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Quasi-superhydrophobic porous silicon surface fabricated by ultrashort pulsed-laser ablation and chemical etching
Accepted Manuscript Title: Quasi-superhydrophobic porous silicon surface fabricated by ultrashort pulsed-laser ablation and chemical etching Author: Huaihai Pan Fangfang Luo Geng Lin Chengwei Wang Mingming Dong Yang Liao Quan-Zhong Zhao PII: DOI: Reference:
S0009-2614(15)00610-7 http://dx.doi.org/doi:10.1016/j.cplett.2015.08.022 CPLETT 33219
To appear in: Received date: Revised date: Accepted date:
6-7-2015 31-7-2015 7-8-2015
Please cite this article as: H. Pan, F. Luo, G. Lin, C. Wang, M. Dong, Y. Liao, Q.-Z. Zhao, Quasi-superhydrophobic porous silicon surface fabricated by ultrashort pulsed-laser ablation and chemical etching, Chem. Phys. Lett. (2015), http://dx.doi.org/10.1016/j.cplett.2015.08.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights We fabricated superhydrophobic surfaces on silicon using pulsed-laser followed by chemical etching.
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Using this simple method, micro-honeycomb-like structures formed on silicon surface.
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The wettability of these nanoporous silicon surfaces can be controlled using pulsed-laser irradiation of different laser fluence followed by chemical etching, whose contact angles between distilled water droplet and silicon surface could reach up to 150 at optimized laser fluence.
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These hydrophobic or quasi-superhydrophobic properties of silicon surfaces can be theoretical supported using the Cassie model.
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Quasi-superhydrophobic porous silicon surface fabricated by ultrashort pulsed-laser ablation
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and chemical etching Yang Liao,a and Quan-Zhong Zhaoa,*
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese
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a
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Huaihai Pan,a,b Fangfang Luo,c Geng Lin,a,b Chengwei Wang,a,b Mingming Dong,a,b
Academy of Sciences, Shanghai 201800, China b
Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576,
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c
University of the Chinese Academy of Sciences, Beijing 100049, China
Singapore
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*[email protected]
Abstract: A silicon surface with distinctive structures is fabricated by ultrashort pulsed-laser
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ablation and chemical etching with acidic fluoride solutions. The surface consists of
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micro/nanostructures that result in the quasi-superhydrophobicity of the silicon surface. By fine tuning a key process parameter (i.e., pulsed laser power), surfaces with different
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wettability are fabricated. The morphology and composition of the surfaces are characterized by scanning electron microscopy, which reveals nanopores. The contact angle of water on these surfaces was measured and found to be as high as 150 at optimized parameters. This work presents a novel process of fabricating a silicon-based quasi-superhydrophobic porous surface.
Key words: ultrashort pulsed laser, silicon, superhydrophobic, chemical etching. 1. Introduction Superhydrophobic surfaces with water contact angles greater than 150 have attracted considerable attention because of special characteristics including self-cleaning, anti-fogging,
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pollutant-repulsive, anti-oxidative, and anti-corrosive properties, among others. The wetting of material surfaces is governed by interplay between the surface morphology and surface energy, and rougher surfaces with lower surface energy are capable of exhibiting superhydrophobicity. In nature, the leaves of various plants such as lotus leaves [1] and rice
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leaves [2] exhibit superhydrophobic properties due to their lower surface energy and rougher morphology. To mimic such superhydrophobic surfaces, many physical and chemical
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methods have been developed to synthesize such surfaces either by constructing
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micro/nanostructures on surfaces exhibiting low surface energy, or by lowering the surface energy of rough surfaces. These methods include phase separation [3, 4], crystal growth [1, 5-
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7], photolithography [8], electron beam lithography [9], electrospinning [10], using anodic aluminum oxide [11], pulsed-laser irradiation [12-17], and chemical etching [18, 19]. Ultrashort pulsed lasers in particular have been widely employed to texture the surfaces of a
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variety of materials, creating all kinds of special surfaces with different functions [20-24]. These ultrashort pulsed lasers can also be used to form micro/nanostructures on surfaces.
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When followed by the coating of these surfaces with chemical materials exhibiting low
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surface energy, such as dimethyldichlorosilane [(CH3)2SiCl2, DMDCS] [25] and CF3(CF2)7CH2CH2SiCl3 [26], the superhydrophobicity of artificial surfaces is induced.
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Among these fabricated superhydrophobic surfaces, silicon is commonly used because of its ability to be employed in a variety of applications, such as in photovoltaic devices, lab-onchips, and micro/nano electromechanical systems. Superhydrophobic surfaces based on silicon are produced by irradiating the surfaces with ultrashort (femtosecond) laser pulses and coating with low surface energy materials [12], obtaining a Nelumbo nucifera-like surface. Although an artificial superhydrophobic surface based on silicon can be fabricated using the methods specified above, we propose a novel process to fabricate quasi-superhydrophobic surfaces on silicon using ultrashort pulsed lasers without the requirements of a chemicalreactive gas atmosphere or extreme vacuum conditions.
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In this study, we demonstrate a novel method of fabricating quasi-superhydrophobic porous silicon surfaces using ultrashort pulsed lasers and chemical etching with acidic fluoride solutions that include hydrofluoric acid (HF), nitric acid (HNO3), and distilled H2O combined in certain proportions. Sub-microscale and nanoscale porous structures are formed
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on the laser-processed surfaces by using this method. By optimizing the pulsed laser fluence
and chemical etching time, a fabricated surface with a contact angle (CA) of approximately
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150 is obtained. These fabricated surfaces can have potential applications not only in micro-
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fluidic devices but also in photodetectors and solar cells to improve their light-collecting efficiency.
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2. Experimental details
To achieve hydrophobic or quasi-superhydrophobic surfaces, the fabrication process was
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carried out in two steps. First, the silicon sample was treated by a pulsed laser. A regenerative, amplified Ti:Sapphire laser at a central wavelength of 800 nm that emits a train of 120 fs
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mode-locked pulses at 1 kHz was used as the light source. The average power employed in
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our work was in the range of 5 to 200 mW. The laser beam was polarized horizontally to the optical table (along the x-axis) and was focused normally on the sample by an objective lens
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(5 , NA = 0.15). The focused laser spot diameter was approximately 13 m. The silicon wafers used in the experiment were two side-polished n-type (111) single crystals with a resistivity of 0.7 -1 cm-1 to 1.4 -1 cm-1 and a thickness of 300 m. The silicon wafers were prepared by rinsing in ethanol and distilled H2O, ultrasonic cleaning, and drying them for
laser processing. After cleaning, the samples were mounted on a three dimensional positioning stage which was precisely controlled by a computer. Microgrooves with 6 mm length and 30 m pitch were produced on the wafers in ambient air using a direct laser
writing technique. Large areas (i.e., 6 6 mm2) of the wafers were prepared for detailed
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sample analysis. The writing speed was fixed at 1 mm/s and the laser fluence was changed from 3.75 to 150 J/cm2 during processing to obtain a series of textured silicon wafers. Only after the laser processing were the textured silicon wafers processed in the second step. Aiming for full chemical etching using chemical solutions, the textured silicon wafers
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were immersed in a mixed solution of HF (40 %), HNO3 (67 %) and distilled H2O in a ratio of 1:3:6 for a fixed time of 2 h.
