中華大學

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中華大學博士論文

以奈米球微影術製備奈米結構及其應用 Fabrication of Nanostructure by Nanosphere Lithography Technology and Its Applications

系所別：工程科學博士學位學程學號姓名：D09624006 吳泓均指導教授：簡錫新博士

馬廣仁博士

中華民國 102 年 1 月

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Abstract

Periodic nanostructures have recently gained widespread attention because of their unique properties and promising applications in numerous fields, such as sensors, photonic crystals, and optoelectronic devices. Various techniques can be used to fabricate periodic arrays of nanostructures, including X-ray lithography method, electron-beam lithography (EBL), and interference lithography (IL). Although these lithographic techniques have been used to control the morphology of the periodic structure arrays, they are time consuming and costly.

Hence, researchers have focused on the study of simple, high-resolution and low-cost nanosphere lithography (NSL) technique to fabricate periodic nanostructure arrays. This dissertation investigates the structure, morphology, wettability, and optical properties of Cr, CrN, and polycarbonate (PC) nanostructure patterns using technology based on NSL. The content of this dissertation is divided into three sections. Firstly, this study shows the effect on their capillary force and convective ﬂux properties associated with controlling the spin speed of the spin coater and concentration of the polystyrene (PS) nanosphere solution. A monolayer of nanospheres with a hexagonal close-packed array structure was formed on glass substrates using the spin-coating method. The size and shape of the size-tunable ordered PS nanosphere arrays structure can be manipulated using the reactive ion etching (RIE) process. The second section of this dissertation investigates the fabrication of hierarchical porous Cr nanoring array patterns using a magnetron sputtering process and NSL-based technique. To study the optical effects of the substrate and the Cr ring-shaped nanostructure film, different nanosphere sizes and deposition thicknesses are introduced to adjust the size and shape of the ring-shaped nanostructures. The optical transmittance of porous Cr nanoring arrays can be enhanced using surface plasmon resonance. The luminous enhancement at a special wavelength of 525 nm and better color purity were observed in this Cr nanoring structure. This new approach has

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various potential applications in optoelectronic devices and optical sensors. The third section of this dissertation shows that the surface of the nanomold of CrN nanohole arrays has a low surface energy. The optical properties of the antireflective PC-tapered nanopillar layer were successfully replicated from the CrN nanomold. These antireflective surfaces are promising for the fabrication of antireflective surface structures with various bands, and these antireflective optical materials have wide applications in various optoelectronic products.

Keywords: Nanoring arrays; Reactive ion etching; Magnetron sputtering; Antireflective nanostructure; Nanosphere lithography

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摘要

週期性之奈米結構由於其獨特的性質，在光子晶體、傳感器和光電子元件等之應用已受到廣泛的關注。目前許多種微影技術可製備週期性陣列之奈米結構，如 X 光微影技術、電子束微影術和干涉微影技術等；但因為成本高、製程耗時及複雜，已限制了它們的用途。因此近來越來越多的研究者開始發展簡單、低成本及高解析度的奈米球微影技術製備週期性之奈米結構。本論文利用奈米球微影術為基礎之技術在玻璃及矽基材上製備鉻環狀奈米結構、氮化鉻孔狀奈米結構和聚碳酸酯錐形奈米柱狀結構，探討其結構、

形貌、潤濕性及光學性能。本論文的內容主要分為三個部分。首先，本研究中對聚苯乙烯奈米球的毛細作用力和對流作用力與控制溶液的濃度和旋轉塗佈機的旋轉速度之相關聯性做了探討，藉由旋轉塗佈方法在玻璃基板上形成具有六方最密堆積結構之單層奈米球陣列結構，並可經由後續之反應性離子蝕刻製程控制其尺寸和形狀。在本論文之第二部分，我們首次研究利用磁控濺射及奈米球微影技術製備多層次之鉻奈米環陣列圖案，利用不同尺寸的奈米球和不同的沉積厚度來調整環狀奈米結構的尺寸和形狀，並研究鉻奈米環狀奈米結構所產生之光學效應。結果顯示具有週期性多孔鉻奈米環之基板，

由於發生表面等離子體共振效應，使其在 525 nm 波段穿透率明顯增強。此種新製程方法將會促進金屬奈米環未來在光學感應器和光電元件之應用。在本論文之第三部分，利用奈米球微影術可製備出氮化鉻奈米模具，由於氮化鉻模具表面具有低的表面能，有助於改善在脫模時之粘黏問題。研究使用氮化鉻奈米模具在聚碳酸酯表面成功地壓印出抗反射錐形奈米柱結構，未來可延伸此技術用於製備不同波段之抗反射表面結構，廣泛應用於不同的光電產品。

關鍵字: 奈米環陣列; 反應性離子蝕刻; 磁控濺鍍; 抗反射奈米結構; 奈米球微影術

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Acknowledgement

回首博士班期間的研究生活，雖忙碌緊湊辛苦，但卻令人懷念。在這段兢兢業業的日子裡，有良師及研究伙伴的指導与相互鼓勵，使得我的學術研究生活既充實且愉快。

首先要感謝我的指導教授簡錫新博士及共同指導教授馬廣仁博士，在博士班求學期間，不僅在學術研究上的殷切指導與啟發，給我自由的空間及全力的支持，而且在專業領域的開導及訓練，使我受益良多，我將永遠銘記於心。而在口試期間承蒙考試委員趙崇禮博士、劉道恕博士、陳大同博士及葉明勳博士的悉心指導與建議，使得本論文更臻理想。

接下來感謝寧波工程學院材料所鮑明東教授對我在寧波工程學院期間的學術研究指導以及生活上的照顧，由衷感謝。這段期間也感謝校長高浩其博士、校長特助鐘小斐老師及外事處阮東波老師在平時生活上的關心及照顧。此外，感謝寧波工程學院材料所邵双喜老師、楊為佑老師、徐雪波老師及鄔寧昆老師等給予諸多的指導及建議，使我獲益良多。感謝中國科學院寧波材料技術與工程研究所實驗員對我實驗測試的幫助。另外，感謝寧波工程學院材料所的學弟妹們在實驗研究中給予的支持與幫助。以上表示誠摯的感謝。

感謝林君明老師在校期間給予諸多的指導及建議，也感謝林育立老師在實驗上的協助與指導，藉此機會表示誠摯的感謝。感謝中央研究院應用科學研究中心何羽軒及台北科技大學彭凱鈺在實驗上的幫助。同時感謝實驗室 Johnny、品瑀、錦宏、松銓、時瑞、

世昌、維傑、書瑋平日熱心的幫忙及協助。

當然還要特別感謝我這位高中同窗摯友目前任教於台北科技大學的魏大華博士，在論文及儀器上提供與實驗技術及經驗的指導，讓我的實驗研究進行更為順利。

最後，將此論文獻給自小辛苦培育我的父母親、摯愛的家人及所有的朋友，感謝他們於博士班求學期間給我的支持、關心與付出，使我能順利完成博士班學業。

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List of Tables

Table 2-1 Conditions for forming the various shapes of moth-eye structures . ... 42 Table 3-1 Specifications of polystyrene nanospheres with 540nm diameter. ... 52 Table 3-2 Concentration and corresponding spin speed for nanospheres with diameter of

