Enhancement of reflectance of densified vertically aligned carbon nanotube forests

Vertically aligned carbon nanotubes (VACNTs), also known as a carbon nanotube (CNT) \nforest, are a porous material that is well known for its exceptional optical absorbance property. \nThe reflectance from a VACNT forest has been reported to be as low as 0.045% [1,2]. \nIt is known as the darkest material on Earth. Because of its remarkable material properties, \nit has various other applications as gas sensors [3], pressure sensors [4], temperature sensors \n[5], and strain sensors [6]. Recently, various efforts have been made to mechanically manipulate \nthe vertical structure of the nanotubes in the CNT forest and to conduct their optical \ncharacterization [7,8]. Optical reflection from bare VACNTs has also been investigated at \ndifferent wavelengths by Wąsik et al. [9]. Controlled densification by wetting of the CNT \nforest is another post processing technique that has been reported by other researchers [10]. \nA densification process is necessary to make the CNT forest useful as a future electronics \ninterconnect [10]. However, no study has been done so far on the optical behavior of CNT \nforests densified by a wetting process. In this letter, for the first time, we investigate and \nexplain the nature of the optical reflectance of densified VACNTs. \nFig. 1 illustrates how the CNT forest is able to absorb most incident light. It was reported \nelsewhere that VACNT arrays are highly porous [11]. As a result, when incident light enters \nthe bare CNT forest, it goes through several internal reflection-absorption cycles via individual \nnanotubes and finally makes its way out of the CNT forest as shown in Fig. 1b. Hence, \na very low amount of light bounces back (approximately 0.045%) [1,2]. \nMathematically, a simple model can be developed to estimate the final amount of light \ncoming out of a CNT forest after several internal reflections; this process is explained \nby eq (1):