저작자표시

Photonic crystals (PCs) can manipulate electromagnetic waves by designing artificial periodic dielectrics, which provide various applications such as sensor, LED, laser, and optical computers. Recently, many studies have focused on display applications by designing photonic band gaps, while the fabrication of photonic crystals at a nanoscale is still challenging. This work describes a simple method for the fabrication of mono-layer self-assembled photonic crystals (SAPCs) using inkjet material printers capable of injecting nanoparticle suspension onto a substrate in contrast to conventional methods such as photolithography, two-photon patterning and direct-write assembly.

To make homogeneous monolayer structure, crucial factors affecting the micropatterns of a colloidal assembly of the inkjet-printed droplet were investigated, including the wettability of the substrate and chemical composition of ink to see what effect they might have on producing structure color. These patterned photonic crystals provide stealth capability to avoid pattern detection by counterfeiters under daylight illumination and generate multiple colorful holograms at different viewing angles for complex cryptography. In addition, the number of SAPCs in a matrix format controls the optical intensities, providing an additional anti-counterfeiting feature.

Because the inkjet-printed SAPC platforms offer simple and unique optical properties, it is believed that the approach can be widely used for anti-counterfeiting systems. A weak light is used for the three images on the left, while a strong light is used for the three images on the right.

Introduction

Motivation

Structure of the Thesis

Characterization Techniques

Background and Literature Review

Inkjet Printing Technology

Inkjet printing techniques
Required properties of inks

The formulation of suitable inks is a critical part of inkjet printing because the ink and its viscosity and surface tension determine the dotting characteristics and pattern quality[23]. However, the formulation for the ink depends on the printer used and the material to be printed. In the case of surface tension affecting drop shape, typical surface tension values range from 28 mNm-1 to 32 mNm-1[28].

In addition, the time stability of the suspension is an important parameter, because the particles in suspension must maintain a uniform dispersion without aggregation during the entire time of the printing process.

Photonic Crystal for Encoding

Experimental

Printing of Nano Silica Suspension

Material printer system
Preparation of SiO 2 suspension inks
Substrate pretreatments

Monodisperse regular silica particles (10% w/v) with diameters of 400 nm and 500 nm (Polysciences, Warrington, USA) were used. The following centrifugal steps were used to manipulate the final working particle concentration. First, 1000 µL of the delivered silica particle solution was centrifuged at 4000 rpm for 12 min, and then the solution was removed for high particle suspension concentration (>20% w/v) or diluted with deionized water for low particle concentration. suspension (< 10% w/v).

Next, the particle suspension was mixed with FA (Sigma Aldrich, Korea) in a ratio of 4 to 1 in volume to control evaporation time for better self-assembly[18]. The prepared, mixed particle suspension was ultrasonicated for 10 minutes for complete dispersion (5510E-DTH, Bransonic, USA). Four different substrates were used to investigate the effects of hydrophobicity and hydrophilicity of substrates on crystallization of nano-silica suspension such as slide glass, Si wafer, polydimethylsiloxane (PDMS), polypropylene (PP) and the substrates were cut with a diamond knife.

All substrates were carefully cleaned in acetone and IPA, washed with deionized (DI) water, and then dried with nitrogen gas.

Patterns from Inkjet Printing

Formation of dot and line pattern
Behavior of a droplet on the substrate
Mono-layered, self-assembled structure using Marangoni effect
Multiple injection on glass and PDMS

Once the suspension was injected onto the surface, it appears to spread due to the force of the drop. In the case of a hydrophilic surface, the water-based drop becomes asymmetric due to the high surface energy: the edge of the drop becomes thin, while the center becomes relatively thicker (Fig. 3.3a). Because the evaporation rate at the edge is much faster than in the center region of the drop, capillary flow is generated to replenish liquid at the edge.