960 nm and 540 nm. ... 53 Table 3-3 Parameters for the manipulation of polystyrene nanospheres with diameters of

960 nm via reactive ion etching system. ... 56 Table 3-4 Parameters for the manipulation of polystyrene nanospheres with diameters of

540 nm via reactive ion etching system. ... 57 Table 3-5 Parameters of sputtering deposition. ... 58

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List of Figures

Fig. 2-1 Schematic diagrams of nanosphere lithography. (a) A monolayer of polystyrene nanospheres is formed on the clean glass substrate by spin coating process. (b) Reduce the nanospheres by oxygen plasma etching. (c) Using a closed field unbalanced magnetron sputtering ion plating system (CFUBMIP) to deposit the metal to fill the gap between nanospheres. (d) The periodic metallic nanostructure is formed after the lift-off process... 7 Fig. 2-2 Electron micrograph showing a random array of 91 nm polystyrene nanospheres

colloidally coated on an aluminum film. ... 8 Fig. 2-3 (a) The monolayer nanosphere masks and the corresponding periodic particle

array surfaces. (b) The bilayer nanosphere masks and the corresponding periodic particle array surfaces. ... 8 Fig. 2-4 (a) Au nanodot arrays and inset shows the monolayer of Au coated hexagonally

packed PS nanospheres as the mask on GaN. (b) The tilted view of ZnO nanowire arrays obtained using a monolayer of PS spheres as the template. (c) The tilted view of ZnO nanowire arrays obtained using a bilayer of PS nanospheres as the template. (d) TEM image shows the vertical alignment and the presence of Au tips (indicated by the circle), the inset is the electron diffraction pattern recorded from the circled area and indicating the growth direction along ＜0001＞. ... 11 Fig. 2-5 (a) SEM image shows the SiNW growth on the Si substrate. (b) Using an in-lens

secondary electron detector. (c) Using a back-scattered electron detector [39]. ... 12 Fig. 2-6 (a) Schematic diagram of diameter and spacing achieved by nanosphere

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Magniﬁed SEM image of the nanospheres monolayer. (d) SEM of the large

metallic nanostructures area on substrate. ... 13

Fig. 2-7 SEM images of the fabricated CNTs. (a) Top view of the grown CNT using thermal CVD. (b) Top view of the grown CNT using PECVD. (c) Cross sectional view of the dense CNTs grown using PECVD. The TEM images of the CNTs were shown the inset. (d) Raman spectrum of the grown CNTs. ... 14

Fig. 2-8 Device structures of two types of OLEDs with scattering medium. ... 15

Fig. 2-9 The photographs of the conventional OLED. (a) Turn-off state under normal room light. (b) Under driving in dark. (c) Type I device under driving. (d) Type II device under driving. ... 15

Fig. 2-10 Two spheres partially immersed in a liquid layer on a horizontal solid substrate. The deformation of the liquid meniscus gives rise to interparticle attraction. ... 18

Fig. 2-11 (a) Flotation lateral capillary forces. (b) Immersion lateral capillary forces. ... 18

Fig. 2-12 Comparison between immersion and flotation capillary forces. ... 19

Fig. 2-13 Schematics drawing of the convection flow force... 20

Fig. 2-14 Schematics of the drop-coating method. The inset shows a cross section of the bath, demonstrating the transfer of a monolayer onto a solid substrate. ... 21

Fig. 2-15 Schematic diagram of the particle and water fluxes in the vicinity of monolayer particle arrays growing on the substrate plate that is being withdrawn from a suspension. The inset shows the menisci shape between neighboring particles. ... 22

Fig. 2-16 The preparation procedures of monolayer PS nanospheres on the substrate. (a) The nanospheres onto the water surface. (b) Addition of sodium dodecyl sulfate solution to consolidate the nanospheres. (c) Lift off the ordered monolayer by the substrate. ... 24

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Fig. 2-17 Image of the convective self assembly continuous process instruments. ... 24 Fig. 2-18 Schematic diagram of the convective self assembly process. ... 25 Fig. 2-19 Schematic diagram of the convective self assembly instruments. ... 25 Fig. 2-20 Schematic illustration of the template-directed self-assembly of nanospheres on

the patterned metal electrode. ... 27 Fig. 2-21 Schematic graph of the spin speed in region X is changed by 100 rpm from 100

to1000 rpm. ... 28 Fig. 2-22 Schematic diagram of the collective oscillations of free electrons. (a) The

metal–dielectric interface: the propagating surface Plasmon. (b) The spherical gold colloid: the localized surface plasmon. ... 30 Fig. 2-23 Effects of parameters on zero-order transmission spectra. (a) Spectra for various

square arrays as a function of λ/a0. Solid line: Ag, a0 = 0.6 um, d = 150 nm, t = 200 nm; dashed line: Au, a0 = 1.0 um, d = 350 nm, t = 300 nm; dashed-dotted line: Cr, a0 = 1.0 um, d = 500 nm, t = 100 nm. (b) Spectra for two identical Ag arrays with different thicknesses. Solid line: t = 200 nm; dashed line: t = 500 nm (this spectrum has been multiplied by 1.75 for comparison). For both arrays: a0

= 0.6 um; d = 150 nm... 33 Fig. 2-24 Focused ion beam image of the silver two-dimensional hole arrays ﬁlm, with ﬁlm

thickness t = 200 nm, period a0 = 900 nm , and hole diameter d = 150 nm. ... 33 Fig. 2-25 Transmission spectrum of the miniarray holes (dashed curve) and the long-range

hexagonal array with a0 = 1 um (solid curve). ... 34 Fig. 2-26 SEM image of PS nanospheres and hole arrays. (a) The PS nanospheres with

diameter of 300 nm arrays monolayer. (b) The reduced PS nanospheres on the p-GaN layer after etching with oxygen for 60 sec. (c) A cross section of the ITO

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deposited on the p-GaN layer and the PS nanospheres. (d) The hole patterned ITO after removing PS nanospheres. ... 34 Fig. 2-27 SEM image of the hole and pillar patterned layer. (a) The p-GaN hole patterned

layer. (b) The cross sectional image of the hole patterned p-GaN after depositing the ITO contact layer. (c) The pillar patterned ITO contact layer. (d) The cross sectional image of the pillar patterned ITO contact layer. ... 35 Fig. 2-28 The light output power and injection current (L-I) of the LEDs with the p-GaN

hole patterned layer (▲), the ITO pillar patterned contact layer (●) , and the conventional LED (■). ... 35 Fig. 2-29 SEM image of the surface structures found covering the eyes of night flying

moths. The texture consists of cone structures of packed hexagonal array with a spacing of 200 nm about high 200 nm. ... 37 Fig. 2-30 Schematic drawing of the light-wave propagation through the antireﬂective

subwavelength structures surface simulated by the FDTD method. ... 38 Fig. 2-31 Simulation values of the reﬂectance of light propagating from the air into the

polymer material ns = 1.54 with and without conical subwavelength nanostructures surface. ... 38 Fig. 2-32 (a) The conical shapes of the antireﬂective subwavelength surfaces. (b) The

pyramidal shapes of the antireﬂective subwavelength surfaces. (c) The transmittance of the conical type with different aspect ratios as a function of wavelength. (d) The transmittance of the pyramidal type with different aspect ratios as a function of wavelength. ... 39 Fig. 2-33 (a) AFM image of the three-dimensional topography surface. (b) The height

profiles of two-dimensional periodic nanostructures. (c) FE-SEM image of the

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concave array on Ni nanomould. (d) Contact angle image of water drop on Ni nanomould without atmospheric-pressure plasma (AP) surface treatment. ... 41 Fig. 2-34 The measured results of transmittance in the spectral range from 300 nm to 800

nm for imprinted ORMOCER/PET and PET ﬁlms. ... 41 Fig. 2-35 (a) Transmittance of the moth-eye patterned LED and the un-patterned reference

