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Abstract
Biomimetic hierarchical structures are an important branch among bio-inspired materials. These structures are responsible for rich physical and chemical phenomena. In this paper, we briefly reviewed facile methods to fabricate large-area biomimetic hierarchical structures via laser processing. One is based on femtosecond laser direct writing, and the other is based on two-beam interference lithography. In details, the first method is laser ablation on ZnS thin films via femtosecond direct writing combined with a phase mask. The second method is a multi-exposure lithography by two-beam interference on a photoresist with varied exposure angles to acquire structural hierarchies with periodicity. Owing to the outstanding patterning ability of the two laser-processing approaches, a diversity of large-area hierarchical structures can be obtained and show rich biomimetic functions such as superhydrophobicity.
© 2017 Optical Society of America
1. Introduction
During the fast development of the designing and fabrication of biomimetic materials, they have long been inspired by the wonderful and multifarious biological features in nature. The large number of life forms show even larger number of unique features evolved from their endless fighting with the nature, including the “lotus leaf effect”, the structural color of butterflies, fireflies luminescence, etc [1–13]. A majority of them have been found to be structure-related effects [4, 12, 14–16]. Among the structures to mimic biomaterials, hierarchical structures (2D or 3D) are a most important branch. Various properties can be derived from those hierarchical structures which usually exist on the surface of organisms, such as super-hydrophobicity and structural color [5–12]. Here the biomimetic hierarchical structures refer to hybrid structures with not only multiple scales (micro-to-nano), but also multiple periods (or quasi-periods) and dimensions in many circumstances.
In order to fabricate these bio-inspired hierarchical structures, one can consider either “top-down” or “bottom-up” approach to build structure hierarchy by hierarchy [17–20]. It should be noted that, given the periodical characteristics of the biological material surfaces to mimic, outstanding patterning ability is required for the fabrication technique to choose. In other scopes such as energy storage, hierarchical structures have been widely exploited and plenty of them have been developed mainly based on chemical synthesis (chemical vapor deposition, self-assembly, etc). In most cases, chemical synthesis that usually lead to random or dispersive products, cannot realize the periodicity arrangement in the structures on its own [17–25]. Moreover, the combination of more than one kind of fabrication techniques tends to make the whole procedure complex and time-consuming. In this strategy, laser micro-nanofabrication techniques with the great power of patterning, should be the first choice for realizing the desired biomimetic hierarchical structures through a “top-down” approach [4, 26–34].
In this paper, we make a short review on how to realize large-area biomimetic hierarchical structures with biomimetic functionalities based on our recent results. Two typical techniques in laser micro-nanofabrication have been introduced here for building up the desired structure hierarchies through a “top-down” approach, respectively. The two techniques involved are femtosecond laser direct writing and two-beam laser interference lithography. Both are able to acquire structure hierarchies and periodicities for the prepared material surfaces according to the bio-mimic design. Moreover, the two fabrication methods are governed by independent mechanisms and result in different structure scales. Based on the high efficiency and scalability such laser-processing methods, they are promising candidates for constructing large-area hierarchical structures with bio-inspired functions. By this short review, we intend not only to provide novel methodologies for biomimetic structure fabrication, but also to inspire new breakthroughs for the technique of laser processing itself.
2. Femtosecond laser induced large-area hierarchical structures
Laser-induced periodic surface structures (LIPSS) are a unique phenomenon in light-matter interaction when a single laser beam is irradiated on the surfaces of a series of materials [35–37]. Such phenomena have been found in metals, semiconductors and some transparent materials as well. And the formation of LIPSS has been well ascribed to the interference between the incident light and the scattered one [38], mostly with a period on the order of half the laser wavelength. When femtosecond lasers are concerned, the period is further reduced to less than a third of the incident light wavelength [39–46]. Therefore, femtosecond laser direct writing (FLDW) is a facile method for introducing periodic nanostructures to a variety of materials. Two key points need consideration: how to boost the low scanning rate and how to introduce the hierarchies to the periodical structures.
In order to take good advantage of FLDW in the biomimetic construction, we finally introduced a phase mask to the scanning system. The core function of the phase mask here is to tailor the light field of the incident wavefront, transferring it from a Gaussian distribution into a series of wavelet arrays. Such a lateral mask aided scanning finally leads to a direct interference ablation of the incident femtosecond lasers on the target material surface. Instead of the limited dimension of LIPSS, the assisted scanning with a phase mask actually multiplies the scanning area as well as the rate by superimposing the micrograting hierarchy region. Meanwhile, the microgratings formed during the interference ablation introduce a larger hierarchy to the whole structure and turn it into a hierarchical one spontaneously.
