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Abstract
Bioinspired structural colors are attracting increasing attention in photonics, display, labeling and so forth. High-resolution and stable coloration is significant but is challenging to be fabricated in a facile and low-cost way. Herein, multilayer architecture containing an internal nanocavity as the structural color unit is obtained conveniently by direct nanosecond laser printing in atmosphere condition. Arbitrary colorful patterns with submicron accuracy can be realized only by a single step. And such structural colors induced by inner structures in the interlayer are antipollutive, antioxidative and easy to clean.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Colour is the most significant carrier of visual information that has been intensively studied by human for thousands of years. Actually, vivid colours of many insects, birds and flora are originated from natural photonic structures [1,2]. Inspired by nature, artificial structural colours have been investigated based on different mechanisms including plasmonic resonances [3], metasurfaces [4–7], interference effect [8,9] photonic crystal [10–12], optical gratings [13], and other optical structures [14]. Considering the practical need, high-resolution and stable colour generation with low-cost fabrication process is highly desired [15].
Recently, assembly of colloidal particle 2D/3D array attracts much attention for preparing structural colour patterns and fibers [16–19]. However, the stability of such particle arrays is doubtful to a certain degree, especially when contacting with the liquid pollutant. Plasmonics resonances is also able to produce brilliant colour, however, the colour patterns usually need to be pre-designed and manufactured by expensive and time-consuming e-beam lithography (EBL) or focused ion beam (FIB). In addition, although the elegant surface nanostructures built by expensive techniques including metasurfaces, optical gratings, and plasmonics patterns are capable to generate rich colours, these surface architectures are always delicate and not easy cleaning.
In nature, butterflies are distinguished colour magicians, e.g. P. blumei and P. palinurus [10]. The wing scales of these butterflies consist of multilayer stacks made from internal alternating layers of cuticle and air, which are inherently stable and antipollution, simultaneously creating intense structural colours due to complex optical interference and scattering. Although the P. blumei wing scales have been used as a template for nanofilm deposition [20], such a strategy is very difficult to copy the internal multilayer stacks especially the internal air layer.
Recently, the laser printing has been utilized to fabricate surface plasmonics structures for colour generation [3,21] due to its advantage of programmability in energy control and pattern designing [22–24]. However, laser printing interlayer nanostructures to obtain structural colours is rarely reported so far. In this work, we develop a low-cost, facile, reproducible and programmable laser printing method that can directly build submicron internal nanocavities in a sandwich layer in a nanosecond pulse width, and thus creating structural colour patterns. Such structural colours with submicron pixel size are antipollution, antioxidative and easy cleaning.
2. Experimental setup
2.1 Pulsed laser printing of structural colours
Sn nano interlayer and SiO2 capping nanolayer were successively deposited onto the same glass substrate by radio-frequency magnetron sputter method (Kurt J. Lesker PVD75) with the Argon gas flow velocity of 25 SCCM and the deposition power of 50 W and 250W, respectively. The deposition times for Sn and SiO2 nanolayers were 270 s and 4650 s, respectively. A laser direct writing system equipped with a vibrating mirror system (HWN LDW-P1500, with a wavelength of 405 nm, objective lens of NA 0.9, laser pulse width of 200 ns, laser power from 40 mW to 10 mW, laser spot diameter of submicron) was employed to carry out the structural colour printing.
2.2 Characterization of the structural colors
The optical images of the laser-printed structural color patterns were obtained with a laser scanning confocal microscope (LSCM, Olympus, LEXT-OLS4000). Fabrication and characterization of the cross-section sample of SiO2-Sn-substrate multilayer were accomplished by a SEM/FIB two-beam system (Nova200 NanoLab). Surface topography of the laser-printed structural color was obtained with an atomic force microscope (AFM, Veeco Multimode). The reflectance spectrum was obtained by a microspectrophotometer (Nikon-Ti-U, Spectra Pro-500).
2.3 Calculation of the SiO2 capping layer deformation and reflectance spectrum
The Finite Element Method was applied to simulate the deformation process and Von Mises stress distribution of the SiO2 layer under the vapor pressure of Sn inside the internal nanocavities using ANSYS. In the three dimensional numerical model, linear elasticity was considered and a fixed constraint was applied at the edge of model. Based on Comsol Multiphysics, the reflectance spectrum of the SiO2/air/glass multilayer structure was also simulated using Finite Element Method, where the thickness of the SiO2 layer and the air layer are 100 nm and 60 nm, respectively. The axial symmetry model and periodic boundary condition were adopted.