LED. (b) Photoluminescence spectra of the moth-eye patterned LED and the un-patterned reference LED... 42 Fig. 2-36 SEM images of various nano/micro multiscale structures. (a) The nanopores

structure (Ø 200 nm) of AAO nanomold. (b) Nickel intermediate film mold using LIGA process (Ø 30 lm, height 100 um. (c) Nano/ micro multiscale structures IFMI hot embossing process with intermediate film mold fabricated by LIGA process. (d) Nano/ micro multiscale structures via the chemically etched intermediate film mold by IFMI hot embossing process. ... 45

Fig. 2-37 SEM images of the micro lens of nanostructures array. ... 46 Fig. 2-38 The measured reﬂectivity of the bare PC ﬁlm and the fabricated PC with

nanostructures. ... 46 Fig. 2-39 (a) AFM image of periodic of 1.1 um PS spheres array monolayer. (b)

Comparison of the normal incidence semispherical reﬂection of the ﬂat Si wafer (solid) and 1.1 um PS sphere masked Si array (dot). Inset is the SEM image of the Si nanopillar arrays structure. ... 47 Fig. 2-40 SEM and AFM images of the AR nanostructures array were nanoimprinted on

the glass substrate using the UV-NIL process. ... 47 Fig. 2-41 Transmittance of the AR nanostructures nanoimprinted on the glass substrate

measured by the UV–visible spectrophotometer. ... 48

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Fig. 3-1 The flowchart for nano-patterning lithography study... 50

Fig. 3-2 The graph of (a) 960 nm and (b) 540 nm size of nanospheres for spin speed versus spin time. ... 54

Fig. 3-3 SEM images of the PS nanosphere arrays monolayer. (a)Top view of the 960 nm monolayer PS nanosphere arrays (volume ratio of surfactant to nanospheres = 1:6) on the glass substrate. (b) The cross sectional image of f the 960 nm monolayer PS nanosphere arrays. (c) Top view of the 540 nm monolayer PS nanosphere arrays (volume ratio of surfactant to nanospheres = 1:2) on the glass substrate. (d) The cross sectional image of f the 540 nm monolayer PS nanosphere arrays. ... 55

Fig. 3-4 Spin coater. ... 59

Fig. 3-5 Reactive ion etching system. ... 60

Fig. 3-6 Closed field unbalanced magnetron sputtering ion plating system (CFUBMIP). ... 62

Fig. 3-7 Schematic illustration of the magnetic field configuration and sample fixturing in the CFUBMIP system. ... 62

Fig. 3-8 Schematic diagram of contactless gas assisted pressing system. ... 63

Fig. 3-9 HITACHI-S-4800 Scanning electron microscope (SEM). ... 64

Fig. 3-10 Dimension 3100V atomic force microscopy (AFM). ... 65

Fig. 3-11 X-ray diffraction (XRD). ... 66

Fig. 3-12 Surface tension contact angle meter. ... 67

Fig. 3-13 UV visible NIR Spectrophotometer (Hitachi, U-4100). ... 68

Fig. 3-14 Schematic illustration of the light enhancement measurements system. ... 69

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Fig. 4-1. Schematic illustration and SEM images of PS nanospheres distribution (diameter:

540 nm, spin speed: 1200 rpm) under various concentrations. (a-c) The appropriate concentrations (volume ratio of surfactant to nanospheres = 1:2) with a corresponding higher magnification micrograph in the inset. (d-f) A lower concentration (volume ratio of surfactant to nanospheres = 1:1). (g-i) A higher concentration (volume ratio of surfactant to nanospheres = 1:4). ... 75 Fig. 4-2. SEM images of nanosphere distribution (diameter: 540 nm, volume ratio of

surfactant to nanospheres = 1:2) at various spin speeds. (a-b) A higher rotational speed of 3000 rpm. (c-d) A lower spin speed of 500 rpm. (e-f) A lower rotational speed of 300 rpm. (g-h) The appropriate spin speed of 1200 rpm, containing two dimension defects. ... 78 Fig. 4-3. Patterns of polystyrene nanospheres at various etching powers (a-b) 20 W, 8 min,

(c-d) 100 W, 8 min, and (e-f) 50 W, 8 min. ... 80 Fig. 4-4. Patterns of the polystyrene nanospheres bilayers produced at various RF etching

powers: (a-b) 50 W, 5 min + 30 W, 20 min and (c-d) 50 W, 5 min + 30 W, 24 min. ... 80 Fig. 4-5. SEM images of polystyrene nanospheres patterns produced at various etching

times: (a-b) 0 min; (c-d) 50 W, 5 min; (e-f) 50 W, 5 min + 30 W, 4 min; (g-h) 50 W, 5 min + 30 W, 8 min; (i-j) 50 W, 5 min + 30 W, 12 min; (k-l) 50 W, 5 min + 30 W, 16 min; and (m-n) 50 W, 5 min + 30 W, 20 min, respectively. ... 84 Fig. 4-6. Polystyrene nanospheres with diameters of 540 nm at various cumulative etching

times. ... 84

Fig. 5-1 Schematic illustration of fabrication procedures for ordered Cr porous nanoring

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surface. (b) A monolayer of polystyrene nanospheres is formed on the glass substrate by spin coating process. (c) Reduce the nanospheres by oxygen plasma etching. (d) Using three-axis rotation of CFUBMIP system to deposit the metal on surface. (e) The ordered Cr porous nanoring nanostructure is formed after the lift-off process. ... 90 Fig. 5-2 Schematic illustration of fabrication process for Cr ordered high-porosity periodic

nanostructured films by NSL. (a) Directional sputtering deposition using CFUBMIP system. (b) The formation of Cr triangular-shaped nanodot patterns by lift off. (c) Plan-view SEM morphology of the Cr triangular-shaped nanodot arrays. (d) Non-directional sputtering deposition using three-axis rotation of CFUBMIP system. (e) The porous Cr nanoring arrays by lift off. (f) Plan-view SEM morphology of the porous Cr nanoring arrays. ... 96 Fig. 5-3 SEM images of sizes-varied Cr nanoring arrays and size statistics. Images (a), (b),

(c) and (d) were obtained by the templates shown in Fig. 4-5 (g-h) 50 W, 5 min + 30 W, 8 min (i-j) 50 W, 5 min + 30 W, 12 min (k-l) 50 W, 5 min + 30 W, 16 min, and (m-n) 50 W, 5 min + 30 W, 20 min, respectively. ... 97 Fig. 5-4 (a) The nanoring sizes were fabricated at various cumulative etching time. (b) The

inter-particle spacing statistics of a series of samples at various cumulative etching time. ... 98 Fig. 5-5 AFM images of the Cr nanoring arrays fabricated by colloidal template using RIE

with an oxygen source with 50 W for 5 min and 30W for 4 min on glass substrates. (a) Two-dimensional topography surface of the nanoring array has a period of approximately 540 nm. (b) Three-dimensional topography surface of prepared nanoring arrays clearly presented that the original PS nanosphere array is well copied into the surface of Cr film. ... 99

