Elsevier

Synthetic Metals

Volume 288, August 2022, 117103
Synthetic Metals

Improving carbon nanotube/copper film composite electrical performances by tailoring oxygen interface through gaseous ozone treatment of carbon nanotube films

https://doi.org/10.1016/j.synthmet.2022.117103Get rights and content

Highlights

  • Beneficial impact of O-interface in carbon nanotube(CNT)/Cu films(CCF) is reported.

  • CCF O-interface tailoring was done by CNT film O3(g) exposure before Cu deposition.

  • Varying O3(g) treat times led to various amounts of interfacial sp3C-O & sp2 C=O.

  • Optimal O-interface inclusion (content and type) improved CNT-Cu wetting in CCF.

  • Optimal O-interface improved CCF electric conductivity & its temperature stability.

Abstract

We experimentally establish for the first time the influence of oxygen at carbon nanotube (CNT)-Cu interface on CNT/Cu composite electrical performances. We fabricated CNT/Cu films (CCF) with systematically varied oxygen interface characteristics through Cu electron beam deposition of CNT films functionalized by O3(g) exposure. By altering O3(g) exposure times (30–1000 s), different contents and types of interfacial O-functionalities (sp3C-O and sp2Cdouble bondO) were obtained. CCF with optimal interfacial oxygen type and content showed enhanced CNT-Cu wetting and improvement in 4-probe electrical conductivities and their temperature stability. For e.g., Cu/CNT with interface sp3C-O and sp2Cdouble bondO and O:C at% ratio ~4.6 × 10–1 showed 1.6 × and 1.8 × higher electrical conductivity at room temperature and 110 °C, respectively than composites with only sp3C-O interface and O:C at% ratio ~2.7 × 10–1. Favorable effects of interfacial oxygen in previous literature have chiefly focused on CNT/Cu mechanical performances. Systematic studies on electrical performances vs. interfacial oxygen content and type are absent. Our results fill this knowledge-gap and provide experimental evidence on benefits of optimally tailored interfacial oxygen functionalities for enhancing Cu-nanotube interactions and composite electrical performances. We believe our work will add to existing CNT/Cu interface tailoring toolkits in literature, aiding fabrication and application-development of lightweight composites with rivaling performances.

Introduction

Lighter alternatives to Cu electrical conductors with better performances are in high demand for next-generation technologies in transportation and electronics sectors [1], [2], [3]. Automobiles and aircrafts, especially in the forthcoming electric vehicles era, require lighter electrical and data cables than Cu to enhance fuel efficiencies and travel range per charge. Future smaller more powerful electronics need electrical interconnect materials of superior current- and heat-stabilities than Cu. To fulfill this rising demand for Cu-substitutes, composites of lightweight high-performance multifunctional nanocarbons like carbon nanotubes (CNTs) and Cu i.e., CNT/Cu with rivaling electrical, thermal, mechanical, heat- and current- stable performances are touted as promising materials [1], [2], [3]. Competitive CNT/Cu performances have been ascribed to nanotube participation as weight reducers, while bringing in excellent robustness, favorable electron/phonon transport behaviors as well as heat- and current- stabilities into composite properties. For instance, with high CNT vol% (up to 45 vol%), composites lighter than Cu (2/3rd as light) exhibiting electrical conductivities of the same order of magnitude as that of Cu have been achieved [4], [5], [6], [7]. In addition, CNT contribution to electron transport has been shown to reduce the typical metal-like electrical conductance-fall with temperature, leading to more temperature-stable electrical conductivities in CNT/Cu [4], [5], [8], [9]. Composites with high-temperature conductivities even surpassing that of Cu have been reported [4], [5], [8]. This temperature-stable high conductivity of CNT/Cu fits requirements for electrical conductors operating in hot environments, such as motor windings, wiring in devices/cables proximal to car and aircraft engines, high-power device interconnects, etc. Besides the demonstration of promising properties, CNT/Cu has been fabricated in various practically applicable forms including macroscale sheets, wires, as well as microscale interconnect structures [4], [5], [6], [7], [9], [10], [11].

Theoretical studies predict CNT/Cu performances to surpass Cu [12], [13], with effective medium calculations forecasting composite electrical conductivities as high as 2 × that of Cu [12]. A key requirement to reach the true potential of CNT/Cu is to enable seamless CNT-Cu interactions that facilitates beneficial participation of both nanotubes and Cu [3]. However, CNT-Cu interactions are poor by default due to inferior affinity between the two materials. Fully filled Cu d orbitals of Cu preclude chemical reactions with Cu, such as carbide formation. Further, Cu wetting of CNTs is impeded by massive surface energy differences (Cu: ~1800 mJ/m2 [14] vs. CNTs: ~30–45 mJ/cm2 [15]) as well as high Cu mobility on nanotubes [16]. Limited CNT-Cu interactions not only place restraints on realizing true performance potential of CNT/Cu but also pose fabrication issues in making composites with uniform nanotube-Cu distribution/mixing [3].