3. Results and discussion
3.1 Programmable laser printing of high-resolution structural color
Herein, the SiO2-Sn-glass multilayer structure was used to realize the structural color by laser printing. In order to create internal nanostructures by a laser beam, it is preferable that the interlayer material has low phase-transition temperature, and thus Sn and some polymers might be suitable. Besides, in principle, Si wafer is also able to replace the glass substrate if a short-pulse laser is used in color printing. As shown in Fig. 1(a), the thickness of SiO2 capping layer and Sn interlayer on the glass substrate are 100 nm and 20 nm, respectively, constituting a sandwich structure with a glass substrate. Flexible and precise movement of a focused laser beam can be controlled by a computer, thus arbitrary patterns can be printed without a photomask in atmosphere. As shown in Fig. 1(b), after laser printing on the surface of SiO2-Sn-glass multilayer stack with laser power of 40 mW and pulse width of 200 ns, a high-precision bright dot array is reproducibly fabricated. The dot spacing is 4 µm and the dot size is about only 1 µm with a perfect circular shape (inset of Fig. 1(b)). In order to obtain the reflectance spectrum of this bright color, a color block (inset of Fig. 1(c)) is printed by laser scanning where the spatial step of adjacent two laser pulses is 100 nm. As shown in Fig. 1(c), the reflectance signal after laser printing is concentrated in the blue-green band. As described in Fig. 1(d), owe to the advantage of laser direct writing in patterning, a colourful butterfly is printed by high-resolution laser scanning, where the incident laser energy varies at different scanning positions. Although vivid structural colours have been realized by existing strategies, obtaining submicron colour pixel size in a facile and low-cost way is not easy yet. Laser printing method proposed in this work might provide a promising solution to this issue.
Fig. 1. Laser printing high-resolution structural colours. (a) Schematic diagram of laser printing. (b) High-resolution bright dot array (the inset scale bar is 4 µm). (c) Reflectance spectrum of the structural colour (the inset scale bar is 10 µm). (d) Laser printed colourful butterfly pattern.
3.2 Stability of the laser-printed structural colour
Satisfactory stability including anti-polluting and inoxidizability is also critical to push this kind of structural colour to practical applications. As shown in Fig. 2(a), colourful patterns of wolfs are fabricated by laser printing on a SiO2-Sn-glass multilayer, where the wolf image details have been clearly expressed.
Fig. 2. Anti-polluting and inoxidizability performances of the laser-printed structural color. (a) Laser-printed structural color wolf patterns with different laser energy. (b) Optical image of dropping R6 g aqueous solution onto the wolf patterns. (c) Microscopic optical image of wolf patterns covered by a drop of R6 g solution. (d) Optical image of wolf patterns covered by dry R6 g molecule pollutants. (e) Optical image of wolf patterns after washed by deionized water. (f) Oxidation operation of the structural color wolf patterns. (g) Optical image of wolf patterns after oxidation for 48 hours. Scale bars in a, d, e and g are all 50 µm.
In order to test the anti-polluting performance of the structural colour patterns, about 1 µL red Rhodamine 6 g (R6 g)’ aqueous solution with concentration of 10−3 Mol/L was dropped onto the colourful wolf patterns (Figs. 2(b) and (c)). After the evaporation of solvent, a lot of R6 g molecules remained on the surface of wolf patterns, forming the dry pollutants (Fig. 2(d)). Then, we attempted to remove these pollutants by washing the patterns covered by R6 g molecules using slow-moving deionized water. Compared Fig. 2(a) with Fig. 2(e), almost all the pollutants have been removed after such a simple washing step, demonstrating an excellent stability in liquid environment, as well as good antipollution and easy-cleaning. In order to test the inoxidizability performance of the structural colour patterns, we put the sample into a vacuum tank, and injected pure O2 at pressure of about one atmosphere into the tank. After 48 hours, any change cannot be observed in the structural colour patterns (Fig. 2(g)), compared to the sample before O2 injection (Fig. 2(e)). Therefore, the laser-printed structural colour has good inoxidizability.
3.3 Investigation of the laser-printed color-showing structure
In order to investigate the detailed structural changes in the SiO2-Sn-glass multilayer system under laser printing, further experiments were implemented. As shown in Fig. 3(a), a bright blue-green line is printed on the surface of multilayers with a pulsed laser power of 35 mW and a spatial step of 100 nm, and the line possesses satisfactory consistency. Surface morphology of the line was measured using an atomic force microscope (AFM) (Fig. 3(b)), that has slight uplift of about 25 nm in height and 1.2 µm in width. In order to figure out the internal structural change of the multilayer stack in the laser-printed area, a cross-section sample was prepared by focused ion beam (FIB) milling. As shown in Figs. 3(c) and (d), a semicircle cavity forms in the Sn interlayer with dimension of about 250 nm in width and 60 nm in height. It is worth noting that, the nanocavity has a very smooth outline (Fig. 3(d)), which can be hardly realized by existing advanced fabrication means at such a nanoscale. The ultra-narrow internal nanocavity (250 nm) guarantees the submicron pixel size of structural colour.
Fig. 3. Morphology characterization of laser-printed structural color. (a) Laser-printed bright blue straight line. (b) AFM cross-section analysis of the straight line in a. (c) SEM cross-section characterization of the straight line in a. (d) Enlarged SEM image of c.