Experimentally, the substrates used for femtosecond laser ablation here were as-fabricated polycrystalline ZnS (General Research Institute for Nonferrous Metals, Beijing, 100088, China) with a thickness of 3 mm and a surface roughness below 100 nm. The setup of the FLDW system is shown in Fig. 1. In the figure, a schematic illustrates the setup of the femtosecond laser ablation system of wavelet interference. The laser pulses were generated from a Ti:sapphire regenerative amplifier laser system (Spectra Physics), operated at a routine wavelength of 800 nm. The pulse duration of the laser was 100 femtoseconds within a tunable duration of 1‒1000 Hz. The laser beam was first led through a cylindrical lens and then a phase mask, the two of which acted together to generate the lateral interference. In detail, the focusing lens was a cylindrical one with a focal length of f = 110 nm and provided the laser beam a compression of rectangular shape, i.e. 1 cm × (2 ‒ 10) μm.
While the phase mask was a silica zero-order-nulled one with a pitch of 3.33 μm. The mask diffracted the laser beam into + 1 and ‒1 order. In this way, the ultimate ablation beam was fine tailored into a “light-line” packed with a series of microscale fringes. These fringes directly endowed the resulting structure a microscale hierarchy with a single scan. In the ablation process, the ZnS subastrate was scanned at an optimized speed under control of a step motor to ensure a uniform large-area fabrication. For the morphology characterization, the samples were observed by FESEM ((JSM-7500F, JEOL, Japan).
Figure 2 shows a schematic, the morphology and a photograph of the resulting hierarchical structures. Figure 2(a) and 2(b) illustrate together the overall configuration of the hierarchical structures built from the wavelet interference ablation. The first hierarchy is a series of microgratings directly resulting from the wavelet interference ablation, with a period of 3.33 μm corresponding to the phase mask period as mentioned above. The second hierarchy is a series of nanogratings induced by the high-power femtosecond laser pulses. These nanogratings are located in the trenches between adjacent micro gratings and scale down to deep-subwavelength. In this way, the target hierarchical structures have been well constructed by the combinations of microgratings and nanogratings. Different from the origin of the microgratings, the formation of the nanogratings is believed a unique phenomenon of light-matter interaction in the case of femtosecond lasers with high-energy pulses. It has been argued that these deep-subwavelength-scale fringes are most probably induced by the interference of the femtosecond laser wavelets with themselves as well as with induced surface plasmon polaritons (SPPs) [40–42]. Further evidence is still needed to demonstrate these speculations [47–49]. Figure 2(c) shows the macroscopic appearance of the ZnS hierarchical structures and the featured rainbow-like colors owing to its diffraction of light. Because of the light field manipulation via the cylindrical lens together with the phase mask, the time duration for a complete scanning of a 5 mm × 5 mm region is reduced to less than 100 s.
Fig. 2 Microscopic structure configuration and macroscopic appearance of the hierarchical structures fabricated by femtosecond laser interference ablation. (a) and (b) are a schematic and an SEM image of the hierarchical structures, respectively; (c) is a photograph of a sample with two regions of hierarchical structures fabricated by the femtosecond laser interference ablation.
Since the larger period (micro-hierarchy) of the hierarchical structures has already been defined directly by the phase mask, the overall structures can be further tuned by either changing the laser power or the laser polarization to tailor the nano-hierarchy. Figure 3 shows the dependence of the duty ratio of the nanogratings on the laser pulse power. The duty ratio is defined as the width of the nanorating region relative to that of the whole hierarchical region. It turns out that the period of the nanogratings shows little change with the increase of the laser pulse power. Instead, tuning the laser power leads to a significant change to the duty ratio of the nanogratings. For example, a low laser power of 0.16 mJ/cm2 near the ablation threshold results in a duty ratio of 0.3, which is defined by the width of the nanograting region over that of the whole hierarchical region. When the laser power is raised to 0.30 mJ/cm2, the duty ratio increses to 0.7, correspondingly. Increasing the power to 1 mJ/cm2 changes the ratio to 0.9. Figure 4 shows the orientation tunability of the nanogratings under the control of laser polarization. Figure 4(a)-(c) addresses a significant polarization dependence of the nanogratings' orientation, shifting from 0° to 90° when varying the polarization of the incident beam. This interesting phenomenon is not only meaningful to structure control, but also indicates something fundamental about the origin of the nanogratings. As is argued above, it has been speculated that these LIPSS are due to induced SPPs. Here the polarization dependence of the nanogratings seems to provide a side proof for this speculation and the whole picture is undoubtedly worthy of more investigations.
Fig. 3 Dependence of the duty ratio of the nanograting area on the pulse energy over the whole hierarchical region. The repetition rate is 1000 Hz.
Fig. 4 Orientation tunability of the nanogratings relative to that of the microgratings by varying the polarization of the incident laser beam. The red arrows represent different laser field polarized at (a) perpendicular, (b) 45° tilted, and (c) parallel to the micrograting orientation. All the structures are fabricated with a laser pulse energy of 0.2 mJ/cm2 and a scanning speed of 100 μm/s. The scale bars for the three images are the same as in (a), i.e. 500 nm.
