Doping Of Single-walled Carbon Nanotubes Research Articles

(Invited) Probing Charge Carrier-Induced Changes in the Complex Dielectric Function in Carbon Nanotube Dispersions

The injection of charge carriers into pi-conjugated semiconductors, whether by electrochemical injection or chemical charge transfer, is a powerful approach to tune their electrical conductivity. Recently, the concept of dielectric catastrophe, where the charge carrier density is sufficiently large to significantly alter the intrinsic dielectric properties, has been demonstrated at high doping levels in organic semiconductor thin films. The observed increase in the dielectric constant leads to reduced coulombic interactions between charge carriers and the associated counterions, promoting enhanced free carrier generation and transport.Here, we employ a combination of steady state optical spectroscopy, quantitative 19F NMR studies, and microwave dielectric loss spectroscopy to probe charge carrier density and transport in doped single-walled carbon nanotube dispersions. We demonstrate that a similar dielectric catastrophe phenomenon can be observed even in isolated single-walled carbon nanotubes dispersed in a low dielectric constant solvent AND, perhaps more surprisingly, at carrier densities less than one carrier per nanotube. The precise details of the observed changes in complex dielectric function are dependent on the chemical structure of the charge-transfer dopant, with a near-instantaneous increase in the dielectric constant and electrical conductivity observed for low carrier densities using bulky benzyloxy-substituted icosahedral dodecaborane cluster dopants, DDBs. We attribute the observations to weak coulombic interactions between the injected charge carrier and the dopant counterion, along with the large polarizability afforded by the unique physicochemical properties of single-walled carbon nanotubes. If time permits, we will show results from recent studies where we extend this approach to the doping of other nanocarbons.

π-Conjugated metallo-copolymer/SWCNT composites for high performance thermoelectric generators

Organic materials have attracted extensive attention in flexible thermoelectric generators due to their solution processability, low thermal conductivity and high flexibility. However, their thermoelectric performances are still far behind those of the inorganic counterparts (e.g. Bi2Te3, PbTe and other related alloys). Here, by virtue of the dual advantages of both inorganic and organic materials, three Pt (II)-containing diketopyrrolopyrrole (DPP) based donor−acceptor (D−A)-type π-conjugated metallo-copolymers (P1Pt, P2Pt and P3Pt) are designed and synthesized, and they are compared with their control copolymers without Pt (II)-coordination (P1, P2 and P3). It is shown that Pt (II) centers and copolymerized molecular structures play important roles in tuning the bandgap and coplanarity. A large Fermi level shift is observed after doping with single-walled carbon nanotubes (SWCNTs) in the copolymers. As expected, the introduction of Pt (II)-side chains and doping of SWCNTs into copolymers could significantly improve the power factor (PF), which reaches the highest value of 276.5 ± 26.6 μW m−1 K−2 for P3Pt/60 wt% SWCNTs. The corresponding p-type thermoelectric generator exhibits a large output power of up to 0.55 μW under a 109 K temperature gradient. Our results can provide a pathway for designing and preparing efficient thermoelectric materials and flexible thermoelectric generators.

Facile Doping of 2,2,2-Trifluoroethanol to Single-Walled Carbon Nanotubes Electrodes for Durable Perovskite Solar Cells

Perovskite solar cells with an indium tin oxide (ITO)/SnO2/CH3NH3PbI3/Spiro-OMeTAD/2,2,2-trifluoroethanol (TFE) doped single-walled carbon nanotube (SWCNT) structure were developed by dropping TFE onto SWCNTs, which replaced the metal back electrode, and a conversion efficiency of 14.1% was achieved. Traditionally, acidic doping of the back electrode, SWCNT, has been challenging due to the potential damage it may cause to the perovskite layer. However, TFE has facilitated easy doping of SWCNT as the back electrode. The sheet resistance of the SWCNTs decreased and their ionization potential shifted to deeper levels, resulting in improved hole transport properties with a lower barrier to carrier transport. Furthermore, the Seebeck coefficient (S) increased from 34.5 μV/K to 73.1 μV/K when TFE was dropped instead of EtOH, indicating an enhancement in the behavior of p-type charge carriers. It was observed that hydrophilic substances adhered less to the SWCNT surface, and the formation of PbI2 was suppressed. These effects resulted in higher conversion efficiency and improved solar cell performance. Furthermore, the decrease in conversion efficiency after 260 days was suppressed, showing improved durability. The study suggests that combining SWCNTs and TFEs improves solar cell performance and stability.