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After these two steps, the prepared samples were dried in air. Finally, the effect of varied
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laser fluence from 3.75 to 150 J/cm2 on these samples immersed in the mixed solution was compared. The various structures produced were analyzed using a JSM 6700F model
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scanning electron microscope (SEM; JEOL, Japan). The elemental composition of the silicon surface was measured by an energy-dispersive X-ray spectroscope (EDX). The static contact angle (SCA) of the silicon surface was measured with a contact angle measurement system
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(SL200B) using the sessile drop method. A micro-syringe was used to drop 2 μL of distilled H2O, and a charge-coupled device (CCD) was used to capture an image of the contacted
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3. Results and discussion
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droplets on the sample surface during the measurement of the CA.
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Figures 1(a)-1(b) show the SEM images of the silicon surface laser-processed at a fluence of 150 J/cm2 in air without any chemical etching. Numerous microgrooves with a certain depth were formed on the laser-processed surface with a period of 30 m. This period was determined by the pitch between two successive scanning lines. The ratio of the area not processed by the pulsed laser to the projected area of the silicon surface is approximately 1:3, as shown in Fig. 1(a). The magnified views of the microgrooved structures are shown in Fig. 1(b), where the dual-scale structures (submicro- and nanostructures) contain many such microgrooves. These dual-scale structures consist of submicro-protrusions and submicrocavities. The diameters of the tips of the submicro-protrusions range from 300 to 1000 nm. When the distilled H2O droplet was dropped on these silicon surfaces, the distilled H2O
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spread out quickly to reach a static CA of approximately 0, as shown in Fig. 1(c), indicating that the superhydrophilc property was achieved. The behavior of H2O uphill against gravity was also observed, which has been reported by Vorobyev and Guo and can be explained by
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the capillary effect [27]. Figure 2 shows the SEM images of artificial silicon wafer surfaces that were fabricated using a pulsed laser at a laser fluence of 150 J/cm2 and chemical etching for 2 h in acidic
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fluoride solutions. The multiple parallel micro-honeycomb-like structures are distributed
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uniformly with a period of 30 m, corresponding to the pitch between two successive scanning lines. There are several nanoporous structures in every honeycomb-like structure.
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These nanoporous structures have different diameters ranging from 200 to 300 nm. Figures 2(c)-2(d) show the images of a water droplet sitting on the artificial silicon surface, showing
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that the static CA of the water droplet reaches as high as 150. It is evident that chemical etching in an acidic fluoride solution creates nanoholes on the laser-processed silicon surfaces, consequently achieving quasi-superhydrophobicity of these surfaces.
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Figures 1 and 2 show a comparison of the microstructures formed in laser-processed
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silicon with those formed with additional chemical etching, respectively. The CAs of water
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droplets on these two types of tailored silicon surfaces reveal the conversion from superhydrophilicity to quasi-superhydrophobicity, which may be attributed to the formation of micro-honeycomb-like structures on silicon. Chemical etching plays a key role in the formation of these structures.
Similar formation mechanisms for porous silicon have been proposed in previous studies
[28-30]. For example, D. Vanmaekelbergh et al. have proposed the mechanism of (photo)anodic dissolution of silicon in a solution containing HF [30], and Kurt W. Kolasinski
et al. have also measured the formation rate of porous silicon in these chemical solutions [28, 29]. In this process, the laser-produced SiO2 layer was preferentially etched by HF, forming the –F terminated silicon surface instead of –H terminated silicon, due to the higher strength
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of the Si-F bond (i.e., 5 eV) than that of the Si-H bond (i.e., 3.5 eV). Subsequently, the high electronegativity of fluorine strongly polarizes the Si-Si bond, weakening this Si-Si bond and removing the -F terminated Si atom, forming H2SiFx molecules. Finally, the stable –H
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terminated silicon is formed, which would not be displaced by HF as it is less strongly polar. However, the existing HNO3 would then oxidize the –H terminated silicon and induce
electron holes in the silicon valence band in the form of electron-deficient Si-Si bonds, and
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such electron holes can transfer from these bonds to surface Si-H bonds, forming electron-
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deficient Si-H bonds that are easily displaced by HF to form Si-F bonds. Again, for the same reason mentioned above, this –F terminated silicon was removed by HF to form H2SiFx
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molecules. The overall reaction can be represented as follows:
Si 6 HF hvB H 2 SiF6 H 2 2 H e1
(1.1)
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where the H2SiF6 is predicted to be the final-etched product in the overall reaction. The micro/nanostructures that form on the silicon surface because of ultrashort pulsed-laser
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irradiation enhance the roughness of silicon in our experiment, thereby reducing the different
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reaction rates in the different surface regions when these surfaces are immersed in acidic fluoride solutions; this induces the formation of porous silicon with micro-honeycomb-like
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structures.
Figure 3 shows the SEM images of porous silicon surfaces that were irradiated at
different laser fluences and chemically etched for 2 h with acidic fluoride solutions. At lower laser fluences (from 3.75 to 7.5 J/cm2), the holes of the porous silicon were formed and distributed uniformly on the surface with hole diameters ranging from 200 to 300 nm. The use of higher laser fluences (from 11.25 to 150 J/cm2) changes the silicon surface morphology on the micro-scale before the honeycomb-like structures are formed with the further help of chemical etching. From the insets with higher magnifications in Figs. 3(a)-3(f), we see that the diameters of the nanoholes remain unchanged with varying laser fluences. In
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this case, air is trapped in these micro/nano holes when the water droplet is placed on these surfaces. Therefore the total solid area of the silicon surface is reduced. In other words, the roughness of the silicon surface is improved with increasing laser fluence and chemical etching.
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To study the wettability of the porous silicon, the water contact angle (WCA) on artificial
surfaces versus laser fluence was plotted, as shown in Fig. 4. The corresponding SEM images
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of these artificial surfaces are shown in Fig. 3. The SCAs of H2O on these artificial surfaces
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were measured at five different sites as well as the average values of the SCAs. The SCAs become larger with increasing laser fluence. When the laser fluence is lower than 15 J/cm2,
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the SCAs of water droplets on such artificial surfaces increase linearly with laser fluence. However, when the laser fluence is higher than 38.8 J/cm2, the SCAs attain a nearly constant value that is close to 150.
