Japanese researchers develop novel, promising alternative
The electronics industry is increasingly looking ‘beyond silicon’, to materials and architectures that can yield ever-improving efficiencies and performance. At the heart of many ongoing research efforts is the carbon nanotube field-effect transistor; a device that relies on aligned, uniform, low-defect nanotubes. However, synthesis of such nanotubes often includes the use of metal catalysts that can contaminate the lattice, or aid in the formation of defects. Writing in Carbon [DOI: 10.1016/j.carbon.2023.118309], researchers from Osaka University report on an alternative approach that replaces metals with nanodiamonds, and produces >100-μm-long aligned CNTs without impurities.
The basis of the method is gas flow-directed growth, which is also known as kite growth, within a standard chemical vapor deposition (CVD) chamber. In the kite mechanism – first proposed in 2004 – the catalyst–substrate interaction is overcome by convective currents caused by vertical temperature gradients. As these currents lift the catalyst off the substrate, the attached, and growing, CNT tip moves with it, like the tail tied to a rising kite. The horizontal laminar flow of the gas carries and aligns the growing CNT, allowing it to reach a considerable length. Once growth is completed, strong van der Waals interaction fixes the originally-floating CNT onto the substrate. This method has been widely adopted and can grow CNTs using nanoparticles of iron, cobalt, and copper as catalysts. But this new study presents the first successful use of non-metallic seeds for kite growth. The authors chose nanodiamond (ND), a material with negligible catalytic activity, but which offers excellent thermal stability at high temperatures. ND can also facilitate CNT growth on any substrate type, and the CNTs grown from it are low-defect and have no metal impurities.
They started by depositing purified ND onto silicon surfaces before placing them into tubular CVD furnace for pre-treatment at 600 °C in air to shrink the nanodiamonds to a diameter < 4 nm. Next, the furnace temperature was steadily increased to the synthesis temperature (850 °C− 950 °C) under continuous flow of a H2/Ar gas mix. To initiate kite growth, this flow rate was increased, and a high-speed flow from an additional gas line was injected. After 30 minutes, the growth reaction was terminated, and the furnace allowed to cool. This produced many CNTs that were longer than 100 μm. While optimising their process, the team investigated the dependence of temperature and gas flow rate on the growth of these CNTs. They found that the highest growth yield was obtained at 930 °C and 455 sccm (standard cubic centimeters per minute).
In order to characterise the as-grown CNTs with a variety of tools, the team added registration marks to the substrates using photolithography and metal deposition. The samples first underwent SEM analysis, in which secondary electrons were used to investigate if there were any metal impurities present in the CNTs. By comparing their ND-deposited samples to a control sample made with an iron nanocatalyst, they confirmed that there were no metals present on the CNTs made with nanodiamond – the ND CNT tips looked ‘dark’ to the electrons, while those made with iron looked ‘bright’. AFM analysis of the same CNT tips showed that the ‘dark’ spot was actually a 5-nm-high protrusion, suggesting the presence of a nanoparticle. This, they said indicated “…that the CNT was grown from non-metallic nanoparticles using the tip-growth mode.” The authors also found that the lengths of CNTs grown from nanodiamond were in the range of 50 to 900 μm; ten to 180 times longer than previous attempts in the literature, “highlighting the significant advantages of the kite-growth mechanism in the synthesis of longer CNTs.” Structural characterization of their CNTs revealed them to be highly crystalline with relatively small diameters.
Because nanodiamond has no catalytic effect, this method produces a lower CNT yield than is currently achievable via metal-catalyzed kite growth. The authors say that “…improving the efficiency of non-metallic NPs in CNT growth is one of the important aspects of future research,” but that if that challenge is resolved, “…non-metallic NPs will be more suitable for growing aligned CNT arrays.” They conclude, “We believe that the realization of kite-grown CNTs from non-metallic NPs would assist in the practical applications of aligned CNT-based devices.”
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Yuanjia Liu, Taiki Inoue, Mengyue Wang, Michiharu Arifuku, Noriko Kiyoyanagi, Yoshihiro Kobayashi. “Gas flow–directed growth of aligned carbon nanotubes from nonmetallic seeds,” Carbon 214 (2023). DOI: 10.1016/j.carbon.2023.118309