Authors: D.B. Kaul, J.B. Coles, K.G. Megerian, M. Eastwood, R.O. Green, T. Pagano, P.R. Bandaru
Affilation: Jet Propulsion Laboratory, United States
Pages: 104 - 107
Keywords: optical absorbers, energy harnessing, thermal detectors, carbon nanostructures
High-Efficiency Optical Absorbers Derived from Carbon Nanostructures Anupama B. Kaul,1 (*)James B. Coles,1 Krikor G. Megerian,1 M. Eastwood,1 R. O. Green,1 T. Pagano,1 and Prabhakar R. Bandaru2 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 2Jacobs School of Engineering, University of California, San Diego, CA Novel properties often emerge in low-dimensionality materials at the nanoscale which, in many instances, can be exploited to enhance the performance of devices and components for a wide variety of electronic and optical applications. , In particular, the ability of nanomaterials to trap light effectively has important implications for their use in energy harnessing, optical blacks for radiometry, as well as detectors. For example, surface plasmon modes in 50-100 nm diameter spherical, metallic nanoparticles on amorphous Si, scatter light more effectively by coupling to incident electro-magnetic radiation, and increase the optical conversion efficiency of solar cells. In this paper, we report on another type of nanomaterial which is exceptional at trapping incoming light as a result of its unique physical structure, a structure comprised of porous arrays of thin (8-15 nm diameter) vertically oriented multi-walled carbon nanotubes (MWCNTs). By virtue of the photo-thermal transduction mechanism, such absorbers have promise in energy harnessing, high sensitivity thermal detectors, and in serving as a reference for quantifying absolute optical power in optoelectronics. Other potential applications include their use in radiative cooling, thermography, antireflection coatings, and optical baffles to reduce scattering. Here we describe MWCNT-based optical absorbers, synthesized using electric-field assisted growth and appropriate choice of buffer layer (Fig. 1), which show an ultra-low reflectance, 100X lower compared to Au-black from wavelength  ~ nm – 2.5 m (Fig. 2a). A bi-metallic catalyst layer was shown to catalyze a high site density of MWCNTs on metallic substrates and the optical properties of the absorbers were engineered by controlling the synthesis conditions using dc plasma-enhanced chemical vapor deposition (PECVD). Total hemispherical measurements revealed a reflectance of ~ 1.7 % at  ~  m, and at longer wavelengths into the infrared (IR), the specular reflectance was ~ 2.4 % at  ~  m. The MWCNT site density with a thin catalyst layer (~ 0.9 nm) was determined to be as high as ~ 4 x 1011/cm2 whereas the site density of the thick catalyst (5 nm) was lower, ~ 6 x109/cm2, which impacts the optical response of the respective samples, as shown in Fig. 2b. We then present an analytical model which was used to shed insight into the optical absorption mechanisms as a function of the physical characteristics of the MWCNT absorber arrays. The high optical absorption efficiency of the MWCNT absorbers, synthesized on metallic substrates, over a broad spectral range suggests they have promise in solar energy harnessing applications, as well as thermal detectors for radiometry.
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