edited by Biomimetic Architectures by Plasma Processing Fabrication and Applications Surojit Chattopadhyay

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1 edited by Surojit Chattopadhyay Biomimetic Architectures by Plasma Processing Fabrication and Applications

3 Biomimetic Architectures by Plasma Processing

4 About the Cover The cover image shows a photograph (Canon EOS 7D, Canon 100 mm, f 2.8, macro, flash) of a Cicada, collected dead, placed in front of a polished silicon wafer (right) and a black silicon nanotip wafer (left). The cicada wing is highly transparent, indicating low reflection, due to the presence of conical burls on their wings. The efficacy of such conical structures in anti-reflection is demonstrated in this image. The part of the cicada in front of the polished wafer produced crisp and clear image of the cicada since the wafer is highly reflecting and mirror-like. However, the part of the insect in front of the black silicon nanotip (similar to the anti-reflecting nanostructures in cicada wings or the moth eyes) produced no image at all, even under a strong camera flash, demonstrating the anti-reflection property of the silicon nanotips (discussed in Chapter 2 of this book). Next, as we know, the conical nanostructures in the cicada wings make their wings highly hydrophobic with water contact angles around 150. This is demonstrated in the near-spherical water droplets that sit on its wings and stick to it even when the wing is not horizontal. In fact, the cicada wing can hold the water droplets in its near-perfect spherical shape even when it is turned upside down. However, if the water droplets make an impact with the wings with some velocity, as in raindrops, they bounce off the wings keeping it perfectly dry. Such wetting properties are discussed in Chapter 4 of this book. In short, the image shows, in a single photographic shot, how nature designs nanostructures for certain specific functionalities and how we are able to mimic them in the laboratories to a certain extent.

5 Biomimetic Architectures by Plasma Processing Fabrication and Applications edited by Surojit Chattopadhyay

6 Published by Pan Stanford Publishing Pte. Ltd. Penthouse Level, Suntec Tower 3 8 Temasek Boulevard Singapore editorial@panstanford.com Web: British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Biomimetic Architectures by Plasma Processing: Fabrication and Applications Copyright 2015 by Pan Stanford Publishing Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the publisher. For photocopying of material in this volume, please pay a copying fee through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA. In this case permission to photocopy is not required from the publisher. ISBN (Hardcover) ISBN (ebook) Printed in the USA

7 Contents Preface Introduction ix xi 1. Photonic Structures in the Animal Kingdom: Valuable Inspirations for Biomimetic Applications 1 Bodo D. Wilts and Doekele G. Stavenga 1.1 Introduction Principles of Animal Colouration Pigmentary Colouration Structural Colouration Note on Order and Disorder One-Dimensional Photonic Structures Thin Films Multilayers Iridescence Two-Dimensional Photonic Structures Three-Dimensional Photonic Structures Iridescence Quasi-Ordered Photonic Structures Combination of Pigmentary and Structural Colouration Increasing the Optical Contrast Increasing the Chromatic Contrast Spectral Tuning Biological Relevance of Animal Colouration Sex Recognition and Mate Choice Warning Colouration Camouflage Bio-Inspired Applications Conclusion: Design Principles for Bio-Inspired Photonic Structures 26

8 vi Contents 2. Anti-Reflecting Nanostructures 35 Abhijit Ganguly, Surojit Chattopadhyay, Pradip Kumar Roy, Li-Chyong Chen, and Kuei-Hsien Chen 2.1 Introduction Anti-reflective Coating Layer-Structured AR Coating Gradient Refractive Index AR Coating Towards Perfect AR Coating Anti-reflective Surfaces Porous AR Surfaces Moth-Eye AR Surfaces Surface-Relief Grating AR Surfaces Textured AR Surfaces Anti-reflecting Nanostructures Fabrication of AR Nanostructures Bottom-up technology Top-down technology Fabrication of wafer scale AR surfaces Biomimetic Photonic Nanostructures Silicon Nanotips for Broadband, Omnidirectional AR Non-Silicon AR Nanostructures Future Trends and Conclusion Plasma-Processed Biomimetic Nano- and Micro-Structures 91 Choonsup Lee 3.1 Introduction Silicon Nanotips Formation Two-Step Technique Single-Step Techniques Room-temperature etching using an ICP-RIE Cryogenic-etching using an ICP-RIE Conclusions 106