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Fig. 5-6 Relationship between the lateral size of nanoring and contact angles with 540 nm initial diameter of the PS nanospheres. ... 100 Fig. 5-7 Transmission spectra of the porous Cr nanoring arrays with different lateral size. .... 100 Fig. 5-8 SEM images for various size of PS nanosphere arrays monolayer. (a) Top view of

the 960nm monolayer PS nanosphere arrays on the glass substrate. (b) Top view of the PS nanosphere thinned by RIE process with 60 W for 7 min and then etched with 40 W for 12 min. (c) Top view of the 540nm monolayer PS nanosphere arrays on the glass substrate. (d) Top view of the PS nanosphere thinned by RIE process with 50 W for 5 min and then etched with 30 W for 12 min. The inset shows a cross-sectional view of the PS nanosphere respectively. .... 105 Fig. 5-9 SEM images of sizes-varied Cr porous nanoring array structures. Images (a), (b),

(c) and (d) were obtained by the templates shown in Fig. 2(a), (b), (c) and (d), respectively. Images (d) shows the 540 nm PS nanospheres were not completely removed on the glass substrate. The inset shows (c) the height profiles of periodic Cr nanoring structure is around 20 ± 1.2 nm. ... 106 Fig. 5-10 AFM image of the 2D topography surface patterns and height profiles of varying

the ordered Cr nanoring structures, arrayed with height of (a) 20 ± 1.2 nm, (b) 30

± 1.6 nm, (c) 40 ± 2.2 nm and (d) 50 ± 2.8 nm... 107 Fig. 5-11 Relationship between the diameter of nanoring and contact angles with (a) 960

nm and (b) 540 nm initial diameter of the nanospheres. (c) Relationship between the height of Cr nanoring and contact angles with 540 nm initial diameter of nanosphere. ... 108 Fig. 5-12 Transmission spectra of the 960 nm and 540 nm periodic Cr porous nanoring

arrays structure with different fill-factors. ... 109

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Fig. 5-13 SEM images of arrayed Cr nanorings prepared by nanosphere lithography where the Cr thin film with deposition duration of (a) 1min, (b) 2 min, (c) 3min, (d) 4 min and (e) 5min, and (f) high magnification of cross-sectional view of the Cr nanoring observed in (e). ... 112 Fig. 5-14 AFM of three-dimensional topography surface of ordered Cr nanoring arrays. (a)

The mean values of cross-sectional of Cr nanoring arrays measured around 7.8 nm high for Cr deposition time of 1 min. (b) The Cr nanoring arrays measured around 40.8 nm high for Cr deposition time of 5 min. ... 113 Fig. 5-15 The histograms show lateral size distribution of Cr nanoring with Cr thin film

deposition time for (a) 1min, (b) 2 min, (c) 3 min, (d) 4 min, and (e) 5 min, respectively. (f) Relationship between dispersion of lateral size and Cr deposition time. ... 114 Fig. 5-16 Effect of deposition time on the lateral size and height of Cr nanorings. ... 115 Fig. 5-17 Photoluminescence of Alq3 layer deposited on the testing substrate. ... 115

Fig. 6-1 Schematic diagrams of the fabrication procedures for a nanomold with an ordered CrN nanohole array structure using nanosphere lithography. ... 120 Fig. 6-2. SEM images for various size of nanosphere arrays. (a) The top view of the spin

coated monolayer PS nanosphere arrays with 540 nm diameter on a clean glass substrate. (b) The top view of the PS nanosphere colloidal ion etched by O2 RIE at 50 W for 20 min. The inset shows a cross-sectional view of the PS nanosphere.

... 125 Fig. 6-3. SEM images of sizes-varied ordered CrN nanohole arrays and size statistics of the

nanomold. Images (a), (b), and (c) were produced by the colloidal templates and thinned by the RIE process at 50 W for 20, 25, and 35 min, respectively. The

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inset shows (a) a high-magnification cross-sectional view of the CrN nanohole.

Image (d) shows the diameter of the nanohole and spacing statistics for a series of samples. ... 126 Fig. 6-4. The AFM images and depth profiles of the ordered CrN nanohole arrays of a

nanomold fabricated by colloidal templates using O2 RIE at 50 W for 10 min on Si substrates, followed by CrN deposition and lift-off PS nanosphere processes. ... 127 Fig. 6-5. The XRD spectrum of the ordered CrN nanohole array structure fabricated using

a closed-field unbalanced magnetron sputtering ion plating system. ... 127 Fig. 6-6. The relationship between the nanohole diameter and the contact angles of three

test liquids. ... 128 Fig. 6-7. The relationship between the nanohole diameter and the surface-free energy. ... 128

Fig. 7-1 Schematic illustration of the process for fabricating 540 nm periodic nanohole arrays of an anti-sticking CrN nanomould and ordered the tapered antireflective nanopillars arrays structure on PC film surface using nano-patterning lithography. ... 134 Fig. 7-2 SEM images of (a) and (b) top view of the polystyrene nanosphere colloid with a

540 nm diameter ion etched by O2 RIE of 50 W for 5 and 20 min, respectively.

Images (c) and (d) show the cross-sectional view of the polystyrene nanospheres, respectively. ... 139 Fig. 7-3 SEM images of size-varied ordered CrN nanohole arrays and diameter statistics of

nanomould. Images (a), (b), and (c) were obtained by the colloidal template and thinned by the RIE process at 50W for 5 min, 10 min, and 15 min, respectively.

Image (d) shows the diameter of nanoholes statistics of a series of samples. ... 140

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Fig. 7-4 (a) AFM 2D topography surface of the nanomould of ordered CrN nanohole arrays which were fabricated by colloidal templates using the RIE process with O2 plasma of 50 W for 20 min on Si substrates. The depth profiles show the mean values of depth of nanoholes to be around 100 ± 6 nm. (b) XRD spectrum of the ordered CrN nanohole arrayed structures fabricated by using a closed field unbalanced magnetron sputtering ion plating system. ... 141 Fig. 7-5 (a) Water droplet in contact with ordered CrN nanohole array’s patterned surfaces.