Probing iodide/iodonium salt interactions with single-walled carbon nanotubes for resilient electrochemical capacitor

Self-standing films from single-walled carbon nanotubes (SWCNTs) were successfully used as electrochemical capacitors’ (ECs) electrode material, which exhibited state-of-the-art characteristics. The surface of SWCNTs was modified by iodonium salt molecules that substantially and permanently improved their electrical conductivity. The addition of pyridinium iodine chloride salt (IPyCl) quadrupled the value of the electrical conductivity of the SWCNT film, which reached values as high as 1022 S cm−1. Subsequently, the electrochemical performance of the newly designed self-standing SWCNTs with deposited IPyCl in aqueous neutral 1 M Li2SO4 and redox-active 1 M KI electrolytes was thoroughly analysed. Because redox activity of iodine species was observed mainly on the positive electrode, activated carbon with a developed surface area was used as a negative electrode to balance the faradaic charge. Due to the notable stability of iodonium salt dopant (instead of labile halogen-containing compounds typically used for SWCNT doping), robust charge/discharge performance of EC was demonstrated (> 10 000 cycles) in Li2SO4 electrolyte. Moreover, a significant improvement of capacitor metrics (from 49 to 75 F g−1) was obtained when electrodes modified by previously unexplored iodonium salt were utilized in 1 M KI electrolyte. Extensive analysis of the obtained data led to the elucidation of the doping mechanism responsible for the aforementioned achievements. The novel SWCNT-based electrodes enhanced with iodonium salt are auspicious materials, which can be used to build EC systems of appreciable capacitance.

High-temperature adsorption of nitrogen dioxide for stable, efficient, and scalable doping of carbon nanotubes

This work is devoted to a novel efficient strategy for single-walled carbon nanotube doping employing heat treatment with nitrogen dioxide (NO2). Unlike numerous reports of unstable NO2 doping at room temperature, our method combines high efficiency and stability, enabled by its temperature-dependent adsorption on the nanotube surface. We reveal that doping stability increases with the treatment temperature reaching maxima at 300 °C avoiding any detrimental effect on nanotube structure and optical transmittance. As a result, we demonstrate doped carbon nanotube transparent conductive films exhibiting competitive performance (in respect to films treated with other dopants) with a less than 50% drop in conductive characteristics for over a year. Thermo-programmed desorption analysis and X-ray photoelectron spectroscopy confirm the preferential formation of long-living adsorbed nitrogen species, such as NO3-groups, as a result of high-temperature treatment. We believe the current work provides a basis for the robust and technologically efficient doping of single-walled carbon nanotubes and related structures at industrial scales, as the developed method could be easily coupled with a continuous technology of carbon nano materials production.

Aerosol doping of SWCNT films with p- and n-type dopants for optimizing thermoelectric performance

We investigate the aerosol doping method for a thermoelectric performance enhancement of single-walled carbon nanotube (SWCNT) films. A study of doping behavior by depositing an aerosolized dopant solution and real-time optimization of thin film properties focuses on thermoelectric characteristics. Achieving precise control over doping results, we demonstrated the thermoelectric power factor of 657 and 708 μW/mK2 for, respectively, p-type and n-type doped SWCNT films. The proposed doping strategy offers superior flexibility for device design and comparable performance to solution-based doping, paving the way to efficient and scalable thermoelectric generator production based on carbon nanotube materials.

Highly stable phosphotungstic acid/Au dual doped carbon nanotube transparent conductive films for transparent flexible heaters

Phosphotungstic acid (PTA) is a highly stable dopant. In this research, transparent conductive films (TCFs) with polyethylene terephthalate as a substrate and PTA/Au secondary p-type doped single-walled carbon nanotubes (SWCNTs) as a conductive layer were prepared by spraying and employed as heating devices. The films had excellent optoelectronic properties (sheet resistance of 46.7 Ω/sq with 87.1% transmittance), mechanical properties (only 0.03 change in sheet resistance after 5000 bending cycles), stability (29.4% change in sheet resistance after one week of exposure to 95% humidity) and electrical heating properties. Transparent flexible heaters (TFHs) made of PTA/Au dual p-type doped SWCNT TCFs have a quick response to temperature changes and outstanding cycling stability. At an operating voltage of 20 V, the temperature of the TFH rapidly reached its maximum steady-state temperature (Tmax) of ∼120 °C within less than 90 s. After 50 cycles at 15 V in the voltage-on-off cycle test, the Tmax achieved by the TFH did not change appreciably. The results of the performance evaluation indicate that the film has a bright future in flexible wearable devices for warmth, physiotherapy, or deicing/defrosting applications.

