Optical and IR absorption of multilayer carbon nanowalls

Abstract

Composition, optical and IR absorption spectroscopy, and complex characterization of multilayer carbon nanowall (CNW) coatings are studied. CNW coatings are made using multistep chemical vapor deposition (CVD) growth by DC discharge plasma enhancement method. Each of the growth steps leads to the formation of structurally and optically similar CNW layers with the average thickness of 1 μm. Optical properties of CNW films reveal no polarization dependence. The absorption of a single CNW layer with the average thickness of 1 μm has record values of 96–99% in the wavelength range from 0.4 to 10 μm, while the mirror reflection from the CNW films decreases steadily with the films thickness. We also demonstrate that the designed CNW films are stable with respect to water and alcohol environment.

Introduction

Coatings with low reflectivity in visible and infrared spectral ranges are now being actively developed and studied. Such materials can be implemented as anti-reflection coatings in optical devices, in solar energy and other alternative energy applications, for the creation of bolometers and infrared thermographs, etc. Depending on the application purpose of such materials (coatings), they should possess certain series of important properties. For example, modern nanostructured materials used as anti-reflection coatings for solar cells and optical devices operating in transmission geometry should have low absorption coefficient values. Commonly they are built from the same material as the device itself, for example, from silicon [1] or SiO₂ [2], and exploit the principle of graded index anti-reflection coatings [3]. On the other hand, optical coatings for bolometers, solar thermal plants or thermocouples should effectively absorb radiation in a wide wavelength range regardless of the angle of incidence or the polarization of incident radiation. In other words, optical characteristics of such coatings should be close to those of a black body. From the technological point of view, the fabrication methods of the coating materials and of the device itself should be compatible, which brings about additional stringent constraints on the choice of fabricated material and technology.

The problem of the composition of a thin film coating that would effectively absorb infrared and visible radiation has emerged in recent years in the context of a search for new effective media for bolometers. Recently, carbon-based materials such as single-walled [4] and multi-walled [5] carbon nanotubes (CNTs), as well as graphene-based films [6] were proposed. Single and multilayer graphene reveals high absorption values in a wide spectral range which originate from its unique dispersion law [7]. At the same time, an increase in the number of layers leads to a sharp rise of its reflection index [8]. It has been revealed that an array of vertically oriented carbon nanotubes shows a record high absorption efficiency in a wide wavelength range [9], [10]. However, the absorption coefficient of such a structure is strongly sensitive to the polarization of incident radiation [11], which is an undesirable property for an absorber. Moreover, the CNTs’ growth technology requires the presence of a catalyst, which is almost impossible to remove afterwards without destroying the film structures. Another difficulty in the usage of CNTs is that they tend to collapse into bundles, especially in the presence of moisture, which leads to changes in their optical properties. As a good competitor to CNTs, carbon nanowalls consisting of graphene sheets reveal very promising absorption properties in the wavelength range from 400 nm up to 1000 nm and were first suggested as an efficient absorber in [12].

In this paper, we propose a modified mechanism of precise step-by-step synthesis of CNW films and demonstrate the unique optical properties of this material in a wide spectral range, from visible up to far infrared. Moreover, we show that the absorption properties of CNW films is independent of the polarization of incident radiation. We also demonstrate a high stability of the composed CNW films with respect to thermal heating and humidity. These properties are very promising for potential application of CNW as absorbers in bolometers and solar concentrators.