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.

Pavel Sorokin was awarded the Moscow Government Prize for Young Scientists (Physics and Astronomy no

The prize was recognized for his contribution to the study of the spintronic and electronic properties of nanomaterials with a specific atomic structure. Together with colleagues, Sorokin developed a theory of a chemically induced phase transition, which makes it possible to transform graphene into two-dimensional diamond. This theory became the basis for the synthesis of ultra-thin diamond films around the world. The award also recognized his work in the study of heterojunction from graphene and Heusler alloy, which is extremely promising as a basis for the development of various spintronic devices based on graphene, such as readout sensors, MRAM, and high efficiency spin transistors.

Atomically thin films: current progress (Review)

A review has been published in Physics-Uspekhi (with 423 references ) devoted to the current state of research of atomically thin films. The structure and properties of atomically thin monoelemental films, such as 2D Fe, Au, Li, as well as Si, Ge, B etc., are described in detail. Two-dimensional films of metallic compounds like FeO, CuO, ZnO and FeC, CoC and CuC are considered. The main approaches to the stabilization of monoatomic films inside pores or between layers of other 2D materials are presented, and the exfoliation mechanism of ionic-covalent films with a polar surface into weakly bounded monolayers is described.

Paper have published in Physics-Uspekhi 64, 1, 28 (2021) ​ The isolation and subsequent detailed study of graphene have shown its significant prospects and potential for use in a wide range of technologies, such as composite materials, low-dimensional catalysts, touch screens, conductive ink, electronic paper, and organic light-emitting diodes. In 10-20 years, the introduction of graphene-based transistors and other logic units is expected. ​ The main obstacles to the wide use of graphene in electronics are the requirements of high quality of the synthesized atomic structure and the absence of a bandgap. The latter is a fundamental problem, to which no universal solution has been found yet. Each of the approaches proposed, e.g., functionalization, the introduction of defects into the structure, or graphene splitting into separate ribbons, has its drawbacks. Indeed, the chemical adsorption of atoms on graphene leads to a change in the hybridization of carbon atoms from sp2 to sp3 accompanied by destruction of the p-system responsible for graphene conductivity. The proposed partial functionalization by forming hydrated or fluorinated regions (in a limiting case, separate periodically arranged chains of hydrogen) solves the problem, because a bandgap opens between the regions due to the dimensional effect. However, in spite of experimental confirmation of the effect, this technique requires adsorption with atomic precision, which at present is hardly a solvable problem. ​ Using monolayers of different compositions can be an alternative way, especially as pioneering study has demonstrated the flexibility of the micromechanical exfoliation approach for obtaining planar two-dimensional structures from any crystals with weakly bound layers. The resulting materials have a planar structure as thin as one or a few atomic layers, whereas the lateral size can exceed a few micrometers. The atomic-size thickness, quantum dimensional effects, and high anisotropy of the optical properties of two-dimensional nanomaterials, as well as prospects of their application, have continuously attracted great interest of the world scientific community. ​ Studies on finding new quasi-2D films turned out to be so efficient that up to 2020 they numbered a few hundred. This led to a paradoxical situation of a lack of resources in the scientific community sufficient for their thorough investigation. Moreover, the latest theoretical studies predict the possibility of five to six thousand more compounds in a quasi-2D state, which ultimately makes 2D materials one of the most vast and poorly studied areas of up-to-date materials science. ​ The aim of the present review is to inform a wide range of readers about the current state of materials science in the field of noncarbon 2D structures. However, because of the great number of discovered and predicted two-dimensional crystals mentioned above, the discussion is focused on atomically thin films only. This gives rise to a certain difficulty in classifying the compounds described; for example, whether the films of phosphorene or silicene are atomically thin or not? Indeed, both structures have a thickness of a single atom and differ only by the degree of lattice corrugation. Therefore, a criterion should be introduced, according to which the present review will consider only those compounds with the corrugation degree (the out-of-plane displacement of atoms D) much smaller than the lattice parameter of the crystal a, i.e., D<<a. Notably, this criterion excludes phosphorene from consideration, but requires describing silicene and other allied materials. The review consists of three main sections. Section 2 considers single-layer films consisting exclusively of metal atoms (Fe, Au, etc.) and the possibility of their stabilization using other two-dimensional materials (mainly graphene). Section 3 presents a detailed description of various two-dimensional metal compounds, in particular, oxides and carbides of transition metals. Section 4 is devoted to films consisting of elements of groups 13-16 of the periodic table: silicene, borophene, and allied materials. ​