Large area metal micro-/nano-groove arrays with both structural color and anisotropic wetting fabricated by one-step focused laser interference lithography

Hao Wu a, Yunlong Jiao a, Chenchu Zhang b, Chao Chen a, Liang Yang a, Jiawen Li *a, Jincheng Ni a, Yachao Zhang a, Chuanzong Li c, Yiyuan Zhang a, Shaojun Jiang a, Suwan Zhu a, Yanlei Hu a, Dong Wu *a and Jiaru Chu a
aCAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230026, China. E-mail: dongwu@ustc.edu.cn; jwl@ustc.edu.cn
bInstitute of Industry and Equipment Technology, Hefei University of Technology, Hefei 230009, China
cSchool of Instrument Science and Opto-Electronics Engineering, Hefei University of Technology, Hefei 230009, China

Received 3rd December 2018 , Accepted 12th February 2019

First published on 14th February 2019


Abstract

Artificial bioinspired surfaces are attracting increasing attention because of their fascinating characteristics, such as the structural color of a butterfly wing and the anisotropic wetting of a rice leaf. However, realization of the multicolor biomimetic metal surfaces with controlled anisotropy by using a simple, inexpensive and efficient method remains a challenge. Herein, we propose a focused laser interference lithography processing method, which has sufficient energy density and high processing efficiency to directly fabricate the groove structures on the metal surface. The surface is multicolor due to the diffraction grating effect of the regular groove structures, and exhibits anisotropic wetting due to its single-direction morphology. The influence of the observation angle on the diversity of colors and the anisotropic wetting under different heights and periods of grooves have been quantitatively investigated. A variety of patterns (e.g., leaf, crab, windmill, letter and so on) can be processed on various metals (e.g., stainless steel, Ti, Ni, Cu, Fe, Zn and so on) by this focused laser interference lithography because of its excellent flexibility and wide range of suitable materials. This multi-functional metal surface has broad applications in identification code, decorative beautification, anti-counterfeiting, information storage, bionic application design and so on.


1. Introduction

Bioinspired surfaces have continuously attracted enormous interest due to their unusual physical and chemical properties, which can be utilized for biological media,1 drag reduction,2 microfluidic devices,3 decorative beautification,4 solar cells,5 and so on. Among the above, two typical properties of bioinspired surfaces are structural color and anisotropic wetting. The structural color is inspired by insect wings or bird feathers, and the anisotropic wetting is inspired by rice leaf, trionum flower, duck feather, or shark skin.6–12 These two fascinating functions are all derived from the surface micro/nanostructures of biological species. In order to mimic biological species, a variety of micro/nano-processing methods are used to fabricate micro/nanostructures to realize bioinspired surfaces. For example, Wang et al. used a femtosecond laser to print nanovoids on metal films, which can reproduce different pure colors based on plasmonic coloration.13 Xiao et al. reported a bioinspired structural color film fabricated by self-assembly of synthetic melanin nanoparticles to reproduce colors of bird feathers.14 Vorobyev et al. used femtosecond laser surface structuring techniques to induce periodic surface structures on metals (LIPSS), which are colorful due to the grating effect.15 By multiple exposure of two beam laser interference with angle variation and period modulation, Abid et al. realized three levels of biomimetic hierarchical structures with structural color and superhydrophobicity on a polymer.16 Yong et al. realized an anisotropic wetting surface by using a line-by-line femtosecond laser scanning to process microgroove arrays on polydimethylsiloxane (PDMS).17 Cheng et al. prepared arrays of micropillars on the shape memory polymer via replica-molding method to mimic the structures on the rice leaf.18 Liu et al. adopted femtosecond laser single-point scanning to realize large grooves (∼50 μm) with anisotropic wetting on a copper surface.19 However, these fabrication processes are complex multistep or one-step with low efficiency (single point scanning) processes. Some of the fabricated microstructures are irregular due to their self-induced formation mechanism.15 Therefore, it still remains a significant challenge to simultaneously realize multifunctional surfaces with structural color and anisotropic wetting on hard materials, such as metal which has broader application rather than a polymer, with the use of a simple, inexpensive and efficient method. Laser interference lithography (LIL) has been used for decades and constantly improved as one of the most powerful yet relatively inexpensive methods for creating large-area patterns that range from sub-micron to micron scales.16,20,21 By adjusting the amount of laser interference beams, light intensity, phase of the laser, incident angle, polarization, etc., 1D, 2D, and even 3D periodic structures can be obtained. Nevertheless, the processing materials of the LIL in the existing reports are almost all based on soft materials such as polymers.

