Studies on nanostructured anti reflective coatings

STUDIES ON NANOSTRUCTURED ANTI-REFLECTIVE COATINGS HEMANT KUMAR RAUT NATIONAL UNIVERSITY OF SINGAPORE 2014 STUDIES ON NANOSTRUCTURED ANTI-REFLECTIVE COATINGS HEMANT KUMAR RAUT (B. Tech., College of Engineering and Technology, Odisha, India) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2014 ACKNOWLEDGEMENTS These four years of graduate studies at NUS have been tremendously valuable for me, both professionally and personally. All the encouragement and help I received along the way was absolutely crucial in completion of this doctoral thesis. I will begin by expressing my sincere gratitude to Professor Seeram Ramakrishna, my supervisor, for his constant support and guidance during the four years of graduate research. The freedom he gave me to formulate original research ideas and implement them was deeply enriching. His motivation was crucial in overcoming the challenges encountered during these four years and he would always remain a positive inspiration in my life. I would also like to thank my co-supervisor Dr. M. S. M Saifullah, from the Institute of Materials Research and Engineering (IMRE), for allowing me to undertake some of my research in his laboratory. I am also thankful to Dr. G Ramakrishnan from Birla Institute of Technology and Science for engaging in scientific discussions regarding my research. Special thanks are due to Dr. E. Leong from IMRE and Dr. K. E. Huat from Institute of High Performance Computing (IHPC) for their assistance in numerical simulations as well as to Dr. Timothy Walsh from the Solar Energy Research Institute of Singapore (SERIS) for providing guidance on fabrication and characterization of photovoltaics. I would like to express special thanks to Dr. J. Venugopal for his valuable inputs and suggestions in regard to the thesis and the immense encouragement he gave during the compilation of the same. I Thanks are due in large measures to my friends Anand, Saman, Kwadwo, Naveen, Rajeshwari and Deepak who have been immensely helpful all these years and made my time at NUS a memorable experience. I would like to acknowledge the prestigious NUS research scholarship that enabled me to undertake my Ph.D. research. I would also like to thank Teo Lay Tin Sharen and Azzlina Binte Shaharuddin from the Department of Mechanical Engineering and the Registrar’s office respectively, for their cordiality and cooperation in administrative matters. Lastly, I would like to dedicate this thesis to my parents who have given me unfathomable love and affection throughout my life and have made great sacrifices to let me pursue the path I wanted to take. II TABLE OF CONTENTS Acknowledgements I Table of Contents III Summary VIII List of Publication X List of Tables XII List of Figures XIII List of Abbreviations XXI 1. Introduction 1.1 Background and motivation 1.2 Biomimetics 1.3 Biomimetic nanostructured ARC 1.4 Scope and objectives of the thesis 1.5 Organization of the thesis 2. Literature Review 2.1 Introduction 10 2.2 The physics of ARC 11 2.3 Computational tools for the analysis of ARC 13 2.3.1 Effective medium theory (EMT) 13 2.3.2 Rigorous coupled wave analysis (RCWA) 14 2.3.3 Finite-difference time-domain (FDTD) 15 2.4 Characteristics of perfect ARC 17 2.4.1 Broadband anti-reflection 17 2.4.2 Omni-directional anti-reflection 17 III 2.5 Types of ARC 19 2.5.1 First generation ARC (based on number of layers) 19 2.5.1(a) Single layer ARC 19 2.5.1(b) Multiple layer ARC 19 2.5.1(c) Gradient refractive index ARC 20 2.5.2 Second generation ARC (based on surface topography) 23 2.5.2(a) Porous ARC 23 2.5.2(b) Biomimetic “moth's eye” nanostructures 24 2.6 Techniques for fabrication of ARC 30 2.6.1 Sol–gel processing 30 2.6.2 Vapor deposition techniques 31 2.6.3 Etching 33 2.6.4 Lithography 35 2.7 Materials used in fabrication of ARC 39 2.7.1 Silicon-based ARC 39 2.7.2 Oxide-based ARC 40 2.7.3 