTs do not show noticeable differences as compared to the pristine nanotubes. The XPS data of the CoI2and nickel carbidefilled SWCNTs show the down-and upshift of the C 1s XPS peak, respectively, as well as its broadening as compared to the pristine nanotubes. This was attributed to the downand upshift of the Fermi level of SWCNTs, respectively [6]. By combining the Raman spectroscopy and XPS data, it was shown that the encapsulated CoI2 and nickel carbide lead to p-and n-doping of SWCNTs, respectively.

Conclusions
The mixed metallicity SWCNTs were filled with CoI2 and nickel carbide (Ni3C). The electronic properties of the filled SWCNTs were investigated in detail by Raman spectroscopy and XPS. It was shown that the embedded CoI2 causes p-doping of SWCNTs with downshifting of the Fermi level of nanotubes. Ni3C results in n-doping of SWCNTs with upshifting of the Fermi level of nanotubes. These results allow for implementation of the filled SWCNTs in devices for various applications, such as nanoelectronics, energy storage, sensors, catalysis and biomedicine.