Structural Colour Images Inkjet Printed on Polymer Substrates Patterned with Nanostructural Pixels

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Structural Colour Images Inkjet Printed on

Polymer Substrates Patterned with Nanostructural

Pixels

by

Sheida Alan

B.Sc., Kermanshah University, 2010

Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of

Master of Applied Science

in the

School of Engineering Science

Faculty of Applied Sciences

© Sheida Alan 2016

SIMON FRASER UNIVERSITY

Fall 2016

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Approval

Name: Sheida Alan

Degree: Master of Applied Science

Title: Structural Colour Images Inkjet Printed on Polymer Substrates Patterned with Nanostructural Pixels

Examining Committee: Chair: Dr. Parvaneh Saeedi

Dr. Bozena Kaminska Senior Supervisor Professor, School of Engineering Science Dr. Ash Parameswaran Supervisor Professor, School of Engineering Science Dr. Ash Parameswaran Supervisor Professor, School of Engineering Science

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Abstract

This work demonstrates a novel means of manufacturing nano-optical devices, functioning according to the principle of structural colouration, by inkjet printing silver nanoparticle ink on nanostructured surfaces. The structural colouration is created by a surface containing micro- or nanofeatures interacting with light. In this study, we use a polymer substrate patterned with different types of nanostructure arrays as structural pixels that give red, green, and blue primary colours. We utilize nanoimprint lithography to replicate the nanostructured substrate from a prefabricated stamp.

These days, inkjet printing has become a scalable micropatterning technology due to its precise, flexible control and cost-effective additive process. In this current work, inkjet printing technology is employed to selectively activate pixels by printing silver nanoparticle ink on the nanosubstrate surface to gain colour mixing.

In the experiments performed, a nanostructured substrate patterned with diffractive nanostructure arrays was implemented to print full-colour images. The effect of surface wettability, different concentrations of silver nanoparticle ink, drop and line spacing, and polymer nanostructures on the optical properties of the subpixels of dried silver printed dots, are investigated to achieve high printing resolution.

This method is a key to achieving full-colour, scalable, high-throughput, and flexible printing of structural colour images. The printed pictures demonstrate unique optically variable effects that distinguish from security products using pigments or printing inks in their manufacturing processes. Therefore, this technique is an ideal candidate for security and authentication applications that require customizable anti-counterfeiting features.

Keywords: Structural colour; Nanostructured substrate; Inkjet printing; Optically

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Dedication

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Acknowledgements

I would first like to thank my supervisor Prof. Bozena Kaminska giving me the opportunity of taking part in Master of Applied Science program. She supported me greatly and was always willing to help me. Under her supervision, I have learnt how to deal with challenges and reach to succeed.

In addition, I would like to thank Dr. Hao Jiang for his valuable guidance. He definitely provided me with the tools that I needed to choose the right direction and successfully complete my thesis. I would also thank Dr. Jasbir N. Patel in Dr. Kaminska’s research group at Simon Fraser University for his valuable comments and guidance.

Above ground, I am indebted to my family and beloved ones for their great patience, support and understandings in all aspects of my life.

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List of Tables

Table 4.1.

Comparison of performance between bright silver mode and dark

silver mode. ... 45

Table 5.1

Different concentration of silver nanoparticles ... 52

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List of Figures

Figure 2.1

The electromagnetic spectrum demonstrating wavelengths that can

be observed by the human eye. ... 7

Figure 2.2

(a) Photograph of a Morpho butterfly (Morpho didius), SEM images

of the wing microstructure in cross-sectional (b) and top (c) views. Schematic of the Morpho-blue wing microstructure in (a) side and (b) top view: (1) Interference in a single multilayer (shelf). (2) Diffraction on narrow structure. (3) Randomness in height suppresses the multicolour. (4) Narrow gap results in high

reflectivity. ... 8

Figure 2.3

Diffraction from an aperture and Fresnel and Fraunhofer regions ... 9

Figure 2.4

Diffraction by a grating consisting of a periodically corrugated surface. The incident angle is the diffracted angle is, and the

grating period is d. ... 10

Figure 2.5

The representation of categories of inkjet printers. ... 13

Figure 2.6

(a) Fujifilm-Dimatix DMP2831 Dimatix Materials Printed. (b) a platen that a substrate can be place on and (c) a disassembled cartridge

and printhead assembly. Reprinted with permission. ... 15

Figure 3.1

Illustration of a nanostructured substrate. ... 20

Figure 3.2

Details of a nanostructured substrate. (a) the diagram illustrates the layout of R,G and blue colour pixel bands as well as the SEM images display the structural details of the nanowells array pixels and the scale bar is 500nm in length. (b) physical process to

display a refection colour in nanowells structure. ... 21

Figure 3.3

Details of a nanaosubstrate. (a) the diagram illustrates the layout of R,G and blue colour pixel bands as well as the SEM images display the structural details of the nanocones array pixels. The scale bar is 500nm in length and the scale bar is 500nm in length. (b) physical process to display a refection colour in nanocones

structure. ... 22

Figure 3.4

Schematic diagram of NCAs nanostructured substrate fabrication,

(a) cleaning of the quartz substrate, (b) evaporation of pf Cr, (c) coating of PMMA, (d) electron beam lithography, (e)developing of the resist,(f) Cr etching,(g) Ti/Ni deposition, (h) lift off on NI/Ti (i) removal of Cr by wet etching, (j) reactive ion etching, (k) NCAs on

quartz. ... 23

Figure 3.5

Schematic of (a) Thermal nanoimprinting and Schematic of (b)

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UV-Figure 3.8

Cross-sectional view of the printing and index-matching processes. ... 27

Figure 3.9

(a) Images of printed OVD camera photos captured at the correct

angle showing the colour patterns that match the original image and at different angle. (b) SEM images of the silver dot printed on

the nanocone arrays on the blue band. ... 29

Figure 3.10

The generated pattern printed on a blank PET film. The black/white

picture is the camera photo of the printed sample under white light illumination from the back. Two inset images are microscope images under reflection mode to show the structural details of the printed silver dots. The graph shows the height profile

scanned across a single silver dot. ... 30

Figure 3.11

Pictures of four printed OVDs with varying the βº viewing angle

while the αº remained constant. ... 32

Figure 4.1

Different shapes of liquid drop impact on to a dry surface[58].

