Anisotropic materials, like carbon nanotubes (CNTs), are the perfect substitutes to overcome the limitations of conventional metamaterials; however, the successful fabrication of CNT forest metamaterial structures is still very challenging. In this study, a new method utilizing a focused ion beam (FIB) with additional secondary etching is presented, which can obtain uniform and fine patterning of CNT forest nanostructures for metamaterials and ranging in sizes from hundreds of nanometers to several micrometers. The influence of the FIB processing parameters on the morphology of the catalyst surface and the growth of the CNT forest was investigated, including the removal of redeposited material, decreasing the average surface roughness (from 0.45 to 0.15 nm), and a decrease in the thickness of the Fe catalyst. The results showed that the combination of FIB patterning and secondary etching enabled the growth of highly aligned, high-density CNT forest metamaterials. The improvement in the quality of single-walled CNTs (SWNTs), defined by the very high G/D peak ratio intensity of 10.47, demonstrated successful fine patterning of CNT forest for the first time. With a FIB patterning depth of 10 nm and a secondary etching of 0.5 nm, a minimum size of 150 nm of CNT forest metamaterials was achieved. The development of the FIB secondary etching method enabled for the first time, the fabrication of SWNT forest metamaterials for the optical and infrared regime, for future applications, e.g., in superlenses, antennas, or thermal metamaterials.
Electronic supplementary materialThe online version of this article (doi:10.1007/s40820-017-0145-5) contains supplementary material, which is available to authorized users.
In this work, shape-dependent mid-infrared properties of novel split ring resonator (SRR) metamaterials composed of single-walled carbon nanotube (CNT) forest are investigated. The introduction of the gap and dip shape to the closed ring geometry reduced the total reflectance by 15%, due to the generation of circular currents and LC resonances in SRRs. The increase of the SRR height reduced the total IR reflectance by 25%. Unique one-dimensional anisotropic electric and photonic properties of CNTs, combined with an artificial refractive index induced in SRR circuits, will stimulate the development of new optoelectronics applications.
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