The chromogenic mechanism of nanometer thin films
If any material can be obtained by means of vapor deposition, the film will be transparent during the initial deposition phase, i.e. the transition time from island to membrane.
The color mechanism of nanofilms mainly depends on two optical constants: n value and k value. When the deposited film reaches a certain thickness and remains unchanged, the larger the k value is, the less light energy is reflected back from the second interface of the film, and the less obvious the interference phenomenon formed between the two interfaces. What the film presents is the color of the material at the time of intrinsic absorption. When k value is smaller, the reflected light energy at the second interface of the film increases, and the interference phenomenon formed by the two interfaces becomes more obvious. What the film presents is the interference color of the two interfaces of the film.
Specific description: when the first period coherence condition within 380-760 nm wavelength is satisfied, the color of the film on the transparent substrate is reflection, transmission and absorption. On the opaque substrate, the color of the film is reflection and absorption; When with the increase of deposition film thickness, film reflected by two interfaces of coherent waves can produce interference phenomena, and the film thickness is the first to reach the thickness of 1/4 wavelength light blue light, means that the blue first be increased through or be absorbed, so the rest of the only green and red light, within 0 to 20 nanometers initial process of film color characterization as pale yellow to yellow,Later, as the thickness increases, the film will appear yellow, blue, amaranth and other interference colors. Once the thickness of the film enters the second cycle, the film will appear seven colors due to the uneven deposition thickness. The above discussion is limited to the case where k value is zero or k value is very small. At this point, in fact, there is a color phenomenon covered by interference phenomenon, is the material's own color. When k value is small, the intrinsic absorption is very weak. When k value reaches a certain value, the intrinsic absorption increases and the interference becomes weak. Therefore, the film shows the same characteristics as the body absorption, which is the characteristic of the film showing color.
The common thin film materials are mainly nitride, oxide and carbides. The colors of these semiconductor materials are different. This is related to the band gap. With less doping, the color changes little, with more doping there are changes. Impurities will form different energy levels in the forbidden zone (depending on the type of impurities, mixing more impurities will form the local band of impurities, which is called the intermediate band, or linked with the guide band bottom, valence band top, etc.), which will affect the absorption of visible light and thus change the color.
Color table of metallic compound films for opaque substrates
Color table of metallic compound films with transparent substrate
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