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The variations in the wettability of laser-processed silicon with chemical etching, as observed in Fig. 4, are attributed to the formation of typical micro-honeycomb-like structures
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on the surface. The nanoholes are formed by chemical etching and are governed by the
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interface between the acidic fluoride solution and the silicon surface. Therefore, increasing the laser fluence during irradiation increases the roughness of silicon by creating
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micro/nanostructures on the surface that consist of submicro-protrusions and submicrocavities with diameters ranging from 300 to 1000 nm. The rougher the silicon surface, the more the silicon surface is in contact with the acidic fluoride solution. Thus, the typical micro-honeycomb-like structures are formed at higher laser fluences, producing the quasisuperhydrophobicity of the silicon surface. An elemental analysis of the EDX spectra was performed to determine the influence of
the composition of the silicon surface on the wettability. Figure 5(a) shows the SEM image of a laser-processed silicon surface with chemical etching, while Figure 5(b) shows the SEM image of a silicon surface treated only by chemical etching. The corresponding EDX spectra
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of Figs. 5(a)-5(b) are shown in Figs. 5(c)-5(d), respectively. Analysis results show that the concentration of elemental F is very low in both cases. The corresponding WCA was measured to investigate the effect of elemental F on the wettability of the artificial surfaces. The CA in Fig. 5(a) is approximately 151° while the CA in Fig. 5(b) is approximately 64°.
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This result shows that elemental F has a minor influence on the realization of the quasisuperhydrophobicity of silicon. Therefore, the difference in the multi-porous properties of the
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textured silicon surfaces is mainly responsible for the drastic change from hydrophilicity to
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surperhydrophobicity.
The effect of multi-porous structures on the wettability of a silicon surface is taken into
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account to better explain the properties of the surface. It is well known that the contact angle
e of a liquid droplet on a flat surface is determined by Young’s equation: (1.2)
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cos e ( sv sl ) / lv
where sv , sl and lv are the surface tensions of solid-vapor, solid-liquid and liquid-vapor
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contact, respectively. For rough surfaces, the CA can be calculated by two different
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theoretical models [31, 32]. When a liquid penetrates the rough surface completely, the Wenzel model [32] is proposed to explain this case, which can be expressed in the equation
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as follows:
cos eW cos e (1.3)
where eW is the apparent CA, and is the ratio of actual rough area to the projected area. Here, e is found to be 64° from the wettability of the silicon surface treated only by chemical etching, as shown in Fig. 5(b). Since is always larger than 1, it is obvious from Eq. (1.3) that eW will become smaller, given that e is 64°. However, from Fig. 4, we can
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see that the CAs of H2O on the silicon surface are near 150° when the laser fluence is 112.5 J/cm2; thus, the Wenzel model is not consistent with the experimental data. In contrast to the Wenzel model, the Cassie model [31] was proposed to explain the apparent CA on rough
cos e C f cos e f 1
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surfaces via another mechanism, which can be expressed in the equation as follows: (1.4)
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where e C is the apparent CA on the rough surface and f is ratio of the area fraction wetted
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by liquid to the projected area, in which f is always smaller than 1. We can deduce from Eq. (1.4) that the apparent CA increases with the decrease of the solid fraction, f . That is,
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decreasing f can enhance the hydrophobic properties of the silicon surface. As f decreases from 1 to 0, the apparent CA e C can increase from 64° to 180°. Therefore it is more suitable
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to use the Cassie model instead of the Wenzel model to explain the quasi-superhydrophobic properties in our experiment. For example, when the apparent CA e C is 150°,
f can be
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calculated to be 0.0931 using Eq. (1.4), which fits the requirement that f be smaller than 1.
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The values of f are 0.1645 and 0.2511 in the cases when the silicon wafers were irradiated
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with laser fluences of 12.5 J/cm2 and 5 J/cm2, respectively. Since the artificial silicon surfaces for this experiment consist of double-scale structures, f can be replaced by f1 f 2 , in which
f1 and f 2 represent the fractional area wetted by liquid at micro- and nanoscales,
respectively. In this case, the equation for the Cassie model can be rewritten as follows:
cos e C f1 f 2 cos e f1 f 2 1
(1.5)
The discussion above shows that the quasi-superhydrophobic properties of artificial
silicon surfaces can be described with the Cassie model, in which the surface can be considered to be composed of a solid area and an air area when a distilled water droplet sits on this surface. Additionally, from the CA comparisons of silicon surfaces treated only by
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chemical etching with those fabricated by a combination of pulsed laser irradiation and chemical etching, we find that the formation of micro/nanoporous structures plays a crucial role in the realization of quasi-superhydrophobic surfaces.
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4. Conclusions Quasi-superhydrophobic porous silicon is fabricated by pulsed laser direct writing and
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chemical etching with acidic fluoride solutions. Such artificial surfaces exhibit quasi-
superhydrophobicity when irradiated by a pulsed laser at 112.5 J/cm2 and immersed in acidic
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fluoride solution for 2 h, after which the CA of a water droplet on the surface can approach approximately 150. The wettability of the artificial surface is controlled by varying the laser
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fluence. Elemental analysis of the EDX spectra shows that the micro/nanoporous structures formed on the surface, rather than elemental F, play a key role in realizing the quasi-
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superhydrophobicity of such artificial surfaces based on silicon. The favorable structures produced on silicon using pulsed laser-processing and chemical etching have various
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potential applications, such as in photovoltaic devices, microfluidic devices, and micro/nano
Acknowledgements
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electromechanical systems among others.
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This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61178024 and 11374316) and partially supported by the National Basic Research Program of China (2011CB808103 and 2010CB923203). Q. Zhao acknowledges the sponsor from the Shanghai Pujiang Program (Grant No. 10PJ1410600). References 1.
L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, and D. Zhu, "Super-Hydrophobic Surfaces: From Natural to Artificial," Adv. Mater. 14, 1857-1860 (2002).
2.
D. Wu, J.-N. Wang, S.-Z. Wu, Q.-D. Chen, S. Zhao, H. Zhang, H.-B. Sun, and L. Jiang, "Three-level biomimetic rice-leaf surfaces with controllable anisotropic sliding," Adv. Funct. Mater. 21, 2927-2932 (2011).
Page 12 of 17
3.
L. Yao, M. Zheng, S. He, L. Ma, M. Li, and W. Shen, "Preparation and properties of ZnS superhydrophobic surface with hierarchical structure," Appl. Surf. Sci. 257, 2955-2959 (2011).
4.
N. J. Shirtcliffe, G. McHale, M. I. Newton, and C. C. Perry, "Intrinsically superhydrophobic organosilica sol−gel foams," Langmuir 19, 5626-5631 (2003). R. Mohammadi, J. Wassink, and A. Amirfazli, "Effect of surfactants on wetting of super-hydrophobic
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5.
surfaces," Langmuir 20, 9657-9662 (2004). 6.
B. Zheng, J. D. Tice, L. S. Roach, and R. F. Ismagilov, "A droplet-based, composite PDMS/Glass capillary
cr
microfluidic system for evaluating protein crystallization conditions by microbatch and vapor-diffusion methods with on-chip x-ray diffraction," Angew. Chem. Int. Ed. 43, 2508-2511 (2004).
X. Guo, Q. Zhao, R. Li, H. Pan, X. Guo, A. Yin, and W. Dai, "Synthesis of ZnO nanoflowers and their
us
7.
wettabilities and photocatalytic properties," Opt. Express 18, 18401-18406 (2010). 8.
