Color of Copper/Copper Oxide

Abstract

Stochastic inhomogeneous oxidation is an inherent characteristic of copper (Cu), often hindering color tuning and bandgap engineering of oxides. Coherent control of the interface between metal and metal oxide remains unresolved. Coherent propagation of an oxidation front in single-crystal Cu thin film is demonstrated to achieve a full-color spectrum for Cu by precisely controlling its oxide-layer thickness. Grain-boundary-free and atomically flat films prepared by atomic-sputtering epitaxy allow tailoring of the oxide layer with an abrupt interface via heat treatment with a suppressed temperature gradient. Color tuning of nearly full-color red/green/blue indices is realized by precise control of the oxide-layer thickness; the samples cover ≈50.4% of the standard red/green/blue color space. The color of copper/copper oxide is realized by the reconstruction of the quantitative yield color from the oxide "pigment" (complex dielectric functions of Cu₂ O) and light-layer interference (reflectance spectra obtained from the Fresnel equations) to produce structural color. Furthermore, laser-oxide lithography is demonstrated with micrometer-scale linewidth and depth through local phase transformation to oxides embedded in the metal, providing spacing necessary for semiconducting transport and optoelectronics functionality.

Keywords: atomic sputtering epitaxy (ASE); coherent oxidation; color control; interfaces; laser-oxide lithography; single-crystal copper thin films.

Figure 1

Improvement in copper (Cu) film crystallinity using ASE. a) Schematic diagram of ASE equipped with electrical single‐crystal Cu wiring and mechanical noise‐elimination system. b) SEM (left) and AFM (right) images of samples obtained from ASE, intermediate‐grade film using a single‐crystal Cu target, and a conventional sputtering system. c) Cross‐sectional TEM image of 50 nm SCCF (upper) after thermal treatment with an electron diffraction pattern near the interface in the inset. d) High‐resolution TEM image of marked area in (c) and a comparison of interplanar spacing profiles between Cu₂O and Cu (right). e) TEM image of a PCCF with corresponding electron diffraction patterns.

Figure 2

Photograph of color maps in SCCFs. a) Photograph of representative SCCFs. b) Color‐wheel photograph composed of the representative colors of actual samples. Samples are enumerated with hue in the clockwise direction of the wheel; lightness and saturation are demonstrated in the inward direction. c) Commission Internationale de L'Eclairage (CIE) xy chromaticity diagram with our samples (>300) and reference colors. The gray triangle represents sRGB color space of a typical computer monitor. The blue ellipse indicates range covered by the current work, as an area ratio relative to the gray triangle.

Figure 3

Reflectance spectra on Cu thin films. a) Schematic diagram of multiple reflections at optically abrupt interface of Cu₂O/SCCF for thin (left) and thick (right) Cu₂O layers. b) Reflectance spectra of SCCF annealed at 330 °C for annealing times of 0, 20, 40, and 60 s (solid lines) and simulated reflectance spectra extracted from the Fresnel equation, rationalizing the mechanism of color control of SCCF (dashed lines). c) (a1–a4) Variation of SCCF (d = 200 nm) by changing T from 250 to 340 °C for t = 60 s and (b1–b4) by changing t from 50 to 80 s at T = 350 °C with red/green/blue (RGB) digital color codes. d) Simulated reflectance spectra of RGB as a function of Cu₂O‐layer thickness marked by layer thickness for (a1–a4) (between 5 and 30 nm) and for (b1–b4) (between 50 and 70 nm). e) Photographs of colored samples (left) and color histograms (right) after oxidation of samples shown in Figure 1b, f) After further aging for 24 h at 120 °C (of the samples shown in (e)). g) Seven representative colors with oxidation temperature (T) and duration time (t). Dots and half‐solid circles indicate respective thicknesses of 200 and 380 nm.

Figure 4

Laser‐oxide lithography and line profiles. a) Photographs and RGB‐color line profiles for focally irradiated SCCFs and PCCFs at varying optical fluence (laser beam power density: 5 kW mm⁻², pulse width: 1 s, number of pulses: N). b) Focal oxidation of SCCF and PCCF samples with varying irradiation time and power density. c,d) schematic diagram and experimental result of optical patterning by laser‐oxide lithography with irradiance power. Focal oxidation (zero dimension, as a dot) can be concatenated to form lines (in one dimension) with depth control indicated by coloration, as well as rectangular patterns (two dimensions).