Elsevier

Carbon

Volume 93, November 2015, Pages 574-594
Carbon

Temperature dependent separation of metallic and semiconducting carbon nanotubes using gel agarose chromatography

https://doi.org/10.1016/j.carbon.2015.05.036Get rights and content

Abstract

Post-synthesis separation of metallic (m-SWNTs) and semiconducting (s-SWNTs) single-wall carbon nanotubes (SWNTs) remains a challenging process. Gel agarose chromatography is emerging as an efficient and large scale separation technique. However, the full (100%) separation has not been achieved yet, mainly due to the lack of understanding of the underlying mechanism. Here, we study the temperature effect on the SWNTs separation via gel agarose chromatography, for four different SWNT sources. Exploiting a gel agarose micro-beads filtration technique we achieve up to 70% m-SWNTs and over 90% s-SWNTs, independent of the source material. The process is temperature dependent, with yields up to 95% for s-SWNT (HiPco) at 6 °C. Temperature affects the sodium dodecyl sulfate surfactant-micelle distribution along the SWNT sidewalls, thus determining the effectiveness of the SWNTs sorting by electronic type. The sorted SWNTs are then used to fabricate transistors with very low OFF-currents (∼10−13 A), high ON/OFF current ratio (>106) and charge carriers mobility 40 cm2 V−1 s−1.

Introduction

Carbon nanotubes (CNTs) attract much attention in view of potential applications that can exploit their transport properties [1], ranging from large area networks [2], to sensors [3] and individual SWNT devices [4], [5], [6], [7], just to cite a few.

However, these often require SWNTs of specific electronic type: either metallic (m-SWNTs) or semiconducting (s-SWNTs). To date, the vast majority of synthesized SWNTs are composed of a mixture of the two electronic species. Growth of pure (100%) m-SWNTs or s-SWNTs is still being pursued. Refs [8], [9], [10], [11], [12], [13], [14], [15], [16] reported SWNT enrichments based on electronic type [8], [9], [10], [11], [12] and specific chiralities [15], [16]. However, achieving on-demand control of SWNTs type is still far from being solved. The rise of graphene [17], [18], [19] and other two-dimensional (2d) crystals [19], [20], [21], [22], has added further pressure to realize the promise of SWNTs in electronic devices, since 2d crystals do not require a sorting process to define their electronic nature (although some 2d crystals, e.g. MoS2, show indirect band gap in bulk and direct in monolayer configuration [20], [21]), a step otherwise necessary in the case of SWNTs [23], [24], [25], [26], [27]. On the other hand, selectivity of SWNTs is not always required and, e.g., the heterogeneity of un-sorted SWNTs can be exploited in ultrafast and mode-locked lasers [28], [29], allowing wideband tunability, due to the presence of a variety of tube diameters and chiralities in a given sample [30].

Post-synthesis separation of m-SWNTs and s-SWNTs [23], [24], [25], [26], [27], which we will refer to as ms-separation, is an effective alternative and/or complementary tool to selective growth [8], [9], [10], [11]. A variety of different methods such as: ac-dielectrophoresis [23], DNA wrapping [24], selective breakdown of m-SWNTs [25], density gradient ultracentrifugation (DGU) [26], [31], [32], amine extraction [33] and gel agarose electrophoresis [27] have been exploited for such goal. These separation strategies have been reviewed in detail in Refs. [34], [35].

Although DGU is by far the most complete approach, and to date has permitted the separation by length [36], diameter [37], metallic vs. semiconducting [26], chirality [31], and handedness [38], it is time and resource consuming, and has low throughput at laboratory scale [37], [39]. On one hand it is possible to scale-up the DGU process and improve yield using large capacity centrifuges, but the costs involved are high. Routinely, DGU separations require up to 20 h [26], [37], [32] centrifugation time, which reflects on energy consumption. Other separation methods based on chemical approaches [40] involve irreversible covalent functionalization [41] with chemicals or biological compounds that not only affect the intrinsic SWNT properties [34], [42], [43], but may also have health and environmental impacts [44], [45].