Y. Xia, D. Qin, and Y. Yin, "Surface patterning and its application in wetting/dewetting studies," Curr. Opin.
9.
an
Colloid Interface Sci. 6, 54-64 (2001).
J. Feng, M. T. Tuominen, and J. P. Rothstein, "Hierarchical superhydrophobic surfaces fabricated by dual-scale
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electron-beam-lithography with well-ordered secondary nanostructures," Adv. Funct. Mater. 21, 3715-3722 (2011).
10. R. Asmatulu, M. Ceylan, and N. Nuraje, "Study of superhydrophobic electrospun nanocomposite fibers for
d
energy systems," Langmuir 27, 504-507 (2010).
te
11. Y. Lai, X. Gao, H. Zhuang, J. Huang, C. Lin, and L. Jiang, "Designing superhydrophobic porous nanostructures with tunable water adhesion," Adv. Mater. 21, 3799-3803 (2009).
Ac ce p
12. V. Zorba, E. Stratakis, M. Barberoglou, E. Spanakis, P. Tzanetakis, S. H. Anastasiadis, and C. Fotakis, "Biomimetic artificial surfaces quantitatively reproduce the water repellency of a lotus leaf," Adv. Mater. 20,
4049-4054 (2008).
13. M. Barberoglou, V. Zorba, E. Stratakis, E. Spanakis, P. Tzanetakis, S. H. Anastasiadis, and C. Fotakis, "Bioinspired water repellent surfaces produced by ultrafast laser structuring of silicon," Appl. Surf. Sci. 255, 54255429 (2009).
14. V. Zorba, L. Persano, D. Pisignano, A. Athanassiou, E. Stratakis, R. Cingolani, P. Tzanetakis, and C. Fotakis, "Making silicon hydrophobic: wettability control by two-lengthscale simultaneous patterning with femtosecond laser irradiation," Nanotechnology 17, 3234 (2006). 15. Y. Shen, D. Liu, W. Zhang, G. f. Dearden, and K. Watkins, "Ultrafast laser surface wettability modification on alumina surface," Chin. Opt. Lett. 11, 021403 (2013).
Page 13 of 17
16. E. L. Papadopoulou, V. Zorba, A. Pagkozidis, M. Barberoglou, E. Stratakis, and C. Fotakis, "Reversible wettability of ZnO nanostructured thin films prepared by pulsed laser deposition," Thin Solid Films 518, 12671270 (2009). 17.
M. Tang, V. Shim, Z. Y. Pan, Y. S. Choo, and M. H. Hong, "Laser Ablaton of Metal Substrates for Super-
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hydrophobic Effect," J. Laser Micro Nanoeng. 6, 6-9 (2011). 18. B. Qian and Z. Shen, "Fabrication of Superhydrophobic Surfaces by Dislocation-Selective Chemical Etching on Aluminum, Copper, and Zinc Substrates," Langmuir 21, 9007-9009 (2005).
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19. Z. Chen, Y. Guo, and S. Fang, "A facial approach to fabricate superhydrophobic aluminum surface," Surf. Interface Anal. 42, 1-6 (2010).
formation on metals," Appl. Phys. Lett. 95, 123111 (2009).
us
20. T. Y. Hwang, A. Y. Vorobyev, and C. Guo, "Ultrafast dynamics of femtosecond laser-induced nanostructure
21. A. Y. Vorobyev and C. Guo, "Colorizing metals with femtosecond laser pulses," Appl. Phys. Lett. 92, 041914
an
(2008).
22. Z. K. Wang, H. Y. Zheng, and H. M. Xia, "Femtosecond laser-induced modification of surface wettability of
M
PMMA for fluid separation in microchannels," Microfluid. Nanofluid. 10, 225-229 (2011). 23. M. H. Hong, Y. Lin, G. X. Chen, L. S. Tan, Q. Xie, B. Lukyanchuk, L. P. Shi, and T. C. Chong, "Nanopatterning by pulsed laser irradiation in near field," J. Phys.: Conf. Ser. 59, 64 (2007).
d
24. J. Yang, F. Luo, T. S. Kao, X. Li, G. W. Ho, J. Teng, X. Luo, and M. Hong, "Design and fabrication of
(2014).
te
broadband ultralow reflectivity black Si surfaces by laser micro/nanoprocessing," Light Sci. Appl. 3, e185
Ac ce p
25. V. Zorba, E. Stratakis, M. Barberoglou, E. Spanakis, P. Tzanetakis, and C. Fotakis, "Tailoring the wetting response of silicon surfaces via fs laser structuring," Appl. Phys. A 93, 819-825 (2008).
26. T. Baldacchini, J. E. Carey, M. Zhou, and E. Mazur, "Superhydrophobic surfaces prepared by microstructuring of silicon using a femtosecond laser," Langmuir 22, 4917-4919 (2006).
27.
A. Y. Vorobyev and C. Guo, "Laser turns silicon superwicking," Opt. Express 18, 6455-6460 (2010).
28. L. Koker and K. W. Kolasinski, "Laser-assisted formation of porous Si in diverse fluoride solutions: reaction kinetics and mechanistic implications†," J. Phys. Chem. B 105, 3864-3871 (2001).
29. K. W. Kolasinski, "The mechanism of Si etching in fluoride solutions," Phys. Chem. Chem. Phys. 5, 1270-1278 (2003). 30. E. S. Kooij and D. Vanmaekelbergh, "Catalysis and pore initiation in the anodic dissolution of silicon in HF," J. Electrochem. Soc. 144, 1296-1301 (1997). 31. A. B. D. Cassie and S. Baxter, "Wettability of porous surfaces," Trans. Faraday Soc. 40, 546-551 (1944).
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32. R. N. Wenzel, " Resistance of solid surfaces to wetting by water," Ind. Eng. Chem. 28, 988-994 (1936).
Figure Captions Figure 1. (a) SEM image of silicon treated by pulsed laser at a fluence of 150 J/cm2 (b)
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magnification of (a) and (c) CA of distilled H2O on the processed silicon. Figure 2. (a) SEM image of a treated silicon surface magnified by 2000 (b) same image as
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(a), magnified by 10000 (c) photo of a water droplet sitting on a treated silicon surface and
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(d) side-view of a water droplet sitting on a treated silicon surface for CA analysis.
Figure 3. SEM images of a silicon wafer treated by laser processing and chemical etching.
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The proportion of solution used is constant. The laser fluences used for the treated surfaces in (a–f) are 3.75, 7.5, 15, 75, 112.5, and 150 J/cm2, respectively, and the chemical etching time
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in the acidic fluoride solutions is fixed at 2 h.
Figure 4. Variations in the WCA of pulsed laser-processed silicon as a function of laser
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fluence (i.e., from 3.75 to 150 J/cm2) with a fixed chemical etching time of 2 h.