3. Two-beam laser lithography printed 3D hierarchical structures
To build hierarchical structures with biomimetic purpose, as we have discussed, “top-down” methods such as laser-based ones are highly efficient and suitable for large-area fabrications. Now the question is, can we make it simple for integrating structure levels using such methods? The answer is positive. As long as a powerful fabrication technique is explored, its patterning ability will be able to define a series of structure hierarchies with considerable scale difference. One can simply start the construction from the fabricating of a microscale structure (with a larger period), i.e. the primary hierarchy. Then a nanoscale structure (with a smaller period) should follow in the basis of the microstructure, making itself the secondary hierarchy and so on.
Following this simple route, we succeeded to provide another method for even facile fabrication for large-area 3D hierarchical structures. This mask-free method is based on a multi-exposure lithography with two-beam interference. As is known, a single exposure of two beam interference on a photoresist leads to a 1D periodical structure with the same scale to its period, e.g. usually micro- or nanogratings. Now, if one does not develop the sample and keeps giving it a second exposure dose, two kinds of structures can result. In detail, the resulting structures depend on whether a revolving angle (0° ‒ 180°) is applied or not to the substrate within its own plane. If a zero revolving angle and a different scale of structure period are applied for the second exposure dose, the resulting structures remain 1D but with a new hierarchy (period). If a nonzero revolving angle and an identical structure period are applied, the resulting structures become 2D with a certain symmetry. By a careful combination of the revolving angle and the structure period, can we build up the structure hierarchies with designable dimension and symmetry.
The experimental details are as follows. The glass substrates were sonicated for 30 min in acetone and ethanol successively and then fully rinsed by de-ionized water. The selected photoresist was BP-212 positive photoresist (Beijing Institute of Chemical Reagents) and spin-coated at 3000 rpm onto the as-cleaned substrates. The resulting film thickness was 3 μm or so. The photoresist films were then prebaked at on a hotplate at 110 °C for 1 min and naturally cooled down to room temperature in ambient atmosphere. The light source used here was a beam of frequency-tripled and Q-switched single mode Nd:YAG laser (Spectra-physics) with a pulse width of ~10 ns and a wavelength of 355 nm. This beam was split into two beams for interference lithography through a simple setup, which is shown in Fig. 5. The laser power was set according to the period of each structure level, e.g. 300 mW for the first level with a 3 μm period and 200 mW for the second level with a 300 nm period in the case of a laser beam size of ~9 mm in diameter. The exposure time was kept to be 1 s throughout the fabrication. The as-exposed samples were finally developed in an aqueous solution of NaOH (0.2% w/v) for 3 ‒ 5 minutes. For the morphology characterization, the structure were observed by FESEM ((JSM-7500F, JEOL, Japan) and AFM (Digital Instruments Nanoscope IIIA) in the tapping mode. For surface wettability characterization, the contact angles were measured on an OCA 20 system (Data physics GmbH, Germany) at room temperature with a droplet volume of about 0.5 µl.
Fig. 5 Schematic of the two-beam laser lithography system with both varied exposure angle and revolving angle. On the left, the intersected angle between the two split beams (exposure angle) can be tuned for building structures hierarchy by hierarchy. On the right, the substrate with a photoresist is mounted on a turn table for applying a revolving angle φ to obtain different dimensions and patterns.
Figure 5 shows the schematic of the two-beam lithography system. The incident laser beam, usually chosen to be ultraviolet wavelength of 355 nm, was first led through a broadening lens set and then split into two beams. Here an exposure angle 2θi can be defined as the intersected angle of the two split beams when they are finally reflected onto the sample surface. The structure period of each hierarchy can be well tuned by changing the exposure angle, ranging from 100 nm to 50 μm and hence enables manifest and rich structure hierarchies. A second angle to be defined, as mentioned above, is the revolving angle φ over which the substrate revolves around the normal of its own plane. By setting the revolving angle in the range of 0° ‒ 180°, both the dimension and symmetry of the hierarchical structures can be well defined. In details, the specific value of the revolving angle should be chosen according to the certain symmetry of the 2D structure pattern for each hierarchy, i.e. according to its space group. So far the revolving angle was decided to be 60° and 90° for an easier fabrication. Under the control of these two angles, one can realize a series of hierarchical structures with various dimensions, patterns and hierarchy levels. And the diversity of the hierarchical structures supports smart micro-nanostructure design and multiple biomimetic functions.