Reprinted with permission. ... 34

Figure 4.2

Schematic diagram of an ideal contact angle (θ) of a liquid drop on a

solid (θ) ... 35

Figure 4.3

Contact angles are shown for HMDS coated surface and FDTS

coated surface. ... 37

Figure 4.4

Schematic of (a) Unprinted NCAs (b) bright silver mode and (c)

dark silver mode for printing colours on NCAs. ... 39

Figure 4.5

Silver ink printed with bright silver mode on hydrophilic surface. (a) Optical microscope image of one printed dot on one green subpixel band after laminating the sample. (b) SEM images of the silver dot printed on the hydrophilic surface and (c), (d) and (e) are inset images to show details in different regions and (f) is

measured contact angle. ... 40

Figure 4.6

Silver ink printed with dark silver mode on hydrophobic surface. (a)Optical microscope image of one printed dot on one green subpixel band. (b) SEM image of the silver dot printed on the hydrophobic surface and (c), (d) and (e) are inset images to show

details in different regions and (f) is measured contact angle. ... 41

Figure 4.7

Silver ink printed on hydrophobic surface. (a) SEM image (tilted angle at 30°) of the silver dot printed on the hydrophobic surface.(b) Optical microscope image of one printed dot on one green subpixel band and (c), (d) are inset images (tilted angle at

30°) to show details in different regions. Scale bar: (a) 65µm. ... 42

Figure 4.8.

Effective subpixel brightness vs number of printed silver dots on

hydrophilic and hydrophobic surface with original silver ink. The inset optical microscope images show the patterned subpixels in

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Figure 4.9

Effective subpixel brightness v.s. number of printed silver dots on hydrophobic surface with diluted silver ink (Ag:EG=1:4 ratio). The inset optical microscope images show the patterned subpixels in

the corresponding data points. ... 45

Figure 4.10.

Photographic images of printed samples. (a) Colour patterns

printed with bright silver mode. (b) Zoom-in image of (a) to show the details on the printed colour letter ‘C’. (c) Colour photo printed with dark silver mode. (d) Zoomed in image of (c) to show the

details on the eye. ... 46

Figure 4.11

Photographic images of a printed sample. (a) A colour pattern printed with bright silver mode with diluted silver ink (Ag:EG=1:4 ratio) and captured at correct angle showing the colour patterns that match the original image. (b), (c) and (d) captured under different angle showing the unique optic properties. (d) Zoom-in image of the area enclosed by white box (a) to show the details on

the lip. ... 47

Figure 5.1

Experimental results of printing silver ink with 30-35% nanoparticle weight on a polymer nanocone array for green diffractive colour. (a) the optical microscope image shows the printed dots in green pixels. (b) a SEM image (tilted angle at 30°) of two silver dots printed on the nanocone array. (c) a SEM image (tilted at 30°)of center of printed dot. (d) a SEM image (tilted at 30°) of the region 2 of printed dot. (e) an overview of region 1, 2, and 3. (f) a SEM

image (tilted at 30°)of unprinted area. Scale bar: (a) 56µm. ... 53

Figure 5.2

Experimental results of printing silver ink (sample #1) on a polymer nanocone array for green diffractive colour. (a) the optical microscope image shows the printed dots in green pixels. (b) a SEM image (tilted angle at 30°) of single silver dot printed on the nanocone array. (c) a SEM image (tilted at 30°)of center of printed dot. (d) and (e) are inset images to show details in different regions (tilted at 30°). (f) a SEM image (tilted at 30°) of unprinted

area. Scale bar: (a) 65µm. ... 54

Figure 5.3

Experimental results of printing silver ink (sample #2) on a polymer nanocone array for green diffractive colour. (a) the optical microscope image shows the printed dots in green pixels. (b) a SEM image (tilted angle at 45°) of single silver dot printed on the nanocone array. (c) a SEM image (tilted at 45°) of center of printed dot .(d) and (e) are inset images to show details in different regions (tilted at 45°). (e) a SEM image (tilted at 45°) of unprinted

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Figure 5.4

Experimental results of printing silver ink (sample #3) on a polymer nanocone array for green diffractive colour. (a) the optical microscope image shows the printed dots in green pixels. (b) a SEM image (tilted angle at 45°) of single silver dots printed on the nanocone array. (c) a SEM image (tilted at 45°)of center of printed dots.(d) and (e) are inset images to show details in different regions (tilted at 45°). (e) a SEM image (tilted at 45°) of

unprinted area. Scale bar: (a) 65µm. ... 56

Figure 6.1

Illustration of printed dot with different drop spacing, which reduces

from (a) to (e). (a) isolated drops, (b) scalloped, (c) uniform line (d)

bulging, and (e) stacked coins. Reprinted with permission. ... 60

Figure 6.2

Binary printing pattern generation from a custom written software

program using the original image, pixel layout, and printer

configuration. ... 61

Figure 6.3

(a) A pixel layout that is comprised of a red, green, and blue band. Each band is 70µm wide. The size of entire a single pixel is 210µm × 210µm. (b), (c), and (d) is an illustration of how they (every subpixels) are tuned to produce any colour. (e) RGB

model. ... 63

Figure 6.4

(a) Schematic of effective pixel layout (b) Shows the binary pattern generated for placing silver dots in the specific subpixels to gain a desired colour, drop and line spacing is 20µm and two lines print on each subpixel. (c) Photo of printed colour bar of red, green, blue, cyan, magenta and yellow (d) Microscope image at the bright magenta colour bar. (e) Measured diffraction spectrum from on the red bar with DPSP: 1, 10, and 20 dots. (f) Measured diffraction from the inset panel of optical microscope image inside

the white square. Scale bar: (d) 200µm and (f) 200µm. ... 64

Figure 6.5

Inkjet printed on a polymer nanocone array for green diffractive colour with varying point and line spacing. As showed in (b), (c), (e), (f), (g), and (h) double line printed in each subpixels (green) with different dot and line spacing and their optical microscope images are on their right side, displayed that show the printed dots

in green pixels. Scale bars are 56µm. ... 65

Figure 6.6

Inkjet printed on a polymer nanocone array for green diffractive

colour with varying point and line spacing. As showed in (b), (c), (e), (f), (g), and (h) triple line printed in each subpixels (green) with different dot and line spacing and their optical microscope images are on their right side, displayed that show the printed dots in

green pixels. Scale bars are 56µm. ... 66

Figure 6.7

Inkjet printed on a polymer nanocone array for green diffractive

colour with varying point and line spacing. As showed in (b), (c), (e), (f), (g), and (h) mixed line printed in each subpixels (green) with different dot and line spacing and their optical microscope images are on their right side, displayed that show the printed dots