(b) Relationship between the diameters of ordered CrN nanohole array’s structure and contact angles with 540 nm initial diameter of nanosphere. ... 142 Fig. 7-6 The relationship between the diameters of ordered CrN nanohole array’s structure

and surface free energy. ... 143 Fig. 7-7 AFM images of the order of the tapered antireflective nanopillar arrays on the PC

film surface fabricated by nanoimprint lithography. Images (a) and (b) were obtained by the ordered CrN nanohole arrays of anti-sticking nanomould using the RIE process with O2 plasma of 50 W for 5 and 20 min on Si substrates, respectively. The height profiles show the mean values of nanopillars with height around 85 ± 5 nm. ... 144 Fig. 7-8 Transmission spectra of the 540 nm periodic the tapered structure of antireflective

nanopillar arrays with different diameters on a PC film surface. ... 145

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List of Symbols

R radius of the particles l0 particles layer thickness

L the distance of Laplace equation of capillarity α the value of the contact angle

Jp particle flow Jw water flow

Je water evaporation ζ friction force

γL the droplet’s surface tension γf wetting-film’s surface tension

h wetting film thickness φ particle volume fraction

d diameter of the particles k number of layers

vw the substrate withdrawal rate vc the array growth rate

λ transmission spectrum

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t film thickness ns refractive indices

Tg glass transition temperature

D the diameter of thinned PS nanospheres D₀ the initial diameter of the PS nanospheres

k the constant depending on the etching conditions A^△ the area of the triangular interstice

a^△ the side of the triangle

θr the water contact angles on a rough film surface θ the water contact angles on a native ﬁlm surface

r roughness factor

ε_m the dielectric constants of metals

k_sp surface plasmon wave vector

k_x the component of the incident wave vector G_x, G_y the reciprocal lattice vectors

I, j integers

ΔL standard deviation

(L) the mean value of lateral size δ dispersion

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ΥL surface tension

Υ_S surface free energy of the solid Υ_SL the solid–liquid interfacial energy

θW contact angle values of the distilled water θE contact angle values of the ethylene glycol θD contact angle values of the di-iodomethane

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Chapter 1 Introduction

Recently, the periodic nanostructure arrays have received widespread attention because of their unique optical properties and vast range of potential applications in a variety of optoelectronic fields including sensors and flat panel displays. Ebbesen et al. found that the extraordinary high transmission of light through a periodic structure array of subwavelength holes due to the incident light and a surface plasmon resonant effect [1, 2]. This result has attracted numerous theoretical and experimental studies on the related subjects [3-7]. In particular, ring-shaped nanostructures exhibit an extensive variety of optical phenomena due to their highly tunable plasmon resonance, which depends on the diameter and the thickness of the ring wall [8].The high volume confinement in nanoring structures provides more space for molecular attachment [9]. In addition, the light reflection from surfaces can seriously deteriorate the performance of the optical device by degrading the transmission and causing stray light. Therefore, antireflective nanostructure films are widely used in optoelectronic fields, such as flat panel displays. Antireflective nanostructure films are divided into two types:

multi-layer coatings and subwavelength structures. Both designs are able to create a gradual increase of effective refractive indices along the light path. However, such multilayer coatings demand some complicated processes and high cost. Hence, antireﬂective nanostructure ﬁlms made of subwavelength structures have drawn great attention due to the demand for low cost.

Owing to the fast development of consumer electronics, the demands for thinn, light, and cheaper have become a trend. To combine the technology of subwavelength structure with replication is a good method to meet these requirements. One of the well-known subwavelength structures is called moth-eye structures, the antireﬂective subwavelength

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structures found in nature, on the cornea of night-ﬂying moths [10]. This antireﬂective effect can be achieved by making the nanostructures with the subwavelength pitch and protrusion with the height in the order in wavelength [11].

In this study, we have developed a low cost and high throughput strategy for fabricating size-tunable hierarchical porous Cr nanoring arrays using modiﬁed nanosphere lithography based technology. The work focus on enhancing the optical transmittance of the periodic porous Cr nanoring arrays was enhanced due to a surface plasmon resonance effect. The size-tunable tapered antireflective nanopillars on a polycarbonate (PC) film can be fabricated using nanosphere lithography (NSL) and nanoimprint lithography (NIL). This is promising for antireflective surface structure of the different bands, and antireflective optical materials’

fabrication in many important fields.

1.1 Motivation

The motivation of study is listed as following:

(1) The formation of the hexagonal close-packed nanospheres monolayer is determined by the spin speed and the concentration of the PS nanosphere solution. A systematic study regarding the effects of nanosphere interactions and self-assembly behaviour on the formation of arrays pattern is still lacking. It is worth to understanding the mechanisms on how nanospheres organize on a surface can be controlled. In addition, we could fabricate the high-quality size-tunable hierarchical PS nanosphere arrays structure by reactive ion etching process. The objective of this dissertation is the modification of the nanospheres functionality in order to develop a technology platform for a range of applications (NSL effect).

(2) Most literatures reported on the formation of triangular-shaped nanodots structure using

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literatures mentioned the methods of producing nanoring arrays structure. This study develops a method for the fabrication of Cr nanoring array patterns using magnetron sputtering approach and NSL based technology. The samples were mounted on the three-axis satellite rotation holder using a closed field unbalanced magnetron sputtering ion plating (CFUBMIP ) system, which allowed the active Cr atoms/ions having better chance to diffuse into the crevices between the triangular interstices of the PS nanospheres.

The aggregation of these Cr atoms/ions formed ring-shaped structures was observed after the removal of PS nanospheres. To study the optical effect between the substrate and Cr ring-shaped nanostructure film, nanosphere of different size and deposition thickness were introduced to adjust the size and shape of ring-shaped nanostructures. The effects of various Cr ring nanostructures on optical properties were investigated (Cr layer effect).

(3) It is possible to apply NSL based technology to fabricate nanomoulds. The nanomoulds with size-tunable CrN nanohole arrays structure can be fabricated by using magnetron sputtering approach and NSL based technology. The effects of the size and depth of CrN hole-shaped nanostructure on the surface free energy of nanomoulds were discussed (CrN layer effect).

(4) A high-performance antireflective nanopillar layer structure can be imprinted on a PC film surface using a contactless gas assisted pressing process with a CrN nanomould fabricated by NSL. The effects of size of tapered nanopillars on antireflective properties of PC film were investigated (PC layer effect).

1.2 Outline of the Dissertation

The main part of this dissertation studies the structure, surface morphology, and optical properties of the Cr ring-shaped nanostructure, CrN nanohole structure, and polycarbonate (PC) tapered nanopillar patterns film. Chapter 2 presents the literatures relevant to the discussion in this dissertation. This chapter summarizes earlier experimental studies and the

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development of the nanostructures system. The fabrication of size-tunable periodic nanostructures using both the nanosphere lithography (NSL) and nanoimprint lithography (NIL) based technology are described in Chapter 3. In Chapter 4, we focus on the study of the formation of the hexagonal close-packed nanospheres monolayer and controlled size and shape of PS nanosphere. Chapter 4.3.1 describes the polystyrene nanospheres with hexagonal close-packed structure were formed on glass substrates by spin-coating method. Chapter 4.3.2 describes the reactive ion etching (RIE) process was used to reduce the diameters of nanospheres. In Chapter 5, the work concentrated on the study of the optical effect between the substrate and Cr ring-shaped nanostructure film. Chapter 5.3.1 describes the fabrication of Cr triangular-shaped nanodot arrays and hierarchical porous Cr nanoring array patterns with distinct magnetron sputtering approach and nanosphere lithography based technology..

Chapter 5.3.2 describes the different period and diameter of Cr nanorings can be easily controlled by the initial diameter of nanospheres and the following Cr coating and RIE etching processes. The wettability can be manipulated by changing the pore size and the height of the enamoring. Chapter 5.3.3 describes the fabrication of Cr nanoring arrays to improve light extraction. In Chapter 6, we focus on the anti-sticking effect between the substrate and CrN hole-shaped nanostructure film, especially on the correlation between the surface morphology and surface free energy. In Chapter 7, we focus on the study of the correlation between the optical properties and the antireflective tapered nanopillars layer which was imprinted on a PC film surface using a contactless gas assisted pressing process by NIL along with a CrN nanomould prepared by NSL. The dissertation is summarized in Chapter 8.

