Electrotwisting the helicity of carbon nanotubes

Negatively charging the catalysts can twist the chirality of carbon nanotubes from metallic to semiconducting. Based on this discovery, 99.9%-pure semiconducting carbon nanotube array is obtained.

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The paper in Nature Catalysis is here.

Single-walled carbon nanotubes (SWCNTs) are expected as the most promising material to construct high-performance chip and flexible devices in the future for their nanostructures and extraordinary electronic properties. A prerequisite of practical applications of SWCNTs in electronics is directly synthesizing pure (99.9999%) s-CNT arrays with high quality. Conventionally, there are mainly two strategies of selective growth of s-CNTs. One is using the difference in interfacial energy between the SWCNTs and the catalyst nanoparticles to obtain enriched s-CNTs with specific chirality. At the growth temperature above 1200 K, the thermal fluctuation of which significantly increases the degree of disorder in chirality for the structural similarity of SWCNTs. As a result, the purity of directly-synthesized s-CNTs is hard to be higher than 99%. Another strategy is using the difference in chemical reactivity of SWCNTs to selectively etch m-CNTs or suppress m-CNTs growth. Although ex-situ experiments have proven that the etching selectivity of m-CNTs (1-1.8 nm) is close to 100% (Science 2006, 314, 974-977), etchant-assisted growth at high temperature only deliveries s-CNTs with a purity of about 97%.

In a word, the natural energy difference between m-CNTs and s-CNTs is too small to provide a very high selectivity in growth condition, which has been the essential obstacle to direct synthesis of highly-pure s-CNTs. Is there any method that can amplify the energy difference between m-CNTs and s-CNTs? To answer this question, further study of growth mechanism is necessary. Just like the well-known saying “More is different” in physics, we found that hot is different and new physics will emerge in CVD growth of SWCNTs. Since 2014, we have noticed an unexpected phenomenon that the growing CNTs, both metallic and semiconducting, are negatively charged when hydrocarbon gas and Fe catalyst are used (Nano Lett. 2016, 16, 4102-4109). Theoretical analysis shows that the Fermi level in s-CNTs is higher than that in m-CNTs with the diameter smaller than 2 nm. From then on, we started to study the relationship between the chirality and the Fermi level of SWCNTs during growth and tried to modulate the energy difference between the m-CNTs and s-CNTs by using the dramatic difference in the electronic density of state between m-CNTs and s-CNTs.

In the paper, we show that the free energy difference between the growth of m-CNTs and s-CNTs is amplified by (heavily) negatively charging the catalyst. As a result, m-CNTs tend to be electrotwisted to s-CNTs with an energy barrier through the formation of a chiral junction. To selectively grow s-CNTs, carefully designed electric field pulse is used to negatively charge the catalyst and to introduce a perturbation to overcome aforementioned energy barrier. Based on this method, high quality s-CNT arrays are obtained stably with the purity above 99.9%. Meanwhile, m/s-CNT junctions are mass produced in alignment. Calculation shows that direct synthesis of s-CNTs with a purity of 99.9999% can be achieved if the diameter of CNTs is further confined to about 1.3 nm.

This study not only realizes the growth of highly-pure s-CNT arrays that paves the way for the industrial application of CNT electronics, but also provides a new way of thinking about selective synthesis of other nanomaterials that promotes the development of materials science. 

The paper in Nature Catalysis is here.

Jiangtao Wang

Mr., Tsinghua University