Quasi-superhydrophobic porous silicon surface fabricated by ultrashort pulsed-laser ablation and chemical etching
Introduction
Superhydrophobic surfaces with water contact angles greater than 150° have attracted considerable attention because of special characteristics including self-cleaning, anti-fogging, 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 leaves [2] exhibit superhydrophobic properties due to their lower surface energy and rougher morphology. To mimic such superhydrophobic surfaces, many physical and chemical methods have been developed to synthesize such surfaces either by constructing 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], [6], [7], photolithography [8], electron beam lithography [9], electrospinning [10], using anodic aluminum oxide [11], pulsed-laser irradiation [12], [13], [14], [15], [16], [17], and chemical etching [18], [19]. Ultrashort pulsed lasers in particular have been widely employed to texture the surfaces of a variety of materials, creating all kinds of special surfaces with different functions [20], [21], [22], [23], [24]. These ultrashort pulsed lasers can also be used to form micro/nanostructures on surfaces. When followed by the coating of these surfaces with chemical materials exhibiting low surface energy, such as dimethyldichlorosilane [(CH3)2SiCl2, DMDCS] [25] and CF3(CF2)7CH2CH2SiCl3 [26], the superhydrophobicity of artificial surfaces is induced. 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-on-chips, 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 chemical-reactive gas atmosphere or extreme vacuum conditions.
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 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 150° is obtained. These fabricated surfaces can have potential applications not only in micro-fluidic devices but also in photodetectors and solar cells to improve their light-collecting efficiency.
Section snippets
Experimental details
To achieve hydrophobic or quasi-superhydrophobic surfaces, the fabrication process was 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 mode-locked pulses at 1 kHz was used as the light source. The average power employed in our work was in the range of 5–200 mW. The laser beam was polarized horizontally to the optical table (along the x-axis) and was focused
Results and discussion
Figure 1a and b shows 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 μm. 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 Figure 1a. The magnified views of the
Conclusions
Quasi-superhydrophobic porous silicon is fabricated by pulsed laser direct writing and 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 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 fluence. Elemental analysis of the EDX
Acknowledgements
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).
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