Limitations of blackbody behavior of vertically aligned multi-walled carbon nanotubes arrays M. Wąsik a,n , J. Judek a , M. Zdrojek a , A.M. Witowski b a Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw, Poland b Institute of Experimental Physics, University of Warsaw, Hoża 69, 00-681 Warsaw, Poland article info Article history: Received 11 July 2014 Accepted 26 August 2014 Available online 4 September 2014 Keywords: Carbon materials Carbon nanotubes Spectroscopy Anti-reﬂection coatings abstract Reﬂection spectra for the arrays of vertically aligned multi-walled carbon nanotubes (MWCNTs) in an extremely broadband spectral range (up to half a millimeter) are presented. Using Fourier transform infrared spectroscopy it is shown that in some conditions, the material generally regarded in the literature as a promising candidate for a blackbody, dramatically losses its anti-reﬂection properties when stepping into infrared range of the electromagnetic spectrum. The experimental results conﬁrm that structural parameters of an array are the key factor responsible for the discussed properties and decide about the spectral extent of a low reﬂectance level. & 2014 Elsevier B.V. All rights reserved. 1. Introduction As far as the research on arrays of carbon nanotubes (CNTs) is concerned, a lot of attention has been recently paid to the descrip- tion on how these systems interact with electromagnetic radiation. The literature delivers both theoretical and experimental analysis of such issues like absorption and reﬂection spectra [1–3], antenna effect [4,5], or interference effects [6,7], all referring to arrays of vertically aligned CNTs. Among the frequently addressed topics is blackbody behavior of such systems [1,8]. It was experimentally shown [9] that the total reﬂectance for the vertically aligned MWCNTs ﬁlm for a wavelength of 633 nm can be as low as 0.045%, what is considered to be the lowest ever reported value for any material. Other authors (e.g. [2]) emphasize the fact of a wide spectral range of a low reﬂectance level. However, there are practically no works showing how reﬂective properties of MWCNTs arrays can signiﬁcantly change when a length of the nanotubes is not of an order of hundreds of microns but several ones instead. The ﬁlling of this gap is one the purposes of the letter. 2. Material and methods In our experiments we used commercially available (NanoLab inc.) samples of vertically aligned MWCNTs grown via chemical vapor deposition method on silicon substrates covered with 1 μm thick chromium layer. Three different samples (A, B, C) of various structural parameters were investigated during the measure- ments. The samples A and B were random arrays (no speciﬁed pattern of tubes placement) with approximate nanotubes dia- meter of 70 nm (both samples) and length 1.5 μm (A) and 5 μm (B). With the site densities equal to 10 9 /cm 2 , these parameters correspond to the ﬁlling factor of about 7%. In contrast, the sample C was the array of vertical, mutually supported MWCNTs of length of about 10 μm and diameter of about 20 nm. In this case, the site density of 10 11 /cm 2 gives rise to the high value of ﬁlling factor equal to  50%. The images of all three samples from a scanning electron microscope are presented in Fig. 1. The measurements of reﬂectance (the ratio of reﬂected and incident radiation intensities) were carried out at room tempera- ture using Bruker 113v vacuum Fourier transform spectrometer with the pyroelectric DTGS (deuterated triglycine sulfate) detec- tors. As a radiation source Globar and Hg-ARC lamp were used. For the measurements of all three samples the geometrical conﬁgura- tion of the optical paths were the same. Incidence angle was equal to about 81 and no speciﬁc polarization state of radiation was provided. 3. Results and discussion For all investigated samples the reﬂectance spectra collected for wavelengths from 1.5 μm to about 470 μm are presented in Fig. 2. In the spectral range up to 5 μm the reﬂection level does not exceed 10%, with the lowest value of about 0.4%, reached for the sample C (mutually supported tubes). Moving into the higher values of the radiation wavelength we observe the drastic increase in reﬂectance. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2014.08.141 0167-577X/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel./fax: þ48 22 234 5471. E-mail address: wasik@if.pw.edu.pl (M. Wąsik). Materials Letters 137 (2014) 85–87