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Figure 5. (a) SEM image of a silicon surface irradiated by a laser fluence of 112.5 J/cm2; the chemical etching time is fixed at 2 h (b) SEM image of a silicon surface treated only by
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chemical etching for 2 h (c) EDX spectra of Fig. 7(a) and (d) EDX spectra of Fig. 7(b). Figure 1
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Figure 2
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Figure 4
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Figure 5
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S0009-2614(15)00610-7 http://dx.doi.org/doi:10.1016/j.cplett.2015.08.022 CPLETT 33219
To appear in: Received date: Revised date: Accepted date:
6-7-2015 31-7-2015 7-8-2015
Please cite this article as: H. Pan, F. Luo, G. Lin, C. Wang, M. Dong, Y. Liao, Q.-Z. Zhao, Quasi-superhydrophobic porous silicon surface fabricated by ultrashort pulsed-laser ablation and chemical etching, Chem. Phys. Lett. (2015), http://dx.doi.org/10.1016/j.cplett.2015.08.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Highlights We fabricated superhydrophobic surfaces on silicon using pulsed-laser followed by chemical etching.
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Using this simple method, micro-honeycomb-like structures formed on silicon surface.
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The wettability of these nanoporous silicon surfaces can be controlled using pulsed-laser irradiation of different laser fluence followed by chemical etching, whose contact angles between distilled water droplet and silicon surface could reach up to 150 at optimized laser fluence.
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These hydrophobic or quasi-superhydrophobic properties of silicon surfaces can be theoretical supported using the Cassie model.
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Quasi-superhydrophobic porous silicon surface fabricated by ultrashort pulsed-laser ablation
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and chemical etching Yang Liao,a and Quan-Zhong Zhaoa,*
State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese
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a
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Huaihai Pan,a,b Fangfang Luo,c Geng Lin,a,b Chengwei Wang,a,b Mingming Dong,a,b
Academy of Sciences, Shanghai 201800, China b
Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576,
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c
University of the Chinese Academy of Sciences, Beijing 100049, China
Singapore
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*[email protected]
Abstract: A silicon surface with distinctive structures is fabricated by ultrashort pulsed-laser
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ablation and chemical etching with acidic fluoride solutions. The surface consists of
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micro/nanostructures that result in the quasi-superhydrophobicity of the silicon surface. By fine tuning a key process parameter (i.e., pulsed laser power), surfaces with different
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wettability are fabricated. The morphology and composition of the surfaces are characterized by scanning electron microscopy, which reveals nanopores. The contact angle of water on these surfaces was measured and found to be as high as 150 at optimized parameters. This work presents a novel process of fabricating a silicon-based quasi-superhydrophobic porous surface.
Key words: ultrashort pulsed laser, silicon, superhydrophobic, chemical etching. 1. Introduction Superhydrophobic surfaces with water contact angles greater than 150 have attracted considerable attention because of special characteristics including self-cleaning, anti-fogging,
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pollutant-repulsive, anti-oxidative, and anti-corrosive properties, among others. The wetting of material surfaces is governed by interplay between the surface morphology and surface energy, and rougher surfaces with lower surface energy are capable of exhibiting superhydrophobicity. In nature, the leaves of various plants such as lotus leaves [1] and rice
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leaves [2] exhibit superhydrophobic properties due to their lower surface energy and rougher morphology. To mimic such superhydrophobic surfaces, many physical and chemical
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methods have been developed to synthesize such surfaces either by constructing
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micro/nanostructures on surfaces exhibiting low surface energy, or by lowering the surface energy of rough surfaces. These methods include phase separation [3, 4], crystal growth [1, 5-
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7], photolithography [8], electron beam lithography [9], electrospinning [10], using anodic aluminum oxide [11], pulsed-laser irradiation [12-17], and chemical etching [18, 19]. Ultrashort pulsed lasers in particular have been widely employed to texture the surfaces of a
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variety of materials, creating all kinds of special surfaces with different functions [20-24]. These ultrashort pulsed lasers can also be used to form micro/nanostructures on surfaces.
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When followed by the coating of these surfaces with chemical materials exhibiting low
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surface energy, such as dimethyldichlorosilane [(CH3)2SiCl2, DMDCS] [25] and CF3(CF2)7CH2CH2SiCl3 [26], the superhydrophobicity of artificial surfaces is induced.
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Among these fabricated superhydrophobic surfaces, silicon is commonly used because of its ability to be employed in a variety of applications, such as in photovoltaic devices, lab-onchips, and micro/nano electromechanical systems. Superhydrophobic surfaces based on silicon are produced by irradiating the surfaces with ultrashort (femtosecond) laser pulses and coating with low surface energy materials [12], obtaining a Nelumbo nucifera-like surface. Although an artificial superhydrophobic surface based on silicon can be fabricated using the methods specified above, we propose a novel process to fabricate quasi-superhydrophobic surfaces on silicon using ultrashort pulsed lasers without the requirements of a chemicalreactive gas atmosphere or extreme vacuum conditions.
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In this study, we demonstrate a novel method of fabricating quasi-superhydrophobic porous silicon surfaces using ultrashort pulsed lasers and chemical etching with acidic fluoride solutions that include hydrofluoric acid (HF), nitric acid (HNO3), and distilled H2O combined in certain proportions. Sub-microscale and nanoscale porous structures are formed
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on the laser-processed surfaces by using this method. By optimizing the pulsed laser fluence
and chemical etching time, a fabricated surface with a contact angle (CA) of approximately
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150 is obtained. These fabricated surfaces can have potential applications not only in micro-
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fluidic devices but also in photodetectors and solar cells to improve their light-collecting efficiency.
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2. Experimental details
To achieve hydrophobic or quasi-superhydrophobic surfaces, the fabrication process was
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carried out in two steps. First, the silicon sample was treated by a pulsed laser. A regenerative, amplified Ti:Sapphire laser at a central wavelength of 800 nm that emits a train of 120 fs
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mode-locked pulses at 1 kHz was used as the light source. The average power employed in
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our work was in the range of 5 to 200 mW. The laser beam was polarized horizontally to the optical table (along the x-axis) and was focused normally on the sample by an objective lens
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(5 , NA = 0.15). The focused laser spot diameter was approximately 13 m. The silicon wafers used in the experiment were two side-polished n-type (111) single crystals with a resistivity of 0.7 -1 cm-1 to 1.4 -1 cm-1 and a thickness of 300 m. The silicon wafers were prepared by rinsing in ethanol and distilled H2O, ultrasonic cleaning, and drying them for
laser processing. After cleaning, the samples were mounted on a three dimensional positioning stage which was precisely controlled by a computer. Microgrooves with 6 mm length and 30 m pitch were produced on the wafers in ambient air using a direct laser
writing technique. Large areas (i.e., 6 6 mm2) of the wafers were prepared for detailed
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sample analysis. The writing speed was fixed at 1 mm/s and the laser fluence was changed from 3.75 to 150 J/cm2 during processing to obtain a series of textured silicon wafers. Only after the laser processing were the textured silicon wafers processed in the second step. Aiming for full chemical etching using chemical solutions, the textured silicon wafers
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were immersed in a mixed solution of HF (40 %), HNO3 (67 %) and distilled H2O in a ratio of 1:3:6 for a fixed time of 2 h.