As a result, the particles are transported to the edge by outward flow and produce a coffee ring-shaped pattern in which several nanobead layers are assembled only along the periphery itself (Fig. 3.3c-i) [32]. In contrast, the same nanobead suspension was repeatedly injected onto the PDMS substrate and the self-assembled nanobead structures were dome-shaped as shown in Figure. As a result, the surface area of the self-assembled dome-shaped nanostructure is much smaller. then on a hydrophilic surface (Fig. 3.3c-ii).

In order to obtain mono-layer structure, several experiments were designed to investigate surface wettability, the concentration of nanobeads and effects of solvents as shown in Fig. First, water-based two nano-bead suspensions at 10% (w/v) and 20% (w/v) on a hydrophilic glass with contact angle 10° were studied. This means the manipulation of both the solvent system and the number density of silica particles enabled monolayer structure on hydrophilic substrates.

In fact, these results indicate that solvent system (water/FA mixture) is more crucial than concentration in controlling the evaporation time and making a homogeneous monolayer: FA has higher boiling point than water, while water has higher surface tension than FA. When using solvents containing water/FA mixture, the water preferentially evaporated at the edge. However, the resulting surface tension gradient causes Marangoni flow from the edge to the center because the FA concentration is higher near the periphery, reducing the surface tension [ 18 , 33 ].

With glass, the number of layers stacked on glass is almost equal to the number of injections (Fig. 3.5a).

Fig. 3.3 a-b, Schematic of evaporation process in a droplet and different nanostructures on hydrophilic and hydrophobic surfaces

Coloration of Self-assembled Nanobeads on Various Substrates

Results and Application

Characterization of Self-assembled Photonic Crystal Patterns
Theoretical Analysis on Spectra of Mono-layered Photonic Crystals
Spectrum Measurement
Application of SAPCs for Anti-counterfeiting System

First-order (m = 1) and second-order (m = 2) diffractive beams of photonic crystals with d = 500 nm are obtained by simple numerical calculation, as shown in Fig. However, if the periodicity falls on a sub-wavelength scale below 400 nm, human eyes cannot sense the diffractive rays from the patterned photonic crystals. Specifically, 2D self-assembled, mono-layer photonic crystals were developed by using substrate wetting and chemical composition of silica particle suspension.

Hu, H., et al., Photonic anti-counterfeiting using structural dyes derived from magnetically responsive photonic crystals with dual photonic bandgap heterostructures. Han, S., et al., Lithographically encoded polymer microtag using high-performance and error-correctable QR code to prevent drug counterfeiting. Chen, L., et al., Temperature-sensitive luminescence of (Y, Gd) VO4:Bi3+, Eu3+ and its application to covert anti-counterfeiting.

Arsenault, A.C., et al., From color fingerprinting to control of photoluminescence in elastic photonic crystals. Lee, I., et al., Quasi-amorphous colloidal structures for angle-independent electrically tunable full color photonic pixels. Jeon, S., et al., Fabrication of three-dimensional nanostructures using two-photon lithography in one illumination step.

Cumpston, B.H., et al., Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication. Wang, L., et al., Inkjet-printed colloidal photonic crystal microdot with fast response induced by hydrophobic transition of poly (N-isopropylacrylamide). Jeong, S., et al., Controlling the thickness of the surface oxide layer on Cu nanoparticles for fabrication of conductive structures by inkjet printing.

Kim, D., et al., Direct writing of silver conductive patterns: improvement of film morphology and conductivity by controlling solvent compositions. Hu, H., et al., Invisible photonic printing: computer design of graphics, UV printing and displayed by a magnetic field. Yao, J., et al., Selective appearance of various laser-induced periodic surface structure patterns on a metal surface using structural colors produced by femtosecond laser pulses.

Deegan, R.D., et al., Capillary flow as a cause of ring staining by dried liquid droplets. Bhattacharya, S., et al., Investigation on the surface wettability of poly(dimethyl)siloxane (PDMS) and glass under oxygen plasma treatment and correlation with bond strength.