The unique tailored electronic properties of single-walled carbon nanotubes (SWCNTs) render them promising platforms for nanoelectronics applications.
The Encyclopedia entry “SWCNTs in Nanoelectronics” is dedicated to applications of filled single-walled carbon nanotubes (SWCNTs).
Their unique tailored electronic properties render them promising platforms for nanoelectronics applications. Molecules, simple substances and chemical compounds can be filled inside SWCNTs. Encapsulated molecules can lead to p- or n-doping of SWCNTs. Organic molecules, organometallic molecules result in n-  or p-doping of nanotubes .
In work , authors built and tested field effect transistors (FETs) with C60 filled SWCNT (C60-peapods) as active channels. Electrical resistivity, thermal conductivity and thermopower of unfilled SWCNTs and C60-peapods were measured in Ref. . Also, other authors measured the transport properties of C60-peapods  and theoretical calculations were conducted .
The properties of fullerene-filled SWCNTs and SWCNTs filled with endohedral fullerenes were compared that allowed to directly investigate the effects of an altered filler on the hosting SWCNTs . In Ref. , a temperature-dependent transition between p- and n-type conductivity was observed. The transport properties of C60 fullerene peapods and gadolinium metallofullerene (Gd@C82) peapods implemented in FETs as channels were explored. The fullerene-filled SWCNTs exhibited p-type electronic characteristics, which was also observed for semiconducting SWCNTs . Gd@C82-filled SWCNTs had ambipolar p- and n-type electronic behavior.
Piecewise filling can create p-n junctions along SWCNTs. Demonstrated examples are partially Fe-filled SWCNTs  and the piecewise co-filling of SWCNTs with Cs and I or C60 . Figures 1a and b depict contour plots of IDS as functions of VDS and VG for Cs/I@SWCNTs and Cs/C60@SWCNTs recorded at room temperature, respectively . Calculations of the chemical potential along the tube axis were carried out. In order to account for the different p-n junction occurring in Cs/I@SWCNTs and Cs/C60@SWCNTs. Figure 1c is the calculated potential profile across the p-n junction. If the doping densities are equal (donor density: ND = acceptor density: NA) in the n and p regions, the potential at the depletion layer is symmetric and the depletion lengths (lp and ln) are symmetric (top of Fig. 1c). If the donor density dominates like ND = 10NA (the bottom of Fig. 1c) the depletion layer in the p region becomes much wider than in the n region (ln) and the potential across the junction is very asymmetric .
Figure 1. Contour plots of IDS as functions of VG and VDS for (a) Cs/I@SWCNTs and (b) Cs/C60@SWCNTs measured at room temperature. (c) Calculated potential profiles with depletion regions around the p-n junction area. ND =NA (top) and ND = 10NA (bottom). Reprinted from  with the permission of AIP Publishing.
Encapsulated metals  lead to n-doping of SWCNTs. The transport properties of FETs with channels made of ferrocene-filled SWCNTs and Fe-filled SWCNTs were directly compared . This comparison revealed that, while Fe-filled SWCNTs exhibited high performance unipolar n-type characteristics, ferrocene-filled SWCNTs are indeed ambipolar semiconductors.
The hitherto developments of carbon nanotube-based nanoelectronics have proven fundamental building blocks and prepared the ground for exploring novel types of filled SWCNTs as specialized building blocks. Foreseeable next step would be the realization of devices based on SWCNTs filled with various non-metals , metal oxides , metal halogenides  and metal chalcogenides . With all the available experience on filling of SWCNTs with a wide range of substances many novel transport characteristics are yet to be explored as building blocks for carbon nanotube-electronics.