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After these two steps, the prepared samples were dried in air. Finally, the effect of varied
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laser fluence from 3.75 to 150 J/cm2 on these samples immersed in the mixed solution was compared. The various structures produced were analyzed using a JSM 6700F model
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scanning electron microscope (SEM; JEOL, Japan). The elemental composition of the silicon surface was measured by an energy-dispersive X-ray spectroscope (EDX). The static contact angle (SCA) of the silicon surface was measured with a contact angle measurement system
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(SL200B) using the sessile drop method. A micro-syringe was used to drop 2 μL of distilled H2O, and a charge-coupled device (CCD) was used to capture an image of the contacted
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3. Results and discussion
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droplets on the sample surface during the measurement of the CA.
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Figures 1(a)-1(b) show the SEM images of the silicon surface laser-processed at a fluence of 150 J/cm2 in air without any chemical etching. Numerous microgrooves with a certain depth were formed on the laser-processed surface with a period of 30 m. This period was determined by the pitch between two successive scanning lines. The ratio of the area not processed by the pulsed laser to the projected area of the silicon surface is approximately 1:3, as shown in Fig. 1(a). The magnified views of the microgrooved structures are shown in Fig. 1(b), where the dual-scale structures (submicro- and nanostructures) contain many such microgrooves. These dual-scale structures consist of submicro-protrusions and submicrocavities. The diameters of the tips of the submicro-protrusions range from 300 to 1000 nm. When the distilled H2O droplet was dropped on these silicon surfaces, the distilled H2O
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spread out quickly to reach a static CA of approximately 0, as shown in Fig. 1(c), indicating that the superhydrophilc property was achieved. The behavior of H2O uphill against gravity was also observed, which has been reported by Vorobyev and Guo and can be explained by
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the capillary effect [27]. Figure 2 shows the SEM images of artificial silicon wafer surfaces that were fabricated using a pulsed laser at a laser fluence of 150 J/cm2 and chemical etching for 2 h in acidic
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fluoride solutions. The multiple parallel micro-honeycomb-like structures are distributed
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uniformly with a period of 30 m, corresponding to the pitch between two successive scanning lines. There are several nanoporous structures in every honeycomb-like structure.
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These nanoporous structures have different diameters ranging from 200 to 300 nm. Figures 2(c)-2(d) show the images of a water droplet sitting on the artificial silicon surface, showing
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that the static CA of the water droplet reaches as high as 150. It is evident that chemical etching in an acidic fluoride solution creates nanoholes on the laser-processed silicon surfaces, consequently achieving quasi-superhydrophobicity of these surfaces.
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Figures 1 and 2 show a comparison of the microstructures formed in laser-processed
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silicon with those formed with additional chemical etching, respectively. The CAs of water
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droplets on these two types of tailored silicon surfaces reveal the conversion from superhydrophilicity to quasi-superhydrophobicity, which may be attributed to the formation of micro-honeycomb-like structures on silicon. Chemical etching plays a key role in the formation of these structures.
Similar formation mechanisms for porous silicon have been proposed in previous studies
[28-30]. For example, D. Vanmaekelbergh et al. have proposed the mechanism of (photo)anodic dissolution of silicon in a solution containing HF [30], and Kurt W. Kolasinski
et al. have also measured the formation rate of porous silicon in these chemical solutions [28, 29]. In this process, the laser-produced SiO2 layer was preferentially etched by HF, forming the –F terminated silicon surface instead of –H terminated silicon, due to the higher strength
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of the Si-F bond (i.e., 5 eV) than that of the Si-H bond (i.e., 3.5 eV). Subsequently, the high electronegativity of fluorine strongly polarizes the Si-Si bond, weakening this Si-Si bond and removing the -F terminated Si atom, forming H2SiFx molecules. Finally, the stable –H
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terminated silicon is formed, which would not be displaced by HF as it is less strongly polar. However, the existing HNO3 would then oxidize the –H terminated silicon and induce
electron holes in the silicon valence band in the form of electron-deficient Si-Si bonds, and
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such electron holes can transfer from these bonds to surface Si-H bonds, forming electron-
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deficient Si-H bonds that are easily displaced by HF to form Si-F bonds. Again, for the same reason mentioned above, this –F terminated silicon was removed by HF to form H2SiFx
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molecules. The overall reaction can be represented as follows:
Si 6 HF hvB H 2 SiF6 H 2 2 H e1
(1.1)
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where the H2SiF6 is predicted to be the final-etched product in the overall reaction. The micro/nanostructures that form on the silicon surface because of ultrashort pulsed-laser
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irradiation enhance the roughness of silicon in our experiment, thereby reducing the different
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reaction rates in the different surface regions when these surfaces are immersed in acidic fluoride solutions; this induces the formation of porous silicon with micro-honeycomb-like
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structures.
Figure 3 shows the SEM images of porous silicon surfaces that were irradiated at
different laser fluences and chemically etched for 2 h with acidic fluoride solutions. At lower laser fluences (from 3.75 to 7.5 J/cm2), the holes of the porous silicon were formed and distributed uniformly on the surface with hole diameters ranging from 200 to 300 nm. The use of higher laser fluences (from 11.25 to 150 J/cm2) changes the silicon surface morphology on the micro-scale before the honeycomb-like structures are formed with the further help of chemical etching. From the insets with higher magnifications in Figs. 3(a)-3(f), we see that the diameters of the nanoholes remain unchanged with varying laser fluences. In
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this case, air is trapped in these micro/nano holes when the water droplet is placed on these surfaces. Therefore the total solid area of the silicon surface is reduced. In other words, the roughness of the silicon surface is improved with increasing laser fluence and chemical etching.
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To study the wettability of the porous silicon, the water contact angle (WCA) on artificial
surfaces versus laser fluence was plotted, as shown in Fig. 4. The corresponding SEM images
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of these artificial surfaces are shown in Fig. 3. The SCAs of H2O on these artificial surfaces
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were measured at five different sites as well as the average values of the SCAs. The SCAs become larger with increasing laser fluence. When the laser fluence is lower than 15 J/cm2,
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the SCAs of water droplets on such artificial surfaces increase linearly with laser fluence. However, when the laser fluence is higher than 38.8 J/cm2, the SCAs attain a nearly constant value that is close to 150.
