Enhanced Electrical Transport in Carbon Nanotube Thin Films

Inorganic Chemistry; J; Journal of the American Chemical Society · Journal of .... (11) Literature reports indicate that the inter-CNT links, which in...
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Enhanced Electrical Transport in Carbon Nanotube Thin Films through Defect Modulation Jamie E. Rossi,†,‡ Cory D. Cress,∥ Sheila M. Goodman,† Nathanael D. Cox,‡,§ Ivan Puchades,†,‡ Andrew R. Bucossi,‡,§ Andrew Merrill,†,‡ and Brian J. Landi*,†,‡ †

Department of Chemical Engineering, ‡NanoPower Research Laboratory, and §Department of Microsystems Engineering, Rochester Institute of Technology, Rochester, New York 14623, United States ∥ Electronics Science and Technology Division, United States Naval Research Laboratory, Washington, D.C. 20375, United States S Supporting Information *

ABSTRACT: The electrical properties of single-wall carbon nanotube (SWCNT) thin films were enhanced through defect introduction and subsequent thermal annealing in forming gas. The defect density in the SWCNT thin films was modulated using ion irradiation with 150 keV 11B+ over the fluence range of 1 × 1013 and 1 × 1015 ions/cm2. Following thermal annealing at 1000 °C in forming gas, partial recovery in the optical absorbance and Raman spectra is observed at all fluences studied, with 100% recovery observed in samples exposed to a fluence less than 5 × 1013 ions/cm2. By comparison, annealing yields near complete recovery of the electrical conductivity at all fluences studied (up to 1 × 1015 ions/cm2). Remarkably, radiation exposure up to a fluence of 1 × 1014 ions/cm2 followed by thermal annealing improves the electrical conductivity, exceeding the as-purified value by as much as ∼4×. These results implicate the origin of the enhanced SWCNT network conductance with the formation of transport-enhancing inter-SWCNT bridges that decrease inter-SWCNT junction resistance, thereby enhancing the overall network connectivity.

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INTRODUCTION The unique physical and electrical properties of carbon nanotubes (CNTs) make these materials attractive for use in many electronic applications.1 However, CNTs typically contain inherent defects from synthesis and purification,2 which affect the structural and electrical properties of the material and ultimately impact device performance. Historically, improvements to crystallinity, quality, and physical properties of both as-produced single-wall carbon nanotube (SWCNT) and multiwalled carbon nanotube (MWCNT) materials were achieved using thermal annealing.3−7 These reports indicate that annealing of CNTs near the graphitization temperature (∼1800 °C) removes defects (such as dangling bonds, surface functional groups, and amorphous carbon impurities, etc.) and allows for restoration of the graphitic lattice as evidenced by Raman spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM).3−7 Thermal treatments have included high vacuum annealing, rapid thermal annealing in weak vacuum, or annealing in inert ambient conditions.3−7 Utilization of these techniques leads to the fabrication of CNT-based nano-optoelectronic devices with improved device performance, such as infrared sensors8 and CNT field-effect transistors,9 among others. Thus, it is important to understand the effect of annealing on the SWCNT physical and electrical © XXXX American Chemical Society

properties as the defect density increases from subsequent postprocessing (e.g., plasma etching) or exposure to harsh radiation conditions found in space and terrestrial applications. Recently, thermal annealing has been used in combination with ion or electron irradiation to exploit and enhance the thermal and mechanical properties of CNT materials.10−13 Molecular dynamics simulations indicate that annealing of ion irradiated SWCNTs reduces the number of irradiation-induced defects through the recombination of vacancies and carbon adatoms (C-adatoms) on the SWCNT sidewall and healing of vacancies through dangling-bond saturation to form nonhexagonal rings, which may lead to cross-linking between neighboring SWCNTs or between SWCNTs and the underlying substrate.13 Experimentally, ion irradiation (H+, He+, Ne+) in SWCNTs was used to examine changes in thermal stability based on thermogravimetric analysis.10 At low ion doses, the SWCNT decomposition temperature was found to increase due to inter-SWCNT bond formation created from dangling bonds induced by the radiation damage and demonstrates an increase in both thermal and oxidation stability. At high doses, defects become highly reactive with oxygen, which explains the Received: May 13, 2016 Revised: June 23, 2016

