Ti₃C₂T_z MXene possesses interesting optical properties, such as a high absorption coefficient and complex refractive index profile throughout the UV, visible, and infrared ranges of light. This makes such MXenes candidates for use in optical films and coatings. One phenomenon of importance is structural coloration, or the appearance of color based on nanostructure instead of pigments. In the past, MXenes have been used in structural color-based films as a base layer only to provide a dark background, instead of being incorporated throughout the film as structural color generators. Here, we create structural coloration using MXene nanosheets themselves in the form of thin film polymer nanocomposite heterostructures. We fabricate three different sets of MXene/polyelectrolyte heterostructures using layer-by-layer (Lbl) assembly of two different cation/anion layer pair sets, or blocks. These blocks are composed of poly(diallylmethylammonium) (PDADMA)/Ti₃C₂T_z and PDADMA/polystyrene sulfonate sodium (PSS). The thickness of the PDADMA/PSS block is varied by changing the salt concentration of assembly (0.5 M NaCl or 1.0 M NaCl) or the number of layer pairs (2 or 4) within the blocks. By varying these assembly parameters, the spacing between the MXene-containing blocks changes as does the optical reflectance and structural coloration. We employ profilometry, UV-visible spectrophotometry, spectroscopic ellipsometry (SE), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and CIE colorimetry to quantify the heterostructures. The three different heterostructures have different thickness and absorbance growth profiles, compositions, and reflectance spectra. We also observe the presence of both angle-independent and angle-dependent structural coloration. These results provide more understanding of how changes in individual blocks contribute to the growth profiles and optical properties of a LbL heterostructure.

Natalie N Neal

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