Nathanael D. Cox

Mechanism of chemical doping in electronic-type-separated single wall carbon nanotubes towards high electrical conductivity

Enhanced electrical conductivity of carbon nanotubes (CNTs) can enable their implementation in a variety of wire and cable applications traditionally employed by metals. Electronic-type-separated single wall carbon nanotubes (SWCNTs) offer a homogeneous platform to quantify the unique physiochemical interactions from different chemical dopants and their stability. In this work, a comprehensive study of chemical doping with purified commercial CNT sheets shows that I2, IBr, HSO3Cl (CSA) and KAuBr4 are the most effective at increasing the electrical conductivity of CNT films by factors between 3× and 8×. These dopants are used with electronic-type-separated SWCNT thin-films to further investigate changes in SWCNT optical absorption, Raman spectra, and electrical conductivity. The dopant effects with semiconducting SWCNTs result in quenching of the S11 and S22 transitions, and a red shift of 8–10 cm−1 of the Raman G′ peak, when compared to a purified SWCNT thin-film. The average electrical conductivity of purified semiconducting SWCNT thin-films is 7.3 × 104 S m−1. Doping increases this conductivity to 1.9 × 105 S m−1 for CSA (2.6× increase), 2.2 × 105 S m−1 for IBr (3.1×), to 2.4 × 105 for I2 (3.3×), and to 4.3 × 105 for KAuBr4 (5.9×). In comparison, metallic SWCNT thin-films exhibit only slight quenching of the optical absorbance spectra for the M11 transition, and shifts in the Raman G′-peak of less than 1 cm−1 for I2 and IBr, whereas KAuBr4 and CSA promote red shifting by 4 cm−1, and 7 cm−1, respectively, when compared to a purified control sample. The increase in electrical conductivity of metallic SWCNT thin-films is gradual and depends on the dopant. With an average value of 9.0 × 104 S m−1 for the purified metallic SWCNT thin-films, I2 doping increases the electrical conductivity to 1.0 × 105 (1.1× increase), IBr to 1.5 × 105 S m−1 (1.7×), KAuBr4 to 2.4 × 105 S m−1 (2.6×), and CSA to 3.2 × 105 S m−1 (3.5×). The time-dependent stability of the chemical dopants with SWCNTs is highest for KAuBr4, which remains in effect after 70 days in ambient conditions. The doping-enhanced electrical conductivity is attributed to the relative potential difference between the SWCNT electronic transitions and the redox potential of the chemical species to promote charge transfer. The results of this work reinforce the chemical doping mechanism for electronic-type-separated SWCNTs and provide a path forward to advance SWCNT conductors.

Ion irradiation of electronic-type-separated single wall carbon nanotubes: A model for radiation effects in nanostructured carbon

The structural and electrical properties of electronic-type-separated (metallic and semiconducting) single wall carbon nanotube (SWCNT) thin-films have been investigated after irradiation with 150 keV 11B+ and 150 keV 31P+ with fluences ranging from 1012 to 1015 ions/cm2. Raman spectroscopy results indicate that the ratio of the Raman D to G′ band peak intensities (D/G′) is a more sensitive indicator of SWCNT structural modification induced by ion irradiation by one order of magnitude compared to the ratio of the Raman D to G band peak intensities (D/G). The increase in sheet resistance (Rs) of the thin-films follows a similar trend as the D/G′ ratio, suggesting that the radiation induced variation in bulk electrical transport for both electronic-types is equal and related to localized defect generation. The characterization results for the various samples are compared based on the displacement damage dose (DDD) imparted to the sample, which is material and damage source independent. Therefore, it is poss...

Carbon nanotube metal matrix composites for solar cell electrodes

Metal matrix composites (MMCs) composed of Ag and single-wall carbon nanotubes (SWCNTs) have been developed as an electrode material for inverted metamorphic multi-junction (IMM) solar cells. Scanning electron microscopy analysis reveals that these Ag-SWCNT MMCs may alleviate issues caused by fracture of the fragile IMMs, and that the metal grain structure varies based on the thickness (or weight loading) of the SWCNTs present in the MMCs. Additionally, dynamic mechanical analysis of the Ag-SWCNT MMCs shows that the addition of SWCNTs to the Ag significantly increases the strain accommodation of the MMCs over conventional Ag. The results indicate that SWCNTs may be a promising material for advanced IMM solar cell electrodes for space and terrestrial applications.

Removal of sodium dodecyl sulfate surfactant from aqueous dispersions of single-wall carbon nanotubes

A reagent-based treatment method was developed for the removal of sodium dodecyl sulfate (SDS) from aqueous dispersions of single-wall carbon nanotubes (SWCNTs). Based on a survey of various reagents, organic solvents emerged as the most effective at interrupting the SDS:SWCNT interaction without producing deleterious side reactions or causing precipitation of the surfactant. Specifically, treatment with acetone or acetonitrile allows for the facile isolation of SWCNTs with near complete removal of SDS through vacuum filtration, resulting in a 100x reduction in processing time. These findings were validated via quantitative analysis using thermogravimetric analysis, Raman spectroscopy, 4-point probe electrical measurement, and X-ray photoelectron spectroscopy. Subsequent thermal oxidation further enhances the purity of the reagent treated samples and yields bulk SWCNT samples with >95% carbonaceous purity. The proposed reagent treatment method thus demonstrates potential for large volume SWCNT processing.