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DOI: 10.1021/acs.jpcc.6b04881 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C observed decrease in the SWCNT thermal stability in air.10 Electron irradiation can also produce interstitial C-adatoms that promote inter-SWCNT cross-linking, which increases the mechanical strength of SWCNT ropes by enhancing the bending modulus without affecting a change in the Young’s modulus.12 In-situ TEM analysis has also been used to characterize small bundles of double-wall CNTs (DWCNTs),14 which demonstrates the increase in mechanical strength achieved through shell−shell and DWCNT−DWCNT crosslinking. More recently, ion irradiation and vacuum thermal annealing were utilized to induce inter-CNT cross-links in MWCNTs and were shown to effectively increase the thermal conductivity in bulk CNT mats.11 Literature reports indicate that the inter-CNT links, which increase phonon transport between CNTs, are stable through high temperature annealing processes. 11 Collectively, these studies reveal that ion irradiation and annealing positively impact the thermal conductivity and mechanical strength of CNTs. Thus, new studies are warranted to investigate the electrical conductivity and optical properties of SWCNT materials under similar conditions. In the current work, the defect density in SWCNT thin films is systematically increased using ion irradiation with 150 keV 11 + B . Subsequent thermal annealing in H2/Ar (5%/95% mol/ mol) forming gas at 1000 °C is used to modulate the defects and recover the SWCNT network properties. The efficacy of the thermal annealing treatment on both the physical and electrical properties of the SWCNTs is evaluated using optical absorption and Raman spectroscopy and 4-point probe resistance measurements, respectively. The findings herein demonstrate recovery in the physical properties and enhancement of the electrical properties when subjected to moderate fluences of ion irradiation and healing through thermal annealing.

thermogravimetric analysis (TA Instruments TGA Q5000; balance purge: N 2(g) 20 mL/min; sample purge: air 20 mL/min; ramp rate: 10 °C/min). Likewise, Raman spectroscopy (Jobin Yvon Horiba LabRam spectrometer, 1.96 eV laser energy) was used to evaluate the structural properties. The purified SWCNT material was subsequently dispersed in an aqueous solution of 2.0 wt % sodium dodecyl sulfate (SDS) to achieve a SWCNT concentration of 20 μg/mL via bath (1 h) and horn (5 h) ultrasonication. The dispersion was ultracentrifuged at 110527g for 3 h, and the top 80% of the supernatant was collected. Optical absorbance spectroscopy and Beer’s law analysis were used to determine the final concentration of SWCNTs in dispersion assuming an extinction coefficient of 53.3 mL/(mg cm) (at EM 11), using a previously established procedure.17 A SWCNT thin film with areal density of 4.2 μg/cm2 was fabricated by filtering the dispersion onto a mixed cellulose ester (MCE) membrane (Millipore, JVWP; pore size: 0.1 μm; o.d.: 47 mm) using a previously established procedure.18−20 From this thin film, a 7 × 7 mm sample was cut and transferred to a polished quartz substrate (GM Associates, Inc.) via membrane dissolution in acetone. The sample was purified through thermal oxidation in air at 300 °C for 2 h. The benefit of additional purification through thermal annealing in H2/Ar forming gas (5%/95% mol/mol) was examined, whereby the sample was first degassed at 200 °C for 15 min to allow for the desorption of H2O and O2. The temperature was then ramped to 1000 °C and held for 30 min. This process was iteratively repeated to optimize the annealing protocol, and the test sample was characterized using optical absorption spectroscopy and Raman spectroscopy after each 30 min thermal annealing treatment. Five additional 7 × 7 mm samples were cut from the original SWCNT thin film, were transferred to separate quartz substrates, and were purified using the optimized procedure, which included both a thermal oxidation (air, 2 h, 300 °C) and thermal annealing treatment (H2/Ar 5%/95% (mol/mol), 1 h, 1000 °C). Electrical contacts (Cr/Au, 10/200 nm) were thermally evaporated onto each of the four corners of the purified SWCNT thin films through a shadow mask. The aspurified SWCNT films were characterized via optical absorbance and Raman spectroscopies, and the resistance was evaluated using a 4-point probe measurement. The thickness of the SWCNT films was characterized using a Veeco Wyko NT1100 optical profiling system with white light interferometry. The samples were subjected to ion irradiation in vacuo (1 × 10−6 Torr) with 150 keV 11B+ using a Varian medium current ion implanter and a 10 μA fixed beam current. The sample temperature was maintained at ∼25 °C by water cooling the Cu stage during radiation exposure. The five SWCNT thinfilm samples were irradiated with fluences between 1 × 1013 and 1 × 1015 ions/cm2. Changes imparted by radiation exposure were evaluated by optical absorbance spectroscopy, Raman spectroscopy, and electrical resistance. The irradiated samples were subsequently degassed at 200 °C for 15 min in forming gas, and a second annealing treatment was performed at 1000 °C for 1 h. Repeat characterization of the structural and electrical SWCNT properties was used to assess the annealing efficacy.