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Figure 6.8

(a) A pixel layout that is comprised of a red, green, and blue band. Each band is 25µm wide. The size of entire a single pixel is 75µm × 75µm. (b) illustration of dot and line spacing. (c), (d), and (e) optical microscope images of Inkjet printed on a polymer nanocone array with 25µm wide for green diffractive colour with varying 25µm dot and line spacing. (f) a photo of the coat-of-arms of Simon Fraser University printed on nanostructured substrate with 25µm and (h) zoom-in photo at one white flower. (c), (d), (e)

and (h) scale bars are 56µm and (f) scale bars are 5mm. ... 69

Figure 6.9

Photo of double line printed in each subpixels with different dot and line spacing on a polymer substrate captured with the lighting angle and viewing angle specified in the schematic at the above

panel. ... 70

Figure 6.10

Photo of mixed line printed in each subpixels with different dot and line spacing on a polymer substrate captured with the lighting angle and viewing angle specified in the schematic at the above

panel. ... 71

Figure 7.1

Characterization of silver NCAs and silver NWAs formed by printing silver dots on red subpixels. (a) and (b) are the schematic process of template-stripping. (c) SEM images (tilted angle at 30°) of the silver NCAs. (d) SEM images (tilted angle at 30°) of the stripped silver NWAs. (e) SEM image (tilted angle at 30°) of the stripped NWAs. (f) EDX map of the silver elements, corresponding to the

SEM. ... 76

Figure 7.2

Characterization of silver NCAs and silver NWAs formed by printing silver dots on blue pixel bands. (a) SEM images (tilted angle at 30°) of the silver NCAs. (b) SEM images (tilted angle at 30°) of the stripped silver NWAs. (c) Measured normalized diffraction

efficiency of silver NCAs and silver NWAs. ... 77

Figure 7.3

Schematics of UV-NIL process to produce substrates from a quartz

stamp. OrmoStamp (MicroChem Corp.) is used as UV-polymer and Acrylate Acrylic Resin (AAR) is applied. The imprinting condition is 28 pounds squared inch (PSI) pressure. UV exposure is applied to cure UV-curable polymer, which its dose larger than

1000mJ/cm2 _{for (a) and (b). ... 78}

Figure 7.4

Experimental results of printing silver ink (Ag:EG=1:4 ratio) nanoparticle weight on the AAR polymer NCA for red diffractive colour. (a) the optical microscope image shows the printed dots in red pixels. (b) a SEM image (tilted angle at 30°) of a silver dot printed on the nanocone array. (e) a SEM image (tilted at 30°)of center of printed dot. (d) a SEM image (tilted at 30°) of the edge of a printed dot. (c) a SEM image (tilted at 30°)of unprinted area.

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Figure 7.5

Experimental results of printing silver ink (Ag:EG=1:4 ratio) nanoparticle weight on the OrmoStamp polymer NCA for red diffractive colour. (a) the optical microscope image shows the printed dots in red pixels. (b) a SEM image (tilted angle at 30°) of a silver dot printed on the nanocone array. (c) a SEM image (tilted at 30°)of center of printed dot. (d) a SEM image (tilted at 30°) of the edge of a printed dot. (e) a SEM image (tilted at 30°)of

unprinted area. Scale bar: (a) 56µm. ... 80

Figure 7.6 Photo of coat-of-arms of Simon Fraser University printed on the ARR

polymer substrate with pixelated nanostructures captured with the lighting angle and viewing angle specified in the schematic at It can be observed that printing silver ink (Ag:EG=1:4 ratio) on the OrmoStamp polymer NCAs give brighter and higher contrast colours than the AAR polymer NCAs printing. (a). (c) Photo of coat-of-arms of Simon Fraser University printed on the OrmoStamp polymer substrate with pixelated nanostructures captured with the lighting angle and viewing angle specified in the schematic at (a) . (d) Optical microscope image of (b) at the region enclosed by white box. (e) Optical microscope image of (c) at the enclosed by white box. (f) Measured normalized diffraction efficiency from the regions enclosed by the white dashed circles in

(d) and (e). Scale bars are 56µm. ... 82

Figure 7.7

Characterisation of silver NCAs by printing silver dots on red pixel bands. (a) SEM images of printed silver dot on the NCAs, viewed at top. (b) SEM images of printed silver dot, viewed at bottom or the silver NWAs. (c) Measured diffraction spectrum of the printed silver nanowell array, and the inset microscope image of a single

dot printed in red pixel band. ... 85

Figure 7.8

Optically variable effects of the printed colour pictures. (a) Schematic of four different configurations of lighting and viewing angle. (b) Photos of the printed SFU coat-of-arms captured under different angles specified in (a). (c) Photos of a printed picture of a hot air balloon captured under different angles specified in (a). (d) Angle-dependent diffraction spectra of the printed silver NWAs on

R, G and B band. The angles are specified in (a). ... 86

Figure 7.9

Characterisation of silver NWAs by printing silver dots on red pixel bands. (a) SEM images of printed silver dot on the NWAs, viewed at top. (b)Measured normalized diffraction efficiency of the printed silver nanowell array, and the inset microscope image of a single

dot printed in a pixel (R, G, and B bands). ... 87

Figure 7.10

Optically variable effects of the printed colour picture,

images printed on an Ormpstamp polymer Nanowell arrays with silver ink. (a) The viewing angle that specified schematically for (b) the photo of printed picture. (c) and (d) the photo of the printed picture captured with the lighting angle at different viewing

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Chapter 1. Introduction

Over the last few decades, there has been a significant growth in fraud and counterfeit products and documents being developed, including their security features. Despite novel security technologies having been developed to increase the level of protection for documents and products, they have been confronted by counterfeit and fraud attacks. This situation leads to a quickly rising demand for innovative security technologies.

The engineering of diffractive structures and material systems produce iridescent colour changing for Optically Variable Devices (OVDs) for document security. OVDs have attracted a great deal of attention due to its colour-shifting features for government documents and banknotes. The majority of these are iridescent features, which are seen in the reflection of light where the change of colour occurs as the viewing angle is changed.