Herein, we proposed a focused laser interference lithography processing method, which has sufficient energy density to directly fabricate the groove structures on various metal surfaces. This multifunctional bioinspired surface has both dazzling structural colors and anisotropic wetting function. We quantitatively investigated the influence of the observation angle on the diversity of structural colors on the center of the processed metal surface when an incident white light angle was fixed. We also systemically investigated anisotropic wetting behavior on groove structures with different periods and heights. It was found that the anisotropic wetting is strongly dependent on the height and weakly dependent on the period when the period was about the magnitude of several micrometers. Based on the investigated fabrication parameters, we can realize multicolor biomimetic metal surfaces with controlled anisotropy.

2. Results and discussion

2.1. Construction of focused laser interference lithography system and analysis of groove formation mechanism

Fig. 1a illustrates the experimental setup for different grooves on the mirror polished 316L stainless steel surface by focused two-beam laser interference lithography. After laser beam expansion, the output laser spot was shaped into a 1 × 1 cm2 square by square aperture. Then, the laser was split into two beams which had the same energy. In order to get enough energy density to process the stainless steel surface, two beams were converged into the 1 × 1 mm2 squares by two convex lenses. By tightly adjusting the optical path length, the beams were overlapped on the sample both temporally and spatially to form a periodic light intensity pattern by the interference. As the light intensity distribution simulation (Fig. 1b) shows, the periodic light intensity pattern is composed of stripes with the light intensity changing continuously from 0 I0 to 4 I0 (I0 is defined as the light intensity of each beam). The stripes period, ds = λ/(2sin[thin space (1/6-em)]θ), is determined by the incident angle θ and the laser wavelength λ. Given θ = 7.04°, the theoretical stripe period ds is 1.45 μm. Judging from the top view SEM image (Fig. 1c), the period dg of the grooves on the processed sample is about 1.46 ± 0.01 μm, which is consistent with the theoretical stripes period ds. Due to the strict uniformity of the light intensity distribution caused by the interference, the grooves generated in the single exposure region are more uniform and continuous than that by LIPSS.15,23 As shown in the AFM image (Fig. 1d), the height of the grooves is sub-200 nm and the cross section has a sinusoidal shape. Fig. 1e schematically shows the formation process of the grooves on the stainless steel surface. During laser irradiation, the laser-induced plasma was expanded to produce the shock wave, high temperature and high pressure at the interface.22 The stainless steel surface was melted, and the higher the light intensity the deeper the melting occurred. Under the synergetic effect of shock wave and debris deposition, the grooves with sinusoidal cross sections were formed.
image file: c8nr09747j-f1.tif
Fig. 1 Experimental setup and the formation mechanism of controllable grooves on the stainless steel surface. (a) Schematic of focused two-beam interference fabrication using nanosecond laser. (b) Simulation diagram of light intensity distribution (20 × 20 μm2). (c) SEM image of grooves on the stainless steel surface. (d) AFM image of grooves on the stainless steel surface (20 × 20 μm2). (e) Mechanism of laser shock wave and debris deposition process, which causes the formation of grooves.