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EXPERIMENTAL SECTION Single-wall carbon nanotubes (SWCNTs) were synthesized inhouse via pulsed laser vaporization employing an Alexandrite laser (755 nm) in argon carrier gas at 1150 °C. The laser pulse was rastered over the surface of a graphite (Alfa Aesar, Graphite Flake, median 7−10 μm, 99% metal basis) target doped with 3% w/w Ni (Sigma-Aldrich, 2 nm. This change in Lv also corresponds with a decrease in defect density of ∼75% after thermal annealing and is similar for all fluences examined. Irradiation of SWCNTs with low energy and low mass ions (e.g., B+, C+, etc.) produces mostly single and dual vacancies.30 The recovery in Lv (and reduced vacancy concentration) after thermal annealing F

DOI: 10.1021/acs.jpcc.6b04881 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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CONCLUSIONS SWCNT thin films were irradiated with 150 keV 11B+ with increasing fluence between 1 × 1013 and 1 × 1015 ions/cm2 to introduce varying defect densities in the samples. Changes in the optical and electrical properties were monitored using optical absorbance spectroscopy, Raman spectroscopy, and 4point probe resistance measurements. Thermal annealing was carried out at 1000 °C in H2/Ar (5%/95% mol/mol) forming gas in an effort to heal the radiation induced defects. Partial recovery was observed in the optical absorbance and Raman spectra at all fluences studied, with 100% recovery observed after low dose radiation exposure (fluence ≤5 × 1013 ions/cm2) and subsequent thermal annealing. Although the intrinsic structural properties can be partially improved after ion irradiation and thermal annealing, it has been demonstrated that near complete recovery in the extrinsic electrical properties can be achieved after high dose ion irradiation exposure up to a fluence of 1 × 1015 ions/cm2. Interestingly, an ∼4× improvement in electrical conductivity compared to the aspurified SWCNT control sample can be achieved after moderate ion irradiation exposure (i.e., 1 × 1014 ions/cm2) and thermal annealing, which is likely due to the combination of defect healing and rearrangement to form inter-SWCNT cross-linking. These results suggest that SWCNTs used in harsh radiation environments can be recovered through thermal annealing in an inert ambient environment. Likewise, the enhanced electrical transport achieved through defect modulation and thermal annealing has implications in advanced electronics, sensors, antennas, and other devices, where increased conductivity is expected to lead to improved device performance.

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from the Office of the Director of National Intelligence. C. D. Cress acknowledges funding from the Office of Naval Research 6.1 Base Funding. All statements of fact, opinion, or analysis expressed are those of the authors and do not reflect the official positions or views of the Intelligence Community or any other U.S. Government agency. Nothing in the contents should be construed as asserting or implying U.S. Government authentication of information or Intelligence Community endorsement of the author’s views.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b04881. SWCNT purity assessment (Figure S1); reproducibility in SWCNT thin-film fabrication (Figure S2); measurement of SWCNT film thickness via optical profilometry (Figure S3); characterization of ES22 optical transition (Figure S4); characterization of EM 11 optical transition (Figure S5); relative change in optical absorption properties (Figure S6); Raman analysis of the D, G, and G′ peaks (Figure S7); Raman analysis of the radial breathing mode (Figure S8); analysis of the relative D/G Raman ratio (Figure S9); spatial defect profiling (Figure S10) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel (585) 475-4726; e-mail [email protected] (B.J.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding from the U.S. Government, the Defense Threat Reduction Agency (DTRA) under Grant HDTRA-1-10-1-0122, and the Office of Naval Research under Grant N00014-15-1-2720. This project was also partially supported by a grant from the Intelligence Community Postdoctoral Research Fellowship Program through funding G

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DOI: 10.1021/acs.jpcc.6b04881 J. Phys. Chem. C XXXX, XXX, XXX−XXX