The main focus of this study is to develop a novel method to manufacture structural colour images and OVDs using nanostructures to improve speed and scale to large-area production with high-throughput with respect to conventional techniques. In this current research, a colourful and up-scalable structural colour printing technique is demonstrated based on a Drop-On-Demand (DOD) inkjet printer.

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processes, the range of materials that can be formulated such as inks, the different type of substrates that can be prefabricated and the possibility to cater to various production scales. In fact, printable processes can significantly decrease material waste and manufacturing process steps, while decreasing manufacturing costs providing a much more environmentally friendly products [1]. In this section, a brief review of printing is presented.

1.1.1. Conventional printing

The range of printing technologies has increased since the combination of printing and computerized technology began, leading to their widespread applications. The main types of conventional printing technologies are offset, gravure, screen and flexographic printing. A master-printing plate is used to produce texts, graphics, and images of any kind of information [2]. These technologies require a prepress process before employing them for rapid and large-scale production, but only flatbed screen printing does not follow this rule.

1.1.2. Digital printing

Digital printing does not require mechanical contact to transfer ink onto a substrate, thus digital printing methods are also called non-impact printing (NIP) [2]. The accurate positioning of a liquid droplet with a small volume directly on the desired location corresponding to the binary unit of images can be achieved. These are key advantages of digital printing compared to high-end conventional printing technologies. Digital printing can be incorporated with a variety of techniques, which makes them accessible to a wide range of applications, including printed circuit board [3], book-on-demand, etc.

Both digital and conventional techniques can be used to fabricate electronic structures, circuits, and devices, which is referred to printed electronics as an application of printing techniques. In these techniques, any type of ink and substrate can be used as long as a functional material or ink can be processed from a liquid phase.

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1.1.3. Structural colour printing

Structural colour printing has attracted significant attention since advancement in nanotechnologies. Structural colour refers to materials, which gain their colour from the interaction of micro or nanoscale structures with light. The structural colouration is produced based on the physical phenomena of diffraction, interference, or scattering. In recent years, the idea of structural colour has been developed to produce extremely high-resolution printing. Structural colour printing has a number of advantages over traditional pigment-based colour printing, including colour intensity, chromaticity, resolution, and colour stability due to the absence of a pigment [4]. In fact, colour printing and colour displays using nanostructures as pixels have become intensive fields of research.

1.2. Motivation and objectives

Structural colour possesses many interesting features that have been studied extensively due to being scientifically and practically important. Due to its unique characteristics, there have been numerous efforts to produce structural colour using different technological approaches, such as colloidal crystallization [5]–[7], dielectric layer stacking [8], [9] and direct lithographic patterning [10], [11].

Dielectric layer stacking and lithographic patterning of periodic dielectric materials have been utilized to generate structural colour via directly controlling its structure down to submicrometer size. There have been different kinds of fabrication process, such as replicating natural substrates [10], depositing materials layer by layer [11] and etching a substrate by several lithographic techniques. Although a periodic dielectric structure on the surface can be precisely fabricated through these methods, long manufacturing process times need to be considered to produce a multicoloured pattern. Thus, any mistake or further changes during the fabrication process leads to extra expenses and effort. To overcome these problems, many high-resolution

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The goal of this research is to render a novel technique to manufacture structural colour images based on micropatterning techniques by taking advantage of a generic substrate of nanostructures called “nanostructured substrate” [12]. A nanostructured substrate is composed of subpixels designed to produce red, green, and blue primary colours. In order to achieve the desired colour images, micropatterning techniques have been employed to activate specific coloured pixels in accordance with the original colour image.

In order to overcome the restrictions and challenges associated with existing techniques, we adopted a combination of inkjet printing and nanostructures to print colours. It allows us to have a unique technology of structural colour printing based on inkjet printing. Simplified processing steps, reduced materials wastage, low fabrication costs, high-throughput, flexible, and simple patterning techniques make inkjet printing technology very attractive for cost-effective manufacturing at the microscale. These features lead to a new way to develop electronic and photonic devices such as polymer LED displays, light emitting diodes, polymer solar cells, etc. The inkjet printer performs in either continuous or drop on demand mode, as demonstrated in chapter 2. DOD printing becomes an attractive technology which can be adopted for a number of applications due to its ability to precisely position material on a surface to produce structures without the need for masks. This reduction in manufacturing process steps and material savings makes it an attractive fabrication process, owing to its additive nature. Thus, DOD inkjet printers can pattern structural colour images.

To pattern structural colours at a microscale using inkjet printing technology, there are two approaches that have been used. First, using an inkjet printer, where especially synthesized inks are ejected on the regular surface, where the ink acts as a nanostructured material. There are many examples of inks composed of nanostructured materials that have been utilized such as, optically variable inks composed of multilayered flakes for security application [12], a titania-based colloidal ink inkjet on the surface and the thickness of inks is controlled at the nanoscale by inkjet deposition to manufacture thickness-dependent interference colours [13]. Second, regular ink drop onto the particular surface to tune colours, for instance, the surface is patterned with plasmonic crystals formed by self-assembled nanospheres [14], [15].

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In this current research, an inkjet printer selectively deposits colourless silver nanoparticle ink on the nanostructured substrate to activate or deactivate pixels. Using the proposed methods, a nanostructured substrate is fabricated once and its replications can be used for different given images with no need to employ costly and lengthy nanofabrication techniques for each image stamp. Using this technology, any given colour image and/or covert data can be produced. This new technology makes it possible to practically and efficiently use the nanostructures as a visual information display, and as a high density long-term optical storage medium. The proposed methods aim to improve speed and manufacturing throughput with respect to conventional techniques, and can be employed to store both covert and overt information with good colour definition and acceptable resolution.

1.3. Thesis outline

The thesis is organized as follows:

• Chapter 1 provides the main themes of the thesis.

• Chapter 2 presents the background and literature review of the mechanism in structural colouration. It also gives an overview of inkjet printing technology and its applications. Optically variable devices are introduced in this chapter as well.

• Chapter 3 presents techniques and methods that are employed to fabricate and replicate the nanostructured substrates. Besides, the principle of manufacturing structural colour images is explained and the concept of inkjet printing OVDs is introduced. To demonstrate these OVDs, experimental results at the end of the chapter are shown.