9 Contents vii 4. Wetting Properties of Natural and Plasma-Processed Biomimetic Surfaces 111 Pradip Kumar Roy and Surojit Chattopadhyay 4.1 Introduction Theoretical Approach Young s Equation The Wenzel State The Cassie and Baxter State Droplet Evaporation Impact Velocity Contact Angle Hysteresis and Self-Cleaning Natural Surfaces Lotus Leaf Salvinia auriculata and Lady s Mantle Fish Scale Antifouling Effect Pond Skater Dove s Feather and Sandfish Skink Adhesion Properties of Surface Fabrication of Hydrophobic and Hydrophilic Surface Hydrophilic Surface Hydrophobic Surface Conclusion 143 Acknowledgement Biomimetic Superhydrophobic Surface by Plasma Processing 151 Anyarat Watthanaphanit and Nagahiro Saito 5.1 Introduction Superhydrophobic Surfaces Superhydrophobic Surfaces in Nature Surface Wettability General Introduction to Plasma Strategies to Generate Superhydrophobic Surfaces by Plasma Processing Etching Functionalization 162

10 viii Contents Polymerization Recent Achievements in the Generation of Superhydrophobic Surfaces with Different Plasma Strategies Etching Strategy Functionalization Strategy Polymerization Strategy Superhydrophobic Surfaces with Tuning Wettability Conclusion Biomimetic Interfaces of Plasma-Modified Titanium Alloy 181 Shuilin Wu and Paul K. Chu 6.1 Introduction Plasma-Induced Biomimetic Interfacial Structure and Coatings Gas PIII&D Metal PIII&D Dual PIII&D Plasma-Induced Biomimetic Interphases Plasma Spraying Reactive Plasma-Aided RF Sputtering Deposition Plasma Polymerization Combined Plasma Treatment Conclusion and Outlook 218 Index 227

11 Contents ix Preface Mr. Stanford Chong of Pan Stanford Publishing house seemed to have taken an interest in our work on plasma-processed biomimetic nanostructures that demonstrated extraordinary anti-reflection behaviour. It was his suggestion to write a book on this topic, which we kept rejecting only because the task seemed to be too daunting. Exhaustive Google searches indicated something which I did not know before. Abundant literature is available on plasma-processed nanostructures and also on biomimetic nanostructures ; however, the sub-set of plasma-processed biomimetic nanostructures was insignificantly small compared with those bigger sets. Hence, the motivation behind this book, with an associated challenge of finding contributing authors from the limited pool available out there. Positive responses to the feeler mails sent out to the prominent authors in the field ultimately convinced us. Stanford s perseverance won. As a researcher, I know how difficult it is to find time for a book chapter amidst one s busy schedule where original research articles claim priority. I must thank my contributors, eminent in their respective fields, for their insightful chapters. These chapters outline natural nanostructures that are found aplenty around us but we are either ignorant of or are indifferent to them. However, when the visual or the functional aspects of these nanostructures are properly explained, even a kid, as young as my seven-year-old daughter, Oorja, would take keen interest in them. This book intends to give the readers a general tour of these illuminating details of nature. For the more advanced and avid readers, students, teachers, and researchers alike, technical details and the science thereof has been included to increase the depth of the subject of this book. As I read the contributed chapters, I understood the mathematical descriptions of these nanostructures more than I ever did before. I also learnt various plasma-based techniques of mimicking these natural structures in the laboratory. It provided me with a perspective of my own research, a different viewpoint to look at the tiny insects and creatures with which we co-exist. It

12 x Preface has been a wonderful two-year journey in the knowledge highway, with few speed breakers, such as proof corrections and obtaining copyright permissions, richly educating though, culminating in the present form of the book. In addition to the contributing authors, many students and colleagues have helped along behind the scenes. Dr. Yi-Fan Huang, Dr. Hung Chun Lo, Pradip Roy, Robeth Mannurung, Mau-Shiun Lee, Jia-Min Wang, and Dr. Abhijit Ganguly have helped in manuscript preparation, figure designing, scanning electron microscopy, and proof corrections. Tremendous technical help from the publishing house is also acknowledged. Hoping that this subject (book) would incite interest in many the same way as it has done to Oorja. Since her introduction to the Cicada, collected dead from the gardens of National Yang Ming University, Taiwan, July 2013, Oorja has been tireless in locating the tiniest insects and flowers whenever I threw my camera with a macro lens around my neck. Surojit Chattopadhyay Autumn 2014