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Chapter 2 Literature Review

This chapter introduces some background and important research results related to the discussion in this dissertation.

2.1 Nanosphere Lithography

Nowadays nano-engineering researchs on how to effectively control the nanoscale materials to form a periodic structure has attracted much attention. There are two approaches to create the nanostructurs. The first is the “bottom-up” approach, which is primarily used in gas-phase cluster beam studies [12, 13], and condensed-phase colloid synthesis [14-16]. The second is the “top-down” approach, which applies advance lithographic technologies to control the dimensions of bulk matter. Several standard lithographic processing techniques have been reported for fabricating the periodic metallic nanostructure arrays, and having the ability to achieve high throughput and high resolution of nanostructures pattern such as electron-beam lithography (EBL) [17-19], X-ray lithography (XRL) [20-22], molecular-beam epitaxy (MBE) [23-25], and ion beam lithography (IBL). [26-28]. However, these fabrication lithographic techniques have the drawback of being high-cost and complicated. Recently many researchers have focused on the development of simple and low-cost nanosphere lithography (NSL) techniques to fabricate periodic nanostructure arrays [29-31].

Two-dimensional self-assembled polystyrene nanosphere arrays as deposition mask is involved during NSL techniques [32, 33], has become an increasingly popular topic in nanotechnology.

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2.1.1 Introduction of Nanosphere Lithography

The fabrication procedure of the nanosphere lithography for periodic metallic nanostructure arrays was depicted in Fig. 2-1. This monolayer of polystyrene nanosphere is capable of being the mask for deposition process. The four steps are as follows: (a) to form a monolayer of polystyrene (PS) nanospheres on the glass substrate by spin coating process, (b) to reduce the nanospheres by O₂ RIE process, (c) to fill the air-gap with metal thin film by using a closed field unbalanced magnetron sputtering ion plating system (CFUBMIP), and (d) to form metallic nanostructure arrays by lift-off process. The critical fabrication step for this process is to create the large scale monolayers of close-packed nanosphere arrays. Nanospheres can be controlled in the form of colloidal suspensions to create the monolayers of close-packed arrays structures. These arrays structures can be used either as sacriﬁcial layers for lift-off processing of metals deposited on the substrates, or as etch masks for patterning substrates.

Nanosphere lithography inception by Deckman et al. in 1982 is now recognized as a powerful fabrication technique for inexpensively producing nanoscale self-assembly nanoparticle arrays with controlled size, shape, and interparticle spacing [34, 35]. They presented a new form of fabrication in which colloidal nanospheres are used to define a large area lithographic mask. A monolayer of colloidal nanospheres is deposited in either random or ordered manner over the entire surface of substrate. The ordered arrays or large area random of identical submicron microcolumnar structures are produced by using the colloidal nanospheres as either an etching or deposition mask. Fig. 2-2 is the electron micrograph showing a random array of 91 nm polystyrene spheres colloidally coated on an aluminum film. Hulteen et al. [36, 37] are the first group to report and demonstrate of periodic nanometer scale structure array surfaces formed from molecular materials. In this study, the nanosphere lithography is demonstrated to be a feasible process to produce the periodic nanostructure array on the

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using identical monolayer and bilayer by nanosphere lithography, as shown in Fig. 2-3.

Fig. 2-1 Schematic diagrams of nanosphere lithography. (a) A monolayer of polystyrene nanospheres is formed on the clean glass substrate by spin coating process. (b) Reduce the nanospheres by oxygen plasma etching. (c) Using a closed field unbalanced magnetron sputtering ion plating system (CFUBMIP) to deposit the metal to fill the gap between nanospheres. (d) The periodic metallic nanostructure is formed after the lift-off process.

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Fig. 2-2 Electron micrograph showing a random array of 91 nm polystyrene nanospheres colloidally coated on an aluminum film [34].

Fig. 2-3 (a) The monolayer nanosphere masks and the corresponding periodic particle array surfaces. (b) The bilayer nanosphere masks and the corresponding periodic particle array

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2.1.2 Application of Nanosphere Lithography

The fabrication of large scale ordere self-assembled nanosphere arrays monolayer can facilitate the production of low-cost and high-quality nanostructure that can be used in many important applications.

Fan et al. [38] used the nanosphere lithography to fabricate the ordered ZnO nanowire arrays on GaN substrate. This study involves nanosphere self-assembly arrays and mask transfer, deposition process of Au nanodots arrays, and vapor–liquid–solid growth of ZnO nanowires arrays. Au ﬁlm of 50 nm thick was deposited using thermally evaporation through the nanosphere mask to form ordered arrays of Au nanodots structure. Subsequently, ZnO nanowires are grown via vapor–liquid–solid epitaxy mechanism catalyzed by the Au nanodots.

The lengths and diameters of the nanowires are obviously correlated with the growth time and Au nanodot sizes, respectively. The optimum size for growth of a single wire at individual points of the lattice with height is around 3 nm and diameter is around 50 nm. Fig. 2-4 shows the SEM image of ZnO nanowire arrays and TEM analysis of the nanowires. Lindner et al.

[39] reported the silicon nanowires (SiNWs) were grown using chemical vapour deposition (CVD) via the vapour–liquid–solid mechanism with Au nanoparticles used as seeds on Si substrates. In order to control the diameter of nanowires, the density and orientation on the substrate that would be controlled the size and the distribution of Au seed particles. Fig. 2-5 shows the SiNW growth on Si substrate. Ting Xu et al. [40] represented a approach for the synthesis of nano-pitched vertically aligned multi-walled carbon nanotube arrays structure based on nanosphere lithography technology. A monolayer of polystyrene nanospheres with diameter of 650 nm was coated on silicon oxide layer to create ordered hexagonally close-packed array patterns, as shown in Fig. 2-6. A metal layer was deposited on the patterns using e-beam evaporation method which acted as a catalyst for carbon nanotube growth. The metallic nanostructure arrays were formed by lift off process. Uniform carbon nanotube arrays

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with pitch of 800 nm were synthesized from the metallic nanostructure patterns by plasma enhanced chemical vapor deposition process, as shown in Fig. 2-7. The ZnO nanowire arrays have a mean diameter of 63 nm obtained using a monolayer of PS nanospheres as shown in Fig. 2-7 (b). The ZnO nanowire arrays have a mean diameter of 53 nm obtained using a bilayer of PS nanospheres as shown in Fig. 2-7 (c). The spectrum displays a strong G band at 1594 cm⁻¹ and weak D band at 1378 cm⁻¹ which was obtained at gas (C2H2) ﬂow rate of 60 sccm with a pressure of 9 mbar, as shows in Fig. 2-7 (d). These results demonstrate that the pitch of the single CNT array can be controlled using nanosphere lithography. Hence, the as-grown CNTs have potential applications in advanced nanoscience technology. Yamasaki et al. [41] reported the silica nanoospheres with the diameter of 550 nm of periodic hexagonally closed-packed arrays dielectric structures, and connected into organic light-emitting devices with a conventional two-layer structure made with vacuum-sublimation, as shown in Fig. 2-8.