• Chapter 4 shows how modifying surface wettability affects the behaviour of drops. It is also demonstrated that dots of ink can be precisely controlled to yield high resolution printing. The experimental results are then compared.

• Chapter 5 presents how different concentrations of the silver nanoparticle ink and Ethylene glycol impact film thickness. It is studied how to minimize the size of printed

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• Chapter 6 demonstrates that printed images with higher resolution and uniform droplets that form at the desired locations on the substrate is achievable by controlling the dot and line spacing. Results are shown that drop and line spacing control the applied amount of ink and govern the spreading behavior of the ink on the substrate.

• Chapter 7 investigate different polymers to find the best candidate in order to obtain higher resolution colourful images. Besides, shapes and morphologies of silver printed dots on the different nanostructured substrates are studied to find which one gives better gratings efficiency. The results of this study are demonstrated and compared in each section.

• Chapter 8 gives the summary of the work

• Appendix A shows material and experiments set-up and supplementary information is accessible in Appendix B.

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Chapter 2. Literature review

2.1. Colour of light

Electromagnetic radiation with a wavelength that can be visualized by naked eyes is referred to light. The wavelength of light that human can perceive is between 380nm and 750nm and called visible spectrum, It is showed in Figure 2.1. The different wavelengths are absorbed by the naked eyes and then the colour is recognized by the brain, for instance, red at the longest wavelengths of about 700nm to violet at the shortest wavelengths of about 400nm.

Figure 2.1 The electromagnetic spectrum demonstrating wavelengths that can be observed by the human eye.

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from the interaction of light with the physical structure of a system, which is usually referred to as structural colour.

2.2. Structural Colour

Structural colours were first observed in the natural world, an example of which is seen in Morpho butterflies, beetles, pearls and moth-eyes [16], when light interacts with the microstructure of an object containing features at a scale comparable to the optical wavelength [17]. There are many optical processes to manufacture structural colours, including thin-layer interference, diffraction grating, light scattering, photonic crystals, etc. Its long-term resistance to discolouration allows them to have a wide range of applications due to chemical changes and pigment-free colouration. Specifically pigment-free colouration is desirable from ecological viewpoint. Figure 2.2 shows a Morpho butterfly that is well-known for showing such structural colours. Discrimination of these micro-size patterns cannot be seen by the naked eye and a mixture of these colours appears in our eyes as this moth demonstrates colour mixing in the wing. An analysis of the optical behaviors of the Morpho’s wing has been reported by S.Kinoshita et al [17].

Figure 2.2 (a) Photograph of a Morpho butterfly (Morpho didius), SEM images of the wing microstructure in cross-sectional (b) and top (c) views. Schematic of the Morpho-blue wing microstructure in (a) side and (b) top view: (1) Interference in a single multilayer (shelf). (2) Diffraction on narrow structure. (3) Randomness in height suppresses the multicolour. (4) Narrow gap results in high reflectivity.

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2.2.1. Diffraction

Diffraction happens when a light beam pass through small slits around obstacles, or by sharp edges [18]. Diffraction is the interference effects resulting from combining many waves or continuous sources and it is explained based on Huygens’ principle[19].

Figure 2.3 Diffraction from an aperture and Fresnel and Fraunhofer regions

Diffraction is also defined as any deviation from geometrical optics due to the obstruction of the light’s wavefront by some opening, which leads to being categorized in far-field (Fraunhofer) and near field (Fresnel) diffraction. Assume that the distance from the screen to the aperture is denoted as Z, and the aperture has a diameter of ρ. If light with the wavelength of λ passes through the aperture and diffracts; the Fresnel diffraction occurs when ρ2≥ λz and the wavefronts are not planar. When ρ2<< λz and the wavefronts are planar, is the characteristic of the Fraunhofer diffraction. Figure 2.3 demonstrates the Fresnel and Fraunhofer regions when light passes through apertures and diffracts.

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Figure 2.4 Diffraction by a grating consisting of a periodically corrugated surface. The incident angle is the diffracted angle is, and the grating period is d.

Figure 2.4 illustrates the situation which is ruled with a periodically structured surface or sharp edges of an object occur, that produce diffraction. It is referred to a diffraction grating that separates all the components of incident light. The diffraction pattern created by the grating is given by the equation 2.1.

d(sin θi + sin θm) = mλ Equation 2.1 • θi is the angle of the incoming light,

• d is the spacing of the grooves, and

• m is an integer which can be positive or negative

Diffraction grating can be either reflective or transmissive. A transmission grating has slits ruled onto a transparent material in order to let light pass through the grating and spread into sets with spectra on either side of it; in a reflective mode, a metal is usually coated on the surface to increase the reflection of light [20]; since it doesn’t allow light to pass through the grating, it offers a wider range of spectrum from ultraviolet to infrared. To obtain a bright structural colour patterns, it requires a diffraction grating that are highly periodically arranged [21].

2.2.2. Interference

Overlapping of two waves can be either constructive or destructive. In constructive interference, the amplitude of the two superimposed waves is greater than

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the two original ones. However, the resultant amplitude in destructive interference is smaller than either wave.

In a case of interference in thin film, a plane wave is incident upon a thin film with the thickness of d and the refractive index of nb and the angles of incidence and

refraction of θa and θb, respectively. Interference of light occurs after reflecting from the

two surfaces. Depending on the refractive index value of the material attached to the thin film, the interference condition can be varied. The difference stems from the reflection at a surface resulting in a phase change by 180°. If the light passes through a material with a smaller refractive index to that with a higher one, yet the reverse action does not happen. Here it is an example where incident light passes through a soap bubble. The condition of constructive interference for the soap-bubble is given by:

2nbdcosθb = (m- !_! )λ Equation 2.2

Where m is the integer and

λ

is the wavelength of the maximum reflectivity [10].

When incident light pass through a material with higher reflectance, a destructive interference occurs at the same condition; constructive interference condition obtains where the relation 2nb dcosθb = mλ is satisfied. That is why, it is important to choose proper reflectivity at the surface to lead the thin-film inference serves well as a colouring method [2].