The internal nanocavity was formed within a short period of 200 ns at a high temperature, thus the dynamic process was challenging to be directly observed under existing conditions. In the laser printing process, both the SiO2 surface layer and the glass substrate have high transmissivity for visible light (such as a laser wavelength of 405 nm we used), thus the laser energy will be mainly absorbed by the Sn interlayer and then transforms into thermal energy. Due to the low thermal conductivities of SiO2 and glass, the thermal energy in the Sn interlayer has a sharp accumulation and will stimulate the phase change of the Sn interlayer. According to the smooth outline of the nanocavity (Fig. 3(d)), we infer that the Sn interlayer gasifies and expands after absorbing the laser energy, and then the SiO2 surface layer has an upward bulge induced by the vapour pressure of Sn. In order to verify the above inference, the vapor pressure of Sn inside the nanocavity was calculated and the deformation of SiO2 surface layer was simulated. The vapour pressure of gaseous Sn at 2875 K is calculated to be 730 MPa (for detailed analysis, see Calculation S1 in Supplement 1) according to the following ideal-gas equation:
where p is the vapour pressure generated by Sn inside the nanocavity, $\Delta V$ is the volume of the nanocavity, m refers to the mass of the gaseous Sn, M is the mole mass of Sn, R is the gas constant, and T is the vaporization temperature of Sn. Then, a three dimensional circular disk model with diameter of 1.2 µm was established to simulate the deformation of SiO2 layer under the uniform pressure load of 730 MPa at the central circular area with diameter of 300 nm. Figure 4(a) shows the simulated Von Mises stress distribution on the SiO2 surface layer. As shown in Fig. 4(b), the central zone of the SiO2 layer is uplifted by 50 nm ultimately, which is close to the experimentally measured height of the nanocavity (Fig. 3(d)). After the short laser pulse, the gaseous Sn will cool down and stack on either side, and finally create a nanocavity with a smooth outline (Fig. 4(c)). The forming process of internal nanocavity can be regard as a mechanical self-limiting effect. This effect means that, in the process of laser heating nano multilayers, the morphology change of interlayers can be mechanically restricted by the capping nanolayers with different optical and thermal properties, and thus the fabricated internal nanostructure will be far smaller than the laser thermal interaction area. This mechanical self-limiting effect has great advantages in fabricating ultra-small and smooth internal nanostructures.Fig. 4. Simulation of the SiO2 layer deformation under the vapor pressure of Sn inside the nanocavity. (a) Von Mises stress distribution on the surface of SiO2 layer. (b) Cross-sectional view of the displacement distribution of SiO2 layer. (c) Schematic diagram of the resultant nanocavity after the gaseous Sn cools down.
A SiO2/vacuum-nanocavity/glass multilayer stack appears at the laser-printed area, as shown in Figs. 3(c) and (d). In our opinion, the bright structural colours mainly originate from the light interference in multilayers. We calculated the reflectance spectrum (Supplement 1, Fig. S2) of the simplified SiO2/air/glass multilayer structure by Finite Element Analysis in COMSOL Multiphysics, and the simulated result is qualitatively consistent with the experimentally measured counterpart (Fig. 1(c)). The laser-printed high-resolution and stable structural colour is based on the appropriate SiO2-nanocavity-glass multilayer stack. On the one hand, the ultra-narrow internal nanocavity (< 250 nm) guarantees the submicron pixel size of structural colour. On the other hand, the 100 nm thick SiO2 capping layer not only has a contribution to creating colours but also can be regarded as a natural protective layer. Therefore, our structural colours induced by inner structures have a huge merit in anti-polluting and inoxidizability, compared with the colours induced by surface structures. In addition, according to our preliminary research, the structural color can be controlled by using different laser energy (Supplement 1, Fig. S3), and further detailed research will be performed in the following work.
4. Conclusions
In this work, we present a facile laser printing method to fabricate color-showing structures with an ultra-small internal nanocavity in a sandwich multilayer system. Submicron pixel size of colour and complex vivid patterns can be printed by a focused laser beam. Due to the protective function of the SiO2 surface layer, the laser-printed structural colours possess great merits in anti-polluting and inoxidizability. Theoretical analysis indicates that the formation of the nanocavity structure with smooth outline can be attributed to the vaporization of Sn interlayer induced by laser energy with a mechanical self-limiting effect. And the structural colour mainly originates from the light interference in the SiO2-nanocavity-glass multilayer stack. In principle, this laser printing method are also capable to fabricate internal nanocavity in other multilayer materials, which has potentials to optimize the performance of structural colours. This laser printing method is programmable, low-cost, high-precision and reproducible, indicating a promising route for practical use of structural colours.
Funding
National Key Research and Development Program of China (2016YFA0200403); National Natural Science Foundation of China (11874232, 51971070); Key Technology Research and Development Program of Shandong (2018GGX101008); The Research Start-up Fund of QUST (1203043003591).
Disclosures
The authors declare no conflicts of interest.
Supplemental document
See Supplement 1 for supporting content.
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