2.2. Regulation of groove characteristics

The characteristics (e.g., period, height) of the grooves on the stainless steel surface can be regulated by changing the processing parameters. Four kinds of period 0.62 μm, 0.92 μm, 1.16 μm and 1.45 μm were obtained under θ = 16.67°, 11.10°, 8.83° and 7.04°, respectively, which is consistent with the theoretical calculations. The height of the grooves, as one of the most important parameters of grooves, was studied in detail. Fig. 2d shows the relationship between groove height and laser fluence with the 100 laser shots and the groove period of 1.45 μm. The laser fluence was 0.215 J cm−2, 0.335 J cm−2 and 0.404 J cm−2 to form the groove heights of 29.96 nm (Fig. 2a), 74.64 nm (Fig. 2b) and 128.20 nm (Fig. 2c), respectively. The larger the laser fluence the higher the groove is. Laser fluence that is too small will lead to irregular grooves (Fig. 2a). If the laser fluence increases, the thickness of the molten metal will be greater and more debris will be deposited so that the groove height increases. Fig. 2 h shows the relationship between groove height and number of laser shots with the laser fluence of 0.404 J cm−2, and the groove period of 1.45 μm. The groove heights 41.26 nm (Fig. 2e), 80.20 nm (Fig. 2f) and 193.39 nm (Fig. 2g) were obtained under the 30, 70 and 130 laser shots, respectively. It is obvious that the effect of smaller number of laser shots on the irregular groove morphology (Fig. 2e) is similar that of the insufficient laser fluence. Too large laser fluence (>0.45 J cm−2) or number of laser shots (>140) will lead to poor groove morphology, while the groove height will stay constant (∼200 nm). Hence, by adjusting the incident angle θ, the laser fluence and the number of laser shots, controllable grooves with different periods and heights can be fabricated on the stainless steel surface.
image file: c8nr09747j-f2.tif
Fig. 2 The relationship between groove height and laser fluence or number of laser shots. (a–c) AFM images and height curves of grooves when laser fluence is 0.215 J cm−2, 0.335 J cm−2, 0.404 J cm−2, respectively. (d) The curve of the relationship between groove height and laser fluence. (e–g) AFM images and height curves of grooves when number of laser shots is 30, 70, 130, respectively. (h) The curve of the relationship between groove height and number of laser shots.

Although the sample with micro/nano grooves was obtained, the area of single processing was only 1 × 1 mm2, which seems small for some particular applications. A two-dimensional translation precision stage was used to move the sample to splice the single processing area into multiple large patterns. Larger surface areas (e.g., 6.5 × 7 cm2) can be obtained through the combination of the control of the shutter and the movement of the two-dimensional XY translation stage. Due to the high fabrication efficiency of this interference method, a fully filled 2 × 2 cm2 square can be realized in 400 seconds, which is much faster than the single point femtosecond laser scanning methods (such as LIPSS, 1000 seconds).23 However, the pattern edges are usually jagged as the minimum exposed area is 1 mm2. To get smooth edges, a mask matching the pattern can be used. In this way, large-area patterns (∼50 cm2) with high quality were processed.

2.3. Influence of observation angle on the diversity of structural colors

The grooves have similar optical properties to diffraction gratings due to their periodic structures. When white light irradiates the grooves, the incident light is divided into spectra with different wavelengths. The spectra are reflected differently due to the diffraction angle, which is dependent on the wavelength. This is why different structural colors can be observed at different observation angles when the sample is irradiated by white light. Fig. 3a shows schematic of measuring the optical properties of micro/nano grooves on the stainless steel surface. The sample was vertically irradiated by white light from a LED lamp. A digital optical camera that could move in the plane vertical to the groove direction was used to capture the reflected light at different angles. The angle between the digital optical camera and the Z-axis is defined as the observation angle α. In order to facilitate the study of the relationship between structural color and observation angle α, groove structural model is simplified and grooves are regarded as rectangular reflection grating. Based on the model, a diffraction equation is deduced from the theoretical analysis to reveal the relationship between structural color and observation angle α:24
 