2.2.3. Scattering

Scattering can be defined as the redirection of the incident light out of the original direction due to its interaction with molecules and particles. The wavelength of the incident light, the size of the scattering particle and the practical properties relative to the surrounding medium are parameters that govern scattering. Scattering occurs when light with different wavelengths interferes with each other in either constructive or

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The colours of nature are produced by a different aspect of the interaction of light with irregularities like particles. The interaction can be the result of scattering of particles smaller than the wavelengths of visible light. Rayleigh proposed that the blue colour of the sky is owed by scattering of light via atmospheric molecules. The preferential scattering of short wavelength blue colour compared to the scattering longer wavelength of red light is referred to Rayleigh scattering, which leads to creating the blue colour of the sky [23]. Based on Tyndall scattering, small colloidal particles in a suspension could lead to scattering of blue light (shorter wavelengths) compared to red light. In contrast, Rayleigh scattering is a result of the scattering of small particles with in the size of molecules, while Tyndall scattering is created by small particles down to the size of visible wavelengths. But they are similar in that the intensity of the scattered light depends on the fourth power of the frequency, so blue light is more efficiently scattered than red light.

2.3. Inkjet printing

Printing is a kind of technology that has been utilized for creating patterns and images since ancient times. Till recently, it has been entirely based on physical contact between a transfer part and a substrate. In terms of printing technologies, inkjet printing has the distinctive term for a non-contact method, ejecting material directly from nozzles and deposited on a substrate according to the desired colour, image, or patterns [23]. Inkjet printing technology has been frequently used as a printing technology because of being either fast or versatile. Since the pattern is generated using software, inkjet printing is classified as a digital printing method, which allows for limitless flexibility in pattern definition and rapid pattern adjustment.

2.3.1. Principle

The nozzles and substrate can be moved while small droplets of material from the nozzle are directed toward the substrate in order to print high-quality images. Drop placement is controlled by high-frequency digital signals from the computer and droplets are formed upon the application of a controlled pressure on the material.

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2.3.2. Inkjet technologies

Inkjet printers can be divided into two main classes: continuous inkjet printer (CIJ) and drop-on-demand (DOD) based on the generation of sequences of droplets. A DOD printer only ejects droplets when required, while an inkjet (CIJ) printer forms a continuous stream of droplets. They are further subdivided into a subclass as presented in Figure 2.5 [24].

Figure 2.5 The representation of categories of inkjet printers.

Continuous (CIJ)

CIJ produces a continuous stream of drops using the Rayleigh instability [25] a liquid column ejected through a small nozzle. The nozzle is held at a potential relative to ground that transfers a small charge onto each drop (as shown in Figure 2.6). Individual drops are steered by applying another potential to deflector plates. Drop diameters are normal > 50 µm and are slightly larger than the diameter of the nozzle. CIJ printers produce a continuous stream of drops; unwanted drops (when no printing occurs) are deflected into a gutter and are normally recycled in many graphics applications to prevent waste. Drop generation rate can be > 50 kHz and drops are ejected at velocities > 10 m/s. Although CIJ produces the greatest volume of ink per minute, it is limited in terms of placement accuracy. Its main application is in product marking and coding.

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significant ink wastage and if recirculation is used, there is a potential for ink contamination.

Thermal (DOD)

Thermal inkjet printers use currently to heat ink and apply it to a medium. Nozzles heat up a small amount of ink using a resistor connected to a power supply and expanding it in a bubble (shown in Figure 2.6). The bubble applies a high pressure (>1 Mpa) wave to the ink chamber, causing collapsing the ink out of the nozzle, in this manner a droplet is formed. The bubble pops due to cooling the heater and generate a pressure wave which separates the droplet from the nozzle. Thermal inkjet system is generally used as household printers due to its low cost. The ink properties may modify because of thermal systems which is a drawback of thermal inkjet printing technology. As a result, thermal inkjet printers are more compatible with the conventional inks for graphics and text.

Piezoelectric (DOD)

Piezoelectric printers function by piezoelectric material located in the ink-filled chamber instead of using a thermal element to avoid thermal cycling issues. Therefore, a piezoelectric inkjet printer is better-suited to the printing of a wide range of ink formulation and material; also, with Piezo inkjet printers, no issues have arisen from a location or buildup of ink residue in the nozzle. However, such design printheads are more challenging to manufacture than thermal (DOD) due to using the specialized materials. A piezoelectric crystal causes the deformation of the ink chamber by applying a voltage to force droplets out of the nozzle; not only peak voltage, but also voltage pulse length, and even the shape of the voltage-time waveform applied to each piezo element can play an important role in the drop formation including drop size and shape. During printing using Piezo DOD, integrated software controls the heads to apply between droplets of ink per dot, only where needed. The flexibility and applicability of the piezoelectric inkjet printer to functional materials on various substrates has given rise to a great interest for a study of ink development for the production of electronic components.

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Fujifilm-Dimatix DMP2831 Dimatix Materials Printer

A piezoelectric printer produced by Fujifilm-Dimatix (formerly Dimatix), is the Dimatix Materials Printer 2831 (DMP2831), shown in Figure 2.6. This piezoelectric inkjet printing is a laboratory tool, which is designed to be convenient to use a limitless number of inks and substrates. The printheads’ sixteen 21.5-µm-diameter, 254-µm-spaced nozzles deposit droplets of either 1 pL or 10 pL in volume, depending on the type, contribute to print quality. A voltage waveform plays a critical role in controlling the size, shape, and speed of drops. Also, a printer is capable of controlling drop spacing by angling nozzles along the printhead. It consists of a fiducial camera that allows the pattern alignment and the screening, at the micro-scale, of the printed pattern. A second camera called the drop watcher, provides direct viewing of jetting nozzles, and the actual jetting the fluid; voltage waveforms can be simultaneously modified and can view the changes in jetting characteristics. Then, the substrate can be placed on the platen at the desired location to print more complicated structures. The platen’s temperature can be set floating or raised up to 60 °C. It also provides a vacuum system with many holes in order to keep the substrate fixed during the printing. A software provided with the product allows the DMP2831 to control the substrate’s, cartridge’s and platen’s parameters (the print origin, the platen, drop spacing, layers, temperature or the cartridge height).

Figure 2.6 (a) Fujifilm-Dimatix DMP2831 Dimatix Materials Printed. (b) a platen that a substrate can be place on and (c) a disassembled cartridge and printhead assembly. Reprinted with permission.

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2.3.3. Application

Inkjet printing has gotten the scientific community’s attention because of being able to print fast and flexibly, as well as being cost-effective. Thus, it has been applied in an wide range of applications, such as organic light emitting diodes (O-LEDs)[27], solar cells [28], electronic devices[29]-[30],polymer displays[31], rapid prototyping [32], biochemical arrays [33], biotechnology [34], organic semiconductors [35], etc.

