Highly efficient double-sided doping of single-walled carbon nanotubes

Together with the experimental group of Prof. Albert Nasibulin (Skoltech) we proposed and thoroughly investigated a new approach that comprises simultaneous bilateral (outer and inner surfaces) SWCNT doping after their opening by thermal treatment at 400 °C under an ambient air atmosphere. Doping by a chloroauric acid (HAuCl4) ethanol solution allowed us to achieve the record value of sheet resistance of 31 ± 4 Ω/sq at a transmittance of 90% in the middle of visible spectra (550 nm). The strong p-doping was examined measurements and confirmed by ab initio calculations demonstrating a downshift of Fermi level around 1 eV for the case of bilateral doping.

Work is published in J. Mater. Chem. C, 2021,9, 4514-4521

Although films of single-walled carbon nanotubes (SWCNTs) are the most promising candidates for the creation of transparent and flexible films, they still do not meet the requirements of optoelectronics. In this work, a new approach was proposed and investigated, which consists in the simultaneous two-sided (from the inner and outer sides) alloying of SWCNTs. This result was experimentally obtained by our colleagues, in which SWCNTs were discovered by heat treatment at 400 ° C in ambient air. Doping with ethanol solution of chloroacetic acid (HAuCl4) made it possible to achieve a record value of leaf resistance (31 ± 4) Ohm / m2 with a transmission of 90% in the middle of the visible spectrum (550 nm).

The origin of the more efficient doping of SWCNTs due to the removal of preformed caps by heat treatment is also confirmed by our observations using transmission electron microscopy (TEM). Figure 1 shows TEM images of films doped with HAuCl4, both intact and thermally treated SWCNTs. In the images of the initial SWCNTs and those treated at 300 C, the surface of the nanotubes is decorated with metallic gold nanoparticles approximately 5 to 10 nm in size (Fig. 1a, b). On the contrary, for samples thermally treated at 400 ºC, in addition to decorating the outer surface with Au0 nanoparticles, one can notice the filling of the inner space of SWCNTs (Figure 1c, d). Measurement of the interplanar distance of the encapsulated material from TEM images gave a result of 0.235 nm. It was found to be metallic gold, which has a (111) interplanar spacing of 0.2355 nm. Gold nanoparticles cover the outer surface of SWCNTs, forming as a result of spontaneous reduction of [AuCl4] - anions. When the heat treatment temperature is high enough to oxidize the caps of the nanotubes, for example, 400 ºC, the alloying solution penetrates into the inner space of the SWCNTs, which leads to improved doping and, in addition, to the formation of a metallic gold phase. This is manifested in a higher doping efficiency of the SWCNT film treated at 400 ºC, which explains its record value of R90.

a) SWCNT without pre-heat treatment and with pre-treatment at b) 300 ºC (b) and c) 400 ºC. e, f) STEM images of open SWCNTs doped with 15 mM HAuCl4 with encapsulated gold nanowires

Figure 1 - TEM images of SWCNT films doped with 15 mM ethanol solution of HAuCl4. Arrows show SWCNTs filled with the Au metal phase

To identify the reasons for the effective doping of heat-treated SWCNTs, we performed a theoretical analysis of their electronic properties for individual cases of either only external or simultaneous external and internal doping. Previous calculations using density functional theory (DFT) showed that the p-type character of doping is due to the adsorption of AuCl4. It can be expected that other Au-containing systems will also participate in the doping process. We have carried out a systematic study of the effect of several types of dopants based on AuClx on the electronic structure of carbon nanotubes. The molecular groups of gold chlorides used in the calculations were taken from the crystal structure of Au (III) and Au (I) chlorides, and a pure Au nanowire was cut from bulk fcc gold. SWCNTs with chirality were chosen as a model of the nanotube for the study (10, 10).

Figure 2 - Atomic structures of pure SWCNT, SWCNT doped outside, and doped both inside and outside with various dopants AuClx. The corresponding shift of the Fermi level is indicated under each structure. Gray, pink and yellow balls represent carbon, chlorine and gold atoms, respectively

Calculations show (Figure 2) that SWCNTs doped only outside (SWCNT @ AuCl4) demonstrate a significant downward shift of the Fermi level by 0.79 eV, which characterizes p-type doping. Additional incorporation of Au nanowires into SWCNTs leads to a redistribution of the electron density from the nanowire to SWCNTs and a general shift of the Fermi level by 0.22 eV, which corresponds to n-type doping. However, no significant effect on charge transfer between SWCNTs and Au nanoparticles located on the surface of nanotubes is observed experimentally. In the case of double-sided doping with AuCl4, the largest shift of the SWCNT Fermi level to 0.97 eV is observed in the case of double-sided doping with AuCl4. the largest shift of the SWCNT Fermi level is observed up to 0.97 eV. This indicates a more efficient p-type doping compared to the sample doped only from the outer surface, which reflects the efficient doping of open SWCNTs.