Polymer-based ARC 41 2.8 Durability of ARC 43 3. Porous Nanostructured Magnesium Fluoride Anti-reflective Coatings 3.1 Introduction 46 3.2 Experimental 47 3.2.1 Materials 48 3.2.2 Sol-gel processing 48 3.2.3 Characterization 49 IV 3.3 Results and discussions 50 3.3.1 Film morphology and characterization 50 3.3.2 Optical characterization 54 3.3.3 Durability assessment 59 3.3.4 Water repellence characterization 61 3. Conclusion 61 4. Porous Nanostructured Silicon Dioxide Anti-reflective coatings 4.1 Introduction 62 4.2 Experimental 63 4.2.1 Materials 63 4.2.2 Sol-gel processing 64 4.2.3 Characterization 64 4.3 Results and discussions 66 4.3.1 Film morphology and characterization 66 4.3.2 Optical characterization 70 4.3.3 Durability assessment 74 4.3.4 Water repellence characterization 76 4. Conclusion 76 5. Moth’s Eye Inspired Nanostructure Patterns for Broadband and Omnidirectional Anti-reflective Coatings 5.1 Introduction 78 5.2 Experimental 81 5.2.1 Materials 81 5.2.2 Methacryl POSS resist formulation and characterization 82 5.2.3 Nanoimprint lithography 82 V 5.2.4 Characterization 83 5.2.5 FDTD simulations 85 5.3 Results and discussions 86 5.3.1 Structural characterization 86 5.3.2 Optical characterization 92 5.3.3 Durability assessment 98 5.3.4 Water repellence characterization 100 5. Conclusion 101 6. Moth’s Eye Inspired Multi-scale Ommatidial Arrays for Anti-reflection and Water Repellency Characteristics 6.1 Introduction 103 6.2 Experimental 106 6.2.1 Materials 106 6.2.3 Nanoimprint lithography 106 6.2.4 Characterization 107 6.2.5 FDTD simulations 108 6.3 Results and discussions 109 6.3.1 Fabrication of multi-scale ommatidial arrays 109 6.3.2 Structural characterization 112 6.3.3 Optical characterization 114 6.3.4 Durability assessment 117 6.3.5 Water repellence characterization 117 6. Conclusion 121 7. Conclusion and Future Work 7.1 Conclusion 122 VI 7.2 Key research contributions 123 7.3 Future work 124 Bibliography 127 Appendix A: Supporting information for fabrication of porous MgF2 ARC on glass 153 Appendix B: Supporting information for fabrication of porous SiO2 ARC on glass 154 Appendix C: Supporting information for fabrication of POSS-based moth’s 156 eye nanostructure arrays on glass Appendix D: Supporting information for fabrication of multi-scale ommatidial arrays on glass 158 Appendix E: Peel test methodology for nanostructured ARC glass 160 Appendix F: Durability tests for nanostructured ARC glass 161 VII Bibliography [172] T. S. Haddad, R. Stapleton, H. G. Jeon, P. T. Mather, J. D. Lichtenhan, S. Phillips, Nanostructured hybrid organic/inorganic materials. Silsesquioxane modified plastics, in: Division of Polymer Chemistry - American Chemical Society, vol. 40, pp. 496-497. 1999. [173] G. Li, L. Wang, H. Ni, C.U. 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Suh, 25th Anniversary Article: Scalable Multiscale Patterned Structures Inspired by Nature: the Role of Hierarchy, Advanced Materials, vol. 26, pp. 675-700, 2014. 152 Appendix A: Supporting information for fabrication of porous MgF2 ARC on glass In this Appendix, information pertaining to the optimization of the sol-gel precursor employed for fabrication of the porous MgF2 ARC on glass have been described. Additionally, information supporting the durability tests conducted for the porous MgF2 ARC glass substrates have been included. Sol-gel optimization: The precursor MT11A showed a very good adherence to glass substrate for sol-gel processing compared to the non-polymer precursor version, MT11. Preliminary studies were performed using three sol-gel precursors, MT11A, MT13 and MT31. Details of the precursors are given in Table S1. The thin-films fabricated from the precursors MT11A and MT31 were found to be transparent and exhibited anti-reflective property (Figure A1). ARC fabricated on one side of the glass by MT11A showed a superior transmittance than that of MT31. However, the thin-film fabricated from MT13 turned completely brown and showed a sharp drop in transmittance thus, being discarded from further study. MT11A (molar ratio of MGAC and TFA as 1:1) was selected as the sol-gel precursor to fabricate the porous MgF2 ARC and perform further characterization. 