Black Ag-Mo alloy films were formed spontaneously on silver (Ag) sheets/targets by composite target sputtering. The morphology, composition, phase, crystal structure and reflectivity of black Ag-Mo alloy films were characterized by SEM, EDS, XRD, TEM and UV spectrophotometer. Results show that Ag-Mo alloy films with conical particle arrays are spontaneously formed on sputtered Ag sheets, and the Ag-Mo alloy films exhibit strong light trapping effect in the visible wavelength range, resulting in a black appearance of the Ag sheets. The strong light-trapping properties can be ascribed to the conical particle arrays closely arranged on the Ag-Mo alloy films, which caused the reflectivity of the Ag sheets to reduce substantially from 90% down to 10%. The Ag-Mo alloy films prepared by magnetron sputtering methods provide a new idea for fabrication of high light trapping materials.
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Light trapping materials have important applications in the fields of solar cells, solar water heaters, stealth materials for aircraft, and telescope parts [1–4]. In addition to black paint and iron oxide black, rough surface of a material can also reduce reflectivity and thereby induce a dark effect visually. Micro-nano structures on materials’ surface usually lead to different colors, which inspire researchers to conduct color bionic research. E. Mazur  designed a concave nickel-phosphorus alloy film with butterfly microstructure to prepare a black substance. H.C. Yuan  discovered the “black silicon” with an ideal cone microstructure and excellent light trapping properties, which can effectively reduce the optical loss on the solar cell. High-efficiency single-crystal black silicon with a needle-like structure was prepared by adjusting the etching process parameters . A.Y. Vorobyev  irradiated the surface of Au with laser to prepare a corrugated periodic structure and the light absorption was substantially improved. Three kinds of typical structures were formed on the alloy’ surface by changing the laser energy density, among them, the coral-like microstructure has 90% absorptivity from infrared to ultraviolet light . Y.G. Huang  prepared a ripple-like periodic structure on the pore wall of a Ti metal by femtosecond laser to modify the surface structure. C.L. Guo  used femtosecond laser to prepare aluminum alloy samples with five colors and analyzed the relationship between microstructures and colors. Nevertheless, it is still urgent to develop new black materials with different micro-nano structures.
Ag with a reflectivity of 0.93 is one of the best candidates to reflect sunlight. The reflection properties of Ag film or Ag sheet will be changed by preparing micro-nano structure on the surface. For example, the Ag nano-cone array was prepared by PS microsphere assembly and ion etching methods , which has high refractive index sensitivity and can show different colors in solvents with different refractivity. Based on Ag nanoparticles with different shapes can produce Localized Surface Plasmon Resonance (LSPR) with different wavelengths, T. Huang prepared a colloidal solution of Ag nanoparticles which can cover the entire visible light range . In the past, most of the light-trapping materials were prepared by femtosecond laser or chemical etching.
Because Ag is a good photosensitive material, Mo is a good light-limiting and light-absorbing material, which is widely used in fields such as displays and solar cells. Therefore, in the present work, a composite target composed of Mo target and Ag sheets are adopted to prepare light trapping structure on Ag sheets by magnetron sputtering. The formation mechanism of black Ag-Mo alloy films and the relationship between the light trapping properties and the microstructures were discussed.
JCP-350 magnetron sputtering machine was used to deposit the Ag-Mo alloy films to obtain blackened Ag Sheets. A composite target was composed of Mo target (99.99%, Ø50 × 4 mm) and Ag sheet (99.9%, 10 mm × 10 mm × 1 mm). The vacuum of the chamber was 5×10−4 Pa, the sputtering gas pressure was 0.5 Pa and the sputtering power was 100 W. The film thickness is controlled by changing the sputtering time.
JSM-7800F field emission scanning electron microscope (FESEM), JSM-5610LV scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) were used to characterize the surface morphology and composition of the black Ag-Mo alloy films, respectively. The phase and crystallinity of the Ag-Mo alloy films was characterized by X-ray diffractometer (Bruker-AXS D8 Advance) and JEM-2100 high resolution transmission electron microscopy (HRTEM). UV2600 ultraviolet-visible spectrophotometer was used to test the reflectance of the black films on Ag sheet and pure Ag sheet in the wavelength range of 350-900 nm with the un-sputtered Ag sheet as baseline.
3. Results and discussion
Figures 1(a) and 1(b) show schematic diagram of Ag-Mo composite target before and after sputtering, respectively. As shown in Fig. 1(a), four silvery white Ag sheets were placed on Mo target. After sputtering for a given time, the Ag sheets became black as shown in Fig. 1(b). Furthermore, the appearance of the sputtered Ag sheets was getting darker gradually with increasing sputtering time, as shown in Fig. 1(c) and 1(d), respectively. In fact, our team has previously studied many alloy films with low solid solubility, such as Mo-Cu, Ag-Zr, Ag-Co, etc., through magnetron sputtering, but the composite target used did not appear to be such a status quo.