The arrays nanostructure acted as a two-dimensional diffraction lattice has a function as a light scattering medium for the light propagated in waveguiding modes within the device. In the type I device, the ordered array of silica nanospheres with the monolayers were placed on both sides of the stripe of the ITO electrode. In the type II device, a front side of the glass substrate was covered silica nanospheres with the monolayer. Fig. 2-9 (a) and (b) show the photographs of OLEDs of a reference device taken under normal room lighting and the device turn-on state under darkened conditions, respectively. Fig. 2-9 (c) and (d) display the photographs of type I and II devices under driving, respectively. In the type I device, the waveguiding light originating from zone A is surely present and the propagating light within the ITO and the glass substrates is partially scattered by nanospheres array structure. In the type II device, light was observed from every zone. This indicates that large amounts of light propagating with waveguided modes in the ITO and the glass substrate are effectively scattered out with the array of silica nanospheres attached to the glass substrate. Therefore,

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due to the ordered array of silica nanospheres.

Fig. 2-4 (a) Au nanodot arrays and inset shows the monolayer of Au coated hexagonally packed PS nanospheres as the mask on GaN. (b) The tilted view of ZnO nanowire arrays obtained using a monolayer of PS spheres as the template. (c) The tilted view of ZnO nanowire arrays obtained using a bilayer of PS nanospheres as the template. (d) TEM image shows the vertical alignment and the presence of Au tips (indicated by the circle), the inset is the electron diffraction pattern recorded from the circled area and indicating the growth direction along ＜0001＞[38].

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Fig. 2-5 (a) SEM image shows the SiNW growth on the Si substrate. (b) Using an in-lens secondary electron detector. (c) Using a back-scattered electron detector [39].

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Fig. 2-6 (a) Schematic diagram of diameter and spacing achieved by nanosphere lithography.

(b) SEM image of nanospheres of multilayer and monolayer. (c) Magniﬁed SEM image of the nanospheres monolayer. (d) SEM of the large metallic nanostructures area on substrate [40].

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Fig. 2-7 SEM images of the fabricated CNTs. (a) Top view of the grown CNT using thermal CVD. (b) Top view of the grown CNT using PECVD. (c) Cross sectional view of the dense CNTs grown using PECVD. The TEM images of the CNTs were shown the inset. (d) Raman spectrum of the grown CNTs [40].

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Fig. 2-8 Device structures of two types of OLEDs with scattering medium [41].

Fig. 2-9 The photographs of the conventional OLED. (a) Turn-off state under normal room light. (b) Under driving in dark. (c) Type I device under driving. (d) Type II device under driving [41].

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2.2 The Formation Mechanism of Nanosphere

Self-Assembled Arrays

The nanotechnology research how to effectively control the nano-sized materials to form a periodic ordered structure of the problem that has become increasingly concerned. The traditional top-down technology thinking has reached its inherent physical limitations with decreasing feature sizes and new concepts for the fabrication of nanostructures in the nanometer range are needed. Hence, a very interesting possibility to form such complex nanostructures is to let the structures assemble by themselves in situ. This fabrication type is called a bottom-up approach. Self-assembly is first represented by Whitesides et al., the autonomous organization of components into patterns or structures without human intervention [42]. The Self-assembled process is general throughout nature and technology.

They involve components from the crystals or molecular to the weather or planetary systems scale and many different kinds of interactions. The concept of self-assembly is used increasingly in many nanotechnology fields. Self-assembly is divided into dynamic and static self-assembly process. Dynamic self-assembly system is considering energy loss to achieve the energy balance state of the self-assembly process, static self-assembly system without considering energy loss state to achieve the balance state, while most of the self-assembly belongs to static self-assembly. The nanosphere self-assembly from the nanometer to submicrometer scale is widely used in various technological applications. The nanosphere types currently in use such as polystyrene (PS), silicon dioxide (Silica), and poly (methyl methacrylate) (PMMA) etc [43-47]. This self-assembly approach for the fabrication of nanostructures arrays were started from nanoparticles have become an increasingly popular topic in nanotechnology.

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2.2.1 Nucleation

In 1992, Denkov et al. proposed the formation mechanism of the two-dimensional array [48]. The process is divided into two steps: the first step is the capillary force, [49, 50] which drives the particles together, nucleating a thin ﬁlm assembly, the second step is the convection flow force [51, 52]; nucleation grows from the convection ﬂux of nanoparticles while the substrate is drying at the front part of the hydrophilic surface. Fig. 2-10 shows the two spheres partially immersed in a liquid layer on a horizontal solid substrate. The deformation of the liquid meniscus gives rise to interparticle attraction [48]. The physical nature of these forces can be explained as follows. The two particles of radius (R) and partially immersed in a liquid layer, whose thickness tends to a constant value (l0) at a large distance from the two particles.

The shape of the meniscus obeys the Laplace equation of capillarity and is determined by the distance L = 2s between the particles, the layer thickness (l0), and the value of the contact angle (α), which characterizes the particle wettability. Lateral capillary forces are divided into flotation forces and immersion forces, as shown in Fig. 2-11 [50, 53]. On a flat surface (without particles on it), the lateral capillary force comes from the surface’s deformation; the capillary force is stronger when the interaction between the particles is larger. The two similar particles floating on a liquid interface attract each other are called flotation capillary forces, as shown in Fig. 2-11(a). The capillary forces between particles partially immersed in a liquid layer on the substrate are called the immersion capillary forces, as shown in Fig. 2-11(b).

The two types of capillary interaction are compared for a wide range of particle sizes, as shown Fig. 2-12. When the liquid by the volatile impact and dropped as low as l0 = R, its sphere array will still be re-arranged due by Immersion Force again [48].

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Fig. 2-10 Two spheres partially immersed in a liquid layer on a horizontal solid substrate. The deformation of the liquid meniscus gives rise to interparticle attraction [48].

Fig. 2-11 (a) Flotation lateral capillary forces. (b) Immersion lateral capillary forces [50].

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Fig. 2-12 Comparison between immersion and flotation capillary forces [48].

2.2.2 The Ordered Array of Growth

After colloidal nanoparticles nucleating growth on the substrate, K. Nagayama et al.

represented the convection force [48, 51, 52, 54] for mechanism of the ordered array of growth. Fig. 2-13 shows the convective flux toward the ordered phase due to the water evaporation from the menisci between the particles in the two dimension array. The contact line was defined as the intersection of the extrapolated meniscus with the plane of the wetting film surface. The particle flow (Jp) is generated by water flow (Jw) that is induced by water evaporation (J_e) from the particle array and the wetting film surface. Due to the suspension flow (a mixture of Jp and Jw) is viscous, hence, the friction force (σ), acts on the wetting film surface close to the contact line to prevent shrinkage of the droplet. At the contact line region, the droplet’s surface tension (γL), is in competition with the wetting-film’s surface tension (γf ), and the friction force (σ). The wetting film thickness (h) is assumed to be equal to the particle

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diameter at the contact line. It is possible to manufacture large-area monolayer or multilayer of nanospheres by control Θ. The last of these particles exist in several layers of structure, depending on the contact line by the evaporation of suspension flow rate [52].