Improving CNT-Cu interactions across the interface is a key research challenge, for which interfacial tailoring strategies involving an additive at the interface have been attempted. Typically, oxygen [8], [17], [18] or a third metal like Al [19], Ni [20], Cr [21], Mo [22], Ti [23], etc. (usually, with carbide forming tendencies) [3] is chosen as the interfacial additive. Inclusion of an interfacial additive – oxygen or a metal that displays affinity for both Cu and CNTs has been shown to improve CNT/Cu performances. However, among the two types of additives, oxygen, a lighter element offers the merit of retaining density reductions arising from combining lightweight CNTs with Cu.

Some reports exist in literature that deal with interfacial oxygen inclusion either by covalent or non-covalent functionalization of nanotubes and its benefits for composite fabrication, CNT-Cu interactions and CNT/Cu performances. For improved composite fabrication, Hannula et al. [24] heat-treated (in O2(g) atmosphere) or anodized CNT films to functionalize and render film-surfaces more hydrophilic to facilitate aqueous Cu electrodeposition electrolyte wetting. Park et al.'s [18] theoretical studies suggest that interfacial oxygen can improve CNT-Cu interactions. Their density functional theory (DFT) calculations indicate stronger Cu adsorption on CNTs bearing oxygenated functionalities (with adsorption binding energies ~1.37 eV) than on pristine nanotubes (adsorption binding energy ~0.53 eV). In terms of performance enhancement with interfacial oxygen inclusion, improved hardness [8] and composite strength [17] are reported. These mechanical property enhancements are ascribed to more effective nanotube contribution to load sharing as reinforcements, which arises from stronger CNT-Cu bonding through oxygen. For electrical performances, the only report to the best of our knowledge is by Mendoza et al. [8], who fabricated composites with interfacial oxygen using non-covalently functionalized surfactant-wrapped nanotubes. The composites showed superior electrical conductivities with better temperature-stability. However, mapping CNT/Cu electrical performances and structure as a function of systematically tailored and characterized interfacial oxygen content and type is not reported in literature.

In this study we have tried to methodically establish the impact of interfacial oxygen on composite structure and electrical performances. For this, we fabricated Cu/CNT film (CCF) composites through Cu electron beam deposition of CNT films oxygen functionalized by O3(g) exposure. By altering O3(g) exposure times (30 s, 60 s, 150 s, 300 s and 1000 s), we attempted to tailor interfacial O-functionalities of different contents and types characterized by X-ray photoelectron spectroscopy (XPS). Next, CCF samples made from CNT films exposed to O3(g) for various times were analyzed for structure and electrical performances to correlate variations with interfacial O-functionalization type and content. The results of our study provide evidence for composite structural improvement in terms of CNT-Cu wetting and electrical performance enhancement with optimal interfacial oxygen inclusion.

Section snippets

Experimental

CCF samples were prepared by electron beam (EB) deposition of Cu (10 nm thick) on CNT thin films (~10 nm in thickness) obtained by spin coating SWCNT/PAA/alcohol dispersions. To introduce and tailor oxygen interface, CNT films were exposed to gaseous ozone (O3(g)) for various time intervals prior to Cu e-beam deposition. The experimental concept is summarized in Fig. 1.

CNT films before and after O3(g) exposure for various time intervals were characterized for structural and surface chemical

Effect of O3(g) exposure on CNT films

CNT films undergo surface oxygenation, which intensifies with increase in O3(g) exposure time leading to increase in surface oxygen content. This is evidenced as XPS O:C at% ratio increase with O3(g) exposure time (Fig. 2, data in Table 1). O:C at% ratios were calculated from C and O1s peaks in XPS wide-scan profiles (provided in Fig. S1, supplementary information). From an initial O:C at% ratio of 2.7 × 10−1 ± 3.2 × 10−2 for films without O3(g) exposure, the value approximately doubles after

Conclusions

In this study, we have systematically studied and established the influence of interfacial O-functionalities on CNT/Cu structure and electrical performances. We fabricated film composites (CCF) with different contents and types of interfacial O-bearing groups through Cu EB deposition of CNT films exposed to O3(g) for various time intervals (30–1000 s). CNT film characterization before and after O3(g) treatment indicate increase in oxygen content with exposure time evidenced as XPS O:C at% ratio

CRediT authorship contribution statement

Rajyashree M. Sundaram: Conceptualization, Methodology, Validation, Investigation, Writing – original draft, Writing – review & editing, Project administration. Atsuko Sekiguchi: Conceptualization, Methodology, Investigation, Project administration, Writing – review & editing. Takeo Yamada: Methodology, Project administration, Writing – review & editing. Ken Kokubo: Methodology, Project administration, Writing – review & editing. Kenji Hata: Conceptualization, Methodology, Project

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We are grateful to M. Nishimura, H. Oosako, R. Shiina and S. Nemoto for their technical support. A part of this work was conducted at the AIST Nano-Processing Facility supported by "Nanotechnology Platform Program" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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    Rajyashree M. Sundaram and Atsuko Sekiguchi contributed equally to this work.

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