Direct patterning has emerged to offer an alternative to conventional photolithography for fabricating micro-patterns. Among the existing direct patterning techniques, inkjet printing has become an attractive technology due to low manufacturing costs, efficient use of materials, a decrease in the number of process steps [36]–[38] and a possibility for mass customization [39] .

In this work, inkjet printing technology has been incorporated into manufacturing OVDs that commonly use an anti-counterfeiting application instead of using expensive fabrication techniques such as electron beam lithography. Our group introduced inkjet printed OVD technology that implements high-resolution inkjet printing to pattern a nanostructured substrate to manufacture colour images and OVDs [40]. To fabricate functional device structures via inkjet printing, it may need to pre-pattern substrates at many levels of processing.

2.4. Optically variable devices (OVDs)

One type of optical device commonly used in anti-counterfeiting applications is the optically variable device (OVD) that can display shifting colours under different viewing angles. Advanced nanofabrication methods need to be employed to fabricate these structures. These methods are not accessible everywhere, thus OVDs are preferable from security and authentication viewpoints [16], [41]. The three important types of optically variable devices which are based on structures with dimensions in the order of the wavelength of visible light are including multiple diffraction gratings, holograms, and thin film interference filters. Except for holography, other concepts have been explained in the above section.

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Typically, a hologram is a photographic recording of a light field, rather than of an image formed by a lens, and it is used to display a fully three-dimensional image of the photographed subject, which is seen at a contact angle and upon illumination by light. In this technique, when two beams of coherent light interact, the desired image is recorded on a thick emulsion plate. In detail, in order to create a hologram, one portion of a laser beam is exposed to the emulsion plate and the rest of the laser beam is scattered to the film after it hits an object. Moreover, the interference of two coherent laser beams is responsible for creating interference patterns

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Chapter 3. Principle of molding inkjetted Silver on

nanostructured surfaces

Optical nanostructures can manipulate light at a scale much smaller than the wavelength and allow for engineering of light-matter interaction in unique fashions [34], [42]–[45]. In recent years, colour printing and colour displays using nanostructures as pixels have become fields of intensive research [46], [47]. Colour images printed with nanoscale pixels at ultra-high resolution (100,000 dots per inch) has been demonstrated [46]. High-density nanofeatures that can change colours and intensity by changing the viewing angles have been applied in optically variable devices (OVDs) for optical document security applications [48], [49].

In most existing technologies to construct colour images and OVDs using nanostructures, nanoscale accuracy is required in fabricating such structures. The low throughput of nanofabrication techniques is the bottleneck that limits the achievable scale of images. To overcome such challenges, one route is to develop high-throughput nanofabrication techniques, for example, high-speed parallel nanoscale mask-less optical lithography. However, such types of equipment have not become common in most fabrication facilities yet. An alternative route is to improve the throughput using existing techniques.

A novel technology has been introduced which combines high throughput nanoimprint lithography and mature microscale patterning techniques [40][12] called ’NanoMedia’ under the supervision of Prof. Kaminska. The essential idea of NanoMedia is to replicate nanostructured substrates patterned with structural pixels from a universal master stamp using fast, parallel, low-cost nanoimprint lithography and implement

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microscale patterning techniques to selectively activate or deactivate the pixels to produce colour images.

3.1. Structural colour pixels

Structural colour pixels depend on the interaction of light with a physical structure containing nanoscale features smaller than the wavelength of light [22]. Structural colour pixels, including gratings, interference, and diffractive sub-wavelength structures generate colours due to light diffraction and light-matter interactions. In other words, the colour of the object is the main outcome from the selective reflectance and absorption of the incident light. Most structures can alter from transparent to brightly coloured as the angle of incident light on the structures changes [50], [51]. In general, structural colour can be divided into iridescent and non-iridescent [21]. The colouration of structures that changes with the viewing angles are called iridescent colours [52]. However, there are no shifting colours under different viewing angles for non-iridescent [53].

The structural colours have been extensively used to create coloured images as they present unique optical effects. The most common nanofabrication methods that can be used to generate structural coloured pixels and create a colourful image, are with electron beam lithography (EBL), focused ion beam (FIB) machining, and laser writing methods like laser interference lithography (LIL). As previously illustrated [41], the structural colour pixels fabricated by these methods can be used as a master stamp and large quantity of similar structures can be replicated from the master stamp. Since a master stamp is further used for mass replication of similar structure, it is essential to be mechanically and chemically durable and adoptable with replication methods; for example, nanoimprinting such as quartz, glass, clear plastic sheets and transparent polymer resins curable with heat or UV rays.

As discussed, the essential idea is to replicate nanostructured substrates patterned with structural pixels from a master stamp using fast, parallel, low-cost

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3.2. Nanostructured substrate: A generic pixelated stamp

To manufacture a colour image using the substrate with nanopixels can be very time-consuming and costly with current fabrications, as specific colour pixels need to be located at the corresponding locations of the substrate to create a desired colour image. It is also showed, each colour image requires a completely new master stamp for each colour image, which restricts the applications of nanostructures as effective visual information media [54]. An alternative way to produce structural colour images with embedded covert information has been presented [12]; a substrate is composed of subpixels manufactured by nanostructure and subpixels are originated in red (R), green (G), and blue (B) – primary colours for showing visible colours, one such substrate is referred to as a “nanostructured substrate”. There are various types of nanostructured substrates for displaying primary colours, although only a 2D diffraction grating structure is considered with either NWAs or NCAs geometries. It is the periodicity of NCAs/ NWAs that leads light to diffract certain colours into a certain direction.

Figure 3.1 Illustration of a nanostructured substrate.

In the current research, the pixel bands that give red, green, and blue primary colours are repeated into a 1-D array. Each band is 70µm wide and the size of the experimental nanostructured substrate is 2.0cm × 1.5cm, as shown Figure 3.1. The structural pixels in this study are composed of either dielectric nanocone arrays (NCAs) (nanostructured substrate #1) or nanowell arrays (NWAs) (nanostructured substrate #1) as shown in Figure 3.3 and Figure 3.2, respectively. On the nanostructured substrate #1

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and #2 used in this work, periods for red, green and blue subpixels are 640nm, 545nm, and 455nm, respectively for the 1st grating order and white light is incident at 80° (i.e., θ = 80°).