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The variations in the wettability of laser-processed silicon with chemical etching, as observed in Fig. 4, are attributed to the formation of typical micro-honeycomb-like structures
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on the surface. The nanoholes are formed by chemical etching and are governed by the
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interface between the acidic fluoride solution and the silicon surface. Therefore, increasing the laser fluence during irradiation increases the roughness of silicon by creating
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micro/nanostructures on the surface that consist of submicro-protrusions and submicrocavities with diameters ranging from 300 to 1000 nm. The rougher the silicon surface, the more the silicon surface is in contact with the acidic fluoride solution. Thus, the typical micro-honeycomb-like structures are formed at higher laser fluences, producing the quasisuperhydrophobicity of the silicon surface. An elemental analysis of the EDX spectra was performed to determine the influence of
the composition of the silicon surface on the wettability. Figure 5(a) shows the SEM image of a laser-processed silicon surface with chemical etching, while Figure 5(b) shows the SEM image of a silicon surface treated only by chemical etching. The corresponding EDX spectra
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of Figs. 5(a)-5(b) are shown in Figs. 5(c)-5(d), respectively. Analysis results show that the concentration of elemental F is very low in both cases. The corresponding WCA was measured to investigate the effect of elemental F on the wettability of the artificial surfaces. The CA in Fig. 5(a) is approximately 151° while the CA in Fig. 5(b) is approximately 64°.
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This result shows that elemental F has a minor influence on the realization of the quasisuperhydrophobicity of silicon. Therefore, the difference in the multi-porous properties of the
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textured silicon surfaces is mainly responsible for the drastic change from hydrophilicity to
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surperhydrophobicity.
The effect of multi-porous structures on the wettability of a silicon surface is taken into
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account to better explain the properties of the surface. It is well known that the contact angle
e of a liquid droplet on a flat surface is determined by Young’s equation: (1.2)
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cos e ( sv sl ) / lv
where sv , sl and lv are the surface tensions of solid-vapor, solid-liquid and liquid-vapor
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contact, respectively. For rough surfaces, the CA can be calculated by two different
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theoretical models [31, 32]. When a liquid penetrates the rough surface completely, the Wenzel model [32] is proposed to explain this case, which can be expressed in the equation
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as follows:
cos eW cos e (1.3)
where eW is the apparent CA, and is the ratio of actual rough area to the projected area. Here, e is found to be 64° from the wettability of the silicon surface treated only by chemical etching, as shown in Fig. 5(b). Since is always larger than 1, it is obvious from Eq. (1.3) that eW will become smaller, given that e is 64°. However, from Fig. 4, we can
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see that the CAs of H2O on the silicon surface are near 150° when the laser fluence is 112.5 J/cm2; thus, the Wenzel model is not consistent with the experimental data. In contrast to the Wenzel model, the Cassie model [31] was proposed to explain the apparent CA on rough
cos e C f cos e f 1
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surfaces via another mechanism, which can be expressed in the equation as follows: (1.4)
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where e C is the apparent CA on the rough surface and f is ratio of the area fraction wetted
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by liquid to the projected area, in which f is always smaller than 1. We can deduce from Eq. (1.4) that the apparent CA increases with the decrease of the solid fraction, f . That is,
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decreasing f can enhance the hydrophobic properties of the silicon surface. As f decreases from 1 to 0, the apparent CA e C can increase from 64° to 180°. Therefore it is more suitable
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to use the Cassie model instead of the Wenzel model to explain the quasi-superhydrophobic properties in our experiment. For example, when the apparent CA e C is 150°,
f can be
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calculated to be 0.0931 using Eq. (1.4), which fits the requirement that f be smaller than 1.
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The values of f are 0.1645 and 0.2511 in the cases when the silicon wafers were irradiated
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with laser fluences of 12.5 J/cm2 and 5 J/cm2, respectively. Since the artificial silicon surfaces for this experiment consist of double-scale structures, f can be replaced by f1 f 2 , in which
f1 and f 2 represent the fractional area wetted by liquid at micro- and nanoscales,
respectively. In this case, the equation for the Cassie model can be rewritten as follows:
cos e C f1 f 2 cos e f1 f 2 1
(1.5)
The discussion above shows that the quasi-superhydrophobic properties of artificial
silicon surfaces can be described with the Cassie model, in which the surface can be considered to be composed of a solid area and an air area when a distilled water droplet sits on this surface. Additionally, from the CA comparisons of silicon surfaces treated only by
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chemical etching with those fabricated by a combination of pulsed laser irradiation and chemical etching, we find that the formation of micro/nanoporous structures plays a crucial role in the realization of quasi-superhydrophobic surfaces.
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4. Conclusions Quasi-superhydrophobic porous silicon is fabricated by pulsed laser direct writing and
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chemical etching with acidic fluoride solutions. Such artificial surfaces exhibit quasi-
superhydrophobicity when irradiated by a pulsed laser at 112.5 J/cm2 and immersed in acidic
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fluoride solution for 2 h, after which the CA of a water droplet on the surface can approach approximately 150. The wettability of the artificial surface is controlled by varying the laser
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fluence. Elemental analysis of the EDX spectra shows that the micro/nanoporous structures formed on the surface, rather than elemental F, play a key role in realizing the quasi-
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superhydrophobicity of such artificial surfaces based on silicon. The favorable structures produced on silicon using pulsed laser-processing and chemical etching have various
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potential applications, such as in photovoltaic devices, microfluidic devices, and micro/nano
Acknowledgements
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electromechanical systems among others.
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This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61178024 and 11374316) and partially supported by the National Basic Research Program of China (2011CB808103 and 2010CB923203). Q. Zhao acknowledges the sponsor from the Shanghai Pujiang Program (Grant No. 10PJ1410600). References 1.
L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, and D. Zhu, "Super-Hydrophobic Surfaces: From Natural to Artificial," Adv. Mater. 14, 1857-1860 (2002).
2.
D. Wu, J.-N. Wang, S.-Z. Wu, Q.-D. Chen, S. Zhao, H. Zhang, H.-B. Sun, and L. Jiang, "Three-level biomimetic rice-leaf surfaces with controllable anisotropic sliding," Adv. Funct. Mater. 21, 2927-2932 (2011).
Page 12 of 17
3.
L. Yao, M. Zheng, S. He, L. Ma, M. Li, and W. Shen, "Preparation and properties of ZnS superhydrophobic surface with hierarchical structure," Appl. Surf. Sci. 257, 2955-2959 (2011).
4.
N. J. Shirtcliffe, G. McHale, M. I. Newton, and C. C. Perry, "Intrinsically superhydrophobic organosilica sol−gel foams," Langmuir 19, 5626-5631 (2003). R. Mohammadi, J. Wassink, and A. Amirfazli, "Effect of surfactants on wetting of super-hydrophobic
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5.
surfaces," Langmuir 20, 9657-9662 (2004). 6.
B. Zheng, J. D. Tice, L. S. Roach, and R. F. Ismagilov, "A droplet-based, composite PDMS/Glass capillary
cr
microfluidic system for evaluating protein crystallization conditions by microbatch and vapor-diffusion methods with on-chip x-ray diffraction," Angew. Chem. Int. Ed. 43, 2508-2511 (2004).
X. Guo, Q. Zhao, R. Li, H. Pan, X. Guo, A. Yin, and W. Dai, "Synthesis of ZnO nanoflowers and their
us
7.
wettabilities and photocatalytic properties," Opt. Express 18, 18401-18406 (2010). 8.
Y. Xia, D. Qin, and Y. Yin, "Surface patterning and its application in wetting/dewetting studies," Curr. Opin.
9.
an
Colloid Interface Sci. 6, 54-64 (2001).