w = d(sin[thin space (1/6-em)]α + sin[thin space (1/6-em)]β[thin space (1/6-em)]sin[thin space (1/6-em)]γ),(1)
where the integer n is the order of diffraction. λw is the wavelength of white light ranging from 400 nm to 700 nm, which almost covers visible color spectrum. d is the period of the grooves (1450 nm). β is the angle between the white light and the Z-axis. γ is the angle between the groove direction and the X-axis. In the experimental setup, both angle β and angle γ are designed to be 0 to reduce the difficulty of measurement and calculation. Through calculation, when the angle α changes from 0 to 90°, only the first-order diffraction, second-order diffraction and partial third-order diffraction of the grating can be selected. To simplify the analysis, we ignored partial third-order diffraction. Fig. 3b shows the curves of theoretical calculation of the wavelength of observed light at different α angles and the corresponding color, when n = 1, 2, respectively. The experimental results agree well with the theoretical calculation curves. Fig. 3c–i and j–p show seven representative colors of visible light and corresponding α angles, when n = 1, 2, respectively. Due to the large size of the sample (6.5 × 7 cm2), when the position of the optical camera is fixed, the observation angles of the edge region and the central region of the sample are different, and this leads to a variety of structural colors that can be seen at a certain α angle. Therefore, the central region of the sample is chosen as the object for studying the structural color in the experiment. For example, in Fig. 3f when the observation angle α is 45.0°, the structural color at the center of the sample is green. According to the curves in Fig. 3b, the wavelength of light detected by the optical camera should be 512.7 nm, which means the color is green at this observation angle. The actual observed structural color is consistent with the theoretical calculation. Further, comparing Fig. 3c–i with Fig. 3j–p, it can be clearly observed that the structural color in Fig. 3j–p is brighter, which is caused by the higher diffraction efficiency of the first-order than that of the second-order diffraction. In addition, it should be noted that the existence of the observation angle causes the rectangular sample to look trapezoidal in Fig. 3c–p. Fig. 3q shows other patterns (e.g., leaf, crab, windmill and so on) that can be processed by this focused laser interference lithography system due to its excellent flexibility.

image file: c8nr09747j-f3.tif
Fig. 3 Experimental setup for measuring the optical properties. (a) Schematic of the way to characterize different structural colors of the stainless steel surface in different observation angles. (b) The theoretical curves and experimental points of the relationship between sample center color and observation angle α when diffraction order n is 1, 2, respectively. (c–p) Multi-colors observed on the sample center displayed with different α angles. (q) Multi-patterns displayed with structural colors, such as leaf, crab, windmill and so on. Scale bars are 1 cm in (c–p), 5 mm in (q).