Electrical characterization of carbon nanotube metal matrix composite solar cell electrodes under mechanical stress

Metal matrix composites (MMCs) have been fabricated as potential advanced solar cell electrodes. Test structures have been developed for evaluating electrical performance of the MMC electrodes upon substrate fracture and under subsequent tensile stress to simulate stress fractures and mechanical fatigue of solar cell electrodes. Electrical analysis reveals that MMCs utilizing single-wall carbon nanotubes (SWCNTs) provide electrical continuity for gaps of ~6 μm or less; however, incorporation of longer multi-walled CNTs (MWCNTs) into the MMCs enables bridging of gaps approaching 30 μm. The results indicate that CNT-MMCs may provide a more robust solar cell electrode, particularly for fragile IMM solar cells.

Modification of Silver/Single-Wall Carbon Nanotube Electrical Contact Interfaces via Ion Irradiation.

Introduction of defects via ion irradiation ex situ to modify silver/single-wall carbon nanotube (Ag-SWCNT) electrical contacts and the resulting changes in the electrical properties were studied. Two test samples were fabricated by depositing 0.1 μm Ag onto SWCNT thin films with average thicknesses of 10 and 60 nm, followed by ion irradiation (150 keV 11B+ at 5 × 1014 ions/cm2). The contact resistance (Rc) between the Ag and SWCNT thin films was determined using transfer length method (TLM) measurements before and after ion irradiation. Rc increases for both test samples after irradiation, while there is no change in Rc for control structures with thick Ag contacts (1.5 μm), indicating that changes in Rc originate from changes in the SWCNT films and at the Ag-SWCNT interface caused by ion penetration through the Ag contact electrodes. Rc increases by ∼4× for the 60 nm SWCNT structure and increases by ∼2.4× for the 10 nm SWCNT structure. Raman spectroscopy measurements of the SWCNTs under the contacts compared to the starting SWCNT film show that the degradation of the 10 nm SWCNT structure was less significant than that of the 60 nm SWCNT structure, suggesting that the smaller change in Rc for the 10 nm SWCNT structure is a result of the thickness-dependent damage profile in the SWCNTs. Despite the increase in overall contact resistance, further TLM analysis reveals that the specific contact resistance actually decreases by ∼3.5-4× for both test samples, suggesting an enhancement of the electrical properties at the Ag-SWCNT interface. Irradiation simulations provide a physical description of the underlying mechanism, revealing that Ag atoms are forward-scattered into the SWCNTs, creating an Ag/C interfacial layer several nanometers in depth. The collective results indicate competing effects of improvement of the Ag-SWCNT interface versus degradation of the bulk SWCNT films, which has implications for scaled high-performance devices employing thinner SWCNT films.

Tensile strength of thin film silver-carbon nanotube metal matrix composites for IMM solar cell electrode applications

IMM solar cell materials often suffer from cracking during a traditional life cycle that includes thermal cycling, shock and vibration, as well as other mechanical stressors. These cracks can limit or destroy the effectiveness of the cell by breaking electrical connections to certain areas of the cell. This work investigates the mechanical properties of carbon nanotube metal matrix composite (MMC) thin films to be used as the grid fingers of the cells. The study fabricates free standing thin films to characterize the pure material properties. The study finds that the grain size of the top Ag surface decreases from 180 to 90 nm with increasing areal densities of SWCNTs from 0 to 10 μg/cm2, respectively The samples are characterized by tensile testing, which shows that as-deposited samples progressively decrease in strength with increasing SWCNT loading. Annealing the samples results in a reduction of tensile strength for all samples. However, the reduction is less significant when low areal density films are incorporated into the composite. This leads to a 10% increase in strength compared to the control sample containing only annealed Ag (no SWCNTs). The volume percentage of low areal density thin film structures was increased by thinning Ag layers and increasing the number of SWCNT layers. The results show that increasing the volume percentage of SWCNTs in the composite leads to improvements in mechanical strength for annealed samples.

Cyclopentadienyliron dicarbonyl dimer carbon nanotube synthesis

Well-aligned multiwalled carbon nanotubes (MWCNTs) were synthesized from a cyclopentadienyliron dicarbonyl dimer precursor using chemical vapor deposition and were systematically characterized over a variety of growth conditions. The injection volume of the precursor was found to affect both the MWCNT diameter distribution and the amount of residual iron catalyst found in the sample. Low injection volumes produced relatively low impurity samples. Synthesized materials contained as little as 2.47% catalyst impurity by weight and were grown without predeposition of catalyst materials onto the substrate, reducing the need for damaging purification processes necessary to remove the substrate. Scanning electron microscopy was used to investigate catalyst contamination, synthesized MWCNT diameters, and growth morphology. Additionally, transmission electron microscopy was employed to qualitatively examine nanotube wall formation and sidewall defects. Longer growth times resulted in a higher quality product. Rama...