153 Appendix Table A1: Composition of precursors formulated for preliminary studies to fabricate porous magnesium fluoride (MgF2) ARC Sol-gel Magnesium Precursor Acetate Name Tetrahydrate (mM) 9.97 MT11 4.00 MT13 9.97 MT31 9.97 MT11A Trifluroacetic 2-propanol Acid (mM) (mM) Polyvinyl acetate (mM) 9.92 13.06 3.26 9.92 PVAc: 0.012 PVAc: 0.012 PVAc: 0.012 194.68 194.68 194.68 194.68 Figure A1: Transmittance graphs and pictures of thin-films fabricated from precursors MT11A and MT31 on glass substrates. It can be seen that the transmittance of porous thin-films fabricated from MT11A is higher than that of MT31. The brown thin-film formed after calcination of MT13 on glass substrate can also be seen. 152 Appendix Figure A2: Microscopic images and SEM images of the porous MgF2 ARC glass substrate before and after the peel-off test as described in Appendix E. 153 Appendix Appendix B: Supporting information for fabrication of porous SiO2 ARC on glass In this Appendix, information pertaining to the optimization of electrospinning technique to fabricate porous SiO2 ARC on glass have been provided. Additionally, information supporting the durability tests conducted for the porous SiO2 ARC glass substrates have been included. Optimization of sol-gel processing via Electrospinning: The duration of deposition of the nanofibres on a glass substrate determines the film thickness of the porous SiO2 ARC after calcination. The transmittance of ARCs corresponding to the initial coating duration has been summarized in Table B1. Similarly, the concentration of the SiO2 precursor, tetraethoxysilane (TEOS) also plays an important role in attaining the desired anti-reflection property. Table B2 summarizes the transmittance obtained from glass coated for 30 minutes, with proportionately increasing concentration of TEOS in the sol-gel. 154 Appendix Figure B1: Curve showing the maximum transmittance of the porous SiO2 ARC glass obtained for coating duration from 10 to 45 minutes. Figure B2: Curve showing the maximum transmittance of the porous SiO ARC glass obtained for different concentration of the SiO2 precursor, tetraethoxysilane (TEOS). Figure B3: Microscopic images and SEM images of the porous SiO2 ARC glass substrate before and after the peel-off test as described in Appendix E. 155 Appendix Appendix C: Supporting information for fabrication of POSSbased moth’s eye nanostructure arrays on glass In this Appendix, information pertaining to the preparation and optimization of resist for thermal nanoimprint lithography of moth’s eye nanostructure arrays on glass substrate have been provided. Optimization of resist: Methacryl POSS (MPOSS) resist compositions were formulated by mixing MPOSS with cross-linkers ethylene glycol dimethacrylate, 1,4-butanediol diacylate and 1,6-hexanediol diacrylate in molar ratios of 1:8, 1:10 and 1:12. These were spin-coated on glass and cured at 130 °C for 25 to form thin-films. Transmittance measurement showed that a molar ratio of 1:12 for all the MPOSS-cross-linker combinations gave the highest marginal rise in transmittance. Out of these, the resist formulation comprising MPOSS and 1,6-hexanediol diacrylate, in 1:12 molar ratio showed the highest increase in transmittance to 92.5% as shown in Figure C1. 156 Appendix Figure C1: Transmittance graphs of plane glass and cured MPOSS-HDA thin films of thicknesses 700 nm, 450 nm and 200 nm, respectively on glass. Notice that there is hardly any change in the transmittance in case of the MPOSS-HDA thin films of different thicknesses in comparison to that of plane glass. This shows that the presence of a residual layer of 200 nm does not alter the transmittance of the imprinted moth’s eye ARC glass. 