2.4. Anisotropic wetting under different heights and periods of grooves

Another macroscopic function of this metal surface caused by the microscopic grooves is the controlled anisotropic wetting. As seen in the sketch in Fig. 4a, the water droplet tends to flow parallel to the groove direction because it must overcome a much higher energy barrier to flow vertical to rather than parallel to the groove direction. Hence, when a series of different size droplets were dropped on the sample, they were all oval and elongated in parallel with the direction of the groove (Fig. 4b). θ1 and θ2 were defined as the contact angles vertical and parallel to the groove direction, respectively. Δθ = θ1θ2 was defined to characterize the degree of wetting anisotropy. By adjusting the laser fluence and number of laser shots, the height of the grooves was set to ∼200 nm, and four periods 0.62 μm, 0.92 μm, 1.16 μm and 1.45 μm was obtained under θ = 16.67°, 11.10°, 8.83° and 7.04°, respectively. The Δθ angles of these four periods are all around 6°, which means no obvious difference in anisotropic wetting behaviors. The reason may be that the changes in the groove period are horizontal, and the horizontal variations of the grooves do not obviously change the energy barrier when it is less than 2 μm, which is consistent with the results reported by Mortia et al.25 By setting the incident angle θ to 7.04°, the period of the grooves was correspondingly determined to be 1.45 μm, and different groove heights from 40 nm to 200 nm were obtained under different number of laser shots. As shown in Fig. 4c, when the groove height increases, the angle θ1 increases from 83.0 ± 1.0° to 86.9 ± 2.0°, while the angle θ2 remains ∼80°. This causes the angle Δθ to change from 3.1° to 6.1°, which means the anisotropic wetting increases as the groove height increases. Evidently, by changing the groove height, the degree of the anisotropy of the metal surface can be precisely controlled. In addition, according to XPS analysis, the number of surface hydroxyl groups decreased, which led to the hydrophobicity increase after laser ablation26,27 (Fig. S1). By 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES) modification, the sample was converted from hydrophilic to hydrophobic, and the angle θ1 and angle θ2 were measured again (Fig. 4d). After surface modification, angle θ2 increases, but still does not change significantly as the groove height increases, maintaining at ∼99°. Angle θ1 also increases and the anisotropy still exists, for example, the θ1 changes from 86.9 ± 2.0° to 113.0 ± 2.1° for 193.40 nm height and Δθ is 12°. After PFDTES modification, the metal surface is more comparable to that of natural rice leaf.28
image file: c8nr09747j-f4.tif
Fig. 4 Schematic of controlled anisotropic wetting. (a) Different energy barriers in the directions vertical and parallel to the groove direction. (b) Oval water droplets on the stainless steel surface caused by anisotropic wetting. White arrow reveals the direction of the grooves. (c–d) The strongly dependence between the anisotropy and groove height. θ1 and θ2 were defined as the contact angles vertical and parallel to the groove direction, respectively. The stainless steel surfaces in (d) were modified by the PFDTES.

2.5. Structural colors and anisotropic wetting on various metal surfaces

This focused interference lithography system can also be used to process a wide range of metal materials, not only stainless steel surfaces. Fig. 5 shows the structural colors and anisotropic wetting of five other processed metal surfaces (Ti, Ni, Cu, Fe and Zn) under the incident angle θ of 7.04°, 150 laser shots and the laser fluence of 0.235 J cm−2. As shown in Fig. 5f, starting from 0 J cm−2, increasing the laser fluence by 0.001 J cm−2 each time to expose the different areas of the sample for 50 laser shots, and then observing the damage on the metal surface with an optical microscope, we obtained the minimum laser fluence at the beginning of ablation of different metals. Among the five metals, the ablation threshold of copper is the highest (0.275 ± 0.003 J cm−2). In addition, five metal surfaces were also processed under the same irradiance (0.235 J cm−2, 150), the height of the grooves changed from 26.20 ± 3.73 nm to 63.60 ± 4.29 nm, depending on the type of the metal (Fig. 5g). The height of the grooves on the copper surface is the smallest, so the structural color is the dimmest (Fig. 5c) and the anisotropy is also minimal (Δθ = 1.6°). Comparing Fig. 5f with Fig. 5g, it is noteworthy that they correspond and the high ablation threshold means a small groove height at the same irradiance. The difference in height is probably caused by the material's properties, such as thermal conductivity, melting point, hardness, absorption of the laser and so on. It is obvious that the structural colors in Fig. 5a–e are not uniform, mainly due to the ununiformity of the laser spot and step of splicing the single processing area into large patterns during the fabrication.
image file: c8nr09747j-f5.tif
Fig. 5 Structural colors and anisotropic wetting of different processed metal surfaces. (a–e) Multi-patterns displayed with structural colors on different metals (Ti, Ni, Cu, Fe and Zn) at the same observation angle α. (f) The minimum single laser fluence of ablation threshold for different metals under 50 laser shots. (g) The groove height on different metals and the corresponding contact angles θ1 and θ2 under the incident angle θ of 7.04°, 150 laser shots and the laser fluence of 0.235 J cm−2. Scale bars are 3 mm in (a–e).