Photoinduced electron doping of single-walled carbon nanotubes based on carboxamide photochemical reactions

Photoinduced electron doping of single-walled carbon nanotubes based on the formation of 1,3-dimethyl-2-arylbenzimidazole (DMBI) cations and hydroxy ions by photochemical reaction of DMBI oxides.

Enhancement Effect of the C60 Derivative on the Thermoelectric Properties of n-Type Single-Walled Carbon Nanotube-Based Films.

Obtaining air-stable and high-performance flexible n-type single-walled carbon nanotube (SWCNT)-based thermoelectric films used in wearable electronic devices is a challenge. In this work, the microstructure and thermoelectric properties of n-type SWCNT-based films have been optimized via doping C60 and its derivative into polyethylenimine/single-walled carbon nanotube (PEI/SWCNT) films. The result demonstrated that the dispersity of triethylene glycol-modified C60 (TEG-C60) was better in PEI/SWCNT films than that of pure C60. Among the prepared composite films, TEG-C60-doped PEI/SWCNT (TEG-C60/PEI/SWCNT) films exhibited the highest TE performance, achieving a peak electrical conductivity of 923 S cm-1 with a Seebeck coefficient of -42 μV K-1 at a TEG-C60/SWCNT mass ratio of 1:100. Compared to that of PEI/SWCNT, the power factor was increased significantly from 40 to 162 μW m-1 K-2 after the addition of TEG-C60, which was higher than that of films after the addition of C60. In addition, the n-type doped SWCNT films had good air stability at high temperatures, and the Seebeck coefficients of C60/PEI/SWCNT and TEG-C60/PEI/SWCNT at 120 °C were still negative and remained at 92% and 85%, respectively, after 20 days. Furthermore, a flexible TE device consisting of five pairs of p-n junctions was assembled using the optimum hybrid film, which generated a maximum output power of 3.6 μW at a temperature gradient of 50.2 K. Therefore, this study provides a facile way to enhance the thermoelectric properties of n-type carbon nanotube-based materials, which have potential application in flexible power generators.

Doping of Carbon Nanotubes with Encapsulated Phosphorus Chains

Single-walled carbon nanotubes (SWCNTs) are a perfect host for the formation of one-dimensional phosphorus structures and to obtain hybrid materials with a large P-C ratio. This work presents a procedure for high-yield phosphorus filling of commercial Tuball SWCNTs and efficient removal of phosphorus deposits from the external nanotube surface. We probed white and red phosphorus as precursors, varied the synthesis temperature and the ampoule shape, and tested three solvents for sample purification. High-resolution transmission electron microscopy and Raman spectroscopy indicated crystallization of interior phosphorus in a form resembling fibrous red phosphorus. An aqueous sodium hydroxide solution allowed removing the majority of external phosphorus particles. Thermogravimetric analysis of the product determined ∼23 wt % (∼10 atom %) of phosphorus, and the X-ray photoelectron spectroscopy (XPS) data showed that ca. 80% of it is in the form of elemental phosphorus. Externally purified SWCNTs filled with phosphorus were used to study the interaction between the components. Raman spectroscopy and core-level XPS revealed p-type SWCNT doping. Valence-band XPS data and density functional theory calculations confirmed the transfer of the SWCNT electron density to the encapsulated phosphorus.

Highly efficient doping of carbon nanotube films with chloroauric acid by dip-coating

Single-walled carbon nanotube (SWCNT) based transparent and conductive films (TCFs) are one of the most prospective materials for novel flexible and stretchable electronic devices. Development of reproducible and scalable doping procedure is the key step towards the widespread implementation of SWCNT TCFs. Here, we thoroughly investigate a dip-coating technique for SWCNT doping as a promising approach for the practical manufacturing of SWCNT films with high performance. We examine the effect of dip-coating parameters on optical and electrical properties of the films using HAuCl4 solution in isopropyl alcohol (IPA) and in situ investigate doping effects. This method appeared to easily fine-tune the optoelectronic parameters of SWCNT films and achieve a record sheet resistance value of 36 Ohm/sq. at the 90% transmittance in the middle of visible spectral range by increasing a work function value from 4.8 (for pristine SWCNTs) to 6.0 eV. The proposed approach allows efficient, uniform, and reproducible fabrication of highly conductive and transparent SWCNT films and opens an avenue for precise tailoring of SWCNT Fermi level for optoelectronic devices.

Stable Doping of Single-Walled Carbon Nanotubes for Flexible Transparent Conductive Films.