J. Feng, M. T. Tuominen, and J. P. Rothstein, "Hierarchical superhydrophobic surfaces fabricated by dual-scale
M
electron-beam-lithography with well-ordered secondary nanostructures," Adv. Funct. Mater. 21, 3715-3722 (2011).
10. R. Asmatulu, M. Ceylan, and N. Nuraje, "Study of superhydrophobic electrospun nanocomposite fibers for
d
energy systems," Langmuir 27, 504-507 (2010).
te
11. Y. Lai, X. Gao, H. Zhuang, J. Huang, C. Lin, and L. Jiang, "Designing superhydrophobic porous nanostructures with tunable water adhesion," Adv. Mater. 21, 3799-3803 (2009).
Ac ce p
12. V. Zorba, E. Stratakis, M. Barberoglou, E. Spanakis, P. Tzanetakis, S. H. Anastasiadis, and C. Fotakis, "Biomimetic artificial surfaces quantitatively reproduce the water repellency of a lotus leaf," Adv. Mater. 20,
4049-4054 (2008).
13. M. Barberoglou, V. Zorba, E. Stratakis, E. Spanakis, P. Tzanetakis, S. H. Anastasiadis, and C. Fotakis, "Bioinspired water repellent surfaces produced by ultrafast laser structuring of silicon," Appl. Surf. Sci. 255, 54255429 (2009).
14. V. Zorba, L. Persano, D. Pisignano, A. Athanassiou, E. Stratakis, R. Cingolani, P. Tzanetakis, and C. Fotakis, "Making silicon hydrophobic: wettability control by two-lengthscale simultaneous patterning with femtosecond laser irradiation," Nanotechnology 17, 3234 (2006). 15. Y. Shen, D. Liu, W. Zhang, G. f. Dearden, and K. Watkins, "Ultrafast laser surface wettability modification on alumina surface," Chin. Opt. Lett. 11, 021403 (2013).
Page 13 of 17
16. E. L. Papadopoulou, V. Zorba, A. Pagkozidis, M. Barberoglou, E. Stratakis, and C. Fotakis, "Reversible wettability of ZnO nanostructured thin films prepared by pulsed laser deposition," Thin Solid Films 518, 12671270 (2009). 17.
M. Tang, V. Shim, Z. Y. Pan, Y. S. Choo, and M. H. Hong, "Laser Ablaton of Metal Substrates for Super-
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hydrophobic Effect," J. Laser Micro Nanoeng. 6, 6-9 (2011). 18. B. Qian and Z. Shen, "Fabrication of Superhydrophobic Surfaces by Dislocation-Selective Chemical Etching on Aluminum, Copper, and Zinc Substrates," Langmuir 21, 9007-9009 (2005).
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19. Z. Chen, Y. Guo, and S. Fang, "A facial approach to fabricate superhydrophobic aluminum surface," Surf. Interface Anal. 42, 1-6 (2010).
formation on metals," Appl. Phys. Lett. 95, 123111 (2009).
us
20. T. Y. Hwang, A. Y. Vorobyev, and C. Guo, "Ultrafast dynamics of femtosecond laser-induced nanostructure
21. A. Y. Vorobyev and C. Guo, "Colorizing metals with femtosecond laser pulses," Appl. Phys. Lett. 92, 041914
an
(2008).
22. Z. K. Wang, H. Y. Zheng, and H. M. Xia, "Femtosecond laser-induced modification of surface wettability of
M
PMMA for fluid separation in microchannels," Microfluid. Nanofluid. 10, 225-229 (2011). 23. M. H. Hong, Y. Lin, G. X. Chen, L. S. Tan, Q. Xie, B. Lukyanchuk, L. P. Shi, and T. C. Chong, "Nanopatterning by pulsed laser irradiation in near field," J. Phys.: Conf. Ser. 59, 64 (2007).
d
24. J. Yang, F. Luo, T. S. Kao, X. Li, G. W. Ho, J. Teng, X. Luo, and M. Hong, "Design and fabrication of
(2014).
te
broadband ultralow reflectivity black Si surfaces by laser micro/nanoprocessing," Light Sci. Appl. 3, e185
Ac ce p
25. V. Zorba, E. Stratakis, M. Barberoglou, E. Spanakis, P. Tzanetakis, and C. Fotakis, "Tailoring the wetting response of silicon surfaces via fs laser structuring," Appl. Phys. A 93, 819-825 (2008).
26. T. Baldacchini, J. E. Carey, M. Zhou, and E. Mazur, "Superhydrophobic surfaces prepared by microstructuring of silicon using a femtosecond laser," Langmuir 22, 4917-4919 (2006).
27.
A. Y. Vorobyev and C. Guo, "Laser turns silicon superwicking," Opt. Express 18, 6455-6460 (2010).
28. L. Koker and K. W. Kolasinski, "Laser-assisted formation of porous Si in diverse fluoride solutions: reaction kinetics and mechanistic implications†," J. Phys. Chem. B 105, 3864-3871 (2001).
29. K. W. Kolasinski, "The mechanism of Si etching in fluoride solutions," Phys. Chem. Chem. Phys. 5, 1270-1278 (2003). 30. E. S. Kooij and D. Vanmaekelbergh, "Catalysis and pore initiation in the anodic dissolution of silicon in HF," J. Electrochem. Soc. 144, 1296-1301 (1997). 31. A. B. D. Cassie and S. Baxter, "Wettability of porous surfaces," Trans. Faraday Soc. 40, 546-551 (1944).
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32. R. N. Wenzel, " Resistance of solid surfaces to wetting by water," Ind. Eng. Chem. 28, 988-994 (1936).
Figure Captions Figure 1. (a) SEM image of silicon treated by pulsed laser at a fluence of 150 J/cm2 (b)
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magnification of (a) and (c) CA of distilled H2O on the processed silicon. Figure 2. (a) SEM image of a treated silicon surface magnified by 2000 (b) same image as
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(a), magnified by 10000 (c) photo of a water droplet sitting on a treated silicon surface and
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(d) side-view of a water droplet sitting on a treated silicon surface for CA analysis.
Figure 3. SEM images of a silicon wafer treated by laser processing and chemical etching.
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The proportion of solution used is constant. The laser fluences used for the treated surfaces in (a–f) are 3.75, 7.5, 15, 75, 112.5, and 150 J/cm2, respectively, and the chemical etching time
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in the acidic fluoride solutions is fixed at 2 h.
Figure 4. Variations in the WCA of pulsed laser-processed silicon as a function of laser
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fluence (i.e., from 3.75 to 150 J/cm2) with a fixed chemical etching time of 2 h.
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Figure 5. (a) SEM image of a silicon surface irradiated by a laser fluence of 112.5 J/cm2; the chemical etching time is fixed at 2 h (b) SEM image of a silicon surface treated only by
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chemical etching for 2 h (c) EDX spectra of Fig. 7(a) and (d) EDX spectra of Fig. 7(b). Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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