Agarose gel chromatography has emerged as a promising alternative to DGU. Ref. [46] reported that agarose gel beads can be exploited for ms-separation, due to simplicity, low cost, short process time and scalability [46]. The processing time for gel agarose beads separation is ∼20 min [34] compared to ∼10–20 h [26], [37] needed for the DGU. The agarose beads filtration requires only a single step and, as in the case of DGU [26], [31], the SWNTs dispersion is in aqueous surfactant solutions. Moreover, the separation purity level (i.e. the percentage of m- and/or s-SWNTs) is now comparable (e.g. ∼90% for (6,5), (7,6), (8,6)) [47] with DGU (>95% both for m-SWNTs [31] and s-SWNTs [26], [27]). It is possible to achieve purities up to ∼99% by multiple agarose gel filtration iterations [48], but, also in this case, the process is time and resource heavy. Although the mechanism of separation by hydrogels (e.g., agarose and sephacryl) was demonstrated to be an entropy-driven process [49], it is not fully understood yet. Furthermore, there are still unexplored process parameters, such as the temperature, which is crucial in order to optimize purity.

Here, we report ms-separation of SWNTs from four different sources using gel agarose beads filtration. We investigate the effect of temperature, showing that this affects the purity of sorted SWNTs. The separation is shown to be influenced by the surfactant micelles formation around the SWNTs’ surface, which in turns is affected by temperature. We demonstrate that there is a trade-off between the purity the enriched s-SWNTs and m-SWNTs fractions for separation carried out at varying temperature. The percentage of s-SWNTs is the highest at low temperature (∼6 °C) and gradually drops as the temperature is increased (∼50 °C), whereas for m-SWNTs, the reverse is observed. To demonstrate a direct application, the enriched s-SWNTs are then used to fabricate CNT-based field effect transistors (CNT-FETs) with superior performance in terms of ON/OFF current ratio (>106) with respect to devices based on un-sorted SWNTs (<2).

Section snippets

Nanotube sources

Four types of SWNTs are used. The first is prepared by enhanced direct injection pyrolytic synthesis [50], [51] (DIPS–CNT). These have diameters in the range 0.8–3 nm, and are obtained from Nikkiso Corp. Japan. The second is arc-discharge SWNT (AD-CNT) from Nanocarblab, with diameters in the 1.2–1.4 nm range [52]. The third is CoMoCAT SWNTs, with diameter distribution between 0.7 and 0.9 nm [53], [54]. The fourth is high pressure carbon monoxide (HiPco) SWNTs [55], with 0.8–1.2 nm diameter [55].

Room temperature separation

The optical absorption data of the four different sets of SWNT before and after the separation process at RT is shown in Fig. 2. The absorption peaks are denoted as M11, eh11(S11), eh22(S22), and eh33(S33), and assigned to the first transition of m-SWNTs and the first, second and third excitonic transition of s-SWNTs, respectively [64], [65]. The spectrum for the un-sorted DIPS–SWNTs (Fig. 2a) shows broad bands for the M11, S22, and S33 peaks due to the “nominally” large diameter distribution

Conclusions

We demonstrated the effect of process temperature in the sorting of m- and s-SWNTs via column chromatography, exploiting agarose gel beads. The sorting procedure is of general validity, being almost independent of nanotubes source. The enrichment of s-SWNTs is more effective at low temperatures (6–10 °C), while m-SWNTs are sorted more effectively in the range 30–40 °C. The best performances is achieved for HiPco: ∼95% s-SWNTs at 6 °C. The temperature dependence helps understanding the sorting

Acknowledgments

We thank G. Fiori for useful discussion and M.J. Beliatis for the graphics in Fig. 12. We acknowledge funding from a Royal Society Wolfson Research Merit Award, the European Research Council Grants NANOPOTS, Hetero2D, EU Grants GENIUS, and Graphene Flagship (contract No.604391), EPSRC Grants EP/F048068/1, EP/K01711X/1, EP/K017144/1, EP/L016087/1, Newton International Fellowship, the Government of Malaysia and Universiti Kebangsaan Malaysia (GGPM-2013-073).

The authors acknowledge funding from

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    Currently at the Department of Electrical, Electronic and Systems Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia.

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