Figure 6 shows both the mechanism and the real morphology of the biomimetic hierarchical structures fabricated by angle-varied two-beam lithography. The schematic of Fig. 6(a) illustrates how the exposure angle 2θi together with the revolving angle decides the period of each hierarchy and form typical two-level and three-level structures. In detail, the decision is based on the equation:
where θi referring to half the exposure angle, λ denoting the incident wavelength and Λi denoting the period corresponding to the exposure dose. According to the equation, one just needs to set the specific exposure angle values for defining the structure hierarchies (both periods and scales) and obtain the desired structures hierarchy by hierarchy. For example, a single exposure by 2θ1 or 2θ2 leads to a 1D nanograting structure with the period of Λ1 or Λ2, respectively. Now if one applies two exposures by 2θ1 and 2θ2 sequentially, with holding the substrate still, the resulting structure should be a parallel superimposition of the two single period structures, as shown by Fig. 6(a). For such a structure, the dimension remains 1D since the substrate does not revolve and the structure level reaches two, making it two-period 1D gratings. Figure 6(b1-b4) shows the diversity of the hierarchical structures prepared with this approach. The hierarchical structures level up to three and the dimension can also be arranged from 1D to 3D, with the revolving angle control of the substrate from 0° to 90°. The structure heights can be controlled by the exposure dose as well, ranging from 700 nm to 900 nm for the primary hierarchy and 50 nm to 100 nm for the secondary one.Fig. 6 Illustration of the structure hierarchy production by tuning the exposure angle. (a) is the schematic of the multi-exposure mechanism for acquiring the hierarchical structures. The two beams of L1 and L1’ intersect a smaller angle 2θ1 and form a larger period (primary hierarchy), whereas the two beams of L2 and L2’ intersect a larger angle 2θ2 and form the smaller period (secondary hierarchy) on the photoresist. The sequential combination of two periods finally constructs the hierarchical structure. Panels (b1)-(b4) are SEM images of hierarchical structures. They show various hierarchy periods, dimensions and patterns. (b1) 300 nm 1D grating at 90° orientation w.r.t. 3 µm 1D grating; (b2) 300 nm 1D grating at 0° orientation w.r.t. 3 µm 2D square patterns; (b3) 300 nm 2D hexagonal pattern at 0° orientation w.r.t. 3 µm 2D hexagonal pattern; (b4) 1 µm 1D gratings at 90° orientation w.r.t. 300 nm and 4 µm 1D gratings. The scale bar in all images is 1 µm.
Since these structures have shown significant structure hierarchies with large area, they are ideal for the realization of biomimetic functionalities, such as the “lotus leaf effect”. It has been addressed that the superhydrophobicity of a variety of leaves is a sheer structure effect, i.e. the hierarchical structures ranging from micro- to nanoscale. With the high similarity to the surface structures of the natural leaves, superhydrophobicity can be easily observed on our two-beam lithography printed structures.
Figure 7 shows the evolution of superhydrophobicity with the building-up of structure hierarchies. One can find that neither of the single structures (microcolumns or nanocolumns) supports superhydrophobicity, showing a contact angle a little larger than that of the planar surface. While the combination of a microscale structure and a nanoscale one should give great rise to the contact angle (over 150°) and provide solid superhydrophobicity. This means the biomimetic construction via laser-processing is not only a successful approach for hierarchical structure developing, but also a potential platform for bio-functionalization.
Fig. 7 The evolution of wetting properties with the accumulation of structure hierarchies. (a) - (d) are the schematics of the contact circumstances of single-level structures and a hierarchical structure with images showing their measured contact angles respectively.
4. Conclusions
So far, two kinds of mask-free laser-processing approaches have been briefly reviewed for fabricating large-area biomimetic hierarchical structures, which are based on femtosecond laser ablation and two-beam lithography, respectively. Both have been proved of the great power for preparing structures with large area and well-defined hierarchies. With distinct fabrication mechanisms, either of the two approaches can lead periodical patterns of each hierarchy with certain tunability. The success of such fabrications shows the great potential of laser processing for patterning as well as enlightening rich physics in light-matter interaction. By carefully choosing and matching the parameters of laser power, polarization and exposure dose, can one realize a diversity of hierarchical structures with tunable period, dimension and pattern. And a series of biomimetic functions have been demonstrated for the hierarchical structures mentioned above, such as the “lotus leaf effect” and the structural color. Since these laser-processing methods are able to well define both structure hierarchies and periodicities, they are superior among the current fabricating methods of biomimetic structures. Exploring laser processing techniques will surely bring about more novel structures other than hierarchical ones. With outstanding scalability and pattern ability, we can foresee the great potential of laser-processing methods in other application areas as well, such as micromachining and medical science.
Funding
National Natural Science Foundation of China (NSFC) (Grant No. 51501070).
Acknowledgements
The author sincerely acknowledges the support from Prof. Hong-Bo Sun, Dr. Lei Wang and Dr. Xue-Qing Liu.
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