157 Appendix Appendix D: Supporting information for fabrication of multiscale ommatidial arrays on glass In this Appendix, information supporting the fabrication of multi-scale ommatidial arrays by sacrificial-layer mediated nanoimprinting have been described. Figure D1: A thin-film of Poly(sodium 4-styrenesulfonate) (PSS). Figure D2: (a) anti-reflective nanostructures buckled and deformed after imprinting of micro-lens, without a sacrificial layer to secure the nanostructure arrays (b) A conventional, direct imprinting approach involving imprinting of anti-reflective nanostructures atop previously imprinted micro-lens arrays that could not fully replicate the nanostructure arrays. 158 Appendix Figure D3: Finite-difference-time-domain simulations showing the reflectance obtained for multi-scale ommatidial arrays or diameter µm and 20 µm. 159 Appendix Appendix E: Peel test methodology for nanostructured ARC glass Peel tests were carried out using the peel test apparatus shown in the schematic below (Figure S1 to appear in Appendix). The coated substrate was firmly mounted on a light weight, low-friction horizontal stage. A pressure sensitive tape, (Brand 3M Scotch 600) was used for the adhesive tape, and a length L of the tape was applied uniformly across the surface of the coated substrate and the other end of the tape was connected to a vertical stage via a digital force gauge (ELC09S Tensile Load Cell, Xiamen Elane Electronics). The adhesive tape was pulled vertically from the substrate at a constant speed, with the 90° geometry being maintained by means of a narrow support bar oriented along the inner bend of the adhesive tape. After peel-off of the tape, the coated substrate was examined under the optical microscope for any delamination or cracks on the film surface. Figure E1: Schematic of the coating peel-off testing apparatus. 160 Appendix Appendix F: Durability tests for nanostructured ARC glass The tests conducted to evaluate the durability of the nanostructured ARC glass substrates studied in the thesis, are described in the following table. Name of the test Immersion test Description/Procedure Immersion in water maintained at 85 °C for 100 h. Immersion acid test Immersion in aqueous acid solution (100 mM H2SO4) maintained at 35 °C for 100 h.[180] Immersion base test Immersion in aqueous basic solution (0.1 M NaOH) maintained at 60 °C for 100 h.[180] Saline exposure test Exposure to fog produced by atomization of 5% NaCl aqueous solution at a temperature of 35 °C for 100 h. Scratch hardness test Pencil hardness scratch test involves using a standard scratch hardness tester (Erichsen, Model 291) that enables the test to be complaint with the WolffWilborn Test (ASTM D3359-02). Pencils of various grades of hardness (1H – 6H) are moved over the coating surface at an angle of 45° to the horizontal with a fixed force of 7.5 ± 0.1 N. The specified force and angle remains constant throughout the test. The degree of hardness of the pencil which damages/scratches the coating on glass substrate is taken as a measurement of scratch hardness (e.g., “2H” hardness). 161 [...]... for fabrication of these complex arrays on large-area substrates In addition to studying the broadband and omnidirectional antireflection property of these arrays, their water repellence characteristics are also analyzed  Chapter 7 states the conclusion of the thesis with a summary of the observations made in the individual chapters Recommendations for conducting further research on the nanostructured. .. water immersion test, saline fog test, acid immersion tests and basic solution immersion tests Detectable variation in the transmittance of the porous MgF2 ARC glass was observed after conducting these tests for 100 h Figure 4.1 Fabrication of the porous SiO2 ARC on glass (a) A 65 conventional electrospinning setup (b) Automated electrospinning performed in NANON to fabricate porous SiO2 ARC on 20 × 20... patterns on glass with emphasis on achieving broadband and omnidirectional anti- reflection properties 2 Implement a fabrication technique to enable the fabrication of these nanostructured ARC on large-area glass substrate 3 Investigate these nanostructured ARC for adherence to glass, hardness and mechanical strength, chemical and thermal stability of the ARC on glass substrates 4 Study these nanostructured. .. also important considerations for their practical application In this thesis, nanostructured ARC of two types, namely porous nanostructured ARC and anti- reflective nanostructure patterns have been studied for minimizing reflection losses in glass Porous nanostructured ARC of an optimally low refractive index have been fabricated on large-area glass substrates to impart antireflection properties to... discussed with fabrication of these anti- reflective nanostructures patterns on glass by nanoimprint lithography Complete structural characterization of the patterns, aided with finite difference time domain (FDTD) simulations explain the superior broadband and omnidirectional antireflection properties of the patterns American Society for Testing and Materials (ASTM) based tests are conducted on the ARC glass... selective dissolution of PMMA resulted in a graded distribution of PS domains in the vertical direction (c) Picture of a polystyrene Petri dish coated on the left half with a bilayer porous polymer anti- reflective film.[60] (d) Picture of a glass slide with a selectively patterned nanoporous coating with region 1 showing plane glass and region 2 showing the ARC The anti- reflection region can be switched... omnidirectional anti- reflection properties, the multi-scale arrays exhibit superior water repellency that can help retain the optical properties of the arrays even in wet and humid conditions These multi-functional properties could potentially enhance the performance of optoelectronic devices incorporating the multi-scale arrays and minimize the influence of in-service conditions IX LIST OF PUBLICATIONS Publications... why PV, in some cases are made to automatically track the sun throughout the day, which obviously incurs additional power consumption Planar glass used in architectural or decorative applications, also confront aesthetic limitations due to reflection Moreover, Chapter 1 reflection of light from oncoming vehicles at night causes glare in automotive window shields that severely compromises the visibility... proportion Secondly, moths and certain other nocturnal insects show an unsurpassed capability to see in low-light conditions.[22] A closer examination of these miniaturized eyes reveal an elegant pattern of anti- reflective nanostructures, alternatively called the moth’s eye nanostructures, on their eyes.[23,24] The optical function of these nanostructure patterns is to severely reduce the reflection of... polyhedral oligomeric silsesquioxane-based anti- reflective nanostructures with broadband quasi-omnidirectional properties”, Energy & Environmental Science, vol 6, pp 1929-1937, 2013 3 Hemant Kumar Raut, S S Dinachali, K A Kwadwo, V A Ganesh, S Ramakrishna, “Fabrication of highly uniform and porous MgF2 antireflective coatings by polymer-based sol-gel processing on large-area glass substrates”, Nanotechnology, . STUDIES ON NANOSTRUCTURED ANTI- REFLECTIVE COATINGS HEMANT KUMAR RAUT NATIONAL UNIVERSITY OF SINGAPORE 2014 STUDIES ON NANOSTRUCTURED ANTI- REFLECTIVE COATINGS. 3.3.4 Water repellence characterization 61 3. 4 Conclusion 61 4. Porous Nanostructured Silicon Dioxide Anti- reflective coatings 4.1 Introduction 62 4.2 Experimental 63 4.2.1. repellence characterization 76 4. 4 Conclusion 76 5. Moth’s Eye Inspired Nanostructure Patterns for Broadband and Omnidirectional Anti- reflective Coatings 5.1 Introduction 78 5.2 Experimental