Fig. 2-13 Schematics drawing of the convection flow force [52].

2.3 Self-Assembly Techniques

There are several different production technologies for the formation of nanosphere self-assembled array structure, such as the Drop-coating, Langmuir-Blodgett (LB) -like technique, Convective self assembly, Electric-assisted Spin-coating technology are described as follows.

2.3.1 Drop-Coating Method

Drop-coating method is the basic technology for forming nanosphere arrays. Nagayama et al. [48] proposed the dynamics of two dimensional of ordered the polystyrene colloidal

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thickness of the water layer containing particles becomes around equal to the diameter of particle. It demonstrated that neither the electrostatic repulsion nor the van der Waals attraction between the particles is responsible for the formation of two dimensional nanostructures. In 2007, Weekes et al. [55] utilized the drop-coating method to produce nanosphere templates with significantly improvement for long-range order arrays, as shown in Fig. 2-14. The ordered arrays monolayer over areas greater than 1 cm²have been achieved by assembling nanospheres with the correct surface chemistry on a water/air interface. The polystyrene nanospheres were obtained with diameters in the range of 120-950 nm.

Fig. 2-14 Schematics of the drop-coating method. The inset shows a cross section of the bath, demonstrating the transfer of a monolayer onto a solid substrate [55].

2.3.2 Langmuir-Blodgett (LB) -like technique

Langmuir-Blodgett (LB)-like technique was proposed by K. Nagayama et al. [51]. This technique looks like the well-known LB technique. The main difference is that in this study the film on the substrate is being formed at once from the substances dissolved in the solution bulk, while using LB technique requires the film to be initially formed on the solution surface

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and then to be transferred onto the substrate. The process of this technology and LB process technology are similar process principles using the following equation:

(2.1) Equation (2-1) shows that the growth rate of the dense arrays depends on the particle volume fraction, φ, water evaporation rate, je, diameter of the particles, d, number of layers, k, and an experimentally determined constant, the product βl. From the Fig. 2-15, the vw is the substrate withdrawal rate, vc is the array growth rate, jw is the water influx, jp is the respective particle influx, j_e is the water evaporation flux, and h is the thickness of the array, once by the control so that Vw = Vc, the substrate area had been completed the self-assembled arrays [51].

Fig. 2-15 Schematic diagram of the particle and water fluxes in the vicinity of monolayer particle arrays growing on the substrate plate that is being withdrawn from a suspension. The inset shows the menisci shape between neighboring particles [51].

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2.3.3 Convective Self-Assembly

In 2007, Y.J. Zhang et al. [56] utilized the nanosphere generated aggregation in the gas phase-liquid phase between the lateral capillary forces to form arrays nanostructures. The polystyrene colloidal suspension was spread all over the substrate. Fig. 2-16 shows the fabrication process of the monolayer PS nanospheres on the substrate. The polystyrene nanospheres started to form an unordered monolayer on the water surface while it was slowly immersed into the glass vessel. (Fig. 2-16(a)). Some sodium dodecyl sulfate solution was dipped onto the water surface (Fig. 2-16(b)) and the large monolayer of ordered areas was form on the silicon wafers, as shown in Fig. 2-16(c). This way to obtain arranged nanospheres is quickly and defects even less than usual evaporation process. In 2009, Canpean et al.

proposed this process to be used at a continuous process technology [57]. Fig. 2-17 represents the convective self assembly continuous process. The deposition plate is horizontally moved by means of a screw, which is associated to the motor through a system of wheels with grooves. This performance of system can be improved by the well control of low velocities during the deposition process. The liquid meniscus is withdrawn horizontally across the substrate by translating the deposition plate at controlled speeds (v_w). Fig. 2-18 schematically demonstrates the process of convective assembly driven by the evaporation ﬂux (JE). Ordered polystyrene nanospheres arrays with diameter of 450 nm were successfully deposited on glass substrate. Matsushita et al. [58] utilized the platform moving rate and liquid evaporation rate, the lateral capillary forces as the driving force of nanospheres formed ordered array. This approach can be applied to any substrate, and the size of the nanosphere has no special limitation, which is able to extend its applications. Fig. 2-19 shows the schematic diagram of the convective self assembly instruments. The thickness of the structure array was controlled by adjusting the distance between the substrate and the cell using the z-axis stage Fig. 2-19(a).

The substrate is translated horizontally using the x-axis stage during preparation of the

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ordered structures array. Fig. 2-19(b) shows the formation of the two dimension array from the two component mixture of equal-sized particles with different surface properties by lateral capillary force, corresponding to the dashed rectangle in part a.

Fig. 2-16 The preparation procedures of monolayer PS nanospheres on the substrate. (a) The nanospheres onto the water surface. (b) Addition of sodium dodecyl sulfate solution to consolidate the nanospheres. (c) Lift off the ordered monolayer by the substrate [56].

Fig. 2-17 Image of the convective self assembly continuous process instruments [57].

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Fig. 2-18 Schematic diagram of the convective self assembly process [57].

Fig. 2-19 Schematic diagram of the convective self assembly instruments [58].

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2.3.4 Electric-Assisted Self-Assembly

Electric-assisted self-assembly technique utilizes the electrical interaction force of nanosphere particle surface charge to control the self-assembly process technology. The principle of this assembly method is that the design substrate electrically administered electric field force resulting in electrostatic interaction force between the particles, which produces polarization is greater than the Brownian motion. In 2005, Winkleman et al. [59] utilized the electrophoretic deposition method to control nanospheres to be deposited on the designed area.

Fig. 2-20 shows the illustration of the template-directed self-assembly of nanospheres on the patterned metal electrode. Fig. 2-20 (a, b) depicts the microcontact printing and etching generated a template electrode. Fig. 2-20 (c, d) depicts the nanospheres self-assembled over the template electrodes when a -20 kv potential (relative to ground) was applied to the patterned metal electrode. The excess nanospheres were ejected from the surface of the electrode during this process. Ideally, one nanosphere remained on each window and without nanospheres on the metal surface (Fig 2-20 (e)). The two arrows indicate the two types of defects [59]. Aizenberg et al. [60] represented this assembly colloids deposited using micron contact imprint (μCP) produced anion and cation distributed on substrate, so that with a positive charge particle adsorbed on with negative electric substrate region, negatively charged particles adsorbed on the positively charged substrate area to be uniformly adsorbed particles, then used capillary forces between the particles and executed the next step of self-assembly arrangement.

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Fig. 2-20 Schematic illustration of the template-directed self-assembly of nanospheres on the patterned metal electrode [59].

2.3.5 Spin-Coating Method

The spin coating method is also widely used a self-assembled nanosphere technology. Klein et al. [61] used spin coating method produced a larger periodic monolayer with the area of 1000 µm2 by control the size of nanosphere, spin speed and the stagnation time. In 2002, Ng et al. [62] utilized spin coating method produced a long-range (500 um) periodic dynospheres monolayer area at a spin speed of 800 rpm. Fig. 2-21 represents the schematic diagram of the spin speed in region X, it is shown that it was increased steadily from 0 rpm to the required speed and kept constant for a 30 sec time interval before it was again increased to a final speed. This was done to check whether nanospheres uniformity spread [62]. The dynospheres

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