13 Introduction Surojit Chattopadhyay Institute of Biophotonics, and Biophotonics and Molecular Imaging Research Center, National Yang Ming University, Taipei-112, Taiwan Nature has created myriads of nano- and micro-structures, invisible to the naked eye, that manifest externally as visible effects in colour production, wetting properties, anti-reflection, anti-stick, anti-drag, adhesive, self-cleaning, high surface tension, and so on. Although these structures appear to be inexplicable whims of Mother Nature, actually they are not. These structures could be modelled mathematically and described, nearly completely, for the functionality they demonstrate. These aspects of the natural designs have been studied by Dr. Bodo Wilts and Prof. Doekele Stavenga of the University of Groningen, The Netherlands, and described beautifully in Chapter 1. His chapter deals with micro- and nanostructures found in birds and their external manifestations in colour production, anti-reflection (AR), and other properties. Colour production and AR is being put in the same bracket here due to the fact that the dominance of photons from a particular narrow band of wavelength, which is the colour exhibited by the surface, means the suppression of photons present outside that narrow band in the broad spectrum. This amounts to selective anti-reflection or absorption. Such colour production as detailed in Chapter 1 can be

14 xii Introduction motion structural, non-motion structural, behavioural, seasonal, functional and so on. The mathematical basis of those structures, micro or nanoscale, can be understood via the tour given by Prof. Stavenga in Chapter 1. Natural architectures are generally the ones with the highest efficiency for a particular functionality. For example, the nanotextured surface of the cornea of a moth eye is anti-reflecting in the spectral region of nm. There may be other nanostructures that are anti-reflecting over a broader spectral region, but nature uses the simplest design to suit the purpose. This idea is demonstrated in all of the natural nanostructures that we observe in insects, birds and animals. However, when copying those in the laboratories we are often lured by sweeping functionalities to claim the top spot in that particular area of research. This is not deterministic meaning we did not pre-determine a structure to have such enhanced, though useless, functionalities. These structures just happened to us due to self-assembly, which we understand, modify, and optimise later. In very few cases, we could pre-determine the structure when copying them from nature. Plasma is a tool that we have at our disposal, which can be used in a deterministic way as well as in self-assembling of nanostructures [1]. The deterministic approach involves customising nanoscale organisation for specific functions. This is mostly done in a topdown approach whereby nanoscale organisation is achieved by systematic and designed removal of matter, from a solid, through a plasma solid interaction. Such interaction is non-equilibratory and complex in nature and has kinetics involved that continually keeps the nanostructures in a metastable state. Most dry-etching techniques create nanostructures through the removal of material in a controlled manner from the bulk and approaching the nanoscale. The silicon nanotips [2] mentioned in Chapter 2, 3 and 4 are examples of this. The complex process kinetics and equilibrium thermodynamics restricts the control of the top-down fabrication to a certain size below which it is no longer deterministic, and one has to depend on self-assembly and bottom-up routes for generation

15 Introduction xiii of nanostructures. Plasma is a tool to help us in both top-down and bottom-up approaches for nanostructure synthesis. Growth of nanostructures, upon the interaction of plasma with a solid surface, depends on the transfer of mass and energy between the two and a continually sustained thermodynamical non-equilibrium upon such transfer. The rate of the dynamic re-arrangement of the matter determines the degree of this non-equilibrium. Hence, there is a driving force that runs the re-arrangement and must depend on the chemical potential of the gaseous precursors and the solid phases being formed. A plasma solid system involves different reactions taking place at hierarchical length scales, from big to small, to be described here in Figure 1 that also depicts the strength of plasma in creating nanostructures [1]. Plasma reactors, normally in the scale of 1 m, will use a fraction of its volume in producing the gas phase precursors and allow for heat and matter exchange between the solid surface and the plasma bulk (Fig. 1a). Figure 1 Spatial scales used to study self-organised nanoscale plasma solid systems: (a) plasma in a reactor chamber; (b) plasma sheath near a solid surface; (c) pattern (array) of solid nanostructures; (d) individual nanostructures; (e) catalyst nanoparticle; and (f) atoms and interatomic bonds [1]. At the plasma solid interface, there is a plasma sheath whose thickness varies from few tens of micrometres to few centimetres,