Figure 3.2 Details of a nanostructured substrate. (a) the diagram illustrates the layout of R,G and blue colour pixel bands as well as the SEM images display the structural details of the nanowells array pixels and the scale bar is 500nm in length. (b) physical process to display a refection colour in nanowells structure.

The physical process to display a reflection colour in a grating structure becomes 𝑛⋀(𝑠𝑖𝑛α - sinβ) = 𝑚𝜆 Equation 3.1

Where n is the refractive index and α is the angle of incidence. As the grating structure with a periodicity of ⋀ reflects particular wavelengths λ into the normal direction as mth

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Figure 3.3 Details of a nanaosubstrate. (a) the diagram illustrates the layout of R,G and blue colour pixel bands as well as the SEM images display the structural details of the nanocones array pixels. The scale bar is 500nm in length and the scale bar is 500nm in length. (b) physical process to display a refection colour in nanocones structure.

3.3. Fabrication of a nanostructured substrate

Dr. Hao Jiang has shown the fabrication methods of nanostructured substrate #1 and #2 which are used in current research. The nanostructured substrate #1 and #2 are both fabricated using electron beam lithography (EBL) and reactive ion etching (RIE). The fabrication process of nanostructured substrate #2 is illustrated in Figure 3.4.

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Figure 3.4 Schematic diagram of NCAs nanostructured substrate fabrication, (a) cleaning of the quartz substrate, (b) evaporation of pf Cr, (c) coating of PMMA, (d) electron beam lithography, (e)developing of the resist,(f) Cr etching,(g) Ti/Ni deposition, (h) lift off on NI/Ti (i) removal of Cr by wet etching, (j) reactive ion etching, (k) NCAs on quartz.

3.4. Replication of a nanostructured substrate

Nanoimprint lithography (NIL) has given rise to a lot of interest for fabricating nanoscale photodetectors, silicon-quantum-dot, quantum-wire, and ring transistors [55], due to its throughput, low costs, and high resolution features down to 10nm. Thus, it is a manufacturing technology. Unlike conventional lithography that uses energetic beams, it relies on direct mechanical deformation, and the chemical and physical properties of the resist material. Thus, the effects of wave diffraction, scattering, and interference in the

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nanosized structures in current research through an imprinting stamp covered by nanoscale structures. There is a different kind of NIL process, as shown in Figure 3.5.

Figure 3.5 Schematic of (a) Thermal nanoimprinting and Schematic of (b) UV-curable lithography.

In thermal nanoimprint lithography (T-NIL), a stamp with nanostructures on its surface is pressed into a thin thermoplastic polymeric resist at a temperature above the glass transition temperature (Tg) of the polymer, followed by removal of the mold. This step duplicates the nanostructures on the mold in the resist film. In UV curable lithography method, a UV-curable polymer resists and cast on the substrate and then the pressure is applied on it to fill the substrate by the resist. In the next step, the resist is cured by a UV source, at that point, it can be detached from the stamp. This step transfers the thickness contrast pattern into the entire resist [23].

3.5. Structural colour images

In order to build a structural colour image, a generic stamp of nanostructured substrate which is composed of R, G, B nanopixels is fabricated, followed by the replication patterned nanostructured substrates from the master stamp using fast, parallel, low-cost nanoimprint lithography and implement low-cost micro-scale patterning of the RGB

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nanopixels for each colour image as shown in Figure 3.6. A wide range of patterning technologies has been developed such as optical lithography, electron beam lithography [56], diamond turning, laser ablation, micro-contact printing and nanotransfer printing. There are several different types of micro-patterning techniques including inkjet printing, optical lithography, photography, intensity control layer. In recent years, inkjet printing has increasingly become popular as a scalable micro-patterning technology due to its precise flexible control, and cost-effective additive process [57]. Moreover, it has important applications in fabricating biological, optical, and electrical devices

Figure 3.6 Process of colourful OVDs production.

3.6. Concepts of inkjet printed optically variable devices

In our technology, we need to have a prefabricated substrate and then use inkjet printing to selectively activate or deactivate special pixels to manufacture OVDs or colour images. As discussed, the substrate is patterned with primarily colour pixel bands. It is mainly based on the following steps: (a) fabricate a master stamp that is composed

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a pattern on a nanostructured substrate in accordance with the desired colour image; (d) utilizing index matching material to laminate the result.

The inkjet printer is governed by the software that allows the printer to control substrate's parameters, such as the platen, drop spacing, layers. In this research, in order to inkjet print an OVD or colour image according to an original image, a custom-written MATLAB script is used to generate a pattern according to the colour image and the nanostructured substrate configuration. After generation the binary pattern, the nanostructured substrate is placed on the platen at the desired location and the complicated pattern is printed using an alcohol-based nanoparticle silver ink.

Figure 3.7 Schematic of molding inkjetted silver on nanostructured surfaces processes.

It is clear that the green colour is obtained by activating only green pixels, as illustrated in Figure 3.7. To form a letter 'F' in green, only green pixel bands were occupied with printed silver dots. When the droplets of silver ink dried on the nanostructured substrate, it is laminated with index matching materials to deactivate the background colours. Finally, the printed green letter ’F’ was formed (Figure 3.7) and it can be viewed under a defined viewing angle. In this study, we demonstrate a unique structural colouration technique, referred to as Molded Ink on Nanostructured Surfaces (MIONS), based on molding the shape of the printed ink material on top of the

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surface-supported transparent polymer nanostructures to display the structural colours, as shown in Figure 3.7.

3.6.1. Index matching

Index matching is a simple technique to significantly improve the overall optical clarity of a printed OVD. UV polymer is deposited onto the inkjet printed nanosubstrate and then exposed to UV light to be cured. Figure 3.8 illustrates a schematic cross-sectional view of the MIONS printing process, where (i) demonstrates the bare nanostructured substrate #1, (ii) the inkjet printed one silver dot on the nanosubstrate, (iii) apply Oxygen plasma to avoid separation, and (iv) UV glue are casted on the substrate and then cured by UV source.

Figure 3.8 Cross-sectional view of the printing and index-matching processes.

The NCAs that are occupied with printed silver thin film stay active in diffracting light because silver gives strong reflection from the nanocone surface. While the

Structural Colour Images Inkjet Printed on Polymer Substrates Patterned with Nanostructural Pixels