Possessing excellent electronic and mechanical properties and great stability, single-walled carbon nanotubes (SWCNTs) are exceptionally attractive in fabricating flexible transparent conductive films. Doping is a key step to further enhance the conductivity of the SWCNT films and the reliable doping is highly needed. We developed a feasible strategy that uses solid acids such as phosphotungstic acid (PTA) to dope the SWCNT films stably relying on the nonvolatility of the dopants. The sheet resistance of the films was reduced to around a half of the original value meanwhile with no obvious change in transmittance. The doping effect maintained during a 700 days' observation. The excellent flexibility of the PTA-doped films was demonstrated by a bending test of 1000 cycles, during which the sheet resistance and transmittance was basically unaffected. The blue shifts of G band in the Raman spectra and the increase of work function measured by the Kelvin probe force microscopy both reveal the p-type doping of the films by PTA. The strong acidity of PTA plays a key role in the doping effect by increasing the redox potential of the ambient O2 and thus the Fermi level of the SWCNTs is brought down. The great feasibility and robustness of our doping strategy are desirable in the practical application of SWCNT-based flexible transparent conductive films. This strategy can be extended to the p-type doping of various CNT-based assemblies (such as sponges and forests) as well as other material families, expanding the application spectrum of polyacids.

Solution-phase p-type doping of highly enriched semiconducting single-walled carbon nanotubes for thermoelectric thin films

Single-walled carbon nanotubes (SWCNTs) are attractive materials for next-generation energy-harvesting technologies, including thermoelectric generators, due to their tunable opto-electronic properties and high charge carrier mobilities. Controlling the Fermi level within these unique 1D nanomaterials is often afforded by charge transfer interactions between SWCNTs and electron or hole accepting species. Conventional methods to dope SWCNT networks typically involve the diffusion of molecular redox dopant species into solid-state thin films, but solution-phase doping could potentially provide routes and/or benefits for charge carrier transport, scalability, and stability. Here, we develop a methodology for solution-phase doping of polymer-wrapped, highly enriched semiconducting SWCNTs using a p-type charge transfer dopant, F4TCNQ. This allows doped SWCNT inks to be cast into thin films without the need for additional post-deposition doping treatments. We demonstrate that the introduction of the dopant at varying stages of the SWCNT dispersion process impacts the ultimate thermoelectric performance and observe that the dopant alters the polymer selectivity for semiconducting vs metallic SWCNTs. In contrast to dense semiconducting polymer films, where solution-phase doping typically leads to disrupted morphologies and poorer TE performance than solid-state doping, thin films of solution-doped s-SWCNTs perform similarly to their solid-state doped counterparts. Interestingly, our results also suggest that solution-phase F4TCNQ doping leads to fully ionized and dimerized F4TCNQ anions in solid-state films that are not observed in films doped with F4TCNQ after deposition. Our results provide a framework for the application of solution-phase doping to a broad array of high-performance SWCNT-based thermoelectric materials and devices that may require high-throughput deposition techniques.

The Electronic Characteristic and Optical Properties of Doped Single-Walled Carbon Nanotubes from a First-Principles Study

In this contribution, electronic structures and optical properties of pure and doped single-walled carbon nanotubes (SWCNTs) are investigated by the first-principles calculation. It is found that the B- and N-doped systems have the lowest binding energies of [Formula: see text][Formula: see text]eV and [Formula: see text][Formula: see text]eV, respectively, indicating that the configurations are relatively stable. Interestingly, the indirect bandgap is transforming into a direct bandgap in the two systems, which has a good development prospect in the field of light-emitting devices. The charge difference density diagrams show that the electron deletion around all doped atoms is weakened, indicating that the covalence between the doped atom and carbon atom is weakened, which is consistent with the population analysis result. In the absorption spectrum, there is a “blueshift” phenomenon. In the reflection spectrum, the six doping systems show an obvious “redshift” phenomenon. In addition, the polarization ability of all doping systems has become stronger. Also, due to the different changes in the peak values of the imaginary part of the dielectric function, the doping of Al, Ga and P improved the photoresponse of the low-energy region, while the doping of B, N and As weakens it.