16 xiv Introduction depending inversely upon the electron density of the plasma (Fig. 1b). The electric field across the plasma sheath determines the flux of impinging growth species on the substrate. The plasma exchanges matter and energy with the solid substrate through this sheath, resulting in deposition or etching or simply heating of the substrate. The exchange of matter and energy can happen at the scales of ~1 µm with the formation of nanostructures (Figs. 1c and 2). This can be done deterministically by the use of masks or catalyst nanoparticles (CNP). The CNP, in the scale of tens of nanometres, can determine the location, size distribution, and density of the resultant nanostructures. The CNP provides surface for precursor dissolution and subsequent precipitation, when supersaturated, with a structure determined by minimum energy level and the extent of non-equilibrium determined by the dynamics of the plasma and the growing nanostructures. In case of plasma etching [3], the morphology of the resulting nanostructure depends on the transfer of matter upon energetic bombardment of the particular surface (Fig. 2). Etching rates will be different for different materials under different plasma conditions characterised by its density, power, gas pressure, composition and flow ratios. Typical biomimetic nanostructures are formed by the plasma etching technique detailed in Chapter 2 by Dr. Abhijit Ganguly and others. In some cases, the plasma etching may proceed through a mask of distributed hard nanoparticles (in the scale of nanometre) on a surface as discussed by Ganguly et al. in Chapter 2 and Roy et al. in Chapter 4. In this case, the plasma must compose of multiple gases that are able to deposit the nanomask and simultaneously etch the softer substrate beneath (Fig. 2). A common example will be the etching of a silicon surface by hydrogen plasma. The mask generally determines the size and density of the resultant nanostructures [3]. In both cases mentioned in Chapters 2 and 3, the plasma is highdensity electron cyclotron resonance (ECR) plasma that includes a magnet to confine the electrons/charges in the plasma, courtesy the Lorentz force, within a restricted volume, thereby increasing the electron density of the plasma. Micrometre scale masks can be used

17 Introduction xv to produce such biomimetic nanostructures using lithography and plasma etching in conjunction. Figure 2 Schematic for anisotropic dry-etching/lithography process: (a-i) shows schematic of a mesh-type mask with aperture dimensions of Λ. SEM image of a single layer of close-packed polystyrene spheres (350 nm diameter) or anodic aluminum oxide (AAO), schematic, that can also be used as mask for dryas well as wet-etching process. A direct reactive plasma etching of a substrate (a-ii) through a simple mesh-type mask produces (a-iii) resultant nanostructures. A mask can be used to deposit (b-i) metal pads or RIE resist material on a substrate; (b-ii) The pads can resist the plasma etching, whereas (b-iii) the exposed area is etched, resulting in the nanostructures. (c-i) Ultraviolet light exposure through a mask or direct e-beam writing can be used to cure the photoresist spread on a substrate; (cii) Removal of unexposed photoresist and subsequent plasma etching, produces (c-iii) the nanostructures. Reprinted from Ref. [3], Copyright 2010, with permission from Elsevier. These nanostructures may demonstrate certain characteristics like AR even when as-grown (Chapter 2). The degree of AR may be tuned with morphology, material, and shape of the nanostructures (Fig. 3). The property of AR is demonstrated in Chapters 2 and 3 in conical or wire-like nanostructures that demonstrate extremely small reflectance values. In fact, in Chapter 3, Dr. Choonsup Lee from NASA describes the use of spiked silicon structure as a Sunsensor. The principle, of course, depends on optical AR. This type of structures is now in demand for AR purposes in solar cell devices.

xvi Introduction Figure 3 (a) Schematic of optimal gradient index AR structure, where h, h, and H denotes the height of the cylindrical part, tapered part (shown in the dashed rectangle) and total