3. Conclusion

In summary, we have utilized focused nanosecond laser interference lithography to realize groove structures on various metal surfaces (e.g., stainless steel, Ti, Ni, Cu, Fe, Zn and so on), which exhibited both dazzling structural color and anisotropic wetting. The grooves had similar optical properties to diffraction gratings, so enhanced colors could be formed when the metal surface was irradiated by white light. Compared to reported methods to fabricate structural colors on metals (such as femtosecond laser induced ripples), this approach has significant advantages in terms of processing efficiency and structure uniformity. Moreover, the structural color and anisotropic wetting behaviors were quantitatively investigated, such as the influence of observation angle on the diversity of structural colors and the anisotropic wetting under different heights and periods of grooves. The results show that the anisotropic wetting mainly depends on the height when the period is several micrometers. Based on the investigated parameters, we could precisely control the anisotropy by adjusting the height of the grooves. These properties of the multifunctional metal surface allows for great application prospects in the fields of identification code, decorative beautification, anti-counterfeiting, information storage, bionic application design and so on.

4. Experimental section

4.1. Materials

Mirror polished 316L stainless steel, titanium, nickel, copper, iron and zinc with the thickness of 1 mm were purchased from New Metal Material Tech. Co., Ltd, Beijing, China. The 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES) used for surface modification was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd, China.

4.2. Focused laser interference lithography

The irradiance light source of focused laser interference lithography was a frequency-tripled, Q-switched, single-mode neodymium dope yttrium aluminum garnet nanosecond laser (Spectra-Physics) with 355 nm wavelength, 10 Hz repetition rate and 10 ns pulse width. The square aperture for shaping the output laser spot was homemade with a maximum light transmission of 1 cm2 and the response time of 1 ms.

4.3. Instrument and characterization

Scanning electron microscopy (SEM) images were obtained using a feld-emission scanning electron microscope (JSM-6700F, JEOL, Tokyo, Japan). Atomic Force Microscope (AFM) worked in contact mode to measure the heights of grooves (MFP-3D-Origin, Oxford Instruments plc, Abingdon, UK). The contact angles of 5 μL water were measured by a Contact Angle System CA100C (Shanghai Innuo precision instruments Co., Ltd, China). By measuring five drops at different locations on the same surface at ambient temperature, the average contact angles were obtained.

4.4. Quantitative measurement of structural color

The sample was irradiated vertically by white light from a customized LED lamp with the wavelength ranging from 400 nm to 700 nm. We built a rotating bracket so the digital optical camera (MV-SUA31GC-T, MindVision, Shenzhen, China) could move in the plane vertical to the groove direction and capture the reflected light at different angles. In order to avoid disturbance from stray light from the surrounding environment, the experiment was carried out in the dark.

4.5. Surface modification

After focused laser interference lithography, the sample was immersed in a 0.667% ethanol solution of PFDTES at ambient temperature for 24 hours. The modified sample was subsequently dried on a hot plate at the temperature of 60 °C for 25 minutes.

Author contributions

D. W. and J. W. L. participated in the design of this study, and they both performed the statistical analysis. H. W. and Y. L. J. conducted the experiments and prepared the manuscript. C. C. and S. W. Z. analyzed the XPS data in ESI. C. C. Z., L. Y. and J. C. N. carried out literature search, C. Z. L., Y. Y. Z. and S. J. J. carried out manuscript editing, Y. C. Z., Y. L. H. and J. R. C. performed manuscript review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (2017YFB1104303, 2018YFB1105400), the National Natural Science Foundation of China (No. 51805508, 51675503, 51875544, 61805230, 51805509, 11801126), the Fundamental Research Funds for the Central Universities (WK2090090012, WK2480000002, WK2090090021, 2192017bhzx0003), the China Postdoctoral Science Foundation (No. 2018M642534), and Youth Innovation Promotion Association CAS (2017495). We acknowledge the Experimental Center of Engineering and Material Sciences, USTC.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nr09747j

This journal is © The Royal Society of Chemistry 2019