Colors of Single-Wall Carbon Nanotubes

Abstract

Although single-wall carbon nanotubes (SWCNTs) exhibit various colors in suspension, directly synthesized SWCNT films usually appear black. Recently, a unique one-step method for directly fabricating green and brown films has been developed. Such remarkable progress, however, has brought up several new questions. The coloration mechanism, potentially achievable colors, and color controllability of SWCNTs are unknown. Here, a quantitative model is reported that can predict the specific colors of SWCNT films and unambiguously identify the coloration mechanism. Using this model, colors of 466 different SWCNT species are calculated, which reveals a broad spectrum of potentially achievable colors of SWCNTs. The calculated colors are in excellent agreement with existing experimental data. Furthermore, the theory predicts the existence of many brilliantly colored SWCNT films, which are experimentally expected. This study shows that SWCNTs as a form of pure carbon, can display a full spectrum of vivid colors, which is expected to complement the general understanding of carbon materials.

Keywords: carbon nanotube thin films; color range; nanomaterials; pigments; quantitative coloration model.

Figure 1

Relationship between collection time and color of film. a) Calibrated photographic image of as‐synthesized SWCNT films with six different thicknesses indicated by the duration of collection. The colors range from light gray to obviously green to nearly black. b) Transmittance spectra of the same SWCNT films. Right circles: colors calculated from each corresponding spectrum. c) Top view of the perceptually uniform lightness–chroma–hue space (L*C*h). Color coordinates measured from the calibrated photographs (+) and calculated from the OAS (•). Error bars indicate uncertainty ΔE ≈ 5 for measuring colors in the calibrated photograph. In the top view where the radial (circumferential) direction indicates chroma (hue), the six colors lie on the same line pointing toward the green–yellow hue, which indicates that varying the film thickness does not change the hue. d) Side view of the L*C*h space. The maximum chroma (most vivid color) is obtained at a moderate absorbance, whereas films that are either too thin or too thick are less visually colorful.

Figure 2

Verification of the coloration model by SWCNT film results. a) Simulated OAS of (6,5) SWCNT film and measured OAS of solution‐separated (6,5)‐SWCNT‐enriched film formed by vacuum filtration. Inset: photograph of the film on filter paper and colors calculated from the experimental and simulated OAS. b) Experimental and simulated OAS of FC‐CVD SWCNT dry film with many (n,m) species. Inset: photograph of the green film (identical to the 60 min sample of Figure 1a), calculated color from the experimental OAS, and color calculated by adding up the simulated OAS generated from optical transitions. c) (n,m) composition for simulating the OAS of (a), which is purely (6,5) SWCNTs. d) (n,m) composition for simulating the OAS of (b), which is obtained by electron diffraction in a transmission electron microscope.^{[ 15 ]} In the legend, M and S represent the metal and semiconductor species, respectively, and the graded color represents the count of each (n,m) in the sample statistics.

Figure 3

Relationship between the colors and (n,m) of SWCNTs. a) Map of the most vivid colors of 466 (n,m) CNT species. The smaller‐diameter species tend to have higher chroma (more vivid colors). b) The calculated colors plotted on the CIE chromaticity diagram compared with the sRGB gamut (color range).

Figure 4

Verification of calculated colors by experimental images of single‐chirality‐enriched SWCNT suspensions. Images are cropped and moved for convenience of comparison, but their colors are not altered. a–d) Comparison of: a) aqueous two‐phase separation by DNA; b) simple gel chromatography; purities from the original report are indicated by the percentages; c) temperature‐controlled gel chromatography; d) aqueous two‐phase extraction separated semiconducting CNT suspensions. a) Adapted with permission.^{[ 13 ]} Copyright 2016, American Chemical Society. b) Adapted with permission.^{[ 11 ]} Copyright 2011, Springer Nature. c) Adapted with permission.^{[ 14 ]} Copyright 2013, American Chemical Society. d) Adapted with permission.^{[ 12 ]} Copyright 2016, American Chemical Society. e) Comparison of 23 of 466 calculated colors correlated with experimental data. Metallic (semiconducting) (n,m) SWCNTs are indicated by the white (black) text in the color plate. f) Distribution of experimentally correlated colors arranged by diameter and chiral angle; most of them are semiconducting CNTs with diameters below 1 nm.