Structure Dependence of Photoluminescence Solvatochromic Energy Shifts Based on Exciton Localization in Locally Functionalized Single-Walled Carbon Nanotubes

Defect doping of single-walled carbon nanotubes (SWCNTs) by local chemical functionalization produces locally functionalized SWCNTs (lf-SWCNTs) that emit red-shifted and bright photoluminescence (PL) in near-infrared regions beyond 1000 nm (E11* PL) from localized excitons trapped at doped sites in the tubes. In this study, we use lf-SWCNTs with different doped sites such as proximal aryl-doped sites and oxygen-doped sites (ether-type and epoxide-type) showing largely red-shifted PL than typical E11* PL, which allow us to elucidate the structural effects on PL solvatochromism (via organic solvent injection) and reveal the excitonic property variations of the localized excitons. We correlate PL solvatochromic energy shifts of the lf-SWCNTs with solvent orientation polarization parameters. The proximal aryl-doped sites of bisaryl-modified lf-SWCNTs emit E112* PL (∼1253 nm), and the observed solvatochromic energy shifts appeared between those of the E11 PL (∼980 nm) and E11* PL (∼1125 nm). The epoxide-type doped sites of the oxygen-doped lf-SWCNTs emit E11*b PL (∼1250 nm), with almost no solvatochromism; this was totally different from the trends seen for other PL, including E11*a PL from the ether-type doped sites. The unique structure-dependent solvatochromism is correlated with the (1) localization-induced polarizability or dipole moment changes and (2) different degrees of exciton localization at doped sites.

Single-Walled Carbon Nanotubes and Hetero-Nanotubes for Perovskite Solar Cells

Single-walled carbon nanotubes (SWCNTs), graphene, and fullerene (C60 and derivatives) are very efficiently used in organic-inorganic Perovskite solar cells [1, 2]. A film of carbon nanotubes or graphene can be the practical replacement [3] of ITO for the flexible and/or foldable [4] transparent electrode of inverted perovskite solar cells. Doping of SWCNT is essential for high performance solar cells through increased in-plane film conductivity and energy level adjustment. Since p-doping is easier than n-doping, it is more practical to use SWCNT electrode in the hole-transport side. Hence, we have developed the normal type perovskite solar cells composed of ITO/ETL/MAPbI3/HTL-SWCNT. The use of SWCNT as the top electrode instead of metal enhances the stability of PSCs by removing the metal-ion migration, and considerably reduces the fabrication cost, and is suitable for the development of tandem system. Recently, we have improved the performance with higher concentration of hole-transporting material [5]. For MAPbI3 system, we have obtained the highest PCE of 18.8 % compared with the control device with gold electrode with PCE of 18.1%. Because of transparency of SWCNT film is higher than that of ITO in the NIR region, SWCNT electrode based perovskite–silicon tandem solar cells can have better performance than ITO-based one [6]. In addition to our demonstration of 24.4 % total PCE in our previous work [6], we are now demonstrating the higher preliminary PCE of 27.7 %.Finally, the ultimately inorganic stable doping of SWCNT could be possible by using the one-dimensional van der Waals hetero-nanotubes [7]. We have synthesized the coaxial few-layer hexagonal boron nitride nanotube (BNNT) around a SWCNT; SWCNT@BNNT. Then, the further coating of coaxial MoS2 nanotubes results SWCNT@BNNT@MoS2NT as shown in Fig. 1. The inner SWCNT and outer MoS2NT are electrically coupled through a few layer BNNT. The preliminary PSC device using the heteronanotube film shows the advantage [8].Part of this work was supported by JSPS KAKENHI Grant Numbers JP18H05329, JP20H00220, and by JST, CREST Grant Number JPMJCR20B5, Japan.

Correction: Highly efficient bilateral doping of single-walled carbon nanotubes

Correction for ‘Highly efficient bilateral doping of single-walled carbon nanotubes’ by Anastasia E. Goldt et al., J. Mater. Chem. C, 2021, 9, 4514–4521, DOI: 10.1039/D0TC05996J.

Electron doping of single-walled carbon nanotubes using pyridine-boryl radicals.

Pyridine-boryl (py-boryl) radicals serve as efficient electron-doping reagents for single-walled carbon nanotubes (SWCNTs). The doping mechanism comprises electron transfer from the py-boryl radical to the SWCNT. The formation of a stable py-boryl cation is essential for efficient doping; the captodative effect of the py-boryl cation is important to this process.

Highly efficient bilateral doping of single-walled carbon nanotubes

Bilateral SWCNT doping procedure.

Effective Doping of Single-Walled Carbon Nanotubes with Polyethyleneimine.

More and more electrically conducting materials are required to sustain the technological progress of civilization. Faced with the performance limits of classical materials, the R&D community has put efforts into developing nanomaterials, which can offer sufficiently high operational parameters. In this work, single-walled carbon nanotubes (SWCNTs) were doped with polyethyleneimine (PEI) to create such material. The results show that it is most fruitful to combine these components at the synthesis stage of an SWCNT network from their dispersion. In this case, the electrical conductivity of the material is boosted from 249 ± 21 S/cm to 1301 ± 56 S/cm straightforwardly and effectively.