MXene thin films deliver brighter, more tunable structural colors and a wider color range than conventional materials, enabling scalable high-performance coatings for sensors, displays, and security features.
(Nanowerk Spotlight) Tilt a modern banknote under light and the color shifts. These effects are not created by pigments but by microscopic layers that manipulate light itself. This is structural color. It works by controlling how light reflects and interferes at surfaces engineered at the nanoscale. It is more stable than dyes, harder to counterfeit, and capable of effects that pigments cannot achieve.
Creating structural color in the lab, however, is far from simple. Most designs rely on carefully stacked layers of metal and transparent materials to control how light is absorbed and reflected. These systems depend on precise thickness and material properties to select specific wavelengths. The results often fall short. Colors may appear vivid but look dim, or they may stay bright but occupy only a narrow range of hues.
The challenge is optical balance. Metals reflect strongly but lack selective absorption. Semiconductors absorb light more precisely but scatter or dampen it in ways that reduce clarity. Both are difficult to fabricate over large areas using low-cost methods.
A study published in Advanced Materials (“Tunable Structural Colors with ‘Light‐Lossy’ MXene Paints”) introduces a different approach. The authors use a class of two-dimensional materials called MXenes to create thin-film coatings that reflect bright, tunable colors across a broad spectrum. These materials are composed of metal carbides and nitrides just a few atoms thick. Their optical properties can be tuned by adjusting their composition, structure, and surface chemistry. This allows them to absorb and reflect light in specific ways not easily achieved with conventional materials.
The authors built a three-layer coating. The top layer is a thin film of MXene that absorbs light in a controlled range. Beneath it lies a transparent spacer made of silicon dioxide. At the base is a reflective aluminum film. Light entering the structure reflects multiple times between these layers. Certain wavelengths reinforce each other while others cancel out, producing visible color through interference.
The key is how the MXene layer interacts with light. The materials used—Ti₃C₂Tx, Ti₂CTx, and Nb₂CTx—have refractive indices that closely match their extinction coefficients across the visible spectrum. The refractive index describes how much light is bent as it passes through the material.
The extinction coefficient describes how much light is absorbed. When these values are similar, the material absorbs light evenly across wavelengths, without creating unwanted distortions or color shifts. This balance results in clear, high-saturation colors that remain bright even as they change.
Structural color films with MXenes. a) Schematic of the solution-processed MXene/SiO2/Al film for generating iridescent colors. b) Digital images of the as-fabricated MXene/SiO2/Al films, showing the diverse colors with different MXenes (Sample size: 15 × 15 mm2). c) Illustration of the multilayer interference with stacked MXene flakes as the upper absorbing layer. d) SEM image of the cross-section of MXene/SiO2/Al film on a silicon substrate, showing the layer stacking. e) AFM image of the film surface, showing the smooth MXene layer with stacked MXene flakes. f) Refractive index (n) and extinction coefficient (k) spectra of different MXene films (Ti₃C₂Tx, Ti₂CTx, and Nb₂CTx) and Au as a reference. g) Calculated color variation of X/SiO2/Al multilayers with varying complex refractive indices of the absorbing layer (X), whereas t and d were fixed at 20 and 330 nm, respectively. The refractive indices of MXenes and typical absorbers are marked in the plot for comparison. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
To measure the performance of the MXene films, the team used spectroscopic ellipsometry and calculated their color output on the CIE 1931 chromaticity diagram, which represents all colors visible to the human eye. The coatings covered nearly 88 percent of the sRGB color space. This is wider than what conventional absorbers such as titanium, chromium, or titanium nitride can achieve in similar configurations. In some cases, the color range exceeded the entire sRGB domain.
Reflectance was also strong. The MXene-based coatings produced peak reflectance values near 0.9. This is unusually high for structural color systems, where strong absorption often reduces brightness. The coatings maintained this brightness even as the thickness of the silicon dioxide layer varied. Changes in this layer shifted the reflected wavelength, producing different colors without compromising intensity.
The authors also examined how changing the thickness of the MXene layer affects color performance. As the film thickened, the material absorbed more light, shifting the reflected color toward longer wavelengths. This changed the hue from yellow to red to violet. However, beyond a certain point, brightness declined.
A thickness of about 20 nanometers proved optimal, providing both strong chroma and high reflectance. At greater thicknesses, the film transmitted less light and produced deeper colors but with reduced brightness.
The team investigated how viewing angle affects the appearance of the coatings. Because interference effects depend on the optical path length, which varies with angle, structural color typically shifts with the direction of incoming light. The MXene coatings followed this pattern. At shallow angles, the colors remained stable. As the angle increased from 10 to 60 degrees, the reflected wavelength shifted.
Under both s-polarized and p-polarized light, the coatings produced distinct, predictable changes in hue. This behavior matched well with simulation results and confirmed the films’ angle-dependent tunability.
To show the practical scope of the method, the researchers created several demonstration pieces. These included a wafer-scale pattern of colorful symbols, letters written by hand using MXene ink, and large-area films deposited on flexible plastic. A chameleon-shaped pattern was made by combining all three MXene types, each producing a different part of the color spectrum. These pieces maintained their color and reflectance after 180 days in storage. No significant degradation was observed.
The coatings were produced using spray coating and ink application techniques. These are simple and compatible with low-cost, scalable processing. The underlying aluminum and silicon dioxide layers were deposited using standard methods. No lithography or nanoscale patterning was required.
This is a critical advantage over other structural color systems, which often require complex fabrication techniques to achieve comparable optical performance.
The authors note that long-term environmental stability remains a concern. Exposure to humidity and heat may degrade the optical properties of MXenes over time. However, the results suggest that continued improvement in MXene quality and composition could extend their durability and performance under a wider range of conditions.
This study demonstrates that MXenes offer a practical and versatile platform for structural color engineering. Their optical characteristics, combined with ease of processing, support vivid, angle-dependent coloration with wide tunability and high brightness. These properties make them suitable for applications in sensing, displays, anti-counterfeiting, and visual design, where color must be both precise and reliable.
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