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Efficient Transfer Doping of Carbon Nanotube Forests by MoO3 Santiago Esconjauregui, Lorenzo D'Arsié, Yuzheng Guo, Junwei Yang, Hisashi Sugime, Sabina Caneva, Cinzia Cepek, and John Robertson ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b04644 • Publication Date (Web): 16 Sep 2015 Downloaded from http://pubs.acs.org on September 18, 2015
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Efficient Transfer Doping of Carbon Nanotube Forests by MoO3 Santiago Esconjauregui,*,† Lorenzo D’Arsié,† Yuzheng Guo,† Junwei Yang,† Hisashi Sugime,† Sabina Caneva,† Cinzia Cepek,‡ and John Robsertson† †
Department of Engineering, University of Cambridge, CB3 0FA Cambridge, UK
‡
Istituto Officina dei Materiali – CNR, Laboratorio TASC, I-34149 Trieste, Italy
ABSTRACT
We dope nanotube forests using evaporated MoO3 and observe the forest resistivity to decrease by two orders of magnitude, reaching values as low as ~5×10-5 Ωcm, thus approaching that of copper. Using in-situ photoemission spectroscopy, we determine the minimum necessary MoO3 thickness to dope a forest and study the underlying doping mechanism. Homogenous coating and tube compaction emerge as key factors for decreasing the forest resistivity. When all nanotubes are fully coated with MoO3 and packed, conduction channels are created both inside the nanotubes and on the outside oxide layer. This is supported by density functional theory calculations which show a shift of the Fermi energy of the nanotubes and the conversion of the oxide into a layer of metallic character. MoO3 doping removes the need for chirality control during nanotube growth and represents a step forward towards the use of forests in next-generation electronics and in power cables or conductive polymers.
KEYWORDS: carbon nanotubes, forests, MoO3, doping, resistivity, interconnects
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It has long been predicted that, owing to their extreme current-carrying capacity, carbon nanotube (CNT) forests – and more recently graphene or graphite – are the only known materials suitable to replace copper wiring in future generations of electronics.1-4 Over recent years, there have been significant advances in the development of CNT-based interconnect technology – especially for vertical interconnects.5,6 It is now possible to synthesise nanotube forests at microelectronic-compatible conditions (e.g. at temperatures below 450 °C), with fine control of nanotube diameter, length, and density (up to 1013 CNTs cm-2), and to grow nanotubes on conductive supports and at specific patterned locations.7-13 However, the design of catalysts able to control nanotube chirality and especially the preferential growth of metallic nanotubes is very challenging.14-1819 It is particularly difficult to design catalysts and process conditions that can simultaneously give ultra-high area densities, chiral selectivity, and growth on conductive support layers. Unselective growth gives rise to forests with resistivity values up to three orders of magnitude higher than that of Cu.5,20-21 Thus in an alternative approach, we design the catalyst system to meet the objective of high density and growth on metallic supports, and we dope nanotube forests to achieve the final conductivity. We subsequently observe a dramatic decrease of forest resistivity, achieving values as low as ~5×10-5 Ωcm. This value is the lowest resistivity measured to date on a nanotube bundle while keeping the highly ordered alignment. The doping is achieved by evaporation of sub-stoichiometric molybdenum trioxide (denoted herein as MoO3) on nanotube bundles and systematically evaluated by in-situ photoemission spectroscopy. We determine the minimum necessary MoO3 thickness to dope a forest, and then study the variation of resistivity over various processing conditions. By density functional theory (DFT) calculations, we observe a shift of the Fermi energy of semiconducting nanotubes as well as the conversion of the oxide into a layer of metallic character. We suggest these factors contribute to improving the electrical properties of the forests and thus remove the need for chirality control during CNT growth for application in electronics. The doping of CNTs has previously been assessed on dispersed nanotubes and on nanotube networks (bucky paper),22,23 but not yet on aligned forests. When doping an ensemble of nanotubes, the overall resistivity is lowered in two ways: (i) the free carrier concentration per nanotube is increased, and (ii) the intra-nanotube junction resistance is reduced since carriers are allowed to move more freely between metallic and semiconducting nanotubes. Various materials, other than MoO3, have been explored to improve the electrical properties of nanotubes. These range from alkali metals and halogens to acidic liquids (e.g.
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K,24 I2,25 HNO3,26 H2SO4,27 and SOCl2) and redox dopants (e.g. FeCl328 or AuCl329). The selected
chemical
dictates
the
doping
mechanism
(absorption,
substitution,
or
functionalization), the charge transfer, and thus the kind of doping (n- or p-type), and the doping stability.30 MoO3 is known to strongly and stably p-type dope dispersed CNTs, effectively lowering their resistivity. Examples include the fabrication of MoO3-CNTs bi-layer transparent electrodes, the fabrication of MoO3-nanodots decorated CNT supercapacitors,31,32 and the fabrication of CNT-wired MoO3-nanobelts for lithium storage.33 More recently, MoO3 has also been used to p-dope graphene (or graphite) and other nanomaterials, and has been envisaged as a key component in transparent electrodes, photocromic and electronic devices, gas sensors, and energy storage systems.34-40 Since MoO3 is relatively inexpensive, microelectronic-compatible, and suitable for the large-scale production of devices,41 this report represents a step forward towards the employment of CNTs in interconnects for nextgeneration electronics and in applications where copper is to be replaced, including power cables or conductive polymer fibres.
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RESULTS AND DISCUSSION
Doping nanotube forests with MoO3 allows us to dramatically decrease the forest resistivity by up to two orders of magnitude. The average measured value after doping is ~5×10-5 Ωcm. To extract the resistivity, we use an approach that includes nanotube growth, placement of a bundle of nanotubes on a flat surface, evaporation of MoO3, and bundle compaction (Figure 1a). We first grow forests with heights of either two or four millimetres. The nanotubes are a mix of single- and double-walled with diameters ranging between ~1.5 2.5 nm. Various chiralities are expected. Then, by employing ultra-fine, straight pointed precision tip tweezers, we pull out bundles of nanotubes from the forests and place them (lying horizontally) onto a SiO2 support (Figure 1b). The nanotubes lie unpacked but maintain their longitudinal alignment and exhibit large spacing between neighbouring tubes or bundles (in the order of tens of nanometres). Note that the final curly shape of the bundles (as seen in Figure 1b) is related to the extremely high aspect ratio of the nanotubes and the processing steps, other than nanotube structural defects. Raman analysis shows a D/G ratio of ~0.7, typical of double-walled CNTs (not shown here). Subsequently, we fix one of the extremes of the bundles and then evaporate nominally 3 to 5 nm of MoO3. To ensure the coating is as homogenous as possible, we vary the bundle position during the evaporation, so that the oxide reaches the nanotubes from various angles. Since the nanotubes are unpacked, the evaporated oxide reaches both outermost and innermost nanotubes. After evaporation, the fixing is removed and the nanotubes are soaked with isopropyl alcohol (IPA), causing a compaction of the nanotubes.42,43 Since the solubility of MoO3 in IPA is negligible, the oxide coating is not removed or damaged. When the IPA evaporates, the nanotubes become nearly fully compacted, as assessed by SEM analysis (250k magnification), Figure 1c. The spacing between neighbouring tubes (or bundles) is reduced to