Figure 2 shows surface morphology of the Ag sheets covered with conical particle arrays after sputtering. As displayed in Fig. 2(a), upon sputtering for 5 min, the particles are closely arranged in a conical shape but with non-uniform size (1∼10 µm). As the sputtering time is elongated, particle size is reduced gradually to about 1 µm, and becomes denser and uniform (Fig. 2(b)), which is somewhat similar to the light trapping structure on the surface of some metals prepared by femtosecond laser [1,2], but with a different fabrication method. Previous studies on the reflection and absorption of alloy films have shown that particles/alloy films can enhance light trapping capabilities [11,12]. However, the light trapping performance of these particles is far less than the cone structure on flat silicon . A similar cone-like structure has been formed in the Ag sheet by the magnetron sputtering method, and exhibits the same excellent light-trapping performance. The conical particle arrays can absorb light and have a strong light trapping effect, resulting in black Ag sheet after sputtering. If sputtering time is further elongated, the top of conical particle arrays (1∼3 µm) may be damaged as shown in Fig. 2(c), and the Ag-Mo conical particle arrays on Ag sheet are hollow, which might cause light trapping effect. It can be seen from the EDS spectra in Fig. 2(d) that the conical particle arrays are composed of 88.2 wt% Ag and 11.8 wt% Mo. The structure of Ag-Mo alloy film on Ag sheets is similar to Ag particles/Ag-Mo films on polyimide substrates, but the shape and composition are different . The micron-scale gap between the top of Ag-Mo particles becomes an ideal light-trapping structures, which makes the light undergo multiple reflections and increases the absorption of light .
Figure 2(e) is XRD patterns of Ag-Mo alloy film on Ag sheet after sputtering. Ag (111), (200), (220), (311) and (222) peaks can be observed in the un-sputtered Ag sheet. Upon sputtering, the intensity of Ag (220) diffraction peak is significantly higher than those of others, indicating preferred Ag (220) direction of the conical particle arrays. Mo (110) diffraction peaks also can be seen on the XRD patterns, confirming that the conical particle arrays are composed of Mo and Ag. The nano-Ag films prepared by sputtering methods mostly exhibit the preferential Ag (111) direction, while the Ag-Mo alloy films prepared in this paper show the preferential Ag (220) direction. This indicates that the formation of Ag-Mo alloy film promotes the preferential growth of Ag grains along (220) crystal plane, which is significantly different from that of Ag-Mo alloy films on PI . Figures 2(f) and 2(g) are TEM images of Ag-Mo conical particles. It can be seen from Fig. 2(f) that the conical particles are hollow and polycrystalline, which is consistent with morphology of Ag-Mo alloy films. Figure 2(g) is high resolution TEM image of some grains in the sidewall of conical particles, the existence of grains along Mo (110) and Ag (111) can be identified by the crystal surface spacing, which also indicates that the alloy film is composed of Mo grains and Ag grains.
Figure 3 illustrates surface reflectivity of the Ag-Mo alloy films on Ag sheets sputtered for different time, and un-sputtered Ag sheets was used as the baseline during test. The reflectivity of pure Ag sheet is 90%, while the reflectivity of black Ag-Mo alloy film deposited on Ag sheet is much lower than that of pure Ag sheets in the visible light rang, showing a good antireflection performance. As sputtering time increases from 5 min to 20 min, reflectivity of the black films decreases from 20% down to 10%, owing to the varied particle size and morphology of the alloy film. As sputtering time is elongated, the conical particle arrays in Ag-Mo alloy films gradually become slender and the aspect ratio becomes larger. As a result, surface reflectivity significantly decreases and the Ag-Mo alloy films/Ag sheets appear black.
Ag-Mo alloy films composed of Ag-Mo conical particle arrays were formed spontaneously on Ag sheets during composite Ag-Mo target sputtering. The formation of Ag-Mo alloy film promotes preferential growth of Ag grains along (220) crystal plane. The Ag-Mo alloy films exhibit strong light trapping effect in the visible wavelength range, resulting in black appearance of Ag sheets. Compared to the pure Ag sheets with the reflectivity of 90%, the reflectivity of Ag-Mo alloy films was reduced to 10%. The strong light trapping properties can be ascribed to unique conical particle arrays and the increases of light reflections and transmissions times inside the Ag-Mo alloy films.
National Natural Science Foundation of China (Grant No. U12041869).
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are available from the corresponding author.
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