18 xvi Introduction Figure 3 (a) Schematic of optimal gradient index AR structure, where h, h, and H denotes the height of the cylindrical part, tapered part (shown in the dashed rectangle) and total structure, respectively. S is the inter-structure spacing that controls its density, and Λ is the process controlled diameter of the structure. The apex part can have (a) straight tapered, (b) convex tapered, (c) concave tapered, (d) moth eye or (e) theoretical quintic index profiles. Reprinted from Ref. [3], Copyright 2010, with permission from Elsevier. (f) SEM image of moth eye patterned output surfaces with convextapered sidewall profiles. Reprinted with permission from Ref. [4]. Copyright 2002, AIP Publishing LLC. (g) Biomimetic moth eye profiles defined by simulation software with height (h) and period (d). Reprinted with permission from Ref. [5]. Copyright 2002, AIP Publishing LLC). (h) Top view SEM image of the GaSb nanograting. Reprinted with permission from Ref. [6], Copyright 2002, The Japan Society of Applied Physics. (i) Surface-relief pattern for an equivalent quintic on a substrate with RI of Reprinted from Ref. [7], with permission from The Optical Society, Copyright On the other hand, chemical surface modification or functionalisation is often required to impart desired properties to the created nanostructures. For example, in Chapter 4, P. K. Roy from the National Yang Ming University, Taiwan, describes the switching of wetting behaviour, from hydrophilic to hydrophobic, in silicon

19 References xvii nanostructures when they are coated with a thin layer of titania. In another case, Dr. Anyarat Watthanaphanit and Prof. Nagahiro Saito from Nagoya University, Japan, describes a superhydrophobic surface with halide (mostly fluorine groups) coating or functionalisation in Chapter 5. This is a typical example where surface modification can be done with the help of plasma also. Applications of such plasma-processed biomimetic surfaces are plenty in the field of photonics, as discussed in Chapters 2 and 3. Interestingly, there are other applications that have potential for commercialisation. One such application of plasma-processed titanium surfaces for biomedical implants is provided by Dr. Shuilin Wu and Prof. Paul Chu of City University of Hong Kong (Chapter 6). We believe that, in due course of time, plasma-processed nanostructured implants could render the conventionally used hydroxyapatite coatings, on commercial implants, useless. To end this introduction, I would urge my esteemed readers to go through a 2011 focus issue of Ref. [8], Perspectives in plasma nanoscience, and also Ref. [1], to have a thorough understanding of the processes, scope, chemistry and physics of a plasma. References 1. Ostrikov, K., Neyts, E. C., and Meyyappan, M. (2013). Plasma nanoscience: from nano-solids in plasmas to nano-plasmas in solids. Adv. Phys., 62, pp Chattopadhyay, S., Chen, L.-C. and Chen, K.-H. (2006). Nanotips: growth, model, and applications, Crit. Rev. Solid State Mater. Sci., 31, pp Chattopadhyay, S., Huang, Y. F., Jen, Y. J., Ganguly, A., Chen, K. H. and Chen, L. C. (2010). Anti-reflecting and photonic nanostructures, Mater. Sci. Eng., R, 69, pp Aydin, C., Zaslavsky, A., Sonek, G. J., and Goldstein, J. (2002). Reduction of reflection losses in ZnGeP2 using motheye antireflection surface relief structures, Appl. Phys. Lett., 80, p Boden, S. A., Bagnall, D. M., (2008). Tunable reflection minima of nanostructured antireflective surfaces, Appl. Phys. Lett., 93, p Ohira, T., Segawa, T., Nagai, K., Utaka, K., Nakao, M. (2002). Large area InP submicron two-dimensional (2D) periodic structures fabricated by two-time laser holography, Jpn. J. Appl. Phys., 41, p

20 xviii Introduction 7. Southwell, W. H. (1991). Pyramid-array surface-relief structures producing antireflection index matching on optical surfaces, J. Opt. Soc. Am. A, 8, pp Ostrikov, K., Cvelbar, U., Murphy, A. B. (eds. ) (2011). Perspectives in plasma nanoscience Plasma nanoscience: setting directions, tackling grand challenges. J. Phys. D: Appl. Phys., 44, pp

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